Contaminants that are small, negatively charged anions are not easy to remove from water sources. But now researchers have come up with a method to do just that: Theyve created a hydrogel that can slurp up anions and then be plucked out of the water, rinsed, and reused (J. Am. Chem. Soc. 2018, DOI: 10.1021/jacs.7b13656). Unwanted anions in water cause all sorts of trouble. For example, millions of people worldwide drink well water with dangerously high levels of arsenate, fluoride, or nitrate. Surplus nitrates and phosphates in agricultural runoff lead to overgrowth of algae in ponds and streams. And sulfates hamper the safe storage of radioactive waste. Some of the problems are mind-bogglingly huge, says Arup Sengupta of Lehigh University, who was not involved in the study. Yet no universal solution exists for anion removal. Ion exchange, widely used in water treatment plants and portable water filters, doesnt target individual anions. Liquid-liquid extraction, although very effective in the lab, uses organic solvents, so it cant be applied to drinking water. Jonathan L. Sesslers group at the University of Texas, Austin, has been working for decades to make a kind of sponge that mops up anions and then gets lifted out of the water, offering an easy way to extract the contaminants. He recently collaborated with supramolecular polymer expert Feihe Huang of Zhejiang University and Lingliang Long of Jiangsu University to finally get it right. They synthesised a flexible macrocyclic compound bearing four positive charges to act as an anion receptor. When it encounters anions in water, the compound cradles them inside its positively charged centre. The scientists inserted the macrocycle as a cross-linker in a polyvinyl alcohol polymer network to form a hydrogel. What they ended up with was a hydrogel with integrity, Sessler says. Its more robust than tofu and can be handled with fingers or tweezers. Importantly, picking up anions doesnt disrupt the gel, but strengthens it. The scientists dropped 13 mg of the dried form of the hydrogel into separate 100 µM solutions of nitrate, nitrite, sulfate, hydrogen sulfate, hydrogen sulfide, fluoride, and four negatively charged organic dyes (10 mL each). After 48 hours, the gel captured over 90% of the four organic dyes. Of the harder-to-capture smaller anions, it retrieved a high of 68% of the nitrate and a low of 47% of the fluoride. The researchers found that the gel is reusable; a soak in dilute HCl releases the anions. Hydrogels as purifiers are worth pursuing further, Sengupta says. Yet the greatest challenge, he adds, is to remove hazardous anions at very low concentrations in the presence of other benign anions at much higher concentrations. This particular macrocycle likely couldnt do that yet. Making it more selective is the goal moving forward, I believe. Indeed, Sessler says he and his colleagues are working on tailoring anion receptors to meet different needs, even incorporating multiple receptors into the hydrogel. This particular receptor, he thinks, could be groomed to remove sulfate from nuclear waste. The general strategy, though, could be adapted to many water-based systems. He hopes to inspire others to join our party of creating tailored hydrogels for water purification.
Chemical & Engineering News, 15 March 2015 ; http://pubs.acs.org/cen/news