Nitrate and nitrite reduction by Fe0:: Influence of mass transport, temperature, and denitrifying microbes

To evaluate how mass transport, temperature, and denitrifying micro-organisms affect the relative kinetics of nitrate and nitrite reduction by iron metal (Fe-0), nitrate and nitrite reduction rates were measured over a range of mixing rates and temperatures. The effect of mixing rate was studied at a polished Fe-0 rotating disk electrode (RDE) in an electrochemical cell, and the effect of temperature was studied in batch reactors with granular Fe-0 in the absence and presence of Paracoccus denitrificans. Electrode rotation rate had little influence on the cathodic current measured in the presence of nitrate, whereas higher rotation rates resulted in significant increases in current in the presence of nitrite. The heterogeneous reaction rate coefficient (k(rxn)) for nitrite reduction at the Fe-0 RDE is several orders of magnitude faster than the surface-area normalized rate coefficient (k(SA)) for nitrite reduction by granular Fe-0. Activation energies for nitrate and nitrite reduction by granular Fe-0 were similar (21.7 +/- 9.3 kJ mol(-1) for nitrate and 23.8 +/- 1.8 kJ mol(-1) for nitrite). Addition of P. denitrificans to reactors containing Fe-0 resulted in faster nitrate removal compared to treatments with Fe-0 alone at all temperatures tested (5 to 50degreesC). Nitrite removal rates measured in both batch reactors and the electrochemical cell were typically 1.5 to 15 times faster than those measured for nitrate, depending on the electrode rotation rate, pH, temperature, and presence of microbes. Results from a simple first-order kinetic model based on sequential reduction of nitrate --> nitrite --> ammonium suggest that differences in the relative rates of nitrate and nitrite reduction may explain why nitrite appears as an intermediate product from nitrate reduction in some studies and not in others.