The leaching characteristics of Cd, Zn, and As from submerged paddy soil and the effect of limestone treatment

Abstract

Rice is cultivated in a paddy system, in which there are variations in water depth with the growth of rice, as well as alternating physical and chemical conditions in the subsurface soil such as alternating oxidizing and reducing conditions during periods of drainage and irrigation. Thus, in rice paddies, it is not clear whether the stabilization method is appropriate. In this study, laboratory-scale column tests were performed to simulate the reducing environment of submerged soil to identify the leaching characteristics of Cd, Zn, and As in paddy soil, as well as to examine the stabilization effect of limestone on the contaminants. As the column soils approached a reducing state, the solubility of Mn and then of Fe abruptly increased and continued to increase. This induced the abrupt leaching of Cd and Zn. Thereafter, Cd and Zn were adsorbed and precipitated to the soil surface, and their contents in the leachate decreased as the negative charge on the soil increased due to the increased pH caused by submersion. Meanwhile, the As concentration continued to increase in the form of highly mobile and toxic As(III). In the soil treated with limestone, however, the pH increased throughout the test period, and accordingly, cations, such as Fe, Mn, Cd, and Zn, had significantly lower mobility than in the control soil. The As concentration also continuously decreased due to the large amount of Ca in limestone.