Mass transport effects on the kinetics of nitrobenzene reduction by iron metal

To evaluate the importance of external mass transport on the overall rates of contaminant reduction by iron metal (Fe-0), we have compared measured ra tes of surface reaction for nitrobenzene (ArNO2) to estimated rates of exte rnal mass transport in a permeable reactive barrier (PRB). The rate of surf ace reaction was measured at a polished Fe-0 rotating disk electrode (RDE) in an electrochemical cell, and the rate of mass transport was estimated fr om a correlation for mass transport in packed-bed reactors. The kinetics of ArNO2 reduction were studied in pH 8.4 berate buffer at a potential below which an oxide film would form. The cathodic current measured in this syste m was dependent on the electrode rotation rate, and the measured first-orde r heterogeneous rate coefficient for surface reaction [k(rxn) = (1.7 +/- 0. 2) x 10(-3) cm s(-1)] was about 10 times faster than the first-order mass t ransport rate coefficient (k(mt) approximate to 2 x 10(-4) cm s(-1)) estima ted for PRBs. The similarity between rates of surface reaction and mass tra nsport suggest that it may be important to consider mass transport processe s in the design of PRBs for contaminants such as nitroaromatics that are hi ghly reactive with Fe-0.