KR20040015855A - In-Situ immobilization of heavy-metal contaminated soil by electrokinetic phosphoric acid injection - Google Patents

Abstract

PURPOSE: Provided is an in-situ immobilization method of lead in soil by electrokinetic phosphorous injection, in which lead in contaminated soil is led to a contamination source by electric ion transportation effect and immobilized in the contamination source under ground, so it is applicable to soil remediation without excavation. CONSTITUTION: The method comprises the steps of burying a plurality of positive and negative electrodes(2,4) under ground in vertical or horizontal direction, and applying DC power to the electrodes(2,4), wherein the positive electrodes(2) are supplied with aqueous solution(3) and the negative electrodes(4) are supplied with mixture solution of chlorine acid and phosphoric acid(5) by a metering pump, wherein the positive electrode(2) is connected with a water level adjusting device(6), wherein the electrodes(2,4) are made of inert conductive material.

Description

In situ immobilization of heavy-metal contaminated soil by electrokinetic phosphoric acid injection

An object of the present invention relates to a method for purifying contaminated soil by making lead insoluble by effectively injecting a mixed solution containing phosphorus in a soil contaminated with lead by incorporating phosphorus.

In situ purification techniques currently used for heavy metal contaminated soils and groundwater include solidification / stabilization and in-situ soil flushing. However, in the case of low permeable fine soils, it is difficult to achieve the purification efficiency even by the conventional purification method. The solidification technique, which mainly uses cement or apatite, is difficult to apply when there is a structure on top of the contaminated area by disturbing and mixing the soil. In addition, the conventional electrokinetic method has a disadvantage in that the purification cost is increased by taking a long time for the purpose of extraction only. Therefore, the limitations of existing heavy metal contaminated soil and groundwater purification technology can be supplemented, and a cost-effective purification technology that can be applied to fine grain soil is needed.

The electrokinetic method is evaluated as an effective method for removing contaminants from viscous soils with very low permeability using electroosmotic and ion transport phenomena.

Electrokinetic purification is a method of inserting a conductive electrode into the soil and applying a direct current to remove contaminants of each polarity to the opposite electrode of its polarity and remove it from the electrode portion. The most serious problem in the general electrokinetic purification is that heavy metal ions that have migrated toward the electrical cathode meet hydroxyl ions from the cathode to the anode. will be. In order to compensate for this, various enhancement techniques have been used. In general, a method of inducing a pH reduction using an acid at a cathode and a technique of injecting a complexing agent are used. The most frequently recorded technique of complexing agent injection at the cathode is EDTA injection. However, previous studies have shown that these enhancement techniques also have many problems to completely remove heavy metal contaminants out of the sample. In particular, lead is a heavy metal that is difficult to extract due to its characteristics. Therefore, when extraction is not easy or economical, contaminated soil and groundwater can be purified cost-effectively using an in situ fixing method.

Representative conventional techniques related to the present invention are as follows.

First, as a method of purifying contaminated soil using electric fields, Brodsky and Ho (1995) described two similar patents entitled 'In-situ Purification of Contaminated Soil' and 'In-situ Purification of Inhomogeneous Soil'. 5398756 (March 21, 1995) and 5476992 (December 19, 1995) were acquired. First, the 'in-situ purification of contaminated soil' is to remove contaminants by electroosmotic by forming at least one liquid permeation zone between contaminated soils. The main claims are the formation of liquid permeable zones and the injection of various additives. 'In situ contamination purification of heterogeneous soil' suggests similar contents and methods as the previous one, but the difference is that hydrophobic gradient is applied to anisotropic soil to induce the flow of water. In addition, it is also possible to apply the same inclination periodically to change the direction, and to reverse the direction of the electroosmotic flow. Claims include the application of hydraulic gradients to anisotropic soils and the periodic exchange of hydraulic gradients and electroosmotic flows, and in Korea, the `` Remedy of Contaminated Bicomponent Soils '' (Korea Patent Publication No. 10-0193917 (1999) 02. 05)) under the title. U.S. Patent No. 5,616,235 (April 01, 1997) proposes to inject a cationic material and an anionic material through a cathode and an anode, respectively, to enhance the ground strength. However, there is no matter for in situ fixation of lead and the feature of the present invention is not injected with chlorine.

In Korean Patent Publication No. 10-0341957 (June 12, 2002), 'Method and Apparatus for Purifying Oil-Oiled Soils', Oil Contaminants in Soil by Passing Oxygen Radicals through Contaminated Area by Electro-osmosis of Electrokinetic Was obtained by oxidative decomposition by hydroxyl radical, and the method of injecting nutrients and electron acceptors used for biodegradation of contaminated soil by electric field in Korean Patent Application Laid-Open No. 2001-0088497 (2001. 09. 28) 'Used electrokinesis as a method of injecting microorganisms' nutrients and electron acceptors to decompose pollutants. However, these patents are limited to organic matter. As for inorganic contaminants, the method of removing heavy metals from sewage and wastewater sludge using electric power is described in 'How to remove harmful heavy metals from sludge by electric power' in Korean Patent Publication No. 10-0321857 (2002. 01. 10). In Korean Patent Publication No. 10-0321857 (2001. 05. 09), the method and apparatus for decontaminating contaminated soil in radioactive waste drums by electrokinetic method are used to remove radioactive substances in drums using electrokinetic machines. Patents were obtained, which were all limited to ground treatment methods, not underground treatment.

However, these patents are only for the purpose of extracting contaminants and do not include the technique for in situ fixation of the present invention.

Korean Patent Laid-Open Publication No. 1997-0065687 (October 13, 1997) is a method of stabilizing soil contaminated with heavy metals with chemicals to inactivate heavy metals and adding metal sulfides or alkalis to form metal sulfides or metal hydroxides. And a heavy metal stabilization method for administering an insolubilizing adjuvant to prevent degradation and re-elution of the product. In Korean Patent Publication No. 0134078 (December 27, 1997), 'Method and apparatus for insolubilizing heavy metals in contaminated soil', an aqueous solution of sodium emulsion or iron lactate is injected by electroosmotic using an electric field. The patent was obtained by inducing and insolubilizing a chemical reaction. This patent proposes a method of insolubilizing heavy metal in a chemical reaction mechanism different from that proposed in the present invention, and does not consider environmental property changes such as pH of the soil.

In the case of stabilization, the present invention is limited to the extraction of contaminants using electrokinetic technology and the movement of chemicals using only electroosmosis and their reactions without considering changes in environmental characteristics in the ground. Therefore, in the case of soil conditions that are not easy to extract, it can be an uneconomical method and it takes a very long time to extract the contaminants completely. In addition, it is difficult to prevent re-elution of heavy metals by methods that do not consider changes in soil environment.

The present invention is relatively easy to extract during the purification of soil contaminated with lead and other heavy metals is extracted by the electroosmotic and electrical ion transport phenomenon, in the case of soil or difficult to extract relatively difficult lead It is a method to fix it in place.

At this time, in order to prevent lead immobilization and re-elution, hydrochloric acid is injected together with the phosphate solution to form a lead phosphate having strong bonding strength. In this case, electrokinetic technology is used to smoothly inject ions and prevent ions from diffusing into non-polluted areas. By using the electrokinetic technique, hydrogen ions produced at the anode part desorb heavy metal adsorbed to the particles, and then move to the cathode part by electroosmotic and electrical ion transfer phenomenon. At this time, heavy metals with relatively small phosphorus reactivity can be removed by transporting them to the cathode reservoir, and in the case of lead with slow mobility, the phosphorus and chlorine ions injected into the cathode portion are immobilized in the soil. In addition, it can be used as a method for removing organic matter using electroosmosis.

1 is a configuration diagram of the tester used in the present invention

Figure 2 shows the pH change in the soil in the embodiment

Figure 3 shows the TCLP results of lead concentration in the soil after the experiment is finished in the Example

4 is a distribution of phosphorus concentration in the soil after the experiment is finished in the embodiment

1 is a schematic diagram of an apparatus for purifying contaminated soil according to an embodiment of the present invention, in which a sample contaminated with lead is filled in a soil cell 1, and an end is separated by a porous membrane 9, and a conductive positive electrode 2 is provided. And a positive electrode portion composed of a positive electrode tank (3) and a negative electrode composed of a conductive negative electrode (4) and a cathode electrode tank (5), and a conductive positive electrode (2). ) And the conductive negative (-) electrode (4) is connected to the power supply (8). The positive electrode tank 3 is filled with ultrapure water, the negative electrode bath 5 is filled with ultrapure water in a conventional electrokinetic experiment, and the hydrochloric acid solution and the phosphoric acid solution are mixed in a constant proportion in an advanced electrokinetic stabilization experiment. In order to maintain the constant level of the positive electrode bath (3) during operation of the system, a level control device (6) is installed, and the negative electrode bath (5) is a fixed quantity pump of a solution in which a hydrochloric acid solution and a phosphoric acid solution are constantly mixed. When (7) is provided to the soil by constant circulation, it is mineralized and insolubilized in the form of lead phosphate having strong bonds when it meets lead ions desorbed and moved under an electric field.

The present invention will be described in more detail with reference to the following examples. However, the technical scope of the present invention is not limited by the following examples.

EXAMPLES Conventional Galvanic Purification of Lead-Contaminated Soil and Improved Galvanic Stabilization Proposed by the Present Invention

In the examples, the conventional electrokinetic purification of lead-contaminated soils and the improved electrokinetic stabilization using a mixture of hydrochloric acid and phosphoric acid were compared. Experimental conditions for each experiment are shown in Table 2. The pollutants selected lead, which is more adsorbable to the granules than other heavy metals, is commonly found around shooting ranges, landfills, industrial areas, and oil storage tanks, and is difficult to remove. Phosphoric acid and hydrochloric acid solution which is injected by using a coin for the stabilization of lead have negative polarity, so it is moved to the anode part by electric ion movement phenomenon at the cathode part, and is produced by the electrolysis reaction at the negative electrode. The hydroxide ions are neutralized by the hydrogen ions of the mixed hydrochloric acid. At the positive electrode, hydrogen ions generated by the electrolysis reaction at the positive electrode move toward the negative electrode to desorb lead. The desorbed and moved lead ions, together with the phosphorus and chlorine ions moved from the negative electrode, show a strong bond as follows.

Clay Properties Used in the Examples Characteristic factor value Soil classification according to the Uniform Classification Law (USCS) CL Initial pH 5.53-5.8 Initial moisture content during experiment 33-34.3% Ingredient Illite 41.6 wt.%, Quartz 46.6 wt.%, Kaolinite 11.8wt.% Specific surface area 6.52 m ² / g

Operating conditions of the embodiment division Initial Lead Elution Concentration (ppm) Driving period (days) Voltage gradient (V / cm) Cathode Electrolyte Solution Experiment 1 27.99 6 2 Ultrapure water Experiment 2 31.48 6 2 1.5 L (ultra pure water 1,335 mL + 0.1 N HCl 150 mL + phosphoric acid solution 15 mL), that is, 0.5 N phosphoric acid, 0.01 N hydrochloric acid Experiment 3 30.4 6 2 1.5L (ultra pure water 1,320mL + 0.1N HCl 150mL + phosphoric acid solution 30mL), that is, 1.0N phosphoric acid, 0.01N hydrochloric acid Experiment 4 27.43 10 2 Experiment 5 29.62 10 2 1.5L (ultra pure water 1,290mL + 0.1N HCl 150mL + phosphoric acid solution 60mL), that is, 2.0N phosphoric acid, 0.01N hydrochloric acid

The electrochemical variables in the sample were measured during the experiment. After the experiment was finished, the sample was divided into 10 parts, and the pH change and the eluted lead concentration of each part were pretreated using the TCLP method, followed by ICP-AES (Jovon Yvon JY-24). Analyzed.

2 shows the pH change in the soil of each sample after the end of the experiment. In the case of Experiment 1, which is a conventional electrokinetic experiment, oxygen gas and hydrogen ions are generated at the anode and hydrogen gas and hydroxide ions at the cathode, so that the pH decreases at the anode and increases at the cathode. In Experiment 1, acidification proceeded from the anode to the normalized distance of about 0.8 points, which is the result of the shift of the acid cable from the anode due to the advection of the electroosmotic flow and the movement of the ion, and the increase in pH near the cathode caused the diffusion of the base wire. The result is. On the other hand, Experiment 2, 3, 4, 5, which is a case of an improved electrokinetic experiment in which phosphoric acid and hydrochloric acid were mixed in the negative electrode, buffered hydroxide ions generated in the negative electrode, so that the pH in the sample was acidified in all regions.

3 shows the results of instrument analysis of the solution extracted by using the TCLP method in the lead concentration in the soil after the end of the experiment. In Experiment 1, which is a conventional electrokinetic method, the amount of lead elution increased more rapidly than the initial point at about 0.85 point normalized to the cathode, but in the case of the improvement test, the lead concentration was lower than the initial concentration, and the amount of phosphorus was increased and the operating time was increased. The lead elution amount was smaller. In Experiment 4, which was mixed with 1 / 50-fold dilute phosphoric acid solution and 0.1N hydrochloric acid and operated for 10 days, the TCLP criterion was satisfied in about 80% of the area, and mixed with 1 / 25-fold dilute phosphoric acid solution and 0.1N hydrochloric acid. In Experiment 4, which was injected and operated for 10 days, the lead was effectively fixed by satisfying the TCLP standard in all areas. In addition, the total lead elution decrease in experiment 5 was 95.82%.

Table 3 shows the lead elution reduction efficiency in each experiment.

Lead Elution Reduction Efficiency in Experiments of Examples division Elution Decrease Rate (%) 6 days 10 days Experiment 1 4.48 - Experiment 2 69.03 - Experiment 3 79.16 - Experiment 4 - 88.46 Experiment 5 - 95.82

Figure 4 shows the distribution of phosphorus concentration in the soil after the end of the experiment. In order to detect the stabilization characteristics of lead by phosphorus injection, the sample was ignited for 2 hours, then stirred for 16 hours, centrifuged, and the supernatant was taken to ICP-AES (Jovon Yvon JY-24). Analyzed. Experiment 3, which doubled the solution of phosphorus, was injected with an additional 50% of Experiment 2, and Experiment 4, which was operated for 4 days longer than Experiment 3, was injected with an additional 36% of Experiment 3. In addition, the amount of phosphorus injected in Experiment 5 was increased by about 240% compared to Experiment 2. Therefore, it can be seen that both the time and the concentration of the phosphorus injection affects.

As described above, when the present invention is used, phosphorus is injected by in situ immobilization rather than extraction for purification of soil contaminated with lead, which is relatively difficult to extract. In this case, hydrochloric acid containing chlorine ion is injected for strong bonding with lead. Economical purification can be achieved by using a coin machine. By utilizing the present invention to effectively fix the lead in situ to prevent elution to the surrounding environment it can be used as an effective method of restoring contaminated soil.

Claims (4)

The method of immobilizing lead in situ in contaminated ground consists of: Depending on the area of the site, one or more anode and cathode sections are installed vertically or horizontally on the ground and applied with a direct current electric field.The anode section is filled with an aqueous solution, and the cathode section is mixed with hydrochloric acid and phosphoric acid. The solution is injected and continuously circulated using a metering pump. The electrodes of the anode part and the cathode part are made of an inert conductive material to prevent corrosion, and the anode part is attached with a level control device. The waste liquid which has flowed out through the cathode portion is reinjected or disposed of into the anode portion through a suitable treatment apparatus. The method of claim 1, An in situ fixation method for electrokinetic lead in which a phosphoric acid solution is used in a mixed solution of a phosphoric acid solution and a hydrochloric acid solution, which is used as an enhancement technique, and is not limited thereto. The method of claim 1, The aqueous solution to be filled in the anode portion is not limited to water, and in situ fixation method of electrokinetic lead using a conductive solution such as an acid solution for enhancing the effect. The method of claim 1, DC electric field is a device for generating direct current, the method of fixing the original position of the electrokinetic lead, characterized in that using a constant voltage, a constant current or a power supply method mixed with them.