WO2020115317A1 - Method for oxidising sulfate - Google Patents

Abstract

The present invention provides a method for oxidising sulfate, with the method comprising the steps of oxidising sulfate (SO4 2-) at a working electrode; and providing an ultrasonic field at the working electrode. Also provided is an apparatus for oxidising sulfate, with the apparatus having a working electrode in a reaction space; an electrolyte comprising persulfate (S2O8 2-) and/or sulfate (SO4 2-) in the reaction space; and an ultrasonicator for providing an ultrasonic field at the working electrode.

Description

METHOD FOR OXIDISING SULFATE

Related Application

The present application claims the benefit of, and priority, to GB 1819928.1 filed on

6 December 2018 (06.12.2018), the contents of which are hereby incorporated by reference in their entirety.

Field of the Invention

The present invention provides a method for generating strong oxidising agents, such as persulfate, and the use of the strong oxidising agents to degrade a pollutant. Also provided is an apparatus for generating a strong oxidising agent, the apparatus comprising a working electrode and an ultrasonicator.

Background

The accumulation of persistent organic pollutants in the natural environment is becoming a matter of increasing concern on account of the impact these pollutants may have on both the environment and human health. As such, there is growing interest in methods for removing persistent organic pollutants from industrial waste streams. Advanced Oxidation Processes (AOPs), whereby powerful oxidising agents are generated in situ and used to oxidise the organic pollutants to less harmful species, have garnered considerable interest as a potential method for removing organic pollutants that cannot be degraded by traditional methods.

Persulfate (S2O8² ) has been explored as an agent for the oxidative degradation of hazardous organic wastes in the textile, detergent and electronics industries (Ike et al.). Typically, in these methods, a solution of a perfulfate salt such as sodium or ammonium persulfate is added to a solution containing the organic pollutant. However, although persulfate is a strong oxidising agent, the rate of degradation of many persistent organic pollutants with persulfate is low (Osgerby).

In order to circumvent these slow kinetics, many researches have proposed methods for the activation of persulfate, such as the use of heat, UV light, iron additives or strong bases (Matzek and Carter). More recently, activation of persulfate by the simultaneous application of ultrasound during the oxidation reaction has been proposed (Matzek and Carter). This strategy has been shown to be effective for the degradation (and in some cases, complete mineralisation) of several persistent organic pollutants, including chlorinated aliphatic hydrocarbons (Li et al.), pefluorochemicals (Hao et al.), pharmaceuticals (Zhou et al.] Wang and Zhou; Safari et al .; Monteagudo et al.] Eslami et al.) and dyes (Ferkous et al.).

Persulfate activation for aniline degradation by sono-electrochemical methods has also been reported (Chen and Huang). However, persulfate is prone to decomposition in the presence of moisture and/or at elevated temperatures. Moreover, Price and Clifton have previously reported that the application of ultrasound accelerates the rate of persulfate decomposition. In addition, as a strong oxidising agent, persulfate is itself hazardous if allowed to escape into the

environment.

As such, approaches that generate persulfate at the point of use are attractive.

Electrochemical oxidation of cold, concentrated aqueous sulfate solutions at high potentials at inert anodes is one such method (Radimer and McCarthy). Comninellis and co-workers first proposed the use of boron-doped diamond electrodes for this purpose (Michaud et ai\ Serrano et al.).

However, the degradation of persistent organic pollutants still presents significant challenges regarding the rate and extent of degradation.

Summary of the Invention

The present inventors have developed a process for the degradation of persistent organic pollutants that combines electrochemical persulfate production with the application of an ultrasonic field.

The inventors have found that, contrary to previous reports, ultrasonic irradiation does not adversely affect the Faradic yield of persulfate generated electrochemically from sulfate. Moreover, the inventors have found that the voltage required to generate persulfate from sulfate at a working electrode is lowered by the application of ultrasound. Additionally, the voltage-time profile for electrochemical generation of persulfate from sulfate is smoother and less prone to fluctuations. Without wishing to be bound by theory, the inventors attribute this effect to the removal of bubbles from the working electrode surface.

The inventors have also found that combined sono-electrochemical treatment is extremely effective at degrading pollutants.

Combined sono-electrochemical treatment can degrade pollutants up to three times faster than a method in which persulfate is generated electrochemically and reacted with a pollutant in the absence of an ultrasonic field. Combined sono-electrochemical treatment can also degrade pollutants up to three times faster than a method in which an excess of a persulfate salt is reacted with a pollutant in the presence of an ultrasonic field.

Thus, the inventors have observed a synergistic effect between the electrochemical generation of persulfate and the application of an ultrasonic field during electrochemical generation which leads to enhanced degradation of pollutants. As such, combined son- electrochemical treatments allows the degradation of organic pollutants without the need to use excess persulfate.

Without wishing to be bound by theory, the inventors attribute this effect to improved mass transport of reactive sulfate and hydroxyl radicals (generated by electrochemical oxidation) into bulk solution, where they can react with an organic pollutant. In the absence of an ultrasonic field, these radicals instead combine with one another to form more persistent species (such as persulfate and peroxide) which have no significant effect on pollutant degradation. Moreover, application of ultrasound itself also leads to the generation of sulfate and hydroxyl radicals in bulk solution (for example, by the homolysis of persulfate). Thus, the overall effect of the combined sono-electrochemical treatment is greater than can be achieved by either the individual sonochemical or electrochemical methods alone.

In addition, sulfate is commonly found in industrial waste streams, particularly textiles waste streams. As such, the addition of further sulfate the waste stream can be minimised.

In a first aspect, the invention provides method for oxidising sulfate comprising the steps of:

(i) oxidising sulfate (SO₄ ² ) at a working electrode; and

(ii) providing an ultrasonic field at the working electrode.

The ultrasonic filed is provided at the working electrode with the generation of the persulfate. The ultrasonic field may have a frequency of at most 50 kHz. The ultrasonic field may have a power of at most 500 mW.

Persulfate (S2O8² ) may be generated from the sulfate (SO4² ) ·

The persulfate is generated electrochemically at the working electrode. The method may comprise operating the working electrode at a current density of at least 0.02 Acnr². The method may comprise operating the working electrode at a voltage of at most 3.5 V vs. Ft.

In a second aspect, the invention provides an apparatus for oxidising sulfate comprising:

(i) a reaction space;

(ii) a working electrode in the reaction space;

(iii) an electrolyte comprising persulfate (S2O8² ) and/or sulfate (SO4² ) in the reaction space; and

(iv) an ultrasonicator for providing an ultrasonic field at the working electrode.

The apparatus may be an electrochemical cell. The electrochemical cell may contain a counter electrode in the reaction space, and optionally the electrodes are connectable to or are in connection with a power supply. The ultrasonicator may be any device suitable for generating an ultrasonic field, such as an ultrasonic bath or an ultrasonic probe.

The electrolyte may be an aqueous electrolyte. The electrolyte may be maintained at a certain pH, for example pH 0.0 to 4.0.

The electrolyte may comprise a sulfate salt, such as ammonium sulfate.

The electrolyte may comprise an acid, such as an inorganic acid, such as sulfuric acid.

The working electrode may be or comprise boron-doped diamond.

In a third aspect, the invention provides a method for degrading a pollutant comprising the steps of:

(i) oxidising sulfate (SCV) according to the method of the first aspect of the invention and generating sulfate radicals (SCV); and

(ii) reacting the sulfate radicals (SCV) with a pollutant.

The sulfate radicals (SOT^’) may be reacted with the pollutant in the presence of an ultrasonic field.

The sulfate radicals (SO_4") may be generated from the sulfate (SCV).

Persulfate (S₂O₈ ² ) may be generated from the sulfate (SO₄ ² ) and the sulfate radicals (SCV) may be generated from the persulfate (S2O8² )·

The pollutant may be an organic pollutant, such as a persistent organic pollutant.

The pollutant may be present in an industrial waste stream, such as a pharmaceutical, agrochemical, textile, plastics, detergents or electronics waste stream. The waste stream is typically an aqueous waste stream.

The method may comprise the step of pre-treating the industrial waste stream, such as: a) adjusting the pH of the industrial waste stream to at most 7.0; and/or

b) adjusting the concentration of sulfate salt in the industrial waste stream to at least 0.1 M and/or;

c) adjusting the temperature of the industrial waste stream to 0 °C.

In a fourth aspect, the invention provides an apparatus for degrading a pollutant comprising:

(i) an apparatus for oxidising sulfate (SCV) according to the second aspect of the invention;

(ii) sulfate radicals (SOT-) in the reaction space of the apparatus; and (iii) a pollutant in the reaction space if the apparatus.

The reaction space may comprise an entry port for connection to an industrial waste stream.

The reaction space may comprise an exit port for discharge of decontaminated waste.

These and other aspects and embodiments of the invention are described in further detail below.

Summary of the Figures

Figure 1 shows the voltage-time curves for the electrolysis of 3.62 M (NFU^SC in 1 M H₂SO₄ at a boron-doped diamond working electrode and an imposed current of 0.1 A (change in potential, V vs. Pt electrode, with change in time, s): (A) unstirred and quiescent starting at 18°C; (B) unstirred starting at 18°C and sonicated at 37 kHz; (C) stirred and quiescent starting at 18°C; (D) unstirred and quiescent starting at 26°C; (E) stirred and quiescent starting at 26°C. Stirring was conducted with a magnetic stir bar at 500 rpm.

Figure 2 shows the change in average absorbance with change in wavelength for a series of experiments where a dye is treated for 5 minutes under the following conditions: (A) shows a control standing at 18°C (uppermost dashed line); under an imposed current of 20 mA and stirred (central dotted line); under an imposed current of 20 mA and sonication at 37 kHz (lower solid line). (B) shows a control standing at 18°C (uppermost dashed line); in the presence of 19 mM (NH^SaOe (uppermost dotted line); in the presence of 19 mM

(NH₄)₂S₂0₈ with sonication at 37 kHz (central solid line); sonication at 37 kHz without added (NH^SaOs (lower mustard line).

Figure 3 shows the average absorbances of the dye/electrolyte solutions (at least 3 repetitions for each experiment) after 5 minutes of treatment under the following conditions:

5 minutes of stirring at 18 °C and then dye added and stirred for 5 minutes more (lower dashed line); simultaneous sonication (37 kHz) and an imposed current density of 23 mAcrrf² for 5 minutes, followed by addition of the dye and stirring for 5 minutes more in the absence of current or sonication (upermost solid line); simultaneous sonication (37 kHz) and an imposed current density of 23 mAcrrr² for 5 minutes, followed by addition of the dye and sonication (37 kHz) for 5 minutes more in the absence of current (central solid line)

Figure 4 shows a series of experiments in which the pollutant (dye) was prevented from accessing the anode using a selective membrane. (A) Schematic of the cell set-up with a cellulose membrane preventing the dye molecule from accessing the working electrode directly; (B) The average (of three repeats) of the absorbance of naphthol blue black when subjected to sono-electrochemistry (current density of 23 mA cm ² in 0.5 M (NH^SCU / 1 M H2SO) in the absence (lowermost solid line at 400 nm) and presence of a cellulose membrane (central solid line at 400 nm). Also shown for comparison is a dye-electrolyte solution that was not subjected to any sono-electrochemical treatment (uppermost solid line).

Detailed Description of the Invention

The invention generally provides a process for the degradation of persistent organic pollutants that combines electrochemical persulfate production with application of an ultrasonic field.

EP 3162768 describes an industrial waste-water treatment method using an oxidising agent produced by stripping ammonia from raw waste water, adding sulfuric acid to the ammonia to produce ammonium sulfate, converting the ammonium sulfate to ammonium persulfate using a diaphragm-type electrical reactor, and then converting the ammonium persulfate to sodium persulfate by reaction with sodium hydroxide. The sodium persulfate is then fed back into the waste water to react with contaminants in the waste water. EP 3162768 describes a number of optional, intermediate and post-treatment steps that can be applied to the waste water, including sonication. However, EP 3162768 does not describe a method that combines electrochemical oxidation of sulfate in the presence of an ultrasonic field.

The voltage values described herein are made with reference to a platinum (Pt), as is common in the art. The two-electron process for persulfate reduction:

S₂0s²- + 2e- 2S0₄ ²

has a voltage of 3.0 to 3.3 vs Pt under the following set-up: a three-electrode set-up comprising a 0.071 cm² boron-doped diamond working electrode, a graphite counter electrode and a Pt wire reference electrode in a single chamber cell with a 3.62 M (NH^SC in 1.0 M H2SO4 electrolyte at an initial temperature of 18 °C.

The two-electron process for persulfate reduction has a standard electrode potential of £° of +2.01 V vs Standard Hydrogen Electrode (SHE).

The Faradic yield (Faradic efficiency) values described herein are calculated with reference to the 2-electron process for sulphate oxidation. Thus, a process in which 90 C of charge produces 0.466 mmol of persulfate has a Faradic yield of 100%.

Persulfate Generation

The invention provides a method for generating persulfate comprising the steps of (i) generating persulfate (S2O8² ) at a working electrode; and (ii) providing an ultrasonic field at the working electrode. The persulfate may be generated electrochemically. Thus, the method may be a method of electrochemically generating persulfate. The method may comprise the step of

electrochemically generating persulfate (S2O8² ) at a working electrode.

The persulfate is generated from a precursor. The precursor may be sulfate (SO4^2') · Thus, the method may comprise the step of generating persulfate (S2O8^2') from sulfate (SO4^2') at a working electrode.

Persulfate is produced from sulfate by a 2-electron oxidation process:

2SO4² S₂0s²- + 2e-

Thus, the method may be a method of oxidising sulfate comprising the steps of (i) oxidising sulfate (SO4^2') at a working electrode; and (ii) providing an ultrasonic field at the working electrode.

Persulfate may be produced from sulfate through one or more intermediate species (Serrano et ai\ Davies et ai). For example, sulfate (SCU^2-) may be oxidised directly at an anode to produce a sulfate radical (SOr-):

S0 ² SO4" + e

Alternatively, water may be oxidised directly at an anode to produce a hydroxyl radical (HO") which may react with sulfate to produce a sulfate radical (SO_4"):

H2O HO^‘ + H⁺ + e- HSO4^· + HO^‘ SOT^’ + H2O

The combination of two sulfate radicals may then generate persulfate:

2SO4- S₂0s²

The method may comprise operating the working electrode at a current density of at least 0.02 Acrrr², for example at least 0.05 Acnr², at least 0.1 Acrrr², at least 0.2 Acnr², at least 0.5 Acnr², at least 0.8 Acnr² or at least 1.0 Acnr².

The method may comprise operating the working electrode at a current density of at most 3.0 Acnr², for example at most 2.5 Acrrr², at most 2.0 Acnr², at most 1.5 Acnr², at most 1.4 Acnr², at most 1.2 Acnr² or at most 1.0 Acnr².

The inventors have found that optimal results are obtained when the working electrode is operated at a current density that is in a range selected from the upper and lower amounts given above, for example in the range 0.1 Acnr² to 3.0 Acnr², such as 0.8 Acnr² to 3.0 Acnr² or 0.8 Acnr² to 1.5 Acnr². The inventors have found that the voltages required to generate persulfate are reduced in the presence of an ultrasonic field. Thus, the method may comprise the step of providing an ultrasonic field at the working electrode during persulfate generation.

Without wishing to be bound by theory, the inventors believe that the ultrasonic field increases the removal of bubbles from the working electrode surface. Thus, the method may comprise the step of applying an ultrasonic field to the working electrode.

The method may comprising the steps of (i) generating persulfate (S2O8² ) at a working electrode; and (ii) simultaneously providing an ultrasonic field at the working electrode.

The method may comprise operating the working electrode at a voltage of at most 4.0 V vs. Ft, for example at most 3.0 V vs. Ft, at most 3.5 V Ft, at most 3.3 V Pt at most 3.2 V Ft or at most 3.1 V vs. Pt.

Pollutant Degradation

The invention provides a method for degrading a pollutant comprising the step of (i) oxidising sulfate (SO4^2') as described above and generating sulfate radicals (SCV); and (i) reacting the sulfate radicals (SCV) with a pollutant.

The invention may provide a method for degrading a pollutant comprising the steps of (i) generating persulfate (S2O8^2') as described above; (ii) generating sulfate radicals (SCV) from the persulfate (S2O8^2') ; and (iii) reacting the sulfate radicals (SCV) with a pollutant.

Sulfate radicals may be produced from persulfate by homolytic cleavage of the 0-0 bond:

S₂0s²- 2SCV

Thus, the method may comprise the step of homolysing the persulfate (S2O8^2') to generate sulfate radicals (SCV).

The rate of generation of sulfate radicals from persulfate is increased in the presence of an ultrasonic field. Thus, the method may comprise the step of generating sulfate radicals (SO4") from the persulfate (S2O8² ) in the presence of an ultrasonic field. Similarly, the method may comprise the step of applying an ultrasonic field to the persulfate (S2CV).

As noted above, sulfate radicals may additionally be generated by direct oxidation of sulfate at an anode, or by reaction of sulfate with hydroxyl radicals. Thus, the method may additionally comprise the steps of generating sulfate radicals (SCV) at a working electrode. For example, the method may comprise the step of generating sulfate radicals (SCV) from sulfate (SCV) at a working electrode. Transport of sulfate radicals from the working electrode surface into bulk solution is increased in the presence of an ultrasonic field. Thus, the method may comprise the step of reacting the sulfate radicals (SOT-) with a pollutant in the presence of an ultrasonic field. Similarly, the method may comprise the step of applying an ultrasonic field to the sulfate radicals (SC ^")·

The method may be carried out in a batch-wise manner. That is, a discrete quantity of pollutant may be added to a reaction vessel comprising the sulfate radicals generated by the method describe above. Degraded pollutant may then be discharged from the reaction vessel.

Alternatively, the method may be carried out in a continuous-feed manner (in flow). That is, pollutant may be continuously added to a reaction vessel comprising the sulfate radicals generated by the method describe above. Degraded pollutant may then be continuously discharged from the reaction vessel.

The pollutant may be present in an industrial waste stream.

The industrial waste stream may be treated prior to processing by the method. Thus, the method may comprise the step of pre-treating the industrial waste stream.

The industrial waste stream may be an aqueous solution (a solution in which the solvent is water). The acidity of an aqueous solution can be specified using the pH scale. Methods for determining the pH of an aqueous solution are known and include, for example,

electrochemical methods (using a pH probe) and titration against an indicator compound such as universal indicator. Typically, the pH of the industrial waste stream refers to the pH of the industrial waste stream prior to treatment with the sulfate radicals.

The pre-treatment may comprise adjusting the pH of the industrial waste stream. Thus, the method may comprise the step of adjusting the pH of the industrial waste steam. The pH of the industrial waste stream may be raised or lowered as appropriate.

The method may comprise adjusting the pH of the industrial waste stream. Thus, the method may comprise the step of adjusting the pH of the industrial waste stream to at most 7.0, for example at most 6.5, at most 6.0, at most 5.5, at most 5.0, at most 4.5 or at most 4.0.

The pH of the industrial waste stream may be lowered by adding a suitable acid to the industrial waste stream, for example sulfuric acid. The pre-treatment may comprise adjusting the concentration of sulfate in the industrial waste stream. Thus, the method may comprise the step of adjusting the concentration of sulfate salt in the industrial waste steam.

The method may comprise the step of adjusting the concentration of sulfate salt in the industrial waste stream to at least 0.1 M, for example at least 0.5 M, at least 1.0 M, at least 1.5 M, at least 2.0 M, at least 2.5 M, at least 3.0 M or at least 3.5 M.

The concentration of sulfate in the industrial waste stream may be adjusted by adding a suitable sulfate salt to the industrial waste steam, for example ammonium sulfate (Nh ^SC .

The pre-treatment may comprise adjusting the concentration of non-sulfate ions in the industrial waste stream. Thus, the method may comprise the step of adjusting the concentration of cations such as sodium, potassium, magnesium or calcium in the industrial waste steam. The method may comprise the step of adjusting the concentration of anions such as chloride, bromide, nitrate or phosphate.

The method may comprise the step of reducing the concentration of sodium, potassium, magnesium or calcium ions in the industrial waste stream. The method may comprise the step of reducing the concentration of chloride, bromide, nitrate or phosphate ions in the industrial waste stream.

The pre-treatment may comprise adjusting the concentration of metal impurities in the industrial waste stream. Thus, the method may comprise the step of adjusting the concentration of metal impurities in the industrial waste steam.

The pre-treatment may comprise filtering the industrial waste stream to remove particulate matter. Thus, the method may comprise the step of filtering the industrial waste stream to remove particulate matter.

The pre-treatment may comprise adjusting the temperature of the industrial waste stream. Thus, the method may comprise the step of adjusting the temperature of the industrial waste steam. The temperature of the industrial waste stream may be raised or lowered as appropriate.

The method may comprise the step of adjusting the temperature of the industrial waste stream to at least 0 °C, for example at least 5 °C, at least 10 °C, at least 15 °C, at least 20 °C, at least 25 °C or at least 30 °C.

The method may comprise the step of adjusting the temperature of the industrial waste stream to at most 60 °C, for example at most 55 °C, at most 50 °C, at most 45 °C, at most 40 °C, at most 35 °C or at most 30 °C. The temperature of the industrial waste stream may be adjusted to a temperature in a range within the lower and upper temperatures selected from the lower an upper temperatures given above.

Apparatus

The invention also provides an apparatus for generating persulfate comprising (i) a reaction space; (ii) a working electrode in the reaction space; (iii) an electrolyte comprising persulfate (S2O8² ) and/or sulfate (SO4^2') in the reaction space; and (iv) an ultrasonicator for providing an ultrasonic field at the working electrode.

As noted above, persulfate may be produced from sulfate by a 2-eletron oxidation process. Thus, the apparatus may be an apparatus for oxidising sulfate comprising (i) a reaction space; (ii) a working electrode in the reaction space; (iii) an electrolyte comprising persulfate (S2O8^2") and/or sulfate (SO4^2') in the reaction space; and (iv) an ultrasonicator for providing an ultrasonic field at the working electrode.

The reaction space may be any space suitable for containing the working electrode and the electrolyte.

The working electrode is suitable for generating persulfate. Persulfate is generated at the working electrode during operation. The working electrode may be an anode or a cathode during operation. Typically the working electrode is the anode during operation.

Suitable working electrode materials are chemically resistant (corrosion resistant) materials. Suitable working electrode materials include platinum and boron-doped diamond.

The apparatus typically comprises a counter electrode in the reaction space.

Suitable counter electrode materials include carbon (graphite) and metals such as aluminium, nickel, steel and titanium.

The counter electrode may be an anode or a cathode during operation. Typically the counter electrode is the cathode during operation.

The electrolyte is suitable for solubilising persulfate (S2O8^2') and sulfate (SO4^2') ions. The electrolyte may be an aqueous electrolyte.

The electrolyte may comprise a sulfate salt. Suitable sulfate salts include sodium sulfate (Na₂SC>₄), potassium sulfate (K2SO4) or ammonium sulfate [(NhUMSC )]. The sulfate salt may be present at a concentration of at least 0.1 M, for example at least 0.5 M, at least 1.0 M, at least 1.5 M, at least 2.0 M, at least 2.5 M, at least 3.0 M or at least

3.5 M.

The electrolyte may comprise an acid. Suitable acids include sulfuric acid (H2SO4).

The acid may be present at a concentration of at least 0.1 M, for example at least 0.2 M, at least 0.5 M, at least 1.0 M, at least 1.5 M, at least 2.0 M or at least 2.5 M.

The electrolyte may be maintained at a certain pH during operation.

The pH of the electrolyte may be at most 7.0, for example at most 6.5, at most 6.0, at most 5.5, at most 5.0, at most 4.5 or at most 4.0.

The pH of the electrolyte may be at least 0.0, for example at least 0.5 at least 1.0 at least 1.5 at least 2.0 or at least 2.5.

The inventors have found that optimal results are obtained when the electrolyte is maintained at a pH that is in a range selected from the upper and lower amounts given above, for example in the range pH 0.0 to 6.0, such as 0.0 to 4.0.

The electrolyte may include non-sulfate ions. For example, the electrolyte may include sodium, potassium, calcium, magnesium or ammonium cations. The electrolyte may include chloride, bromide, nitrate or phosphate anions.

The electrolyte may be maintained at a certain temperature during operation.

The temperature of the electrolyte may be at least 0 °C, for example at least 5 °C, at least 10 °C, at least 15 °C, at least 20 °C, at least 25 °C or at least 30 °C.

The temperature of the electrolyte may be at most 60 °C, for example at most 55 °C, at most 50 °C, at most 45 °C, at most 40 °C, at most 35 °C or at m 30 °C.

The electrolyte may be maintained a temperature that is in a range selected from the upper and lower amounts given above, for example in the range 15 to 60 °C, such as 25 to 60 °C or 25 to 35 °C.

The electrochemical cell may also include a solid porous membrane positioned between the working electrode and the counter electrode. The solid porous membrane may protect the working electrode or the counter electrode from chemical degradation during use. Typically, the solid porous membrane protects the counter electrode. Suitable solid porous membrane materials include porous ceramic materials or porous polymer materials. Suitable polymers include polyethylene, polypropylene, or a copolymer thereof.

The apparatus may be an electrochemical cell. The electrochemical cell may be in electrical connection with a power supply. The electrochemical cell may be in electrical connection with a measurement device, for example an ammeter or voltmeter.

The invention also provides an apparatus for degrading a pollutant comprising (i) an apparatus for generating persulfate (S2O8² ) as described above; (ii) sulfate radicals (SCV) in the reaction space of the apparatus; and (iii) a pollutant in the reaction space of the apparatus.

As noted above, persulfate may be produced from sulfate by a 2-eletron oxidation process. Thus, the apparatus may be an apparatus for degrading a pollutant comprising (i) an apparatus for oxidising sulfate (SO4^2") as described above; (ii) sulfate radicals (SO4") in the reaction space of the apparatus; and (iii) a pollutant in the reaction space of the apparatus.

The reaction space may comprise an entry port for connection to an industrial waste stream.

The reaction space may have an exit port for discharge of decontaminated waste.

Ultrasonic Field

The invention provides a method for generating persulfate and/or degrading a pollutant comprising the step of providing an ultrasonic field at a working electrode.

The ultrasonic field may be continuously applied. Alternatively, the ultrasonic field may be applied intermittently (pulsed).

The ultrasonic field may also be applied to the persulfate (S2O8² ) and/or the sulfate radicals (SOT^’)· The ultrasonic field may be provided to the entire reaction space.

The ultrasonic field may have a frequency of at most 100 kHz, for example at most 90 kHz, at most 80 kHz, at most 70 kHz, at most 60 kHz, at most 50 kHz or at most 40 kHz.

The ultrasonic field may have a frequency of at least 10 kHz, for example at least 15 kHz, at least 20 kHz, at least 25 kHz, at least 30 kHz or at least 35 kHz.

The inventors have found that optimal results are obtained when the ultrasonic field has a frequency that is in a range selected from the upper and lower amounts given above, for example in the range 10 kHz to 80 kHz, such as 20 kHz to 50 kHz. The ultrasonic field may have a power density of at most 35 WL·¹ , for example at most 30 WL¹ at most 28 WL¹at most 26 WL·¹ , at most 24 WL·¹ , at most 22 WL·¹ or at most 20 WL·¹.

The ultrasonic field may have a power density of at least 5 WL·¹ , for example at least 6 WL·¹ , at least 8 WL·¹ , at least 10 WL ¹at least 12 WL·¹ , at least 14 WL·¹ or at least 16 WL·¹.

The inventors have found that optimal results are obtained when the ultrasonic field has a power density that is in a range selected from the upper and lower amounts given above, for example in the range 5 WL·¹ to 30 WL·¹ , such as 12 WL·¹ to 24 WL ¹or 16 WL·¹ to 20 WL·¹.

The ultrasonic field may have a power of at most 1 ,000 mW, at most 900 mW, at most 800 mW, at most 700 mW, at most 600 mW, at most 500 mW, at most 450 mW.

The ultrasonic field may have a power of at least 100 mW, at least 150 mW, at least 200 mW, at least 250 mW, at least 300 mW or at least 350 mW.

The ultrasonic field may have a power that is in a range selected from the upper and lower amounts given above, for example in the range 100 mW to 800 mW, such as 200 mW to 500 mW.

The ultrasonic field may be generated by an ultrasonicator. An ultrasonicator may be any device suitable for generating an ultrasonic field, such as an ultrasonic bath or an ultrasonic probe. For example, the ultrasonic field may be generated by a Fisher Scientific FB15050 ultrasonic bath.

Pollutants

The invention provides a method and apparatus for degrading a pollutant.

The pollutant may be an organic pollutant. The pollutant may be a persistence organic pollutant (POP).

The UK government publishes a lists of persistent organic pollutants

(https://www.qov.Uk/auidance/usina-persistent-oraanic-pollutants-pops#list-of-pops). A persistent organic pollutant may be a member of this list, for example as published 8 August 2016.

The Stockholm Convention on Persistent Organic Pollutants also lists several persistent organic pollutants. Its signatories have agreed to eliminate or restrict the production and use of persistent organic pollutants (POPs). Persistent organic pollutants cannot be degraded by traditional methods. For example, persistent organic pollutants cannot be degraded by natural biological processes, precipitation, adsorption, flocculation or ultrafiltration.

Persistent organic pollutants may be bioactive molecules, such as pharmaceuticals or agrochemicals. Persistent organic pollutants may be chlorinated aliphatic hydrocarbons, perfluorochemicals, brominate flame retardants or dyes.

Persistent organic pollutants may be waste products from the chemical industry, such as waste products from the pharmaceutical, agrochemical, textile, plastics, detergents or electronics industries.

Examples of persistent organic pollutants that are also pharmaceuticals include

dibenzazepine derivatives such as carbamazepine, and antibiotics such as amoxicillin, sulfadiaxine and tetracycline.

Examples of persistent organic pollutants that are also agrochemicals include

dichlorodiphenyltrichloroethane (DDT), endosulfan, chlordane, lindane, dieldrin, endrin, heptachlor, hexachlorobenzene, kepone, aldrin, mirex, toxaphene

Examples of persistent organic pollutants that are also chlorinated aliphatic hydrocarbons (CAHCs) include trichloroethylene and 1 ,1 ,1-trichloroethane.

Examples of persistent organic pollutants that are also perfluorochemicals include perfluorooctane sulfonic acid and peril uorooctane sulfonic acid derivativse such as ammonium perfulorooctanoate.

Examples of persistent organic pollutants that are also brominated flame retardants include tetrabromodiphenyl ether, pentabromodiphenyl ether, hexabromodiphenyl ether,

heptabromodiphenyl ether and hexabromocyclododecane.

Examples of persistent organic pollutants that are also dyes include azo dyes such a napthol blue black.

Additional persistent organic pollutants include pentachlorobenzene, polychlorinated biphenyl (RGB), hexabromobiphenyl, hexachlorobutadiene and polychlorinated

naphthalenes. Other Preferences

Each and every compatible combination of the embodiments described above is explicitly disclosed herein, as if each and every combination was individually and explicitly recited.

Various further aspects and embodiments of the present invention will be apparent to those skilled in the art in view of the present disclosure.

“and/or” where used herein is to be taken as specific disclosure of each of the two specified features or components with or without the other. For example“A and/or B” is to be taken as specific disclosure of each of (i) A, (ii) B and (iii) A and B, just as if each is set out individually herein.

Unless context dictates otherwise, the descriptions and definitions of the features set out above are not limited to any particular aspect or embodiment of the invention and apply equally to all aspects and embodiments which are described. Where technically appropriate embodiments may be combined and thus the disclosure extends to all permutations and combinations of the embodiments provided herein.

Certain aspects and embodiments of the invention will now be illustrated by way of example and with reference to the figures described above.

Examples

The following examples are provided solely to illustrate the present invention and are not intended to limit the scope of the invention, as described herein.

Materials and Apparatus

All reagents and solvents were obtained from commercial sources and used without further purification. Aqueous solutions were prepared using 18.2 MQcm water. Controlled current electrolysis was carried on a CHI760d potentiostat. Cell resistances were measured by applying potentiostatic electrochemical impedance at 0.01 V vs. reference, at a single frequency of a 100 kHz using a Palmsens4. Typical resistances were around 10 W. UV-Vis spectra were recorded on a JASCO V-670 spectrophotometer using 1 cm pathlength cuvettes.

Ultrasonication Methods

A Fisher Scientific FB15050 ultrasonic bath (frequency = 37 kHz) was employed, always filled with 2.2 L of water. A 100 mL beaker was used as the reaction vessel, and this was always submerged to the same depth (1 cm) and clamped in exactly the same position in the bath for each experiment. The volume of solution submerged was 12.5 cm³ and the total surface area of the beaker exposed to the bath was 25 cm². Using this set-up, the consistent temperature rise during sonication of 25 ml_ pure water over 30 minutes (7°C) could be used to gauge the acoustic power dissipated during sonication as 443 ± 83 mW.

Electrochemical Methods

The investigations into the effect of ultrasound on persulfate electrosynthesis were conducted in a three-electrode configuration, at an imposed current of 100 mA (current density of 1.41 Acnr²) for 15 minutes. This corresponds to 90 C of charge, or a theoretical yield of 0.466 mmol of persulfate (assuming a 2-electron process and a 100% Faradaic efficiency). The electrolyte was 3.62 M (NFU^SC solution (25 ml_) in 1 M H₂SO₄. A 0.071 cm² boron-doped diamond button working electrode (produced by chemical vapour diffusion and used as grown after polishing by the supplier, supplied by Windsor Scientific Ltd., UK), graphite counter electrode and Pt wire reference electrode were used, under an atmosphere of air, in single chamber cells, and with an initial temperature of 18 °C. Ultrasonic fields were applied during the electrosynthesis. Controlled current electrolysis at low current density (20 mA, 23 mAcrrr²) were carried out in a similar manner, except that the working electrode was now a boron doped diamond foil (surface area = 0.88 cm²) and the electrolyte was 0.5 M ammonium sulfate solution in 1 M H₂SO₄.

The conditions were chosen in order to make a comparison with the batch-type nature of most of the literature studies on sonochemically-activated organic degradations with persulfate, and also to try and replicate some of the parameters that might be useful for eventual industrial scale-up of this system. Given the harsh reaction conditions, the application of ultrasound to the electrochemical cell, and the presence of organic pollutants within this cell, an inexpensive cell design minimizing the use of membrane separators, pumps and other expensive components may be preferred, at least initially.

Determination of Total Oxidants

To determine the total amount of oxidants made during electrolysis, iodometric titration was used according to general procedures reported by Deadman et al. To the solution to be tested, 200 equivalents of Kl were added (relative to the theoretical maximum yield of ammonium persulfate calculated using Faraday’s law). This turned the solution dark orange. Sodium thiosulfate of known concentration was then added until the solution became colourless. To aid with detecting the endpoint, starch solution may be added, which reacts with any remaining iodide ions to give a dark blue colour. This method allowed the total amount of oxidants present in the analyte solution to be obtained on the basis of the amount (and concentration) of the sodium thiosulfate used in the titration. Determination of SO₅ ² (Caro’s Acid)

100 mM vanadyl sulfate solution was prepared by mixing sodium metavanadate with 1 :3 diluted sulfuric acid to produce a yellow solution. A molar equivalent amount of sodium sulfite was added forming the blue vanadyl sulfate solution. This solution was then degassed to remove sulfur dioxide and used as below.

VOSO₄ (1 mol equivalent relative to the theoretical maximum amount SO₅ ² that could be made based on the charge passed in the electrolysis) was added to the solution to be tested. The vanadylion (V0²⁺) is selectively oxidised by SO₅ ² to the pervanadyl ion (V0²⁺). The absorption at 360nm was measured and was compared to a calibration graph to determine the concentration of pervanadyl. Knowing the concentration of pervanadyl allows the calculation of the concentration of SO₅ ² that was in the solution and thus, the amount of SO₅ ² made during a particular experiment. To produce the calibration curve, known amounts of KMnCU were added to the VOSO₄ solution to produce known amounts of pervanadyl, and then the absorption at 360 nm was measured.

Determination of Hydrogen Peroxide

To determine the concentration of hydrogen peroxide present, the analyte solution was mixed with titanium oxysulfate (TiOSC ; a 1 mol equivalent relative to the theoretical maximum amount of hydrogen peroxide that could be made based on the charge passed). The T1OSO₄ solution was made by dissolving T1OSO₄ in 2 M H₂SO₄ (aided by sonication). When hydrogen peroxide reacts with T1OSO₄, titanic acid is formed, turning the solution yellow. Absorption at 407 nm was taken and compared with a calibration curve to determine the amount of hydrogen peroxide made. To produce the calibration curve, known amounts of H₂O₂ were added to the T1OSO₄ solution and then absorption at 407 nm was measured.

Production of Persulfate Oxidant

Table 1 shows the total amount of oxidant present in solution after 15 minutes (gauged iodometrically). It is well-known that aqueous solutions of persulfate can decompose to generate both H2O2 and SO₅ ² (Caro’s acid; House). Therefore, we also performed specific colorimetric tests for both H2O2 and SO₅ ² as described previously by Deadman et at. These tests indicated that H2O2 accounted for less than 1 % of the total oxidant present in solution, whilst SO₅ ² accounted for about 2% of the total oxidant present (rising to 4% when ultrasound was applied). Persulfate therefore accounts for the overwhelming share of the total oxidants detected iodometrically, and the other oxidants are present at levels within the error limits of the iodometric test (Table 1). During the 15 minutes of sonication, the temperature of the solution would rise to 23-24 °C. To isolate the effect of heat from other factors, some control experiments were performed where the reaction vessel was held at a constant 26 °C. Controls with and without stirring were also performed to examine the effects of mass transport. Table 1 : The molar and Faradic yields of oxidants

aFaradic yields calculated on the basis of oxidant formation being a 2-electron process, as it is for the generation of persulfate, peroxide and Caro’s acid; ^b Initial voltage is 3.00 V and final voltage is

3.04 V; ^clnitial temperature; ^d Initial voltage is 3.34 V and final voltage is 3.67 V; ^e Stirring the solution with a magnetic stirrer bar at 500 rpm had no noticeable effect on oxidant yields or Faradic yields; fHeld at a constant temperature; ⁹initial voltage is 2.18 V and final voltage is 2.75 C; Initial voltage is 2.30 V and final voltage is 2.83 V; 'No current applied; ^J Below detection limit.

In light of the above, it can be concluded that sonication does not impact negatively on the yield of electrochemical persulfate production under these conditions, and indeed the Faradaic yield for the electrosynthesis of oxidants (of which persulfate is by far the main constituent) remains essentially the same (within error) as long as the same current density is imposed. This information is critical to any practical application of this strategy for in situ sono-electrochemical pollutant degradation.

The Faradaic yields for oxidant production under these conditions is not as high as that reported in studies on electrochemical persulfate production under flowed conditions (Zhu et a!.). In this experiment, the inventors have made no attempt to optimize the total amount of oxidant produced in terms of lowering the synthesis temperature, using electrolyte additives, flowing the electrolyte, altering the current density, etc. Instead, the inventors have focussed on a single set of exemplar conditions (batch process, near-room temperature) in order to draw conclusions as to whether or not a combined sono-electrochemical process for organic pollutant degradation might have any potential advantages over a step-wise electrochemical- sonochemical treatment.

Significant gas evolution was evident at the working electrode during these electro-oxidation reactions. The inventors believe that the balance of the charge not going towards making solution-phase oxidants is consumed in performing oxygen evolution reactions (water splitting) under these conditions.

Surprisingly, the inventors noticed that the voltages required to drive the electrochemical processes listed in Table 1 at 0.1 A were lowered (typically by several hundred millivolts) by the application of ultrasound. In the same way, the voltage-time profiles were smoother and less prone to fluctuations (Figure 1 B). The inventors ascribe this to the effectiveness of ultrasound at removing bubbles from the electrode surface and therefore prevents large areas of the electrode from becoming periodically obscured by gas bubbles.

Reduced Current Density and Sulfate Concentration

In order to investigate any link between current density and sulfate concentration in relation to Faradaic yield for persulfate synthesis, the inventors examined the effect of reducing the current density (from 1.41 Acnr² to 23 mAcnr²) at the same time as reducing the total sulfate concentration (from 4.6 M to 1.5 M). These results are shown in entries 4 and 5 of Table 1. Reducing the imposed current leads to a significant increase in the Faradaic yield for the solution-phase oxidants that can be detected by iodometry (entry 5). This is expected, as lower voltages are required to sustain these lower current densities and so the competing oxygen evolution reaction is diminished. However, applying a 37 kHz ultrasound field during this electrosynthesis appeared to negate the effect of reducing the current density in this manner, and the Faradaic yield under simultaneous imposition of ultrasonication and a current density of 23 mAcnr² is almost the same as that achieved at 1.41 Acnr². Meanwhile, the control without imposing any current (entry 6) shows that essentially no iodometrically- detectable solution-phase oxidants are generated by the action of ultrasound alone under these conditions.

There are two possibilities to explain the apparently lower yield of solution-phase oxidants during sonication at an imposed current density of 23 mAcnr². Either sonication disfavours the formation of these oxidants, or these oxidants form to the same extent as under quiescent conditions but are then degraded by the ultrasonic field. The latter possibility appears not to be operating here: iodometry on 19 mM aqueous solutions of ammonium persulfate after 0, 15 and 60 minutes of sonication at 37 kHz revealed no diminution of the amount of solution-phase oxidant. Hence, under these conditions at least, it seems that the ultrasound field does not significantly degrade the solution-phase oxidants, implying that their lower yield during the sono-electrochemical protocol in Table 1 , entry 4 is probably due to the initial formation of these species being suppressed. Meanwhile, the very low level of solution-phase oxidants detected after sonication in the absence of an imposed current (entry 6) suggests that the iodometrically detectable solution-phase oxidants are produced essentially entirely by electrochemical processes.

Price and Clifton have previously reported that the application of ultrasound accelerates the rate of persulfate decomposition. The inventors believe that the comparatively low intensity of the ultrasonic field used in the invention in comparison to this earlier work (where intensities of ~26 W-crrf² were applied using an ultrasonic horn) prevents excessive decomposition of the persulfate as it is formed. Mechanistic Considerations

As noted above, at low current densities, persulfate may be produced from sulfate through one or more intermediate species (Serrano et ai/, Davies et ai). For example, sulfate (SO4^2') may be oxidised directly at an anode to produce a sulfate radical (SO4") , or, water may be oxidised directly at an anode to produce a hydroxyl radical (HO^’) which may react with sulfate to produce a sulfate radical (SCV"). In addition, peroxide may form by combination of two hydroxyl radicals, whilst peroxomonosulfuric (Caro’s) acid may be generated by combination of sulfate and hydroxyl radicals:

SO4- + HO· SO5²-

Furthermore, it is known that sulfate radicals can generate hydroxyl radicals by reaction with water at all pHs, although sulfate radicals tend to predominate at acidic pH.

Returning to the data in Table 1 , this presents an explanation for the difference in solution- phase oxidant yield observed for entries 4 and 5. In the quiescent experiments (entry 5), production of SO4" occurs in the immediate vicinity of the electrode and hence the local concentration of these radicals is rather high. This then facilitates the rapid dimerization of SO4" to give persulfate. Applying an ultrasonic field during this process is expected to lead to more effective dispersal of radicals from the vicinity of the electrode into bulk solution and therefore, decreasing the local concentration of SCV. This would lead to a lower overall yield of persulfate, but not necessarily a lower yield of SO5² , as SO4" dispersed into bulk solution would most likely react with water to produce HO' in the first instance, to produce SO5^2' in bulk solution. In the case of entries 1 and 2, the higher current densities and sulfate concentrations give rise to a much greater rate of production (and hence local concentration) of SCV at the anode in both cases, and so the effect of radical dispersal into bulk solution would be expected to be less pronounced than at lower current densities and sulfate concentrations (i.e. as the current density and concentration of sulfate rise, recombination of sulfate radicals to generate persulfate comes to dominate regardless of any enhanced mass transport effects).

Degradation of Persistent Organic Pollutant

The above rationale is consistent with the data in Table 1 , and in turn predicts that highly reactive radicals generated at the electrode are very rapidly dispersed into bulk solution by the application of the ultrasonic field. Hydroxyl and sulfate radicals have previously been suggested to be the two main species responsible for the degradation of organic pollutants during the sonochemical activation of persulfate (rather than persulfate itself) on account of their more positive reduction potentials than persulfate. However, the rates of reaction of SO4" and HO^’ with organic species tend to be approximately an order of magnitude lower than their rates of reaction with themselves or with each other (Ferkous et ai). Therefore, if the explanation above was correct, an organic probe molecule ought to be more rapidly degraded under the conditions in entry 4 of Table 1 than those of entry 5, as under the conditions in entry 4 there would be a greater dispersal of reactive radicals into bulk solution, whilst in the absence of ultrasound the radicals produced would be more likely to react with each other producing (less oxidising) persulfate, hydroxide or Caro’s acid.

To test this hypothesis, the inventors investigated the degradation of an exemplar organic pollutant (naphthol blue-black) via a combined sono-electrochemical method.

Naphthol blue-black is used widely as a dye in the colouring of fabrics, textiles and soaps, and for the manufacture of wood stains and inks (Onder et ai). It is extremely stable to both light and heat, and so has the potential to be a persistent organic pollutant in the natural environment in cases of uncontrolled release. Its discoloration under ultrasound has previously been explored as a proxy for the degradation of a range of related azo dyes of industrial significance (Ferkous et ai), and so it seemed a suitable compound to use to examine the effectiveness of a combined sono-electrochemical approach for the degradation of persistent organic pollutants.

Accordingly, solutions of 25 mL of 0.5 M (NFU^SCU and 3.2 mM naphthol blue-black in 1 M H₂SO₄ were subjected to a range of conditions and controls, the results of which are summarised in Figure 2. Sulfate concentrations of around this order are sometimes found in dye industry effluent (Zeng et ai). For electrolyses, a 0.88 cm² boron doped diamond electrode (produced by chemical vapour diffusion and used as grown (nucleation side exposed), supplied by Windsor Scientific Ltd., UK) was used. In Figure 2, each trace is the average of five runs and confidence intervals are marked with error bars at the characteristic absorbance of the dye at 620 nm.

Figure 2A shows the effect of passing a current of 20 mA through the dye/electrolyte solution (current density = 22.7 mA cm ²) for 5 minutes with and without simultaneous ultrasonic irradiation (lower solid and central dotted lines respectively; non-irradiated solutions were stirred at 500 rpm). Complete discoloration of the solution occurs within 9 minutes at a current of 20 mA with simultaneous ultrasonic irradiation. Also shown is a control where the dye/electrolyte solution was left at 18 °C without either electrochemical oxidation or sonication (uppermost dashed line). Clearly, in the case with simultaneous ultrasonic irradiation there is a noticeable decrease in dye absorbance (and the solutions are noticeably less coloured to the eye) than in the case where electrochemical oxidation alone is employed (or, indeed, the control).

In a similar fashion, Figure 2B shows a series of comparisons between the absorbance of untreated dye/electrolyte solution after standing at 18 °C for 5 minutes (uppermost dashed line), and the absorbance of an identical solution after 5 minutes of sonication (lower solid line), after standing at 18 °C for 5 minutes in the presence of 19 mM ammonium persulfate (uppermost dotted line), or after being sonicated for 5 minutes in the presence of 19 mM ammonium persulfate (central solid line). These latter two experiments were performed to examine the relative efficacy of a stepwise persulfate-production-followed-by-sonication approach, where a 19 mM solution of ammonium persulfate corresponds to a roughly fifteen fold excess in the amount of persulfate that could be produced electrochemically by our methods at this current density in 5 minutes (assuming a 100% Faradaic yield for persulfate synthesis). Hence it is apparent that the approach leading to the most rapid diminution of dye absorbance under these conditions is indeed the concurrent sono-electrochemical method (lower solid line in Figure 2A), and that sonicating in the presence of even a significant excess of persulfate cannot compete with the combined sono-electrochemical degradation treatment. These results are consistent with a dye degradation mechanism involving hydroxyl and sulfate radicals but in which more stable solution-phase oxidants such as peroxide and persulfate have no significant effect on dye discoloration.

When the combined effects of ultrasound alone (Figure 2B, lower solid line) and

electrochemical oxidation alone (Figure 2A, central dotted line) are compared to the combined sono-electrochemical experiment (Figure 2A, lower solid line), there appears to be synergy when the two processes are used together, beyond that which might be expected by simply combining their individual effects on dye discoloration. Given the comparatively low levels of discoloration evident when persulfate is ultrasonically-activated in situ with the dye (Figure 2B, central solid line), these data suggest that the synergic effect of the sono- electrochemical process is due to the effects of ultrasound on the electrode reactions, rather than activation electrochemically-produced persulfate in bulk solution. Hence, these results are also consistent with the hypothesis that the ultrasonic field aids in the rapid dispersal of reactive radicals (generated by electrochemical oxidation) into bulk solution, where they can react with the dye molecule.

Addition of Dye after Sono-electrochemistry

In further support of this supposition, Figure 3 shows the outcome of experiments where a dye-free electrolyte solution (0.5 M (NH^SCU in 1 M H₂SO₄) was subjected to the conditions shown in Table 1 , entry 4 for 5 minutes, after which the sono-electrochemical experiment was terminated. Only then was dye added to the solution to produce a concentration of 3.2 mM naphthol blue black and this solution was stirred for a further 5 minutes (uppermost solid line). Likewise, the central solid line shows what happens in an otherwise identical experiment, but where after the dye has been added the solution is sonicated for 5 minutes. In neither case is any degradation significantly different from that seen in the absence of a preceding sono-electrochemical process observed. This implies that whatever agent causes the dye discoloration in Figure 2 does not remain in solution long enough to react with a dye molecule subsequently added to the solution. This again points towards highly reactive radicals such as SO4" and HO^’ as the agents interacting with the dye.

Preventing Access of Dye to the Anode

The experiments suggest that the dye is being degraded primarily by sono-chemically and anodically-generated radicals. However, the possibility that the application of ultrasound is simply enhancing mass transport of the dye to the electrode surface (and therefore enhancing the direct electro-oxidation of the dye) cannot be excluded on the basis of the data presented thus far. Hence a final set of experiments was conducted in which the dye was prevented from accessing the working electrode by means of a cellulose membrane using the set-up shown in Figure 4A. This membrane (MEMBRA-CEL) has small pores (nominal molecular weight cut-off of 3500) that would be expected to impede the movement of the dye (MW = 616) but which should provide a lower barrier to the movement of the smaller sulfate and hydroxyl radicals.

When dye was added to the counter electrode compartment of Figure 4A but not to the working electrode compartment), the diffusion of the dye through the membrane over the course of 30 minutes was very slow, supporting the hypothesis that the membrane slows down the movement of the dye and should prevent it from reaching the working electrode in significant amounts over this time period. Sono-electrochemical experiments were then conducted as above (current density of 23 mA cm ² in 0.5 M (NH^SCU / 1 M H₂SO₄) and the absorbance of the dye solution monitored after 5 minutes. The results, shown in Figure 4B, suggest that the level of discoloration of the dye is the same (within error) whether a membrane is used to prevent the dye from accessing the anode or not. This in turn suggests that direct electro-oxidation of the dye at the working electrode is not the main dye discoloration mechanism operating in this system. Instead, these data are consistent with the idea that the dominant pollutant degradation mechanism is by sonochemically and anodically-generated radicals, which the ultrasound field then effectively disperses into bulk solution.

References

A number of publications are cited above in order to more fully describe and disclose the invention and the state of the art to which the invention pertains. Full citations for these references are provided below. The entirety of each of these references is incorporated herein.

Chen and Huang, 2015,“Mineralization of aniline in aqueous solution by electro-activated persulfate oxidation enhanced with ultrasound”, Chem. Eng. J., Vol. 266, pp. 279-288.

Davies et al. , 2014,“Understanding persulfate production at boron doped diamond film anodes”, Electrochim. Ada., Vol 150, pp. 68-74.

Deadman et al., 2017,“A colorimetric method for rapid and selective quantification of peroxodisulfate, peroxomonosulfate and hydrogen peroxide”, React. Chem. Eng., Vol. 2, pp. 462-466.

Eslami et al., 2016,“Optimization of sonochemical degradation of amoxicillin by sulfate radicals in aqueous solution using response surface methodology (RSM)”, J. Mol. Liq. , Vol. 222, pp. 739-744. Ferkous et al., 2017,“Persulfate-enhanced sonochemical degradation of napthol blue black in water: Evidence of sulfate radical formation”, Ultrason. Sonochem. , Vol. 34, pp. 580-587.

Hao et al., 2014,“Intensification of sonochemical degradation of ammonium

perfluorooctanoate by persulfate oxidant”, Ultrason. Sonochem., Vol. 21 , pp. 554-558.

House, 1962,“Kinetics and mechanism of oxidations my peroxydisulfate”, Chem. Rev., Vol. 62, pp. 185-203.

Ike et al., 2018,“Critical review of the science and sustainability of persulphate advanced oxidation processes”, Chem. Eng. J., Vol. 338, pp. 651-669.

Li et al., 2013,“Removal of 1 ,1 ,1-trichloroethane from aqueous solution by a sono-activated persulfate process”, Ultrason. Sonochem. , Vol. 20, pp. 855-863.

Lin et al., 2015,“Enhanced sonochemical degradation of perfluorooctanoic acid by sulfate ions”, Ultrason. Sonochem., Vol. 22, pp. 542-547.

Matzek and Carter, 2016,“Activated persulfate for organic chemical degradation: A review”, Chemosphere, Vol. 151 , pp. 178-188.

Michaud et al., 2000,“Preparation of peroxodisulfuric acid using boron-doped diamond thin film electrodes”, Electrochem. Solid State Lett. , Vol. 3, pp. 77-79.

Monteagudo et al., 2015, “In situ chemical oxidation of carbamazepine solutions using persulfate simultaneously activated by heat energy, UV light, Fe2+ ions, and H202”, Appl. Catal. B-Environ., Vol. 176-177, pp. 120-129.

Onder et al. , 2011 ,“Decolorization of naphthol blue black using the horseradish peroxidase”, Appl. Biochem. Biotechnol., Vol. 163, pp. 433-443.

Osgerby, 2006,“ISCO technology overview: do you really understand the chemistry?”, Contaminated Soils, Sediments and Water, Vol. 10, pp. 287-308, Springer-Verlag, New York, USA.

Price and Clifton, 1996,“Sonochemical acceleration of persulfate decomposition”, Polymer, Vol. 37, pp. 3971-3973.

Radimer and McCarthy, 1979,“Electrolytic production of sodium persulfate”, US 4,144,144, published 13 March 1979.

Safari et al., 2015,“Optimization of sonochemical degradation of tetracycline in aqueous solution using sono-activated persulfate process”, J. Environ. Health Sci. Eng., Vol. 13, pp. 1-15.

Serrano et al., 2002,“Electrochemical preparation of peroxodisulfuric acid using boron doped diamond thin film electrodes”, Electrochim. Acta, Vo. 48, pp. 431-436.

Wang and Zhou, 2016,“Removal of carbamazepine from aqueous solution using sono- activated persulfate process”, Ultrason. Sonochem. , Vol. 29, pp. 156-162. Zeng et al., 2017,“Alkaline textile wastewater biotreatment: A sulfate-reducing granular sludge based lab-scale study”, J. Hazard. Mater. , Vol. 332, pp. 104-111.

Zhu et al., 2016,“T oward a green generation of oxidant on demand: practical

electrosynthesis of ammonium persulfate”, ACS Sustainable Chem. Eng., Vol. 4, pp. 2027- 2036.

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Claims

Claims:

1. A method for oxidising sulfate comprising the steps of:

(i) oxidising sulfate (SO*² ) at a working electrode; and

(ii) providing an ultrasonic field at the working electrode.

2. The method of claim 1 , wherein the ultrasonic field has a frequency of at most 50 kHz.

3. The method of claim 1 or 2 wherein the ultrasonic field has a power of at most 500 mW

4. The method of any of claims 1 to 3 which comprises operating the working electrode at a current density of at least 0.2 Acrrr².

5. The method of any of claims 1 to 4 which comprises operating the working electrode at a voltage of at most 3.5 V vs. Ft.

6. The method of any one of claims 1 to 5, wherein persulfate (S2O8² ) is generated from sulfate (SO4^2') at the working electrode.

7. An apparatus for oxidising sulfate comprising:

(i) a reaction space;

(ii) a working electrode in the reaction space;

(iii) an electrolyte comprising persulfate (S₂O₈ ² ) and/or sulfate (SO₄ ² ) in the reaction space; and

(iv) an ultrasonicator for providing an ultrasonic field at the working electrode.

8. The apparatus of claim 7 wherein the ultrasonicator is an ultrasonic bath.

9. The apparatus of claim 7 or 8, wherein the electrolyte comprises an aqueous solution of ammonium sulfate.

10. The apparatus of any of claims 7 to 9, wherein the electrolyte has a pH of 0.0 to 4.0.

11. The apparatus of claim 10, wherein the electrolyte comprises sulfuric acid.

12. The apparatus of any of claims 7 to 11 , wherein the working electrode comprises boron-doped diamond.

13. A method for degrading a pollutant comprising the step of:

(i) oxidising sulfate (SCV-) according to the method of any of claims 1 to 6 and generating sulfate radicals (SCV); and

(ii) reacting the sulfate radicals (SCV) with a pollutant.

14. The method of claim 13, wherein the sulfate radicals (SCV) are reacted with the pollutant in the presence of an ultrasonic field.

15. The method of claim 13 or 14, comprising generating sulfate radicals (SCV) from the sulfate (SCV)·

16. The method of any one of claims 13 to 15, comprising generating persulfate (S2O8² ) from the sulfate (SO4² ); and generating sulfate radicals (SO4") from the persulfate (S2CV)·

17. The method of any of claims 13 to 16, wherein the pollutant is a persistent organic pollutant.

18. The method of any of claims 13 to 17, wherein the pollutant is present in an industrial waste stream.

19. The method of claim 18 comprising the step of pre- treating the industrial waste stream, preferably wherein the pre-treatment comprises:

a) adjusting the pH of the industrial waste stream to at most 7.0; and/or

b) adjusting the concentration of sulfate salt in the industrial waste stream to at least 0.1 M and/or;

c) adjusting the temperature of the industrial waste stream to 0 °C.

20. An apparatus for degrading a pollutant comprising:

(i) an apparatus for oxidising sulfate (SO4² ) according to any one of claims 7 to 12;

(ii) sulfate radicals (SCV) in the reaction space of the apparatus; and

(iii) a pollutant in the reaction space of the apparatus.