SI23314A - Procedure of electrochemical recycling of edta after washing of ground - Google Patents

Abstract

A procedure of electrochemical recycling of EDTA after washing of the ground according to the invention is solving the problem of reuse of a washing solution of EDTA after extraction or washing of the ground or sediments contaminated first of all by toxic metals, metal radionuclides and also organic pollutants. EDTA forms water soluble complexes with metals so that they can be removed from the ground or sediment by means of a washing solution. Rinse aids in a washing solution (1) remove organic pollutants from the ground (2) or sediments. The used washing solution is treated in an electrolytic cell with a sacrificed Al anode at a basic pH. In this case, Al replaces other metals in complex with EDTA leaving Al-EDTA complex (recycled EDTA) in the solution and the released toxic metals and radionuclides are separated either by electro-precipitation as insoluble hydroxides or by electro-coagulation. Organic pollutants (7) are separated by electro-coagulation too. After removal of Al hydroxide slurry and adjustment of pH, the washing solution with the recycled EDTA has the ability of further removal of toxic metals and radionuclides from the contaminated ground (2) or sediment.

Description

ELECTROCHEMICAL RECYCLING PROCEDURE OF EDTA PO

WASHING FLOOR

The subject of the invention is the process of electrochemical recycling of EDTA after washing the soil for cleaning with toxic metals of contaminated soil and sediments with a washing solution of ethylenediamine tetraacetic acid (EDTA). EDTA forms water-soluble complexes with metals so that they can be removed from the soil or sediment with a washing solution. The process according to the invention enables the electrochemical recycling of EDTA in the washing solution used in the electrolytic cell with the sacrificial aluminum (Al) anode under basic conditions. In doing so, Al replaces the toxic metals in the complex with EDTA, the remaining AI-EDTA complex (recycled EDTA) in the washing solution, and the released toxic metals are eliminated by electro-precipitation, such as insoluble hydroxides or by electro-coagulation. After pH adjustment, the washing solution with recycled EDTA has the ability to further remove toxic metals from

2o contaminated soil or sediment. The invention is in accordance with the international patent classification in B09C 1/02.

The main sources of soil and sediment pollution by toxic metals such as lead (Pb), copper (Cu), zinc (Zn) and cadmium (Cd), and metal radionuclides such as uranium (U) are industrial and transport emissions. , the use of plant protection products, mineral and organic fertilizers in agriculture, and various accidents. Unlike organic pollutants, toxic metals do not decompose in the environment, and radionuclides decompose very slowly. In situ and ex situ technologies for the treatment of toxic metals of contaminated soil and sediments, which underlie the washing (extraction or rinsing) of the soil with the EDTA ligand, is intended to provide a novel recycling process for the used EDTA washing solution of the invention.

The known methods of cleaning soil and sediment contaminated with toxic metals and metal radionuclides so far are:

Solidification and stabilization of metals in soil and sediments by mixing and injecting materials such as phosphates (U.S. Pat. No. 5,202,033; U.S. Pat. No. 6,204,430), cement, lime or various sulfides (U.S. Pat. No. 4,629,509). is We do not remove metals from soil and sediment in these processes, but increase their proportion in the solid phase to make them immobile and less bioavailable.

Electro-kinetic processes include the release of weak electrical currents between the anode and the cathode, which are located in soil or sediments. The electrical gradient then triggers the movement of particles, including toxic metals, that accumulate along and at the electrodes from which they are removed.

Phytoextraction is the cleaning of soil by plants, usually hyperaccumulators, that have a natural ability to accept certain metals (especially nickel and zinc) from the soil and accumulate them in their above ground parts (U.S. Pat. No. 6,313,374). In induced phytoextraction processes, the application of ligands to the soil increases the availability of soil metals (especially lead) to plants and thus increases the efficiency of phytoextraction (U.S. Pat.

No. No. 5,917,117).

Removal of the most polluted soil fractions (most commonly fine clay and colloidal particles and organo-mineral complexes) into hydrocyclones or by flotation with toxic metals.

Washing (rinsing or extraction) of soil and sediments with washable aqueous io solutions in situ and ex situ. Washing solutions with various salts, acids and ligands are most commonly used. Solutions are known to use salt solutions whose washing anions with toxic metals are water-soluble salts that are easier to wash (U.S. Pat. No. 5,744,107) for washing soil and sediments. There are also known solutions for washing soil solutions with mineral and organic acids, which lower the soil pH and thus increase the water solubility of toxic metals (U.S. Pat. No. 5,919,001). Ligands in soil and sediments with toxic metals and metallic radionuclides bind to water-soluble coordination compounds (complexes), thus converting the bulk of metals and radionuclides from the solid to the liquid phase of the soil and sediment that is removed. Solutions are known when using washing solutions of natural origin ligands (U.S. Pat. No. 5,849,567). The most well-known and most commonly used soil-washing ligand is ethylenediamine tetraacetic acid (EDTA) or its salts.

• ·

The process of the invention is intended to recycle EDTA in a washing solution after washing (extracting or rinsing) soil and sediments in situ and ex situ.

When washing soil and sediment contaminated with toxic metals with EDTA, large amounts of used and waste washing solution are generated.

The economics of soil cleaning technology technologies require the recycling of EDTA in the washing solution.

A process is known for separating toxic metals from the EDTA complex with Na ₂ S, which results in the almost complete removal of the metal from the washing solution in the form of insoluble sulfide (JP Pat. No. 4263874). In this process both reagents and products are toxic and dangerous substances.

The process of recycling EDTA from the washing solution by replacing the toxic metal in complex with Fe ³⁺ under acidic conditions is known, followed by the recovery of the metal s as insoluble phosphate at neutral pH. Fe ³⁺ ions can be removed as insoluble hydroxide at basic pH using is NaOH, thereby releasing the active form of EDTA. (Kirn, C., Ong, SK. Recycling of lead-contaminated EDTA wastewater. J. Hazard. Mater.,

1999, B69, pp. 273-286).

According to another known method, toxic metals from the complex with EDTA are removed by binding to an anion exchanger, and thus the metals are removed from the washing solution used (Tejovvulan, RS, Hendershot, WH Removal of gauge metals from contaminated soils using EDTA incorporating resin trapping techniques. Environ. Pollut., 1998, 103, pp. 135-142).

Again, according to another known solution, they use a zero-valent two-metal mixture (Mg ° -Pd °, Mg ° -Ag °) to recover toxic metal from solution, and release EDTA at basic pH (Ager, P., Marshall, W.D. The removal of metals and release of EDTA from pulp wash water J. Wood

Sci. Technol., 2001,21, pp. 413-425). The method is efficient but expensive.

An electrochemical process for the removal of toxic metals from a complex with EDTA and from the washing solution used in a two-chamber electrolytic cell is known, where the electrodes are separated by a cation-selective membrane (Juang, RS, Wang, SW Electrolytic recovery of binary io metals and EDTA from strong complexed Solutions. Water Res., 2000, 34, pp. 3179-3185). The Katina-selective membrane is expensive and sensitive to saturation with particles from the washing solution and to degradation due to aging.

Among the EDTA recycling processes in the washing solution, the electrochemical process using a two-chamber electrolyte cell, where the electrodes are separated by a cation-selective membrane, is most similar to the process of the invention. The essential differences between this known process and the process of the invention are:

- in the process according to the invention, the electrolytic cell is a single-chamber;

- no cation-selective membrane is required in the process of the invention;

- the process according to the invention does not require additional electrolyte solutions like the known process;

- in the process according to the invention, the anode is Al, which during the process is electrocorroded, and in the known process is made of inert materials, such as a titanium electrode coated with iridium;

- in the process according to the invention, Al replaces toxic metals in complex z

EDTA, the AI-EDTA complex remains in the washing solution, and the released toxic metals are eliminated by electro-precipitation on the cathode, such as insoluble hydroxides or by electro-coagulation;

- in the process according to the invention, organic pollutants can also be removed from the washing solution by electrocoagulation. The process according to the invention thus permits the cleaning (washing) of double with toxic metals and organic pollutants of contaminated soil.

The invention will be explained in more detail on the basis of an embodiment and accompanying drawings, of which:

is Fig. 1 is a schematic view of a preferred embodiment of the method of the invention for remediation with toxic metals of contaminated soil and sediments with a EDTA wash solution in a closed process loop.

Figure 2 electrochemical separation of Pb and EDTA in the washing solution used in the electrolyte cell during the process of the invention (Example 1), where Pb is eliminated from the washing solution while the recycled EDTA remains in solution. Mean regularity and standard deviation of the three measurements are indicated;

Figure 3 shows the proportion of Pb removed from contaminated soil after extraction with a newly prepared EDTA wash solution and after extraction with a wash solution with EDTA recycled according to the process of the invention (Example 1). The EDTA concentration was the same in both washing solutions (11.3 g L ' ¹ ). The mean and standard deviation of the three measurements are indicated.

Fig. 4 Electrochemical separation of Cu and EDTA in the washing solution used in the electrolyte cell during the process of the invention io (Example 2). Cu is eliminated from the washing solution while the recycled EDTA remains in the solution.

Figure 5: Cu fraction removed from contaminated soil after extraction with newly prepared washing solution with 8.5 g L ' ¹ EDTA and after extraction with washing solution with recycled EDTA (8.2 g L' ¹ ) according to the method of the invention (Example 2).

Compact material can be removed from contaminated soil or sediment, for example. stones. Toxic metals with EDTA wash solution (Scheme

1,1) is extracted from soil or sediment 2 in mixing reactors or other devices which allow the semi-solid phase of the soil to be mixed. After extraction, the solid phase of the soil is separated from the washing solution by decantation (for example by sedimentation), by filtration (for example by a filter chamber press or tape

press) or by centrifugation (centrifuge decanter). The metals can also be washed with EDTA solution from a pile of soil above the drainage system or from floor columns. The advantage of rinsing is that it is not necessary to separately separate the solid phase of the soil from the washing solution. Rinsing is only possible for floors with a sufficiently high hydraulic conductivity, such as sandy soil. Highly contaminated soil 2 can be extracted / washed 2 ₀ to 2 _{j +} i with washing solution several times in succession. If soil 2 is slightly contaminated, the same EDTA wash solution can be used to rinse 2 ₀ to 2 _{i +} i repeatedly. In both embodiments, either a newly prepared EDTA solution or io or a washing solution with EDTA may be recycled according to the process of the invention. After extraction or washing with EDTA washing solution, the soil can be further extracted or washed with clean water to remove any remaining toxic metals that are in complex with EDTA from the soil. Since the final performance of extraction or leaching of contaminated soil depends on the properties of the soil and the concentration of toxic metals and other pollutants, the final implementation can always be decided on the basis of feasibility tests, as known to the average person skilled in the art.

After extraction or leaching of the soil or sediments according to the method of the invention, the EDTA washing solution 3 used is increased by the addition of NaOH, Ca (OH) ₂ or KOH by a factor of 4, preferably to a value between 8 and 12. The washing solution is then treated in a single-chamber electrolyte cell 5 with sacrificed Al anode and cathode. Preferably, the cathode is stainless steel. The current density at the electrodes is preferably between 5 and 500 mA cm ' ² . Due to the passivation of the Al anode, the voltage between the electrodes at constant current can start to increase, which means higher energy consumption. Passivation can preferably be prevented by adding Cl '(for example as a NaCI or KCI solution), or by changing the pole of the electrodes or AC power.

In electrolytic cell 5, toxic metals (M) are eliminated from the complex with EDTA after a cathode reduction reaction:

M-EDTA ² '-> M ²⁺ + EDTA ⁴ '

The toxic metals in the complex with EDTA replace Al ³⁺ , which is formed by the electro-corrosion of the sacrificed Al anode:

Al Al ³⁺ + 3e '

Unlike most toxic metals, Al has a higher stability of complex formation with EDTA at strongly alkaline pHs than at lower pHs. Thus, AI-EDTA remains in the washing solution, while the released toxic metals (M) are eliminated from the washing solution by electrodeposition on the cathode:

M ²⁺ + 2e '- »M (s) • · ·

They can also be eliminated as insoluble hydroxides formed in an alkaline medium, or precipitated after electro-coagulation into Al hydroxide flakes into which Al ³⁺ ions are transformed:

AI (OH) ₃ + M ²⁺ -> AI (OH) O ₂ M + 2H ⁺

Electro-coagulation also removes any organic pollutants from the washing solution. The end of the washing solution treatment process in electrolyte cell 5 is characterized by the fact that the concentrations of toxic metals in the io washing solution fall to their predicted values, typically between 0 - 50% of their initial values. After electrochemical treatment, sedimentation, flocculation, decantation, filtration or centrifugation 6 remove the Al hydroxide slurry with toxic metals and other contaminants 7. value between 4 and 8.

The end of the process according to the invention is characterized in that the washing solution 9 with recycled AI-EDTA is used for the re-extraction or washing of the soil (2 ₀ to 2 _{i + 1} ) or sediments, replacing the part of the washing solution which is lost during the process with newly prepared EDTA washing solution (1).

The slurry of Al hydroxide, toxic metals and other pollutants 7 can be used in the Hall-Heroult Al extraction process, and the slurry can also be safely deposited after solidification and stabilization with hydraulic binders or other methods known to one of ordinary skill in the art. Cleaned soil 2 or sediment can be returned to its original location, otherwise used or safely deposited.

Washing solution

To prepare the washing solution in water, EDTA is dissolved in the form of acid or various salts, preferably the sodium salt. The concentration of EDTA in washing solution does not normally exceed 150 mmol EDTA per kg of air-dry soil. The washing solution may be adjusted to pH before and during the extraction or leaching process of soil or sediments by adding a base or acid, if this permits greater leaching performance of hazardous metals. Excipients such as surfactants, emulsifiers, detergents (eg Tween 80, Tvveen 20, Triton Χ100) and solvents known to the average person skilled in the art to promote the leaching of toxic metals, metal radionuclides, may be added to the washing solution, as well as organic pollutants from soil and sediments that are doubly polluted (with both toxic metals and organic pollutants).

An electrolytic cell

The process of the invention can be performed in any embodiment of a single-chamber electrolyte cell 5 with mono or bipolar ordered electrodes, as is known to one of ordinary skill in the art.

Materials and other apparatus

In carrying out the electrolytic cell for the process according to the invention, materials known to one of ordinary skill in the art can be used. Methods and apparatus for separating the solid phase of soil or sediment from the washing io solution and for separating the Al hydroxide slurry by sedimentation, flocculation, decantation, filtration or centrifugation 6 are known to one of ordinary skill in the art.

Emissions is Because the extraction or leaching of contaminated soil or sediments together with the process according to the invention is in a closed process loop, no toxic metals, metal radionuclides, ligands and other pollutants are emitted under a properly managed process. Gas emissions are low and do not pose an environmental hazard.

PERFORMANCE EXAMPLES

EXAMPLE 1

The efficiency and usefulness of the process according to the invention "Electrochemical recycling of EDTA after soil washing" was demonstrated in a laboratory experiment with a washable EDTA solution obtained after extraction of soil contaminated with Pb. We measured the proportion of Pb removed from contaminated soil and the concentrations of Pb and EDTA in the washing solution after soil extraction. The washing solution used was electrochemically treated according to the process of the invention, and the concentrations of Pb and recycled EDTA were measured. We compared the effectiveness of the washing solution with recycled EDTA for extracting Pb from the soil with the newly prepared EDTA washing solution.

is Experiment layout

We used garden soil from the Mežica Valley (Crane). The pollution is due to atmospheric emissions due to the centuries of operation of the Pb mine and smelter. The soil was of sandy loam texture class and contained 3980 mg kg ¹ Pb.

Contaminated soil (0.5 kg) was extracted for 72 hours with 875 ml_ of washing solution containing 43 mM EDTA (75 mmol EDTA per kg of dry soil). The extraction was carried out in 1.5 L bottles on a rotary mixer. The electrolytic cell consisted of sacrificed Al anodes placed between two • · · stainless steel cathodes (electrode spacing of 10 mm) with a total surface area of 63 cm ² and an anode to cathode area ratio of 1: 1. The electrodes were placed in a 1 I vessel equipped with a magnetic stirrer. In the electrolyte cell, 500 ml_ of the washing solution used was treated at a constant current density of 96 mA cm < ² &^gt;. The contact time of the electrochemical process was calculated by multiplying the actual process time by the ratio of the volume between the electrodes and the volume of the treated washing solution (30 min of actual running time was 3.8 min of contact time). During the electrochemical process, the pH of the treated solution was maintained at 10 by the addition of 5 M NaOH or HCl. If the voltage between the electrodes exceeded the value of 8 V due to passivation of the Al anode, the voltage was reduced by the addition of NaCI solution. The electrochemical recycling process of EDTA in the washing solution used (according to the invention) was completed in less than 25 min. contact time. is Al hydroxide and other suspended particles were removed from the washing solution by centrifugation at 2880 g.

According to the electrochemical process according to the invention, the pH of the washing solution with recycled EDTA with 5 M HCl was adjusted to 4.3 and the solution was reused for Pb contaminated soil extraction.

Results

After extraction, 67.5% of the initial Pb content was removed from the soil with the washing solution. The washing solution used contained 1535 mg L ' ¹ Pb

and 12.4 g L ' ¹ (33.4 mM) EDTA. The electrochemical process of the invention removed 90% of Pb from the washing solution, which was mainly electro-precipitated at the cathodes, while the concentration of recycled EDTA in the solution remained the same (Figure 2). Washing solution with recycled EDTA (according to the process of the invention) retained 90% of the ability to remove Pb from the soil relative to the newly prepared washing solution (Figure 3).

EXAMPLE 2 io The efficiency and usefulness of the process of the invention “Electrochemical recycling of EDTA after soil washing” was demonstrated in a laboratory experiment with a washable EDTA solution obtained after extraction of Cu-contaminated soil.

Setting up the experiment is We used vineyard soil from the Koper area. Soil contamination with Cu was the result of long-term use of Bordeaux broth fungicide containing CuSO ₄ as the active substance. The soil was of sandy loam texture class and contained 290 mg kg ' ¹ Cu.

Contaminated soil (0.5 kg) was extracted for 24 hours with 875 mL of washing solution containing 23 mM EDTA (40 mmol EDTA per kg of dry soil). The extraction was carried out in 1.5 L bottles on a rotary mixer. The electrolytic cell consisted of sacrificed Al anode placed between two cathodes of stainless steel (10 mm electrode spacing) with

total area of 63 cm ² and anode to cathode ratio of 1: 1. The electrodes were placed in a 1 L vessel equipped with a magnetic stirrer. In the electrolytic cell, 500 m L of washing solution used at a constant current density of 96 mA cm < ² &^gt; were treated. The contact time of the electrochemical process was calculated by multiplying the actual process time by the ratio of the volume between the electrodes and the volume of the treated washing solution (30 min of actual running time was 3.8 min. Of contact time). During the electrochemical process, the pH of the treated solution was maintained at 10 by the addition of 5 M NaOH or HCl. In io cases where the voltage between the electrodes exceeded 8 V due to passivation of the Al anode, the voltage was reduced by the addition of NaCI solution. The electrochemical process of recycling EDTA in the washing solution used (according to the invention) was completed in less than 12 min. contact time. Al hydroxide and other suspended particles were removed from the washing solution and by centrifugation at 2880 g.

According to the electrochemical process of the invention, the pH of the washing solution with recycled EDTA with 5 M HCl was adjusted to 4.6 and reused to extract Cu-contaminated soil.

Results

After extraction, 44% of the initial Cu content was removed from the soil by washing solution. The washing solution used contained 73 mg L ' ¹ Cu and 8.4 g L' ¹ EDTA. By the electrochemical process of the invention, 99% of Cu precipitated from the washing solution was electro-precipitated at the cathodes, while the concentration of recycled EDTA in the solution remained the same (Figure 4). Washing solution with recycled EDTA (according to the process according to the invention) retained more than 80% of the ability to remove Cu from the soil relative to the newly prepared washing solution (Figure 5).

Claims (11)

1. Electrochemical recycling process of EDTA after washing the soil for recycling EDTA in washing solution after extraction or washing of hazardous 5 metals or metal radionuclides and any organic pollutants present from contaminated soil or sediments, characterized in that the washing solution used (1) is electrochemically treated in a single-chamber electrolytic cell with sacrificed Al anode at basic values of the pH of the washing solution. Method according to claim 1, characterized in that the aqueous solution of ethylenediamine is tetraacetic acid (EDTA) or its salts, whose concentration does not exceed 150 mmol per kg of air-dry soil or sediment, is used as a washing solution (1). Method according to Claims 1 and 2, characterized in that 20 that for effective extraction or washing of the soil (2), or for removal of any organic pollutants from the soil or sediments, the pH of the washing solution (1, 9) may be adjusted or auxiliaries such as various surfactants, emulsifiers, detergents and solvents added. • · Method according to Claims 1,2 and 3, characterized in that, after washing or washing the soil (2) or sediment, washing was used Adjust the pH factor (4) to a value between 8 and 12 in solution (3) and treat it in an electrolytic cell (5). Method according to claims 1 and 4, characterized in that the end of the process in the electrolytic cell (5) indicates a decrease in the concentration of toxic metals or radionuclides in the treated washing solution to 0 to 50% of their initial concentration in the washing solution used (3). , while the EDTA in the treated washing solution mostly remains, recycled in the form of AI-EDTA. Method according to claim 5, characterized in that the concentration of toxic metals or metal radionuclides in the treated washing solution is reduced due to their electro-precipitation to 20 cathode, precipitation in the form of insoluble metal hydroxides and electrocoagulation with Al ions and hydroxides. Process according to Claims 5 and 6, characterized in that organic pollutants are also removed from the treated washing solution by electro-coagulation. 5 Method according to claim 5, 6 and 7, characterized in that a slurry (6) of Al hydroxide and other pollutants (7) is removed from the washing solution with recycled EDTA by sedimentation, flocculation, decantation, filtration or centrifugation. Method according to claim 8, characterized in that the washing solution with recycled EDTA adjusts the pH factor (4) to a value between 4 and 8, which allows efficient extraction or leaching of toxic metals or metal radionuclides from the soil or sediments, and is added thereto subsidiary substances. A method according to claim 9, characterized in that 20 to reuse the washing solution (9) with recycled EDTA for the extraction or leaching of toxic metals or metal radionuclides and any organic pollutants present from soil (2) or sediments. • · · · Method according to claim 10, characterized in that after the end of the process according to the invention the cleaned soil or sediments are returned to their original location, otherwise used or safely deposited.