Scientists Mine Sewer Treasure | Technology networks

The wastewater of some is the treasure of others. A new Stanford University study paves the way for the extraction of wastewater for valuable materials used in fertilizers and batteries that could one day power smartphones and airplanes. The analysis, recently published in ACS Engineering ES&Treveals how to optimize electrical processes to transform sulfur pollution and could contribute to affordable, renewable energy-powered wastewater treatment that creates clean drinking water.

“We are always looking for ways to close the loop of chemical manufacturing processes,” said the study’s lead author. Does Tarpehassistant professor of Chemical Engineer at Stanford. “Sulfur is a key elemental cycle with room for improvements in the efficient conversion of sulfur pollutants into products like fertilizers and battery components.”

A better solution

As freshwater supplies dwindle, particularly in arid regions, attention has intensified on the development of technologies that convert wastewater into drinking water. Membrane processes that use anaerobic or oxygen-free environments to filter wastewater are particularly promising because they require relatively little energy. However, these processes produce sulfide, a compound that can be toxic, corrosive and smelly. coping strategies this problem, such as chemical oxidation or the use of certain chemicals to convert sulfur into separable solids, can generate by-products and cause chemical reactions that corrode pipes and make it more difficult to sanitize water.

An enticing solution to dealing with sulfide production from anaerobic filtration is to convert the sulfide into chemicals used in fertilizers and cathode materials for lithium-sulfur batteries, but the mechanisms for doing so are still not well understood. So Tarpeh and his colleagues set out to elucidate a cost-effective approach that would create no chemical byproducts.

The researchers focused on the electrochemical oxidation of sulfur, which requires a low energy input and allows precise control of the final sulfur products. (While some products, like elemental sulfur, can deposit on electrodes and slow chemical reactions, others, like sulfate, can be easily captured and reused.) If it worked efficiently, the process could be powered by renewable energies and adapted to treat waste water collected in individual buildings or entire cities.

Making a novel use of scanning electrochemical microscopy – a technique that facilitates microscopic snapshots of electrode surfaces during reactor operation – the researchers quantified the rates of each step in the electrochemical oxidation of sulfur as well as the types and the quantities of products formed. They identified the main chemical barriers to sulfur recovery, including electrode fouling and the most difficult-to-convert intermediates. Among other things, they found that varying operating parameters, such as reactor voltage, could facilitate low-energy recovery of sulfur from wastewater.

This and other information has clarified the trade-offs between energy efficiency, sulphide removal, sulphate production and time. With them, the researchers defined a framework to inform the design of future electrochemical sulphide oxidation processes that balance energy input, pollutant removal and resource recovery. In the future, the sulfur recovery technology could also be combined with other techniques, such as the recovery of nitrogen from wastewater to produce ammonium sulphate fertilizers. The Codiga Resource Recovery Center, a pilot-scale treatment plant on the Stanford campus, will likely play an important role in accelerating the future design and implementation of these approaches.

“I hope this study will help accelerate the adoption of technologies that mitigate pollution, recover valuable resources, and create clean drinking water at the same time,” said the study’s lead author. Xiaohan Shaodoctoral student in civil and environmental engineering at Stanford.

Reference: Shao X, Johnson SR, Tarpeh WA. Quantification and characterization of sulfide oxidation to shed light on the operation of electrochemical sulfur recovery from wastewater. ACS IS Engineering. 2022. do: 10.1021/acsestengg.1c00376

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