Antimony (Sb) is a metalloid element with potential toxicity and carcinogenicity, and is a global environmental contaminant that is ubiquitously present in the environment as a result of natural processes and human activities. Mine exploitation is an important source of Sb contamination, and very high levels of contamination have been detected around mine sites. Antimony mobilization is closely linked to redox state and adsorption and desorption in nature. Molybdenum (Mo) and Mo isotope can be informative of the redox conditions, moreover, Mo isotope fractionation is mainly controlled by the adsorption and desorption onto Fe/Mn-(hydr)oxides. The Mo isotope compositions were analyzed to elucidate the Sb mobilization in high Sb groundwater from the world’s largest antimony mine, Xikuangshan (XKS) Sb mine.
The Mo isotope values (δ98Mo) in groundwater at XKS Sb mine range from +1.57‰ to +2.49‰ (average of +1.96‰), which are relatively heavier that those reported in fresh waters. The relationships between δ 98Mo values and Fe, Mn and Mo concentrations suggest that compared with Mn, Fe-(hydr)oxides may have a more significant effect on Mo isotope. The increased δ98Mo values with an decrease in Mo concentrations might result from the adsorption of Mo from Fe/Mn-(hydr)oxides, and also might be a consequence of the faster rate of adsorption of Mo from groundwater than the rate of desorption of Mo from Fe/Mn-(hydr)oxides. Importantly, the relationships between δ98Mo values and Sb and As concentrations further demonstrate that adsorption and desorption of Fe/Mn-(hydr)oxides are an important factor for affecting the Sb mobilization in groundwater, more specially, a competitive adsorption with As may occur.
This study is the first to examine the Mo isotope compositions in high Sb groundwater mine, and the results show that the Mo isotope would be an important tool in demonstrating the migration and transformation of antimony in groundwater.