Iron Oxide-Coated Zeolite Composite Granules for Cadmium Stabilization in Contaminated Soils: Synthesis and Assessment

Cadmium is a highly toxic and mobile heavy metal that poses a significant threat to the environment and human health, particularly through its accumulation in agricultural soils. To address this challenge, composite zeolite granules (1-2 mm) coated with an iron (oxyhydr)oxide layer were synthesized using a two-stage process. Scanning electron microscopy revealed that the resulting coating consists of rounded nanoparticles with a morphology characteristic of weakly crystalline 2-line ferrihydrite. The adsorption and stabilizing properties of natural and composite granules were evaluated using three test series: adsorption from a single-component cadmium solution; adsorption from a multicomponent “soil solution”; and an experiment in pots. The adsorption of cadmium by both materials is best described by a pseudo-second-order kinetic model. The maximum cadmium adsorption capacities, as determined by the Langmuir model, were 12.96 mg/g for natural zeolite and 15.37 mg/g for composite zeolite. Cadmium uptake increased with rising pH and declined in solutions of higher ionic strength. Desorption tests suggested weak binding of cadmium to the iron (oxyhydr)oxide surface through outer-sphere complexation over a short adsorption interval. In the multicomponent “soil solution” both materials demonstrated selective uptake of Cd, Pb, and Zn, with enhanced cadmium adsorption by the composite granules. Pot experiments revealed a partial structural change in the iron (oxyhydr)oxide phase, accompanied by the high retention of adsorbed heavy metals (Pb and Zn). These results demonstrate the high potential of the iron (oxyhydr)oxide-coated zeolite granules for the in situ stabilization of cadmium and other heavy metals in contaminated soils.