Excessive anthropogenic production of nitrogen fertilizers combined with rapid industrialization has disrupted the natural balance of the nitrogen cycle leading to increased concentrations of reactive-nitrogen/nutrient species including nitrates (NO₃^-), nitrites (NO₂^-), and ammonium (NH₄⁺) in groundwater, rivers and lakes^1,2. These nutrient imbalances can lead to health risks to human beings by causing methemoglobinemia in infants, also known as “blue baby syndrome,” and may cause cancer in adults, and negatively impact the environment by causing acid rain and algal blooms. Despite the chemical energy that is stored in common dissolved contaminants in water, wastewater treatment poses the challenge of being a highly energy intensive process. In this study we propose a photoelectrochemical approach (see figure below) to oxidize water and reduce nitrates (NO₃^-) to various nitrogen-species including N₂, and other value-added products such as NH₃ and N₂O. NH₃ can be directly used as a fuel and/or for fertilizer production, and N₂O is a powerful oxidant in combustion reactions (used as “nitrox” to supercharge engines of high performance vehicles); in the combustion of methane (CH₄), N₂O increases the heat released in the reaction by 37% as compared to when O₂ is used as an oxidant.

Thermodynamic analyses is first performed to predict the solar energy conversion efficiencies for the various product species. Thermodynamic predictions show that N₂O could be a more promising value-added NO₃^- reduction product compared to NH₃ due to the increased aqueous solubility of the latter, which leads to significant dissolved gas crossover losses. Furthermore, a coupled species transport and reaction kinetics model is developed to predict overall rates of product formation for tandem photoelectrochemical reactor designs. Specifically, the model developed is applied to quantify the influences of competing oxidation and reduction reactions and species mass transport on the energy and nutrient recovery potential of the proposed photoelectrochemical device.

References:

1. Falkowski, P. et al. The Global Carbon Cycle : A Test of Our Knowledge of Earth as a System. Science (80 ). 290, 291–297 (2000).
2. Duca, M. & Koper, M. T. M. Powering denitrification: the perspectives of electrocatalytic nitrate reduction. Energy Environ. Sci. 5, 9726 (2012).