Divergent roles of natural organic matter in iron precipitation and arsenic sequestration in zerovalent iron-based flow-through systems: Resolving homogeneous vs. heterogeneous pathways.

In zerovalent iron (ZVI) flow-through systems used for groundwater arsenic (As) remediation, As is sequestered via heterogeneous (surface) and homogeneous (bulk) precipitation of Fe corrosion products. However, the dynamic partitioning and respective sequestration capacities of these two pathways remain critically unresolved. This gap is significantly complicated by the ubiquitous presence of natural organic matter (NOM), which spatially alters both Fe precipitation and As sequestration dynamics. To resolve this, we utilized a novel magnetic confinement-enabled column reactor (MCCR) to spatially segregate and independently evaluate the homogeneous and heterogeneous pathways. By integrating controlled flow-through experiments and mass transfer modeling, we systematically tracked the co-distribution of Fe, As, and NOM. Our results reveal that NOM exerts fundamentally divergent, spatially dictated roles governed by localized carbon-to-iron (C/Fe) ratios. In the heterogeneous pathway (ZVI surface), NOM indirectly influenced As removal by modulating Fe precipitation, with As partitioning following Fe partitioning. In contrast, in the homogeneous pathway (bulk solution), dissolved NOM suppressed Fe precipitation, generating mobile NOM-Fe complexes, and directly inhibited As sequestration via competitive binding for active sites on Fe (oxyhydr)oxides and the electrostatic repulsion effect. Furthermore, spatial mass transfer modeling demonstrated that these divergent mechanisms are driven by the counter-diffusion between outbound Fe and inbound As/NOM, resulting in the formation of spatially distinct precipitation zones with contrasting localized C/Fe and As/Fe ratios. Ultimately, this study elucidates the intricate, spatially segregated reaction dynamics in ZVI flow-through systems, providing a critical theoretical foundation for optimizing As removal strategies in complex, NOM-rich groundwater environments.