Kinetic and Thermodynamic Modelling of Zinc Adsorption Onto Acid-activated Oyster and Periwinkle Shells

This study investigated the adsorptive removal of zinc ions (Zn 2+ ) from aqueous solution using low-cost biosorbents derived from periwinkle shell char (PSC, Tympanotonus fuscatus) and oyster shell char (OSC, Crassostrea gigas), activated with 1.0 M HCl following calcination at 600°C. Characterization by XRF and FTIR confirmed that both adsorbents are predominantly composed of calcite (CaCO 3 ), with calcium contents of 87.43% (PSC) and 91.26% (OSC), and exhibited surface hydroxyl and carbonate functional groups responsible for Zn 2+ binding. Physicochemical analysis revealed specific surface areas of 186.4 m 2 /g (PSC) and 214.7 m 2 /g (OSC). Batch adsorption experiments were conducted to evaluate the effects of contact time, adsorbent dosage, temperature, initial Zn 2+ concentration, and solution pH. Maximum Zn 2+ removal of 82.6% (PSC) and 85.3% (OSC) was achieved at optimum conditions of pH 6.0, contact time of 60 min, adsorbent dosage of 1.0 g/100 mL, and temperature of 60°C. Adsorption kinetics were best described by the pseudo-first order model (R 2 = 0.927 for PSC; R 2 = 0.951 for OSC), with intraparticle diffusion identified as a contributing mechanism. Both Langmuir and Freundlich isotherm models satisfactorily correlated the equilibrium data; the Langmuir model yielded maximum monolayer adsorption capacities (qm) of 32.26 mg/g and 37.88 mg/g for PSC and OSC, respectively. Thermodynamic analysis revealed negative ΔG°, positive ΔH°, and positive ΔS° values, confirming that the adsorption process is spontaneous, endothermic, and associated with increased surface randomness. OSC consistently outperformed PSC across all parameters, attributable to its higher surface area, calcium content, and pore volume. These results demonstrate that both periwinkle and oyster shell chars are effective, sustainable, and low-cost adsorbents for zinc ion removal from wastewater.