The effects of soil amendments on arsenic concentrations in soil solutions after long-term flooded incubation

We investigated the effects of various soil amendments on the concentration of arsenic (As) in soil solutions under flooded anaerobic conditions. Ten amendments (six As adsorbents and four general agricultural amendments) were used with two soil samples (Aquepts). The time-course changes in the concentrations of As, iron (Fe) and silicon (Si) in soil solutions with or without amendments were measured after incubations of approximately 30, 60 and 100 d. We conclude that a precipitate of polysilicate-iron solution (1.1 mol L^‒1 Fe) shows promise as a novel As adsorbent for flooded anaerobic soils such as paddy soils. The precipitate is present in sludge from water purification plants and is therefore potentially cost effective. Moreover, it could increase the concentration of Si in soil solution, which could decrease As uptake by rice plants. Ferrihydrite and aluminum-substituted ferrihydrite were also effective for decreasing the dissolved As in both soils and nano-sized layered double hydroxides, and magnesium oxides worked in one soil. Poultry manure significantly increased dissolved As in both soils, but composted bark and silicate fertilizers did not change As in either soil. Some amendments had different effects in the two soil samples; further comprehensive studies will need to focus on how soil properties influence the effects of amendments.