Fate and transport of nanotechnology in groundwater by Ian Molnar

10/10/2019

A crystal clear lake in Canada suffering from sulfur dioxide poisonning

Fate and transport of nanotechnology in groundwater by Ian Molnar

In short:

Ian Molnar is a new lecturer to Edinburgh University from Canada with a background in civil engineering. He is a contaminant hydrogeologist currently researching the transport of nanoparticles in groundwater. His study is based on the nanoparticle modelling problem of the currently used Happel sphere model. His experimental work found that 40% of the nanoparticle mass was omitted in this model thus leading to an overestimation of the filtering capacity of soil and sand. His experiment hopes it will help on the underway development of a new model for nanoparticle flow.

Main:

Ian Molnar is a Canadian hydrogeologist with a background in civil engineering. He joined as lecturer at Edinburgh University this summer is is thus the third Ian in the department. While still in Canada, his research as a contaminant hydrogeologist included the study of the crystal clear lakes North of Lake Huron. In this case, some lakes where crystal clear due to the lack of bio-activity caused by the leaching of sulfur dioxide from the nearby industry. In the quartzite regions the lakes where dangerously high in sulfure dioxide and their pH dropped down to values up to 5, whereas in the pink granite regions the buffering system was higher so the lakes where less affected. But now Ian Molnar is focusing on the transport of nanoparticles in groundwater.



The modelling of the flow and capture of nanoparticles in groundwater is important for multiple aspects like the modelling and tracing of contaminants in groundwater or the design water treatment installations. Nanoparticles are defined as any particle with at least one dimension smaller than 100nm. These particles are abundant in our modernized world for example titanium dioxide (a strong UV absorbant), carbon nanotubes, silver nanoparticles (antimicrobial) drug container and so forth. The average production of nanoparticles is averaged to around 320 000 tons/year (Keller 2013) which leads to the question of where do they land afterwards? Most of them get into a landfill (189 000 t/y), soil (51 600 t/y), groundwater (69 200 t/y), and the rest in surface water.

With such a mass of contaminant it is needed to me able to model the transport at a regional scale. Some preliminary experiments where done on a bench-scale experiment, the soil-column and then compare the flow of particles with the now used models. The result is that the model overpredicted the filtering potential for the nanoparticles which can be dangerous as for example water treatment plants could unknowingly release contaminated water. So Ian Molnar tried to improve the model as the Colloid Filtration Theory seemed to be good for micron scale colloids but not for nanoparticles.

The base model is based on the Happel shere in cell model. This model consist of modelling the pore space to a grain with a thin water shell around (proportional to pore space). Then it calculated the probability of a particle injected on one side to collide with the grain and then remain attached to it or to just flow inside the water shell to the other side.

In order to correct this model for nanoparticles, Ian and his team wanted to image the flow of nanoparticles during the soil column experiment by X-ray Tomography. The resolution of X-ray Tomography is of 10 µm which is too big for imaging 100nm particles. So to solve this problem they used absorption-edge imaging which involves a difference imaging on either side of the K-edge energy of the particles. This technique leads to a 3D map of nanoparticles concentration.

So to check to Happel sphere model they checked the concentration of nanoparticles around grains up to 15 µm close (cannot more due to refraction problem). So they found out that 40% of the nanoparticles are outside the Happel fluid envelope. As this mass is outside the fluid envelope it does not interact as predicted in the model leading to overestimation.

The design and testing of a new model is underway and this study hopes to help in the making of it.