Search

Abstract: Iron isotope compositions of various Fe pools in aquifer sediments were measured at a known As-contaminated site in the Datong Basin, China. The δ 56Fe values of HCl-extracted poor-crystalline Fe(III) range widely from −0.41‰ to 0.36‰. We interpret the low Fe(II)/FeExtractable ratios (<50%) and the negative correlation between Fe(II)/FeExtractable and δ 56Fe values in HCl-extracted poor-crystalline Fe to be best explained by redox cycling of Fe induced by microbial Fe(III) reduction. However, the high Fe(II)/FeExtractable ratios (≳70%) and positive correlation between Fe(II)/FeExtractable and δ 56Fe values for HCl-extracted poor-crystalline Fe indicates production of sulfides (FeSs). The δ 56Fe values of crystalline Fe(III) extracted by reductant appears to be comparatively small varying from −0.01‰ to 0.24‰, which is consistent with the δ 56Fe values for ferric oxides/hydroxides having undergone microbial Fe(III) reduction. The Fe isotope composition of various Fe pools shows the transformation between crystalline Fe(III) and poor-crystalline crystalline Fe(III) and the secondary Fe(II) phases has already occurred or is occurring in aquifer sediments. More importantly, there is a significant difference in the As concentrations in crystalline Fe(III) oxides/hydroxides and HCl-extracted Fe phases. The concentrations of As range from 1.6 to 29.9mgkg−1 and from 0.6 to 3.0mgkg−1, for crystalline Fe(III) and HCl-extracted Fe phases respectively. Accordingly, the transformation of Fe minerals induced by microbial Fe(III) reduction can contribute to the mobilization of As. This study is the first to examine the Fe isotope compositions in high As aquifer sediments; the results show that the Fe isotope would be an important tool in demonstrating the enrichment of As in groundwater. [Copyright &y& Elsevier]

Abstract: Several Se-bearing minerals have been identified in Se-rich stone coal, spoils, and their adjacent organic-rich soils in Yutangba of Enshi, China, where human Se toxicity occurred in the 1960s. These minerals mainly include native Se (Se0), krutaite (CuSe2), klockmannite (CuSe), mandarinoite (Fe2(SeO3)3·6H2O), Se-bearing chalcopyrite (CuFe(Se, S)2), and pyrite (Fe(Se,S)2). The assemblage of native Se, krutaite, klockmannite, and Se-bearing pyrite and chalcopyrite is primarily present in the stone coal near a fault plane, while the assemblage of native Se and krutaite is found in the Se-rich carbonaceous mudstone and organic-rich soils which are 60m away from the stone coal exposure. The assemblage of mandarinoite and native Se is present in abandoned stone coal spoils, where natural combustion occurred. Native Se is quite extensive in the stone coal spoils and nearby soils derived from them. The co-occurrence of Cu–Se minerals and native Se indicates that these minerals could have formed under a relatively acidic and reducing environment below 220°C, and suggests that Cu could play a significant role in fixing reduced Se ions in the acidic, organic-rich surface environment. Furthermore, the occurrence of mandarinoite suggests that iron-oxides constrain the geochemical behavior of Se in oxidizing environments. Our observations provide new insights into the mechanisms of Se fixation and accumulation during weathering of Se-rich rocks. [Copyright &y& Elsevier]