Interfacial Chemistry Of Arsenopyrite Bio-oxidized By Acidithiobacillus Ferrooxidans

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.