GB1601138A - Process for the oxidation of harmful material in aqueous solution - Google Patents

Abstract

In a process for the oxidation of oxidizable pollutants contained in aqueous solution, on a catalyst with feed of oxygen to the aqueous solution, the catalyst used is a material which is suitable for the production of cathodes at which oxygen is electrochemically reduced in fuel cells. The catalyst used in this case can be nickel powder or Raney nickel, silver powder or Raney silver. The process can be carried out using nickel- or silver-coated activated charcoal bodies or ceramic bodies of large specific surface area.

Description

(54) PROCESS FOR THE OXIDATION OF HARMFUL MATERIAL IN AQUEOUS SOLUTION (71) We, KERNFoRscHuNGsANLAGE JijLICH GESELLSCHAFT MIT BESCHRANKTER HAFTUNG, of Postfach 1913, 5170 Jülich, Federal Republic of Germany, a Body Corporate organised according to the Laws of the Federal Republic of Germany, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: The invention relates to a process for the oxidation of oxidisable harmful material contained in aqueous solution.

The oxidation of harmful substances in aqueous solutions is known. It is applied to the purification of waste waters, whose content of harmful material must be limited to low values owing to the toxic action of the material. In particular, processes are known for the oxidation of cyanide-containing aqueous solutions which are formed, for example, in the electrochemical industry in the treatment of metal surfaces or of bodies of non-conducting material which are electrically conducting on the surface.

For removing cyanide-containing harmful substances, an aqueous solution can be treated with sodium hypochlorite so that the cyanide is oxidised to form cyanate or further to form carbonate. However, sodium hypochlorite also has a toxic action, and so it must be ensured that the waste water does not receive an excess of sodium hypochlorite.

A disadvantage of this process is that the aqueous solution which is treated is made highly saline with sodium chloride.

An electrochemical process has also been proposed, see 'Fachberichte für Oberflächen- technik', 1973, part 5, pages 151 ff. The cyanide-containing aqueous solution is introduced into an electrochemical cell and the cyanide ions are anodically oxidised with the formation of cyanate ions or of carbonate and nitrogen. However, the detoxication of the aqueous solutions can only be effected with acceptable current yields in the case of high cyanide concentrations in the solution. The current yield falls below 5% while the cyanide concentration is still far above the maximum which can be tolerated in waste waters as having no deleterious effect.

Another proposal has been to treat aqueous solutions containing harmful substances with hydrogen peroxide, to oxidise the harmful substances with the use of catalysts. However, a disadvantage of this is the consumption of very costly chemicals. In 'Galvano-Technik', 1970, part 6, pages 468 ff., it is proposed to -float an activated charcoal mixture in aqueous solutions containing nitrite, cyanide or oxidisable organic harmful substances and to add oxygen of the air to the aqueous solution. The activated charcoal acts as a catalyst in oxidation of the harmful substances. However, it has not hitherto been found possible to obtain a sufficient yield by this process, because the reaction speeds on activated charcoal powder have been too low. It has already been proposed, for the purpose of removing cyanide, also to add heavy metals to the solution in order to accelerate the catalysis on activated charcoal, see 'oberflichen-Sufface', 1969, part 11, pages 798/799.

United States Patent Specification 3,650,949 proposes the use of copper-impregnated activated charcoal particles for removing cyanide from aqueous solutions. In this case, however, the intention is to adsorb the cyanide on the surface of the activated charcoal particles with the formation of a coppercyanide complex. In this process also it is not possible to obtain sufficient reaction speeds.

The present invention seeks to provide a process for oxidising harmful material in aqueous solution, in which it is possible to achieve an economically sufficient conversion in the oxidation of the harmful material in the solution, while avoiding rendering the treated aqueous solution highly saline, and with low consumption of additional chemicals.

According to this invention there is provided a process for the oxidation of oxidisable harmful material contained in aqueous solution comprising supplying oxygen to the aqueous solution, while providing as a catalyst a material, other than phthalocyanins, which is suitable for cathodes, on which oxygen can be electrochemically reduced in fuel cells, the catalyst consisting of solid catalyst ma terial or a coating of the catalyst material on a conductive support. The catalyst material may be metallic. Conveniently the oxygen is supplied as air. Such a catalyst is well suited for the electrochemical reduction of oxygen and on it the oxidation of the harmful material is catalysed. In the presence of oxygen and an oxidisable injurious substance in the aqueous solution there is set up on the catalyst - without any external supply of current - an electrochemical potential which is intermediate between the redox potential of the redox system oxygen/oxygen reduction product and the redoxpotential of the redox system harmful substance/oxidation product thereof. The harmful substance is oxidised on the catalyst with reduction of the oxygen introduced into the aqueous solution. On catalysts of this kind, the conversion in the oxidation of harmful substances can be considerably increased. Suitable cathode materials are, for example, nickel, platinum, silver, tungsten bronzes, and porphyrins. A cathode material known under the name 'Pfeiffer-Complex 1', such as, for example, M-bisXsalicylaldehyde)-diphenyl ethylenedi- amine may alternatively be employed. Essential the choice of these materials should be made in accordance with the chemical compatability of the catalyst with the aqueous solution, which may be either acid or alkaline, with the harmful substances to be oxidised and their oxidation products, as well as taking economic aspects into account.

There is advantageously employed as catalyst nickel powder, Raney nickel, silver powder or Ranely silver. Large effective catalyst surfaces per unit volume are also provided if nickelcoated activated charcoal is used as catalyst, the charcoal providing a conductive support. Likewise, platinum-coated or silver-coated activated charcoal have proved suitable as catalysts.

The term 'harmful material' embraces not only toxic materials, but any substances which are potentially injurious or undesirable, for example any contaminants of waste water which would present a hazard to the ecology of the biological system into which the waste water is to be released.

Examples of processes embodying the invention will now be described. There is illustrated in the drawing, the course of cyanide oxidation reactions as a function of time, the concentration of cyanide in aqueous solution being plotted in mg/l on a logarithmic scale against the reaction time in hours.

EXAMPLE 1 Activated charcoal powder was coated with platinum and heated at 1100 OC in a hydrogen atmosphere. After a heating period of one hour, the activated charcoal powder was cooled in the hydrogen atmosphere. The applied quantity of platinum corresponded to 2% by weight calculated on the total weight of the activated charcoal powder. The activated charcoal thus pretreated was suspended in a concentration of 2% by weight in an aqueous solution which had been adjusted to a pH value of 11 by means of caustic soda. The solution was continuously mixed by blowing-in of air. A harmful substance, namely sodium nitrite, was added to the aqueous solution in a concentration of 100 mg per litre.

During the reaction, the concentration of the sodium nitrite in the aqueous solution steadily decreased. After 10 hours, the added quantity of nitrite had been oxidised to form nitrate to such an extent that the nitrite concentration was below the detectable limit.

EXAMPLE 2 Activated charcoal powder having a grain size of up to 0.1 mm was coated with platinum and heated in Vacua at 1100 OC for one hour.

The activated charcoal powder was cooled in the vacuum. The added quantity of platinum was 5% by weight calculated on the total weight of the activated charcoal powder. 10 g of the activated charcoal thus pretreated were suspended in 1 litre of an aqueous solution which contained as a harmful substance 250 mg of cyanide and which had been adjusted to a pH value of 10 by means of sodium hydroxide. The course of the reaction was measured while air was supplied to the solution continuously. The measured values of cyanide concentration corresponded to the curve a shown in the drawing. After 5ss hours, the cyanide concentration in the aqueous solution had fallen to 1 mg per litre.

EXAMPLE 3 Nickel powder having a grain size in the range up to 5 jim was suspended in an aqueous solution which contained 250 mg of cyanide per litre and which had been adjusted to a pH value of 10 by means of sodium hydroxide. The added quantity of nickel powder was 10% by weight calculated on the weight of the aqueous solution, to which air was supplied continuously. The cyanide concentration in the aqueous solution rapidly fell. The course of the reaction can be seen from curve b of the drawing. After 2.5 hours, the cyanide concentration in the aqueous solution had fallen below a value of 0.1 mg per litre.

EXAMPLE 4 In 10 cc of an aqueous solution adjusted to a pH value of 3 by means of sulphuric acid, there was suspended 5 g of activated charcoal coated with 5% by weight of platinum. Air containing nitric oxide was supplied to the suspension. The nitric oxide content in the air was about 200 parts per million (volume parts). Even after a few minutes, it was possible to detect in the aqueous solution nitrate (NO3l which had been formed on the surface of the platinum-coated activated charcoal by oxidation of dissolved nitric oxide (NO) with simultaneous reduction of oxygen.

EXAMPLES 1 g of silver powder corresponding to 1% by weight was suspended in 100 cc of aqueous solution adjusted to a pH of 11 by means of caustic soda, and air was introduced into it.

1 g of sodium sulphite corresponding to a concentration of lOg per litre was added to the solution and oxidised to sodium sulphate. After about 8 hours, sulphite could no longer be detected in the solution.

The catalyst which may be employed for this invention are not confined to particles suspended in aqueous solution: there may alternatively be employed skeleton structures consisting of catalyst material, through which the aqueous solutions charged with harmful substances are passed for the purpose of cleaning.

WHAT WE CLAIM IS: 1. A process for the oxidation of oxidisable harmful material contained in aqueous solution comprising supplying oxygen to the aqueous solution, while providing as a catalyst a material, other than phthalocyanins, which is suitable for cathodes, on which oxygen can be electrochemically reduced in fuel cells, the catalyst consisting of solid catalyst material or a coating of the catalyst material on a conductive support.

2. A process according to claim 1 wherein the catalyst is metallic.

3. A process according to claim 1, wherein nickel powder or Raney nickel is employed as catalyst.

4. A process according to claim 1, wherein there is employed as catalyst nickel-coated activated charcoal.

5. A process according to claim 1, wherein silver powder or Raney silver is employed as catalyst.

6. A process according to claim 1, wherein there is employed as catalyst silver-coated activated charcoal.

7. A process according to claim 1, wherein there is employed as catalyst platinum-coated activated charcoal.

8. A process according to any one of the preceding claims wherein the harmful material is a cyanide.

9. A process according to any one of the preceding claims which is a process for the treatment of waste water, the said aqueous solution being waste water and the harmful material being a contaminant therein.

10. A process for the oxidation of oxidisable harmful material contained in aqueous solution, substantially as herein described with reference to any one of the Examples.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (10)

**WARNING** start of CLMS field may overlap end of DESC **. caustic soda, and air was introduced into it.

1 g of sodium sulphite corresponding to a concentration of lOg per litre was added to the solution and oxidised to sodium sulphate. After about 8 hours, sulphite could no longer be detected in the solution.

The catalyst which may be employed for this invention are not confined to particles suspended in aqueous solution: there may alternatively be employed skeleton structures consisting of catalyst material, through which the aqueous solutions charged with harmful substances are passed for the purpose of cleaning.

WHAT WE CLAIM IS: 1. A process for the oxidation of oxidisable harmful material contained in aqueous solution comprising supplying oxygen to the aqueous solution, while providing as a catalyst a material, other than phthalocyanins, which is suitable for cathodes, on which oxygen can be electrochemically reduced in fuel cells, the catalyst consisting of solid catalyst material or a coating of the catalyst material on a conductive support.

2. A process according to claim 1 wherein the catalyst is metallic.

3. A process according to claim 1, wherein nickel powder or Raney nickel is employed as catalyst.

4. A process according to claim 1, wherein there is employed as catalyst nickel-coated activated charcoal.

5. A process according to claim 1, wherein silver powder or Raney silver is employed as catalyst.

6. A process according to claim 1, wherein there is employed as catalyst silver-coated activated charcoal.

7. A process according to claim 1, wherein there is employed as catalyst platinum-coated activated charcoal.

8. A process according to any one of the preceding claims wherein the harmful material is a cyanide.

9. A process according to any one of the preceding claims which is a process for the treatment of waste water, the said aqueous solution being waste water and the harmful material being a contaminant therein.

10. A process for the oxidation of oxidisable harmful material contained in aqueous solution, substantially as herein described with reference to any one of the Examples.