Valorizing Rice Husk Waste as a Biosorbent with Gamma-Induced Surface Modification for Enhanced Heavy-Metal Adsorption

This work investigated the effects of gamma irradiation on the adsorption capacities of rice husk (RH) for the removal of Cu2+, Cr3+, and Zn2+ ions from aqueous solutions, with potential applications in wastewater remediation. RH samples were gamma-irradiated at doses up to 40 kGy and characterized using SEM-EDS, XRF, FTIR, XRD, and BET analyses. While morphological and textural changes remained subtle, FTIR and SEM-EDS confirmed the formation and intensification of oxygen-containing functional groups, including –OH, –COOH, and C=O, as well as increased exposure of silica (Si–O) on the surfaces, which substantially enhanced surface reactivity of RH toward metal ions. Batch adsorption experiments revealed that 40-kGy irradiated RH samples (RH-40) exhibited the highest removal efficiencies compared to non-irradiated and lower-dose samples (RH-0, RH-10, RH-20, and RH-30), specifically with improvements of 415% for Cu2+, 502% for Cr3+, and 663% for Zn2+ compared to RH-0, determined at the initial concentration of 10 mg/L. Kinetic studies also showed rapid adsorption within the first 10–15 min, dominated initially by boundary-layer diffusion, followed by chemisorption-driven equilibrium behavior. The pseudo-second-order (PSO) model provided an excellent fit for all metals (R2 = 0.999), indicating maximum model-predicted kinetic capacities of 555.56 mg/g (Cu2+), 769.23 mg/g (Cr3+), and 434.78 mg/g (Zn2+). Langmuir isotherms also fitted well (R2 = 0.941–0.995), with predicted monolayer capacities of 535.33 mg/g (Cu2+), 491.64 mg/g (Cr3+), and 318.88 mg/g (Zn2+). Freundlich modeling further indicated favorable heterogeneous adsorption, with KF values of 42.614 (Zn2+), 20.443 (Cr3+), and 16.524 (Cu2+) and heterogeneity factors (n) greater than 1 for all metals. These overall results suggested that gamma irradiation substantially enhanced RH functionality that enabled fast and high-capacity heavy-metal adsorption through surface oxidation and carbon valorization. Gamma-irradiated RH, therefore, represented a promising, low-cost, and environmentally friendly biosorbent for wastewater treatment applications.