Wastewater sludge as a low-cost effective adsorbent of hexavalent chromium: equilibrium, kinetics, and thermodynamic studies

This manuscript demonstrates the ability to remove toxic hexavalent chromium species from aqueous solutions through adsorption in batch system using a cost-effective raw sludge (RS). The physico-chemical properties of the raw sludge were evaluated by scanning electron micro-scope/energy-dispersive X-ray spectroscopy, Fourier-transform infrared spectroscopy, X-ray diffraction analysis, thermogravimetric analysis, and X-ray fluorescence. The RS was found to be rich in amines and carboxylic groups, which may induce strong surface interactions with Cr(VI) ions in the solution and exhibit a porous morphology suitable for the fixation of Cr(VI) ions. The effects of adsorbent amount, contact time, initial Hexavalent chromium [Cr(VI)] concentration, pH and temperature on the adsorption of Cr(VI) were studied. Under the optimized operating conditions (initial Cr(VI) concentration = 20 mg/L, pH = 2, contact time = 60 min, temperature = 30°C) produced a maximum removal rate of 80.9% and maximum adsorption capacity of 9.84 mg/g. The equilibrium data fitted well to the Langmuir and Dubinin–Radushkevich adsorption isotherm models. The kinetic data showed good compliance with a pseudo-second-order rate model. The thermodynamic parameters indicated that the Cr(VI) removal by RS is an endothermic process with positive entropy was feasible spontaneously by an increase in the temperature. Based on the obtained results, the tested waste product sewage sludge can provide a low-cost means for the removal of Cr(VI) from synthetic effluents.