Floating sponges can recover ammonia from wastewater using the sun

Sunlight shining on specialized floaties can now produce fuel for plants by recovering ammonia from wastewater. Researchers designed a floatable amino-grafted (-NH₂) MXene (Ti₃C₂)-based (AMS) sponge that, when scaled efficiently, can provide two sustainable solutions simultaneously: cleaning up wastewater and providing ammonia (NH₃), an essential nitrogen source for plants, to farmers at a lower cost.

Over a century ago, two scientists saved the world from impending starvation by developing the Haber-Bosch process—a method for converting nitrogen in the air into ammonia fertilizer. Despite Haber, one half of the duo being involved with chemical warfare during World War I, this invention won the scientists a Nobel Prize, which goes on to show the significance of growing food. It is also a foundational feedstock for many chemical industries.

The Haber-Bosch process still remains one of the most common methods for ammonia (NH₃) synthesis. However, the process is energy-hungry and results in considerable carbon emissions—3.27 tons of CO₂-equivalent are emitted for every ton of NH₃ recovery.

Recovering ammonia from agricultural and industrial runoff could help reduce emission values and alleviate the pressure on chemical industries to meet the world’s growing demand, which amounts to over hundreds of millions of tons per year.

According to the findings published in Nature Sustainability, the researchers were able to recover ammonia at the rate of 0.6 mol/m²/h with 99.8% purity using ammonium chloride (NH₄Cl) wastewater under 5-sun light intensity, without any added chemicals or energy.

The MXene-based sponge fully regenerated itself under 15 sun, and produced hydrochloric acid as a by-product, another economically valuable chemical.

In the right places, ammonia is a savior, but when present in run-offs and wastewater, this same chemical acts as a potent pollutant with the ability to disrupt aquatic life. China alone discharges over 10 million tons of NH₄⁺-containing wastewater into the hydrological systems annually.

Recovering NH₃ from NH₄⁺-containing wastewater hinges on the reversible hydrolysis reaction of NH₄⁺ (NH₄⁺ ⇌ NH₃ + H⁺), governed by equilibrium principles.

The aim is to shift the equilibrium towards greater NH₃ production, which is usually done by removing or neutralizing the H⁺ on the product side. Conventional recovery methods require excess alkaline chemicals and electric heating to shift the NH₄⁺ hydrolysis equilibrium toward NH₃ production.

Recent studies have demonstrated that interfacial solar heating is a promising, energy-efficient alternative for ammonia recovery, operating on the principle of the localized photothermal effect.

Tapping into the advantages of interfacial solar heating, the researchers proposed a solar-driven ammonia recovery strategy using floatable AMS.

The sponge created a reversible local alkaline environment and interfacial heat on the water surface under sunlight. When floating on the surface, the ‒NH₂ groups grafted in the sponge captured H⁺ ions without added reagents, enabling NH₄⁺ hydrolysis into NH₃.

The Ti₃C₂ in the floaties helped with efficient absorption of solar energy and conversion into heat required to evaporate NH₃, which was collected later via condensation.

Life-cycle and techno-economic analyses revealed substantial environmental and cost advantages over conventional approaches. For instance, this solar-driven recovery strategy emitted only 0.102 tons of CO₂-equivalent, which is 30 times lower emissions compared to the conventional Haber-Bosch process.

The researchers emphasize the need for further studies to optimize material designs tailored to specific wastewater characteristics, seasons, locations, and industry types.

© 2025 Science X Network