Iron (Hydr)oxide-Enriched Biochar for Arsenic Immobilization Under Microbial Iron Reduction and Fluctuating Redox Conditions

Iron (hydr)oxide-enriched biochar (IEB) is a material with significant environmental applications, demonstrating considerable potential in soil remediation, pollutant immobilization, greenhouse gas emissions, and carbon sequestration. However, the stability of arsenic (As) immobilized by IEBs with different crystalline structures under redox fluctuations, as well as the role of dissimilatory iron-reducing bacteria (DIRB) in mediating the biotransformation of iron (hydr)oxides within IEBs and subsequent As retention, remain poorly understood. This study investigated the effects and mechanisms of Shewanella oneidensis MR-1 (MR-1) on As retention in ferrihydrite- and goethite-enriched biochar (FB and GB) during anaerobic and aerobic transitions and evaluated the passivation capabilities of the IEB in real flooding-drying As-contaminated soils. The results from batch experiments revealed that MR-1 did not significantly affect As retention in crystalline GB. However, during the 9 days anaerobic–aerobic cultivation period, MR-1 colonization of FBs increased ferrihydrite reduction and biotransformation, increasing electrostatic repulsion and leading to a 27.2–88.2% increase in dissolved As compared with the sterile control. Compared with the control, both FB and GB effectively reduced the amount of water-soluble As by 44.8% and 51.0%, respectively, after 100 days of flooding–drying incubation. The more stable passivation effect of GB also mitigated the adverse effects of As on pak-choi seed germination and growth. These findings provide valuable insights into how DIRB influences As retention in IEBs, highlighting the potential of crystalline IEBs as environmentally friendly and effective materials for passivating As in soils experiencing alternating flooding and drying conditions.