Application of acid-activated attapulgite clay on effective immobilization of Cd in paddy soils

This study investigated the effects of different amounts of acid-activated attapulgite clay on soil environmental risks and ecotoxicity after incubation as a soil amendment. The experimental soil was alkaline clay soil with a pH of 7.54. The optimal addition and incubation times for the remediation of Cd-contaminated soil were determined. The results showed that the optimal period for Cd stabilization was 24 days, and the optimal addition amount was 8% (w/w). The addition of 8% attapulgite clay significantly reduced the bioavailable Cd concentration in soil. Water-Cd, DTPA-Cd, TCLP-Cd, and HCl-Cd reduced the bioavailable Cd concentration in the soil by 26.43%, 42.75%, 35.22%, and 25.89%, respectively. Meanwhile, the addition of 8% attapulgite clay converted the chemical speciation of exchangeable-Cd into residual-Cd, and the stabilizing component increased by 48% after 48 days. The ecological and environmental risk indices decreased significantly, reaching 10.89% and 4.22%, respectively. The pot experiment showed that attapulgite clay promoted corn growth while decreasing concentrations of Cd in aboveground parts and roots by 9.01-31.82% and 9.52-40.48%, respectively, and limited Cd transfer from roots to stems, minimizing its presence in edible portions as much as possible. The accumulation of heavy metals in the growth of corn plants was significantly and positively correlated with the bioavailability of Cd extracted from different extractants and exchangeable speciation, and significantly and negatively correlated with soil residual speciation. Attapulgite is a promising soil amendment that can effectively reduce ecological and environmental risks, reduce the ecological toxicity of Cd, and improve soil quality.