Comparative study on cadmium ion removal from acidic solutions using zeolites and bentonite

Adsorption of Cd(II) on synthetic Na-X zeolite was studied by batch method and compared with that on clinoptilolite and bentonite. The study examined the effects of contact time (2.5-1440 min), pH (3.0 and 5.0), anion type (Cl⁻, SO₄²⁻), and initial Cd(II) concentration (0.1-950 mg/L) on adsorption capacity. Adsorption data were fitted to Freundlich, Langmuir, Dubinin-Radushkevich, and Sips isotherm models, as well as pseudo-first-order (PFO) and pseudo-second-order (PSO) kinetic models. The highest adsorption occurred at pH 5.0 in the presence of sulphate ions, while the lowest - at pH 3.0 in the presence of chloride ions. The maximum adsorption capacity followed the trend Na-X (185-268 mg/g) > > bentonite (97-136 mg/g) > clinoptilolite (55-68 mg/g). The PFO model fit well for low Cd(II) concentrations and short contact times, whereas the PSO model described adsorption across all conditions. Binding strength, evaluated using the BCR extraction method, showed that Cd(II) was weakly and reversibly bound in fractions E1 and E2. The study concludes that synthetic Na-X zeolite is a highly effective adsorbent for removing Cd(II) from acidic wastewater, demonstrating significant potential for environmental applications.