New insights into the influence mechanisms of particle size on the aging behavior and TCE removal of biogenic sulfidated mZVI in groundwater

In-situ biogenic sulfidation of microscale zero-valent iron (mZVI) is an efficient and green method to enhance its dechlorination in groundwater. However, for practical field applications, the technology must maintain long-term effectiveness. At present, the performance evolution of aged biogenic sulfidated mZVI (BS-mZVI), especially regarding the influence of particle size, remains unclear and needs further investigation. In this study, BS-mZVI with two particle sizes (7 μm and 30 μm) was aged for 15, 30, and 45 days and systematically compared. The results showed that smaller particles (7 μm) achieved rapid initial trichloroethylene (TCE) removal (95 % in 6 days), but their reactivity declined with prolonged aging ( k SA  = 1.12 × 10 −2  L·m −2 ·d −1 with 45-day aging). Conversely, larger particles (30 μm) exhibited improved reactivity over time, reaching a k SA of 1.57 × 10 −2  L·m −2 ·d −1 after 45-day aging while maintaining over 92 % TCE removal. Surface characterization revealed that aged BS-mZVI (7 μm) developed increased hydro(oxide) passivation layers (Fe 2 O 3 /FeOOH) from 15 to 45 days of aging, which decreased its reactivity. In contrast, aged BS-mZVI (30 μm) gradually formed conductive FeS x shells during aging, retaining hydrophobicity and enhancing reactivity. Metagenomic analysis further revealed that with prolonged aging (15 to 45 days), the 30 μm BS-mZVI enriched sulfate-reducing bacteria (SRB) and iron-reducing bacteria (IRB), accompanied by higher abundances of key functional genes involved in sulfate and iron reduction, thereby promoting sustained TCE degradation. These particle size-dependent results demonstrate that smaller BS-mZVI enables rapid TCE decontamination, whereas larger particles offer prolonged reactivity, helping to choose materials based on the needed remediation timeframe required in applications.