Functionalization of Rice Husks with Ortho-Phosphoric Acid Enhanced Adsorptive Capacity for Anionic Dye Removal

Rice husk is a biodegradable polymer which is quite abundant. However, its low adsorption capacity has limited its use as an adsorbent. This study has described the adsorption of methyl orange (MO) on rice husk (RH) and its H 3 PO 4 -functionalized derivative (FRH). Both adsorbents were characterized by Fourier transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM) and Energy Dispersive X-ray (EDX). Spectra data showed shifts and reduction in intensities of functional groups in addition to appearance of a peak at 963 cm −1 in FRH. SEM and EDX images revealed changes in surface morphology with rougher cavities and increased percentage of carbon content that could influence adsorptive properties. Effects of pH, contact time, MO concentrations, adsorbent dosage, and temperatures on adsorption processes were studied. Adsorptive uptake of MO was maximum at pH 2.0 for RH, pH 5.0 for FRH, and was found to increase with increase in contact time, dosage and concentration. Langmuir isotherm was the most favourable to describe the adsorption process for RH whereas Freundlich was the most appropriate for FRH. The maximum monolayer adsorption capacity improved from 28.7 (RH) to 177.4 mgg −1 (FRH). Energies of adsorption (4.5 and 3.6 kJmol −1 ) indicated a physiosorption removal of MO. Pseudo second order kinetics best described the adsorption process. Both adsorption processes were spontaneous, while it was exothermic for RH, it was endothermic for FRH. Functionalization with H 3 PO 4 enhanced the adsorptive capacity of rice husk by 519%.