Enhanced Cationic Dyes Removal from Aqueous Solution by Oxalic Acid Modified Rice Husk

Abstract

The adsorption of two basic dyes, namely, methylene blue (MB) and malachite green (MG), onto natural rice husk (NRH) and oxalic acid modified rice husk (MRH) was studied in a batch adsorption system. Factors influencing dyes adsorption such as the concentration of the adsorbate, the pH, the salt concentration, the temperature, and the contact time were investigated. The Langmuir and Freundich isotherms were used to fit the equilibrium data, and the results showed that the Langmuir isotherm exhibited a little better fit to the MG adsorption data by both adsorbents, while the Freundlich isotherm seemed to agree better with the MB adsorption. The kinetic experimental data were analyzed using three kinetic equations, via the pseudo-first-order equation, the pseudo-second-order equation, and the intraparticle diffusion model, to examine the mechanism of adsorption and the potential rate-controlling step. The mechanism of the process was found to be complex, consisting of both surface adsorption and pore diffusion. The values of the effective diffusion parameter, Di, were estimated to be of the order of 10−8 cm2·s−1, indicating that the intraparticle diffusion was not the rate-controlling step. Calculated thermodynamic parameters showed that the adsorption of MB and MG onto MRH was feasible, spontaneous, and endothermic. The removal capacities of NRH and MRH adsorbing dyes from aqueous solution were compared. By functionalizing, the adsorption capacity (qe) of rice husk for MB or MG was increased from (19.77 to 53.21) mg·g−1 or (28.00 to 54.02) mg·g−1 at 293 K, respectively. The carboxyl groups on the surface of the MRH were primarily responsible for the sorption of dyes. It is suggested that MRH may be suitable as an adsorbent material for adsorbing MB and MG from aqueous solutions.