Efficient removal of hexavalent chromium through adsorption-reduction-adsorption pathway by iron-clay biochar composite prepared from Populus nigra

An iron-clay biochar composite (PFB) prepared from invasive Populus nigra was employed for Cr(VI) removal. The results suggested that surface coatings of mineral phases could improve the pore structure, redox moieties, and graphene phase of biochar. The adsorption and reduction rates of Cr(VI) at the initial concentration 100 mg·L-1 in aqueous solution by PFB were 100% and 87.18%, respectively, which was 3.1 and 2.1 times higher than the pristine biochar (PB). Based on the model simulation, the Cr(VI) removal by PFB was a mixed adsorption-reduction-adsorption approach including the electrostatic attraction via film and intraparticle diffusion and chemical interactions, in which the adsorption of Cr(III)c was the main control step. A series of characterizations revealed that -C-O functional groups, Fe(II)oct in Fe3O4, Fe(0), and persistent free radicals (PFRs) were the prominent electron donors involved in the Cr(VI) removal. In addition, the PFB with a graphite-like structure could also serve as an electron shuttle to facilitate Cr(VI) adsorption-reduction.