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Hardy hydrogel cleans water

A gel acts as a sponge for anions, removing negatively charged water contaminants

by Louisa Dalton, special to C&EN
March 15, 2018

Photos of a clear cube of hydrogel and bright yellow cube of hydrogel sitting on round plastic bottle caps.
Credit: J. Am. Chem. Soc.
A hydrogel equipped with an anion receptor removes about half of the fluoride (left) after 48 hours and 90% of a bright organic dye (right) from 100 µM solutions of the anions.

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: They’ve 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).

Credit: Adapted from J. Am. Chem. Soc.
The positively charged macrocycle incorporated into a hydrogel binds anions, pulling them out of water.
Structure of a hydrogel consisting of various numbers of polyvinyl alcohol monomers cross-linked by an eight-ring macrocycle bearing four positive charges.
Credit: Adapted from J. Am. Chem. Soc.
The positively charged macrocycle incorporated into a hydrogel binds anions, pulling them out of water.

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, doesn’t target individual anions. Liquid-liquid extraction, although very effective in the lab, uses organic solvents, so it can’t be applied to drinking water.

Jonathan L. Sessler’s 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 synthesized 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 center. 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. “It’s more robust than tofu” and can be handled with fingers or tweezers. Importantly, picking up anions doesn’t 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 couldn’t 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.


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