Use of biogas solid residue from anaerobic digestion as an effective amendment to remediate Cr(VI)-contaminated soils

Chromium (Cr) is one of the most common metal pollutants and has thus attracted considerable attention. In this study, we investigated the potential use of biogas solid residue (BSR) from anaerobic digestion as an effective amendment to decrease the bioavailability of Cr in Cr(VI)-polluted soil using pot experiments. Compared to the no-addition treatment, the addition of BSR (treatments-50, 100, and 150 g kg^-1 soil) increased the soil nutrient levels, microbial diversity and activities, and decreased the redox potential (Eh). BSR treatment of Cr(VI)-contaminated soil caused a reduction in soil Cr(VI) concentration (16.6-52.1%) and the exchangeable Cr proportion (15.2-52.4%), thereby decreasing the available Cr for uptake by plants. BSR treatments resulted in a reduction in the Cr contents of the roots and aboveground biomass of pakchoi plants. The Cr(VI) content in treated soils decreased with increasing BSR addition, with 150 g kg^-1 being the most efficient application. The relative abundance of Cr-reducing groups, such as Pseudomonas, Microbacterium, and Bacillus, increased with the increase in BSR application. The enhancement of soil Cr(VI) immobilization by the addition of the BSR was mostly attributed to the simultaneous effect of organic matter addition, stimulation of microorganisms, and reduced Eh value. Organic matter contributed more to the variation in Cr. The presence of BSR decreased the bioavailability of Cr in the soil and, therefore, lowered the potential mobilization of Cr(VI) from the soils. Our results demonstrated that BSR application may offer a potentially promising solution for enhancing agricultural production in Cr-contaminated soils.