Silver is immobile in Ag-rich mine waste: Isotopic evidence from anthropogenic soils

Abstract

This study presents silver (Ag) stable isotope ratios (expressed as delta Ag-109) combined with mineralogy and speciation data in Ag-contaminated anthropogenic soils from a historic mining area at Kutna Hora, Czech Republic. Results showed large variability of soil Ag, both in concentrations (similar to 0.5-100 mg/kg) and isotopic patterns (delta Ag-109 from -0.5 to +1.2 parts per thousand, +/- 0.1 parts per thousand 2 sigma), resulting from differences in the source substrates, i.e. the composition of the original mining waste or slag. In other words, the data do not reflect the fractionation of Ag isotopes during pedogenesis and weathering processes, but rather the inherited Ag-109/Ag-107 signature of the hydrothermal ore and its genesis history. Examining our endmembers and their potential quantities, a tendency for soils to become isotopically lighter (depleted in the heavy Ag-109 isotope) as the concentration of Ag increases was identified, and vice versa. This finding suggests that Ag-rich sulfides, primarily galena, generally favor the light Ag-107 isotope over Ag-poor phases being enriched in the heavy isotope (Ag-109). Given the negligible post-depositional isotopic effects and the overall high stability of Ag in the studied soils, it is theorized that Ag isotopes can serve as sensitive indicators for monitoring the origin and nature of Ag-containing sulfides and their weathering products. From a general contamination perspective, which is important on many levels, the Ag isotopic and concentration data demonstrate the following: (i) the low mass migration of Ag, and (ii) the well-preserved Ag isotopic patterns in ancient anthropogenic soils (<= 17th century).