GB2449655A

GB2449655A - An electrochemical reactor for aqueous solutions with high electrical resistance

Info

Publication number: GB2449655A
Application number: GB0710243A
Authority: GB
Inventors: Peter Edwin Platt; Peter James Eddowes; Ian Mccrady Dalrymple; Arnold Black
Original assignee: JLJ ENGINEERING SERCICES Ltd
Current assignee: JLJ ENGINEERING SERCICES Ltd
Priority date: 2007-05-30
Filing date: 2007-05-30
Publication date: 2008-12-03
Also published as: GB0710243D0

Abstract

An electrochemical reactor that enables electrochemical reactions to be carried out in poorly conducting electrolyte media with high electrical resistance is described. The cell consists of a polymer electrolyte membrane 1 in direct contact with at least one electrode 2, in water containing little or no supporting electrolyte. A further electrode 2 can be either in direct contact with the membrane 1 or separated from it. The reactor enables electrochemical reactions to proceed without the need to add supporting electrolytes, such as, sulphuric acid or metal halides, because this is not always possible or desirable. The reactor may be used for the electrochemical de-nitrification of potable water. The electrodes may possess a catalytic layer and be of a porous or reticulate construction.

Description

An Electrochemical Reactor Design for Use in Solutions with High

Electrical Resistance Electrochemical processes require a conducting electrolyte between the electrodes to enable electron transfer reactions to occur. When the desired electrochemjcal reaction involves electron transfer between one of the electrodes and a molecule or ion in solution at a low concentration, an ionic supporting electrolyte must be added. This is normally a salt or acid that does not take part in the electron transfer reactions. Without a supporting electrolyte added, the solution has a high electrical resistance and low conductivity, with the result that electrochemical reactions can only proceed slowly and require a high applied voltage, with a high level of energy consumption.

However, it is not always possible or desirable to add a supporting electrolyte. For example, when treating low concentrations of contaminants in potable water that will subsequently be used for human consumption or that water that will be discharged to fresh water courses, the addition of a supporting electrolyte is clearly not acceptable. In cases such as these, the potential advantages offered by electrochemical treatment technologies caunot be realised using conventional electrochemical reactors.

The central topic of this invention is a novel electrochemical reactor design that will solve this problem limiting the scope of applications possible for electrochemical reactors. The new design will enable electrochemical reactions to be carried out in poorly conducting electrolyte media with high electrical resistance, such as water containing low concentrations of contaminants. This will be achieved by the use of solid polymer electrolyte membranes placed between the electrodes, and in intimate contact with at least one of the electrodes, to provide a high conductivity electrolyte. The approach is derived from the generic structure of polymer electrolyte membrane fuel cells (PEMFC) and the use of membrane electrode assemblies (MEA) used in these systems. Unlike fuel cells however, DC power will be applied to the new reactor to carry out electrolysis reactions.

The reduction of nitrate ions in potable water is a real current industrial need and is also an electrochemical reaction that has been well studied. Therefore this application is the focus of the experimental work that has been carried out.

Nitrate salts are found in ground water, mainly due to the run off from agricultural land to which fertilisers have been applied. When ground water is used as a source of potable water, legislation requires the removal of the nitrates, e.g. typically to less than 30 mg/I nitrate ion. Most treatment processes are based on either ion exchange or reverse osmosis to remove the nitrate from the water. However, a secondary effluent stream is produced containing the extracted nitrate in a more concentrated form. Therefore, much research has been carried out on processes to treat the secondary effluent by electrochemical reduction.

Electrochernjcal reduction of nitrate ions is a well-documented reaction. The overall reaction for reduction of nitrate at the cathode is: 2NQ3+l2H+jOe = N2+6H20 However, the reaction mechanism is a complicated, multi-stage reaction, with nitrite ion (N02) as an intermediate product and ammonia as an additional product resulting from greater reduction.

In addition to the application of de-nitrificatjon, the generic reactor design could equally offer advantages when applied to many other electrochemical processes that can be carried out in low-conductivity electrolytes.

Disinfection of water by killing micro-organisms can be achieved by mechanisms that involve the rupture of the cell wall and disruption of the cell metabolic processes. This can be achieved by adding chemical oxidising agents such as active chlorine species (e.g. hypochlorite) to the water. Although the residual effects from the addition of chemicals is beneficial in maintaining disinfection activity throughout a water supply distribution system, the addition of chemicals can be undesirable due to the residual taste and other environmental and health concerns.

The new electrochemjcal reactor described will enable electrochemjcal disinfection to be carried out in water directly without the addition of chemicals. When micro-organisms are brought into the close vicinity of an anode in intimate contact with a solid polymer electrolyte membrane, inactivation and further degradation can be achieved. The effect can be implemented through the oxidising activity of short-lived intermediate species such as hyrdoxyl radicals that are present close to the electrode surface, high levels of acidity produced locally due to the electrolysis of water to produce oxygen and the effects

of the electric field.

Electrochemical degradation of organic compounds is a developing industrial process for the treatment of industrial waste streams. It can be applied to processes containing toxic organic molecules, but must have a background electrolyte present to confer conductivity. In cases where sodium chloride is present, or can be added, the chloride ion acts as a catalyst that promotes the organic compound degradation via the production of active chlorine intermediates produced at the anode surface. When toxic organic compounds such as pesticides are found in low concentrations in ground water, the envirornriental impact can be very severe. However it is generally not possible to treat these waters as there is no background electrolyte present and normally it is not possible to add chemical species such as salts.

The new electrochemical reactor described will allow the strongly oxidising properties of the anode to be applied to the degradation of organic molecules in water avoiding the need to add a supporting electrolyte. Degradation will be result from oxidation by short-lived intermediates close to the anode surface and also from the direct electron transfer from the organic compound to the anode surface.

The use of membrane separators in electrochemical processing is well known. The membranes act as separators or barriers to prevent the transport of electrolyte components from one electrode to the other. Because electrochemical reactions are often kinetically reversible, transport of the reaction product from one electrode to the other can result in the reverse reaction occurring. The membranes prevent the back-reaction by acting as a barrier between two separate electrolytes (anolyte in contact with the anode and catholyte in contact with the cathode). Either porous or ion-conducting membranes can be used.

The ion-conducting membranes separate conducting electrolyte solutions and complete the electrical circuit by transporting charged ions between electrolytes. In the case of nitrate reduction, membranes can be present to prevent re-oxidation of reduction products, such as intermediate nitrite ions, back to nitrate ions. However, both electrolyte solutions have a supporting electrolyte present and therefore have high electrical conductivity.

De-nitrification is the topic of UK Patent Application No. GB 2 332 210 A. The application is based on the sequence of the treatment regime, i.e. oxidation of ammonia and hydrazine followed by reduction of nitrates and nitrites. Both of these stages are well known as independent electrochemical processes. The electrochemical cell design used is very different from our proposed design. The purpose of the diaphragm/membrane in the application referenced is to keep these two solutions apart, not to reduce the cell voltage, and there is clearly no intimate contact between the electrodes and the membrane.

Patent DE 3838181 relates to a specific electrochemical design for nitrate waste treatment. The key aspect of the invention appears to be based on the use of a particulate bed cathode with a high surface area. Many other electrochemical reactor designs contain high surface area electrodes. The separator in this cell is a porous material and is not a solid polymer electrolyte membrane. The main function appears to be the physical retention of the porous particulate cathode and is clearly very different from the membrane type and function in proposed research. In any case, the separator is not in intimate contact with the anode. The cylindrical versus planar geometry is a relatively minor consideration. A metallic copper coating on the particles was found to be an effective catalyst for the removal of nitrate. The use of copper as a suitable cathode material for nitrate reduction is well known in public domain literature, for example D. Pletcher and S. Poorabedi, Electrochjmjca Acta, Vol. 24, pp 1253-1256.

A number of patents deal with the use of reactors containing solid polymer electrolytes and MEAs for the electrolysis of water. Patent JP 8325771A describes the use of electrochemical cells containing solid polymer electrolyte MEAs to produce high purity oxygen gas through water electrolysis. Impurity gases are removed from the water by a separate degasification membrane process (not electrochemical) prior to electrolysis to produce pure oxygen and hydrogen. US 2003/0057088 Al describes a typical PIEMFC type construction used for water electrolysis to produce oxygen and hydrogen gas. The novelty appears to be the mixture of the precious metals comprising the anode catalyst, not the cell design. JP 3079783 is also describes a typical fuel cell construction used for water electrolysis to produce oxygen and hydrogen, or to produce ozone. The novelty seems to be the maimer in which current is supplied to the cell.

In conclusion, there appear to be no disclosures of electrochemical reactors with membranes as solid polymer electrolytes for application to effluent treatment processes such as de-nitrification and to disinfection processes in low conductivity electrolytes, including natural, potable and industrial water. Other disclosures are related to the production of hydrogen and oxygen gases via water electrolysis, where the water contains high concentrations of supporting electrolytes. These disclosures are not applicable to the conversion of soluble species dissolved in low conductivity water.

According to the present invention, a novel electrochemical reactor design has been developed that will enable electrochemical reactions to be carried out in poorly conducting electrolyte media with high electrical resistance. In conventional electrochernical processes, a supporting electrolyte is usually present or specifically added to provide electrical conductivity, but not to take part in the electron transfer reactions occurring at the electrodes. This is particularly important when the electrochemically active components are present at low concentrations. The design that is proposed in this invention allows reactions to proceed without the need to add supporting electrolytes, because this is not always possible or desirable, or alternatively requires the application of very high voltages to achieve even low electrical currents, and therefore reaction rates, at high energy costs.

The reactor design has been proven using a specific application of real industrial interest; the electrochemical de-nitrification of potable water. Using the new reactor, the de-nitrification treatment has been demonstrated for both raw water sources and secondary effluent streams from other treatment processes such as ion exchange and reverse osmosis.

The generic structure is derived from the basic design of a PEMFC. These contain solid polymer electrolyte membranes with electrodes on either side, including catalysts applied directly to one or both sides of the membrane (MEAs). Of course, fuel cells are used for the generation of energy from fuels such as hydrogen and methanol. Another established use of solid polymer electrolytes is the application of DC power for the electrolysis of water to produce hydrogen and oxygen gases, for example to store energy from renewable sources as gaseous hydrogen. However, the applicants are not aware of any use of these types of electrode structures in de-nitrification and other effluent treatment and disinfecting applications.

The generic cell design that is central to this invention consists of a polymer electrolyte membrane (PEM) in intimate contact with at least one electrode, i.e. the working electrode carrying out the electrochemical reaction in water containing no supporting electrolyte. The counter electrode can be either in direct contact with the membrane or separated from it.

The working electrodes can be either coated directly onto the surfaces of the membrane, or can be held in intimate physical contact with the membrane. The design uses the membrane, which has a very low electrical resistance, as a solid polymer electrolyte. The water to be treated, with its high electrical resistance, is not positioned between the electrodes and therefore does not impede the electrolysis process and does not require the application of very high cell voltages, leading to high power consumption. In this way, very dilute effluents or contaminated water can be passed over the working electrodes for electrochemical treatment. In conventional electrochemjcal reactors, with the resistive water positioned between the electrodes or between the electrode and membrane, electrolysis would be either impossible or would incur very high power costs.

Both oxidation and reduction processes are possible using the generic design. Nitrate treatment is of course a reduction reaction. However, oxidation reactions have also been identified as possible applications, for example the degradation of low concentrations of toxic organic compounds in industrial or potable water, and disinfecting water by killing bacteria and other micro-orgamsms.

In Figures 1 to 4, four variations of the unit cell arrangement are given. In each case, this unit is repeated throughout the electrochemical reactor stack. The relative positions of the anode and cathode feeder electrodes, i.e. those connected to the external power supply, are given. The number of repeating cell units that can be used in each arrangement can be varied from one up to a number normally limited by safety regulations governing the total cell voltage that can be applied across the whole cell stack.

Two reactor designs that are suitable for electrochemical reduction reactions are given in Figures 1 and 2, and for electrochemical oxidation reactions in Figures 3 and 4.

Description of Drawings

Figure 1: Reactor Design 1 The cell arrangement is suitable for electrochemical reduction reactions in water. The water to be treated is pumped through the catholyte compartment in which the cathode is held in direct contact with the solid polymer electrolyte membrane. The cathode must be an open structure such as a mesh or a porous material such as felt or foam, through which the water with low conductivity can be pumped.

I Solid polymer electrolyte 2 Bipolar plate 3 Catholyte compartment through which water for treatment is pumped.

4 Anode and anolyte compartment through which a liquid solution containing a supporting electrolyte is pumped.

Figure 2: Reactor Design 2 The cell arrangement is suitable for electrochemical reduction reactions in water. The cathode is a single-sided membrane electrolyte assembly, i.e. the cathode is a catalyst layer bonded directly to the solid polymer electrolyte membrane. A bipolar plate flow-field structure is present to distribute the water flow to the MEA cathode.

1 Solid polymer Electrolyte 2 Bipolar plate 4 Anolyte compartment through which a liquid solution containing a supporting electrolyte is pumped.

Flow field distributor for water to be treated at the cathode 6 MEA cathode catalyst layer on the solid polymer electrolyte membrane to carry Out reduction reactions of soluble components in water to be treated Figure 3: Reactor Design 3 The cell arrangement is suitable for electrochemical oxidation reactions in water. The water to be treated is pumped through the anolyte compartment in which the anode is held in direct contact with the solid polymer electrolyte membrane. The anode must be an open structure such as a mesh or a porous material such as a felt or a foam, through which the water with low conductivity can be pumped.

I Solid polymer Electrolyte 2 Bipolar plate 3 Cathode and catholyte compartment through which a liquid solution containing a supporting electrolyte is pumped.

4 Anolyte compartment through which water for treatment is pumped Figure 4: Reactor Design 4 The cell arrangement is suitable for electrochemical oxidation reactions in water. The anode is a single-sided membrane electrolyte assembly, i.e. the anode is a catalyst layer bonded directly to the solid polymer electrolyte membrane. A bipolar plate flow-field structure is present to distribute the water flow to the MBA cathode.

I Solid polymer Electrolyte 2 Bipolar plate electrode 3 Cathode and catholyte containing supporting electrolyte

Flow field distributor for water to be treated

6 MBA anode catalyst layer Figure 5 Ceneral System Design The diagram illustrates a typical overall treatment system, using the reduction of nitrate in potable water as an example.

7 Electrochernical Reactor 8 Contaminated Water Tank 9 Supporting Electrolyte Tank Pumps 11 Flow Meters 12 DC Power Supply

Particular Examples

Experimental studies were carried out on electrochemical de-nitrification, with the aim of treating both raw water and secondary concentrate from reverse osmosis. The typical concentrations of nitrate ions were 100 mg/I in raw water and 1000 mg/I in secondary effluent. Simulated solutions of 100 mg/I nitrate as sodium nitrate in deionised water were used to optimise cell design and operating parameters, such as flow rate and current density. Further tests were then conducted on both types of water from industrial sources.

Example 1

Water (1 litre) containing 100 milligrams per litre nitrate ion as sodium nitrate was pumped from an external reservoir through the catholyte compartment of a laboratory electrochemica] reactor. The cathode working electrode was a rectangular sheet of carbon felt of dimensions 7 cm x 7 cm x 0.8 cm. The carbon felt was mounted vertically in the cell, sandwiched tightly between a solid polymer electrolyte membrane and a bipolar graphite plate, to which the negative terminal of a DC power supply was connected. The water flow was vertically upwards through the carbon felt cathode at a rate of 200 litres per hour, at right angles to the direction of current flow between the electrodes. The anolyte (0.02 N sulphuric acid) was circulated through the anolyte chamber, and was in contact with the other side of the solid polymer electrolyte membrane and the anode secondary electrode.

A current of 0.5 Amps was passed, equivalent to a current density on the carbon felt of Amps per geometrical square metre. Over a period of 22.5 hours, the nitrate concentration was reduced from 98 to 6 milligrams per litre. The final product of the reaction was ammonium ion, with nitrite ion produced as an intermediate, before being further reduced. The cell voltage during electrolysis was approximately 5 Volts, and the temperature varied between 25 C and 30 C.

Table 1 Mass balance of nitrogen species for 2001/hr experiment % Nitrogen START After 5 hours electrolysis Nitrate ioo 3 Nitrite 0 1 Ammonium 0 39 Nitrogen gas 0 57

Example 2

The experimental conditions employed were identical to that described in Example 1. In this case, a direct comparison was made between two carbon felt materials of different thickness, but otherwise identical; 8 millimetres (as used in Example I) and 6 millimetres. A current of 0.5 Amps was passed, equivalent to a current density on the carbon felt of 100 Amps per geometrical square metre. On using the 6 millimetre thick carbon felt over a period of 5 hours, the nitrate concentration was reduced from 99 to 26 milligrams per litre. In the case of the 8 millimetre thick carbon felt, the nitrate concentration was reduced from 99 to 6 milligrams per litre over the same time period.

The final product of the reaction was ammonium ion, with nitrite ion produced as an intermediate, before being further reduced.

Example 3

The experimental conditions employed were identical to that described in Example 1. In this case, the carbon felt cathode was compared with carbon felt coated with copper. Two cathodes with different quantities of copper (1 mg/cm2 and 5 mg/cm2) were prepared by electro-deposition onto the carbon felt from an electroplating solution in a separate electrolysis cell prior to use.

The experiment was then repeated replacing the untreated carbon felt with 5mg/cm2 copper loaded carbon felt. A current of 0.5 Amps was passed, equivalent to a current density on the carbon felt of 100 Amps per geometrical square metre. Over a period of 6 hours, the nitrate concentration was reduced from 99.8 to 2.7 milligrams per litre. The final product of the reaction was animonium ion, with nitrite ion produced as an intermediate, before being further reduced. The cell voltage during electrolysis was approximately 4.5 Volts, and the temperature varied between 25 C and 30 C.

The experiment was then repeated replacing the untreated carbon felt with 1mg/cm2 copper loaded carbon felt. A current of 0.5 Amps was passed, equivalent to a current density on the carbon felt of 100 Amps per geometrical square metre. Over a period of 5 hours, the nitrate concentration was reduced from 98 to less than 1 milligrams per litre.

The final product of the reaction was ammonium ion, with nitrite ion produced as an intermediate, before being further reduced. The cell voltage during electrolysis was approximately 5.5 Volts, and the temperature varied between 25 C and 30 C.

Table 2 Composition of catholyte at end of experiments with different copper loading on carbon felt cathode. Figures given are % Nitrogen at end of electrolysis.

Cu Loading Nitrate Nitrite Ammonium Nitrogen gas I NoCu 6.3 1.3 18.5 73.9 Lcu 1m9/cm' 09 3.1 33.7 62.2 [Cu 5mg/cm' 2.7 9.9 13.6 73.8

Example 4

The results in this example were obtained by using a larger version of the electrochemical reactor and associated apparatus. Three experiments were carried out to demonstrate the increasing rate of nitrate reduction achieved in the catholyte water, by increasing the acid concentration in the anolyte chamber.

Water (10 litre) containing 1000 milligrams per litre nitrate ion as sodium nitrate was pumped from an external reservoir through the catholyte compartment of a laboratory electrochemical reactor, The cathode working electrode was a rectangular sheet of carbon felt of dimensions 30 cm x 20 cm x 0.8 cm. The carbon felt was mounted vertically in a 4mm thick PVC compartment, sandwiched tightly between a solid polymer electrolyte membrane and a bipolar graphite plate (with 1mm EPDM gaskets), to which the negative terminal of a DC power supply was connected. The water flow was vertically upwards through the carbon felt cathode at a rate of 350 litres per hour, at right angles to the direction of current flow between the electrodes. Slitres of sulphuric acid solution were circulated through the anolyte chamber. The solution was in contact with the other side of the solid polymer electrolyte membrane and the anode secondary electrode.

Using an anolyte solution containing 0.02N sulphuric acid, a current of 12 Amps was passed, equivalent to a current density on the carbon felt cathode of 200 Amps per geometrical square metre. Over a period of 7 hours, the nitrate concentration was reduced from 965 to 450 milligrams per litre, before reaching less than 1 milligrams per litre after hours. The final product of the reaction was ammonium ion, with nitrite ion produced as an intermediate, before being further reduced. The cell voltage during electrolysis was approximately 5 Volts, and the temperature varied between 25 C and 30 C.

The experiment was then repeated using 5litres of 0.04N sulphuric acid as anolyte. A current of 12 Amps was passed, equivalent to a current density on the carbon felt of 200 Amps per geometrical square metre. Over a period of 7 hours, the nitrate concentration was reduced from 942 to 7 milligrams per litre. The final product of the reaction was amrnonium ion, with nitrite ion produced as an intermediate, before being further reduced. The cell voltage during electrolysis was approximately 4.5 Volts, and the temperature varied between 25 C and 30 C.

The experiment was then repeated using 5litres of 0.08N sulphuric acid as anolyte. A current of 12 AmpS was passed, equivalent to a current density on the carbon felt of 200 Amps per geometrical square metre. Over a period of 6 hours, the nitrate concentration was reduced from 911 to less than 1 milligrams per litre. The final product of the reaction Jo was anmionium ion, with nitrite ion produced as an intermediate, before being further reduced. The cell voltage during electrolysis was approximately 4.0 Volts, and the temperature varied between 25 C and 3 0 C.

Claims

Claims 1. An electrochemical reactor incorporating individual unit

cells in which a solid polymer electrolyte membrane is in direct contact with either an anode or a cathode to carry out oxidation reactions or reduction reactions respectively in liquid media containing low levels of supporting electrolyte and therefore high electrical resistance.
2. An electrochemjcal reactor as described in claim 1, in which either the anodes carrying out the oxidation reaction or the cathodes carrying out the reduction reaction consist of a catalyst layer or layers bonded to the solid polymer electrolyte membrane.
3. An electrochemical reactor as described in c]aim 1, in which either the anodes carrying out the oxidation reaction or the cathodes carrying out the reduction reaction consist of an open structure such as a mesh or a porous material.
4. An electrochemical reactor as described in claims 1 to 3 in which a solution containing a supporting electrolyte consisting of an acid, a base or a salt, is in contact with the opposite side of the polymer electrolyte membrane from the liquid media containing no supporting electrolyte.
5. An electrochemical reactor stack incorporating multiple unit cells as described in claims 1 to 4 positioned between current feeder electrodes that are connected to an external power supply.
6. Application of the electrochemical reactor described in claims 1 to 5 to electrolytic oxidation or reduction reactions in natural, potable or industrial water sources containing low levels of supporting electrolyte and high electrical resistance.
7. Application as described in claim 6 whereby removal of contaminants in water is achieved by electrochemical reduction reactions.
8. Application as described in claim 6 whereby removal of contaminants in water is achieved by electrochemical oxidation reactions.
9. Application as described in claim 6 whereby disinfection of water is achieved by killing microorganisms by electrochemical oxidation and reduction reactions.