Carbon Cloth Electrodes Could Reduce the Cost of Seawater Desalination

Desalinating seawater has long been pitched as one possible route to addressing water insecurity around the globe. However, the process has remained cost-prohibitive at large scales because it requires enormous amounts of energy, as well as expensive and resilient equipment and infrastructure. In addition, one of the primary costs associated with desalination is the high cost of the necessary chemicals for acid and base treatments. In a new study published in the journal Nature Water, researchers developed a novel method of removing boron from seawater, a critical step in making seawater safe to drink, using carbon cloth electrodes — a woven fabric of carbon fibers that function as an electrode.

Boron is naturally occurring in the form of boric acid in seawater, appearing at concentrations twice as high as the most lenient World Health Organization guidelines for drinking water and five to twelve times higher than is tolerant for agriculture. These high concentrations make boron removal a necessity for any large-scale desalination plant.

Standard reverse osmosis (RO) processes remove salts via a semi-permeable membrane that allows water molecules to pass through but blocks larger and/or charged molecules like salt ions. This poses a problem, as boric acid (B(OH)₃) is uncharged at neutral pH and therefore passes through RO membranes more easily than salts. To get around this, desalination processes increase the pH by adding a base, thereby increasing the concentration of hydroxyl groups, which bond with boric acid to create negatively charged borate ions, B(OH)₄⁻. These negatively charged ions are better rejected by the RO membrane. After filtration, acid is added to neutralize this base.

“The cost associated with acid and base treatment for the second pass of reverse osmosis in seawater reverse osmosis (SWRO)...