Mechanisms of Cr(VI) removal by FeCl3-modified lotus stem-based biochar (FeCl3@ LS-BC) using mass-balance and functional group expressions

A novel lotus stem-based biochar modified by ferric chloride (FeCl3@LS-BC) was synthesized as a low-cost adsorbent for the removal of Cr(VI) from aqueous solutions. The optimum preparation conditions, including carbonization temperature, carbonization time, activation temperature and activation time, were chosen using orthogonal design. Mass ratios of biochar to FeCl3 were optimized. FeCl3@LS-BC removed Cr(VI) very efficiently within the pH range from 2.03 to 10.04 by the surface sorption and reduction mechanisms. The percentage of Cr (VI) removed by reduction and surface sorption was 81.70% and 18.30%, respectively, at the initial Cr(VI) concentration of 10.02 mg/L. The following two-step mechanism of the Cr(VI) removal by the dominant reduction process step is : (1) the hydrolysis of Fe(III) supplied sufficient H+ to create a favorable acidic environment for Cr(VI) reduction in aqueous solution; (2) Cr(VI) was reduced to Cr(III) in the aqueous phase by contact with the electron donor groups of FeCl3@LS-BC (C=O, -OH and C=C). The present study demonstrated a promise of FeCl3@LS-BC as an adaptable, eco-friendly and effective sorbent for the Cr(VI) removal.