Sustainable sequestration of Pb(II) and Cd(II) by South African clinoptilolite: mechanistic insights, nonlinear modeling, and multi-cycle reusability

Abstract The discharge of toxic heavy metals into aquatic systems poses serious risks to ecosystems and public health. In this study, a South African clinoptilolite-rich natural zeolite was evaluated for Pb(II) and Cd(II) removal from aqueous media. Physicochemical characterization confirmed a multiphase, clinoptilolite-dominated material with a Brunauer–Emmett–Teller (BET) surface area of 14.37 m $$^2$$ g $$^{-1}$$ , a pH-responsive, negatively charged surface, and high thermal stability. Batch adsorption experiments demonstrated strong pH dependence, rapid uptake kinetics, and a consistent selectivity sequence of Pb(II) > Cd(II), governed primarily by ion-exchange affinity and hydrated ion properties. Equilibrium data were best described by the Sips isotherm ( $$R^2_{\text {adj}} > 0.998$$ ; 0.002 ≤ $$\chi ^2_{\text {red}}$$ ≤ 0.013; 0.017 ≤ RSS ≤ 0.091), with maximum adsorption capacities ( $$q_m$$ ) of 10.94 mg g $$^{-1}$$ for Pb(II) and 7.70 mg g $$^{-1}$$ for Cd(II). These results indicate heterogeneous adsorption sites with a transition toward monolayer saturation at higher surface loading. Low Temkin binding energies ( $$< 1.6$$ kJ mol $$^{-1}$$ ) confirmed a predominantly physical and reversible adsorption mechanism, supporting efficient regeneration. Kinetic data were best represented by the pseudo-first-order model, suggesting concentration-driven, surface-controlled uptake. Importantly, the zeolite retained approximately 50% of its initial adsorption capacity after eight consecutive adsorption-desorption cycles using mild alkaline regeneration (1.0 M $${\textrm{Na}_2\textrm{CO}_3}$$ , pH 8), highlighting its structural stability and reusability. These findings demonstrate that naturally occurring clinoptilolite is an effective, regenerable, and low-cost adsorbent for heavy-metal removal, with practical relevance for sustainable water treatment in resource-constrained settings.