A stepwise processing strategy for treating manganese residue and the remediation of hexavalent chromium in water and soil by manganese residue-derived (Fe,Mn)C2O4

• Manganese residue was effectively treated and utilized for environmental remediation. • Cement material and (Fe,Mn)C 2 O 4 composite were obtained in different processing steps. • (Fe,Mn)C 2 O 4 composite showed excellent remediation of Cr(VI) in both water and soil. • Synergistic effect of Mn and Fe resulted in higher removal rate of Cr(VI). • The Cr(VI) was removed through adsorption, reduction, and co-precipitation. This work provides a new approach for effective treatment and utilization of manganese residue (MR), including leaching, desulfurization, and recycling of iron and manganese. The secondary residue and the acidic filtrate (pH = 0.94) composed of H 2 SO 4 and H 2 C 2 O 4 could be used as a cement material and the medium and reductant for manganese sulfate production system, respectively. Approximately 94.5% of Fe and 97.5% of Mn were recovered to produce (Fe,Mn)C 2 O 4 composite, which had a capacity of 122.7 mg g −1 for removing Cr(VI) and a good performance in a wide pH range of 2–9. The removal of Cr(VI) by (Fe,Mn)C 2 O 4 was attributed to adsorption, reduction, and co-precipitation. What's more, the control experiment of FeC 2 O 4 and MnC 2 O 4 affirmed the synergism between Mn and Fe in (Fe,Mn)C 2 O 4 composite, attributed to that the accelerated reduction of Mn(III) by Fe(Ⅱ) and the co-reduction effect of Fe with Mn on Cr(VI) synergistically promoted the reduction of Cr(VI). Furthermore, the ability of (Fe,Mn)C 2 O 4 for immobilizing Cr(VI) in the soil was investigated, and the result proved that (Fe,Mn)C 2 O 4 significantly reduced the leaching and releasing risk of Cr(VI). This strategy for treating and utilization of MR has great economic and ecological benefits.