How does soil heterogeneity shape the reductive dechlorination behavior of SZVI? - A study on the component-reactivity-pathway relationship based on soils from 20 provinces in China.

Sulfidized zero-valent iron (SZVI) represents a promising approach for chlorinated hydrocarbon remediation in soil and groundwater. Current understanding of SZVI dechlorination comes mainly from homogeneous liquid-phase studies, with limited evidence for its performance in heterogeneous soils. Therefore, this study utilized soil samples collected from 20 representative provinces across China to systematically compare the dechlorination performance of SZVI on trichloroethylene (TCE) across five degradation systems: SZVIwater (aqueous phase system), SZVIsoil (soil system), SZVIpre-soil (soil system pre-adsorbed with TCE), SZVIde-soil (soluble-component-removed soil system) and SZVIex (soil extract system). Redundancy analysis (RDA) was implemented to elucidate the mechanism of soil physicochemical properties in SZVI dechlorination. The influence of the addition of cetyltrimethylammonium bromide (CTAB) on these patterns was subsequently evaluated. The pseudo-first-order rate constants of SZVI in TCE degradation followed the order: SZVIpre-soil (0.10-1.33 h-1) > SZVIsoil (0.11-0.98 h-1) ≈ SZVIde-soil (0.10-0.76 h-1) > > SZVIwater (0.133 h-1) > SZVIex (0.009-0.083 h-1). The material reactivity was predominantly enhanced by soil particles, whose promoting effect far outweighed the inhibitory effects exerted by other solid-phase components and nearly all soluble components. Moreover, direct electron transfer between SZVI and TCE was facilitated by soil particles, promoting β-elimination as the primary pathway. Coupled with the inhibitory effect of soluble components on hydrogenation, acetylene emerged as the primary product (>75 % in SZVIsoil compared to 30.9 % in SZVIwater). This study elucidates the physicochemical mechanisms how soil heterogeneity influences SZVI dechlorination in soil-aqueous matrix, providing critical insights for optimizing SZVI-based remediation strategies at chlorinated hydrocarbon-contaminated sites.