Thioctic acid-based adsorbent for heavy metal removal (Cd2+, Pb2+, and Cu2+): isotherms, kinetics, and thermodynamic evaluation.

Low-molecular-weight organosulfur compounds possess well-defined metal-binding functionalities; however, their direct application as unsupported adsorbents remains insufficiently characterized. Previously reported thioctic-acid-based sorbents have generally employed thioctic acid as an immobilized ligand or as a polymeric component incorporated into supported composite materials, particularly for precious-metal recovery. In the present study, commercially available, unsupported thioctic acid (α-lipoic acid) was directly evaluated, without grafting onto a carrier or incorporation into a composite, for the removal of Cd(ii), Pb(ii), and Cu(ii) from aqueous media. Its adsorption performance was systematically investigated through equilibrium, kinetic, and thermodynamic analyses, supported by physicochemical characterization using SEM-EDX, FTIR spectroscopy, XRD, BET surface-area analysis, and zeta-potential measurements. The equilibrium data were best described by the Langmuir model, indicating predominantly monolayer adsorption at energetically comparable binding sites, while the Freundlich and Temkin models provided complementary information regarding surface heterogeneity and adsorbate-adsorbent interactions. The pseudo-second-order model provided the best description of the kinetic data, consistent with adsorption controlled by site-dependent interactions. Combined FTIR, SEM-EDX, and zeta-potential results supported the involvement of carboxyl/carboxylate and cyclic disulfide functionalities in metal-ion binding through electrostatic attraction and coordination interactions. Thermodynamic analysis indicated that adsorption was spontaneous and exothermic under the investigated conditions. Under optimized conditions, the maximum uptake capacities reached 130, 120, and 100 mg g-1 for Cd(ii), Pb(ii), and Cu(ii), respectively. The adsorbent retained more than 89% removal efficiency after five consecutive adsorption-desorption cycles, demonstrating satisfactory regeneration performance and operational stability. Application to real industrial wastewater resulted in removal efficiencies of 92%, 75%, and 70% for Cd(ii), Pb(ii), and Cu(ii), respectively. Accordingly, the principal contribution of this work is the systematic establishment of the intrinsic adsorption behaviour, regeneration potential, and real-wastewater applicability of unsupported thioctic acid toward three environmentally relevant divalent metal ions.