Environ. Sci. Technol. 2010, 44, 8649–8655
Use of Dithionite to Extend the Reactive Lifetime of Nanoscale Zero-Valent Iron Treatment Systems YANG XIE AND DAVID M. CWIERTNY* Department of Chemical and Environmental Engineering, University of California-Riverside, A242 Bourns Hall, Riverside, California 92521, United States
Received July 19, 2010. Revised manuscript received October 2, 2010. Accepted October 6, 2010.
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 g 8 quickly exhausted NZVI reactivity despite preservation of significant Fe(0) 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) produced aqueous Fe(II) concentrations corresponding to as much as 50% of the available NZVI initially in suspension. We therefore attribute the scatter in Figure 1b, particularly at high dithionite loadings, to system-to-system variations in the iron concentration remaining in suspension after reaction with dithionite. As a result, Cr(VI) removal capacities were normalized to the initial mass of NZVI in suspension. Characterization and Reactivity of NZVI Aged in Air at pH 8 after Reaction with Dithionite. Suspensions of NZVI aged in air at pH 8 also reacted nearly instantaneously with dithionite (Figure S7b, Supporting Information). Again, the dithionite loading was critical in determining the nature of the reaction products. Diffraction patterns indicated that R-Fe remained the dominant identifiable phase after reaction with low concentrations of dithionite (
