Removal of Cd(II) from aqueous solutions using modified synthetic sorbents: Sorption and kinetic study

Cadmium contamination in aquatic environments remains a serious environmental challenge due to its high toxicity, persistence, and bioaccumulation potential. In this study, a synthetic polymeric sorbent was chemically modified using two azo-containing organic reagents, 3-((1,5-dimethyl-3-oxo-2-phenyl-2,3-dihydro-1 H-pyrazol-4-yl)diazenyl)pentane-2,4-dione (R1) and 4-((2,4-dioxopentan-3-yl)diazenyl)benzoic acid (R2), to enhance its sorption performance toward Cd(II) ions. The successful functionalization of the sorbent was confirmed by FTIR analysis, which revealed the contribution of oxygen- and nitrogen-containing groups to Cd(II) binding. The influence of pH, ionic strength, contact time, and initial Cd(II) concentration on sorption behavior was systematically investigated. The optimum sorption performance was achieved at pH = 5, where Cd(II) exists predominantly as free Cd2+ ions and interacts effectively with the introduced functional groups. The maximum sorption capacities of the unmodified sorbent S, S+R1, and S+R2 were 229, 260, and 310 mg g−1, respectively, demonstrating the significant enhancement achieved through chemical modification. The S+R2 sorbent exhibited the fastest sorption kinetics, reaching equilibrium within 90 min. Equilibrium data were best described by the Langmuir model, indicating monolayer sorption on relatively homogeneous active sites, while kinetic analysis showed that the pseudo-second-order model provided the most suitable description of Cd(II) uptake. The calculated sorption energies from the Dubinin–Radushkevich model suggested that specific surface interactions contribute to the sorption mechanism. The modified sorbent demonstrated excellent selectivity toward Cd(II) in the presence of common interfering ions and retained approximately 98% of its initial performance after five regeneration cycles using 0.5 mol·L−1 HNO3. The practical applicability of the S+R2 sorbent was confirmed through Cd(II) determination in a real soil sample using a microcolumn preconcentration procedure coupled with ICP-OES analysis. The developed functionalized sorbent provides a simple, efficient, and reusable approach for Cd(II) removal and preconcentration in environmental applications.