HALOGENATED ORGANIC COMPOUNDS USING A COMBINATION

OF LIGHT ENERGY AND ULTRASONIC ENERGY

IN THE PRESENCE OF A PHOTOCATALYST

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a method of treating an aqueous liquid containing halogenated organic impurities. More particularly, this invention relates to a method of treating an aqueous liquid containing halogenated organics using a combination of light energy and ultrasonic energy in the pres¬ ence of a photocatalyst to decompose the halogenated organic impurities in the liquid.

2. Description of the Related Art

It is desirable to remove halogenated organic materials from aqueous liquids such as water con¬ taining chlorinated hydrocarbons, e.g., chlorinated phenols. Prior art removal techniques have included the use of ultraviolet light radiation to decompose halogenated organic compounds. For example, Chou et al. U.S. Patent 4,764,278 discloses a method for reducing the concentration of haloorganic compounds in water by first extracting the haloorganic com- pounds from the water using a water immiscible al ane hydrocarbon solvent. The solvent is then separated from the water and regenerated by exposing the solvent to ultraviolet light to degrade the haloorganic compounds.

Ultraviolet light energy has also been used in combination with a photocatalyst, sometimes also

referred to as a semiconductor photocatalyst, such as Ti0 ₂ , to remove halogenated organic materials from aqueous liquids by decomposure of the organic contaminant. For example, Barbeni et al., in "Photodegradation of 4-Chlorophenol Catalyzed by Titanium Dioxide Particles", Nouveau Journal de Chimie, Vol. 8, (1984), pp. 547-550, describes the decomposition of 4-chlorophenol in an aqueous solution containing a suspension of Ti0 ₂ exposed to radiation of UV wavelength or sunlight to form C0 ₂ and HC1.

D'Oliveira et al., in "Photodegradation of 2- and 3- Chlorophenol in Ti0 ₂ Aqueous Suspensions", Environ. Sci. Technol., Vol. 24, No. 7, (1990), pp. 990-996, discuss the use of Ti0 ₂ in combination with radia¬ tion of >290 nm. , and preferably >340 nm. , to remove 2-chlorophenol and 3-chlorophenol.

Olliε, in "Contaminant Degradation in Water", Environ. Sci. Technol., Vol. 19, No. 6, (1985), pp. 480-484, discloses the removal of trichloromethane (chloroform) or ethylene dibromide from an aqueous solution by simultaneous presence of both Ti0 ₂ and near-UV light of 300 nm. to <400 nm. In both cases, use of either the catalyst or the UV illumination alone did not produce the degradation.

Zepp, in "Factors Affecting the Photochemical Treatment of Hazardous Waste", Environ. Sci. Technol., Vol. 22, No. 3, (1988), pp. 256-257, discusses various photoreactions, pointing out that direct photoreactions of ionizable compounds, such

as chlorophenols, are often very sensitive to pH. He• lso observes that the combination of UV light and ozone is effective for oxidizing pollutants. He states that irradiated semiconductors are versatile reagents that show promise for treatment of hazard¬ ous wastes and that titanium dioxide has been shown to effectively photocatalyze the reduction of chlorinated organics.

Ultrasonic energy has also been used in the removal of halogenated organics from an aqueous liquid. For example, Sittenfield U.S. Patent 4,477,357 describes a process for removal of contaminants such as halo¬ genated organics from a liquid. Halogenated organic materials in oil or water are mixed with an equal amount of an alkaline agent, such as a hydroxide or a carbonate of an alkali metal or an alkaline earth metal, and then exposed to ultrasonic energy to decompose the halogenated organic contaminant. The presence of the alkaline agent is said to signifi- cantly accelerate the dehalogenation and decompo¬ sition of organic ring structures.

Sierka et al., in "Catalytic Effects Of Ultraviolet Light And/Or Ultrasound On The Ozone Oxidation Of Humic Acid and Trihalomethane Precursors", describe the catalytic effects of the use of both UV irradia¬ tion and ultrasound, either singly or in combina¬ tion, on the ozone oxidation of organic materials, such as humic acid, -in aqueous solutions. It is indicated that the most effective reactor conditions for both the destruction of nonvolatile total or¬ ganic carbon and trihalomethane formation potential

utilized both ultrasound and UV irradiation in combination with ozone.

While these methods have been shown to be successful in removing halogenated organic contaminants from aqueous liquids and decomposing such organic mater¬ ials, usually the reaction times are sufficiently slow to reduce the economic attractiveness of such processes, especially for continuous or on line treatment systems.

SUMMARY OF THE INVENTION

It is, therefore, an object of this invention to provide an improved method for treating an aqueous liquid contaminated with one or more halogenated organic compounds which will rapidly decompose such halogenated organic contaminants.

It is another object of this invention to provide an improved method for treating an aqueous liquid contaminated with one or more halogenated organic compounds using a combination of exposure of the contaminated liquid to light energy and ultrasonic energy in the presence of a photocatalyst.

It is a further object of this invention to provide an improved method for treating an aqueous liquid contaminated with one or more halogenated organic compounds using a combination of exposure of the contaminated liquid to light energy and ultrasonic energy while contacting the contaminated liquid with a particulated photocatalyst.

It is a still another object of this invention to prbvide an improved method for treating an aqueous liquid contaminated with one or more halogenated organic compounds using a combination of exposure of the contaminated liquid to ultraviolet light energy and ultrasonic energy while contacting the contami¬ nated liquid with a suspension of particulated photocatalyst.

These and other objects of the invention will be apparent from the following description and accom¬ panying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 is a flowsheet illustrating the process of the invention.

Figure 2 is a diagrammatic layout of typical appara¬ tus which may be used in carrying out the method of the invention.

Figure 3 is a graph showing the purification of a contaminated aqueous liquid containing 2,4-dichloro- phenol with and without the use of ultrasound in combination with UV light.

Figure 4 is a graph showing the purification of a contaminated aqueous liquid containing pentachloro- phenol with and without the use of ultrasound in combination with UV light.

Figure 5 is a graph plotting the decomposition of 3- chlorobiphenyl in an aqueous liquid against time

using a Ti0 ₂ photocatalyst with both ultraviolet light and ultrasound and with only ultraviolet light.

Figure 6 is a graph plotting the absorption of light at 240 nm. by 4-chlorophenol against time using a ZnO photocatalyst with both ultraviolet light and ultrasound, as well as with only ultrasound, and with only ultraviolet light.

DETAILED DESCRIPTION OF THE INVENTION

The invention provides a novel process for effic¬ iently removing halogenated organic compounds from contaminated aqueous liquids by contacting the contaminated liquid with a particulated photocata¬ lyst, while simultaneously exposing the contaminated liquid to acoustic energy and light energy to decom¬ pose the halogenated organic compounds.

Photocatalytic degradation requires a relatively inexpensive, safe, and nontoxic catalyst. It produces no toxic by products, can utilize concen- trated solar energy to reduce light energy costs, exhibits reasonable yields, and is applicable to a wide variety of organic (many organics will react with hydroxyl radicals at appreciable rates) .

When such catalytic degradation is combined with ultrasound, in accordance with the invention, the resulting process has the potential to offer im¬ proved rates and efficiencies (hence throughputs) under optimized conditions; the ability to use impure or cheaper forms of the photocatalyst;

significant increases (as much as fivefold) in the degradation rate of aqueous organics without poison¬ ing of the catalyst; improvements in conventional suspended catalyst separation methods such as ultrafiltration; and for applications involving treatment of waste water containing suspended solids, the use of sonication may also assist in the release of hydrophobic organics adsorbed on soil particles, for subsequent reaction at the photocata- lyst surface.

Examples of photocatalyst materials which may be used in the practice of the invention include, for example, titanium dioxide (Ti0 ₂ ) , zinc oxide (ZnO) , cadmium sulfide (CdS) , iron oxide (Fe ₂ 0 ₃ ) , ^' gallium phosphide (GaP) , tin oxide (Sn0 ₂ ) , silicon carbide (SiC) , and tungsten oxide (W0 ₃ ) . By way of illus¬ tration, and not of limitation, the photocatalyst will be hereinafter referred to as a Tiθ ₂ catalyst.

By use of the term "photocatalyst" is meant any com- pound in which irradiation of such compound with electromagnetic radiation of visible or ultraviolet wavelength will result in the generation of conduc¬ tion band electrons (e' _cb ) and valence band holes (h ⁺ _vb ) that can then undergo redox reactions at the catalyst surface with species such as water or inorganic and organic compounds. The initiating step in this photocatalytic process requires illumi¬ nation with light of energy higher than the band gap of the semiconductor photocatalyst (e.g., <380 nm. for anatase Ti0 ₂ , the most effective and widely studied photocatalyst) . Electromagnetic radiation

within a wavelength range of from about 250 nano¬ meters (nm.) to about 450 nm. will usually have such an energy level.

While it is not desired to be bound by any theories of the mechanism of the degradation reaction, for degradation of organics in water, the reaction of adsorbed water with these electrons and holes to yield hydroxyl radicals OH ^* and hydrogen radicals H ^* is the generally accepted initial event in the degradation. The organic substrate degradation then occurs via reaction principally with hydroxyl radicals.

The photocatalyst used in the process is preferably a particulated photocatalyst which may be provided in a wide distribution of average particle sizes ranging from an average particles size of as small as, for example, from about 0.05 microns to as large as, for example, +5 mesh (Tyler) , which is about 4000 microns in diameter. Since large surface areas are desired for the photocatalyst to provide a large amount of active sites, small particles will be desired. It will be appreciated, however, that larger size particles could be used, i.e. , particles larger than 4000 micron in size.

The particulated photocatalyst, such as Ti0 ₂ , may be added in dry form to the contaminated liquid and mixed together to form a suspension prior to expo¬ sure of the contaminated liquid to the ultraviolet light and acoustic energy. Alternatively, the dry particulate catalyst may be premixed as a slurry or suspension with an aqueous liquid miscible with the

contaminated liquid, and this premix may then be added to the contaminated liquid prior to exposure of the contaminated liquid to the ultraviolet light and acoustic energy. The photocatalyst may also be incorporated into an inert matrix such as, for ex¬ ample, by coating glass beads with the photocata¬ lyst.

In either instance, the concentration of the partic¬ ulate photocatalyst in the contaminated liquid should range from about 100 milligrams per liter of contaminated liquid to about 2 grams per liter of contaminated liquid. Higher concentrations may be used but are not deemed to be necessary and only would add expense to the process, both from the standpoint of additional material costs as well as additional processing costs, e.g. , if the catalyst must be separated from the purified liquid after the halogenated organic compound has been decomposed.

Typically, the concentration of the photocatalyst in the contaminated liquid will be about 1 gram per liter. Lower concentrations may be used, but this may result in a lowering of the decomposition rate. In this regard, it will be appreciated that the actual amount of catalyst used will depend upon the surface area of the catalyst which will, in turn, depend upon the porosity and/or particle size of the particulate catalyst. A typical surface area range will be from about 1 to about 50 meter ² /gram of photocatalyst such as Ti0 ₂ .

It should be noted that it may be desirable to pretreat the photocatalyst, for example, by heating

to greater than 400"C under a reducing atmosphere, or by washing the catalyst with a concentrated acid, to enhance the efficiency of the photocatalyst.

As stated above, the electromagnetic radiation source should be a light source which provides energy higher than the band gap of the semiconductor catalyst. To accomplish this, the electromagnetic radiation or light source will be within the wave¬ length range of from about 250 nm. to about 450 nm. For a photocatalyst such as Ti0 ₂ , for example, the wavelength range will be from about 290 nm to about 400 nm. , and preferably from about 300 nm. to about 360 nm., i.e., an ultraviolet light source. The intensity of the light source will be inversely proportional to the amount of time which it will takes to affect the desired deco posure of the halogenated organic compounds, i.e., the weaker the light source, the longer the decomposure time will be. This, in turn, will affect the overall effi- ciency of the process and will be of particular importance when the process is being run on a continuous basis rather than as a batch operation.

For a light source which is coupled directly to an optical cell containing the contaminated liquid, the intensity of the light source, at the selected wavelength, e.g., 360 nm. for Ti0 ₂ , should be at least about 1000 microwatts per inch ² of exposure area per liter of liquid to achieve complete decom¬ posure within a reasonable exposure period. Such light sources are commercially available, e.g., a mercury lamp source, or sunlight may be used as the source of light radiation of ultraviolet wavelength.

The distance that the light source is located from the' liquid to be irradiated is important since much of the light energy may be absorbed within a few cm. of the liquid surface. Therefore, depending upon the size and geometry of the reaction vessel, as well as the intensity of the light source, it may be appropriate to use a plurality of such light sources dispersed around the perimeter of the vessel, as well as above and below the respective top and bottom surfaces of the vessel.

In general, it may be stated that the relationship of the intensity of the light source (or sources) to the dimensions of the reaction vessel should be such that the light entering the reaction vessel will still have sufficient intensity or energy to initi¬ ate photocatalytic decomposure of the organic material in the liquid when it reaches the farthest extremity of the volume of the vessel. For a single light source, for example, this would mean the opposite wall of the vessel. However, if a plurali¬ ty of such light sources were to be spaced around the perimeter of the reaction vessel, this distance would mean the distance to the center of the vessel.

It should be noted in this regard that the decompo- sition rate will be dependent upon not only the intensity of the light source and the dimensions of the reaction vessel, but also on the type and concentration of cat-alyst, the intensity of the sonication source, and the type and concentration of the organic material being decomposed.

The acoustic energy is provided by one or more sonication apparatuses or acoustic generators operating at between from about 1 KHz to about 1 MHz, preferably from about 10 KHz to about 100 KHz, and most preferably about 20 KHz, at a power level which may be within a range of from about 10 watts to about 2500 watts.

It should be noted here that such a power range will be dependent upon the size of the vessel into which the acoustic energy will be coupled. If the vessel is sufficiently large, it may be preferable to utilize more than one generator rather than increas¬ ing the power level of an individual generator to more than about 2500 watts. A single acoustical generator having a power range of from about 10 to about 2500 watts will usually provide sufficient power for a reaction vessel up to about 2 liters in volume. Thus, the total power range of the acousti¬ cal source may be expressed as a power range equiva- lent to from about 10 to about 2500 watts for a 2 liter vessel.

The sonication apparatus is coupled, through an appropriate transducer or sonication tip, directly into the optical cell or vessel which contains the contaminated liquid. That is, the transducer or sonication tip is immersed directly in the liquid to be decomposed within the cell. Such an acoustic generator may comprise any commercially available apparatus capable of operating within these ranges such as, for example, Ultrasonic Processor W-2500, available from the Heat Systems Company.

It should be noted here that the intensity of the sonication, like the intensity of the light radia¬ tion, will be proportional to the distance of the liquid being acted upon from the transducer or sonication tip. Therefore, depending upon the size of the vessel, it may be preferable to utilize a plurality of transducers or sonication tips, with the power level of the sonication source adjusted accordingly, as well as using more than one acousti- cal generator as discussed above.

As in the previously discussed relationship between the intensity of the light source (or sources) to the dimensions of the reaction vessel, the power level of the sonication source (or sources) should be such that the acoustic energy imparted to the liquid in the reaction vessel will still have suffi¬ cient energy to accelerate photocatalytic decompos¬ ure of the organic material in the liquid when it reaches the farthest extremity of the volume of the vessel. This, for a single transducer or sonication tip immersed in the liquid would mean the farthest point in the vessel from the location of the trans¬ ducer or tip, i.e., probably the opposite wall of the vessel. However, if a plurality of such sonica- tion tips were to be immersed in the liquid, for example, around the perimeter of the reaction vessel, this distance would mean the distance to the center of the vessel, as in the case of the use of multiple light sources.

While, as stated above, there is not the intention to be bound by theories of operation of the process of the invention, it is believed that by combining

sonication and photocatalytic degradation reaction enhancement is provided due to: cavitational effects which lead to dramatic increases in temperature and pressure at the localized microvoid implosion sites; cleaning or sweeping of the photocatalyst surface due to acoustic microstreaming which allows or pro¬ vides more active sites; increased mass transport of reactants and products at the catalytic surface and in solution; increased photocatalyst surface area due to fragmentation or pitting of the photocatalyst particles by the sonication; cavitational inducement of radical intermediates which become involved in the destruction of the organic compounds; and reac¬ tion of the organic substrate directly with the photogenerated surface holes and electrons.

The halogenated organic compounds present in the contaminated liquid to be purified may comprise halogenated aromatic compounds such as, for example, chlorinated phenols (e.g., 2,4-dichlorophenol, 4- chlorophenol, pentachlorophenol) , chlorinated bi- phenyls (e.g., 3-chlorobiphenyl and 4,4'-dichloro- phenyl) , brominated biphenylε, and halogenated benzene derivatives (e.g., chlorobenzene, 4-chloro- toluene, chlorinated dioxin, and halogenated benzo- furans) .

Examples of halogenated aliphatic organics include halogenated hydrocarbons such as fluoromethanes, ethanes, propanes, etc.; chloromethanes, ethanes, propanes, etc. ; bromomethanes, ethanes, propanes , etc.; and mixtures of same; halogenated alkenes, such as trichloroethylene; halogenated alcohols, such as l-chloro-2-propanol; halogenated ketones;

halogenated aldehydes, such as aldrin aldehyde; halogenated carboxylic acids, such as trichloroace- tic acid; and halogenated ethers, such as bis(2- chloroisopropyl)ether.

The concentration of such halogenated organic compounds in the aqueous liquid may range from as little as 2 ppm to as much as 2000 ppm. After purification by the process of the invention, the concentration of such halogenated organic impurities in the previously contaminated liquid may be reduced to less than 1.0 ppm.

Exposure of the contaminated aqueous liquid con¬ taining such halogenated organic compounds to both UV radiation and acoustic energy, in the presence of the particulated Ti0 ₂ catalyst, results in decompos¬ ure of the halogenated organic compounds into HX (where X is the particular halogen) and C0 ₂ gases, which may then be subsequently removed from the liquid. A separate source of oxygen is usually not necessary for the formation of the C0 ₂ decomposition product, since there will normally be sufficient dissolved oxygen in the aqueous liquid. However, a separate source of oxygen may be optionally provided if desired, which could be introduced into the contaminated liquid through a sparger ring or the like.

Turning now to Figure 2, the process of the inven¬ tion is schematically illustrated. A contaminated liquid source 10 such as water containing halogen- ated organic contaminants, in a concentration which may range from about 2 ppm to about 2000 ppm, is

mixed with a source of particulated Ti0 ₂ catalyst 20 which may be in dry form or, preferably, in a previously formed suspension or slurry. The amounts of each source may be adjusted, respectively, through valves 12 and 22 to adjust the proportions to provide the desired concentration range of from about 100 milligrams to about 2 grams of catalyst per liter of contaminated liquid flowing through valve 26 into optical cell 30.

Optical cell 30 may comprise any reaction vessel having sidewalls (or one or more openings in the sidewalls) transparent to electromagnetic radiation, e.g., UV light radiation of from about 300 nm. to about 360 nm. for a Ti0 ₂ photocatalyst, to permit the liquid in optical cell 30 to be irradiated by light energy from a light source 40 which is prefer¬ ably coupled directly to the sidewall of cell 30 to permit the most efficient coupling of the light energy to cell 30 from source 40. While Figure 2 illustrates light source 40 as a single source, it will be readily appreciated that the light energy may be passed through the transparent sidewalls or windows through a plurality of such light sources arranged around the periphery of optical cell 30, as previously discussed, to uniformly illuminate the liquid therein with light of the proper wavelength during the decomposure of the halogenated organic compounds in the contaminated liquid in cell 30.

Also mounted on the sidewall of optical cell 30 is an acoustic energy source 50 which provides acoustic energy to the contaminated liquid in cell 30 simul¬ taneous with irradiation of the liquid with the

light source through a transducer 54 immersed in the contaminated liquid in cell 30.

The dimensions of cell 30 are selected, with respect to the location and energy levels of the light and acoustical energy sources, so that all of the liquid in cell 30 will be exposed to both light energy and acoustical energy at an energy level sufficient to cause decomposure of the halogenated organic mole¬ cules in the liquid.

Cell 30 may be further provided with temperature controlling means 32 to maintain the liquid being treated within cell 30 within a temperature range of from about 15"C to about 60'C. Pressure control means 34 may also be provided to increase the hydrostatic pressure within cell 30 from atmospheric pressure up to a pressure of about 500 psi.

The halogenated organic compounds in the aqueous liquid decompose in optical cell 30 upon exposure to both the light radiation and acoustic energy in the presence of the particulated photocatalyst. Because of the synergistic effect of exposing the halogenat¬ ed organic compounds to both of said energy sources in the presence of the photocatalyst, the compounds quickly break down at a rate faster than when alone is used, although it will be appreciated that this rate will be dependent upon the cell volume of contaminated liquid "being treated, the type of contaminated liquid, the intensity of the light and ultrasound energy sources and, when the process is being run on a continuous basis, the throughput or

flow rate of the contaminated liquid through the cell.

After decomposure of the halogenated organic contam¬ inants in the liquid, the liquid passes out of cell 30 through valve 36 to a separation zone 60 wherein the catalyst and any other solid residues which may remain from the decomposed organic compounds may be separated from the liquid, by any suitable separa¬ tion means, which may, for example, include the addition of a flocculating agent, as shown at 62. Such separation means can include, for example, filtration, centrifugation, or settling.

The now purified liquid then flows into a reservoir 70, while the solid residues or sludge passes into a receptacle 80, from which the photocatalyst may be later either completely or partially recovered and recycled if desired. Volatile gases from the decomposition step, such as HCl and C0 ₂ , may be removed from the apparatus at exhaust port 90.

When it is desired to operate the process as a batch process, valve 26 may be opened to fill optical cell 30 while drain valve 36 is closed. If the process is to be operated on a continuous basis, valves 26 and 36 are both opened and adjusted to provide the same amount of flow into and out of optical cell 30.

As an example of the operation of the process of the invention as a batch process, when optical cell 30 has a volume of about 25 liters, the process can be successfully operated by exposing the contaminated liquid, e.g., an aqueous liquid contaminated with

2,4-dichlorophenol, to both the light radiation and the acoustic energy for a period of 20 minutes with the intensity of light source 40 (within the 300-360 nm. range) maintained at about at least 10 illi- watts/cm ² , and acoustic source 50 maintained at a power level of at least about 500 watts within the previously recited frequency range, in the presence of about 25 grams of a photocatalyst such as Ti0 ₂ , to provide a purified liquid having a level of halogenated organic impurities of less than about 1.0 ppm.

The following examples will serve to further illus¬ trate the practice of the process of the invention:

Example I

About 25-30 ml. of a contaminated aqueous solution containing 9 x 10^ molar 2,4-dichlorophenol was placed in a 40 ml cylindrical jacketed glass cell having about a 3 cm. inner diameter. A sufficient amount of particulated titanium dioxide (Degussa P25, anatase), having a particle size of about 2 microns, was added to provide a concentration of about 0.2 weight percent of catalyst, based on total weight of the solution, including the catalyst.

The cell was irradiated by a Blak-Ray B-100A 100 watt mercury bulb ultraviolet light source having a nominal intensity of 7000 microwatts/cm. ² at 350 nm. and which was positioned 5 inches from the outer jacket wall of the cell. The solution was sonicated using a 1/2" titanium horn immersed 1 cm. into the solution in the cell and powered by a Heat Systems

XL2020475 watt ultrasonic processor. The amplitude of the ultrasonic vibration at the tip of the horn was set to the maximum value of 120 micrometers, which for this cell and contents required an output power of approximately 130 watts.

During the sonication and photolysis, the cell was water cooled to maintain the solution temperature at 35 ^* C. After sonication and photolysis, the sample was analyzed for chloride ion production by means of a chloride ion selective electrode, and for the disappearance of the particular chlorinated compound by UV/visible spectrophotometric analysis after filtration through a 0.20 micron Teflon filter. A second sample of the same contaminated liquid was also treated in the same manner except that no ultrasound was used. The results are shown in the graph of Figure 3 wherein the amount of chlorine released is shown plotted against time. The graph shows the marked improvement in removal of chlorine from the solution when both UV light irradiation and ultrasound are used in combination with the photo¬ catalyst, with complete destruction of the 2,4- dichlorophenol within 120 minutes using both ultra¬ sound and UV light in contrast to destruction of only about 25% of the chlorinated phenol when UV light radiation was used without ultrasound. When ultrasound was used without UV light, it had no effect on the degradation of the chlorinated phenol.

Example II

Several contaminated water samples containing a 2.5 millimolar concentration of pentachlorophenol were

treated in the same manner as the samples treated in Example I. As shown in the graph of Figure 4, when the sample was exposed to both UV light and ultra¬ sound, rapid and quantitative destruction of the pentachlorophenol was noted within 2 hours, whereas the use of UV light without ultrasound resulted in a slower initial rate of destruction of the penta¬ chlorophenol and a maximum degradation efficiency of only about 30%.

Example III

To an aqueous solution containing about 75 ppm (4 x 10"*M) of 3-chlorobiphenyl was added sufficient particulated Ti0 ₂ , as described in Example I, to provide a concentration of 0.2 wt.%, based on total weight of the solution. 30 ml. aliquots of this solution containing the photocatalyst were then respectively subjected to UV light irradiation alone, and combined UV light and ultrasound irradia¬ tion, in the same manner as previously described in Example I. The results are shown in the graph of Figure 5. The rate of appearance of chloride (decomposition of the organic compound) was fairly linear over time for both treatments. However, it can be seen that the rate using the combination of UV light and sonication was approximately 3 times greater than without sonication.

Example IV

Several contaminated water samples containing 0.001 molar 4-chlorophenol were treated in the same manner as the samples in Example I, except that zinc oxide

was used as the photocatalyst. The results are shown in the graph of Figure 6, which plots the absorbance by 4-chlorophenol of light at 240 nm. against time. As shown in the graph cf Figure 6, when the sample was exposed to both UV light and ultrasound, rapid destruction of the 4-chlorophenol was noted and complete degradation occurred within about 100 minutes. The use of only UV light in the presence of the zinc oxide photocatalys- resulted in a much slower rate of degradation, while the use of ultrasound alone, i.e. without UV light, did not result in any appreciable degradation of the 4- chlorophenol in the sample. Similar results can be obtained using cadmium sulfide, tungsten oxide, iron oxide, gallium phosphide, tin oxide,, or silicon carbide as the photocatalyst.

Thus, the invention provides an improved process for the removal of halogenated organic impurities from an aqueous liquid by the simultaneous exposure of the contaminated liquid to both ligh radiation and acoustic energy in the presence of a photocatalyst to cause the efficient decomposure cf t e halogenat¬ ed organic compounds in the contaminated liquid.

Having thus described the invention whar is claimed is: