LAUSR

Letter to the editor The above review has received wide attention since it was summarised in the July 2016 edition of the European Commission’s ‘Science for Environment Policy’. Such reviews are incredibly helpful to the wider scientific and practitioner community in distilling and evaluating relevant literature. However, given the currently contested status of engineered nanoparticles in the field of polluted groundwater remediation, the conclusions and headline message from the paper would have benefitted from a more nuanced terminology in the conclusions that more obviously emerged from the literature. One of the contentions in this paper is that the use of nanoremediation is potentially problematic for soil functions. This assertion is made in part based on a limited set of publications about impacts of nano zero valent iron (nZVI) on denitrifiers, which the authors then extend to signify the entire nitrogen cycle. This assertion is flawed for a number of reasons. (1) nZVI use in remediation is a saturated zone treatment and the likelihood of (highly oxidisable) nZVI being present in the aerobic A-horizons of soil where the N cycle is important is to say the least low. (2) It is fallacious to extend concerns over the entire N cycle based on one small microbial component. (3) Nowhere in the paper is there any attempt made to distinguish entirely predictable effects that might be due to perturbations in pH or redox potential as a result of adding nZVI. (4) Nowhere in the paper is there any attempt made to specifically link observed impacts with iron nanoparticles as opposed for example to dissolved iron species. (5) No controlled experimentation comparing micro-Fe and nano-Fe addition is reported. The conclusions reached about the potential environmental impacts of nanoremediation are therefore reached on the basis of no clear supporting evidence. The review is entirely silent about the known and widely reported biostimulatory effects of saturated zone injection of nZVI on dehalorespiration. Hence, the paper is postulating a problem in the A-horizon of agricultural soils—a subsurface environment unlikely to ever ‘see’ nZVI whilst ignoring a benefit in the subsurface environment most likely to see nZVI, namely the remediation of aquifers impacted by dense non-aqueous phase liquids (DNAPL). Ironically, terminological inexactitude is the root of the problem in two ways. The authors group specific metals, but exclude iron, into a general ‘metal and metal oxide’ category while at the same time using the all-encompassing ‘carbon nanoparticles’ to refer specifically to fullerene and carbon nanotubes (CNT). The narrative has a tendency to treat nanoparticles as a coherent group and assumes that the effects on soil microbial communities of one nanoparticle have something to say about the likely effects of other, different nanoparticles. However, this is not borne out by the literature. To say “in response to very low concentrations of Ag-NPs illustrates the high toxic potential of some metal NPs” (p. 13174) is an extrapolation that almost beggars belief. What the results show is that nanosilver is highly ecotoxic—hardly surprising given what is well known about its widely commercially exploited biocidal properties. But there is no reason given whatsoever to ascribe to other metals’ silver potency in such a cavalier manner. Responsible editor: Philippe Garrigues