Adsorptive removal of Cr(VI) ions using nitrogen-doping activated carbon: influence of pH, kinetics, isotherm models,...

Hexavalent chromium ion [Cr(VI)] poses serious environmental risks due to its high toxicity and ability to cross biological membranes, leading to organ damage and genetic mutations. This study presents a new practical method for removing Cr(VI) ions from wastewater using nitrogen-doped activated carbon (AC600) prepared by pyrolyzing sawdust with ZnCl₂ at 600 °C. Thereafter, batch experiments were conducted using different adsorbent doses (0.5-2.5 g L^-1) and Cr(VI) ion concentrations (100-400 mg/L) at pH ranging from 1 to 12. Results revealed that Cr(VI) ions sorption on AC600 followed both linear and adsorption isotherm models at low sorbent doses (< 1.5 g/L). At higher doses, sorption shifted toward a hyperbolic adsorption pattern. Overall sorption data fitted well with both the Freundlich and Temkin isotherm models. Cr(VI) ions removal efficiency increased with contact time, following a sharp rise within the first 20-40 min of contact, while equilibrating within approximately 60-80 min. However, this efficiency decreased as pH increased. The highest efficiency (95-99%) was achieved using 2.5 g of AC600 to treat 1 L of water containing 100-150 mg/L of Cr(VI) ions within 2 h. For higher Cr(VI) ion concentration (200-250 mg/L), the efficiency decreased to 88-90%. The kinetics of Cr(VI) ion sorption on activated carbon followed the Pseudo-Second-Order model at low to moderate AC600 doses and low Cr(VI) ion concentrations. At higher doses and Cr(VI) levels, the power function and intraparticle diffusion models provided better fits. A maximum adsorption % could be achieved by using 2.5 g of AC600 and a Cr(VI) ion solution at 100 mg/L, according to results from an optimization of adsorption parameters using response surface methodology (RSM).