Adsorption Behavior of Lead (II) and Chromium (VI) by Chitosan Functionalized Iron Oxide-Biochar Composites in Single and Binary Metal Ion Systems

This study aimed to utilize Chitosan functionalized Iron Oxide-Biochar Composites (CH@PSB/γ-Fe2O3) to eliminate chromium and lead ions from aqueous solutions. Adsorption behavior and sorption mechanisms for Cr (VI) and Pb (II) were investigated in both single and binary ion systems. Adsorption complied well with the Freundlich isotherm model in the single metal system. Maximum adsorption capacities were calculated as 8.78 mg/gm for Chromium and 6.56 mg/gm for lead. The Temkin linear isotherm revealed an exothermic nature, supported by a high regression coefficient and the positive value of the constant. Pseudo-second-order kinetics were suggested for the adsorption process. The optimal contact time was determined as 120 minutes for both ions. The multi-linear interpretation of plots unveiled distinct kinetic phases defined by the intra-particle diffusion model. Lead ions exhibited the highest adsorption capacity at pH 7.5, limited to 97.20 mg/gm, while chromium ion adsorption initially increased up to pH 4.5, peaking at 79.10 mg/gm before declining. There is apparent competitive adsorption behavior recognizable in the binary metal ion system beyond 30 mg/L ion concentration, with maximum capacities lower than those in single-metal systems. Adsorption isotherms favored the Freundlich model for both ions in binary systems, indicating the adsorption mechanism's dependency on the adsorbent's affinity, with chromium ions displaying a higher affinity than lead ions. The process was supported well with the pseudo-second-order and Freundlich isotherm models.