This may be due to the presence of excess
amount of phosphate, which, during soil washing, can replace As
bound to soils by competition between As and phosphate
(Hongshao and Stanforth, 2001). In addition, the less harsh condition
created after washing with the neutral phosphate solution,
compared to when the acid-based washing solutions were used,
is less likely to dissolve soil fractions including metallic fractions
(e.g., hydrous oxides of Fe and Al), thus, more soil fractions for
resorption of the extracted As may be available (Tokunaga and
Hakuta, 2002). Nonetheless, the sum of the
(NH4)2SO4-extractable and NH4H2PO4-extractable As was reduced
after soil washing (Fig. 1(b)).