Solution and surface chemistry of the Se(IV)-Fe(0) reactions: Effect of initial solution pH

- Selenite (Se(IV)) removal rate with nZVI decreased with increasing pH.
- The reductive products of Se(IV) by nZVI were dependent on initial solution pH.
- Element selenium (Se(0)) was the dominant product at higher solution pH.
- nZVI was oxidized to mostly Fe3O4 and amorphous FeOOH in acidic solutions.
- The corrosion products were mainly Fe(OH)2 with some Fe3O4 in alkaline conditions.

Aspects of solution and solid-phase reactions between selenite (Se(IV)) and nanoscale zero-valent iron (nZVI) were investigated. Experimental results on the effects of initial solution pH, formation and evolution of nZVI corrosion products, and speciation of selenium in nZVI were presented. In general, the rate of Se(IV) removal decreases with increasing initial pH. The observed rate constants of Se(IV) removal decreased from 0.3530 to 0.0364 min−1 as pH increased from 4.0 to 10.0. Composition and morphology of nZVI corrosion products and selenium species were characterized using scanning electron microscopy (SEM), X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS). Results confirmed that Se(IV) was reduced to Se(0) and Se(-II) by nZVI. Lower solution pH favored further reduction of Se(0) to Se(-II). Amorphous FeOOH, magnetite/maghemite (Fe3O4/γ-Fe2O3) and ferrous hydroxide (Fe(OH)2) were identified as the main corrosion products. Under alkaline conditions, the corrosion products were mainly of Fe(OH)2 along with small amounts of Fe3O4, while nZVI in acidic solutions was oxidized to mostly Fe3O4 and amorphous FeOOH. Furthermore, these corrosion products acted as intermediates for electron transfer and reactive/sorptive sites for Se(IV) adsorption and reduction, thus played a crucial role in the removal of aqueous Se(IV).