Electrochemical oxidation of contaminated solutions

Abstract

A method and reactor for electrochemical oxidation of per - and polyfluoroalkyl substances, PFAS, in a solution. An electrochemical reactor comprising at least one boron-doped diamond electrode is provided. The PFAS-containing solution is passed over the boron-doped diamond electrode. The electrochemical reactor is operated under first conditions of a current-limited mode with an electrode current density of at least 2000 A m-2. It is then subsequently operated under second conditions of a voltage of at least 20 V and an electrode current density of no more than 200 A m-2. The first operating conditions are optimised to remove organics from the solution, the second operating conditions are optimised to remove PFAS from the solution.

Description

[0001] ELECTROCHEMICAL OXIDATION OF CONTAMINATED SOLUTIONS

[0002] FIELD OF THE INVENTION

[0003] The present invention relates to the electrochemical oxidation of per- and polyfluoroalkyl substances (PFAS) in solution, and to electrochemical reactors for the electrochemical oxidation of PFAS in solution.

[0004] BACKGROUND

[0005] Per- and polyfluoroalkyl substances (PFAS) are synthetic organofluorine chemical compounds that have multiple fluorine atoms attached to an alkyl chain, terminated at one end with a hydrophilic acidic group, typically either sulphonic or carboxylic in nature. Figure 1 shows an exemplary PFAS chemical structure. Extremely stable and effective fluoro-surfactants, their use is extremely widespread as low surface energy coatings in consumer products such as waterproofing and self-cleaning coatings, low friction lubricants, polishes and release agents, coatings for food packaging, and industrial uses include aqueous film forming foam (AFFF) used in flammable liquid fires. However, PFAS are of increasing environmental concern as their chemical and thermal stability renders traditional waste infrastructure ineffectual at preventing them from reaching the environment. Once in the food chain they are found to bioaccumulate, and, via biomagnification, reach levels where adverse health outcomes can result. In humans PFAS is now linked to diseases including cancers, liver damage, elevated cholesterol levels, infertility, asthma, thyroids conditions and lower immunity. As a result of their ability to bioaccumulated, PFAS contamination limits are being set at much lower concentrations, typically low parts per trillion, than for other types of dissolved organic contaminates.

[0006] In the environment, contamination is often linked to manufacturing sites, waste management infrastructure, including landfills, municipal sewage and incinerator plants, major fire incident sites and fire-fighting training centres such as at airports and military bases. Technology to remediate contaminated water sources, using separation technology such as reverse osmosis, ion exchange, absorption via activated carbon and nanofiltration are rapidly being developed. However, while these separation processes can mitigate further downstream contamination e.g. for drinking water, they can create relatively large volumes of PFAS contaminated reject streams and spent filtration media that may create a significant future legacy contamination if they are disposed of via conventional waste disposal routes. Often, contaminated sites, e.g. spent fire-fighting foam, ground waters around fire training sites and fire incidents, leachates seeping out of landfill sites are contaminated with a combination of dissolved organics, in the range of 100 to 10,000 mgL'1and PFAS in the range from 100 ngL'1to 10 mgL'1.

[0007] PFAS compounds have varying carbon chain lengths, and while specific long chain (C atoms >8) compounds such as perfluorooctanoic acid (PFOA) and perfluorooctanesulfonate (PFOS) were already phased out in 2006 and 2001 respectfully, they remain as significant contaminates and initially were replaced with shorter chain (C<6) PFAS compounds with similar surfactant properties. PFAS destruction technology must be able to mineralise all types of PFAS, including short chain (C<6) variants into fluoride ions and carbon dioxide otherwise there is a significant risk of re-distribution in the environment of them at even lower concentrations. In 2023, the US Department of Defence (DoD) imposed a moratorium on thermal incineration of legacy AFFF fire suppressant that contained PFOS, over concerns that the more volatile short chain PFAS could be transported via the exhaust gas emissions into the atmosphere. Typical PFAS contaminated waste streams also contain significant quantities of dissolved organics. For example, humic substances from soil remediation processes, organic acids from landfill leachates and animal proteins are part of many AFFF formulations. Even after PFAS concentration steps, effluents typically have total organic carbon (TOC) of ~ 300 to 5000 mgL'1.

[0008] Electrochemical advanced oxidation at the surface of boron doped diamond (BDD) is recognised as having the ability to break the carbon fluorine bonds and break down PFAS into the constituent elements emitting e.g.CO2 gas, fluorine ions and sulphates. While electrochemical processing is more practical than thermal processes such as supercritical water oxidation, where corrosion, energy consumption are seen as drawbacks, however there are a number of concerns over its practicality as a PFAS oxidation technology. Examples of electrochemical reactors that use BDD include WO 2008029258 and WO 2012049512.

[0009] Schaefer et al. “Electrochemical treatment of perfluorooctanoic acid and perfluorooctanesulfonate: Insights into mechanisms and application to groundwater treatment”, Chemical Engineering Journal 317 (2017) 424-432 describes that several studies have shown that PFOA oxidation via electrochemical treatment proceeds via a stepwise mechanism in which C-C bond cleavage occurs between the carbon chain and the carboxylate group, coupled with fluoride ion elimination. This process is repeated, resulting in the intermediate generation of shorter chain perfluorocarboxylic acids. Consistent with this mechanism, generation of low concentrations of shorter chain perfluorinated carboxylates was observed (perfluorobutanoic acid, perfluoropentanoic acid, perfluorohexanoic acid, and perfluoroheptanoic acid), but at net concentrations of 25% of the total fluorine balance at all times during the experiments. The observation of short chain perfluorinated carboxylates as intermediate products were generally greater in the experiments conducted at low current density, and the generation of these short chain carboxylic acids was up to 50% greater in the presence of chloride.”

[0010] Electrochemical breakdown at the surface of BDD electrodes is one of the most effective and industrially mature techniques for dealing with concentrated dissolved PFAS contaminated effluents. As it is a low temperature process, one major advantage over thermal incineration is the ability to control and manage the gas emissions from the process, while at the same time being able to mineralise the carbon content. However a well-established drawback of using a surface chemistry driven electrochemical process is that it is limited by mass transport of the species of interest to within a short distance of the electrode surface. The technique is none-selective, and when breaking down long chain PFAS compounds, with carbon chain lengths >8, that these can generate short chain PFAS species, with carbon chain lengths of <6.

[0011] Other competing oxidation reactions occur, including those for other none PFAS species dissolved organics in the effluent. Species that are present in higher concentrations can diffuse to the electrode at a faster rate than species at a lower concentration, and so much of the electrode area and electrical energy is wasted on electrolysis of water or other dissolved organics rather than PFAS oxidation.

[0012] SUMMARY

[0013] In the scenario described above, the destruction process could be said to be currentlimited as the rate of mineralisation is dependent on the number of radicals generated at the electrode surface. The current-limited region is defined as when the rate of species of interest removal is linear, as shown in the example of Figure 2, for perfluorobutanoic acid (PFBA) at relatively high concentrations of 60 & 20 mgL'1operating at 5000 Am-2. The rate of destruction is limited by the arrival of oxidation sites at the surface of the electrode.

[0014] A higher current density leads to a faster rate of removal of the desired species. However these processes are competing with the oxidation of dissolved organics that are present in higher concentrations that may be several orders of magnitude higher. Furthermore, when combined with dissolved organics there are a shortage oxidation sites that gives rise to incomplete mineralisation, as shown in Figure 3.

[0015] Once the TOC has been essentially removed from the PFAS effluent, at < 1 rngL'1, the oxidation process enters a different phase, where this process is now limited by the arrival of PFAS species at the surface of the electrode, or mass transport limited process, as shown in Figure 4 in which the PFAS concentration is < 1 rngL-1at an operating current density of 5000 Am-2.

[0016] It might be expected that smaller molecules with a shorter carbon chain are more mobile in the solution and thus able to reach the surface of the electrodes more easily. However, what is seen more often is that the removal rate of these short chain species slows and becomes exponential. PFAS molecules are terminated with either sulphonic or carbonyl groups. In electrochemical destruction, the end products of breaking down PFAS contaminated waste streams comes down to two short chain species, typically perfluorobutane sulfonate (PFBS) and perfluorobutanoic acid (PFBA). These two molecules typically make up more than 99% of the remaining effluent contaminant, and electrochemical processes have been reported to be unable to achieve full PFAS mineralisation as measured by the fluoride ion mass balance.

[0017] At low concentrations, PFBA and PFBS have different characteristics despite having similar carbon chain lengths. PFBS is more difficult to break down than PFBA and is often the species that remains after an electrochemical treatment process.

[0018] PFAS molecules are slightly polar in nature (see, for example, Guan et al, “Nearcomplete destruction of PFAS in aqueous film-forming foam by integrated photoelectrochemical processes”, Nature Water volume 2, pages443-452 (2024)), with an acidic group giving them their characteristic hydrophilic properties and, as such, the slight ionic charge can be exploited to drive the negatively charged ion towards the anode of the electrochemical cell. Extreme potentials can only be applied when the cell is limited in current otherwise gas generation self-limits the active sites on the electrode surface available for direct oxidation of PFAS.

[0019] An object of the invention is to provide a more efficient methodology and system for electrochemical oxidation of per - and polyfluoroalkyl substances. According to a first aspect, there is provided a method of electrochemical oxidation of per - and polyfluoroalkyl substances, PFAS, in a solution. An electrochemical reactor comprising at least one boron-doped diamond electrode is provided. The PFAS- containing solution is passed over the boron-doped diamond electrode. The electrochemical reactor is operated under first conditions of a current-limited mode with an electrode current density of at least 2000 A r2. It is then subsequently operated under second conditions of a voltage of at least 20 V and an electrode current density of no more than 200 A nr2. The first operating conditions are optimised to remove organics from the solution, the second operating conditions are optimised to remove PFAS from the solution.

[0020] As an option, the method further comprises monitoring a total organic carbon, TOC, content of the solution. The electrochemical cell is operated under the first conditions when the TOC is greater than a predetermined concentration, and then under the second conditions when the TOC is less than or equal to the predetermined concentration.

[0021] Optionally, the predetermined concentration is 1 mg I’1.

[0022] In most optional embodiments, the electrochemical reactor comprises a plurality of boron-doped diamond electrodes.

[0023] The boron-doped diamond electrode is optionally a monolithic, unbacked boron-doped diamond electrode.

[0024] As an option, the method further comprises operating the electrochemical reactor under first conditions at an electrode current density selected from any of at least 2 000 Am-2, at least 5 000 Am-2; and at least 10 000 Am-2.

[0025] The conductivity of the solution is optionally selected from any of at least 5 000 pScrrr1; at least 10 000 pScrrr1; and at least 20 000 pScm-1.

[0026] As an option, the solution further comprises a supporting electrolyte. An example of a supporting electrolyte is sodium hydroxide.

[0027] The method optionally further comprises adding sulphate ions to the solution to create persulfates. The method optionally further comprises, prior to electrochemical oxidation, performing a separation operation on the PFAS-containing solution. In this case, the separation operation optionally comprises any of forming a foam from the solution and separating the foam from the solution; and using a condensing column to collect volatile short-chain PFAS that are entrained in the exhaust gases.

[0028] The boron-doped diamond electrode is optionally selected from any of a CVD boron doped diamond electrode, a high-pressure high temperature (HPHT) boron doped diamond electrode, and a boron doped diamond electrode formed from hot compacted boron doped diamond grit.

[0029] According to a second aspect, there is provided an electrochemical reactor configured for electrochemical oxidation of per - and polyfluoroalkyl substances, PFAS, in a solution. The electrochemical reactor comprises at least one boron-doped diamond electrode and a channel for passing the PFAS-containing solution over the at least one boron-doped diamond electrode. The electrochemical reactor is configured to operate under first conditions in a current-limited mode with an electrode current density of at least 2000 A rrr2, and is further configured to operate under second conditions of a voltage of at least 20 V and an electrode current density of no more than 200 A nr2.

[0030] As an option, the electrochemical reactor further comprises a sensor configured to monitor a total organic carbon, TOC, content of the solution. A controller is configured to operate the electrochemical cell under the first conditions when the monitored TOC is greater than a predetermined concentration, and operate the electrochemical cell under the second conditions when the TOC is less than or equal the predetermined concentration.

[0031] As an option, the electrochemical reactor further comprises a plurality of boron-doped diamond electrodes.

[0032] The boron-doped diamond electrode is optionally a monolithic, unbacked boron-doped diamond electrode.

[0033] The boron-doped diamond electrode is optionally selected from any of a CVD boron doped diamond electrode, an HPHT boron doped diamond electrode, and a boron doped diamond electrode formed from hot compacted boron doped diamond grit. According to a third aspect, there is provided a controller for an electrochemical reactor configured for electrochemical oxidation of per - and polyfluoroalkyl substances, PFAS, in a solution, the electrochemical reactor comprising at least one boron-doped diamond electrode and a channel for passing the PFAS-containing solution over the at least one boron-doped diamond electrode. The controller is configured to control the electrochemical reactor to operate under first conditions in a current-limited mode with an electrode current density of at least 2000 A rrr2, and is configured to further control the electrochemical reactor to operate under second conditions of an electric field of at least 20 V per cell and an electrode current density of no more than 200 A rrr2.

[0034] BRIEF DESCRIPTION OF THE DRAWINGS

[0035] The invention will now be more particularly described, by way of example only, with reference to the accompanying drawings, in which:

[0036] Figure 1 shows an exemplary PFAS chemical structure;

[0037] Figure 2 is a graph of concentration with respect to time for perfluorobutanoic acid removal at a fixed current;

[0038] Figure 3 is a graph of PFOA mineralisation as a function of total energy;

[0039] Figure 4 is a graph of PFBA and PFBS concentration with respect to time, showing a mass transport-limited process;

[0040] Figure 5 illustrates schematically in a block diagram an exemplary electrochemical reactor;

[0041] Figure 6 is a flow diagram illustrating steps for electrochemical oxidation of PFAS using the electrochemical reactor of Figure 5; and

[0042] Figure 7 illustrates schematically in a block diagram an exemplary controller for controlling the operation of an electrochemical reactor.

[0043] The figures are not drawn to scale. Throughout the description, similar parts have been assigned the same reference numerals, and a detailed description is omitted for brevity. DETAILED DESCRIPTION

[0044] An exemplary reactor that uses boron dope diamond (BDD) electrodes is described in WO 2008029258. The reactor described therein has an acrylic container that holds an electrolyte and three solid diamond bipolar electrodes spaced apart and disposed parallel to each another. The bipolar electrodes are located between and anode and a cathode. In an example given, the electrolyte is wastewater, to which salts may be added to ensure electrical conductivity. In use, a potential difference is applied between the anode and the cathode.

[0045] The type of reactor described in WO 2008029258 is quite simple, and subsequent developments include electrochemical reactors that can be operated at higher temperatures and pressures. However, the basic principles remain the same. Figure 5 illustrates schematically in a block diagram an exemplary electrochemical reactor 1. The electrochemical reactor 1 has an anode 2 and a cathode 3, which may be formed from BDD. In this example, three bipolar electrodes 4, 5, 6 are disposed between the anode 2 and the cathode 3.

[0046] Channels 7 pass between the electrodes and in use a PFAS-containing solution is passed through the channels in proximity to the electrodes. One or more seals 8 ensure the channels can operate at elevated pressure. A heater source may be 9 provided that allows the temperature of the PFAS-containing solution to be elevated. This may be provided as a separate heat source, but note that the heat source may be the combination of the PFAS-containing solution and the applied electrical power. It will be appreciated that the PFAS-containing solution has some resistivity, and so when applying electrical power to effect electrochemical oxidation, some of the applied power is dissipated as heat.

[0047] In an optional embodiment described below, a sensor 10 is provided for measuring the total organic content of the PFAS-containing solution.

[0048] As described above, a higher current density leads to a faster rate of removal of the desired species, but these processes are competing with the oxidation of dissolved organics that are present in higher concentrations that may be several orders of magnitude higher. The inventor has devised realised that destruction of PFAS is more efficient if the TOC is removed first, in order to prevent the destruction of organic contaminants from taking place preferentially to the destruction of PFAS.

[0049] Figure 6 is a flow diagram illustrating steps for electrochemical oxidation of PFAS using the electrochemical reactor of Figure 5. The following numbering corresponds to that of Figure 6:

[0050] S1. An electrochemical reactor that has at least one boron-doped diamond electrode is provided. In practice, most reactors will have a plurality of BDD electrodes as shown in Figure 5. Various types of boron-doped diamond electrodes may be used. For example, the BDD electrode may be a monolithic, unbacked boron-doped diamond electrode, and it may be made using a CVD process, an HPHT process or formed from hot compacted BDD grits.

[0051] Increasing the porosity of the BDD electrodes increases the effective surface area of the BDD electrodes and therefore increases the efficiency of PFAS oxidation. This may be done in CVD BDD electrodes by, for example, laser machining holes in the surface of the BDD electrode. It is known that nickel etching can also create porosity in BDD electrodes. This type of process can be used to create a porous electrode structure for a cell. In combination with the use of bipolar electrodes between an anode and a cathode, the PFAS-containing solution can be passed through the electrodes rather than just over the surface.

[0052] Improvements to the efficiency of oxidation of PFAS-containing solvents can be made by engineering or selecting suitable electrodes. BDD grits can be formed in an HPHT process rather than by CVD, see for example GB2582942 and W02020207978. Such electrodes have been found to have a much higher capacitance than CVD electrodes, which is indicative of a higher surface area and higher porosity. The electrodes can be engineering to increase the porosity further by selecting suitable process parameters during manufacture or by post-synthesis acid leaching to create a network of pores in the electrode. This gives permeable access to the electrode and effectively increases the surface area of the electrode, thereby increasing the efficiency of PFAS oxidation. Finally, when dealing with PFAS contamination, even very low concentrations of PFAS can be a problem. The electrochemical reactor can be designed to minimise the risk of any PFAS compounds leaking into the solution from the electrochemical reactor components. For example, some reactors may contain polytetrafluoroethylene (PTFE) parts and Viton fluoropolymer O-rings that can shed PTFE into the solution. These parts can be eliminated where ultra-high purity is required.

[0053] The subsequent process to remove PFAS can be energy intensive, so it is efficient to remove as much PFAS as possible from the solution prior to electrochemical oxidation. This may be, for example, by separating a proportion of the PFAS from the solution by forming a foam and separating the foam, or using a condensing column to collect volatile short-chain PFAS.

[0054] 52. The PFAS-containing solution is passed over the boron-doped diamond electrode.

[0055] Prior to this, the solution may be modified to improve its conductivity, for example to at least 5 000 pScm-1, at least 10 000 pScrrr1, or at least 20 000 pScnr1. The solution may be modified to include a supporting electrolyte, such as sodium hydroxide. Sulphate ions may be added to the solution to create persulfates. These types of modification can improve the efficiency of the electrochemical oxidation of the PFAS.

[0056] 53. The electrochemical reactor is operated under first conditions of a current-limited mode with an electrode current density of at least 2000 A nr2. These conditions are optimised to remove organics from the solution but are not optimised to remove PFAS. The electrode current density may be selected from any of at least 2000 Am'2, at least 5000 Am-2; and at least 10,000 Am'2.

[0057] 54. In the case where the electrochemical reactor 1 includes a sensor 10 to monitor the TOC, the TOC is monitored. While the TOC is above a predetermined level, the electrochemical reactor continues to operate under the first conditions described in step S3. This is to ensure that as much organic content as possible is removed from the solution. Once the monitored TOC reaches the predetermined level or below (for example, 1 mg I’1), then the mode of operation is switched to the second conditions, described below in step S5. The advantage of removing organic content first is that the process becomes more efficient; rather than inefficiently trying to remove PFAS but instead spending energy removing organic content, the PFAS removal step S5 primarily targets the removal only of PFAS rather than other impurities.

[0058] S5. In order to target removal of PFAS, once the organic content of the solution has been substantially removed, the electrochemical reactor is operated under second conditions of a voltage of at least 20 V and an electrode current density of no more than 200 A nr2. In an environment where there is a limited concentration of the target species e.g. PFAS to be oxidized, there will be an increased turnover of electrolytes e.g. sulphate ions and water forming oxygen. Increasing the applied voltage while limiting the current will limit the volume of oxygen generated, while attracting the slightly polar PFAS species to the anode surface.

[0059] Operating the electrochemical reactor 1 at elevated pressure and temperature, in addition to reducing the operating voltage required via increased conductivity of the PFAS-containing solution, also improves the efficiency of the electrochemical oxidation process. Mass transport of the PFAS-containing solution is increased at higher temperatures, meaning that the PFAS is more likely to be exposed to an electrode.

[0060] Turning now to Figure 7, there is illustrated schematically in a block diagram an exemplary controller 11 for controlling the operation of an electrochemical reactor 1 . The controller 11 is provided with an input 12 for receiving a signal from the electrochemical reactor 1 relating to an aspect of process control. For example, the signal could be received from the sensor 10 that measures the TOC. The controller 1 is provided with a processor 13 for processing the received data signal. The controller 1 is further provided with an output 14 for sending a control signal to the electrochemical reactor 1 in response to the processed data signal. A database 15 may also be provided, accessible by the processor 13.

[0061] As described above, in an exemplary mode of operation, the controller 13 is configured to control the electrochemical reactor 1 to operate under first conditions in a currentlimited mode with an electrode current density of at least 2000 A nr2when the TOC is above a predetermined level, and to switch to control the electrochemical reactor 1 to operate under second conditions of an electric field of at least 20 V per cell and an electrode current density of no more than 200 A nr2. While this invention has been particularly shown and described with reference to embodiments, it will be understood by those skilled in the art that various changes in form and detail may be made without departing from the scope of the invention as defined by the appended claims.

Claims

CLAIMS:

1. A method of electrochemical oxidation of per - and polyfluoroalkyl substances, PFAS, in a solution, the method comprising: providing an electrochemical reactor, the electrochemical reactor comprising at least one boron-doped diamond electrode; passing the PFAS-containing solution over the boron-doped diamond electrode; and operating the electrochemical reactor under first conditions of a current-limited mode with an electrode current density of at least 2000 A nr2; and subsequently operating the electrochemical reactor under second conditions of a voltage of at least 20 V and an electrode current density of no more than 200 A r2.

2. The method according to claim 1 , the method further comprising: monitoring a total organic carbon, TOC, content of the solution; and operating the electrochemical cell under the first conditions when the TOC is greater than a predetermined concentration; and operating the electrochemical cell under the second conditions when the TOC is less than or equal the predetermined concentration.

3. The method according to claim 2, wherein the predetermined concentration is 1 mg h1.

4. The method according to any one of claims 1 to 3, wherein the electrochemical reactor comprises a plurality of boron-doped diamond electrodes.

5. The method according to any one of claims 1 to 4, wherein the boron-doped diamond electrode is a monolithic, unbacked boron-doped diamond electrode.

6. The method according to any one of claims 1 to 5, further comprising operating the electrochemical reactor under first conditions at an electrode current density selected from any of at least 2 000 Am-2, at least 5 000 Am-2; and at least 10 000 Am-2.

7. The method according to any one of claims 1 to 6, wherein the conductivity of the solution is selected from any of at least 5 000 pScm-1; at least 10 000 pScm-1; and at least 20 000 pScnr1.

8. The method according to any one of claims 1 to 7, wherein solution further comprises a supporting electrolyte.

9. The method according to claim 8, wherein the supporting electrolyte comprises sodium hydroxide.

10. The method according to any one of claims 1 to 9, further comprising adding sulphate ions to the solution to create persulfates.

11. The method according to any one of claims 1 to 10, further comprising, prior to electrochemical oxidation, performing a separation operation on the PFAS-containing solution.

12. The method according to claim 11, wherein the separation operation comprises any of forming a foam from the solution and separating the foam from the solution; and using a condensing column to collect volatile short-chain PFAS.

13. The method according to any one of claims 1 to 12, wherein the boron-doped diamond electrode is selected from any of a CVD boron doped diamond electrode, an HPHT boron doped diamond electrode, and a boron doped diamond electrode formed from hot compacted boron doped diamond grit.

14. An electrochemical reactor configured for electrochemical oxidation of per - and polyfluoroalkyl substances, PFAS, in a solution, the electrochemical reactor comprising: at least one boron-doped diamond electrode; a channel for passing the PFAS-containing solution over the at least one boron- doped diamond electrode; the electrochemical reactor being configured to operate under first conditions in a current-limited mode with an electrode current density of at least 2000 A r2; and the electrochemical reactor being further configured to operate under second conditions of a voltage of at least 20 V and an electrode current density of no more than 200 A m-2.

15. The electrochemical reactor according to claim 14, further comprising: a sensor configured to monitor a total organic carbon, TOC, content of the solution; anda controller configured to operate the electrochemical cell under the first conditions when the TOC is greater than a predetermined concentration, and operate the electrochemical cell under the second conditions when the TOC is less than or equal the predetermined concentration.

16. The electrochemical reactor according to claim 14 or claim 15, further comprising a plurality of boron-doped diamond electrodes.

17. The electrochemical reactor according to any one of claims 14 to 16, wherein the boron-doped diamond electrode is a monolithic, unbacked boron-doped diamond electrode.

18. The method according to any one of claims 14 to 16, wherein the boron-doped diamond electrode is selected from any of a CVD boron doped diamond electrode, an HPHT boron doped diamond electrode, and a boron doped diamond electrode formed from hot compacted boron doped diamond grit.

19. A controller for an electrochemical reactor configured for electrochemical oxidation of per - and polyfluoroalkyl substances, PFAS, in a solution, the electrochemical reactor comprising at least one boron-doped diamond electrode and a channel for passing the PFAS-containing solution over the at least one boron-doped diamond electrode; the controller being configured to control the electrochemical reactor to operate under first conditions in a current-limited mode with an electrode current density of at least 2000 A m-2; and the controller being configured to further control the electrochemical reactor to operate under second conditions of an electric field of at least 20 V per cell and an electrode current density of no more than 200 A rrr2.

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