Environ. Sci. Technol. 2000, 34, 3749-3755
Selective Colloid Mobilization Through Surface-Charge Manipulation JOHN C. SEAMAN* AND PAUL M. BERTSCH Advanced Analytical Center for Environmental Sciences, Savannah River Ecology Laboratory, The University of Georgia, Drawer E, Aiken, South Carolina 29802
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.
Introduction The strong partitioning (i.e., precipitation, sorption, etc.) of groundwater contaminants to the immobile solid aquifer matrix typically limits the success of remediation efforts. The slow release of contaminants can make it inefficient to reclaim an aquifer by simply capturing and treating the groundwater (1). Recent studies suggest that soil and groundwater colloids * Corresponding author phone: (803)725-0977; fax: (803)725-3309; e-mail:
[email protected]. 10.1021/es001056w CCC: $19.00 Published on Web 07/29/2000
2000 American Chemical Society
mobilized as a result of chemical perturbation may actively facilitate the subsurface migration of sparingly soluble contaminants, such as radionuclides, transition metals, metalloids, and hydrophobic organics (2-8). Therefore, enhancing the migration of mobile colloids and their associated contaminants has been proposed as a means of increasing the efficacy of contaminant extraction systems, such as pump-and-treat, with the obvious recognition that formation damage due to pore clogging may be an important obstacle to overcome (2, 9). Despite such assertions, little progress has been made in the effective management of colloidal migration as a reclamation strategy. Considerable research has focused on elucidating the processes controlling mobile colloid generation. Previous studies suggest that mobile colloids may be produced in the environment by a number of mechanisms including clay dispersion due to changes in groundwater pH, ionic strength, and/or Na/Ca ratios (3, 6, 10-18); dissolution of carbonate or iron cementing agents resulting in the release and transport of silicate clays (19-21); and the precipitation of colloidal particulates resulting from changes in local groundwater chemistry (22). Physical perturbation due to increased shear velocities associated with elevated pumping rates or bailing groundwater samples can artifactually mobilize colloids that would otherwise remain in place (23-25). Colloid dispersion resulting from a change in ionic strength, pH, or the sodium adsorption ratio (SAR) of the soil or geologic material has been the focus of most controlled laboratory studies due to the deleterious impact of Na+ on the physical properties of agricultural soils and geologic formations as it influences hydraulic conductivity (13, 14, 26, 27). To a degree, these factors have been emphasized because they are easier to experimentally control than factors such as redox potential or the partial pressure of CO2. Previous studies have clearly demonstrated the importance of iron and aluminum oxides in controlling aggregation/filtration behavior (10, 19, 28-33). The inability to predict colloid deposition rates under so-called “unfavorable” capture conditions has been attributed to surface charge heterogeneities associated with the presence of iron and aluminum oxides (33-35). Thus, colloid mobility in low-carbon, oxidecoated systems has generally been thought to be quite limited due the favorable conditions for attachment of negatively charged clays (33). To overcome the flocculating effect of iron oxides, studies often resort to conditions that highly favor the generation and transport of negatively charged phyllosilicates, such as the use of sodic solutions as surrogates for native or contaminated pore waters (3, 6, 10, 11). In general, these studies demonstrate that a high exchangeable sodium percentage (ESP) and an elevated pH, which reduces positive charge by inducing oxide charge reversal, are required in addition to low ionic strengths to observe significant colloid generation. In a series of column experiments in which coarsetextured, oxide-coated Atlantic Coastal Plain sediments were leached with solutions containing various Na/Ca ratios at different ionic strengths and pH values, Seaman et al. (12, 36) observed that colloidal dispersion resulted from minor changes in solution chemistry, many of which are considered highly flocculating based on current thoughts regarding colloid generation in the subsurface environment (25). The introduction of dilute CaCl2 solutions resulted in colloid mobilization and a decrease in effluent pH attributed to Al3+ exchange and hydrolysis reactions as well as specific cation VOL. 34, NO. 17, 2000 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 1. Mechanisms responsible for shifts in solution pH (A) and the generation of positive surface charge (B) resulting in iron oxide dispersion (based on data from ref 47). sorption reactions on hydrous oxide surfaces that can impart greater net-positive charge (Figure 1). The resulting colloids displayed positive electrophoretic mobilities, and electron microscopy and thermal analysis confirmed that they consisted primarily of iron oxides (goethite) rather than the predominate phyllosilicate, kaolinite (12, 36, 37). The ability to produce stable positively charged colloids has been confirmed at Darcy flow velocities ranging from 0.73 to 3.7 m d-1 using multiple sediment samples displaying a range of iron oxide/kaolinite contents, with the production of iron oxide colloids generally increasing with increasing iron oxide content (12, 36). According to the conceptual model presented in Figure 1, such materials are at or near the zero point of net charge (ZPNC) for the combined mineral assemblage (i.e., variable charge oxides and constant charge phyllosilicates) and thus remain flocculated regardless of the SAR or the ionic strength of the suspending solution. This behavior is consistent with the low ionic strength native solutions to which such materials are typically exposed (IS < 0.5 mM, pH ≈ 5.0) (38). Reactions that alter pH, however, including an increase in the concentration of polyvalent cations which lowers pH, can increase net matrix charge. Although the low pH mechanism of colloid generation may be more common than previously recognized, the mobilization of significant iron oxide coatings can expose negatively charged surfaces, which then act as deposition sites. To significantly increase colloid mobility, one must block the negatively charged sites in a manner similar to the way previously deposited colloids inhibit the subsequent filtration of additional particles (33, 34). On the other hand, widespread colloid dispersion can induce formation damage due to pore clogging, which may limit further particle migration. Once the hydraulic conductivity of a given aquifer region has been reduced, the efficacy of any remediation technique is compromised. Therefore, the ability to control these two disparate processes is critical to the use of colloid mobilization as an enhanced contaminant extraction technique. 3750
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Cationic surfactants such as the quaternary amine hexadecyltrimethylammonium bromide (HDTMA) are generally poor detergents because they readily flocculate clay suspensions at surface coverages equal to or less than the CEC by shielding negative charge sites and favoring the face-to-face association of opposing clay surfaces (39, 40). At relatively high surface coverages, however, the cooperative adsorption of HDTMA in excess of the cation exchange capacity (CEC) can induce charge reversal and the eventual stabilization of a suspension as positively charged colloids (39, 41). The objective of the current study was to evaluate the use of HDTMA to enhance and control the mobilization and subsequent transport of native colloidal iron oxides by selectively blocking negatively charged sites within the aquifer matrix.
Materials and Methods A surface soil horizon (Ap) (Orangeburg Series; Typic Paleudult, fine-loamy, siliceous, thermic) and an aquifer sediment (Tobacco Rd. Formation) were collected on the Department of Energy’s Savannah River Site (SRS), near Aiken, SC (Table 1). These materials are typical of the coarsetextured, highly weathered soils and sediments of the Atlantic Coastal Plain. The surface soil was collected from an exposed profile located within a mixed deciduous/coniferous forest. The aquifer sediment was collected from a deep erosional exposure of sediments typical of the water table aquifer (Tobacco Rd. Formation) and the first confined aquifer (Barnwell Formation). Prior to sample collection, the dried surface crust and overburden present on the exposure were removed to reveal the moist homogeneous material that was stored in a field-moist state (typically 0.5-5%) at 4 °C until column packing. Typical specimens of both materials have been characterized extensively and described in previous studies (36, 42). The clay fractions of both materials are primarily kaolinitic with the surface soil also containing hydroxy-interlayered vermiculite (HIV) and gibbsite, while the remaining clay fraction of the aquifer material consists
TABLE 1. Chemical and Physical Characteristics of Samples Used in Column and Batch Studies. Tobacco Rd. sedimenta
Orangeburg Seriesb
5.30 4.98 0.02 0.19 g 100 g-1 87.2 4.5 8.3 BDL k, goe, m
5.52 4.43 0.76 0.74 g 100 g-1 85.5 7.8 6.6 4.5 k, HIV, gibb
pHwater pHKCl TOC (g/100 g)c CDB Fe (g/100 g)d PSDe sand silt clay WDC f clay mineralogyg
a 2:1 solution/soil ratio in DIW. b 2:1 solution/soil ratio in 1 M KCl. TOC, total organic carbon, dry combustion method (55). d CDB, citratedithionite-bicarbonate extraction (54). e PSD, particle size distribution (44). f WDC, water-dispersible clay determined without the aid of a dispersing agent (i.e., sodium hexametaphosphate) (44). BDL, below detection limits. g Clay mineralogy determined by X-ray diffraction: k, kaolinite; HIV, hydroxy-interlayered vermiculite; gibb, gibbsite; goe, goethite; m, mica (illite). c
TABLE 2. Composition of Artificial Groundwater Based on Routine Monitoring Data from the Department of Energy’s Savannah River Site (38)a component
concn (mg L-1)
component
concn (mg L-1)
Ca2+ Mg2+ K+
1.00 0.37 0.21
Na+ SO42pH
1.40 0.73 5.2
a Na SO was used to make up the sulfate component, and then 2 4 chloride salts were used for the remaining Na and the other cations as well. The pH was adjusted using HCl.
mainly of goethite and mica (illite). Analysis of exchangeable cations indicated that both materials have a high degree of Al3+ saturation on the exchange complex (g80%) (12, 43). The water-dispersible clay (WDC) content for each soil was determined as an indicator of inherent clay dispersion potential. Four grams of each soil was weighed into three replicate 50-mL centrifuge tubes and 40 mL of deionized water (DIW) was added to each tube prior to shaking overnight. After being shaken, each soil suspension was allowed to settle undisturbed for approximately 2 h before the dispersed colloidal fraction (3 mL) was sampled by slowly pipetting at a fixed, predetermined depth within the tube below which particles >2 µm would be expected to have settled (44). The dispersed clay present in the sampled aliquot was quantified by placing the pipetted suspension in preweighed, oven-dried aluminum tins and heating at 110 °C to dry the sample prior to reweighing the pan. Column Methods. Field-moist samples were packed in 10-cm-long columns with interior diameters of 5 cm to a uniform bulk density of ≈ 1.5 g cm-3. Above and below the sample, 1-cm layers of Ottawa sand were included to disperse flow throughout the entire cross-section of the column and reduce the impact of turbulent flow on colloid formation. The packed columns were oriented vertically and slowly saturated from the outlet in an upward manner with an artificial groundwater solution (AGW) based on the composition of groundwater collected from several nonimpacted wells located on the SRS (Table 2). The saturated columns were then oriented horizontally and leached at a constant Darcy velocity of ≈ 0.72 m d-1 with one of several treatment solutions: 0.001 M NaCl or 0.0005 M CaCl2 (inlet pH ∼ 6.0); acidic 0.005 M CaCl2 (adjusted to a pH of 3.0 with HCl); 0.001 M Na-hexameta-polyphosphate (NA-P); and 0.00025-0.001 M HDTMA. HDTMA breakthrough in the organic poor Tobacco Rd. material was evaluated based on dissolved organic carbon in the effluent (Shimadzu, Inc.).
The pH, electrical conductivity (EC), and turbidity of the column effluents were continuously monitored, and turbid samples were collected for surface charge and chemical characterization. When effluent turbidity exceeded the 100 NTU linear range of the instrument (DRT 200B, Hialeah, FL), an aliquot of the turbid sample was diluted with DIW to within the linear range. A linear regression comparing the mass (g L-1) of colloids mobilized as a function of NTU based on the diluted samples had an r2 of 0.974. Column plugging, as influenced by solution treatment, was monitored using a 0-1 PSI pressure transducer (OMEGA Engineering, Stamford CT) calibrated with a piezometer tube at the column inlet. Each repacked column was used for only one leaching treatment. The electrophoretic mobilities of suspended colloids produced in the column experiments were determined by laser doppler velocimetry with a Delsa 440 (Coulter Electronics, Hialeah, FL). Thermal Analysis of Column Suspensions. Suspensions generated by the HDTMA and Na-P treatments were dialyzed against DIW to remove soluble salts, quick-frozen in liquid N2, and then freeze-dried prior to thermal analysis. Suspensions generated by the other treatments, such as 0.001 N CaCl2, required preconcentration prior to analysis. The dilute suspensions were filtered through a 0.1 µm pore-size polycarbonate filter. The filter was then placed in a 250 mL Nalgene bottle, filled to half its volume with DIW, and shaken to resuspend the captured particulates. The clean filter was removed from the bottle and the resulting suspension was freeze-dried as described above. Approximately 10 mg of freeze-dried clay were analyzed by thermal-gravimetric analysis using the dynamic-rate, high-resolution mode (TGA 2950, TA Instruments Inc.). Postcolumn Flocculation Studies. Suspensions generated by HDTMA and Na-P treatments were used in flocculation studies as an indication of particle charge and colloidal stability. The ability to flocculate the captured suspensions efficiently is critical for the development of a practical groundwater remediation scheme. Bulk suspension samples generated by each treatment were split into several equivalent fractions and subjected to the following flocculation treatments: (1) control (i.e., no chemical adjustment); (2) pH adjustment to 7.5 with NaOH; (3) addition of CaCl2 for a final concentration of 0.001 M; and (4) pH adjustment to 7.5 plus addition of CaCl2 for a final concentration of 0.001 M. After chemical treatment, all of the tubes were physically agitated to resuspend the colloids and then periodically photographed as a function of settling time.
Results and Discussion The Orangeburg surface soil was included in the present study to illustrate how the evaluated treatments affect the dispersion of materials which are similar in mineralogy (i.e. kaolinitic) and texture (≈90% sand) to the oxide-coated subsurface sediments, but display little positive charge due to the higher organic matter content and the presence of more-negatively charge clays such as HIV. As a result, the Orangeburg soil can be easily dispersed when shaken in DIW, i.e., water-dispersible clay (WDC), in contrast with the oxidecoated Tobacco Rd. material which readily flocculates in batch dispersion tests (Table 1). Column Results. Initial column effluents from the Orangeburg soil display extremely high turbidities that depend on inlet solution flow rate, regardless of the composition of the treatment solution. As the higher ionic strength treatment solution breaks through, however, the response in terms of mobile colloid generation is consistent with numerous studies in that Ca2+-rich or acidic treatments tend to inhibit colloid transport compared to highpH, sodic solutions (data not shown). In fact, the 0.001 M HDTMA solution was more effective than Na+ and even Ca2+ in decreasing effluent turbidity during leaching. A significant VOL. 34, NO. 17, 2000 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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decrease in effluent solution pH was observed for the surface soil resulting from Al3+ exchange and hydrolysis described in Figure 1A, but the mobile colloids display negative electrophoretic mobilities regardless of influent treatment. Thermal analyses of the initial suspensions resulting from the Orangebeurg soil are consistent with the bulk clay and WDC generated in batch studies. These results are somewhat conflicting with the rather striking differences between the mineralogy of the WDC fraction from the surface horizon and mobile colloids which were collected at the bottom of rebuilt Orangeburg soil profiles (16). These results suggest that small-scale column studies may not accurately reflect other particle retention and filtration processes that may be active at the pedon and field scale, especially when the pedon consists of distinctly different textural and mineralogical horizons. In contrast to the surface soil, the subsurface material is generally stable (i.e. well-flocculated) in low ionic strength solutions, with little turbidity observed when initially saturated with AGW and even less when exposed to 0.001 M NaCl (Figure 2). This is consistent with other studies in that very little colloidal material is mobile under the dilute native groundwater conditions (pH ≈ 5.0, IS < 0.5 mM) until the pH is raised above 7.0 or the iron oxides have been removed (10, 19). However, leaching dilute CaCl2 solutions through the oxide-rich material results in a stable turbid suspension that coincides with a spontaneous drop in effluent pH, resulting in colloids that display positive electrophoretic mobilities (Figure 2). When acidified CaCl2 solutions are leached through the columns, a continuous stable suspension is generated until the treatment is switched back to DIW, at which time the generation of mobile colloids ceases (12, 36). In contrast, NaCl solutions generate little or no mobile colloids unless pH-adjusted to >8.0 prior to leaching, with exposure to high SAR solutions generally reducing the potential for dispersion of iron oxides unless combined with high pH conditions. Calcium exchange for native Al3+ and subsequent Al hydrolysis apparently lowers the pH below the effective ZPNC for the highly weathered, oxide-coated sediments (Figure 1). Greater colloidal stability in the presence of polyvalent cations such as Al3+ and Ca2+ as compared to Na+ and K+ is consistent with batch dispersion tests for goethite at pH values below the ZPC in which polyvalent cations increase positive surface charge (45). In addition, polyvalent cations are more effective at shielding attractive forces between iron oxides and negatively charged phyllosilicates (46). The surface charging mechanisms described in Figure 1 are likely to be active in many highly weathered systems that are exposed to dilute acidic wastes; however, the ability to generate and transport a positively charged suspension through a kaolinite/quartz-based matrix possessing negatively charged sites is a self-limiting process. As the oxide coating is stripped away by the loss of readily mobile iron oxides, negatively charged sites become exposed and active in the physicochemical filtration process. This is generally confirmed by the relatively minor colloid yields (
