US20090152123A1

US20090152123A1 - Methods and Apparatus for Generating Oxidizing Agents

Info

Publication number: US20090152123A1
Application number: US11/994,967
Authority: US
Inventors: Dean Butler; Robert Lewis Clarke
Original assignee: Applied Intellectual Capital
Current assignee: APPLIED INTELLECTUAL CAPITAL Ltd
Priority date: 2005-07-07
Filing date: 2006-07-07
Publication date: 2009-06-18
Also published as: AU2006269410B2; WO2007008591A3; WO2007008591A9; AU2006269410A1; WO2007008591A2; WO2007008591B1

Abstract

Contemplated devices and methods include an electrolytic cell having a cathode and a carbon felt anode, wherein the carbon felt anode is configured as a flow-through anode for an aqueous solution in which a contaminant is dissolved or dispersed. The cell is operated at a current density that promotes formation of oxidizing species in neutral pH to thus destroy the contaminant and at a flow rate sufficient to prevent oxidative damage of the carbon felt.

Description

This application claims priority to our copending U.S. provisional patent application with the Ser. No. 60/697,455, which was filed Jul. 7, 2005.

FIELD OF THE INVENTION

The field of the invention is devices and methods for electrochemical generation of oxidizing species, especially as they relate to anodic in situ generation of such species in a fluid.

BACKGROUND OF THE INVENTION

Oxidative species, including ozone, hypochlorite, hydrogen peroxide, persulfate, chlorine dioxide, and hydroxyl radicals are frequently used to sterilize water and destroy toxic organic compounds. Among such oxidative species, some exhibit sufficient chemical stability in water to keep the water sterile over relatively long periods, while other oxidative species (e.g., ozone) have relatively short half lives in water, which potentially allows re-infection of previously sterilized water.
Among the most common sterilizing agents, hypochlorite (OCl⁻) is typically prepared by adding chlorine gas or sodium hypochlorite solutions to water. Hypochlorite is relatively stable, non toxic to humans, and thought to oxidize bacterial cell membranes. Hypochlorite is also known to bleach numerous colored substances, typically by oxidation of aromatic ring structures in the colored substance. Unfortunately, hypochlorite is also known to react with naturally occurring organic materials, including fulvic and humic acids (e.g., present in soil and sediments in lakes, rivers, and ground water) to form chloromethanes and other toxic byproducts. Moreover, storage of chlorine gas or solutions of sodium hypochlorite on site for dosing into water streams presents a potential hazard, and dosing requires an at least somewhat experienced operator. Yet another undesirable effect of chlorination of water is that hypochlorite produces a relatively bad taste in the treated water.
To overcome most of the aforementioned disadvantages of chlorination, ozone may be employed as a sterilization agent. Ozone is chemically more aggressive as a sterilizing agent, but is often more expensive and has a relatively short half life in water. Therefore, ozone sterilization typically requires in situ production at the point of use. Alternatively, hydrogen peroxide may be employed as a sterilizing agent via photolytic cleavage of the H₂O₂to generate the OH radical as active species. While the hydroxyl radical is an even stronger oxidizing agent as compared to ozone, the hydroxyl radical also has the shortest half life in water. Unfortunately, photolytic hydroxyl radical generation is often limited by turbidity or other compounds that absorb and/or scatter incident light.
In still further known approaches, selected configurations for direct electrochemical oxidation of water to oxygen and other oxidizing species were reported. In a typical configuration, the reactive species is generated from water at the anode in the anode compartment of an electrochemical cell (if chloride ions are present, hypochlorite may be generated). In one such approach, Electrosynthesis Corp. developed a device based on precious metal coated niobium electrodes which was installed in a water stream. In another approach, several groups in Switzerland and Germany have developed diamond coated electrodes as an alternative to precious metal coated niobium. Diamond coatings are characterized as having very high oxygen overpotentials, which is thought to be a prerequisite for generating hydroxyl radicals and/or oxidizing organic materials at high efficiency. In a still further approach, Magneli phase titanium suboxide electrodes have been employed to create a device that was effective as a sterilizing device in contaminated waters (Magneli phase titanium oxide is characterized by its high oxygen overvoltage).
However, while most of known devices provided at least some satisfactory results, other disadvantages still remain. Among other things, electrode materials are either often relatively expensive, difficult to replace by an inexperienced user, and/or typically require relatively large amounts of energy to provide efficient sterilization/decontamination. Thus, there is still a need to provide improved configurations and methods for electrochemical generation of oxidizing agents.

SUMMARY OF THE INVENTION

The present invention is directed to various devices and methods of electrochemical generation of oxidative species in an aqueous solution having typically neutral pH using a carbon felt flow-through anode under conditions that reduce, and more typically eliminate oxidative damage to the anode while generating the oxidative species in an amount effective to reduce and/or destroy a contaminant.
Therefore, in one preferred aspect of the inventive subject matter, a method of treating an aqueous solution is contemplated in which an electrolytic cell is provide having an anode in an anode compartment, a cathode in a cathode compartment, and a diaphragm separating the anode compartment from the cathode compartment. Most preferably, the anode comprises a carbon felt that is conductively coupled to an electrical connector such that a flow path is formed to allow flow of the aqueous solution through the carbon felt. In another step, the aqueous solution is moved through the anode compartment such that substantially the entire solution passes through the carbon felt from one side to another side, and in yet another step, the electrolytic cell is operated at a current density effective to generate oxidative species in an amount sufficient to oxidize a contaminant in the aqueous solution.
Most preferably, the carbon felt is prepared from carbonized organic textile fibrous felts and has a surface area of about 0.1-5 m²/g to about 1200 m²/g and even higher (where the carbon felt is activated). While the exact configuration is of the carbon felt may be variable, it is typically preferred that the carbon felt configuration will allow for a flow path having a length of between 0.1 cm and 10 cm, and even more preferably between 0.5 cm and 5 cm.
Suitable aqueous solutions will have an approximately neutral pH of between 5.5 and 8.5, and more typically between 6.5 and 7.5, and pass through the carbon felt at a flow rate of 0.1 ml/cm³*min to 10 ml/cm³*min, and more preferably at a flow rate of 0.5 ml/cm³*min to 3 ml/cm³*min. Depending on the particular aqueous medium and current densities, it should be recognized that the oxidative species may vary considerably. However, preferred oxidative species include ozone, hydrogen peroxide, hydroxy radicals, oxygen ions, singlet oxygen, and superoxide anions. Similarly, the contaminant in the aqueous solution may vary from source to source, and suitable contaminants include bacteria, spores, viruses, eukaryotic cells and fragments of thereof, optionally halogenated aromatic organic compounds, and dyes. Where desirable, it is also contemplated to recycle at least part of the aqueous solution back to the anode compartment after the solution has passed through the carbon felt.
Preferred current densities are typically between 10 mA/cm²and 500 mA/cm², and more preferably between 30 mA/cm²and 100 mA/cm², and it should be noted that the cell can be continuously operated under such conditions for at least 6 hours, more typically at least 24 hours, and most typically at least 48 hours without apparent oxidative damage (visual inspection using 20× optical magnification) to the carbon felt anode.
Various objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of preferred embodiments of the invention, along with the accompanying drawing.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a schematic view of one exemplary device according to the inventive subject matter.

FIG. 2 is a schematic view of another exemplary device according to the inventive subject matter.

DETAILED DESCRIPTION

The inventors surprisingly discovered that high-surface area carbon felts provide for highly effective in situ flow-through anodes for sterilization and chemical decontamination, especially when operated in substantially neutral (pH of between about 5.5 and 8.5) aqueous streams. Most typically, sterilization and chemical decontamination is achieved by production of oxidizing species rather than by direct anodic oxidation of the contaminants. Remarkably, the high-surface area carbon felts remained undegraded, even at current densities ordinarily expected to destroy the carbon felt anode to carbon dioxide.
Such finding is particularly unexpected as it has been well recognized that if carbon is used as an anode in an electrochemical cell, the carbon is subject to an oxidative attack which leads to its complete destruction over a period of time which tends to be directly related to the cross-sectional area. Typically, such destruction occurs at a relatively rapid rate if the carbon is present in a fibrous configuration and employed as an anode rather than a cathode. These findings are reflected in various attempts to mitigate anode destruction as, for example, described by Tiedemann in U.S. Pat. No. 3,471,383. Here a consumable carbon-containing anode is passed through an electrolytic cell. In another example, U.S. Pat. No. 4,360,417 to Reger et al, the carbon anode is substantially completely covered by a protective mixed crystal coating consisting essentially of ruthenium oxide and titanium oxide (Reger et al. describe and depict oxidative destruction of uncoated materials as comparative examples). Still other carbonaceous electrodes are described in, for example, U.S. Pat. Nos. 3,071,637; 3,072,558; 3,214,647; 3,459,917; 3,471,383; 3,476,604; 3,619,382; 3,637,468; 3,759,805; 3,764,499; 3,811,943; 3,827,964; 3,829,327; 3,852,113; 3,915,822; 3,923,629; 3,953,313; 4,046,663; 4,046,664; 4,061,557; 4,108,754; 4,108,755; 4,108,757; 4,445,990; 4,556,469; 5,662,789; 5,725,752; 6,315,886; 6,984,303; and WO 92/12931. However, these electrodes are either used as cathodes, or where used as anodes, are found to be subject to relatively rapid oxidative degradation, especially where the surface area is relatively high.
Remarkably, it appears that in all known carbonaceous anodes, the anode is only used as a ‘static electrode’ that is in contact with the anolyte. Viewed from a different perspective, the electrolyte is in substantially non-moving contact with the internal surfaces of the porous or otherwise high-surface anode and thus subject to oxidation of locally generated oxidative species, especially where the anode is operated at conditions that promote formation of such oxidative species. Without wishing to be bound by any particular theory or hypothesis, the inventors contemplate that by continuously passing the anolyte through the entire volume (including the internal surfaces) of the anode, oxidative damage from oxidative species to the anode is avoided as the oxidative species either react with a passing contaminant in the electrolyte, are removed from direct contact with the anode by the moving anolyte, and/or otherwise decay due to their chemical instability. Viewed from yet another perspective, the inventors contemplate that the oxidizing species may be ‘washed off’ the anode before reaching residence time and quantities sufficient to damage the anode.
As used herein, the term “carbon felt” refers to a textile material that predominantly comprises randomly oriented and intertwined carbon fibers, which are typically fabricated by carbonization of organic felts (see e.g., IUPAC Compendium of Chemical Terminology 2nd Edition (1997)). Most typically, organic textile fibrous felts are subjected to pyrolysis at a temperature of at least 1200° K, more typically 1400° K, and most typically 1600° K in an inert atmosphere, resulting in a carbon content of the residue 90 wt %, more typically 95 wt %, and most typically 99 wt %. Furthermore, contemplated carbon felts will have a surface area of at least about 0.01-100 m²/g, and more typically 0.1-5 m²/g, most typically 0.3-3 m²/g, and where the carbon felt is activated, will have a surface area (BET) of more than 100-500 m²/g, more typically at least about 500-800 m²/g, even more typically at least about 800-1200 m²/g, and most typically at least about 1200-1500 m²/g, or even more. Depending on the organic textile material and carbonization conditions, the carbon felt may be graphitic, amorphous, have partial diamond structures (added or formed by carbonization), or a mixture thereof. In contrast, reticulated or vitreous (glassy) carbon is formed from carbonized thermosetting organic polymer foams that generally have a non-fibrous, open or closed cellular architecture.
In one exemplary aspect of the inventive subject matter, an electrolytic device is made as schematically depicted in FIG. 1. Here, the monopolar parallel plate reactor 100 includes a pair of backing plates 102 that confine the plastic cell body panels 104, typically including one or more flow channels for feeding and/or circulating anolyte or catholyte to the respective anode and cathode compartments 110 and 120, respectively. Anolyte and catholyte ports 106 and 108 are fluidly coupled to the body panels to deliver the electrolyte to the respective compartments. The anode compartment includes the carbon felt anode 112 that is conductively coupled to the electrical connector 114. Most preferably, the connector has a grid shape or otherwise provides openings such that anolyte flowing from the panel 104 passes through the connector 114 and carbon felt anode 112 into the cell gap 130. Treated anolyte that has flown through the carbon felt anode 112 is then withdrawn from the anode compartment through a port (not shown) and recirculated for a desirable number of passes. The arrows indicate the direction of flow of the anolyte and catholyte. Feeding the anolyte that is to be treated is typically performed from a tank or other reservoir (not shown), but may also be implemented by direct feed from a process. Within the cell gap 130 is the diaphragm 132 that defines the boundary of the cathode and anode compartments in the electrolyte. Cathode 122 is typically mechanically coupled to body panel 104 to complete the electrochemical cell. As with the anode compartment 110, it is generally preferred that the catholyte may be circulated through the cathode compartment, which may include a porous or otherwise open-structured cathode to permit catholyte flow through the cathode. Alternatively, the cell may also be constructed as a flow through cell in which the solution to be treated enters the anode compartment and exits the cathode compartment.
With respect to the particular configuration of contemplated cells, it should be noted that all known cell configurations are deemed suitable so long as such configurations allow flow of the anolyte through the carbon felt anode. As used herein, the term “flow through” means that the anolyte enters one side of the anode at a flow rate and exits the anode at that flow rate on another side, typically traversing the entire cross section of the anode. Viewed from another perspective, substantially the entire volume (i.e., greater than 90 vol %, more typically greater than 95 vol %, even more typically greater 99 vol %) of anolyte to be treated will enter the anode on one side and pass through a volume of the anode and exit on the other side. In contrast, tangential or turbulent flow passing across one side of the anode (which will likely diffuse to at least some extent through the anode) is not considered as flowing through the anode as not substantially the entire volume passes through the anode. Viewed from a different perspective, cells with a flow-through anode will typically have an anolyte feed port on one side of the anode and an anolyte withdrawal port on the other side of the anode. Alternatively, the anolyte may also be withdrawn from the cathode compartment (e.g., where the anolyte is in at least temporary fluid communication with the catholyte).
Therefore, suitable electrolytic cells may be configured as monopolar cells or bipolar cells, each or which may be stacked or configured independently of each other. Furthermore, it is generally preferred that the cell includes a body panel on at least the anode side that is configured such that anolyte is (preferably fed into the body panel and) evenly distributed over one side of the anode. Thus, the anolyte may be fed in a continuous flow to the anode compartment or in batches. Where the flow to the anode compartment is discontinuous, it is generally preferred that the anolyte is circulated within the anode compartment such that the anolyte flows through the anode. With respect to suitable containers, it should be appreciated that all containers are deemed suitable that receive the solution to be treated directly from an industrial source (typically via a pipe or other fluid conduit) or from a reservoir that holds the fluid. Suitable containers may have various volumes, and it is generally contemplated that the container (which at least partially encloses the anode and cathode) will have a volume of between about 50 ml to several 100 liters (and even more). Among other things, it should be recognized that the volume of a container will be determined by the volume flow of the solution, the concentration of the contaminant, and the current/voltage applied to the electrodes.
Regardless of the particular mode of feeding, it is contemplated that the anolyte and the catholyte may be provided to the electrolytic cell in all known manners. Thus, continuous and discontinuous flow are both deemed appropriate. Where desired, at least one of the anolyte and catholyte may also be circulated. However, it is preferred that during operation at least the anolyte will substantially continuously (e.g., at least 90% of the time, and more typically at least 95% of the time) flow through the anode. It should be appreciated that the electrolytes may be moved using any manners known in the art. Most preferably, the electrolytes are pumped at a predetermined flow rate, wherein the electrolyte may be pumped from a reservoir or directly from a process operation (e.g., dying bath, rinsing bath, etc.). Alternatively, at least one of the electrolytes may also be moved by gravity.
The flow rate of the anolyte through the anode and/or anode compartment may vary considerably and will typically depend on various factors, including the concentration of the contaminant, total anode surface, conductivity of the anolyte, and current density. However, it is generally preferred that the anolyte will pass through the anode at a flow rate of between about 0.01 ml/cm³*min to about 100 ml/cm³*min, more preferably between about 0.1 ml/cm³*min to about 10 ml/cm³*min, and most preferably between about 0.5 ml/cm³*min to about 3 ml/cm³*min (cm³reflects bulk volume of carbon felt anode) where the anode is operated under conditions that allow formation of oxidative species. Where relatively low current densities and/or multiple passes of the anolyte are contemplated, lower flow rates are deemed suitable. On the other hand, higher flow rates may be desirable where high current densities are applied, low contaminant concentrations are encountered, and/or incomplete destruction is desired. Of course, multiple anodes (or anodes with multiple layers of carbon felt) are also contemplated through which the treated anolyte may be passed to complete or increased efficiency of destruction of the contaminant. It is further preferred that the flow of the anolyte through the anode is unidirectional. However, in alternative aspects of the inventive subject matter, bidirectional, multi-directional, and even tortuous flow paths are also deemed suitable.
The anolyte is preferably fed to one side of the anode via a distributor structure, which may be integral to the body panel, or may be a dedicated distribution device. Once the anolyte exits the anode, it is contemplated that all manners of withdrawing the anolyte from the anode compartment are deemed suitable for use herein. Typically, treated anolyte can be removed from the anolyte compartment using one or more fluid ports that are configured to receive the treated anolyte in a continuous or intermittent manner. The treated anolyte may then be recirculated to the anode for further treatment, stored in a tank, or can be discharged.
Similarly, the flow rate of the catholyte will be determined at least in part by the current density, conductivity, gap width, and other factors that are well within the scope of the person of ordinary skill in the art. The flow rate for the catholyte will be between 0.01 vol % (of the total cathode compartment volume) per hour and 50 vol % per hour (or even more), more typically between 0.1 vol % per hour and 20 vol % per hour, and most typically between 0.5 vol % per hour and 10 vol % per hour. Alternatively, and especially where the cathode compartment is relatively large, circulation of the catholyte may not be needed or may only be temporary. With respect to feeding and withdrawing the catholyte, the same considerations as for the anolyte apply.
It is especially preferred that the anode material comprises carbon felt produced from an organic textile material via carbonization (see above). Preferably, the carbon felt will have a thickness of between about 0.1 cm and 10 cm (and in some cases even more), and even more preferably between about 0.5 cm and 5 cm, while the width and length are generally dependent on the particular electrolytic cell configuration. As the anolyte will typically (but not necessarily) flow through the thickness of the felt, appropriate anode thicknesses and material parameters will at least in part be determined by the backpressure generated by the anode and desired flow rate. However, in further contemplated aspects, it should be recognized that numerous alternative materials and configurations are also suitable, and especially preferred alternative configurations and methods include those in which the anode is fabricated from porous carbonaceous materials, including glassy carbon and similar materials so long as such materials allow anolyte flow through the carbonaceous material (e.g., via network of interconnecting pores or channels) at a rate sufficient to reduce or even eliminate oxidative carbon degradation when the anode is operated under conditions that generate oxidative species. Suitable alternative anode materials include fabrics/webbings that include activated carbon fibers, graphite felt, and any reasonable combination thereof. Still further contemplated anode materials also include composite materials that include the felt or other carbonaceous materials. For example, suitable anodes may be manufactured from a conductive polymer that is coated, or in which is embedded carbon felt or other carbonaceous materials.
Similarly, suitable cathodes may vary substantially, and a particular choice for the cathode material and configuration will typically depend on the particular solution and/or contaminant that is to be treated. However, it is generally preferred that appropriate cathode materials are electrochemically relatively inert. Therefore, especially preferred cathode materials include platinum-coated titanium. However, numerous other metals, metal alloys, and even carbon are considered suitable for use herein. Furthermore, where it is desirable that the polarity of the electrochemical cell is switched (e.g., to prevent build-up of oxidized materials at the anode), it should be recognized that the cathode material and configuration may be identical with the anode material and configuration. Therefore, the cathode may have numerous configurations, and may include materials and configurations in which the solution can pass through the cathode, as well as impermeable materials and configurations.
With respect to contemplated anolytes, it is generally preferred that the anolyte that includes the contaminant is an aqueous solution having a substantially neutral pH, typically between about 5.5 and 8.5, more typically between about 6.0 and 8.0, and most typically between about 6.5 and 7.5). However, more alkaline or more acidic solutions may also be desired, especially where the so desired pH will increase solubility of the contaminant or oxidation product(s) of the contaminant. Similarly, anolytes need not be restricted to purely aqueous solutions, and non-aqueous solutions are also expressly contemplated, including those comprising emulsifiers, organic solvents, and even liquefied gases. However, generally preferred solutions especially include waste streams form an industrial process, wherein such solutions are either circulated between the process and the electrochemical cell (which may further include use of a reservoir), or be directly fed from the process to the electrolytic cell. Exemplary processes and sources include rinsing or washing operations (e.g., from fruit or animal processing plant, or from metal plating operations), sterilization, cooling/heating water, aqueous solvents for chemical (e.g., chromatography supplies, buffers, etc.) and/or biological processes (e.g., fermentations, enzymatic reactions, etc.).
Thus, the nature and concentration of a contaminant may vary substantially. However, especially contemplated contaminants include bacteria (including spores), viruses, eukaryotic cells, halogenated (typically aromatic) organic compounds, dyes or otherwise colored compounds, and all organic matter directly or indirectly derived from contact of the organic matter with water. Therefore, it should be recognized that the pH of suitable solutions may vary, and it is generally preferred that electrolysis according to the inventive subject matter will be in the neutral pH range. Suitable pH values may be adjusted by adding acid or base, or a buffer system to the anolyte.
Furthermore, depending on the chemical composition of the solution, the solution may further be modified to increase and/or decrease conductivity. Where conductivity is increased, all known salts (preferably with insignificant interference [e.g., electroplating] to the electrolytic process) are considered suitable herein. Similarly, the solution may also be diluted, or otherwise reduced in conductivity (e.g., precipitation, chemical modification, or filtration of conductive species in the solution).
Consequently, it should be recognized that the current and/or voltage of contemplated systems will vary substantially, and all currents and/or voltages suitable for reduction of the contaminant are considered appropriate for use herein. However, typical voltages will be in the range of 0-100 Volt, and more typically between 10 Volt and 50 Volt, at currents of between about 1 mA (or even less) and 100 A and higher, and more typically between about 0.1 A and 10 A. Viewed from a different perspective, current densities will preferably be in the range of about 1 mA/cm²and 1000 mA/cm², more preferably between about 10 mA/cm²and 500 mA/cm², and most typically between about 30 mA/cm²and 100 mA/cm². As used herein in the following examples, the term “about” in conjunction with a numeral refers to a range of that numeral starting from 10% below the absolute of the numeral to 10% above the absolute of the numeral, inclusive. For example, the term “about 10 A/cm²” refers to a range of 9 mA/cm²to 11 mA/cm².
Therefore, and depending on the solution, contaminant, current and/or voltage, it should be appreciated that the contaminant may be oxidized directly at the anode, or indirectly via an oxidative species. However, it is generally preferred that the current density will be adjusted such that oxidative species are formed in the particular anolyte. Among other oxidative species, preferred species include ozone, hydrogen peroxide, hydroxy radicals, oxygen ions, superoxide anions, singlet oxygen, etc. It is contemplated that formation of such oxidative species will provide for reactive molecules that then oxidize the contaminant. The inventors therefore contemplate that the contaminant is predominantly oxidized (i.e., at least 51% of the contaminants, more typically at least 70% of the contaminants, and most typically 85-90% of the contaminants) via such reactive species produced from the aqueous solution rather than being directly oxidized on the anode.
Using an exemplary device as shown in FIG. 1, the inventors performed numerous experiments to generate oxidative species, and in some cases to also perform direct oxidation of various compounds. While generation of oxygen was relatively simple to observe, the detection of hydroxyl radicals, ozone, peroxide, and other oxidative species typically requires expensive and sophisticated equipment. Therefore, the inventors decided to indirectly detect oxidative species by adding materials to the water that react with or capture these species. Initially it was believed that small amounts of chloride ions were being converted to hypochlorite, which could act as the oxidizing agent. However, subsequent experiments with de-ionized water conclusively demonstrated that hypochlorite was not formed in an amount sufficient to react with oxidizable molecules. Instead, it was found that all or almost all of the contemplated configurations and methods allowed for the generation of reactive species that indirectly oxidized contaminants. In some cases, direct oxidation of the contaminant was also observed.
Among other unexpected things, the inventors discovered that carbon felt materials made from dehydrogenation of long chain organic structures remained intact, even when continuously operated for prolonged periods (e.g., more than 2 hours, more typically more than 6 hours, even more typically more than 24 hours, and most typically more than 48 hours) at currents of 50 mA/cm²(cm²representing the gross dimensions of anode felt portion, not actual surface area), which indicates that oxygen and other oxidizing species were generated at higher overpotentials than carbon dioxide formation from the carbon at the electrodes. In control experiments, certain carbon black and graphitic structures were oxidized at significant rates (especially where no flow-through configuration was implemented).
Electrochemical evolution of oxygen from water starts theoretically at +1.23 volts, but depending on conditions and choice of electrode material, the voltage required for generation of oxygen is often significantly higher at practical current densities (the electrode potential for hydrogen peroxide generation is +0.64 volts, ozone generation requires +1.6 volts). Both reactions are very inefficient in water at neutral pH and ambient temperature, and thus high oxygen overpotential electrodes (e.g., lead dioxide, Magneli phase titanium suboxides, doped tin dioxide) are required. Diamond coatings have been found to work at very high oxygen overpotentials indicating that the structure and stability of diamond carbon resists oxidation. When oxygen overpotentials are high, above 1.7 volts versus standard hydrogen electrodes, other oxidative reactions that can occur at lower potentials are promoted (including oxidation of organic species, including the destruction of organisms or pesticide residues). The following experiments should illustrate some of the inventive aspects:

Experiments

An electrochemical cell was constructed as a monopolar parallel plate reactor similar to the device of FIG. 1. The anode was made by attaching a carbon felt pad to a predrilled graphite feeder plate. The carbon felt was 10 cm by 4 cm, and 1 cm thick. The cathode was a flat titanium plate coated with platinum. The cell gap (distance between carbon felt and the cathode) was 14 mm. The flow through the cell was arranged so that the treated water flowed out of the top of the cell after flowing through the felt in the anode compartment.
The flow rate was adjusted to 30 ml/min to correspond with a once through treatment which removed the color and odor from the water in the exiting stream. The water was dosed with 2 g/liter of sodium sulfate to increase the conductivity. The experiments were conducted at room temperature. The water was first pumped through the anode compartment with the current off and samples taken to check for concentrations of the target contaminant, This ensured that the inventors would not observe effects based on adsorption onto the high surface area felt. When current was applied to the cell, a constant current of 3.0 A was used at various voltages, depending on the particular experiment. Typical voltages were about 12 volts, with the actual voltage being predominantly determined by the cell gap and electrolyte conductivity.

EXAMPLE 1

Acid violet 7B, a common triphenylene dye identified by reference to the Color Index as number 42745 was added to deionized water at a concentration of 100 milligrams per liter. Deionized water was made conductive with addition of 2 grams per liter of sodium sulfate. This solution was pumped through the anode compartment described above with the current off. The dye concentration remained the same. A 3.0 amps current was then applied to the cell and the solution pumped through at the rate of 30 ml per minute. Remarkably, the solution was completely decolorized as it exited the cell. The experiment was repeated with Allura Red AC, a common foodstuff dye identified as CI # 16035, and later with Brilliant Green #42040 in the Color Index with the same results.

EXAMPLE 2

Perchlorobiphenyl (PCB) was extracted from transformer oil by emulsification with a commercially available emulsifying agent LA8 sold by Imperial Chemical Industries Ltd. The PCB extract was added to water producing a solution that was found by analysis to be 12 mg/liter of PCB. The solution was pumped through the anode compartment at no current, and the exiting solution was found to be 11 mg/liter of PCB. When the current was turned on, the solution leaving the anode compartment contained no detectable levels of PCB.

EXAMPLE 3

DDT (1,1-Bis-(p-chlorophenyl)-2,2,2-trichloroethane) extracted from an old disused spray containing the organophosphate pesticide Methidathion (O,O-Dimethyl S-(5-methoxy-1,3,4-thiadiazolinyl-3-methyl)dithiophosphate) was added to the test solution producing a solution which on analysis gave 53 ppm of DDT. After passing through the anode compartment without electrolysis, the concentration was found to be 54 ppm, while no DDT was detected and after electrolysis of the solution in the electrolytic anode compartment.

EXAMPLE 4

Winery waste water was treated from a local winery in the following manner: A first portion was treated with 1 ppm Cyquest N2100 flocculant (neutral polymeric flocculant, food grade), which removed all visible suspended solids. A second portion was ran through a sand filter to remove suspended solids. Both clarified liquors from the procedures above had dark brown color with little difference in the clarity of the liquors from the two methods. Both clarified liquors had a strong odor.
To each of the above liquors sodium sulfate (2 grams per liter) was added as a current carrier. The cell was run at 3 amps, and the resultant treated liquors became clear on standing for 30 minutes with brown sediment of insoluble oxidized organics at the bottom of the liquors. There was no odor from the treated liquors either immediately after treatment or after 4 weeks. It should be noted that the addition rate of sodium sulfate could be lessened or entirely omitted by closer spacing of the electrodes.

EXAMPLE 5

Wash water from an apple packing plant containing pesticide residues, organic acids and fermentation products was treated in the cell in an arrangement as schematically depicted in FIG. 2. The initial solution was blue green in color with a strong odor. When pumped through the anode compartment without a current, there was no change in appearance or smell. Once the current was applied, the color and odor disappeared. After settling the treated solution a brown sediment precipitated, which was found to be sterile.

EXAMPLE 6

A solution of water inoculated with E. coli having a starting concentration of 1200 cfu (colony forming units) was pumped through the anode compartment. Remarkably, the exiting count was 1140 cfu without current, and with current turned on, no E. coli survived. No sign of wear or distress was observed on the felt electrode apart from the initial loss of fibers not completely secured.
It should be recognized that high surface area felt anodes are not only suitable for the destruction of organic materials such as DDT and PCB, but have also many other uses in the treatment of waste waters. For example, contemplated devices and methods may be employed for the (preferably continuous) sterilization of the effluent from septic tanks without the many problems otherwise associated with hypochlorite handling. In another example, the devices and methods according to the inventive subject matter may also be employed for self cleaning swimming pool water sterilizers in which both the anode and cathode are made from carbon felt and are electrochemically reversed to inhibit growth of calcium salts on the cathode.
Thus, specific embodiments and applications of methods and apparatus for generating oxidizing agents have been disclosed. It should be apparent, however, to those skilled in the art that many more modifications besides those already described are possible without departing from the inventive concepts herein. The inventive subject matter, therefore, is not to be restricted except in the spirit of the appended claims. Moreover, in interpreting both the specification and the claims, all terms should be interpreted in the broadest possible manner consistent with the context. In particular, the terms “comprises” and “comprising” should be interpreted as referring to elements, components, or steps in a non-exclusive manner, indicating that the referenced elements, components, or steps may be present, or utilized, or combined with other elements, components, or steps that are not expressly referenced. Furthermore, where a definition or use of a term in a reference, which is incorporated by reference herein is inconsistent or contrary to the definition of that term provided herein, the definition of that term provided herein applies and the definition of that term in the reference does not apply.

Claims

1. A method of treating an aqueous solution, comprising:

providing an electrolytic cell having an anode in an anode compartment, a cathode in a cathode compartment, and a diaphragm separating the anode compartment from the cathode compartment to thereby fluidly isolate the anode compartment from the cathode compartment;

wherein the anode comprises a carbon felt that is conductively coupled to an electrical connector such that a flow path is formed to allow flow of the aqueous solution through the carbon felt;

moving the aqueous solution through the anode compartment such that substantially the entire solution passes through the carbon felt from one side to another side; and

operating the electrolytic cell at a current density effective to generate oxidative species from the aqueous solution in an amount sufficient to oxidize a contaminant in the aqueous solution.

2. The method of claim 1 wherein the carbon felt is prepared from carbonized organic textile fibrous felts.

3. The method of claim 1 wherein the carbon felt has a surface area of at least about 800-1200 m²/g.

4. The method of claim 1 wherein the flow path has a length of between 0.1 cm and 10 cm.

5. The method of claim 1 wherein the flow path has a length of between 0.5 cm and 5 cm.

6. The method of claim 1 wherein the aqueous solution has a pH of between 5.5 and 8.5.

7. The method of claim 1 wherein the aqueous solution has a pH of between 6.5 and 7.5.

8. The method of claim 1 wherein the aqueous solution passes through the carbon felt at a flow rate of 0.1 ml/cm³*min to 10 ml/cm³*min.

9. The method of claim 1 wherein the aqueous solution passes through the carbon felt at a flow rate of 0.5 ml/cm³*min to 3 ml/cm³*min.

10. The method of claim 1 wherein the current density is between 10 mA/cm²and 500 mA/cm².

11. The method of claim 1 wherein the current density is between 30 mA/cm²and 100 mA/cm².

12. The method of claim 1 wherein the cell is continuously operated for at least 6 hours without apparent oxidative damage to the carbon felt.

13. The method of claim 1 wherein the cell is continuously operated for at least 24 hours without apparent oxidative damage to the carbon felt.

14. The method of claim 1 wherein the cell is continuously operated for at least 48 hours without apparent oxidative damage to the carbon felt.

15. The method of claim 1 wherein the oxidative species is selected from the group consisting of ozone, hydrogen peroxide, a hydroxy radical, an oxygen ion, singlet oxygen, and a superoxide anion.

16. The method of claim 1 further comprising a step of recycling at least part of the aqueous solution back to the anode compartment after the solution has passed through the carbon felt.

17. The method of claim 1 wherein the contaminant in the aqueous solution is selected from the group consisting of a bacterium, a spore, a virus, a eukaryotic cell, an optionally halogenated aromatic organic compound, and a dye.