Effect of redox zonation on the reductive transformation of p-cyanonitrobenzene in a laboratory sediment column

The reductive transformation of a model compound, p-cyanonitrobenzene (pCNB ), was investigated in a laboratory sediment column that had been character ized with respect to redox zonation. Characterization of the redox zones wa s assessed by measurement of the solution phase concentrations of NO3-, NO2 -, Mn(ll), Fe(ll), and SO42-. Monitoring iron concentrations over time show ed that the redox zones were not under steady-state conditions. The iron-re ducing zone migrated in the direction of the flow at an initial rate of abo ut 0.2 cm per pore volume resulting in more oxidizing conditions with time, pCNB was rapidly reduced at the head of the column to p-cyano-N-hydroxylan iline (pCNH), which was reduced further to p-cyanoaniline (pCNA) in the iro n-reducing zone. The same reaction-product distribution was observed in bat ch experiments with Fe(ll)/goethite. Modeling of the column reaction kineti cs demonstrated, however, that reduction of pCNB occurred at least an order of magnitude faster on the column than predicted by reaction parameters fr om the batch experiment. Aging of the column had a significant effect on th e reduction kinetics of pCNB. After elution of 240 pore volumes, pCNB under went facile reduction directly to pCNA under nitrate-reducing conditions su ggesting a change in reaction mechanism from a chemical to a biological pro cess. After elution of 283 pore volumes, the column was completely oxic, an d reduction of pCNB was no longer observed.