WO1999058452A2

WO1999058452A2 - Electrodes for electrolytic removal of nitrates from water, methods of making same, and apparatus incorporating said electrodes

Info

Publication number: WO1999058452A2
Application number: PCT/US1999/010674
Authority: WO
Inventors: Sjef Otten; Mark Lubin
Original assignee: Upscale Water Technologies, Inc.
Priority date: 1998-05-14
Filing date: 1999-05-14
Publication date: 1999-11-18
Also published as: AU3991699A

Abstract

A cathodic carbon fiber electrode plated with metallic iridium and an anodic titanium or DSA electrode is plated with metallic iridium or first coated with iridium oxide and then plated. According to a presently preferred embodiment, a cathodic electrode is manufactured by submersing a PAN carbon fiber electrode in an aqueous solution of 0.1 % to 3.0 % H2IrCl6 by weight and subjecting it to an electroplating voltage. An apparatus for removing nitrates from water according to the invention includes an electrochemical flow cell or a holding tank cell and an electrode system including an anodic electrode as described above, a cathodic electrode as described above, and a silver/silver chloride reference electrode. Nitrate laden water is introduced into the holding tank or flows through the flow cell. All of the electrodes are immersed in the nitrate laden water and are coupled to an electronic control circuit which impresses a voltage across the electrodes such that the voltage causes electrochemical reduction/oxidation reactions on the surface of the electrodes.

Description

ELECTRODES FOR ELECTROLYTIC REMOVAL OF NITRATES FROM WATER, METHODS OF MAKING SAME, AND APPARATUS INCORPORATING SAID ELECTRODES

FIELD OF THE INVENTION

The invention relates to electrochemical reactions and electrodes used therein. More particularly, the invention relates to electrodes for use in the electrochemical removal of nitrates and nitrites from water, methods for making such electrodes, and apparatus incorporating such electrodes.

BACKGROUND OF THE INVENTION Nitrates and nitrites, collectively referred to as nitrogen compounds, are unwanted contaminants in many different water sources and their concentration in water is regulated by the Environmental Protection Agency, (EPA). These nitrogen compounds, as contaminants in water, can be harmful to the environment and to organisms exposed to the water. Several different technologies have been suggested for removal of such nitrogen compounds from water. These technologies include biological denitrification, ion-exchange resins, and thermal decomposition.

Biological water treatment systems involve the use of bacteria that are capable of breaking down the nitrogen compounds to nitrogen and water. While being effective at removing nitrogen compounds, these systems must be operated over narrow pH and temperature ranges to permit the bacteria to operate. Additionally, there are costs associated with disposing of the waste sludge resulting from the process.

Ion-exchange resin systems involve passing the water through an ion-exchange resin which includes a resin designed to selectively remove the nitrogen compounds and replace them with other harmless ions such as chlorides. However, these systems are not extremely efficient at removing the nitrogen compounds. Additionally, once the resin is spent, it must be disposed of as a toxic waste or recharged, both of which are expensive. Thermal decomposition systems involve thermally decomposing the nitrogen compounds at elevated temperature and pressure. However, other compounds, such as nitrous oxide, hydrazine, chlorine and ammonia are also formed. While being very effective at removing some nitrogen compounds, these systems are expensive to operate due to the elevated temperatures and pressure, and they produce toxic byproducts.

Co-owned U.S. Patent Applications Serial Number 09/079,071 , and Serial Number 08/457,040, and U.S. Patent No. 5,614,078, all of which are hereby incorporated herein by reference, disclose methods and apparatus for the removal of nitrates from water. Although not limited thereto, the apparatus generally includes an electrochemical flow cell through which the aqueous solution containing nitrates flows or a holding tank cell into which the solution is introduced and then released after processing, and an electrode system including a carbon fiber cathodic electrode, a carbon fiber anodic electrode and a reference electrode. All of the electrodes are immersed in the aqueous solution and coupled to an electronic control circuit which impresses a voltage across the electrodes such that the voltage causes electrochemical reduction/oxidation reactions on the surfaces of the cathodic and anodic electrodes. According to the method, the electrodes are at a potential wherein nitrates are reduced to gaseous products but hydrogen, oxygen, chlorine, and other noxious substances are not produced. According to the disclosed preferred embodiment, the reference electrode is a silver/silver- chloride electrode, the cathodic and anodic electrodes are carbon fibers based on poly aery lonitrile (PAN), and the surface area ratio of the anodic electrode to the cathodic electrode is preferably in the range of 40: 1 to 120: 1. Additionally, all voltages referred to in this application are versus the silver/silver-chloride reference electrode.

As disclosed in the U.S. Patent No. 5,614,078 and '040 applications, the anodic to cathodic surface area ratio must be large in order to prevent a chlorine evolution reaction from taking place in salt water and to prevent oxygen evolution reactions and changes in pH in fresh water. In practice, it has been discovered that, in seawater, an anodic to cathodic surface area ratio of up to 150:1 is desirable to prevent chlorine formation under any circumstances. In addition, the anodic voltage (relative to the reference electrode) must be kept below +800 mV to prevent chlorine formation in seawater. Moreover, in flow-through systems, an increased flow rate (flow velocity relative to the cathode) increases the overall background current required and, as a consequence, causes an increase in the anodic voltage.

Accordingly, what is needed are electrodes and and methods of making and using electrodes that remove nitrogen compounds from water in an efficient and cost-effective manner without producing harmful byproducts or the need to dispose of spent treatment media.

SUMMARY OF THE INVENTION

The present invention is directed to electrodes and systems of electrodes and methods of making and using such electrodes and systems that are effective at removing nitrates and nitrites from water. These electrodes, systems and methods may be used to treat water from a wide variety of water systems including, but not limited to, private and municipal drinking water; domestic and municipal wastewater; slaughter, feedlot, and other food processing operations; munitions manufacturing or destruction waste streams; polluted surface waters such as rivers, lakes and streams; polluted run-off from commercial agricultural operations; aquaculture farms; and aquarium water. The present invention includes systems which electrochemically treat the water from these types of systems to remove the above-mentioned nitrogen compounds.

The present invention is preferably directed to cathodic carbon fiber electrodes plated with a noble metal, such as iridium, and anodes of the DSA type. As used herein, a DSA (Dimensionally Stable Anode) electrode is an electrode made of a titanium base metal which is coated with one or more oxides. DSA electrodes are commercially available from a number of sources, including Exceltec International Corp., Sugar Land, TX. According to a presently preferred embodiment, a cathodic electrode is manufactured by submersing a PAN carbon fiber electrode in an aqueous solution of 0.1% to 3.0% H₂IrCl₆by weight and subjecting it to a triangular electroplating voltage in the range of 0 mV to -2100 mV. Electroplating is performed for a period ranging from 0.5 to 3 hours at room temperature while the plating solution is stirred. According to other embodiments, electroplating is accomplished with a pulsed voltage or a sweeping voltage.

The present invention also comprises apparatus for removing the described nitrogen compounds from water. A preferred embodiment includes an electrochemical flow cell or a holding tank cell and an electrode system including an anodic electrode as described above, a cathodic electrode as described above, and a silver/silver-chloride reference electrode. Water containing the nitrogen compounds is introduced into the holding tank or flows through the flow cell. All of the electrodes are immersed in the water with nitrogen compounds and are coupled to an electronic control circuit which impresses a voltage across the electrodes such that the voltage causes electrochemical reduction/oxidation reactions on the surface of the electrodes. The electrodes are at a potential wherein nitrogen compounds are reduced to gaseous products but hydrogen, oxygen, chlorine, and other noxious substances are not produced. The flow cell or holding tank is preferably made of an inert material which is non-reactive and non-conductive. The surface area ratio of the anodic electrode to the cathodic electrode is preferably in the range of 1:1 to 5: 1. The cathodic voltage is preferably from about -400 mV to about -1000 mV. The anodic voltage is preferably from about +300 mV to about +800 mV and preferably below +800 mV in seawater or solutions with high chloride content. The water flow velocity in feet per minute is preferably from about 0.5 to about 30 feet per minute.

It was discovered that plating the cathodic electrode in different ways produces different results in the nitrate reducing apparatus. For example, the cathodic electrode which is plated as described above gives excellent results for both nitrate and nitrite reduction in both salt and freash water. However, electrodes plated with different voltages than those described above will preferentially work better in one type of water than the other

Accordingly, it is an object of the invention to provide an improved apparatus for removing nitrogen compounds from water.

It is yet an object of the invention to provide improved electrodes for removing nitrogen compounds from water.

Another object of the invention is to provide methods for making improved electrodes for use in removing nitrogen compounds from water.

Still another object of the invention is to provide electrodes for removing nitrates from fresh water which reduces nitrogen compounds to nontoxic compounds.

A further object of the invention is to provide electrodes for removing nitrogen compounds from both fresh water and saltwater.

It is an object of the invention to provide apparatus that can be used to remove nitrogen compounds from fresh water.

A further object of the invention is to provide apparatus that can be used to remove nitrogen compounds from salt water.

These and other objects, features and advantages of the present invention will become apparent after a review of the following detailed description of the disclosed embodiments and the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 is a perspective view of an exemplary embodiment of a nitrate reducing apparatus incorporating electrodes according to the invention.

Figure 2 is an upside down perspective view of the lid and electrode system of the apparatus of Figure 1.

Figure 3 is a perspective schematic view of a presently preferred embodiment of a nitrate reducing apparatus incorporating electrodes according to the invention.

Figure 4 is an enlarged detail of one of a cathodic electrode from Figure 3.

Figure 5 is an exploded view of an electrode assembly from Figure 3. Figure 6 is a schematic sectional view of a Ag/AgCl electrode with a self-contained chloride solution according to a preferred embodiment of the invention.

DETAILED DESCRIPTION OF THE INVENTION

The present invention is directed to electrodes and electrode systems and methods of making and using the same which are effective at removing nitrogen compounds from water. These electrodes, systems and methods may be used to treat water from a wide variety of water sources. The present invention utilizes electrochemical treatment systems and methods to break down the described nitrogen compounds to gaseous nitrogen. The electrochemical system utilizes novel cathodes, anodes and reference electrodes to provide a system which efficiently removes nitrogen compounds in an economical manner without the generation of harmful by- products.

One of the novel aspects of the present invention is a cathodic electrode based on a carbon fiber electrode which is treated as described below. According to a presently preferred embodiment of the invention, the carbon fiber electrode is preferably based on polyacrylonitrile (PAN) although it may also be carbon fibers based on petroleum derivatives and/or phenolic resins. Carbon fiber densities ranging from Ik to 320k fibers per tow are commercially available, although an exemplary embodiment employs a 12k carbon fiber.

According to a first cathodic electrode embodiment, a carbon fiber electrode is submersed in an aqueous solution of 0.1% to 3% H₂IrCl₆ by weight and subjected to a sweeping electroplating voltage vs. a Ag/AgCl reference electrode. Electroplating is performed for 0.5 to 3 hours at room temperature while the plating solution is stirred. The voltage continuously sweeps from the range of +200 mV to -200 mV to the range of -900 mV to -1300 mV at a sweep rate of 2- 100 millihertz.

According to a second cathodic electrode embodiment, a carbon fiber electrode is submersed in an aqueous solution of 0.1% to 3% H₂IrCl₆ by weight and subjected to a pulsed electroplating voltage vs. a Ag/AgCl reference electrode. Electroplating is performed for 0.5 to 3 hours at room temperature while the plating solution is stirred. The voltage is pulsed from the range of -700 mV to -1000 mV to the range of -1500 mV to -2000 mV in cycles ranging from 2-20 minutes.

According to a third cathodic electrode embodiment, a carbon fiber electrode is submersed in an aqueous solution of 0.1% to 3% H₂IrCl₆ by weight and subjected to a constant electroplating voltage in the range of

-1500 mV to -2000 mV. Electroplating is performed for 0.5 to 3 hours at room temperature while the plating solution is stirred.

It is to be understood that even though the above embodiments plate the cathodic electrode with iridium, the present invention includes cathodic electrodes having almost any noble metal as the coating. Noble metals useful in the present invention include, but are not limited to, platinum, iridium, rhodium, rhenium, osmium, silver, gold, and mixtures thereof. According to the invention, the cathodic electrodes described above are useful in nitrogen compound reducing apparatus as described in the previously incorporated co-owned patent applications and patents. In addition, according to the present invention, the cathodes described above are useful in nitrogen compound reducing apparatus which incorporate anodic electrodes based on titanium or DSA.

Still another novel aspect of the present invention is the use of a silver/silver chloride reference electrode. These electrodes are fully described U.S. Patent No. 5,833,825, which is incorporated herein by reference. The Ag/AgCl electrode preferably includes a rigid tube within which a Ag/AgCl wire is mounted by epoxy. An insulated wire lead is coupled by a solder joint to the Ag/AgCl wire. An optional plastic cap is placed on the upper end of the tube and the wire lead extends out of the upper end of the tube and through the plastic cap. A sodium chloride liquid or gel solution is provided in the lower end of the tube such that the Ag/AgCl wire is bathed in the solution. The solution is maintained within the tube by a porous glass, ceramic, or polymeric plug which is press fit into the lower end of the tube. The lower end of the tube including the plug can be coated with a water soluble non-toxic substance. The water soluble substance is preferably applied by dip coating and air drying. The water soluble substance forms a hard durable protective coating which prevents the sodium chloride solution from evaporating during the time between manufacture and use of the electrode. In addition, the water soluble substance quickly dissolves once in contact with flowing water. The electrode according to the present invention meets all of the electrochemical requirements of the nitrate removal systems described above in the co- owned patent applications.

According to a presently preferred embodiment of the Ag/AgCl electrode, the water soluble coating is preferably made from a concentrated solution of polyvinyl alcohol (PVA) in deionized or RO (reverse osmosis) water. The solution is made by slowly dissolving 30-35% by weight PVA in water at 70-80°C with vigorous stirring. After the PVA is dissolved, the syrupy solution is allowed to stand for several hours so that air bubbles dissipate. The solution can be stored at room temperature for up to two months. The Ag/AgCl electrodes are then dipped into the prepared solution and then allowed to dry.

Referring now to Figures 1 and 2, an exemplary embodiment of a nitrate reducing apparatus is generally illustrated by reference numeral 10. The nitrate reducing apparatus 10 includes a flow cell 11 which includes a container 12 and a lid 14, both preferably formed of polystyrene or other non-conductive material. The lid 14, which is provided merely to prevent foreign material such as dirt from entering the cell, is optionally bolted to container 12 by screws 15. The nitrate reducing apparatus 10 further includes a solution inlet 16 through the side of container 12 and a solution outlet 18 which is spaced apart from the solution inlet 16. A silver/silver-chloride reference electrode 21, an iridium plated carbon fiber cathodic electrode 22, and a plurality of anodic electrodes 24, as described above, are immersed in an aqueous solution in the container 12 and coupled to a voltage source as described below when the nitrate reducing apparatus 10 is operational. The electrodes of the nitrate reducing apparatus 10 are connected to an electronic control circuit 30 via cables 20, 28, 32, 34, 36, 38, and 40. The electronic control circuit 30 controls the voltage pattern and magnitude applied to the cathode, as described in the previously incorporated co-owned applications. When a sufficient voltage is applied to the cathode (e.g. -400 to -1000 mV), a current flows through the anode and the voltage at the anode rises. The voltage at the cathode produces the nitrate removing reactions and determines the level of current flowing through the anode. As the current through the anode rises, the voltage at the anode rises. If the voltage at the anode is too high, oxygen and/or chlorine can be produced. As explained in the co-owned applications, the anodic voltage is maintained low by increasing the surface area of the anodic electrodes and controlling the cathodic voltage.

Referring now to Figures 3-5, a presently preferred embodiment of a nitrate removing apparatus 110 includes a tank 112 having an inlet 116 and an outlet 118, a plurality of cathodes 122 and anodes 125, and at least one reference electrode 121. The cathodes 122 are mounted on one or more plates or sheets 123 and the anodes are similarly mounted on plates or sheets 125. According to the presently preferred embodiment, the plates 123, 125 are made from 0.25" thick plastic. The plates 123, 125 are provided with holes, e.g. 127, 129, within which the electrodes are mounted.

As shown best in Figures 4 and 5, the electrodes are preferably made from a woven fabric, e.g. 122a which is a woven carbon fiber cloth. Fabric 122a is placed over the hole 127 and a plastic ring 131 is pressed over the fabric and into the hole 127 making a force fit. The ring 131 is, prior to insertion, provided with a outer layer of titanium foil 133 (preferably applied as an adhesive tape) which terminates with a bent tab end 135. Then the ring is force fit into the hole, the foil 133 makes a good electrical contact with the fabric 122a. The tab 135 is used to make electrical connection with a cable 120 to the power supply (not shown). After the electrodes have been formed, the electrode, including the woven fabric, is then plated with a noble metal using one of the methods previously described to form the final electrodes. According to a presently preferred embodiment, the rings 131 are made from the plugs cut out of the plates 123 to form the holes 127. It will be appreciated, however, that other means may be used to fit the electrodes over the holes. As seen in Figure 4, a reference electrode 121 is mounted on the plate 123 with a glued clip 137 adjacent to a cathode 122, and is coupled to the control circuitry (not shown) by a cable 119.

Referring now to Figure 6, a Ag/AgCl electrode 220 according to the invention includes a rigid tube 212 within which a Ag/AgCl wire 214 is mounted by epoxy 216. The Ag/AgCl wire 214 is electrically coupled to an insulated wire lead 218 by a solder joint 220. An optional plastic cap 222 seals the upper end of the tube 212 and the wire lead 218 extends out of the upper end of the tube 212 and through the cap 222. A sodium chloride liquid or gel solution 224 is provided in the lower end of the tube 212 such that the Ag/AgCl wire 214 is bathed in the solution. The solution 224 is maintained within the tube 212 by a porous glass, ceramic, or polymeric plug 226 which is press fit into the lower end of the tube 212. In order to prevent the sodium chloride solution 224 from dissipating during the time between manufacture and use of the electrode, the lower end of the tube 212 including the plug 226 can be coated with a water soluble non-toxic substance 228.

From the foregoing it will be appreciated that water entering the apparatus 110 via the inlet 116 must contact the electrodes in order to reach the outlet 118. It will be appreciated that the shape and the number of electrodes provided on each plate may be varied according to the size of the tank and other variables in the electrochemical reaction. For example, the tanks may be made small for domestic aquarium use or large for industrial use. It will also be understood that in lieu of a tank, the electrodes of the invention may be placed in a flowing water formation in such a way that water passes through the electrodes. To provide modularity in the construction of different sized tanks, the presently preferred electrodes are made one per plate, such as shown in Figure 4, and are arranged on racks to form a "mosaic" of electrodes. In order to provide a reasonable seal between the edges of adjacent plates, the edges of the plates are provided with rabbets. Finally, it will be understood that the water may contact the electrodes using any desired flow path. The water may pass directly through the electrodes, or the electrodes may be arranged such that they are parallel to the flow path of the water. The electrodes may even be arranged such that they form a serpentine flow path and the water may either follow this path or a portion thereof. One advantage of the present system is that it is not size or shape limited and may be modified as needed depending on the volume of water to be treated and the nitrate/nitrite concentration of the water. However, in another embodiment of the present invention, the electrodes are constructed as concentric cylinders composed of layers of polyethylene or polypropylene sheet, which is overlain with a flat rectangular piece of treated carbon cloth. A polyethylene or polypropylene "fishnet" is then overlain and finally a thin, preferably multi-layer, metallic screen is placed on top. A metallic connection, preferably Ti or some other similarly inert metal, is made along the long edges of the rectangle thereby forming a "seam" down the length of the cylinder.

The treated carbon referred to above functions as the cathode. The "fishnet" functions as a non-conductive separator and as a turbulence promoter in the thin layer between the electrodes. The metallic screen functions as the counter electrode, or anode. Its construction is best achieved using thin flexible screening made from an appropriate anodic catalyst metal or a suitable base metal coated with an appropriate anodic catalyst.

The basic group forms an electrochemical flow cell if the fluid electrolyte is forced down the length of the cylinder between the described electrodes. The concentric cylindrical electrodes may also be constructed such that the cathode connection is located at one end and the anode connection at the other end.

One advantage of this embodiment is that the cell voltage is dramatically reduced since the anode and cathode are close together, approximately 1-3 mm, although larger gaps may be used if needed. Another advantage is that the system could be contained in a pipe or other pressure-containment system, thus allowing water to be forced through at a controlled rate and greatly reducing or eliminating the use of gravity feed and clogging. The disadvantage of very thin films of treating water may be overcome by the large number of films treatable at one time and the even flow.

Voltage control and current control methods may be employed, depending on the concentration of the nitrates/nitrites in the water. Additionally, another aspect of the present invention is the ability to clean the electrodes such that they can be reused, thereby helping to lower the cost of operating the system. The catalytic activity of electrodes towards one or more electrochemical reactions can be decreased due to a number of factors including adsorption of mostly organic substances, deposition of mostly inorganic substances, and adsorption of reaction products.

Adsorption of organic substances including, but not limited to, grease, dirt, epoxies, finger prints and soap residues, which usually occurs during manufacturing of the electrodes, may be cleaned using standard cleaning procedures.

Deposition of inorganic substances includes such substances as calcium and magnesium carbonates, hydroxides and oxides. This usually occurs when the pH near the cathodic electrode surface increases to allow the deposition of the above-mentioned inorganics. One possible cleaning method to clean the electrodes of these deposited inorganics is to reverse the electrode potential of the cathode for a short period of time. The prefened pulse burst as from 1-200 seconds at a voltage in the range from +850 to +1100 mV. During this time, the pH near the electrode becomes slightly acidic due to the generation of oxygen, thereby removing these inorganics.

Adsorption of reaction products is a common problem with most electrochemical reactions. Normally, desorption is used to remove these reaction products. However, by employing short voltage or cunent pulses (0.1 to 10 Hz) extending into the hydrogen and oxygen region will cause a short burst of hydrogen and oxygen, which results in a very efficient method of electrode cleaning, including the removal of adsorbed species. The preferred voltage waveform for this cleaning operation to remove adsorbed species is a pulse train from 1-20 seconds at 0.5 to 15 Hz in the anodic range from +850 to +1100 mV and in the cathodic range from -850 to -1100 mV.

There have been described and illustrated herein several embodiments of anodes and cathodes for electrolytic removal of nitrates from water. While particular embodiments of the invention have been described, it is not intended that the invention be limited thereto, as it is intended that the invention be as broad in scope as the art will allow and that the specification be read likewise. Thus, while particular plating techniques have been disclosed as advantageous, it will be appreciated that other coating techniques might yield similar or better results subject to experimentation. For example, it is believed that CVD and other sputtering techniques may yield similar results. Also, while no particular geometry for the electrodes has been specified, it will be recognized that the electrodes could be formed as sheets, perforate sheets, expanded mesh, etc. as described in the previously incorporated co-owned applications. It will therefore be appreciated by those skilled in the art that yet other modifications could be made to the provided invention without deviating from its spirit and scope as so claimed.

Claims

1. An electrode for use in an electrochemical reaction, said electrode comprising: a) a substrate selected from the group consisting of carbon fiber and titanium; and b) a layer of a noble metal coated on the substrate.

2. An electrode according to Claim 1, wherein said noble metal layer is plated on said substrate.

3. An electrode according to claim 1, wherein said noble metal is selected from the group consisting of platinum, iridium, rhodium, rhenium, osmium, silver, gold, and mixtures thereof.

4. An electrode according to claim 1 , wherein said substrate is PAN carbon fibers, and said noble metal is electroplated on said fibers.

5. A method of making an electrode for use in an electrochemical reaction, comprising: a) dissolving a noble metal salt compound in water to form a noble metal salt solution; b) placing a substrate in the solution; and c) applying an electroplating voltage to the substrate so as to cause noble metal to be electroplated onto the substrate.

6. The method of claim 5, wherein the substrate comprises carbon fiber.

7. The method of claim 5, wherein the noble metal salt compound includes a noble metal selected from the group consisting of platinum, iridium, rhodium, rhenium, osmium, silver, gold, and mixtures thereof.

8. A method according to claim 5, wherein said solution is approximately 0.1% to 3.0% H₂IrCl₆by weight at room temperature.

9. A method according to claim 8, further comprising: d) stirring the solution during the plating process.

10. A method according to claim 9, wherein the electroplating voltage applied to the substrate is swept from the range of approximately

+200 mV to -200 mV to the range of approximately -900mV to -1300 mV at a sweep rate of approximately 2-100 millihertz for approximately 0.5 to 3 hours.

11. A method according to claim 9, wherein the electroplating voltage applied to the substrate is pulsed from the range of approximately -700 mV to -1000 mV to the range of approximately -1500 mV to -2000 mV in cycles ranging from 2-20 minutes for approximately 0.5 to 3 hours.

12. A method according to claim 9, wherein the electroplating voltage applied to the substrate is a constant electroplating voltage in the range of -1500 mV to -2000 mV for approximately 0.5 to 3 hours.

13. An apparatus for reducing nitrates in an aqueous solution comprising: a) an anodic electrode; b) a reference electrode; c) an noble metal coated cathodic electrode; and d) a control circuit which impresses a voltage across said electrodes.

14. An apparatus according to claim 13, wherein the cathodic electrode comprises carbon fiber.

15. An apparatus according to claim 13, wherein the noble metal is selected from the group consisting of platinum, iridium, rhodium, rhenium, osmium, silver, gold, and mixtures thereof.

16. An apparatus according to claim 13, further comprising: e) an electrochemical cell in which the aqueous solution containing nitrates an nitrites is treated.

17. An apparatus according to claim 13, wherein the anodic electrode to cathodic electrode surface area ratio is in the range of 1 : 1 to 5:1.

18. An apparatus according to claim 13, wherein upon immersion of said electrodes in the aqueous solution, said voltage causes electrochemical reduction/oxidation reactions on the surfaces of said cathodic and anodic electrodes such that nitrates are reduced to gaseous products and hydrogen, oxygen and chlorine are not produced.

19. An apparatus according to claim 13, wherein said cathodic electrode is a carbon fiber fabric mounted on a frame.

20. An apparatus according to claim 19, wherein said carbon fiber electrode is mounted in a plate having a hole within which said frame is force fit.

21. An apparatus according to claim 13, further comprising a silver/silver chloride reference electrode which comprises: a) a hollow member having a first end and a second end; b) a silver/silver chloride conductor disposed within said hollow member; c) an electrical wire lead means for electrically coupling to said conductor and for extending out of said first end of said hollow member; d) a porous plug at said second end of said hollow member, said plug having an outer surface; e) a chloride solution disposed within said hollow member between said plug and said conductor, such that said conductor is bathed in said solution; and f) a water soluble protective coating covering said outer surface of said plug.

22. An apparatus according to claim 21, wherein said coating covers a portion of said hollow member adjacent to said plug.

23. An apparatus according to claim 21 , wherein said coating comprises a poly vinyl alcohol.

24. An apparatus according to claim 21, further comprising: g) a cap on said first end of said hollow member for sealing said first end of said hollow member.

25. An apparatus according to claim 21, wherein said wire lead means is electrically coupled to said silver/silver chloride conductor by a solder joint.

26. An apparatus according to claim 21, wherein said solution is a gel.

27. A method of cleaning an electrode used in a system for the removal of nitrogen compounds, the method comprising: applying a short sequence of voltage bursts to generate short bursts of hydrogen and oxygen to thereby clean the electrode.

28. The method according to claim 27, wherein the short voltage burst comprises an anodic pulse at +850 to +1100 mV for between 1 to 200 seconds to remove carbonate; and which is followed by a 10 Hz/┬▒ IV alternating square pulse for between 1 to 20 seconds to remove adsorbed species.