Interaction of pyrite with zerovalent iron with superior reductive ability via Fe(ii) regeneration

Abstract

Sulfidation of zero-valent iron (SZVI) emerged as a promising approach to enhance both the reactivity and selectivity of nano zero-valent iron (ZVI). However, the dense base layer of the Fe⁰ core–FeS_x shell structure inhibited the Fe(II) regeneration and the applicable pH range was narrow (only acidic conditions), thus limiting the application of SZVI. In this work, a structure with a substrate of pyrite and well dispersed ZVI (FeS₂/ZVI) was developed. The interaction of pyrite with Fe⁰ significantly promoted the generation of Fe²⁺. More than 99.9% of Cr(VI) (50 mg L⁻¹) was removed within 2 h, which was 3.7–8.9 times higher than that of the precursor alone. During Cr(VI) removal, Fe⁰ and FeS₂ reacted with Fe³⁺ to regenerate Fe²⁺ and H⁺, promoting the Cr(VI) reduction and extending the applicable pH range to 3.04–9.20. Moreover, the effects of S species (S₂²⁻, S²⁻, and S⁰) in sulfide precursors on the crystal structure, electron transfer capacity, and Fe(II) cycling ability of SZVI were further revealed. The introduced S₂²⁻ promoted the formation of a galvanic couple between FeS₂ (+0.35 V) and Fe⁰ (−0.44 V), thus accelerating the electron transfer from ZVI and releasing more Fe(II) for Cr(VI) reduction. This study developed an efficient approach for ZVI activation and contributed to the insight into the enhanced mechanism of incorporated sulfur.