Enhanced Adsorption Removal of Aqueous Hexavalent Chromium by an Acid-Treated Biochar Coupled with Calcium Chloride Activation

An acid-treatment biochar (AcBC) was prepared by coupling acid-boiling with CaCl 2 activation. The preparation of AcBC was optimized by means of response surface methodology (RSM) via central composite design (CCD) with two factor and five levels. The RSM models fitted Cr(VI) and Cr(tot) with r 2 =0.9883 and r 2 =0.6990, respectively. The RSM analysis showed that optimal preparation parameters are pyrolytic temperature (PyT) 400℃ and CaCl 2 /biomas mass ratio (B/Ca) 1.6. The CaCl 2 activation could increase the specific surface area of biochar because of the dehydration and catalyst effects. The acid (3M HCl solution) boiling modified the biochar with carboxyl-rich surface. The carboxyl-rich AcBC showed predominant performance on Cr(VI) adsorption, Cr(VI) reduction, and Cr(III) adsorption compared with pristine biochar (P-BC). Second-order kinetic and Freundlich isotherm could explained the adsorption of Cr(VI) and Cr(tot) well. The Langmuir isotherm evaluated the AcBC had a maximum adsorption capacity ( q e ) of 176.8 mg·g -1 on Cr(tot) adsorption. Benefitted from the rich carboxyl, Cr(VI) was reduced during adsorption with a high q e of 419.9 mg·g -1 determined by Sips isotherm. A multi-steps adsorption-reduction-adsorption kinetic model was developed to evaluate the removal process of Cr(VI). Results showed that ~62% of Cr(VI) was reduced by AcBC. Reaction rate of Cr(VI) reduction (8.39±1.37E-03 L n -1 ·mg n -1 ·min -1 , where n =1.27±0.47) in aqueous phase was faster than the adsorption ( k ad =1.07±2.44E-03 L·mg -1 ·min -1 ). The reduction of Cr(VI) on AcBC surface was slower with rate constant of =1.76±3.46E-04 L n -1 ·mg n -1 ·min -1 ( n =1.27±0.47).