Experimental and theoretical study into 3-aminopropyl triethoxysilane-modified MWCNTs immobilised in chitosan hydrogels for Cd2+ and Zn2+ removal.

In this study, amine-modified multi-walled carbon nanotubes (MWCNTs-APTES) were incorporated into a chitosan hydrogel to develop a robust adsorbent for Cd2+ and Zn2+ ions removal from aqueous solution. The amino-rich CNTs/chitosan hydrogels were designed to enhance surface reactivity, stability, and adsorption capacity. The structural and textural analyses confirmed the homogenous CNT dispersion within the chitosan matrix, accompanied by increasing porosity and improved hydrogel stability. Thermodynamic study revealed that adsorption process is spontaneous and exothermic characteristics of the adsorption process, which refers to the strong affinity between the ions and the hydrogels. Kinetic modelling shows that the adsorption data were best described by the pseudo-second-order (PSO) and Elovich models suggesting a chemisorption-dominated mechanism involving the coordination with amine and hydroxyl groups. The optimised hydrogel exhibited a high maximum adsorption capacity of 216 mg/g for Cd2+ and 187 mg/g for Zn2+ in a single-component system. Also, Zn2+ showed faster adsorption kinetics and higher binding affinity. These experimental findings are consistent with DFT calculations, which predicted shorter equilibrium interaction distances for ZnN (∼2.7 Å) compared to CdN (∼2.8 Å), supporting the preferential binding of Zn2+ ions. Isotherm analysis indicated adsorption occurring on a heterogeneous surface involving both monolayer and multilayer uptake. Competitive adsorption exhibited preferential Zn2+ uptake due to stronger electrostatic and coordination interactions between Zn2+ and the active sites. Overall, the current study demonstrates that the APTES-CNT/chitosan composite hydrogels exhibited high selectivity, excellent structural stability and strong reusability (˃95% after 5 cycles) highlighting their potential for efficient and simultaneous removal of heavy metal ions from contaminated water.