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Thermodynamics, Transport, and Fluid Mechanics
Colloid transport in aggregated porous media with intra- and inter-aggregate porosities Hongjuan BAI, Laurent Lassabatere, and Edvina Lamy Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.8b00501 • Publication Date (Web): 17 Apr 2018 Downloaded from http://pubs.acs.org on April 18, 2018
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Colloid transport in aggregated porous media with intra- and inter-aggregate porosities H.J. Baia,b, L. Lassabaterec, E. Lamya,* a
Sorbonne Université, Université de Technologie de Compiègne, Département de Génie des Procédés
Industriels, (UTC/ESCOM), EA 4297 TIMR, Centre de Recherche de Royallieu, Compiègne F-60205, France. b
Henan University of Technology, School of Chemistry, Chemical and Environmental Engineering,
Zhengzhou, 450001, PR China. c
Université de Lyon; UMR5023 Ecologie des Hydrosystèmes Naturels et Anthropisés Université Lyon 1;
ENTPE; CNRS; 3, rue Maurice Audin, Vaulx-en-Velin, F-69518, France. * Corresponding author. E-mail:
[email protected] Abstract Column tracer and colloid transport experiments were performed under both saturated and unsaturated steady state flow conditions in an aggregated porous medium with bimodal pore size distribution (PSD): intra-aggregate porosity with a pore radius between 10-2 and 10-1 m and inter-aggregate porosity ranged between 101-103 m (inter-porosity). All experiments were carried out under unfavorable conditions for physicochemical attachment to solid-water interfaces, using negatively charged porous media and latex microspheres (1 µm). Both porous media and colloids used in this work were hydrophobic. The results obtained through experimental observations and numerical simulations in the aggregated medium were confronted with those obtained for a sandy medium, characterized by a narrow unimodal PSD with a pore radius ranged between 101-102 m (inter-porosity), to explore the relative importance of the PSD on water flow, colloid transport and deposition. Physicochemical interactions between colloids and porous media, calculated according to the DLVO theory, showed no primary minimum and low secondary minimum depth, suggesting reversible colloid retention and possibility for colloid detachment by hydrodynamics drag for both sand and aggregated media. Hydrodynamic drag forces were slightly greater than the resisting adhesive DLVO forces in the secondary minimum, indicating a possibility for colloid detachment. For the same flow rate, more non-uniform transport of colloids were obtained in the bimodal aggregated medium compared to the unimodal sand. If the non-uniform and preferential transport of colloids should contribute in decreasing of colloids retention,
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particularly under saturated flow conditions, surprisingly greater overall colloid retention was obtained in the aggregated medium. Based on the PSD, colloid exclusion from smaller pores has higher probability to occur in the aggregated medium compared to the sand. Therefore, size exclusion contributed to the overall preferential transport in this dual porosity medium, and it was expected that this non-uniform transport would disfavor colloid retention. However, colloid retention efficiency at the column scale was higher for the aggregated medium compared to the sand under saturated flow conditions, despite a more uniform flow and transport in the later. This means that the presence of the intra-porosity in the aggregated medium and related small pores contributed not only in colloid size exclusion, but it had also an opposite effect, resulting in additional deposition sites of colloid particles. Under unsaturated flow conditions, capillary forces governed colloid retention, independently from the PSD.
Keywords Colloid, aggregated media, intra-porosity, inter-porosity, transport, deposition
1. Introduction Groundwater contamination is a great cause of concern in prevention of drinking-water contamination and emphasizes the importance of source-water management. For that purpose, the understanding of the mechanisms governing flow and contaminant transport through porous media may be useful in determining the extent of contamination in aquifers. The structure heterogeneity of natural soils renders difficult to predict flow and contaminant transport to groundwater systems. This difficulty is compounded in soils that are prone to preferential and non-equilibrium flows, due to a variety of small-scale heterogeneities such as aggregates, cracks, root channels1. The presence of these small-scale heterogeneities in natural soils involves complicated pore-size distributions. For example, soils containing macropores (fractures or cracks) have a bimodal PSD. A two-domain approach is necessary, to account for a fast (macropores) and a slow (matrix) transport region, when modeling solute transport. The aggregate soils represent another example of the soil heterogeneity. They exhibit a continuous PSD at macroscopic scale and the pore system is partitioned into intraaggregate and inter-aggregate pores. Although inter-aggregate pores cannot be treated as real macropores, the rate of flow in inter-aggregate pores is substantially higher than that in the boundary layer around the grains and in the matrix (the intra-aggregate pores) and a relatively faster flow region can develop2. A consequence of this physical heterogeneity for both
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aggregate and macroporous soils is variation in fluid velocity at REV (representative Elementary Volume) scale, causing non-uniformity of the flow field. A common approach to deal with this difficulty is to apply the two-region flow concept, with consideration of physical non-equilibrium transport3. This approach has widely been applied to describe nonreactive solute transport in macroporous or aggregated soils. The examination of the preferential flow through macroporous soils4 has permitted to conclude that macropores enhance conservative solute transport, leading to earlier solute breakthrough curves (BTCs) with a substantial tailing. Water flow in heterogeneous macroporous or aggregated porous media has been intensively studied, because fluid flow impact contaminant transport2, 5-7. In natural and anthropic subsurface environment, many contaminants occur as colloids or are associated with colloids8. These ubiquitous colloidal particles are of wide variety: inorganic, organic, and microbiological9. The transport of colloidal contaminants is of particular concern because they can be transported very quickly through macroporous soils and can lead to contamination of aquifers. Colloid transport in porous media involves several basic processes, partitioned into mechanisms controlling the movement of colloids through the pore space (advection, dispersion, diffusion) and physical or physicochemical processes of release and capture onto the solid surfaces or at the pore constrictions between grains. Extensive explanation of these mechanisms is provided in the reviews carried out by many authors10. Colloid transport in porous media has extensively been studied in the existing literature. Various factors (pore water velocity, water content, pH and ionic strength of the solution, size and hydrophobicity of colloids…) that could influence colloid transport and deposition in homogenous porous media have been investigated by experiments conducted with uniform sand packs11-16. Many studies related to heterogeneous porous media have also widely been performed, in most cases, by mixing of sand grains of various sizes17-19. These studies are often restricted to saturated systems. An increasing number of studies dealing with colloid transport in unsaturated porous media20 have been carried out, because the presence of air in partially saturated media introduces additional mechanisms for colloidal interactions that are not well understood21 and potentially disturb flow pathways. Grain and pore size effect on colloid and bio-colloid transport has widely been investigated and breakthrough and retention of (bio)colloids were affected by changes in grain and pore sizes of the porous media20, 22, 23. Many authors reported higher colloid or bacteria recovery in coarse compared to fine sands under saturated conditions, as the results of preferential transport. Several researchers have
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also observed this mode of transport, when discussing preferential transport occurring through the macropores or artificial fractures inserted in sand matrix20,
24-27
. This physical non-
equilibrium transport in soils containing macropores is mainly dominant under saturated conditions. Once water pressure is reduced and the larger pores are deactivated, most of the water flows through smaller pores. On one hand, preferential flow pathways are no active and flow and colloid transport occurs through the matrix, since macropores are deactivated. On the other hand, water and colloid pathways become more tortuous with decreasing water content, because of the presence of the air-water-interfaces and flow discontinuities in the matrix. Whereas conservative solute transport in aggregated media has been intensively studied, that of colloid transport through porous media presenting intra- and inter porosities (dual or bimodal porosity) has garnered much less attention. Studies about aggregated media are mostly performed with aggregates, made up of sands of different grain sizes, presenting only inter-porosity between grains25,
28, 29
. Leij and Bradford suggested that the complex flow
pattern in such composite systems makes difficult to give conclusions on colloid transport30. This difficulty is enhanced in porous media, when intra-porosity is present. Grain intraporosity may affect not only water and colloid flow pathways, but also it may constitute supplementary sites of colloid deposition or exclusion. This study aims to investigate the influence of pore size distribution (PSD) between intra- and inter-porosities on flow and colloid transport and retention processes under both saturated and unsaturated conditions. Packed transport experiments were performed on a dual porosity aggregated porous medium with a high intra-porosity of the grains and inter-porosity between grains. The results obtained from these experiments were confronted with those performed on a reference sandy medium with an unimodal PSD (inter-porosity between grains). Negatively charged porous media and latex microspheres (1 µm) were used in this study, creating unfavorable conditions for physicochemical attachment to solid-water interfaces, using both porous media and colloids used in this work were hydrophobic. The breakthrough data of water tracer and colloids were measured and simulated using the modified MIM (mobile-immobile) model, implemented on HYDRUS-1D code. Water flow and colloid transport parameters were determined by fitting HYDRUS model to observed BTCs. As water saturation may enhance or reduce the role of intra- and inter-porosity, all experiments have been performed under both saturated and unsaturated flow conditions. DLVO, hydrodynamic and capillary interactions were calculated
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to describe colloid deposition to the solid-water and air water interfaces and to distinguish the relative importance of intra- and inter-porosity on colloid transport and deposition.
2. Materials and methods 2.1. Porous media Two porous materials, sand and aggregated medium, were used for transport experiments performed in Plexiglas columns (10 cm in diameter and 27 cm in length). The sand used in this work was composed of highly pure quartz sand. Mineralogical analysis by X-ray diffraction indicated 98% of SiO2 with negligible, quantities of metal impurities (
