Mineralogical speciation of arsenic in soils above the Mokrsko-west gold deposit, Czech Republic

Natural soil profiles strongly contaminated by arsenic (As) have been studied. Soils were characterised by its pH, chemical composition, carbonate, humus, exchangeable cations and H+, and oxalate extractable Fe contents. Mineralogical and chemical speciation of the As was studied by mineralogical methods and sequential extraction. Results were compared and discussed regarding the two different types of soil environment: i) soil developed above the flat unforested granodiorite bedrock and ii) soil developed above the volcanosedimentary bedrock in a sloping forested area. As-bearing minerals were concentrated from soils using heavy liquid and determined using XRD, SEM-EDS/WDS, and Raman spectroscopy. Iron (III) oxyhydroxides (FOHs); K(-Ba) pharmacosiderite, arseniosiderite, scorodite, and jarosite were identified as products of arsenopyrite and/or pyrite oxidation. Arsenates of varying chemical compositions dominate the soil above the granodiorite, while goethite, minor hematite and other indistinguishable FOHs are observed in the soil above the volcanosediments. The diversity and stability of the As secondary minerals in the studied soils are influenced partly by variation in the bedrock composition and mainly by the presence/absence of vegetation cover which mirrors in various contents of exchangeable Ca2+, content of Fe oxalate, and pH. The results of the sequential extraction show an average to high As retention by these soils, when As was extracted during the third and fourth extraction steps (NH4-oxalate buffer, ascorbic acid) in most samples. This finding indicates that As is more mobile in soils where arsenates dominate over well crystallized FOHs.