In the 17th-century, a German alchemist named Hennig Brandt was on a quest to find the “philosopher’s stone”, a mysterious object that could turn any ordinary metal into gold. When he realised the golden colour of urine, he believed it contained tiny particles of the precious metal.
Brand then collected urine from himself, his family, and beer drinkers at his local pub and began distilling it in the hope of finding gold at the end. There was no gold, alas, but the silver lining was that Brandt discovered the element phosphorus.
In fact, some call urine “liquid gold” because it is loaded with elements that plants desire. The waste product is packed with phosphorus, potassium, and nitrogen in the form of urea, the “Big Three” nutrients that fuel plant growth and form the backbone of commercial fertilisers.
Two birds, one stone
A new electrochemical technique published in the journal Nature Catalysis now proposes to separate urea from urine in its solid form via a greener, less energy-consuming process. This method converts urea, a nitrogen-rich compound in urine, into a crystalline peroxide derivative called percarbamide.
Thus it strikes two targets at once: enabling the treatment of urine in urban wastewater and transforming it into a useful resource.
Humans obtain nitrogen from food, convert it into urea, and excrete it through urine. Since urea is rich in nitrogen, it has the potential to be a natural fertiliser. In theory, returning the nutrients to the soil could complete the nitrogen cycle, but scientists currently lack efficient methods to extract urea directly from urine, leaving a crucial gap in this cycle, Xinjian Shi, a researcher at Henan University, China, and the first author of the new study, said.
“Our team’s research fills this gap.”
Pee-cycling to close the loop
An adult produces around 450-680 litres of urine, researcher Björn Vinnerås estimated in a 2002 study. The substance is 95% water yet the annual output also contains around 4 kg of nitrogen and 0.3 kg of phosphorus, enough to grow wheat for one loaf of bread every day for a whole year.
If it’s so valuable, why flush it down the toilet? The answer is that urine is a complex system and many of its components, especially salts, interfere with processes that can extract urea alone from wastewater, Shi said. The team’s study claims to have jumped this barrier.
Urea is made up of nitrogen, oxygen, and hydrogen, and is prone to forming hydrogen bonds with other molecules, including of urea itself. When these bonds form, the compound’s physical and chemical properties tend to change. This tendency turned out to be a game-changer in the separation process.
For example, when urea forms hydrogen bonds with hydrogen peroxide, it forms percarbamide, a white, crystalline solid that can be precipitated out from urine with high purity.
Percabadmide is known for its ability to steadily release active oxygen, making it a valuable candidate for processes that need to supply oxygen for other chemical reactions. Another key trait of this substance is its ability to accelerate the recovery of urea from urine.
To take advantage of this property, the researchers developed an in-situ electrochemical technique that uses graphitic carbon-based catalysts to convert urea in urine into percarbamide. Achieving almost 100% purity, the team used this process to effectively extract percarbamide from both human and animal urine.
While the result was promising, the true focus of the researchers was something else.
A eureka moment
At first, the researchers focused on one problem: keeping hydrogen peroxide stable in liquid form at higher concentration. They began exploring whether it could be solidified directly within a solution with a compatible material. Urea seemed like a promising option — but commercially made urea is quite expensive.
“Then, we suddenly thought, if we could use it within the urine system, it would not only achieve the original goal but also address the issues of urine treatment and nitrogen cycling. Wouldn’t that be a win-win?” Shi asked.
With this insight, the researchers designed an activated graphitic carbon catalyst. Graphite is a soft crystal made of carbon atoms. Activated graphitic carbon is a porous form of graphite subsequently modified to further increase its surface area, making it more reactive.
In this case, it was engineered to enhance two chemical reactions, or pathways, that produce solid percarbamide.
In pathway I, urea reacts directly with hydrogen peroxide in the presence of a catalyst that facilitates interactions between the two molecules. In pathway II, urea binds to a hydroperoxyl (-*OOH) intermediate, a highly reactive and short-lived molecule. Then it gains hydrogen ions (H⁺) and triggers a reaction to form percarbamide in the presence of a catalyst that enhances hydrogen bonding.
The activated graphitic carbon catalyst was suitable for both pathways.
Waste is golden?
After several rounds of trial and error, the researchers found they could maximise percarbamide production by holding the concentration of urea between 15% and 38%. They also found that maintaining temperatures just above freezing at a slightly acidic pH of around 4 works best for the process.
According to the researchers, the pure percarbamide extracted from this new process combines the best of both worlds: the nitrogen-rich benefits of urea and the oxidative power of hydrogen peroxide, unlocking new possibilities for sustainable applications.
“When the solid product is collected and used as fertiliser, nitrogen is slowly released, while also promoting root respiration and facilitating crop growth,” Shi said. “This process fully addresses the missing link in the nitrogen cycle that exists in human society.”
The team has also expressed excitement about bringing together resource-recovery and recycling with wastewater treatment in future. They believe this innovative approach can change how we think about and use waste.
Sanjukta Mondal is a chemist-turned-science-writer with experience in writing popular science articles and scripts for STEM YouTube channels.
Published – March 12, 2025 05:30 am IST
