High-efficiency removal of hexavalent chromium from contaminated water using nitrogen-doped activated carbon: kinetics and isotherm study

Abstract

Hexavalent chromium (Cr6+) is a carcinogenic pollutant found in different wastewater streams. The current study investigates the potential removal of Cr6+ from aqueous solutions by nitrogen-doped coconut granulated activated carbon (N-GAC). The raw GAC was chemically and thermally treated with urea (1:1 ratio) in an ethanol medium and calcined under nitrogen atmosphere at temperatures 400 °C and 600 °C. The optimized sample (N-GAC 400 °C) possessed a remarkable surface area (474.62 m2/g) while the chemical composition analysis revealed successful doping of nitrogen with no significant changes on the mesoporous morphology of the raw GAC. The adsorption studies revealed that Cr6+ adsorption capacity was higher for N-GAC 400 °C. The removal efficiency was significantly decreased with increasing the initial solution pH (optimum pH 2.2). The temperature has no significant effect on the adsorption capacity. The equilibrium experimental results were best fitted with Sips and Redlich-Peterson isotherm models with R2 ≥ 0.987 and a maximum adsorption capacity of 15.15 mg/g. The kinetic data agreed with the pseudo 2nd order model (R2 > 0.997). The isotherm and kinetic studies revealed that physisorption and chemisorption processes are involved in the removal process. The adsorption mechanism was mainly controlled by electrostatic interactions, ion exchange and reduction of Cr6+ to trivalent chromium. Accordingly, the N-GAC can be considered a promising material for the adsorptive removal of heavy metals from water.