Chemistry: electrical and wave energy – Processes and products – Processes of treating materials by wave energy
Reexamination Certificate
2000-11-22
2003-02-25
Wong, Edna (Department: 1741)
Chemistry: electrical and wave energy
Processes and products
Processes of treating materials by wave energy
C204S157150, C204S157300, C588S253000, C422S186300, C210S748080
Reexamination Certificate
active
06524447
ABSTRACT:
FIELD OF THE INVENTION
The present invention generally relates to a method and apparatus for the purification and disinfection of water. More specifically, the present invention relates to an apparatus and method of use of a semiconductor material for the photocatalytic degradation of organic and inorganic pollutants and microorganisms in water and ultrapure water
1
. The present invention is an apparatus and method incorporating a rigid, open cell, three dimensionally reticulated, fluid permeable, photocatalytic semiconductor unit.
1
Ultrapure water as used herein refers to water that is pretreated by methods know to those skilled in the art to remove suspended and dissolved inorganic and organic mater from municipal, well water and any other water source.
BACKGROUND OF THE INVENTION
Heterogeneous photocatalysis is the general term that describes the technical approach, [Mills, A.; Le Hunte, S.; “An Overview of Semiconductor Photocatalysis,” J. PhotoChem. & PhotoBio. A: Chemistry 108 (1997) 1-35] and [Hoffman, M. R.; Martin, S. T.; Choi, W.; Bahnemann, D. W.; “Environmental Applications of Semiconductor Photocatalysis,” Chem Rev 1995, 95, 69-96]. The specific process is properly described as semiconductor-sensitized photomineralization of organics by oxygen. It may be summarized as:
Organic pollutant+O
2
CO
2
+H
2
O+mineral acid
hv>E
bg
where hv represents the energy of a photon and E
bg
is the bandgap energy separating electrons in the valence band of the semiconductor from those in its conduction band.
The process is driven by photons having more energy than the bandgap of the semiconductor they irradiate. Each such photon absorbed by the semiconductor will promote an electron from the valence band producing a conduction band electron (e−) and a valence band hole (h+). When the resultant electron-hole pair migrates to the semiconductor/solution interface, oxidation-reduction processes are initiated. These include:
Holes:
Acidic or neutral solutions: H
2
O+h
+
OH.+H+
Alkaline solutions: OH—+h
+
OH.
Electrons:
Uncertain reaction pathway resulting in the reduction of oxygen to various reactive species including
O., O
2
., O
2
H., HO
2
—, H
2
O
2
and OH.
Of particular importance is the formation of OH., the hydroxyl radical. The hydroxyl radical is an extremely potent oxidizing agent (redox potential of +2.8 V), capable of oxidizing almost all organic compounds. By comparison, the redox potentials for the more conventional oxidants chlorine and ozone are +1.36 and +2.07 V, respectively. Hydroxyl radicals also kill and breakdown microorganisms. The reactive species created by the reduction of oxygen will also oxidize organic compounds. All active species are created from water, and decay back to water. Light is the only reagent required.
Semiconductor photocatalysts that have been demonstrated for the destruction of organic contaminants in fluid media include but are not limited to: TiO
2
, ZnO, CaTiO
3
, SnO
2
, MoO
3
, Fe
2
O
3
, and WO
3
. TiO2 is the most widely investigated because it is chemically stable, has a suitable bandgap structure for UV/Visible photoactivation, and is relatively inexpensive.
TiO
2
exists in two principal crystalline forms: rutile and anatase. The rutile form of TiO2 is widely used as a pigment and can be found in almost anything white—paint, paper, textiles, inks, plastics and cosmetics. Anatase, the low temperature form (stable below ~600° C.) is the most photoactive form. Nanoscale (5-50 nm) anatase particles with very high surface areas (50-500 m
2
/gm) show high photoactivity when irradiated with UV light (<390 nm) in the presence of water.
The deposition of a transition metal (e.g., platinum, palladium, silver) on the surface of the anatase increases the photocatalytic activity by approximately a factor of two. A variety of methods improve the quantum efficiency of TiO
2
by doping with various metals to extend the spectral response into the more efficient visible light wavelengths, [Borgarello, E. et al. “Visible Light Induced Water Cleavage in Colloidal Solutions of Chromium-Doped TiO2 Particles,” J. Am. Chem. Soc. 1982, 104, 2996-3002] or to increase the minority carrier diffusion length, [Augustynski, J.; Hinden, J. Stalder, C.; J. Electrochem. Soc. 1977, 124, 1063] or achieve efficient charge separation to increase carrier lifetimes, Vogel, R.; Hoyer, P; Weller, H.; “Quantum-Sized PbS, CdS, Ag2S, Sb2S3 and Bi2S3 Particles as Sensitizers for Various Nanoporous Wide-Bandgap Semiconductors,” J. Phys. Chem. 1994, 98, 3181-3188].
Most of the early research on semiconductor photocatalysis concerned methods using titanium dioxide (TiO2) slurries or TiO2 wash coatings onto or inside a glass tube and the photodegradation of organic compounds and their intermediates in water. These methods of using TiO2 have limitations for commercial applications. For example, although TiO2 slurry has tremendous surface area and has acceptable quantum yields, there are serious limitations to the removal of the TiO2 particles from the purified water. While wash coating TiO2 onto glass avoids the removal limitations of the slurry approach, it has its own problems in that insufficient surface area is presented for effective destruction of organics within a reasonable time period. Additionally, the wash coat is not firmly attached to the glass resulting in a loss of TiO2 to the water stream and a concomitant reduction in photocatalytic activity.
Kraeutler and Bard made a photocatalytic reactor of water slurry of suspended TiO2 powder, in the anatase crystalline form, and studied the decomposition of saturated carboxylic acid, [J. ACS 100 (1978) 5985-5992]. Other researchers used UV-illuminated slurries of TiO2 for the photocatalyzed degradation kinetics of organic pollutants in water.
Mathews created a thin film reactor by wash coating TiO2, (Degussa P25™), particles to the inside of a 7 millimeter long borosilicate glass tube wound into a 65-turn spiral. The reactor was illuminated with a 20 watt, black light UV fluorescent tube. He monitored the destruction of: salicylic acid, phenol, 2-chlorophenol, 4-chlorophenol, benzoic acid, 2-naphthol, naphthalene, and florescin in water, [J. Physical Chemistry 91 (1987) 3328-3333].
As an improvement over the prior art approaches, U.S. Pat. No. 4,892,712 to Robertson et al. disclosed the attachment by the sol-gel process of anatase TiO2 to a fiberglass mesh substrate. This mesh was wrapped around a light source contained within a quartz glass cylinder and emitting UV radiation in a wavelength range of 340 to 350 nanometers (nm). The entire structure was placed within a stainless steel cylinder containing fluid inlet and outlet ports thereby creating a reactor. Polluted water was passed through this reactor for purification. Unlike the present invention, Robertson's mesh is not rigid, open cell, three dimensionally reticulated and lacks permanent bonding of the semiconductor to the mesh.
Professor I. R. Bellobono prepared photocatalytic membranes immobilizing 23% of Titanium Dioxide (Degussa P-25). Controlled amounts of appropriate monomers and polymers, containing the semiconductor to be immobilized and photoinitiated by a proprietary photocatalytic system was photografted onto a non-woven polyester tissue. The final porosity of the photosynthesized membrane was regulated at 2.5-4.0 microns. He trade named this membrane “Photoperm”™. A fluid containment structure surrounded the membrane creating a reactor. The reactor volume occupied by the fluid was 2.5 liters (l) and the membrane surface area was 250 square centimeters (cm
2
). The reactor was illuminated with a cylindrical high-pressure mercury arc lamp at a power of 2 kilowatts (kW) and at a wavelength of 254 nm. Water flowed into the center of the reactor and moved out tangential to the lamp through the membrane. This system was used to destroy phenol in water, [“Effective Membrane Processes. New Perspectives”
Carmignani Gary M.
Frederick Lee W.
Titan Technologies
Wong Edna
LandOfFree
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