Transport and toxicity for bacterial communities of reactive iron nanoparticles used for nanoremediation
Résumé
Highly toxic contaminants (e.g. As, chlorinated hydrocarbons, aso…) persist in the environment because of their low volatilization and/or hydrolysis and/or biodegradation. This situation is critical for public health and raises industrial and economic issues. Remediation techniques for these contaminants in groundwater are often technologically impossible, extremely expensive or hardly effective. Nanoparticles (e.g nZVI (Zero-Valent Iron), iron oxides, ferrate) applicable as in-situ reduction or oxidation agents for groundwater treatment show an extremely high reactivity leading to effective transformation of many contaminants into less toxic or benign products.
However, these nanoparticles (NP) may also represent an additional contamination. Moreover, the different processes controlling the fate of NP and their toxicity towards microbial communities remain poorly understood. This study, part of the EU-project NanoRem (Taking Nanotechnological Remediation Processes from Lab Scale to End User Applications for the Restoration of a Clean Environment; EU FP7/2013-2017), aims to (i) study the mobility of NP in the presence or absence of biofilm by columns transport assays mimicking real aquifer conditions and (ii) to evaluate NP toxicity on bacterial communities (planktonic and biofilm grown onto sand).
Results show that the mobility of nZVI widely depended on the water flow and/or NP concentration. The recovery at the column outlet was 1% for a 100 m.d-1 and 40 % for 10 m.d-1 water flow. The presence of biofilm in the column decreased its total porosity from 35% to 25%. Though the recoveries of nZVI in the presence or absence of biofilm were similar, the analysis of the sand showed that the variation of labile Fe content is concomitant with that of TOC, strongly suggesting NP-biofilm interactions. That was confirmed by CryoSEM images.
Otherwise, measures of the denitrifying activity showed a toxicity of NP on planktonic bacteria starting at 50 mg Fe.L-1 while cellular viability evaluated by Live and Dead method highlighted toxicity starting at 10 mg Fe.L-1.
It appears therefore that reactive NP, very useful for in situ groundwater treatment, can represent a source of emerging contamination. Other studies about mobility and toxicity of such particles will have to be conducted before a prospective large-scale application.