Silica nanoparticles flow in (and out of) waste
STEPHEN KILL
late on outcomes at the moment easy to track with small-angle neuAs nanomaterials continue to enwith their particular nanoparticles. tron scattering. This well-estabter the market embedded in fabEither way, the authors say, these lished detection technique was rics, medicines, and more, results hint at possible managerecently introduced to environmenresearchers are watching where ment solutions, such as modifying tal nanocolloid research because of these particles might surface. One the step in which the solids in its aptness for quantifying the variplace they will pop up is in the wastewater are allowed to settle out ous characteristics of nanoparticles waste stream, washing out in to optimize the removal of in liquids. laundry, flowing down the drain nanoparticles. along with cosmetics, and Bernd Nowack of the coming from other domesSwiss Federal Laboratories tic uses. Researchers pubfor Materials Testing and lishing in ES&T (2009, DOI Research (Empa) comments 10.1021/es901399q) examthat the team used very ined silica-shelled nanoparhigh starting concentrations ticles, and their preliminary of nanoparticles, more than results show that these 2000 milligrams per liter. In might pass through some “real-world conditions,” stages of traditional wasteNowack says, concentrawater treatment, depending tions are not likely to exon their outer coatings. ceed more than a few The laboratory-based exmicrograms per liter of periments highlight how Jarvie and colleagues tested the behavior of silica nanonanoparticles in wastewa“changes in surface chemis- particles during wastewater treatment. ter; this has been shown in try will have an important The researchers added their previous modeling of other nanoeffect on where these materials go particlessaround 56 nanometers materials such as titanium dioxin nature,” comments Mark in diametersto both raw and ide. But one possible scenario in Wiesner, an environmental engilightly filtered wastewater from a which such high concentrations neer at Duke University and dilocal utility serving communities of nanomaterials might be found rector of the Center for the in south central England. They deis an industrial accident or some Environmental Implications of termined that all of these particles kind of spill, Wiesner comments. NanoTechnology. Wiesner, who stayed suspended in the waste efThe researchers say that they says the new work is interesting fluent for several hours. However, needed such high concentrations colloidal chemistry, also notes after introducing a commercial surfor adequate detection levels at that the team has adopted trackfactant to coat or “functionalize” timescales that simulated the resiing methods for tracing nanomathe silica nanoparticles, the team dence time of wastewater during terials quantitatively that will be found that the nanoparticles settled treatment. “We wanted to be able of future use for environmental out alongside particles of waste to reproduce the behavior of nanoresearch. But he and other outwithin seconds. particles over that critical time peside experts say that much work The researchers hypothesize riod,” Jarvie says. King adds that remains to elucidate what might that the coated nanoparticles will the small-angle neutron scattering happen to these and other nanonot pass through to the next stage detection method is sensitive particles in the real world. of the waste-treatment process if enough to show that there were no The experiments were jointly they are allowed to settle out. The interactions among the nanopartiled by Helen Jarvie of the U.K.’s “naked” nanoparticles, however, cles themselves or dissolution of Natural Environment Research seem to remain in sewage efflutheir silica shells; he says this Council’s Centre for Ecology and ent, and the team has plans to means that the nanoparticles used Hydrology and Stephen King of test what will happen once the by the team would have behaved the U.K. government’s Science materials go to the next level of like commercial silica and Technology Facilities Council, treatment, which usually involves nanoparticles. along with researchers from King’s microbes. Wiesner comments that the College London and the University Jarvie notes that previous restructure of the nanoparticles of Oxford (U.K.). The team used search has shown the effects of used in the team’s experiments lab-synthesized silica-shelled nanonanoparticles on microbes, but she may not accurately represent particles with iron-oxide cores; the says that it is impossible to specuthose that would be used in comiron centers made the particles 8708 9 ENVIRONMENTAL SCIENCE & TECHNOLOGY / December 1, 2009
10.1021/es9031024
2009 American Chemical Society
Published on Web 11/12/2009
mercial products or pure silica nanoparticles. “Having said that,” Wiesner continues, “the thinness of the [silica] shell is of interest [for] the possible effects it might have.” Thin gold shells around quantum dots, for example, are used in particles for tumor treatments, and the surface interactions with the gold govern the behavior of the nanoparticle. If, for example, the core is suddenly exposed, the particle would behave very differently.
How these particles would fare in wastewater is a question for further exploration, Wiesner says: “It’s hard to say what would happen in a full-scale waste-treatment process.” Add bacterial growth, increased nanoparticlebacteria interactions, and the generation of material by microbes during secondary treatment, and those microbes are “likely to have an enormous impact on the material,” he says.
The true contribution of the new work may be advancing the use of small-angle neutron scattering methods that are capable of measuring nanomaterials in complex matrices such as sewage effluent. If the team has made quantitative measurements of their test particles in wastewater, recording exactly what they put into the complex matrix, Wiesner says, “that would be a significant accomplishment.” —NAOMI LUBICK
December 1, 2009 / ENVIRONMENTAL SCIENCE & TECHNOLOGY 9 8709
