Enhanced adsorptive removal of Pb²⁺ and Cd²⁺ ions from aqueous solution using phosphoric acid–activated rice husk

Adsorption is an efficient method for removing toxic metals from contaminated water, and this study investigates the performance of raw rice husk (RRH) and phosphoric acid – activated rice husk (ARH) for the removal of Pb(II) and Cd(II) ions from simulated aqueous solutions. The adsorbents were characterized using Fourier transform infrared spectroscopy (FTIR), Field emission scanning electron microscopy (FESEM), and Energy dispersive X-ray (EDX), confirming the presence of functional groups and porous surfaces suitable for adsorption. Batch experiments examined the effects of initial metal concentration (10 – 200 mg/L), contact time (0 – 60 min), solution pH (2 – 8), adsorbent dosage (0.5 – 3.0 g/L), and temperature (303 – 323 K). The optimum pH for Pb(II) adsorption was 4 for both RRH and ARH, while for Cd(II), the optimum pH was 5 on ARH and 7 on RRH. Adsorption equilibrium was achieved within 50 and 20 min for Pb(II), and 50 and 30 min for Cd(II) on ARH and RRH, respectively. Maximum adsorption capacities (qₘ) determined from the Langmuir model were 133.33 mg/g for Pb(II) on ARH, 35.07 mg/g on RRH, 28.82 mg/g for Cd(II) on RRH, and 17.67 mg/g on ARH. Equilibrium data were best described by the Freundlich isotherm, with constants n ranging from 1.958 to 2.182, indicating favorable adsorption. Dubinin – Radushkevich analysis yielded mean free energies below 8 kJ/mol, confirming a physisorption-dominated mechanism. Kinetic studies followed the pseudo-second-order model, and thermodynamic parameters (ΔG° = – 3.12 to – 3.75 k J/mol; ΔH° = – 18.04 to – 16.53 kJ/mol; ΔS° = +55.28 to +61.84 J/mol·K) indicated that the adsorption process is spontaneous and exothermic. These results demonstrate that rice husk and its activated form are effective, low-cost, and sustainable adsorbents for heavy metal remediation in aqueous environments.