Removal Cr(VI) from tunneling wastewater by Fe-containing sludge biochar: The role of endogenous iron

Fe-containing sludge biochar produced by pyrolysis of Fe-containing residual municipal sludge was utilized for removing Cr(VI) from aqueous solutions. It was observed that increasing the solution pH decreased the adsorption performance of FSBC for Cr(VI), while raising the adsorption temperature enhanced Cr(VI) removal on FSBC. The coexisting ions of Na+, K+, SO42- and NO3- had minimal impact on removing Cr(VI) by FSBC, whereas the divalent cation of Ca2+ and Mg2+ exhibited a slight inhibitory influence on removing Cr(VI). Both the pseudo-first-order and Langmuir model were found to better describe the Cr(VI) removal process on FSBC from aqueous solutions. The mechanisms involved in removing Cr(VI) by FSBC may include complexation, electrostatic interaction and reduction processes. The Fe species present on FSBC were identified as both Fe2O3 and Fe3O4, with Fe3O4 disappearing after removing Cr(VI). The Fe species on FSBC provided Fe-O groups as site for removing Cr(VI), while Fe3O4 facilitated the Cr(VI) reduction by FSBC. This findings offered further viewpoints into removing Cr(VI) from aqueous solutions using Fe-containing sludge biochar.