WO2010000798A1

WO2010000798A1 - Waste water treatment

Info

Publication number: WO2010000798A1
Application number: PCT/EP2009/058321
Authority: WO
Inventors: Johannes Wietse De Boer; Stephen John Flowers; Ronald Hage; Tristan Conway Soh; Alan William Wakeling
Original assignee: Hindustan Unilever Ltd; Unilever NV
Current assignee: Hindustan Unilever Ltd; Unilever NV
Priority date: 2008-07-04
Filing date: 2009-07-02
Publication date: 2010-01-07
Anticipated expiration: 2011-01-04
Also published as: AR072687A1

Abstract

The present invention concerns the treatment of industrial waste with transition metal catalysts, hydrogen peroxide and a reactive radical.

Description

WASTE WATER TREATMENT

FIELD OF INVENTION

The present invention relates to the catalytic treatment of waste water with hydrogen peroxide.

BACKGROUND OF INVENTION

Disposal and treatment of industrial aqueous waste is significant. Problems encountered in the industry include excessive coloured waste streams, waste streams with high Biological Oxygen Demand (BOD) or Chemical Oxygen Demand (COD) , chlorinated phenolic materials, low levels of pharmaceutical intermediates or end products or a variety of other organic materials that may give bio-accumulation or problems with aquatic environments. Various industries may have concerns with the dislodging of the waste streams, such as textile/dyeing industries, pulp/paper industries, food producers, chemical industries, or pharmaceutical industries. A variety of methods exists to deal with these issues, including Advanced Oxidation Processes (AOP) . Different AOP processes include ozone/hydrogen peroxide, UV/H2O2, electron beams, TiO2/H2O2, sonication/ultrasonic devices, and Fenton's processes.

One of the systems applied is the Fenton reactors (based on Fe salts and hydrogen peroxide giving rise to hydroxyl radicals that are extremely powerful to destroy any organic matter) . Disadvantages include low selectivity, and therefore high levels of hydrogen peroxide and iron salts are needed to furnish complete breakdown of the organic matter. This leads then to high chemical costs. UV/H2O2 also generates hydroxyl radicals efficiently and this method is also widely applied. Often large molecules can be then degraded into smaller, biodegradable molecules. Using this technique in conjunction with a biological treatment unit, can be very advantageous. A disadvantage may be in the high electricity costs when a full degradation of the organic waste molecules need to be obtained. Also TiO2/H2O2/UV is sometimes employed to destroy organic matter. Further, high- energy E-beam reactors can be used to generate selected radicals, such as hydroxyl radicals.

Also degradation by ozone is being applied, but that leads to significant costs for equipment and electricity to generate ozone. Ozone dissociates in water to generate hydroxyl radicals. Whilst ozone itself is also reactive, often the benefits are seen upon hydroxyl radical formation. For example direct oxidation of ethers by ozone is slow, the hydroxyl radicals react diffusion limited with substrates. Often mixtures of ozone and hydrogen peroxide are employed to generate hydroxyl radicals efficiently.

Also various methods are known in the art to remove organic matters by physical methods, such as flocculation, after which the material can be separated from the bulk aqueous layer. Biological treatment processes are commonly used, although not all chemicals can be removed that way. EP 1078033 and Science, 296, 326 (2002) disclose the use of an anionic iron-based transition metal catalyst for treating waste water, particularly so in the area of waste streams from the paper/pulp industry where bleaching processes with chlorine-based oxidants are common practice.

WO98/39098 discloses that X-bridged macrocycles may be used for the oxidative destruction of waste materials or effluents .

The macrocyclic triazacyclic molecules have been known for several decades, and their complexation chemistry with a large variety of metal ions has been studied thoroughly. The azacyclic molecules often lead to complexes with enhanced thermodynamic and kinetic stability with respect to metal ion dissociation, compared to their open-chain analogues.

EP 0458397 discloses the use manganese 1, 4, 7-Trimethyl-

1 , 4 , 7-triazacyclononane (Me₃-TACN) complexes as bleaching and oxidation catalysts and use for paper/pulp bleaching and textile bleaching processes. 1, 4, 7-Trimethyl-l, 4, 7- triazacyclononane (Me₃-TACN) has been used in dishwashing for automatic dishwashers, SUN™, and has also been used in a laundry detergent composition, OMO Power™. The ligand (Me₃- TACN) is used in the form of its manganese transition metal complex, the complex having a counter ion that prevents deliquescence of the complex.

United States Application 2001/0025695A1, Patt et al, discloses the use of PF₆ ^~ salts of 1, 2, -bis- (4, 7, -dimethyl- 1, 4, 7, -triazacyclonon-1-yl) -ethane and Me₃-TACN (Me4-DTNE) . -A-

United States Application 2002/010120 discloses the bleaching of substrates in an aqueous medium, the aqueous medium comprising a transition metal catalyst and hydrogen peroxide .

WO 2006/125517 discloses a method of catalytically treating a cellulose or starch substrate with a Mn(III) or Mn(IV) preformed transition metal catalyst salt and hydrogen peroxide in an aqueous solution. The preformed transition metal catalyst salt is described as having a non- coordinating counter ion and having a water solubility of at least 30 g/1 at 20 ⁰C. Exemplified ligands of the catalysts described in WO 2006/125517 are 1, 4, 7-Trimethyl-l, 4, 7- triazacyclononane (Me₃~TACN) and 1, 2, -bis- (4, 7, -dimethyl- 1, 4, 7, -triazacyclonon-1-yl) -ethane (Me₄-DTNE).

EP 0733594 discloses the use of a transition metal catalyst for removing noxious compounds in water or gas.

SUMMARY OF INVENTION

We have found that the use hydroxyl radicals together with a transition metal catalyst and hydrogen peroxide are effective in the aggressive treatment of aqueous industrial waste.

In one aspect the present invention provides a method of treating aqueous industrial waste comprising the following steps : (i) storing the aqueous industrial waste in a holding vessel, the holding vessel a batch or a continuous vessel; (ii) adding to the aqueous industrial waste in the holding vessel a transition metal catalyst of a tridentate, tetradentate, pentadentate or hexadentate nitrogen donor ligand or precursor thereof and hydrogen peroxide to provide a concentration of the transition metal catalyst in a concentration from 0.1 to 100 micromolar and a concentration of the hydrogen peroxide from 1 to 1500 mM; and, (iii) releasing the effluent after treatment for further processing or directly into the environment, wherein the aqueous industrial waste is subjected to a hydroxylation step during or prior to step (ii) .

Most preferably the method is other than that of a cleaning or bleaching process. The method is other than that applied in a laundry washing machine or dishwasher. In this regard, the term industrial waste is other than that of laundry or dish water. That is to say, most preferably the process is other than that applied by default to an effluent stream arising from treating a solid or particulate substrate with a preformed transition metal catalyst or ligand thereof together with hydrogen peroxide.

DETAILED DESCRIPTION OF INVENTION

HYDROXYLATION STEP

Hydroxylation is any chemical process that introduces one or more hydroxyl groups (-OH) into a compound (or radical) thereby oxidizing it. In biochemistry, hydroxylation reactions are often facilitated by enzymes called hydroxylases . The hydroxylation step is most preferably provided in an aqueous environment.

The hydroxylation step may be provided by enzymatic, chemical reagents or electrochemical means.

Examples of chemical oxidisers that may also provide hydroxylation processes are permanganate, ammonium cerium (IV) nitrate, chromic acid, dichromic acid, chromyl chloride, chromium trioxide, pyridinium chlorochromate and chromate salts, dichromate salts, hypohalite, chlorate, chlorine dioxide, perchlorate, osmium tetraoxide, potassium osmate dihydrate (K₂Osθ2 (OH) ₄) , ruthenium tetroxide, nitric acid, alkyl peroxide (particularly in combination with transition metal complexes), Co(OAc)₂, Pb(OAc)₄, SeO₂.

The hydroxylation step is most preferably provided by hydroxyl radicals.

SOURCE OF HYDROXYL RADICALS

There are many ways of providing hydroxyl radicals to the aqueous environment.

In a preferred aspect, the treatment of the aqueous industrial waste with hydroxyl radicals is stopped before treatment with the transition metal catalyst. This is to reduce degradation of the transition metal catalyst by hydroxyl radical attack. In this regard, the aqueous industrial waste is subjected to a hydroxylation step prior to step (ii) and not during step (ii) . A carbonate buffer may also be used reduce degradation of the transition metal catalyst. In this regard the transition metal catalyst is added to the aqueous industrial waste in a carbonate buffer to trap hydroxyl radicals. This mode is particularly advantageous where the hydroxyl radicals are generated chemically rather than by physical processes such as UV irradiation and electrolysis which may simply be turned off.

UV-irradiation

Using UV-irradation, hydrogen peroxide is homolytically split into hydroxyl radicals. As hydrogen peroxide absorbs UV-light only at wavelengths less than 300 nm, only lamps generating UV-light between 200 and 300 nm can be used. A wavelength of typically 254 nm is used. A typically UV-light exposure dose is 1-10 J/cm², although much higher values can be obtained in large-scale commercial applications. For example a large reactor may contain twelve 20 kW UV lamps. A typical residence time of a flow system within the cell containing the UV-lamp is less than 1 minute. Hydrogen peroxide can be applied via a single-dose unit or at various points into the system. A disadvantage of such processes may be that turbidity of the solution diminishes the efficiency of these systems. Leading suppliers for UV disinfection or units include, but are not limited to, Calgon Carbon Inc., US Filter, Berson-UV, Wedeco, and Trojan Technologies. See for example Photochemical Purification of Water and Air, T. Oppenlaender, Wiley, VCH, 2003.

Most preferably, UV is provided to the aqueous environment via a UV transmissive -¹. ^Λ v >Λ to irradiate the aqueous environment. The UV transmissive \::.α^ may be a finger insert into which a UV source is inserted or a window which forms part of the vessel in which the aqueous environment is travelling or is held. Preferably the UV transmissive V¹^S ^, is a quartz window.

Ozone

Alternatively, one may use UV-irradiation to enhance the formation of hydroxyl radicals using ozone or ozone in the presence of hydrogen peroxide.

Ozone is generated using compressed dried air or oxygen. Usually 0.5-2.0% of ozone is generated this way. If air is used, then complete dehumidification is needed, which makes the process more costly. Therefore often pure oxygen is used to generate ozone. Ozone gas is then transferred into the liquid phase using spargers, porous pipes or plates, or Venturi-type injection at dosages of about 1-2 mg/1 ozone per mg/1 dissolved COD.

See for example J. -W. Kang, An Advanced Monitoring and

Control System for Optimization of the Ozone-AOP (Advanced Oxidation Process) for the Treatment of Drinking Water, part 3, 2007, Spinger.

E-beam reactors generate high-energy electrons that irradiate contaminated water. The electrons are generated by a hot cathode and are accelerated by a voltage differential. In water these electrons react with water to form reactive species, such as hydroxyl radicals, hydrated electrons, and hydrogen atoms. The high-energy electrons do not penetrate the water for more than one centimeter. An advantage of this method is that when compounds react relatively slowly with hydroxyl radicals (like acetone) ; the reaction with the hydrated electrons may be much higher, leading to efficient removal of such substrate. See for example M. B. Ray, Advanced Physicochemical Treatment Processes, Humana Press, 2006.

Electrode

Electrolytic treatment of wastewater is known in the art; see for example: US 4,014,766; US 4,399,020; US 4,308,122; US 4,839,007; and US 5,160,417.

Anodic oxidation of wastewater leads, however, to problems concerning corrosion of the electrode materials. Typically electrodes include platinum, ruthenium dioxide, lead dioxide, tin dioxide. Other frequently-employed electrodes are titanium coated with mixed metal oxides such as ruthenium and iridium, especially suitable for ammonia removal. <.. ; x ,, fouling of the electrode may be a problem during electrolytic oxidation of substrates (like phenols) . A considerable improvement has been made by using crystalline diamond electrodes that have been doped with boron to increase electrically conductivity. The conductivity should be 100 ohm cm or less, whilst the optimal current level is between 50 and 600 mA/cm². It has been shown that the rate and extent of COD removal is much higher than using a conventional Pt electrode, see US 5,399,247 for a full description and examples. Also diamond electrodes using silicon carbide as a conducting material are known in the art. As the charge on the electrode can be very high (more than 12 V, as shown in P. Canizares, et al . , J. Chem. Techn. Biotechn., 81, 1331 (2006)), different oxidation processes take place parallel. First there is the direct anodic oxidation. Further, the large electrochemical window ensures formation of hydroxyl radicals in the system, thereby yielding efficient waste degradation.

It is preferred that the electrode used is a boron-doped diamond electrode. Boron-doped diamond electrodes work particularly well, due to good conductivity properties, good stability and efficiency. BDD electrodes are made of polycrystalline diamond formed by Chemical Vapour Deposition

(CVD) in a high temperature microwave process. In the tetrahedral diamond lattice, each carbon atom is covalently bonded to its neighbours forming an extremely robust crystalline structure. Some carbon atoms in the lattice are substituted with boron to provide electrical conductivity. Boron acts as an electron acceptor due to its electron deficiency in its outer shell. Most suitable levels of B in the diamond lattice are in the range 10¹⁹ to 10²¹ atoms/cm³ (up to 8000 ppm) .

It is preferred that the current per surface area is at least 500 A/m², more preferabbly at least 1000 A/m² and most preferably at least 2000 A/m².

It is preferred that the voltage applied to the electrode is at least 5 V, more preferably at least 10 V and most preferably at least 15 V. In order to provide a high rate of degradation of waste material, it is particularly useful to apply a plurality of electrodes parallel in a vertical set-up. Most preferably up to 20 electrodes are applied. These electrodes are typically spaced by 5 mm each. The effluent is mixed continuously, so there is a clear solvent flow near the electrodes. This mixing is most preferably achieved using a recirculation pump. A typical surface area per electrode will be between 0.005 and 0.05 m².

Recirculation is preferably in the range from 5 to 30 m³ per hour .

Fenton reactor Fenton chemistry may also used to generate hydroxyl radicals. A typical Fenton reactor contains 50-500 mM hydrogen peroxide and an iron level of 5-100 mM. The pH of the system is between 2 and 5. Fe salts react with hydrogen peroxide to generate hydroxyl radicals. Also sacrificial steel electrodes can be employed, wherein the electrodes slowly release the ferric ions. Often this system is also employed in combination with UV-irradiation to enhance the flux of hydroxyl radicals. The temperature of the Fenton reactor may be between ambient conditions and more than 100⁰C, when a pressured vessel is used, see for example M.J. Fesse et al . , J. Hazardous Materials, 147, 167 (2007) for a description of the Fenton processes without and with UV- irradiation .

The Fenton derived hydroxyl radicals are preferably provided when the pH of the aqueous environment is in the range 2 to 5. It is then preferred that the pH of the aqueous environment pH is then increased to 7.5 to 11.5; aqueous sodium hydroxide is particularly suitable to provide this pH change .

Pulse Radiolysis and Sonolysis

Pulse radiolysis or sonolysis may also used to generate hydroxyl radicals and other reactive species. In pulse radiolysis apart from hydroxyl radicals also H atoms and hydrated electrons are formed. See M. G. Gonzales, et al . ,

Journal of Photochemistry and Photobiology C: Photochemistry Reviews, 5, 225 (2004) as an example for pulse radiolysis and Vinodgopal, K.; Peller, Julie, Research on Chemical Intermediates, Volume 29, Number 3, 2003 , pp. 307-316(10) (VSP publisher) .

TRANSITION METAL CATALYST

The preformed transition metal catalyst or precursor thereof is formed from or provided by a tridentate, tetradentate, pentadentate or hexadentate nitrogen donor ligand.

The transition metal catalyst is preferably provided as a preformed transition metal catalyst. When provided as a precursor the ligand is added to sequester adventitious transition metals or transition metals salts are added. Where one particular transition metal is preferred to be part of the complex then the addition of a particular transition metal salt, with respect to the transition metal, is preferably employed. The tridentate, tetradentate, pentadentate or hexadentate nitrogen donor ligand may be built up within any organic structure which will support coordinating nitrogen atoms. For example one can take a basic tridentate ligand such as 1 , 4 , 7-triazacyclononane and have further nitrogen coordination groups, e.g., -CH2-CH2-NH2, -CH2-Py, covalently bound to one or more of the cyclic nitrogens or aliphatic groups .

Preferably the iron ion is selected from Fe(II) and Fe(III) and the manganese ion is selected from Mn(II), Mn(III), and Mn (IV) .

Preferably the ligand is present in one or more of the forms [MnLCl₂]; [FeLCl₂]; [FeLCl]Cl; [FeL(H₂O)] (PFg)₂; [FeL]Cl₂,

[FeLCl]PF₆ and [FeL (H₂O) ] (BF₄) ₂. However water soluble counter ions conferring increasing solubility, say over PF₆, are also preferred.

The following are preferred classes of catalyst that are iron or manganese complexes of tetradentate, pentadentate or hexadentate nitrogen donor ligands.

If unspecified the length of any alkyl chain is preferably Cl to C8-alkyl chain and preferably linear. If unspecified the aryl group is a phenyl group.

BISPIDON

The bispidon class are preferably in the form of an iron transition metal catalyst. The bispidon ligand is preferably of the form:

wherein each R is independently selected from: hydrogen, F, Cl, Br, hydroxyl, Cl-C4-alkylO-, -NH-CO-H, -NH-CO-C1-C4- alkyl, -NH2, -NH-Cl-C4-alkyl, and Cl-C4-alkyl; Rl and R2 are independently selected from: Cl-C24-alkyl, C6-C10-aryl, and, a group containing a heteroatom capable of coordinating to a transition metal;

R3 and R4 are independently selected from hydrogen, C1-C8 alkyl, Cl-C8-alkyl-O-Cl-C8-alkyl, Cl-C8-alkyl-O-C6-C10-aryl, C6-C10-aryl, Cl-C8-hydroxyalkyl, and - (CH2) _nC (0) 0R5 wherein R5 is independently selected from: hydrogen, C1-C4- alkyl, n is from 0 to 4, and mixtures thereof; and, X is selected from C=O, -[C(R6)2]y~ wherein Y is from 0 to 3 each R6 is independently selected from hydrogen, hydroxyl, Cl-C4-alkoxy and Cl-C4-alkyl.

Preferably R3 = R4 and selected from -C(O)-O-CH3, -C(O)-O- CH2CH3, -C (0) -O-CH2C6H5 and CH2OH. Preferably the heteroatom capable of coordinating to a transition metal is pyridin-2-ylmethyl optionally substituted by -C0-C4-alkyl .

Preferably X is C=O or C (OH) 2.

Preferred groups for Rl and R2 are CH3, -C2H5, -C3H7, benzyl, -C4H9, -C6H13, -C8H17, -C12H25, and -C18H37 and pyridin-2-yl . A preferred class of bispidon is one in which at least one of Rl or R2 is pyridin-2-ylmethyl or benzyl, preferably pyridin-2-ylmethyl.

A preferred bispidon is dimethyl 2, 4-di- (2-pyridyl) -3- methyl-7- (pyridin-2-ylmethyl) -3,7-diaza-bicyclo[3.3.1]nonan- 9-one-l, 5-dicarboxylate (N2py3o-Cl) and the iron complex thereof FeN2py3o-Cl which was prepared as described in W002/48301. Other preferred bispidons are one in which instead of having a methyl group (Cl) at the 3 position have longer alkyl chains, namely isobutyl, (n-hexyl) C6, (n- octyl) C8, (n-dodecyl) C12, (n-tetradecyl) C14, (n- octadecyl) C18, which were prepared in an analogous manner.

Preferred tetradentate bispidons are also illustrated in WO00/60045 and preferred pentadentate bispidons are illustrated in WO02/48301 and WO03/104379.

N4py type

The N4py are preferably in the form of an iron transition metal catalyst.

The N4py type ligands are preferably of the form:

wherein each R¹ , R² independently represents -R⁴-R⁵,

R³ represents hydrogen, optionally substituted alkyl, aryl or arylalkyl, or -R⁴-R⁵, each R⁴ independently represents a single bond or optionally substituted alkylene, alkenylene, oxyalkylene, aminoalkylene, alkylene ether, carboxylic ester or carboxylic amide, and each R⁵ independently represents an optionally N- substituted aminoalkyl group or an optionally substituted heteroaryl group selected from pyridinyl, pyrazinyl, pyrazolyl, pyrrolyl, imidazolyl, benzimidazolyl, pyrimidinyl, triazolyl and thiazolyl.

Preferably R¹ represents pyridin-2-yl or R² represents pyridin-2-yl-methyl . Preferably R² or R¹ represents 2-amino- ethyl, 2- (N- (m) ethyl) amino-ethyl or 2- (N, N-di (m) ethyl) amino- ethyl . If substituted, R⁵ preferably represents 3-methyl pyridin-2-yl. R³ preferably represents hydrogen, benzyl or methyl .

The preferred ligands are N4Py (i.e. N, N-bis (pyridin-2-yl- methyl) -bis (pyridin-2-yl) methylamine) which is disclosed in WO95/34628 and MeN4py (i.e. N, N-bis (pyridin-2-yl-methyl-l, 1- bis (pyridin-2-yl) - 1- aminoethane, as disclosed in EP0909809.

TACN-Nx The TACN-Nx are preferably in the form of an iron transition metal catalyst.

The ligands possess the basic 1, 4, 7-triazacyclononane structure but have one or more pendent nitrogen groups that complex with the transition metal to provide a tetradentate, pentadentate or hexadentate ligand. Preferably, the basic 1, 4, 7-triazacyclononane structure has two pendent nitrogen groups that complex with the transition metal (TACN-N2) . The TACN-Nx is preferably of the form:

R20

(III) wherein each R20 is selected from: an alkyl, cycloalkyl, heterocycloalkyl, heteroaryl, aryl and arylalkyl groups optionally substituted with a substituent selected from hydroxy, alkoxy, phenoxy, carboxylate, carboxamide, carboxylic ester, sulphonate, amine, alkylamine and N⁺(R2i; , wherein R21 is selected from hydrogen, alkanyl, alkenyl, arylalkanyl, arylalkenyl, oxyalkanyl, oxyalkenyl, aminoalkanyl, aminoalkenyl, alkanyl ether, alkenyl ether, and -CY2-R22, in which Y is independently selected from H, CH3, C2H5, C3H7 and R22 is independently selected from an optionally substituted heteroaryl group selected from pyridinyl, pyrazinyl, pyrazolyl, pyrrolyl, imidazolyl, benzimidazolyl, pyrimidinyl, triazolyl and thiazolyl; and wherein at least one of R20 is a -CY2-R22.

Preferably R22 is selected from optionally substituted pyridin-2-yl, imidazol-4-yl, pyrazol-1-yl, quinolin-2-yl groups. Most preferably R22 is either a pyridin-2-yl or a quinolin-2-yl .

CYCLAM AND CROSS BRIDGED LIGANDS

The cyclam and cross bridged ligands are preferably in the form of a manganese transition metal catalyst.

The cyclam ligand is preferably of the form:

wherein : Q i s independently selected from : R ? N [ CR₁R₂CR₃R₄ ) _{and N} [ CR₁R₂CR₃R₄CR₅R₆ ] . p i s 4 ;

R is independently selected from: hydrogen, Cl-C6-alkyl, CH2CH2OH, pyridin-2-ylmethyl, and CH2COOH, or one of R is linked to the N of another Q via an ethylene bridge; Rl, R2 , R3, R4, R5 and R6 are independently selected from: H, Cl-C4-alkyl, and Cl-C4-alkylhydroxy . Preferred non-cross-bridged ligands are 1,4,8,11- tetraazacyclotetradecane (cyclam) , 1, 4, 8, 11-tetramethyl- 1, 4, 8, 11-tetraazacyclotetradecane (Me4cyclam) , 1,4,7,10- tetraazacyclododecane (cyclen) , 1, 4, 7, 10-tetramethyl- 1, 4, 7, 10-tetraazacyclododecane (Me4cyclen) , and 1,4,7,10- tetrakis (pyridine-2ylmethyl) -1,4,7, 10-tetraazacyclododecane (Py4cyclen) . With Py4cyclen the iron complex is preferred.

A preferred cross-bridged ligand is of the form:

wherein "R¹" is independently selected from H, and linear or branched, substituted or unsubstituted Cl to C20 alkyl, alkylaryl, alkenyl or alkynyl; and all nitrogen atoms in the macropolycyclic rings are coordinated with the transition metal .

Preferably Rl = Me, which is the ligand 5, 12-dimethyl- 1, 5, 8, 12-tetraaza-bicyclo [ 6.6.2] hexadecane of which the complex [Mn (Bcyclam) Cl₂] may be synthesised according to WO98/39098. Other suitable crossed bridged ligands are also found in WO98/39098.

TRIDENTATE LIGANDS WITH MANGANESE A suitable class of tridentate ligands is based on terpyridine-type ligands, depicted below.

Also terpyridine derivatives could be employed, such as bispyridylpyrimidine or bispyridyltriazine . Preferred classes include the ones disclosed in WO2002088289; WO2004007657; WO2004039933 ; WO2004039934 ; WO2005068075 ; and WO2005068074; WO2005105303.

TETRADENTATE LIGANDS WITH MANGANESE AND IRON Another suitable class of molecules contains a substituted bipyridine-alkylamine-alkylpyridine unit, depicted below with m, n =1 or 2 as disclosed in WO07/090461.

TRISPICEN-type

The trispicens are preferably in the form of an iron transition metal catalyst.

The trispicen type ligands are preferably of the form: R17R17N-X-NR17R17 (VI),

wherein : X is selected from -CH₂CH₂-, -CH₂CH₂CH₂-, -CH₂C(OH)HCH₂-; and,

R17 independently represents a group selected from: R17 and alkyl, cycloalkyl, heterocycloalkyl, heteroaryl, aryl and arylalkyl groups optionally substituted with a substituent selected from hydroxy, alkoxy, phenoxy, carboxylate, carboxamide, carboxylic ester, sulphonate, amine, alkylamine and N⁺(R19)3 , wherein R19 is selected from hydrogen, alkanyl, alkenyl, arylalkanyl, arylalkenyl, oxyalkanyl, oxyalkenyl, aminoalkanyl, aminoalkenyl, alkanyl ether, alkenyl ether, and -CY₂-R18, in which Y is independently selected from H, CH3, C2H5, C3H7 and R18 is independently selected from an optionally substituted heteroaryl group selected from pyridinyl, pyrazinyl, pyrazolyl, pyrrolyl, imidazolyl, benzimidazolyl, pyrimidinyl, triazolyl and thiazolyl; and wherein at least two of R17 are -CY₂-R18.

The heteroatom donor group is preferably pyridinyl optionally substituted by -C0-C4-alkyl .

Other preferred heteroatom donor groups are imidazol-2-yl, l-methyl-imidazol-2-yl, 4-methyl-imidazol-2-yl, imidazol-4- yl, 2-methyl-imidazol-4-yl, l-methyl-imidazol-4-yl, benzimidazol-2-yl and l-methyl-benzimidazol-2-yl .

Preferably three of R17 are CY₂-R18. The ligand Tpen (i.e. N, N, N', N' -tetra (pyridin-2-yl- methyl) ethylenediamine) is disclosed in WO97/48787.

Other suitable, and preferred, trispicens are found in WO02/077145 and WO08/003652.

COLLIN' S CATALYST

A class of oxidatively stable iron (III) complexes with dianionic tetradentate nitrogen donor ligands have been disclosed by Collins and co-workers. References are made to WO1999058634; WO A 9803625 1996; Ace. Chem. Res. 2002, 35, 782; J. Am. Chem. Soc. 1990, 112, 5637; J. Am. Chem. Soc. 1989, 111, 4511; J. Am. Chem. Soc. 1990, 112, 899; Inorg Chem. 1992, 31, 1550; J. Am. Chem. Soc. 1998, 120, 11540; J. Am. Chem. Soc. 1998, 120, 4867; J. Am. Chem. Soc. 2003, 125, 12379; J. Am. Chem. Soc. 2005, 127, 2505.

ME3-TACN AND RELATED COMPOUNDS

A more preferred transition metal catalyst for the method is as described in EP 0458397 and WO06/125517; both of these patents disclose the use of manganese 1, 4, 7-Trimethyl-l, 4, 7- triazacyclononane (Me3-TACN) as related compounds as complexes. The PF₆ ^~ ligand of Me3-TACN has been commercialised in laundry detergent powders and dish wash tablets. It is preferred that a preformed transition metal of Me3-TACN and related compounds is in the form of a salt such that it has a water solubility of at least 50 g/1 at 20 ⁰C. Preferred salts are those of chloride, acetate, sulphate, and nitrate. Most preferred are the acetate and sulphate salts. The catalyst is most preferably a mononuclear or dinuclear complex of a Mn H-V transition metal catalyst, the ligand of the transition metal catalyst of formula (I) :

R N [CR₁R₂CR₃R₄; wherein: Q = p is 3;

R is independently selected from: hydrogen, Cl-C6-alkyl,

C2OH, ClCOOH, and pyridin-2-ylmethyl or one of R is linked to the N of another Q via an ethylene bridge;

Rl, R2 , R3, and R4 are independently selected from: H, Cl-

C4-alkyl, and Cl-C4-alkylhydroxy .

R is preferably independently selected from: hydrogen, CH3, C2H5, CH2CH2OH and CH2COOH.

R, Rl, R2, R3, and R4 are preferably independently selected from: H and Me.

1, 4, 7-Trimethyl-l, 4, 7-triazacyclononane (Me3-TACN) and 1,2,- bis- (4 , 7 , -dimethyl-1 , 4 , 7 , -triazacyclonon-1-yl) -ethane (Me4- DTNE) are most preferred.

When a preformed transition metal of Me3-TACN is used it is preferred that it is used in a carbonate buffer. STORAGE OF THE AQUEOUS INDUSTRIAL WASTE DURING TREATMENT

The aqueous industrial waste is stored during treatment as a batch or a continuous process for treating waste water. In a batch process, material is placed in the vessel at the start and removed at the end of the process. In a continuous process, material flows into and out of the process during the duration of the process. In this regard the holding vessel may be a conduit or discreet vessel.

Often biological treatment units are in use to treat most of the waste streams. The catalytic degradation solution may be used in combination with such a biological treatment unit. The vessel may be located before or after a biological treating system. When located before the biological treatment system, one can destroy most of the undesired molecules from the effluent in a more concentrated form, i.e. just after the process where it has been formed (e.g. dyeing bath or chemical reactor) . This will lead to an increased selectivity of destruction, as there are less or no components other than the component that needs to be degraded. Further, the volumes to treat the solutions are much lower, which leads to a reduced chemical (catalyst and hydrogen peroxide) demand. Another advantage of carrying out the reaction before the biological treatment unit is that there is a reduced chance of destroying hydrogen peroxide by catalase enzymes, which may be found in biological treatment systems. Optionally to the with catalyst/hydrogen peroxide treated waste stream, catalase enzyme may be added to destroy hydrogen peroxide before it enters the biological treatment unit (excessive hydrogen peroxide may lead to reduced biological activity) . The catalytic oxidation treatment unit may be placed after the biological treatment tank. An advantage can be that most of the undesired waste products are degraded by the biological treatment unit and therefore less of the catalyst and hydrogen peroxide is needed to remove the last traces of the undesired components .

The extent of degradation of the pollutant by the catalyst and hydrogen peroxide depends on many factors, including time, pH of the reaction, optionally addition of sequestrant, temperature of the reaction, type of pollutant, level of pollutant, level of other organic materials, level of transition-metal ions, like Fe or Cu, which could give hydrogen peroxide decomposition, and presence of catalase enzyme, which gives hydrogen peroxide decomposition. For each application these optimal conditions need to be assessed.

The solvent is not necessarily limited to water; also organic solvents may be employed to degrade the pollutant, such as methanol, ethanol, or acetone. This could be the case where industrial processes to produce fine or bulk chemicals. In the case where an organic solvent is used, it is desired to use the same solvent (s) for the oxidative degradation process as used in the industrial processes. After the treatment with the catalyst and hydrogen peroxide, the organic solvent is preferably removed. ADDITION OF CATALYST AND HYDROGEN PEROXIDE

The preformed transition metal catalyst or precursor thereof may be added in one batch, multiple additions, or as a continuous flow. The use of a continuous flow is particularly applicable to continuous processes.

The hydrogen peroxide may also be generated electrochemically or using oxidase enzymes, including glucose oxidase, methanol oxidase and the like.

MONITORING OF TREATMENT

The aqueous industrial waste is preferably monitored during treatment by UV or UV-visible spectroscopy. The wavelength of monitoring depends upon the nature of the waste. When coloured dyes or lignin residues are being treated it is preferred that monitoring is conducted with UV-visible spectroscopy. The absorption of the aqueous industrial waste is preferably linked to a set threshold value that when met results in an automated step taking place. This step may be step (iii) or the addition of further actives to the aqueous industrial waste. Also the monitoring device may include HPLC or other chromatographic methods to analyse the degradation of the undesired product. This may include an automatic feedback loop as described above.

The invention is illustrated by the list of non-limiting examples . EXPERIMENTAL

Example 1

Degradation of formaldehyde A reactor containing 19 bi-polar boron-doped diamond (BDD) electrodes and 2 monopolar BDD electrodes as end plates bonded to Stainless steel plates with electrical connection was used for all experiments. The electrodes sit in acrylic plate holders with silicon seals, each plate holder has a 5mm gap which allows the electrode to be in contact with the effluent at all times. The electrodes are mounted vertically in the reactor and the effluent is circulated though the system for the required period; a current is applied typically in the region of 3,000 A/m² to 5,000 A/m² depending on conductivity.

BDD electrodes are made of polycrystalline diamond formed by Chemical Vapour Deposition (CVD) in a high temperature microwave process. In the tetrahedral diamond lattice, each carbon atom is covalently bonded to its neighbours forming an extremely robust crystalline structure. Some carbon atoms in the lattice are substituted with boron to provide electrical conductivity. Boron acts as an electron acceptor due to its electron deficiency in its outer shell.

Boron doping levels are in the range 10¹⁹ to 10²¹ atoms/cm³ (up to 8000 ppm) .

In general there are three routes of oxidation at the electrodes: direct anodic oxidation of the waste, formation of hydroxyl radicals, that turn degrade the organic waste molecules, and formation of secondary oxidants, such as chlorine, hypochlorite, peroxodisulfate, peroxodicarbonate, hydrogen peroxide and ozone. These latter reactions are considered to be of minor importance.

Using a reactor as described above, a solution of industrial waste containing 3.1 g/1 formaldehyde was treated.

The temperature was set at around 5O⁰C, the voltage set on the electrode was 31.5 V and the current was between 42 and 45 A. The amount of formaldehyde was determined using a MBTH Method 8110 (Hach) set up, using a colour blue colour upon reaction with 3-methyl-2-benzothiazoline hydrazone.

After 4 h there was no formaldehyde left in the waste water solution.

This experiment shows that the reactor is capable to remove formaldehyde satisfactorily.

A similar experiment was conducted in the same reactor without a charge, but with 60 g/1 of 35% H₂O₂ added. The pH was between 3.7 and 4.5.

After 5 h the level of formaldehyde was reduced from 3.1 g/1 to 2.6 g/1.

This experiment shows that only hydrogen peroxide is not capable to remove formaldehyde satisfactorily.

A similar experiment was conducted in the same reactor with a charge set as shown above, with 60 g/1 of 35% H₂O₂ added. The pH was between 3.7 and 4.0. A similar experiment was done starting at alkaline pH (12), and leaving the pH drop to 4.5-5.0.

After 5 h the level of formaldehyde was reduced from 3.1 g/1 to 0 g/1. Also in the higher pH system, there was a similar effect found.

This experiment shows that the reactor in the presence of H2O2 is capable to remove formaldehyde satisfactorily, but that the H2O2 alone is not aiding in the formaldehyde depletion .

A similar experiment was conducted in the same reactor with a charge set as shown above, with 10 g/1 of 35% H2O2 and 0.5 g/1 of 1% buffered aqueous solution of

[Mn₂O₃ (Me₃tacn)₂] (CH₃COO)₂ (disclosed in (WO 2006/125517), yielding a concentration of 8 μM, added. The pH was around 10. After only 1 h the level of formaldehyde was reduced from 3.1 g/1 to less than 3mg/l.

A similar experiment was conducted in the same reactor without a charge, with 10 g/1 of 35% H2O2 and 0.5 g/1 of 1% buffered aqueous solution of [Mn₂O₃ (Me₃tacn) ₂] (CH₃COO)₂ (disclosed in (WO 2006/125517), yielding a concentration of 8 μM, added. The pH was around 10 and slowly decreased during 3 h to pH 8. After 3 h the level of formaldehyde was reduced from 2.3 g/1 to 1.3 g/1.

This experiment shows that the reactor in the presence of H2O2 and Mn catalyst is capable to rapidly remove formaldehyde, whilst in the presence of catalyst and hydrogen peroxide only, the rate of degradation is much lower. This shows a synergistic effect between the reactor and the manganese catalyst with hydrogen peroxide to degrade formaldehyde waste.

Example 2

Degradation of hydraulic cutting and cooling oil

Using a reactor containing a boron-doped diamond electrode, as discussed in example 1, a solution of industrial waste containing hydraulic cutting and cooling oil was treated. The COD level of this system was 159.6 g/1. The temperature was set at around 40 ⁰C, the voltage set on the electrode was around 30 V and the current was between 25 and 30 A between 0 and 4 h and at 52.5 A between 4 and 10 h.

The COD level was determined using a Hach Reactor Digestion Method 8000 set up, using a Mn-based COD determination.

After 5 h the COD reduction was 20.6%. After 10 h the COD reduction was 38.3%.

This experiment shows that the reactor is only slowly reducing COD in these waste streams.

A similar experiment was conducted in the same reactor with a charge between 40 and 50 A at 31.5V, with now 10 g/1 of 35% H₂O₂ and 8 μM of [Mn₂O₃ (Me₃tacn) ₂] (CH₃COO)₂ added (see example 1 for more detailed description) . The pH was between 8.5 and 11. After only 1 h the COD level was reduced by 83.6%. This experiment shows that the reactor in the presence of H2O2 and Mn catalyst is capable of rapidly reducing COD in this waste stream.

Example 3

Degradation of wash bay waste

Using a reactor containing a boron-doped diamond electrode, as discussed in example 1, a solution of wash bay waste mainly containing ethane-1, 2-diol (27%), 2-butoxy-ethanol (25%), triethylenglycol (16%) and N- (3-chloropropyl) -N- methyl-benzenesulfonamide (11%) was treated. The COD level of this solution was 13.7 g/1. The temperature was set at around 60 ⁰C, the voltage set on the electrode was between 20 and 30 V and the current was between 37 and 52 A. The pH was between 7.0 and 9.7.

A similar experiment as shown in examples 1 and 2 was conducted in the same reactor without a charge, but with 10 g/1 of 35% H₂O₂ added and 8 μM of [Mn₂O₃ (Me₃tacn) ₂] (CH₃COO)₂ added (see example 1 for more detailed description) . The pH was around 10.0.

After 4 h the COD value has been reduced by only 6.4 %, showing that the catalyst with H₂O₂ without the reactor switched on is very slowly degrading COD in this system.

A similar experiment was conducted as above, with for 3 h the reactor switched on in the absence of H₂O₂ and Mn catalyst. The charge was about 74A, the voltage between 180 and 240 V and the temperature increasing from 4 to 50 oC. After 3 h the COD has been reduced from 12.8 to 11.1 g/1, showing that reactor switched on, in the absence of the catalyst and H2O2 is also very slowly removing COD.

Upon addition of g/1 of 35% H₂O₂ and 8 μM of

[Mn₂O₃ (Me₃tacn) ₂] (CH₃COO)₂ (see example 1 for more detailed description) , and allowing the reactor being switched on for another hour, gives now a COD reduction from 11.1 to 4.4 g/1 in only one h.

This experiment shows that the combination of the reactor yielding highly reactive oxygen species and the Mn catalyst with H₂O₂ gives a large acceleration in removing COD above the reactor only or catalyst + H₂O₂ only.

Example 4

Dye bleaching

The progress of the bleaching was followed by UV-Vis spectroscopy using a Hewlett-Packard 8453 UV-Vis diode array spectrophotometer following the absorbance at around 540 nm in time. The dye solution was contained in a reactor consisting of a 10 mm quartz cuvette.

Using a setup as described above, a solution containing the dye Reactive Red 239 and a 50 mM carbonate buffer was treated at around 40 oC. The concentration of the dye was such that the initial absorbance at 540 nm was around 1.

Sequestrant (60 μM Dequest 2066) and hydrogen peroxide (10 mM at the start) were added together with [Mn₂O₃ (Me₃tacn) ₂] (PF₆) ₂ (10 μM) . The solution was irradiated with 254 nm light (a Vilber VL-8.LC power 400-520 μW/cm²) . After 15 min, nearly complete bleaching (98%) was observed.

A similar experiment was conducted using the same setup, but without irradiation with UV light. Only partly bleaching was observed (7% after 20 min) .

A similar experiment was conducted using the same setup, but without [Mn₂O₃ (Me₃tacn) ₂] (PF₆) 2, but with irradiation at 254 nm. Only partly bleaching was observed (15% after 20 min) .

A similar experiment was conducted using the same setup, but without [Mn₂O₃ (Me₃tacn) ₂] (PF₆) ₂ and without hydrogen peroxide, but with irradiation at 254 nm. Less than 1% bleaching was observed after 20 min.

Therefore, these experiments show that the combination of UV-irradiation in combination with the manganese catalyst and hydrogen peroxide furnishes a better Reactive red dye bleaching effect, than either UV/H2O2 alone or [Mn₂O₃ (Me₃tacn) ₂ ] ( PF₆) ₂ with H2O2 .

Example 5

Dye Bleaching

The progress of dye bleaching was followed as described above, monitoring an absorbance at 500nm. The setup is as described previously (Example 4). A solution containing the dye Reactive Orange 16 and 5OmM carbonate buffer was treated at around 60⁰C. The initial concentration of the dye was such that the initial absorbance at 500nm was around 1. Sequestrant (60μM Dequest 2066) and hydrogen peroxide (1OmM at the start) were added together with [MnIIIMnIV (Me4- DTNE) (D-O) 2 (D-CH₃COO) ] (PF₆) ₂ (2μM) . [MnIIIMnIV (Me4-DTNE) (D- O) 2 (D-CH₃COO) ] (PFe)₂ was prepared as disclosed elsewhere (K. Schaefer et al . , J. Am. Chem. Soc, 120, 13104 (1998). The solution was irradiated as described above. Nearly complete bleaching (90%) is attained after 16 min; 98% bleaching is obtained after 25 min.

A similar experiment was conducted using the same setup, but without irradiation with UV light. Only partial bleaching was observed (54%) in the same time scale (16 min) . Almost complete bleaching, 98%, is only reached after 50 min.

A similar experiment was conducted using the same setup, but without [MnIIIMnIV (Me4-DTNE) (D-O) 2 (D-CH₃COO) ] (PFe)₂, but with irradiation at 254nm. Only 20% bleaching was observed after 16 min.

A similar experiment was conducted using the same setup, but without [MnIIIMnIV (Me4-DTNE) (D-O) 2 (D-CH₃COO) ] (PFe)₂ and hydrogen peroxide, but with irradiation. Less than 5% bleaching was observed after 16 min.

These experiments have demonstrated that [MnIIIMnIV (Me4- DTNE) (D-O) 2 (D-CH₃COO) ] (PF₆) ₂ and hydrogen peroxide, combined with UV irradiation, have an enhanced bleaching effect on Reactive Orange 16 than UV/H2O2 alone or [MnIIIMnIV (Me4- DTNE) (D-O) 2 (D-CH₃COO) ] (PFg)₂ /H2O2 alone. Hence, a synergy between the two processes. Example 6

Dye Bleaching

N,N-bis (pyridin-2-yl-methyl-l, 1-bis (pyridin-2-yl) - 1- aminoethane , hereafter referred to as MeN4Py, and the corresponding iron (II) complex, [Fe (MeN4py) Cl] Cl, were prepared as described in EP0909809.

The progress of dye bleaching was followed as described in example 4. The setup is as described as example 5. A solution containing the dye Reactive Orange 16 and 5OmM carbonate buffer was treated at around 60⁰C. The initial concentration of the dye was such that the initial absorbance at 50OnM was around 1. Sequestrant (60μM Dequest 2066) and hydrogen peroxide (1OmM at the start) were added together with [Fe (MeN4py) Cl] Cl (20μM) . The solution was irradiated as described above. After 40 min, a high level of bleaching (75%) is obtained, reaching 90% in 60 min.

A similar experiment was conducted using the same setup, but without irradiation with UV light. Only 52% bleaching is obtained after 40 min, and 75% in 60 min.

A similar experiment was conducted using the same setup, with irradiation at 254nm, but without the presence of [Fe (MeN4py)Cl]Cl. Only 38% bleaching was observed after 40 min .

A similar experiment was conducted using the same setup, now without [Fe (MeN4py) Cl] Cl and hydrogen peroxide, but with UV irradiation. Less than 9% bleaching was observed after 40 min . These experiments have shown that the combination of

[Fe (MeN4py) Cl] C1/H2O2 with UV irradiation provides a better bleaching effect than [Fe (MeN4py) Cl] C1/H2O2 or UV/H2O2 alone .

Example 7

Dye Bleaching

Chloro (dimethyl 2, 4-di- (2-pyridyl) -3-methyl-7- (dimethylamino-ethan-2-yl) -3,7-diaza-bicyclo[3.3.1]nonan-9- one-1, 5-dicarboxylate) iron (II) - chloride hydrate (abbr hereafter as [N2py2-amineFeCl] Cl was synthesised as described in WO 03/104379.

The progress of dye bleaching was followed as described in example 4. The setup is as described as example 5, again using Reactive Orange 16. Sequestrant (60μM Dequest 2066) and hydrogen peroxide (1OmM at the start) were added together with [N2py2-amineFeCl] Cl (20μM) to the Dye/Buffer solution. The solution was irradiated as described above.

After 75 min, a level of 76% bleaching is attained.

A similar experiment was conducted using the same setup, but without irradiation with UV light. Only around 40% bleaching is obtained after 75 min.

A similar experiment was conducted using the same setup, but without [N2py2-amineFeCl] Cl, but with hydrogen peroxide and irradiation. Around 59% bleaching was observed after 75 min .

A similar experiment was conducted using the same setup, but without [N2py2-amineFeCl] Cl and hydrogen peroxide, but with UV irradiation. Less than 18% bleaching was observed after 75 min.

This experiment has shown that there is an enhancement in bleaching efficacy when [N2py2-amineFeCl] Cl is used in conjunction with UV and H2O2, over when UV/H2O2 and [N2py2- amineFeCl] C1/H2O2 are used alone.

Example 8 Dye Bleaching

4, 11-dimethyl-1, 4, 8, 11-tetraazabicyclo [6.6.2] hexadecane , hereafter referred to as Bcyclam, and the corresponding manganese (II) complex, [Mn (Bcyclam) CI2] , were prepared as described in WO98/39098 and J. Am. Chem. Soc . , 122, 2512 (2000) . A similar experiment to those in examples 4-7 was performed, again using Reactive Orange 16. Sequestrant (60μM Dequest 2066) and hydrogen peroxide (1OmM at the start) were added together with [Mn (Bcyclam) Cl₂] (50μM) to the Dye/Buffer solution. The solution was irradiated as described above. After 60 min, a level of 72% bleaching is attained, rising to 78% after 75 min.

A similar experiment was conducted using the same setup, but without irradiation with UV light. Only around 49% bleaching is obtained after 60 min.

A similar experiment was conducted using the same setup, though without [Mn (Bcyclam) Cl₂] , but with hydrogen peroxide and irradiation. Around 49% bleaching was observed after 60 min. A similar experiment was conducted using the same setup, but without the catalyst [Mn (Bcyclam) CI2] , and hydrogen peroxide, but with UV irradiation. Around 11% bleaching was observed after 60 min.

Therefore, this experiment has shown the combination of UV irradiation with the catalyst [Mn (Bcyclam) CI2] and H2O2 furnish a better bleaching effect on this dye than when [Mn (Bcyclam) Cl₂] /H2O2 and UV/H2O2 are used alone.

Claims

We claim:

1. A method of treating aqueous industrial waste comprising the following steps: (i) storing the aqueous industrial waste in a holding vessel, the holding vessel a batch or a continuous vessel; (ii) adding to the aqueous industrial waste in the holding vessel a transition metal catalyst of a tridentate, tetradentate, pentadentate or hexadentate nitrogen donor ligand or precursor thereof and hydrogen peroxide to provide a concentration of the transition metal catalyst in a concentration from 0.1 to 100 micromolar and a concentration of the hydrogen peroxide from 1 to 1500 mM; and, (iii) releasing the effluent after treatment for further processing or directly into the environment, wherein the aqueous industrial waste is subjected to a hydroxylation step during or prior to step (ii) .

2. A method according to claim 1, wherein the hydroxylation step is provided by hydroxyl radicals.

3. A method according to claim 1, wherein the hydroxyl radicals are provided by a generation process selected from: Fenton chemistry; ozone generation; ultra violet irradiation of hydrogen peroxide; and electrolysis.

4. A method according to claim 3, wherein the hydroxyl radicals are provided by a generation process selected from: a) ultra violet irradiation provided thorough a UV transmissive window to the aqueous industrial waste; b) the provision of gaseous ozone via a conduit to the aqueous industrial waste; c) the provision of iron(II/III) to the aqueous environment, the aqueous environment having a pH in the range 2 to 5; and, d) Pulse radiolysis or sonolysis.

5. A method according to claim 3, wherein the hydroxyl radicals are provided by an electrode system, the electrode system a conducting diamond electrode.

6. A method according to claim 5, wherein an electrode system comprises a boron-doped diamond electrode

7. A method according to claim 6, wherein the electrode is a boron-doped diamond electrode has levels of Boron in the diamond lattice in the range 10¹⁹ to 10²¹ atoms/cm³, and during treatment, current per surface area applied is at least 500 A/m², and voltage applied to the electrode is at least 5 V.

8. A method according to any preceding claim, wherein the transition metal catalyst or precursor thereof transition metal catalyst is a mononuclear or dinuclear complex of a Mn(III) or Mn(IV) transition metal catalyst wherein the ligand of the transition metal catalyst is of formula (I) :

R N [ CR₁R₂CR₃R₄ wherein : Q = p is 3;

R is independently selected from: hydrogen, Cl-C6-alkyl, CH2CH2OH, and CH2COOH, or one of R is linked to the N of another Q via an ethylene bridge; Rl, R2 , R3, and R4 are independently selected from: H, Cl- C4-alkyl, and Cl-C4-alkylhydroxy .

9. A method according to claim 8, wherein Rl, R2, R3, and R4 are independently selected from: H and Me.

10. A method according to claim 8, wherein the catalyst is derived from a ligand selected from the group consisting

1, 4, 7-Trimethyl-l, 4, 7-triazacyclononane (Me₃-TACN) and 1,2,- bis- (4,7, -dimethyl-1, 4,7, -triazacyclonon-1-yl) -ethane (Me₄- DTNE) .

11. A method according to any one of claims 8 to 10, wherein the preformed transition metal catalyst salt is a dinuclear Mn(III) or Mn(IV) complex with at least one O^2~ bridge.

12. A method according to any preceding claim, wherein the method is a continuous process.

13. A method according to any preceding claim, wherein the method is a batch process.

14. A method according to any preceding claim, wherein the aqueous medium is monitored in the holding vessel by UV, UV- visible or chromatography and the monitoring is determinative of the time of treatment in the holding vessel before step (iii) .