Chitosan–humic acid composite cryogels for heavy metal adsorption: insights from single and binary aqueous systems

Abstract

The persistent contamination of water by heavy metals poses a significant threat to ecosystems and human health due to their toxicity, non-biodegradability, and bioaccumulative nature. In this study, we report the design and fabrication of an innovative chitosan–humic acid (Ch–HA) composite cryogel with adequate porosity and surface area, which is rich in functional groups, for the efficient removal of Pb²⁺ and Mn²⁺ ions from aqueous solutions. Comprehensive characterization using FTIR, FE-SEM, BET, and pH_pzc analyses confirmed the successful integration of humic acid into the chitosan matrix, enhancing adsorption sites and structural stability. Batch adsorption experiments revealed that the cryogel exhibited rapid and selective uptake, with adsorption strongly influenced by solution pH, initial metal concentration, and contact time. Single-component adsorption followed the Freundlich isotherm, suggesting heterogeneous multilayer adsorption, with maximum capacities of 0.31 mmol g⁻¹ for Pb²⁺ and 4.03 mmol g⁻¹ for Mn²⁺. Notably, in competitive binary systems, Pb²⁺ ions were preferentially adsorbed, demonstrating the cryogel's potential for selective remediation in complex wastewater. Kinetic studies indicated that adsorption proceeded via a pseudo-second-order mechanism, with intraparticle diffusion contributing to the rate-determining step. Mechanistic insights suggest that adsorption arises from synergistic effects of electrostatic interactions, hydrogen bonding, and metal–ligand complexation on the cryogel surface. This work highlights the Ch–HA cryogel as a sustainable, eco-friendly, and high-performance adsorbent for heavy metal removal, bridging the gap between laboratory-scale research and real-world wastewater treatment applications. These findings provide a strategic framework for developing advanced biomaterial-based adsorbents, offering a promising pathway toward cleaner water and environmental preservation.