[Professor Wonyong Choi’s research team at POSTECH has developed a photocatalyst material for denitrification that directly converts nitrate ions to dinitrogen using sunlight and water only.]

As the temperature rises in summer, the accumulated influx of fertilizers, domestic sewage, and industrial wastewater that contain a large amount of nitrogen compounds can seriously contaminate rivers and lakes where algal blooms may occur. The strong sun lights in summer can heat up the water and provide a perfect condition for the overgrowth of algae or bacteria. As the dissolved oxygen level is decreased due the increase of algae in the water, the water quality cannot be preserved. Denitrification system that converts nitrate ions into nitrogen has been studied and considered as one of the efficient methods to prevent this phenomenon. In this process, nitrate ions are converted to harmless dinitrogen, which leaves no inorganic nitrogen residues in water and also prevents the algal blooms and eutrophication.

Professor Wonyong Choi and Ph.D. candidate Shinbi Lee from Division of Environmental Science and Engineering (DESE) at POSTECH have developed the world’s first photocatalytic material that quantitatively converts nitrate ions (NO3-) to dinitrogen (N2) without using any chemical reducing agents. The findings from this study were published by the Royal Society of Chemistry (RSC) as the back-cover paper of Energy & Environmental Science, the most prestigious journal in the field of energy environmental science. Chemistry World, a magazine published by RSC, has also highlighted these findings as an innovative denitrification process.

The conventional denitrification process requires high energy inputs as well as reducing agents such as hydrogen gas, methanol, or formic acid. Although there have been attempts to utilize sunlight, most of them require reducing agents which limits economic feasibility and applicability. Using water as a reducing agent has remained as an unsolved problem, which Professor Choi’s team challenged.

The research team synthesized a photocatalyst composite composed of titanium dioxide (TiO2), bimetallic (Cu-Pd) nanoparticles and graphene oxide (rGO). This photocatalyst achieved near 100% reduction of 600 ppm of nitrate ions (NO3-) without using any chemical reducing agent and 98% of the removed nitrate was converted into nitrogen (N2). The synergic action of the various components in the system enables the in situ generated H2 gas from water splitting to be consumed immediately for reducing nitrate into dinitrogen. It is also interesting to note that the photocatalyst completely hinders the generation of nitrite ions (NO2-) which is a dominant by-product in the conventional denitrification process.

This study was financially supported by the Leading Researcher Program of the National Research Foundation of Korea (NRF).