ENVIRONMENTAL ENGINEERING SCIENCE Volume 21, Number 2, 2004 © Mary Ann Liebert, Inc. Nitrate and Nitrite Reduction by Fe 0 : Influence of Mass Transport, Temperature, and Denitrifying Microbes Jennifer L. Ginner, 1 Pedro J.J. Alvarez, Sharon L. Smith, and Michelle M. Scherer * Department of Civil and Environmental Engineering The University of Iowa Iowa City, IA 52242-1527 ABSTRACT To evaluate how mass transport, temperature, and denitrifying micro-organisms affect the relative rates 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 elec- trode (RDE) in an electrochemical cell, and the effect of temperature was studied in batch reactors with gran- ular 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 signif- icant increases in current in the presence of nitrite. The heterogeneous reaction rate coefficient (k rxn ) for ni- trite reduction at the Fe 0 RDE is several orders of magnitude faster than the surface-area normalized rate co- efficient (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 50°C). Nitrite removal rates measured in both batch reactors and the electrochemical cell were typically 1.5 to 15 times faster than those measured for nitrite, 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 rela- tive 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. Key words: iron metal; zero-valent iron; nutrients; rotating disk electrode; reactive barrier; denitrifiers 219 1 Present address: Hussey, Gay, Bell & DeYoung, Inc., P.O. Box 14247, Savannah, GA 31416. * Corresponding author: Department of Civil and Environmental Engineering, The University of Iowa, Iowa City, IA 52242- 1527. Phone: 319-335-5654; Fax: 319-335-5660; E-mail: michelle-scherer@uiowa.edu INTRODUCTION I N A RECENT SURVEY OF 1,255 drinking-water wells and 242 public supply wells, nitrate was the compound most frequently detected at concentrations exceeding a regulatory standard [such as U.S. EPA’s maximum con- centration level (MCL)] or a health advisory (Squillace et al., 2002). Of the 1,497 wells tested (for volatile or- ganic compounds, pesticides, and nitrate), water in 9.4% of the wells exceeded the U.S. EPA’s nitrate MCL of 10