LABORATORY EXPERIMENT pubs.acs.org/jchemeduc
Laboratory Experiment on Electrokinetic Remediation of Soil Alya H. Elsayed-Ali,†,§ Tarek Abdel-Fattah,*,† and Hani E. Elsayed-Ali‡ †
Department of Biology, Chemistry, and Environmental Science, Christopher Newport University, Newport News, Virginia 23606, United States ‡ Department of Electrical and Computer Engineering, Old Dominion University, Norfolk, Virginia 23529, United States
bS Supporting Information ABSTRACT: Electrokinetic remediation is a method of decontaminating soil containing heavy metals and polar organic contaminants by passing a direct current through the soil. An undergraduate chemistry laboratory is described to demonstrate electrokinetic remediation of soil contaminated with copper. A 30 cm electrokinetic cell with an applied voltage of 30 V is used to demonstrate the redistribution of copper in sand initially contaminated with 0.24 M copper chloride solution. The copper content in sand is measured by acid extraction followed by complexometric titration. The measurement of the pH across the cell is used to demonstrate the difference in mobility between the Hþ and OH ions during the electrokinetic process. KEYWORDS: Second-Year Undergraduate, Analytical Chemistry, Environmental Chemistry, Laboratory Instruction, Hands-On Learning/Manipulatives, Electrochemistry, Geochemistry, Oxidation/Reduction, Quantitative Analysis, Titration/Volumetric Analysis
E
lectrokinetic remediation involves the application of an electric field across a volume of soil to transport contaminants to a localized area where they can be effectively extracted.15 Electrokinetic remediation uses the three principles of electromigration, electroosmosis, and electrophoresis:6 electromigration is the transportation of metallic ions by attraction to the oppositely charged electrode;7 electroosmosis is the motion of water molecules from the anode to the cathode during electrolysis;8 and electrophoresis is the movement of charged particles under an electrical current.9 These factors facilitate movement of contaminant ions to the cathode where the ions can be precipitated and reclaimed.9 Electrokinetic remediation is a proven technology for soil and water volumes and has demonstrated both promising prospects and challenges.1014 A laboratory experiment that introduces students to the basic principles of electrokinetic remediation of soil contaminated by heavy metals is described. This experiment best fits in an analytical chemistry laboratory or environmental chemistry laboratory that trains students in the fundamentals of electrochemistry and its application in environmental remediation. In addition, students gain experience in metal extraction from soil and complexometric titration as a method to measure ion concentration in a solution. This experiment requires two 4-h laboratory sessions. The concept of electrokinetic remediation was previously described in this Journal.15 These experiments were focused on the destruction of organic wastes in electrochemical cell and classroom demonstration of the transport of inorganic contaminants in silica gel using an electric field based on observation of color changes.15 Here we discuss an electrokinetic remediation experiment with detailed quantitative analysis of ion transport, based on complexometric titration method. The concentration of Cu2þ containment in a cell Copyright r 2011 American Chemical Society and Division of Chemical Education, Inc.
containing sand is measured along with pH variation across the cell. The role of pH gradient development on the electrokinetic remediation process is tested.
’ PRINCIPLE The basic processes involved in electrokinetic remediation of soil contaminated with metal ions (Cu2þ in this experiment) are oxidation at the anode and reduction at the cathode. The oxidation of water at the anode results in the formation of Hþ ions, which migrate to the cathode:7 2H2 O f O2 þ 4Hþ þ 4e
ð1Þ
At the cathode, the reduction of water results in the formation of hydroxide ions, which migrate to the anode:7 4H2 O þ 4e f 2H2 þ 4OH
ð2Þ
Hydrogen ions move more quickly than do hydroxide ions.9 Consequently, as hydroxide ions created at the cathode move toward the anode, they encounter the acidic front created by Hþ ions, resulting in the precipitation of increased amounts of copper oxide closer to the cathode.9 This causes precipitation of insoluble Cu(OH)2 close to the cathode, followed by oxide formation, inhibiting the reclamation of copper ions:16 Cu2þ þ 2OH f CuðOHÞ2
ð3Þ
CuðOHÞ2 f CuO þ H2 O
ð4Þ
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The addition of an acid to the soil neutralizes some of the OH ions formed at the cathode. Neutralizing hydroxide ions would aid in effectively transporting Cu2þ ions to the cathode by minimizing the amount of Cu2þ ions precipitated from the soil as hydroxide, thus allowing for more Cu2þ ions to be transported to the cathode.9,17
’ HAZARDS The hazards associated with this experiment arise mainly from the corrosive nature of copper chloride, which is toxic, reacts with exposed skin, and is an extreme irritant to the eyes and the respiratory tract. The 0.1 M HCl solution, used by the students, is corrosive and an irritant. It is strongly advised that preparation of the copper chloride solution be done under a fume hood. Proper eye protection must be worn at all times. Polyvinyl protective gloves must be used during the experiment. Upon conclusion of the experiment, all solid and liquid waste must be placed in designated containers for proper disposal. ’ EXPERIMENTAL PROCEDURE Instructions for the Instructor
To preparation the soil (sand), place approximately 1 kg of sand (e.g., Pavestone high desert play sand) in a covered polycarbonate container. Create slurry of the sand in 10% HCl solution (cover the sand in the container) to dissolve trace metals. Close the container and shake well and then allow the sand to settle. Pour out the HCl and wash with enough water repeatedly until the pH of the wet soil rises to ∼4.9. Once the soil has reached this suitable pH, pour out excess wash water. Heat the sand for several hours in the oven at ∼150 °C until it becomes completely dry. For the preparation of the electrokinetic cell, obtain polyvinyl tubing (∼1.15 cm i.d.) and cut it to 30 cm length. The tube length and dimensions are chosen because they are suitable to demonstrate the redistribution of copper by the electrokinetic processes and they can be easily assembled. With plastic ties, attach the polyvinyl tube to a wooden plank with similar dimensions as the polyvinyl tubing to keep the it from bending. Obtain a graphite rod with ∼1.1 cm i.d. and cut to ∼2 cm lengths for use as electrodes. Drill and tap each electrode on one side to allow for the use of a stainless steel screw for making an electrical contact. Procedure Done by Students during Laboratory Session 1
A group of three students works on each setup. In the first laboratory session, the students assemble the electrokinetic cell using the precut polyvinyl tubing and graphite electrodes as shown in Figure 1. Wrap Teflon tape around the graphite electrodes so that they make a good seal when inserted in the polyvinyl tube. Insert one of the graphite electrodes into the polyvinyl tube and then pack the tube with a specified quantity of sand. A copper chloride solution with a specified molarity (0.10.4 M) is prepared. Then, a certain volume is used to contaminate the sand by holding the tube vertically and dripping the solution directly into the tube. In the present example, a 0.24 M CuCl2 solution is used. The above-suggested range of molarity results in ion current flow in the cell that is sufficient to observe the redistribution of ions in a few hours with 30 V applied across the cell. To ensure a uniform distribution of the copper chloride solution across the tube, load the sand in increments of ∼5 cm, saturate with copper chloride solution, and then add another
Figure 1. Setup of electrokinetic cell, power source, and multimeter. The dark brown color in the electrokinetic cell is due to precipitation of copper oxide.
increment of sand and saturate the top layer with copper chloride. Continue this process for the full length of the tube. Several regions between the cathode and anode are marked on the tube. After placing both electrodes, the electrokinetic cell is operated for 3 h at a specified dc voltage while measuring the current across the cell at 15 min or fewer intervals. The sand from each separate region is scraped out into different glass jars and stored for use in the second laboratory session. Procedure Done by Students during Laboratory Session 2
In this laboratory, the students measure the pH of the sand from each region. Copper from each region is extracted and measured by complexometric titration with disodium ethylenediaminetetraacetic acid (Na2EDTA) (see details in the Supporting Information). The sand from each region is dried and its weight measured to determine copper-to-sand weight ratio.
’ RESULTS AND DISCUSSION The voltage across the cathode and anode is maintained at 30 V using a regulated power supply. The produced electric field across the cell is enough to observe the redistribution of copper ions in a few hours. These power supplies are readily available in undergraduate laboratories and are safe to use. The current through the cell is measured with an ammeter. In a typical result, the starting current is 8.5 ( 0.1 mA, while at the end of the experiment, the current is 8.3 ( 0.1 mA. During the experiment, the current rises to 9.3 ( 0.1 mA. This current is determined by the applied voltage across the cell and the conductivity of the cell due to the copper chloride solution. The current trends vary from one run to another depending on many factors, such as formation of bubbles and precipitation of copper(I) oxide, and packing of the cell with sand may vary from one cell to another. The initial pH of the contaminated sand prior to electrokinetic treatment was 3.7. This acidic pH is due to the copper chloride solution. These observations can be explained from the hydrolysis reactions of divalent metal cations by the general reaction:18 M2þ þ nH2 O a ½MðOHÞn 2 n þ nHþ
ð5Þ
However, based on the equilibrium constants, only the first hydrolysis reaction (n = 1) is considered significant for copper ions: Cu2þ þ H2 O a CuOHþ þ Hþ
ð6Þ
A general trend can be seen in Figure 2 depicting the changes in pH throughout the cell. The distance in Figure 2 refers to the distance from the cathode to the center of the region (midpoint) from which the sand was collected. The pH changes gradually at a distance greater than 10 cm away from the cathode. Because Hþ ions are more mobile than OH ions, the amount of ions in the 1127
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Figure 2. pH across the electrokinetic cell measured after 5 h of operation.
Figure 3. Ratio of copper-to-sand relative to distance from cathode.
acid front moving toward the cathode affects the rate of movement of the acid front. The error bars represent the margin of error, estimated at (0.2, from reading pH paper. The ratio of copper-to-sand obtained after operating the electrokinetic cell is shown in Figure 3. Error bars in Figure 3 represent the margin of error in the titration, estimated at 0.2 mL. The results show the drift of copper ions as milligrams of copper per gram of sand (mg/g) toward the cathode and its precipitation near the cathode.
’ AUTHOR INFORMATION
’ CONCLUSIONS This experiment covers the fundamentals of oxidationreduction reactions and one of the applications of electrochemistry in environmental remediation. The experiment focuses on (i) metal ion migration and distribution across the electrokinetic cell, and (ii) pH distribution across the electrokinetic cell due to the movement of an acidic (Hþ ions) and a basic (OH ions) front. The experiment also introduces students to a quantitative method of measuring a metal contaminant in soil.
’ ACKNOWLEDGMENT We gratefully acknowledge M. Mahmoud of Alexandria University in setting up the complexometric titration method.
’ ASSOCIATED CONTENT
bS 1 Supporting Information 13
H NMR and C NMR spectra of divanillyl oxalate. This material is available via the Internet at http://pubs.acs.org.
Corresponding Author
*E-mail:
[email protected]. Present Addresses §
Department of Chemistry, University of North Carolina at Chapel Hill, Chapel Hill, North Carolina 27515, United States. E-mail:
[email protected].
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