Abstract

The purpose of this experiment was to determine the influence of texture on the effect of electroosmosis (EO) on a hydraulic gradient imposed flow of an ionic solution through soil. It was hoped that by determining this, it would be possible to determine the effect of texture on the remedial potential of electroosmosis in a particular soil. A number of soil samples were gathered to provide a variety of attributes for a basis of comparison and analysis of trends. Soil samples were characterized for texture, pore space, particle density, and organic matter.

Using PVC pipe, a test chamber was designed to maintain an open-flow arrangement with a bottle used at the side of the anode to maintain a constant and relatively small external hydraulic gradient. The ionic solution used was a sodium chloride solution (3g/L).

For each test, an air-dried sample was loaded into the test chamber and the hydraulic gradient applied. Measurements of flow output were taken at ten-minute intervals for a total of four hours. Electroosmosis tests were run with a direct current of eight volts applied and control tests were run in the absence of electric current.

Of all variables, texture was found to be most directly related to the efficacy electroosmosis, with clay influencing EO negatively; the relationship identified substantiated the trend identified by the previous year’s study and contradicted the theoretical basis for electroosmotic efficacy.

Also available, in condensed + stylized form, in PDF.

BACKGROUND INFORMATION

Electroosmosis is one of a group of several “electrokinetic” phenomena which relate to the motion of charged electrolyte solutions and the motion of charged particles in solution or suspension; it is defined as “the movement of a liquid relative to a stationary charged surface (e.g., a capillary or porous plug) by an electric field.” (Probstein 195).

Since the discovery of these phenomena by F.F. Reuss in 1809, the potential of electroosmosis for various applications has been demonstrated numerous times, with electroosmosis being first utilized in the dewatering and stabilizing of soils in the 1930s. Electroosmosis has since been shown to have applicability in the separation of organic and inorganic contaminants, in construction of leak detection systems in clay liners, in the diversion of waste “plumes,” in the injection of nutrients or microorganisms into subsoil layers, in the increasing of pile strength, in the dewatering of foams, sludges, and dredgings, and in systems and barriers for lowering ground-water. The focus of this project, however, was the application of electroosmosis to the remediation of contaminated soil.

Electroosmosis and Soil

When in contact with an ionic solution the mineral particles present in soil give rise to a negative charge referred to as the soil’s zeta potential. This potential arises for many reasons, including chemical and physical adsorption within the soil and lattice imperfections. The negative charged surface of the particles attracts positive ions from the solution, creating a super-thin layer of water (sometimes referred to as double-layer or Debye sheath) in the pore walls. This layer of water moves much like a single ion under the influence of the electric field, and the viscous drag of the water causes the water in the pores to also gain a velocity component, generally in the direction of the cathode.

Soil Remediation

Despite the growing ecological awareness in society today, contamination of soil (and the resulting ground-water contamination) by both industrial and consumer chemicals is becoming a more prevalent concern. Minimization of chemical by-products and pollutants is obviously an important step in minimizing ecological impact, but identification of better methods of soil remediation is also an important focus given that conditions of our society will continue to favor the escape of certain levels of chemical pollutants into the environment regardless of what measures are taken to limit them.

Identification of methods for contaminant removal which could be most effectively be utilized given the conditions of a site and considering the resources available must take into account the nature of the pollutant and soil as well as economic factors, since prohibitive costs can curtail the implementation of a method regardless of the benefits of the method.

Though the study of electroosmosis for its potential in soil remediation has been largely limited to laboratory studies and very limited field tests, indications are that electroosmosis may prove to be an extremely valuable addition to conventional methods of soil remediation for both its efficacy and cost.

Application of Electroosmosis to Soil Remediation

The focus of this project was electroosmosis; however, in applying electroosmosis to soil remediation other electrokinetic phenomena are involved, and characterization of the movement of the contaminant only in terms of electroosmotic convection and electromigration is still inadequate to fully describe the motion of the contaminant. The chemical reactions that take place at the electrodes, in the fluid itself, desorption, dissolution, and chemical interactions within the soil itself all have bearing upon the contaminant’s motion.

In effecting the removal of soluble or slightly soluble contaminants from soil both electroosmosis and electromigration would be involved, electroosmosis effecting the movement of the purge solution through the soil and electromigration effecting the movement of the contaminant within the purge solution. In cases where electroosmosis would be the dominant mechanism, the percentage of contaminant removal would be directly correlated to pore volumes of purge solution flushed through the soil.

One group of weakly ionized compounds commonly associated with ground water contamination and which to which electroosmosis has great applicability is the aromatics, which includes such compounds as benzene, toluene, and xylene. These compounds are known to ionize within a range that should give rise to a sufficient zeta potential on the soil for electroosmosis to take place. Electromigration, unlike electroosmosis, is independent of zeta potential and soil particle size and depends primarily on the charge of the contaminant itself.

For compounds such as heavy hydrocarbons, which are essentially not ionized, the primary benefit of electroosmosis is restricted to purging the compound indirectly by pushing it with a water “front” in the soil unless surfactants can be utilized within the purge solution to solubize the contaminants.

Electroosmosis research has been largely limited to the laboratory, and only in certain locations have large-scale experiments been conducted in a natural setting. The majority of laboratory experiments concerning the remediary potential of electroosmosis has been limited to tests on kaolinite clay samples; one of the main goals of this project was to extend tests of electroosmosis past uniform samples to tests on soils of various texture.

Advantages of Electroosmosis

Although electroosmosis (in theory) should be more effective in soils with a lower hydraulic conductivity (i.e., soils with a greater percentage of micropores and that are more difficult to move water through by means of a pressure-driven device), this research sought to determine how great an effect texture has on the efficacy of electroosmosis by experimental means.

Conventional pressure-driven methods inevitably channel water through the larger-sized pores; electroosmosis-driven flow, however, should allow for a more uniform channeling of water (or purge solution) through pores and a much greater degree of control of the motion exhibited by the purge solution. Additionally, conventional methods of soil remediation rely primarily on various extraction processes which can range in cost from $50 to $1,500 per cubic yard of contaminated soil, compared to $2.00/ton of effluent removed by EO (Shapiro and Probstein 1993).

INTRODUCTION

Quite a number of laboratory studies have been conducted in regards to the determination of the feasibility of electroosmosis as a method of soil remediation. Whereas a large proportion of these studies published in scientific journals have dealt with the nature of various contaminants, very few were concerned with the textural properties of the soil. Most used a kaolinite clay sample for use in electroosmosis tests, and those that did involve different textures were limited to very pure silt and clay mixtures. There is a large deal of theoretical basis for the assumption that electroosmosis would have a greater efficacy in soils of low hydraulic permeabilities (i.e., clays), however, the effect of electroosmosis is still veiled in a certain level of practical uncertainty, with most studies still restricted to laboratory studies.

Phase One

The first phase of this research conducted during 1997-1998 involved studying what relationships might exist between soil texture and the efficacy of electroosmosis in effecting the removal of organic contaminants from soil. This research attempted to identify a relationship by tracing the movement of tannic acid through test samples under the influence of electroosmosis. Tannic acid was chosen as the model for the experiment since it is relatively safe and more importantly because it is structurally similar to the aromatics, a group of organic chemicals for which there exists a potentially wide range of applicability for electroosmosis.

The boundary conditions for flow in this experiment were a closed electrode configuration, with a slight current applied across the test sample by means of a DC power supply. Samples were taken over time at three points along the test chamber in order to establish the change in concentration over time as effected by electroosmosis.

The data produced by this phase of the research indicated several potential relationships involving electroosmotic efficacy in the removal of organic contaminants from soil. Significant among these trends is the positive relationship of sand to electroosmotic efficacy; that is, as sand content increases, electroosmotic efficacy was found to also increase. This contradicts the theoretical range of applicability of electroosmosis, which is in fine-grained soils of low hydraulic permeabilities (i.e., soils with high clay content). To explore this contradiction, a second phase of research was devised.

Phase Two

The second and current phase of research was devised with several new considerations in addition to those made in the first phase of experimentation. One consideration was the process of adsorption of the contaminant by the soil; because concentration readings of a contaminant present in a soil sample could be drastically altered by adsorption of the compound, it was decided that a more accurate measure of electroosmotic efficacy could be determined by looking at electroosmosis-induced flow rather than contaminant migration. By removing the variable of the contaminant, it was hypothesized that a more accurate comparison of electroosmosis in various soils could be made with a wider range of applicability of the results.

This phase of the project attempted to measure electroosmotic efficacy by flow rate in order to obtain a more reliable measure of how texture affects electroosmosis. Also of interest is the apparent trend that the first phase of this research established, which seemed to contradict theory concerning the soils in which electroosmosis would have greatest applicability; this research seemed to indicate a correlation between sandy soils and greater electroosmosis efficacy, which was a primary reason for a continued and intensified investigation of this process. It was hoped that the data provided by additional research could better quantify the relationships indicated by the first phase of research.

METHODS

Design and Construction of Test Chamber

The experimental set-up was designed to allow the effects of electroosmosis to be observed separate from other the electrokinetic phenomena associated with the processes that would be involved in the electroosmotic purging of a contaminant through soil; for this reason, the purpose of the test chamber was to allow measurement of water flow through the soil. The test chamber was designed to maintain an open-flow arrangement, with a bottle used at the side of the anode to maintain a constant and relatively small external hydraulic gradient.

The main body of the test chamber was designed to separate into two pieces to allow easy loading of samples and was based on a 2″ PVC pipe. Also comprising the main body were a cap, coupler, and reducer (2″ to ¾”). The hydraulic gradient from the elevated salt solution was maintained from a bottle and 2″ coupler through a series of connecting pieces of PVC pipe. The entire apparatus was also designed with the intention that unused salt solution in the bottle at the conclusion of a test could be reclaimed for use in later tests.

An outlet for the water was drilled through the cap at the cathode end of the electroosmosis test chamber with the hydraulic gradient being supplied to the sample through a reducer on the anode side of the chamber. Electrodes for the chamber consisted of iron washers that were placed in the cap (at the cathode end) and the reducer (at the anode end). These electrodes were connected to the external power supply (8 volts, DC) by means of metal screws that contacted the washers and breached the walls of the test chamber.

Characterization of Soil Samples

Soil samples with a wide range of attributes for a basis of comparison and analysis of trends were gathered. Each sample was characterized by its textural grouping, pore space, bulk and particle density, and organic matter content.

Texture was determined for each sample by means of a texture test kit which separated the soil into its three basic mineral fractions of sand, silt, and clay based on the amount of time required for each to settle in solution.

Fifteen milliliters of a soil sample were added to one 50-mL soil separation tube, and the tube was tapped firmly on a hard surface to eliminate air spaces. One milliliter of Texture Dispersing Reagent was added to the tube, and the sample was then diluted to 45 mL with water. The tube was capped and shaken for two minutes, and then allowed to settle for 30 seconds. The solution from the tube was poured into another tube and allowed to settle for 30 minutes. The amount of sediment in the first tube was divided by the initial amount of soil (15 mL) to calculate the percentage of sand in the soil. The amount of sediment in the second tube was then divided by the initial amount of soil (15 mL) to calculate the percentage of silt in the soil. The percentage of clay in the sample was calculated by subtracting the sum of the other two percentages from 100 %. This was done to obtain a more accurate reading of clay percentage than the measurement of the volume of clay, as the colloidal nature of clay causes it to swell in water. This procedure was repeated for each soil sample.

Pore space was determined by gradually adding water to a dried soil sample of a known volume. The volume of water used by the point at which the soil reached saturation was noted, and this volume was divided by the total volume of the soil sample to determine percent pore space.

Density for each sample was determined by weighing a known volume of a soil sample. Bulk density was calculated as the sample’s overall density while particle density was calculated by factoring out the volume of the pore space in the soil.

Percent organic matter content for each soil sample was determined by heating a small oven-dried sample of the soil over a Bunsen burner to a constant mass. Though the values obtained by means of this method are approximate, the technique is accurate enough to give a fairly good relative indication of organic matter present in the samples.

Testing of Soil Samples

A double-chamber set-up was devised to allow simultaneous testing of electroosmosis-affected flow and control flow, which consisted of an identical set-up without the current applied.

Each half of the test-chamber was loaded and the two halves were joined. To minimize leakage through the coupler, electrical tape was utilized to seal the chamber. A weak purge effluent of sodium chloride was prepared to a concentration of 3 g/L (~ 0.05 M). The hydraulic gradient was then applied; for the electroosmosis tests, a direct current of 8 volts was applied via the external power supply, while the control tests maintained the hydraulic gradient in the absence of electric current. Volume readings were taken at ten-minute intervals to determine the amount of effluent discharged at the cathode and individual tests were carried out for a total of four hours. The comparatively short duration of the tests was intentional to minimize the chemical interactions within the soil during the tests.

DISCUSSION

Many factors are known to have an effect on the potential benefit of electroosmosis in soil remediation, and for this reason many recent studies have investigated numerous variables involved in the process including electric potential; contaminant type, concentration, and distribution; and electrode material, arrangement, and configuration. The focus of this investigation was soil texture, a variable given little consideration or attention outside the range of what is theoretically assumed to be the applicable range of electroosmosis (i.e., very fine-grained soils-specifically, soils with high clay content). Other variables examined in this investigation for their potential influence on electroosmosis were pore space, particle density, and organic matter. Several variables of significance to the electroosmotic flow which were kept constant during this experiment were voltage, concentration of the ionic purge solution, temperature of soil sample, hydraulic head, and electrode configuration.

This experiment was designed with the intent that relative electroosmotic efficacy in various soils could be determined by measuring the change in flow rate of an ionic solution through a sample of the soil; it was expected that electroosmosis would impose an additional velocity vector on the advection of the effluent through the test chamber. It was hoped that this focus would yield a more quantitative understanding of the relationships existing between soil texture and electroosmosis than the previous year’s research, which focused on contaminant migration.

One difficulty in interpreting the data was that the flow varied greatly even within several tests of a single sample. Despite this variability, it was found that third-order approximations for the data of total flow over time had high R² values: of 67 tests conducted overall, R² values for third-order polynomial trendlines of only three tests fell below 0.98. Another observation of note to this research was that a slight disturbance of the test chamber during testing could result in a radical change of the flow rate; this suggests that minor changes within the structural character of a soil sample could have disproportionately large influences on flow through the medium, contributing to the idea that radical variations of flow through various tests of a single soil sample might be expected. This variability of flow rate was one reason that a large number of trials for a relatively small number of soil samples was favored over a smaller number of trials for a larger number of soil samples; while a larger number of soil samples would allow for a more diverse texture base, it was not felt that the results obtained would reflect the true relationships as well as the average of a large number of trials of a smaller number of soil samples would.

Flow rates achieved during electroosmosis tests averaged significantly less than those of the control tests did, and overall flow rates for both control and EO tests decreased over time. Additionally, the decrease in flow rate for EO tests was initially found to average significantly greater than the decrease found in the control tests (i.e., flow rate in EO tests decreased more rapidly than flow rate in control tests). The overall decrease in flow rate for both sets of tests can be partially attributed to the saturation of the soils over time by the effluent solution. The additional decrease in flow rate for the EO tests presented a difficulty for analyzing the data collected, since it was originally hypothesized that the relative strength of electroosmotic influence on flow rate in various soils could be determined by calculating the proportion of increase in flow rather than decrease. This decrease could be attributed to the effects of the electrode reactions on the pH of the soil (to decrease pH at the anode and increase pH at the cathode), which would tend to have a negative effect on the flow rate over time (Shapiro and Probstein 1993).

While this offers an explanation for the initially more rapid decrease in flow rate (with both soil saturation with the purge solution and electrochemical changes within the soil contributing to a greater decrease in flow rate than just soil saturation), this does not account for the difference between control and EO flow rates initially. The significance of this observation is also uncertain, and additional experimentation may be necessary in order to explain the origins of this effect. Hamed et al. noted a delay in the initiation of EO-induced flow rate in experiments designed to measure EO-induced flow over time, though reasons for this delay were not hypothesized (1991).

For the purposes of this investigation, comparative values for trend analyses were obtained by averaging the differences between control and EO flow rates and change in flow rates (referred to in this paper as “rate change index” [RCI]). The convention used for this was to subtract the EO values for rate and rate change from the control values.

Textural composition of each of the samples was represented in several ways, in an attempt to give an accurate analysis of the correlations existing between texture and electroosmosis. Texture was broken into its distinct particle size groupings and the rate change index was plottted against each of these individual fractions (i.e., separate graphs were created that plotted rate change index against percent sand, percent silt, and percent clay). For an additional graph, samples were given numerical rankings based on their location on the textural triangle, with sand being given a ranking of 1 and sandy clay loam being given a ranking of 4; following this convention, however, resulted in data being grouped into only three categories (2, 3, and 4 corresponding to loamy sand, sandy loam, and sandy clay loam).

In an effort to consolidate data and provide means for better analyzing trends, averages were obtained for final RCI values (taken from the last five data points of 200, 210, 220, 230, and 240 minutes) and plotted against texture. Lower [negative] values would correspond to data points for which EO rate change exceeded control rate change and EO flow decreased less rapidly than control flow. Lower values would therefore seem to indicate samples for which flow rate was most strongly influenced by electroosmosis.

The apparent correlation between sand content and RCI index was of a lesser RCI index with higher sand content; this would suggest that electroosmosis had a stronger influence on the flow rate established by advection in soils of high sand content. Additionally, negative relationships were indicated between silt and RCI index and clay and RCI index, suggesting that higher clay and silt content would have an adverse effect on EO in application to soil remediation. The R² value for the correlation between clay content and the RCI index was significantly greater than the correlation between silt and the index.

While the R² values produced for these relationships are relatively low, it is significant that they reproduce the results indicated by the first phase of this research, which also indicated a correlation between sand content and electroosmotic efficacy as well as between clay content and a decrease in electroosmotic efficacy. Theoretical basis for the EO phenomenon is of higher efficacy in fine-grained soils, a trend opposite that indicated by the data produced by this research. For this reason, the data produced by this research appear to contradict the theoretical basis for EO efficacy. Additional significance can be found in the duplication of these results in two separate experiments of different experimental design (phase one and phase two).

Several possibilities have been identified by this researcher for the existence of this contradiction found in the data.

While previous studies have been devised to investigate the effects of a variation of texture within a limited range of silt and clay mixtures, none have studied the effects of texture within the range studied by this project. For this reason, it is possible that EO efficacy increases at the extremes of the textural triangle. In order to investigate this possibility, it may be helpful to obtain larger quantities of soil samples for purposes of mixing; with only a minimally larger sample base of soil samples, it would then be possible to create a much wider range of textures for purposes of testing by mixing samples.

The possibility also exists that the discrepancy between the data and the theoretical applicability of electroosmosis is linked to the relatively short duration of trials in this experiment. While future research may want to focus on flow over a more extended time period, it was felt that for the purposes of this particular experiment more exact measures of rate and change in rate (obtained by more frequent readings of flow volume) would be desirable over less frequent and less accurate measures of rate and change in rate.

Finally, there exists the possibility that theoretical models of electroosmotic efficacy in application to soil remediation do not serve as accurate representations of the processes at work. Though many studies have been conducted recently to investigate electroosmotic phenomenon, the processes at work are still not very well understood and the potential exists that present theoretical models may be in error.

The remaining variables in addition to texture were considered by this research were systematically assumed to be the independent variables in order to establish whether a stronger correlation existed between the strength of EO-induced flow and a variable other than texture (table follows). Based on the data, however, clay content remained the variable with the greatest likelihood of having a direct effect on EO-flow.

CONCLUSION

Based on the findings of the previous year’s research, this phase of experimentation sought to refute or quantify the correlations between texture and electroosmotic efficacy that were indicated. It is significant that both phase one and two of this research indicated the same potential trend of clay content having a negative influence upon electroosmotic efficacy. Additionally, it is important to note that this trend is contrary to the theoretical range of greater electroosmotic-applicability, which is of clay content to greater efficacy. Three possibilities were suggested to explain this contradiction-a greater efficacy with extremes of sand and clay content, a strong time dependence which differs over more extended periods of time greatly from the initially established trends, or an inaccuracy of theoretical models at explaining the true processes taking place under the influence of electroosmosis.

REFERENCES

Acar, Y. et. al. Phenol removal from kaolinite by electrokinetics. J. Geotechnical Eng. 118: 1837; 1992.

Anderson, J.; Idol, W. Electroosmosis through pores with nonuniformly charged walls. Chem. Eng. 38: 93-106; 1985.

Bouma, J., Brown, R.B., and P.S.C. Rao. “Movement of Water: Basics of Soil-Water Relationships – Part III.” University of Florida Cooperative Extension Service. http://hammock.ifas.ufl.edu/txt/fairs/16804 (11-Nov-97).

Bruell, C. et. al. Electroosmotic removal of gasoline hydrocarbons and TCE from clay. J. Geotechnical Eng. 118:68; 1992.

Buttler, Tasha, Martinkovic, Wendel, and O. Nesheim. “Factors Influencing Pesticide Movement to Ground Water.” University of Florida Cooperative Extension Service. http://hammock.ifas.ufl.edu/txt/fairs/18887 (11-Nov-97).

Catsimpoolas, Nicholas, ed. Isoelectric Focusing. New York: Academic Press, 1976.

Chapelle, Francis H. The Hidden Sea. Tucson, AZ: Geoscience, 1997.

Hamed, J. et. al. PB(II) removal from kaolinite by electrokinetics. J. Geotechnical Eng. 117: 241-271; 1991.

“Introduction to Soil Water Potential.” Department of Agronomy and Horticulture, New Mexico State University. http://taipan.nmsu.edu/aght/soils/soil_physics/tutorials/wp/wp_tut.html (14-Nov-97).

M.Mizoguchi, T.Ito and K.Matsukawa. Movement of water and ions in frozen clay by electroosmosis.

Probstein, Ronald F. Physicochemical Hydrodynamics. New York: Wiley, 1994.

Removal of contaminants from soils by electric fields. Science. 260:498; 1993.

Segall, B. et. al. Electroosmotic contaminant-removal processes. J. Env. Engineering. 118: 84-100; 1992.

Shapiro, A. and R. Probstein. Removal of Contaminants from Saturated Clay by Electroosmosis. Envirn. Sci. Technol. 27: 283-291; 1993.

“Soil Bulk Density.” http://www.soils.umn.edu/academics/classes/soil3125/doc/5chap2.htm (12-Nov-97).

“Soil Texture.” http://www.soils.umn.edu/academics/classes/soil3125/doc/3chap1.htm (12-Nov-97).

Webber, M.D. and S.S. Singh. “Contamination of Agricultural Soils.” Soil Health. http://res.agr.ca/CANSIS/PUBLICATIONS/HEALTH/chapter09.html (12-Nov-97).

Acknowledgments

(This is more or less the list of acknowledgments posted with my project for the 1998-1999 fair)
I will say that I feel that my independent science projects over my four years of high school have been an enormous success and that I have quite a number of people to thank who have make this possible:

• I would first and foremost like to thank my parents for, well, everything.
• Thanks must also go out to my grandparents for support among other things.
• Profuse thanks and applause to my science-fair adviser of two years, Mrs. Patricia Wee, whose amazing encouragement and influence have been of more help to me than I could possibly imagine, I am sure.
• Thanks also go out to Mr. Larry Hess, science department head at my high school, whose support and encouragement have also led my ideas to become realized through my science fair projects.
• I would like to thank Mr. Wilmer Nolt, whose seemingly endless energy and encouragement have also served as an inspiration to me and allowed me to make my projects a reality.
• I would be remiss if I were not to acknowledge the help of Colleen LeFevre, whose insight and ideas have helped to direct me in my studies of electroosmosis.

Additional thanks to (in no particular order):

Wendy Zwally; Mr. James Collier; Mr. Bruce Lasala; Mr. Wayne Boggs; Mr. Michael Vavreck; Mr. Pat Ross; Mrs. Tamme Westbrooks; Mr. Michael Eyster; Mrs. Filitea Dean; The numerous teachers who have had enough foresight and understanding to allow classes to be missed “in the name of science”; Numerous colleages/classmates, especially fellow independent science “associates”; Science surplus catalogs; A hearty thanks to velcro, foam-board, double-stick tape, and formica.

Abstract

The purpose of this experiment was to determine whether texture plays a significant role in determining the efficacy of electroosmosis in effecting the removal of organic, water-soluble contaminants from soil. Soil samples were collected which provided a wide assortment of attributes for a basis of comparison and analysis. The soil samples were oven-dried to assure the equal saturation of all the soil samples by equal concentrations of tannic acid.

A test chamber was constructed from a fluorescent tube protective cover and the end contacts of a fluorescent lamp. Three holes equidistant from one another were drilled in the tube in order to allow a measurement of tannic acid at regular intervals along the chamber. The test solution of tannic acid was 100 mg/L.

Each sample was saturated with tannic acid solution to the point that all pores were occupied. The test chamber was then filled with the saturated soil and the end caps sealed. An initial sample was taken from the test chamber in order to substantiate the initial concentration. A direct current of eight volts was then applied to the test chamber across the end contacts by means of a power supply. Every two hours for a total of six hours water samples were taken from each of the testing ports by means of pipettes. Each water sample was tested for tannic acid concentration.

The data collected show a tentative relationship of texture to electroosmotic efficacy and a distinct correlation between pore space and electroosmotic efficacy.

INTRODUCTION

Electroosmosis

Electroosmosis is a term applied to the process in which a liquid containing ions moves relative to a charged stationary surface . The phenomenon of electroosmosis has been applied in numerous ways, including as a means of dewatering soils for construction purposes and dewatering mine tailings and waste sludges. Recently, electroosmosis has been investigated as a potentially valuable in situ (i.e., on-site) method of purging contaminants from soil. In this respect, electroosmosis operates by causing an effluent (water) to move relative to a stationary charged surface, which is in this case the collective capillary pores of the soil. Another element of electroosmosis is the migration of the contaminant particles themselves relative to the liquid and the soil profile. This specific element is sometimes called electromigration and viewed separately from electroosmosis, although the two processes cannot be separated from one another due to the nature of their operation.

Electroosmosis research has been largely limited to the laboratory, and only in certain locations have large-scale experiments been conducted in a natural setting. The majority of [published] laboratory experiments concerning the remediary potential of electroosmosis has been limited to tests on kaolinite clay samples; one of the main goals of this project was to extend tests of electroosmosis past uniform samples to tests on soils of various texture. Although electroosmosis (in theory) should be more effective in soils with a lower hydraulic conductivity (i.e., soils with a greater percentage of micropores and that are more difficult to move water through by means of a pressure-driven device), this research sought to determine how great an effect texture has on the efficacy of electroosmosis by experimental means.

Soil Contamination and Remediation

As the technological level of our society continues to increase rapidly, the variety of new kinds of chemical wastes that we produce has also increased. As these chemical wastes continue to multiply and diversify, it is becoming more and more important that our methods of minimizing the impact of these wastes improve at the same rate. Although one important step in this process is the minimizing of the production of such wastes, some chemical by-products are inevitably produced and some inevitably escape into the environment. Most cases of direct contamination of the environment by chemicals include contamination of the soil. To worsen matters, soil is, in one way or another, of great importance to every plant and animal on this planet. When contaminated, soil can also serve as a means of spreading contamination throughout the environment. It is for these reasons that it is important that effective methods of remediating contaminated soil be developed.

An important aspect of the process of soil remediation is the ability to correctly recognize the dominant mechanism for the removal of the contaminants-that is, the method of soil remediation that can most effectively (both environmentally and economically) be used. In making this decision, it is necessary to take into account the nature of the contaminant itself, the characteristics of the soil, and the characteristics of the various methods of remediation. The main goal of this research was to determine whether or not texture should be a factor in considering the use of electroosmosis as a method of soil remediation.

Optimal remediation of contaminated soil also demands that unnatural (or extreme quantities of natural toxins) be removed effectively with minimal processing or alteration of the natural chemistry and morphology of the soil and surrounding environment. The investigation of new technologies and methods of remediation is necessary to attain these goals. Additionally, demonstration of the relationships of efficacy to the variables present in both soil and the contaminant will be required.

Soil Contaminants

In this research the contaminant chosen was tannic acid mainly because it is structurally similar to a widespread group of organic, water-soluble contaminants known as the aromatics. Organic contaminants are more difficult to remove by electrokinetic means than metallic compounds, namely due to the fact that they have a low polarity. However, electroosmosis has still been demonstrated to be a potentially valuable method of effecting the removal of such aromatics due to the migration coefficient of the effluent.

Soil Contaminants: The Aromatics

Aromatics are some of the most important organic chemicals both in industrial processes and to the consumer. The principal members of this group are toluene, benzene, and xylene, which are liquid hydrocarbons that yield a number of derivatives and combine with other organic chemicals to produce compounds such as styrene (which is a product of benzene, a member of the aromatics, and ethylene). One of the compounds that aromatics makes possible is motor gasoline. Benzene is used primarily to produce intermediate chemicals such as styrene, cumene, and cyclohexane, which are then used in industrial processes to produce polystyrene, phenol, and nylon. Mixed xylenes are utilized in many industrial processes as well, both as solvents and as chemical intermediates to produce a number of plastics, resins, and synthetic fibers. Additional products of the aromatics are synthetic rubber, detergents, latex, and polyurethanes. It is this wide variety of uses and applications that makes the aromatics particularly ubiquitous, both in industry and household, and therefore, common contaminants of soil. This fact, coupled with the potential hazard of these chemicals, makes it even more important that we develop effective methods of dealing with such chemicals.

Advantages of Electroosmosis over Conventional Methods of Soil Remediation

In situ methods of soil remediation such as electroosmosis hold obvious advantages over more conventional methods which can involve physically removing or greatly disturbing a large portion of the contaminated soil. Although the addition of pure water to an ecosystem can in itself be disturbing in some cases, electroosmosis is much less destructive to the natural environment than conventional methods which use pressure-driven water flow to remove contaminants. Because electroosmosis operates on a particle level rather than on a macro level, it has virtually no impact on the structure of the soil.

Many contaminants tend to “stick” to soil particles by the process of adsorption. For this reason, even conventional, pressure-driven methods of soil remediation can only remove a fraction of the initial amount of contaminant. Electroosmosis, by operating on a particle level, is able to overcome this limitation to a certain extent. Remediation by electroosmosis is dependent upon the level of adsorption of the contaminant and the nature of the soil itself.

An additional benefit of electroosmosis over conventional methods of soil remediation is the fact that electroosmosis overcomes macropore flow, the tendency of water that is applied to soil faster than it can be absorbed and dispersed to move into and through the soil profile mainly through the macropores, bypassing the micropores. This preferential flow, depending upon the particular soil (sandy soils have more macropores, clayey soils have more micropores), means that conventional methods of soil remediation leave a considerable amount of the contaminant behind after treatment.

For these reasons, a deeper exploration of the relationships that affect the efficacy of electroosmosis are important if we wish to most effectively deal with contamination of soil.

PROCEDURE

Preparation

Soil samples with a wide range of attributes for a basis of comparison and analysis of trends were gathered were oven dried to a constant mass (to assure that all moisture was evaporated in the process). Tannic acid solution was prepared by dissolving tannic acid (VWR Sargent Welch; Buffalo Grove, IL) in distilled water to a concentration of 100 mg/L.

The end contacts of a fluorescent lamp were used to construct the anode and cathode of the test chamber. To prepare the anode, two holes were drilled in a circular metal slug from an electrical box. A rubber washer was placed over the filament wires and the wires were threaded through the holes in the metal slug. The end contact, the rubber washer, and the metal slug were fastened together with epoxy glue. Care was taken to keep the ends of the filament wires and the exposed surface of the metal slug free of glue. The filament wires were soldered to the exposed surface of the slug. The end contact of the fluorescent lamp was fastened in the end cap of a fluorescent tube protective cover with epoxy glue. This process was repeated to prepare the cathode for the testing chamber. A clear, plastic fluorescent tube protective cover was cut to a length such that distance from contact to contact would be thirty centimeters. One hole was drilled at the center of the tube (138 mm from both ends of the tube), and two more holes were drilled at 76 mm on either side of the first hole, the total length of the tube being 276 mm.

Soil Analysis

Determining Texture

Fifteen milliliters of a soil sample were added to one 50 mL soil separation tube. The tube was tapped firmly on a hard surface to eliminate air spaces. One milliliter of Texture Dispersing Reagent (LaMotte; Chestertown, MD) was added to the tube, and the sample was then diluted to 45 mL with water. The tube was capped and shaken for two minutes, and then allowed to settle for 30 seconds. The solution from the tube was poured into another tube and allowed to settle for 30 minutes. The amount of sediment in the first tube was divided by the initial amount of soil (15 mL) to calculate the percentage of sand in the soil. The amount of sediment in the second tube was then divided by the initial amount of soil (15 mL) to calculate the percentage of silt in the soil. The percentage of clay in the sample was calculated by subtracting the sum of the other two percentages from 100 %. This was done to obtain a more accurate reading of clay percentage than the measurement of the volume of clay, as the colloidal nature of clay causes it to swell in water. This procedure was repeated for each soil sample. To determine the texture grouping, a soil texture triangle was used, although it was the specific fractions (of mineralogical size group) that were used to sort the data.

Determining Percent Organic Matter

A crucible was weighed, a small amount of an oven-dried soil sample was added, and the crucible was weighed again, to obtain the mass of the soil. The crucible and the soil were heated over a Bunsen burner for one hour. After cooling, the crucible and soil were weighed again and the change in mass calculated. This process was repeated until two consecutive equal readings of mass were obtained. In this procedure, the values obtained for organic matter are not precise, but approximate, as all weight loss upon ignition of the soil samples is assumed to be due to loss of organic matter. The following reaction is that which occurs in this process:

C₆H₁₂O₆ (O.M.) + 6O₂ + heat -> 6CO₂ + 6 H₂O

(Where O.M. is organic matter)

Determining Percent Pore Space

A known volume of water was added to a known volume of oven-dried soil. The total volume was then recorded. From this, the percentage pore space was calculated using the following formula:

(((VW + VS) – VT)/(VS)) * 100

where VW is the volume of the water, VS is the volume of the soil, and VT is the total volume of the combined water and soil

Determining Densities

The bulk density of each soil sample was calculated as the weight of a known volume of the oven-dried sample. The particle density was then calculated by the following formula:

(M)/(SS* V)

where SS is percent particle space, or ((100 % – % pore space)/(100)), M is mass, and V is volume

Testing Samples

An oven-dried soil sample was saturated with the tannic acid solution so that all pore space was occupied by the solution. One end cap of the test chamber was fastened in place with electrical tape and the testing ports were sealed with tape to prevent leakage of soil or water prior to testing. The soil was added to the tube and the other end cap positioned and fastened in place with electrical tape.

An initial sampling of water was taken prior to the application of electricity in order to establish the initial concentration of tannic acid in the event that it was changed by factors influenced by soil variables.

A direct current of eight volts was then applied to the soil in the test chamber across the end contacts. Every two hours for a total of six hours water samples were taken from each of the testing ports by means of pipettes and tested for tannic acid concentration.

Quantitative comparison of tannic acid was made of 0.5 mL samples of water from each testing port (diluted to 5 mL with deionoized water). One drop of TanniVer 3 Tannin-Lignin Reagent (Hach; Loveland, CO) and one milliliter of a sodium carbonate solution (Hach) were added to each of the three test tubes and each tube was gently swirled to thoroughly mix the solution. Color development in each of the tubes was recorded after 25 minutes via a color comparator (Hach).

DISCUSSION

There are many factors that are known to have an important role in determining the efficacy of electrokinetic methods of soil remediation such as electroosmosis. The charge, solubility, and level of adsorption of both the contaminant and the soil are known to have a certain degree of influence on the efficacy of electroosmotic-related mechanisms in effecting removal of contaminants. The main purpose of this research was to establish whether texture plays a role in determining the efficacy of electroosmosis in removing organic, water-soluble contaminants.

The variables involved in this research are soil texture, organic matter, pore space, and particle density. Initially, the change in concentration of the contaminant at testing ports one and three was used to attempt to establish any relationships that might exist. However, an increase in concentration at port three did not consistently correspond to a decrease in concentration at port one. An attempt was then made to determine which sets of data were more consistent, based on the average skewness and standard deviation of the data points for each set of data (the data for port three being one set and the data for port one being the other set). From this, it was determined that the set of data with the most consistency was the set of data for port three, so it was that set of data which was used as a base for a comparison of the relative strengths of relationships established.

In an attempt to establish which variable was the dominant factor in determining the efficacy of electroosmosis in effecting the removal of organic, water-soluble contaminants from soil, each variable was systematically assumed to be the independent variable. Once making this assumption, it was possible to plot the data against percent concentration change. With the data plotted, trend analyses were made based on the graphs, and the existence of a definite relationship was either supported or refuted. This was done for each of the soil variables, and then the correlations for each variable were compared in order to establish which variable could be most strongly related to electroosmotic efficacy. For the purposes of graphing, “electroosmotic efficacy” was determined by the percent change in concentration of the contaminant over the time elapsed in the experiment. The larger the percent change in concentration (in this case, the percent change being an increase), the greater the efficacy of the electroosmotic effect.

All variables showed a tentative relationship to electroosmotic efficacy; however, some of the correlations were more distinct than others. The strength of the correlations was determined based on the r² value for the relationships, and the nature of the relationships was determined from the Pearson’s correlation coefficient and a graphical analysis of the data. The weakest correlation was found to be between particle density and electroosmotic efficacy (r² = 0.0246). A correlation between organic matter and electroosmotic efficacy was found to be not much greater (r² = 0.0311). Pore space was found to be more closely related to electroosmotic efficacy, with an r² value of 0.0810; the Pearson’s correlation coefficient for this relationship was found to be -0.2846, indicating that increasing pore space might have a negative effect on electroosmotic efficacy (i.e., efficacy might degrease with an increase in pore space). Of all the variables, however, texture showed the greatest correlation to the electroosmotic efficacy with an r² value of 0.1007. The texture categories used to calculate the correlation were based on the soil groups defined by the textural triangle. On the numerical scale used to determine this correlation, smaller numbers were used to indicate soils with greater portions of sand and lesser portions of silt and clay. The Pearson’s correlation coefficient for this relationship was -0.3174, a value that indicates a decreasing electroosmotic efficacy with increasing textural category number. From this, then, of all variables the greatest basis for a correlation is between soil texture and electroosmotic efficacy, that correlation being of decreasing efficacy corresponding to a decreasing portion of sand and increasing portions of silt and clay.

To substantiate this trend, analyses were made of correlations between individual size fractions and the electroosmotic efficacy (percentage clay, percentage silt, and percentage sand vs. efficacy comprising three separate sets of data). The Pearson’s correlation coefficient for the relationship of clay to efficacy was found to be -0.2219, indicating a decreasing efficacy with increasing clay percentage and supporting the correlation produced using textural categories based on the textural triangle. The Pearson’s correlation coefficient found between silt percentage and efficacy was -0.2027, indicating the same type of relationship and also corroborating the correlation established earlier. Finally, the Pearson’s correlation coefficient for the relationship of sand to efficacy was 0.2811, indicating increasing efficacy with increasing sand percentage, helping to further validate the initial textural relationship established. Pure mathematical percentages were not used as the primary method of comparing the efficacy of electroosmosis since it is the ratios of sand, silt, and clay that have the greatest impact on the properties of a soil.

The relationship of texture to electroosmotic efficacy established seems contradictory to the relationship established by theory of increasing efficacy to increasing, rather than decreasing, clay percentage. There are several possibilities why the data seem to indicate a relationship contradictory to theory. One possibility is that the relationship of texture to the efficacy of electroosmosis is closer to a parabolic relationship than a linear relationship; if this were the case, electroosmotic efficacy might increase approaching the “extremes” of the soil textural triangle (as clay, silt, or sand percentage approaches 100%).

Another possibility is that the time that was allowed in this research for the movement of the contaminant was not enough to compensate for differing rates and duration of electroosmosis among soil samples. It is possible that a soil with a high electroosmotic rate initially might only maintain that high rate of contaminant movement for a short amount of time, while another soil with a lower electroosmotic rate might be able to maintain that rate for a longer amount of time, resulting in a higher overall efficacy.

The purpose of this investigation was to establish whether or not texture does play a role in determining electroosmotic efficacy, and in that regard, this research has accomplished its goal. It is significant to note that the data produced by this research do support the existence of a correlation between texture and electroosmotic efficacy. Although the data contradict rather than support the relationship of texture to the efficacy of electroosmosis and are therefore not definitive in what they reveal, it is notable that the data do support a correlation which warrants further investigation.

In investigating the potential of electroosmosis as a valuable method of in-situ soil remediation, several factors should be considered in addition to the efficacy of electroosmosis. An investigation of the efficacy of electroosmosis versus conventional methods of soil remediation might help to determine the most applicable texture range of electroosmosis. Further tests including a wider array of soil samples might help to pinpoint a more precise relationship.

Additionally, as it is the goal of any method of soil remediation to cleanse the soil with the minimal amount of disturbance possible, another subject of future investigation should involve determining the effect of electroosmosis not only on the contaminant in the soil, but on other facets of the soil profile and environment. A study of how electroosmosis affects nutrients and how well various plants retain nutrients in their roots under the influence of electroosmosis could become important in determining if a contamination site is a likely candidate for the use of electroosmosis or not. Another aspect which may be important in determining the method of soil remediation in any particular instance is the fauna inhabiting the soil profile and how electroosmosis influences them. One valuable investigation of this subject might attempt to determine whether the electrical current used for electroosmosis interferes with electrical impulses in the bodies of simple organisms such as earthworms. All of these factors have the potential to influence a decision made as to the most effective method of soil remediation, and need to be seriously considered as such.

CONCLUSION

In conclusion, there appears to be a correlation between texture and the efficacy of electroosmosis in effecting the removal of organic, water-soluble contaminants from soil. In order to more clearly establish a trend, however, it is necessary to collect more data in order to smooth the curves of the data. Another trend which became made apparent through the data was that as pore space increases, electroosmotic efficacy decreases. It is significant that this trend is partially independent of texture.

Awards won by this project

Reserve Champion at Lancaster Science and Engineering Fair (March 1998)

Intel Science Talent Search Semifinalist (formerly Westinghouse STS)