Unraveling iron speciation on Fe-biochar with distinct arsenic removal mechanisms and depth distributions of as and Fe

Tailored manipulation of iron speciation has become a critical challenge for the further development of Fe-biochar as an economical and eco-friendly amendment for arsenic (As) immobilization. Herein, a series of Fe-biochars with manipulated iron speciations were fabricated by controlling the carbon structures and pyrolysis conditions. Results revealed that abundant labile-/amorphous-C induced more reductive-Fe(0) formation (10.9 mg g⁻¹) in the Fe-biochar. The high Fe(0) content resulted in the effective As immobilization (4.34 mg g⁻¹ As(V) and 7.72 mg g⁻¹ As(III)) as evidenced by Pearson correlation coefficient (PCC) analysis. The hierarchical depth distributions of As and Fe on the Fe-biochar caused by the redox reaction and concomitant sorption of As proved the decisive role of Fe(0). An iron-oxide shell (~10–20 nm) with a high arsenic accumulation was revealed on the surface, while deeper within the particles, Fe(0) was found to be associated with elemental As (As(0), up to 19.4%). By contrast, pyrolysis with the stable-/graphitic-C generated more amorphous-Fe (61.9 mg g⁻¹) on the Fe-biochar, which accounted for the high As removal (10.1 mg g⁻¹ As(V) and 7.70 mg g⁻¹ As(III)) despite the limited Fe(0) content. In comparison to the reductive Fe(0), distinct depth distribution was observed that the As/Fe ratio was marginally changed within 200 nm depth of the amorphous-Fe biochar after As decontamination. Co-precipitation of As with Fe released from amorphous-Fe contributed to this depth distribution, as evidenced by the high correlation between released-Fe and As immobilization capacity (PCC as 0.84–0.95). This study unveiled a crucial role of iron speciation on distinct mechanisms for As removal, guiding the application-oriented design of multifunctional Fe-biochar for broad environmental remediation.