Yale Chemists Use Seafood Waste to
Remove Arsenic from Ground Water
It was already well-known that arsenic-tainted water can be treated by adding tiny particles of titanium dioxide. The particles are photo-oxidants that will, if exposed to ultraviolet light, tightly bind molecules of arsenic to their surfaces, simultaneously converting the more-toxic form of the poison, arsenic(III), into less-toxic arsenic(V).
The good news, according to Zimmerman, is that the process “takes something toxic and hard to remove and turns it into something less toxic and easier to remove.” The bad news, at least in the context of the developing world, is that once treated, the cloudy water normally needs to be mechanically pumped through filters to remove the miniscule, contaminated particles, adding expense and complexity and demanding large amounts of energy.
Miller and Zimmerman’s new version of the process instead embeds nanosized particles of titanium dioxide in beads of chitosan, the biopolymer that gives crab, shrimp and other crustacean shells their strength. Rather than particles the size of finely sifted flour, the new process yields nuggets of “about the size of Grape-Nuts,” says Miller.
The much-heavier beads settle at the bottom of a container after the embedded titanium dioxide binds up the poison. “You can literally pour clean drinking water off the top,” says Zimmerman.
So far, the researchers have used simple glass containers to expose the beads to the ultraviolet in ordinary sunlight to prove the concept, but they foresee somewhat larger systems, perhaps long plastic or glass tubes exposed to the sun on a rooftop, to provide more-centralized treatment at a village or neighborhood level.
Miller says she found that it was possible to use chitosan after some trial and error in the lab with other compounds. She discovered that she could dissolve small flakes of chitosan in a weak acid solution and simply stir in the titanium dioxide, leaving a substance that looks like white glue. The new substance is then extruded through a needle into beads and dried.
Part of the advantage of using chitosan as a medium is the sheer abundance of its source and the appeal of transforming this seafood industry waste into a valuable resource. “Go to the shore somewhere, like in Maryland,” says Zimmerman. “You can find piles of it. People will pay to have it hauled away.”
But there’s another sustainability advantage. The titanium dioxide-impregnated chitosan beads are themselves recyclable. After clean water is poured off, the beads can be exposed to a moderately alkaline solution—pH 9 to 10, or about the alkalinity of milk of magnesia. That will release the arsenic back into the solution, leaving uncontaminated beads still impregnated with titanium dioxide, for more arsenic removal.
“We still end up with a classic problem,” admits Zimmerman. “We’ve recovered the arsenic. Now what do we do with it?”
The answer to that question is part of the reason the project has won $230,000 in funding from the National Science Foundation (NSF), according to Bruce Hamilton, who leads the environmental sustainability program at the foundation’s engineering directorate.
He says the NSF was especially impressed that the Yale chemists intend to find safe ways to turn the arsenic itself into a resource, to “get arsenic out of the biosphere and lock it up somewhere in, for example, the technosphere,” says Hamilton, pointing out that sustainability advocates increasingly are looking for such “cradle-to-cradle” approaches.
Arsenic is already used in manufacturing, for instance, to make high-efficiency gallium arsenide microchips and solar photovoltaic cells. (Not that moving a waste into a usable product solves everything. Miller and Zimmerman both point to a growing worldwide “e-waste” problem. Electronics components can be recycled and remaining wastes disposed of securely, but that’s not yet being done consistently. It’s a sustainability problem beyond the scope of their project, but of concern nevertheless.)
Miller acknowledges that finding ways to make the waste arsenic a viable raw material for industry will be a challenge. “We face a big hurdle in turning an effluent into the kind of pristine material industry needs,” she says.
If it works, adds Zimmerman, “It could mean industries getting a feedstock that they need by paying for a clean-water project in the developing world.”