Drinking water containing arsenic is linked to serious health issues such as cancer and diabetes. Unfortunately, millions of people around the world are at risk of water-related arsenic poisoning. While it is relatively easy to remove arsenic from otherwise pure water, natural water sources contain other competing anions, negatively charged molecules. Yale chemists Lauren Pincus and Predrag Petrović investigated the ability of various transition metal chitosan complexes (TMCs) to selectively adsorb—stick to—arsenate and arsenite over phosphate, their biggest competitor.
TMCs are complexes of metal bound to chitosan, a waste product of the shellfish industry. Previous research demonstrated that copper(II)-chitosan selectively adsorbs arsenate, but not arsenite. This study found that iron(III)-chitosan had the highest rates of selectively adsorbing arsenate and arsenite, reducing concentrations below World Health Organization benchmarks.
The key is the TMC’s molecular structure: iron(III) and chitosan formed a less rigid bidentate complex—two bonds holding iron, as opposed to monodentate—than other studied transition metals. X-ray spectroscopy data showed that this diffusion of iron’s positive charge allowed iron(III)-chitosan to bind more tightly to negatively charged arsenic, whose electron cloud is more easily distorted than phosphorus’s.
The researchers’ findings address the global need for efficient water purification systems. “There aren’t many adsorbents that are selective for arsenic over phosphate,” Pincus said. Importantly, the scientists are furthering the incorporation of computational techniques in green chemistry, helping save time and resources. “We can do a lot of good things by combining computational and experimental work,” Petrović said. Their research advances an environment-friendly approach while solving environmental problems.
Pincus, L. N., Petrović, P. V., Gonzalez, I. S., Stavitski, E., Fishman, Z. S., Rudel, H. E., … Zimmerman, J. B. (2021).
Selective adsorption of arsenic over phosphate by transition metal cross-linked chitosan. Chemical Engineering Journal, 412, 128582. https://doi.org/10.1016/j.cej.2021.128582