Researchers of the University of California – Santa Cruz (US) recently published promising results on the development of a new material for water purification; potentially, it could be used to treat certain nuclear wastes. This new compound is based on the principle of ion exchange. Here is an explanation of the ion exchange mechanism, how this is already used to clean wastewaters, and about the novelties of this material.
The contamination of waters with dangerous pollutants is a very serious problem, affecting both the environment and human health. Pollutants can come from many different sources, such as domestic, agricultural or industrial waste.
Before releasing the waters into the environment or using them in domestic activities, a treatment is necessary to remove the hazardous compounds from them. The treatment(s), however, will have to be different, depending on the nature of the pollutants. In some cases, these molecules are converted into less dangerous ones through chemical reactions; in the majority of cases, however, they are adsorbed into an appropriate solid material, with an ion exchange process.
In this process, an ion-containing liquid is placed in contact with a porous solid material, which also contains ions with similar characteristics. With the contact, there is an exchange between the ions of the liquid and the solid.
During the exchange process, the solid material acts as filter, by adsorbing the ions from the liquid. Toxic ions or ions present in very high concentration are thus removed from water samples and they will be replaced by less toxic species from the solid material.
The ion exchange process is used for many applications in different fields. Considering the purification of liquids, one of the most common uses is the removal of calcium (Ca2+) or magnesium (Mg2+) to obtain “soft water”. In this case, these ions are replaced with other ones, such as sodium (Na+) or hydrogen (H+). The compounds used to make the filters in water softeners are generally polymers; the bulk of the material, which acts as a substrate, has a negative charge and it is bonded very weakly to positive-charged ions.
Heavy metals in industrial wastewaters
Industrial waste water from different process can be contaminated with heavy metals such as lead (Pb2+), cadmium (Cd2+), zinc (Zn2+), arsenic (As3+), mercury (Hg2+), chromium (Cr3+), etc. A detailed report was published by the European Commission on the presence of these metals in the environment, their sources, and their effect on both human health and the whole ecosystem.
Ion exchange-based resins are commercially available to remove these metals from the waters and to lessen their negative impact on the environment.
Ion exchange materials for negative ions
The ion exchange principle can also be applied to negative ions; in this case, the bulk of the material will have a positive charge with weak bonds to negative ions such as OH– or CO32-. They could be used to remove anionic pollutants from waters, for instance perchlorate (ClO4–), chromate (CrO42-) or pertechnetate (TcO4–).
This last ion is particularly dangerous due to its radioactivity. Technetium (Tc) is a nuclear element; it is a by-product that gets formed in nuclear reactors. As it is not stable as an element, it tends to react with the oxygen in the surroundings to form the pertechnetate ion. Therefore, the removal of this ion from nuclear waste is a very important issue for the environment.
Materials for negative ion exchange and removal, however, are not yet in common use as their performances need to be improved. The degree of ion removal is not very high, and furthermore the compounds are not very stable; therefore they are not suitable for industrial processes.
Latest developments in California
Interesting results in this field were recently achieved at the University of California – Santa Cruz (US). Professor Scott Oliver and his co-worker Hongan Fei developed SLUG26, a new material for anion removal; it is based on the mixture of two different compounds, copper hydroxide and ethanedisulfonate ([Cu4(OH)6] [O3SCH2CH2SO3]·2H2O). Copper hydroxide is the substrate, with a positive double charge; the ethanedisulfonate, on the other hand, is the anion which can be exchanged.
The performance of SLUG26 was tested for the removal of the permanganate ion (MnO4-): this was chosen because it has the same structure as the pertechnetate ion. Therefore, it is reasonable to assume that the behavior of the material towards the two species will be comparable.
Professor Oliver said, “We compared SLUG26 to hydrocalcite, a commercial anion exchange material; we were very pleased to see that SLUG26 was much more efficient at permanganate removal – up to 5 times better. Furthermore, SLUG26 showed good thermal and pH stability. The next stage for us is to see whether the material works or not with the pertechnetate ion, and to check its stability in a radioactive environment.”
The results obtained by Professor Oliver and his co-worker were published in “Angewandte Chemie”.
European Commission. “Heavy Metals in Waste. Final Report.” Accessed September 2011.
H. Fei, S. Oliver. “Copper Hydroxide Ethanedisulfonate: A Cationic Inorganic Layered Material for High-Capacity Anion Exchange.” Angew. Chem Int. Ed., 50, 1, 2011.
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