Enhanced Cr(VI) reduction using highly conductive material synthesized by modified chitosan coated with natural iron-manganese minerals

•Chitosan based biochar coated magnetite/rhodochrosite composites were prepared and evaluated.•Cr(VI) removal by MaRh@Ch composite were highly pH-dependent.•MaRh@Ch-500 showed the best Cr(VI) removal performance (104.5 mg/g) at pH 2.•99.33% of Cr(VI) species was reduced to CrxFe1-x(OH)3/Cr2O3 and immobilized on the MaRh@Ch-500.•The remarkable electrical conductivity of MaRh@Ch-500 enhanced the Cr(VI) removal efficiency.