Cutting acid and base treatments from conventional desalination plants could save billions of dollars globally, making seawater a more affordable option for drinking water
Study: A highly selective and energy efficient approach to boron removal overcomes the Achilles heel of seawater desalination (DOI: 10.1038/s44221-024-00362-y)
Water desalination plants could replace expensive chemicals with new carbon cloth electrodes that remove boron from seawater, an important step of turning seawater into safe drinking water.
A study describing the new technology has been published in Nature Water by engineers at the University of Michigan and Rice University.
Boron is a natural component of seawater that becomes a toxic contaminant in drinking water when it sneaks through conventional filters for removing salts. Seawater's boron levels are around twice as high as the World Health Organization's most lenient limits for safe drinking water, and five to 12 times higher than the tolerance of many agricultural plants.
"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.
"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.
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.
But 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.
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.
The electrodes were studied at the Michigan Center for Materials Characterization.
