【作者】 朱婷婷；

【导师】 陆现彩；

【作者基本信息】 南京大学 ， 地球探测与信息技术， 2012， 硕士

【摘要】 矿业开发在带来经济效益的同时,其排放的废矿石也引发严重的环境污染问题。当废弃矿石和尾矿暴露在空气或水中发生风化时,有害元素(如重金属元素)易被释放出来,并随形成的酸性矿山排水(AMD)远距离迁移,严重污染地表水、地下水及土壤。在众多重金属元素中,砷因其具有多种价态,且分布广泛、危害巨大,成为最受关注的矿山污染元素。毒砂(FeAsS)是自然界中最重要的含砷矿物,它广泛分布于金属硫化物矿床中,毒砂氧化能释放Fe、 As和S等元素,并形成H3As03、H3As04和H2SO4等含氧酸。而毒砂的分解与微生物密切相关。大量研究表明嗜酸菌在含砷硫化物的分解、砷的化学态转化、迁移和沉淀过程中起着重要的作用,显著影响着砷的地球化学循环。本文选取毒砂为研究对象,与AMD中最常见的细菌——Acidithiobacillus ferrooxidans反应,拟研究毒砂中Fe、 As和S元素的氧化行为,查明氧化过程中次生产物的类型及其生成顺序,以此来探讨毒砂的微生物氧化机理。为了考察环境流体中大量存在的Fe3+的作用,实验设置含铁和无铁两种反应体系。着重探讨含铁体系中毒砂的微生物氧化过程及机理,辅以无铁体系对比不同晶面的溶解速率及初始铁离子的作用。通过扫描电镜形貌观察对比无铁体系中不同晶面上溶蚀坑的大小、数量及形态的变化,得出毒砂晶面{510}的溶解速率大于晶面{230}。通过SEM、 XPS、 TEM、 XRD、 IR及拉曼等实验方法,对比两种体系中的次生沉淀类型,发现初始溶液中的铁离子对其有重大影响。无铁体系中生成的次生沉淀主要为磷酸铁,还有极少量的单质硫与黄钾铁矾,沉淀物中几乎不含砷,从毒砂中溶出的As均以游离态释放到溶液中。而含铁体系中生成的次生沉淀以黄钾铁矾(及施氏矿物)和单质硫为主,毒砂氧化层以铁的硫酸盐、砷酸盐、亚砷酸盐及铁的氧化物或氢氧化物为主,其中铁的硫酸盐、黄钾铁矾及施氏矿物能够吸附砷。在A. ferrooxidans作用下,毒砂氧化层化学组成剖面中单质硫的含量随深度并非单向增加或减少,表明单质硫为一种中间产物。另外A. ferrooxidans使沉淀中毒性更强的As(Ⅲ)含量增加而As(V)含量减少。毒砂的氧化机制是直接作用与间接作用相结合的机制,部分A. ferrooxidans附着在毒砂表面对其进行侵蚀,侵蚀后的残余物被释放至溶液中由浮游的微生物继续作用。在A. ferrooxidans作用下,初始溶液中的Fe2+被氧化为Fe3+,从而进一步氧化毒砂。毒砂中Fe(Ⅱ)最先被氧化为Fe(Ⅲ),而后Fe(Ⅲ)从(AsS)基团获得电子还原为Fe(Ⅱ),失去电子的(AsS)基团与水中的羟基结合,并与Fe(Ⅱ)断开,毒砂中的Fe以Fe2+的形式进入溶液,在微生物作用下开始新的循环。而断键后的(AsS)-OH暴露在溶液中被继续氧化,其中As经过-1、0、+1价,最终被氧化为+3价,其中部分被继续氧化为+5价,而S经过-1价、多硫化物、单质硫,最终被氧化为亚硫酸盐及硫酸盐。和化学氧化相比,在微生物作用下,毒砂中S的氧化速率大于As。更多还原

【Abstract】 As the mining industry brings economic benefits to society, it also causes severe environmental problems due to the mine wastes. The tailings will be weathered when exposed to air or water, releasing toxic elements such as heavy metals. The discharge of toxic elements containing water bodies, such as acid mine drainage (AMD), can cause serious contamination of soil, groundwater, and surface water. Among these heavy metals, arsenic is the most concerned one for its variable chemical states and widespread distribution. Arsenopyrite (FeAsS) is a commonly discarded arsenic-bearing sulfide mineral in mine wastes. Its dissolution in aerobic aqueous conditions is a major contributor to acid (e.g., H3ASO3, H3ASO4and H2SO4) and toxic metal (e.g., arsenic) release. The oxidation of arsenpyrite is greatly enhanced by microbes. Acidophilic bacteria play an important role in the geochemical cycle of arsenic through decomposition of arsenic-bearing sulfide minerals, and oxidation, transportation and precipitation of arsenic.In this study, arsenopyrite was chosen to interact with Acidithiobacillus ferrooxidans, the most common bacteria in AMD. To determine the oxidation mechanism of arsenopyrite, we observed the oxidation behavior of Fe, As and S elements, identified the types of secondary products and resolved their formation sequence. To investigate the influence of iron ions that widely distribute in AMD, two experimental systems were designed:one with0.016M Fe2+initially, and the other without iron ions. The investigation of ferrous system reveals the oxidation mechanism of arsenopyrite. In the supplementary study of non-ferrous system, we compared the dissolution rates of two different crystal planes and disclosed the role of iron ions.By observing the surface morphology of two crystal planes ({510} and {230}) of arsenopyrite in none-ferrous system, comparing the size, quantity and shape of the pitches on them, we found that the plane{510} is more favourable to be attacked either by Fe3+or A.ferrooxidans.With the assistance of SEM, XPS, TEM, XRD, IR and Raman Microscopy, the types of secondary products in two systems were identified. Distinct differences were displayed in the precipitates of the two systems. Ferric phosphate was the main precipitate in non-ferrous system, while elemental sulfur and jarosite were identified in minor contents. None arsenic-bearing precipitates were detected, meaning all dissolved arsenic was released into solution as free ions. By contrast, the secondary products in ferrous system were mainly ferric sulfate, ferric arsenate, ferric arsenite, and ferric oxide or ferric hydroxide. Among them, ferric sulfate such as jarosite and schwertmannite can absorb arsenic. In the presence of A. ferrooxidans, the change of the content of elemental sulfur in oxidation layers of arsenopyrite was none-monotonic, indicating that elemental sulfur was one of the intermiediates. In addition, with the mediation of A. ferrooxidans, the content of As(Ⅲ) increased while that of As(Ⅴ) decreased. However, As(Ⅲ) is more toxic than As(Ⅴ).Experimental results help to resolve the reaction mechanism. The oxidation of arsenopyrite is a combination of direct and indirect process. Some A. ferrooxidans attach to the surface of arsenopyrite, and release some insoluble particles into solution while attacking the mineral. These particles are further oxidized by suspended bacteria. In the presence of A. ferrooxidans, initial Fe2+in solution is oxidized to Fe3+, thus oxidizing arsenopyrite. Fe(Ⅱ) in arsenopyrite is the first to be oxidized to Fe(Ⅲ), then obtains an electron from (AsS) group. As a result, Fe(Ⅲ) is transformed to Fe(Ⅱ) again. After losing an electron,(AsS) binds with OH in water, and breaks up with Fe(II). Finally, Fe in arsenopyrite is released into solution as Fe2+, and enters a new cycle with the presence of bacteria. The bonded (AsS)-OH is then exposed to solution and undergoes further oxidation. Arsenic is oxidized through-1,0and+1to+3valences, and some of which is further oxidized to+5valence. In the meanwhile, sulfur is oxidized through-1valence, polysulfides and elemental sulfur to sulfite and sulfate. Unlike chemical oxidation, the oxidation rate of sulfur is faster than that of arsenic with the presence of bacteria.更多还原