Alkali-treated carbonized rice husk for the removal of aqueous Cr(VI)

Abstract

Rice husk was chemically modified for the preparation of activated carbon. Rice husk was treated with nitric acid and carbonized at 700 C. After carbonization, the resulting rice husk char was treated with NaOH at room temperature. The 5 M NaOH-treated rice husk had the highest surface area (750 m2/g). Proximate analysis of activated carbon confirmed that NaOH treatment removed silica completely. Temperature programmed decomposition (TPD) graphs showed that the total gas contents (CO and CO2) liberated by CRH and H2O-treated CRH and CRH5M were 2l5 μmol/g, 390 μmol/g, and 970 μmol/g, respectively. The adsorption studies of the activated carbon during Cr(VI) removal from the aqueous medium indicated that CRH5M showed the highest rate of adsorption. The effect of adsorbent dosage, Cr(VI) concentration, pH, and temperature were studied to determine the best removal efficiency. With a decrease in pH from 4.4 to 2, the adsorption capacity increased from 3 mg/g to 25.2 mg/g. The adsorption of Cr(VI) followed pseudo-second-order behaviour. The changes in Gibbs free energy, enthalpy, and entropy affected by thermodynamic parameters were found to be negative, which confirmed that the adsorption of Cr(VI) on CRH5M is spontaneous, exothermic, and favours low temperatures.