Polarity-driven Fe phase transformation in biochar-based electrocoagulation: Toward selective metal recovery from...

The efficient, economical, and sustainable treatment of acid mine drainage (AMD) remains a major challenge. This study developed a low-cost biochar electrode for electrocoagulation (EC) that not only reduces material cost but also enables contaminant-specific control and resource recovery. Utilizing polyvinyl acetate (PVAC) as a binder, biochar as a catalyst, and Al mesh as a current collector, the optimized electrode (36 mg/cm² biochar loading, 6 mA/cm² current density) achieved high removal efficiency from field-collected AMD: 99% for Fe and Mn, 96.8% for Zn, and 91.4% for SO₄^2--at substantially lower costs than conventional carbon electrodes. Beyond its performance, this system revealed element-specific behaviors and controllable Fe mineralogy. Mn predominantly accumulated on the electrode surface but largely redissolved (> 80%) upon polarity reversal, whereas Zn showed only partial release and Fe remained predominantly insoluble. X-ray absorption fine structure (XAFS) and complementary analyses revealed a sequential Fe phase transformation-from ferrihydrite through schwertmannite and lepidocrocite, ultimately to stable goethite, with polarity reversal capable of reversing this sequence and modulating Fe-SO₄ bonding, thereby enabling precise control over Fe speciation and laying the groundwork for understanding associated contaminant interactions. These insights establish a tunable platform for synchronizing heavy-metal removal, sludge stability, and selective metal recovery, transforming EC from a simple purification method into a predictive, resource-circular technology.