Epichlorohydrin-Crosslinked Nano-TiO₂/Plum Kernel Shell/Chitosan Hydrogel Beads for the Efficient Removal of Hexavalent Chromium from Aqueous Solutions

The removal of hexavalent chromium [Cr (VI)] from industrial wastewater remains a significant environmental challenge due to its extreme toxicity and serious risks to both human health and the ecosystem. This study aims to develop and evaluate a sustainable and efficient adsorbent for Cr (VI) removal from aqueous solutions. To achieve this, composite hydrogel beads composed of nano-TiO₂, plum kernel shell, and chitosan were synthesized using a sol-gel method and crosslinked with epichlorohydrin (nTiO₂–PKS–Cts@ECH). The morphological and structural characteristics of the biosorbent were analyzed using scanning electron microscopy (SEM) and Fourier-transform infrared spectroscopy (FTIR). Important operational factors, such as pH, adsorbent dose, contact time, and starting Cr (VI) concentration, were optimized using batch adsorption experiments. The equilibrium data were evaluated using the Langmuir, Freundlich, and Dubinin–Radushkevich isotherm models, with the Langmuir model showing the best fit, indicating monolayer adsorption and a maximum capacity of 97.09 mg/g. Kinetic analysis demonstrated that the adsorption process followed a pseudosecond-order model, suggesting that chemisorption, electrostatic attraction, and reduction collectively constitute the rate-limiting steps. Overall, the findings highlight the high potential of nTiO₂–PKS–Cts@ECH as a promising and eco-friendly biosorbent for the effective removal of Cr (VI) from contaminated water systems