The present invention relates to a reactor comprising a solid diamond bipolar electrode for use in a method of treating waste water.
All documents referred to herein are hereby incorporated by reference.
Waste water contains a number of pollutants which may be organic or inorganic in nature e.g. cyanides and phenols. Electrochemical oxidation of waste water is a well known method for reducing the amount of pollutants present.
Electrochemical processes are preferred as compared to the use of powerful chemical oxidants on the basis that they are safer and more environmentally friendly.
It is known that the size of the electrochemical reaction surface in a reactor is key to the rate of electrochemical reaction that occurs. Therefore, the larger the available surface area, the greater the rate of electrochemical oxidation. With this in mind, bipolar electrode arrangements are of particular interest. A bipolar electrode is created by placing a third electrode between a cathode and an anode. Upon application of a potential between the anode and cathode, the bipolar electrode functions both as an anode and a cathode, vastly increasing the available anode and cathode surface area while still requiring only two electrical connections.
Diamond electrodes, in particular, boron-doped diamond electrodes are useful in electrochemical applications owing to a number of properties, which are significantly different to the properties of other electrode materials such as glassy carbon or platinum. These properties include the high hardness, high thermal conductivity and chemical inertness associated with diamond and the wide electrochemical potential window of conductive diamond.
The use of both solid diamond electrodes and diamond coated electrodes in electrochemical systems has been described. For example, EP 0 659 691 and U.S. Pat. No. 5,399,247 describe solid diamond electrodes and coated diamond electrodes used as the anode in a method of treating a solute in a liquid solution. In general, diamond coated electrodes are preferred because they are cheaper to make, with the absolute cost of a solid diamond electrode being significantly higher than that of a diamond coated electrode. There are a number of other advantages to diamond coated electrodes taught by the prior art, including enhanced toughness provided to the electrode by the substrate, for example where this is a metal.
The use of diamond coated bipolar electrodes in an electrochemical cell has been described in U.S. Pat. No. 6,306,270.
In the context of electrochemical processes, there is a continuing need for electrodes with increased operational lifetimes. Diamond coated electrodes suffer from the problem of pin-holes which allow the liquid being treated to penetrate the coating and electrochemically attack the interface between the diamond coating and substrate resulting in delamination. This is a problem that can be reduced by increasing the thickness of the diamond coating. However, to increase the thickness of the diamond coating is generally understood to be undesirable as it significantly increases production time and material costs. The problem of short operational lifetimes of electrodes is one which is exacerbated where the electrodes are driven at high current densities.
Solid diamond electrodes have longer lifetimes, however, a disadvantage of such electrodes is achieving the required conductivity as compared to a diamond coated electrode where the substrate which is coated with the diamond provides the conductivity and hence the conductivity of the diamond layer is less of a concern.
Generally, in order to achieve the required conductivity, heavy doping of diamond is required. It has, however, been found that heavily doped regions in diamond electrodes tend to be eroded more quickly through etching by organic solvents than lightly doped regions.
In order to overcome this problem, WO2006/013430 describes that erosion of a solid diamond electrode can be reduced by coating the working surface(s) of the electrode with a thin layer of lightly doped diamond (i.e. a passivation layer). This has the effect of reducing erosion at the working surface(s) while maintaining the required conductivity in the bulk of the diamond layer. However, it adds an additional coating step to the production process or an additional step during deposition wherein the boron concentration has to be adjusted.
WO 2006/061192 describes a method and a device for treating waste water containing pesticides. In the method described, the waste water to be treated is passed through an electrochemical cell comprising a boron doped diamond electrode.
US 2004/003176 describes the electrolytic disinfection of drinking water using an electrochemical cell comprising an anode positioned between two gas diffusion electrodes. The anode may be a boron doped diamond electrode.
An object of the present invention is to provide a reactor which maximises the available electrochemical reaction surface and to obtain a long operational lifetime without requiring additional production steps.
The present invention provides a reactor comprising an anode, a cathode and at least one bipolar electrode having first and second major working surfaces positioned therebetween wherein the at least one bipolar electrode consists essentially of diamond and the diamond comprises a dopant such that the diamond is conductive and has an electrical resistivity of 1 MΩcm or less and wherein the average concentration of the dopant in a region of at least one of the major working surfaces, to a depth of 50 nm, is at least 8×1019 atoms/cm3.
In this way the electrochemical cell has solid diamond electrode(s) in a bipolar arrangement. This results in an increase in the operational lifetime of the at least one bipolar electrode while avoiding the need for additional production steps.
Advantageously, solid diamond can be used as a bipolar electrode without the need to alter the concentration of dopant present at the major working surfaces which will be in contact with the electrochemical environment to form a passivation layer. Surprisingly long operational lifetimes are observed, even at high current densities
The term “bipolar electrode” as used hereinafter refers to an electrode which, when placed between an anode and a cathode across which a potential is applied, will behave as both an anode and cathode. Thus a bipolar electrode necessarily has two major working surfaces in contact with the electrolyte. Furthermore, a bipolar electrode does not require a separate electrical connection, although one or more may be provided for monitoring purposes, for example.
The present invention also provides an electrode consisting essentially of diamond wherein the diamond comprises a dopant such that the diamond is conductive and has an electrical resistivity of 1 MΩcm or less and wherein the average concentration of dopant in a region of a least one of the major working surfaces, to a depth of 50 nm, is at least 8×1019 atoms/cm3 and wherein the electrode has at least one of the following features:
a) the concentration of dopant atoms in any 1 mm3 volume does not vary from the concentration of dopant atoms in any other 1 mm3 by more than 50%,
b) the uniformity of doping through the thickness of the electrode when measured by SIMS at at least five points approximately uniformly spaced through the thickness is such that the maximum dopant concentration is less than about 150% of the mean value and the minimum concentration is greater than about 50% of the mean value,
c) a thickness in the range 0.2 mm to 5 mm,
d) at least one lateral dimension of at least 10 mm, and,
e) a surface area of at least 10 cm2.
Preferably the electrode of the present invention has at least two, preferably at least three, preferably at least four, preferably all five of features a) to e) above.
The electrode of the present invention may be used as the bipolar electrode in the reactor of this invention. All features of the electrode of the invention described herein may also be present in the bipolar electrode of the reactor of this invention. As used herein, the term electrode must be understood to refer to the characteristics of the bipolar electrode of the reactor of this invention.
The term “diamond” includes but is not limited to diamond which has been made by a chemical vapour deposition (CVD) process, preferably a microwave plasma CVD process, diamond made by a high temperature—high pressure process and natural type fib diamond. The diamond may be polycrystalline or single crystal diamond. Preferably the diamond is polycrystalline diamond, preferably made by CVD
The term “consisting essentially of” as used herein requires that the functional behaviour of the electrode is provided by diamond and the dopants within it, and in particular that there is no other identifiable material such as a substrate, providing useful function to the electrode. This term is not intended to preclude the possibility that other components or features may be added to the electrode, for example one or more electrical connections may be added using metallization, brazing or other bonding means.
An advantage of the invention is that the need for a passivation layer at the surface of the bipolar electrode is avoided. Passivation layers are known in which the working surface of the electrode is only lightly doped compared to the bulk. In contrast, it is a feature of the present invention that the average concentration of the dopant in a region of a major working surface to a depth of 50 nm is at least about 8×1019 atoms/cm3. In this way, the region of the diamond at the at least one major working surface is doped sufficiently highly for the diamond in this region to be conductive.
Preferably the average concentration of dopant in a region of both major working surfaces to a depth of 50 nm is at least 8×1019 atoms/cm3.