By Eileen Thorsos
As Jeff mentioned in our last post, using nanoparticles to clean up contaminated sites more quickly and cheaply than conventional methods shows a lot of promise. In fact, one of the earliest such nanoremediation projects was carried out about 10 years ago at a pharmaceutical site here in the Research Triangle Park (NC), using nano zero-valent iron to break down contaminants that contain chlorine – specifically di-, tri-, and tetrachloroethylene. (For the interested: A technical description of that project and a global map of sites already using nanotech cleanup.)
These nano techniques still have kinks and questions to work out, though, which Duke SRC’s Project 4 is investigating for two types of nanoparticles: nano titanium dioxide (TiO2) and nano zero-valent iron (nZVI).
So, how do nanomaterials help decontaminate pollutants?
Nano-sized particles are very reactive.
Nanoparticles are small – really small. One nanometer is the width of just two to six atoms, and nanoparticles are about one to 100 nanometers across. (Because of their size, some nanomaterials experience unusual changes in their chemical and physical properties.)
Super tiny particles have more surface area for their weight than large particles, giving them more contact with things they touch and more opportunities to interact physically and chemically. Imagine you take a round rock, a cup of sand, and a cup of clay powder, all dry and the same weight, and pour water over each. The rock is solid, and the water runs off the outside surface. The sand grains have more surface area but are big enough to feel gritty in your hand, and water drains straight through. The clay is so fine (though often not quite in nano dimensions) that you can’t feel one particle from the next, and the tiny gaps in the clay soak up the water, holding the clay together. Similarly, the monumentally high surface area of nano zero-valent iron and titanium dioxide seems to drive their nano properties – in this case, much greater chemical reactivity.
Cleaning up contaminated sites with nanomaterials.
At a polluted site, contaminants can be found anywhere, including the soil surface, deep soil, groundwater, stream or river sediment, and surface water such as streams, rivers, or lakes. The risks to human health are the greatest when people come into contact with a contaminated part of the site, such as by playing in the soil or drinking the water.
Different pollutants and contaminated materials require different strategies for cleanup,
and ToxInsider will cover this topic in January. For now, know that remediation with nanoparticles must bring the contaminant and the nanoparticles together to react – for example, by pumping up contaminated groundwater or by injecting nanoparticles underground. And, underground, tiny nanoparticles can work their way into ultra-small crevices, reaching target materials that bigger particles pass right by.
Meet the actors.
Zero-valent iron (ZVI): At both macro and nano scales, ZVI provides electrons for chemical reactions that break apart contaminants like chlorpyrifos and PBDEs – for example, by removing halogens like chlorine or bromine. Remediators will often bury macro ZVI in the flow path of contaminated groundwater, which is labor-intensive. Nanoparticles, however, can be injected into the water plume underground with very little digging.
Titanium dioxide (TiO2): In sunlight and other sources of UV light, TiO2 makes hydroxyl radicals – small, reactive molecules of the type that antioxidant foods help neutralize in our bodies. Outside of our bodies, these hydroxyl radicals can break apart organic pollutants like PBDEs, PAHs, and pesticides. Because this process requires UV light, nano TiO2 can’t be injected underground, but it shows promise for cleanup at the soil surface.
Real world challenges with nanoremediation:
- Nanomaterials are so reactive that they may react with other materials before reaching the target contaminant. Coatings may prevent such neutralization but then also not prevent them from reacting with target contaminants.
- Nanoparticles might also chemically react with beneficial micro-organisms that independently help break down pollutants. Do nanoparticles harm these microbial helpers?
- Because of their reactivity, nanomaterials are unlikely to travel far. But, if they disperse off the remediation site, their general reactivity may be risky for human health or the environment.
- And, sometimes breaking down pollutants can produce compounds that are more toxic than the original contaminant – while we want remediation to leave a site safer.
Investigating these possible kinks and designing ways to clean up contaminants safely is a large part of Duke SRC’s nano research.