Properties and reactivity of sulfidized nanoscale zero-valent iron prepared with different borohydride amounts

Abstract

The liquid-phase reduction method with NaBH₄ as the reductant is the most widely used method for sulfidized nanoscale zero-valent iron (SNZVI) synthesis. However, it is unclear how the reductant amount (e.g., NaBH₄/Fe ratio) affects the physicochemical properties (i.e. Fe⁰ and S content, surface S speciation, and lattice constant of the Fe BCC structure) and reactivity of SNZVI. Herein, we synthesized SNZVI with different NaBH₄/Fe ratios to assess how the NaBH₄/Fe ratio affects their properties, and how these properties correlated to the reactivity and selectivity. The S/Fe molar ratio in the particles of SNZVI decreased from 0.15 to 0.02 when the NaBH₄/Fe ratio increased from 1.5 to 5, respectively; meanwhile, the yield of SNZVI increased from 20% to 97%. The Fe⁰ content and the surface S²⁻/S₂²⁻ molar ratio ([S²⁻/S₂²⁻]_surface) increased with the increase of the NaBH₄/Fe ratio, which enhanced the electron transfer of SNZVI. The trichloroethylene reactivity increased ∼8.5-fold when the [S²⁻/S₂²⁻]_surface of SNZVI increased from 0.85 to 1.62, while the selectivity decreased from 82.8% to 64.4% accordingly, indicating that [S²⁻/S₂²⁻]_surface could be an indicator for predicting the reactivity and selectivity of SNZVI. These results indicate that the NaBH₄/Fe ratio is a possible reason for the lab-to-lab variability of SNZVI reactivity. A higher NaBH₄/Fe ratio was favorable for the TCE dechlorination reaction while a lower NaBH₄/Fe ratio could enhance the electron selectivity.