Biochar and microorganism assisted phytoremediation of severely molybdenum-contaminated soil: Efficacy, mechanisms and the impact of low temperatures

Soil pollution owing to excess molybdenum (Mo) poses a significant threat to the ecosystem and environmental health. However, current knowledge about its effective remediation remains limited. This study comprehensively assessed the phytoremediation of Mo assisted by biochar and microorganisms. The results showed that ryegrass could be used for the phytoremediation of Mo, achieving bioaccumulation factors (BCF) and translocation factors (TF) of 0.69 ± 0.04–0.94 ± 0.01 and 0.83 ± 0.11–1.57 ± 0.05, respectively. Mo was converted into a soluble state (6.56%–58.72%) and a cell wall-bound state (11.48%–48.56%), which were its main fates once it entered ryegrass. The increase in the activities of superoxide dismutase, peroxidase, and catalase alleviated the toxicity of Mo in ryegrass. Biochar and microorganisms, as passivators and conditioners, immobilized the remaining Mo, while facilitating its extraction by plants. Compared with the control, the removal rates of Mo from the soil increased from 7.63% to 24.78% and 25.59% under the treatments of ryegrass/R.ornithinolytica A1/biochar (T5) and ryegrass/S.marcescens A2/biochar (T6), respectively. Simultaneously, residual Mo (RES-Mo) proportions increased by 30.12% and 31.24%, respectively. Furthermore, the proportion of RES-Mo increased by 6.22% after the T7 treatment at a lower temperature (10 °C) compared to T8. The redundancy analysis results revealed that all the treatments resulted in increased Mo immobilization and confirmed the more significant positive correlation of RES-Mo with R. ornithinolytica A1 application than S. marcescens A2. The study proposes an effective strategy for the remediation of severely Mo-contaminated soils, particularly in regions with low temperatures.