Magnetic Biochar–Nanoscale Zero-Valent Iron Composites for Arsenic Immobilization and Magnetically Assisted Separation From Aqueous Matrices in Mining-Impacted Environments

A magnetic biochar–nanoscale zero-valent iron (MBC-nZVI) composite was synthesized from pine bark via an in-situ liquid-phase reduction method to immobilize arsenic in mining-impacted soils. The optimized MBC-40 exhibited uniformly dispersed Fe(0)@Fe3O4 nanoparticles (50–80 nm) anchored on a graphitic biochar scaffold, combining a high surface area (180.9 m2 g-1) with strong magnetization (32.5 emu g-1). Batch experiments revealed outstanding performance, achieving 98.5% As(V) and 96.5% As(III) removal under neutral conditions, with equilibrium reached within 120 min. Adsorption kinetics were well described by the pseudo-second-order model (R2 >0.998) and equilibrium data were consistent with the Langmuir model, yielding maximum capacities of 65.4 mg g-1 for As(V) and 82.1 mg g-1 for As(III). Based on the observed pH dependence, zeta-potential/point of zero charge (PZC) context, and competitive anion effects, the arsenic immobilization behaviour is consistent with contributions from electrostatic interactions (particularly for anionic As(V) under pH < PZC) and specific interactions with iron (hydr)oxide surfaces. Potential redox involvement of Fe(0) is discussed as mechanistically plausible for nZVI-based systems, but arsenic/iron speciation was not directly measured in this study. The composite maintained >81% efficiency after five regeneration cycles and displayed excellent selectivity in the presence of competing anions, with minimal interference from sulfate and chloride but moderate competition from phosphate. When tested in simulated mining wastewater containing 5.0 mg L-1 total arsenic and high levels of Pb2+, Cd2+, and SO42-, the MBC-40 achieved 97.0% arsenic removal, >99.9% Pb2+ elimination, and 95.6% Cd2+ reduction. The rapid magnetic separation from treated water within 30 s enables simple recovery of the sorbent using an external permanent (neodymium) magnet, without requiring electrically powered separation equipment. These findings demonstrate that the MBC-nZVI composite mitigates key practical limitations of bare nZVI (aggregation/passivation and post-treatment recovery) by integrating a biochar scaffold with a magnetically recoverable Fe(0)@Fe3O4 architecture. Consistent with prior reports of nZVI/biochar (BC)-type systems, the present work contributes an application-focused validation primarily: performance is quantified across Fe-loading levels and confirmed in a mining-relevant mixed-ion/multi-metal matrix (high sulfate and co-occurring Pb2+/Cd2+), together with reusability supported by permanent-magnet separation.