| Acid mine drainage (AMD) is a hot research topic in environmental science area because of its extremely low pH and relatively high contents of heavy metals and metalloid. These characteristics of AMD can have a negative effect on the ecological environment. At present, the treatment of AMD is mainly focused on neutralization method, artificial wet land method, microbe method and so on. Microbe method has attracted much academic attention because of its low cost, high applicability and non-secondary pollution. Previous studies showed that iron sulfate minerals can precipitate in the AMD environment through the microbial or abiotic activity. Jarosite is the most common and stable secondary iron mineral in the AMD environment. As a new environment mineral material, jarosite has drawn special attention because of its strong adsorption and isomorphism replacement capacity.to heavy metals or metalloid.In this study, experiments were conducted in laboratory to study how to efficiently precipitate secondary minerals and its influence mechanism.1. In order to study effects of different growth environment and conditions on A.ferrooxidans LX5activity and secondary minerals formation, the effects of inoculating amount, the storage time of resting cells, initial pH of solution,(NH4)2SO4, NH4HCO3and KHCO3on precipitates, Fe2+oxidation rate and soluble Fe removal efficiency were investigated. The results showed that:(1) when inoculating quantity was more than75%, the precipitate weight and soluble Fe removal efficiency had remained stable;(2) although the oxidability of A.ferrooxidans had weakened after conservation in4℃for7d, there was no significant difference in precipitate weight between A.ferrooxidans cells before and after condervation;(3) similar precipitate weight was observed when solution initial pH value varied between2.25to3.0, but it would be significantly decreased when initial pH value was below2.0;(4) addition of appropriate low concentration NH4+could facilitate the formation of secondary mineral for providing nitrogen resources, but high concentration of NH4+would contrarily restrain the formation of secondary mineral;(5) because of their capacity in providing N and CO2resources and adjusting solution pH value, addition of NH4HCO3could facilitate the formation of secondary mineral. 2. Secondary minerals were mainly formed through chemical and biological methods, in order to study the effects of the two methods on secondary mineral formation, Fe2+was oxidated through both biological methods and addition of H2O2. The results showed that:(1) the precipitate synthesized by bacterial method was pure jarosite and has a high crystallinity, while the precipitates synthesized by chemical method were mixture of jarosite and schwertmannite;(2) the Fe2+oxidation rate in chemosynthesis system was much faster than that in biosynthesis system;(3) in the biosynthesis system, the soluble Fe removal efficiency was about71.58%at72h, while in the chemosynthesis system, it was only26.46%. The mineral particles formed in chemosynthesis system displayed an appearance of small spheroids with a diameter of2μm, while the mineral particles formed in biosynthesis system were angular with a diameter of1.5-2μm. Otherwise, the precipitate formed in biosynthesis system indicates an obvious tendency for its particles to agglomerate.3. Although secondary minerals can be formed through chemical methods, there are plenty of impurities in minerals for quick oxidation rate. In this part, H2O2was added slowly to investigate the effects of H2O2oxidation rate on Fe2+oxidation rate and precipitate weight. The results showed that:(1) The XRD patterns and element composition of precipitates synthesized through the biooxidation and the slow oxidation treatments were well coincide with that of potassium jarosite, while precipitates at the initial stage of incubation in the rapid oxidation treatment showed a similar XRD patterns to schwertmannite. With the ongoing incubation, XRD patterns and element composition of the rapid oxidation treatment were transformed to that in the biooxidation and the slow oxidation treatments;(2) the precipitate weight was30%higher in H2O2slow oxidation treatment than in biological treatment;(3) in contrast with chemical oxidation treatment, there was plenty of EPS in biological treatment. That might be the reason for the decreased precipitate formation. In order to analysis the effects of EPS on Fe2+oxidation rate and precipitate weight, A. ferrooxidans was centrifuged to peel off EPS. The results showed that, there was no significant difference in Fe2+oxidation rate between A. ferrooxidans with and without EPS, but the precipitate weight formed by A. ferrooxidans without EPS was higher than formed by that with EPS. Therefore, with the inhibition of A. ferrooxidans and their EPS, the amount of precipitates and soluble Fe removal efficiency were lower in the biooxidation treatment than in the slow oxidation treatment.4. In order to reconfirm our hypothesis that EPS can inhibit precipitate formation, different concentrations of dextran were supply and Fe2+oxidation rate and precipitate weight were investigated. The results showed that:(1) dextran restrained the formation of secondary minerals;(2) with increasing dextran concentrations, The content of Fe in secondary minerals was decreased, while the content of S did not vary significantly. The content of K in all treatments was extremely low;(3) XRD pattern of secondary minerals formed without dextran were well coincide with that of jarosite, while those formed with dextran were coincide with that of schwertmannite. But the crystallization of secondary minerals in all treatments was not very good;(4) with increasing dextran concentrations, the particle sizes of secondary minerals were decreased, while specific surface area was increased.(5) as a macromolecular organic substance, dextran can form complex with Fe3+which lead to the decrease of Fe3+participated in the formation of secondary minerals. Therefore, dextran depressed the formation of secondary minerals by forming complex with Fe3+, and restrained the transformation of schwertmannite to jarosite.5. Except for dextran, there is also plenty of glucose in EPS. In this part, the effects of different glucose concentrations on the formation of secondary iron minerals were also investigated. The results showed that:(1) addition of KOH instead of K2SO4could not only provide monovalent cations, but also increased pH value of Fe2(SO4)3solution, which would hence facilitate the transformation of Fe3+to jarosite;(2) glucose restrained the formation of precipitate from both Fe2+oxidation system and Fe3+system. The inhibitory effect was observed in the first72h in Fe3+system, while it was observed between72and168h in Fe2+oxidation system. Therefore, glucose can efficiently inhibit secondary minerals formation.It was concluded that Fe2+oxidation rate can efficiently affect the mineral phase of precipitates. When Fe2+oxidation rate is relatively high, amorphous minerals such as schwertmannite were the precursor of jarosite. The carbohydrates in the EPS can inhibit the precipitate weight through forming complex with Fe3+, separating crystal nucleus and preventing the transformation of mineral phase. By contrast, slow oxidation treatment is an effective method in jarosite formation with less environmental restriction. |
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