Nuclear mattersPac-Man-like molecule chews up uranium contamination

Published 18 January 2008

Uranium leaches into groundwater from natural deposits of its ore, depleted uranium munitions, nuclear facilities, and the detritus of uranium mining; removing uranium from groundwater is very difficult: Not only does uranium bind very strongly to oxygen — it is also soluble, making dissolved uranium virtually impossible to remove; British scientists find an innovative solution

A molecule which can bite a uranium-containing ion between its (the molecule’s) “jaws,” not unlike the munching blob in the arcade game Pac-Man, could one day lead to a way to clean up groundwater contaminated with the toxic metal. Uranium leaches into groundwater from natural deposits of its ore, depleted uranium munitions, nuclear facilities, and the detritus of uranium mining. It occurs most commonly in the form of the water-soluble uranyl ion, (UO2)2+, in which the uranium atom is linked to two oxygen atoms by double bonds. Allowing uranyl to react with other substances may change it into a different, insoluble ion, which can be filtered out. There is a problem to overcome here, though: Uranium binds very strongly to oxygen — the bonds it forms are 25 per cent stronger than typical double bonds — making the uranyl ion very stable. Combined with its solubility, this makes dissolved uranium virtually impossible to remove. “It’s a very problematic, persistent groundwater contaminant,” Polly Arnold, a chemist at the University of Edinburgh, told the New Scientist’s Paul Marks.

Which brings us to Pac-Man. Arnold’s colleague Jason Love had been working on improving catalysts for fuel cells using a large organic molecule known as a macrocycle, which can fold in half to form a structure like a pair of jaws. Love was using the gap between the jaws to capture a pair of cobalt ions, but Arnold realized that it was just the right size and shape to clamp onto a uranyl ion. When she added the macrocycle molecule to uranyl ions dissolved in an organic solvent, she found that it did indeed capture them in its jaws, leaving one oxygen atom protruding (see the fascinating Nature’s article listed below). What is more, a silicon-containing compound present in the mixture was able to bind to the protruding oxygen atom, a sign that the uranyl’s stubborn bonds with oxygen had been weakened. Now, because the macrocycle is destroyed by water, it cannot be used to remove uranium from contaminated water, but Arnold’s team believes their demonstration that the uranyl ion’s bonds can be loosened is a first step toward finding substances that can transform dissolved uranyl into an insoluble compound. “No one has been able to do this before,” says Arnold. “We might now be able to develop and suggest new pathways for uranium removal from solution.”

Robin Taylor, a chemist with Nexia Solutions, the research arm of British Nuclear Fuels in Sellafield in the United Kingdom (see article elsewhere in the issue about a Nexia Solution’s newly developed nuclear mapping device), says the findings will help develop better processes for uranium sequestration and boost confidence in dealing with nuclear materials and waste. The Edinburgh team will also investigate how some bacteria and iron-rich minerals reduce uranium concentrations naturally in contaminated water, and whether the macrocycle is able to loosen bonds in ions containing plutonium.

-read more in Polly L. Arnold et al., “Reduction and Selective Oxo Group Silylation of the Uranyl Dication,” Nature 451 (17 January 2008) (doi:10.1038/nature06467): 315-17 (sub. req.)