Coffee processing waste as a reinforcing material in the preparation of chitosan-based magnetic composite for the removal of chromium (VI) from aqueous solution

• Coffee husk fiber was used as a reinforcement for chitosan-based magnetic composites. • A higher percentage of chitosan to coffee husk fiber resulted in increased removal efficiency. • The composite material exhibited a higher specific surface area. • Maximum chromium (VI) removal was achieved at lower initial concentrations and higher adsorbate doses. • The composite demonstrated stable efficiency as investigated in different cycle tests . This study aimed to synthesize and characterize a coffee-husk fiber-reinforced magnetic chitosan composite for the removal of hexavalent chromium (VI) from synthetic wastewater. The composite adsorbent was synthesized using a co-precipitation method with different mixing ratios of chitosan to coffee husk fibers (27:75, 50:25, and 75:25). The surface functional groups, surface morphology, specific surface area, and crystal structure were investigated using Fourier transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM), Brunauer-Emmett-Teller (BET) analysis, and X-ray diffraction (XRD). The adsorption operational parameters were initial concentration (1.5-2.5 mg/L), initial pH of the solution (4-10), contact time (60-120 minutes), and adsorbent dosage (1-3 g/L) were selected as operational parameters. A statistical experimental technique known as response surface methodology, specifically the Box–Behnken experimental design, was applied to investigate the interaction effects and identify the optimum parameters for the adsorption process of Cr (VI) removal from aqueous solutions. Statistical analysis was performed using analysis of variance (ANOVA). Accordingly, the maximum adsorbent chromium removal efficiency was found to be 98.04% at a pH of 7.25, adsorbent dosage of 2.6 g/L, initial concentration of hexavalent chromium 1.5 mg/L, and contact time of 116 min. Based on this analysis, all selected parameters were found to be equally significant for the performance of the synthesized composite adsorbent. The reusability of the composite under optimal conditions showed minimal loss in removal efficiency through the fourth cycle but a rapid decline from 78.2% to 60% after the fourth cycle.