WO1993001136A1

WO1993001136A1 - Enhanced oxidation treatment of contaminated effluents and groundwaters

Info

Publication number: WO1993001136A1
Application number: PCT/CA1992/000273
Authority: WO
Inventors: Keith G. Bircher; Stephen R. Cater; Tony De Marco; Wayne Mcphee; Ali Safarzadeh-Amiri
Original assignee: Solarchem Enterprises Inc
Current assignee: Solarchem Enterprises Inc
Priority date: 1991-07-05
Filing date: 1992-06-26
Publication date: 1993-01-21
Anticipated expiration: 1994-01-05
Also published as: AU2181892A

Abstract

A method is provided for enhancing the rate of oxidation of organic contaminants in a liquid effluent by hydrogen peroxide and ions of at least one transition metal, the method comprising promoting conversion of the transition metal ions from an oxidized to a reduced form.

Description

ENHANCED OXIDATION TREATMENT OF CONTAMINATED EFFLUENTS AND GROUNDWATERS

This invention relates to methods for treating liquid effluents or contaminated groundwaters to remove organic contaminants. More particularly, the invention relates to processes for removing organic contaminants by treatment with hydrogen peroxide and transition metal ions.

Background of the Invention

Fenton's reagent, which comprises hydrogen peroxide and a source of ferrous ion, is a strong oxidising agent and its use for the removal of certain organic contaminants from industrial effluents and groundwaters has been reported.

Canadian Patent No. 646,440 discloses a process for treating solutions containing phenolic compounds with hydrogen peroxide and metallic iron or ferrous ions. The patent does not disclose treatment of actual industrial wastes containing phenolic compounds.

Eisenhauer, H. (J. Water Pollut. Control Fed., vol. 36, p. 1116 (1964)) has also reported the use of Fenton's reagent to treat solutions containing phenolic compounds. Using laboratory prepared solutions of pure phenolic compounds, he found that substituted phenols reacted more slowly than unsubstituted phenols, the resistance to oxidation increasing with substitution until, with the fully substituted compound pentachlorophenol (PCP) , no reaction occurred. When actual industrial wastes were treated, the treated waste had an intense colouration, making it unsuitable for disposal without additional flocculation treatment to remove the colour.

The inventors have also devised a treatment method combining Fenton's reagent and UV irradiation for destruction of groundwater contaminants. This method is advantageous for treating low levels of contaminants, ie. parts per billion (ppb) to low parts per million (ppm) range. For heavily contaminated effluents or groundwaters, UV light cannot penetrate deeply enough into the solution for the treatment to be fully effective. It is desirable to use a non-photochemical pre-treatment method to reduce the concentrations of contaminant to a level at which ultraviolet light becomes effective. One possible treatment is the use of Fenton's reaction. There are, however, some drawbacks to Fenton's reaction as currently practised.

As indicated above, the processes suggested in the literature are not effective for treatment of organic contaminants such as PCP.

Furthermore, while use of Fenton's reagent at ambient temperatures may give effective treatment of pure solutions of organic contaminants such as benzene and toluene, this same process carried out at ambient temperatures often fails to destroy these contaminants when they occur in industrial wastes such as gasoline wastes, where many other substances are present.

Summary of Invention

In accordance with one embodiment of the invention, a method is provided of enhancing the rate of oxidation of organic contaminants in a liquid effluent or groundwater by hydrogen peroxide and ions of at least one transition metal, the method comprising promoting conversion of the transition metal ions from an oxidised to a reduced form. In accordance with a further embodiment of the invention, a method is provided of enhancing the rate of oxidation of organic contaminants in a liquid effluent or groundwater by hydrogen peroxide and ions of at least one transition metal, by promoting conversion of the transition metal ions from an oxidised to a reduced form, the method comprising the steps of: (a) providing a liquid effluent or groundwater containing organic contaminants;

(b) adjusting the pH of the liquid effluent or groundwater to an acidic pH if not already acidic; (c) contacting the liquid effluent or groundwater at an effective temperature with hydrogen peroxide and ions of at least one transition metal, the hydrogen peroxide and metal ions being present in effective amounts based on the initial concentration of the organic contaminants in the liquid effluent or groundwater to destroy the organic contaminants. In accordance with a further embodiment of the invention, a method is provided of enhancing the rate of oxidation of organic contaminants in a liquid effluent or groundwater by hydrogen peroxide and ions of at least one transition metal, by promoting conversion of the transition metal ions from an oxidised to a reduced form, the method comprising the steps of:

(a) providing a liquid effluent or groundwater containing organic contaminants;

(b) adjusting the pH of the liquid effluent or groundwater to an acidic pH if not already acidic;

(c) contacting the liquid effluent or groundwater with hydrogen peroxide, ions of at least one transition metal and a promoter compound, the hydrogen peroxide, metal ions and promoter compound being present in effective amounts based on the initial concentration of the organic contaminants in the liquid effluent or groundwater to destroy the organic contaminants.

Summary of Drawings

The invention, as exemplified by preferred embodiments, is described with reference to the following drawings: Figure 1 shows toluene concentration as a function of time when BTX water was treated with Fenton's reagent at 20°C.

Figure 2 shows reaction time at various temperatures when PCP contaminated liquid effluent was treated by the process of the invention.

Figure 3 shows toluene destruction as a function of time when BTX water was treated by the process of the invention at various temperatures. Figure 4 shows the destruction of benzene, toluene and xylenes by Fe²⁺/H₂0₂ at 70°C and pH 3 as a function of time; concentration of H₂0₂ and Fe²⁺ are also shown.

Figure 5 shows the effect of initial Fe²⁺ concentration on the destruction of toluene at 50°C. Figure 6 shows the effect of temperature on the rate of toluene destruction by Fe²⁺/H₂0₂ (20 ppm : 150 ppm) .

Figure 7 shows the destruction of toluene at 50°C and pH 3 by Fe²⁺/H₂0₂ and Fe³⁺/H₂0₂.

Figure 8 shows the temperature dependence of the decomposition rate constant of H₂0₂ by Fe³⁺ in distilled water.

Figure 9 shows the effect of peroxide and resorcinol on the destruction of total BTX at 50°C and pH 3.

Figure 10 shows the destruction of toluene at 50°c and pH 3 as a function of time, with and without addition of resorcinol.

Figure 11 shows the destruction of toluene at 60°C and pH 3 by Fe³⁺/H₂0₂, 65:300 ■ or by Fe³⁺/H₂0₂/resorcinol, 62:300:15 ♦. Fe²⁺ concentrations during these treatments are shown as D and 0 respectively.

Figure 12 is a schematic diagram of a suitable apparatus for treating liquid effluents by the process of the invention.

Figure 13 shows the effect of pH on the destruction rate constant of toluene by Fe²⁺/H₂0₂ (20:150) at 50°C. Detailed Description of the Invention

Liquid effluents and groundwaters include wastes resulting from industrial processes and contaminated groundwaters resulting from spills or leakage at industrial or storage sites. Typically, such liquid effluents and groundwaters may be contaminated with organic compounds including aromatic hydrocarbons such as benzene, toluene and xylene, substituted analogues of these compounds, phenol and substituted phenols such as pentachlorophenol (PCP), ethers such as 1,4-dioxane or tetrahydrofuran, ketones such as acetone or methylethylketone or chlorinated organics such as chloroform.

It has been found that many organic contaminants in liquid effluents and groundwaters are resistant to oxidative treatment with Fenton's reagent at ambient temperatures. For example, if liquid effluents from industrial processes containing benzene, toluene or xylene are treated with Fenton's reagent at ambient temperatures, there is an initial sharp drop in concentration of the contaminant, but the rate of oxidation quickly declines to insignificant levels, leaving unacceptable concentrations of contaminant, as seen in Figure 1. As benzene, toluene and xylene in pure solution are readily destroyed at ambient temperatures by Fenton's reagent, the resistance of these compounds when present in industrial wastes suggested that other contaminants in the industrial effluent were inhibiting the oxidation process, perhaps by complexing with the iron ions and/or by preventing conversion of Fe³⁺ ions back to Fe²⁺ ions.It is thought that the following reactions occur during oxidation by Fenton's reagent: Fe⁺ + H₂0₂ → Fe³⁺ + OH- + OH* RH + OH* → R* + H₂0

Fe³⁺ + H₂0₂ → Fe²⁺ + H0₂* + H⁺ where RH represents an organic contaminant. Transition metal ions other than iron may also be used in a similar oxidative process, copper and zinc being preferred, although iron is especially preferred.

The present inventors have found, surprisingly, that it is possible to promote conversion of the oxidised form of the transition metal ions to the reduced form, thereby enhancing the rate of oxidation of contaminants and overcoming the above-described problems.

This promotion was achieved by the application of heat during the oxidation or by addition to the reaction mixture of a promoter compound.

When thermal enhancement was used, it was found that the temperature required to give enhanced oxidation of organic contaminants, referred to herein as the "effective temperature", varied depending on the particular organic contaminant to be treated, and also on the other constituents present in the liquid effluent or groundwater.

Figure 2 shows the treatment of wood treatment process water containing PCP. At 50°, the rate of destruction of PCP was too slow to be monitored. At 60°C, the rate had increased and at 70°C the reaction was complete in less than 60 minutes.

Enhanced oxidation of PCP required an effective temperature of at least about 60°C.

Figures 3 and 4 show the treatment of gasoline wastes at different temperatures.

Gasoline wastes typically contain benzene, toluene, ethylbenzene and xylene (the term "xylene" includes the three possible isomers of the compound) and are referred to herein as "BTX waters". These waters also contain other aromatic hydrocarbons along with methyl t-butyl ether. The data on toluene destruction described herein are representative of the destruction of all the above- listed components of BTX waters.

Enhanced oxidation of the contaminants in BTX waters required an effective temperature of at least about 40°C. In contrast, tap water spiked to the same concentration with benzene, toluene and xylene (BTX) was readily treated at 25°C.

These results suggested that inhibitory complexes inhibiting conversion of Fe³⁺ to Fe²⁺ were present in BTX waters.

When the concentration of Fe²⁺ was monitored during the treatment of BTX contaminated groundwater, an initial steep drop was noted, after which Fe²⁺ concentration remained low and relatively constant, as seen in Figure 4.

The destruction of toluene as a function of Fe²⁺ concentration was examined and the results are shown in Figure 5. The amount of the initial steep drop in toluene concentration is dependent on initial Fe²⁺ concentration but the destruction rate of the second phase is not.

These results are consistent with most of the iron being in the Fe³⁺ state after the initial phase. The destruction rate of toluene in the second phase was shown to be temperature dependent, as seen in Figure 6, suggesting that increased temperature does facilitate conversion of Fe³⁺ to Fe²⁺.

If Fe³⁺ is used instead of Fe²⁺ in the treatment of toluene, there is a much smaller initial drop in toluene concentration, followed by a second phase rate comparable to that seen with Fe²⁺, as seen in Figure 7. The effect of Fe³⁺ concentration on toluene destruction and the effect of temperature at a given Fe³⁺ concentration were found to be similar to those observed for Fe²⁺.

Temperature also affects the rate of H₂0₂ decomposition in the presence of Fe³⁺, with a linear relationship between the logarithm of the rate constant versus the reciprocal of temperature, as seen in Figure 8. This suggests that one effect of temperature is to improve the reduction of Fe³⁺ to Fe²⁺ : heat Fe³⁺ + H₂0₂ → Fe²⁺ H0₂. + H⁺

It would have been expected that addition of further organic compounds to liquid effluents or groundwaters containing organic contaminants would reduce the rate of oxidation of the organic contaminants by increasing the load of material to be oxidised. The present inventors have found, surprisingly, that when oxidation by hydrogen peroxide and transition metal ions is used to treat liquid effluents or groundwaters containing organic contaminants, addition of promoter compounds, such as phenols or their quinone oxidation products, enhances the rate of oxidation of the contaminants. It is believed that these promoter compounds act by promoting conversion of the oxidised form of the transition metal ions to the reduced form.

Promoter compounds include phenol, catechol, resorcinol, hydroquinone and their quinone oxidation products.

Figures 9, 10 and 11 show the enhanced oxidation of BTX when resorcinol was added as promoter compound. The enhanced oxidation was seen when either Fe²⁺ or Fe³⁺ was initially added as transition metal ion.

Figure 11 shows that Fe²⁺ concentration increased, to a level of 10-12 ppm, as toluene concentration dropped, giving support to the idea that addition of resorcinol facilitates the conversion of Fe³⁺ to Fe²⁺. The process of the invention also gave enhanced oxidation of the contaminants 1,4-dioxane, acetone, chloroform and methylethylketone (MEK) .

As seen in Table 1, treatment with Fenton's reagent at 25°C gave virtually no oxidation of 1,4-dioxane. At the same temperature, when 100 ppm phenol was added as promoter compound to the solution, 1,4-dioxane was reduced quickly to discharge levels, the phenol also being destroyed. The results are shown in Table 1.

TABLE 1

ppm Dioxane with 100 ppm Phenol

96.9 2.9 1.3 0.75

0.49

1,4-dioxane was also destroyed by thermal enhanced oxidation by Fenton's reagent, as seen in Table 2, as was MEK. Thermal enhanced oxidation of 1,4-dioxane was seen at an effective temperature of at least about 60°C and MEK at at least about 55°C.

TABLE 2

The disappearance rate constant of 1,4-Dioxane and methylethylketone (MEK) as a function of temperature with 20 ppm of Fe²⁺ and 500 ppm of H₂0₂. Temperature K(l/hr) c

1,4-Dioxane MEK

30 0.159 0.172 40 0.139 0.499

50 0.309 1.972

60 0.637 5.420

Enhanced oxidation of acetone by Fenton's reagent was also obtained either by heat or by addition of a promoter compound as seen in Table 3. TABLE 3

The rate constant of disappearance of acetone (100 ppm) at various temperatures with and without 100 ppm phenol added. [Fe²⁺] = 20 ppm, [H₂0₂] = 500 ppm

Temp,°C k(l/hr) K(l/hr) with phenol

Thermal enhanced oxidation of acetone was seen at an effective temperature of at least about [ ? ] .

Table 3 shows the observed rate constant, k, for disappearance of acetone as a function of temperature in the presence and absence of 100 ppm of phenol. Similar observations were also made for chloroform. Under similar conditions and at room temperature, the rate of destruction of chloroform increased by a factor of two in the presence of 100 ppm of phenol (k=1.41 1/hr) as promoter compound as compared to that in the absence of phenol (k=0.72 1/hr).

The present invention provides a novel and convenient method for treating liquid effluents or groundwater containing many organic contaminants which were heretofore resistant to treatment by Fenton's reaction, as described above.

Classes of organic contaminants which may be treated by the process of the invention include ethers, aromatics, polyaromatics, nitroaromatics, chlorinated aromatics, phenols and chlorinated phenols, ketones, aldehydes, chlorinated alkanes and alkenes and alcohols. In particular, the organic contaminants may be compounds selected from the group of compounds consisting of an alkyl or alkenyl which may be linear, branched or cyclic preferably having 1 to 20 carbon atoms which may be substituted by one or more of fluorine, chlorine, bromine, nitro, sulfo, carboxyl, hydroxyl or c,-C₁₀-alkoxy, preferably trichloroethane, trichloroethylene, and chloroform, an aromatic or polyaromatic compounds which may be substituted by one or more of alkyl or alkenyl which may be linear or branched and preferably having 1 to 10 carbon atoms, fluorine, chlorine, bromine, nitro, sulfo, carboxyl, hydroxyl or c,-C,₀-alkoxy, preferably benzene, toluene, phenolics which may be substituted by one or more atoms of fluorine, chlorine, bromine, biphenyl which may be substituted by the above-mentioned substitutents for aromatics or polyaromatics, xylene, chlorobenzene, trinitrotoluene, PCBs, naphthalene and anthracene; fused phenols, preferably dioxins; and ethers, preferably dioxane, aliphatic ketones and aldehydes having preferably 1 to 20 carbon atoms and cyclic ketones and aldehydes.

Particularly preferred organic contaminants which may be treated by the process of the invention include benzene, toluene, xylene, ethylbenzene, methyl-t-butyl ether, chlorobenzenes, polyaromatic hydrocarbons (naphthalene, anthracene) , trichloroethane, trichloroethylene, dioxane, ketones, phenols and chlorinated phenols, alcohols, PCBs, chloroform and trinitrotoluene. A suitable apparatus for treatment of liquid effluents or groundwaters by the process of the invention is illustrated in Figure 12.

Other possible arrangements of apparatus will be known to those skilled in the art. In accordance with one embodiment of the invention, the liquid effluent or groundwater is placed in the mixing tank and is heated to the effective temperature before being contacted with hydrogen peroxide and transition metal ions and is maintained at that temperature during their addition and throughout the period of oxidation of the contaminants. The liquid effluent or groundwater is contacted with the hydrogen peroxide and transition metal ions at an acidic pH.

In accordance with a preferred embodiment of the invention, the liquid effluent or groundwater is adjusted to an acidic pH in the range of between 1 and 5. In accordance with an especially preferred embodiment, the pH is adjusted to 3.

Hydrogen peroxide is preferably added to the liquid effluent or groundwater as an aqueous solution. A sufficient amount of hydrogen peroxide is added based on the concentration of the organic contaminants in the liquid effluent or groundwater. It is appreciated that not all of the organic contaminants have to be removed from a liquid effluent or groundwater to provide an environmentally acceptable liquid effluent or groundwater. To determine the amount of H₂0₂ to employ, the total organic content of the liquid effluent or groundwater may be measured by known techniques and the amount of H₂0₂ needed in the method of the invention to remove the desired organic contaminant portion thereof can be readily calculated. Typically for purposes of this invention between 1 and 10 ppm of H₂0₂ are added per 1 ppm of organic contaminants in the liquid effluent or groundwater.

Transition metal ions are added in the form of a suitable salt. Copper, zinc and/or iron compounds are preferably used as sources of transition metal ions, iron compounds being most particularly preferred. Iron compounds such as Fe(0H)₃, Fe₂0₃, FeCl₃, Fe₂(S0₄)₃, FeO,

Fe(OH)₂, FeCl₂, FeC0₃ or FeS0₄ may be used in the process of the present invention. Preferably, FeS0₄.7H₂0 may be used as the source of iron ions.

The concentration of transition metal ions is selected based on the concentration of the organic contaminants in the liquid effluent or groundwater, and desired degree of removal of the organic contaminants. The hydrogen peroxide and transition metal ions should desirably be mixed into the liquid effluent or groundwater as effectively as possible in order to maximize the effectiveness of the hydrogen peroxide and iron ions in the method.

The liquid effluent or groundwater is continued in contact with the hydrogen peroxide and transition metal ions at the effective temperature with mixing until the desired level of contaminant is reached. The order in which the steps of heating, pH adjustment and reagent addition are carried out is not critical and appropriate sequences of steps will be known to those skilled in the art.

In accordance with a further embodiment of the invention, the liquid effluent or groundwater is adjusted to an acidic pH, as described for thermal enhancement, and contacted with hydrogen peroxide, transition metal ions and a promoter compound with effective mixing.

Hydrogen peroxide and transition metal ion additions are as described above for thermal enhancement.

Typically for purposes of this invention, between 10 and 100 ppm of promoter compound is added to the liquid effluent or groundwater.

In accordance with a further embodiment of the invention, enhanced oxidation of organic contaminants by the method of the invention may be used as a pre- treatment, to reduce the level of contaminant partially, the remaining contaminant being reduced to discharge levels by exposure of the liquid effluent or contaminated groundwater to ultra violet light in the range 200 - 400 nm.

In accordance with a further embodiment of the invention, enhanced oxidation of organic contaminants may be combined with recycling of the transition metal used in the oxidation process.

When the desired discharge levels for organic contaminants have been achieved, the liquid effluent or groundwater is adjusted to a pH in the range of about 6 to 9 which precipitates the transition metal present as the hydroxide eg. Fe(OH)₃ if iron is used.

The precipitate is recovered, for example by settling or filtration and acidified to a pH of 0 to 2 to redissolve the transition metal which is re-used in the process of the invention.

The following examples are illustrative only, for the purpose of describing the process of the invention and the invention is not limited thereto.

Example 1

Liquid effluent from a wood treatment process which had a chemical oxygen demand of 4000 mg/L and was contaminated with PCP was obtained.

29L of effluent was placed in the mixing tank of the batch re-circulation system of Figure 12 and pH adjusted to 3 by addition of sulphuric acid. The effluent was heated to 50°C, ferrous sulphate was added to give 100 ppm and then 4000 ppm H₂0₂ was added.

PCP concentration was monitored at desired time intervals using high pressure liquid chromatography (HPLC) .

Further samples of effluent were similarly treated at 60°C and 70°.

The results are shown in Figure 2.

Example 2

A sample of BTX water was obtained from an industrial source. 29L portions were actified to pH 3 and treated, at various temperatures, with ferrous sulphate (20 ppm) and H₂0₂ (350 ppm) , as described in Example 1.

BTX concentrations were monitored by HPLC. The results are shown in Figures 1, 3 and 4. Example 3

29L tap water was placed in the mixing tank of Figure 12, pH was adjusted to 3 by addition of sulphuric acid and the liquid was heated to 50°C. Toluene was added to give a concentration of 100 ppm, the solution being circulated at 60Lpm for 60 minutes to dissolve the toluene.

Fe²⁺ was added to the desired level (levels of 10, 20, 30 and 40 ppm were tested) and H₂0₂ was added to give 150 ppm. Samples were taken at regular time intervals and analysed for H₂0₂, Fe²⁺ and toluene.

H₂0₂ was measured by titration with eerie sulphate and Fe²⁺ was measured colorimetrically by complexing with o-phenanthroline. The results are shown in Figure 3.

The same general procedure was used to follow toluene destruction at various temperatures with 150 ppm H₂0₂ and 20 ppm Fe²⁺, with the results shown in Figure 6, and to examine the effect of pH on the destruction rate of toluene at 50°C and the same H₂0₂ and Fe²⁺ levels, the results being shown in Figure 13.

Example 4

The procedure of example 3 was followed, samples of toluene solution being treated at 50°C and pH 3 with either Fe²⁺ (20 ppm) or with Fe³⁺ (20 ppm) , both added as the sulphate salt. H₂0₂ concentration was 150 ppm. The results are shown in Figure 7.

Fe³⁺ treatment was repeated at 60°C and 70°C and the rate of H₂0₂ decomposition was found at each temperature. The results are shown in Figure 8.

Example 5

Four samples of water spiked with 100 ppm each of benzene, toluene and xylene were treated at 50°C and pH 3, as in Example 2, with the following additions: 1. Fe²⁺/H₂0₂ ( 54 ppm/ 350 ppm)

2 . Fe²⁺/H₂0₂/resorcinol (56/350/20)

3 . Fe²⁺/H₂0₂ ( 58/ 600 )

4 . Fe²⁺/H₂0₂ /resorcinol (58/ 600/30) Samples were taken at regular intervals for toluene analysis. The results are shown in Figure 9.

Example 6

Samples of toluene solution were treated with Fe³⁺/H₂0₂ in the presence or absence of resorcinol as in Example 5 and Fe²⁺ concentration was monitored along with toluene concentration. The results are shown in Figure 10 and 11.

Example 7

Water spiked with 100 ppm 1,4-dioxane was treated at 25°C, pH 3, 20 ppm Fe²⁺ and 500 ppm H₂0₂, in the presence or absence of 100 ppm phenol. Phenol and dioxane levels were measured by gas chromatography in samples taken at regular intervals. The results are shown in Table 1.

Portions of the same spiked solution were similarly treated at various temperatures in the absence of phenol. Results are shown in Table 2.

Water spiked with MEK (100 ppm) was similarly treated at various temperatures and the results are shown in Table 2.

Example 8

Water spiked with 100 ppm acetone was treated at 25°C, pH 3, 20 ppm Fe²⁺ and 500 ppm H₂0₂ in the presence or absence of 100 ppm phenol. Acetone concentrations were measured by gas chromatography at regular time intervals.

The experiment was repeated at 40°C and 50°C. The results are shown in Table 3. The present invention is not limited to the features of the embodiments described herein, but includes all variations and modifications within the scope of the claims.

Claims

1. A method of enhancing the rate of oxidation of organic contaminants in a liquid effluent or groundwater by hydrogen peroxide and ions of at least one transition metal, the method comprising promoting conversion of the transition metal ions from an oxidised to a reduced form.

2. A method in accordance with claim 1 wherein conversion of the transition metal ions from oxidised to reduced form is promoted by heating the liquid effluent or groundwater during oxidation treatment.

3. A method in accordance with claim 2 comprising the steps of:

(a) providing a liquid effluent or groundwater containing organic contaminants;

(c) contacting the liquid effluent or groundwater at an effective temperature with hydrogen peroxide and ions of at least one transition metal, the hydrogen peroxide and metal ions being present in effective amounts based on the initial concentration of the organic contaminants in the liquid effluent or groundwater to destroy the organic contaminants.

4. A method in accordance with claim 3 wherein the pH is adjusted to a pH in the range of about 1 to about 5 and the transition metal ions are iron, copper or zinc ions.

5. A method in accordance with claim 4 wherein the transition metal ions are iron ions supplied by at least one compound selected from the group consisting of FeS0₄.7H₂0, Fe(OH)₃, Fe₂0_3/ FeCl₃, Fe₂(S0₄)₃, Fe(OH)₂, FeCl₂, FeC0₃ and FeS0₄.

6. A method in accordance with claim 5 wherein the organic contaminant is pentachlorophenol and the effective temperature is at least about 60°C.

7. A method in accordance with claim 5 wherein the organic contaminant is one or more compounds selected from the group consisting of benzene, ethylbenzene, toluene, xylene and methyl-t-butyl ether and the effective temperature is at least about 40°C.

8. A method in accordance with claim 5 wherein the organic contaminant is 1,4-dioxane and the effective temperature is at least about 60°C.

9. A method in accordance with claim 5 wherein the organic contaminant is methylethylketone and the effective temperature is at least about 55°C.

10. A method in accordance with claim 1 wherein conversion of the transition metal ions from oxidised to reduced form is promoted by addition of a promoter compound to the oxidation reaction.

11. A method in accordance with claim 10 comprising the steps of:

(a) providing a liquid effluent or groundwater containing organic contaminants;

(b) adjusting the pH of the liquid effluent or groundwater to an acidic pH if not already acidic; (c) contacting the liquid effluent or groundwater with hydrogen peroxide, ions of at least one transition metal and a promoter compound, the hydrogen peroxide, metal ions and promoter compound being present in effective amounts based on the initial concentration of the organic contaminants in the liquid effluent or groundwater to destroy the organic contaminants.

12. A method in accordance with claim 11 wherein the promoter compound is a phenolic compound or a quinone.

13. A method in accordance with claim 12 wherein the promoter compound is selected from the group consisting of phenol, catechol, resorcinol, naphthol, hydroquinone, p-benzoquinone, 1,4-naphthaquinone, anthraquinone, o-quinone and -quinone.

14. A method in accordance with claim 13 wherein the pH is adjusted to a pH in the range of about 1 to about 5 and the transition metal ions are iron, copper or zinc ions.

15. A method in accordance with claim 14 wherein the transition metal ions are iron ions supplied by at least one compound selected from the group consisting of FeS0₄.7H₂0, Fe(OH)_3/ Fe₂0₃, FeCl₃, Fe₂(S0₄)₃, Fe(0H)₂, FeCl₂, FeC0₃ and FeS0₄.

16. A method in accordance with claim 3 wherein step (b) is continued only until partial destruction of the contaminants is obtained and further comprising after step (b) the step of irradiating the liquid effluent or groundwater with ultra violet light of wavelength in the range of about 200 to 400 nm to complete destruction of the contaminants.

17. A method in accordance with claim 11 wherein step (b) is continued only until partial destruction of the contaminants is obtained and further comprising after step (b) the step of irradiating the liquid effluent or groundwater with ultra violet light of wavelength in the range of about 200 to 400 nm to complete destruction of the contaminants.

18. A method in accordance with claim 3 further comprising after step (b) the step of recycling the transition metal.

19. A method in accordance with claim 11 further comprising after step (b) the step of recycling the transition metal.

20. A method in accordance with claim 18 or 19 wherein the recycling step comprises the following steps:

(i) adjusting the pH of the liquid effluent or groundwater to a pH in the range of about 6 to 9 to precipitate the transition metal as a hydroxide;

(ii) recovering the precipitated metal hydroxide from the liquid effluent or groundwater;

(iii) dissolving the metal hydroxide in acid solution at a pH in the range of about 0 to 3 and employing the resulting solution as a source of transition metal ion for step (c) of the process.

21. A method in accordance with claim 1 wherein the organic contaminants are one or more compounds selected from the group consisting of an alkyl or alkenyl which may be linear, branched or cyclic having 1 to 20 carbon atoms which may be substituted by one or more of fluorine, chlorine, bromine, nitro, sulfo, carboxyl, hydroxyl or C-- C₁₀-alkoxy; or an aromatic or polyaromatic compound which may be substituted by one or more of alkyl or alkenyl which may be linear or branched having 1 to 10 carbon atoms, fluorine, chlorine, bromine, nitro, sulfo, carboxyl, hydroxyl or C,- C,₀-alkoxy, or phenolics which may be substituted by one or more atoms of fluorine, chlorine or bromine.

22. A method in accordance with claim 1 wherein the organic contaminants are one or more compounds selected from the group consisting of benzene, toluene, xylene, ethylbenzene, methyl-t-butyl ether, naphthalene, anthracene, chlorobenzene, trinitrotoluene, trichloroethane, trichloroethylene dioxane, methylethylketone and pentachlorophenol.