New “Super Sand” Makes for a Better Water Filter
"Super sand" can deal with at least five times the concentration of heavy metals as pure sand, and it could be relatively cheap to produce.
As a filtration device, plain old sand can be magical. Thousands of years back, people realized that running water through filters of tightly packed sand and gravel could rid it of filth and bacteria and make it safer to drink. Across the world, large-scale municipal water filtration systems depend on sand filters; anyone who’s ever been curious enough about the inner workings of a Brita filter to pry those capsules open know that those ubiquitous pitchers work on much the same principle.
The “magic sand” that Wei Gao, a graduate student at Rice University, has been showing off does everything that regular sand can, only better. Sand alone cannot capture every bit of nastiness from water that flows through it, and it can quickly become saturated with captured particles. This sand (it’s also been dubbed “super sand”) can deal with at least five times the concentration of heavy metals or organic dye that pure sand can, according to a new study, on which Gao is the lead author. She and other researchers ran mercury through samples of pure sand and of magic sand and found that the pure sand was saturated within 10 minutes. The magic sand took more than 50 minutes to reach its saturation point and produced water with mercury levels that could have doubled and still been below the EPA’s maximum standard level for safe drinking water.
What makes the sand “magic” is a coating of graphite oxide nanoparticles that arrange themselves around each grain of sand. The samples that the researchers used turned from almost clear, yellowy crystals into dark gray seeds that look like poppy seeds. (The study does note that the graphite oxide might not cover each grain in its entirety, but that likely it coats the majority of each granule.)
One of the reasons that sand works so well as a filtration device is that all those tiny grains collectively have an enormous surface area. The nanoparticle coating creates an even more extensive surface area, enhancing the sand’s ability to capture unwanted particles.
This isn't the first time scientists have improved upon the filtration properties of run-of-the-mill sand. But the most commonly used alternative is a system of activated carbon granules, which have an electric charge to them that enhances their particle-capturing abilities. These granules are actually what comes in those Brita filter capsules, for instance, and they’re relatively expensive. A three-pack of Brita filters, for instance, can cost almost $45. But the process for creating coated sand is simple enough that the researchers who created it expect that filters using this technology could cost much less—making basic water filtration more readily available to people living in lower-income countries, where it’s more needed. They wouldn’t be sacrificing quality, either: Gao and her colleagues found that their coated sand did as good a job as filtering out dyes and heavy metals as activated carbon. In the future, it could do an even better job at targeting dangerous particles: The next step for the super-sand creators is to create coatings that could specialize in capturing particular contaminants, like arsenic.
Photo courtesy of flickr user Ben+Sam