Hyderabad: Engineers at the University of Michigan and Rice University have come up with a new method that could save billions of dollars when it comes to turning seawater into safe drinking water. The study, published in Nature Water, describes "a highly selective and energy-efficient approach to boron removal" using new carbon cloth electrodes.
Boron in treated seawater
Plants use reverse osmosis for water desalination, but the process fails to remove an element called boron, a natural component of seawater that becomes a toxic contaminant in drinking water when it sneaks through conventional filters for removing salts. According to the University of Michigan, seawater's boron levels are around twice as high as the World Health Organisation's most lenient limits for safe drinking water, and five to 12 times higher than the tolerance of many agricultural plants.
Jovan Kamcev, U-M assistant professor of chemical engineering and macromolecular science and engineering and a co-corresponding author of the study, explained that most reverse osmosis membranes don't remove much boron. Therefore, desalination plants typically need to perform additional post-treatment to eliminate boron, which can be costly.
In seawater, boron exists as neutral boric acid, allowing it to pass through reverse osmosis membranes that remove salt. To address this, desalination plants typically add a base to make boric acid negatively charged, enabling its removal in another stage of reverse osmosis, and then neutralising the base with acid. These additional steps make the process expensive.
Carbon cloth electrodes to remove boron
Weiyi Pan, a postdoctoral researcher at Rice University and a co-first author of the study, mentioned that their device (carbon cloth electrode) reduces the chemical and energy demands of seawater desalination, significantly enhancing environmental sustainability and cutting costs by up to 15 per cent, or around 20 cents per cubic meter of treated 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, maximising the amount of boron they can capture," the University of Michigan explained.
These new boron-catching structures still require the element to have a negative charge, but instead of adding a base, water is split between two electrodes, creating positive hydrogen ions and negative hydroxide ions.
The hydroxide attaches to the boron, giving it a negative charge and allowing it to stick to capture sites in the positive electrode. This process allows treatment plants to avoid a costly additional stage of reverse osmosis, resulting in neutral, boron-free water.
New avenues of safe drinking water
The new technology, if applied globally, could save around $6.9 billion annually and significantly reduce costs for large plants like San Diego’s Claude “Bud” Lewis Carlsbad Desalination Plant. These savings could make seawater a more accessible source of drinking water and help address the growing water crisis.
Notably, a 2023 report from the Global Commission on the Economics of Water claims that freshwater supplies are expected to meet only 40 per cent of demand by 2030.
