Optimizing removal of chromium (VI) ions from water by coagulation process using central composite design: Effectiveness of grape seed as a green coagulant

In the past decades, considerable work has been devoted to remove chromium (VI) ions from the aqueous environment, owning to their serious health concerns to humans and the aquatic ecosystem. Recently, natural coagulants have attracted increasing attention from researchers due to their great ability to remove pollutants from industrial wastewater. In the present study, we focused on the utilization of a grape seed powder (GSP) biocoagulant as low-cost and eco-friendly material for the effective removal of hexavalent chromium, Cr(VI) ions from synthetic wastewater using coagulation process. The GSP was characterized using familiar techniques like phytochemical analysis, scanning electron microscopy (SEM) coupled with EDS mapping, microelectrophoresis, Fourier transform infrared spectroscopy (FTIR) and X-ray photoelectron spectroscopy (XPS). The impacts of some independent parameters such as coagulant dosage, initial pH, initial Cr(VI) concentration, agitation time, and sedimentation time on the removal of Cr(VI) ions by using GSP were examined and the process was optimized by using the response surface methodology (RSM) based central composite design (CCD) design. In addition, the relationship between process variables was described. Results demonstrated that the removal efficiency of Cr(VI) from aqueous solution was decreased by increasing the initial pH of solution and initial Cr(VI) concentration. Furthermore, the removal efficiency is favored by increasing GSP dosage, sedimentation time and agitation time. Optimized values of pH, coagulant dosage, initial Cr(VI) concentration, agitation time and sedimentation time were found to be 4.53, 0.5 g/L, 5.12 mg/L, 15 min and 3.5 h, respectively. The removal mechanism study indicated that the Cr(VI) removal was reached via esterification reaction between the functional groups of GSP and Cr(VI) ions, and reduction reaction of Cr(VI) to Cr(III) species. The high removal capacity validated the effective applicability of GSP as a useful natural coagulant for the removal of Cr(VI) ions from synthetic wastewater.