By Peter Polacek
Ammonia, a precursor for fertilizers, is produced via the Haber process, one of the world’s most important industrial processes. It consumes around 1 to 2% of the world’s energy usage, mostly in the form of natural gas. However, the process is a significant source of carbon dioxide, producing 245 million tonnes of CO2 annually (Lan et al., 2013). In nature, ammonia production is carried out by some prokaryotes, such as cyanobacteria (blue-green algae). Considering the limited supply of natural gas and the environmental impact of the process there is a need for a new method of production, preferably based on living organisms or biomaterials.
Producing ammonia simply by growing cyanobacteria has proven ineffective, mostly due to the difficulty of extracting the product from the cells. Paschkewitz (2012) proposes the use of cyanobacteria modified to release produced NH3 to the extracellular space. NO2- and NO3- ions were used as a nitrogen source and enzymatically reduced by the cells to form ammonia. These ions are commonly found in sewage from animal farms, bringing the possibility of coupling the production method with wastewater treatment.
Despite the need to stimulate the microorganisms with electrical current to maximize yields, the process uses significantly less energy than the Haber process. This is because the latter requires high pressure and temperature, while cyanobacteria carry out the reaction at atmospheric conditions. Statistical analysis has even showed that this method may be more feasible for individual farmers than buying ready-made fertilizer. On the other hand, the presence of photosynthesizing cells makes the system dependent on sunlight or artificial light.
An alternative process has also been described by Milton et al. (2017). With the use of methyl viologen as an electron source and the enzymes nitrogenase and hydrogenase (Fig. 1), the experiment was successful in producing NH3 from H2 and N2 gases. The reaction took place within an enzymatic fuel cell, generating electrical current as a by-product.
Even though some (Schrock, 2006) suggest that, considering its impact, the Haber-Bosch process is unlikely to be replaced in the near future, a new, cleaner production method is undoubtedly worth exploring. With recent advances in biotechnology and the eco-friendliness of algae-based production methods, these may prove to be more affordable and more attractive than purely chemical methods.
LAN, R., IRVINE, J., TAO, S., 2013. Synthesis of ammonia directly from air and water at ambient temperature and pressure. Scientific Reports 3:1445.
Milton, R. D, Cai, R., Abdellaoui, S., Leech, D., De Lacey, A. L, Pita, M., Minteer, S. D., 2017. Bioelectrochemical Haber–Bosch Process: An Ammonia-Producing H2/N2 Fuel Cell. Angewandte Chemie International Edition 2017, 56, 2680 –2683.
PASCHKEWITZ, T. M., 2012. Ammonia Production at Ambient Temperature and Pressure: An Electrochemical and Biological Approach. PhD (Doctor of Philosophy) thesis, University of Iowa.
SCHROCK, R. R., 2006. Reduction of dinitrogen. Proceedings of the National Academy of Sciences, November 14, 2006, vol. 103, no. 46.