GB2499025A - Decontamination of a system and treatment of the spent decontamination fluid - Google Patents

Abstract

A method for the decontamination of a contaminated system, comprising the steps of selecting a decontaminating agent, applying the decontaminating agent to the system, collecting the used decontaminating agent and electrochemically treating the used decontaminating agent. The spent decontamination fluid is treated by electrolysis wherein the electrolysis cell may comprise a boron doped diamond electrode, metal oxide coated titanium electrode or platinum electrode. The decontaminating agent may comprise hydrochloric acid (HCl), nitric acid (HNO3), sodium hydroxide (NaOH), sodium chloride (NaCl), hydrogen peroxide (H2O2) or a complexant such as EDTA or citric acid. The method may further include an initial characterisation step, a chemical dosing step prior to the electrochemical treatment of the used decontaminating agent to remove contaminant via precipitation, a subsequent characterisation step wherein the system is re-evaluated in order to determine whether the decontamination objectives have been successfully accomplished, and a final step of post-treatment of the effluent remaining after the electrochemical treatment. The methods of the invention may find particular application in the treatment of systems comprising radioactivity.

Description

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NOVEL DECONTAMINATION SYSTEM

Field of the Invention

[0001] This invention relates to a novel system for the decontamination of surfaces and 5 treatment of the resulting effluent. More specifically, the system provides a process which allows for the removal of contaminants with suitable decontaminating media and the subsequent treatment and safe disposal of the contaminants, decontaminating species and associated products.

10 Background to the Invention

[0002] Industrial processes are frequently associated with the requirement for surfaces to be cleaned and decontaminated, either in order to promote efficient working of the processes, or for reasons associated with environmental or health and safety considerations. However, it is generally found that aggressive decontaminants, such as

15 hydrochloric acid, can only be used in limited circumstances in view of the potential corrosion impact of residual halide on downstream infrastructure. In general, therefore, the use of chloride bearing decontamination solutions is prohibited or severely limited, with disposal typically being achieved through extensive dilution. Similarly, the use of complexants is also limited, due to the reduction in the efficacy of abatement plant or 20 increased mobility of contamination in the environment.

[0003] Nevertheless, situations are frequently encountered where it is necessary that such aggressive decontaminants should be employed in order that efficient decontamination of surfaces contaminated with particularly stubborn or toxic contaminants may be achieved. Consequently, there is an evident requirement for the development of

25 procedures which may facilitate efficient decontamination in such circumstances.

[0004] Specifically, the effectiveness of aggressive acids such as HCI has previously been demonstrated but, as with various complexants, downstream processing issues have limited their use. For example, during the decommissioning of nuclear facilities large amounts of pipework and vessels require processing; for a typical nuclear plant,

30 approximately 300,000 metres of steel pipework may be involved. Hence, more rapid and lower volume decontamination approaches are clearly desirable and can provide significant savings in terms of time (years of operational costs may be associated with maintaining active plant and resource) and burden on infrastructure (i.e. the amount of supporting infrastructure required to permit operations).

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[0005] Consequently, it would be desirable to provide an integrated treatment system that couples decontamination with both treatment of the resulting effluent and subsequent reagent recycling, and it is this issue that the present invention seeks to address.

[0006] The electrolysis of nitric acid streams using platinum anodes as a means of 5 removing chloride contaminants has previously been reported in connection with platinum mining by R.G. Wilkinson of Eldorado Mining and Refining Limited, Port Hope, Ontario ("Removal of Chloride Contaminants from Nitric Acid: Electrolytic Process uses Platinum Anodes", R. G. Wilkinson, Platinum Metals Rev.,1961, 5(4), 128), and the same process has been used as a treatment stage in a nuclear silver(ll) process) to prevent silver 10 wastage through precipitation as silver chloride, as detailed by F.J Poncelet, M.H. Mouliney and M. Lecomte, RECOD 1994. Vol. 2, Industrial use of electrogenerated Ag(ll) for Pu02 dissolution. Furthermore, electrochemical oxidation has previously been employed for the destruction of complexants in a range of industries, particularly including the nuclear industry, as noted for example in PNNL-11590, Electrochemical Destruction of 15 Organics and Nitrates in Simulated and Actual Radioactive Hanford Tank Waste, M.R. Elmore and W.E. Lawrence, September 1996.

[0007] It is noteworthy, however, that none of these prior art treatment methods was in any way associated with decontamination processes, nor did any of them address situations which involved combinations of halides and organic materials. It has previously

20 been established that electrochemical processing involving combinations of chloride-bearing streams with organic materials accelerates oxidation of the organic species. It is also known that the use of boron doped diamond electrodes offers increased resilience to complexing organic molecules and other organic materials and facilitates the application of higher current densities, thereby allowing for the use of electrodes having smaller 25 dimensions and, hence, more compact cells.

[0008] Nevertheless, the prior art is silent regarding the possible pre- or post-treatment of solutions in order to remove species such as Fe and Cr and none of the known processes have previously been utilised in systems which seek to accelerate decontamination procedures whilst limiting effluent volumes and the possible recycling of materials such as

30 chlorides or nitric acid is never considered. The present inventors seek to address these omissions.

Summary of the Invention

[0009] The present invention, therefore, seeks to provide an integrated treatment system 35 that couples decontamination with both treatment of the resulting effluent and, typically,

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subsequent reagent recycling, and thereby to permits the use of previously restricted or prohibited reagents through the removal of residual components which have previously proved to be problematic in terms of effluent handling and disposal. Most particularly, the invention seeks to address the problems associated with the handling of chloride from 5 spent decontamination solutions and also considers the possibility of reagent recycling and the destruction of organic materials, including complexants.

[0010] Thus, according to a first aspect of the present invention, there is provided a method for the decontamination of a contaminated system, the method comprising at least the steps of:

10 (a) selecting a decontaminating agent;

(b) applying said decontaminating agent to said system;

(c) collecting the used decontaminating agent; and

(d) electrochemically treating the used decontaminating agent.

[0011] Certain embodiments of the invention envisage an initial characterisation step, 15 wherein the system is examined in order that the nature of the contaminant may be determined and evaluated so as to assist in optimising the selection of a decontaminating agent.

[0012] In various embodiments of the invention, a chemical dosing step may be employed between steps (c) and (d), prior to electrochemical treatment of the used

20 decontaminating agent. This step may facilitate the precipitation of certain materials and thereby allow their removal by e.g. deposition or filtration.

[0013] Further embodiments of the invention envisage a subsequent characterisation step, wherein the system is re-evaluated in order to determine whether the decontamination objectives have been successfully accomplished. In the event that the

25 said objectives have not been achieved, the sequence of steps comprised in the method of the invention may be refined or repeated as many times as necessary in order to successfully complete the decontamination operation.

[0014] Still further embodiments of the invention envisage a final step comprising post-treatment of the effluent remaining after the electrochemical treatment.

30 [0015] The method of the first aspect of the invention may comprise one, some or all of said optional additional steps.

[0016] Thus, embodiments of the method according to the first aspect of the invention may comprise the following steps:

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(i) optionally characterising the system;

(ii) selecting a decontaminating agent;

(iii) applying said decontaminating agent to said system;

(iv) collecting the used decontaminating agent;

5 (v) optionally chemically dosing the used decontaminating agent;

(vi) electrochemically treating the used decontaminating agent;

(vii) optionally re-characterising the system;

(viii) optionally repeating, at least once, each of steps (ii), (iii), (iv), (v) (if present) and (vi);

10 (ix) optionally repeating steps (vii) and (viii) as necessary; and

(x) optionally post-treating the effluent.

[0017] In typical embodiments of the invention, the system which is to be treated comprises at least one contaminated surface. Contaminated surfaces may, for example comprise contaminated pipework. Said surfaces are typically concrete surfaces or metal

15 surfaces formed of, for example, stainless steel.

[0018] The disclosed method therefore provides a decontamination system coupled to an effluent treatment process and thereby facilitates the use of, for example, chloride bearing reagents such as HCI and NaCI, as well as nitric acid and complexants - and combinations thereof - for decontamination operations by virtue of the fact that it allows for

20 the removal and destruction of problematic species such as chlorides and complexants from the waste solution whilst also facilitating the treatment of removed contamination.

[0019] Furthermore, the system which is provided permits the local treatment of effluent, e.g. using trolley based units, thereby in certain embodiments allowing decontamination operations to be decoupled from the central plant infrastructure. Alternatively, in other

25 embodiments, the system can be used as an addition to an existing centralised decontamination facility.

[0020] The present inventors have established that the electrochemical treatment, chemical dosing and post-treatment steps of the method of the present invention are also applicable to the treatment of process liquors in order to remove chloride and organic

30 materials which are present in process or environmental streams, as opposed to streams generated through decontamination operations.

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[0021] Thus, a second aspect of the present invention envisages a method for the removal of contaminants from a process or environmental stream which comprises the steps of:

(a) optionally chemically dosing the used decontaminating agent; 5 (b) electrochemically treating the used decontaminating agent; and

(c) optionally post-treating the effluent,

wherein the method comprises at least one of steps (a) and (c).

[0022] The methods of the invention are typically operated as batch procedures but may, if required be operated as continuous processes in certain embodiments.

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Brief Description of the Drawings

[0023] Embodiments of the invention are further described hereinafter with reference to the accompanying drawings, in which:

Figure 1 is a flow chart which provides an overview of the method according to 15 the invention.

Figure 2 is a flow chart which illustrates the process stapes associated with the electrochemical treatment of the used decontaminating agent.

Figure 3 illustrates a typical electrochemical cell for use in a method according to the invention.

20 Figure 4 illustrates graphically the efficiency of removal of chloride and organic carbon from a decontamination solution according to the method of the invention.

Description of the Invention

[0024] The present invention provides a decontamination/local effluent treatment process 25 which includes multiple steps and comprise at least the steps of:

(a) selecting a decontaminating agent;

(b) applying said decontaminating agent to said system;

(c) collecting the used decontaminating agent; and

(d) electrochemically treating the used decontaminating agent.

30 [0025] Embodiments of the invention include at least one additional step selected from the steps of:

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• initially characterising the system prior to steps (a), (b), (c) and (d);

• chemically dosing the used decontaminating agent prior to step (d);

• re-characterising the system after steps (a), (b), (c) and (d);

• depending on the outcome of the re-characterisation of the system, repeating each 5 of steps (a), (b), (c) and (d) - and, where appropriate, the chemical dosing;

• repeating the re-characterisation and repetition of steps (a) (b), (c) and (d) - and, where appropriate, the chemical dosing - until satisfactory decontamination is achieved; and

• post-treating the effluent.

10 [0026] A typical process is illustrated in Figure 1 and is made up of the steps of:

1. Characterising the system.

2. On the basis of the characterisation, selecting a suitable decontaminant.

3. Delivering the decontaminant to the system.

4. Collecting and electrochemically treating the spent decontaminant.

15 5. Re-characterising the system to evaluate the success of the treatment.

6. Post-treating the spent decontaminant to remove contamination.

[0027] As previously stated, certain of the above steps are optional, but the process must comprise at least steps 2, 3 and 4 and will usually comprise at least steps 2, 3, 4 and 6.

[0028] Thus, in an exemplary process according to the invention, a system is 20 characterised (Step 1) in order to facilitate selection of at least one suitable decontamination agent (Step 2). Thereafter, the process involves the delivery of decontamination agent(s) (Step 3) to remove surface contamination and/or remove contaminated surface (e.g. surfaces comprising stainless steel and concrete), following which the waste decontamination agent(s) is/are subjected to an electrochemical treatment 25 (Step 4) in order to remove residual corrosive species and/or complexant species so as to make them compatible with downstream plant or the environment. Thereafter, the system may be re-characterised (Step 5) to determine if the decontamination objective has been achieved; if not, then steps 2, 3 and 4 may be repeated. Finally, the spent decontaminant is post-treated in order to remove the contaminants by, for example, precipitating the 30 removed contamination/contaminated surface components.

[0029] Characterisation of the system may be carried out by any of a number of techniques known to those skilled in the art. Characterisation may be carried out remotely

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where required - notably, for example, when the system comprises a hazardous environment. A knowledge of parameters such as the severity, type and location of the contaminant is important to an efficient decontamination process, so this step is typically included when one or more of these parameters is not known.

5 [0030] Suitable characterisation techniques may, for example, include simply using an established knowledge of plant history, or may involve active methods such as radiation and chemical fingerprinting techniques, including the use of devices for the detection of elevated levels of radiation in remote locations, such as those described in WO-A-2011/018657 which comprise scintillators in combination with fibre optic cables, or 10 methods such as Laser Induced Breakdown Spectroscopy (LIBS) or Raman Spectroscopy which may involve the collection and analysing of samples. Devices such as those described in W0-A-2011/018657 may be used to establish the intensity and spatial distribution of radiation, whilst Laser Induced Breakdown Spectroscopy or Raman Spectroscopy can provide elemental composition data remotely.

15 [0031] On the basis of the determination provided by the characterisation step, selection of a suitable decontamination reagent composition is facilitated and other parameters such as the concentration, volume and sequence of reagent delivery may also be established in the light of the characteristics of the contamination.

[0032] In the context of the present invention, it is generally the case that aggressive 20 decontaminants are utilised, typical examples of which may be selected from, for example,

hydrochloric acid, nitric acid, sodium hydroxide, sodium chloride, hydrogen peroxide, or various complexants, or suitable mixtures of one or more of these components.

[0033] Following selection of the decontaminant and suitable parameters relating thereto as detailed above, this reagent is delivered to the system to be decontaminated. Delivery

25 may be achieved via any convenient means. Thus, for example, delivery may simply involve flooding and draining the system and this may be carried out with the decontaminant in the form of, typically, a foam, gel, bulk liquid, mist, spray, an aerosol or an atomised reagent. In certain embodiments of the invention, delivery is by means of atomisation and misting, which allows the reagent volume to be kept to a minimum and 30 thereby reduces the inventory in use and the amount of reagent to be prepared.

[0034] In decontaminants which comprise mixtures of more than one decontaminating reagent, the relative proportions of these reagents may effectively be varied by varying the duration of the misting and the relative concentrations. Thus, in a particular embodiment wherein the process is applied to the decontamination of stainless steel surfaces, e.g. in

35 pipework, HCI (or NaCI) may initially be utilised for a rapid decontamination phase, then HN03 may be deployed to (a) wash down the HCI (or NaCI), (b) provide passivation of the

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steel in order to prevent an unintentional breach of containment, and (c) optimise the postprocessing of the spent decontamination solution in an electrochemical cell.

[0035] Typically, the dissolution rate of steel in high concentrations of HCI (10M) may vary from 0.07 micron h"1 under passivating conditions to more than 70 micron h"1 for the

5 corrosion rate of bare steel. In the case of 2M NaCI and 0.041 M HN03, the dissolution rate was found to be 1.94 micron h"1, whilst for 0.03M NaCI and 4M HN03 the dissolution rate was 0.037 micron h"1 and for 1M NaCI +10M HN03 the dissolution rate was measured as 0.57 micron h"1.

[0036] In typical embodiments of the invention, the time taken to remove 10 micron of a 10 steel surface using HCI or NaCI mixed with HN03 is in the range of 10 minutes to 5 hours,

assuming that the decontaminating reagent is present in excess.

[0037] Particularly favourable results may be achieved with atomised reagents having droplet sizes in the range of from 0.5-20 micron, more specifically from 1-10 micron, most specifically from 2-5 micron, which are useful in achieving high surface area distribution of

15 the decontaminating reagent. The proportion of liquid to air in the atomised droplets may advantageously be increased to give droplet sizes in the region of 50 micron for the purpose of surface flooding and washing down of surfaces.

[0038] Following delivery of the decontaminant to the system, and its interaction with the system, the spent decontaminant is collected and electrochemically treated and this stage

20 of the process is illustrated in Figures 2 and 3.

[0039] The electrochemical treatment is carried out in at least one electrochemical cell. In typical embodiments of the invention, a single electrochemical cell is employed. However, embodiments are envisaged wherein a multiplicity of electrochemical cells may be used. In said embodiments, the multiplicity of cells may comprise a cell stack

25 comprising multiple repeat units. Alternatively, said multiplicity of cells may comprise a multiplicity of duplicate identical cells or a multiplicity of cells of different design, wherein each cell is optimised for a specific electrochemical process, for example the treatment of high or low chloride-containing process streams.

[0040] In the embodiment illustrated in Figure 2, it is seen that a stream of spent 30 decontaminant is fed to a receipt vessel, following which any coarse solids are removed,

e.g. by filtration, and the stream is fed to a single electrochemical cell. As previously discussed, a chemical dosing step may optionally be employed prior to treatment in the cell, this step facilitating the precipitation of certain materials and allowing their removal by e.g. deposition or filtration.

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[0041] Treatment of the spent decontaminant in the electrochemical cell facilitates the removal of further contaminants. Thus, for example, chloride may be removed by electrolysis and liberated as chlorine gas which may then be treated in an off-gas scrubber prior to release of any remaining harmless gases to the atmosphere, whilst any

5 complexants - typically organic compounds such as EDTA or citric acid - which are present in the stream are oxidised in the electrochemical cell. In addition to direct oxidation at the anode, chloride ions may optionally be added to the electrolyte in order to enhance oxidation of organic materials through the generation of chlorine, or chlorine-containing ions, such as chlorate, hypochlorite or hypochlorate, on or in the vicinity of the

10 anode.

[0042] Suitable off-gas systems which may be used to scrub the liberated chlorine may, for example, use NaOH as the scrubbing agent. This would then produce NaCI, which may be re-used as a decontamination agent. In alternative embodiments of the invention, solid state scrubbers can be employed.

15 [0043] A more detailed illustration of an electrochemical cell is seen in Figure 3, wherein there is shown a typical example of such a cell. This cell seeks to perform two electrochemical tasks:

1. Evolution of chlorine (or other halides e.g. bromine and iodine, but not fluorine in aqueous systems) at the anode in order to reduce the chloride content of

20 the electrolyte; and

2. Destruction of the complexing function of any organic complexants through direct oxidation at the anode or indirect oxidation through the formation of oxidising chlorine containing species e.g. chlorine, hypochorite, hypochlorate, chlorate, etc.

25 [0044] Typically, the cathode reaction is hydrogen evolution and it is generally beneficial to separate the anode and cathode reactions through the use of a separator or membrane (porous or ion selective) for the following reasons:

1. Preventing parasitic redox couples (e.g. Fe2+/Fe3+ and N03~/N02~) from reducing the current efficiency;

30 2. Keeping metal ions away from the cathode in order to minimise the risk of electro-deposition, even though this should be minimal in such very acidic electrolytes; and

3. Separating gas streams (oxygen and/or chlorine evolving from the anode and hydrogen at the cathode).

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[0045] The catholyte is typically nitric acid which prevents any problems from occurring as a consequence of cross-over from the anolyte (electrolyte in the anode circuit) and catholyte compartments. The catholyte will gradually accumulate cations, e.g. metal ions from the anolyte, but it could be re-used periodically as a make-up solution for the nitric

5 acid based decontamination solution.

[0046] Typical separators/membranes are polymeric in nature and may comprise any of a number of commercially available alternatives which would be apparent to a skilled person such as, for example, a Nafion® (sulpbonated tetrafluoroethyiene based fluoropolymer copolymer) cationic selective membrane or a microporous polyethylene

10 separator. These components, under normal conditions, have a finite lifetime (typically 2 or 3 years) before requiring replacement. Polymeric membrane lifetimes are likely to be reduced in environments with a high radiation dosage during use, which can lead to accelerated damage and/or embrittlement. On occasions, however, replacement of the membranes/separators may not be possible so that, in certain embodiments of the 15 invention, the use of radiation resistant materials, such as ceramics (for example porous silicon nitride or porous alumina) may be necessary.

[0047] A cell such as that which is illustrated in Figure 3 for use in a method according to the invention is unlike a conventional electrochemical chlorine generator, in that the majority reaction changes during the treatment of a batch of effluent. The cell would

20 normally operate in batch mode but could be operated in a continuous flow-through mode if required. In the initial stages of operation, the predominant reactions result in mainly chlorine evolution and oxidation of organic materials. However, as the concentration of these species decreases and they begin to become mass transport limited, then the degree of oxygen evolution increases until the stage at which the chloride has almost been 25 removed from the system, at which point more than 99% of the anodic current is utilised for oxygen evolution. Mechanisms for the enhancement of the mass transport are found to be beneficial to the system, and such mechanisms may include, for example, high fluid velocities, the use of 3D electrodes, and the incorporation of inert mesh turbulence promoters.

30 [0048] Referring specifically to Figure 3, there is seen a cell 1 which comprises an anode 2 and a cathode 3 with membrane 4 being placed between the anolyte circuit and the catholyte circuit. The cell additionally comprises anolyte tank 5 and catholyte tank 6, valves 7, 8, 9 and 10 and pumps 11 and 12. Thus, in operation, effluent (spent decontaminant) flows through valve 7 into anolyte tank 5 through which it passes before 35 being pumped by pump 11 into cell 1. Following electrolysis, the anolyte then flows out of cell 1 and returns to anolyte tank 5 from which it is discharged via valve 9 for optional

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further treatment, whilst purge gas (for example, nitrogen or air) enters the tank and liberated chlorine and oxygen is vented therefrom. (In certain embodiments of the invention the liberated gases may be extracted from the vessel under negative pressure and diluted at a different location.) Concurrently, in the catholyte circuit, fresh nitric acid is 5 fed through valve 8 into catholyte tank 6 through which it passes before being pumped by pump 12 into cell 1. Following electrolysis, the catholyte then flows out of cell 1 and returns to catholyte tank 6 from which it is discharged via valve 10 for re-use in the decontamination solution, whilst purge gas enters the tank and liberated hydrogen is vented therefrom. Typically, the processes according to the invention are batchwise 10 processes wherein the liquors in the anolyte and catholye circuits are recirculated throughout the process and only discharged from valves 9 and 10 following completion of the process.

[0049] The components of the cell, including the electrodes and fluid circuits are adapted to handle the gas evolution, such that the evolved gases may be disengaged from the fluid

15 streams and then suitably post-treated.

[0050] Effluent which is released from the cell following the electrochemical treatment may optionally be further characterised and/or post-treated as previously indicated. Such treatment of the waste stream may, for example, involve the destruction of organic materials which do not form complexes in chloride/radioactive waste streams.

20 [0051] It is necessary that the anode material of the cell should show stability in the electrolyte and in the presence of complexants. In addition, the material should be suitable for both oxygen and chlorine evolution and demonstrate low wear rates for both reactions, as well as low overpotentials for the reactions which are comprised in the method of the invention - specifically, for example, chlorine evolution and the oxidation of organic 25 materials. It is also desirable that the anode material should show higher overpotentials for other reactions, such as oxygen evolution. Typical materials for use as anode materials include boron-doped diamond, coated titanium (coated with oxides of metals, e.g. iridium oxide, mixed iridium/ruthenium oxide and tin oxide) and bulk platinum.

[0052] The selection of materials for the cathode materials is not as critical, with the main 30 requirement being for stability in the electrolyte. In this context, both stainless steel and titanium are found to be particularly suitable, but this in no way limits the number of available materials, and a wide range of other materials which would be readily apparent to a person skilled in the art may also be employed for this purpose.

[0053] It should also be emphasised that, whilst the cell depicted in Figure 3 is divided 35 between the anolyte and catholyte compartments by a microporous separator or membrane, an undivided cell may also be used for the purposes of the present invention.

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This alternative approach provides the benefit of a simpler cell design with only one fluid circuit and, of course removes the requirement for separator which is stable to radiation in those instances where a radioactive spent decontaminant requires to be treated.

[0054] As will be appreciated from the earlier discussion, however, such an approach is 5 associated with certain potential disadvantages including, for example, the following:

• The use of an undivided cell could probably result in a lower efficiency for chlorine reduction and organic destruction due to the possible presence of parasitic redox couples (such as Fe2+/Fe3+ and N037N02~) in the system;

• There is the possibility of metal electro-deposition (most likely involving iron or 10 nickel) on the cathode; and

• There is the possibility that the off gas mixture of hydrogen, chlorine and oxygen may be present in proportions having the potential to be explosive

These problems, however, could be overcome if necessary by, for example, precipitating most of the metals before electrochemical treatment in the cell and/or by the use of smaller 15 area - and hence high current density - cathodes which are able to maximise hydrogen production, and/or by ensuring that the pH is maintained at a value which is much less than 2.

[0055] In certain embodiments of the invention, the optional chemical dosing step prior to feeding to the electrochemical cell may, for example, be employed for the removal of a

20 contaminant, such as chloride, via chemical precipitation. Such an approach could be valuable in situations wherein the off-gas management facilities are inadequate to handle and clean up toxic gas discharges. If necessary, further polishing of chloride may be achieved through precipitation with silver nitrate to form silver chloride.

[0056] In alternative embodiments of the invention, removed contaminant/contaminated 25 surface dissolved in a spent decontaminant stream which comprises nitric acid may be precipitated through the addition of NaOH and, optionally, other reagents. Thus, for example, streams which result from the treatment of stainless steel surfaces will typically contain iron, nickel and chromium, and will thereby produce an iron hydroxide floe on treatment with NaOH, thus allowing for the iron to be removed as a precipitate. Such 30 treatments are also successfully used for the removal of actinides, heavy metals and other metals such as strontium, which may either be removed as co-precipitates or by means of sorption onto floes.

[0057] Such chemical precipitation procedures may also be augmented by the addition of at least one ion exchange material which could be tailored to the fingerprint of the

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contamination; thus, for example, hexacyanoferrate ion-exchange material may be used in instances wherein the contaminant comprises Cs-137. In such cases, delivery of the relevant precipitation/neutralisation reagent can be achieved via in-line mixing in order to achieve rapid mixing and good floe characteristics, and to reduce space demands in the 5 equipment. Such an operation could also most conveniently be applied before the treatment in the electrochemical cell in order to remove Fe from the feed to the cell, which would then eliminate the requirement for the use of a membrane (by removing the Fe2+/Fe3+ redox couple).

[0058] In any event, the mixing of reagents at the optional chemical dosing step may be 10 achieved using in-line mixing controlled by algorithm.

[0059] The optional system re-characterisation procedure may be employed in order to establish if further treatment is required, and the techniques employed would typically be the same as those which are appropriate to the initial system characterisation step.

[0060] The final stage of the procedure involves post-treatment of the effluent produced 15 from the electrochemical cell, and an embodiment of this step may be gleaned from Figure

2, wherein the stream is further treated locally to precipitate additional waste solids prior to discharge of the remaining liquid effluent. In alternative embodiments of the invention, the effluent discharged from the cell may be bowsered to a centralised treatment facility where post-treatment of the material may be effected.

20 [0061] The method of the first aspect of the invention may typically be applied to nuclear decontamination processes which involve the treatment of surfaces to remove contamination as part of routine operations and plant shut downs, post-operational clean-out (POCO) procedures, and decommissioning. They are also used for the cleaning of non-nuclear surfaces, in applications such as the treatment of stainless steel to remove 25 contaminants e.g. surface treatment post fabrication.

[0062] The successful application of the method according to the first aspect of the invention may be gleaned from Figure 4 which illustrates the successful removal of organic carbon and chloride from a used decontamination solution. The Figure shows the reduction in total organic carbon content and chloride concentration against the electrolytic

30 charge which is passed (in Farads). The starting solution was 0.75 I of 0.1 M HCI, 3M HN03 and 1 g/l of SGD3 (a commercial complexant mixture containing the chelators citric acid and EDTA). The applied current was 8A and a divided cell was used with boron doped diamond electrodes.

[0063] It can be seen that both the chloride content and the total organic carbon (TOC) 35 content initially dropped rapidly and then the rate of reduction reduced as their

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concentration dropped. Both TOC and chloride content continued to fall until the chloride content was reduced to around 5 ppm and the TOC content reduced to around 50 mg/l. It should be pointed out that the remaining organic carbon content is probably not the chemicals which were originally present but, rather, comprises degradation products 5 without the complexing functionality.

[0064] Methods according to the second aspect of the invention are applicable to a range of process effluent treatment operations where the effluent has become contaminated, and are particularly successfully applied to the destruction of organics (e.g. complexants), the destruction of organics from trade or pharmaceutical wastes, the removal of chloride from

10 process effluent, and the removal of chloride from radioactive contaminated oils from drilling.

[0065] Methods according to the invention allow for the use of reagents to facilitate increases in decontamination rate/efficiency to minutes/hours compared with hours/weeks in the case of the methods of the prior art. Furthermore, the relative volumes of reagents

15 which are employed is dramatically reduced to tens of litres compared with thousands of litres in the case of prior art methods. The processes also allow for reagents to be recycled.

[0066] Furthermore, the electrochemical treatment may be carried out using cells which are portable, so that decommissioning activities may be decoupled from infrastructure and

20 adapted to produce an effluent that is compatible with existing infrastructure. The method of the invention is, therefore, flexible and adaptable to the system to be treated.

[0067] In specific embodiments, the method of the first aspect of the invention finds particular applicability in the treatment of substrates which are contaminated with radioactive contaminants and they bring together the ability to use nitric acid and chloride

25 bearing reagents as decontamination agents in combination with complexants by facilitating the removal of components which have downstream processing implications and providing a post-treatment option to precipitate the stripped material and remove radioactivity.

[0068] Furthermore, when the method is used in combination with low volume delivery of 30 reagent (e.g. using an atomised reagent), the volume of reagent is substantially reduced when compared to the methods of the prior art, thereby allowing for fine tuning of decontamination and minimisation of effluent volume, and consequently limiting the size of treatment unit which is required. The method also provides the facility to deploy aggressive reagents in limited controlled quantities where plant integrity is of concern to 35 such decontamination operations.

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[0069] Throughout the description and claims of this specification, the words "comprise" and "contain" and variations of them mean "including but not limited to", and they are not intended to (and do not) exclude other moieties, additives, components, integers or steps. Throughout the description and claims of this specification, the singular encompasses the

5 plural unless the context otherwise requires. In particular, where the indefinite article is used, the specification is to be understood as contemplating plurality as well as singularity, unless the context requires otherwise.

[0070] Features, integers, characteristics, compounds, chemical moieties or groups described in conjunction with a particular aspect, embodiment or example of the invention

10 are to be understood to be applicable to any other aspect, embodiment or example described herein unless incompatible therewith. All of the features disclosed in this specification (including any accompanying claims, abstract and drawings), and/or all of the steps of any method or process so disclosed, may be combined in any combination, except combinations where at least some of such features and/or steps are mutually

15 exclusive. The invention is not restricted to the details of any foregoing embodiments. The invention extends to any novel one, or any novel combination, of the features disclosed in this specification (including any accompanying claims, abstract and drawings), or to any novel one, or any novel combination, of the steps of any method or process so disclosed.

20 [0071] The reader's attention is directed to all papers and documents which are filed concurrently with or previous to this specification in connection with this application and which are open to public inspection with this specification, and the contents of all such papers and documents are incorporated herein by reference.

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Claims (1)

1. A method for the decontamination of a contaminated system, the method comprising at least the steps of:

5 (a) selecting a decontaminating agent;

(b) applying said decontaminating agent to said system;

(c) collecting the used decontaminating agent; and

(d) electrochemically treating the used decontaminating agent.

10 2. A method as claimed in claim 1 which additionally comprises a chemical dosing step between steps (c) and (d), prior to electrochemical treatment of the used decontaminating agent.

3. A method as claimed in claim 2 wherein said chemical dosing step comprises the 15 removal of a contaminant via chemical precipitation.

4. A method as claimed in claim 2 or 3 wherein said chemical dosing step comprises the addition of at least one ion exchange material to the used decontaminating agent.

20 5. A method as claimed in any one of claims 1 to 4 which additionally comprises an initial characterisation step, prior to step (a), wherein the system is examined to determine the nature of the contaminant.

6. A method as claimed in any preceding claim which additionally comprises a 25 subsequent characterisation step, after step (d), wherein the system is re-evaluated in order to determine whether the decontamination objectives have been successfully accomplished.

7. A method as claimed in claim 5 or 6 wherein characterisation of the system is 30 carried out remotely.

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8. A method as claimed in claim 5, 6 or 7 wherein characterisation of the system is carried out by means of radiation and chemical fingerprinting techniques.

9. A method as claimed in claim 8 wherein said technique involves the use of devices 5 which comprise scintillators in combination with fibre optic cables.

10. A method as claimed in claim 8 wherein said technique comprises Laser Induced Breakdown Spectroscopy or Raman Spectroscopy.

10 11. A method as claimed in any one of claims 1 to 10 which additionally comprises a final step comprising post-treatment of the effluent remaining after the electrochemical treatment.

12. A method as claimed in claim 11 wherein said post-treatment of the effluent 15 comprises the precipitate of waste solids prior to discharge of the remaining liquid effluent.

13. A method as claimed in claim 6 wherein the objectives have not been achieved, and the preceding sequence of method steps is repeated at least once.

20 14. A method as claimed in any preceding claim which comprises the following steps:

(i) optionally characterising the system;

(ii) selecting a decontaminating agent;

(iii) applying said decontaminating agent to said system;

(iv) collecting the used decontaminating agent;

25 (v) optionally chemically dosing the used decontaminating agent;

(vi) electrochemically treating the used decontaminating agent;

(vii) optionally re-characterising the system;

(viii) optionally repeating, at least once, each of steps (ii), (iii), (iv), (v) (if present) and (vi);

30 (ix) optionally repeating steps (vii) and (viii) as necessary; and

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(x) optionally post-treating the effluent.

15. A method as claimed in any preceding claim wherein said contaminated system comprises at least one contaminated surface.

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16. A method as claimed in claim 15 wherein said at least one contaminated surface comprises contaminated pipework.

17. A method as claimed in claim 15 wherein said at least one contaminated surface 10 comprises a concrete or metal surface.

18. A method as claimed in claim 17 wherein said metal surface is formed of stainless steel.

15 19. A method as claimed in any preceding claim wherein said decontaminating agent comprises at least one of hydrochloric acid, nitric acid, sodium hydroxide, sodium chloride, hydrogen peroxide, or at least one complexant.

20. A method as claimed in claim 19 wherein said complexant comprises at least one 20 of EDTA or citric acid.

21. A method as claimed in any preceding claim wherein the decontaminating agent is delivered to the system to be decontaminated by flooding and draining the system.

25 22. A method as claimed in any preceding claim wherein the decontaminating agent is delivered to the system in the form of a foam, gel, bulk liquid, mist, spray, an aerosol or an atomised reagent.

23. A method as claimed in claim 22 wherein said decontaminating agent is in the form 30 of an atomised reagent having droplet sizes in the range of from 0.5-20 micron.

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24. A method as claimed in claim 22 or 23 wherein said droplet size is from 1-10 micron.

25. A method as claimed in claim 22, 23 or 24 wherein said droplet size is from 2-5 5 micron.

26. A method as claimed in any preceding claim wherein said electrochemical treatment of the used decontaminating agent is carried out in an electrochemical cell which comprises as the anode, a boron doped diamond electrode, a coated electrode or a

10 platinum electrode.

27. A method as claimed in claim 26 wherein said coated electrode comprises a coated titanium electrode.

15 28. A method as claimed in claim 27 wherein said electrode is coated with at least one metal oxide.

29. A method as claimed in claim 28 wherein said at least one metal oxide is selected from iridium oxide, mixed iridium/ruthenium oxide and tin oxide.

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30. A method as claimed in any preceding claim wherein said electrochemical treatment of the used decontaminating agent is carried out in an electrochemical cell which comprises as the cathode a stainless steel or titanium electrode.

25 31. A method as claimed in any preceding claim which is applied to nuclear decontamination processes including plant shut downs, post-operational clean-out procedures and decommissioning.

32. A method as claimed in any one of claims 1 to 31 which is used for the cleaning of 30 non-nuclear surfaces in applications including the treatment of stainless steel to remove contaminants.

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33. A method for the removal of contaminants from a process or environmental stream which comprises the steps of:

(a) optionally chemically dosing the used decontaminating agent;

(b) electrochemically treating the used decontaminating agent; and 5 (c) optionally post-treating the effluent,

wherein the method comprises at least one of steps (a) and (c).

34. A method as claimed in claim 33 which is applied to the destruction of organics from trade or pharmaceutical wastes, the removal of chloride from process effluent, or the

10 removal of chloride from radioactive contaminated oils from drilling.

35. A method as claimed in any preceding claim wherein said electrochemical treatment comprises treatment in a single electrochemical cell.

15 36. A method as claimed in any one of claims 1 to 34 wherein said electrochemical treatment comprises treatment in a multiplicity of electrochemical cells.

37. A method as claimed in any preceding claim which comprises a batchwise process.

20 38. A method as claimed in any preceding claim which comprises a continuous process.

39. A method for the decontamination of a contaminated system as hereinbefore defined and with reference to the accompanying description and drawings.

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40. A method for the removal of contaminants from a process or environmental stream as hereinbefore defined and with reference to the accompanying description and drawings.

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