Enhanced Cr(VI) and nitrate reduction using rGO/nZVI coupled hydrogen autotrophs under weak magnetic field: Performance and mechanisms

The compound pollution of nitrate and hexavalent chromium (Cr(VI)) in groundwater poses a serious hazard to human health and ecology. The use of H₂-connected graphene oxide loaded nano zero-valent iron (rGO/nZVI) chemical reduction coupled with hydrogen autotrophic bioreduction system is expected to reduce the economic cost and alleviate the problem of gas blockage. In this study, we investigated the process and mechanism of the removal of NO₃^–/Cr(VI) compound pollution by rGO/nZVI coupled HAM under weak magnetic field (WMF). The results showed that rGO/nZVI coupled HAM system had the highest removal of NO₃^– (93.8 %), and the allocation ratio of chemical reduction to biological reduction of nitrate was about 4:1. Under the compound pollution conditions, the removal of Cr(VI) in the coupled system could reach 100 %. The abiotic reaction mechanism should be the main pathway for Cr(VI) removal, and the ratio of chemical reduction to biological reduction was 99:1 within 24 h. The results showed that the chemical reduction and biological reduction of Cr(VI) in the coupled system was the most effective way to remove Cr(VI). High N₂ conversion of rGO/nZVI coupled with HAM system at 30 mT was obtained (66.98 % and 43.17 %) at NO₃^– and NO₃^–/Cr(VI) composite contamination systems, respectively. The presence of WMF corroded rGO/nZVI towards lepidocrocite moving the denitrification process towards harmlessness (high N₂ selectivity). This finding provides a theoretical basis of the coupled system of rGO/nZVI and HAM for the groundwater compound pollution removal.