Synergistic effects between adsorption and chemical reduction towards the effective Cr(Ⅵ) removal in the presence of the sulfur-doped biochar material

Abstract

Abstract As the major by-products of municipal sewage treatment, the waste activated sludge (WAS) gives rise to secondary pollution to the environment and threatens human health without proper disposal. On the other side, hexavalent chromium (Cr(Ⅵ)) is among the 17 chemicals that pose the most detrimental effects to human beings. Herein, the anaerobic sludge withdrawn from thickener in a sewage treatment plant served as the precursor for sludge-based bichoar fabrication, which was further modified via the sulfur (S) heteroatom doping (i.e. S-BC). The S atom doping resulted in the adjustment of the physicochemical properties towards the carbon material, endowment of abundant functional groups on biochar surface, as well as increasing the binding sites between biochar and Cr(Ⅵ). Compared to the primary biochar (i.e. biochar without heteroatomic doping, named BC), S-BC exhibiting the rough surface, had remarkable advantages in ash content, specific surface area, and pore volume. and its specific surface area was 16.7 m2/g, which was significantly larger than BC-500 (9.4 m2/g). The existence of graphene carbon crystal structure for S-BC was confirmed through S-BC by XRD and FTIR analysis. The removal of Cr(Ⅵ) efficiency by S-BC could reach 91.8% under initial Cr(Ⅵ) concentration of 30 mg/L, S-BC amount of 0.8 g and pH of 3.0. The analysis of adsorption kinetics and isotherms showed that pseudo-second-order kinetics and Langmuir model more fitted the results of of Cr(Ⅵ) removal behavior in the presence of S-BC. Therefore, the chemisorption and monolayer adsorption were the primary mechanisms involved in the Cr(Ⅵ) removal process. Additionally, XPS analysis results illustrated the aqueous Cr(Ⅵ) was efficiently eliminated through the synergistic effect of chemisorption and reduction to Cr(Ⅲ) in the presence of S-BC. Moreover, S-BC could still achieve the Cr(VI) eliminating efficiency of 85.31% after five cycles with functional group and crystal structure almost unchanged.