DE3428582A1 - Method for generating hydroxyl radicals, and application of the method - Google Patents

Abstract

A method is described for generating hydroxyl radicals by carrying out a direct-current electrolysis in aqueous electrolytes, using an anode which has a high deposition potential for oxygen and whose active coating does not promote the immediate further reaction of hydroxyl radicals. The method can be used advantageously for sterilising drinking water and bathing water and for carrying out oxidations.

Description

Process for generating hydroxyl radicals and

Use of the method The invention relates to methods of production of hydroxyl radicals and the application of such procedures.

In some areas of technical chemistry is the use of oxidizing agents common to a large extent. In organic chemistry, e.g.

Oxygen compounds, such as acids etc., produce hydrogen atoms split off, double bonds broken etc. and in inorganic chemistry thus, among other things, metals are transferred to a higher valency level. In the Environmental technology takes the breakdown of disruptive or toxic substances with oxidizing agents an extremely large room.

Examples are: exhaust air scrubbing with ozone, cyanide detoxification with peroxides, Hypochlorite and ozone, water disinfection with chlorine, chlorine dioxide and ozone, degradation of the constituents of the wastewater, such as phenols, humic substances, lignins, etc. Chlorine and ozone.

As is well known, when looking for oxidizing agents, one is possible aimed at high oxidation effect. The strength of the oxidizing effect is generally read from the level of the oxidation potential E in volts. Some examples like make this clear: (1) ClO2 = ClO2 + e +0.95 V (2) 2C1 = C12 + 2e +1.40 (3) Cl2 + 4H20 = 2HOC1 + 2e + 2H30 +1.63 (4) H20 = H2O2 + 2e: + 2H # 0 + +1.77 (5) O2 + H20 = ° 3 + 2e + 2H +2.07 (6) 2F = F2 + 2e +2.85 (7) H20 = OH + e- + H + +2.85 The table shows that the hydroxyl radical next to fluorine with +2.85 V has the highest oxidation potential and thus one compared to chlorine around 104% (equation 2), compared to ozone one shows 38% higher oxidizing power (equation 5).

The production of hydroxyl radicals is known in principle. So is used when using Fenton's reagent, i.e. one for oxidation to hydroxy compounds used mixture of hydrogen peroxide and iron ((II) salts believed to be the Reaction leads to such radicals. Hence, the reaction also becomes to carry out applied by hydroxylations, which proceed similar to the enzymatic ones.

In addition, the appearance of hydroxyl radical is also in the case of decay known from ozone.

The invention is based on the object of a simple method for the targeted creation of hydroxyl radicals. In particular a process is to be shown that does not depend on the existence of special starting chemicals is dependent on the use of the hydroxyl radicals in an undesirable manner must be brought into the treatment system. According to another aspect the object of the invention should be simple processes for carrying out oxidations, Hydroxylation and oxidative disinfection are available.

This task is accomplished by creating processes of the type mentioned at the beginning Type solved, which are characterized in that in the aqueous electrolyte a Direct current electrolysis is carried out with an anode that has a high deposition voltage for oxygen and whose active coating has the immediate further reaction not favored by hydroxyl radical.

In the context of the method according to the invention, anodes are in particular with an oxygen deposition voltage in the range of more than +1.5 V compared Normal hydrogen electrode (NWE) can be used successfully, measured at pH 6 or below in 1 m Na2 SO4 solution at 200C at a current density of 10 mA cm. In particular, it is preferred that Anodes with an oxygen separation voltage of more than +1.9 V compared to NWE, measured under the above conditions.

When using the anodes proposed according to the invention, it is usually those whose active coating contains metal oxides. By the possibly sole presence of metal oxides as active component should the oxygen deposition voltage in the desired high deposition area be mistaken for oxygen at which hydroxyl radicals can be formed. Metal oxides, which are at least partially in rutile structure, are particularly suitable for this purpose are present.

Suitable metal oxides are in particular ß-PbO2, but also alpha-PbO2, MnO2, which is molecularly + trochemically deposited e.g. from Mn2 solutions, as well as others Metal oxides in question that have the desired high deposition voltage for oxygen result. In this context, particular attention is paid to antimony oxide-containing resp. tin oxide-containing systems or active coatings, both components in the controlled Weight ratio included in order to obtain a high oxygen overvoltage.

It has also been found to help prevent immediate Further reduction of the OH radicals formed is beneficial if the electrode does not have any has too much active surface development.

The electrode can be made of the metal oxides mentioned in solid form be educated. However, it is preferred that the electrode consists of a core with an active coating form which at least partially covers the surface of the core. As core materials come metals that are stable under the applied treatment conditions and in particular valve metals such as titanium, niobium, tantalum and zirconium or their alloys in question. Among the core materials mentioned are in particular titanium and its Alloys preferred. Alternatively, however, the core can also consist of conductive Carbon materials and preferably made of graphite.

An electrode which can preferably be used in the method according to the invention is described in DE-OS 24 44 691.

Anodes with a β-PbO2 coating are within the scope of the invention particularly preferred, the ß-PbO2 having an average crystal size of at least 0.5 / µm, but particularly favorable in the range from 5 to 30 / µm.

However, the method can be used in devices of conventional type Use of the proposed anode can be carried out, in which the anode and cathode compartment by separators or suitable flow guidance or other measures are appropriately separated from each other.

The method according to the invention can be carried out in such a way that work is carried out in a current density range in which a current flow under oxidation or hydroxylation of the substrate to be treated occurs, but at the same time a noticeable occurrence of anodic gas evolution is avoided.

The method according to the invention can advantageously be carried out in this way that work is carried out in a current density range that is above the range the direct anodic oxidation of the substrate to be treated. The for the current density characteristic of the transition from direct to indirect oxidation could be determined with -2 -l i (mA cm) = C (mmol 1) x 0.3, where C is the concentration is on substrate, which can react with hydroxyl radicals.

Below the characteristic current density, anodic current occurs Direct oxidation of the substrate, thus leading to the formation of organic radicals and in the This often results in the formation of polymer layers on the anode surface. the Electricity and energy yield, based on the breakdown of organic substances in water, drops sharply in this area.

In the area above the characteristic current density, the generated hydroxyl radicals almost quantitatively for the oxidation of organic matter exploited, which leads to high electricity yields and low gas development comes.

Be in the range well above the characteristic current density Hydroxyl radicals are generated in large excess, which leads to a strong increase in the (competing) Oxygen formation and correspondingly low degradation current yields result. On the other hand, the high excess of OH enables good space-time yields. In addition, the primary reaction now predominates (hydroxylation by electrophilic Addition (with aliphatic double bonds) or by electrophilic substitution (for aromatics)), which means, for example, for chlorinated aromatics a rapid quantitative Cleavage of the chlorine substituents is made possible and thus haloforms as electrolysis products be excluded.

In the process according to the invention, it is expedient to work in such a way that that as far as possible there is no significant O2 formation. Much more the current density remains so low that OH radicals can arise. The inventive Method can be used with particular advantage in the case of anode current densities in the range of 0.5 to about 50 mA cm 2 can be carried out. In the area of higher current densities, However, depending on the system used, for example from 20 mA cm ~ 2, it can lead to a noticeable or significant O2 development come, so that such a process management is less cost effective.

Therefore, in the context of the method according to the invention, above all Current densities in the range from 0.5 to -2 -2 15 mA cm and in particular 0.5 to 10 mA cm preferred.

In the process according to the invention it is advantageous if an electrolyte with inert anions strongly adsorbing on lead dioxide in a concentration of more than 50 mg 1 liter is used. Such inert anions are in particular phosphate, Fluoride or sulfate, these anions in a concentration of> 100 mg 1-1 are expediently added.

Phosphates represent a particularly favorable combination of such anions in a concentration of up to 2 g 1-1 in combination with 100 to 2,000 mg 1-1 fluoride represent.

The inventive method can be used in a variety of technical Oxidation processes are used. It is also suitable for performing Hydroxylations. It has been shown that when using the inventive Method, the current densities when performing oxidation processes at considerably can be lower values than with conventional electrochemical procedures the case is. So much for the method according to the invention for reasons of expediency be carried out at current densities at which electrochemical oxidation processes run using other electrode materials, according to the invention, the required response times an applied active coating described from ß-PbO2, which are produced according to the methodology of DE-OS 24 44 691 what is. However, as can be seen from the above, there are also other electrodes equally suitable for carrying out the method according to the invention, insofar as they meet the conditions defined at the outset, which is why the present invention in no way on the use of the electrode described below as preferred is restricted.

Whether a specific electrode is used to produce hydroxyl radicals suitable in the method according to the invention, can easily be carried out using conventional OH radical detection or detect catcher reactions.

Evidence of the formation of OH, e.g. on special lead dioxide electrodes, is possible by electrolysis in aqueous solution of p-nitrosodimethylaniline (RNO) is carried out. RNO is a common means of detection in biochemistry for Hydroxyl radicals. The oxidation of RNO by OH can be followed by the decrease in the absorption of the RNO at 440 nm was observed in the UV spectrometer. Control attempts are possible through the addition of ethanol, a "catcher" for the hydroxyl radical. A reaction caused by OH is stopped immediately when ethanol is added.

With the electrolysis of RNO solutions with special lead dioxide anodes 5 mA.cm 2 a continuous decrease in RNO absorption can be observed, Which can be stopped immediately at any time by adding ethanol.

Electrolyses under the same conditions using electrode materials with relatively low oxygen separation tensions, which are considerably below +1.5 V (such as platinum, graphite, steel, etc.), on the other hand, result in significantly lower values RNO degradation rates which are not influenced in any way by the addition of ethanol permit. Therefore, there is apparently no formation of hydroxyl radicals in the latter electrodes instead of.

Evidence for hydroxyl radicals as the cause of the oxidizing effect, e.g. with special lead oxide or MnO2 anodes, is also possible by e.g. the degradation reaction of phenol is examined: During the electrolysis of aqueous Solutions (0.4 g.l Na2SO4) with or without the addition of -l 100 mg.l 1 phenol show this Electrodes have the same electrochemical behavior regardless of the phenol content (current density potential characteristic, Impedance spectra, etc.). This includes a direct oxidation of the phenol on the Lead dioxide resp.

Mn02-Anod # off. In the case of platinum, graphite, steel and other anodes on the other hand, there are strong dependencies on the phenol content. This is where direct oxidation lies before.

The only oxidizing agents used are the products of water electrolysis (the anodic side) in question: peroxide, oxygen, ozone and hydroxyl radical.

Peroxide evolution on lead dioxide anodes is at room temperature not detectable. Oxygen does not have an oxidizing effect on phenol at room temperature. Ozone can be detected (in the phenol-free electrolyte) under these conditions. The determined values show, however, that the detectable amounts of ozone are responsible for the oxidation of phenol are far too low. This proves that the breakdown of phenol runs on ß-lead dioxide or MnO2 anodes via the hydroxyl radical.

The effect of the procedure is based on the current understanding the invention ensures that the anodes used meet the above conditions suffice, a high excess of OH can be produced continuously. Because Due to its high oxidizing power and reactivity, it is relatively short-lived and reacts quickly: either oxidatively with reducing water constituents of all kinds, or among each other or on the electrode surface with formation of oxygen and ozone.

The oxidation effect is either titrimetric with the known KJ method determined or read from the amount of oxidized substance (e.g. phenol).

As mentioned above, the method according to the invention can be implemented in a large number of processes application Find. These range from sterilization of drinking water and bathing water, the implementation of technical oxidation reactions up to special hydroxylation reactions. Illustrative and preferred uses result from the following examples, which, however, are not intended to be limiting are to be seen.

One of the particular advantages of the method according to the invention lies also in the fact that when using hydroxyl radicals in addition to the high oxidizing power of the hydroxyl radical this is quite short-lived. This allows the oxidizing agent possibly to be used in uncontrolled quantities, without subsequently like this E.g. when using chlorine, ozone etc., this is the case in a complex process step, e.g. to avoid damage to the health of the end user, a possible Having to destroy excess. In addition, there are high current yields and conventional oxidation reactions can be carried out at comparatively low levels Carry out current densities, as can also be seen from the examples below is. In these, the cathode materials are not specifically indicated in each case. Here however, it can be conventional cathodes, e.g. sheets made of valve metal, expanded metal grids etc., act. The lead dioxide-titanium anodes given in the examples were produced in the manner described in DE-OS 24 44 691. Instead of the lead dioxide titanium anodes However, anodes made from electrochemically deposited MnO2, from alpha-PbO2, can also be used etc., can be used with success.

Example 1 A cell with 500 cm lead dioxide-titanium anodes and through The cathode compartment in the Nafion S is filled with 1 1 IN H2SO4. It will be at room temperature Electrolyzed for 15 minutes at 2.5 A (= 5 mA.cm # 2).

Then with KJ / Na2S2O3 (for OH + J --- OH- + 1 / 2J2) a total of 6.2 mg. 1-1 OH radical or secondary product detected.

Example 2 A cell according to Example 1 is filled with 0.5M H3PO4. It is electrolyzed at room temperature for 30 minutes at +2.5 A (= 5 mA.cm-2). Then, as in Example 1, a total of 15.2 mg.l 1 OH radical is detected.

Example 3 A cell according to Example 1 is filled with 0.005M phosphate buffer (pH 3.0) filled. It is at room temperature for 15 minutes at +2.5 A (= 5 mA.cm electrolyzed. Thereafter, as in Example 1, a total of 5.2 mg.l -1 OH radical proven.

Example 4 A cell according to Example 1 is filled with 0.005M Na2SO4. It is electrolyzed at room temperature 30 mA.cm # for 2 minutes at +2.5 A (= 5 mA.cm). Then, as in Example 1, a total of 6.9 mg.l 1 OH radical is detected.

Example 5 A cell according to Example 1, but without a separated one Cathode compartment is filled with 1 1 0.005M Na2SO4 and 100 mg phenol is added. It is electrolyzed at room temperature with 250 mA (= 0.5 mA.cm # 2). The phenol will degraded approximately linearly with a half-life of 50 minutes. Specific energy consumption: 0.014 Wh.mg phenol.

Example 6 A cell according to Example 5 is as in that mentioned Example filled. It is at room temperature with 2,500 mA (= 5 specific energy: 0.067 -l Wh.mg 1 phenol.

Example 7 A cell according to Example 5 is as in that mentioned Example filled, but 100 mg of p-chlorophenol are added. It will be at Room temperature electrolyzed with 250 mA. The p-chlorophenol becomes approximately linear degraded with a half-life of 50 minutes. Specific energy: 0.014 Wh.mg Example 8 A cell according to Example 5 is filled as in Example 7. It is electrolyzed at room temperature with 2,500 mA. The p-chlorophenol becomes logarithmic degraded with a half-life of 10 minutes.

-l Specific energy: 0.041 Wh.mg Example 9 A cell according to Example 5 is filled with 1 liter of dye-containing laundry waste water. It will be at Electrolyzed at room temperature with 2,500 mA (= 5 mA.cm # 2). The dye becomes logarithmic degraded with a half-life of 5 minutes.

Example 10 An undivided cell with 36 cm² anode area and 1.5 mm distance anode-cathode is filled with 15 ml.s. of tap water (400 / uS.cm ) flows through, -l which contains a population of 10 E. coli.ml 1. It will be at Electrolyzed at room temperature with 200 mA (= 6 mA.cm # 2). Residence time of the electrolyte at the electrodes was 0.5 s, after which time the E. coli population is quantitative killed. There are still about 0.01-0.05 mg. L # 1 oxidizing agent to be detected.

Example 11 Water colored strongly brown due to the content of humic substances des Bansees (Upper Bavaria) was in the undivided 2.

Cell with electrode areas of 40 cm conductive with 1.4 g.l -1 Na2SO4 made. 600 ml of it were at room temperature and 200 mA (= 5 mA.cm) and pH 7 electrolyzed. The degradation was logarithmic with half-lives for the three Dye proportions (436 nm, 276 nm and 254 nm) from 2 to 3 hours. The DOC (share of the dissolved organically bound carbon) increased from an initial 5.85 mg.l -1 Cab to 4.01% Benzene emulsified, current density 40 mA.cm. At a terminal voltage from 4.6 to 5.8 V a current efficiency of 36 to 40% of p-benzoquinone is achieved (40 "C).

Example 13 In order to oxidize β-picoline to nicotinic acid, a cell according to Example 12 used. Area of the lead dioxide anode as in Example 12, current density -2 50 mA.cm, terminal voltage 5.2 V, content of ß-picoline approx. Ln, room temperature. Current efficiency 62% for nicotinic acid.

Claims (23)

Process for generating hydroxyl radicals and application of the process PATENT CLAIMS 1. Process for the generation of hydroxyl radicals, whereby in the aqueous Electrolyte a direct current electrolysis with an anode is carried out, thereby NOTICE that the anode has a high deposition voltage for Oxygen, whose active coating has the immediate further reaction of hydroxyl radicals not favored. 2. The method according to claim 1, characterized in that g e -k e n n z e i c h n e t that an anode with an oxygen deposition voltage in the range of greater than +1.5 V compared to normal hydrogen electrode (NWE), measured at pH 6 or below is used in 1 m Na2SO4 solution at 200C at a current density of 10mA cm. 3. The method according to claim 2, characterized in that g e -k e n n z e i c h n e t that the anode has an oxygen deposition voltage of +1.9 V or higher. 4. The method according to claims 1 to 3, characterized in that g e k e n n z e i c N e t that an anode is used, the active coating of which is metal oxides contains. 5. The method according to claim 4, characterized in that g e -k e n n z e i c h n e t that an anode is used, its metal oxides in the active coating are at least partially in a rutile structure. 6. The method according to one or more of the preceding claims, as a result, that an anode with a core made of valve metal and an active coating of ß-lead dioxide is used. 7. The method according to claim 6, characterized in that g e -k e n n z e i c h n e t that the ß-lead oxide has an average crystal size of at least 0.5 / µm. 8. The method according to claim 7, characterized in that g e -k e n n z e i c h n e t that the mean crystal size is in the range of 5 to 30 µm. 9. The method according to one or more of the preceding claims, as a result, that the electrolysis in the presence of substances contained in water, which react with OH radicals, largely avoiding the development of O2 will. 10. The method according to one or more of claims 1 to 9, characterized g e k e n n n z e i c h -n e t that the current density i in the presence of opposite OH radical reactive substrate with a concentration C has a minimum value of i (mA cm 2) = C (mmol 1) x0.3, which is not less than 0.03 mA cm. 11. The method according to claim 10, characterized in that g e -k e n n z e i c h n e t that the electrolysis with an anode current density in the range of 0.5 to 50 mA cm 2 is carried out. 12. The method according to one or more of the preceding claims, This means that an electrolyte with lead dioxide is strong adsorbing inert anions used in a concentration of more than 50 mg 1 1 will. 13. The method according to claim 12, characterized in that g e -k e n n z e i c h n e t that phosphate or fluoride are used in a concentration of 2100 mg l each will. 14. The method according to claim 12 or 13, characterized g e k e n n z e i n e t that phosphate is used in a concentration of up to 2 g l. 15. The method according to claim 14, characterized in that g e -k e n n z e i c h n e t that an additional 100 to -l 2,000 mg 1 fluoride are used. 16. The method according to one or more of the preceding claims 10 to 15, it is indicated that for the submission of an optimal Electricity and energy yield, based on the breakdown of organic substances in water, the current density used not more than 50% above that defined in claim 10, concentration-dependent minimum value of the current density. 17. The method according to one or more of the preceding claims 10 to 15, thereby noting that to achieve a high Hydroxylation rate the current density more than ten times that defined in claim 10, concentration-dependent minimum value of the current density. 18. The method according to one or more of the preceding claims, This means that electrolysis is carried out in one device is carried out in the anode and cathode compartment by separators or suitable Flow guidance are separated from each other. 19. Application of the method according to claim 1 to 18 for implementation of technical oxidation processes and especially of hydroxylation. 20. Application of the method according to claim 1 to 18 for disinfection of aqueous systems, in particular of utility and waste water. 21. Application of the method according to claims 1 to 18, for the degradation of Pollutants in aqueous systems. 22. Application of the method according to claim 19, 20 or 21, characterized it is not noted that in the system to be treated there is a direct hydroxyl radical is produced. 23. Application of the method according to claim 19, 20 or 21, characterized it is not noted that the system to be treated contains a concentrate of oxidizing agent is supplied, which was previously produced in a separate manner.