Contents
1. Aim
2. Overview
3. Additional Resources on the NanoRem Web Site
4. Feedback and Opinion
1 Aim
The aim of this page is to summarise the state of current knowledge on the fate and transport of nZVI particles within the subsurface, and on the consequent risks posed by these particles to human health and the environment. More detailed information is available from the NanoRem Tool Box (http://www.nanorem.eu/toolbox/index.aspx#TB1).
2 Overview
In terms of the source-pathway-receptor paradigm used in Risk Based Land Management (RBLM), renegade NPs are presumed to represent a hazard. Receptors in the form of not yet polluted groundwater are assumed to be present.
During the early stages of the NanoRem project, a qualitative risk assessment protocol was developed for the NPNPs that were to be investigated in the laboratory and in the field. The protocol applied to renegade NPs. This protocol was based on an expert elicitation workshop hosted by Land Quality Management Ltd (LQM) in Nottingham and an extensive review of the literature. It found that the NPs being investigated by NanoRem are likely to have low toxicity and ecotoxicity; are likely to interact with aquifer matrix, each other and groundwater to, often rapidly, cease to be mobile NPs and; are likely to be difficult to penetrate into the aquifer more than a few metres from the point of injection. While there were considerable uncertainties particularly with regards to the transport of NanoRem NPs the ability of NPs to penetrate far into the formation was likely to be very limited. Further information on this workshop can be found in Annex 2 of DL9.2 “Final Exploitation Strategy, Risk Benefit Analysis and Standardisation Status”, downloadable from the NanoRem Tool Box (http://www.nanorem.eu/toolbox/project-deliverables.aspx).
NanoRem laboratory and field work has helped refine our understanding of the transport of NPs. Most of the upscaling (large containers and field sites) was for porous materials. The results from the large containers and field trials showed maximum travel distances of 2.5m and 5m respective-ly. WP4 (Mobility and Fate of NPs) reported LT99.9% values which are predicted maximum travel dis-tances calculated using the results of the column experiments. Early experiments show predicted transport distances to just over 20m (21.8m). Column experiments on optimised particles in field relevant conditions had predicted distances (LT99.9%) of just over 30m (32.2m).
The Neot Hovav NanoRem site is in an industrial zone in southern Israel over fractured chalk with high permeability fractures and a low permeability matrix. The aim of this trial was to look at transport in fractured rock. Ben Gurion University (BGU) reported that the NPs travelled from the injection point to the pumping well, a distance of 47m (Pers comm Noam Weisbrod). A maximum distance for NP transport in fractured rock has not been calculated, so could be in excess of 47m; further work would be required to evaluate likely transport distances in fractured rock.
WP5 (Environmental Impact of Reactive NPs) reported results of toxicity testing of NanoRem NPs and generally found that toxicity was low; typically the limiting concentration was 100 mg/l.
A more detailed Risk Screening Model (RSM) for application of NanoRem NPs to groundwater remediation has been developed. The RSM includes conceptual exposure scenarios, consideration of fate, transport and toxicity and a spreadsheet based model to estimate transport distances. The RSM has been developed with only the NanoRem NPs in mind but may inform risk assessment for other NPs as well.
The risk model for NP applications considers the macro-scale transport of NPs within saturated me-dia and is based on a modified advection-dispersion equation as described within NanoRem DL7.1 (eq. 10a and 10b, Tosco et al., 2016) and the MNMs user manual (Eq 5-1, Bianco et al., 2015), i.e. from DL7.1. The Environment Agency Remedial Targets Methodology, RTM (Environment Agency, 2006) has been used as the basis for deriving the transport element of the risk model that estimates a screening level NP concentration versus distance from the NP source (injection) zone. The RTM is accompanied by a MicrosoftTMExcel spreadsheet tool for four Levels of assessment. The RTM spreadsheet model has been modified at the Level 3 stage (i.e. saturated zone transport) by incorporating some of the key NP parameters into one of the analytical solutions (currently the Ogata Banks equation) used to describe the advection-dispersion including degradation and retardation of solutes downstream of the source term. The model has been compared against the numerical solution currently included within the MNMs 2015 (v 1.012) model . The methodology depends on calculating values of attachment (katt) and detachment (kdet) using the MNMs model (micro-and NP transport, filtration and clogging model suite) developed by WP7 (Modelling Tool for NP Mobility and Interaction with Contaminants).
For the continuous injection scenario the modified RTM model can be used to estimate the time at which ‘breakthrough’ (very low but non-zero concentration) occurs at a distance 100m downstream (23 years), with the NP concentration distance profiles at specific times (1-50 years) also shown. Clearly, a continuous injection for the lengths of time assumed is unrealistic but even for such a cautious assumption the travel time is predicted to be relatively high and travel distance limited. The density of NPs per litre can also be modelled for various distances downstream of the injection point. After one year very low concentrations are estimated only 20m downstream from the injection point. These findings compare well against evidence from the NanoRem field trials, notably at the Hungary field pilot site (Balassagyramat).
The comparison of the modified RTM model (analytical solution) output with that provided by the MNM’s (numerical solution) output provides an indication that the simplified models can provide similar outputs for the same inputs. A number of key limitations and assumptions have been identified but it is considered that our approach provides a useful basis for a suitably cautious risk assessment methodology.
Nano particles below a certain size begin to behave in ways that seem to be different to their micro equivalents. Irrespective of that, the increasing surface area to mass ratio means that particles below 100nm can improve currently deployed in situ groundwater remediation technologies and have the potential to deal with presently recalcitrant substances. The UK government supported a joint Royal Society/ Royal Academy of Engineering recommendation that the use of novel manufactured NPs be prohibited for use in environmental remediation while uncertainties about the risks such particles posed were being addressed. NanoRem has included work on the risks posed by NPs injected into polluted groundwater for remediation purposes. This work has drawn on published literature as well as field trials and laboratory studies by NanoRem partners to inform a qualitative and semi quantitative risk assessment protocol on the magnitude of risks posed by NPs that escape the zone of polluted groundwater they were intended to remediate. Such renegade particles have been found not to migrate distances significant enough to pose a credible risk to unpolluted groundwater, surface waters or ecosystems.
NanoRem’s experimental work has helped inform our understanding of the levels of risk that could be posed by deploying NPs for remediation: NanoRem results suggest that the toxicity of its tested NPs is low;with predictions of maximum transport distances (up to 30m) which are greater than the results of the upscaling and field trials work.
While there are considerable uncertainties particularly with regards to the transport of NanoRem NPs the ability of NPs to penetrate far into the formation is likely to be very limited. Their ability to escape dissolved phase plumes is likely to be even more limited. Research goes on for ways to in-crease the migration distance and a simplified quantitative approach to estimating transport distances has been developed.
This reinforces the view that it seems reasonable to conclude that overall environmental and human health risks of discharging NanoRem particles into polluted groundwater are extremely low.
Information in detail is available from the Risk Screening Model report downloadable from the NanoRem Tool Box (http://www.nanorem.eu/toolbox/nanoparticles_and_tools.aspx#TB1).
3 Additional Resources on the NanoRem Web Site
Comprehensive resources are available from the NanoRem Tool Box, shown below (http://www.nanorem.eu/toolbox/index.aspx):
