Fig. 1(a) compares the As extraction efficiency (i.e., proportion
of the extracted As to the total As in soils) for different washing
solutions. The As extraction efficiency was the greatest when
H3PO4 (0.5 M) was used as a washing solution, and this was followed
by 2% Na dithionite in HCl (0.01 M) and HCl (1 M). The
acid-based washing solutions extracted more As than the neutral
phosphate solution, which removed approximately 32% of As in
soils (Fig. 1(a))