Desalination — the removal of salt — is only one step in the process of producing drinkable water from ocean or wastewater. Either before or after the removal of salt, the water often has to be treated to remove boron, which is toxic to plants, and heavy metals like arsenic and mercury, which are toxic to humans. Often, the process leaves behind a toxic brine that can be difficult to dispose of.
Chemists have discovered a way to simplify the removal of toxic metals, like mercury and boron, during desalination to produce clean water, while at the same time potentially capturing valuable metals such as gold.
The new technique, which can easily be added to current membrane-based electrodialysis desalination processes, removes nearly 100% of these toxic metals, producing a pure brine along with pure water and isolating the valuable metals for later use or disposal. Desalination or water treatment facilities typically require a long series of high-cost pre- and post-treatment systems that the water must go through. The new process enables several of the steps to be done at once.
The chemists synthesized flexible polymer membranes, like those currently used in membrane separation processes, but embedded nanoparticles that can be tuned to absorb specific metal ions — gold or uranium ions, for example. The membrane can incorporate a single type of tuned nanoparticle if the metal is to be recovered, or several different types, each tuned to absorb a different metal or ionic compound if multiple contaminants need to be removed in one step. The polymer membrane laced with nano-particles is very stable in water and at high heat, which is not true of many other types of absorbers including most metal-organic frameworks (MOFs) when embedded in membranes.
The researchers hope to tune the nano-particles to remove other types of toxic chemicals including a common groundwater contaminant — polyfluoroalkyl (PFA) substances — found in plastics. The new process, called ion-capture electrodialysis, also could potentially remove radioactive isotopes from nuclear power plant effluent. The polymer membranes are highly effective when incorporated into membrane-based electrodialysis systems — where an electric voltage drives ions through the membrane to remove salt and metals — and diffusion dialysis, which is used primarily in chemical processing.
While reverse osmosis and electrodialysis work well for removing salt from high-salinity water sources, such as seawater, the concentrated brine left behind can have high levels of metals including cadmium, chromium, mercury, lead, copper, zinc, gold, and uranium.
Calculations suggest that a kilogram of the polymer membrane could strip essentially all of the mercury from 35,000 liters of water containing 5 parts per million (ppm) of the metal before requiring regeneration of the membrane. The membranes can be reused many times — at least ten but likely more — without losing their ability to absorb ionic metals. And membranes containing PAFs tuned to absorb metals easily release their absorbed metals for capture and reuse.