A method of forming a heap structure from acid producing rock material

Abstract

A method of forming a heap structure from acid producing rock particles such as coal mining waste rock, the method comprising mixing acid producing rock particles with acid neutralizing rock particles such as coal fine waste to provide a mixture, adding a bacteria culture to the mixture and irrigating the mixture with a cementation solution. The bacteria may be a urease producing bacteria such as sporosarcina pasteurii and the cementation solution may comprise urea and calcium chloride. The method results in calcite precipitates which form cementation bonds between the particles which improves compressive strength and stability of the heap and reduces permeability of oxidants therefore reducing acid rock drainage (ARD). Ideally, the mixture is formed by alternating layers of acid producing rock and acid neutralising rock in a mass ratio of 3:2 of acid rock particles to acid neutralising rock particles. Irrigation by the cementation solution is preformed by a gravity drip system with drip emitters spaced about 0.5m to 2m.

Description

FIELD OF THE INVENTION This invention relates to a method of forming a heap structure from acid producing rock material, and in particular acid producing waste rock particles, in order to render the waste rock particles more environmentally compatible. In this specification “rock particles” has its widest meaning and includes fine and coarse particles alike. BACKGROUND TO THE INVENTION Conventional mining practice consists of excavating mineral borne rock from the ground and extracting the desired minerals from the rock, usually by chemical processes. In the process of mining, substantial amounts of rock that does not contain sufficient minerals to warrant extraction must first be removed. This rock is called waste rock. Waste rock from mining operations commonly consists of sulphide minerals of which pyrite is typically the most abundant. When natural elements, rainwater and oxygen, come into contact with these minerals, it is converted into sulphuric acid by hydrolysis and oxidation. The acidic leachate produced is called Acid Rock Drainage (ARD) or Acid Mine Drainage (AMD) and is detrimental to the environment. There are several methods to control or mitigate ARD including remediation and prevention. Remediation strategies include treating acid mine drainage until it is neutralised. Hydrated lime is commonly used by the coal mining industry because it is effective and relatively inexpensive. However, the major disadvantages are the significant volumes of sludge produced that requires further storage or disposal. ARD prevention strategies aim to eliminate the process of sulphide oxidation at source. The most cost-effective ARD prevention strategy is to use a cover, either wet (water) or dry (soil) which is placed over the waste heap to minimize penetration of air and water through the heap. The use of water covers requires significant volumes of water to be monitored and maintained, which is not always achievable in water scarce areas, where mines are often located. Further, these covers cannot always completely stop the oxidation process and generation of ARD. There is accordingly scope to address the aforementioned problems and deficiencies at least to some extent. The preceding discussion of the background to the invention is intended only to facilitate an understanding of the present invention. It should be appreciated that the discussion is not an acknowledgment or admission that any of the material referred to was part of the common general knowledge in the art as at the priority date of the application. SUMMARY OF THE INVENTION According to an aspect of the present invention there is provided a method of forming a heap structure from acid producing rock particles, the method comprising: mixing acid producing rock particles with acid neutralizing rock particles to provide a mixture, adding a bacteria culture to the mixture, and irrigating the mixture with a cementation solution, wherein the bacteria culture is selected to cause calcite precipitates to form cementation bonds between rock particles which increase overall compressive strength of the structure and reduce permeability of oxidants therethrough. Further features provide for the mixture to be formed by a layer of acid producing rock particles on top of a layer of acid neutralizing rock particles or vice versa. Further features provide for acid neutralizing rock particles to have a relatively smaller particle size distribution compared to the size distribution of the acid producing rock particles. Further features provide irrigation to be provided by a gravity drip system which includes spaced apart drip emitters for allowing the cementation solution to drip onto the structure. The drip emitters may be placed between 0.5 m and 2 m apart and preferably 1 m apart. Further features provide for the acid producing rock particles to be non-hygroscopic and acid neutralizing rock particles to be hygroscopic. The acid neutralizing rock particles may be less pervious to water and air than the acid producing rock particles. The acid neutralizing rock particles may further be a mine wastes with a low sulphide content. The acid neutralizing rock particles may contain neutralizing minerals including calcite, kaolinite, or gypsum. Further features provide for the quantity of acid producing rock particles to be greater than or equal to the quantity of acid neutralizing rock particles in the mixture. The quantities of acid producing rock particles and acid neutralizing rock particles in the mixture may be related by a mass ratio of between 1:1 to about 5:1 and preferably a mass ratio of about 3:2. Further features provide for the quantity of acid producing rock particles to be less than the quantity of acid neutralizing rock particles in the mixture. The quantities of acid producing rock particles and acid neutralizing rock particles in the mixture may be related by a mass ratio of about 2:3. Further features provide for the bacteria culture is urease producing bacteria and preferably, Sporosarcina pasteurii. Further features provide for the cementation solution to be made up of urea and calcium chloride and preferably 1 M urea, 0.5 M calcium chloride and 3% ATCC® 1376 media. Even further features provide for the method to include irrigating the mixture daily, or preferably weekly to allow the cementation solution to permeate through the structure. Still further features provide for the acid producing rock to be coal waste rock and the acid neutralizing rock to be coal fine waste. The coal waste rock may have a particle size distribution of 2-4 mm and the coal fine waste may have a particle size distribution of 0.01-1 mm. Embodiments of the invention will now be described, by way of example only, with reference to the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS In the drawings: Figure 1 is a schematic diagram which illustrates an example embodiment of four different bioreactor packing configurations according to aspects of the present disclosure; Figure 2 is a bar graph which illustrates the day on which the bioreactors sealed and no effluent was observed; Figure 3 is a bar graph which illustrates calcite content from the continuous irrigation for all bioreactors, as determined via acid wash tests; Figure 4 is a graph which illustrates effluent volume obtained from I RR and control bioreactors over the 60-day treatment period; Figure 5 is a graph which illustrates effluent volume obtained from AGG and control bioreactors over the 60-day treatment period; Figure 6 is a graph which illustrates effluent pH for I RR and control bioreactors over the 60-day treatment period; Figure 7 is a graph which illustrates effluent pH for AGG and control bioreactors over the 60-day treatment period; Figures is a graph which illustrates effluent redox potential for I RR and control bioreactors over the 60-day treatment period; Figure 9 is a graph which illustrates effluent redox potential for AGG and control bioreactors over the 60-day treatment period; Figure 10 is a bar graph which illustrates calcite content from the non-continuous irrigation, as determined via acid wash tests; Figures 11A to 11C are top-down and cross-sectional images of AGG-WR; Figures 12A to 12C are top-down and cross-sectional images of AGG-LAY; Figures 13A to 13B are top-down and cross-sectional images of IRR-FW; Figures 14A to 14B are top-down and cross-sectional images of IRR-LAY; Figures 15A to 15C are top-down and cross-sectional images of IRR-BLND; Figures 16A to 16C are top-down and cross-sectional images of CONTROL-BLND; Figures 17 to 17B are top-down and cross-sectional images of CONTROL-LAY; Figure 18 is a graph which illustrates effluent volume obtained from I RR and control bioreactors over the 60-day cementation period; Figure 19 is a graph which illustrates effluent volume obtained from AGG and control bioreactors over the 60-day cementation period; Figure 20 is a graph which illustrates effluent pH for I RR and control bioreactors over the 60-day cementation period; Figure 21 is a graph which illustrates effluent pH for AGG and control bioreactors over the 60-day cementation period; Figure 22 is a graph which illustrates effluent volume obtained from all bioreactors over the 90-day SAR treatment; Figure 23 is a graph which illustrates effluent volume obtained from the co-disposed bioreactors over the 90-day SAR treatment; Figure 24 is a graph which illustrates effluent pH obtained from all bioreactors over the 90-day SAR treatment; Figure 25 is a graph which illustrates effluent pH obtained from the co-disposed bioreactors over the 90-day SAR treatment; Figure 26 is a graph which illustrates effluent volume obtained from all bioreactors over the second 90-day SAR treatment period; Figure 27 is a graph which illustrates effluent volume obtained from the co-disposed bioreactors over the second 90-day SAR treatment period; Figure 28 is a graph which illustrates effluent pH obtained from all bioreactors over the second 90-day SAR treatment period; Figure 29 is a graph which illustrates effluent pH obtained from the co-disposed bioreactors over the second 90-day SAR treatment period; Figure 30 is a graph which illustrates effluent pH obtained from the headspace of sealed bioreactors over the first 90-day SAR treatment period; Figure 31 is a graph which illustrates effluent pH obtained from the headspace of sealed bioreactors over the second 90-day SAR treatment period; Figure 32 is a bar graph with illustrates expected bioreactor neutralisation lifespan over the 180-day SAR treatment period; Figure 33 is a bar graph with illustrates Fe2+, Fe3+, and Total Fe content in the leachate of each bioreactor over the cementation period; Figure 34 is a bar graph with illustrates Fe2+, Fe3+, and Total Fe content in the leachate of bioreactors over the cementation and 180-day SAR periods; Figure 35 is a bar graph with illustrates Fe2+, Fe3+, and Total Fe content in the leachate of all bioreactors besides CONTROL-WR over the cementation and 180-day SAR periods; Figure 36 is a graph which illustrates effluent volume obtained from bioreactors over the cementation period of the daily irrigation study; Figure 37 is a graph which illustrates effluent volume obtained from bioreactors over the cementation period of the 4-day intermittent irrigation study; Figure 38 is a graph which illustrates effluent volume obtained from bioreactors over the cementation period of the 7-day intermittent irrigation study; and Figure 39 is a bar graph with illustrates final effluent pH of the bioreactors from the daily, 4-day, and 7-day irrigation groups. DETAILED DESCRIPTION WITH REFERENCE TO THE DRAWINGS Aspects of the present disclosure relates to a method of forming a heap structure from acid producing rock particles. The method may include mixing acid producing rock particles with acid neutralizing rock particles to provide a mixture. The mixture may be formed by a layer of acid producing rock particles on top of a layer of acid neutralizing rock particles or vice versa. The heap structure may have multiple alternating layers. Alternately, the mixture may be formed by blending acid producing rock particles with acid neutralizing rock particles. A bacteria culture may be added to the mixture and the mixture may be irrigated with a cementation solution. The bacteria culture may be selected to cause calcite precipitates to form cementation bonds between rock particles in the mixture which increase overall compressive strength of the structure and reduce permeability of oxidants therethrough. For example, the bacteria culture may be urease producing bacteria, preferably, Sporosarcina pasteuhi. The cementation solution may be made up of urea and calcium chloride and preferably 1 M urea, 0.5 M calcium chloride and 3% ATCC® 1376 media. Irrigation may be provided by a gravity drip system which may include spaced apart drip emitters for allowing the cementation solution to drip onto the structure. The drip emitters may be placed between 0.5 m and 2 m apart and preferably placed 1 m apart. This may maximise dispersion causing little or no dead volume in the structure. The method may include irrigating the mixture daily, or preferably weekly to allow the cementation solution to permeate through the structure. The acid producing rock particles may be non-hygroscopic and acid neutralizing rock particles may be hygroscopic. The acid neutralizing rock particles may be less pervious to water and air than the acid producing rock particles. The acid neutralizing rock particles may further be a mine waste with a low sulphide content. The acid neutralizing rock particles may have a relatively smaller particle size distribution compared to that the particle size distribution of the acid producing rock particles. The quantity of acid producing rock particles may be preferably greater than the quantity of acid neutralizing rock particles in the mixture. For example, the quantities may be related by a mass ratio of about 3:2 (acid producing rock particles:acid neutralizing rock particles). The particle size distribution in a mixture of this ratio may enable the smaller acid neutralizing rock particles to migrate or be embedded in voids or pore spaces between the relatively larger acid producing rock particles. This may provide an increase in uniformity in distribution of calcite precipitates in the structure, increase physical and chemical stability of the structure and may further enhance long term neutralization of the structure. In an exemplary embodiment, the acid producing rock particles may be derived from coal waste rock and the acid neutralizing rock particles may be derived from coal fine waste. The coal waste size distribution of 0.01-1 mm. Forming a heap structure using the method as disclosed herein may have the advantage of preventing formation of acid rock drainage. Specific examples are now described with reference to the accompanying figures. EXAMPLE 1 This example illustrates the use of microbially induced calcite precipitation (MICP) in heterogeneous co-disposal waste systems. MICP involves the use of urease-producing bacteria that naturally form calcite via the following three processes: Hydrolysis: CO(NH2)2 + 2H2O 2IW + CO32- (Eq. 1) Dissociation: CaCI2 + H2O -> Ca2++ 2CI' (Eq. 2) Precipitation: Ca2+ + COa2' CaCOa (Eq. 3) Combined with co-disposal, this leads to reduced oxidant ingress, improved structural stability, and an improved neutralization capacity. Twelve bioreactors were setup with different packing configurations to determine the optimum physical conditions for MICP. Coal samples and their characterization The coal waste rock (WR), as an acid producing rock, and coal fine waste (FW), as an acid neutralizing rock, both in particulate form, were used in this example and obtained from coal mines in Mpumalanga, South Africa. The coal waste rock used has a particle size distribution of 2-4 mm and the coal fine wasted used has a particle size distribution of 0.01-1 mm. Acid-base accounting using net acid generating tests and a LECO sulfur analyser showed that the WR (1.32% S) was potentially acid-forming and the FW (0,50% S) was non-acid-forming with high acid-neutralizing capacity as shown in Table 1. Even though the WR was classified as potentially acid forming, the acid-neutralizing capacity and the low net acid-producing potential of the FW was sufficient for use as co-disposal material. Table! Acid and base accounting results for the waste rock (WR), fine waste (FW), and blends of the two materials Sample Acid neutralising capacity (KgH2SO4 / Tonnes) Nett acid producing potential ARD Classification Sulphur (%) WR 29,51 10,88 PAF 1,32 FW 56,55 -41,21 NAF 0,50 3CD:2FW 32,04 -2,28 NAF 0,97 2CD:3FW 24,82 -21,04 NAF 0,81 Bacterial culture Sporosarcina pasteurii, was used in this example. The S. Pasteurri was cultured in ATCC®1376 ammonia-yeast (20 g / L yeast extract, 10 g / L ammonium sulphate, and 0.13 M pH 9 Tris buffer) until the cell density reached 1 x 109 cells / mL (approximately 24 h) and was then used to inoculate the bioreactors. All reagents were obtained from Sigma Aldrich. Cementation solution Cementation media (1 M urea and 0.5 M CaCI2) was made by adding urea (30 g / L), CaCl2’2H2O (73.5 g / L), and ATCC®1376 ammonia-yeast media (3 g / L) to 1 L of deionized water. A fresh batch of cementation media was made every three days. Bioreactor setup The bioreactors were constructed using 60-mL syringes with the plunger removed, and a 1-mm stainless steel mesh placed at the bottom of the syringe barrel. The reactors were packed with a width to height ratio of 1:1.16. The bioreactors were packed in four different configurations that included bioreactors containing only WR (10), only FW (12), a mass ratio of 3:2 WR:FW (14) in a layered configuration (LAY) with alternating layers of WR and FW, and a mass ration of 3:2 WR:FW (16) in a blended configuration (BLND), where WR and FWwere thoroughly mixed prior to packing, as shown in Figure 1. Ten bioreactors were used for the continuous irrigation experiment and a total of 12 bioreactors were used for the intermittent irrigation. Reactors 1-4 were used to investigate MICP formation when inoculated via irrigation, where 20 mL of 1 x 109 cells / mL S. Pasteurii were introduced into the packed reactors over a period of 2 h. Reactors 5-8 involved inoculation via agglomeration, where the coal waste was inoculated and mixed with 20 mL of 1 x 109 cells / mL S. Pasteurii prior to packing the bioreactors. Reactors 9-12 (9-10 for the continuous irrigation) were not inoculated with S. Pasteurii and served as the control. After inoculation, the reactors were left for 24 h before treatment with the cementation solution (1 M urea and 0,5 M CaCh). The first MICP experiment with the co-disposed bioreactors made use of continuous irrigation using a gravity drip system. The cementation solution was irrigated into all bioreactors at a flowrate of 5 mL / h (equivalent to 1.17 L / m2 / h). This was done to minimize disturbance to the waste bed and avoid pressure build up from excess solution volume in the headspace of the bioreactor. Since the reactors sealed too quickly for insightful effluent analyses, an experiment with a slower cementation solution irrigation rate was conducted. Over a 60-days, only 20 mL of cementation solution was added daily over a period of 2 h to ensure that the sealing rate of the bioreactors were slowed down such that effluent could be obtained over a longer period of time and thus provide more information on MICP in these heterogenous coal waste beds. Additionally, since an excess of cementing solution was used in the continuous irrigation method, this slower rate would consume less reagents and thus result in a more feasible method of cementation. Bioreactor monitoring and effluent analyses The effluent from each bioreactor was collected every 24 h and the volume, pH, and redox potential were measured and recorded. The Fe2+, Fe3+, and total iron concentration was determined using the 1,10-phenanthroline assay (American Public Health Association; 1976), where Fe2+ is quantitatively complexed by 1,10-phenanthroline in the pH range between 3 to 9. Additionally, the SO42' content was determined using the spectrophotometric sulfate assay (American Public Health Association; 1975), which is regarded as the most reliable method for microbiological investigations due to its ease of use. Direct cell counting was used to determine the cell concentration in the effluent of each reactor using a Helber Cell Counting Chamber with a Leica DM500 compound microscope at 100x magnification (Leica Microsystems, Switzerland). Bioreactor deconstruction and acid wash tests Following 60 days of treatment with cementation solution, the bioreactors containing the codisposed coal waste were air-dried for a further 60 days. To observe the calcite formation, the airdried bioreactors were then opened axially, using a soldering station (Power Sonic Industrial, South Africa) to minimize agitation to the coal waste packing, and were medially separated into two fractions, where the calcite content of the top and bottom half of the bioreactors were subsequently analyzed separately. The treated samples were then sectioned, further dried at 80°C, and weighed (mbeforewash), before being washed with 1 M hydrochloric acid. The acid served to dissolve any calcite produced during the 60-day cementation period. The washed sample was then dried once more at 80°C and weighed (mafterwash). The weight difference between the sample before and after washing (mbeforewash and mafierwash) was calculated to be the weight of calcite present in the sample (Eq. 1). Since acid-neutralising tailings were used in this example, the calcite already present in the sample was factored into the acid wash calculation to prevent overestimation of the MICP-generated during the treatment stage. CaC03(wt. %) = x 100 (Eq. 4) mafter wash Results and discussion Since the hybrid MICP-co-disposal method aimed to prevent ARD generation by limiting oxidant ingress, cementing solution was irrigated through the beds daily and the effluent was collected and analysed. Following the treatment phase and drying, the bioreactors were then opened to determine the calcite distribution throughout the bed. Continuous irrigation Continuous irrigation of cementing solution into the co-disposed beds represented the first attempt at preventing ARD via the hybrid MICP-co-disposal system. This irrigation method (5 mL / h) yielded sealed co-disposed beds with restricted access to oxidants in the shortest amount of time. Figure 2 shows the day on which the bioreactors were sealed and no effluent was observed thereafter. Continuous irrigation led to the sealing of all inoculated bioreactors that contained FWwith the exception of the agglomerated layered bioreactor (AGG-LAY) and the control FW bioreactor (CONTROL-FW); although the effluent volumes for the AGG-LAY bioreactor were significantly reduced to 5 mL over the 10-day treatment period. As can be seen in Figure 2, the AGG-FW bioreactor was the first to seal on Day 1, followed by the IRR-LAY and AGG-BLND, which sealed on Days 3 and 4, respectively. The IRR-FWand IRR-BLND bioreactors both clogged on Day 6. It is well-known that pore space plays a significant role in MICP; therefore, it was expected that reactors containing FW clogged easily. The AGG-FW bioreactor sealed first and it was attributed to fact that the agglomerated inoculation allowed for an even distribution of microbes and nutrients throughout the bed and thus led to quicker calcite formation. Both of the BLND bioreactors sealed but for the LAY bioreactors, only the IRR-LAY reactor sealed. However, it is important to note that this experiment was only run for 10 days, and this is likely a too short period to draw any significant conclusions. After the 10 days of treatment, the bioreactors were opened and dried, and acid wash tests were conducted on the top and bottom halves of their contents. Figure 3 shows the calcite content of the bioreactors. It displays the calcite concentrations for the top and bottom of the bioreactors, as well as the total calcite formed. This provides insight into the uniformity of the calcite formed within the coal waste and assists in determining whether the calcite could provide the desired effect. It can be clearly seen that the calcite content in the top half of the bioreactors were significantly higher than that in the bottom half for all bioreactors except IRR-CD and AGG-CD. This was observed in many previous studies, where calcite production leads to clogging, which limits oxygen and nutrient availability for S. Pasteurii further away from the surface. This is clearly seen in the AGG-FW bioreactor, which sealed on Day 1 and presented very low levels of calcite, especially in the bottom half of the bioreactor. This is similar for the AGG-BLND, which also sealed early in the experiment. For IRR-LAY, which clogged early (Day 3), significant volumes of calcite were formed; however, the majority of the calcite formed was in the top half of the bioreactor and very little calcite was present in the bottom half. Capillary barrier effect (CBE) prevented the access of oxidants to the lower portions of codisposed LAY beds, and this can be seen in both LAY bioreactors in this example, where the top half of the LAY bioreactors have significantly higher calcite than their bottom halves. A similar calcite distribution was observed for the AGG-BLND bioreactor, but since this bioreactor sealed very early in the experiment, the limited distribution of nutrients and oxygen was attributed to the clogging by calcite instead of the CBE. With regards to the inoculated CD bioreactors, since there are significant pore spaces in the bioreactor, oxygen and nutrient access was not limited throughout the bed, and since the nutrient solution may sit in pockets at the bottom of the bioreactor, there is an increased likelihood of MICP occurring at the bottom of these bioreactors compared to the top. Thus, the calcite content at the bottom of these beds were higher than expected for the FW, BLND, and LAY bioreactors. The IRR-BLND bioreactor, which sealed on Day 6, contained the most calcite from all bioreactors (219,13 mg / g), especially when compared to AGG-BLND (43,47 mg / g), which sealed on Day 4. This suggests that inoculation via irrigation leads to a slower clogging time but a more even distribution of calcite throughout the bioreactor, since the bottom half of IRR-BLND displayed the highest calcite content from all bioreactors (96,48 mg / g). AGG-LAY showed the highest calcite content in the top half of co-disposed bioreactors (185,11 mg / g) but very small volumes of calcite in the bottom half of the bioreactor (10,88 mg / g). This can be attributed to the CBE. However, it is also important to note that AGG-LAY did not clog even with significant formation of calcite (196,00 mg / g). It is possible that preferential flow paths were formed in this bioreactor which allowed for a reduced flow of effluent to pass through the bed and out the bioreactor without being obstructed. It may even be likely that the calcite cemented and strengthened these flow paths and prevented clogging from occurring. The formation of calcite can occur in two ways, either by encapsulating the ore particles or by forming bridges between ore particles in the pore space between them. Calcite formed as bridges between particles leads to improved shear strength and reduced permeability compared to calcite formed around ore particles. It was also hypothesized that calcite formed via the ore encapsulation route in the heterogenous co-disposed AGG-LAY bioreactor. Thus, a reduced effluent flowrate but no sealing was observed in AGG-LAY. Conversely, the reduced pore space in the homogenous AGG-FW bioreactor led to bridge-forming calcite precipitation, and ultimately clogging of the bed. The I RR-LAY sealed while the AGG-LAY did not. This is another indication that the inoculation method plays a significant role in MICP. Initially, it was thought that inoculation via agglomeration would lead to more rapid MICP with an increased penetration depth of calcite formation. However, since these heterogenous co-disposed systems have many variables, including the packing configuration, leaching of metals, and generation of ARD, MICP formation in these systems require more in-depth study. Additionally, it is not known if the AGG-LAY bioreactor would seal if the treatment period was increased. Due to the rapid rate at which the bioreactors clogged, it was also difficult to observe any trends when analysing the pH, redox conductivity, Fe3+, SO42; and cell counts. Thus, for the next set of experiments, the flowrate of the cementation solution was significantly decreased to delay MICP formation and the onset of sealing. Non-continuous irrigation The intermittent irrigation study was run for 60 days and yielded significant insight into the bioreactor conditions under cementation treatment. The delay in sealing allowed for trend analyses to be conducted on the effluent from all bioreactors, including trends in effluent volume, pH, redox conductivity, iron and sulphate leaching, and cell density. Effluent volume and reactor sealing Figure 4 shows the effluent volume obtained from the I RR and control bioreactors and Figure 5 shows the effluent volume obtained from the AGG and control bioreactors, both treated with non-continuous cementation solution irrigation. When the effluent volume reached 0 mL, the bioreactor was considered sealed. Table 2: Day on which the bioreactors sealed and no effluent was observed. Bioreactor Sealing day IRR-FW 29 &46 IRR-WR N / A IRR-BLND 21 IRR-LAY N / A AGG-FW 29 AGG-WR N / A AGG-BLND 11 AGG-LAY N / A CONTROL-FW 54 CONTROL-WR N / A CONTROL-BLND 32 CONTROL-LAY 42 The effluent volume and sealing day yielded noteworthy results in all bioreactors with a FW component. The first bioreactor to seal was AGG-BLND, followed by IRR-BLND. This may occur due to the “just fit” phenomenon when 3:2 WR:FW (by mass) is used in a blended co-disposal system, which may provide the limited pore space required for bridge-forming MICP to occur and ultimately lead to efficient clogging of these systems. AGG-BLND sealed after just 11 days and IRR-BLND sealed on day 21, which indicates that the inoculation method has a significant effect on MICP in the blended system. Inoculation by agglomeration allows for S. Pasteurii to be present throughout the bed before irrigation with cementation solution. In terms of the blended system, this seems to be advantageous by either allowing cementation to occur evenly through the bed or by providing more consistent calcite-seeding locations in the bed, since the microbes themselves act as calcite-seeding sites. The inoculated bioreactors containing only FW (IRR-FW and AGG FW) both sealed on the same day and were tied for third in terms of quickest sealing time. IRR-FW was sealed for 4 days when effluent was observed again, which indicated breakthrough of the bioreactor bed. This is an unwanted phenomenon since breakthrough of the bed would lead to oxidant ingress and ultimately ARD generation in these systems; however, impressively, the bed sealed again on day 46 and stayed that way until the end of the treatment phase of the study. This self-healing ability observed in the bioreactor is promising in terms of long-term application of the MICP-co-disposal system, since it improves on a common weakness in the use of traditional co-disposal systems, that is, the long-term stability of heterogenous waste beds. pH and redox conductivity In the MICP mechanism, urease breaks down urea into ammonium and carbonate. The released ammonium increases the pH, which creates an environment that is suitable for S. Pasteurii (pH 7-9). Therefore, monitoring the pH of the effluent provides insight into the urease activity as the pH increases to 8 or 9. Figure 6 shows the pH of the effluent for IRR and control bioreactors and Figure 7 shows the pH of the effluent for AGG and control bioreactors. As seen in Figures, all inoculated columns sealed when their pH value was close to pH 8. Besides the visible signs of calcite, the increased pH proves that the clogging was as a result of MICP. Interestingly, the control columns sealed at pH values closer to 7, and when the effluent microbes were observed under the microscope, they were round-shaped, which points to the fact that some native MICP bacteria may be present in the ore, which produces calcite at around pH 7. However, the Control-BLND bioreactor sealed when the pH was closer to pH 9, which was similar to the calcite-rich AGG-WR bioreactor. Both bioreactors showed rod-shaped bacteria, which could have either been S. Pasteurii, or another native rod-shaped MICP species. Additionally, since these bioreactors were irrigated with solution consistently, it is impressive that the pH was kept relatively high once MICP occurred. This is pronounced in AGG-WR, which contains only acid-generating WR, yet it produced effluent that displayed the highest pH values (~pH 9). This is important since it reveals that MICP is able to keep the pH high and prevent the pH from reaching the values (~pH 2) where a self-perpetuating cycle of Fe3+-driven pyrite oxidation, sustained by the regeneration of Fe3+ through Fe2+-oxidizing bacteria, is initiated, which results in the production of acid until either the pyrite source or the Fe3+ is exhausted. Figures 8 and 9 show that the pH and redox conductivity are linked to MICP. Except for Control-BLND (85 mV), which followed AGG-WR, all the bioreactors were sealed between redox conductivity values of 100-150 mV. Although no studies have investigated the redox conductivity of MICP reactions, it can be assumed that the calcite forming reactions occur within the 100-150 mV band. The exact redox couple that causes this phenomenon is unknown and studies are being conducted by the authors to elucidate the relation between redox conductivity and calcite generation. Taken together, it can be said that the MICP occurs in these mine waste bioreactors at pH values between 8-9 and redox conductivity values between 100-150 mV. Acid wash tests Once the bioreactors were dried, they were opened and visually inspected in terms of calcite formation and structural changes. Figure 10 shows the calcite content in the bioreactors and Figures 11-17 depict the most important dried and opened bioreactors. After acid wash tests were conducted, unsurprisingly the long term non-continuous irrigation method yielded more calcite precipitate than the 10-day continuous irrigation method. The stand out bioreactor in this experiment was AGG-CD, which yielded a staggering 769,12 mg / g of calcite (Figures 11A to 11D). As with all bioreactors that only contained CD, the calcite content was expected be the lowest since the larger pore size significantly decreases the retention time of the cementing solution and thus reduces the likelihood of calcite formation, especially towards the top of the bioreactor. As discussed earlier, since the nutrient solution may sit in pockets at the bottom of the bioreactor, it was expected that MICP would occur at the bottom and would not yield such high amounts of calcite. Due to the large pore sizes present in the CD bioreactors and since there was sufficient oxygen for bacteria to thrive, it was likely that in order to obtain significant amounts of calcite, a small pocket of cementation solution was upheld and this led to crystal seeding, which then grew to impede solution flow through the bioreactor, as seen in the non-continuous irrigation experiment. However, the crystal seeding in the CD reactors are unpredictable due to the packing limitations and large pore sizes associated with this material. In terms of the inoculated systems, the rest of the bioreactors followed a similar trend as seen in the continuous irrigation study, where AGG-LAY contained the most calcite (377,90 mg / g; Figures 12A to 12C). As can be seen in Figures 12A to 12C, visually, there were significant volumes of calcite on the top of the bioreactor (337,65 mg / g) compared the bottom (40,25 mg / g). The top half of the bioreactor presented a solid column while the calcite in the following alternating layers was not observable. Critically, Figures 12A to 12C provides insight into the effect of flow path formation in the bioreactors, as can be seen by the “crack” in the column between the top and bottom halves (Figure 12C). As discussed earlier, it was hypothesized that preferential flow paths were formed in the AGG-LAY bioreactor of the 10-day experiment since the effluent flow decreased but never stopped, and the same occurred in the 60-day experiment. It was also hypothesized, that calcite can form around these flow paths and promote the access of oxidants through these co-disposed beds, and this can be seen by the calcite formation in the middle of the column (Figure 12C). Additionally, wall effects are thought to play a part in terms of preventing the complete clogging of bioreactors when using the LAY configuration, since it provides an easy path for solution to flow through the bioreactor. The BLND bioreactors contain waste that is mixed thoroughly and packed tightly so as to minimize migration of fine particles through the bioreactor; however, since the LAY bioreactors have layers of CD between layers of FW, even though they too are tightly packed, the pore spaces between the CD layers may allow for FW migration to the bottom layers of the bed, which leads to the formation of preferential flow paths and bed instability, and ultimately may promote slumping and increased flow via wall effects. The bioreactors with the next highest calcite contents were IRR-FW (290,13 mg / g), IRR-LAY (262,14 mg / g), and IRR-BLND (231,32 mg / g). Figures 13A-13B, 14A-14B, and 15A-15C demonstrate that calcite content alone does not translate into an improvement in structural stability. Although IRR-LAY (262,14 mg / g; Figures 14A-14B) contained more total calcite than IRR-BLND (231,32 mg / g; Figures 15A-15C), IRR-BLND contained almost five-times more calcite in the bottom half of the bioreactor (74,37 mg / g) than IRR-LAY (16,94 mg / g). In the top half of IRR-LAY (Figure 14B), a solid column can be observed; however, the alternating layers below show no visible calcite formation and no structural improvement was seen, especially in the CD layers, which slumped out when the bioreactor was opened. This also shows the CBE in action and the paradox that occurs when trying to increase penetration depth in these systems. In traditional co-disposal systems, it is beneficial to prevent oxidant ingress throughout the bed. However, in these MICP-co-disposal systems, it is beneficial for the inoculum and cementing solution to penetrate as deep as possible through the bed to facilitate uniform calcite formation. It is important to note that although the LAY bioreactors did not seal and effluent was collected throughout the 60-day period, the hypothesis for as to why limited calcite formation was observed in the bottom of these bioreactors was the potential formation of preferential flow paths, which prevented the even distribution of cementing solution throughout the bed and provided an avenue for the unrestricted flow of cementing solution, which did not provide the high retention times required for successful calcite formation. In this sense, the LAY packing configurations are seemingly unsuitable in terms of the hybrid MICP-co-disposal system. The bottom of IRR-BLND (Figure 15C) showed the highest calcite concentration from all inoculated bioreactors, and visually, the top half displayed a solid column. Although the bottom half did not present a solid column, the heterogenous ore particles were held together in place and did not show obvious slumping. The improved penetration depth of calcite suggests that the BLND packing configuration is more suitable for MICP-co-disposal systems, even though the total calcite content was lower than their LAY counterparts. Additionally, the bioreactors that sealed the quickest showed the least amount of calcite in the bottom half of the bioreactors. This is expected since calcite clogs the upper part of the reactor first, which prevents access of nutrients and oxygen to the bottom half of the bioreactors; the quicker the bioreactors sealed, the less calcite formed at the bottom of the bioreactor. This is clearly seen in the control bioreactors, which took the longest to seal and displayed significantly higher calcite concentrations in the bottom halves of both CONTROL-BLND (98,77 mg / g) and CONTROL-LAY (105,53 mg / g) than in their inoculated counterparts. Visually, CONTROL-BLND looked very similar to IRR-BLND with a solid column forming in the top half of the bioreactor and a well-bound bottom half, as shown in Figures 16A-16C. The calcite content in the bottom half of the BLND bioreactors followed the order CONTROL-BLND (98,77mg / g) >IRR-BLND (74,37 mg / g) >AGG-BLND (16,54 mg / g). The days required for sealing also followed the same order: CONTROL-BLND (Day 42) >IRR-BLND (Day 21) >AGG-BLND (Day 11). For all bioreactors, those that sealed the quickest showed the least amount of calcite in the bottom half of the bioreactors. This is expected since calcite clogs the upper part of the reactor first, which prevents access of nutrients and oxygen to the bottom half of the bioreactors; the quicker the bioreactors sealed, the less calcite formed at the bottom of the bioreactor. This is clearly seen in the control bioreactors, where the calcite concentrations in the bottom halves of both CONTROL-BLND (98,77 mg / g) and CONTROL-LAY (105,53 mg / g) were significantly higher than in their inoculated counterparts. It may also be possible that the native MICP bacteria at the bottom of the control bioreactors are actually anaerobic or facultatively anaerobic, which may allow calcite formation even at the bottom of the bioreactors. Unlike other LAY bioreactors, the CONTROL-LAY sealed. When visually comparing CONTROL-LAY (Figure 17B) to its inoculated counterparts (Figures 12B and 14B), the two FW layers formed stiff columns and no obvious “cracks” were observed. Thus, it is likely that no preferential flow paths were formed in CONTROL-LAY, and under these conditions, clogging is possible when using the LAY packing configuration. In CONTROL-LAY, the CD layers did not contribute structurally, as was evidenced by the CD particles slumping out of the bioreactor. The formation of calcite successfully prevented oxidant ingress in co-disposed beds packed in the blended configuration, and a significantly reduced flow was observed in beds packed in the layered configuration. The calcite that prevents oxygen ingress on the top of the bioreactors results in a desirable outcome since prevention of oxygen in mine waste beds may lead to the prevention of ARD. Uniform calcite formation throughout bioreactors may provide increased bed stability and prevent failure in co-disposed columns; hence, inoculating bioreactors with native or anaerobic MICP bacteria may lead to optimal ARD prevention conditions. Additionally, the IRR-FW bioreactor displayed a self-healing ability once breakthrough was observed, which is promising in terms of long-term stability of these systems. The results presented here suggest that the total concentration of calcite in the bioreactor is not as important as the distribution of calcite throughout the system. Although LAY bioreactors showed higher total calcite content than their BLND counterparts, the BLND bioreactors showed higher calcite content in their bottom halves and thus a better calcite penetration depth. The better calcite penetration depth led to bioreactors with structural improvements that were visually observable, especially when compared to LAY bioreactors. Thus, in terms of the inoculated bioreactors, the IRR-BLND was the most promising for our hybrid MICP-co-disposal system, in terms of sealing, calcite content, and calcite penetration depth. For the CONTROL bioreactors, where calcite generation was attributed to native MICP-generating bacteria, both the blended and layered bioreactors were sealed with high calcite concentrations in the bottom half of the bioreactors, which was attributed to the fact that these bioreactors allowed sufficient nutrient and oxygen access due to the delayed sealing time. Additionally, the native bacteria in these bioreactors may be anaerobic or facultatively anaerobic, and thus are able to survive in the low oxygen conditions present lower down the bioreactors. EXAMPLE 2 In this example, synthetic acid rain is used to stress-test the hybrid MICP co-disposal system to determine its ARD prevention potential. The coal waste used was the same as the previous example in order to maintain a comparable matrix. It involved the use of acid generating coal waste rock (WR) and acid-neutralizing coal fine waste (FW). The same MICP protocol was used to generate calcite in the co-disposed coal bioreactors, which were then subjected to aggressive acidic conditions for two 90-day periods with continuous monitoring to determine the performance of this hybrid system. Twelve 60-mL syringes were packed in four packing configurations: (1) FW, (2) WR, (3) 3:2 WR:FW (by mass) in a layered configuration (LAY) with separate layers of WR and FW, and (4) 3:2 WR:FW (by mass) in a blended configuration (BLND), where the WR and FW were mixed together in the bioreactor. Sporosarcina pasteurii was cultured in ATCC®1376 ammonia-yeast (20 g / L yeast extract, 10 g / L ammonium sulphate, and 0.13 M pH 9 Tris buffer) until a cell density of 1 x 109 cells / mL was obtained. A fresh batch of cementation solution (1 M urea, 0.5 M CaCh, and 3% ATCC®1376 media) was prepared daily and used to irrigate the co-disposed bioreactors. Bioreactor monitoring and effluent analyses The Twelve bioreactors were considered as three groups of four (FW, WR, BLND, and LAY). The first set of bioreactors were inoculated via irrigation, where the S. Pasteurii was irrigated through the bioreactor (IRR-FW, IRR-WR, IRR-BLND, and IRR-LAY). The second group were inoculated via agglomeration, where the coal waste was agglomerated with the S. Pasteurii culture, mixed, and then packed into the bioreactors (AGG-FW, AGG-WR, AGG-BLND, and AGG-LAY). The third group of bioreactors were not inoculated (UNINOC-FW, UNINOC-WR, UNINOC-BLND, and UNINOC-LAY). The bioreactors were then irrigated with 20 mL of cementing solution daily for 60 days. The daily effluent was analyzed in terms of volume, pH, and redox potential. Additionally, the Fe2+, Fe3+, and total iron concentration was determined using the 1,10-phenanthroline assay (American Public Health Association; 1976), and the SO42’ content was determined using the spectrophotometric sulfate assay (American Public Health Association; 1975). Finally, the cell concentration was determined using a Helber Cell Counting Chamber with a Leica DM500 compound microscope at 100* magnification (Leica Microsystems, Switzerland) Synthetic Acid Rain Experiment Bioreactor Setup After the 60-day cementing period, any irrigant in the head space of the bioreactors was removed and the 12 bioreactors were left to air dry for a further 60 days. Four additional bioreactors were then packed (CONTROL-FW, CONTROL-WR, CONTROL-BLND, and CONTROL-LAY) and used as the control since they did not undergo the cementation process. Synthetic Acid Rain A synthetic acid rain (SAR) solution was prepared using a mixture of HNO3 and H2SO4 (1:3) and adjusted to pH 4,5 using deionized water. pH 4,5 was chosen since it is the pH of acid rain in the Mpumalanga region where the coal waste samples were obtained. Bioreactor Monitoring and Effluent Analyses The 16 bioreactors were irrigated with 20 mL of SAR weekly, and the same effluent analyses were conducted as in the cementing period (volume, pH, redox potential, Fe2+, Fe3+, total iron concentration, SO42; and cell concentration). In the clogged bioreactors where no effluent was observed, the irrigant present in the head space was removed and the pH was measured. The bioreactors underwent SAR stress testing for 90 days, followed by a 60-day air-drying period, and then stress-tested with SAR for a further 90 days. The effluent analyses between the MICP-co-disposed bioreactors and the co-disposed controls were compared to determine the robustness of the hybrid MICP-co-disposal method in terms of ARD mitigation. Results and discussion MICP-Co-Disposal Cementation Effluent flow during cementation The cementation results obtained were similar to those obtained in Example 1. Figures 18 and 19 show the effluent flow volume obtained. Table 3: Sealing day of the bioreactors, when no effluent was observed, for examples 1 and 2. Bioreactor Sealing Day Example 1 Sealing Day Example 2 IRR-FW 29 26 IRR-WR N / A N / A IRR-BLND 21 32 IRR-LAY N / A N / A AGG-FW 29 36 AGG-WR N / A N / A AGG-BLND 11 32 AGG-LAY N / A N / A CONTROL-FW 54 N / A CONTROL-WR N / A N / A CONTROL-BLND 32 N / A CONTROL-LAY 42 N / A The behaviour of the co-disposed bioreactors was similar to the previous experiment (Examplel), where the BLND bioreactors clogged and the LAY did not clog, but rather showed a significant reduction in effluent volume. The timeframe of clogging were similar too, as shown in Table 3. The bioreactors took longer to seal in this experiment compared to the last, which is clearly seen for AGG-LAY. Additionally, the uninoculated bioreactors did not clog in this experiment. The sealing of the uninoculated bioreactors was thought to be due to native MICP-generating bacteria in the coal packing and since it was not observed here, the consistency of the packing may play a part in terms of allowing oxygen and nutrient supply evenly throughout the bed. Furthermore, bioprospecting tests will be conducted to determine the native bacteria in this system and the optimum environmental conditions for their success. Effluent pH during cementation pH is an excellent indicator in terms of calcite generation during cementation and a clear increase in pH is observed. Figures 20 and 21 show the pH data obtained during the cementation period. The increase in pH between 7-9 observed in all inoculated bioreactors was rapid. For BLND, it remained in this range until the reactors were sealed and no effluent was observed. For IRR-LAY there was a decrease in pH between days 20 and 32, from around pH 8 to pH 7. At this point, the effluent volume decreased to such an extent that no pH data could be obtained. For AGG-LAY, no pH data could be obtained from 32 as well; however, the pH in this bioreactor remained constant around pH 8,5. This difference may be attributed to the fact that inoculation via agglomeration yields a more consistent distribution of S. Pasteurii, and hence more uniform calcite distribution throughout the bioreactor; whereas for inoculation via irrigation may yield a less consistent distribution of calcite, which leads to preferential flow paths developing through sections with less resistance, that is, sections with less calcite, which results in the decrease in pH. Since the bioreactors behaved similarly to the previous experiment, a similar amount of calcite was expected to be produced and the bioreactors were left to air-dry for 60 days before stresstesting with SAR. Synthetic Acid Rain Stress Tests 90-day Effluent flow under synthetic acid rain irrigation After the 60-day cementation and 60-day air-drying period, four uninoculated, untreated reactors were packed as controls (CONTROL-FW, CONTROL-WR, CONTROL-BLND, and CONTROL-LAY) to determine the ARD mitigation potential of the hybrid MICP-co-disposal system. Since ARD is SAR acid rain, and any effluent observed was collected the next day and analysed. Reactors that did not yield any effluent and remained clogged, showed a build-up of SAR in the headspace, which was removed after 24 h and analysed for pH. All reactors were then allowed to air-dry for 6 days, before repeating SAR irrigation. Figure 22 shows the effluent flow for the first 90 days of SAR treatment for all bioreactors, and Figure 23 shows the effluent flow for the co-disposed bioreactors. The effluent flow can be used to determine the exposure of the mine waste to oxidants. For example, in the IRR-FW bioreactor formed a calcite layer on the surface that prevented SAR from penetrating the bed and hence no effluent was observed. In terms of co-disposed beds, IRR-BLND, which yielded an excellent calcite yield in the previous experiment (231,32 mg / g), showed a gradual increase in effluent flow over the 90-day SAR treatment. This may be due to the formation of preferential flow paths within the blended bioreactor, or due to the dissolution of calcite by the SAR. Interestingly, AGG-BLND did not show any effluent throughout the 90-day period, showing the benefit of inoculation via agglomeration in terms forming a reactor bed that prevent access to oxidants. The UNINOC-BLND bioreactor behaved similarly to CONTROL-BLND with no restriction in effluent flow. The best performing packing configuration in terms of effluent flow was the layered bioreactors. All layered bioreactors that underwent cementation, including the UNINOC-LAY, showed a significant restriction in effluent flow, with IRR-LAY and AGG-LAY sealing and not producing any effluent from day 78 onward. In traditional co-disposal, the layered configuration also outperforms the blended configuration in terms of effluent flow. This is attributed to the formation of capillary barriers within the bed that allows for cascading neutralization throughout the bed. Combining the capillary-barrier effect with calcite formed via MICP, a bed with improved structural stability and limited oxidant access is achieved. 90-day Effluent pH under synthetic acid rain irrigation Figure 24 shows the effluent pH for the first 90 days of SAR treatment for all bioreactors, and Figure 25 shows the effluent pH for the co-disposed bioreactors. It is important to note that since the effluent flow in the treated layered bioreactors was restricted for almost the entire 90-day SAR treatment period, the effluent pH data is limited. Figure 24 shows that all bioreactors that underwent cementation maintained circum-neutral to basic conditions, including those that only contained the acid-generating WR. This indicates that the calcite formed in all the treated reactors was sufficient to prevent the pH dropping low enough for the solubilization of ferric iron and the generation of ARD. IRR-BLND, which showed a steady increase in effluent volume, also showed a steady increase in pH. This resulted in the highest pH obtained, together with the UNINOC- BLND bioreactor. The CONTROL-LAY and CONTROL-WR bioreactors showed the lowest pH values, particularly CONTROL-WR, which showed a consistent pH of around 2,5. This is within the range of ferric solubilization and also the range where acid-generating bacteria that promote ferric regeneration and ferric-driven oxidation occurs. Since the treated bioreactors prevent this from occurring, at least over the 90-day treatment, ARD generation was mitigated in the treated bioreactors by the reduced effluent flow and increased pH of the system. A significant increase above neutral conditions may not be ideal though, since the increase in pH is due to calcite dissolution. Thus, a high rate of calcite dissolution may shorten the mitigation lifespan of the MICP-co-disposal system. Fortunately, S. Pasteurii was shown to have a calcite self-healing ability, where lost calcite can be regenerated over time. The bioreactors were allowed to air-dry for 60 days before undergoing a further 90-day SAR treatment. The air-drying allowed time for regeneration of calcite, but also allowed for gaseous oxidants to permeate the bioreactors; thus, calcite generation would have occurred under unfavourable conditions that mimic seasonal changes, further stress-testing the MICP-co-disposal system. 180-day Effluent flow under synthetic acid rain irrigation After the 60-day air-drying period, the same SAR irrigation protocol was continued for a further 90-days. Figure 26 shows the effluent flow for days 90-180 of SAR treatment for all bioreactors, and Figure 27 shows the effluent flow for the co-disposed bioreactors. After an initial decrease in effluent flow, the volume increased from day 99 to similar levels as observed in the first 90-day SAR treatment for most bioreactors; however, there were exceptions. The AGG-FW bioreactor showed a varying effluent volume, decreasing to as low as 6,22 mL. This may be due to calcite forming over the drying period that altered the flow within the bed. In terms of the co-disposed bioreactors, IRR-BLND and UNINOC-BLND showed unrestricted flow and behaved similarly to CONTROL-BLND in terms of effluent flow. Promisingly, AGG-BLND remained sealed and showed no effluent throughout this second 90-day SAR treatment period, which shows that calcite formed via MICP successfully prevented oxidant ingress and was stable over 90 days of aggressive SAR treatment. The layered bioreactors were once again the stellar performers. Both the I RR-LAY and AGG-LAY bioreactors remained sealed and showed no effluent over this second 90-day SAR treatment, with UNINOC-LAY showing a significant reduction in effluent volume. These results indicate that the layered configuration, which promotes the capillary-barrier effect, synergises well with MICP yielding a bed that limits oxidant ingress over long periods of time even under hyper-aggressive conditions. 180-day Effluent pH under synthetic acid rain irrigation The pH of the effluent provides insight into the reactor bed environment; Figure 28 shows the effluent pH for the second 90 days of SAR treatment for all bioreactors, and Figure 29 shows the effluent pH for the co-disposed bioreactors. A decrease in pH is observed across the board for all bioreactors except CONTROL-BLND and CONTROL-WR. The pH of the CONTROL-BLND bioreactor was between 5-6 for the first 90-day SAR treatment but decreased significantly to between 3-4 for the second 90-day treatment. Without calcite to assist in preventing gaseous and aqueous oxidant ingress, the combination of 90-day SAR treatment and the 60-day air-drying period led to a decrease in pH and a rise in the acidity of the reactor environment and the generation of ARD. As expected, the reactor with the worst performance was CONTROL-WR. There was no decrease in pH between the first and second 90-day periods. The pH remained highly acidic, between 2-3, for the entire treatment, signifying unrestricted acidic reactor conditions and ARD generation. In terms of the treated bioreactors, in the first 90 days, the general effluent pH values were between 8-9 but for the second 90 days the general pH dropped to between 7-8. The pH in all treated bioreactors was still neutral to basic, even after 180 days of aggressive acidic conditions. Even UNINOC-WR, which was not inoculated with S. Pasteurii and contained only acid-generating WR, produced a neutral effluent. As mentioned previously, for reactors that were sealed and produced little to no effluent, the irrigant in the headspace of the reactors were removed and the pH was measured. The irrigant pH of the sealed bioreactors are shown in Figures 30 (first 90-day SAR treatment) and 31 (second 90-day SAR treatment). The pH of the headspace irrigant for all sealed bioreactors remained consistent over the 180-day SAR treatment period between pH 7,5-8,5. This shows that all treated bioreactors had the ability to neutralise a relatively high volume of acidic pH 4,5 SAR for over 180 days. The relative volume of acid rain to the amount of co-disposed coal waste and calcite generated via MICP bodes well for the real-world application of a hybrid MICP-co-disposal system. Since the increase in pH results from the dissolution of calcite, the calcite content and its regeneration of is key to the longterm prevention of ARD. Using the calcite content data from the previous example and the change in pH obtained from this example, a neutralisation lifespan was obtained per bioreactor (Figure 32). The expected neutralisation lifespan presented here is based on 35,67 mm of consistent weekly acid rainfall. This is four times the weekly rainfall expected in South Africa (450 mm per annum which equates to approximately 8,65 mm per week for the sake of our calculation; and is applied consistently every week. Additionally, this estimation assumes that all rainfall is acid raid, which results in the conditions used in our experiment being significantly more aggressive than those found in nature. Thus, even under these hyper-aggressive conditions, the inoculated codisposed bioreactors will maintain neutralising conditions for between 52-104 years, not including calcite regeneration that may occur. In terms of the co-disposed bioreactors, AGG-LAY (104 years) had the longest lifespan followed by I RR-LAY (72 years), once again highlighting the superior performance of the layered configuration over the blended. IRR-BLND (68 years) had a longer expected lifespan than AGG-BLND (52 years). AGG-LAY was the standout bioreactor with almost double the lifespan of AGG-BLND. In terms of ARD prevention, the hybrid MICP-co-disposal system was successful in preventing ARD generation, particularly using a layered packing configuration. As mentioned previously, although traditional co-disposal showed short term success in terms of ARD prevention, exposure to oxidants over the long term led to bed failure and ultimately, ARD generation. The results obtained from this example show that combining co-disposal with MICP leads to successful ARD prevention over extended periods even under hyper-aggressive conditions. Iron and Sulfur Content of Effluent The neutralization capacity is not the only factor that should be taken into consideration when determining the success of the hybrid MICP-co-disposal method. The iron and sulfur content paints an important picture in terms of the leachate quality, particularly, the Fe3+ (ferric iron) content can be used as indicator of ARD generation since Fe3+ is soluble at low pH values and facilitates the generation of ARD at greater rates than those associated with oxygen driven oxidation. Since the cementing solution can be considered as an aqueous oxidant, it is important to factor in the iron and sulphur and leached during the cementing period and the SAR treatment period to determine the total iron and sulphur leached throughout the lifetime of the bioreactors. Table 4 and Figure 33 show the Fe2+, Fe3+, and Total Fe content in the leachate of each bioreactor over the cementation period. Table 4: The Fe2+, Fe3+, and Total Fe leachate content over the cementation period. Bioreactor Fe2+ (mg / L) Fe3+ (mg / L) Fe Tot (mg / L) IRR-FW 6,10 1,30 7,40 IRR-WR 27,63 18,24 45,87 IRR-BLND 17,84 19,94 37,77 IRR-LAY 23,54 7,38 30,92 AGG-FW 17,68 2,96 20,64 AGG-WR 41,04 32,97 68,81 AGG-BLND 19,24 11,76 31,00 AGG-LAY 20,74 10,57 31,07 UNINOC-FW 68,96 0,79 68,73 UNINOC-WR 425,02 100,53 449,57 UNINOC-BLND 246,32 2,59 248,91 UNINOC-LAY 62,79 5,52 68,30 Over the cementing period, the iron content in the leachate of inoculated bioreactors were substantially lower than those of the uninoculated bacteria. Even the bioreactors only containing WR showed minimal iron in the leachate, which suggests that the calcite successfully prevented leaching of metal or immobilised any metal that was leached. MICP can be used to immobilize metals in soils by chemical precipitation between CO32’ and Ca2* or other metals present Additionally, heavy metal ions with an ion radius close to Ca2+ may be incorporated into the calcite crystal Ca2+ substitution or by direct precipitation into the CaCOs crystal lattice. The uninoculated reactors showed a substantial amount of iron in the WR reactor, with UNINOC-BLND showing almost four times more total iron than UNINOC-LAY, again showing the positive effects of the layered configuration over the blended. Fortunately, the Fe3+ content remained low in both, and in fact in all inoculated bioreactors as well. Even though the uninoculated reactors ultimately formed calcite and even yielded neutral conditions over the SAR treatment period for 180 days, the rate at which calcite formed in the absence of S. Pasteurii was too slow to prevent the leaching of iron in the cementing period. To determine the total iron leached throughout the experiment, we analysed the leachate during SAR treatment, and compared the iron content with that found in the control bioreactors that did not receive cementing solution. Table 5 and Figure 34 show the Fe2+, Fe3+, and Total Fe content in the leachate of all bioreactors over the cementation and SAR treatment periods. Figure 35 shows the same data excluding that of CONTROL-WR to facilitate comparison. Table 5: The Fe2+, Fe3+, and Total Fe leachate content over the cementation 180-day SAR treatment periods. Bioreactor Fe2+ (mg / L) Fe3+ (mg / L) FeTot (mg / L) IRR-FW 6,10 1,30 7,40 IRR-WR 76,27 121,47 197,74 IRR-BLND 86,01 49,82 135,83 IRR-LAY 24,09 7,45 31,54 AGG-FW 44,05 5,41 49,45 AGG-WR 97,81 32,92 130,72 AGG-BLND 19,24 11,76 31,00 AGG-LAY 23,28 10,44 33,72 UNINOC-FW 110,13 24,36 122,94 UNINOC-WR 484,01 43,37 527,38 UNINOC-BLND 295,17 7,87 303,04 UNINOC-LAY 68,57 5,65 74,22 CONTROL-FW 37,77 45,72 83,49 CONTROL-WR 1322,17 11356,96 12679,13 CONTROL-BLND 503,48 41,38 544,86 CONTROL-LAY 417,89 187,56 605,45 As expected, CONTROL-WR showed the highest leachate iron content (12679,13 mg / L), with approximately 90% consisting of Fe3+ (11356,96 mg / L). Thus, ARD generation is assumed to be promoted by ferric-driven oxidation in this reactor, yielding unrestricted acid generation as can be concluded from other effluent data. This shows the deleterious effects of exposing acidgenerating coal mine to oxidising elements and in contrast, highlights the success of the hybrid MlCP-co-disposal method in preventing these conditions from prevailing. The CONTROL-BLND and CONTROL-LAY contained the next highest iron content (Figure 19). In the CONTROL-LAY reactor, Fe3+ comprised 31,5% (187,56 mg / L) more of the total iron content than in CONTROL-BLND (41,38 mg / L). This was not expected since the layered configuration outperformed the blended configuration in a previous co-disposal example. However, in terms of treated bioreactors, the layered configuration did, in fact, outperform the blended in all aspects, and as can be seen in Table 4 and Figure 35, even in terms of iron content in the leachate. This is especially seen between IRR-BLND (135,83mg / L) and IRR-LAY (31,54 mg / L), as well as UNINOC-BLND (303,04 mg / L) and UNINOC-LAY (74,22 mg / L), where in both instances the layered reactors contained approximately four times less iron. With regards to AGG-BLND (31,00 mg / L) and AGG-LAY (33,72 mg / L), the leachate from both bioreactors contained similar iron contents. This outcome should be expected, since the total iron content in the control and uninoculated bioreactors compared to inoculated bioreactors was significantly different and indicates that the sooner that calcite forms, the less likely it is for iron to leach out of the reactor. Therefore, since the agglomerated bioreactors received a more consistent application of inoculum throughout the bed, its stands that more even and consistent calcite formation should occur and thus reduce iron leaching and ARD formation in these beds. Traditional co-disposal performs well in terms of preventing ARD generation in the short-term; however, the long-term success is uncertain. The hybrid MICP-co-disposal method sought to alleviate the shortcomings of traditional co-disposal using calcite to improve structural strength, prevent oxidant ingress, and increase the pH of the microenvironment. Example 1 showed that MICP can occur in co-disposed coal mine waste beds with high calcite yields. In Example 2, the robustness of the MICP-co-disposal beds was tested using pH 4,5 synthetic acid rain. All reactors that received cementing solution maintained neutral conditions over 180 days of SAR treatment, with the layered bioreactors outperforming its blended counterparts in all facets, especially in terms of reactor sealing and iron leaching. Reactor sealing limits oxidant ingress and prevents acidic conditions from prevailing inside reactor beds. According to estimates, neutral conditions may be maintained for between 72-104 years for layered reactors under hyperaggressive conditions, with continuous weekly acid rain four times more than found in nature, excluding any calcite regeneration that may occur. Additionally, any iron leached may also be trapped within the generated calcite and limit heavy metal contamination in the environment. Thus, the hybrid MICP-co-disposal method, especially the layered configuration, shows great success in mitigating ARD generation under long-term oxidant exposure. EXAMPLE 3 In this example, varying cementation solution irrigation rates were investigated to determine a more cost-effective and optimized rate of irrigation for the formation of calcite in co-disposed beds. The coal waste used was the same as the previous examples in order to maintain a comparable matrix. It involved the use of acid generating coal waste rock (WR) and acid-neutralizing coal fine waste (FW). Sporosarcina pasteurii, was cultured in ATCC®1376 ammonia-yeast (20 g / L yeast extract, 10 g / L ammonium sulphate, and 0.13 M pH 9 Tris buffer) until a cell density of 1 x 109 cells / mL (approximately 24 h) was achieved and inoculated into the bioreactors. The bioreactors were constructed in the same manner as those in the 60-day daily irrigation of example 1. Briefly, two sets of twelve 60-mL syringes were packed with coal waste rock (WR) and coal fine waste (FW) in four different configurations: (1) FW, (2) WR, (3) 3:2 WR:FW (by mass) in a layered configuration (LAY) with layers of WR and FW, and (4) 3:2 WR:FW (by mass) in a blended configuration (BLND), where the WR and FW were carefully mixed prior to packing. Four bioreactors packed in this way were then inoculated via irrigation, where 20 mL of 109 cells / mL S. pasteurii were added into the already packed columns; another set of four bioreactors were inoculated via agglomeration, where 20 mLof the 109 cells / mL S. pasteurii culture was mixed with the packing material prior to packing; and the last set of four bioreactors were not inoculated and acted as the controls, to give a total of 12 bioreactors. The various inoculation methods yielded differences in terms of calcite formation, clogging time, and calcite distribution previously, and these were unchanged in this example to determine the effect of the irrigation rate on these inoculation methods and for comparative purposes. In example 1, the 12 bioreactors received of 20 mL of cementation solution (1 M urea, 0.5 M CaCh, and 3% ATCC media) daily

[19] ; in this example, the first set of 12 bioreactors received 20 mL of cementation solution every 4 days and the second set of 12 bioreactors received 20 mL of cementation solution every 7 days over a period of 60 days. Effluent analyses Effluent was collected from each bioreactor 24 h after irrigation with cementation solution, and the volume and pH were measured. Additionally, the cell concentration in the effluent was determined via direct cell counting using a Helber Cell Counting Chamber with a Leica DM500 compound microscope at 100x magnification (Leica Microsystems, Switzerland). Acid wash tests The co-disposed bioreactors were air-dried for 60 days after the cementation treatment period, the syringes were cut open and the contents were separated into two halves (top and bottom). The halves were then homogenized using a pestle and mortar and dried at 80°C until a consistent weight was achieved (3 days). Next, the samples were weighed (m before wash) and washed with 1 M hydrochloric acid, which dissolved the calcite produced during the cementation period, and were then dried once more at 80°C and weighed (mafterwash). The difference between the sample before and after washing (mbeforewash and mafterwash) was calculated to be the weight of calcite generated (Eq. 4). Provisions were made for the calcite already present in the acid-neutralizing FW used in this example. Results and discussion The results of the 4-day and 7-day irrigation studies were compared to the previous daily irrigation study in terms of effluent volume, reactor sealing day, pH, and final calcite content, to determine the optimum method for calcite generation in these systems in terms of cost and calcite yield. Effluent volume and reactor sealing As calcite is generated in these beds via MICP, a reduction in effluent volume was expected, with some bioreactors completely sealing during cementation, as observed in the daily irrigation study. Figures 36-38 show the effluent volumes obtained from the daily, 4-day, and 7-day irrigation studies, respectively. Table 8 shows the day upon which the bioreactors sealed, and no further effluent was observed thereafter. Table 8: Sealing day of the co-disposed bioreactors when no effluent was observed. Bioreactor Sealing day Daily Irrigation 4-day Intermittent Irrigation 7-day Intermittent Irrigation IRR-FW 29 &46 13 50 IRR-WR N / A N / A N / A IRR-BLND 21 33 N / A IRR-LAY N / A 41 N / A AGG-FW 29 21 29 AGG-WR N / A N / A N / A AGG-BLND 11 41 36 AGG-LAY N / A N / A N / A CONTROL-FW 54 N / A N / A CONTROL-WR N / A N / A N / A CONTROL-BLND 32 N / A N / A CONTROL-LAY 42 N / A N / A The comparison between the decrease in effluent flow followed general trend of Daily >4-day > 7-day. Bioreactors from the daily irrigation group sealed earlier than in the 4-day irrigation group, except for the bioreactors containing only FW, where IRR-FW and AGG-FW sealed on days 29 and 29, respectively, for the daily irrigation group and IRR-FW and AGG-FW sealed on days 13 and 21 respectively. Like in the daily irrigation group, AGG-FW from the 7-day irrigation group also sealed on day 29; however, IRR-FWonly sealed on day 50 compared to days 29 and 13 for the daily and 4-day irrigation groups, respectively. Interestingly, AGG-BLND sealed earlier than that observed on the 4-day irrigation group, on day 36 compared to day 41. Only three reactors sealed in the 7-day irrigation group compared to the five that sealed in the 4-day group. Control reactors (CONTROL-FW. CONTROL-BLND, AND CONTROL-LAY) were only sealed in the daily irrigation group since, as explained previously, the calcite formed here was likely due to native urease-producing bacteria, and thus required significant and constant volumes of cementation solution to thrive and establish themselves in the bioreactors before facilitating MICP. Generally, it could be concluded that although the co-disposed bioreactors sealed earlier in the daily irrigation group, the sealing days weren’t too disparate between the daily irrigation and 4-day irrigation groups. Additionally, even though only three bioreactors sealed during the irrigation phase, it should be noted that calcite growth, as with all crystal growth, does not thrive in a wet environment, with calcite crystal growth preferring humid conditions since it facilitates amorphous calcium carbonate-to-calcite transformation via dissolution and precipitation; thus, the sealing day should not be seen as a success or failure but rather should be considered together with the final calcite content after drying in order to establish the effectiveness of a particular treatment. Effluent pH analyses pH analyses are important in MICP studies since an environmental increase to pH 8-9 indicates the breakdown of urea to ammonia via the action of urease, thus indicating the activity of ureaseproducing bacteria and signalling the first step of MICP. Figure 39 shows the final effluent pH for the daily, 4-day, and 7-day irrigation groups, respectively. The effluent pH was used as an indicator for MICP in the bioreactors, and for all irrigation groups, the pH of all bioreactors approached neutral conditions by the end of the 60-day treatment period. However, the inoculated bioreactors, particularly the co-disposed bioreactors, showed significantly increased effluent pH values approaching pH 9. Thus, in terms of final pH, no significant differences were observed between irrigation protocols and MICP was thought to occur in all co-disposed bioreactors. Bioreactor calcite content After receiving cementation solution at varying irrigation rates for 60 days, the bioreactors were allowed to air-dry for a further 60 days before undergoing acid wash tests to determine their total calcite content. For ease of reference, only the calcite content for the co-disposed bioreactors are presented here. Table 9 shows the calcite content of the co-disposed bioreactors of the daily, 4-day, and 7-day irrigation studies, respectively. Table 9: Calcite content for the co-disposed bioreactors obtained from the daily, 4-day, and 7-day irrigation studies, respectively, as determined by acid wash tests. Bioreactor Daily Irrigation Total Calcite (mg / g) 4-Day Irrigation Total Calcite (mg / g) 7-Day Irrigation Total Calcite (mg / g) IRR-BLND Top 156,96 141,51 152,28 IRR-BLND Bottom 74,37 4,49 64,36 IRR-BLND Total 231,32 146,00 216,64 1 RR-LAY Top 245,20 180,49 141,85 1 RR-LAY Bottom 16,94 51,60 67,72 IRR-LAY Total 262,14 232,09 209,57 AGG-BLND Top 170,98 183,47 166,09 AGG-BLND Bottom 16,54 3,14 4,60 AGG-BLND Total 187,52 186,62 170,69 AGG-LAY Top 337,65 199,68 169,99 AGG-LAY Bottom 40,25 94,87 70,05 AGG-LAY Total 377,90 294,55 240,04 CONTROL-BLND Top 105,07 20,60 30,64 CONTROL-BLND Bottom 61,42 -8,29 12,59 CONTROL-BLND Total 166,48 12,31 43,23 CONTROL-LAY Top 95,42 14,87 61,38 CONTROL-LAY Bottom 60,85 27,79 30,49 CONTROL-LAY Control 156,27 42,66 91,86 5 The calcite content data shows (Table 9) that the daily irrigation group had the highest total calcite yield from all treatments. Since there was no trend between the 4-day and 7-day irrigation groups in terms of total calcite content, in order to see an increased calcite yield, a substantial volume of cementation solution would be required to make a significant difference. The 4-day irrigation group showed higher calcite yields compared to the 7-day irrigation group with regards to the 10 inoculated bioreactors, except for IRR-BLND, where the 7-day calcite yield (216,64 mg / g) was very similar to the daily calcite yield (231,32 mg / g). In this way, besides the controls and a few exceptions, the bioreactors did not perform significantly different from each other. Interestingly, the calcite content in the bottom half of both inoculated bioreactors packed in the layered configuration (I RR-LAY and AGG-LAY) of the 4-day and 7-day groups was superior to that of the 15 daily irrigation group. The early sealing of beds is an important consideration here, since the quicker a bioreactor seals, less oxygen and nutrients become available throughout the bioreactor. This is especially true for the layered bioreactors where the capillary barrier effect limits oxidant ingress through the bed and thus limit the optimal conditions required for MICP to occur. Calcite crystal growth is optimal at 85% relative humidity, which likely occurs during the 60-day airdrying period. However, if reactors seal early, oxygen and nutrient ingress is limited; thus, the conditions are not favourable for MICP and the system relies on crystal growth using the available nutrients. In systems that do not seal early, or are irrigated intermittently, cementation solution is allowed to permeate through the bed and allow crystal seeding to occur more uniformly, thus allowing crystal formation throughout the bed during the drying period. Therefore, even though less reagent was used in the 4-day and 7-day irrigation groups, the larger intervals between irrigation are beneficial since crystal seeding and crystal formation is allowed to occur with less disturbance to the bioreactor bed, leading to similar calcite yields for almost all inoculated bioreactors. The cost implications between these irrigation protocols are also substantial. The 4-day and 7-day irrigation protocols utilized four- and eight-times less cementation solution for the experiment, respectively. These cost implications are critical when considering the upscaling of this hybrid MICP-co-disposal system for use in established mines. Optimizing the irrigation protocol is one way of improving feasibility of these systems. The layered system shows promise in terms of obtaining a structurally improved co-disposed bed with limited oxygen ingress and uniformly improved neutralization capacity. Considering the proficiency of calcite generation via MICP, the 4-day and 7-day irrigation protocols performed similarly to the daily irrigation group, and even outperformed the latter in the inoculated bioreactors packed in the layered configuration. Combined with the associated cost reduction, the intermittent irrigation groups provide a feasible and promising way forward for ARD prevention using MICP and co-disposal. Application of microbially induced calcite precipitate (MICP) in mine waste typically focuses on mine dust suppression and seem to only produce calcite in the top of mine waste systems. In these systems, only water-retaining, hygroscopic mine tailings are used. This does not allow for easy oxygen access to the MICP-bacteria lower in these beds and also does not allow for crystal seeding to occur in the limited pore spaces available in this type of fine material. Further, only non-water-retaining mine waste rock are used. This allows easy access of nutrients and oxygen throughout the bed but does not provide the required retention time of the inoculum and cementing solution to establish themselves, which also leads to limited crystal seeding, and thus non-uniform crystal formation. Additionally, the large pore sizes make it unlikely for bridge-forming MICP to occur, which leads no noticeable increase in structural stability. These systems are prone to fractures over time, thus making the underlying waste vulnerable to oxidation once in contact with natural elements. With a view to preventing the formation of ARD in mine waste, the present disclose uses a mixture of non-hygroscopic mine waste rock and hygroscopic mine waste tailings at ratio of 3:2 by mass, inoculated with ureolytic bacteria. The hygroscopic mine tailings, which are fine, water-retaining particles, allow for water hold-up and thus promotes lateral dispersion, whereas the larger non-water-retaining waste rock particles allow for easier flow through and thus axial dispersion. Calcite produced via microbially induced calcite precipitate (MICP) was used to create a reactive barrier to oxidants that promote the onset of ARD in mine waste; thus delaying the emergence of acid producing reactions. The calcite precipitate bifunctionally provided structural stability, reduced acid seepage, and promoted neutralization. The layered configuration, where alternate layers of mine waste rock and mine tailings are packed leads to limited oxidant ingress and enhanced ARD prevention in heaped waste beds, as well as long-term neutralization capacity. The irrigation further enhances the system. Even though the blended packing typically shows more uniformity when using daily irrigation of cementing solution (over a period of 60 days), when using weekly irrigation (over a period of 60 days), greater uniformity is observed in reactors packed in the layered configuration, with increased calcite concentration in the lower halves of beds, as compared to daily irrigation. This intermittent irrigation allows for less reagent to be used, but importantly, allows for oxygen access through the bed and undisturbed crystal formation. Together with heterogenous packing, bridge forming MICP can occur and thus yield beds with uniform calcite formation, long-term structural stability, and long-term ARD prevention with its enhanced neutralization capacity. When using unrealistically high volumes of synthetic acid rain (59 times more than expected in nature) to stress-tests the reactors, it was found that neutralizing conditions may persevere for over 75 years in the present hybrid MICP-co-disposal system. It will be appreciated that other applications are envisioned, for example, microbially induced calcite precipitate (MICP) may be applied in mine waste heap management. In practice, a heap structure may be formed by providing alternating layers of acid producing rock particles and acid neutralizing rock particles, adding a bacteria culture to the mixture so produced, and then irrigating with a cementation solution. Alternately, acid producing rock particles may be blended with acid neutralizing rock particles and formed into a heap, adding a bacteria culture to the mixture so produced, and the mixture then irrigated with a cementation solution. The heap may otherwise be constructed in accordance with known techniques for mine or waste heap or dump construction. The bacteria culture may be selected to cause calcite precipitates to form cementation bonds between rock particles in the mixture which increase overall compressive strength of the structure and reduce permeability of oxidants therethrough. For example, the bacteria culture may be urease producing bacteria, preferably, Sporosarcina pasteurii. The bacterial culture may be added to the mixture using the drip emitters of the gravity drip system. The heap structure so produced has the advantages of being stable and also having a much reduced, possibly eliminated, propensity for ARD production. The cementation solution may be made up of urea and calcium chloride and preferably 1 M urea, 0.5 M calcium chloride and 3% ATCC® 1376 media. Irrigation may be provided by a gravity drip system on the top of the heap and may include spaced apart drip emitters for allowing the cementation solution to drip onto the structure. The drip emitters may be of conventional configuration and may be between 0.5 m and 2 m apart and preferably 1 m apart. The mixture may be irrigated regularly, such as daily, or preferably weekly, to allow the cementation solution to permeate through the structure. Microbially induced calcite precipitate (MICP) may also be applied in ARD-preventing mine waste landfill by using MICP bio-cement instead of construction cement. Construction cement leaves a huge footprint on the environment since it is one of the two largest producers of carbon dioxide (CO2). In this application, coal waste rock, coal fine waste, cementing materials, and water may be mixed in a specific proportional basis. The mass ratio of the coal waste rock to coal fine waste may be maintained as 3:2 as disclosed herein. The mixture may be stirred to form coagulable paste slurry which may then be piped either down a decline, shaft, or surface borehole into the area of the mine that requires backfilling. It will be appreciated that, in this application, irrigation with the cementation solution occurs together with the mixing of the other materials. Microbially induced calcite precipitate (MICP) may also be applied in metal extraction or immobilisation: In this application, metal immobilisation under leaching conditions may be achieved. This opens the possibility of using MICP to extract metals (ideally rare earth elements or critical raw materials) from mine wastes while preventing ARD formation at the same time. Microbially induced calcite precipitate (MICP) may also be applied in treatment of ARD: In this application, mine waste can be used as a solid matrix to immobilise MICP that has the ability to neutralise acid generated from sulphidic mine waste. The foregoing description has been presented for the purpose of illustration; it is not intended to be exhaustive or to limit the invention to the precise forms disclosed. Persons skilled in the relevant art can appreciate that many modifications and variations are possible in light of the above disclosure. The language used in the specification has been principally selected for readability and instructional purposes, and it may not have been selected to delineate or circumscribe the inventive subject matter. It is therefore intended that the scope of the invention be limited not by this detailed description, but rather by any claims that issue on an application based hereon. Accordingly, the disclosure of the embodiments of the invention is intended to be illustrative, but 5 not limiting, of the scope of the invention to be set forth in any accompanying claims. Finally, throughout the specification and any accompanying claims, unless the context requires otherwise, the word ‘comprise’ or variations such as ‘comprises’ or ‘comprising’ will be understood to imply the inclusion of a stated integer or group of integers but not the exclusion of any other 10 integer or group of integers.

Claims

1. A method of forming a heap structure from acid producing rock particles, the method comprising: mixing acid producing rock particles with acid neutralizing rock particles to provide a mixture, adding a bacteria culture to the mixture, and irrigating the mixture with a cementation solution, wherein the bacteria culture is selected to cause calcite precipitates to form cementation bonds between rock particles which increase overall compressive strength of the structure and reduce permeability of oxidants therethrough.

2. The method of forming a heap structure as claimed in claim 1, wherein the mixture is formed by a layer of acid producing rock particles on top of a layer of acid neutralizing rock particles or vice versa.

3. The method of forming a heap structure as claimed in claim 1 or claim 2, wherein the acid neutralizing rock particles have a relatively smaller particle size distribution compared to the size distribution of the acid producing rock particles.

4. The method of forming a heap structure as claimed in any one of the preceding claims, wherein irrigation is provided by a gravity drip system which includes spaced apart drip emitters for allowing the cementation solution to drip onto the structure.

5. The method of forming a heap structure as claimed in claim 4, wherein the drip emitters are placed between 0.5 m and 2 m apart.

6. The method of forming a heap structure as claimed in claim 4 or claim 5, wherein the drip emitters are placed 1 m apart.

7. The method of forming a heap structure as claimed in any one of the preceding claims, wherein the acid producing rock particles are non-hygroscopic and acid neutralizing rock particles are hygroscopic.

8. The method of forming a heap structure as claimed in claim 7, wherein the acid neutralizing rock particles are less pervious to water and air than the acid producing rock particles.

9. The method of forming a heap structure as claimed in any one of the preceding claims, wherein the acid neutralizing rock particles are mine wastes with a low sulphide content.

10. The method of forming a heap structure as claimed in any one of the preceding claims, wherein the quantity of acid producing rock particles is greater than the quantity of acid neutralizing rock particles in the mixture.

11. The method of forming a heap structure as claimed in claim 10, wherein the quantities of acid producing rock particles and acid neutralizing rock particles in the mixture are related by a mass ratio of about 5:1.

12. The method of forming a heap structure as claimed in claim 10, wherein the quantities of acid producing rock particles and acid neutralizing rock particles in the mixture are related by a mass ratio of about 3:2.

13. The method of forming a heap structure as claimed in any one of claims 1 to 9, wherein the quantities of acid producing rock particles and acid neutralizing rock particles in the mixture are equal.

14. The method of forming a heap structure as claimed in any one of claims 1 to 9, wherein the quantity of acid producing rock particles is less than the quantity of acid neutralizing rock particles in the mixture.

15. The method of forming a heap structure as claimed in claim 14, wherein the quantities of acid producing rock particles and acid neutralizing rock particles in the mixture are related by a mass ratio of about 2:3.

16. The method of forming a heap structure as claimed in any one of the preceding claims, wherein the bacteria culture is urease producing bacteria.

17. The method of forming a heap structure as claimed in claim 16, wherein the bacteria culture is Sporosarcina pasteurii.

18. The method of forming a heap structure as claimed in any one of the preceding claims, wherein the cementation solution is made up of urea and calcium chloride.

19. The method of forming a heap structure as claimed in claim 18, wherein the cementation solution is made up of 1 M urea, 0.5 M calcium chloride and 3% ATCC® 1376 media.

20. The method of forming a heap structure as claimed in claim 19, including irrigating the mixture weekly.

21. The method of forming a heap structure as claimed in any one of the preceding claims, 5 wherein the acid neutralizing rock particles contain neutralizing minerals.

22. The method of forming a heap structure as claimed in claim 21, wherein the neutralizing minerals include calcite, kaolinite, or gypsum.10 23. The method of forming a heap structure as claimed in any one of the preceding claims,wherein the acid producing rock is coal waste rock and the acid neutralizing rock is coal fine waste.

24. The method of forming a heap structure as claimed in claim 23, wherein the coal waste 15 rock has a particle size distribution of 2-4 mm.

25. The method of forming a heap structure as claimed in claim 23, wherein the coal fine waste has a particle size distribution of 0.01-1 mm.

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