JP4565182B2 - Method for decomposing refractory organometallic complexes - Google Patents

Description

  The present invention relates to decomposition of hardly decomposable organometallic complexes such as ethylenediaminetetraacetic acid (hereinafter referred to as EDTA) and nitrilotriacetic acid. The present invention also relates to a method for recovering the central metal of the hardly decomposable organometallic complex as fine particles.

  There are many organic metal compounds generally called complexes or complex salts that are stable and difficult to decompose.

  In general, an organic compound that can be a ligand forms a stable complex with a transition metal or the like, and thus is used as a trace metal scavenger.

  For example, EDTA is a softener for hard water, antioxidants for rubber and vitamin C, metal deactivators such as pharmaceuticals, anti-degradation agents due to trace metals in foods, sequestering metal ions or precipitation of heavy metal ions It is widely used for inhibitors, boiler cleaners, and for the purpose of preventing the influence of metal ions in enzyme reactions and analytical chemistry.

  Among these metal complexes, particularly difficult-to-decompose ones, which are usually discarded as wastewater, flow into rivers, lakes, or seas, leading to an increase in COD and BOD, which is one of the causes of eutrophication. In particular, heavy metal complexes can themselves cause pollution as soil and water pollutants.

  Therefore, wastewater containing complexes must be subjected to various chemical treatments and microbial treatments as well as other harmful chemical substances, and discarded after being decomposed and removed.

  However, there are many complexes that are stable and hardly decomposable. The stability of the complex is determined by the properties of the metal ions and the ligands contained therein. In general, those having multiple coordination sites in one molecule, such as ethylenediamine, in other words, those that form chelate compounds are more stable than those having one coordination site in one molecule such as ammonia. It becomes a hardly decomposable organometallic complex. In the case of the central metal, for example, when a divalent transition metal ion is shown as an example, it is generally known that there is the following relationship regarding stability.

  Furthermore, the stability of the complex can also be determined by actual measurement and calculation as a stability constant.

  For example, considering a reaction in which one kind of complex (MLn) is generated from a metal ion M and a ligand L, when the following formula (1) is satisfied, the molarity is set at a constant ionic strength (activity coefficient constant). By measuring the apparent equilibrium constant depending on the concentration, it can be obtained as equation (2).

（但し、Ｍは金属イオン、Ｌは配位子、Ｋは見かけの平衡定数（安定定数））

(However, M is a metal ion, L is a ligand, K is an apparent equilibrium constant (stability constant))

  In the present invention, what can be regarded as a hardly decomposable organometallic complex is a complex having a stability constant of 5 or more, and generally a chelate compound corresponds to this. In particular, iron, cobalt, nickel, and copper complexes are used as the central metal, and these are usually compounds having a stability constant of 7 or more.

  The disposal of the stable, ie, hardly decomposable organometallic complex as described above has hitherto not been particularly effective. For example, even in the case of a complex with EDTA, which is a typical complexing agent used in industry, The treatment method is to adsorb to porous iron or the like and bury it in the ground and dispose of it, or to dilute it and use it to flow into the river. There is no assurance that will not leak.

  In view of the above technical background, the present invention provides a method for decomposing a hardly decomposable organometallic complex by simple means. In addition, the present invention provides a means by which the central metal constituting the organometallic complex, particularly heavy metals such as copper, can be recovered as fine particles instead of being plated on the negative electrode.

In the present invention, at least one of an electrode composed of a positive electrode and a negative electrode is a carbon electrode, an electrolyte solution containing a hardly decomposable organometallic complex is present between both electrodes, and electrolysis is performed while periodically changing the polarity of the positive electrode and the negative electrode. This is a method for electrolyzing a hardly decomposable organometallic complex.

  One of the characteristics of the present invention is that the negative metal is extracted from the hardly decomposable organometallic complex and a reduction reaction is performed in the negative electrode. Thus, since the ligand that has lost the central metal can be easily decomposed by oxidation, the ligand can be oxidized and decomposed on the anode.

  These reactions can be performed continuously on both the positive and negative electrodes by performing electrolysis while continuously or intermittently supplying an electrolyte solution containing a hardly decomposable organometallic complex.

  In the present invention, one of preferred embodiments is that at least a carbon electrode is used as the negative electrode, so that the reduced metal is extracted from the surface of the negative electrode without being deposited on the negative electrode without being deposited on the negative electrode. Precipitates at the bottom of the electrolytic cell. This tendency is particularly remarkable in heavy metals such as copper. Thus, the metal particles precipitated on the bottom of the electrolytic cell can be recovered very easily by a sorting means such as filtration.

  Of course, the carbon electrode may be amorphous carbon, but from the viewpoint of durability, crystalline carbon is preferable, and a diamond electrode is particularly excellent. As other carbon electrodes, diamond-like carbon, glassy carbon, or the like is also preferably used.

  In the present invention, the electrode (voltage) applied between the positive electrode and the negative electrode for the electrolysis is naturally required to be equal to or higher than the voltage causing electrolysis of the hardly decomposable organometallic complex, and generally 2.5 volts or higher. Is used.

  According to the present invention, it is possible to very easily decompose a hardly-decomposable organometallic complex that has been difficult to decompose and remove, particularly a heavy metal complex of EDTA, and thus reduce the amount of organic matter contained in wastewater. Can contribute to the reduction of COD and BOD in the waste water.

  Furthermore, the present invention can be effectively used for removing harmful heavy metals and recovering valuable metals only by simple means such as filtration.

  The present invention is characterized by using an electrolysis method as a means for decomposing a hardly decomposable organometallic complex.

  According to the knowledge of the present inventors, even if the organometallic complex is electrically neutral and has no charge, the central metal has a charge as a cation, Although it is in a weak ionic bond state, metal ions are extracted from the ligand on the negative electrode and reduced by electrolytic treatment. This reaction proceeds not only when the hardly decomposable organometallic complex is completely dissolved in the electrolyte solution, but also in a state where it is suspended and dispersed in the electrolyte solution in the form of colloids or fine particles.

  Further, such a reaction on the negative electrode is possible regardless of the degree of stability of the organometallic complex, but in the present invention, it can be used particularly meaningfully for a hardly decomposable organic complex.

  Thus, the metal obtained by the negative electrode reaction has a very fine shape of about submicron to several microns when deposited.

  Moreover, these fine metals tend to desorb from the negative electrode without depositing on the negative electrode. This is particularly noticeable when a carbon electrode is used as the negative electrode. The carbon electrode is not particularly limited, and the negative electrode made of a substance having a crystal structure such as amorphous carbon, glassy carbon, graphite or diamond-like, at least a part of which has a crystal structure of diamond, causes deposition of the metal. Since it is excellent not only in the absence but also in durability, it is a particularly preferable electrode material.

As such a diamond electrode, a conventionally known one can be used without any limitation. For example, a diamond electrode in which a thin layer of diamond is formed on a substrate such as silicon or niobium by chemical vapor deposition (CVD method) is used. Diamond usually has no conductivity, but becomes a conductor by doping with boron or the like. Usually, the doping concentration of atoms such as boron is in the range of about 1 × 10 ¹⁹ to 10 ²³ per cm ³ of diamond.

That is, when the doping amount is less than 1 × 10 ¹⁹ , the diamond electrode is difficult to become a uniform semiconductor film, and when it becomes 1 × 10 ²³ or more, it becomes an electrical conductor, but the characteristics of the diamond surface gradually increase. At the same time, the durability as an electrode is lost.

  Further, the thin diamond layer formed on the substrate by the CVD method is substantially polycrystalline, but the crystal structure may be broken as long as chemical stability as diamond is maintained. Furthermore, the diamond surface obtained by the CVD method has mainly C—H bonds and is hydrophobic, but when used as an electrode, it gradually increases by applying an electrolytic voltage of the complex, generally 2 to 5 volts. Oxidized to form a C—OH bond, efficiently causing a metal extraction / reduction reaction from the complex, and the metal can be released from the electrode as fine particles.

  Of course, the surface of the diamond electrode obtained by CVD is treated with a mixed solution of chromic acid or oxygen plasma so that oxygen is bonded to the surface of the diamond. It is also a preferred embodiment to apply water to the AgCl electrode (hereinafter the same in the voltage display) for about 10 minutes to 1 hour to electrolyze water and oxidize the surface of the diamond electrode.

In the present invention, a conductor such as platinum, iron, nickel, and graphite is generally used as the counter electrode (positive electrode) . However, the carbon electrode is preferably used because the electrode is periodically changed .

  Next, the ligand from which the central metal has been extracted loses stability, is easily oxidatively decomposed on the positive electrode, and eventually becomes carbon dioxide gas, which virtually disappears. Such oxidative decomposition reaction can also be sufficiently achieved with a potential difference of 2 to 5 volts applied between the positive and negative electrodes.

  The electrolytic cell used for the electrolysis of the hardly decomposable organometallic complex of the present invention is not particularly limited, and a diaphragm electrolyzer used for a normal electrochemical reaction can be used. Preferred embodiments of the electrolytic cell include those in which the liquid flow rate in the vicinity of the electrode can be sufficiently increased, and those having a stirrer inside and those having an external circulation device for the electrolytic solution.

  As described above, the energization method is generally performed by applying a voltage so as to form a potential difference of 2 to 5 volts, preferably 2.5 to 4 volts. That is, when the voltage is 2 volts or less, the decomposition of the complex hardly occurs.

  In addition, the electrolytic solution generally dissolves or suspends a hardly decomposable organometallic complex in a concentration of 1 to 1 mol, and usually adjusts conductivity by about 0.05 to 2 N electrolyte substance, generally a salt or acid, for example, Add potassium chloride, hydrochloric acid or sulfuric acid.

  Furthermore, the most effective electrolysis method in the present invention is to periodically change the polarity in electrolysis. That is, in the mechanism of the present invention, since the target is generally an electrically neutral hardly-decomposable organometallic complex, the central metal is first extracted and reduced at the cathode, and then becomes unstable. Since the ligand undergoes oxidative decomposition on the anode, in direct current electrolysis, it is essential that the hardly decomposable organometallic complex moves from the cathode surface to the anode surface.

  However, in order for the above-mentioned hardly decomposable organometallic complex having substantially no electric charge or a ligand from which the central metal is extracted from the complex to move to the electrolyte solution and reach each electrode, diffusion in the solution Or turbulent flow due to convection or liquid agitation. However, by periodically changing the polarity of the electrode, the refractory organometallic complex or the ligand from which the central metal has been extracted from the complex can undergo reduction and oxidation sequentially even if it remains on the electrode. Therefore, it is decomposed without moving between the electrodes.

  More preferably, since the central metal deposited on the cathode is peeled off from the electrode at the moment when the electrode becomes the anode, the metal can be recovered in a very fine and uniform state.

  For this reason, in the present invention, it is an extremely effective means to operate while changing the polarity of the electrode during electrolysis.

  In the present invention, the electrode conversion is 0.1 times / minute to 4000 times / minute, preferably 0.5 times / minute to 100 times / minute.

  Further, when the polarity is changed frequently, for example, when the rate is changed to 1000 times / minute or more, it can be used by being superimposed on the direct current. Thus, the metal deposited on the electrode can be miniaturized.

  The current change at the time of polarity change is generally performed so as to have a sinusoidal waveform, but can be switched on and off.

  The conversion of the electrode will be described with an EDTA copper complex as an example. When a diamond electrode is used as one electrode and the counter electrode is platinum, the maximum potential of the applied voltage is 2.5 volts, and the voltage change is 10 mV to The electrolysis is performed while appropriately changing to a sinusoidal waveform at a speed of 500 mV / second, preferably 50 mV to 300 mV / second. Thus, copper and the like are separated from the surface of the diamond electrode without depositing on the diamond electrode and separated from the surface of the diamond electrode, and deposited as a precipitate on the bottom of the electrolytic cell.

In the present invention, the hardly decomposable organometallic complexes to be treated are those having a stability constant of 5 or more, particularly 7 or more, and are generally chelate compounds. The ligands belonging to these include polymethylenediamines such as ethylenediamine and diaminopropane, polyethylenepolyamines such as diethylenetriamine and triethylenetetramine, amino acids such as glycine, polycarboxylic acid amides such as ethylenediaminetetraacetic acid and nitrilotriacetic acid, and cytochromes. And metal complexes such as porphyrins such as chlorophyll and femoglobin, bipyridine, dibenzene, dicyclopentadiene, and crown ethers. Of these complexes, it is particularly effective for the treatment of industrial EDTA complexes, especially copper complexes.
[Comparative Example]

  A glass electrolytic cell with an inner volume of 150 ml having a circular window with a diameter of 18 mm at the lower side of one side wall, a conductive diamond electrode installed in the window through an O-ring, and a coiled platinum wire (φ0 .5 mm, 5 cm). Further, an Ag / AgCl electrode (HS-205C manufactured by TOADenpa) was used as a standard electrode, and an electrolytic treatment was performed by applying a constant potential of 3 V to the diamond electrode while stirring the electrolytic solution with a magnetic stirrer. At this time, a potential of −3 V is applied to the counter electrode.

  A solution containing 20 mmol of Cu-EDTA and 0.1 mol of sodium sulfate was electrolyzed at the above-mentioned potential for 80 hours. At this time, reduction and precipitation of copper occurs on the platinum wire, and EDTA is oxidatively decomposed on the diamond electrode.

  Check the status of electrolysis over time. That is, the electrolyte was sampled, the solute was separated by high performance liquid chromatography using Inertsil ODS-3 (GL, Science) as a column, and ultraviolet analysis was performed at λ = 200 nm and 250 nm.

  The results are shown in FIG.

From FIG. 1, it can be seen that the reduction of Cu ²⁺ first proceeds, the EDTA complex decreases with the passage of time, and conversely, the decomposition products of EDTA increase. It took 35 hours for the EDTA complex concentration to drop to about 10% of the initial concentration.

Further, copper was reduced and deposited on the platinum wire and plated with copper. This copper could be removed by dissolving in a sulfuric acid solution.
[Example 1]

  Using a glass electrolytic cell with a volume of 25 ml having a circular window with a diameter of 10 mm at the lower part of one side wall, a conductive diamond electrode is installed in the window part via an O-ring, and a coiled platinum wire ( φ0.5 mm, 100 cm) was used. In addition, an Ag / AgCl electrode (HS-205C manufactured by TOADenpa) was used as a standard electrode, and a potential of −1.5 to 3 V was applied to the diamond electrode at an insertion speed of 50 mV / s while stirring the electrolyte solution with a magnetic stirrer. Applied to the click, electrolytic treatment was performed.

  A solution containing 1 mmol of Cu-EDTA and 0.13 mol of potassium chloride was electrolyzed for 100 hours in the above-mentioned potential region. At this time, reduction precipitation of copper occurs at 0 to -1.5 V, and oxidative decomposition of EDTA occurs only on the diamond electrode at 2.5 V or more.

  Check the status of electrolysis over time. That is, the electrolyte was sampled, the solute was separated by high performance liquid chromatography using Inertsil ODS-3 (GL, Science) as a column, and ultraviolet analysis was performed at λ = 200 nm and 250 nm.

  The results are shown in FIG.

The results are shown in FIG.
From FIG. 2, it was possible to reduce the Cu-EDTA concentration to 10% of the initial concentration in 50 hours. Although this time is about 1.5 times that of the comparative example , the processing speed is higher because the constant potential application process has a longer high potential application time. On the other hand, in Example 1 , copper adhesion to the platinum wire as in the comparative example was not observed, and after copper was reduced and deposited on the diamond electrode, it was peeled off from the electrode surface as fine particles having a diameter of about 100 nm, and in the solution. To be released. The fine particles could be recovered as an aggregate of fine particles of about 1 μm by filtering the treated solution with a filter paper (pore diameter 0.2 μm Anodisc, Whatman). In other words, the potential can be continuously used without contaminating the electrode in the electrolytic treatment by cyclically inserting the potential.
(Industrial applicability)

  According to the present invention, the EDTA complex can be easily decomposed and removed. For this reason, it is thrown away as conventional waste water and can remove one of the causative substances of river pollution.

  In addition, since the central metal of the EDTA complex, particularly harmful heavy metals, can be removed, it can be effectively used in environmental conservation related industries.

FIG. 1 is an ultraviolet ray analysis graph showing a situation in which Cu-EDTA and Cu ²⁺ are reduced by direct current electrolysis, and instead, a peak of decomposition products of EDTA gradually increases. FIG. 2 is a graph showing a temporal degradation state when Cu-EDTA is decomposed while changing the polarity of the electrode.

Claims (9)

  Refractory property characterized in that at least one of the electrodes is a carbon electrode, an electrolyte solution containing a hardly decomposable organometallic complex is present between both electrodes, and electrolysis is performed while periodically changing the polarity of the positive electrode and the negative electrode Method for electrolysis of organometallic complex.   2. The method for electrolyzing a hardly decomposable organometallic complex according to claim 1, wherein the carbon electrode is one selected from conductive diamond, conductive diamond-like carbon, glassy carbon, and amorphous carbon.   The method for electrolyzing a hardly decomposable organometallic complex according to claim 1 or 2, wherein the hardly decomposable organometallic complex is a chelate compound.   The method for electrolyzing a hardly decomposable organometallic complex according to claim 3, wherein the chelate compound is an organometallic complex having at least one selected from alkylenediamine, ethylenediaminetetraacetic acid, crown ether, porphyrin and bipyridine as a ligand.   The method for electrolyzing a hardly decomposable organometallic complex according to any one of claims 1 to 3, wherein the central metal of the hardly decomposable organometallic complex is at least one metal selected from iron, cobalt, nickel and copper.   The method for electrolyzing a hardly decomposable organometallic complex according to claim 1, wherein the central metal ion of the hardly decomposable organometallic complex is reduced at the negative electrode and recovered as fine particles.   2. The method for electrolyzing a hardly decomposable organometallic complex according to claim 1, wherein the hardly decomposable organometallic complex is a heavy metal complex of ethylenediaminetetraacetic acid. The method for electrolyzing a hardly decomposable organometallic complex according to claim 7 , wherein the heavy metal complex of ethylenediaminetetraacetic acid is a copper complex of ethylenediaminetetraacetic acid.   2. The method for electrolyzing a hardly decomposable organometallic complex according to claim 1, wherein the electrolysis is carried out by applying a voltage of 2.5 to 4 volts between the positive electrode and the negative electrode.

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