Frontiers: Nanotechnology in the Subsurface

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As a broad scientific field, Earth science has always employed interdisciplinary methods of research to answer the many complex questions it has posed since time immemorial. As demonstrated by a seminar delivered by Dr Ian Molnar in early October, a background in civil engineering can most certainly lead to a specialisation in contaminant hydrogeology and risk reduction. Although an interesting field of study, engineering has never been a subject I found easy to understand. Even so, the presentation given by Dr Molnar on investigating the risk that engineered nanoparticles pose to our drinking water was an intriguing one to follow—or at least in my case, attempt to follow.

For context, it is due to the advances of modern technology that engineered nanoparticles are becoming more ubiquitous in consumer products that we use in our daily lives. Some of these consumer products include TiO2 and ZnO nanoparticles used in sunscreen for UV absorption, and even silver nanoparticles embedded into clothes and bandages for anti-microbial properties1. Like most consumer products, nanoparticles tend to end up in landfills, but their miniscule size and the high rate at which they accumulate due to its $1 trillion industry2 also present a huge risk to groundwater systems.

Thus, the question posed in this seminar is as follows:

How far can nanoparticles travel in soils before they start to contaminate groundwater resources, and therefore our drinking water?

Photo by Dylan de Jonge on Unsplash

The key component to answering such a question is to understand the mechanisms involved in the transport of nanoparticles through soils in the subsurface environment. Because these mechanisms require some specialist knowledge to understand, I will do my best to explain some of the more unfamiliar terms along the way.

Some readers may not have heard of the word “colloid”. In one explanation, a colloid is essentially a homogeneous substance consisting of a medium in which particles of macro- to nanometre-scale sizes3 are evenly dispersed4. In another explanation, an example of a colloid is the milk you drank this morning. In essence, milk consists of proteins dispersed in water; it is an emulsion-type colloid. “Colloid” is simply a label that encompasses a broad group of substances, including smoke (an aerosol-type colloid), whipped cream (a foam-type colloid) and even coloured glass (a sol-type colloid). Engineered nanoparticles fall into various classes of colloids as this minute substance can be dispersed within mediums like aerosols, landfill leakages and wastewater sludge.

The study used sol-type colloids that employed a fixed type of nanoparticle dispersed in a liquid at varying concentrations. These nanoparticle solutions were placed into bench-scale columns of uniform quartz sand, so that their filtration rate in an idealised soil can be examined. As an analogy to real life, this experiment helps visualise the rate at which nanoparticles would percolate through soil.

In conjunction with these experiments, Colloid Filtration Theory (CFT) was also used to estimate how much of the nanoparticle colloids were retained as they passed through the column. For readers without an engineering background, CFT is a mathematical model that estimates the trajectory of a colloid near a collector without relying on parameters that require actual observation5. This enables the model to predict the rate of colloid transport through a porous medium, such as our idealised soil column. For each grain in our column, the CFT idealises them using the Happel sphere in-cell model. This model visualises liquid passing by as a continuous, thin sheath that completely surrounds a spherical collector6, which in this study is used to analyse the behaviour of the nanoparticle colloids that pass through the column.

While it was found that CFT estimates fit well with experimental observations for micro-sized colloids, such was not the case for nanoparticle colloids. Rather, the CFT actually overestimated the retention rates of nanoparticle colloids. Thus even with an established mathematical theory, there is still a lot of uncertainty around what is actually happening to nanoparticles inside the column.

Photo by Samara Doole on Unsplash

Thus by using x-ray tomography to image cross-sections of the column experiments alongside the CFT and additional mathematics7, the study was able to visualise the concentration of nanoparticles within the pore spaces of the column. This method produced several observations, the key ones being that:

  1. Nanoparticle concentration increased over distance from grain surfaces,
  2. 40% of nanoparticles passing through the column diffused further away from grain surfaces, and
  3. 50% of nanoparticle mass flow occured outside of the idealised Happel in-sphere fluid envelope.

Altogether, the study concluded that retention rates of nanoparticles in the idealised soil column were being overpredicted, which is largely due to the limitations of Colloid Filtration Theory. Alongside relying on an oversimplified grain and pore geometry8, this model also largely focuses on nanoparticle flow along grain surfaces at the expense of nanoparticle activity in pore spaces away from grains.

All in all, the short answer to the question posed earlier is, “We don’t know, yet.”

In the greater context, the study tells us that current models of colloidal filtration are currently not robust enough to accurate predict how nanoparticles travel through idealised soil columns, let alone soils in real life. This is not surprising given that models are ultimately mathematical constructs. Much of scientific research, including this study, involves the testing and improvement of existing models and the subsequent designing of new ones to reflect the complexity of Earth systems so that we may answer questions such as the one posed in this seminar.

Photo by Martijn Baudoin on Unsplash

In further discussion after the seminar, another contemporary issue was brought to the fore: How can this research be made relevant to microplastics?

For graduates looking for research opportunities, Dr Molnar intends to probe this question in an upcoming project to investigate microplastic contamination of groundwater and its risk to drinking water and coastal resources, by using methods and findings such as the ones in this study. This project is just one of 150 research opportunities offered by the Edinburgh Earth, Ecology and Environmental Doctoral Training Partnership (E4 DTP). Led by the School of Geosciences at the University of Edinburgh, the E4 DTP is currently looking to recruit and train five cohorts of PhD students between 2019 and 2023 on fully funded studentships by the National Environmental Research Council (NERC)9.

To find out more, follow the links below:

About the E4 DTP

Deadline: 9th January 2020

Projects
The following is a sample of the opportunities available:

Follow NERC on Twitter here!

Follow E4 DTP on Twitter here!

References

1 Silver Nanoparticle Transport Through Soil: Illuminating the Governing Pore-Scale Processes (2015)
I.L. Molnar, Electronic Thesis and Dissertation Repository. 3394.

2,7,8 The impact of immobile zones on the transport and retention of nanoparticles in porous media (2015)
I.L. Molnar, J.I. Gerhard, C.S. Willson, D.M. O’Carroll

3,5 Colloid Transport in Porous Media: A Review of Classical Mechanisms and Emerging Topics (2019)
I.L. Molnar, E. Pensini, M.A. Asad, C.A. Mitchell, L.C. Nitsche, L.J. Pyrak-Nolte, G.L. Miño, M.M. Krol

4 Colloids (2019)
Chemistry LibreTexts

6 Significance of straining in colloid deposition: Evidence and implications (2006)
S.A. Bradford, J. Simunek, M. Bettahar, M.T. van Genuchten, S.R. Yates

9 Funding and Eligibility (2019)
The University of Edinburgh

(Photo by Dylan de Jonge on Unsplash)

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  1. Pingback: Frontiers in Earth Science – Student Blog

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