NaOH-treated rice husk, a lignocellulosic biomaterial for removal of Cr(VI) from polluted water

Rice husk is an inexpensive and readily available adsorbent for heavy metal ions, though its natural form lacks sufficient adsorption capacity. This study focuses on improving its ability to adsorb Cr(VI) by treating it with NaOH. After treatment with 1N NaOH at 90 °C for 4 h, the adsorption efficiency of rice husk for Cr(VI) increased by almost 30%. Field emission scanning electron microscopy and energy dispersive X-ray spectroscopy revealed an increase in the cellulosic content of the treated rice husk, while X-ray diffraction indicated the formation of new polymorphs and enhanced crystallinity of the cellulose fraction. Among the applied isotherms, the Redlich-Peterson and Langmuir models best described the adsorption, suggesting both monolayer and multilayer adsorption. The maximum monolayer adsorption capacities were determined to be 18.9, 21.1, and 22.9 mg g⁻¹ at 303 K, 313 K, and 323 K, respectively. The kinetic data fit well with pseudo-second-order and Elovich models, confirming the chemical nature of the adsorption process. Activation energy, along with other kinetic and thermodynamic parameters, showed that the process is endothermic, involving both physical and chemical interactions. FT-IR analysis identified various functional groups involved in the chemical interaction with chromium, while XPS confirmed that all adsorbed chromium was in the + 3 oxidation state. The presence of interfering anions reduced the adsorption efficiency of NaOH-treated rice husk for Cr(VI), regardless of the anion type, while regeneration studies showed that the adsorbent could be reused for up to five adsorption–desorption cycles.