In a crude sense of matters, nanotechnology is all about an extremely small dimension, one billionth of a meter, to be precise. At the nanoscale, the rules of the game are not the same as those we have been playing till now with respect to conventional chemicals and substances. On the one hand, nanotechnology has revolutionized advancements in numerous industries leading to large scale manufacture and consumerization of nanoforms. On the other hand, an increased use inevitably resulted in nanomaterials entering the environment via various routes as shown in the figure. Nanomaterials can be aerosolized during industrial practices, in wastewater treatment plants and even during natural processes, such as, volcanic eruption, leading to air pollution. Nanomaterials entering water, however, poses another concern as they can directly interact with aquatic life forms. What happens to nanomaterials in water? Do they dissolve generating ions? Perhaps, perhaps not. Are they mobile? Could be, could be not. Are they toxic? Possibly, possibly not.

In the late 2000s, nanotoxicology research opened a “black box”, which brought to light hitherto unknown effects of nanomaterials, some of which were detrimental to environmental and aquatic health, while others were largely benign. To completely assess nanomaterial behavior and toxicity in water, an array of information has to be evaluated simultaneously, such as,

characteristics of the nanoparticle (NP) itself: morphology, size, surface functionality, surface area, density etc., and not only chemical composition, and
– Water quality: nature, chemistry and concentration of dissolved and particulate organic matter (DOM and POM), composition and concentration of ions, pH, temperature, salinity etc.

A few examples may be helpful to put this information into perspective. For instance, in a typical freshwater environment, spherical Ag NPs behave differently compared to Ag nanorods. Titania NPs, Ag NPs, ZnO NPs and silica NPs, all of the same size and shape may exhibit different dispersion or aggregation dynamics in water. Changing the water matrix from freshwater to saline or brackish water also affects nanomaterial behavior. Although this behaviour of nanomaterials may appear frustrating from a regulatory perspective, these varied responses oddly and largely stem from a single fundamental quality of the nanomaterial: dispersion stability. In water, NPs can interact with themselves (homoagglomeration), with DOM or POM in water (heteroagglomeration), with anions and cations in water (electrostatic stabilization or destabilization depending on NP surface charge) or, they can dissolve, resulting in ions leaching into water. Quantifying dispersion stability is key information to adequately evaluate mobility, settling (on to soil or sludge), and bioavailability and/or bioaccumulation (toxicity), finally enabling robust hazard and risk assessment of nanomaterials in water.

The new OECD guideline No. 318 , is the first standardized test guideline (TG) to quantify the dispersion stability of nanomaterials in aquatic environment. In a typical test system, a freshwater environment is simulated, with DOM, electrolyte and pH conditions that are observed in 95% of freshwater sources, and the nanomaterial dispersion stability is studied over time. To compensate for differences arising from specific physicochemical characteristics of nanomaterials, the guideline offers a simple solution: consider nanomaterial “number concentration”, rather than the typically used “mass concentration”. Number concentration determination requires environmentally relevant physicochemical data defining the nanomaterial, such as, particle shape, size, surface area and density, while mass concentration determination only requires the chemical formula of the substance. This approach ensured the inclusion of nanomaterial characteristics and water quality parameters within a single umbrella of testing protocol. Results from this test primarily classifies nanomaterial as highly stable, least stable and intermediately stable in the aquatic environment, which can be further used for risk assessment.
We, at nEcoTox GmbH conduct nanoparticle dispersion stability analysis in accordance to OECD TG-318. The OECD GD-318 also provides information on quantifying dissolution of nanomaterials in water, which is also a service that we provide. We can also help you with characterization of your nanoforms, by providing relevant information required for REACH registration.
To learn more on how we conduct these tests at our facility, participate in the short but informative online webinar with Q&A, by Dr. Ricki Rosenfeldt on “Dispersion Stability of Nanomaterials – OECD TG-318” on 16th February 2021 at 10 am (CET). Register now here.

For more information, contact me by phone: 06346/9661490 or via email: menon@necotox.de