CA2444385A1

CA2444385A1 - Apparatus and method for phtocatalytic purification and disinfection of water and ultrapure water

Info

Publication number: CA2444385A1
Application number: CA002444385A
Authority: CA
Inventors: Gary M. Carmignani; Lee W. Frederick
Original assignee: Individual
Current assignee: Titan Technologies
Priority date: 2001-04-12
Filing date: 2001-04-12
Publication date: 2002-10-24
Also published as: WO2002083570A1; EP1434737A1; EP1434737A4; JP2004528171A

Abstract

An apparatus and method for the photocatalytic purification and ultrapurification of water. Water containing organic, inorganic, and/or biological contaminants is directed through a rigid, three-dimensionally open celled, fluid permeable, semiconductor unit (18). Within the unit, a semiconductor surface capable of promoting electrons from its valence band to its conduction band, when exposed to a photoactivating light source (38, 40), removes the contaminants through a photocatalytic reaction.

Description

APPARATUS AND METHOD FOR PHOTOCATALYTIC PURIFICATION
AND DISINFECTION OF WATER AND ULTRAPURE WATER
BACKGROUND OF THE INVENTION
Technical Field 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 waters. The present invention is an apparatus and method incorporating a rigid, three-dimensionally open celled, fluid permeable, photocatalytic semiconductor unit.
Background Art 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:
Semiconductor Organic pollutant + OZ ~ C02 + HBO + mineral acid hV > Ebg where by represents the energy of a photon and Eb~ 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 ' Ultrapure water, as used herein, refers to water pre-treated by methods known to those skilled in the art to remove suspended and dissolved inorganic and organic matter.

interface, oxidation-reduction processes are initiated. These include;
Holes:
Acidic or neutral solutions: H20 + h+ ~ OH~ + H+
Allcaline solutions: OH- + h+ ~ OH~
Electrons:
Uncertain reaction pathway resulting in the reduction of oxygen to various reactive species including:
O~, 02~, OZH~, HOZ-, H202 and OH~.
Of particular importance is the formation of OH~, the hydroxyl radical. The hydroxyl r adical 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 and endotoxins.
Semiconductor photocatalysts that have been demonstrated for the destruction of organic contaminants in fluid media include but are not limited to: TiOa, ZnO, CaTi03, Sn02, Mo03, FezO3, and W03. Ti02 is the most widely investigated because it is chemically stable, has a suitable bandgap structure for UV/Visible photoactivation, and is relatively inexpensive.
Ti02 exists in two principal crystalline forms: rutile and anatase. The rutile form of TiO~ is widely used as a pigment and can be found in almost anything white --paint, paper, textiles, inlcs, 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 m2/gm) show high photoactivity when irradiated with UV
light (<
390nm) 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 Ti02 by doping with various metals to extend the spectral response into the more efficient visible light wavelengths, [Borgarello, E.
et aI. "Visible Light Induced Water Cleavage in Colloidal Solutions of Chromium-Doped Ti02 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;

Welter, 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 (Ti02) slurries or TiO~ wash coatings onto or inside a glass tube and the photodegradation of organic compounds and their intermediates in water. These methods of using Ti02 have limitations for commercial applications. For example, although Ti02 slurry has tremendous surface area and has acceptable quantum yields, there are serious limitations to the removal of the TiO~ particles from the purified water. While wash coating Ti02 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 Ti02 to the water stream and a concomitant reduction in photocatalytic activity.
Kraeutler and Bard made a photocatalytic reactor of water slurry of suspended Ti02 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 slmTies of Ti02 for the photocatalyzed degradation kinetics of organic pollutants in water.
Mathews created a thin film reactor by wash coating TiOz, (Degussa P25TM), 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 sot-gel process of anatase Ti02 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. Unlilce the present invention, Robertson's mesh is not rigid, three-dimensionally open celled 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"TM. A fluid containment structure stuTOUnded the membrane creating a reactor. The reactor volume occupied by the fluid was 2.Sliters (1) and the membrane surface area was 250 linear centimeters (cm2). The reactor was illuminated with a cylindrical high-pressure mercury arc lamp at a power of 2 lcilowatts (1cW) and at a wavelength of 254nm. 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" (R. Paterson, ed.) BHR, Mech. Eng.
Publ., London (1993), pg 257-274]. The process was patented in Italy in 1995, Italiaxi Pat.
No. IT1252586. Unlike the present invention, Bellobono's apparatus is not inert, not three-dimensionally open celled, and not durable.
Cittenden, et al. discloses a method and apparatus for destroying organic compounds in fluids [The 1995 American Society of Mechanical Engineers (ASME) International Solar Energy Conference, Maui, Hawaii, USA]. Ti02 was attached by wash coating to a 35x60-mesh silica gel substrate. The substrate was placed within a plastic tube that allowed the penetration of UV light. Organic pollutants in a water stream passed axially through the tube. Natural light and/or artificial UV light oxidize the investigated organic pollutants.
Unlike the present invention, Cittenden's invention is not three-dimensionally open celled, not durable, and has very limited fluid pemneability.
Anderson discloses a method to make ceramic titanium membranes by the sol-gel process. [J. Membrane Science 30 (1988) 243-258]. These membranes are porous and transparent to UV illtunination. They are made from a titanium allcoxide and then fired to form the anatase crystalline structure. Unlike the present invention, Anderson's invention is not open celled, not three-dimensionally reticulated, not durable, and has very limited fluid permeability.
Thus, while attempts were made in the prior art to enhance quantum yields by increasing semiconductor surface area and improving UV light penetration, serious limitations remain to the coriunercial development of an efficient, durable photocatalytic purification apparatus and method for its use. In Robertson, in addition to the severe limitations already above noted, the flexible strands of fiberglass precluded the permanent attachment of Ti02 because, as water passed by, the fiberglass strands bent and flexed releasing TiOz particles, particularly at high fluid flow rates. For Bellobono, in addition to all the limitations also above noted, the photocatalytic process gradually oxidized the organic membrane reducing its activity over time. In addition to all the limitations also above noted, Cittenden's Ti02 sloughed-off because it was wash coated to the silica gel substrate. In 5 addition, the void space between silica particles was so small that flow through the system was restricted malting the structure unsuitable for commercial applications.
In Andersen's membrane, in addition to the limitations above noted, limitations on the structural integrity of these membranes exist particularly at high fluid velocities needed for efficient industrial applications.
Disclosure of Invention The object of the present invention is to substantially improve upon the prior art to produce an effective, quantum efficient, durable, economic, commercial apparatus for the rapid photocatalytic purification and disinfection of water and ultrapure water. At the present time in the semiconductor processing industry, current technology struggles to achieve 2 parts-per-billion (ppb) in Total Organic Carbon (TOC). This represents a limit on the industry's ability to achieve further improvements in the chip density and speed. The present invention, which achieves 500 parts-per-trillion (ppt) in TOC, or better, represents a breakthrough for both the water purification and semiconductor industry. The invention also has profound implications for other water purification systems, including those related to environmental cleanup.
The apparatus of the present invention involves a reactor apparatus and a method for its use for photo-promoted, catalyzed degradation of compounds in a fluid stream. The effectiveness of the process is determined in part by the mass transfer efficiency, which is the rate at which the contaminant is transported from the fluid stream to the photocatalytic surface where it can be destroyed. Mass transfer is greatly aided by proximity. The photocatalyst is widely and uniformly distributed in the volume of water to be treated, such that a contaminant is never far from a catalyst surface Another feature of the present invention is the uniform illumination of the catalyst within the vohune of water to be treated. Since the catalyst itself absorbs the light, its concentration in the volume is limited to allow sufficient penetration of the activating photons. In addition, in the preferred embodiments, the support structure does not block illumination of the volume of water to be treated. Thus, the volume fraction of support material is minimized andlor has high transparency to the activating photons.
To enhance volumetric illumination, in an embodiment which employs a substrate, the substrate material is preferably made from glass or other materials transparent or semitransparent to the photoactivating wavelengths between 180nm and 700nm. This is possible using a rigid, three-dimensionally open celled photocatalytic semiconductor unit. In an embodiment which bonds or chemically integrates the substrate with the semiconductor, the unit is also preferably made from transparent or semitransparent materials.
The water flow through the catalyst is turbulent to improve mixing and mass transfer rates between the organic contaminants and the oxidizing species generated at the catalyst surface. Laminar flow is largely avoided.
The open celled structure utilized in a first preferred embodiment of the present invention substantially represents a breakthrough over the prior art and allows for the commercial use of photocatalytic technology in ultrapure water production because it optimizes mass transfer, surface area, illumination, water flow, durability, rigidity, and so forth. The photocatalytic semiconductor unit provides a high surface area, rigid structure on which the photocatalyst is deposited or into which it is incorporated. The interstitial struts forming the open celled structure of the photocatalytic semiconductor unit are relatively thin, so volume fraction of substrate support material is low and flow is not significantly restricted. The ramification and alignment of the struts with respect to the flow direction will generate tortuous flow paths and enhance mass transport. The rigidity of the support structure provides a stable base to permanently attach or incorporate a highly active Ti02 surface.
Brief Description of the Drawings FIG. 1 is a partial cross-sectional side view in elevation showing a first preferred embodiment of a point-of use reactor with LED's as the source of photoactivating light.
FIG. 2A is a partial cross-sectional side view in elevation of a cylindrical tube reactor in which water flows in and passes radially through the open celled photocatalytic substrate and axially past the UV light source;
FIG. 2B is a cross-sectional end view of the reactor of FIG. 2A;
FIG. 3 is a schematic drawing of a purification system that includes an air injection system for injecting gas into the water before it is introduced into the photocatalytic system;
FIG. 4 is a view showing detail of the open celled photocatalytic semiconductor unit of the first preferred embodiment of the present invention;
FIG. 5 is a partial cross-sectional perspective view of a reactor tube having an alternative semiconductor unit substrate structure;
FIG. 5A is a partial cross-sectional side view in elevation showing detail of the surface topography of the substrate structl~re shown in FIG. 5;
FIG. 6 is a schematic drawing of an experimental test system used to evaluate the performance of the present invention;
FIG. 7 shows the results of a flow rate optimization study;
FIG. 8 shows the comparison of the photocatalytic destruction of acetic acid over time for a fiberglass mat substrate and a three-dimensionally open celled photocatalytic semiconductor unit utilized in an embodiment of the present invention; and FIG. 9 shows a comparison of the photocatalytic destruction of acetic acid over time for LTV photolysing/mixed bed ion exchange system compared to the LJV
photolysing/mixed bed ion exchange plus an open celled semiconductor unit:
Best Mode for Carrying Out the Invention The present invention is directed to the use of a photocatalytic semiconductor unit photo-actively charged with a semiconductor for use in a reactor apparatus and method for the purification and disinfection of water for the semiconductor industry, environmental cleanup, and for the home point-of use market.
In a first preferred embodiment, the present invention discloses an apparatus and method for purifying water and ultrapure water that solves problems of the prior art by transporting water through a rigid, three dimensionally open-cell material characterized by an inert, porous, photoactivating light semitransparent, fluid permeable, high surface area substrate onto which a photocatalytic semiconductor layer is permanently bonded, into which it is incorporated, or of which it is fabricated. The material described in the present invention and the apparatus and method for its use in photocatalytic purification and disinfection of water and ultrapure water is further characterized by high contact efFciency turbulent fluid flow with relatively low pressure drop. In a second and third preferred embodiment, the photocatalytic substrate is not open celled but nonetheless presents a large surface area over which the water flows and that also induces turbulent flow of the water through the system.
It will be readily appreciated by those skilled in the art that the current invention can be used to purify water in manufacturing semiconductors and pharmaceuticals, in biotechnology, power plant water, bottled water, municipal water supplies, point-of use, to name just a few examples.
Although never before used for the present purpose, three dimensionally open-cell substrates made from a variety'of materials are scientifically described and commercially available. Such materials, all of which may be suitable for use in the present invention, include alumina, titania, aluminum, gold, copper, metal alloys, carbon, silica, glass, quartz, organic polymers, silicon carbide, silicon nitride, boron nitride, zirconium, tungsten carbide, and many more. One of many methods of making an open celled substrate is described in the prior art - U.S Pat No. 3,052,967 to Fischer; 3,946,039 to Walz; 4,568,595 to Morris; and 5,441,919 to Park et al. Custom substrates may also be made utilizing the stereolithograhic process or selective laser sintering or other methods familiar to those experienced in the art.
The rigid, three-dimensionally open celled substrate utilized in a first preferred embodiment of the cL~rrent invention possesses a highly variable surface, with an easily controlled surface roughness and a huge macro surface area, depending on the overall pore size from approximately 4 to 96 pores per linear centimeter (ppc), approximately 10 to 240 pores per linear inch (ppi). The concentrated yet compact surface area opens the possibility of using a great variety of attachment methods; such as, without being limited to, sol-gel process, ion assisted gun deposition ion beam sputtering, chemical vapor deposition, aerosol application, evaporation deposition, etc.
Literature and the prior art explain the procedures necessary for the permanent bonding of Ti02 to a substrate. For example for sol-gel process refer to: U.S.
Pat. No.
4,892,712 to Robertson; U.S. Pat. No. 6,013,372 to Hayalcawa , et al., and U.S. Pat. No.
6,093,676 to Heller, et al., or in literature, Preparation, Microstructure and Photocatalytic Activity of Porous Ti02 Anatase Coatings by sol-gel Processing, [J Sol-Gel Science and Technology 17 (2000) 163-171] by Jiaguo Yu, et al; Nanocrystallite Titanium Dioxide Films Made by the Sol-Gel Method Using Reverse Micelles, [J Sol-Gel Science and Technology 10 (1997) 83-89] by E. Stathaios, et al. For chemical vapor refer to: U.S. Pat.
No. 5,389,401 to Gordon, or in Metal Organic CVD of Nanostructured Composite Ti02-Pt Thin Films: A
Kinetic Approach, [Chern. Vapor Deposition 5 (1999) 13-20] by Giovanni, et al.
Yet another method condenses from aerosolized semiconductor droplets, as described in Deposition of Multifunctional Titania,Films by Aerosol Routes, [J. Am. Ceramic Soc. 82 (1999) 10] by G.
Yang and Pratim Biswas. While these are some of the popular methods for attaching semiconductor films, we do not limit ourselves to variations on them and other methods that are to be found in prior art.
The semiconductor layer may also be formed chemically in situ by oxidation of the underlying metal, either electrochemically or thermally or by chemical reaction. See for example Titanium Dioxide Film Electrodes Prepared by Thermal Oxidation, [J.
Electrochem.
Soc. 139, no. 7, (1992) 1803 by Choi Yong-lcoolc et. al. and In Situ Raman Spectra of Anodically Formed Titanilun Dioxide Layers in Solutions of H2S04, I~OH and HN03, [J.
Electrochem. Soc. 138 no. 10 (1991) 2964].
In a further embodiment the substrate is made of the same material as the semiconductor layer and the two materials are chemically integrated. This creates stable surface capable of withstanding tremendous t<ubulent flow.
Photocatalytic activity of many semiconductor surfaces is enhanced by a process of doping or coating these surfaces with a variety of metals, including transition metals such as, but not limited to, platinum, palladium, ruthenium, iridium, rhodium, gold, silver, copper, tin, iron, cobalt, vanadium, niobium, and zinc. Combinations of these metals and their oxides, sulfides or other compounds are known to those experienced in these arts. By altering the doping of Ti02 the band gap energy can be shifted to the visible spectrum (400nm-700nm).
Zang. et al. showed that the addition of platinum (IV) halide shifted the band gap energy required for TiO2 from 335nm to 366mn to 400rim into the visible spectrum.
[Amorphous Microporous Titania Modified Platinum (IV) Chloride - A New Type of Hybrid Photocatalyst for Visible Light Detoxification. J Phys. Chem. B 102 (1998) 10765-10771].
Doping with iron or chromium produces similar results. [Visible Light Induced Water Cleavage in Colloidal Solutions of Chromitun-doped Titanium Dioxide Particles. J ACS 104 (1982) 2996-3002, by E. Borgarello, et al.]
An enhancement of the preferred embodiment is a film made from the anatase form of Ti02 in a usable grain size for particles from 1 to 30 nanometers in diameter.
An active surface thickness can vary from 1 to 190 micrometers. Platinum was found to be effective dopant to increase activity when applied in the range of from 0.025 to 3% by weight of the titanium dioxide, though a range of 0.05 to 1 % may be optimal. Platinum as specified above increased the TOC destruction activity by a factor of 2 to 3. A further enhancement of the dopant is a vanadium compound, such as vanadium pentoxide (0.1 to 15% by weight of the Ti02) on a semiconductor such as Ti02, when used in combination with ozone. It increases the rate of destruction of TOC by a factor of 2-8 times. This enhancement applies in the dark 5 as well as under illumination. This means that if light does not penetrate to the interior of the substrate, TOC will still be destroyed.
A further enhancement of the first preferred embodiment of the apparatus and method of the present invention consists using a combination or set of open celled substrates, each with its own particular variety of parameters and enhancements and each designed to operate 10 on a particular component of the TOC. For instance, one set may work on polar/non-polar components, while others work on hydrophobic/ hydrophilic components, aromatic/aliphatic components, alcoholic/acidic components and chemical/biological components.
The members of the set are used in a series combination where water flows thru first one member and then another member. This enhancement enlarges the scope of the invention by bringing a complete collection of destruction capabilities to bear on combinations of contaminants, even though individual members of the set are alone incapable of achieving acceptable overall TOC
destruction levels.
Preferred light sources include, without being limited to low, medium and high-pressure mercury lamps, xenon lamps, and conventional and ultraviolet emitting LED's, or any other light source that activates the semiconductor by producing light at a wavelength of between 180 to 700 nm.
Drawing attention to U.S. Pat No. 5,116,582, to Cooper, et al. entitled Photocatalytic Slurry Reactor Having Turbulence Generating Means, the creation of turbulence has been recognized in prior art as a necessary condition for effective TOC
destruction. Effective TOC
destniction requires that organic molecules present in the water come into close proximity to the active surface. The open celled photocatalytic semiconductor unit, and the alternative substrate structures described for use in the apparatus and method of the present invention, behave superbly in this regard. Each cause turbulent water flow, and the open celled photocatalytic semiconductor wit, in particular, causes dramatically turbulent water flow, causing the water flowing through its pores to shear, thrashing from side to side within the pores, and to speed up and slow down according to the cross section of the pore openings.
Further, it causes microturbulence within the pores themselves. Open celled photocatalytic semiconductor units provide many ways to control turbulence by adjusting pores sizes and pore distributions, pore wall formations and surface textures.
As a further enhancement of the invention, known methods exist to grade the size of the poxes so that they start large near the surface at approximately 4 ppc (~10 ppi), and then diminish in size towards the photocatalytic semiconductor unit interior at approximately 96 ppc 0240 ppi), thus providing tailored light guides. Additionally, the water itself may be modified such as by adding microscopic gas bubbles (such as gaseous oxygen, ozone, or peroxides) to guide the light into the interior. The materials of construction of the photocatalytic semiconductor unit can be varied from reflective (metals) to opaque (TiOa, carbon, metals) to transparent (silica, alumina) to provide further control over the penetration of the photoactivating light. Innovative designs can incorporate light guides including, but not limited to, light fibers, quartz blocks, voids, gaps and separations.
Although particular embodiments of the present invention have been described and illustrated herein, it should be recognized that modifications and variations might readily occur to those skilled in the art and that such modifications and variations may be made without departing from the spirit and scope of our invention. Consequently, our invention as claimed may be practiced otherwise than as specifically described.
Referring now to Figs. 1-10, wherein like reference numerals refer to like components in the various views, FIG. 1 shows an example of a point-of use reactor 10 with LED's 12 as the photoactivating light. Contaminated source water flows into the reactor housing 14 through inlet 16. The water then flows through the open celled semiconductor unit 18 that is photoactivated by LED's 12. A support/wiring plate 20 holds the LED lights. A
transparent plate 22 is provided to isolate the LED lights from the water flow. Purified water exits the reactor through outlet 24. The point-of use reactor housingl4 can be constructed from a variety of thermoplastics (polyproplylene, etc), or metals (304 stainless steel, 316 stainless steel, etc), or other materials that are both inert to degradation by the LED
light source and resistant to corrosion by water. Further, the enclosure may either be integral with the semiconductor unit or separable, the latter configuration preferable in cases where the removal and installation of a replaceable semiconductor unit is desired. The semiconductor unit defines a fluid passage 26 in fluid communication with inlet 16 and outlet 24.
The point-of use reactor can use an open celled semiconductor unit 'that is photoactivated by LED's that emit UV energy at 390 nm or lower. The point-of use reactor can also use an open celled semiconductor unit that is doped to shift the band gap to visible wavelengths. In this reactor, an LED that emits visible wavelengths is utilized. This latter configuration enables a more efficient use of the LED energy.
The point-of use reactor is designed to be commercialized into markets defined by low and intermittent demand for purified water, such as potable water in the home.
This reactor is superior to existing technologies because it uses only a small percentage of energy and it does not transmit heat to the product water while not in use (eliminating the need to rinse the system to ambient temperature prior to using product water). In addition, the reactor only requires low power electrical energy per LED, making it both safe for the user in an environment that includes water and electricity and enabling the reactor to be utilized in portable applications (e.g., battery or solar powered).
FIG. 2A is a partial cross-sectional side view in elevation of a cylindrical tube reactor 30, suitable for commercial/industrial applications, having a generally elongate housing 32 into which water flows through inlet 34 and then passes radially through the open celled photocatalytic unit 36 and axially past a IJV light source comprising tube type lamps 38 and 40 as the photoactivating light. FIG. 2B is an end view of the same commercial/industrial reactor 30. The open celled photocatalytic semiconductor unit 36 is a cylinder. Both the exterior photoactivating lights 38 and interior photoactivating lights 40 are tubes enclosed by cylindrical quartz sleeves 42. After contaminated source water flows into the reactor through inlet 34, it flows radially through the open celled semiconductor unit and passes by the exterior the photoactivating lights. The contaminated water flows axially through the reactor, through the substrate and over the photoactive surface, where the photoreactive surface is activated by both the exterior photoactivating lights and by the interior photoactivating lights.
Purified water flows out through outlet 44.
The commercial/industrial reactor is designed to be commercialized into marleets defined by high and continuous demand for pl~rified water. The configuration of the reactor is designed to be modular so that longer and/or parallel reactors can be employed for higher flows. Series reactors with different sets of open celled photocatalytic semiconductor unit specifications and/or different wavelengths for the exterior and interior photoactivating lights can be employed for custom purification of source water with different polar/non-polar components, hydrophobic/hydrophilic components, aromatic/ahiphatic components, ahcoholic/acidic components, and chemical/biological components.

FIG. 3 is a schematic drawing of a water purification system 50 that includes a gas injection system 52 for injecting gaseous oxygen, ozone, or peroxides, and thereby modifying the water to facilitate the passage of light into the interior of the semiconductor unit. The gas injection system includes a gas supply, tank, or reservoir 54 in fluid communication with a mixing chamber 56 through a gas line 58. The mixing chamber is 'preferably a venturi.
Interposed between the gas tank and the mixing chamber are one or more flow control valves 60 for regulating the gas flow into the mixing chamber, where it is injected into water flowing into the chamber. After gas is introduced into the water, the water is then processed in the photocatalytic system 62 as described above.
FIG. 4 is a sectional view showing detail of an open celled photocatalytic semiconductor unit 70. The unit includes a plurality of differentially sized pores 72, with pore sizes ranging from 4 to 96 ppc (~10 to 240 ppi).
FIG. 5 is a partial cross-sectional perspective view of a reactor tube having an alternative semiconductor unit substrate structure. In this embodiment, the photocatalytic system 80 includes a reactor housing 82 having a water inlet end 84 and an outlet end 86.
Running axially substantially the entire length of the housing are a plurality of tube-type lights 88 encased.within quartz sleeves 90. Water flows into the housing and over a substrate 92 having a large surface area 94. Unlike the open celled semiconductor unit of the first embodiment, wherein water flows through the semiconductor unit structure, the semiconductor unit of this embodiment promotes fluid flow over and around the photoactive surface area. The principle of action, however, remains the same, as the substrate is coated or impregnated with a catalyst that promotes hydroxyl radical migration to the surface of the substrate when exposed to light of selected wavelengths. Contaminant molecules exposed to the surface are thus oxidized.
It will be recognized that there are inmunerable possible configurations of the semiconductor substrate. However, as a general rule it is most advantageous to provide a geometry that induces turbulent fluid flow over the substrate surface as well as providing a maximum surface area. Such configurations may include, for example, a helical screw substrate 92 surrounding an axially disposed rod 96, as shown. To maximize the surface area of the substrate, it is preferable to include surface contours or topography 98, including bumps, protrusions, corrugations, ridges, fins, flanges, mesh, three-dimensional matrices, as shown in Fig. 5A. The thrust of the surface features is to enhance turbulent flow by creating counter-rotating vortices, cross-current mixing, division and recombination of water, and otherwise mixing and agitating the water stream.
FIG. 6 is a schematic drawing of a laboratory water purification system 120 utilized in evaluating the present invention. The water system utilized for laboratory testing is configured t~ provide the flexibility required for a wide range of laboratory experiments. The exact volume of the water system is carefully measured. The feedwater for any experiment is added through a covered storage tank 122. Feedwater can range from typical point-of use water to ultrapure water. An exact amount of organic impurities is also added through the storage taut. Since the water volume of the system is precisely known, the level of organics in an experiment can be mixed to a predetermined level and verified with the TOC
analyzer 134.
The system includes a pump 124, a rotometer 126, a throttling valve 128 to control system flow, an ultraviolet (UV) photolysing unt 130 with 185/254 nm UV lamps, a test chamber with a photocatalytic surface 132, a TOC analyzer 134, and a mixed bed ion exchange (MBIX) unit 136. Valves 138 are provided to isolate the UV
photolysing unit 130;
Valves 140 isolate the test cell and the test chamber 132; valves 142 isolate the MBIX unit;
and shunt valves 143a-c allow the photolysing unit130, the TOC analyzer 134, and the MBIX
unit 136 to be selectively bypassed, either individually or in any combination. The TOC
analyzer 134 measures TOC, temperature, and resistivity. The water from the TOC analyzer can be returned to the storage tank, or the same stream can be diverted to drain from the TOC
analyzer if desired.
The system enables testing of variables including, but not limited to feedwater water quality (including analysis, conductivity, temperature), feedwater TOC, system flow rate, choice of applying either 185/254 nm, 254 nm UV energy or no light energy at all, choice of applying the MBIX unit (including the choice of resins installed), a choice of the TOC
analyzer utilized, the choice of the light source utilized to illuminate the photocatalytic surface (including wavelength, power, and the option to illuminate from multiple locations including 180 degrees), the choice to add microbubbles in the feedwater to the photocatalytic surface, and all of the possible choices and variations associated with the photocatalytic surface, including, but not limited to, material, surface preparation, surface coating, doping, size of pores, pore dispersion matrix, thickness of the ligaments, and the thickness of the photocatalytic surface.

EXAMPLES:
EXAMPLE 1 - Fig. 7 depicts TOC removal rate as a function of flow rate. The rigid 3-dimensional open celled semiconductor unit (99.5% alumina, 45 ppi, 1.5 inches in diameter by 5 0.50 inches thick) coated with a 2:1 mixture of alumina sol and 35 11m particle Ti02 in the anatase form was placed in the photocatalytic reactor of the test loop of FIG.
6. Acetic acid was spiked through the tank 122 and the flow was adjusted with valve 128 and monitored with flow meter 126. The photocatalytic substrate was illuminated with 365 nm light at 3 milliwatts/cm2. The water was shunted past the ultraviolet (LTV) photolysing unit 130 and 10 MBIX unit 136 and passed through the photocatalytic reactor cell 132 which contains the photocatalytic semiconductor unit. The rate of oxidation of the acetic acid was monitored with a TOC analyzer 134 over time comparing two different flow rates. First order rates are compared among the different surfaces that have been tested to create a useful ranking of different surfaces and geometries. The first order rates are found to be significantly dependent 15 on flow rates, which is related to the degree of turbulence and mixing that occurs. It is clear from FIG. 7. that peak effectiveness in this sample is found at a flow rate of 0.8 gpm with 14.3% TOC reduction in one hour compared to 9.2% TOC reduction at 0.5 gpm.
EXAMPLE 2 - FIG. 8 compares the performance of a prior art substrate and the open celled semiconductor unit utilized in the present invention. It uses the same water loop configl~ration of FIG. 6. That is the water is shunted by the ultraviolet (IJV) photolysing unit 130 and the MBIX unit 136. The photocatalytic surfaces compared are fused silica (20 ppi, 1.5 inches in diameter by 0.25 inches thick) and fiberglass mat. Ti02 in the anatase form was deposited via sol gel techniques to the fused silica and the fiberglass mat. Both samples were platinum doped. Water was passed through the open celled fused silica and fiberglass mat at 1 gallon per minute (gpm) and illuminated at 3milliwatts/crnz at 365 nm wavelength. The water was spiked with 100 ppb acetic acid. The water was monitored with a TOC analyzer.
Each photocatalytic semiconductor unit is independently evaluated according to the previously described procedures and the data was graphed.
FIG. 8 compares the fiberglass mat substrate to the fused silica open celled semiconductor unit. The open celled fused silica underwent 57% mineralization in 13 minutes while fiberglass mat had 11% mineralization in 13 minutes. Turbulent flow of water through the open celled semiconductor unit utilized in the present invention explains the better results.
Even though the fiberglass mat is semitransparent to ITV light and has more surface area than the open celled photocatalytic semiconductor unit, it does not induce or enhance turbulent mixing.
In "Guidelines for Ultrapure Water in Semiconductor Processing"( Sematech Co~sortizem, National Technology Roadn2ap for Semiconductors: Technology Needs, 1997 Ed., p. 170)and the "Standard Guide for Ultrapure Water in the Electronics and Semiconductor Industry"( ASTlIl Standard D5127-98, 'Standard Gzcide for Ultrapure Water Used in the Electrov~ics and Semiconductor Industry, Tlol. 1l. 02) water purity level is related to process line width. For line widths >0.5 microns total organic concentration (TOC) levels of < 2.0 ppb are recommended. For line widths in the range 0.35 - 0.18 microns recommended TOC levels are below 1 ppb. Current technology struggles to achieve the 2 ppb level and has not come close to achieving the 1 ppb level. The International Road Map for Semiconductors shows the following schedule for achieving these water purity goals:
Year: 2000 2001 2002 2003 2004 2005 Max. TOC Level: 2ppb 2 ppb lppb lppb <lppb <lppb <lppb Current ultrapure water treatment systems utilize carbon and multimedia adsorption beds, various filtration units, reverse osmosis, and ion exchange membranes to remove inorganic contaminants and reduce TOC levels to the 10 - 20 ppb range. To bring TOCs down further, photolysing is used. This process requires deep UV irradiation (185nm and 254nm) using massed banks of UV lamps to decompose organics in water. The process is terribly inefficient, but is the only technology available to bring organic contamination down to marginally acceptable levels of 2 - 5 ppb. A common experience in the semiconductor industry is that at these levels, photolysing reaches a barrier at which point the curve of TOC
versus total expended energy flattens out. This barrier, evidently, is due to one or more molecules present in low concentrations that are particularly difficult to destroy by photolysing.
EXAMPLE 3 - In FIG. 9 water in laboratory water purification system 120 of FIG. 6 was spiked with acetic acid (10 ppb) through tank 122 and passed through an ultraviolet (UV) photolysing unit 130 and then a test chamber with a photocatalytic surface 132 utilized in the present invention and then through the MBIX unit 136 and monitored with a TOC
analyzer 134. The test chamber with a photocatalytic surface 132 was illuminated with a 365nm (3 mW/cm2) light. The flow was 0.7 gpm.
FIG. 9 shows the results. First the destruction of acetic acid was evaluated with the ultraviolet (LTV) photolysing mut 130 and the MB1X unit 136 in operation. Next the destruction of acetic acid was evaluated with the same configuration plus a test chamber with a photocatalytic surface 132 containing the open celled semiconductor unit utilized in the present invention. With the ultraviolet (LTV) photolysing unit 130 and the MBIX unit 136 in operation, the water reached a steady state barrier around 1.5 ppb TOC and then started to climb. While TOC in the water passing through the ultraviolet (LTV) photolysing unit 130 and the MBIX unit 136 in operation plus the photocatalytic open celled reactor with good light penetration, turbulent flow, and high surface area was reduced to 0.50 ppb.
The essential method of using the above-described apparatus for photocatalytic degradation of organic, inorganic, and microbiological contaminants in a fluid stream, involves the following steps: (1) providing a reactor enclosure having a water inlet and a water outlet;
providing at least one semiconductor unit, disposed within the reactor enclosure and interposed between, and in fluid communication with, the water inlet and the water outlet, and with which the fluid stream comes into contact, wherein the semiconductor unit includes a substrate having a photoreactive semiconductor surface fabricated of semiconductor material;
(2) providing a light emitting means in optical proximity to the semiconductor surface for promoting electrons from the valance band to the conduction band of the semiconductor material;
and (3) directing a fluid stream over the semiconductor surface while engaging the light emitting means to photactivate the semiconductor surface, whereby contaminants are removed from the fluid stream by photocatalytic reaction.

Claims

What is claimed as invention is:

1. An apparatus for photocatalytic degradation of organic, inorganic, and microbiological contaminants in a fluid stream, said apparatus comprising:

a reactor enclosure (14) having a water inlet (16) and a water outlet (24);

at least one semiconductor unit (18), interposed between, and in fluid communication with, said water inlet and said water outlet, said semiconductor unit including a substrate having a photoreactive semiconductor surface with which the fluid stream comes into contact;
and light emitting means (12) in optical proximity and communication to said at least one semiconductor unit;

characterized in that said light emitting means and said semiconductor surface work cooperatively to remove contaminants from the fluid stream by photocatalytic reaction.

2. The apparatus of Claim 1, wherein said semiconductor surface and said substrate are fabricated from the same material and are chemically integrated

3. The apparatus of Claim 1, wherein said semiconductor unit comprises a substrate into which a photoreactive semiconductor material is incorporated.

4. The apparatus of Claim 1, wherein said semiconductor surface comprises a layer of semiconductor material bonded to said substrate.

5. The apparatus as in any one of Claims 2, 3 or 4, wherein said photoreactive semiconductor material includes at least one doping material.

6. The apparatus of Claim 5 wherein said at least one doping material is a transition metal.

7. The apparatus of Claim 5 wherein said at least one doping material is selected from the group consisting of platinum, palladium, ruthenium, iridium, rhodium, silver, gold, copper, iron, cobalt, vanadium, niobium, tin, and zinc.

8. The apparatus of Claim 5 wherein said doping material is vanadium pentoxide.

9. The apparatus of Claim 8, wherein said vanadium pentoxide is included in an amount of 0.1 - 15% by weight of the semiconductor material.

10. The apparatus of Claim 5, wherein the doping material is platinum.

11. The apparatus of Claim 10, wherein said platinum is included in amount of between 0.025 and 3% by weight of the semiconductor material.

12. The apparatus of Claim 4 wherein said photoreactive semiconductor surface is selected from the group consisting of wash coating, sol-geling, vapor condensing, sintering, evaporation deposition, ion assisted gun deposition, ion beam sputtering, molecular beam epitaxy, ionized cluster beam deposition, reactive electron-beam evaporation, spray pyrolysis, DC magnetron sputtering, metal-organic vapor deposition, and aerosolized droplet condensing.

13. The apparatus of Claim 1 wherein said semiconductor surface is selected from the group consisting of TiO2, ZnO, CaTiO3, SnO2, MoO3, Fe2O3, and WO3.

14. The apparatus of Claim 13 wherein said semiconductor surface is TiO2.

15. The apparatus of Claim 14, where said TiO2 is in the anatase crystalline form having a grain size from approximately 1 to 30 nanometers.

16. The apparatus of Claim 1, wherein said semiconductor unit substrate is a porous, three-dimensionally open celled, fluid permeable structure.

17. The apparatus as in Claim 16 wherein said at least one semiconductor unit has a pore size ranging from 4 to 96 pores per linear centimeter.

18. The apparatus of Claim 16 wherein said at least one semiconductor unit has a pore size tapering from 4 pores per linear centimeter at the semiconductor surface of said at least one semiconductor unit to about 96 pores per linear centimeter at the interior of said at least one semiconductor unit.

19. The apparatus of Claim 1 wherein said semiconductor substrate comprises a helical screw substrate 92 surrounding an axially disposed rod 96.

20. The apparatus of Claim 19, wherein said helical screw substrate includes surface topography 98 selected from the group consisting of bumps, protrusions, corrugations, ridges, fins, flanges, mesh, and three-dimensional matrices.

21. The apparatus of Claim 1, wherein said at least one semiconductor unit is fabricated from material selected from the group consisting of alumina, titania, aluminum, gold, copper, metal alloys, carbon, silica, fused silica, glass, quartz, organic polymers, silicon, silicon carbide, silicon nitride, boron nitride, zirconium, and tungsten carbide.

22. The apparatus of Claim 1 wherein said at least one semiconductor unit is partially transparent to light having a wavelength between 180nm and 700nm.

23. The apparatus of Claim 1 wherein said light emitting means is selected from the group consisting of light emitting diode, low pressure mercury lamp, medium pressure mercury lamp, high pressure mercury lamp and xenon lamp.

24. The apparatus of Claim 1 wherein said light emitting means produces light at a wavelength ranging from 180 to 700 nm.

25. The apparatus of Claim 1 further comprising gas injection means in fluid communication with the fluid stream, characterized in that said gas injection means introduces an oxidizing agent into the fluid stream before the fluid stream comes into contact with said photoreactive semiconductor surface.

26. The apparatus of Claim 25, wherein said gas injection means comprises:

a mixing chamber in fluid communication with said semiconductor unit;

a gas reservoir in fluid communication with said mixing chamber; and a control valve to regulate the flow of gas from said reservoir into said mixing chamber.

27. The apparatus of Claim 26, wherein said gas injection means introduces an oxidizing agent selected from the group consisting of oxygen, ozone, and peroxides.

28. The apparatus of Claim 26, wherein said mixing chamber is a venturi.

29. A method of degrading organic, inorganic, and microbiological contaminants in a fluid stream by photocatalytic reaction, comprising the steps of:

providing a reactor enclosure having a water inlet and a water outlet;

providing at least one semiconductor unit, disposed within the reactor enclosure and interposed between, and in fluid communication with, the water inlet and the water outlet, and with which the fluid stream comes into contact, wherein the semiconductor unit includes a substrate having a photoreactive semiconductor surface fabricated of semiconductor material;
and providing a light emitting means in optical proximity to the semiconductor surface for promoting electrons from the valance band to the conduction band of.said semiconductor material;

photoactivating the semiconductor surface with the light emitting means; and directing a fluid stream over the semiconductor surface, whereby the contaminants are removed from the fluid stream by photocatalytic reaction.

30. The method of Claim 29, wherein the semiconductor unit substrate is a porous, three-dimensionally open celled, fluid permeable structure.