Chromium removal using a magnetic corncob biochar/polypyrrole composite by adsorption combined with reduction: Reaction pathway and contribution degree

Cr(VI)-containing wastewater is drawing more and more attention due to its high toxicity. In this study, a novel magnetic adsorbent for Cr(VI) removal was synthesized by decorating corncob biochar with polypyrrole (PPy) (MBC/PPy). Scanning electron microscopy (SEM), fourier transform infrared spectra (FTIR), X-ray photoelectron spectroscopy (XPS) and vibrating sample magnetometer (VSM) were used to determine the removal mechanism of Cr(VI). The MBC/PPy composite had an outstanding Cr(VI) removal ability within a wide pH range (2.0-10.0). The adsorption process followed the pseudo-second-order kinetic and Langmuir isotherm models, and the maximum adsorption capacity was speculated as 19.23 mg/g at 20 degrees C. The mass and charge balance calculation of N showed that the PPy was four-layer deep, and only the outermost layer was available. The detailed mechanism was demonstrated via three steps: (1) Cr(VI) anions were adsorbed on the surface of the MBC/PPy composite, followed by a partial anion exchange with Cl-; (2) Adsorbed Cr(VI) anions were reduced to Cr(III) by amine (-NH-) and hydroxyl (C-OH) functional groups; (3) Most of the generated Cr(III) cations were immobilized by precipitation and chelation, and a small part of them was released into the liquid phase due to the electrostatic repulsion process.