| China is a major consumer of sulfide mine.With the global depletion of sulfide mine resources,the secondary utilization of polymetallic sulfide mine tailings is an inevitable choice to ensure the security of mineral resources supply in our country.The polymetallic sulfide mine tailings with high content of heavy metals and reduced sulfide mineral,is easily oxidized under the condition of water and air,resulting in producing a lot of heavy metal ions and sulfuric acid,etc.The dissolved heavy metals are prone to migration and transformation,which can cause pollution to the surrounding environment of mining area,such as water and soil,posing huge potential environmental risk.Compared with other physical and chemical methods,bioleaching has many advantages,such as recovering valuable metals,reducing environmental toxicity of tailings,mild reaction and environmental friendliness.Therefore,the lead-zinc sulfide mine tailings from lead-zinc mine factory in northern Guangdong province was as the research object in this study.The acidophilic bacteria with higher oxidation activity were selected as bioleaching microorganisms,which were artificially designed into mixed culture s to leach lead-zinc sulfide mine tailings according to the principle of complementary metabo lic characteristics.The mechanism of microbial leaching in different combinations was discussed and the effect of bioleaching on the environmental toxicity of tailings was clarified.First,iron-oxidizing bacterium Leptospirillum ferriphilum(L.ferriphilum)and sulfur-oxidizing bacterium Acidithiobacillus thiooxidans(A.thiooxidans)with single metabolic function were artificially rebuilt into mixed cultures to leach lead-zinc sulfide mine tailings.The effects of mixed cultures with different initial inoculation ratios on lead-zinc sulfide mine tailings were investigated.The results show that the proportion s of L.ferriphilum and A.thiooxidans in the mixed cultures had no significant effect on the leaching efficiency.More importantly,there was no significant difference in the leaching efficiency between the mixed cultures and pure culture of L.ferriphilum.At this time,the zinc leaching rate was about 94.5% after 20 days.Analysis of the reasons for above results shows that only after the chemical bond between metal and sulfur in sulfide minerals was opened by L.ferriphilum or oxidizing agent,could A.thiooxidans directly oxidize sulfide minerals in lead-zinc sulfide mine tailings,resulting in L.ferriphilum playing a leading role in the mixed culture.The contents of elemental sulfur membrane and jarosite substances that blocked the leaching of iron-oxidizing bacterium were less,which did not block the oxidation of minerals by L.ferriphilum.Therefore,only considering the metabolic characteristics of microorganisms,the leaching efficiency of lead-zinc sulfide mine tailings by the combination of iron-oxidizing bacteria and sulfur-oxidizing bacteria is not necessarily improved significantly.Iron/sulfur-oxidizing bacteria Acidithiobacillus ferrooxidans(A.ferrooxidans)and Sulfobacillus thermosulfidooxidans(S.thermosulfidooxidans)with dual metabolic functions of ferrous iron and reduced sulfur were mixed to leach lead-zinc sulfide mine tailings,which was aimed at improving the leaching efficiency.The results show that the leaching effect was better than the mixed culture of L.ferriphilum and A.thiooxidans.It took only 15 days to reach 94.5% leaching rate,which shortened the bioleaching cycle by 25% and achieved 96.4% zinc leaching rate in the end.Compared with the previous leaching process of native bacterium A.ferrooxidans in our team,the time to reach the same zinc leaching rate was reduced by 64%,significantly improving the leaching efficiency.Analysis of microbial community shows that the contents of the two bacteria were no less than 15% during bioleaching of the mixed culture.It's also found that the A.ferrooxidans with a strong ferrous-oxidizing ability was the dominant bacteria in the early stage,while S.thermosulfidooxidans with a strong sulfur-oxidizing ability had a competitive advantage in the later stage.Hence,the two bacteria played the oxidation function and co-leached lead-zinc sulfide mine.The characteristic changes of EPS mediated by microorganisms was analyzed by 3DEEM.The results show that the characteristic changes of EPS were correlated with the microbial physiological characteristics.The higher content of polysaccharides in EPS meant that the oxidation activity of microorganisms was stronger,contributing to tailings bioleaching.Finally,the effect of bioleaching on tailings toxicity was evaluated by analyzing the changes of heavy metal form,leaching toxicity and ecological risk.The results show that the lead-zinc sulfide mine tailings was hazardous waste.The acid-soluble/exchangeable and reducible fraction in heavy metals were preferentially leached or converted to the oxidizable and residual fraction by bio-oxidation,effectively reducing the leaching toxicity of tailings.It's proved that bioleaching could effectively mitigate the potential ecological risks of tailings to the environment in this experiment.In this study,microorganisms with different metabolic functions were artificially mixed to leach lead-zinc sulfide mine tailings,which improved the leaching rate and shorten the bioleaching cycle significantly.These results would provide technical support for promoting the secondary utilization of polymetallic sulfide mine tailings.When the iron-sulfur-oxidizing bacteria with different metabolic functions are combined for bioleaching,the microorganisms with dual metabolic functions should be selected in preference,so that the metabolic functions of ferrous iron and sulfur are present in each stage.At the same time,the proportion of bacteria with a strong ferrous-oxidizing ability in the early stage were increased,while the proportion of bacteria with a strong sulfur-oxidizing ability were increased in the later stage,which can effectively improve the bioleaching efficiency of polymetallic sulfide mine tailings. |
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