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Mechanistic study on adsorption and reduction of hexavalent chromium by iron-clay biochar composite prepared from Populus nigra

Populus nigra; Iron-clay biochar; Cr(VI); Adsorption-reduction; Electron shuttle.

Populus nigra was introduced to Dongting Lake area in central China for economic profitability, which caused great harm to the wetland ecosystem. In this study, an iron-clay biochar composite (PFB) prepared from invasive Populus nigra was employed for highly effective removal of Cr(VI). The results suggested that surface coatings of mineral phases on the biomass could promote the electron-mediating capacity of biochar for its application in the Cr(VI) adsorption-reduction. The adsorption and reduction rate of Cr(VI) by PFB was 100% and 87.18%, respectively, which was 3.1 and 2.1 times higher than the pristine biochar (PB). The pseudo-second-order kinetic, Langmuir, and intraparticle diffusion model fitted the adsorption datas well. The maximum adsorption capacity of PFB was 91.13 mg/g. The simulation results of the adsorption-reduction-adsorption process by apparent kinetic model showed that the adsorption of Cr(VI)^* and Cr(III)_c was the control step for the removal of Cr(VI). A series of characterizations proved 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 reduction of Cr(VI). Besides, the PFB with a graphite-like structure could also act as an electron shuttle to facilitate the reduction of Cr(VI) to Cr(III). This sutdy provides a strategy for forestry waste utilization and new insights into the mechanisms of Cr(VI) adsorption-reduction from aqueous solution by PFB.