$6.9 Billion Solution: Engineers Unveil Cleaner, Cheaper Way to Desalinate Seawater

Jovan Kamcev Places a Filter Membrane Between Two Electrodes — Jovan Kamcev, an assistant professor of chemical engineering and macromolecular science and engineering at U-M, places a filter membrane between two electrodes, which measure how well the membrane conducts electricity. This helps his team predict how well it can purify water. Credit: Marcin Szczepanski, Michigan Engineering

Eliminating acid and base treatments from conventional desalination plants could save billions of dollars worldwide, making seawater a more cost-effective source of drinking water.

Water desalination plants may soon be able to eliminate the use of costly chemicals by using new carbon cloth electrodes that remove boron from seawater, an essential step in making seawater safe to drink.

The technology is detailed in a study published in Nature Water by engineers from the University of Michigan and Rice University.

Boron, a naturally occurring element in seawater, becomes a toxic contaminant in drinking water when it passes through standard salt-removal filters. Its concentration in seawater is about twice the World Health Organization’s most relaxed safety limit for drinking water and five to twelve times higher than what many crops can tolerate.

Jovan Kamcev Inserts a Carbon Cloth Electrode Into a Flow Cell — Jovan Kamcev, an assistant professor of chemical engineering and macromolecular science and engineering at U-M, inserts a carbon cloth electrode into a flow cell for water desalination. Credit: Marcin Szczepanski, Michigan Engineering.

Why current filters fall short

“Most reverse osmosis membranes don’t remove very much boron, so desalination plants typically have to do some post treatment to get rid of the boron, which can be expensive,” said Jovan Kamcev, U-M assistant professor of chemical engineering and macromolecular science and engineering and a co-corresponding author of the study. “We developed a new technology that’s fairly scalable and can remove boron in an energy-efficient way compared to some of the conventional technologies.”

In seawater, boron exists as electrically neutral boric acid, so it passes through reverse osmosis membranes that typically remove salt by repelling electrically charged atoms and molecules called ions. To get around this problem, desalination plants normally add a base to their treated water, which causes boric acid to become negatively charged. Another stage of reverse osmosis removes the newly charged boron, and the base is neutralized afterward by adding acid. Those extra treatment steps can be costly.

Acid Treatment Enhances Boron Capturing — When treated with acid, the carbon cloth fibers in the researchers’ electrodes gain oxygen-containing features that can trap boron. Credit: Jovan Kamcev, Kamcev Research Lab, University of Michigan

“Our device reduces the chemical and energy demands of seawater desalination, significantly enhancing environmental sustainability and cutting costs by up to 15 percent, or around 20 cents per cubic meter of treated water,” said Weiyi Pan, a postdoctoral researcher at Rice University and a study co-first author.

Global savings and water access

Given that global desalination capacity totaled 95 million cubic meters per day in 2019, the new membranes could save around $6.9 billion annually. Large desalination plants—such as San Diego’s Claude “Bud” Lewis Carlsbad Desalination Plant—could save millions of dollars in a year.

Those kinds of savings could help make seawater a more accessible source of drinking water and alleviate the growing water crisis. Freshwater supplies are expected to meet 40% of demand by 2030, according to a 2023 report from the Global Commission on the Economics of Water.

The new electrodes remove boron by trapping it inside pores studded with oxygen-containing structures. These structures specifically bind with boron while letting other ions in seawater pass through, maximizing the amount of boron they can capture.

Water Purifier Diagram — This diagram shows how boron is removed by the researchers’ electrodes. First a majority of the salt ions are removed with reverse osmosis. Then the water flows into a cell containing a membrane with positive (pink) and negative (orange) layers. Similarly charged electrodes face the membrane layers, and when a current is applied, water molecules at the interface of the membranes split into hydrogen and hydroxide ions. The hydroxide ions stick to boron, causing it to stick to the positive electrode. Credit: Jovan Kamcev, Kamcev Research Lab, University of Michigan, and Weiyi Pan, Elimelech Research Lab, Rice University

However, the boron-catching structures still need the boron to have a negative charge. Instead of adding a base, the charge is created by splitting water between two electrodes, creating positive hydrogen ions and negative hydroxide ions. The hydroxide attaches to boron, giving it a negative charge that makes it stick to the capture sites inside the pores in the positive electrode. Capturing boron with the electrodes also enables treatment plants to avoid spending more energy on another stage of reverse osmosis. Afterward, the hydrogen and hydroxide ions recombine to yield neutral, boron-free water.

“Our study presents a versatile platform that leverages pH changes that could transform other contaminants, such as arsenic, into easily removable forms, “said Menachem Elimelech, the Nancy and Clint Carlson Professor of Civil and Environmental Engineering and Chemical and Biomolecular Engineering at Rice University, and a co-corresponding author of the study.

“Additionally, the functional groups on the electrode can be adjusted to specifically bind with different contaminants, facilitating energy-efficient water treatment,” Elimelech said.

Reference: “A highly selective and energy-efficient approach to boron removal overcomes the Achilles heel of seawater desalination” by Weiyi Pan, Debashis Roy, Betül Uralcan, Sohum K. Patel, Arpita Iddya, Eungjin Ahn, Amir Haji-Akbari, Jovan Kamcev and Menachem Elimelech, 20 January 2025, Nature Water.
DOI: 10.1038/s44221-024-00362-y

The research is funded by the National Alliance for Water Innovation, the U.S. Department of Energy, the U.S. National Science Foundation, and the U.S.-Israel Binational Science Foundation.