Heat and Nanoparticles in Water

As nanoparticles become more commonly used in everyday products  it becomes increasingly important to understand " nanoparticle aggregation in the aqueous environment . . .  for assessing the fate, transport and toxicity of nanomaterials".  In an effort to increase the body of scientific knowledge in this area, Dongxu Zhou, Samuel W. Bennett, and Arturo A. Keller, all of the University of California Santa Barbara Bren School of Environmental Science and Management, in an article published on the PLOS One website "report for the first time . . . temperature variations can cause either agglomeration or disagglomeration . . . depending on the heating and cooling paths. This finding is very relevant . . . , since it indicates that ambient temperature change, constantly occurring in open waters, can alter nanoparticle mobility." Following  studies cited in the article's references, the authors define aggregates  as "particle clusters bound by irreversible chemical bonds", while agglomerates are "clusters" held together by weak physical interactions. " Once released in the environment, nanoparticles will very likely exist as agglomerated aggregates, i.e. aggregate clusters that have weaker bonds between them. "

In experiments on clusters of three types of metallic oxides - titanium dioxide, zinc oxide, and cerium oxide - lead the authors to conclude

. . . that in open water these soft (weakly bonded) agglomerates can be disagglomerated by common environmental stimuli, such as exposure to sunlight or an increase in temperature from diurnal variations. Although not evaluated, it is likely that mechanical shocks may also result in temporary disagglomeration. The released aggregates can be much more mobile and bioavailable while the stimuli is present. Although in our experimental setting we observe reagglomeration once the stimuli are removed, in the environment it may be that the probability of interacting with another nanoparticle aggregate is much lower. . . . The effect of disagglomeration on toxicity has not been considered, or systematically evaluated. This phenomenon warrants attention since it is likely that these metal oxide nanoparticles will experience these natural stimuli during their transport in the environment.

Danish EPA Releases Report on Environmental & Health Risks for Selected Nanoparticles

Denmark's Environmental Protection Agency (DEPA)  recently released "Survey on basic knowledge about exposure and potential environmental and health risks for selected nanoparticles". The survey was written by Sonja Hagen Mikkelsen, Erik Hansen and Trine Boe Christensen of COWI A/S, Anders Baun and Steffen Foss Hansen of DTU Environment and Mona-Lise Binderup of DTU Food, all working under contract with DEPA.

Noting that "There is no single source of information that provides an overview of nanomaterials and products in Denmark or in the EU for that matter",

DEPA has therefore initiated this project to provide an overview of the existing knowledge about seven of the most common nanomaterials, their environmental and health properties, the use of those nanomaterials and the possibility of exposure of humans and the environment.

The seven nanomaterials selected as the focus of the survey are

1 - Titanium dioxide

2 - Cerium dioxide

3- Fullerenes (aka carbon balls or 'buckeyballs")

4- Nanosilver

5- Zero-valent iron

6 - Silicium dioxide

7 - Nanoclay

These nanomaterials were selected based on

1- Application volums

2- Potential human exposure

3- Potential direct discharge into the environment

4 - Expected biological effect (human and/or environment), persistence or bioaccumultion

The authors of the survey developed "profiles" for the nanomaterials, focusing on " the general characteristics and manufacture of the nanomaterial, their current uses (mainly focused at consumer products) and hazard profiles (ecotoxicity and human toxicity) . . . . The profiles included sections discussing relevant exposures from consumer products and considerations regarding the related risk."

The first two chapters of the survey, "Introduction" and "Nanomaterials Survey" discuss the nature of the nanomaterials, their use in industries, general availability of products incorporating the nanomaterials and brief summaries of earlier studies. Chapters 3-9 focus on the individual nanomaterials selected for this survey, discussing the general characteristics of a specific nanomaterial, how it is manufactured, which consumer products available in Denmark, either via a website or a bricks and mortor shop, review of toxicity studies, possible scenarios where humans and the environment might be exposed to the nanomaterials, ranging from disposal of products containing nanomaterials in landfills to the use of nanomaterial ensconced cosmetics, such as sunscreens containing titanium dioxide, and brief discussions of "risk profiles" for the selected nanomaterials. Summary sheets are found at the end of the chapters.

Chapter 10, "Exposure and risk potential", raises a point that critics of this survey will note:

. . . the range of toxicological and ecotoxicological studies available is not sufficient to allow firm conclusions with regard to the toxicity of the nanoparticles compared to their bulk counterparts. . . (emphasis added)

As one might then expect, the authors of this survey come to the conclusion that, in order to answer the questions regarding nanomaterials and risk more information and research is needed.