Study of Kinetics and Thermodynamics of Removal of Phosphate from Aqueous Solution using Activated Red Mud

Red mud is the industrial waste by-product produced in alumina refineries. The red mud (RM) was activated using 1 N hydrochloric acid (hereafter referred as RMA). In the present communication, RM and RMA are used to study the kinetics and thermodynamics of effective adsorption for removal of phosphate from aqueous solution. The characterization of red mud is supported by BET, SEM, TG, DTG, XRD, and FTIR analysis. Batch experiments are carried out as a function of initial concentration, contact time, adsorbent dose and pH. It was observed that maximum adsorption capacity of phosphate (112 mg g −1 of RMA) is obtained using Langmuir adsorption isotherm in case of RMA in comparison to raw RM (56 mg g −1 of RM) at pH 2. Analysis of adsorption data indicates that the Langmuir isotherm provides a better fitting than Freundlich isotherm. Adsorption data were used to study first- and second-order kinetics and intra-particle diffusion models. Second-order kinetics is seen to be the best fitted for removal of phosphate. Thermodynamic parameters such as Δ H^0 , Δ S^0 and Δ G^0 were obtained. Negative value of Gibbs energy suggests the spontaneity of the adsorption phenomena which is in good agreement with the negative value of entropy indicating decrease in randomness of adsorbents at the solid and liquid interface. The characterization results and separation factor ( R L ) suggest that RMA and RM can be used as commercial adsorbent for removal of phosphate from aqueous solution.