Engineers reveal how to optimize processes for transforming sulfur in wastewater to valuable materials

Date:
March 2, 2022
Source:
Stanford University
Summary:
Promising technologies for converting wastewater into drinkable
water produce a chemical compound that can be toxic, corrosive
and malodorous.

An analysis of one possible solution reveals ways to optimize it for
maximum energy efficiency, pollutant removal and resource recovery.



FULL STORY ==========================================================================
One person's wastewater is another person's treasure. A new Stanford
University study paves the way to mining sewage for valuable materials
used in fertilizers and batteries that could someday power smartphones
and airplanes. The analysis, published recently in ACS ES&T Engineering, reveals how to optimize electrical processes for transforming sulfur
pollution, and could help lead to affordable, renewable energy-powered wastewater treatment that creates drinkable water.


==========================================================================
"We are always looking for ways to close the loop on chemical
manufacturing processes," said study senior author Will Tarpeh, an
assistant professor of chemical engineering at Stanford. "Sulfur is a
key elemental cycle with room for improvements in efficiently converting
sulfur pollutants into products like fertilizer and battery components."
A better solution As fresh water supplies dwindle, particularly in arid regions, focus has intensified on developing technologies that convert wastewater to drinkable 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 malodorous. Strategies for dealing with that problem, such as chemical oxidation or the use of certain chemicals to convert the sulfur into
separable solids, can generate byproducts and drive chemical reactions
that corrode pipes and make it harder to disinfect the water.

A tantalizing solution for dealing with anaerobic filtration's sulfide
output lies in converting the sulfide to chemicals used in fertilizer and cathode material 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 electrochemical sulfur oxidation, which
requires low energy input and enables fine-tuned control of final sulfur products. (Whereas some products, such as elemental sulfur, can deposit
on electrodes and slow down chemical reactions, others, like sulfate,
can be easily captured and reused.) If it worked effectively, the process
could be powered by renewable energy and adapted to treat wastewater
collected from individual buildings or entire cities.

Making novel use of scanning electrochemical microscopy -- a technique
that facilitates microscopic snapshots of electrode surfaces while
reactors are operating -- the researchers quantified the rates of each
step of electrochemical sulfur oxidation along with the types and amounts
of products formed. They identified the main chemical barriers to sulfur recovery, including electrode fouling and which intermediates are hardest
to convert.

They found, among other things, that varying operating parameters, such
as the reactor voltage, could facilitate low-energy sulfur recovery
from wastewater.

These and other insights clarified trade-offs between energy efficiency, sulfide removal, sulfate production and time. With them, the researchers outlined a framework to inform the design of future electrochemical
sulfide oxidation processes that balance energy input, pollutant
removal and resource recovery. Looking toward the future, the sulfur
recovery technology could also be combined with other techniques, such
as recovery of nitrogen from wastewater to produce ammonium sulfate
fertilizer. The Codiga Resource Recovery Center, a pilot-scale treatment
plant on Stanford's campus, will likely play a large role in accelerating future design and implementation of these approaches.

"Hopefully, this study will help accelerate adoption of technology that mitigates pollution, recovers valuable resources and creates potable
water all at the same time," said study lead author Xiaohan Shao, a PhD
student in civil and environmental engineering at Stanford.

Video: https://www.youtube.com/watch?v=pFEOR9E01iA Tarpeh is also an
assistant professor (by courtesy) ofcivil and environmental engineering,
a center fellow (by courtesy) of theStanford Woods Institute for the Environment, an affiliated scholar with Stanford'sProgram on Water,
Health and Development, and a member ofStanford Bio-X. Additional author
Sydney Johnson was an undergraduate student in chemical engineering at
Stanford at the time of the research.

The research was funded by Stanford'sDepartment of Chemical Engineering,
the National Science FoundationEngineering Research Center for
Re-inventing the Nation's Urban Water Infrastructure (ReNUWIt)and
the Stanford Woods Institute for the EnvironmentEnvironmental Venture
Projects program.

========================================================================== Story Source: Materials provided by Stanford_University. Original written
by Rob Jordan.

Note: Content may be edited for style and length.


========================================================================== Journal Reference:
1. Xiaohan Shao, Sydney R. Johnson, William A. Tarpeh. Quantifying and
Characterizing Sulfide Oxidation to Inform Operation of
Electrochemical Sulfur Recovery from Wastewater. ACS ES&T
Engineering, 2022; DOI: 10.1021/acsestengg.1c00376 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2022/03/220302131330.htm

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