Bio-Composite Materials for Cadmium Removal: Insight Ion Transport Behavior and Functional Group Interactions

The cadmium (Cd2+) adsorption performance of bio-composites comprising cellulose, bentonite, chitosan, and biochar was investigated under batch conditions at 30 and 70 °C with initial Cd2+ concentrations of 500 and 1000 ppm. Adsorption capacity (q) was evaluated as a function of the square root of time (√t) to assess ion transport dynamics. Formulation B (2:1:1:1) exhibited non-Fickian diffusion behavior, whereas formulations A (3:1:1:1) and C (1:1:2:1) displayed linearity (R2 > 0.75), consistent with Fickian diffusion. Kinetic analysis using the pseudo-second-order model confirmed chemisorption as the dominant mechanism, supported by FTIR evidence of functional group interactions. Despite limited equilibrium data, both the Langmuir and the Freundlich isotherms achieved high correlation coefficients (R2 > 0.9). Transmission Electron Microscopy (TEM) revealed dense agglomerates and collapsed pores in formulation B, while formulations A and C maintained layered microstructures that facilitated ion migration. These findings demonstrate that formulation ratios and thermal conditions critically govern ion transport and adsorption mechanisms, providing a structural–kinetic framework for the rational design of bio-composite adsorbents in heavy metal remediation.