Time-Course Changes in Speciation and Solubility of Cadmium in Reduced and Oxidized Paddy Soils

A prolonged flooding during rice (Oryza sativa L.) cultivation is recommended to immobilize soil Cd by the formation of CdS under anoxic conditions. The problem with this recommended practice is that it reduces rice yields compared with intermittent flooding, which is less effective at immobilizing CdS through reductive precipitation. This study investigated the solubility of Cd in relation to chemical speciation of Cd and S and soil redox potential through a time series of measurements during a 29-d reduction period followed by a 20-d oxidation period using X-ray absorption fine structure spectroscopy and chemical extraction. Two Aquent soils with different S levels in water-and oxalate-extractable fractions (hereafter low-S and high-S soils) were used to investigate the formation and dissolution of CdS. In the reducing period, the CdS proportion in the high-S soil rapidly increased to 30% at Day 4 and reached 90% at Day 29, whereas CdS in the low-S soil did not exceed 35%. In the following oxidizing period, CdS in the soils underwent oxidative dissolution but was not completely dissolved, remaining at <20% of the total Cd. A two-piece segmented linear relationship existed between HCl-extractable Cd and soil Eh, and the regression slope was 28-fold greater for the low-S soil at Eh >435 mV and 38-fold greater for the high-S soil at Eh >529 mV. This result indicates that each soil has a threshold Eh value at which the solubility of Cd is drastically altered during the course of soil reduction and oxidation processes.