Arsenic really isn’t very good for you, in fact, it’s a perennial favourite of your amateur – or indeed, professional – poisoner. Even if your loved ones are not surreptitiously sprinkling it on your cornflakes, you may still be ingesting a little bit too much of the stuff. Vicki Colvin from Rice University gave a talk this afternoon addressing the problem of arsenic contamination in drinking water – not only is this a problem in poorer third-world nations, but a map of the North Eastern USA was covered with an alarming number of large red dots. Red dots are never a good thing.
Arsenic finds its way into the water supply from both natural (it’s in the soil) and anthropogenic sources – and has been linked with bladder cancer. Although some ferns naturally sequester arsenic, it’s a slow process and not everyone grows them in their back garden. It turns out, however, that iron oxide is good at binding arsenic – an alloy structure forms on the surface, which continues to irreversibly adsorb arsenic in a multilayer fashion. Colvin exploits this phenomenon for the removal of arsenic compounds from water by using iron oxide nanoparticles.
Why go nano? Well, there are two reasons: (i) smaller particles have a higher surface area-to-volume ratio, i.e., you can bind more arsenic, and (ii) the magnetic properties of these nanoparticles are quite handy – you suck up the arsenic and then drag the whole nasty mess out of solution with a magnet. There is, however, a Goldilocks effect: if the nanoparticles are too small you need a fancy bad-science-fiction-movie super magnet to get the job done, if the nanoparticles are too large, they become magnetized and can’t be pulled away from the magnet even after it is turned off. 12 nm is just right.
The most intriguing part came near the end of the presentation, when Colvin suggested that there could be an alternative and renewable route for making the iron oxide nanoparticles. Bugs! Certain bacteria produce iron oxide nanoparticles with a very narrow size distribution and could, in theory, be used in a biosynthetic process. Ultimately, this research could result in an inexpensive point-of-use product for the removal of arsenic from water – I’ll drink to that!
Stuart Cantrill (Associate Editor, Nature Nanotechnology)