Immobilization of Cd In a Paddy Soil Using Zero-Valent Iron (ZVI).

Zero-valent iron (ZVI) can immobilize heavy metals in contaminated soil because it provides effective surfaces for metal sorption and induces soil reduction to form insoluble precipitates. This study assessed the effect of ZVI on reduction of Cd solubility and its speciation in soil-solid phase in a 25-cm column system under a flooding condition (150 mg Cd kg^-1, as Cd(NO₃)₂). The soils were collected from three types of long-term managed soil with chemical fertilizer, cattle manure, and plant residue applications. The addition of ZVI (1% soil wt.) to all soils rapidly reduced the soil Eh value around -250 mV at day 1^st. In the manure-amended soil, for example, the soil solution concentration of Cd was below 0.01 mg L^-1 by adding ZVI at day 24^th, but it remained above 0.01 mg L^-1 without ZVI until 40^th day. Sequential extraction of soil determined that exchangeable Cd [Ca(NO₃)₂] was the most dominant fraction in the soil with chemical fertilizer (~60%) followed by manure (55%) and plant residue (25%) managements. The addition of ZVI reduced the exchangeable Cd fraction and transformed the acetic-acid extractable fraction in all soils. Linear combination fit on Cd-Kedge XANES spectrum using CdS and Cd(NO₃)₂ was performed to determine the contribution of CdS on soil Cd immobilization. Formation of CdS found to be 65% in the soil with plant residue, 35% in the manure soil, and 30% in the chemical fertilizer soil.