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

This study presents silver (Ag) stable isotope ratios (expressed as δ 109 Ag) combined with mineralogy and speciation data in Ag-contaminated anthropogenic soils from a historic mining area at Kutná Hora, Czech Republic. Results showed large variability of soil Ag, both in concentrations (~0.5–100 mg/kg) and isotopic patterns (δ 109 Ag from −0.5 to +1.2 ‰, ±0.1 ‰ 2σ), 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 109 Ag/ 107 Ag 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 109 Ag 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 107 Ag isotope over Ag-poor phases being enriched in the heavy isotope ( 109 Ag). 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). • Silver has a low leaching and toxicity potential in soils. • Silver isotopic composition in old (source) wastes is well preserved. • Silver isotopes can be used as a proxy indicator to monitor Ag origin in soils.