Arsenic geochemistry in forested soil profiles as revealed by solid-phase studies

Arsenic concentrations in soils may be elevated either because of anthropogenic activity or because of a high natural abundance of the parent material. In the unsaturated zone of seven forest soils in northern Sweden, inorganic As(V) generally dominated the solid-phase speciation while non-NaBH4-reducible organic As associated with isolated humic substances (humic As) was present in low amounts. In unpolluted soils, absorbed As(V) was more or less constant through the B and C horizons and did not show any obvious relationship with secondary short-range ordered Fe or Al minerals-this suggested that most As(V) had formed early during pedogenesis as a result of sulphide weathering. When a small amount of As(V) was added to the mineral soils, adsorption was almost complete and the amount of remaining As(V) in solution depended on the ratio of pyrophosphate-C to oxalate-(Fe + Al). On higher As(V) additions, the amount of adsorption sites governed the As solubility. As regards the humic As, the XAD-4 acid fulvates were more enriched with As as compared to the hydrophobic acids. The As content of the forest floor was highly dependent on the distance from the Rönnskärsverken non-ferrous metal smelter, but did not reflect the As content of the underlying horizons; thus, biological uptake of As from the mineral soil appeared to be very low.