Alleviating the reactivity-sustainability dilemma in groundwater dechlorination: Particle size regulation in stabilizer-assisted biogenically sulfidated mZVI systems

Particle size regulation balances reactivity and sustainability in XG-BS-mZVI. • Larger mZVI particles sustain reactivity by reducing passivation and cytotoxicity. • Larger particles enhance chemical dechlorination and biological denitrification. • A size-mediated abiotic–biotic feedback loop enables sustained TCE dechlorination. • Multi-criteria assessment guides sustainable and high-performance system design. In situ biogenically sulfidated microscale zero-valent iron (BS-mZVI) has shown promise for selective dechlorination in groundwater systems containing competing electron acceptors such as nitrate (NO 3 − ). However, practical application is critically limited by the unresolved trade-off between initial reactivity and sustainability. This trade-off is further complicated by the engineering requirement for stabilizer-assisted delivery (e.g., xanthan gum, XG) to enhance subsurface mobility. Here, we demonstrate that iron particle size is not merely a reactivity-related parameter but also a system-level governing factor controlling the long-term co-evolution of chemical reactivity, microbial community structure, and byproduct selectivity in XG-stabilized BS-mZVI systems. Using continuous-flow column experiments exceeding 106 pore volumes (PVs) to trichloroethylene (TCE) and NO 3 − , we show that larger particles (23 μm, XG-BS-mZVI 23 ) achieve sustained dechlorination performance, with a high cumulative TCE removal (62.86 mg C), an 87.82% complete dechlorination rate, and >97% N 2 selectivity. Mechanistic analyses reveal that appropriate particle size mitigates surface passivation and alleviates cytotoxicity (validated via a cross-study data synthesis) under XG-stabilized conditions, thereby establishing a biologically favorable niche that sustains functional microbial communities. This enables a particle-size-mediated abiotic-biotic feedback loop, in which sulfate-reducing bacteria maintain reactive FeS x surface phases to support electron transfer-driven dechlorination, while denitrifying bacteria concurrently achieve efficient biological nitrate reduction. A multi-criteria performance assessment integrating technical performance, environmental outcomes, and material costs further confirms that design toward an intermediate-to-large particle size (represented here by 23 μm) alleviates the intrinsic reactivity-sustainability dilemma in stabilizer-assisted mZVI systems. Collectively, this study highlights particle size as a central design principle for engineering long-term sustainable groundwater remediation. Download: Download high-res image (160KB) Download: Download full-size image