Magnetic triiron tetraoxide/biochar–loaded nanoscale zero-valent iron for chromium(VI) removal from aqueous solution

To inhibit the agglomeration of nanoscale zero-valent iron (nZVI or Fe0), increase its stability and facilitate electron transfer, magnetic triiron tetraoxide/biochar (MBC) was used as the carrier to load nZVI. Consequently, a new pathway was created for electron transfer from nZVI to triiron tetraoxide (Fe3O4) and then to contaminants, thereby improving the chemical reactivity of nZVI. Magnetic triiron tetraoxide/biochar–loaded nanoscale zero-valent iron (nZVI@MBC) composites were prepared for chromium(VI) adsorption. Different influencing aspects were studied, including the loading percentage of nZVI, solution pH, co-existing anions(PO43−, SO42−, Cl−, NO3−), temperature and reusability. Furthermore, the adsorption isotherms, kinetics and thermodynamics for Cr(VI) removal were performed. nZVI@MBC exhibited an adsorption amount of 117.25 mg g−1 at the optimal conditions (dosage 0.4 g L−1, initial concentration 50 mg L−1, temperature 30 °C, nZVI loading percentage onto MBC 5.0 wt% and pH 2). The adsorption data obeyed the pseudo-second-order kinetics model and followed the Langmuir isotherm, thermodynamic characteristics revealed that Cr(VI) adsorption onto nZVI@MBC was a spontaneous endothermic process. After four recycles, the Cr(VI) removal efficiency remained higher than 70 %, exhibiting a good reusable ability. The mechanisms were confirmed to be physical adsorption, chemical reduction and co-precipitation processes.