The adsorption and reduction of anionic Cr(VI) in groundwater by novel iron carbide loaded on N-doped carbon nanotubes: Effects of Fe-confinement

•Pyrolysis temperature induced formation of unconfined/confined CNTs/Fe composites.•Unconfined CNTs/Fe composites showed better stability, reactivity and reusability.•Removal mechanisms of Cr(VI) were demonstrated by XPS, XANES and DFT calculation.•Quantitative calculation of Cr(VI) removal was conducted.•Cr(VI) treatment cost on unconfined material was 430 times less than commercial CNTs.