James E. Saiers

Colloid Movement in Unsaturated Porous Media: Recent Advances and Future Directions

Investigations of colloid movement through geologic materials are driven by a variety of issues, including contaminant transport, soil-profile development, and subsurface migration of pathogenic microorganisms. In this review, we address recent advances in understanding of colloid transport through partially saturated porous media. Special emphasis is placed on features of the vadose zone (i.e., the presence of air–water interfaces, rapid fluctuations in porewater flow rates and chemistry) that distinguish colloid transport in unsaturated media from colloid transport in saturated media. We examine experimental studies on colloid deposition and mobilization and survey recent developments in modeling colloid transport and mass transfer. We conclude with an overview of directions for future research in this field.

First- and second-order kinetics approaches for modeling the transport of colloidal particles in porous media

We present results from experiments on the migration of inorganic colloids through laboratory columns containing clean quartz sand. Particle retention on the quartz collectors was found to be substantially less in experiments using negatively charged silica (SiO2) colloids than in experiments using positively charged anatase (TiO2) or boehmite (AlOOH) colloids. Analysis of these data with respect to two different advection-dispersion models indicates that deposition of colloidal silica follows a first-order, reversible kinetics process, while deposition of both anatase and boehmite is more closely depicted by second-order kinetics. Fitted values of the rate constant used to describe particle attachment vary consistently with the mean grain size of the sand and, for anatase and boehmite, are within a factor of 2 of the values predicted on the basis of colloid filtration theory.

Colloidal silica transport through structured, heterogeneous porous media

Abstract In order to better understand factors controlling the migration of inorganic colloids through heterogeneous porous media, the transport of colloidal silica through columns containing a single preferred flow path was investigated. The preferred flow path, which consisted of a tubule of coarse-grained sand, was embedded in a matrix of fine-grained sand. The relative importance of advection, dispersion, deposition (removal of particles from suspension), entrainment (resuspension of deposited particles), and mass exchange between the two pore water regions was evaluated by comparing experimental data to calculations of a two-dimensional advection-dispersion model. Results of this comparison indicate that silica movement could be understood in terms of advective-dispersive transport in the preferred flow path and surrounding matrix with small interaction between the adjacent regions. Deposition of silica was found to follow a first-order kinetic process and was at least partially reversible.

The influence of ionic strength on the facilitated transport of cesium by kaolinite colloids

We report results of laboratory experiments on the co-transport of 137Cs by inorganic colloids composed of kaolinite. We find that under conditions of low pore water ionic strength, the kaolinite colloids significantly accelerate 137Cs transport through columns packed with quartz sand, kaolinite mobility and the affinity of kaolinite for binding 137Cs diminish with increasing ionic strength. As a result, kaolinite exerts a progressively smaller influence on 137Cs transport as the ionic strength increases from 0.002 to 0.1 m. The 137Cs breakthrough data are used to test a model that incorporates advection-dispersion equations for the movement of kaolinite colloids, dissolved 137Cs, and kaolinite-associated 137Cs and mass transfer equations for kaolinite deposition, 137Cs adsorption by kaolinite, and 137Cs adsorption by quartz sand; The partition coefficient for137Cs retention by kaolinite colloids and the first-order rate coefficient for kaolinite deposition vary in a discernible fashion with changes in ionic strength. The adsorption rate coefficient and the sorption capacity term of the second-order rate law taken to describe 137Cs adsorption to the quartz sand are independent of ionic strength; however, the magnitude of the desorption coefficient varies logarithmically with ionic strength. This work indicates the need to account for enhanced movement of sorbing solutes by inorganic colloids and provides a basis for quantifying the response of colloid-associated solute transport to changes in pore Water chemistry.

Chromium transport, oxidation, and adsorption in manganese-coated sand.

We examine how the processes of advection, dispersion, oxidation-reduction, and adsorption combine to affect the transport of chromium through columns packed with pyrolusite (beta-MnO2)-coated sand. We find that beta-MnO2 effectively oxidizes Cr(III) to Cr(VI) and that the extent of oxidation is sensitive to changes in pH, pore water velocity, and influent concentrations of Cr(III). Cr(III) oxidation rates, although initially high, decline well before the supply of beta-MnO2 is depleted, suggesting that a reaction product inhibits the conversion of Cr(III) to Cr(VI). Rate-limited reactions govern the weak adsorption of each chromium species, with Cr(III) adsorption varying directly with pH and Cr(VI) adsorption varying inversely with pH. The breakthrough data on chromium transport can be matched closely by calculations of a simple model that accounts for (1) advective-dispersive transport of Cr(III), Cr(VI), and dissolved oxygen, (2) first-order kinetics adsorption of the reduced and oxidized chromium species, and (3) nonlinear rate-limited oxidation of Cr(III) to Cr(VI). Our work supplements the limited database on the transport of redox-sensitive metals in porous media and provides a means for quantifying the coupled processes that contribute to this transport.

Advection, dispersion, and filtration of fine particles within emergent vegetation of the Florida Everglades

[1] The movement of particulate matter within wetland surface waters affects nutrient cycling, contaminant mobility, and the evolution of the wetland landscape. Despite the importance of particle transport in influencing wetland form and function, there are few data sets that illuminate, in a quantitative way, the transport behavior of particulate matter within surface waters containing emergent vegetation. We report observations from experiments on the transport of 1 mm latex microspheres at a wetland field site located in Water Conservation Area 3A of the Florida Everglades. The experiments involved line source injections of particles inside two 4.8-m-long surface water flumes constructed within a transition zone between an Eleocharis slough and Cladium jamaicense ridge and within a Cladium jamaicense ridge. We compared the measurements of particle transport to calculations of two-dimensional advection-dispersion model that accounted for a linear increase in water velocities with elevation above the ground surface. The results of this analysis revealed that particle spreading by longitudinal and vertical dispersion was substantially greater in the ridge than within the transition zone and that particle capture by aquatic vegetation lowered surface water particle concentrations and, at least for the timescale of our experiments, could be represented as an irreversible, first-order kinetics process. We found generally good agreement between our field-based estimates of particle dispersion and water velocity and estimates determined from published theory, suggesting that the advective-dispersive transport of particulate matter within complex wetland environments can be approximated on the basis of measurable properties of the flow and aquatic vegetation.

Migration of 137Cs through quartz sand: experimental results and modeling approaches

We present results from experiments on the migration of 137Cs through columns containing quartz sand. Times for 137Cs movement through these columns and the quantity of 137Cs adsorbed by the sand decreased as the ionic strength of the pore water increased from 0.002 to 0.1 m. The breakthrough curves were characterized by a slow approach towards steady-state concentrations as well as by long tails, indicating that 137Cs adsorption to the sand grains was, at least in part, controlled by rate-limited reactions. Various formulations for solute mass transfer were tested for their ability to fit the experimental breakthrough curves. Based on a statistical analysis, a nonlinear, two-site model was identified as the most appropriate for describing the suite of experimental data. Variation in the model parameter that describes the rate of 137Cs adsorption to the sand showed no consistent pattern with changes in ionic strength. In contrast, model parameters describing the sorption capacity of the sand grains and the fraction of kinetic sorption sites on the sand decreased with increasing ionic strength. The parameter describing the rate of 137Cs desorption varied directly with changes in ionic strength.

Colloid-facilitated transport of cesium in vadose-zone sediments: the importance of flow transients.

Colloid-sized particles are commonly detected in vadose-zone pore waters and are capable of binding chemicals with sorptive affinities for geologic materials. Published research demonstrates that colloids are capable of facilitating the transport of sorptive contaminants under conditions of steady pore water flow, when volumetric moisture content and pore water velocity are constant. Less is known about the role of colloids in governing contaminant mobility under transient-flow conditions, which are characteristic of natural vadose-zone environments. The objective of this study is to elucidate the influences of flow transients on the mobilization and transport of in situ colloids and colloid-associated contaminants. We conducted column experiments in which the mobilization of in situ colloids and (137)Cs was induced by transients associated with the drainage and imbibition of (137)Cs contaminated-sediments. Our results demonstrate that substantial quantities of in situ colloids and colloid-associated (137)Cs are mobilized as volumetric moisture content declines during porous-medium drainage and as volumetric moisture content increases during porous-medium imbibition. We also find that the colloid-effect on (137)Cs transport is sensitive to changes in pore water ionic strength. That is, the quantities of colloids mobilized and the capacity of the these colloids to bind (137)Cs decrease with increasing ionic strength, leading to a decrease of the mass of (137)Cs eluted from the columns during porous-medium drainage and imbibition.

Colloid-Facilitated Mobilization of Metals by Freeze–Thaw Cycles

The potential of freeze-thaw cycles to release colloids and colloid-associated contaminants into water is unknown. We examined the effect of freeze-thaw cycles on the mobilization of cesium and strontium in association with colloids in intact cores of a fractured soil, where preferential flow paths are prevalent. Two intact cores were contaminated with cesium and strontium. To mobilize colloids and metal cations sequestered in the soil cores, each core was subjected to 10 intermittent wetting events separated by 66 h pauses. During the first five pauses, the cores were dried at room temperature, and during last five pauses, the cores were subjected to 42 h of freezing followed by 24 h of thawing. In comparison to drying, freeze-thaw cycles created additional preferential flow paths through which colloids, cesium, and strontium were mobilized. The wetting events following freeze-thaw intervals mobilized about twice as many colloids as wetting events following drying at room temperature. Successive wetting events following 66 h of drying mobilized similar amounts of colloids; in contrast, successive wetting events after 66 h of freeze-thaw intervals mobilized greater amounts of colloids than the previous one. Drying and freeze-thaw treatments, respectively, increased and decreased the dissolved cesium and strontium, but both treatments increased the colloidal cesium and strontium. Overall, the freeze-thaw cycles increased the mobilization of metal contaminants primarily in association with colloids through preferential flow paths. These findings suggest that the mobilization of colloid and colloid-associated contaminants could increase when temperature variations occur around the freezing point of water. Thus, climate extremes have the potential to mobilize contaminants that have been sequestered in the vadose zone for decades.

Temperature and hydrologic controls on dissolved organic matter mobilization and transport within a forest topsoil.

Variations in concentrations of dissolved organic matter (DOM) in streams and rivers reflect, in part, coupled biogeochemical and hydrological processes that govern DOM mobilization and transport through soils. We explore the effects of temperature and rainfall characteristics on the quantity and composition of DOM mobilized from laboratory columns packed with an unsaturated forest soil. Our observations demonstrate that changes in temperature and a five-month drought period affect the mass and structure of DOM mobilized during soil-water infiltration, while changes in rainfall intensity and hourly to daily changes in the frequency of rainfall affect only the mass of DOM mobilized. The spectroscopic analyses indicate that DOM mobilized at low temperature is less humified and tends to be less condensed and lower in molecular weight. The C:N ratios of effluent DOM decline with cumulative rainfall volume during successive rainfall events and decrease by an average of 36% owing to the extended drought period. A two-region model that accounts for rate-limited DOM desorption from soil-water interfaces present within inter-aggregate and intra-aggregate pore spaces closely describes the time-series data on total concentrations of DOM eluted from the soil columns during the rainfall events.