Adsorption of congo red from aqueous solution using rice husk, calcined kaolin clay, and microwaved rice husk clay hybrid

The present study focused on the comparative efficiency of rice husk (RH), calcined kaolin clay (CKC), and microwave rice husk clay hybrid (MRHCH) in the adsorption of congo red (CR) from aqueous solution. The rice husk was locally sourced and pulverized, the kaolin clay was calcined at 650 °C for two hours, while the rice husk clay hybrid was generated by microwave-impregnating an equal mixture of RH and CKC for 15 min at a medium temperature. The resulting adsorbents were characterized using scanning electron microscope (SEM) and Fourier transform infrared (FTIR) spectroscopy to evaluate their surface morphology and functional groups before and after adsorption. The effect of process variables comprising initial dye concentration, contact time, pH, and adsorbent dosage were investigated and the obtained adsorption equilibrium data were modelled using Langmuir, Temkin, and Freundlich isotherms. The adsorption kinetics were computed using Pseudo first order and Pseudo second-order reaction kinetics. The results of the study indicated that adsorption using all three adsorbents increased with increasing adsorbent dosage and contact time. On the other hand, adsorption using MRHCH and RH increased at a high initial concentration while CKC exhibited high adsorption at a low initial concentration. Isotherm studies indicated that Temkin isotherm best fits the adsorption equilibrium data while kinetic studies showed that the pseudo-second-order kinetic model best described CR adsorption, indicating chemisorption as the mechanism of adsorption. MRHCH demonstrated its exceptional adsorption capacity with a maximum adsorption capacity from the Langmuir isotherm of 4.008 mg/g. The adsorption process was found to be spontaneous, endothermic, and more random at the solid-solution interface by thermodynamic investigations. The study concludes that MRHCH has great promise for wastewater treatment applications as a very efficient adsorbent for CR removal.