Abstract
Precipitation of highly insoluble metal sulfide minerals like acanthite (β-Ag 2S) or red cinnabar (HgS) is in principle an effective means to reduce metal availability and toxicity in contaminated soils. Unfortunately, experiments have shown that red cinnabar may be solubilized in the presence of dissolved organic matter or thiol ligands. To determine whether the same applies to acanthite, a laboratory synthesized β-Ag 2S mineral was incubated for up to 3 weeks in the presence of KNO 3, dissolved humic acids, cysteine, methionine and thiosulfate. XPS analysis identified Ag 2O (52%), Ag 2SO 4 (8%) and Ag 2S (40%) on the particle surfaces. Ag was released into solution in the presence of KNO 3 and methionine, presumably from mixed-oxidation surface layers. Contrary to earlier results with cinnabar, however, humic acids reduced Ag concentrations in solution by about 75%, and cysteine and thiosulfate, each containing a free –SH functional group, almost completely suppressed Ag release into solution.
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