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.
Life-cycle
and global potential analysis. Credit: Nat Sustain (2025). DOI: 10.1038/s41893-025-01609-6
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.
Source: Floating sponges can recover ammonia from wastewater using the sun
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