Fabrication and characterization of magnetic eucalyptus carbon for efficient Cr(VI) removal in aqueous solution and its mechanisms

For an exhaustive removal of Cr(VI), three different types of biochar were prepared, modified and evaluated for Cr(VI) removal from wastewater. Magnetic eucalyptus biochar (MBC) was synthesized via a facile pyrolysis process of eucalyptus biochar pretreated with FeCl3 firstly coupled with K2CO3 activation. As a comparison, raw biochar (BC) and FeCl3 modified biochar (FBC) were prepared. The physicochemical property of adsorbent synthesis and removal mechanism was examined using BET, SEM with EDS, zeta potential, XRD, FTIR, XPS and Vibrating sample magnetometer (VSM), and the effect of various reaction conditions and parameters were evaluated for Cr(VI) removal. Results demonstrated that compared to BC and FBC, MBC has a larger specific surface area (870.3264 m2/g), higher content of Fe mainly existed as γ-Fe2O3 and Fe3O4, and higher zeta potential (10.66). This is due to the significant alleviation of pore blockage caused by iron oxides and more efficient conversion of iron oxides with the participation of K2CO3 during modification process. Upon batch tests, the removal efficiency could achieve more than 91% for 200 mg/L Cr(VI) solution with optimum adsorbent dosage (0.01 g) at desired acid condition (pH = 2) within 9 h. The separation behavior of MBC for Cr(VI) was highly identical with elovich and Freundlich model. MBC showed better adsorption performance up to three cycles and under various co-existing ions. Besides, the removal ability of MBC gained an obvious increase of 2–5 folds and 3–10 folds at different initial Cr(VI) concentrations, compared with BC and FBC. Characterization analyses unraveled that physicochemical removal process was responsible for the efficient elimination of Cr(VI) in an aqueous solution, resulting from adsorption, electrostatic attraction, pore filling, complexation, ion exchange, redox reaction and precipitation due to the fabrication of iron oxides into biochar.