Optimal soil Eh, pH for simultaneous decrease of bioavailable Cd, As in co-contaminated paddy soil under water management strategies.

The co-contamination with cadmium (Cd) and arsenic (As) in the paddy soil is the most seriously combined pollution of toxic elements in China, and it is rather difficult to decrease bioavailable Cd and As levels in soil because of the opposite ionic forms of bioavailable Cd (cation) and As (anion). This study explored the optimal conditions of Eh and pH in different soils for simultaneous decrease of Cd and As bioavailabilities in the soil-rice system through soil culture and rice pot experiments under water management strategies. The results showed that near neutral soil pH (7.0) were eventually observed under long-term flooding conditions. Under unflooded conditions, soil pH is the dominant factor influencing bioavailabilities of Cd and As, while under flooded conditions, Eh becomes the most important factor. Pot experiments showed that flooding significantly reduced the Cd concentration in rice grains from 54.5% to 95.5%, but concomitantly increased rice As concentration substantially (214%-302%). By evaluating the trade-off value between the bioavailabilities of Cd and As in the soil, the minimal trade-off value was obtained when the soil Eh was -130 mV and the pH was 6.8.