The black box opened: Non-invasive observation of nanoparticle transport in rock pore systems

Groundwater is used by approximately 2 billion people worldwide. It is thus imperative that we develop the tools to protect this valuable resource from pollutants. A key tool in this endeavour is a reliable transport model for the pollutant of concern. Without this, we cannot predict the movement of the pollutant through the aquifer, which is essential for risk assessment and the design of remediation strategies. Manufactured nanoparticles present a new and poorly understood threat to this resource, with increasing numbers of nanoparticles found to exhibit toxicity. This is of particular concern as the global demand for nanoparticles continues to grow due to their use in a wide range of commercial applications. Problematically, large scale production and use of manufactured nanoparticles will inevitably lead to release into groundwater. In addition, manufactured nanoparticles are also being designed for in situ groundwater remediation of a range of both organic and inorganic pollutants. Effective delivery of these nanoparticles, however, requires the ability to predict their movement within the aquifer and contaminated zone.
Critically, however, we are at present unable to predict reliably nanoparticle transport due to significant limitations in current transport models. To date, most nanoparticle transport models have been developed using data from columns containing glass beads or sand, where nanoparticles are injected at one end and the breakthrough of nanoparticles at the other is measured. As it stands, models based on these data all too often fail to predict nanoparticle transport. This is because we must use the breakthrough curves to infer the transport processes which occur inside the column, rather than actually seeing them in action. The column remains an elusive black box. To open this black box, we must be able to look inside the column and image the movement of nanoparticles within.

Here, we will achieve this using a novel combination of magnetic resonance imaging (MRI) and magnetic susceptibility measurements (MSM). MRI is most renowned for its use in hospital settings, where it is used to image inside patients in a non-invasive manner, the patient unharmed by analysis. This same technology can be used to image inside the columns of porous media. Moreover, when we use nanoparticles that are labelled with a paramagnetic tag, the molecule becomes easily visible with MRI. This technology is already applied in clinical research, where, for example, tagged nanoparticles are used to image drug delivery.
By imaging nanoparticle transport with MRI, we will be able to create high resolution movies of nanoparticle migration through the porous media. With this dramatically enhanced dataset, we will develop far more robust models of nanoparticle transport. While MRI affords us considerable advantage by generating high spatial and temporal resolution transport datasets, it does not work so well on rocks which contain high concentrations of paramagnetic impurities, such as Fe or Mn. For these rocks, we will use magnetic susceptibility measurements (MSM). Indeed, this is a novel application of MSM, which is traditionally used to examine porosity and the alignment of magnetic fabric in rocks. This technique does not give us the detailed spatial resolution of MRI, but it does provide essential data on nanoparticle concentration and the shape of the nanoparticle plume as it migrates through the porous media. These data will enable us to test if the enhanced models developed using MRI datasets are applicable to MRI-incompatible rock. Using this 2-pronged approach we are able to test our enhanced models on a much wider range of rock types.
By the end of this research, we aim to deliver far more robust and reliable nanoparticle transport models which are sorely needed for nanoparticle risk assessment and in the design of techniques for targeting nanoparticle delivery in remediation applications.

The long term beneficiaries are societies in general (both in the UK and abroad) who will enjoy better protection of public and environmental health, via improved management of groundwater resources.

More immediately, a better understanding of nanoparticle transport is needed to help develop regulatory frameworks to protect groundwater resources. Thus, the work undertaken here will be of direct benefit to the agencies charged with developing and implementing these policies. The work will also help regulatory agencies develop policies for the use of nanoparticles in the remediation of other pollutants in groundwater. In the UK, the main regulatory agencies include the Scottish Environment Protection Agency (SEPA), the Environment Agency of England and Wales, and the Northern Ireland Environment Agency, and for this reason SEPA is a partner on the project. Moreover, impact is not restricted to the UK as regulatory bodies which will be able to utilize this data exist in most countries.

In the longer term, manufactured nanoparticles will be released into groundwater, despite policy implementation. For each groundwater contamination, a risk assessment and remediation strategy must be developed. The research on nanoparticle transport in this proposal will be needed for these risk assessments and the development of remediation strategies. In this context, the research will be of direct benefit to the regulatory authorities, water supplying organizations (in the UK, the water companies), and consultants who will be employed to undertake this task.

This work will also assist groups responsible for nuclear decommissioning and disposal, such as the UK Nuclear Decommissioning Authority (and their overseas equivalents), by improving prediction of radioactive nanoparticle movement in the subsurface.

A far more robust understanding of nanoparticle transport would also be valuable in environmental forensic investigation of nanoparticle pollution, where transport models must be to the satisfaction of a court of law.

Our better understanding of manufactured nanoparticle transport is not just of relevance to manufactured nanoparticles per se, but also to natural nanoparticle migration including viruses, (e.g. avian 'flu, foot and mouth, blue tongue, and plant viruses) and to the role that both natural and manufactured nanoparticles have in transport of sorbed pollutants. Thus the models we propose to develop here have the potential to assist regulators and environmental consultancies in regulation and risk assessment in these areas.

Less directly, future development of the MRI and MSM techniques here have the potential to focus upon the movement of larger particles in porous media. This is off particular relevance considering new, recently lowered, turbidity standards for public water supply. In future, development of our methods to examine bacterial transport can assist with regulation and risk assessment of, for example, Cryptosporidium transport.
In addition, our advancement of MRI techniques will add to the knowledge base, and tools available, for those who already use MRI for analysis of rock and rock fluid dynamics - in particular the oil industry.

Development of the AMS approach will enhance the capabilities of this technique for those who use it for detecting mineral fabrics in rocks. Additional benefits are in the development of commercial applications related to MagPoreTM - for instance there may be potential to develop techniques for understanding permeability pathways. These added benefits will feed back into both academic and industrial applications of magnetic susceptibility techniques, especially in the oil and gas industry. Indeed, Fugro, who utilize this approach for permeability analysis to maximise hydrocarbon recovery, are onboard as a project partner.