Enhancement of chromium contaminated soil remediation by ferrous sulfate with the addition of biogas residue

In this study, a cost-effective and eco-friendly method was developed to remediate soil contaminated by hexavalent chromium (Cr[VI]) using ferrous sulfate and biogas residue (BR). The results show that the addition of BR at mole ratio of Fe(II)/Cr(VI) = 3:1 and mass ratio of BR (dry mass)/Cr(VI) = 70:1 significantly improved the reduction of Cr(VI) and reduced its mobility. Moreover, the reduction efficiency of Cr(VI) was higher than 85% at day 1 and reached a maximum of 96.7% at day 7. The X-ray photoelectron spectroscopy, scanning electron microscopy, and microbial diversity analyses indicated that the microbial mechanism created by the BR addition was crucial for the long-term and stable reduction of Cr(VI). The enriched Cr(VI)-reducing bacteria genera in the BR directly reduced Cr(VI), whereas the reducing bacteria in the BR indirectly reduced Cr(VI) to Cr(III) through the electron transport system in the reaction system. The addition of BR can lead to a high reduction efficiency and long-term stabilization of Cr(VI)-contaminated soils. This study provides reference values for the combined remediation technology for Cr(VI)-contaminated soils.