Study of colloidal transport in saturated clay sediments using taylored nanostructures (PhD)

Thick clay-rich aquitard systems are of interest to both industry and government because of their potential for geological storage of toxic wastes and for their role as protective covering for regional aquifer systems. Consequently, a thorough understanding of the physical and geochemical processes that control and impact the transport of solutes in aquitard deposits is required to assess the suitability of these deposits to contain contaminants for time-frames in excess of thousands of years (Hendry et al., 2003). An increasing number of investigations document that contaminants may associate with colloidal particles (1 nm – 1 µm) dispersed in the fluid phase (McCarthy and Zachara, 1989; Kersting et al., 1999). These studies provide ample evidence that suspended colloids act as contaminant carriers, and represent a rapid transport pathway. Such an enhanced spreading of hazardous chemicals is generally referred to as “colloid facilitated transport” (Figure).

Figure: Schematic representation of colloid-facilitated contaminant transport in porous media. A contaminant (•) can be present as a dissolved species, sorbed to the solid matrix, or bound to colloidal particles that move with the flowing water. For strongly sorbed contaminants, such mobile colloids may serve as "carriers" and provide a rapid transport pathway (Kretzschmar et al., 1999)

 The mobility of colloids in sediments is limited by attachment-detachment processes at the solid-water interface, and by physical straining which is the retention of colloids in the smallest regions of the soil pore space formed adjacent to points of grain-grain contact, the so-called pore throats (Bradford et al., 2007; Reszat and Hendry, 2009). Straining is influenced by physical factors such as the relative size of the colloid and the porous medium, but also by solution chemistry and system hydrodynamics. Although several studies on colloid migration through coarse textured and fractured fine textured media have been conducted (e.g., Bradford et al., 2003; Grolimund et al., 1996), the knowledge of colloidal transport through massive, fine-grained geological media (aquitards) is limited.

Pore diameters are difficult to measure in fine-grained media, but have been estimated using geometric calculations assuming colloidal particles are spherical and the sediment has uniform grain size distribution (Bradford et al., 2003). They have also been estimated using Hg-injection porosimetry and micro-imaging methods. This has been done for Boom Clay, and resulted in the largest pores having reported diameters at 100-200 nm, and median pores between 10 and 20 nm diameter (BoivinJahns et al., 1996). Unfortunately, physical pore diameter measurements may not be directly related to colloid movement through clay-rich aquitards because the distribution of interconnected pore throat diameters is not uniform and pore throat diameters can be much smaller than the pore diameters (Foppen et al., 2005; Reszat and Hendry, 2009).

References

Boivin-Jahns, V.; Ruimy, R.; Bianchi, A.; Daumas, S.; Christen, R. “Bacterial diversity in a deep-subsurface clay environment”, Appl. Environ. Microbiol., 62, 1996, 3405-3412

Bradford, S.A., Simunek, J., Bettahar, M., Van Genuchten, M.T., Yates, S.R. “Modeling colloid attachment, straining, and exclusion in saturated porous media”, Environ. Sci. Technol., 37, 2003, 2242-2250

Bradford, S.A., Torkzaban, S., Walker, S.L. "Coupling of physical and chemical mechanisms of colloid straining in saturated porous media", Water Research, 41, 2007, 3012-3024

Foppen, J.W.A., Mporokoso, A., Schijven, J.F. “Determining straining of Escherichia coli from breakthrough curves”, J. Contam. Hydrol., 76, 2005, 191-210

Grolimund, D., Borkovec, M., Barmettler, K., Sticher, H. "Colloid-facilitated transport of strongly sorbing contaminants in natural porous media: A laboratory column study", Environ. Sci. Technol., 30, 1996, 3118-3123

Hendry, M.J., Ranville, J., Boldt-Leppin, B.E.J., Wassenaar, L.I. «Geochemical and transport properties of dissolved organic carbon in a clay-rich aquitard system”, Water Resour. Res., 39, 2003, 1194-1203

Kersting, A.B., Efurd, D.W., Finnegan, D.L., Rokop, D.J., Smith, D.K., Thompson, J.L. “Migration of plutonium in ground water at the Nevada Test Site”, Nature, 397, 1999, 56-59

Kretzschmar, R., Borkovec, M., Grolimund, D., Elimelech, M. "Mobile subsurface colloids and their role in contaminant transport", Advances in Agronomy, 66, 1999, 121-193

Matson, J.B., Grubbs, R.H. “Synthesis of Fluorine-18 functionalised nanoparticles for use as in vivo molecular imaging agents”, J. Am. Chem. Soc., 130, 2008, 6731-6733

McCarthy, J.F., Zachara, J.M. “Subsurface transport of contaminants”, Environ. Sci. Technol., 23, 1989, 496-502

Put, M.J., Dierckx, A., Aertsens, M., De Cannière, P. « Mobility of the dissolved organic material through intact Boom Clay », Radiochim. Acta, 82, 1998, 375-378

In nuclear medicine, labelled nanostructures have been used extensively as contrasting agents for in vivo molecular imaging. In particular, nanoparticles of specific size ranging between 1 and 100 nm and based on polyethylene glycol (PEG) can be synthesised and labelled with a radioactive tracer (Matson and Grubbs, 2008). These nanoparticles are amphiphilic polymers, and are therefore good representatives for natural organic matter (NOM) colloids. In this project, we will aim to synthesise such labelled nanocolloids, and to study their transport behaviour in saturated clay sediments. The Boom Clay formation, which has been thoroughly studied as a candidate host formation for the deep geological disposal of high radioactive waste, will hereby be used as a reference. The plastic clay formation is saturated with water up to 42 volume percent and has an organic matter  content of up to 5% of its dry weight. About 0.05% of this NOM is mobile in the interstitial clay water, and mobility is strongly dependent on the size distribution (Put et al., 1998). With the help of suitable imaging techniques (fluorescence, PET, NAA, depending on the radioactive tracer used), the transport pathways of these colloids through the clay can be visualised. Moreover, transport modelling of their migration profiles will help to constrain and predict the possibility for colloid-facilitated transport through the formation. This project will also make use of complementing studies on the pore structure of the Boom Clay using cryo-SEM analysis which are performed in collaboration with RWTH Aachen.