Comparative Assessment of Sorption Capability of Natural and Fe(III)-Modified Zeolite for Cobalt(II) Removal—Sorption Performance Optimization and Mechanism Investigation

The performance of natural zeolites for the removal of divalent metal ions is strongly dependent on the nature of the target ion and the surface properties of the sorbent. Building on previous studies of natural and Fe(III)-modified zeolites, this study investigates whether Fe(III) modification can enhance Co(II) uptake and, importantly, how the modification alters the dominant sorption processes and the subsequent retention of Co(II) within the zeolite structure. Natural zeolite (NZ) and Fe(III)-modified zeolite (FeZ) were systematically compared under controlled batch conditions, with particular emphasis on sorption optimization, kinetics, ion-exchange contributions, surface interactions, and Co(II) retention after saturation. Fe(III) modification substantially enhanced Co(II) sorption, increasing the maximum sorption capacity from 0.071 mmol/g for NZ to 0.230 mmol/g for FeZ. Although ion exchange remained the predominant mechanism for both sorbents, involving primarily Ca2+ release from NZ and Na+ release from FeZ, SEM-EDS and elemental mapping demonstrated preferential Co(II) association with Fe-containing surface sites formed after modification. Thus, Fe(III) modification not only increased the number of available sorption sites but also introduced additional surface interactions that contributed to Co(II) uptake. Kinetic analysis indicated that intraparticle diffusion controlled the overall sorption rate, while leaching experiments showed that FeZ retained >99% of sorbed Co(II) at equilibrium pHo > 4. Overall, the study demonstrates that Fe(III) modification provides a distinct advantage for Co(II) immobilization and establishes a mechanistic basis for selecting Fe(III)-modified zeolite for the remediation of Co-contaminated aqueous systems.