Fe/N co-doped magnetic porous hydrochar for chromium(VI) removal in water: Adsorption performance and mechanism investigation

Four kinds of Fe/N co-doped porous hydrochar were prepared by one/two-step N-doping schemes using microwave/traditional pyrolysis methods for removing Cr(VI) from aqueous phase. Heterocyclic-N was introduced through CO(NH2)2-based hydrothermal carbonization process, which could adjust the electronic structure of the hydrochar framework. Furthermore, Fe0 and Fe3O4 were embedded into hydrochar via carbothermal reduction reaction using FeCl3 as the precursor, which improved the reducibility and magnetism of the material. The modified hydrochar exhibited pH-dependency and rapid kinetic equilibrium, and the maximal adsorption amount of magnetic porous hydrochar obtained by microwave-assisted one-step N-doping (MP1HCMW) reached 274.34 mg/g. Meanwhile, the modified hydrochar had a high tolerance to multiple co-existing ions and the removal efficiency maintained above 73.91 % during five regeneration cycles. Additionally, MP1HCMW efficiently removed Cr(VI) via pore filling, electrostatic attraction, ion exchange, reduction, complexation, and precipitation. Summarily, Fe/N co-doped porous hydrochar was a feasible adsorbent with outstanding remediation potential for Cr(VI)-contaminated water.