Nanoparticles are being used in more and more consumer products. They can make materials stronger and lighter and make surfaces cleaner and more wear-resistant. They are increasing the performance of road vehicles, and are increasingly used in medicine to increase the efficacy of medication.
As these products reach their end of life, it will be very difficult to prevent the release of nanoparticles into the environment. As the size of these particles allows them to interact strongly with biological structures, there are all sort of potential human and environmental health issues associated with the build up of nano-waste in ecosystems.
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Figure 1. Nanoparticles can interact directly with biological matter, potentially causing long-term health issues which we are yet to fully understand. Image Credits: Argonne National Laboratory.
Studying Nanoparticles in the Environment
Studies of real nanoparticle concentrations in the environment are few and far between. A Swiss study published in 2010 attempted to model the concentrations of some common types of nanoparticles which end up in the environment. Their results agreed with the limited real-world data which is available.
The study found that the destination of the nanoparticles depends on their usage - for example, zinc oxide nanoparticles accumulates in sewage sludge, which is often spread on fields as a fertilizer, but carbon nanotubes are more likely to enter the waste stream from the manufacturing process, eventually ending up in landfill.
Due to the difficulties in detecting nanoparticles in the environment directly, modelling studies like this will be very useful in coming years, to identify problem areas where large amounts of nanoparticles are entering a particular natural system. To make this type of modelling process more accurate and useful, more data is required on the exact properties of the nanoparticles which are used in different products.
It would also be very useful to correlate nanoparticle concentration data with more detailed toxicity data on nanoparticles. The data which exists is mostly from lab-based studies on small samples, which use unrealistically high doses to force a response in a short time frame. Our understanding of the effects of long-term exposure to nanoparticles is very limited. This is a great concern, and many research groups around the world are working to better understand how ubiquitous low concentrations of nanoparticles will affect humans and ecosystems - particularly aquatic ecosystems.
Recovering Nanoparticles From Products
Given our limited understanding of how nanoparticles behave when released into the environment, most advice from expert committees suggests that manufacturers and retailers of nano-enhanced products should build plans for recovering and recycling the nanomaterials into the product lifecycle. Ideally, products should be designed so that nanoparticles can be separated out and re-used as easily as possible.
As of yet, there is no specific legistation to control the release of nanoparticles into the environment. Many conventional waste treatment processes (incineration, scrubbing/filtering etc.) do not decompose nanoparticles, allowing their direct release into the environment. This means that new methods must be developed for recovering nanoparticles from products at the end of their life.
Recovering Nanoparticles from the Environment
Even with recoverability built into nano-products, it is inevitable that some nano-waste will end up in waste streams. In recent years, researchers have proposed a number of ways to remove these nanoparticles, which are incredibly difficult to filter out by conventional methods due to their small size, from the waste before it is released into the environment.
Nanoparticle Recovery Using a Microemulsion
In 2010, researchers from Bristol University published work on the separation of cadmium and zinc nanoparticles using a special solvent. The solvent is a stable microemulsion of oil in water, which breaks down into two layers when heated. All of the nanoparticles in the solution end up in one of the layers, allowing simple separation.
Nanoparticle Recovery by Cloud Point Extraction
A similar process was reported in 2011, by a research team from Pakistan. They used a technique called cloud point extraction (CPE) to separate gold and palladium nanoparticles from an aqueous solution. The cloud point of an emulsion is the point when the two phases are on the verge between mixing fully and forming two layers, causing "clouding" of the solution. In their method, the nanoparticle solution is heated to this point, then centrifuged to fully separate the layers, extracting the nanoparticles.
Recovery and re-use of these expensive materials could speed up commercial application of nanotechnology to a variety of fields, by alleviating worries about nano-waste, whilst also decreasing production costs.
Sources and Further Reading