Deciphering the roles of iron sulfides in sulfidated nanoscale zero-valent iron for arsenite sequestration under anoxic water

Sulfidized nanoscale zero-valent iron (S-nZVI) has exhibited remarkable efficacy in the sequestration of arsenic (As) from aquatic ecosystems. However, the interfacial reaction mechanisms between post-sulfidation-prepared S-nZVI (S-nZVI^post) and arsenic, particularly the functional roles of iron sulfide phases (FeS, FeS₂, FeSₓ) within S-nZVI, remain inadequately elucidated. Herein, the specific phases and elemental distribution, especially iron sulfides in S-nZVI^post, were proved to exert a pivotal influence on mediating interfacial catalytic processes and As sequestration efficiency. Sulfur-defect sites in iron sulfides activated adsorbed water molecules into surface-bound hydroxyl radicals (•OH_ad), thereby facilitating partial oxidation of As(III) under anoxic condition. The metallic iron core provided the driving force for As(III)/As(V) penetration into the inner core region of S-nZVI^post, while iron sulfides promoted the formation of low-solubility As–S precipitates (e.g., As₂S₃/As₂S₅) and homogeneous distribution of As-S throughout the particle. Column breakthrough experiments proved the long-term stability and high performance. S-nZVI^post maintained almost complete removal of As for 14.5 hours in column experiment, whereas pristine nZVI only sustained this performance for 2.5 hours. Recycling tests further validated its long-term operational stability. These findings highlight S-nZVI^post as an advanced nanomaterials with exceptional longevity and As immobilization capacity for groundwater remediation and waste water treatment.