Chemical forms of As (i.e., fractionation of As) in soils were
assessed using the sequential extraction procedure of Wenzel
et al. (2001), which divides As in soils into five fractions, with a
slight modification – (1) ammonium sulfate ((NH4)2SO4;
0.05 M)-extractable As (i.e., non-specifically sorbed As); (2)
NH4H2PO4 (0.05 M)-extractable As (i.e., specifically sorbed As);
(3) ammonium oxalate ((NH4)2C2O4; 0.2 M)-extractable As (i.e.,
As associated with amorphous hydrous oxides of Fe and Al); (4)
(NH4)2C2O4 (0.2 M)/ascorbic acid (0.1 M)-extractable As (i.e., As
associated with crystalline hydrous oxides of Fe and Al); and (5)
residual As fraction extracted with the USEPA method 3052, which
digest siliceous and organically based matrices such as soils and
sediments in a mixture of nitric acid, hydrofluoric acid, HCl, and
hydrogen peroxide using microwave heating (US Environmental
Protection Agency, 1996).