CA1287829C

CA1287829C - Composite photocatalyst for refractory waste degradation

Info

Publication number: CA1287829C
Application number: CA000519650A
Authority: CA
Inventors: Cooper H. Langford; Mark K.S. Mak; Andrew M. Crouch
Original assignee: Canadian Patents and Development Ltd; Concordia University
Current assignee: Canadian Patents and Development Ltd; Concordia University
Priority date: 1986-10-02
Filing date: 1986-10-02
Publication date: 1991-08-20
Also published as: US4806514A

Abstract

Abstract of the Disclosure The invention is concerned with a composite photocatalyst for refractory waste treatment, comprising particles of a wide band gap semiconductor material selected from the group consisting of titanium oxide, cadmium sulfide and cadmium selenide, the particles being coated with a polymer film capable of absorbing a refrac-tory waste substrate to be treated and comprising a pyridine-containing polymer and a divalent metal porphyrin or metal phthalocyanine-based dye. The dye is molecularly dispersed throughout the film and chemically bonded to the pyridine-containing polymer. Upon mixing of the photo-catalyst with the refractory waste substrate and irradi-ation with light having a wavelength of about 300 to about 400 nm, the photocalyst of the invention generates in the polymer film thereof reactive species which are sufficient-ly oxidizing to degrade the refractory waste substrate absorbed in the polymer film.

Description

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The present invention relates to a composite photocatalyst for the photochemical degradation of refrac~
tory waste materials and to a method of using the pho-toca-talyst for treating refractory waste materials which are known to withstand high temperature oxldation, such as chlorinated aromatic compounds and metal cyanide complexes, to degrade same. The invention is particularly direc~ed to the detoxification of PCB's (polychlorinated biphenyls).
Previous reports have described the dechlorination of PCB's involving a photocatalytic process using titanium dioxide. The scope of the potential for ph~tocatalysis in - waste treatment was reported in a paper by Oliver and Carey, ~ -Water Poll. Res. J. Can., 1980, Vol. 15, p. 157. In this report, the authors were able to demonstrate laboratory potential for all of the following reactions using titanium dioxide:
l.Oxidation of cyanide.

2.Decoloration of pulp mill black liquors.

3.Dechlorination of chlorobezoate to benzoate.

4.Increase of biodegradability of a lignin model.

5.Detoxification of AROCLOR 1254 (trademark; a polychlorinated biphenyl).
Titanium dioxide has been the most popular of metal oxide semiconductors for study of photocatalysis for nearly 50 years. It has a band gap of 3.2 eV and absorbes light starting from 350 nm and continuing toward higher energy. Consequently, there has been continuing interest in methods for the sensitization of titanium dioxide to allow ~ -~ use of longer wavelength.
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Japanese Patent No. 57-166,175, for example, describes the use of titanium dioxide with 5~ platinum - deposited thereon as a photocatalyst for decomposing Pcs's under visible and ultraviolet light. Such a pho-tocatalyst, however, is effective for all~practical purposes on PCB in water only, and no-t on PCB in oil, the more normally enc-ountered form of PCB waste.
; It is therefore an object of the present invention ~ to overcome the above drawback and to provide a composite ;~ 10 photocatalyst for the photochemical degration of refractory ; waste materials such as PCB's, which photocatalys-t has increased reactivity and is capable of efficiently degrading ; the refractory waste when in an organic phase rather than in water.
In accordance with the invention, there is pro--~ vided a composite photocatalyst for refractory waste - treatment, comprising particles of a wide band gap semicon-... ..
ductor material selected from the group consisting of titanium dioxide, cadmium sulfide and cadmium selenide, the particles being coated with a polymer film capable of absorbing a refractory waste substrate to be treated and comprising a pyridine-containing polymer and a divalent metal porphyrin or metal phthalocyanlne-based dye. The dye is molecularly dispersed throughout the film and chemically bonded to the pyridine-containing polymer. Upon mixing of the photocatalyst with the refractory waste substrate and irradiation with light having a wavelength of about 300 to about 400 nm, the photocalyst of the invention generates in the polymer film thereof reactive species which are suffi-ciently oxidizing to degrade the refractory waste substrate ~ absorbed in the polymer film.

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~2~7~3~9 The present invention also provides, in another aspec~ thereof, a method of photochemically degrading a refractory waste substrate, which comprises the steps of:
a) dispersing particles of a composite photoca-talyst as defined above in a polar liquid medium to form a slurry of the photocatalyst particles;
- b) mixlng the slurry o photocatalyst particles obtained in step (a) with the reractory waste substrate;
and c) irradiating the mixture of substrate and - slurry of photocatalyst particles under agitation with light ;- having a wavelength of about 300 to about 400 nm to cause degradation of the refractory waste substrate.
The development of a photocatalyst for practical applications such as the detoxification of PCB waste ~ involves three aspects. First, a catalyst must be effective -~ for initiating, photochemlcally, the deslrèd reactlons by ~-generation of reactive species. Thus, the reactive species - must somehow be prevented from undergoing undesirable~recom-bination. Second, the catalyst must be stàble and fabri-~ ;
cated from relatively~inexpensive materials, Third, it must~
absorb light over a usefuI region of the spectrum.
The composite photocatalyst according to the . . .
invention meets all of the above requirements. Indeedr it has been observed that the photoreaction is initiated by~ -hole scavenging of the porphyrin or phthal~ocyanine dye ~to produce the strong}y oxidizing cation radical of the porphyrin or phthalocyanine. Hole-electron recombination~is~

~; prevented by the semiconductor providing a~potential gra-.`~ 30 d1ent across the space charge layer between the dye and the~

semiconductor, so that long lived reactlve species, i.e.

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.~,' ~ ~37~3~9 highly oxidizing porphyrin and phthalocyanine radicals, are produced. It would appear that the electron from the dye would be attracted to and be trapped in localized dopiny sites of the semiconductor. Thus, the semiconductor would play the role to render the photoreaction irreversible.
The presence of pyridine groups in the polymer film is also essential since pyridine has been fo~nd to substantially increase the reactivity of the oxidized porphyrin or phthalocyanine. The catalysis of reac~ions of substrates difficult to oxidize such as PCB's and metal - cyanide complexes would depend upon efficient charge transfer between the dye excited under blue or ultraviolet light and the semiconductor via the pyridine groups of ~he polymer film.
The wide band gap semiconductor materials which can be used in accordance with the invention are titanium dioxide (TiO2), cadmium sulfide (CdS) and càdmium selenide (CdSe). These materials are readily availa~le at low cost, are stable under light and are non-toxic. They preferably have a particle size ranging from about 1 to about 10 ~u.

- Ferric oxide (Fe203) and cadmium telluride (CdTe), on the other hand, which are also wide band gap semiconductors, are not effective to provide the desired photocatalyst and are thus inoperative in the context of the present invention.
Coated over the semlconductor materia1 is a polymer film comprising a pyridine-containin~ polymer,~ such as polyvinylpyridine. The polymer may be a polymer blend~
. :
involving for example the co-polymerisation of yol~vinyl- ~

, pyridine with styrene to control the substrate absorption ;-and hydrophobicity of the film and/or the mixing of the polyvinylpyridine with a cationic ionomer to allow the ~L2~ 9 incorporation of electrolytes in the film. The polymer ilm generally has a thickness ranging from about 0.5 to about 2 u, preferably 1 u.
Examples of suitable divalent metal porphyrin and phthalocyanine dyes which can be incorporated in-to the polymer film include zinc, palladium and magnesi~m tetraphe-nylporphyrins and copper phthalocyanine. The dye must contain a divalent metal for stability, with or without ionic groups such as sulfonates on the periphery of the organic ring.
According to a particularly preferred embodiment, the polymer film comprises zinc tetraphenylporphyrin and polyvinylpyridine or a copolymer of vinylpyridine with styrene in a zinc to pyridine mole ratio of about 1:6. A
photocatalyst with'such a film is especially suited for the dechlorination of mixed polychlorinated biphenyls such as AROCLOR 1254* to release ionic chloride. The PCB's may be dissolved in an organic phase while the photocatalyst is slurried in a polar liquid medium, such as water, methanol or acetonitrile, the sample then being stirred with the ; catalyst slurry and irradiated with light in the 300-400 nm range.
According to another preferred embodiment, one part by weight of polyvinylpyridine is mixed wlth two parts - by weight a cationic ionomer of the formuIa:

~ ~ n :~ 30 N(C2H \ N(CH2cH2O~l)3 Cl~ Cl~
- * Trade mark ~ ~ .

~ ~ ~t~ 9 where n is the number of recurring units per molecule of ionomer, to provide a cationic film which incorporates tetrasulfonated zinc tetraphenylporphyrin at cationic sites with the polyvinylpyridine available for coordination. A
photocatalyst with such a polymer film is effective for the oxidative degradation of Fe(CN)6 3 without the release of free cyanide at wavelengths of 300-400 nm.
The following non-limiting examples further ~

illustrate the invention. -A solution of polyvinylpyridine (PVP~ copolyme-rised with 10~ polystyrene was prepared by dissolving 104 mg of the polymer in 100 ml of CHC13. To this sol~ltion, 24 mg of zinc tetraphenylporphyrin (ZnTPP) was added. Prepara~ion of the photocatalyst was accomplished by dispersing 100 mg of 10 ~u size titanium dioxide (TiO2) particles into 5 ml of - the prepared ZnTPP/PVP solution along with 25 ml of CH2C12 and sonicating for 24 hours in a Petri dish. The final produc~ was oE a greenish colour. -The procedure of Example 1 was repeated, except that the ZnTPP was replaced by palladium tetraphenylpor- ~
phyrin (PdTPP). A composite photocalyst comprising parti- ~ `
cles of TiO2 coated with a PdTPP/PVP film wa;s obtained~
having a similar greenish colour.

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The procedure of Example 1 was repeated, except that the ZnTPP was replaced by magnesium tetraphen~lpor-, . .
phyrin (MgTPP). A composite photocatalyst comprising particles TiO2 coated with a MgTPP/PVP film was obtained, having a similar greenish colour.

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, ~ , ;, ~37~29 Three types of substrates, namely o-dichoroben-zene, an AROCLOR sample and 2,2',3,3',4,4',6-heptachlorobi-phenyl isomers (HCB), were used to evaluate the effeckive-ness of the photocatalyst obtained in Example 1 as compared with Ti02 alone. The o-dichlorobenzene used was reayent grade and found to have a single sharp peak in the gas chromatogram (retention time was 2.38 minutes at 140C).

AROCLOR pesticide samples were gas chromatography standards supplied by Chromatographic Specialities Limited and were - used without further purifications. 2,2',3,3',4,4',6-hepta-~; chlorobiphenyl monomers were supplied by RFR Corporation and found to have a single peak in the gas chromatogram (reten-tion time 26.20 minutes at 180C).
A stock solution of o-dichlorobenzene in hexane was prepared. The concentration was 168.36 ppm. The AROCLOR pesticides stock in hexane as supplied was 100 ppm.
The stock solution prepared of 2,2',3,3',4,4',6-heptachloro~

biphenyl in hexane was 74.70 ppm in concentration.
Photolysis of each sample was generally performed according to the following procedure. A portion of 0.03 g of the prepared catalyst was weighed into a 50 ml conical ; Pyrex flask. This was followed by 10 ml of distilled and deionized water. The organic phase was made up of 1 ml of the substrate stock plus 5 ml of hexane (in experiments involving paraffin, the 5 ml of hexane was replaced wlth 5 ml of paraffin). The conical flask was then assembled with~

- a condenser that was stoppered at the top. The final assembly was a closed system. The thermosta~ed cell was ; ~30 fabricated in Pyrex with a collapsed and flattened area ~ which served as the window for irradiation. The light . ~ . ., -7~
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37~3~9 source in all the experiments was a Xenon lamp uni-t supplied by Photochemical Research Associates Inc. with a 150 W ~BO
lamp. The duration of irradiation was generally 3 hours unless specified otherwise. The sample solution was s-tirred with a magnetic stirrer continually during irradiation.
Another 5 ml of hexane was added to the organic phase after the irradiation was over. A 2 ml aliquot was carefully pipetted from the organic phase and further diluted with 25 ml of hexane before it was subjected to gas chromatographic analysis. A control in the dark was prepared in exactly the same manner and left to equilibrate under stirring for 3 hours. All the hexanes used were of pesticide grade.
The column used in the gas chromatography (GC) was 2% OV -~ 3% QF. Carrier gas was 5% methane -~ 95% argon at a flow rate of 60 ml/minute. The column was either programmed at 180C or 200C. The detector for the GC was an electron capture detector (ECD) at 180C, the parent peak `
of HCB referred to hereinbelow appeared at approximately 19 `
minutes. The intermediate peak referred to appeared at approximately 12 minut2s and at 200C, at approximately 10 minutes. Finally, the short retention time peak referred to appeared at approximately 5.5 minutes while at 200C, the same peak appeared at approximately 5 minutes. -Preliminary experiments evaluated the effective-ness of TiO2 alone. The standard aqueous slurry was 10 ml of distilled water hexane or paraffin oil (1 ml hexane stock plus 5 ml diluent).
Four hours of irradiation of o-dichlorobenzene in paraffin oil resulted in 34% loss of the dichlorobenzene GC
peak as opposed to zero loss from samples irradiated without TiO2 or dark controls. A parallel experiment employing ~.
; -8-s,. -~3~7~3~9 AROCLOR 1016 revealed very little degradation. The same system in hexane yielded no significant increase of photode-gradation. Thus, Ti02 alone is very much less eficient when the substrate is in an organic phase than has been ; reported in the literature for aqueous experiments.
With -this result in hand, the Ti02 was replaced with the same weight of the composite photocatalyst ZnTPP/-PVP/Ti02 obtained in Example 1. On irradiation of the AROCLOR 1016 mix-ture, there were decreases in all GC peaks associated wi-th component PCB's. Some decreased by as much - as 54~ in three hours. A solution of the pure isomer, HCB, in hexane could be photodegraded with zero order kinetics -- over a period of about four hours. GC results indicate the loss 95~ of all species which have well defined retention ' times and are detectable with the ECD. Addition of AgN03 to the aqueous phase indicates a relèase of approximately stoichiometrlc amounts of chloride ion. In the course of the degradation, intermediate retention time peaks are seen - which are assignable as partially dechlorinated PCB's. HCB was used to study the reAction in more detail.
The conditions ~ust described utilize white light containing 300-400 nm UV, visible light and IR which raised `
the temperature to approximately 40C. Thermostating the reaction vessel to 25C greatly reduced the rate of the reaction, but there was no detectable thermal reaction in - ~ the dark at 40C. Similarly, the use of a 410 nm cutoff `
filter reduced the rate of reaction to an unobservably small value. When the~heavy paraffin oil was substituted for~
hexane as the solvent for HCB, the rate dropped by a factor ~ 30 of approximately two and the four hour conversion was 50~.
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Carrying out the irradiation test described in Example 4, but using instead the composite photocatalys~
PdTPP/PVP/TiO2 obtained in Example 2, gave equivalent results.

Carrying out the irradlation test set forth in Exarnple 4, but using instead the composite photocatalyst MgTPP/PVP/TiO2 obtained in Example 3, provided a 35% reduc- :
- 10 tion in reaction rate as compared with the photocatalyst ZnTPP/PVP/TiO2 1. Preparation of cationic ionomer.
A 100 ml solution of benzene containing 3.166 g of styrene, 18.374 g of chloromethylstyrene and 0.5188 g of 2,2'-azobis(isobutyronitrile) (AIBN) was de~gassed with nitrogen. The polymerisation process was then allowed to proceed for 24 hours under a blanket of nitrogen. The random copolymer was precipitated from n-hexane to remove unreacted monorners. The precipitate was then rediss~lved in 100 ml benzene and 13 ml of triethylamine was added. The resulting solution was refluxed for 1-1/2 hours at 80 C and ~ then allowed to cool. 10 ml of triethanolarnine was then ; added and refluxing was allowed to continue for an addi-tional 90 min. The resulting copolymer was then precipi--~ tated by isopropanol and the light-yellow ~oly~er was dried~ ;
under vacuum at 30C for 24 hours. 2 grams of this p~olymer which has the formula (I) defined hereinabove (hereinafter referred to as ionomer ~I)), was dissolved in 100~ ml of methanol to give a working solution of effectively 2 ionomer (I).

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~ ~f~7~ 9 2. Preparation of Doly(4-vinylpyridine) (PVP) solu-tion.
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A 2% solution of PVP was prepared in methanol by dissolving 2 g of PVP (lO~ costyrene) in 100 ml of methanol.
3. Preparation of tetrasulfonated porphyrin dye.
Q.7 g of tetrasodium-mesotetra(4-sulfonatophenyl) ~- porphine(12-hydrate) (TPPS 4) was refluxed with Zn metal (which was etched with HCl and washed thoroughly with distilled water) in distilled water for 24 hours. After cooling, the solut on was decanted from the excess Zn metal, the water evaporated, and the dried product stored under vacuum at 60 C for 24 hours. UV-Visible spectra of the ; dissolved dye (ZnTPPS 4) show complete conversion of TPPS to ZnTPPS.
~ 4. Preparation of dye-polymer blend (casting solution?
- 53~92 mg of ZnTPPS 4 dissolved in lO ml of methanol was added to 10 ml of 2% PVP in methanol and lO ml of 2% ionomer (I) in methanol. This solution gave an - effective concentration of 165 mM ZnTPPS , 0.67% PVP and ; 20 0.67% ionomer in methanol.
5. Preparation of composite photocatalyst _ The semiconductor powders TiO2, CdS and CdSe were coated in the following way. l.00 g of semiconducting powder was added to a beaker containing 5 ml of the casting solution and 50 ml of methanol. These suspensions~ were sonicated to dryness (usually for 3-4 hours). Sonication allows equal dispersion of the semiconducting particle-s in the polymer-dye solutions. The effective loading of the semiconducting particles with dye is about a . 75-0.85% (w/w).
The dried catalysts which adhere to the sides of the beaker were scrapped off with a spatula. The physical appearance ~ .

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of the particulate catalysts ranges from green-yellow for the coated Ti02 to yellow for the coated CdS. Further drying of the catalyst took place in a vacuum oven a~ 30C
for 24 hours.

, The three composite photocatalysts obtained in Example 7 were irradiated as 0.2% slurries in a 100 ml three-necked round bottom flask. The solution was a 75 ppm K3Fe(CN)6 in 0.1 M NaOH. The irradiation source was a 75 watt Xe lamp and the light beam was passed through a 5 cm water filter to remove IR and a 400 nm cutoff filter to remove UV light, respectively. The incident radiation at the focal point inside the solution slurry was 200 mW/cm2 (Coherent power meter). The flask was equipped with a water condenser and the slurry was stirred continuously during irradiation.
- The reaction was monitored in two ways:
(i) Measuring the absorption at 300 nm for the disappearance of K3Fe(CN)6.
(ii) Measuring the absorption at 578 nm for the appearance of CN (by the standard ASTM colorimetric method for CN determination).
1.5 ml samples were taken at different timè
intervals with a syringe. The samples were filtered through a no. 1 Whatman filter paper to remove the catalyst ~and then testing proceeded.
:~ .
~ All three coated catalysts were compared to ~he ,~ corresponding bare semiconductor particles. In all cases, degradation was observed to be greater for the coated c~talyst. Approximately 4 hours irradiation was required for complete degradation by coated Ti02 or CdS. Coated CdSe .

- .

~ ': . " ,, 1 , , ~f~'7~:9 completely degraded the ferricyanide in about 150 minutes.
Kinetics are not simple. In no case was free cyanide detected as a degradation product. The extent of degradation of ferricyanide after 60 minutes is reported in the following table:
Ferricyanide remalning Photocatalyst after 60 mins. irradiation _ - (ppm) bare TiO2 58 coated TiO2 37 bare CdS 43 coated CdS 24 bare CdSe 50 coated CdSe 31 :~ .
As it is apparent from the table, the composite ; photocatalyst of the invention is significantly more effective th\n the corresponding bare semi-onduc or.

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Claims

1. A composite photocatalyst for refractory waste treatment, comprising particles of a wide band gap semicon-ductor material selected from the group consisting of titanium dioxide, cadmium sulfide and cadmium selenide, said particles being coated with a polymer film capable of absorbing a refractory waste substrate to be treated and comprising a pyridine-containing polymer and a divalent metal porphyrin or metal phthalocyanine-based dye, said dye being molecularly dispersed throughout said film and chemic-ally bonded to said pyridine-containing polymer, whereby upon mixing of said photocatalyst with said refractory waste substrate and irradiation with light having a wavelength of about 300 to about 400 nm, said photocatalyst generates in the polymer film thereof reactive species which are suffi-ciently oxidizing to degrade the refractory waste substrate absorbed in said polymer film.

2. A composite photocatalyst according to claim 1, wherein said semiconductor material is titanium dioxide.

3. A composite photocatalyst according to claim 1, wherein said pyridine-containing polymer is polyvinyl-pyridine or a copolymer of vinylpyridine with styrene.

4. A composite photocatalyst according to claims 1, 2 or 3, wherein said dye is a divalent metal porphyrin selected from the group consisting of zinc tetraphenylpor-phyrin, palladium tetraphenylporphyrin and magnesium tetra-phenylporphyrin.

5. A composite photocatalyst according to claims 1, 2 or 3, wherein said dye is copper phthalocyanine.

6. A composite photocatalyst according to claims 1 or 2, wherein said polymer film comprises zinc tetra-phenylporphyrin and polyvinylpyridine or a copolymer of vinylpyridine with styrene in a zinc to pyridine mole ratio of about 1:6.

7. A composite photocatalyst according to claims 1 or 2, wherein said polymer film comprises polyvinylpyri-dine or a copolymer of vinylpyridine with styrene blended with a cationic ionomer to provide a film having cationic sites, said film incorporating tetrasulfonated zinc tetra-phenylporphyrin at the cationic sites thereof.

8. A composite photocatalyst according to claim 1, wherein said particles of semiconductor material have a size of about 1 to 10 u.

9. A composite photocatalyst according to claim 1, wherein said polymer film has a thickness of about 0.5 to about 2 u.

10. A method of photochemically degrading a refractory waste substrate, which comprises the steps of:
a) dispersing particles of a composite photo-catalyst as defined in claim 1 in a polar liquid medium to form a slurry of the photocatalyst particles;
b) mixing the slurry of photocatalyst particles obtained in step (A) with the refractory waste substrate;
and c) irradiating the mixture of substrate and slurry of photocatalyst particles under agitation with light having a wavelength of about 300 to about 400 nm to cause degradation of said refractory waste substrate.

11. A method according to claim 10, wherein said polar liquid medium is selected from the group consisting of water, lower alkanols and acetonitrile.

12. A method according to claim 10, wherein use is made of a composite photocatalyst in which the semicon-ductor material is titanium dioxide.

13. A method according to claim 10, wherein use is made of a composite photocatalyst in which the pyridine-containing polymer is polyvinylpyridine or a copolymer of vinylpyridine with styrene.

14. A method according to claims 10, 12 or 13, wherein use is made of a composite photocatalyst in which the dye is a divalent metal porphyrin selected from the group consisting of zinc tetraphenylporphyrin, palladium tetraphenylporphyrin and magnesium tetraphenylporphyrin.

15. A method according to claims 10, 12 or 13, wherein use is made of a composite photocatalyst in which the dye is copper phthalocyanine.

16. A method according to claim 10, wherein said refractory waste substrate is a polychlorinated biphenyl or a mixture of polychlorinated biphenyls.

17. A method according to claim 10, wherein said refractory waste substrate is a metal cyanide complex.

18. A method according to claim 10, wherein said metal cyanide complex is an alkali metal ferricyanide complex.

19. A method according to claim 16, wherein use is made of a composite photocatalyst in which the polymer film comprises zinc tetraphenylporphyrin and poly-vinylpyridine or a copolymer of vinylpyridine with styrene in a zinc to pyridine mole ratio of about 1:6.

20. A method according to claim 17, wherein use is made of a composite photocatalyst in which the polymer-film comprises polyvinylpyridine or a copolymer of vinylpy-ridine with styrene blended with a cationic ionomer to provide a film having cationic sites, said film incorporat-ing tetrasulfonated zinc tetraphenylporphyrin at the catio-nic sites thereof.