Application of Nanotubular Halloysite for Heavy Metal Immobilization and Reduction in the Environment

This study evaluated natural halloysite from the Dunino deposit as a sorbent for Pb2+, Cd2+, Cu2+, Zn2+, Hg2+, and Cr3+ ions from aqueous solutions. Batch experiments were conducted to examine the effects of solution pH, initial metal concentration, contact time, and multicomponent conditions. Equilibrium data were evaluated using the Langmuir and Freundlich models, whereas adsorption kinetics were analyzed using pseudo-first-order, pseudo-second-order, and Weber–Morris intraparticle diffusion models. Metal uptake increased markedly between pH 3 and 5, while changes between pH 5 and 7 were smaller. Nonlinear Langmuir fitting yielded maximum adsorption capacities ranging from 47.68 to 63.87 mg g−1, with the highest values obtained for Hg2+ and Pb2+. The pseudo-first-order model provided a closer empirical description of the kinetic data than the pseudo-second-order model, and the adsorption profiles approached a plateau after approximately 480–720 min. SEM, FTIR, and nitrogen adsorption measurements revealed changes in surface morphology, the chemical environment of surface functional groups, and nitrogen-accessible surface area after metal loading. These observations are consistent with the involvement of surface sites but do not identify a unique molecular adsorption mechanism. Acidic desorption using 0.1 M HCl released 75.0–92.5% of the retained metals, compared with 25.0–47.5% under alkaline conditions. The results support further evaluation of natural halloysite as a mineral sorbent, including matched competition experiments and repeated adsorption–desorption cycles.