Contaminated water as a source of fertilizer

It can also remove nitrates from contaminated waters and at the same time produce ammonia in a sustainable way, according to PhD candidate Phebe van Langevelde and Professor Marc Koper of the Leiden Institute of Chemistry at Joule on January 26, 2021. Together with other colleagues from Germany, they conducted a literature study on the possibility.

Two birds, one stone

Nitrate is a pollutant that comes from traffic, industry, and agriculture. It is a nitrogen compound: NO3-. Environmental technologists are looking for a way to convert this into nitrogen gas, N2, but it turns out to be difficult. Ammonia, the NH3 nitrogen compound, is an important chemical raw material, for example in the production of fertilizers. It is made from nitrogen gas, but the process takes place at high pressures and temperatures and requires the use of fossil fuels.

The solution to both problems is to combine these processes and convert nitrate directly into ammonia, according to Phebe van Langevelde. With his friends, he evaluates his choices. “The total picture was new,” he said.

Bacteria against electrochemistry

The current way to convert nitrates into nitrogen gas is to use bacteria. Electrochemistry can be a cheaper and more robust alternative. The initial material is dissolved in a liquid in which two electrodes are stored, coated with a catalyst. When a voltage is applied to an electrode, a chemical reaction takes place at its surface, depending on the catalyst applied. “So far, it is not possible to convert nitrate into pure nitrogen gas in this way. Too many other products are formed,” said Van Langevelde. “It turns out it’s easier to convert nitrate into ammonia, a suitable catalyst that was designed a few years ago, based on new ingredients.”

Sustainable and safe

This is thought to be not only a better way to remove nitrates from contaminated water, but also an environmentally friendly alternative to producing ammonia. The process of consuming energy based on nitrogen gas is the Haber-Bosch process, a mature technology that has been applied for more than a century. It will be difficult to succeed. But the electrochemical approach of nitrate can be cheaper than the Haber-Bosch method. The process takes place at room temperature and is powered by electricity, which can be produced continuously. They are also small scale and can be applied locally. “So, you do not have to store a lot of products, with the associated security risks,” said Van Langevelde. “The explosion in Beirut, for example, involved fertilizer storage.”

Additional research

Before the electrochemical conversion of nitrate to ammonia can be applied in practice, additional fundamental research is still needed, he said, for example research on catalysts that are relevant to the formation of by-products. And the question remains: which wastewater flow is appropriate? Waste flow from nuclear power plants and industry is a possibility but contains other substances that can interfere with the process. “And the amount of nitrate in contaminated groundwater is too small for this application. Additional steps for nitrate concentration will be needed. So, there are still some obstacles to overcome, but the approach seems promising. It can contribute to the restoration of the global nitrogen cycle.”

References: “Electrocatalytic Nitrate Reduction for Sustainable Ammonia Production” by Phebe H. van Langevelde, Ioannis Katsounaros, Marc TM Koper, January 26, 2021, Joule.
DOI: 10.1016 / j.joule.2020.12.025

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