Polychlorinated biphenyl (PCB)-contaminated sediments remain a significantthreatto humans and aquatic ecosystems. Dredging and disposal is costly, so viable in situ technologies to dechlorinate PCBs are needed. This study demonstrates that nanoscale zerovalent iron (ZVI) dechlorinates PCBs to lower-chlorinated products under ambient conditions, provides insight into structure-activity relationships between PCB isomers, and compares the reactivity of nanoscale ZVI to that of palladized microscale ZVI. Six PCB congeners were studied (22', 34', 234, 22'35', 22'45', and 33'44') to compare the initial rate of dechlorination of each and to monitor the order in which chlorines are removed. Using 200 g/L of nanoscale ZVI in a 30% MeOH/water mixture, observed surface-area-normalized pseudo-first-order PCB dechlorination rate constants ranged from 1 x 10(-6) to 5.5 x 10(-4) L yr(-1) m(-2) depending on the PCB congener tested. Using 200 g/L of palladized (0.05 wt %) microscale ZVI, surface-area-normalized pseudo-first-order PCB dechlorination rate constants were significantly faster and ranged from 3.8 x 10(-2) to 1.7 x 10(-1) L yr(-1) m(-2), but these rates were not sustainable. For nanoscale ZVI, nonorthosubstituted congeners had faster initial dechlorination rates than orthosubstituted congeners in the same homologue group. Chlorines in the para and meta position were predominantly removed over chlorines in the ortho position, which suggests that more-toxic coplanar PCB congeners are not likely to form from less-toxic noncoplanar, orthosubstituted congeners. Complete dechlorination was not observed over the course of the experiments. PCB dechlorination is rapid enough that nanoscale ZVI may offer novel in situ remedial alternatives for PCB-contaminated sediments.

To evaluate the importance of external mass transport on the overall rates of contaminant reduction by iron metal (Fe0), we have compared measured rates of surface reaction for nitrobenzene (ArNO2) to estimated rates of external mass transport in a permeable reactive barrier (PRB). The rate of surface reaction was measured at a polished Fe0 rotating disk electrode (RDE) in an electrochemical cell, and the rate of mass transport was estimated from a correlation for mass transport in packed-bed reactors. The kinetics of ArNO2 reduction were studied in pH 8.4 borate buffer at a potential below which an oxide film would form. The cathodic current measured in this system was dependent on the electrode rotation rate, and the measured first-order heterogeneous rate coefficient for surface reaction [krxn = (1.7 +/- 0.2) x 10(-3) cm s-1] was about 10 times faster than the first-order mass transport rate coefficient (kmt approximately 2 x 10(-4) cm s-1) estimated for PRBs. The similarity between rates of surface reaction and mass transport suggest that it may be important to consider mass transport processes in the design of PRBs for contaminants such as nitroaromatics that are highly reactive with Fe0.

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PLATES 1-1819. Miscellaneous Investigations Series Map 1-1819, Geologic map of the Challis I°x2°q uadrangle, Idaho, compiled by F.S. Fisher, D.H. Mclntyre, and K.M. Johnson 1-2. Maps showing:1. Principal geographic features and boundaries, mining districts, and selected mines and prospects in the Challis I°x2° quadrangle 2. Geologic terranes of the Challis I°x2° quadrangle

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