Ammonia is an important compound to many industries around the world. Most of the fertilizers used by crop growers have ammonia as an essential ingredient. It can also be useful as a fuel source, offering greater energy density per unit than hydrogen and greater safety. Currently, the predominant method for producing ammonia on an industrial scale is by the Haber-Bosch process. This process uses steam evolution of methane to provide H2 gas, which is then combined with N2 gas over an iron catalyst to form NH3. This process requires large amounts of energy as well as high temperatures and pressures.
Here, an alternative method for ammonia production is explored. With Anabaena Variabilis, a photosynthetic cyanobacteria, on a carbon electrode, ammonia can be generated at ambient temperatures and pressures at little energy cost, a few tenths of a volt. A bioelectrocatalytic device has been constructed by immobilizing whole cell a. variabilis in a Nafion film modified with a trimethyl octadecyl ammonium bromide (TMODA) salt at an electrode surface [3]. The polymer modified electrode provides the driving force and reductive microenvironment to facilitate production of NH3 by nitrogenase and nitrate/nitrite reductase enzymes present in a. variabilis. Ammonia production by cyanobacteria were increased from basal levels of 2.8 ± 0.4 µM produced over a two week period, to 22 ± 8 µM produced in 20 minutes under mild voltage perturbation, roughly 104% increase in rate.
Control of ammonia producing structures (nitrogenase in heterocystic cells or nitrate/nitrite reductase in vegetative cells) can be accomplished by growing the algae with and without fixed sources of nitrogen in the growth media. With the addition of various nitrogen-containing gases to the electrolyte solution during cyclic voltammetry, there is evidence that biofilms containing a mixture of cell types increases ammonia production above controls when the nitrogen is present as NO2-, NO, or N2O. Chronoamperometric perturbation studies show increased ammonia production at near +600 mV and -300 mV vs SCE. In cyclic voltammetric studies, nitrate/nitrite reductase in vegetative-only biofilms responds favorably to positive voltage ranges, while isolated heterocyst biofilms containing nitrogenase can be effectively targeted with the application of a negative voltage profile.
References:
[1] Johna Leddy and Timothy M. Pashkewitz, Ammonia Production Using Bioelectrocatalytic Devices, US Patent Application 20140011252
[2] Timothy M. Paschkewitz, Ammonia Production at Ambient Temperature and Pressure: An Electrochemical and Biological Approach, Ph.D., University of Iowa, 2012.