Cheap electrochemistry process can turn urine into powdered fertiliser

A cheap and simple electrochemical process has been developed that captures urea – a vital nitrogen-rich ingredient to make fertiliser – from human and animal urine. The approach, which converts liquid urea into a retrievable solid peroxide derivative, could allow waste urine from households or farms to be turned into fertilisers, disinfectants and even batteries, say the researchers.

The idea of tapping urine to obtain valuable urea is not new. However, existing technologies that extract urea from urine have struggled to be economically viable and environmentally sustainable. This is because they involve multiple steps and require a lot of energy, while failing to produce high-purity urea due to limitations on separation selectivity. The upshot is that it’s less work to synthesise urea using traditional industrial processes to meet demand.

Now, Xinjian Shi at Henan University, China, and his collaborators at Stanford University, US, have devised an efficient electrocatalytic technique that can turn waste urine into a valuable supply of urea without complex extraction or purification steps. The team were able to harness a specific characteristic of urea, namely its ability to combine with hydrogen and oxygen to form percarbamide, a peroxide.

‘Percarbamide is more easily precipitated from liquid than urea, enabling in situ solid–liquid separation and extraction,’ explains Shi. Urea, he says, is the only component among the many others in urine capable of undergoing this process. ‘This unique property naturally ensures the purity of the obtained product. At the same time, the electrochemical method requires very mild conditions, with lower energy consumption and costs,’ Shi adds.

As carbon-based materials have been widely used as catalysts in redox reactions, as well as being abundant and cost-effective, the team explored graphite carbon catalysts to drive the electrochemical process. These catalysts allow the addition of oxygen and hydrogen to urea through the reduction of oxygen, which makes them ideal for turning urea into percarbamide.

Experiments revealed that during the electrochemical process, there were two possible pathways for converting urea into percarbamide. The first followed the traditional oxygen reduction pathway involving oxygen undergoing a two-electron reduction and hydrogenation to produce hydrogen peroxide. This hydrogen peroxide then formed strong hydrogen bonds with urea, resulting in percarbamide crystals.

The second pathway involved oxygen undergoing a one-electron reduction and hydrogenation to generate hydroperoxide, which combined with urea to produce an intermediate that could be further reduced and hydrogenated to yield percarbamide crystals.

Via both routes, the researchers were then able to extract the percarbamide in powder form, which they say could have diverse applications including water treatment, pathogen disinfectant, batteries and crop fertilisers.

The team calculated that daily production of 1 tonne of percarbamide would require only 100m² of land and the urine from about 6400 houses or a farm of 3800 cows. However, collecting enough concentrated urine could be tricky.

The researchers suggest that urine would need to be diverted at source to prevent mixing with general sewer systems. ‘One option is to develop small-scale devices for use within households,’ Shi says. However, the challenge here is that individual households may lack the motivation to treat urine and produce percarbamide on their own.’

Another option is to invest in large-scale processing using dedicated urine collection channels in communities or farms. ‘In this case, urine and faeces would be separated at the user level and then transported through specialised pipelines to centralised treatment and stabilisation pools,’ adds Shi.

‘The fact that the percarbamide precipitates could be both a blessing and a curse: it makes it easy to harvest, but precipitation could also lead to fouling or blocking of flow systems,’ comments Mark Symes, an electrochemist at the University of Glasgow, UK. ‘Perhaps the most attractive application is as a fertiliser. Their initial results look promising, but a much more detailed study of the effects on plant growth and soil health will be required before this becomes a possible market.’