Indeed, the transformations of nanomaterials and how those transformations affect their potential for harm is an important topic for study.  Also, long term multigenerational impacts have not been assessed well.  This is a goal of CEINT and will be featured in forthcoming publications of Ben Colman and others.  Stay tuned.

Here are a few papers that begin to look at transformations of nanomaterials and how those transformations affect behavior and toxicity.

1. Lowry, Gregory V., Benjamin P. Espinasse, Appala Raju Badireddy, Curtis Richardson, Brian Reinsch, Lee Bryant, Audrey Bone, Amrika Deonarine, Soryong Chae, Mathieu Therezien, Melanie Auffan, Ben Colman, Helen Hsu-Kim, Emily Bernhardt, Cole W. Matson, Mark Wiesner. (2012). Long-Term Transformation and Fate of Manufactured Ag Nanoparticles in a Simulated Large Scale Freshwater Emergent Wetland. Environ, Sci. Technol. 46 (13), pp 7027–7036.

2. Ma, Rui, John Stegemeier, Tiffany Yang, Clement Levard, Gordon E. Brown, Jr., Gregory V. Lowry. (2014) Sulfidation of copper oxide nanoparticles and properties of resulting copper sulfide. Environmental Science: Nano. 1, 347-357. 10.1039/C4EN00018H.

3. Colman, B., Espinasse, B., Richardson, C., Matson, C., Lowry, G. V., Hunt, D., Wiesner, M. R., Bernhardt, E. (2014). Emerging contaminant or an old toxin in disguise? Silver nanoparticle impacts on ecosystems Environmental Science & Technology 48 (9), 5229–5236. 10.1021/es405454v.

4. Ma, R., Levard, C., Judy, J. D., Unrine, J., Durenkamp, M., Martin, B., Jefferson, B., Lowry, G. V., (2014) Fate of zinc oxide and silver nanoparticles in a pilot waste water treatment plant and in processed biosolids. Environmental Science & Technology 48 (1) 104–112. DOI: 10.1021/es403646x

5. Clément Levard, Ernest M. Hotze, Benjamin P. Colman, Lisa Truong, X. Y. Yang, Audrey Bone, Gordon E. Brown, Jr., Robert L. Tanguay, Richard T. Di Giulio, Emily S. Bernhardt, Joel N. Meyer, Mark R. Wiesner, Gregory V. Lowry (2013). Sulfidation and Chlorination of Silver Nanoparticles: Natural antidotes to their toxicity Environmental Science & Technology. 47, 13440 doi.org/10.1021/es403527n.

6. Dale, A., Lowry, G. V., Casman, E. A (2013). Modeling Nanosilver Transformations in Freshwater Sediments. Environ. Sci. Technol. 47 (22) 12920. DOI: 10.1021/es402341t.

7. Ma, Rui, Levard, Clement, F. Marc Michel, Brown, Jr., Gordon E., Lowry, Gregory V. (2013). Sulfidation Mechanism for Zinc Oxide Nanoparticles and the Effect of Sulfidation on Their Solubility. Environ. Sci. Technol. 47 (6), 2527–2534. DOI: 10.1021/es3035347

8. Lowry, G. V., Gregory, K. B., Apte, S., Lead, J. R. (2012). Environmental Transformations of Nanomaterials. Environ. Sci. Technol. 46 (13) 6893−6899.

9. B.C. Reinsch, C. Levard, E. Li, R. Ma, A. Wise, K. B. Gregory, G.E. Brown, Jr., and G.V. Lowry. (2012). Sulfidation decreases Ag NP toxicity toward E. coli. Environ. Sci. Technol. 46 (13), pp 6992–7000.

10. Levard, C., Hotze, E., Lowry, G. V., Brown, Jr, G. E. (2012). Environmental Transformations of Silver Nanoparticles: Impact on Stability and Toxicity Environ. Sci. Technol. 46 (13) 6900–6914.

More aspects, eventually the whole NPs circulation in long term studies should be carefully evaluated. E.g. the NPs transport in soil/water, NPs interaction with plants (direct uptake, dissolution) translocation in plants (NPs and ions) and finally the NPs and/or ions influence on plants growth. 

Additional perspective: Long term transformations of NPs may be done in batch studies in isolation, just considering chemical changes. However, we know that in natural systems, where these long term transformations will occur, biota (micro and macro; flora and fauna) abound and will play some roles in transforming NPs. Our transformation studies should consider both chemically- and biologically-induced "metamorphosis" of NPs, in addition to see how the transformations affect biota.

Silver nanoparticles (AgNPs, nanosilver) are used as an antimicrobial agent in many

consumer products such as fiber coating, shoe odor control, detergents, hydrogels, and plastics. These nanoparticles are often released during washing and eventually enter wastewater treatment plants (WWTPs).silver nanoparticles would affect wastewater treatment and anaerobic digestion.nitrifying bacteria are especially susceptible to inhibition by silver nanoparticles.

That is indeed a very good suggestion, we may also learn a lot from naturally occurring nanomaterials

Thank you Gregory (and others), this is helpful. I've scanned the papers mentioned and a common theme is "Sulfidation of Ag-NPs results in a significant decrease in their toxicity due to the lower solubility of silver sulfide."  I wonder, after exposure, does the biota and  root growth return to pre-exposure levels or might there be lingering "scars"?

It seems to me that the shortcomings of most of these evaluations is to not consider whether effects are due to the particles or the ions released. As far as I know in cases like nanosilver the effect is due to the ions. Interesting point is the effect on soil microbiota. Of course this has to be expected but may have also have then an effect on the plants. As also seen in the links of the original post measurable effect on plant biomass production is more an exception than a rule.

It would be very important to consider that after aging in environment nanoparticles will be present as agglomerate and likely covered by an adapted biofilm, i.e. release of the active ions maybe considerably less as well as the chance to have an effect as nanoparticle.

Thanks Titus,

Yes we do need to study the effect of ions especially on soil biota as you say. I am not a botanist but I have come across work suggesting a symbiotic relationship of roots with soil microbiota such as

• http://www2.mcdaniel.edu/Biology/botf99/imagesfor%20questions/stemrtimagesf/mycorhiz.html and
• https://www.boundless.com/microbiology/textbooks/boundless-microbiology-textbook/microbial-ecology-16/microbial-symbioses-196/the-legume-root-nodule-symbiosis-993-7114/.

A focus on rhizobia makes sense to me. 

Uptake of nanoparticles (as nanoparticles and ions) by plants has been shown [e.g. Hernandez-Viezcas, J. A. et al 2013. ACS Nano. 7(2):1415-1423. DOI:10.1021/nn305196q] as well as effects of nanoparticles on nutritional value of food crops [e.g. Peralta-Videa, J. R. et al. Plant Physiol. Biochem. 80: 128-135.]. One would wonder if these nanoparticles can also be passed down to the next plant generation or other receptors (such as humans) through fruits/seeds on the long run.

"Ion effect" is mostly expected from soluble nanoparticles. But effects from particles like CeO2, TiO2 will be mostly "nanoparticle-effect" due to their relative poor solubility in aqueous media.

In addition to uptake, a major concern should be the effect on mycorrhizae. For example:

• http://scholarsresearchlibrary.com/ABR-vol3-iss8/ABR-2012-3-8-4239-4245.pdf 
• https://www.researchgate.net/publication/259495478_Influence_of_silver_and_titanium_nanoparticles_on_arbuscular_mycorrhiza_colonization_and_accumulation_of_radiocaesium_in_Helianthus_annuus
• http://pubs.acs.org/doi/abs/10.1021/es402109n 

Article Influence of silver and titanium nanoparticles on arbuscular...

Water Sci Technol. 2017 Jan;75(2):462-473. doi: 10.2166/wst.2016.541.

Long-term effect of metal oxide nanoparticles on activated sludge.

https://www.researchgate.net/publication/279537072_Toxicity_of_Mixture_of_Metal_Oxide_Nanoparticles_on_Activated_Sludge

Chapter Toxicity of Mixture of Metal Oxide Nanoparticles on Activated Sludge