Immobilization of Cadmium by Molecular Sieve and Wollastonite Is Soil pH and Organic Matter Dependent

The excessive cadmium (Cd) concentration in agricultural products has become a major public concern in China in recent years. In this study, two amendments, 4A molecular sieve (MS) and wollastonite (WS), were evaluated for their potential passivation in reducing Cd uptake by amaranth (Amaranthus tricolor L.) in six soils with different properties. Results showed that the responses of amaranth biomass to these amendments were soil-property-dependent. The effects of MS and WS on soil available Cd were in turn dependent on soil and amendment properties. The application of WS and MS at a dose of 660 mg·kg−1 Si produced the optimum effect on inhibiting Cd accumulation in amaranth shoots (36% and 34%, respectively) and did not affect crop yield. This was predominantly attributed to the marked increase in pH and exogenous Ca or Na, which facilitated the adsorption, precipitation, and complexation of Cd in soils. The immobilization effects of WS and MS were dependent on soil properties, where soil organic matter may have played an important role. In conclusion, MS and WS possess great potential for the remediation of Cd-contaminated acidic soils.