Subproject Ecotoxicological assessment of iron-oxide-NP using groundwater-relevant organisms: nematodes

Aims of the joint project

Aim of the project that is funded by the German Ministry of Education and Research (BMBF) is a new technology for a biological in situ remediation of benzine and creosote contamination in groundwater. Specific iron-oxide-nanoparticles (iron-oxide-NP) are applied to stimulate the biological metabolization of contaminants.Experiments at the Institute of Groundwater Ecology (HMGU) showed that iron-oxide-NP are able to strongly stimulate the microbial iron-reduction. This was shown in small-scale static experiments, however, also in larger-scale column experiments. It is assumed that the nanoparticles have a shuttle function between the external membrane of the microorganisms and the inert, ferreous surface of the matrix, thus stimulating the degradation. This catalytic activation will be developed for the application in remediation actions.In contaminated aquifers the potential of natural purification by microorganisms can often not by used, because the abundant electron acceptor iron(III) is present as solid phase or coating of minerals and thus simply not bioavailable. By adding a readily bioavailable iron(III) surface, in form of parametrically defined and ecotoxicologically assessed iron-oxid-NP, this project wants to carry out and validate an innovative in situ remediation on site.The stimulation of the decontamination will take place by injecting the nanoparticles at the source or the plume of contamination. Due to the nano-size and the ability to suspend in water, the iron-oxide-NP partition in the same pore system as the contaminants. Their mobility is dependent on their surface characteristics and the environmental conditions, that influence agglomeration, coagulation and (re)dispersion. The success of remediation will be proofed using innovative isotope methods and evaluated in terms of cost effectiveness. In principle, the new remediation technology is considerably more (cost) efficient, environmentally sound and sustainable compared to conventional techniques (e.g. pump-and-treat; H2O2-Injection).

The joint project has following goals:

• simple production and practicable handling of NP (HMGU-M),
• Control of reactions and mechanisms of decontamination (HMGU-M),
• optimal application of NP in the subsoil (FSUJ, ARCADIS),
• Assessment of ecotoxicological effects and risks (HMGU-L/IBN),
• Supervising and optimization of the decontamination rates in the field (Isodetect),Implementation,
• cost-benefit-optimization and analysis of cost-effectivess of the remediation technology (ARCADIS, Isodetect).

Results of the subproject:

The results that have been worked out in close cooperation with the project partners, provide a good basis for evaluating the ecological risks of iron oxide (FeOx) injections in aquifers. Ecotoxicity could be used as one criterion for the selection of the most suitable FeOx-nanoaggregates (NA) for the purpose of groundwater remediation. For various FeOx-NA toxicity threshold were defined, which showed to be dependent on the aggregate size, the specific surface area, and the quality of the organic coating. Goethite showed the lowest toxicity on the nematode Caenorhabditis elegans compared to ferrihydrite and akaganeite. Also ferrihydtite colloids extracted from natural soils showed a significantly higher toxicity than the synthetic goethite-NA. If coating the Goethite-NA with humic acids (HA), the toxicity again decreased by a factor of 4 to 5. Thus, also from the ecotoxicological perspective, the use of goethite-NA with HA-coating for the field trial was a good choice. In model ecosystems that were spiked with ferrihydrite NA, for micro- and meiobenthos no irreversible change in community structure could be observed, even at relatively high FeOx concentrations (500 mg Fe kg-1 sediment dry weight). Bacterial activity, however, was strongly inhibited by all treatments till the end of the experiment. With the exception of bacterial community structure, no micro- or meiobenthic community parameter responded specifically to nano-sized material. It has to be noted that goethite-NA (+HA), used in the field trial, showed a considerably lower toxicity (factor of 20) in the on C. elegans compared to ferrihydrite NA that were used in the microcosm study. Therefore, considering the rapid dilution of the injected FeOX NA in the aquifer, the risk of FeOx with low toxicity (e.g. goethite + HA) are estimated to be low. Toxicity assessment of groundwater samples after Fe injection have confirmed this risk estimation.