Colloid-enhanced transport

VIGNETTE 1.1 ENVIRONMENTAL SCIENCES Colloid-Enhanced Transport in Unconsolidated Media... 

Ouyang, Y. et ah, Colloid-enhanced transport of chemicals in subsurface environments A review, Crit. Rev. Environ. Sci. Technol., 26, 189, 1996. 

It would lie far beyond the aim of this chapter to introduce the state-of-the art concepts that have been developed to quantify the influence of colloids on transport and reaction of chemicals in an aquifer. Instead, a few effects will be discussed on a purely qualitative level. In general, the presence of colloidal particles, like dissolved organic matter (DOM), enhances the transport of chemicals in groundwater. Figure 25.8 gives a conceptual view of the relevant interaction mechanisms of colloids in saturated porous media. A simple model consists of just three phases, the dissolved (aqueous) phase, the colloid (carrier) phase, and the solid matrix (stationary) phase. The distribution of a chemical between the phases can be, as first step, described by an equilibrium relation as introduced in Section 23.2 to discuss the effect of colloids on the fate of polychlorinated biphenyls (PCBs) in Lake Superior (see Table 23.5). 

An additional process that enhances transport is due to the typical velocity distribution within the pores of the aquifer. Since colloids mainly move in the central parts of the... 

The objectives of this study were (1) to assess the effect of colloid mineralogical composition on colloid-mediated transport of metals in subsurface soil environments, and (2) to establish physicochemical gradients and conditions enhancing or inhibiting colloid-mediated transport. The following case studies were used to demonstrate the effects of mineralogy on colloid-mediated transport of metals. 

Figures 2.3 and 2.4 show breakthrough curves for total and soluble metal fractions, respectively, eluted in the absence and presence of colloids. In the absence of colloids (controls) practically none of the metals exhibited any meaningful breakthrough, suggesting nearly complete sorption by the soil matrix (Figure 2.3). The presence of colloids enhanced considerably total metal elution and in most cases even soluble metal elution, thus providing strong evidence for colloid-mediated metal transport.

Metal elution was essentially zero for all control treatments, suggesting total attenuation by the soils matrix (Figures 2.11 and 2.12). The presence of colloids enhanced drastically the elution of both soluble and total metal levels, showing an excellent correlation with colloid elution patterns. This confirms the strong association between metals and colloids and their role as carriers or facilitators in the transport process. 

Because mobile colloids may enhance the mobility in porous medias of hydrophobic pollutants such as PAHs, Jenkins and Lion (1993) tested bacterial isolates from soil and subsurface environments for their ability to enhance transport of phenanthrene in aquifer sand. The most mobile isolates tested significantly enhanced the transport of phenanthrene, as a model PAH, in sand. 

Kaplan et al. [100] measured nuclide distribution coefficients for Pu, Am, Cm, and U in groundwater colloids. This study showed the extent to which each actinide was associated with groundwater colloids, a potential cause for the apparently enhanced transport of the contaminants. 

The idea that colloidal forms of contaminants may exhibit enhanced transport through geologic media, relative to dissolved forms, has been the subject of much concern and debate for the last 15 or so years. It is interesting that a phenomenon so generally accepted in principle, and subjected to such intense scrutiny, should have yielded so little incontrovertible evidence at scales of interest, i.e., the field. 

The simulation results of Fig. 7-8 (Corapcioglu et al., 1999) show that, for this case, the presence of colloids enhances contaminant transport relative to the colloid-free case. If the first-order rate constant for the desorption of the contaminant from the mobile colloids, is small in value, the simulation results for the local equilibrium assumption (LEA) case and the kinetic case are the same. In the case of an intrinsic colloid, K m will be the rate of dissolution of the colloid however, if the rate of desorption of the contaminant is relatively slow (in these cases 0.012 vs. 0.0005 s" ), desorption kinetics has the effect of enhancing colloid-facilitated contaminant transport. Note that in the case of intrinsic colloids, Ks would 0. 

Evidence for colloid-facilitated radionuclide or metal ion transport come primarily from two sources (i) the deduction that a mobile colloid-phase must be present to explain the apparent enhanced transport of otherwise highly relarded species (e.g., Buddemeicr Hunt, 1988) and (ii) laboratory stuilies of model syslems (e.g.,... 

The Principles of Colloid Facilitated Radionuclide Transport outlined in the first section support the conclusion that colloid facilitated radionuclide transport is a phenomenon that will probably take place only under limited system conditions. Systems that we define here as being symmetrical are probably the dominant systems under natural conditions they also are the systems for which colloid-facilitated radionuclide transport will be minimal. In addition, radionuclides that are subject to colloid-facilitated transport have a dissolved phase component that is, in many cases, negligible. Kersting et al. (1999), for example, evaluated Pu at levels of around Laboratory studies provide incontrovertible examples that colloidal materials can enhance the transport of low-solubility radionuclide species but field studies remain ambiguous because the colloids, themselves, are not traced. 

Sonoelectrochemistry has also been used for the efficient employment of porous electrodes, such as carbon nanofiber-ceramic composites electrodes in the reduction of colloidal hydrous iron oxide [59], In this kind of systems, the electrode reactions proceed with slow rate or require several collisions between reactant and electrode surface. Mass transport to and into the porous electrode is enhanced and extremely fast at only modest ultrasound intensity. This same approach was checked in the hydrogen peroxide sonoelectrosynthesis using RVC three-dimensional electrodes [58]. 

Microbes are ubiquitous in the subsurface environment and as such may play an important role in groundwater solute behavior. Microbes in the subsurface can influence pollutants by solubility enhancement, precipitation, or transformation (biodegradation) of the pollutant species. Microbes in the groundwater can act as colloids or participate in the processes of colloid formation. Bacterial attachment to granular media can be reversible or irreversible and it has been suggested that extracellular enzymes are present in the system. Extracellular exudates (slimes) can be sloughed-off and act to transport sorbed materials [122]. The stimulation of bacterial growth in the subsurface maybe considered as in situ formation of colloids. 

In contrast, transport of chemicals through groundwater systems is generally enhanced by colloids. Though colloids may become immobile due to attachment to the solid matrix, part of them are moving with the water. Thus, chemicals that are sorbed on colloids and not on the solid matrix are not (or not as strongly) retarded as one would calculate by using the relative dissolved fraction,, . 

Humic substances (HS) are polymeric oxidation products that result from the decomposition of plant and animal residues. As a consequence of their colloidal state in natural waters, they play an important role in the transport of organic pollutants. Thus hydrophobic organic pollutants such as polycyclic aromatic hydrocarbons, DDT, and PCBs are known to bind well to humic substances, thereby enhancing the former s water solubility. One important characteristic of... 

In recent years the study of mobile soil and groundwater colloids has received considerable attention because of concerns that such a vector may enhance the mobility of strongly sorbing contaminants, a process that is often referred to as facilitated transport. 15-16 However, our ability to predict colloid movement and deposition is often confounded by the complexities of surface interactions in such dynamic, unstable systems. The lack of universally accepted analytical techniques and failure to realize instrumental limitations have made it difficult to compare and critically evaluate the results of different studies. Artifacts associated with ground-water sampling, filtration, and storage, and the dilute nature of most soil and ground-water suspensions further hamper characterization efforts.17-21... 

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