Sunlight-Driven Device Recovers Ammonia from Wastewater for Fertilizer and Water Purification

As solar energy applications expand, scientists have developed a cost-effective technology that uses sunlight to recover ammonia from wastewater. The innovation, published in Nature Sustainability, offers a low-cost source of nitrogen fertilizer while simultaneously purifying agricultural and industrial effluents.

Ammonia, a key component of nitrogen fertilizer, has an annual global production of about 240 million tons. Its manufacture typically depends on energy-intensive fossil fuels, while residual ammonia in agricultural runoff pollutes waterways and triggers algal blooms. Existing wastewater treatment methods can reclaim ammonia but are prohibitively expensive, leading most facilities to destroy it instead.

To address this, the research team combined solar distillation with a novel material to create a high-efficiency ammonia recovery system. The setup consists of a wastewater container covered by a transparent dome. Sunlight heats the water, causing it to evaporate, while the vapor condenses into pure water. Since most ammonia in wastewater exists as ammonium salts, which cannot evaporate directly, the system employs a floating sponge coated with light-absorbing titanium carbide and functionalized with alkaline amine groups. These groups capture protons from ammonium salts, converting them into gaseous ammonia that evaporates along with water vapor for collection.

Once the sponge becomes saturated with protons, sunlight drives a reaction with chloride ions in the wastewater to produce hydrochloric acid—a recoverable by-product—while regenerating the sponge. Economic analysis suggests that sales of recovered ammonia and hydrochloric acid could offset the device's cost within 3.5 years.

Having completed laboratory validation, the system is poised for scalable application, as it requires no rare or costly materials. This approach offers a sustainable solution to wastewater treatment and could reduce agriculture's reliance on conventional fertilizers.