Leaching effect on arsenic mobility in agricultural soils

The stability of soil arsenic during long-term leaching was studied in four soils from an agricultural area. Two identical columns simulating soil profiles of three layers were leached with As-free natural rainwater (<3.10−3 mg L−1 As) to test As mobility and the same rainwater enriched with As(V) (2.5 mg L−1 As) for the study of As accumulation. The relative As flow (μg g−1 day−1) showed a comparable run for all soils, with the peak corresponding to maximum As release in the first leaching stage, and then with a tendency to equilibrate. The amount of released As was controlled by the saturated hydraulic conductivity Ksat and free Fe oxides, and the kinetics of the leaching process correlated with the content of organic matter (OM). An overall stability and accumulation of soil arsenic were mostly affected by soil properties (Ksat, particle size, clay fraction), while the chemical composition (Fe, OM content) and surface properties (specific surface area SBET, theoretical adsorption capacity Qt) were of marginal significance. The distribution of As forms was performed by sequential extraction (SEP), which indicated negligible transformation (<12%) of As species in upper soil layers.