Sulfidation methods determine longevity and performance of zerovalent iron: Mechanistic insights into trichloroethylene removal and aging resistance

Although sulfidation has been widely applied to enhance the removal performance of trichloroethylene (TCE) by nanoscale zerovalent iron (nZVI), the comparative effects of sulfidation methods, especially their long-term efficacy, remain unclear due to incomparable conditions in literature. Here, three sulfidated nZVI (S-nZVI) prepared using typical sulfidation methods (solid-solid (SS), aqueous-aqueous (AA), and aqueous-solid (AS)) were compared for TCE removal in terms of k_obs, electron efficiency (EE), and performance evolution during aging. S-nZVI(SS) exhibited the highest initial k_obs and EE, attributed to its abundant, crystalline FeS_x layer that facilitates electron transfer. It also maintained the best long-time performance despite a rapid decline in reactivity due to structural instability of the non-uniform FeS_x phase. In contrast, the AA method generated uniform FeS_x shells with improved long-term stability but moderate initial performance, whereas the AS method produced thin, less crystalline FeS_x layers with the lowest but most stable k_obs. Correlation analyses revealed that Fe⁰ content, electrochemical impedance, and surface Fe²⁺/S²⁻ were the key physicochemical properties that collectively control the Fe⁰-FeS_x-TCE electron transfer pathway. The crystallinity, coverage, and quantity of the FeS_x layer determine the its stability and Fe⁰ protection, leading to distinct aging resistances across sulfidation methods. These findings not only highlight the SS method as the current optimal sulfidation strategy for balancing short- and long-term performance but also demonstrate the key considerations for developing new sulfidation strategies to enhance aging resistance and address the trade-off between k_obs and EE.