Method for heavy metal removal in oligotrophic environments using semiconductor mineral-strengthened microorganisms

Abstract

The application discloses a method for removing heavy metal in oligotrophic environment by using semiconductor mineral reinforced microorganisms, which utilizes light energy to replace organic matter to drive sulfate-reducing bacteria metabolism to remove heavy metal ions. The sphalerite produced by microbial metabolism or natural sphalerite or chemically synthesized sphalerite is excited to produce photoelectrons under light conditions, and the sulfate-reducing bacteria with extracellular electron transfer capacity utilize the photoelectrons to carry out sulfate reduction metabolism to produce hydrogen sulfide, which reacts with heavy metal ions (such as lead ions) in wastewater to form heavy metal sulfide precipitates such as lead sulfide, so as to remove the heavy metal ions from the water phase. The method cooperatively utilizes light energy and chemical energy to efficiently treat heavy metal pollution, and simultaneously avoids problems such as secondary pollution, active sludge production and greenhouse gas emission in the treatment process, has low treatment cost, is environment-friendly, and has a wide application prospect.

Description

TECHNICAL FIELD

[0001] The present application relates to a treatment technology of heavy metal ions (such as Cu 2+ , Zn 2+ , Pb 2+ , Cd 2+ , etc.) in wastewater, in particular to a method for removing lead ions by using a photosulfate-reducing bacteria to replace organic matter as an electron / energy source to reduce sulfates to produce hydrogen sulfide under light conditions, using photoelectrons generated by a semiconductor mineral—zinc blende (ZnS), which belongs to the field of environmental engineering and water treatment. BACKGROUND

[0002] With the development of industry and mining, more and more wastewater containing heavy metals is discharged into the environment, causing environmental pollution and ecological damage. Long-term accumulation of heavy metals in water bodies can enter the bodies of animals and plants through the food chain and ultimately harm human health. Current methods for removing heavy metals from wastewater include chemical, physical, and biological treatment methods. Biological treatment refers to a method that uses organisms (such as microorganisms, plants, etc.) to absorb, transform, degrade, or stabilize heavy metal pollutants. This method has the advantages of low energy consumption, green sustainability, etc. Sulfate-reducing bacteria (SRB) are widely used in the treatment of heavy metal pollution in industrial wastewater and mine drainage due to their environmental adaptability and active metabolism. SRB can oxidize various organic matter to reduce sulfates to produce negative divalent sulfur, which reacts with metal ions (such as Cu 2+ , Zn 2+ , Pb 2+ , Cd 2+ , Hg 2+ , Sn 2+ , etc.) in solution to form water-insoluble metal sulfide precipitates, thereby achieving the removal of heavy metal ions. However, the concentration of organic matter in mine drainage and other contaminated wastewater is usually low (oligotrophic environment), and the energy source required by SRB is insufficient, resulting in weak metabolic activity. Therefore, a large amount of organic matter needs to be added to drive SRB to metabolize during wastewater treatment, which has the problems of high cost, easy secondary pollution, greenhouse gas emissions, etc. Therefore, how to use energy sources (electron donors) other than organic matter to drive sulfate-reducing bacteria to metabolize and treat heavy metals is the key and difficulty of current heavy metal wastewater treatment.

[0003] Choudhary et al. used manure, sawdust, millet feed and sugarcane residue as substrates to drive sulfate-reducing bacteria to reduce sulfate and remove iron, copper, nickel, zinc, cobalt and manganese ions from water bodies, and found good removal effect. However, complex organic matter not only inhibits the activity of sulfate-reducing bacteria, but also easily causes secondary pollution and causes the massive reproduction of other heterotrophic microorganisms, thereby affecting the removal of heavy metals by the system (Choudhary, R. P. & Sheoran, A. S. Performance of single substrate in sulphate reducing bioreactor for the treatment of acid mine drainage. [J]. Minerals Engineering, 39, 29-35.).

[0004] Su et al. used the electrode electrons of a bioelectrochemical system (bias -0.4 V vs. Ag / AgCl) to drive sulfate-reducing bacteria to reduce sulfate, and the removal efficiency of sulfate can reach 72%, and a large amount of hydrogen sulfide is produced for heavy metal treatment. However, the bioelectrochemical system has problems such as high energy consumption, electrode corrosion, byproduct production, and poor long-term stability and stability. And there are certain requirements for the water quality of wastewater, and high initial investment and high-cost maintenance and operation are required (Su, W., Zhang, L., Tao, Y., Zhan, G., Li, D., and Li, D. (2012). Sulfate reduction with electrons directly derived from electrodes in bioelectrochemical systems. [J]. Electrochemical Communication. 22, 37-40.).

[0005] Guo et al. used zero-valent iron (ZVI) as an electron donor to drive sulfate-reducing bacteria to reduce sulfate, and found that the removal rate of heavy metals in the experimental group with the addition of zero-valent iron reached more than 99%. However, the addition of zero-valent iron will lead to the generation of a large amount of corrosion products such as iron oxide and iron hydroxide, forming difficult-to-handle secondary waste, increasing the complexity and cost of subsequent treatment. Not only that, but zero-valent iron will be consumed during the reaction process, and its regeneration process is complex and costly, so it is usually used in a disposable manner in actual operation, but this greatly increases the operating cost of the system (Guo, J., Kang, Y. & Feng, Y. a Bioassessment of heavy metal toxicity and enhancement of heavy metal removal by sulfate-reducing bacteria in the presence of zerovalent iron. [J]. Journal of Environmental Management. 203, 278-285.).

[0006] Microbial photoelectron metabolism refers to the process in which some microorganisms that can perform extracellular electron transfer use membrane surface proteins or secrete redox-active substances to uptake photoelectrons generated by semiconductor minerals for intracellular metabolism and energy synthesis, thereby realizing the conversion of light energy and chemical energy into biomass energy. SUMMARY

[0007] The purpose of the present application is to provide a method for treating heavy metals in oligotrophic wastewater by using light energy to replace organic matter to drive sulfate-reducing bacteria, in order to solve the problems of low treatment efficiency, serious secondary pollution, poor environmental compatibility, greenhouse gas emission, high treatment cost and other problems in the current method. The present application utilizes sphalerite and sulfate-reducing bacteria with extracellular electron transfer capacity that exist in the natural environment and are synthesized by microbial metabolism, to realize the synergistic use of light energy and chemical energy, and to efficiently treat heavy metal pollution, while avoiding problems such as secondary pollution, activated sludge production and greenhouse gas emission during treatment.

[0008] The photo-generated holes and photo-generated electrons of the semiconductor mineral used in the application have oxidation and reduction properties respectively, and will separate, but will recombine in a short time. When there are reducing substances such as divalent iron and small molecule organic acids in the system to fill the photo-generated holes, the photoelectrons and photo holes of the semiconductor mineral can be better separated, and the reducing photoelectrons can be taken up by the microorganisms with extracellular electron transfer ability for intracellular metabolism, including sulfate-reducing bacteria. The sulfate-reducing bacteria uptake of semiconductor mineral photoelectrons can be used for the reduction of sulfate to produce sulfide ions (under neutral conditions), and the sulfide ions are easy to react with metal cations (such as lead ions) in the system to form insoluble metal sulfides (such as lead sulfide). Subsequently, the metal cations can be separated from the liquid phase by centrifugation to collect the sulfide precipitate.

[0009] The method of the application is suitable for the removal of various heavy metals in wastewater, including Cu 2+ , Zn 2+ , Pb 2+ , Cd 2+ , Hg 2+ , Sn 2+ , etc. In the embodiment of the application, the removal of Pb 2+ in industrial / mining wastewater is simulated, and the process can be adjusted according to the actual wastewater conditions.

[0010] The technical solutions provided by the application are as follows:

[0011] A method for treating heavy metal ions in oligotrophic wastewater, which utilizes the synergistic effect of sphalerite and sulfate-reducing bacteria capable of extracellular electron transfer under light conditions to form insoluble metal sulfides from heavy metal ions in wastewater, and separate them from the liquid phase.

[0012] In the above method for treating heavy metal ions in oligotrophic wastewater, the heavy metal ions include but are not limited to Cu 2+ , Zn 2+ , Pb 2+ , Cd 2+ , Hg 2+ , Sn 2+ , etc., which can react with sulfide ions to form insoluble metal sulfides.

[0013] In the above method for treating heavy metal ions in oligotrophic wastewater, the sulfate-reducing bacteria capable of extracellular electron transfer include Desulfovibrio, Desulfococcus, Desulfosarca, etc., and Desulfovibrio desulfuricans G20 (ATCC 29577) is preferred.

[0014] In the method for treating heavy metal ions in oligotrophic wastewater, the amount of the sulfate-reducing bacteria added to the wastewater is generally controlled to obtain a final bacterial amount OD 600 between 0.08 and 0.1; the concentration of the sphalerite in situ synthesized or added in the wastewater is preferably 10-50 mg / L; and the total concentration of the heavy metal ions in the treated wastewater is preferably in the range of 50 μM-0.5 mM.

[0015] In the method for treating heavy metal ions in oligotrophic wastewater, the pH of the wastewater is preferably adjusted to 6-6.5 before the addition of the sulfate-reducing bacteria, and if the pH is lower than 6, the growth of the sulfate-reducing bacteria is not favorable. A hydrogen phosphate-dihydrogen phosphate buffer can be added to the system to maintain the pH of the system at 6-6.5.

[0016] In the method for treating heavy metal ions in oligotrophic wastewater, a certain amount of hole-trapping agent is added to better separate the photoelectrons and photoholes of the sphalerite under light conditions. The hole-trapping agent can be, for example, ascorbic acid, sodium oxalate, humic acid, divalent iron ions, etc. In some embodiments of the present application, 1-20 mM ascorbic acid is added as the hole-trapping agent for the sphalerite at the same time when the sphalerite and the sulfate-reducing bacteria are added, and preferably 10 mM ascorbic acid is added.

[0017] In the method for treating heavy metal ions in oligotrophic wastewater, the light conditions can use natural sunlight or artificial light sources, such as placing under an artificial mercury lamp light source, irradiating mainly in the wavelength range of 365-500 nm, which can excite the separation of photoelectrons and photoholes of the sphalerite, and the irradiation light intensity is 35 mW / cm 2 ; or under natural sunlight, maintaining the temperature in the range of 30-35 °C.

[0018] The sphalerite used in the method of the present application can be synthesized by microorganisms themselves, artificially synthesized by chemical synthesis, or natural sphalerite. The added sphalerite preferably has a nano-scale particle size, and more preferably has an average particle size of 1 nm to 10 nm.

[0019] In the method for treating heavy metal ions in oligotrophic wastewater, if the concentration of zinc ions (Zn 2+ ) in the wastewater is higher than 10 mg / L, the sphalerite can not be added to the wastewater, and only the sulfate-reducing bacteria capable of extracellular electron transfer are added. After 5 days of cultivation, the sulfate-reducing bacteria can synthesize nano-scale sphalerite by themselves.

[0020] If the zinc ion concentration in the wastewater is low, the artificial chemically synthesized wurtzite or natural wurtzite can be added at the same time as the sulfate-reducing bacteria are added, wherein the wurtzite concentration can range from 10 mg / L to 50 mg / L; or the wurtzite can be biosynthesized by the sulfate-reducing bacteria in advance to obtain a compound of the sulfate-reducing bacteria and the wurtzite, and then the compound is added to the wastewater.

[0021] The method for biosynthesizing wurtzite by using sulfate-reducing bacteria and the method for chemically synthesizing wurtzite are both known in the art, and will not be described here.

[0022] The method for cooperatively treating heavy metal ions in oligotrophic wastewater by using wurtzite and sulfate-reducing bacteria under light conditions improves the tolerance of the microorganisms to the heavy metal ions, promotes the reduction of the sulfate by the microorganisms, and thus improves the removal effect of the heavy metal ions, thereby saving the operation cost. BRIEF DESCRIPTION OF DRAWINGS

[0023] Figure 1 Figure 6 is an X-ray diffraction (XRD) spectrum of the biosynthesized wurtzite sample and the chemically synthesized wurtzite sample in Example 1 compared with a standard wurtzite spectrum (bottom) for identification.

[0024] Figure 2 Figure 7 is a kinetic curve of the cooperative removal of lead ions by the sulfate-reducing bacteria and the biosynthesized or chemically synthesized wurtzite under light or dark conditions when the lead ion concentration is 50 mg / L in Example 1.

[0025] Figure 3 Figure 8 is the lead ion removal rate after 54 hours of reaction under the conditions of no addition of wurtzite (wurtzite removal group), addition of dead bacteria and wurtzite (dead cell group), and no addition of bacterial cells (cell removal group) under light conditions when the lead ion concentration is 50 mg / L in Example 2.

[0026] Figure 4 Figure 9 is the lead ion removal rate after 54 hours of reaction of the sulfate-reducing bacteria and the biosynthesized or chemically synthesized wurtzite under light conditions when the lead ion concentration ranges from 10 mg / L to 200 mg / L in Example 3.

[0027] Figure 5 Figure 10 is an XRD spectrum of the reaction product after the cooperative removal of lead ions by the sulfate-reducing bacteria and the chemically synthesized wurtzite under light conditions when the lead ion concentration is 50 mg / L in Example 3.

[0028] Figure 6 Figure 11 is a scanning electron microscope (SEM) image (left) and an energy dispersive spectroscopy (EDS) image (right) of the reaction product after the cooperative removal of lead ions by the sulfate-reducing bacteria and the chemically synthesized wurtzite under light conditions when the lead ion concentration is 50 mg / L in Example 3.

[0029] Figure 7 is the removal rate of lead ions by the biosynthesis group and the chemical synthesis group in Example 4 under light conditions at a lead ion concentration of 50 mg / L for 3 cycles.

[0030] Figure 8 is the removal rate of lead ions by the biosynthesis group and the chemical synthesis group at a lead ion concentration of 50-150 mg / L under light / dark conditions after 54 hours of reaction. DETAILED DESCRIPTION

[0031] The application will be described in greater detail below with reference to specific embodiments, but the application is not limited thereto.

[0032] In the following examples, the experimental methods are described below, and if not specifically stated, they are all conventional methods; the reagents and materials described below can be obtained from commercial channels if not specifically stated.

[0033]

Example 1

[0034] Desulfovibrio desulfuricans G20 (ATCC 29577, purchased from China General Microbiological Culture Collection Center) was mixed with 50% glycerol at a volume ratio of 1:1 in a cryo tube and stored at -80°C. When used, the stored bacteria were added to the MS medium (composition: 0.5 g / L KH2PO4, 1.0 g / L NH4Cl, 2.0 g / L MgSO4·7H2O, 0.1 g / L CaCl2·2H2O, 1.0 g / L Na2SO4, 10 mL / L vitamin solution (2 mg / L biotin folic acid, 2 mg / L pyridoxine HCl, 10 mg / L thiamine HCl, 5 mg / L riboflavin, 5 mg / L nicotinic acid, 5 mg / L calcium D-(+)-pantothenate, 0.1 mg / L cyanocobalamin, 5 mg / L p-aminobenzoic acid, 5 mg / L thioctic acid), 10 mL / L trace metal solution (2.8 g / L nitrilotriacetic acid, 0.24 g / L FeSO4·7H2O, 0.1 g / L MnCl2·4H2O, 0.17 g / L CoCl2·6H2O, 0.02 g / L CuCl2·2H2O, 0.21 g / L ZnSO4·7H2O, 0.01 g / L Na2MoO4·2H2O), 2.0 g / L sodium lactate, 1.0 g / L yeast extract, 0.1 g / L ascorbic acid) at a 5% inoculation amount, and then activated and subcultured at the same ratio. To avoid the influence of extracellular secretions of sulfate-reducing bacteria on the removal of lead ions, the bacteria were washed with PBS buffer multiple times, the supernatant was discarded, and the bacteria were resuspended to achieve the washing effect. Desulfovibrio desulfuricans G20 is a strict anaerobic microorganism, and all operations were carried out under anaerobic conditions and the solution was deoxygenated to allow the bacteria to grow faster.

[0035] The cultured Desulfovibrio desulfuricans G20 bacteria were subcultured into a zinc-containing DMS medium (composition similar to the MS medium, with the addition of 0.19 g / L ZnCl and 5.5 g / L HEPES) at a volume ratio of 10%, and cultured at 30°C for 5 days. After centrifugation at 4°C to collect the mineral-microorganism aggregates, the aggregates were added to a deoxygenated salt solution to prepare reaction solution 1 (obtained biosynthetic sphalerite containing microbial cells, with a sphalerite concentration of 18 mg / L).

[0036] Weigh 0.58 g of Na₂S solid and dissolve it in 500 mL of water to prepare a 15 mM Na₂S solution; weigh 0.136 g of ZnCl₂ solid and dissolve it in water to prepare a 2 mM ZnCl₂ solution; use a peristaltic pump to pass 500 mL of Na₂S solution into 500 mL of ZnCl₂ solution at a flow rate of 30 mL / h while continuously stirring; collect the resulting suspension by centrifugation, and wash the resulting sphalerite precipitate with ultrapure water until the conductivity of the water after washing is similar to that of pure water, then bottle and seal it, and store it at 4°C protected from light (to obtain chemically synthesized sphalerite). Collect the cultured desulfurized Vibrio G20 by centrifugation at 4°C, add it to a deoxygenated salt solution, and then add chemically synthesized sphalerite to bring the final concentration to 21 mg / L, to obtain reaction solution 2.

[0037] Figure 1 The XRD patterns of sphalerite synthesized by the above-mentioned biosynthesis method and chemical coprecipitation method are shown. Comparison with the standard XRD pattern of sphalerite confirms that the minerals synthesized by the two methods are both sphalerite.

[0038] The above reaction solution 1 (containing biosynthetic zincblende) and reaction solution 2 (containing chemically synthesized zincblende) were separately added to anaerobic simulated wastewater containing lead ions (50 mg / L) (pH = 6, containing 10 mM ascorbic acid as a cavitation scavenger). The mixed solution was then placed under a mercury lamp with an irradiation intensity of 35 mW / cm². 2 The samples in the dark group were wrapped in aluminum foil to protect them from light and maintained at a temperature between 30-35℃. Figure 2 As shown, after 54 hours of reaction, the lead ion removal efficiency of both the biosynthetic group and the chemical synthesis group reached over 95% under light conditions, while only 10% of lead ions were removed under dark conditions. Under light conditions, the lead ion removal rate and amount of both the biosynthetic group and the chemical synthesis group were significantly improved compared to the dark group.

[0039]

Example 2

[0040] The chemical synthesis of zincblende and the bacterial culture method described in Example 1 were used. Zincblende, live bacteria, and dead bacteria were added to simulated wastewater containing lead ions (50 mg / L) according to experimental requirements, resulting in three groups: a group with zincblende added only (cell removal), a group with zincblende added only (zincblende removal), and a group with both zincblende and dead bacteria. The zincblende concentration was 21 mg / L, and the bacterial OD... 600 The value was 0.1, and the ascorbic acid concentration was 10 mM. The mixture was then irradiated under light (irradiation conditions were the same as in Example 1). After 54 hours of reaction, as... Figure 3As shown, the lead ion removal efficiency in the cell removal group, the zinc sphalerite removal group, and the dead cell group was all less than 15%. This indicates that the microbial-zinc sphalerite synergistic removal of lead ions in Example 1 requires a combination of microbial metabolism and the photoelectric conversion ability of zinc sphalerite.

[0041]

Example 3

[0042] The biosynthesis / chemical synthesis of zincblende and the bacterial culture method described in Example 1 were employed. Synthetic zincblende, bacterial culture, and ascorbic acid were added to simulated wastewater containing different concentrations of lead ions (10-200 mg / L), and the final OD of the bacterial culture was adjusted accordingly. 600 The value was 0.1, the ascorbic acid concentration was 10 mM, and the zincblende concentration was 18 mg / L for the biosynthetic group and 21 mg / L for the chemically synthesized group. The mixture was then irradiated under light conditions (same as in Example 1). After 54 hours of reaction, as... Figure 4 As shown, when the lead ion concentration is below 100 mg / L, both the biosynthetic and chemical synthesis groups can achieve a lead ion removal rate of over 95% under light conditions. However, when the lead ion concentration is 100-150 mg / L, the biosynthetic group exhibits better lead ion removal performance. When the lead ion concentration reaches 200 mg / L, the removal efficiency of both the biosynthetic and chemical synthesis groups drops significantly to below 15%. At this concentration, lead ions have strong biotoxicity to microorganisms, and the removal of lead ions depends on the adsorption by microorganisms and minerals. A portion of the reaction product from the 50 mg / L lead ion concentration reaction group was collected by centrifugation at 4°C after 54 hours of reaction, dried, and the phase composition of the reaction product was determined by XRD. Figure 5 Another portion was fixed with 2.5% glutaraldehyde for 1 hour, then washed with PBS buffer, centrifuged and dehydrated with ethanol at concentration gradients of 20-100%, and the morphology and elemental composition of the reaction product were analyzed by SEM. Figure 6 The reaction products were found to form a stable lead sulfide (galena) precipitate containing added sphalerite particles.

[0043]

Example 4

[0044] The biosynthesis / chemical synthesis of zincblende and the bacterial culture method described in Example 1 were employed. Synthetic zincblende, bacterial culture, and ascorbic acid were added to simulated wastewater containing lead ions (50 mg / L), and the final OD of the bacterial culture was adjusted. 600The value is 0.1, the ascorbic acid concentration is 10 mM, the sphalerite concentration (biological synthesis group is 18 mg / L, chemical synthesis group is 21 mg / L). Then the mixture is irradiated under light (the irradiation condition is the same as in Example 1), after 54 h of reaction, the precipitate is collected by centrifugation and then added into the simulated wastewater containing 50 mg / L lead ions for reaction, after 24 h, the precipitate is collected again by centrifugation and then added into the simulated wastewater containing 50 mg / L lead ions for reaction for 24 h. As shown in Table 2, it is found that the biological synthesis group still has more than 50% removal rate of lead ions after the third cycle, while the chemical synthesis group has only 20% removal efficiency of lead ions after the third cycle, which indicates that the biological synthesis group exhibits better removal effect for the recycling and long-time treatment of lead ions. Figure 7

[0045] Finally, it should be noted that the above examples are only used to further understand the present application and not to limit it. However, those skilled in the art can understand that various substitutions and modifications are possible without departing from the spirit and scope of the present application and the appended claims. Therefore, the present application should not be limited to the disclosed content of the examples, and the scope of the present application claimed is defined by the scope of the claims.​

Claims

1. A method for treating heavy metal ions in oligotrophic wastewater, which utilizes the synergistic effect of wurtzite and sulfate-reducing bacteria capable of extracellular electron transfer under light conditions to form insoluble metal sulfides from heavy metal ions in wastewater, and separate them from the liquid phase.

2. The method of claim 1, wherein, The heavy metal ions include one or more of Cu 2+ , Zn 2+ , Pb 2+ , Cd 2+ , Hg 2 + , Sn 2+ ; the sulfate-reducing bacteria capable of extracellular electron transfer are selected from one or more of Desulfovibrio, Desulfococcus, Desulfonema.

3. The method of claim 2, wherein, The sulfate-reducing bacteria capable of extracellular electron transfer are Desulfovibrio G20.

4. The method of claim 1, wherein, The amount of the sulfate-reducing bacteria added to the wastewater is between 0.08 and 0.1 of the final bacteria amount OD 600 The sphalerite is biologically synthesized in-situ by the sulfate-reducing bacteria in the wastewater and / or is artificially added to the wastewater, wherein the added sphalerite is natural sphalerite, chemically synthesized sphalerite and / or biologically synthesized sphalerite.

5. The method of claim 4, wherein, The average particle size of the added wurtzite is 1-10 nm, and the concentration added to the wastewater is 10-50 mg / L.

6. The method of claim 4, wherein, When the concentration of zinc ions in the wastewater reaches 10 mg / L or more, no wurtzite is added to the wastewater, only the sulfate-reducing bacteria are added.

7. The method of claim 1, wherein, The wurtzite is biosynthesized in advance using the sulfate-reducing bacteria to obtain a composite of sulfate-reducing bacteria and wurtzite, and then the composite is added to the wastewater.

8. The method of claim 1, wherein, The light conditions are irradiation using natural sunlight, or the addition of artificial light sources, the main wavelength range of which is 365-500 nm.

9. The method of claim 1, wherein, The pH of the wastewater is controlled to be 6-6.

5.

10. The method of claim 1, wherein, A hole-trapping agent is added to the wastewater.

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