A new nuclear reactor could turn wastewater into drinking water while producing one of the world's most sought-after chemicals.
The new reactor, published August 12 in the journal Nature Catalysis, produces ammonia gas from water contaminated with nitrate ions.
Ammonia (NH3) is a crucial industrial chemical. It is one of the main components of fertilizers and is also essential in chemical manufacturing processes. More than 180 million tons (163 million metric tons) are produced annually, most of it via the century-old Haber-Bosch process, a high-temperature, high-pressure reaction of hydrogen and nitrogen. According to the study, this single chemical reaction consumes about 2% of the world's energy.
Nitrates, on the other hand, pollute rivers and streams when excess runoff from fertilized farmland enters local waterways. Nitrates destroy aquatic ecosystems and can be harmful to health if present in high concentrations in drinking water. Water must be thoroughly treated to remove the nitrates before it can be safe to drink.
Existing commercial processes use bacteria to convert nitrate directly into nitrogen, but this is an expensive process that also produces nitrous oxide, which per pound is 265 times more potent as a greenhouse gas than carbon dioxide.
To avoid this climate impact, scientists are working on ways to use electricity to convert nitrates into ammonia, but early systems have suffered from unwanted side reactions.
Fenyang Chen holds a prototype reactor system that could dramatically improve the efficiency of electrically powered ammonia production. (Image credit: Jeff Fitlaw/Rice University)
These devices have a positive and a negative end, with different charges on each end. Chemical reactions occur at both ends: at the negative end of the reactor, water is split into oxygen gas and hydrogen ions, and at the positive end, a second reaction converts nitrates into ammonia and hydroxide ions (OH-).
Unfortunately, the hydrogen ions produced on one side tend to diffuse to the other, where they undergo a chemical reaction to form hydrogen. Even highly polluted water still has traces of nitrates remaining, so this hydrogen reaction dominates, preventing the main reaction of nitrates to ammonia from occurring as efficiently. Scientists have tried to get around this by adding additives to the mixture, but this is impractical for practical applications in water treatment.
The new reactor produces ammonia gas from water contaminated with nitrate ions. Here, scientists fine-tune a prototype of the reactor. (Image credit: Jeff Fitlow/Rice University)
In the new study, the researchers partially circumvented this problem by adding an intermediate chamber to create a three-chamber reactor, study lead author Feng-Yan Chen, a researcher at Rice University in Texas, told Live Science in an email. In the first chamber, nitrates are converted into ammonia gas and hydroxide ions. These combine with sodium ions already present in the water to form sodium hydroxide. When purified water leaves the first chamber and is pumped into the intermediate chamber along with this sodium hydroxide, the newly formed ammonia gas bubbles up. Meanwhile, in the third chamber, hydrogen ions produced by the splitting of water diffuse throughout the cell into the intermediate chamber. Here, hydrogen ions from the sodium hydroxide combine with hydroxide ions to form water. The remaining sodium ions return from the intermediate chamber to the first chamber to repeat the cycle.
Crucially, hydrogen ions do not reach the other side of the reactor and interfere with the nitrate reaction. During a 10-day test run, more than 90 percent of the current in the researchers' cell went to making ammonia, compared with about 20 percent in their previous system.
Wang's design is still in the experimental stage, and several issues need to be ironed out before the technology can be commercially deployed.
One major challenge, Chen said, was making the reaction work in the presence of impurities such as magnesium and calcium ions that are commonly found in water.
