Cadmium sorption behavior of natural and synthetic zeolites

Abstract

The sorptive properties of zeolites (a natural zeolite and 3 synthetic zeolites) for cadmium (Cd) were investigated with the intention of selecting suitable materials, for use as amendments in contaminated soils, to reduce the uptake of Cd by field crops. The Cd sorption data were well described by the Langmuir and the Freundlich equations and the coefficients values indicated significant sorptive capacities for Cd by these minerals. Synthetic zeolites have 100 to 500 times greater sorption capacities than the natural zeolite. In all cases, sorption increased with increasing pH and this relationship was linear over the pH range between 3 and 10 for the natural zeolite whereas for the synthetic zeolites a sharp increase was observed between pH 3 and 6. The critical pH above which maximum Cd sorption occurred on synthetic zeolites coincided with the pH where mineral dissolution ceases. The natural zeolite showed an inverse relationship between particle size (<0.15–2.0 mm) and Cd sorption. In desorption experiments with natural zeolite, 10 sequential extractions with 0.01 MNaC104 removed 19% Cd from the lowest sorption treatment and 44% of the metal from the highest level, showing that Cd was bound more strongly with decreasing concentrations. Desorption was greater with 0.01 MKC104than 0.01 MNaC104 especially in the first 5 extractions.