Removal of cod from real textile wastewater using three low-cost adsorbents - its kinetic models and adsorption isotherms

In this work, batch adsorption studies were carried out using three low-cost adsorbents like rice husk ash (RHA), corncob ash (CA) and sugarcane bagasse fly ash (BFA) with the aim of removing COD from textile wastewater (TWW) having an initial concentration of 1026 mg/L. Batch experiments were conducted to study the adsorption behaviour through the effect of adsorbent dosage, pH and contact time on the adsorption of COD removal. The results showed that the removal percentage of COD could be over 70–75% at the optimal adsorbent dosage of 6 g/L, at a pH of 8.0 to 8.5 and a contact time of 40 min for RHA and BFA. At the same time, less COD removal was observed while using CA as an adsorbent giving only 50–55% COD removal. The applicability of the Langmuir, Freundlich, Temkin and Dubinin–Radushkevich isotherm models for the three low-cost adsorbents adsorption data was tested. Langmuir isotherm plots indicated that the adsorption process is favourable and best fitted. In the Temkin adsorption isotherm, the values of 'B' (heat of adsorption) showed positive values, which indicates that the reaction with all three adsorbents was an exothermic reaction. From the results of the Dubinin–Radushkevich adsorption isotherm, the constant E (mean free energy) values lying within 8 indicated that the adsorption process is physical adsorption. Kinetics data showed that the pseudo-second-order kinetic model showed a good correlation for the adsorption than the pseudo-first-order. Therefore, RHA and BFA can be potentially used as low-cost adsorbents for treating TWW.