Uranium pollution in-situ remediation method and application thereof

Abstract

The invention discloses an in-situ remediation method for uranium pollution and application of the in-situ remediation method. According to the in-situ remediation method for uranium pollution, pyrite is driven to be hydrolyzed, oxidized, subjected to atomic rearrangement and biomineralized through interaction of microorganisms and pyrite, uranium pollutants are fixedly sealed and stored in reconstructed product iron oxide lattices, uranium pollution areas are remedied, and the microorganisms comprise one or more of brucella, geobacter and clostridium. If the uranium polluted area contains microorganisms, natural humus is used as an electron donor to stimulate in-situ microorganisms. The natural humus solution is injected into the uranium polluted area in a circulating pumping and injecting mode, in-situ microorganisms are stimulated, the concentration of the natural humus solution is 0.05-5 g / L, and the dosage of the natural humus solution is 5-500 g / day. According to the method, in-situ minerals, microorganisms and geochemical resources are fully considered and reasonably utilized, the engineering cost is saved by means of the in-situ occurrence interaction relation of the minerals and the microorganisms, and the remediation efficiency is high.

Description

Technical Field

[0001] The present application relates to the technical field of heavy metal pollution remediation, and specifically to an in-situ remediation method for uranium pollution and its application. Background Art

[0002] Uranium resources are an important material basis for supporting the development of nuclear power, and their efficient development is the key to achieving the "dual carbon" goal. With the increasing global demand for nuclear energy, the development and utilization of uranium resources are in a rapid advancement stage. The development of uranium resources not only brings about an increase in energy supply, but also brings potential threats to the environment and ecosystem. During uranium mining and processing, soil, surface water and groundwater may be polluted to a certain extent, which may have a potential impact on the health of surrounding residents and regional ecological security. Due to its unique chemical properties, uranium has both heavy metal toxicity and radiotoxicity. It can be absorbed by the human body through the food chain, and then enriched in organs such as bones, liver and kidneys, directly affecting the body's growth and development and various physiological and biochemical functions, inducing hair loss, leukemia, and even death.

[0003] In view of the indispensable role of uranium resources in social economy, energy, national defense and science and technology, as well as the serious health risks and hazards that uranium pollution may cause, potential uranium pollution must be controlled and dealt with. At the same time, many uranium mines have entered the decommissioning stage after years of mining, and there is an urgent need to develop effective and economically reasonable mining area decommissioning and remediation technologies.

[0004] At present, the remediation technology of uranium pollution can be divided into two categories according to the treatment method: in situ remediation and ex situ remediation. Ex situ remediation requires the extraction or transfer of contaminated water or soil to a specific area for secondary treatment. Its treatment cost is high, the remediation cycle is long, and the treatment effect is difficult to sustain. In situ remediation refers to the direct remediation work at the site of pollution, without the need to transfer the contaminated water or soil, with less impact on the environment and lower treatment cost. At present, in situ remediation mainly includes physical (adsorption, ion exchange and membrane separation technology, etc.), chemical (chemical precipitation, oxidation-reduction, etc.) and biological (microbial remediation, plant remediation, biological adsorption, etc.) methods. Physical methods often have high material costs, high consumption, complex regeneration treatment and environmental sensitivity, which are easy to cause secondary pollution. Chemical methods have poor low-concentration treatment effects and large environmental interference, which is not conducive to long-term environmental stability; reagent residues cause new pollution problems, and the precipitates generated during the treatment process need to be further disposed of. The biological disposal process faces the disadvantages of uranium toxicity inhibition, low remediation efficiency, long remediation cycle, poor product stability, and strong condition dependence.

[0005] In summary, the current uranium pollution remediation technologies each have their own advantages and limitations, and a single technology often cannot meet the dual requirements of efficient removal of uranium pollution and long-term stability of the product. Therefore, from the perspective of multidisciplinary intersection, this application explores a composite remediation method that integrates the advantages of multiple technologies, develops more environmentally friendly, economical and efficient remediation materials and processes, and provides innovative solutions for uranium pollution control. This is of great significance for ensuring the environmental safety of mining areas and promoting the sustainable development of the nuclear energy industry. Summary of the invention

[0006] The purpose of the present application is to provide an in-situ remediation method for uranium contamination and its application, which can seal the uranium pollutants adsorbed on the surface of pyrite in the reconstructed product iron oxide lattice through biologically induced in-situ / exogenous pyrite hydrolysis, oxidation, atomic rearrangement, and biomineralization, isolate the external environmental interference, and prevent secondary pollution.

[0007] The present application provides an in-situ remediation method for uranium contamination, which drives the hydrolysis, oxidation, atomic rearrangement and biomineralization of pyrite through the interaction between microorganisms and pyrite, fixes and seals the uranium contaminants in the reconstructed product iron oxide lattice, and repairs the uranium contaminated area.

[0008] The microorganisms include one or more of Brucella, Geobacter or Clostridium.

[0009] Specifically, if the uranium contaminated area contains the microorganisms, natural humus is used as an electron donor to stimulate the microorganisms in situ.

[0010] Specifically, a natural humus solution is injected into the uranium contaminated area by circulating pumping to stimulate the in-situ microorganisms. The concentration of the natural humus solution is 0.05-5 g / L, and the dosage is 5-500 g / day.

[0011] Specifically, if the iron content of pyrite in the uranium contaminated area is less than 15 g / m 3 , then the pyrite needs to be added until the iron content of the pyrite is ≥ 15g / m 3 .

[0012] Specifically, if the uranium contaminated area does not contain the microorganism, the pyrite iron content in the uranium contaminated area is less than 15 g / m 3 , the in-situ repair method comprises the following steps:

[0013] The microorganisms are selected to prepare a bacterial suspension, wherein the microorganisms include one or more of the Brucella, the Geobacter or the Clostridium, cultured to the logarithmic phase, and resuspended in the sterilized uranium-contaminated water for 8 to 12 hours to obtain the bacterial suspension, wherein the final concentration of the bacterial suspension is 1×10 5 ~1×10 10 CFU / mL;

[0014] preparing a pyrite suspension having a concentration of 1 to 100 g / L;

[0015] The bacterial suspension and the pyrite suspension are put into the uranium contaminated area.

[0016] Specifically, the final concentration of the bacterial suspension is 1×10 8 CFU / mL.

[0017] Specifically, the particle size of the pyrite is 75 to 1700 μm.

[0018] Specifically, the bacterial suspension and the pyrite suspension are added in the following manners:

[0019] Putting the bacterial suspension and the pyrite suspension into the uranium contaminated area simultaneously;

[0020] Alternatively, the pyrite suspension is firstly put into the uranium contaminated area, and after adsorption equilibrium is reached, the bacterial suspension is added.

[0021] The present application also provides the use of any of the above-mentioned in-situ remediation methods for uranium contamination in the remediation of uranium contamination.

[0022] Uranium mining sites and their surrounding areas often contain pyrite, as well as microbial monomers and bacterial communities with metal reduction functions, such as Brucella, Geobacter, Clostridium, etc. Fully considering and making good use of in-situ minerals, microorganisms and geochemical resources can not only save engineering costs, but also make the remediation effect more effective. In the real environment, these metal-reducing microorganisms continuously drive biomineralization, pyrite hydrolysis, oxidation, atomic rearrangement, and biomineralization processes, but the rate is slow and cannot reach the level of repairing polluted water bodies. This application considers and makes good use of in-situ minerals, microorganisms and geochemical resources as much as possible, and proposes a green, efficient and durable uranium pollution remediation method based on the in-situ microbial-pyrite interaction mechanism. This method achieves in-situ stable storage of uranium pollutants by using microorganisms to mediate pyrite hydrolysis, oxidation, atomic rearrangement, and biomineralization processes. Biomineralization can also adsorb free uranium pollutants in the environment, reduce their chemical toxicity and radioactive hazards, and enhance microbial reaction activity. Furthermore, by driving the pyrite hydrolysis, oxidation, atomic rearrangement and biomineralization processes through microorganisms, the uranium pollutants are sealed in a stable iron oxide lattice, isolating them from the impact of the external environment and achieving long-term environmental restoration.

[0023] The advantages of this application compared with the prior art are:

[0024] (1) This application fully considers and rationally utilizes in-situ mineral, microbial and geochemical resources, relying on the interaction between in-situ minerals and microorganisms to save engineering costs and improve restoration efficiency.

[0025] (2) According to the actual situation of uranium contaminated sites, this application directly stimulates the in situ microbial flora or selects one or more of exogenous Brucella, Geobacter or Clostridium. These species are widely available, environmentally safe, non-toxic and harmless; they are commonly found in heavy metal mining environments, are tolerant to a variety of heavy metals, and can be used for heavy metal pollution remediation.

[0026] (3) Based on the actual situation of uranium contaminated sites, this application directly utilizes the in-situ mineral pyrite. Pyrite is widely distributed in nature, cheap and easy to obtain, and has a simple preparation method and can be applied on a large scale.

[0027] (4) Microbial-induced biomineralization can significantly reduce the in situ free heavy metal uranium concentration, protect the physiological and biochemical activities of microorganisms from the chemical toxicity and radioactive hazards of uranium pollution, and in situ pyrite and iron sulfide mineral aggregates can provide trace elements and nutrients for microorganisms.

[0028] (5) Pyrite has a large specific surface area, which not only reduces the apparent free heavy metal uranium concentration, but also allows uranium pollutants to be quickly sealed in the mineral lattice during the pyrite hydrolysis, oxidation, atomic rearrangement, and biomineralization processes, isolating them from the external environment. The stability is greatly improved and is not easily affected by subsequent changes in environmental conditions.

[0029] (6) The uranium pollutants sealed in this application have significant stability. As long as the iron oxides of the reconstructed biomineralization products do not dissolve, the uranium pollutants sealed in the lattice cannot be released. Stable iron oxides (such as goethite, hematite, and limonite) can be stably present in 0.5M HCl (pH0.3) and 1M NaOH (pH14) systems, that is, the uranium in the lattice can almost always be stably present in nature and not released. BRIEF DESCRIPTION OF THE DRAWINGS

[0030] Figure 1 These are the results of determination of the mineral composition of the uranium contaminated site.

[0031] Figure 2 This is the result of U concentration measurement in uranium contaminated sites.

[0032] Figure 3 These are the results of in-situ determination of microbial composition at uranium contaminated sites.

[0033] Figure 4 These are the results of U concentration measurement before and after stimulating the in situ microbial reaction in Example 1.

[0034] Figure 5 To repair the stability of the product.

[0035] Figure 6 The results of U concentration measurement before and after the exogenous microorganism induction reaction in Example 2 are shown.

[0036] Figure 7 The results of U concentration measurement before and after the exogenous microorganism induction reaction in Example 3 are shown. DETAILED DESCRIPTION

[0037] The present application is further described in detail below in conjunction with specific implementation methods. The examples given are only for illustrating the present application, not for limiting the scope of the present application. The experimental methods in the following examples are all conventional methods unless otherwise specified.

[0038] An in-situ remediation method for uranium contamination comprises the following steps:

[0039] Microorganisms are selected to prepare a bacterial suspension, wherein the microorganisms include one or more of Brucella intermedia LMG3301, Geobacter sulfurreducens ATCC 51573 or Clostridium intestinale DSM 6191, cultured to a logarithmic phase, and resuspended in sterilized uranium-contaminated water for 8 to 12 hours to obtain a bacterial suspension, wherein the final concentration of the bacterial suspension is 1×10 5 ~1×10 10 CFU / mL, preferably 1×10 8 CFU / mL.

[0040] Select pyrite to prepare pyrite suspension. Mix pyrite and water to obtain pyrite suspension. The concentration of pyrite suspension is 1-100 g / L, preferably 5 g / L. The particle size of pyrite is 75-1700 μm, preferably 150 μm. If the particle size of pyrite is too large, the reaction activity is reduced and the efficiency of uranium pollution control is reduced.

[0041] The bacterial suspension and pyrite suspension are added to the uranium contaminated area. The addition methods include:

[0042] The bacterial suspension and the pyrite suspension are simultaneously added to the uranium contaminated area, and the volume ratio of the bacterial suspension, the pyrite suspension and the uranium contaminant (water / soil / sediment) is 1:1:100;

[0043] Alternatively, the pyrite suspension is first placed in the uranium contaminated area for 8 to 12 hours, the volume ratio of the pyrite suspension to the uranium pollutant is 1:100, and after adsorption equilibrium, the bacterial suspension is added, and the volume ratio of the bacterial suspension to the uranium pollutant after the above adsorption equilibrium is 1:101. Adsorption equilibrium means that the adsorption rate and desorption rate of uranium pollutants on the surface of pyrite are equal, the concentration and adsorption amount of uranium pollutants remain constant, and no obvious changes occur on a macro scale, and the system reaches equilibrium.

[0044] Example 1

[0045] 1. In situ mineral composition analysis of uranium contaminated sites.

[0046] The samples were collected from the bottom sediments of a uranium-contaminated water body in South my country, dried and ground to 200 mesh, and their mineral composition was monitored using an X-ray diffractometer.

[0047] Depend on Figure 1 It can be seen that the main mineral compositions of the uranium contaminated site are quartz, pyrite, goethite, mica and zeolite, and no typical uranium minerals are found. In situ pyrite and its reconstructed biomineralization product goethite are widely developed.

[0048] 2. Analysis of the occurrence form and content of in-situ uranium elements in uranium contaminated sites.

[0049] The sample was also collected from fresh sediments (dry weight 5 g) at the bottom of a uranium-contaminated water body in South my country. The sample was extracted with HCl (pH 0.3) at a final concentration of 0.5 M for 48 hours, and the U concentration in the extract was detected.

[0050] Depend on Figure 2 It can be seen that the in-situ uranium pollutants in the contaminated site are mainly present in the form of adsorbed uranyl ions, which can be extracted by 0.5M HCl to reach 160ppm.

[0051] 3. Analysis of in situ microbial composition at uranium contaminated sites.

[0052] The samples were also collected from fresh sediments at the bottom of a uranium-contaminated water body in South my country. The samples were collected under sterile conditions and transported back to the laboratory under dry ice freezing conditions. The bacterial community was separated according to the separation steps: the following reagents and utensils were sterilized in advance, and the entire experiment was performed in a sterile safety cabinet. 50mL of 4mM CaCl2 solution was mixed with 20g of fresh sediment, and then quartz sand was added to make the mass ratio of sediment: quartz sand: CaCl2 solution 1:1:9. Then vortex at the maximum speed for 2h to elute the microorganisms in the sediment into the solution. Then centrifuge at 1000g and 20℃ for 12min to remove larger stones and particles. Wash with sterile CaCl2 three times to wash away soluble organic matter. Then centrifuge at 4000g and 20℃ for 10min, take the precipitate, and resuspend it with CaCl2 to obtain the in situ microbial community of the uranium contaminated site. The total DNA of the in situ microbial community was extracted using a soil DNA kit, and the extracted DNA fragments were amplified using 16Sr RNA V3-V4 region primers 338F (SEQ ID NO: 1: 5'-ACTCCTACGGGAGGCAGCA-3') and 806R (SEQ ID NO: 2: 5'-GGACTACHVGGGTWTCTAAT-3'), and high-throughput sequencing was performed on the Illumina sequencing platform to detect the in situ microbial community of the uranium contaminated site.

[0053] Depend on Figure 3 It can be seen that the top 10 major categories of the in situ microbial community in the uranium contaminated site are Verrucomicrobiota (~5%), Spirochaetota (~8%), Kryptonia (~6%), Acidobacteriota (~12%), Desulfobacterota (~13%), Crenarchaeota (~4%), Bacteroidota (~15%), Firmicutes (~20%), Chloroflexi (~8%) and Proteobacteria (~10%). The Proteobacteria, Firmicutes, Spirochaetota, and Desulfobacterota phyla have numerous known metal-reducing microorganisms, such as Pseudomonas sp., Geobacter sp., Clostridium sp., Desulfovibrio sp., and Desulfomonas sp.

[0054] 4. Stimulate in situ functional microorganisms to induce pyrite hydrolysis, oxidation, atomic rearrangement, and biomineralization to repair uranium contamination.

[0055] The sample was also collected from fresh sediments (dry weight 5g) at the bottom of a uranium-contaminated water body in South my country, and a natural humus solution with a concentration of 1g / L was injected into it for one week. The solids before and after the reaction were extracted with 0.5M HCl to compare the U pollution remediation effect.

[0056] Depend on Figure 4 It can be seen that the concentration of uranium extractable by 0.5M HCl is significantly reduced from ~166ppm to ~97ppm, fixing nearly 42% of the uranium contaminants.

[0057] 5. Stability analysis of uranium pollution remediation products induced by in situ functional microorganisms: pyrite hydrolysis, oxidation, atomic rearrangement and biomineralization strategy.

[0058] The above-mentioned remediation products were exposed to the air for 1 month, during which the sediment suspension was continuously stirred and aerated. A control was set up, and a batch of U(IV) precipitates were prepared by pre-bioreduction using Geobacter sulfurreducens ATCC 51573, and exposed to the same aeration experimental conditions for one month. The solids before and after a month of aeration were extracted with 100mM NaHCO3 to explore the stability of the U pollution remediation effect. Here, 100mM NaHCO3 was used to extract the solids instead of 0.5M HCl, because 100mM NaHCO3 can only extract U(VI) but not U(IV). In the absence of pyrite to reconstruct the iron oxide lattice of the biomineralization product, 0.5M HCl can directly dissolve the batch of U(IV) precipitates prepared by bioreduction, which loses the significance of comparison.

[0059] Depend on Figure 5 It can be seen that continuous oxygen exposure for one month does not release the uranium contamination products fixed by the in-situ functional microbial-induced pyrite hydrolysis, oxidation, atomic rearrangement, and biomineralization strategy, but can significantly oxidize the U(IV) precipitate prepared by bioreduction, showing the superiority of the strategy of this application in the practice of uranium contamination remediation. In addition, the values ​​of the remediation products before NaHCO3 extraction and oxygen exposure are similar to Figure 4 The extraction values ​​of HCl in the solution are comparable, indicating that this part of the uranium pollutants is the U(VI) that has not entered the mineral lattice, which indirectly confirms that the uranium remediation product of the present application has excellent stability against acid and oxidation.

[0060] Example 2

[0061] Exogenous functional microorganisms induce hydrolysis, oxidation, atomic rearrangement and biomineralization of exogenous pyrite to repair uranium contamination.

[0062] The use of exogenous single pure functional microorganisms to induce hydrolysis, oxidation, atomic rearrangement and biomineralization of exogenous pyrite monomers to repair uranium pollution and construct the above-mentioned in-situ remediation process is more conducive to explaining the uranium pollution remediation mechanism proposed in this application.

[0063] Geobacter sulfurreducens ATCC 51573, purchased from American Type Culture Collection, was selected to prepare bacterial suspension, cultured to logarithmic phase, and resuspended in sterilized uranium-contaminated water for 12 hours to obtain bacterial suspension. The final concentration of the bacterial suspension was 1×10 8 CFU / mL.

[0064] The samples were collected from the bottom sediment of a uranium-contaminated water body in South my country. The collection location was different from that in Example 1. The iron content of the samples in this example was <15 g / m 3. Add 1L of U(VI) pollutant with a final concentration of 100ppm to 10mL of freshly prepared pyrite suspension with a concentration of 5g / L, stir for 2 hours to achieve full adsorption, then add 10mL of the above bacterial suspension, and set up a control test without adding any microorganisms after adsorption equilibrium, and react for one week.

[0065] Depend on Figure 6 It can be seen that in the control system without adding Geobacter, no significant pyrite hydrolysis, oxidation, atomic rearrangement, and biomineralization occurred during the experimental period, and the uranium concentration in the 0.5M HCl extraction state did not decrease significantly, that is, the uranium pollution was not significantly repaired. Correspondingly, in the experimental system of the present application, pyrite underwent significant hydrolysis, oxidation, atomic rearrangement, and biomineralization. The uranium pollutants adsorbed on the surface of pyrite were brought into the lattice of the biomineralization reconstruction product and were sealed, and the uranium concentration in the water body decreased, thereby controlling the uranium pollution.

[0066] Example 3

[0067] Exogenous mixed functional microorganisms induce hydrolysis, oxidation, atomic rearrangement and biomineralization of exogenous pyrite to remediate uranium contamination.

[0068] The use of exogenous mixed functional microorganisms to induce hydrolysis, oxidation, atomic rearrangement, and biomineralization of exogenous pyrite monomers to repair uranium pollution and construct the above-mentioned in-situ remediation process is more conducive to explaining the uranium pollution remediation mechanism proposed in this application. Brucella intermedia LMG 3301 (purchased from the Belgian Bacteria Collection BC / LMG Bacteria Collection) and Clostridium intestinale DSM 6191 (purchased from the German Microbial Culture Collection Deutsche Sammlung von Mikroorganismen und Zellkulturen GmbH) were selected, mixed in a ratio of 1:1, and a bacterial suspension was prepared. The mixture was cultured to the logarithmic phase and resuspended in the sterilized uranium-contaminated water for 8 hours to obtain a bacterial suspension. The final concentration of the bacterial suspension was 1×10 5 CFU / mL.

[0069] Sample collection was the same as in Example 2. 1L of U(VI) pollutant with a final concentration of 100ppm was added to 10mL of freshly prepared pyrite suspension with a concentration of 5g / L, stirred for 2 hours to achieve full adsorption, and then 10mL of the above bacterial suspension was added thereto and reacted for one week.

[0070] Depend on Figure 7It can be seen that in the control system without microorganisms, no significant pyrite hydrolysis, oxidation, atomic rearrangement, and biomineralization occurred during the experimental period, and the uranium concentration in the 0.5M HCl extract did not decrease significantly, that is, the uranium pollution was not significantly repaired. In contrast, in the experimental system with the addition of mixed bacteria (Brucella and Clostridium), pyrite underwent significant hydrolysis, oxidation, atomic rearrangement, and biomineralization. The uranium pollutants adsorbed on the surface of pyrite were brought into the lattice of the biomineralization reconstruction product and were sealed, and the uranium concentration in the water body decreased, thereby controlling the uranium pollution.

[0071] Although the present application has been described in detail above by means of general description, specific implementation methods and experiments, it is obvious to those skilled in the art that some modifications or improvements may be made to the present application. Therefore, these modifications or improvements made without departing from the spirit of the present application are within the scope of protection claimed in the present application.

Claims

1. A method for in-situ remediation of uranium contamination, characterized in that: Through the interaction between microorganisms and pyrite, the pyrite is driven to hydrolyze, oxidize, rearrange atoms, and biomineralize, so that the uranium pollutants are fixed and sealed in the reconstructed iron oxide lattice, thereby repairing the uranium contaminated area. The microorganisms include one or more of Brucella, Geobacter or Clostridium.

2. The in-situ remediation method for uranium contamination according to claim 1, characterized in that: If the uranium contaminated area contains the microorganisms, natural humus is used as an electron donor to stimulate the microorganisms in situ.

3. The in-situ remediation method for uranium contamination according to claim 2, characterized in that: The natural humus solution is injected into the uranium contaminated area by circulating pumping to stimulate the in-situ microorganisms. The concentration of the natural humus solution is 0.05-5g / L and the dosage is 5-500g / day.

4. The in-situ remediation method for uranium contamination according to claim 1, characterized in that: If the iron content of pyrite in the uranium contaminated area is less than 15 g / m 3 , then the pyrite needs to be added until the iron content of the pyrite is ≥ 15g / m 3 .

5. The in-situ remediation method for uranium contamination according to claim 1, characterized in that: If the microorganism is not contained in the uranium contaminated area, the iron content of pyrite in the uranium contaminated area is less than 15g / m 3 , including the following steps: The microorganisms are selected to prepare a bacterial suspension, wherein the microorganisms include one or more of the Brucella, the Geobacter or the Clostridium, cultured to the logarithmic phase, and resuspended in the sterilized uranium-contaminated water for 8 to 12 hours to obtain the bacterial suspension, wherein the final concentration of the bacterial suspension is 1×10 5 ~1×10 10 CFU / mL; preparing a pyrite suspension having a concentration of 1 to 100 g / L; The bacterial suspension and the pyrite suspension are put into the uranium contaminated area.

6. The in-situ remediation method for uranium contamination according to claim 5, characterized in that: The final concentration of the bacterial suspension was 1×10 8 CFU / mL.

7. The in-situ remediation method for uranium contamination according to claim 5, characterized in that: The particle size of the pyrite is 75 to 1700 μm.

8. The in-situ remediation method for uranium contamination according to claim 5, characterized in that: The bacterial suspension and the pyrite suspension are added in a manner as follows: Putting the bacterial suspension and the pyrite suspension into the uranium contaminated area simultaneously; Alternatively, the pyrite suspension is firstly put into the uranium contaminated area, and after adsorption equilibrium is reached, the bacterial suspension is added.

9. Use of the in-situ remediation method for uranium contamination according to any one of claims 1 to 8 in the remediation of uranium contamination.

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