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Electroosmotically driven water flow in sediments
Pergamon
0043-1354(95)00224-3
Wat. Res. Vol. 30, No. 4, pp. 811-818, 1996 Copyright© 1996ElsevierScienceLtd Printed in Great Britain.All rights reserved 0043-1354/96 $15.00+ 0.00
ELECTROOSMOTICALLY DRIVEN WATER FLOW IN SEDIMENTS TIM G R U N D L * and PAUL M I C H A L S K I Geosciences Department, University of Wisconsin-Milwaukee, Milwaukee, WI 53201, U.S.A. (First received February 1995; accepted July 1995)
Abstract--The electroosmotically driven flow of water through a fine-grained glacial till in response to the application of an electric field is examined. The till contains large amounts of calcite and clay minerals of the illite and smectite groups. The presence of these minerals cause the system to respond much differently to the application of an electric field than kaolinite systems. The mode of current conduction differs from that in kaolinite and the presence of calcite prevents the formation of low pH conditions in the sediment pore water. These results are applicable to the wide variety of naturally occurring fine-grained sediment that commonly contain calcite and expandable clays. Key words---electroosmotic flow, groundwater
INTRODUCTION The application of low level direct current (d.c.) electric fields to fine-grained sediments has long been used as a technique to de-water these materials (Casagrande, 1953). Recently the technique has been applied to the remediation of contaminated sediments and a growing body of literature has appeared (see Acar and Alshawabkeh, 1993). This body of experimental work has almost exclusively used kaolinite as the fine-grained sediment (e.g. Acar et al., 1990; Bruell et al., 1992; Segall and Bruell, 1992; Shapiro and Probstein, 1993; Pamukcu and Wittle, 1992; Gray and Mitchell, 1967). Some studies use water contents above the liquid limit of kaolinite (Acar et aL, 1991; Bruell et al, 1992; Segal and Bruell, 1992). Other solid media have been investigated including quartz sand (Runnels and Wahli, 1993; Runnels and Larson, 1986), dredge spoils (Segall et aL, 1980), illite (Gray and Mitchell, 1967), montmorillinite (Pamukcu and Wittle, 1992) or mixtures of these minerals (Gray and Mitchell, 1967; Segall and Bruell, 1992; Pamukcu and Wittle, 1992, Thomas and Lentz, 1990). The montmorillinite study (Pamukcu and Wittle, 1992) used water contents of 271%, well above the liquid limit. The illite study (Gray and Mitchell, 1967) applied electric fields for short periods of time only. Although natural sediments in which kaolinite is the dominant clay mineral are common in warm humid climates, other more surface active clay minerals are usually present. Large amounts of fine-grained glacial sediments in *Author to whom all correspondence should be addressed, present address: EAWAG, t2berlandstrasse 133, CH8600 Diibendorf, Switzerland.
north central United States are dominated by surface active clays such as illite and smectite and commonly contain carbonate minerals such as calcite. Application of an electric field to a saturated porous media induces several interrelated effects. If the solid media is sufficiently permeable and its surfaces are sufficiently inactive (little or no surface charge), electric conduction is accomplished by ionic migration (anions move toward the anode; cations toward the cathode). If the solid media is sufficiently impermeable to hydraulic flow and its surfaces are sufficiently active (significant surface charge), electric conduction is accomplished by a one way movement of counter ions (ions with a charge opposite the surface charge). Under the pH regimes normally encountered in natural waters (5 < pH < 9), clay minerals exhibit a negative surface charge. The resultant electric double layer extends into, or in some cases completely across, the pore spaces of the sediment and effectively excludes co-ions from the pore water. For a given water content, co-ion exclusion is proportional to the surface charge of the mineral and inversely proportional to the ionic strength of the water. The ratio of co-ions to counterions in a water of 1.0 mM ionic strength and a water content of 20% is nearly infinite in illite and reaches 22,000 in kaolinite in spite of the low surface charge (Gray and Mitchell, 1967). In this system the only mobile charged species are cationic counter-ions. As these cations move under the influence of the electric field, a net flow of water is induced as both waters of hydration and bulk water are carried along with the cations. This flow of water from anode to cathode is known as electroosmotic flow. A number of theoretical treatments on electroosmotic flow have been advanced including
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Tim Grundl and Paul Michalski
Helmholz-Smoluchowski (Smoluchowski, 1914) for pores that are large with respect to double layer thickness and Schmid (1950, 1951) for pores that are small with respect to the double layer. Esrig (1964) and Spiegler (1958) developed separate models both of which are independent of pore size. Electric field strengths of 100 V/m or less are used to generate electroosmotic flow. Voltages required to reach these field strengths are sufficient to electrolyze water. The half reactions for water hydrolysis are: H20--~ 2H ÷ + ½02(g) + 2e- (anode) 2H20 + 2e- ~ H2(g ) + 2OH- (cathode). Water becomes acidic near the anode and basic near the cathode. Hydronium ions formed by the oxidation of water at the anode move into the media. The resulting low pH front migrates through the column because of the bulk water flow and the high ionic mobility of H ÷ (nearly twice that of O H - ) (EPRI, 1994). The magnitude of this pH front as well as its velocity is a function of the buffering capacity of the system. Both buffer intensity and capacity of sediment-water systems are dominated by the solids present, not by the pore water (Stumm and Morgan, 1981). The total buffer capacity will be strongly dependent on the mineralogy of the sediment. This migrating front of low pH water has been the focus of much of the previous work on electroosmotic remediation of metals contaminated soils (e.g. Hamed et al., 1991; Aear and Alshawabekeh, 1993). Low pH conditions solubilize the metals, allowing movement toward the cathode where basic conditions cause precipitation as hydroxides. The movement of two soluble organic molecules (phenol and acetic acid) with acidic functional groups is also strongly affected by pore water pH (Shapiro and Probstein, 1993; Acar et al., 1992). Hydraulic flow through porous media under a gravitational potential is described by Darcy's law: Oh =
KhiA,
(l)
where Qh = hydraulic flow (cm3/s), Kh = hydraulic conductivity (cm/s), i = hydraulic gradient (unitless), A --- area (cm~). Electroosmotic flow, under the influence of an electrical potential, can be described in a similar manner
Qeo=KooieoA,
(2)
where Q,o=electroosmotic flow (cm3/s), /~o= electroosmotic conductivity (cm2/V-s), leo = electrical gradient (V/cm). Hydraulic conductivity (Kh) is a constant for any given sediment. Unless an outside process changes the pore water chemistry or water content, Kh uniquely describes the characteristics of the sediment with respect to the gravitationally driven flow of water. By contrast, the application of an electric field not only generates electroosmotic flow, but also the
associated acid-base, redox and precipitation reactions. These reactions directly change the electroosmotic conductivity (Ko) of a sediment over time. These chemical changes migrate through the media from anode to cathode therefore the Keo also varies on a spatial scale. The Keo is not a unique descriptor of the sediment with respect to electroosmotically driven flow of water (see also Hamed et al., 1991). Eykholt and Daniel (1994) present a model of electroosmotic flow with time variant Keo. The purpose of the present study is to investigate electroosmotic phenomena caused by the application of low level d.c. fields to a natural fine-grained sediment at geologically reasonable water contents. The clay-sized fraction of the sediment was dominated by illite and smectite and was collected in southeastern Wisconsin. The actual clay present was probably a mixed-layer illite/smectite phase. The water used was calcium bicarbonate water similar in ionic strength and chemical composition to the natural groundwater associated with this sediment. METHODS
Sedimentcharacterization: the Oak Creek Till sample was collected as auger drill cuttings from a site on the University of Wisconsin-Milwaukeecampus from a depth of between 2 and 15 m. The sample was air dried and crushed until the sediment passed through a 600pm sieve (Sieve No. 30). Grain size analysis was determined by Stokes Law settling velocities (Klute, 1986). The percentage of clay in some of the trials was decreased by the addition of extra material from the silt-sand fraction. Cation exchangecapacity (CEC) was determined by extraction with magnesium nitrate (Rhoades, 1982). Percent carbonate was determined by weight loss after reaction with 10% hydrochloric acid. Kaolinite was obtained commerciallyand used directly with no further treatment. Sediment pH was determined with a glass electrode in a 1:1 mix of deionizedwater and sediment. Over 40 column experiments were performed at 40, 80 and 100 V/m using the Oak Creek till and at 80 V/m using kaolinite. All experiments were performed in plastic columns with an inside diameter of 7.6cm. Sediment samples were between 5 and 6 cm in length. Spring loaded end plates held the sample in place. Perforated graphite sheet electrodes extended over the entire diameter of the column at each end. Constant pressure heads on the column of between 71 and 13 cm were maintained throughout each experiment. Both ends of the column were open to the atmosphere to allow evolved gases (from the electrolysisof water) to escape. De-aerated tap water was used a pore fluid. This water was a calcium bicarbonate water with total dissolved solids content of 250 ppm and an ionic strength of 4.8 mM. Air dried sediment was brought to a water content of approximately 28% and manually placed in the column. Experiments were run for periods between 100 and 300 h. Figure 1 shows the experimental setup. Experiments were run at constant voltage and hydraulic head while effluent volume, effluent pH and amperage were monitored over time. At the end of each experiment, columns were disassembledand the sediment sliced into one centimeter sections for a section by section analysis of sediment pH. RESULTS AND DISCUSSION The till unit used is typical of the highly impermeable, glacially deposited tills found in southeastern
Electroosmotic water flow in sediments
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Wisconsin. The water used was City of Milwaukee tap water which is t a k e n directly from Lake Michigan. Lake M i c h i g a n water is similar in c o m p o s i t i o n a n d c o n c e n t r a t i o n to shallow groundwater f o u n d in the area. M a j o r ion analysis of the water along with bulk soil characteristics are given in Table 1. Mineralogy o f the clay fraction o f this sediment is approximately 72% illite, 15% expand-
able clays and 13% kaolinite and chlorite (Schneider, 1983). All experiments were p e r f o r m e d at a water content typical o f natural clay rich sediments a n d is well below the liquid limit of illite or kaolinite ( G o l d m a n et al., 1990). Soil columns were assembled a n d a pressure head applied. Equilibration periods of approx. 24 h were needed to obtain c o n s t a n t flow o f hydraulically
Table 1. Bulk sediment characteristics of the Oak Creek Till and ionic composition of influent water. Clay sized particles are ~<2gm, silt sized are between 2 and 20#m and sand sized are >20pm Water composition Influent ( m g / I ) (raM/I) Till characteristics Ca2+ 37 0.92 Percent clay sized (%) 41 Mg2÷ l0 0,41 Percent silt sized (%) 47 Na ÷ 5.8 0.25 Percent sand sized (%) 12 K+ 3.8 0.10 Percent carbonate (%) 37 HCOf 150 2.46 Water content (%) 28 CI 12 0.34 CEC (meq/100g) 23.4 SO~ 28 0.29 Jr("h ( c m / s ) 6.5 × 10-s NOj 0.7 0.01 pH 6.8 --
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Time (h) Fig. 2. Representative water flow rate in till columns versus time since current application at differing field strengths. Time is measured from the start of electrolysis. (r-l) 100 V/m, (V) 80 V/m, (A) 40 V/m, (O) 0 V/re. Current was turned off at 20t)h in the 100 V/m trial and was followed by a relatively large and variable water flow.
driven water. Pressure heads of up to 71 cm were applied and the resultant hydraulic flow was 0.02-0.07 ml/h. Control columns with no voltage applied maintained flows of this magnitude for the entire course of each experiment. This corresponds to a mean K h value 6.5 x 10 -8 cm/s. Application of an electric field immediately produces an electroosmotically driven flow of water of as much as 2.4 ml/h, nearly 2 orders of magnitude higher than background water flow rates (Fig. 2). Several features of the electroosmotic flow can be distinguished. First, the peak flow rate was a function of the applied field up to a maximum of 100 V/m. At this field strength, visible fracturing of the sediment became evident in the latter stages of the experiment, These fractures were millimeter scale in width and centimeter scale in lateral extent. Fractures were seen throughout the column and were oriented both perpendicular and parallel to the axis of the column. Fractured samples exhibit a large and highly variable hydraulic flow after the electric field was shut off, indicating possible fracture dominated flow. The underlying cause of this fracturing was not investigated but may be due to desiccation. Ekyholt and Daniel (1994) report development of negative pore pressures during electroosmosis in kaolinite systems which could lead to dessication. At all field strengths tested, electroosmotic flow in the till diminished over time and reached zero after approx. 200 h. This variable flow rate through time is a result of the fact that the electroosmotic permeability (K,o) changes through time [see equation (2)]. At peak flow the K,o of the till was between 1.2 and 0.4 x 10-Scm2/V-s for applied voltages between 100 and 40V/m, respectively. However, the cumulative volume of electroosmotically driven water that passed through the column increased as the field strength increased. An average
of 19, 114 and 143ml of water passed through the column at field strengths of 40, 80 and 100V/m, respectively. This corresponds to between 1/4 and 2 pore volumes. Although the fact that electroosmosis moved no more than 2 pore volumes seems unimpressive, over 95% of dissolved contamination is removed from a sediment after the passage of two bulk pore volumes (VanDoren and Bruell, 1987). This is because each pore passes many individual pore volumes during the passage of one bulk pore volume through the sediment as a whole. As noted by Acar and Alshawabekeh (1993), additional removal can be accomplished by ionic migration of contaminants. A plot of peak flow rates vs applied electric field (Fig. 3) for a total of 14 experiments using till shows an increase in peak flow rate with increasing field strength. Error bars depict the maximum and minimum flowrate at each potential field. Also included in this plot are the results of a series of ancillary experiments at 80 V/m in which successive portions of the clay fraction were removed. Experiments were run at 21% and 13% clay sized fraction with no apparent change in the electroosmotic behavior. Peak flow rates appear to be independent of the percent clay present. Apparently the presence of as little as 13% clay is sufficient to produce electroosmotic flow. The levelling off of the electroosmotic flow below a field strength of 50 V/m may indicate the approach of a threshold electric field below which eleetroosmotic flow cannot overcome the opposing frictional forces. The existence of threshold hydraulic gradients in fine grained sediments below which water ceases to flow has been discussed in the literature. Tavenas et al. (1983) performed laboratory tests on glaciolacustrine clays and found it hard to accurately measure threshold gradients but determined it must be less than 0.07. Desauliniers and Cherry (1989) invoked a threshold gradient of 0.07 to explain the
Electroosmotic water flow in sediments isotopic and geochemical data collected in a study of a thick sequence of Lake Champlain clays. Law and Lee (1981) conducted laboratory experiments on clay rich glacial tills and reported threshold gradients that range from 0.1 to 3. Conversion of gradients that range from 0.07 to 3 to basic units (force per unit mass) yields values of 1 x 10-26 to 43 x 10-26N (Newton) per molecule of water. A similar conversion of the threshold electric field to units of force per unit charge yields a value of 1.6 x 10-17N per divalent ion. The electroosmotic force is expressed in terms of divalent ions because the pore water is dominated by Ca 2+ and to a lesser extent Mg 2÷. The ion cloud associated with each ion in water is a function of temperature, dielectric constant of water, charge of ion and ionic strength of the solution. For water at 25°C the radius of the ion cloud surrounding a divalent ion is (Pankow, 1991)
1/r = (6.2 × 10-7)(1) 1/2, where r = radius in cm and I = ionic strength in tool/1. For the ionic strength of the solution used in this study (4.8 mM), the radius of the ion cloud is 2.2 × 10 -7 cm. The volume of this spherical ion cloud is 4.5 x 10-Z°cm3. The number of water molecules contained in this volume is approximately 1500. Each ion carries 1500 molecules with it plus any additional molecules that are moved from momentum gained by collision with this large ion cloud. The maximum electroosmotic force needed to initiate water flow can then be expressed in terms equivalent to hydraulic flow (N per molecule water): 1.6
x 10 -17
N/divalent ion
1500 molecules/divalent ion = 1.1 × 10-2o N/molecule. This force is much larger than any estimate of hydraulic threshold forces. The large apparent
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difference between the underlying force needed to initiate water movement hydraulically and electroosmotically (a maximum of 6 orders of magnitude) may be due to the fact the double layer extends well into the pore spaces and solution ions are experiencing a net attraction to the clay surface. This attraction forms an additional frictional force that must be overcome in order to initiate flow. In addition, the estimate of electroosmotic threshold is a maximum because it does not include water that is moved by momentum exchange with the ion cloud and the actual difference between electroosmotic and hydraulic threshold would be smaller. Previous workers have shown a similar decrease in electroosmotic flow over time (Hamed et al., 1991). These workers used kaolinite with a dilute (0.1 mM ionic strength) sodium sulfate water to which 3-5 ppm Pb 2+ was added. They attributed the cessation of electroosmotic flow to ion depletion caused by precipitation of Pb 2+ in the basic portion of the column near the cathode as well as changes in conductivity brought on by the high H + content of the pore fluid. As ions were removed from solution, the conductivity of the sediment column decreased and the passage electricity via electroosmosis became increasingly difficult. The cause of electroosmotic flow cessation in illite dominated is entirely different, A plot of current flow rate and electroosmotic flow rate vs time for till and kaolinite is shown in Figs 4 and 5. The overall electric conductivity of the glacial till (Fig. 4) increases over time as the electroosmotic flow decreases. This is in sharp contrast to the behavior of kaolinite in which the overall electric conductivity and electroosmotic flow both decrease over time (Fig. 5). The continuing passage of electricity through till as electroosmotic flow ceases is an indication that the basic mechanism of electric conduction has changed over time.
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Potential field (V/m) Fig. 3. Peak water flow rate in till columns versus applied field. Data points includes four ancillary experiments in which the percent clay sized fraction of the till was reduced from 40 to 13%. No difference in eleetroosmotic behavior was observed.
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Tim Grundl and Paul Michalski 10
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Time (h) Fig. 4. Representative plot of electroosmotic flow rate and current flow rate during the course of an 80 V/m experiment for till. (E3) Electroosmotic flow rate and (V) current flow rate.
There are four different modes in which electricity can be transmitted in a sediment/water system: (1) two way ionic migration and redox reactions at electrode surfaces; (2) electroosmosis and redox reactions at electrode surfaces; (3) conduction along sediment surfaces; and (4) electrolysis of water. In a constant voltage system at hydraulic steady state, electroosmosis will continue as long as a supply of redox active ions can be maintained and the permeability of the sediment remains unaltered. Whether either or both of these conditions are met is largely dependent upon heterogenous reactions
between the pore water and sediment. Several heterogenous reactions that affect pure clay systems have been suggested, in particular precipitation of metal hydroxides near the cathode and desorption of metal cations in the acidic region near the anode. In addition, acidity generated near the anode dissolves both metal oxide coatings and attacks clay minerals directly. Hamed et al. (1991) suggest ion starving as a result of metal hydroxide precipitation as the cause for the cessation of electroosmosis in a kaolinite/water system. Sethi et al. (1973) report the formation of amorphous iron oxide and hydrated calcium silicate in a plug of kaolinite and silica glass beads. Precipitation of these phases would diminish the permeability of the plug.
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Time (h) Fig. 5. Representative plot of ¢lectroosmotic flow rate and current flow rate during the course of an 80 V/m experiment for kaolinite. (El) Eleetroosmotic flow rate and (A) current flow rate,
Electroosmotic water flow in sediments Table 2. Representative sectional pH values for kaolinite and till at the termination of column experiments Section number Influent chamber 1
2 3 4 5 Effluent chamber
Kaolinite pH
Till pH
3.0 3.1 3.4 4.2 4.8 5.8 11.4
3.0 7.0 7.3 7.2 8.3 11.7 10.4
These same reactions will also cause changes in the bulk conductivity of the system itself. Ion exchange, dissolution, precipitation reactions as well as dewatering and pH changes all profoundly effect the nature of the clay surfaces and therefore effect the conductivity of the solid. Acid conditions near the anode not only lower or reverse the surface charge of the clay minerals, but also change clay fabric to a more flocculated and stable form (Thomas and Lentz, 1990; Goldman et al., 1990). The increase in apparent conductivity of till over time must be due to a combination of presently undefined physicochemical changes that alter the system such that bulk conductivity increases sufficiently to offset the decreasing electroosmotic conduction. This is in sharp contrast to kaolinite systems. Surface conduction in kaolinite does not change and electroosmotic water flow and electroosmotic conduction both decrease in a parallel manner (Fig. 5). The fundamentally different nature of the illite and kaolinite surface leads to fundamentally different modes of electric conduction late in the experiment. Another important difference between the till sample and pure kaolinite is the presence of calcite. This causes a fundamentally different pH behavior. Kaolinite systems are poorly buffered in the time scales of column studies, and typically exhibit sharp, migrating fronts of low pH that are the basis for the removal of heavy metals via electroosmosis. Table 2 shows the pH conditions in both a kaolinite and till column at the completion of an experiment after electroosmotic water flow has ceased. Values are given for each I cm section of the sediment as well as the influent and effluent chambers. The highly acidic influent conditions (pH 3-5) persist throughout the kaolinite column until the fifth section where the pH rises to 5.8. The acidic front has moved to within a centimeter of the cathode. It is interesting to note that the pH of zero charge (ZPC) for kaolinite is 4.6 (Stumm and Morgan, 1981) and that the majority of the kaolinite in the column has a positive or neutral surface charge. This is undoubtedly a contributing factor in the cessation of electroosmotic flow. Calcite contained in the till dissolves rapidly enough to act as a buffer in the time scale of these column experiments. Alkalinity measurements (to pH 4.5) on 1:1 sediment:water slurries of both till and kaolinite yielded values of 4082 and 5.6 meq/kg, respectively.
817
These alkalinity measurements were made on a time scale of hours and for kaolinite represent nonequilibrium conditions similar to those encountered in column experiments. Acidic influent water is immediately buffered to pH 7 and remains relatively constant until the last section. This section exhibits a high pH similar to that seen in the effluent chamber. Nowhere within the soil column are acidic conditions encountered nor is the ZPC of any clay mineral approached. The acid-base response of these two sediments to the effects of electrolysis is fundamentally different. The presence of only a few percent calcite is sufficient to buffer the acidity produced at the anode.
CONCLUSIONS
This study presents data to show that electroosmotically driven flow of water through a naturally occurring calcite and illite-smectite rich till differs markedly from the behavior of the often studied kaolinite-water system. Although both systems cease to electroosmotically move water after approx. 2 pore volumes, the mechanism of flow stoppage in kaolinite is due to the cessation of current flow whereas electric current continues to flow through the till after water ceases to move. An undetermined set of physicochemical changes occur in the till as the result of electrolysis to prevent the continued transmission of water. Work is in progress to define the nature of these physicochemical changes. Additionally, the buffer capacity of solid calcite is sufficient to prevent strong pH gradients from forming in the till. This buffering prevents the large pH range, and associated changes in both pore water chemistry and surface charge that is seen in kaolinite systems. These results will impact the design of full scale electroosmotic remediation systems. Encouraging results include the production of electroosmotic flow in sediments with as little as 13% clay sized minerals, and the fact that approximately 2 pore volumes is moved which is sufficient to remove most dissolved contaminants. Additional removal may be accomplished by ionic migration of contaminants. Discouraging results include the fact that 2 pore volumes is insufficient to remove contamination that results from dissolution of sorbed or precipitated constituents or residual non-aqueous phase liquids. Another concern is that the presence of even small amounts of solid calcite will buffer the system and prevent the sharp pH drop that is necessary to remove heavy metals.
REFERENCES
Acar Y. B. and Alshawabkeh A. N. (1993) Principles of electrokinetic remediation. Environ. Sci. Technol. 27, 2638-2647. Acar Y. B., Gale R. J., Putnam G. A., Hamed J. and Wong
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R. L. (1990) Electrochemical processing of soils; theory of pH gradient development by diffusion, migration and linear convection. J. environ. Sci. Hlth A25, 698-714. Acar Y. B,, Li H. and Gale R. J. (1992) Phenol removal from kaolinite by electrokinetics. J. Geotech. Engng 18, 1837-1852. Bruell C. J., Segall B. A. and Walsh M. T. (1992) Electroosmotic removal of gasoline hydrocarbons and TCE from clay. J. environ. Engng 118, 68-83. Casagrande L. (1953) Harvard Soil Mechanics Series No. 45 Review Of Past and Current Work On Electro-Osmotic Stabilization of Soils, pp. 1-20. Harvard University. Desaulniers D. E. and Cherry J. A. (1989) Origin and movement of groundwater and major ions in a thick deposit of Champlain Sea clay near Montreal. Can. Geotech. J. 26, 80-89. EPRI (1994) Electrokinetic removal of coal tar constituents from contaminated soils. Electric Power Research Institute Project 2879-21 Final Report. Esrig M. I. (I 964) Report of a Feasibility Study of Electrokinetic Processes for Stabilization of Soils for Military Mobility Purposes. Res. Rep. No. I, Cornell University, Ithaca, N.Y. Eykholt G. and Daniel D. (1994) Impact of system chemistry on electroosmosis in contaminated soil. J. Geotech. Engng 120, 797-815. Goldman L. J., Greenfield L. I., Damle A, S., Kingsbury G. L., Northeim C. M. and Truesdale R. S. (1990) Clay Liners for Waste Management Facilities, Chap. 2, pp. 10-51. Gray D. H. and Mitchell J. K. (1967) Fundamental aspects of electroosmosis in soils. Soil Mech. Foundations Div., Proc. Am. Soc. civil Engrs 93(6), 209-236, Hamed J. Acar Y. and Gale R. (1991) Pb(II) removal from kaolinite by electrokinetics. J. Geotech. Engrs 117, 241-271. Klute A. (Editor) (1986) Methods of Soil Analysis, part 1. Physical and Mineralogical Methods, 2nd edn, p. 383. Am. Soc. Agron., Agron. Ser. No. 9, Madison, Wisc. Law K. and Lee C. (1981) Initial gradient in a dense glacial till. Proc. lOth Int. Conf. Soil Mech. Foundation Engng, Stockholm, Sweden, pp. 441-446. Rotterdam, The Netherlands. Pamukcu S. and Wittle J. K. (1992) Electrokinetic removal of selected heavy metals from soil. Environ. Prog. 11(13), 241-250. Pankow J. F. (1991) Aquatic Chemistry Concepts. Lewis, Chelsa, Mich. Rhoades J. D. (1982) Cation exchange capacity. In Methods
of Soil Analysis, Part 2, Chemical and microbiological properties--Agronomy Monogr. No. 9 (Edited by Page A. L.), 2nd edn, Chap. 8, pp. 149-154. Am. Soc. Agron. & Soil Soc. Am. Madison, Wisc. Runnels D. R. and Larson J. L. (1986) A laboratory study of electromigration as a possible field technique for the removal of contaminants from groundwater. Ground Wat. Monitoring Rev., summer, pp. 85-91. Runnels D. R. and Wahli C. (1993) In situ electromigration as a method for removing sulfate, metals, and other contaminants from ground water. Ground War. Monit. Remed. winter, pp. 121-129. Schmid G. (1950) Zur electrochemie feinporiger kapillarsysterns. Zhurn. Electroch. 54, 424. Schmid G. (1951) Zur electrochemie feinporiger kapillarsystems. Zhurn. Electrochem. 55, 684. Schneider A. F. (1983) Wisconsinan stratigraphy and glacial sequence in southeastern Wisconsin. Geosei. Wisc. pp. 59-85. Segall B. A. and Bruell C. J. (1992) Electroosmotic contaminant removal processes, J. environ. Engng 118, 84-101. Segall B. A., O'Bannon C. E. and Mathias J. A. (1980) Electroosmosis chemistry and water quality. Proc. Am. Soc. chem. Engrs 106 (GTI0), 1148-1152. Sethi A. J., Herbillon A. J. and Fripiatt J. J. (1973) Electrochemical modifications in kaolinite-glass bead plugs. Clays Clay Miner. 21, 210-227. Shapiro A. P. and Probstein R. F. (1993) Removal of contaminants from saturated clay by electroosmosis. Environ. Sci. Technol. 27, 283-291. Smoluchowski M, (1914) Handbuch der Elektrizitat and des Magnetismus (Edited by Graetz L.), Vol. 2. Barth, Leipzig, Spiegler K. S. (1958) Transport processes in ionic membranes. Faraday Soc. Trans. 54, 1408-1428. Stumm W. and Morgan J. (1981) Aquatic Chemistry. Wiley, New York. Tavenas F., Leblond P., Jean P. and Lereoueil S. (1983) The permeability of natural soft clays. Part I: Methods of laboratory measurement. Can. Geotech. J. 20, 629-644. Thomas T. J. and Lentz R. W. (1990) Changes in soil plasticity and swell caused by electro-osmosis, PhysicoChemical Aspects of Soil and Related Material, ASTM STP 1095 (Edited by Hoddinott K. B. and Lamb R. O.), pp. 108-117. Am. Soc. Test. Mater. Philadelphia, Pa. VanDoren E. •. and Bruell C. J. (1987) Electroosmotic removal of benzene from a water saturated clay. N W W A / A P I Hydrocarbon Chemicals in Groundwater Conference, Houston, Tex. pp. 107-126.
0043-1354(95)00224-3
Wat. Res. Vol. 30, No. 4, pp. 811-818, 1996 Copyright© 1996ElsevierScienceLtd Printed in Great Britain.All rights reserved 0043-1354/96 $15.00+ 0.00
ELECTROOSMOTICALLY DRIVEN WATER FLOW IN SEDIMENTS TIM G R U N D L * and PAUL M I C H A L S K I Geosciences Department, University of Wisconsin-Milwaukee, Milwaukee, WI 53201, U.S.A. (First received February 1995; accepted July 1995)
Abstract--The electroosmotically driven flow of water through a fine-grained glacial till in response to the application of an electric field is examined. The till contains large amounts of calcite and clay minerals of the illite and smectite groups. The presence of these minerals cause the system to respond much differently to the application of an electric field than kaolinite systems. The mode of current conduction differs from that in kaolinite and the presence of calcite prevents the formation of low pH conditions in the sediment pore water. These results are applicable to the wide variety of naturally occurring fine-grained sediment that commonly contain calcite and expandable clays. Key words---electroosmotic flow, groundwater
INTRODUCTION The application of low level direct current (d.c.) electric fields to fine-grained sediments has long been used as a technique to de-water these materials (Casagrande, 1953). Recently the technique has been applied to the remediation of contaminated sediments and a growing body of literature has appeared (see Acar and Alshawabkeh, 1993). This body of experimental work has almost exclusively used kaolinite as the fine-grained sediment (e.g. Acar et al., 1990; Bruell et al., 1992; Segall and Bruell, 1992; Shapiro and Probstein, 1993; Pamukcu and Wittle, 1992; Gray and Mitchell, 1967). Some studies use water contents above the liquid limit of kaolinite (Acar et aL, 1991; Bruell et al, 1992; Segal and Bruell, 1992). Other solid media have been investigated including quartz sand (Runnels and Wahli, 1993; Runnels and Larson, 1986), dredge spoils (Segall et aL, 1980), illite (Gray and Mitchell, 1967), montmorillinite (Pamukcu and Wittle, 1992) or mixtures of these minerals (Gray and Mitchell, 1967; Segall and Bruell, 1992; Pamukcu and Wittle, 1992, Thomas and Lentz, 1990). The montmorillinite study (Pamukcu and Wittle, 1992) used water contents of 271%, well above the liquid limit. The illite study (Gray and Mitchell, 1967) applied electric fields for short periods of time only. Although natural sediments in which kaolinite is the dominant clay mineral are common in warm humid climates, other more surface active clay minerals are usually present. Large amounts of fine-grained glacial sediments in *Author to whom all correspondence should be addressed, present address: EAWAG, t2berlandstrasse 133, CH8600 Diibendorf, Switzerland.
north central United States are dominated by surface active clays such as illite and smectite and commonly contain carbonate minerals such as calcite. Application of an electric field to a saturated porous media induces several interrelated effects. If the solid media is sufficiently permeable and its surfaces are sufficiently inactive (little or no surface charge), electric conduction is accomplished by ionic migration (anions move toward the anode; cations toward the cathode). If the solid media is sufficiently impermeable to hydraulic flow and its surfaces are sufficiently active (significant surface charge), electric conduction is accomplished by a one way movement of counter ions (ions with a charge opposite the surface charge). Under the pH regimes normally encountered in natural waters (5 < pH < 9), clay minerals exhibit a negative surface charge. The resultant electric double layer extends into, or in some cases completely across, the pore spaces of the sediment and effectively excludes co-ions from the pore water. For a given water content, co-ion exclusion is proportional to the surface charge of the mineral and inversely proportional to the ionic strength of the water. The ratio of co-ions to counterions in a water of 1.0 mM ionic strength and a water content of 20% is nearly infinite in illite and reaches 22,000 in kaolinite in spite of the low surface charge (Gray and Mitchell, 1967). In this system the only mobile charged species are cationic counter-ions. As these cations move under the influence of the electric field, a net flow of water is induced as both waters of hydration and bulk water are carried along with the cations. This flow of water from anode to cathode is known as electroosmotic flow. A number of theoretical treatments on electroosmotic flow have been advanced including
811
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Tim Grundl and Paul Michalski
Helmholz-Smoluchowski (Smoluchowski, 1914) for pores that are large with respect to double layer thickness and Schmid (1950, 1951) for pores that are small with respect to the double layer. Esrig (1964) and Spiegler (1958) developed separate models both of which are independent of pore size. Electric field strengths of 100 V/m or less are used to generate electroosmotic flow. Voltages required to reach these field strengths are sufficient to electrolyze water. The half reactions for water hydrolysis are: H20--~ 2H ÷ + ½02(g) + 2e- (anode) 2H20 + 2e- ~ H2(g ) + 2OH- (cathode). Water becomes acidic near the anode and basic near the cathode. Hydronium ions formed by the oxidation of water at the anode move into the media. The resulting low pH front migrates through the column because of the bulk water flow and the high ionic mobility of H ÷ (nearly twice that of O H - ) (EPRI, 1994). The magnitude of this pH front as well as its velocity is a function of the buffering capacity of the system. Both buffer intensity and capacity of sediment-water systems are dominated by the solids present, not by the pore water (Stumm and Morgan, 1981). The total buffer capacity will be strongly dependent on the mineralogy of the sediment. This migrating front of low pH water has been the focus of much of the previous work on electroosmotic remediation of metals contaminated soils (e.g. Hamed et al., 1991; Aear and Alshawabekeh, 1993). Low pH conditions solubilize the metals, allowing movement toward the cathode where basic conditions cause precipitation as hydroxides. The movement of two soluble organic molecules (phenol and acetic acid) with acidic functional groups is also strongly affected by pore water pH (Shapiro and Probstein, 1993; Acar et al., 1992). Hydraulic flow through porous media under a gravitational potential is described by Darcy's law: Oh =
KhiA,
(l)
where Qh = hydraulic flow (cm3/s), Kh = hydraulic conductivity (cm/s), i = hydraulic gradient (unitless), A --- area (cm~). Electroosmotic flow, under the influence of an electrical potential, can be described in a similar manner
Qeo=KooieoA,
(2)
where Q,o=electroosmotic flow (cm3/s), /~o= electroosmotic conductivity (cm2/V-s), leo = electrical gradient (V/cm). Hydraulic conductivity (Kh) is a constant for any given sediment. Unless an outside process changes the pore water chemistry or water content, Kh uniquely describes the characteristics of the sediment with respect to the gravitationally driven flow of water. By contrast, the application of an electric field not only generates electroosmotic flow, but also the
associated acid-base, redox and precipitation reactions. These reactions directly change the electroosmotic conductivity (Ko) of a sediment over time. These chemical changes migrate through the media from anode to cathode therefore the Keo also varies on a spatial scale. The Keo is not a unique descriptor of the sediment with respect to electroosmotically driven flow of water (see also Hamed et al., 1991). Eykholt and Daniel (1994) present a model of electroosmotic flow with time variant Keo. The purpose of the present study is to investigate electroosmotic phenomena caused by the application of low level d.c. fields to a natural fine-grained sediment at geologically reasonable water contents. The clay-sized fraction of the sediment was dominated by illite and smectite and was collected in southeastern Wisconsin. The actual clay present was probably a mixed-layer illite/smectite phase. The water used was calcium bicarbonate water similar in ionic strength and chemical composition to the natural groundwater associated with this sediment. METHODS
Sedimentcharacterization: the Oak Creek Till sample was collected as auger drill cuttings from a site on the University of Wisconsin-Milwaukeecampus from a depth of between 2 and 15 m. The sample was air dried and crushed until the sediment passed through a 600pm sieve (Sieve No. 30). Grain size analysis was determined by Stokes Law settling velocities (Klute, 1986). The percentage of clay in some of the trials was decreased by the addition of extra material from the silt-sand fraction. Cation exchangecapacity (CEC) was determined by extraction with magnesium nitrate (Rhoades, 1982). Percent carbonate was determined by weight loss after reaction with 10% hydrochloric acid. Kaolinite was obtained commerciallyand used directly with no further treatment. Sediment pH was determined with a glass electrode in a 1:1 mix of deionizedwater and sediment. Over 40 column experiments were performed at 40, 80 and 100 V/m using the Oak Creek till and at 80 V/m using kaolinite. All experiments were performed in plastic columns with an inside diameter of 7.6cm. Sediment samples were between 5 and 6 cm in length. Spring loaded end plates held the sample in place. Perforated graphite sheet electrodes extended over the entire diameter of the column at each end. Constant pressure heads on the column of between 71 and 13 cm were maintained throughout each experiment. Both ends of the column were open to the atmosphere to allow evolved gases (from the electrolysisof water) to escape. De-aerated tap water was used a pore fluid. This water was a calcium bicarbonate water with total dissolved solids content of 250 ppm and an ionic strength of 4.8 mM. Air dried sediment was brought to a water content of approximately 28% and manually placed in the column. Experiments were run for periods between 100 and 300 h. Figure 1 shows the experimental setup. Experiments were run at constant voltage and hydraulic head while effluent volume, effluent pH and amperage were monitored over time. At the end of each experiment, columns were disassembledand the sediment sliced into one centimeter sections for a section by section analysis of sediment pH. RESULTS AND DISCUSSION The till unit used is typical of the highly impermeable, glacially deposited tills found in southeastern
Electroosmotic water flow in sediments
813
~dedT gas vent
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~I
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perforated cathode plate nut
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Fig. 1. Schematic diagram of experimental column. Water is input to the column through one arm of the threaded T connection. The anode lead exits out the other arm. Pressure head is applied by maintaining appropriate water levels in the gas vent. Funnel leads to a collection vessel for measurement of effluent volume.
Wisconsin. The water used was City of Milwaukee tap water which is t a k e n directly from Lake Michigan. Lake M i c h i g a n water is similar in c o m p o s i t i o n a n d c o n c e n t r a t i o n to shallow groundwater f o u n d in the area. M a j o r ion analysis of the water along with bulk soil characteristics are given in Table 1. Mineralogy o f the clay fraction o f this sediment is approximately 72% illite, 15% expand-
able clays and 13% kaolinite and chlorite (Schneider, 1983). All experiments were p e r f o r m e d at a water content typical o f natural clay rich sediments a n d is well below the liquid limit of illite or kaolinite ( G o l d m a n et al., 1990). Soil columns were assembled a n d a pressure head applied. Equilibration periods of approx. 24 h were needed to obtain c o n s t a n t flow o f hydraulically
Table 1. Bulk sediment characteristics of the Oak Creek Till and ionic composition of influent water. Clay sized particles are ~<2gm, silt sized are between 2 and 20#m and sand sized are >20pm Water composition Influent ( m g / I ) (raM/I) Till characteristics Ca2+ 37 0.92 Percent clay sized (%) 41 Mg2÷ l0 0,41 Percent silt sized (%) 47 Na ÷ 5.8 0.25 Percent sand sized (%) 12 K+ 3.8 0.10 Percent carbonate (%) 37 HCOf 150 2.46 Water content (%) 28 CI 12 0.34 CEC (meq/100g) 23.4 SO~ 28 0.29 Jr("h ( c m / s ) 6.5 × 10-s NOj 0.7 0.01 pH 6.8 --
814
Tim Grundl and Paul Michalski 2.0
1.5
--
1.o--
0.5
0 -20
20
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Time (h) Fig. 2. Representative water flow rate in till columns versus time since current application at differing field strengths. Time is measured from the start of electrolysis. (r-l) 100 V/m, (V) 80 V/m, (A) 40 V/m, (O) 0 V/re. Current was turned off at 20t)h in the 100 V/m trial and was followed by a relatively large and variable water flow.
driven water. Pressure heads of up to 71 cm were applied and the resultant hydraulic flow was 0.02-0.07 ml/h. Control columns with no voltage applied maintained flows of this magnitude for the entire course of each experiment. This corresponds to a mean K h value 6.5 x 10 -8 cm/s. Application of an electric field immediately produces an electroosmotically driven flow of water of as much as 2.4 ml/h, nearly 2 orders of magnitude higher than background water flow rates (Fig. 2). Several features of the electroosmotic flow can be distinguished. First, the peak flow rate was a function of the applied field up to a maximum of 100 V/m. At this field strength, visible fracturing of the sediment became evident in the latter stages of the experiment, These fractures were millimeter scale in width and centimeter scale in lateral extent. Fractures were seen throughout the column and were oriented both perpendicular and parallel to the axis of the column. Fractured samples exhibit a large and highly variable hydraulic flow after the electric field was shut off, indicating possible fracture dominated flow. The underlying cause of this fracturing was not investigated but may be due to desiccation. Ekyholt and Daniel (1994) report development of negative pore pressures during electroosmosis in kaolinite systems which could lead to dessication. At all field strengths tested, electroosmotic flow in the till diminished over time and reached zero after approx. 200 h. This variable flow rate through time is a result of the fact that the electroosmotic permeability (K,o) changes through time [see equation (2)]. At peak flow the K,o of the till was between 1.2 and 0.4 x 10-Scm2/V-s for applied voltages between 100 and 40V/m, respectively. However, the cumulative volume of electroosmotically driven water that passed through the column increased as the field strength increased. An average
of 19, 114 and 143ml of water passed through the column at field strengths of 40, 80 and 100V/m, respectively. This corresponds to between 1/4 and 2 pore volumes. Although the fact that electroosmosis moved no more than 2 pore volumes seems unimpressive, over 95% of dissolved contamination is removed from a sediment after the passage of two bulk pore volumes (VanDoren and Bruell, 1987). This is because each pore passes many individual pore volumes during the passage of one bulk pore volume through the sediment as a whole. As noted by Acar and Alshawabekeh (1993), additional removal can be accomplished by ionic migration of contaminants. A plot of peak flow rates vs applied electric field (Fig. 3) for a total of 14 experiments using till shows an increase in peak flow rate with increasing field strength. Error bars depict the maximum and minimum flowrate at each potential field. Also included in this plot are the results of a series of ancillary experiments at 80 V/m in which successive portions of the clay fraction were removed. Experiments were run at 21% and 13% clay sized fraction with no apparent change in the electroosmotic behavior. Peak flow rates appear to be independent of the percent clay present. Apparently the presence of as little as 13% clay is sufficient to produce electroosmotic flow. The levelling off of the electroosmotic flow below a field strength of 50 V/m may indicate the approach of a threshold electric field below which eleetroosmotic flow cannot overcome the opposing frictional forces. The existence of threshold hydraulic gradients in fine grained sediments below which water ceases to flow has been discussed in the literature. Tavenas et al. (1983) performed laboratory tests on glaciolacustrine clays and found it hard to accurately measure threshold gradients but determined it must be less than 0.07. Desauliniers and Cherry (1989) invoked a threshold gradient of 0.07 to explain the
Electroosmotic water flow in sediments isotopic and geochemical data collected in a study of a thick sequence of Lake Champlain clays. Law and Lee (1981) conducted laboratory experiments on clay rich glacial tills and reported threshold gradients that range from 0.1 to 3. Conversion of gradients that range from 0.07 to 3 to basic units (force per unit mass) yields values of 1 x 10-26 to 43 x 10-26N (Newton) per molecule of water. A similar conversion of the threshold electric field to units of force per unit charge yields a value of 1.6 x 10-17N per divalent ion. The electroosmotic force is expressed in terms of divalent ions because the pore water is dominated by Ca 2+ and to a lesser extent Mg 2÷. The ion cloud associated with each ion in water is a function of temperature, dielectric constant of water, charge of ion and ionic strength of the solution. For water at 25°C the radius of the ion cloud surrounding a divalent ion is (Pankow, 1991)
1/r = (6.2 × 10-7)(1) 1/2, where r = radius in cm and I = ionic strength in tool/1. For the ionic strength of the solution used in this study (4.8 mM), the radius of the ion cloud is 2.2 × 10 -7 cm. The volume of this spherical ion cloud is 4.5 x 10-Z°cm3. The number of water molecules contained in this volume is approximately 1500. Each ion carries 1500 molecules with it plus any additional molecules that are moved from momentum gained by collision with this large ion cloud. The maximum electroosmotic force needed to initiate water flow can then be expressed in terms equivalent to hydraulic flow (N per molecule water): 1.6
x 10 -17
N/divalent ion
1500 molecules/divalent ion = 1.1 × 10-2o N/molecule. This force is much larger than any estimate of hydraulic threshold forces. The large apparent
815
difference between the underlying force needed to initiate water movement hydraulically and electroosmotically (a maximum of 6 orders of magnitude) may be due to the fact the double layer extends well into the pore spaces and solution ions are experiencing a net attraction to the clay surface. This attraction forms an additional frictional force that must be overcome in order to initiate flow. In addition, the estimate of electroosmotic threshold is a maximum because it does not include water that is moved by momentum exchange with the ion cloud and the actual difference between electroosmotic and hydraulic threshold would be smaller. Previous workers have shown a similar decrease in electroosmotic flow over time (Hamed et al., 1991). These workers used kaolinite with a dilute (0.1 mM ionic strength) sodium sulfate water to which 3-5 ppm Pb 2+ was added. They attributed the cessation of electroosmotic flow to ion depletion caused by precipitation of Pb 2+ in the basic portion of the column near the cathode as well as changes in conductivity brought on by the high H + content of the pore fluid. As ions were removed from solution, the conductivity of the sediment column decreased and the passage electricity via electroosmosis became increasingly difficult. The cause of electroosmotic flow cessation in illite dominated is entirely different, A plot of current flow rate and electroosmotic flow rate vs time for till and kaolinite is shown in Figs 4 and 5. The overall electric conductivity of the glacial till (Fig. 4) increases over time as the electroosmotic flow decreases. This is in sharp contrast to the behavior of kaolinite in which the overall electric conductivity and electroosmotic flow both decrease over time (Fig. 5). The continuing passage of electricity through till as electroosmotic flow ceases is an indication that the basic mechanism of electric conduction has changed over time.
3.0 2.5 "~ 2.0
~
1.0 0.5
0
I
I
I
10
20
30
I 40
50
60
70
80
90
100
110
120
Potential field (V/m) Fig. 3. Peak water flow rate in till columns versus applied field. Data points includes four ancillary experiments in which the percent clay sized fraction of the till was reduced from 40 to 13%. No difference in eleetroosmotic behavior was observed.
816
Tim Grundl and Paul Michalski 10
1.0
8I-
~
"
~
0.8
-
6
- 0.6 --~
4
--
0.4
2
--
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0
20
40
60
80
100
120
140
160
180
200
o
0 220
Time (h) Fig. 4. Representative plot of electroosmotic flow rate and current flow rate during the course of an 80 V/m experiment for till. (E3) Electroosmotic flow rate and (V) current flow rate.
There are four different modes in which electricity can be transmitted in a sediment/water system: (1) two way ionic migration and redox reactions at electrode surfaces; (2) electroosmosis and redox reactions at electrode surfaces; (3) conduction along sediment surfaces; and (4) electrolysis of water. In a constant voltage system at hydraulic steady state, electroosmosis will continue as long as a supply of redox active ions can be maintained and the permeability of the sediment remains unaltered. Whether either or both of these conditions are met is largely dependent upon heterogenous reactions
between the pore water and sediment. Several heterogenous reactions that affect pure clay systems have been suggested, in particular precipitation of metal hydroxides near the cathode and desorption of metal cations in the acidic region near the anode. In addition, acidity generated near the anode dissolves both metal oxide coatings and attacks clay minerals directly. Hamed et al. (1991) suggest ion starving as a result of metal hydroxide precipitation as the cause for the cessation of electroosmosis in a kaolinite/water system. Sethi et al. (1973) report the formation of amorphous iron oxide and hydrated calcium silicate in a plug of kaolinite and silica glass beads. Precipitation of these phases would diminish the permeability of the plug.
10
1.2 1.0
8
0.8
6 O.6
9 4
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2
--
0 0
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I
[
I
I
I
I
I
I
I
20
40
60
80
100
120
140
160
180
200
~'
0.2
0 220
Time (h) Fig. 5. Representative plot of ¢lectroosmotic flow rate and current flow rate during the course of an 80 V/m experiment for kaolinite. (El) Eleetroosmotic flow rate and (A) current flow rate,
Electroosmotic water flow in sediments Table 2. Representative sectional pH values for kaolinite and till at the termination of column experiments Section number Influent chamber 1
2 3 4 5 Effluent chamber
Kaolinite pH
Till pH
3.0 3.1 3.4 4.2 4.8 5.8 11.4
3.0 7.0 7.3 7.2 8.3 11.7 10.4
These same reactions will also cause changes in the bulk conductivity of the system itself. Ion exchange, dissolution, precipitation reactions as well as dewatering and pH changes all profoundly effect the nature of the clay surfaces and therefore effect the conductivity of the solid. Acid conditions near the anode not only lower or reverse the surface charge of the clay minerals, but also change clay fabric to a more flocculated and stable form (Thomas and Lentz, 1990; Goldman et al., 1990). The increase in apparent conductivity of till over time must be due to a combination of presently undefined physicochemical changes that alter the system such that bulk conductivity increases sufficiently to offset the decreasing electroosmotic conduction. This is in sharp contrast to kaolinite systems. Surface conduction in kaolinite does not change and electroosmotic water flow and electroosmotic conduction both decrease in a parallel manner (Fig. 5). The fundamentally different nature of the illite and kaolinite surface leads to fundamentally different modes of electric conduction late in the experiment. Another important difference between the till sample and pure kaolinite is the presence of calcite. This causes a fundamentally different pH behavior. Kaolinite systems are poorly buffered in the time scales of column studies, and typically exhibit sharp, migrating fronts of low pH that are the basis for the removal of heavy metals via electroosmosis. Table 2 shows the pH conditions in both a kaolinite and till column at the completion of an experiment after electroosmotic water flow has ceased. Values are given for each I cm section of the sediment as well as the influent and effluent chambers. The highly acidic influent conditions (pH 3-5) persist throughout the kaolinite column until the fifth section where the pH rises to 5.8. The acidic front has moved to within a centimeter of the cathode. It is interesting to note that the pH of zero charge (ZPC) for kaolinite is 4.6 (Stumm and Morgan, 1981) and that the majority of the kaolinite in the column has a positive or neutral surface charge. This is undoubtedly a contributing factor in the cessation of electroosmotic flow. Calcite contained in the till dissolves rapidly enough to act as a buffer in the time scale of these column experiments. Alkalinity measurements (to pH 4.5) on 1:1 sediment:water slurries of both till and kaolinite yielded values of 4082 and 5.6 meq/kg, respectively.
817
These alkalinity measurements were made on a time scale of hours and for kaolinite represent nonequilibrium conditions similar to those encountered in column experiments. Acidic influent water is immediately buffered to pH 7 and remains relatively constant until the last section. This section exhibits a high pH similar to that seen in the effluent chamber. Nowhere within the soil column are acidic conditions encountered nor is the ZPC of any clay mineral approached. The acid-base response of these two sediments to the effects of electrolysis is fundamentally different. The presence of only a few percent calcite is sufficient to buffer the acidity produced at the anode.
CONCLUSIONS
This study presents data to show that electroosmotically driven flow of water through a naturally occurring calcite and illite-smectite rich till differs markedly from the behavior of the often studied kaolinite-water system. Although both systems cease to electroosmotically move water after approx. 2 pore volumes, the mechanism of flow stoppage in kaolinite is due to the cessation of current flow whereas electric current continues to flow through the till after water ceases to move. An undetermined set of physicochemical changes occur in the till as the result of electrolysis to prevent the continued transmission of water. Work is in progress to define the nature of these physicochemical changes. Additionally, the buffer capacity of solid calcite is sufficient to prevent strong pH gradients from forming in the till. This buffering prevents the large pH range, and associated changes in both pore water chemistry and surface charge that is seen in kaolinite systems. These results will impact the design of full scale electroosmotic remediation systems. Encouraging results include the production of electroosmotic flow in sediments with as little as 13% clay sized minerals, and the fact that approximately 2 pore volumes is moved which is sufficient to remove most dissolved contaminants. Additional removal may be accomplished by ionic migration of contaminants. Discouraging results include the fact that 2 pore volumes is insufficient to remove contamination that results from dissolution of sorbed or precipitated constituents or residual non-aqueous phase liquids. Another concern is that the presence of even small amounts of solid calcite will buffer the system and prevent the sharp pH drop that is necessary to remove heavy metals.
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