[Mechanism of Cr( VI) removal from aqueous solution using biochar promoted by humic acid]

Biochar derived from excess sludge was used for hexavalent chromium [Cr( VI)] removal from water as a sorbent, and impacts of co-existing humic acid on performance of Cr( VI) sorption onto biochar were studied. The experimental results indicated that humic acid facilitated biochar adsorption of Cr(VI), which enhanced capacity of Cr sorption and shortened reaction time reaching equilibrium remarkably. The sorption kinetic process could be described with the pseudo second order model. On the condition of initial pH 4.0, biochar concentration 20 g x L(-1), initial concentration of Cr(VI) in the range of 50-800 mg x L(-1), the Langmuir model fitted adsorption isotherm better than the Freudlich model. The Langmuir Q0 values of biochar with and without existing humic acid were 10.10 mg x g(-1) and 5.56 mg x g(-1), respectively. In the pH range of 2.0-8.0, sorption capacity of all sorbents decreased with increasing initial pH value. Ascending concentration of humic acid in solution promoted sorption capacity of biochar. Fourier transform infrared spectroscopy (FTIR) analysis showed that hydroxyl, carboxyl, ester, aromatic C-H stretch and ring C=C on the biochar were responsible for Cr(VI) sorption. Combined with X-ray photoelectron spectroscopy (XPS) analysis, mechanism of promoting Cr( VI) sorption onto biochar was speculated that humic acid enhanced concentration of Cr( VI) ions aggregating on the surface of biochar and benefited Cr(VI) adsorption coupled with subsequent reduction by biochar functional groups. Meanwhile, humic acid also increased amount of Cr(VI) and Cr(II) removal from aqueous solution.