Chemists at the University of California at Berkeley have found a way to simplify the removal of toxic metals. like mercury and boron. during desalination to produce clean water, while potentially capturing precious metals, such as gold.
Desalination – the removal of salt – is just one step in the process of producing drinking water, or water for agriculture or industry, from the ocean or wastewater. Whether before or after salt removal, water often needs to be treated to remove boron, which is toxic to plants, and heavy metals like arsenic and mercury, which are toxic to humans. . Often times, the process leaves behind a toxic brine that can be difficult to remove.
The new technique, which can easily be added to current membrane electrodialysis desalination processes, removes almost 100% of these toxic metals, producing pure brine with pure water and isolating the precious metals for later use or storage. elimination.
“Desalination or water treatment plants typically require a long series of expensive pre and post-treatment systems that all the water has to pass through, one by one,” said Adam Uliana, a graduate student at the UC Berkeley who is the first author of an article describing the technology. “But here we have the option of doing several of these steps in one, which is a more efficient process. Basically you can implement it in existing configurations.”
Chemists at UC Berkeley have 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 are to be removed in one step.
The polymeric membrane laced with nanoparticles is very stable in water and at high temperatures, which is not the case with many other types of absorbers, including most organometallic reinforcement (MOF), when incorporated. in membranes.
Researchers hope to be able to fine-tune the nanoparticles to remove other types of toxic chemicals, including a common groundwater contaminant: PFAS, or polyfluoroalkyl substances, found in plastics. The new process, which they call ion-capture electrodialysis, could also remove radioactive isotopes from nuclear power plant effluents.
In their study, to be published this week in the journal Science, Uliana and senior author Jeffrey Long, professor of chemistry at UC Berkeley, demonstrate that polymer membranes are very effective when incorporated into membrane electrodialysis systems – where an electrical voltage drives ions through the membrane to remove salt and metals – and diffusion dialysis, which is mainly used in chemical processing.
“Electrodialysis is a known method of doing desalination, and here we are doing it in a way that incorporates these new particles into the membrane material and captures the targeted toxic ions or neutral solutes, like boron,” said said Long. “So while you are conducting ions through this membrane, you are also decontaminating the water for, for example, mercury. But these membranes can also be very selective in removing other metals, like copper and iron, at high capacity. “
Global water shortages require reuse of wastewater
Water shortages are becoming commonplace around the world, including California and the American West, exacerbated by climate change and population growth. Coastal communities are increasingly installing factories to desalinate seawater, but inland communities are also looking for ways to turn contaminated sources – groundwater, agricultural runoff and industrial wastes – into clean water. and safe for crops, homes and factories.
While reverse osmosis and electrodialysis work well to remove salt from high salinity water sources, such as seawater, the concentrated brine left behind can contain high levels of metals, including cadmium, chromium, mercury, lead, copper, zinc, gold and uranium.
But the ocean is increasingly polluted by industry and agricultural runoff, and inland sources even more.
“This would be especially useful for areas that have low levels of contaminants that are still toxic at those low levels, as well as for different wastewater sites that have a lot of types of toxic ions in their waterways,” Long said.
Most desalination processes remove salt – which largely exists as sodium and chlorine ions in water – using a reverse osmosis membrane, which allows water to pass through, but not ions, or an ion exchange polymer, which allows ions to pass through, but not water. The new technology simply adds porous nanoparticles, each about 200 nanometers in diameter, which capture specific ions while letting sodium, chlorine and other untargeted charged molecules pass through.
Long designs and studies porous materials that can be decorated with unique molecules that capture targeted compounds from liquid or gas streams: carbon dioxide from emissions from power plants, for example. The nanoparticles used in these polymer membranes are called porous aromatic frames, or PAFs, which are three-dimensional networks of carbon atoms linked by compounds made up of multiple ring-shaped molecules – chemical groups called aromatic compounds. The internal structure is related to that of a diamond, but with the bond between the carbon atoms lengthened by the aromatic linker to create a lot of internal space. Various molecules can be attached to aromatic linkers to capture specific chemicals.
To capture mercury, for example, sulfur compounds called thiols, known to bind tightly to mercury, are attached. Added methylated sulfur groups allow copper capture and oxygen and iron containing sulfur capture groups. The altered nanoparticles represent about 20% of the weight of the membrane, but, because they are very porous, represent about 45% of the volume.
Calculations suggest that one kilogram of the polymeric membrane could strip essentially all of the mercury in 35,000 liters of water containing 5 parts per million (ppm) of the metal, before requiring regeneration of the membrane.
Uliana has shown in her experiments that boric acid, a boron compound toxic to crops, can be removed by these membranes, but with diffusion dialysis which relies on a concentration gradient to drive away the chemical – which does not is not ionic, like metals – – through the membrane to be captured by the PAF nanoparticles.
“We have tried different types of high salinity water – for example, groundwater, industrial wastewater and also brackish water – and the method works for all of them,” he said. “It seems to be versatile for different water sources; that was one of the design principles we wanted to put into it.”
Uliana also demonstrated that membranes can be reused multiple times – at least 10, but probably 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.
“It’s a technology where, depending on your toxic impurities, you can customize the membrane to deal with that type of water,” Long added. “You may have issues with lead, say, in Michigan, or iron and arsenic in Bangladesh. So you target the membranes for specific contaminated water sources. These materials really drop it to levels often. immeasurable. “