Adsorption of Cr(VI) on bamboo bark-based activated carbon in the absence and presence of humic acid

Understanding the effect of humic acid (HA) on the adsorption of Cr(VI) by activated carbon (AC) is practically significant to assess Cr(VI) adsorption behavior of AC in natural water. This study investigated the adsorption of Cr(VI) onto bamboo bark-based AC as a function of contact time, adsorption isotherm, and initial pH in the absence and presence of HA by means of batch experiments. Results indicate that the Cr(VI) adsorption in both the absence and presence of HA most appropriately followed the pseudo-second-order kinetic model (R-2 > 0.99) and adsorption isotherm is well fitted to the Freundlich equation (R-2 > 0.99). In addition, the adsorption of Cr(VI) was promoted in the presence of HA in the pH ranging from 2 to 9, and when the initial concentration of HA increased from 10 mg L-1 to 20 mg L-1, a more pronounced promotion effect was observed. Excitation-emission matrix spectra and X-ray photoelectron spectroscopy were employed to understand the interactions among chromate species, HA, and AC. Results reveal the existence of several possibilities for the promotion mechanism when adsorption occurred in the presence of HA including binding interactions between Cr(VI) and HA, reduction of Cr(VI) induced by HA, Cr(III) complexation by surface-bound HA, and adsorption of Cr(III)-HA complexes. (C) 2015 Elsevier B.V. All rights reserved.