Use of Dithionite to Extend the Reactive Lifetime of Nanoscale Zero-Valent Iron Treatment Systems

Nanoscale zero-valent iron (NZVI) represents a promising approach for source zone control, but concerns over its reactive lifetime might limit application. Here, we demonstrate that dithionite (S2O42-), a reducing agent for in situ redox manipulation, can restore the reducing capacity of passivated NZVI. Slurries of NZVI were aged in the presence (3 days) and absence (60 days) of dissolved oxygen over a range of pH values (6-8). Upon loss of reactivity toward model pollutants {1,1,1,2-tetrachloroethane, hexavalent chromium [Cr(VI)], nitrobenzene}, aged suspensions were reacted with dithionite, and the composition and reactivity of the dithionite-treated materials were determined. NZVI aging products generally depended on pH and the presence of oxygen, whereas the amount of dithionite influenced the nature and reducing capacity of products generated from reaction with aged NZVI suspensions. Notably, air oxidation at pH >= 8 quickly exhausted NZVI reactivity despite preservation of significant Fe(O) in the particle core. Under these conditions, formation of a passive surface layer hindered the complete transformation of NZVI particles into iron(III) oxides, which occurred at lower pH. Reduction of this passive layer by low dithionite concentrations (<1 g/g of NZVI) restored suspension reactivity to levels equal to, and occasionally greater than, that of unaged NZVI. Multiple dithionite additions further improved pollutant removal, allowing at least a 15-fold increase in Cr(VI)) removal [similar to 300 mg of Cr(VI)/g of NZVI] relative to that of as-received NZVI mg of Cr(VI)/g of NZVI].