Mobilization of arsenic in aquifers from the Datong Basin, China: Evidence from geochemical and iron isotopic data

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 δ56Fe 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 δ56Fe 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 δ56Fe values for HCl-extracted poor-crystalline Fe indicates production of sulfides (FeSs). The δ56Fe 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 δ56Fe 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.