Electrochemical technique gathers valuable nutrients from manure

Researchers have used a material similar to the pigment Prussian Blue as an electrode to efficiently extract ammonia and potassium from livestock manure (Nat. Sustainability 2023, DOI: 10.1038/s41893-023-01252-z) . Besides recovering those key fertilizer ingredients, the new electrochemical method can also generate either hydrogen for fuel or the disinfectant hydrogen peroxide. This could reduce the environmental impact of livestock farming while helping farmers generate additional revenue, the researchers say.

Farmers traditionally spread manure on fields as a fertilizer. But excess nutrients from the roughly 140 million metric tons of manure produced annually by the U.S. livestock industry can contaminate groundwater and harm aquatic life in nearby water bodies. Farm runoff also leads to climate-warming, odorous emissions.

So scientists are working on ways to gather valuable nutrients from manure. But most processes require heating the waste or adding large amounts of water and chemicals. Newer technologies such as membranes, meanwhile, are easily fouled by organic matter, says Mohan Qin, a civil engineer at the University of Wisconsin-Madison. “We wanted to target a membrane-free system that is compact and easy to use.”

Qin, chemist Song Jin, and their colleagues found that potassium nickel hexacyanoferrate (KNiHCF), a material with a porous Prussian Blue crystal structure that has shown promise for battery electrodes, is good at selectively taking up ammonium and potassium ions. When they put the material into manure-laden wastewater, they had “a bit of a lucky discovery,” Jin says. The electrode oxidized the organic matter, and the resulting positive charge drove the ammonium and potassium ions into the electrode.

Next, they used the ion-soaked KNiHCF as the anode in an electrochemical cell, where it gets oxidized, releasing ammonium and potassium ions into a potassium sulfate electrolyte solution. Depending on the cathode, either platinum or carbon, the redox reactions produce hydrogen or hydrogen peroxide respectively. Farmers could use the hydrogen peroxide to disinfect the remaining wastewater before discharging it into fields.

Evaporating the electrolyte solution gives a powdery mix of ammonium sulfate and potassium sulfate for making fertilizer. The team could reuse the KNiHCF at least 100 times. Their preliminary calculations show that a model 1,000-cow dairy farm using the system could make a profit of $200,000 a year. The main costs would be the electrochemical reactor and chemicals needed for operations. The biggest varying factor would be electricity price, Jin says, but, as solar and wind farms spread in the Midwest, “this economic solution looks more and more promising.”

The strategy could be used for nutrient recovery from municipal wastewater and sewage sludge as well, the researchers say. They now plan to scale it up and test in real livestock operations.

Xiao Su, a chemical engineer at the University of Illinois, calls this “a very clever use of redox chemistry for sustainability. The purely electricity-driven process should be easy to use in a decentralized fashion.” While scale-up and manufacturing of the electrodes could be a challenge for widespread use, the work is an interesting proof-of-concept, he says, with potential for promising impact on dairy farm operations.