Mobile Colloids Research Articles

Physical and chemical determinants of colloid transport and deposition in water-unsaturated sand and Yucca Mountain tuff material.

Colloid mobility was determined in a system consisting of quartz sand or crushed Yucca Mountain tuff, simulated groundwater (J-13), and hydrophilic latex particles. Water content (theta) and ionic strength (I; DI water, 0.1x, 1x, 10x groundwater dilution) were manipulated to define limiting conditions for colloid transport atthe Yucca Mountain site. Colloid transport, measured with a centrifuge method at relatively high theta (saturation >36% for sand, >62% for crushed tuff) in DI water, was equivalent to transport at 100% saturation measured with conventional columns. When variables were isolated, increasing I and decreasing theta resulted in a greater extent of colloid deposition; I was more important at higher theta; physical properties were more important at lower theta. I and theta had an interactive effect on colloid deposition whereby synergism was generally observed, especially for simulated groundwater (1x); antagonism was observed at 10x groundwater dilution. At 19% moisture saturation on the crushed tuff, a decreasing rate of colloid deposition was observed. This corresponded to a hydrodynamic condition of 79% immobile water where solute tracers were excluded from a fraction of the pore volume. This suggests that a portion of the favorable sites for deposition were associated with the excluded or immobile water domain and were not accessible to colloids.

Electrically Switchable Colloidal Ordering in Confined Geometries

A novel field-induced approach has been developed that allows the controlled ordering of colloidal particles in confined systems. In this approach, an electric field is used to create a strong lateral dipole−dipole interaction that induces colloidal crystallization in two-dimensional geometries. Because the particle density is relatively low and the interaction of long range, colloidal mobility is high enough to allow rapid crystallization upon the application of the electric field. The observed phase transitions are fully reversible and can be readily controlled by adjusting either the applied field strength or particle confinement.

Functional characterization of colloidal phosphorus species in the soil solution of sandy soils.

The impact of mobile colloids on the transport of phosphorus in the subsurface environment is not well understood. We hypothesized that interactions between metals, organic matter, and P control the dynamics of mobile colloidal P species in excessively fertilized sandy soils. The effect of UV irradiation and additions of 32P, orthophosphate, Fe, Al, and NaF on the concentration of colloidal P was examined using gel filtration chromatography. In addition, molybdate unreactive P (MUP) was characterized using phosphomonoesterase assays. The high molecular mass reactive P (HMMRP) fraction did not react to orthophosphate additions, increased upon Al and Fe additions and decreased upon NaF addition and UV irradiation. These results support the hypothesis that HMMRP is present as organic matter-metal-orthophosphate complexes. The concentration of high molecular mass unreactive P (HMMUP) decreased upon UV irradiation. The MUP concentration slightly decreased upon incubation with phytase and acid phosphatase. These observations fitted well to the "protection" hypothesis, where hydrolyzable P bonds are protected from monoesterase attack through occlusion in colloidal material. Taken together, this study indicates the high potential for subsurface P loss by colloidal particles in soils excessively fertilized with animal manure.

Effect of colloids on radionuclide migration for performance assessment of HLW disposal in Japan

Abstract Colloidal effect is one of the major factors to enhance the migration of radionuclides in groundwater. The experimental and theoretical studies of colloid mobility and colloid-facilitated radionuclide transport for the performance assessment of high-level radioactive waste (HLW) geological disposal is presented in this paper. The major aims of the study are (1) to study the filtration effect on colloids by the engineered barrier system, (2) to study bentonite colloid generation by erosion of the engineered barrier system, and (3) to calculate radionuclide migration with groundwater colloids through fractured rock systems. Alternative coagulants based on prehydrolyzed forms of aluminium or iron can be more effective than the traditional materials in many cases, but their mode of action is not completely understood, especially with regard to the role of charge neutralization and hydroxide precipitation. Basic principles of colloid stability and metal ion hydrolysis are briefly reviewed, and the action of hydrolyzing metal coagulants is then discussed, with some examples from recent experimental studies. Although it is possible to interpret results reasonably well in terms of established ideas, there are still some uncertainties that need to be resolved

Colloids in the mortar backfill of a cementitious repository for radioactive waste

Colloids are present in groundwater aquifers and water-permeable engineered barrier systems and may facilitate the migration of radionuclides. A highly permeable mortar is foreseen to be used as backfill for the engineered barrier of the Swiss repository for low- and intermediate-level waste. The backfill is considered to be a chemical environment with some potential for colloid generation and, due to its high porosity, for colloid mobility. Colloid concentration measurements were carried out using an in-situ liquid particle counting system. The in-house developed counting system with three commercially available sensors allowed the detection of single particles and colloids at low concentrations in the size range 50–5000 nm. The counting system was tested using suspensions prepared from certified size standards. The concentrations of colloids with size range 50–1000 nm were measured in cement pore water, which was collected from a column filled with a highly permeable backfill mortar. The chemical composition of the pore water corresponded to a Ca(OH) 2-controlled cement system. Colloid concentrations in the backfill pore water were found to be typically lower than ∼0.1 ppm. The specific (geometric) surface areas of the colloid populations were in the range 240 m 2 g −1 to 770 m 2 g −1. The low colloid inventories observed in this study can be explained by the high ionic strength and Ca concentrations of the cement pore water. These conditions are favourable for colloid–colloid and colloid–backfill interactions and unfavourable for colloid-enhanced nuclide transport.

Assessment of Colloid Filtration in Natural Porous Media by Filtration Theory

Mobile colloids in groundwater aquifers can act as carriers for sorbing contaminants and thereby facilitate contaminant transport. Therefore, the removal efficiency of two natural porous media for colloids with regard to the ionic strength of the solution and the counterion valence has been investigated. Short-pulse experiments were conducted using fluorescence labeled latex colloids in order to investigate their transport behavior. The predictions of the filtration theory concerning the filtration of colloids were confirmed qualitatively. The removal efficiency of a filter bed is influenced by both the type of electrolyte and its concentration as well as the type of the porous medium. An attachment parameter α, which was introduced into Happel's Sphere-in-Cell-Model, describes the dependence of the removal efficiency on the electrolyte concentration qualitatively. The normalization of α by the valence of the dominant counterion describes the removal efficiency independent of the kind of electrolyte.

Selective Colloid Mobilization Through Surface-Charge Manipulation

The objective of the current study was to evaluate the use of amine hexadecyltrimethylammonium bromide (HDTMA) to enhance the mobilization and subsequent transport of colloidal iron oxides by selectively blocking negatively charged sites within soil or aquifer sediments. Two materials were used in a series of column leaching studies, a surface soil (Orangeburg Series) and an iron oxide-rich subsurface sediment (Tobacco Rd. Formation) both from Aiken, SC. For comparison, the same materials were leached with sodium hexametaphosphate (Na−P) as a nonselective dispersing agent. As a cationic surfactant, HDTMA is generally considered a strong flocculent for soils because of its ability to shield the electrostatic repulsion on opposing negatively charged clays, which was observed in leaching experiments for the surface soil material. Leaching HDTMA solutions through the iron oxide-coated aquifer sediments resulted in the selective dispersion and transport of iron oxides, relative to the more abundant kaolinite. Despite effluent colloid levels in excess of 6 g L-1, no column plugging was observed for the HDTMA treatments. The Na−P treatment, however, produced effluent turbidity levels that were less than HDTMA but induced rapid column plugging. Thermal characterization of the Na−P-derived colloids indicated that they were similar to the same as the bulk clay fraction of the aquifer sediment, indicating that dispersion was nonselective. HDTMA appears to block negatively charged filtration sites that limit iron oxide transport, thus enhancing colloid dispersion without inducing column plugging observed for nonselective dispersants. Iron oxides have been demonstrated to be the resident phase for many inorganic and organic contaminants within highly weathered, organic matter-poor systems. This suggests that selective mobilization of colloidal iron oxides and their associated contaminants can potentially enhance subsurface remediation activities via implementation of pump-and-treat technologies.

Field Investigations and Modeling of Particle‐Facilitated Pesticide Transport in Macroporous Soil

AbstractPhysical and chemical non‐equilibrium processes may facilitate the transport of pesticides and other chemicals through structured and macroporous soils. For sorbing pesticides, transport associated with a mobile colloidal or particulate phase represents an additional transport mechanismin structured soils that is not well understood. We investigated particle‐facilitated transport of a sorbing pesticide (prochloraz, N‐propyl‐N‐[2‐(2,4,6‐trichlorophenoxy)ethyl]imidazole‐1‐carboxamide) in a 5‐ by 5‐m subsurface‐drained field plot in a structured sandy loam in Denmark. Following pesticide application, three simulated rainfall events during an 8‐d period were monitored in terms of drainage flow rate, content of particulate matter (>0.24 µm), and pesticide concentration in the solution and in the particulate phases in the drainage water. The fraction of pesticide loss to the drain was 0.2% of the applied mass, of which 6% was associated with the particulate phase. Macroporous flow paths appeared to be major routes of pesticide and particle transport. Preferential sorption to particles in the drainage water relative to bulk soil, and possibly also slow desorption from the particles, were assumed to influence the pesticide leaching in the particulate fraction. Based on experimental and reported data, the dual‐porosity model MACRO, modified to account for particle mobilization and transport, could be calibrated to simulate the observations. Sensitive parameters for the particle and pesticide descriptions were identified.

Colloid Movement through Stable Soils of Low Cation-Exchange Capacity

Factors controlling the transport of mobile colloids through soils are poorly understood yet have major environmental impacts. This study attempted to identify the relative significance of two physical factors controlling movement of different illitic clays through columns of various stable, nonreactive soils: (i) pore size distribution of the soil matrix and (ii) average size of mobile colloids. The columns were leached with various clay suspensions, and colloid concentrations were measured in relation to effluent solution chemistry. No single factor dominated colloid mobility through the soils tested: pore size distribution exerted some control, but so too did size of the mobile colloids (partly affected by cation status). The most mobile colloids in low-charge soils may simply be fine enough to escape physical filtration by the soil matrix. This applies mainly to near-saturated soils, because as soils dry, greater salt concentrations and particle interactions increase mobile-particle sizes and thus i...

Prediction of colloid detachment in a model porous media: hydrodynamics

Understanding the mobility of colloids through porous media is important in both engineered and natural applications. This study employed a model system to explore physically based colloid detachment. Polystyrene colloids were attached in the primary minimum to glass beads in a packed column, and a residually attached fraction was subsequently detached via hydrodynamic shear. Colloid fractions released from the surfaces in the porous media were experimentally quantified. A flowrate of 75 ml/min detached 50% of the residual colloid fraction from the surface. The residual colloid fraction released was predicted with a model incorporating an interaction energy distribution and system physics. In this model, detachment is realized when the applied rolling moment from hydrodynamic shear overcomes the resistance associated with rolling. With the exception of one parameter, the hysteresis loss factor, system characteristics were described a priori. Using the hysteresis loss factor and interaction energy distribution developed from extended-DLVO theory, the detachment of the residual colloid fraction from the packed bed was well predicted. This work decomposes colloid detachment into the constitutive mechanisms dependent upon thermodynamics and hydrodynamics.

Processes of colloid mobilization and transport in macroporous soil monoliths

Transport of pesticides, PAH and other hydrophobic or surface-complexing contaminants in soils may be enhanced by colloid-facilitated transport. A prerequisite for colloid-facilitated transport is the release and transport of colloids. The mechanisms for colloid mobilization and transport in a macroporous Alfisol have been evaluated by measuring the amount and type of colloids leached in two large soil monoliths during long duration simulated rain events. The soil was irrigated with water having a chemical composition close to natural rainwater and at intensities as expected under natural conditions. The results showed that the colloids were primarily mobilized and transported in the macropores and that the source of colloids was not exhausted for extended rainfall duration. The first flush of water mobilized loosely bound colloids that had a high organic content relative to the bulk soil. After the initial release, the high ionic strength in the percolating water limited the mobilization. For prolonged leaching, the diffusion of colloids from the macropore walls appeared to rate-limit the mobilization process. During the late leaching phase, the rate of colloid mobilization was positively correlated with flow velocity.

Origin and mobility of humic colloids in the Gorleben aquifer system

The origin and mobility of humic colloids in the Gorleben aquifer system have been examined. For this purpose, the distribution of humic colloids and relevant hydrological and geochemical parameters were examined. An investigation area was selected where sediments have been disturbed by salt dome uplift and glacial events. It is shown that, on a local scale, considerable groundwater movement and intermixing takes place from the surface down to the salt dome. Consequently no effective separation of groundwater layers occurs. Two different humic colloid sources are identified: influx from the humus horizon with recharge water and continuous in situ generation via mineralization of sedimentary organic carbon (SOC). The in situ generation leads to groundwaters with humic colloid concentrations approaching 0.4 g/L, compared to concentrations of less than 0.005 g/L in recharge waters. Young groundwaters (no 14C decay detected) between approximately 50 and 200 m depth exhibit these highly elevated humic colloid concentrations. At greater depth, salt brines are found with low humic colloid concentrations. This can be attributed to precipitation of humic acid and/or hampering of the in situ generation process due to the high salt content. There is no indication of retention or decomposition of humic colloids. The fate of in situ generated humic colloids cannot be precisely evaluated due to analytical limitations and insufficient understanding of groundwater movement.

Evaluation of the effects of natural dissolved and colloidal organic ligands on the electrochemical lability of Cu, Pb and Cd in the Arran Deep, Scotland

The physicochemical speciation of Cu, Pb and Cd was determined by voltammetric titration procedures on filtered (F; 0.4-μm) and cross-flow filtered (CFF; 3000 Da) fractions of 15 samples collected at three sites in the Firth of Clyde, on Scotland's west coast. The colloidal material — defined on the basis of size (size <0.4 μm) and molecular weight (NMW>3000 Da) — was characterized by dynamic light scattering in terms of its electrophoretic behaviour with changing pH in a 0.01 M NaCl medium. The particulate material (size >0.4 μm) was analysed for its total metal content (Cu, Pb, Cd, Mn, Fe) by atomic absorption spectrometry. The top 20 m of the water column in the Arran Deep was influenced by freshwater inputs (32.9<S<33.2/Sbottom=33.7) and showed elevated organic complexation for Cu, Pb and Cd. Dissolved organic ligands were responsible for producing pCu (−log [Cu2+]) values of 10.2–11.9 and pCd values of 11.2–11.6. Colloidal ligands were largely responsible for the elevated pPb values (11.9–12.5) observed in the top 20 m of the water column. Unexpected features in (a) the speciation and size fractionation of Cu and Pb, and (b) the electrophoretic mobility (uE) of colloids, occurred at depths situated 35 m above the bottom at the three sites. At these depths, uE measured at any given pH showed minima, coinciding with maxima in the concentrations of colloidal ligands with strong affinity for Cu and Pb, and with maxima in the colloid/solution partitioning of Cu. No such anomalies were observed in the vertical profiles representing the acid–base properties of the colloids or in any of the key physicochemical variables such as S, T, pH or suspended particulate matter (SPM). These observations suggest that (1) the deep water (80–150 m) cycle of Cu and Pb may be partly coupled to that of their organic ligands via the in situ formation of colloids at a depth situated around (Zbottom−35) m, and (2) these colloids contain weakly bound (cation-bridged) assemblages made up from the same dominant organic components as found in solution at other depths in the water column.

5846434 In-situ groundwater remediation by selective colloid mobilization : Seaman John C; Bertch Paul M New Ellenton, SC, United States assigned to University of Georgia Research Foundation

5846434 In-situ groundwater remediation by selective colloid mobilization : Seaman John C; Bertch Paul M New Ellenton, SC, United States assigned to University of Georgia Research Foundation

Field studies of in situ colloid mobilization in a southeastern coastal plain aquifer

The release of colloids to groundwater was investigated in situ in an iron‐oxyhydroxide‐rich, sandy aquifer. Groundwater amended with various solutes was injected into and immediately withdrawn from the shallow aquifer. Turbidity and colloid composition were monitored in the retrieved injectate. The response of the aquifer material to the amendments generally mimicked that observed in an earlier study using packed columns containing the sediment, demonstrating the viability of the single‐well method for testing colloid mobilization in situ. The decline of turbidity in the retrieved injectates with increasing withdrawal volume was analyzed to determine a “reaction order” n, describing the redeposition of mobilized colloids to the immobile matrix. Differences in the reaction order for the amendments tested presumably indicated the effectiveness of these amendments to generate repulsive colloid‐immobile matrix interactions.

Colloidal and dissolved metals in leachates from four Danish landfills

Landfill leachate samples were collected at four Danish landfills. The samples were collected anaerobically for characterization of the leachate in terms of pH, total alkalinity, total solids, total organic carbon, cations, anions and heavy metals. The sample procedure was designed to accurately reflect the in situ mobile colloid and associated contaminant concentrations. The leachate samples were separated into size fractions in order to obtain information about size distribution of colloids and associated heavy metals using filters with the pore sizes: 1.2, 0.40 and 0.001 μm. The general characterization of the four leachate samples showed large variation in the leachate quality for the investigated landfills ranging a factor of 2–60 among the four landfills for individual parameters. The dominant part of the colloidal material was in the smallest colloidal size fraction (0.001–0.40 μm) with total organic carbon (TOC) as one of the major components of the colloidal mass. Also Si, Al, Ca, Fe and S were found in the colloidal fractions. This indicated that both clay type particles, organic matter and precipitates (most likely sulfides) were present in the leachate samples. The distribution of the investigated elements varied from Cl being primarily in the dissolved fraction to Fe which mostly was found in the colloidal fractions. The concentrations of heavy metals in the leachate samples were 0.2–3.6 μg Cd/L, 28–84 μg Ni/L, 85–5310 μg Zn/L, 2–34 μg Cu/L, 0–188 μg Cr/L and 0–16 μg Pb/L with a considerable but highly varying part of the heavy metals associated with the colloidal fractions. However, when the heavy metal content in leachates is low, it is probably not of major environmental concern that part of the heavy metal is associated with the colloids. However, if the metal content is high and, as was the case for Zn in one of the leachate samples, nearly exclusively associated with the colloidal fractions, the behavior of the heavy metal in the leachate treatment facilities and in the environment may differ from the behavior expected for truly dissolved heavy metal.

Soluble Organic Carbon in a Biosolids‐Amended Mollisol

AbstractOrganic matter can be mobilized from many sources and transported in soil by precipitation, snowmelt, and water table fluctuations. Knowledge of the potential fluxes of soluble organic C in different kinds of soil is essential to improve global C budgets. We characterized the soluble and suspendable organic matter in a biosolids‐amended fine, smectitic, mesic Cumulic Vertic Endoaquoll under two vegetation treatments: switchgrass (Panicum virgatum L.) and cottonwood‐poplar hybrid (Populus × euramericana—clone NC‐5326). Soil organic C (OC) that was dispersed in 0.1 mM CaCl2 was fractionated according to molecular weight and polarity using dialysis membranes and XAD‐8 resin chromatography. Under both types of vegetation, addition of biosolids increased the amount of soluble OC (SOC) in the soil, particularly under switchgrass. Biosolids application shifted the molecular‐weight distribution of SOC to lower weights (&lt;14 000 Da). The concentrations of hydrophilic OC fractions exceeded those of hydrophobic OC in all treatments, although biosolids application increased the levels of both fractions. As the rate of biosolids addition increased, the 0 to 5 cm zone yielded smaller concentrations of extractable colloidal organic compounds than the 5 to 25 cm zone, and we attributed this to the high amounts of soluble salts introduced by biosolids application. A sequential extraction experiment showed that large amounts of colloidal organic compounds could be mobilized from the soil only after removal of the soluble salts. Our results suggest that biosolids amendments could increase the mobilization of hydrophilic, low‐molecular‐weight compounds in a soil while at the same time limiting the mobility of organic colloids.

Impact of Ionic Strength on Colloid Mobility in Saturated And Unsaturated Porous Media

AbstractA model system consisting of well-characterized quartz sand, negatively charged carboxyl-modified latex microspheres, and synthetic J- 13 groundwater was used as a first-step in defining limiting conditions for colloid transport at the proposed Yucca Mountain Repository for radioactive waste. Colloid (280-nm diameter) and tracer (bromide) transport was determined using column methods under steady-state flow conditions for both saturated and unsaturated moisture conditions. Experiments were conducted at three water contents (13%, 70%, and 100% saturated) and four ionic strengths (deionized water, and 0.1 x, 1x, and 10x the ionic strength of J-13 groundwater, which has an ionic strength of 0.0116). Colloid mobility in the 13% saturated columns was appreciably less than in the 70% and 100% saturated columns, whereas colloid mobility in the 70% and 100% saturated columns were quite similar. Ionic strength had a significant impact on colloid retention, becoming more pronounced in the drier systems. Colloids were mobile in deionized water and the 0.1x J-13 groundwater. Retention was approximately 20% in lx J-13 groundwater and 100% in the 10x J-13 groundwater. Compared with the 70% and 100% saturated conditions, colloid mobility decreased for the 10% saturated condition, with greater than 50% retention for the 1x J-13 groundwater system. As observed for the 70% and 100% saturated conditions, colloids in the 10x J-13 solution were essentially immobile. Colloid mobility is greatly affected by the ionic strength of the soil solution and this effect is moisture saturation dependent.

Migration of plutonium in ground water at the Nevada Test Site

Mobile colloids—suspended particles in the submicrometre size range—are known to occur naturally in ground water1, 2 and have the potential to enhance transport of non-soluble contaminants through sorption3. The possible implications of this transport mechanism are of particular concern in the context of radionuclide transport. Significant quantities of the element plutonium have been introduced into the environment as a result of nuclear weapons testing and production, and nuclear power-plant accidents. Moreover, many countries anticipate storing nuclear waste underground. It has been argued that plutonium introduced into the subsurface environment is relatively immobile owing to its low solubility in ground water4 and strong sorption onto rocks5. Nonetheless, colloid-facilitated transport of radionuclides has been implicated in field observations6, 7, but unequivocal evidence of subsurface transport is lacking3, 8, 9. Moreover, colloid filtration models predict transport over a limited distance resulting in a discrepancy between observed and modelled behaviour3. Here we report that the radionuclides observed in groundwater samples from aquifers at the Nevada Test Site, where hundreds of underground nuclear tests were conducted, are associated with the colloidal fraction of the ground water. The 240 Pu/239 Pu isotope ratio of the samples establishes that an underground nuclear test 1.3 km north of the sample site is the origin of the plutonium. We argue that colloidal groundwater migration must have played an important role in transporting the plutonium. Models that either predict limited transport or do not allow for colloid-facilitated transport may thus significantly underestimate the extent of radionuclide migration.

Transport of reactive colloids and contaminants in groundwater: effect of nonlinear kinetic interactions

Transport of reactive colloids in groundwater may enhance the transport of contaminants in groundwater. Often, the interpretation of results of transport experiments is not a simple task as both reactions of colloids with the solid matrix and reactions of contaminants with the solid matrix and mobile and immobile colloids may be time dependent and nonlinear. Further colloid transport properties may differ from solute transport properties. In this paper, a one-dimensional model for coupled colloid and contaminant transport in a porous medium (COLTRAP) is presented together with simulation results. Calculated breakthrough curves (BTC's) during contamination and decontamination show systematically the effect of nonlinear and kinetic interactions on contaminant transport in the presence of reactive colloids, and the effect of colloid transport properties that differ from solute transport properties. It is shown that in case of linear kinetic reactions, the rate of exchange of mobile and immobile colloids have a large impact on the shape of BTC's even if the solid matrix is saturated with respect to colloids. BTC's during the contamination and decontamination phase have identical shapes in this case. Moreover, the slow reactions of contaminants and colloids may lead to unretarded breakthrough of contaminants. Independent of reaction rates, nonlinear reactions lead to BTC's that are steeper during contamination than in the linear case. A characteristic aspect of nonlinear sorption is that shapes of BTC's differ during the contamination and decontamination phase. It has been observed that shapes of some of the simulated adsorption and desorption curves are similar as shapes found in experiments reported in literature. This stresses the importance of incorporating both kinetics and nonlinearity in models for coupled colloid and contaminant transport and the capability of COLTRAP to interpret experimental results. Finally, to figure out whether nonlinear processes play a role, it is very important to consider both contamination and decontamination in transport experiments.