WO2008100228A1

WO2008100228A1 - Process for removing organic contaminants from aqueous media

Info

Publication number: WO2008100228A1
Application number: PCT/SG2008/000051
Authority: WO
Inventors: Yi-Fan Han; Kanaparthi Ramesh
Original assignee: Agency For Science, Technology And Research
Priority date: 2007-02-13
Filing date: 2008-02-13
Publication date: 2008-08-21

Abstract

A process for treating an aqueous medium having an organic contaminant therein, the process comprising the step of exposing a gold catalyst to the aqueous medium in the presence of an oxidizing agent to thereby form free radicals capable of at least partially degrading the organic contaminant in the aqueous medium.

Description

PROCESS FOR REMOVING ORGANIC CONTAMINANTS FROM AQUEOUS

MEDIA

Technical Field The present invention generally relates to a process for degrading organic contaminants from an aqueous media.

Background

Remediation of water containing contaminants such as organic compounds is of great importance because the organic compounds may impose toxic effects on the health of humans and animals. Some examples of such organic compounds include formaldehyde, phenol and alcohols. Currently, due to climate change and environmental pollution, the occurrence and duration of droughts in the world is increasing. Accordingly, improving the supply of water that is suitable for human consumption and industrial use is an urgent global problem.

There are currently several processes available for the removal of toxic organic compounds from wastewater, contaminated groundwater or surface water. These processes include oxidation, with or without solid catalysts ^[1~4] , biological oxidation, supercritical oxidation, and adsorption¹⁵' ^6] . However, the above processes present a number of disadvantages. For example, in wet oxidation with air as the oxygen source, the reaction conditions are extremely stringent such that high temperatures (>200°C) and high pressures (>0.5 MPa) are required. Furthermore, it is an energetically unfavorable process, especially for the treatment of organic contaminants at low concentrations.

One disadvantage of the supercritical oxidation and adsorption process is that disposal of the adsorbents is expensive and can cause secondary pollution. Another disadvantage associated with most of the above-mentioned processes is the high cost required for treating wastewater containing organic compounds at low concentration levels, for example, less than 1000 ppm. Another known process is to use a Fenton reagent, that consists of homogenous ferric ions (Fe²⁺) and hydrogen peroxide (H₂O₂) , as an oxidant for treating industrial effluents containing organic compounds. This process is suitable for treating industrial effluents containing organic compounds at low concentration levels in the range of 10^"2 ~ 10^~3 M^[7] . This process has proven to be effective in the removal of odour and/or noxious components from atmospheric effluents.

In the Fenton reaction, radicals such as hydroxyl (OH), superoxide radical (O₂ ^") and perhydroxyl radicals ('HO₂) can be produced by the decomposition of H₂O₂ as expressed, in equations 1 and 2 below (where S and S+ represent reduced and oxidised metal ions, such as Fe²⁺) : S + H₂O₂ → S⁺ + OH^" + -OH (1) S⁺ + H₂O₂ → S + -HO₂ + H⁺ (2)

As a hydroxyl radical is one of the most robust oxidants known in the art, it can oxidize or degrade organic compounds rapidly in aqueous medium. One of the advantages of such a process is that the final products are water (H₂O) and carbon dioxide (CO₂) , which can be released into the environment without the need for any post-treatment steps. Additionally, hydroxyl radicals can be used for the treatment of synthetic organic compounds and chemicals that are resistant to biological degradation.

However, several problems exist in homogeneous Fenton systems. Firstly, the waste product is inevitably iron- containing sludge, which tends to be environmentally problematic to dispose of. Secondly, in order for the above process to work effectively, the process must be carried out under relatively high acidic conditions. This entails the need for pH adjusting chemicals to be added to the process in order to achieve the workable pH range of 2.0 to 5.0. These additional chemicals must be removed from the treated water in additional post-treatment steps before use. Furthermore, due to the acidic conditions, acid-resistant reactors must be used for the treatment process. Such rectors tend to be expensive and require extensive maintenance, adding to the cost of the treatment process. Thirdly, as seen from equations (1) and (2) above, the metal ion, such as Fe²⁺, participates in the reactions and becomes depleted as the process continues. Furthermore, due to the reaction between the metal ion and the intermediates, or products of the process, the activity of the metal ion decreases irreversibly. Therefore, the metal ion must be continuously added to the treatment process, resulting in more costs and the need for more raw materials. In heterogeneous Fenton systems, such as H₂θ₂/Fe-based solid catalyst systems, it is believed that as H2O₂ is decomposed by the iron catalysts, the concentration of useful reactive intermediates does not increase with increasing concentration of H₂O₂. This means that the heterogeneous Fenton system cannot effectively produce the reactive intermediates, leading to a decrease in the efficiency of the process. In addition, leaching of iron during the process will deteriorate the catalytic performance . Thus, there is an urgent need to remediate wastewater from industries and purify freshwater from rain, which is usually contaminated by organic substances from various sources . There is a need to provide a process for degrading organic contaminants from an aqueous media. The conditions of the process may be substantially mild as compared to known processes.

Summary

According to a first aspect, there is provided a process for treating an aqueous medium having an organic contaminant therein, the process comprising the step of exposing a gold catalyst to the aqueous medium in the presence of an oxidizing agent to thereby form free radicals capable of at least partially degrading the organic contaminant in the aqueous medium.

Advantageously, the inventors have found that the above process can be carried out at mild conditions of pH and temperature as compared to conventional treatment processes.

More advantageously, the above process may be used for treating water contaminated with low concentrations of organic compounds. Even more advantageously, the gold catalyst may not substantially leach into the water during the treatment process.

In one embodiment, the gold catalyst comprises a gold metal portion and a support material portion. According to a second aspect, there is provided a water treatment process for degradation of organic contaminants comprising the step of exposing a gold catalyst to the water in the presence of a peroxide to thereby form free radicals capable of at least partially degrading the organic contaminants in the water.

According to a third aspect, there is provided use of a gold catalyst and an oxidizing agent for removing an organic contaminant from water. According to a fourth aspect, there is provided a water treatment catalyst comprising gold metal and being capable of degrading an organic contaminant present in water when in the presence of an oxidizing agent.

Definitions

The following words and terms used herein shall have the meaning indicated:

The term ^λλFenton-like reaction" is to be interpreted broadly to refer to variations of the conventional Fenton reaction. In a conventional Fenton reaction, the reactants are typically Fe²⁺ ions and hydrogen peroxide. In Fenton-like reactions as disclosed herein, the Fe²⁺ cations are replaced with the gold catalysts disclosed herein. Although a gold catalyst is used in the Fenton- like reaction compared to traditional Fenton reaction, the reactive intermediates generated in a Fenton-like reaction may still include hydroxyl radicals (-OH).

The term "aqueous medium" as it is used herein does not necessarily include pure water but covers water that may include incidental additional constituents such as organic liquids.

The term "organic contaminant (s) " is to be interpreted broadly to include any organic compound that can be degraded via oxidation and which is not wanted in an aqueous media. The organic compounds may include, but are not limited to, alcohols, aromatic compounds, ketones, aldehydes, nitrogen-containing organic compounds, and halogenated organic compounds. Exemplary organic compounds include phenols, benzene, ethanols, propanols, acetones, formaldehydes, quinolines or p-chlorophenol .

The term "reactive intermediate" is to be interpreted broadly to refer to radicals that react with and oxidize the contaminant species, or to radicals or intermediate compounds that react with hydrogen peroxide to form radicals that react with and oxidize organic compounds.

The terms "radicals" or "free radical" are to be interpreted broadly to include any species that contains one or more unpaired electrons. For example, hydroxyl radicals are one type of radicals formed from a Fenton or

Fenton-like reaction.

The terms "degrading", "degrade" and grammatical variations thereof refer to the breaking of covalent bonds of an organic compound. The breaking of bonds need not be complete in that not all breakable bonds are necessarily cleaved. Breakage of bonds may also result in the release of fragments differing from one another, depending on the chemical composition of the organic compound being degraded.

The word "substantially" does not exclude

"completely" e.g. a composition which is "substantially free" from Y may be completely free from Y. Where necessary, the word "substantially" may be omitted from the definition of the invention.

Unless specified otherwise, the terms "comprising" and "comprise", and grammatical variants thereof, are intended to represent "open" or "inclusive" language such that they include recited elements but also permit inclusion of additional, unrecited elements.

As used herein, the term "about", in the context of concentrations of components of the formulations, typically means +/- 5% of the stated value, more typically +/- 4% of the stated value, more typically +/- 3% of the stated value, more typically, +/- 2% of the stated value, even more typically +/- 1% of the stated value, and even more typically +/- 0.5% of the stated value.

Throughout this disclosure, certain embodiments may be disclosed in a range format. It should be understood that the description in range format is merely for convenience and brevity and should not be construed as an inflexible limitation on the scope of the disclosed ranges. Accordingly, the description of a range should be considered to have specifically disclosed all the possible sub-ranges as well as individual numerical values within that range. For example, description of a range such as from 1 to 6 should be considered to have specifically disclosed sub-ranges such as from 1 to 3, from 1 to 4, from 1 to 5, from 2 to 4, from 2 to 6, from 3 to 6 etc., as well as individual numbers within that range, for example, 1, 2, 3, 4, 5, and 6. This applies regardless of the breadth of the range.

Disclosure of Optional Embodiments

Exemplary, non-limiting embodiments of a process for treating an aqueous medium having one or more organic contaminants therein will now be disclosed. The process comprises the step of exposing a gold catalyst to the aqueous medium in the presence of an oxidizing agent to thereby form free radicals capable of at least partially degrading the organic contaminant in the aqueous medium.

The aqueous medium containing contaminants may be effluents from pharmaceutical, electronic, metal processing, food or chemical industry, ground water, rainwater or tank water.

The inventors have surprisingly found that gold catalyst is a suitable reagent for the Fenton-like reaction. In the Fenton-like reaction, one type of free radicals generated is hydroxyl radicals. The hydroxyl radicals may at least partially degrade or oxidize the organic contaminants, leading to at least a reduction and possibly complete removal of the organic contaminants from the aqueous medium. The organic contaminants may be degraded by the free radicals generated in the Fenton-like reaction to form products such as water and carbon dioxide, which can be released to the environment without the need for additional post-treatment steps to remove organic contaminant degeneration products.

The gold catalyst may be in particle form. The gold catalyst particle may be in the nano-sized range. In one embodiment, the nano-sized gold catalyst particle may have a particle size in the range selected from the group consisting of about 1 nm to about 100 nm, about 1 nm to about 50 nm, about 1 to about 20 nm, about 1 nm to about 10 nm, about 1 nm to about 5 nm, about 3 nm to about 5 nm and about 10 nm to about 15 nm. The gold catalyst may comprise a gold metal portion and a support material portion.

The gold metal portion may be substantially pure gold or a gold alloy.

In one embodiment, the gold metal portion may be mounted on the support material. In another embodiment, the gold metal portion may be provided within the structure of the support material.

The support material may be single or binary metal oxides with a high surface area. The metal of the metal oxides may be selected from the group consisting of aluminum, titanium, manganese, iron, tin, zirconium, molybdenum, tungsten, cerium and combinations thereof.

The metal oxides may be selected from the group consisting of Al₂O₃, TiO₂, Mn_x0_y, Fe₂O₃, SnO₂, ZrO₂, MoO₃, WO₃, CeO₂ and combinations thereof.

The support material may be carbon in the form of nanotubes, fiber or ball. The support material may be a silicate. The silicate may be SiO₂. The support material may be titania-silicas or zirconia-silicas .

The support material may be a porous material. The support material may be a metal silicate. The metal silicate may be a zeolite. The zeolite may be, but is not limited to, MCM-41, SBA-15 or ZSM-5. It is to be appreciated that other kinds of silicate or metal siicate may be employed as the support material.

The support material may be an apatite mineral. The apatite mineral may be hydroxylapatite.

The gold catalyst may have a surface area in the range selected from the group consisting of about 1 m²/g to about 1000 m²/g, about 1 m²/g to about 500 m²/g, about 1 mVg to about 200 m²/g, about 50 m²/g to about 200 m²/g and about 100 m²/g to about 200 mVg.

The use of supported gold catalysts can stabilize the size and morphology of the gold particles, and modify the electronic property of the gold particles due to the interaction between the gold particles and the support materials. In addition, the use of supported gold catalysts can aid in the adsorption of contaminants to the support material such that the contaminants are in proximate contact with the gold catalyst. The free radicals produced from the reaction between the gold catalyst and oxidizing agent can oxidize the adsorbed contaminants, leading to an increase in the degradation efficiency.

The gold catalyst may comprise about 0.1 wt% to about 10 wt%, about 0.1 wt% to about 5 wt%, about 0.1 wt% to about 3 wt% or about 0.1 wt% to about 1 wt% of gold metal portion relative to the support material.

The gold catalyst comprising the gold metal portion and support material portion may be made from a number of processes known in the art. These processes may include, but are not limited to, a kneading process, an impregnation process, an adsorption process, a spraying process or an ion exchange process.

The shape of the gold catalyst is not limited and may include pellet-like, particle-like, spherical-like, ring- like, honeycomb-like shapes etc. When the gold catalyst is used in the treatment process, the honeycomb-like shape is selected in one embodiment due to the possibility of formation of a layer of contaminants over the surface of the gold catalyst. The use of the honeycomb-like shape substantially protects the gold metal portion therein from being blocked or clogged and hence rendered unusable by the layer of contaminants.

The volume of reaction solution versus the gold catalyst amount, in the case of 100 ppm contaminants, may be in the range selected from the group consisting of about 0.1 to about 10.0 L_(soi_ution) /<3catr about 0.5 to about

5.0 L(_Solution)/gcat and about 1.0 to about 2.0 L_(solution) /g_Cat-

The oxidizing agent may be a peroxide. The peroxide may be hydrogen peroxide. The gold catalyst may catalyze the conversion of hydrogen peroxide to form hydroxyl radicals in a Fenton-like reaction.

In one embodiment, the oxidizing agent, such as hydrogen peroxide, may be supplied in solution to the aqueous medium.

The concentration of the oxidizing agent used may be chosen for safe handling during transportaion. The concentration of the oxidizing agent used may be of the range selected from the group consisting of about 20 wt% to about 60 wt%, about 20 wt% to about 30 wt%, about 25 wt% to about 35 wt%, about 30 wt% to about 60 wt% and about 35 wt% to about 60 wt%.

In another embodiment, the oxidizing agent may be generated in situ. For example, hydrogen peroxide could be produced in the aqueous medium by a direct synthesis from hydrogen and oxygen in the same reaction system.

The exposing step may be undertaken under conditions to substantially degrade the contaminants present in the aqueous medium.

In order to substantially minimize the scavenging of hydroxyl radicals by the gold catalyst, the oxidizing agent may be added to the aqueous medium at time intervals in the exposing step. This may be advantageous in embodiments where a batch reactor is used. The time duration of said intervals may be in a range selected from the group consisting of about 1 minute to about 30 minutes, about 1 minute to about 15 minutes, about 1 minute to about 10 minutes and about 1 minute to about 5 minutes.

The temperature of the aqueous medium may be in a range selected from the group consisting of less than about 100⁰C, less than about 80⁰C, less than about 75°C and less than about 70⁰C. The pH of the aqueous medium may be in a range selected from the group consisting of about 1.0 to about 9.0, about 2.0 to about 7.0, about 4.0 to about 7.0, about 5.0 to about 7.0 and about 4.0 to about 6.0. The pH of the aqueous medium may be maintained at a non-alkaline pH. The pH of the aqueous medium may be in the acidic range or substantially neutral range. The pH of the aqueous medium may be about 6.5, about 6.8 or about 7.0. It is to be appreciated that a pH-adjusting chemical may be used to obtain and maintain the desired pH value. For example, a mineral acid solution such as hydrochloric acid or sulphuric acid may be used to acidify the aqueous medium. On the other hand, ammonia solution may be used to increase the pH of the aqueous medium. In one embodiment, the pH of the aqueous medium may be adjusted and maintained at a desired value after the gold catalyst has been added to the aqueous medium. In another embodiment, the temperature of the aqueous medium may be adjusted after the pH has been adjusted. In a further embodiment, the temperature of the aqueous medium may be adjusted just after the catalyst is added to the aqueous medium. After the operational conditions have been adjusted to required values, the oxidizing agent may be added to the aqueous medium. As mentioned above, the oxidizing agent may be added at intervals.

The process may be used for removing low concentrations of contaminants from an aqueous medium. The concentration of the contaminants may be in the range selected from the group consisting of about 1 ppm to about 10,000 ppm, about 1 ppm to about 1,000 ppm, about 1 ppm to about 500 ppm, about 1 ppm to about 100 ppm, about 300 ppm to about 500 ppm and about 400 ppm to about 500 ppm.

The rate of degradation of the contaminants may be dependent on the concentration of reactive intermediates in the aqueous medium. This may be in turn dependent on the intial concentration of the oxidizing agent. The amount of oxidizing agent used may be dependent on the type of support material used, the reaction temperature, the reaction pH and nature of the contaminants.

In one embodiment, the total mole ratio of oxidizing agent : contaminant is of the range selected from the group consisting of about 10 to about 1000, about 50 to about 500 and about 100 to about 400. The production rate of hydroxyl radicals may be dependent on the reaction time, amount of gold catalyst used or type of support material used in the process. It is to be appreciated that the reaction time should be long enough to effectively produce the free radicals and yet, be as short as possible in order to substantially minimise the scavenging of the radicals by the gold catalyst. The reaction time may also be used to determine the time duration between addition of oxidizing agent to the aqueous medium. In one embodiment, the reaction time may be about 1 hours to about 3 hours for a batch reaction. In another embodiment, the reaction time may be about 1 hours to about 2 hours.

The reaction vessel or reactor that may be used for the process is not limited and may be a batch reactor or a continuous flow reactor. The batch process may be interrupted at intervals in order to add in reagents as described above.

The process may include a recycling step for passing partially treated aqueous medium back to the reactor in order to ensure a substantially complete degradation and hence removal of the contaminants.

Brief Description Of Drawings The accompanying drawings illustrate disclosed embodiments and serve to explain the principles of the disclosed embodiments. It is to be understood, however, that the drawings are designed for purposes of illustration only, and not as a definition of the limits of the invention.

Fig. 1 is a schematic diagram of the reactor used in the process of Example 1 below.

Fig. 2 is a graph showing the degradation of hydrogen peroxide over time under the experimental conditions described in Example 1 below.

Fig. 3 is a graph showing the degradation of phenol over time when Au (1.5wt%) /TiO₂ catalyst is used at three temperature values as described in Example 3 below. Fig. 4 is a graph showing the degradation of phenol over time when Au (2.4wt%) /HAp catalyst is used at three temperature values as described in Example 3 below.

Fig. 5 is a graph showing the degradation of phenol over time at five pH values as described in Example 4 below.

Fig. 6 is a graph showing the degradation of various types of organic contaminants over time as described in Example 7 below. Fig. 7 is a graph showing a comparison of active metal leaching at various pH values for Au (2.4 wt%) /HAP and Fe²⁺ (1.2wt%) /ZSM-5 catalysts as described in Comparative Example 3 below.

Fig. 8a and Fig. 8b are graphs showing stability results of the Au (2.4 wt%)/HAP and Fe (1.2wt%) /ZSM-5 catalysts in weakly acidic conditions (pH = ~5.0) and strongly acidic conditions (pH = -2.0), respectively, as described in Comparative Example 3 below.

Examples

Non-limiting examples of the invention and comparative examples will be further described in greater detail by reference to specific Examples, which should not be construed as in any way limiting the scope of the invention.

Example 1

An aqueous medium made up of deionized water with 100 ppm phenol dissolved therein was used in this example. Referring to Fig. 1, 20 ml of the above phenol-water solution and 2 mg of a supported catalyst of Au(1.5wt%, d_Au:3.5 nm) /TiO₂ from the World Gold Council of London of the United Kingdom was placed in a glass batch reactor 1 connected to a condenser 2 with continuous stirring using a magnetic stirrer 3. The speed of the magnetic stirrer 3 was¹ set to 400 rpm.

The conditions of the reaction were set at a pH value of 6.8 and a temperature value of 70 °C. Then, 1 ml of 30 wt% hydrogen peroxide solution from Sigma-Aldrich of St. Louis of Missouri of the United States of America was dropped into the glass batch reactor 1 via glass stopper 4 at intervals of ~0, 10 minutes, 20 minutes, 30 minutes and 50 minutes. The reaction was carried out for 60 to 120 minutes .

In this example, the amount of phenol converted at 10 minutes was ca. 30%; the amount of phenol converted at 30 minutes was ca. 40%; and the amount of phenol converted at 60 minutes was ca. 45%.

The conversion of phenol during reaction was detected using gas chromatography coupled with mass spectroscopy

(GC-MS 6890 N; from Agilent Technologies, Inc of Santa

Clara, California of the United States of America) , equipped with HP-5 capillary column. There is no other species but phenol determined in the reaction system as evidenced by the GC-MS. Phenol is supposed to be completely oxidized into CO₂ and H₂O.

The amount of decomposed hydrogen peroxide as a function of time is shown in Fig. 2. Hydrogen peroxide is decomposed in the presence of the supported catalyst to form reactive intermediates such as hydroxyl radicals (^•OH), superoxide radical (O₂ ^") and perhydroxyl radials (-HO₂) From Fig. 2, the amount of hydrogen peroxide decomposed or converted at 10 minutes is ca. 40%; the amount of hydrogen peroxide converted at 30 minutes is ca. 84%; the amount of hydrogen peroxide converted at 60 minutes is ca. 90%; and the amount of hydrogen peroxide converted at 120 minutes is ca. 92%. Fig. 2 shows that the supported gold catalyst is an effective and efficient catalyst for decomposing hydrogen peroxide.

The variation of H₂O₂ concentration during reaction was analyzed colorimetrically using a UV-Vis spectrophotometer (Epp2000; from StellarNet Inc. of Tampa,

Florida of the United States of America) after complexation with a TiOSO₄ZH^SO₄ reagent.

Example 2 The same reaction conditions as in Example 1 were used for this example, except that the amount of supported catalyst used was increased from 2.0 mg to 10.0 mg. As the amount of supported catalyst was increased, the conversion of phenol at 60 minutes decreased from ca. 90% to ca. 31%. This could be due to the increased incidence of scavenging of the free radicals (hydroxyl radicals) by the catalyst to form hydroxide ions. Therefore, due to the decrease in the amount of free radicals available for oxidation of phenol, the conversion of phenol decreased as the amount of catalyst used increased.

Example 3

The same reaction conditions as in Example 1 were used for this example, except that the reaction was carried out at two additional temperature values of 25°C and 40°C. The results of this example are shown in Fig. 3. When the reaction time was 60 minutes, the percentage conversion of hydrogen peroxide increased from 27% to 35% to 45% as the temperature increased from 25°C to 40°C to 70⁰C. Therefore, increasing the temperature has the effect of increasing the conversion of hydrogen peroxide. Hence, the amount of free radicals available for oxidizing phenol is also increased, leading to an increase in the amount of phenol degraded from the reaction. Another temperature study was carried out using Au (2.4wt%) /HAp catalyst at the same temperature values of 298 K (-25⁰C) , 313 K (~40C) and 343 K (~70°C) .

The results of this example are shown in Fig. 4. When the reaction time is 120 minutes, the conversion of phenol at 298 K is 50%, the conversion of phenol at 313 K is 70% and the conversion of phenol at 343 K is 83%. Therefore, it was observed that reactivity significantly accelerated with an increase in temperature.

Example 4

The same reaction conditions as in Example 1 were used for this example, except that the reaction was carried out under varying pH values from 2.0 to 9.0 (pH values of 2.0, 4.0, 6.8, 8.0 and 9.0).

In order to make the reaction solution more acidic, 0.01 M hydrochloric acid was used. In order to make the reaction solution more basic, 0.1 M ammonia solution was used. The results of this example are shown in Fig. 5. From Fig. 5, it was observed that conversion of phenol varied inversely with pH over the range of 2.0 to 9.0. When the reaction time is 120 minutes, the conversion of phenol at pH of 2.0 is about 96%, conversion of phenol at pH of 4.0 is about 89%, conversion of phenol at pH of 6.8 is about 78%, conversion of phenol at pH of 8.0 is about 65% and conversion of phenol at pH of 9.0 is about 21%. Therefore, the reaction is favourably carried out under acidic and pH neutral conditions. The lower conversion of phenol at pH 6.8 when compared to the conversion of phenol at pH values of 4.0 and 2.0 can be compensated by reduction in the need for special acid-resistant reactors or additional pH-adjusting chemicals to create the acidic reaction environment. By carrying out the reaction at neutral pH, this can lead to decreased cost and fewer process steps as compared to conducting the reaction in an acidic environment.

Under Inductively Coupled Plasma (ICP) analysis using Vista-Mpx from Varian, Inc. of Palo Alto of California of the United States of America, there was no substantial leaching of gold in the reaction solution at 60 minutes when the pH of the solution was higher than 4.0. When the pH was adjusted to 2.0 using 0.01 M hydrochloric acid, trace amounts (about less than 1%) of gold were detected in the reaction solution. This showed that the gold catalyst is substantially stable in the reaction solution.

Example 5 The same reaction conditions as in Example 1 were used for this example, except that the concentration of phenol dissolved in the deionized water was increased to 500 ppm. As the concentration of phenol increased to 500 ppm, the conversion of phenol remains. This indicates that the reaction rate is dependent on the concentration of phenol dissolved in the aqueous medium.

Example 6

The same reaction conditions as in Example 1 were used for this example, except that different supports are used for the gold catalyst. The various supported gold catalysts are Au (2.4wt%) /Ct-Fe₂O₃, Au (1.5wt%) /TiO₂ and

Au(O.8wt%) /C. All of the above supported gold catalysts were obtained from the World Gold Council of London of the United Kingdom.

In this example, the reaction rate is dependent on the type and property of the support material used. Under the same reaction conditions as in Example 1, the conversion of phenol at 60 minutes in the presence of Au(2.4wt%) /α-Fe₂O₃ catalyst is 85%. The conversion of phenol at 60 minutes in the presence of Au(0.8wt%)/C catalyst is 19%. Additional details can be seen in Table 1 further below.

Example 7

Four individual samples of an aqueous medium made up of deionized ^' water with 100 ppm of phenol, acetone, ethanol and formaldehyde separately dissolved therein were used in this example.

20 ml of the above respective organic compound-water solution was placed in the glass batch reactor 1 of Fig. 1, followed by 2.0 mg of Au (2.4wt%) /α-Fe₂O₃ catalyst. The conditions of the reaction were set at a pH value of 6.8 and a temperature value of 70°C. Then, 1 ml of 30 wt% hydrogen peroxide solution was dropped into the glass batch reactor 1 via glass stopper 4 at intervals of ~0, 10 minutes, 20 minutes, 30 minutes and 50 minutes. The reaction was carried out for 120 minutes. The results of this example are seen in Fig. 6. The catalytic reactivity varied with the property of the organic compounds. The conversion of organic compounds when time is 120 minutes is of the following order: phenol

(80%) > acetone (73%) > ethanol (55%) > formaldehyde (44%) .

Example 8

An aqueous medium made up of deionized water with 100 ppm phenol dissolved therein was used in this example. 500 ml of the above phenol-water solution was placed in a reaction beaker. 50 mg of Au (2.4wt%) /0,-Fe₂O₃ catalyst was placed in the reaction beaker. The conditions of the reaction were set at a pH value of 6.8 and a temperature value of 70°C. Then, 25 ml of 30 wt% hydrogen peroxide solution was dropped into the reaction solution at intervals of ~0, 10 minutes, 20 minutes, 30 minutes and 50 minutes. The reaction was carried out for 60 minutes. The amount of phenol converted at 60 minutes was 85%.

Example 9

The same reaction conditions as in Example 1 were used for this example, except that an Au (2.4wt%) /Hap catalyst used was recycled from a previous reaction for 5 times. The amount of phenol converted at 60 minutes was 76%. This indicates that the recycled catalyst retains its catalytic activity.

Comparative Example 1

The same reaction conditions as in Example 1 were used for this comparative example, except that specific reactants were removed from the reaction. It was observed that phenol was not converted after the reaction was carried out for 120 minutes in the absence or presence of the support materials such as Hap, Tiθ₂, α-Fe₂θ₃ or graphite C. Also, under similar conditions in a hydrogen peroxide free solution, phenol was not converted over all the catalysts used. These results showed that (i) the support materials are inert in the reaction; and (ii) a trace amount of oxygen dissolved in water or potential dissociation of phenol due to adsorption does not have any contribution to the conversion of phenol under reaction conditions .

Comparative Example 2

The same reaction conditions as in Example 1 with supported catalysts such as Au (2.4wt%) /HAp, Au (2.4wt%) /α- Fe₂O₃, Au(1.5wt%) /TiO₂, Au (5.0wt%) /Fe₂O₃ and Au(0.8wt%)/C were used for this comparative example, except that Fe (1.2wt%) /ZSM-5 was used as a comparative catalyst at a pH value of 4.0.

As seen in Table 1 below, when Fe (1.2wt%) /ZSM-5 was used (pH of 4.0), the amount of phenol converted at 60 minutes was 60%. The amounts of phenol converted at 60 minutes when Au (2.4wt%) /HAp and Au (2.4wt%) /Ot-Fe₂O₃ were used were higher than that using Fe (1.2wt%) /ZSM-5. This shows that the Au (2.4wt%) /HAp and Au (2.4wt%) /α-Fe₂O₃ catalysts are more effective than Fe (1.2wt%) /ZSM-5.

Further, it is to be noted that although Au (5.0wt%) /Fe₂O₃ appears to have a lower conversion of phenol as compared to that using Fe (1.2wt%) /ZSM-5, Au (5.0wt%) /Fe₂O₃ may be a superior or at least, comparable, catalyst as compared to Fe (1.2wt%) /ZSM-5 because these reactions were carried out at different pH values. As noted in Example 4 above, the conversion of phenol is inversely proportional to the increase in pH. Therefore, if Au (5.0wt%) /Fe₂O₃ was used at a lower pH value of 4.0, the conversion of phenol will be higher than the present value of 50%.

Table 1 demonstrates that the catalytic performance varied significantly depending on the properties of the support materials even though the average size of the gold particles (d_Au) was very close, generally <5.0 nm, except for the Au(0.8wt%)/C catalyst, where the particles are 10.5 nm. Table 1. Comparison of various supported gold catalysts in the conversion of H2O2 ^Ca] .

Supported Au Temp [Phenol] time Conv. STC^[b] catalyst particle [K] (ppm) (min) [%] [rnmolhf¹!/¹] size^[cl

(d_Au:nm)

Au(2.4wt%)/HAp 4.9 343 100 60 80 0.53

Au(2.4wt%)/α- 4.9 343 100 60 85 1.06

Fe₂O₃

Au(1.5wt%)/TiO₂ 3.5 343 100 60 45 0.48

Au(5.0wt%)/Fe₂O₃ 4.0 343 100 60 50 0.64

Au(0.8wt%)/C 10.5 343 100 60 19 0.24

Fe(1.2wt%)/ZSM- NA 343 100 60 60 0.74

₅[d]

[a] Conditions: 2 mg catalyst, 20 ml of 100 ppm aqueous phenol solution, 1 ml of 30(wt)% H₂O₂ solution, pH of 6.8. [b] Space-time conversion, [c] Size of Au particles was determined by TEM. [d] pH value of 4.0 when using Fe/ZSM-5 catalyst.

Comparative Example 3

The same reaction conditions as in Example 1 were used for this comparative example, except that the reaction was carried out at four additional pH values of 2.0, 3.0, 4.0 and 5.0 using Au (2.4wt%) /HAp as the catalyst and Fe (1.2wt%) /ZSM-5 as a comparative catalyst.

Fig. 7 shows the active metal leaching of Au(2.4wt%) /HAp and Fe (1.2wt%) /ZSM-5 across the pH range tested. The results suggest that there was almost no loss of gold in acidic conditions when pH value approached about 2.0. However, the leaching of iron from Fe (1.2wt%) /ZSM-5 increased with gradually decreasing pH value such that a loss of 50% iron was measured at a pH of 2.0.

Fig. 8a and Fig. 8b shows the stability of the Au (2.4wt%) /HAp and Fe (1.2wt%) /ZSM-5 catalysts after several recycles at two pH values of 5.0 and 2.0, respectively. In Fig. 8a, the Au (2.4wt%) /HAp catalyst not only showed a comparable stability with the Fe (1.2wt%) /ZSM-5 catalyst at pH ~ 5.0 after several recycles for the removal of phenol, but also exhibited a high stability in the strong acidic solution of pH ~ 2.0 as observed in Fig. 8b. Almost no decay had been observed after the Au (2.4wt%) /HAp catalyst had been recycled five times. However, poor stability was observed for the Fe (1.2wt%) /ZSM-5 catalyst at strong acidic conditions and the conversion of phenol was decreased from 60% to 10% after the Fe (1.2wt%) /ZSM-5 catalyst had been recycled five times. This is probably due to the leaching of iron, as depicted in Fig. 7 above.

Although it had been established in the art that the use of iron as a catalyst in a Fenton reaction results in a high conversion rate of organic contaminants in acidic conditions, the leaching of iron into the reaction solution and the low stability of iron at low pH are disadvantages of the conventional Fenton reaction. It is to be appreciated that the above disadvantages are not observed in the Au (2.4wt%) /HAp system. Further, the conversion rate of organic compounds when Au (2.4wt%) /HAp is used is higher than that when Fe (1.2wt%) /ZSM-5 is used. This shows that Au (2.4wt%) /HAp is a more superior catalyst as compared to Fe (1.2wt%) /ZSM-5.

Applications

The disclosed catalyst may be used in the treatment of an aqueous medium having organic contaminants therein. Organic contaminants may pose serious threats to human health due to their toxicity and persistence in industrial effluents or wastewater. Accordingly, the disclosed treatment process aids in the remediation of water containing contaminants such as formaldehyde, phenol or alcohols in order to substantially increase the amount of water that is suitable for industrial use or human consumption.

The use of the disclosed catalyst provides for an alternative treatment process that is substantially superior to known treatment processes. For example, the disclosed treatment process can be carried out at mild conditions of pH and temperature as compared to conventional processes. This may cut down on the need for expensive reactors and pH adjusting chemicals that are usually required in conventional processes, leading to an inexpensive and uncomplicated process.

Another advantage of the disclosed treatment process is that the production of waste products that require disposal is substantially minimized as the disclosed catalyst does not substantially participate in the Fenton- like reaction. Since the catalyst does not participate in the Fenton-like reaction, the catalyst can be recycled for use in a number of process cycles, leading to raw material savings. Moreover, the need to stop the treatment process in order to replenish the depleted catalyst is substantially negated.

Still another advantage of the disclosed process is that it can be used for treating wastewater containing organic compounds at low concentration levels, for example, less than 1000 ppm.

The disclosed process can be used to purify rain water in order to obtain freshwater that is suitable for human consumption. It will be apparent that various other modifications and adaptations of the invention will be apparent to the person skilled in the art after reading the foregoing disclosure without departing from the spirit and scope of the invention and it is intended that all such modifications and adaptations come within the scope of the appended claims. References

1. Y.I. Matatov-Meytal, M. Sheintuch, Ind. Eng. Chem. Res. 37 (1988) 309.

2. J. Donglagic, J. Levee, Enviro. Sci. Technol. 32 (1998) 1294.

3. J. Levee, A. Pintar, Catal. Today. 24 (1995) 51.

4. A. Santos, P. Yustos, A. Qiutanilla, G. Ruiz, F. Garcia-Ochoa, Appl . Catal. B 61 (2005) 323.

5. S. Gopalan, P. E. Savage, AIChE J. 41 (1995) 1864. 6. T. D. Thornton, P. E. Savage, Ind. Eng. Chem. Res. 31 (1992) 2451. 7. A. Pintar, Catal. Today. 77 (2003) 451.

Claims

1. A process for treating an aqueous medium having an organic contaminant therein, the process comprising the step of exposing a gold catalyst to the aqueous medium in the presence of an oxidizing agent to thereby form free radicals capable of at least partially degrading the organic contaminant in the aqueous medium.

2. The process of claim 1, wherein the gold catalyst comprises a gold metal portion and a support material portion.

3. The process of claim 2, wherein the gold metal portion is substantially pure gold or a gold alloy.

4. The process of claim 2, wherein the gold metal portion is mounted on the support material.

5. The process of claim 2, wherein the gold metal portion is provided within the structure of the support material .

6. The process of claim 1, wherein the exposing step is undertaken under conditions to substantially degrade the organic contaminant present in the aqueous medium.

7. The process of claim 1, comprising the step of providing the gold catalyst with a particle size in the range of 1 nm to 100 nm.

8. The process of claim 2, wherein the support material is at least one of a metal oxide, a silicate, a metal silicate, an apatite mineral, carbon and combinations thereof.

9. The process of claim 8, wherein the support material is selected from the group consisting of SiO₂,

Al₂O₃, TiO₂, Mn_xOy, Fe₂O₃, SnO₂, ZrO₂, MoO₃, WO₃ and CeO₂, zeolite and hydroxylapatite .

10. The process of claim 2, wherein the catalyst comprises 0.1 wt% to 10 wt% gold metal portion relative to the support material.

11. The process of claim 1, wherein the catalyst has a surface area of 1 m²/g to 1000 m²/g.

12. The process of claim 1, wherein the oxidizing agent is a peroxide.

13. The process of claim 12, wherein the peroxide is hydrogen peroxide.

14. The process of claim 1, wherein the oxidizing agent is added over time intervals to the aqueous medium during the exposing step.

15. The process of claim 14, wherein the time duration of the intervals is 1 minute to 30 minutes.

16. The process of claim 1, comprising the step of maintaining the pH of the aqueous medium between 1 to 9.

17. The process of claim 16, comprising the step of maintaining the aqueous medium at a non-alkaline pH.

18. The process of claim 17, comprising the step of maintaining the aqueous medium in substantially- neutralized pH conditions.

19. The process of claim 1, wherein the mole ratio of the oxidizing agent to the organic contaminant is 10 to 1000.

20. A water treatment process for degradation of organic contaminants comprising the step of exposing a gold catalyst to the water in the presence of a peroxide to thereby form free radicals capable of at least partially degrading the organic contaminants in the water.

21. Use of a gold catalyst and an oxidizing agent for removing an organic contaminant from water.

22. A water treatment catalyst comprising gold metal and being capable of degrading an organic contaminant present in water when in the presence of an oxidizing agent.