The fate of arsenic in contaminated paddy soil with gypsum and ferrihydrite amendments

Ferrihydrite and gypsum (CaSO_{4 2}H₂O) are two chemicals which were suggested to reduce the bioavailability of arsenic (As) in agricultural field. The efficiency of those chemicals added in the paddy field was investigated by pot culture in this study. The results showed As uptake by rice under ferrihydrite treatment was significantly lower than that in the control (36% in shoot), however, plants grown in gypsum show no significant difference in As content compared with those in the control. Sequential extraction showed that As was mainly adsorbed to amorphous and crystalline hydrous oxide. The addition of gypsum did not change the As fractionation in comparison with the control, which may be due to restrained sulphur (S) reduction by the high microbially reducible Fe(III) (150-250 mg kg^-1), which precipitates with sulphide in preference to As. These results suggest that the chemical Fe oxides efficiently reduce As risk in the paddy.