WASTE TREATMENT

Abstract: A process for decomposing one or more organic species in aqueous solution, the organic species having at least a limited solubility in water, the process comprising reacting the organic species with an aqueous solution of hydrogen peroxide in the presence of a transition metal catalyst comprising chromium, the hydrogen peroxide being added progressively to the aqueous solution of the organic species and the reacting being carried out within a temperature range of ambient temperature to reflux temperature of the aqueous solution. All the organic species present in the aqueous solution are decomposed to small molecules and other species having no significant complexing activity.

Description: WASTE TREATMENT The present invention relates to a process for the destruction of waste organic liquids. Waste organic liquids, either in the free phase or in aqueous solutions, are necessary by-product of many operations in chemical, nuclear and related industries. Such wastes are increasingly becoming a disposal problem since their traditional methods of disposal, for example, incineration, landfill and discharge to sewer, are becoming unacceptable due to increasingly stringent environmental protection legislation. In particular, organic complexing agents are used in many chemical processes and in the nuclear industry for decontamination and other purposes. The spent complexing agent solutions often contain toxic heavy metals and/or radioactive nuclides. The chemical nature of these solutions mean that they cannot, in significant concentrations and without some form of initial treatment, be directed to normal effluent treatment plants, such as precipitation, ion exchange or evaporation plants. In fact, the addition of these solutions to such effluent treatment plants, could well cause the operation of the plants to be less efficient. In the past, various photo-oxidation processes have been used for the destruction of the aforementioned wastes. However, such processes have generally been found to be inefficient and require optical windows which are susceptible to fouling by the wastes to be treated. In a known process, described in European Patent Application No 0342876 A2, alkylphosphates, either undiluted or dissolved in hydrophobic organic solvents, can be destroyed by reaction with hydrogen peroxide in the presence of a chromium catalyst. The two phase decomposition reaction is carried out at a temperature of at least 60°C and the pH of the reaction mixture is controlled within specified limits. The alkylphosphates are present in an organic phase and are decomposed by the hydrogen peroxide which is in aqueous solution. During the reaction the alkylphosphates are decomposed to inorganic phosphate, carbon dioxide and water, whilst the hydrophobic solvent remains unaffected. This process is known specifically for use in treating alkylphosphates which are a particular class of organic compounds and which are used for specific purposes such as solvent extraction or as pesticides. The process described, therefore, selectively destroys alkylphosphates in the presence of other organic compounds, thereby leaving the other organic compounds intact. According to the present invention there is provided a process for decomposing one or more organic species in aqueous solution, the organic species having at least a limited solubility in water, the process comprising reacting the organic species with an aqueous solution of hydrogen peroxide in the -presence of a transition metal catalyst comprising chromium, the hydrogen peroxide being added progressively to the organic species and the reaction being carried out within a temperature range of ambient temperature to reflux temperature of the aqueous solution. The aqueous solution to be treated may be an aqueous solution containing significantly less than 1% of the organic species. For example, an aqueous solution to be treated may contain from 0.05% by volume of the organic species. Preferably, the organic species may comprise organic liquids. Advantageously, the catalyst used in the process may be a chromate, particularly sodium chromate or potassium chromate. The process is suitable for the treatment of a wide range of organic substrates but typically the organic species may include complexants in dilute aqueous solution or waste solvents, such as dibutoxydiethyl ether (Butex) , lubricating oils or cutting oils. The process is particularly useful for destroying complexants, including carboxylic acids, such as citric acid, and ethylene dia ine tetra-acetic acid (EDTA) which are major constituents of decontamination solutions widely used in the nuclear industry. As mentioned previously, the decomposition reaction may be carried out over the temperature range of ambient temperature to reflux temperature (about 100°C) and preferably, the hydrogen peroxide may be in a concentration range of 10% to 50% v/v and may be added over a period of 1 hour. However, the inventors have found that the precise temperature, hydrogen peroxide concentration, or rate of addition of hydrogen peroxide are not crucial to the decomposition reaction. It is desirable, but not essential to control or maintain the pH of the reaction mixture within the range pH 3.5 to pH 9.0. In many cases, during the decomposition of organic species by the process, the pH of the reaction mixture may fall due to the formation of acidic intermediates. Should this fall in pH occur to an undesirable extent (for example, below pH 3.5), the pH may be corrected by the addition of an appropriate quantity of dilute alkali, such as sodium hydroxide, to the reaction mixture. In the process, a reaction between dissolved chromium ions and the added hydrogen peroxide is postulated as a switching of the chromium between its III and VI oxidation states. This switching between oxidation states is thought to catalyse the breakup of the hydrogen peroxide into hydroxyl radicals which, in turn, attack the organic ' species. In contrast to the prior art process described in European Patent Application No 0342876 A2, the process of the present invention may be used for the destruction of a class of chemical compounds having properties quite different from those of alkylphosphates (as treated in the prior art) , which compounds having significantly different uses to those of alkylphosphates. By the process of the present invention, two or more different organic species, present together as a mixture or in solution, may each be destroyed. For example, citric acid and EDTA may be treated in this way, such species typically being present together in some commercially available decontamination agents. The process converts organic complexing agents into small molecules such as carbon dioxide, water and other species with no significant complexing activity, and hence makes the streams containing them suitable for further treatment, to remove heavy metals and radionuclides, by conventional aqueous treatment plants, such as precipitation, ion exchange or evaporation plants, and thereby minimises the amount of harmful species discharged to the environment. Use of the process on organic wastes such as solvents, lubricating oils or cutting oils converts these organic materials into small, non-toxic, non-complexing species, and transfers into aqueous solution metal ions and other potentially toxic inorganic species, thereby rendering them suitable for conventional aqueous effluent treatment as above. Following the decomposition reaction, chromium from the catalyst will be present in the resulting aqueous solution. However, if the chromium is conditioned so that it is converted to CrIII, it may be removed by a precipitation process in common with the other heavy metals present. The chromium may also be removed by ion exchange. An advantage of the process of the present invention is that it operates using a stoichiometric quantity or a modest excess of hydrogen peroxide and can be performed in a simple stirred tank reactor. Embodiments of the present invention will now be described, by way of example only, by reference to the following Examples: Example 1 A small scale reactor (1000ml) fitted with an agitator and reflux condenser was charged with 250ml of a 2300ppm solution of citric acid in demineralised water. To this solution was added 0.0132g of sodium chromate and the solution was stirred until homogeneous. The solution was heated to 70°C and continuously stirred. 19ml of a 30% w/w hydrogen peroxide solution was added at a steady rate over a 1 hour period. The pH of the solution was initially adjusted to ~8.5. After the 1 hour duration of the experiment ~40% of the total initial carbon present in the solution remained. Example 2 A small scale reactor (1000ml) fitted with an agitator and reflux condenser was charged with 250ml of a 0.2% solution of dibutoxydiethyl ether, the main component in the commercial solvent "Butex", in demineralised water. To this solution was added 0.1125g of sodium chromate and the solution was stirred until homogeneous. The solution was heated to 30°C and continuously stirred. 14ml of a 30% w/w hydrogen peroxide solution was added at a steady rate over a 1 hour period. The pH of the solution was allowed to vary but the solution was maintained at 30°C. After the 1 hour duration of the experiment ~85.9% of the total initial carbon present in the solution remained. Example 3 The apparatus used was as in Example 2. Again, 250ml of a 0.2% solution of dibutoxydiethyl ether was added to he reactor with 0.1125g sodium chromate. The solution was then heated to 50°C and continuously stirred. 14ml of a 30% w/w hydrogen peroxide solution was added at a steady rate over a 1 hour period. The pH of the solution was allowed to vary but the solution was maintained at 50°C. After the 1 hour duration of the experiment ~85.9% of the total initial carbon present in the solution remained. Example 4 The apparatus was as in Example 2. Again, 250ml of a 0.2% solution of dibutoxydiethyl ether was added to he reactor with 0.ll25g sodium chromate. The solution was then heated to 70°C and continuously stirred. 14ml of a 30% w/w hydrogen peroxide solution was added at a steady rate over a 1 hour period. The pH of the solution was allowed to vary but the solution was maintained at 70°C. After the 1 hour duration of the experiment -37.0% of the total initial carbon present in the solution remained. Example 5 The apparatus was as in Example 2. Again, 250ml of a 0.2% solution of dibutoxydiethyl ether was added to he reactor with 0.1125g sodium chromate. The pH of the solution was then adjusted to ~7 by the addition of 0.1M sodium hydroxide solution. The solution was then heated to 70°C and continuously stirred. 14ml of a 30% w/w hydrogen peroxide solution was added at a steady rate over a 1 hour period. The pH of the solution was allowed to vary but the solution was maintained at 70°C. After 1 hour duration of the experiment ~26.6% of the total initial carbon present in the solution remained. Example 6 The apparatus was as in Example 2. Again, 250ml of a 0.2% solution of dibutoxydiethyl ether was added to he reactor with 0.1125g sodium chromate. The pH of the solution was then adjusted to ~7 by the addition of 0.1M sodium hydroxide solution. The solution was then heated to 70°C and continuously stirred. 14ml of a 30% w/w hydrogen peroxide solution was added at a steady rate over a 1 hour period. The pH of the solution was continuously adjusted by the addition of 0.1M sodium hydroxide solution, such that the pH of the solution remained at ~7. The temperature of the solution was maintained at 70°C. After 1 hour duration of the experiment ~29.6% of the total initial carbon present in the solution remained. Example 7 The apparatus was as in Example 2. 500 ml of a 2900 pp Total Organic Carbon (TOC) solution of SDG3 in ultra-pure water was added to the reactor. (SDG3 is a commercially available decontamination agent containing around 40% of organic material, mainly as sodium citrate and sodium salts of EDTA (Na-EDTA).) To this solution was added a 0.0446g of sodium chromate and the solution was stirred until homogeneous. The solution was heated to 70°C and continuously stirred. 40 ml of a 30% w/w hydrogen peroxide solution was added at a steady rate over a 5 hour period. The pH of the solution was allowed to vary but the temperature was maintained at 70°C. After the 5 hour duration of the experiment ~75% of the total initial TOC present in the solution remained. Example 8 The apparatus was as in Example 2. 250 ml of a 2390 ppm -TOC solution of SDG 3 in ultra- pure water was added to the reactor. To this solution was added 0.223g of sodium chromate and the solution was stirred until homogeneous. The solution was heated to 30°C and continuously stirred. 22 ml of a 30% w/w hydrogen peroxide solution was added at a steady rate over a 1 hour period. The pH of the solution was allowed to vary but the temperature was maintained at 70°C. After the 1 hour duration of the experiment ~60% of the total initial TOC present in the solution remained. Example 9 The apparatus was as in Example 2. 250 ml of a 670 ppm TOC solution of SDG3 in ultra-pure water was added to the reactor. To this solution was added 0.0557g of sodium chromate and the solution was stirred until homogeneous. The solution was heated to 70°C and continuously stirred. 16 ml of a 10% w/w hydrogen peroxide solution was added at a steady rate over a 5 hour period. The pH of the solution was allowed to vary but the temperature was maintained at 70°C. After the 5 hour duration of the experiment ~75% of the total initial TOC present in the solution remained. Example 10 The apparatus was as in Example 2. 500 ml of a 2550 ppm TOC solution of SDG3 in ultra- pure water was added to the reactor. To this solution was added 0.446g of sodium chromate and the solution was stirred until homogeneous. The solution was heated to 30°C and continuously stirred. 131 ml of a- 10% w/w hydrogen peroxide solution was added at a steady rate over a 1 hour period. The pH of the solution was initially adjusted to ~6.5 by addition of sodium hydroxide solution but the temperature was maintained at 30°C. After the 1 hour duration of the experiment ~60% of the total initial TOC present in the solution remained. Example 11 The apparatus was as in Example 2. 500 ml of a 2230 ppm TOC solution of SDG3 in ultra- pure water was added to the reactor. To this solution was added 0.446g of sodium chromate and the solution was stirred until homogeneous. The solution was heated to " 30°C and continuously stirred. ~302 ml of a 30% w/w hydrogen peroxide solution was added at a steady rate over a 2.1 hour period. The pH of the solution was initially adjusted to ~6.5 by addition of sodium hydroxide solution but the temperature was maintained at 30°C. After the 2.1 hour duration of the experiment, only ~27% of the total initial TOC present in the solution remained. The present invention therefore provides a cost effective and environmentally acceptable process for converting waste organic liquids, having at least a limited solubility in aqueous solutions, into a form acceptable for discharge (to the environment) and renders any associated metals, be they toxic, radioactive or any other undesirable species, into a form readily treatable by conventional aqueous effluent treatment facilities.

Claims: Claims 1. A process for decomposing one or more organic species in aqueous solution, the organic species having at least a limited solubility in water, the process comprising reacting the organic species with an aqueous solution of hydrogen peroxide in the presence of a transition metal catalyst comprising chromium, the hydrogen peroxide being added progressively to the aqueous solution of the organic species and the reaction being carried out within a temperature range of ambient temperature to reflux temperature of the aqueous solution. 2. A process as in Claim 1 and wherein the aqueous solution to be treated contains at least 0.05% by volume of the organic species. 3. A process as in Claim 1 or Claim 2 and wherein the organic species comprise organic liquids. 4. A process as in any one of Claims 1 to 3 and wherein the organic species comprise one or more complexants dissolved in dilute aqueous solutions. 5. A process as in Claim 4 and wherein the complexants comprise carboxylic acids including citric acid or ethylene diamine tetra-acetic acid or mixtures thereof. 6. A process as in any one of Claims 1 to 3 and wherein the organic species comprise solvents or lubricating oils or cutting oils. 7. A process as in any one of preceding Claims and wherein the catalyst comprises a chromate including sodium chromate or potassium chromate. 8. A process as in any one of the preceding Claims and wherein the hydrogen peroxide concentration is in the range of 10% to 50% v/v. 9. A process as in any one of the preceding Claims and wherein the pH of the reaction is in the range pH 3.5 to pH 9.0.

Inventors: HUTSON GRAHAM VICTOR, JORDAN PHILIP HENRY, RUMNEY NORMA, HUTSON, GRAHAM, VICTOR, JORDAN, PHILIP, HENRY, RUMNEY, NORMA
Applicants: BRITISH NUCLEAR FUELS PLC, HUTSON GRAHAM VICTOR, JORDAN PHILIP HENRY, RUMNEY NORMA, HUTSON, GRAHAM, VICTOR, JORDAN, PHILIP, HENRY, RUMNEY, NORMA
Application year: 1995
Main classification: A62D3
IPC codes: A62D3, A62D101, B01J23, C10M175