KR100767339B1 - Electrokinetic remediation of fluorine-contaminated soil - Google Patents

Abstract

A method for purifying soil polluted with fluorine is provided to remove effectively the fluorine contained in the soil, by selecting an electrolyte for eluting the fluorine and using the relation between concentration of the electrolyte and current density of kinetic electricity. A current supply unit is prepared. An anode tank and a cathode tank are distanced from soil polluted with fluorine, wherein the soil is disposed between the anode tank and the cathode tank. An alkaline solution is inputted into the anode tank, which is connected to the current supply unit, using a circulation pump. An alkaline solution is inputted into the soil to elute fluorine while electric current is passed through the soil, thereby removing the eluted fluorine in the anode tank and the cathode tank by electro-migration and electrical osmosis.

Description

ELECOMTROKINETIC REMEDIATION OF FLUORINE-CONTAMINATED SOIL

1 is a schematic diagram of an electrokinetic purification system for the purification of fluorine contaminated soil according to the present invention.

2 is a graph showing the fluorine extraction and removal efficiency according to the concentration of NaOH electrolyte.

3 is a graph showing a change in voltage over time.

4 is a graph showing the accumulated electroosmotic flow over time.

5A and 5B are graphs showing water content and pH change after electrokinetic treatment.

6A and 6B are graphs showing fluorine concentration and fluorine removal rate in soil after electrokinetic treatment.

The present invention relates to a soil purification method using electrokinetic for soil contaminated with fluorine. More specifically, the present invention utilizes alkaline solutions, preferably sodium hydroxide (NaOH) or potassium hydroxide (KOH) electrolytes to make fluorine contaminated soils in alkaline conditions, and uses kinetic techniques to remove fluorine from the soil. It relates to a soil purification method to remove.

In recent years, heavy metal contamination in abandoned mines and US military units has become a social problem. When soil is contaminated with heavy metals, there are many problems that are difficult to restore and directly damage not only crops but also humans. Among them, fluoride is closely related to human life such as the use of tap water, and it is not only toxic but also causes bone fluoride, which causes bone and nervous system damage when ingested above the standard value. There is this. There are existing methods of purifying soils contaminated with heavy metals, such as after-pumping and soil washing. Recently, innovative technologies include electrokinetic purification (EK) [Zhongming Li, et al. A new approach to electrokinetic remediation of soils polluted by heavy metals, Journal of Contaminant Hydrology, 22, pp. 241-253, 1996. Restoration technology using electrokinetic purification is the technology that can be most effectively applied to the removal of organic matter and heavy metals and is being actively researched not only in developed countries but also in Korea [K. R. Reddy, C. Y. Xu, Supraja. Chinthamreddy, Assesment of electrokinetic removal of heavy metals from soils by sequential extraction analysis, Journal of Hazardous Materials, B84, pp. 279-296, 2001]. In addition, it can be applied to the removal of various substances such as radioactive substances, organic compounds, inorganic matters [Y. B. Acar, et al. Electrokinetic remediation: Basics and technology status, Journal of Hazardous Materials, 40, pp. 117-137, 1995]. The main elimination mechanisms of the electrokinetic purification method (EK) to remove these pollutants include electrophoretic movements of ions and electroosmotic movements of pore water. In the field treatment, EK is widely used because of its advantages over conventional treatment methods. Advantages of EK include the simultaneous removal of organic and inorganic materials, the simultaneous removal of various metals, and high removal efficiencies compared to a short treatment time [Jing-Yuan. Wang, Di-song. Zhang, Olena. Stabnikova, Joo-Hwa. Tay, Evaluation of electrokinetic removal of heavy metals from sewage sludge, Journal of Hazardous Materials, B124, pp. 139-146, 2005, and Ji-Hsing Chang, Zimin Qiang, Chin-Pao Huang, Remediation and stimulation of selected chlorinated organic solvents in unsaturated soil by a specific enhanced electrokinetics, Colloids and surfaces A: physicochem. Eng. Aspects, 2006]. It is also applicable to heterogeneous soils and low permeability clay soils, which is more economical than other methods, and can be applied with other methods such as biological treatment [Ching. Yuan, Chih-huang Weng, Sludge dewatering by electrokinetic technique: effect of processing time and potential gradient, Advances in environmental research, 7, pp. 727-732, 2003, and V. Pomes, A. Fernandez, N. Costarramone, B. Grano, D. Houi, Fluorine migration in a soil bed submitted to an electric field: influence of electric potential on fluorine removal, Colloids and surface A: physicochem. Eng. Aspects, 159, pp. 481-490, 1999]. While there is such an advantage, there is an important disadvantage. When EK is applied, a rapid pH change occurs between the low pH environment and the electrode. This phenomenon causes oxidation and reduction reactions due to electrolysis at the anode and the cathode during the EK process. Because. The redox reactions at each electrode are as follows: Jih-Hsing Chang, Ying-Chih Liao, The effect of critical operational parameters on the circulation-enhanced electrokinetic, Journal of Hazardous Materials, B129, pp. 186-193, 2006].

1/2 H ₂ O  → H ⁺ + 1/4 O ₂  + e ^-  (anode)

H ₂ O  + e ^- → OH ^- + 1/2 O ₂ (cathode)

By electrolysis, hydrogen ions are generated at the positive electrode, hydroxyl ions are generated at the negative electrode, so that the pH is lowered at the positive electrode and a sharp pH difference is generated between the electrodes. As fluorine has a higher pH, solubility in water is increased by ion exchange between fluorine ions and hydroxy ions [Walter W. Wenzel, Winfried E. H. Blum, Fluorine speciation and mobility in F-contaminated soils, Soil Science Vol. 153, No. 5, 1992, and MA Elrashidi, WL Lindsay, Chemical equilibria of fluorine in soils: A Theoretical development, Soil Science Vol. 141, No 4, 1986]. High removal rates can be obtained.

In the present invention, an alkali solution, preferably NaOH or KOH electrolyte, is injected into a fluorine contaminated soil to elute fluorine in the soil, and then a soil purification method is provided by removing a fluorine by adding a galvanic iron having a specific current density.

In the present invention, a specific electrolyte for eluting fluorine is selected in the soil purifying method using a voltaic cell, and the correlation between the concentration of the electrolyte and the current density of the galvanic phase is selected to select a suitable condition. It provides a method for efficient purification.

In the present invention, using an alkaline solution, NaOH or KOH electrolyte to increase the solubility of fluorine in restoring fluorine contaminated soil by applying the electrokinetic purification method, and to study the removal rate of fluorine according to the strength of the current and the concentration of the electrolyte It provides a way to efficiently restore fluorine contaminated soil.

Preferably, the present invention provides a step of providing a current supply, providing a positive electrode and a negative electrode tank having a positive electrode and a negative electrode, respectively, a predetermined distance between the fluorine contaminated soil, connected to the current supply using a circulation pump Injecting an alkali solution into the anode tank, and injecting an alkaline solution into the soil to maintain the pH of the soil at about 11 or more, and passing current while eluting fluorine to remove the eluted fluorine from the cathode and anode tanks. Provides a method for purifying fluorine contaminated soil.

Suitable current densities for feeding the soil in the method of the present invention are about 0.01-10 mA / cm ² in consideration of power consumption, fluorine removal efficiency and the like.

In the method of the present invention, the concentration of the alkaline solution injected into the soil is about 0.01M-1.0M (OH-ion standard) in consideration of power consumption and fluorine removal efficiency.

The pH of the soil in the process of the invention is suitably maintained at about 9-12 for proper elution of fluorine.

In the method of the present invention, the distance between the anode tank and the cathode tank is 0.5m to 5m for proper current flow.

The electrokinetic purification process used in the present invention is an on-site soil purification technique that removes fluorine by applying a low DC current by installing electrodes in fluorine contaminated soil. In case of using DC voltage supply, voltage gradient of 0.1 V / cm-10 V / cm can be used. In case of using DC current supply, current density of 0.01-10mA / cm ² can be applied. Due to the large difference in the electrical conductivity of the soil, it can be applied in consideration of the site situation within this range.

In the electrokinetic phenomenon, the flow of fluid in the soil by the potential difference is called electroosmosis, and the movement of metals or ions toward the opposite electrode is called electromigration, and colloid or charged particles Is moved in the direction of the opposite electrode is called electrophoresis. The electroosmotic refers to a phenomenon in which a liquid in the medium moves in a specific direction when an electric field is present in a porous medium such as soil. The reason for the occurrence of electroosmotic is that most soil particles come in contact with water and have a weak negative charge. Therefore, a liquid with a large positive charge density collects in the spaces between the particles. Because it occurs. In order to generate the electroosmotic in the soil, the anode and the cathode are installed around the polluted area, and the electrodes are supplied to supply the current to the polluted area from the power source.

According to a preferred embodiment of the present invention, as illustrated in FIG. 1, the fluorine purification method of the present invention provides a current supply and an anode tank and a cathode each provided with an anode and a cathode at a predetermined distance between fluorine-contaminated soil. A tank is provided, and an alkaline solution is injected into the anode tank connected to the current supply using a circulation pump, and an alkaline solution is injected into the soil so that the pH of the soil is kept in an alkaline state (pH 9.0-pH 12.0) while eluting fluorine. A current is passed to purify the fluorine contaminated soil, including the step of removing the fluorine eluted by electroosmotic from the cathode tank and concentrating and finally removing the fluorine eluted by electrophoresis in an alkaline solution.

Hereinafter, the present invention will be described in more detail with reference to examples, but these examples are merely to illustrate the present invention, and thus the present invention is not limited thereto.

< Example >

Electrokinetic purification was applied to purify fluorine contaminated soil. It was confirmed that fluorine was easily eluted under alkaline conditions through fluorine dissolution experiments, and alkali conditions were made using electrolyte (NaOH) to remove fluorine from soil. Fluorine in the soil is removed by electroosmotic movement of pore water toward the cathode and electrophoretic movement of ions toward the anode. In this experiment, four EK experiments were conducted for 14 days under different conditions of electrolyte concentration and current intensity. The change of voltage and electroosmotic flow (EOF) over time were measured and the soil was divided into 10 parts after the end of the experiment to study the water content, pH, residual fluorine concentration and removal rate.

Experimental Materials and Devices

The soil used in the experiment was collected from the fluorine contaminated area less than 2mm in size and the average concentration of fluorine in the soil was about 414mg / kg. The initial pH of the soil was 8.91 and the water content was 15%. As electrolyte, NaOH solution was used.

1. Fluorine dissolution test

To determine the elution of fluorine by electrolyte, 3 g of fluorine contaminated soil was placed in a 50 ml polyethylene bottle, NaOH was added at a concentration of 0.001 M, 0.01 M, 0.1 M, 0.5 M, and 1 M, and then mixed for 30 hours. Supernatant was collected and ion concentration (futex, Korea) was used to measure the concentration of fluorine ion.

2. EK  Experiment

A constant current supply was used for the EK experiment, and the pH was adjusted while injecting NaOH solution to the anode side using a circulation pump. As the electrode, a carbon electrode was used and the size was (4 × 4) cm ² . The size of the reactor used in the experiment was (20 × 4 × 4) cm ³ and was made of acrylic material. The experimental conditions of EK are shown in Table 1. Applied Current is 2mA / cm ² 5 mA / cm ² and the electrolyte solution concentration was used at 0.1M-1.0M. The change of voltage was measured during the experiment, and after the experiment was completed, the soil in the reactor was divided into 10 parts, and the respective pH, water content, and fluorine concentration were analyzed. Followed.

Table 1. EK  Experimental Conditions for Operation

EK  Experiment. Current density mA Of cm ² ) Electrolyte Uptime ( days ) Experiment 1 2 NaOH  0.1M 14 Experiment 2 3 NaOH  0.1M 14 Experiment 3 5 NaOH  0.1M 14 Experiment 4 2 NaOH  0.5M 14 Experiment 5 3 NaOH  0.5M 14 Experiment 6 5 NaOH  0.5M 14 Experiment 7 2 NaOH  1.0M 14 Experiment 8 3 NaOH  1.0M 14 Experiment 9 5 NaOH  1.0M 14

result

1. Fluorine dissolution test

Figure 2 shows the concentration of fluorine and the removal rate according to the electrolyte (NaOH) concentration. The higher the concentration of NaOH, the higher the concentration of fluorine and the removal rate. In this experiment, 0.1 M, 0.5 M, 1.0 M NaOH was used as the electrolyte in the EK experiment.

2. Change of voltage

Figure 3 shows the change in voltage over time. Initially, the voltage became slightly unstable and gradually decreased, and then stabilized after 96 hours. It seems that the voltage is kept constant by decreasing the resistance due to the effect of Na ⁺ ions generated by ionization of NaOH used as electrolyte while the current is constant at V (voltage) = I (current) × R (resistance) (FIGS. 2 and 3). ).

3. Electroosmotic flow ( EOF )

4 shows the electroosmotic flow over time. The higher current density and electrolyte concentration, the higher the EOF.

4. Water content and pH

5a and 5b show the soil water content and pH. Moisture content after 14 days of experiment showed a similar tendency to EOF. The pH was above 11, depending on the current density and electrolyte concentration.

5. Fluorine concentration in soil

6A and 6B show fluorine concentration and removal rate in soil. Since fluorine is an anion, the fluorine concentration in the soil is reversed in the direction of electrophoresis and electroosmosis, so it was removed near the anode and cathode, and the removal rate was higher as the current density and electrolyte concentration were higher.

6. Power Consumption

Table 2 summarizes the results of the EK experiments in this study.

Table 2.

EK  Experiment. Removal rate average (%) Power consumption KWh Of ton ) Experiment 1 51.8 564.9 Experiment 2 50.8 1234 Experiment 3 67.1 1664 Experiment 4 64.4 406.2 Experiment 5 75.6 337.9 Experiment 6 79.4 982.4 Experiment 7 65.1 442 Experiment 8 72.5 564 Experiment 9 76.7 671

result

The electrokinetic (EK) process was used to purify soils contaminated with fluorine using different concentrations of electrolyte and current. The detailed experimental results are summarized as follows.

-In the fluorine dissolution test, 0.001M, 0.01M, 0.1M, 0.5M, and 1M NaOH were used to determine the dissolution rate of fluorine contaminated in soil. In NaOH 1M, fluoride ions were most eluted, and the degree of fluorine dissolution was proportional to NaOH concentration.

-EOF was the highest at 3650mL at current density of 3mA / cm ² and 0.5M NaOH. The higher the EOH concentration was, the higher the current intensity was.

-After 14 days of EK experiment, the soil was divided into 10 sections and the fluorine concentration in the reactor was measured to determine the removal rate, power consumption, and removal efficiency. When the current density was 3mA / cm ² and 0.5M NaOH, the average removal rate of fluorine was 75.6%, and the power consumption was 337.9 KWh / ton.

Purifying fluorine-contaminated soil according to the soil purification method of the present invention efficiently removed fluorine from the soil while significantly reducing costs.

Claims (5)

Providing a current supply, providing an anode tank and a cathode tank each equipped with an anode and a cathode at a predetermined distance between the fluorine-contaminated soil, and injecting alkaline solution into the anode tank connected to the current supply using a circulation pump In the step of injecting an alkaline solution into the soil to maintain the pH of the soil in an alkaline state, eluting fluorine and passing an electric current, the fluorine eluted by electrophoresis and electroosmotic is pumped in the anode and cathode tanks, respectively. Purifying the fluorine contaminated soil comprising the step of removing the same. The method of claim 1 wherein the suitable current density for feeding the soil is 3 mA / cm ² and the voltage gradient is 0.1-10 V / cm. The method of claim 1 wherein the concentration of the alkaline solution is 0.5M. The method of claim 1, wherein the distance between the anode tank and the cathode tank is 0.1m to 5m. The method of claim 1 wherein the alkaline solution is a NaOH or KOH solution.

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