ABSTRACT
References 313
11.1 INTRODUCTION
Surface chemical reactions play a major role in controlling contaminant fate and transport in the environment. To better understand such processes, one often resorts to welldefined laboratory studies using mineral and organic standards or synthetic analogs as surrogates for the more complicated natural systems, either focusing on homogeneous systems or assuming the additivity of the major system components. In reality, such mixtures may display changes in particle size, surface area, and reactivity that differ from the individual surrogate components or the natural diagenetic environment that the investigator wishes to emulate.1-5 For example, natural colloids observed in the electron microscope often appear irregularly eroded or coated with other mineral or organic phases and rarely resemble synthetic or pure mineral particles.6,7 Complex mixtures and the presence of “surface coatings” or surface heterogeneities, often representing only a small fraction of the total suspension or matrix composition, can alter the reactivity of the more abundant components in ways that are difficult to quantify or predict based on the idealized systems.8-11 Even common lab practices, such as homogenization and air-drying of soil materials can alter surface reactivity more than generally recognized.12-14
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
Not surprisingly, elevated concentrations of mobile groundwater colloids are generally associated with a disruption in the native hydrogeochemical environment, including those induced by artificial recharge, groundwater contamination, and even elevated flow rates associated with conventional sampling practices.22,23 When precautions are taken to ensure that groundwater samples are representative of actual geohydrologic conditions within an aquifer, background or control wells outside the influence of the contamination
source generally yield few mobile colloids.24,25 However, artifactual colloids can be introduced during well construction or development (drilling fluids, bentonite, etc.),26
or become suspended from the immobile matrix by the shear forces associated with pumping.17,22,23 Aggregation after sampling and membrane clogging can increase the efficiency of phase separation and reduce the average size and percentage of total suspended solids passing through the filter18,19; thus, the relative percentage of colloidassociated metals in filtered samples may not vary systematically with turbidity, that is, with colloid mass or concentration. In addition, larger size particles may contribute much of the colloidal mass, but reflect a smaller portion of the surface area available for contaminant sorption.20
