GB2558552A

GB2558552A - Method for regenerating ion exchange material

Info

Publication number: GB2558552A
Application number: GB1700039.9A
Authority: GB
Inventors: Waite Michael
Original assignee: IONEX SG Ltd
Current assignee: IONEX SG Ltd
Priority date: 2017-01-03
Filing date: 2017-01-03
Publication date: 2018-07-18
Also published as: WO2018127679A1; GB201700039D0

Abstract

A method of regenerating an ion exchange material, which can be a strong base anion (SBA) exchange resin, for an ion exchange bed. The method comprising the steps of: a) preparing a regenerant which comprises a salt solution preferably 7.4-12% w/v of sodium chloride or potassium chloride; b) modifying the regenerant by 1) increasing the pH, preferably to 9.1-9.3 by adding a metal hydroxide which can be an alkali metal hydroxide or an alkali earth metal hydroxide such as sodium hydroxide, lithium hydroxide, potassium hydroxide, magnesium hydroxide and or calcium hydroxide or a mixture thereof and/or 2) increasing the oxidation potential by adding an oxidising agent such as hydrogen peroxide, ozone or hypochlorite to promote conversion of trivalent chromium hydroxide to soluble hexavalent chromium ions; and c) passing the regenerant through the ion exchange bed to achieve the conversion and remove the hexavalent chromium ions from the ion exchange material. Preferably the salt solution has a concentration of at least 2M and at least 2.5 or 3-3.5 bed volumes of the regenerant is passed through the bed. The method can be used to provide potable drinking water.

Description

(54) Title of the Invention: Method for regenerating ion exchange material Abstract Title: Method for Regenerating Ion Exchange Material (57) A method of regenerating an ion exchange material, which can be a strong base anion (SBA) exchange resin, for an ion exchange bed. The method comprising the steps of: a) preparing a regenerant which comprises a salt solution preferably 7.4-12% w/v of sodium chloride or potassium chloride; b) modifying the regenerant by 1) increasing the pH, preferably to 9.1-9.3 by adding a metal hydroxide which can be an alkali metal hydroxide or an alkali earth metal hydroxide such as sodium hydroxide, lithium hydroxide, potassium hydroxide, magnesium hydroxide and or calcium hydroxide or a mixture thereof and/or 2) increasing the oxidation potential by adding an oxidising agent such as hydrogen peroxide, ozone or hypochlorite to promote conversion of trivalent chromium hydroxide to soluble hexavalent chromium ions; and c) passing the regenerant through the ion exchange bed to achieve the conversion and remove the hexavalent chromium ions from the ion exchange material. Preferably the salt solution has a concentration of at least 2M and at least 2.5 or 3-3.5 bed volumes of the regenerant is passed through the bed. The method can be used to provide potable drinking water.

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Method For Regenerating Ion Exchange Material

The present invention relates to a method of regenerating an ion exchange material for an ion exchange column, particularly but not necessarily for the regeneration of a strong base anion exchange resin for use in the removal of hexavalent chromium from potable water.

In order to provide potable drinking water, contaminants often need to be removed in order for the water to be safe for human consumption. Chromium ions are a common contaminant arising most frequently from leaching of the underlying bedrock or from industrial sources, and there are strict requirements as to the safe level of various chromium ions which can be present in water. It is therefore useful to remove as much chromium from water as is possible, and ideally to treat the waste fraction in such a manner as to permit safe disposal thereof

The removal of contaminants is commonly achieved by running the water through an ion exchange column containing an ion-exchange material such as a strong base anion (SB A) exchange resin, particularly for the treatment of drinking water. Treatment plants comprising such ion exchange columns may operate on a proportion of the total water flow or on the total water flow in order to improve the quality of the water so as to meet the relevant potable standard.

SBA exchange resins operate by binding contaminants contained within the water at functional sites within the resin. Contaminant ions will be captured at the functional sites, allowing decontaminated water to pass through the ion exchange column.

Over time, the functional sites within the resin will become exhausted or saturated with contaminant, and therefore the SBA exchange resin will need to be regenerated in order to become usable once more. Regeneration of the SBA exchange resin is typically achieved by passing a strong brine solution through the ion exchange column, with ions from the brine solution displacing the captured contaminant anions, reforming the active SBA exchange resin functional sites.

Hexavalent chromium maybe present in groundwater as chromate or dichromate ions, having a valence of six. Chromium in a lower valence state, such as trivalent chromium, is insoluble at pH values greater than 3, whereas most groundwaters have a pH in the range of 6.5 to 8, at which chromium is only soluble in the hexavalent form. Traditional treatment methods have used weak base anion resins for its removal; however, this method also uses significant quantities of acid to reduce the pH of the feed water and removes a large amount of other anions from the water, resulting in very high operational costs and significant waste volumes.

SBA exchange resins are used in the removal of hexavalent chromium contaminants from water, such as CrCri²' anions. It has, however, been found that the regeneration of the SBA exchange resin becomes less efficient over time, resulting in a reduced hexavalent chromium capture by the SBA resin, and therefore a higher incidence of hexavalent chromium in the treated water. This could lead to dangerous levels of hexavalent chromium being present in the potable water despite treated.

It has been theorised that the decreased efficiency of the regeneration process is due to the conversion of a proportion of the hexavalent chromium to insoluble trivalent chromium hydroxide (Cr(OH)3) as the water is processed by the SBA resin, which blinds the active sites on the resin. Chromium hydroxide is insoluble in the brine solution, and therefore cannot be removed by the regenerant. Over time, an increasing proportion of the SBA exchange resin functional sites become blocked or blinded by the trivalent chromium hydroxide, with a corresponding apparent reduction of the capacity of the SBA resin to remove hexavalent chromium.

The present invention seeks to provide an improved regeneration process for an SBA exchange resin which overcomes the blinding of the SBA exchange resin by trivalent chromium hydroxide over time.

According to a first aspect of the invention, there is provided a method of regenerating an ion exchange material for an ion exchange bed, such as an ion exchange column, the method comprising the steps of: a] preparing a regenerant which comprises a salt solution; b] modifying the regenerant by increasing the pH and/or increasing the oxidation potential thereof to promote conversion of trivalent chromium hydroxide to soluble hexavalent chromium ions; and c] passing the regenerant through the ion exchange bed to at least partially remove hexavalent chromium ions from the ion exchange material.

The present inventors have found that either increasing the pH or oxidation potential (Eh) of the salt solution promotes the conversion of trivalent chromium hydroxide to hexavalent chromate ions within the ion exchange material, effectively solubilising the trivalent chromium hydroxide to permit its removal as hexavalent chromium. It is thought that the pH of raw water which is processed is around 7 to 8, with the Ehbeing in the range 400 to 500 mV, typically. This is very close to the equilibrium change between trivalent chromium hydroxide and hexavalent chromate ions. The increase in the pH or the increase in the oxidation potential moves the position of the equilibrium towards the hexavalent chromate during the regeneration, and thus the insoluble trivalent chromium converted back to hexavalent chromium can be extracted from the ion exchange material.

Preferably, during step a], the salt solution may comprise sodium chloride or potassium chloride.

Said salt solution may comprise from 7.4 to 12.0 % w/v of sodium chloride or potassium chloride.

The provision of a regenerant having such a concentration has been shown to have excellent regeneration properties for SBA exchange resin materials under normal circumstances, notwithstanding the issue of blinding by trivalent chromium hydroxide.

Preferably, during step b] the regenerant may be modified to increase the pH of the salt solution.

The alteration of the pH of the salt solution is one mechanism by which the efficiency of the regenerant can be improved. Data for chromium ions in water suggests that the equilibrium boundary between hexavalent chromium and trivalent chromium hydroxide exists in a region for oxidation potential of between 0.2V and 0.5V and for pH of between 6.5 and 10.5, which may be depicted on a Pourbaix diagram. The pH of the raw water passed through the ion exchange material is typically between 7 and 8, whilst having an oxidation potential of around 400 to 500mV. It is postulated that, under local conditions in the SBA resin bed, there is a significant proportion of hexavalent chromium ions converted to trivalent chromium hydroxide in the water which insolubly blocks the active sites of the SBA exchange resin material. It is theorised that an increase in pH or oxidation potential shifts the equilibrium back towards hexavalent chromium, which is soluble in the regenerant solution.

Optionally, the pH may be increased by adding a metal hydroxide, and said metal hydroxide added may be an alkali metal hydroxide or an alkali earth metal hydroxide. In particular, the metal hydroxide added may be any one of: sodium hydroxide; potassium hydroxide; lithium hydroxide; magnesium hydroxide; and/or calcium hydroxide; or a mixture thereof.

Addition of a hydroxide into the regenerant solution can significantly increase the pH without significantly altering the oxidation potential thereof. This allows a user to redress the balance between trivalent and hexavalent chromium easily.

In a preferred embodiment, the pH may be increased to between 9.1 and 9.3.

A pH of approximately 9.2 has been found to render the regenerant near to 100% efficient, in that repeated regeneration of an SBA exchange resin with the regenerant at a pH of 9.2 maintains the effective capacity of the resin for hexavalent chromium.

As an alternative, during step b], the regenerant may be modified by addition of an oxidising agent to increase the oxidation potential of the regenerant.

As an alternative to the pH treatment of the regenerant to alter the equilibrium between hexavalent and trivalent chromium, the oxidation potential can be increased; this is most readily achieved by introducing an oxidising agent into the regenerant solution. As discussed above, the boundary between trivalent and hexavalent chromium for aqueous chromium favours trivalent chromium hydroxide at lower oxidation potentials. As such, an oxidising agent should shift the balance back to soluble hexavalent chromium.

The oxidising agent added may be hydrogen peroxide, ozone, or hypochlorite, for example, in the form of hypochlorous acid or a salt thereof.

Hydrogen peroxide, ozone and hypochlorite are mild oxidising agents which, in small concentrations will readily alter the oxidation potential of the regenerant by a sufficient degree to render the equilibrium balance to favour hexavalent chromium instead of the insoluble trivalent chromium hydroxide.

In a preferred embodiment, the ion exchange material may be a strong base anion 5 (SBA) exchange resin.

SBA exchange resins are effective ion exchange materials for chromium removal. Traditional techniques which have relied on weak base anion exchange for removal require significant volumes of acid to reduce the pH of the water prior to treatment and have a tendency to remove a large amount of other anions from the processed water, and therefore the process produces a significant volume of waste, and therefore cost. SBA exchange resins do not require pH adjustment of the water prior to treatment, which significantly reduces the operational costs of the process.

Preferably, during step a], the salt solution of the regenerant may have a concentration of at least 2M.

Optionally, during step c], at least 2.5 bed-volumes (BV) of the regenerant may be passed through the ion exchange bed, and more preferably, at least 3 to 3.5 bed volumes of the regenerant may be passed through the ion exchange bed.

For reference, one bed volume is the volume of ion exchange resin contained within the ion exchange vessel. In one particular set-up, a fractionated set-up may be provided in which only a proportion of the regenerant is provided so as to be pH-treated. It is preferable to introduce such a volume of regenerant following the removal of the majority of the hexavalent chromium using untreated brine solution.

In one preferable embodiment, the regenerant may comprise at least one first fraction of an untreated salt solution and at least one second fraction of the said salt solution, and wherein, during step c], the at least one first and second fractions of the regenerant are passed through the ion exchange bed sequentially.

By fractioning the salt solution passed through the ion exchange bed, it is possible to cleanse the SBA exchange resin of the vast majority of the hexavalent chromium using the traditional method, without pH-adjusting or changing the oxidation potential of the regenerant. Instead, a second, modified fraction of salt solution can be passed through the ion exchange bed subsequent to one or more initial fractions specifically so as to target the trivalent chromium hydroxide which blinds some of the binding sites of the

SBA exchange resin.

Preferably, the at least one second fraction may be passed through the ion exchange bed either before or after the or each first fraction has been passed through the ion exchange bed. Furthermore, the at least one second fraction may comprise no more than 1 bed volume of regenerant. Alternatively, the proportion of the second fraction may be between 1 bed volume and the total volume of regenerant.

In an alternative embodiment of the invention, the entire volume of regenerant may comprise the said salt solution which is treated.

It will be appreciated that the user is able to only modify a small proportion of the regenerant to be passed through the ion exchange bed, which may have significant cost and/or effi ci ency b enefits.

The use of a salt solution to remove trivalent chromium hydroxide from an ion exchange material loaded with chromate ions, wherein the salt solution has a pH of between 9.1 and 9.3.

According to a second aspect of the invention, there is provided a use of a salt solution 20 to remove trivalent chromium hydroxide ions from an ion exchange material loaded with chromate ions, wherein the salt solution has a pH of between 9.1 and 9.3.

The term loaded is intended to refer to an ion exchange material in which at least some of the functional sites thereof are bound to chromate ions, and does not necessarily reflect whether a predetermined percentage of the functional sites are bound.

Preferably, the ion exchange material may be a strong base anion exchange resin.

Optionally, the salt solution may comprise sodium chloride or potassium chloride, in which case, the salt solution may comprise from 7.4 to 12.0 % w/v of sodium chloride or potassium chloride.

In one embodiment, the pH may be increased by adding a metal hydroxide. Said metal hydroxide added may be an alkali metal hydroxide or an alkali earth metal hydroxide. Furthermore, the metal hydroxide may be any one of: sodium hydroxide; potassium hydroxide; lithium hydroxide; magnesium hydroxide; and/or calcium hydroxide; or a mixture thereof.

The present disclosure will now be described further. In the following passages, different aspects or embodiments of the disclosure are defined in more detail. Each aspect or embodiment so defined may be combined with any other aspect(s) or embodiment(s) unless clearly indicated to the contrary. In particular, any feature indicated as being preferred or advantageous may be combined with any other feature or features indicated as being preferred or advantageous.

The present invention relates to a method for regenerating an ion exchange material used in the removal of chromate ions from a water supply. As used herein, the term “chromate ions” includes any hexavalent chromium species present in aqueous solution. These include hydrated and non-hydrated forms of CrCE²', HCrOT, CnCE²', HCnCE', and mixtures of two or more thereof.

The method comprises the regeneration of an ion exchange material. An “ion exchange material” or “ion exchange resin” is an insoluble material or support structure normally in the form of small beads, typically having a diameter of around 0.5 to 1.0mm, fabricated from a, preferably organic polymer, substrate. The beads may typically be porous, providing a high surface area. The process of ion exchange involves passing a solution through a resin, such that the ions present in the solution displace ions that initially form part of the resin. For example, the order of selectivity of an SBA exchange resin is Cl', HCO3', NO3', and SO4²'. Nitrate selective resins are also known, in which the selectivity of sulphate and nitrate is reversed. Both SBA exchange resins and nitrate selective resins may be regenerated by elution with, for example, brine (concentrated sodium chloride or potassium chloride solution), as the chloride ions are able to displace HCO3', NO3', and SO/'ions adsorbed on the resin.

The term “ion exchange” material as used in the context of the present application refers to an ion exchange material contained within an ion exchange bed, such as an ion exchange column. The ion exchange material may be present preferably in a fixed bed, but may alternatively be in a moving bed or fluidized bed.

The ion exchange resin may be an SBA exchange resin. Examples of suitable SBA exchange resins are Purolite A600E/4149 supplied by Purolite International Limited,

Amberlite (TM) PWA7 supplied by Rohm & Haas Limited, Resintech SGB1 and Resintech SGB2 both supplied by Resintech, Inc., and Lewatit ASB 1 supplied by Lanxess Deutschland GmbH.

The term “ion exchange effluent” as used in the context of the present application refers to the waste solution obtained from an ion exchange column outlet upon elution of the ion exchange material with a regenerant. Preferably, the ion exchange effluent comprises sodium chloride or potassium chloride. As explained above, chloride ions are able to displace ions adsorbed onto an ion exchange resin, thus regenerating the resin and providing an effluent containing ions that were previously adsorbed onto the resin.

As will be demonstrated in the Examples below, a greater proportion of the chromate ions adsorbed onto the ion exchange resin can be eluted in the ion exchange effluent by the present method when compared with existing regeneration methods.

Many techniques are known in the art for the determination of total chromium in a sample, including Inductively Coupled Plasma Mass Spectrometry (ICPMS), absorption spectroscopy, emission spectroscopy, X-ray fluorescence, and neutron activation analysis. For determining chelated chromium or the hexavalent or trivalent form only, such methods as gas chromatography (with various detection techniques), liquid chromatography (LC), polarography, and spectrophotometry can be used. Preferably, hexavalent chromium is determined by LC-ICPMS. Alternatively, Environmental Protection Agency method 218.6 may be used (“Determination of Dissolved Hexavalent

Chromium in Drinking Water, Groundwater and Industrial Wastewater Effluents by Ion Chromatography”).

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

Figure 1 shows a graph indicating the chromate concentration in water treated by an ion exchange column versus a cumulative flow of regenerant passed through the ion exchange column in mega-gallons, indicating the chromate concentration both before and after pH adjustment of a regenerant

Figure 2 shows an ion exchange bed regeneration system comprising a plurality 5 of regeneration tanks so as to provide a sequential regeneration process;

Figure 3 shows a graph indicating the chromate concentration in the ion exchange effluent with respect to the bed volume of regenerant passed through the ion exchange material, both with and without pH adjustment; and

Figure 4 shows an enlarged section of the graph of Figure 3 centred around 2.6 10 bed volumes indicating the second chromate elution peak.

The present disclosure will now be described in relation to the following non-limiting examples.

Example 1

An ion exchange column or similar ion exchange bed is loaded with a strong base anion 15 resin for hexavalent chromium removal. Regeneration of the strong base anion resin uses approximately 3 bed volumes of a strong salt solution, typically 7.4 to 12% solution by weight of sodium chloride or potassium chloride. It will, however, be appreciated that stronger or weaker salt solution could be used.

After repeated regenerations using this method, the strong base anion resin exhibited a 20 significantly decreased capacity for hexavalent chromium, leading to higher hexavalent chromium leakage from the ion exchange column, eventually up to a level which was above the desired compliance level.

In this embodiment of the invention, the regenerant may be supplied from a tank or reservoir which is in fluid communication with the ion exchange bed. If a single tank is provided, then the entire regenerant would be provided as an adjusted or modified regenerant.

Instead of regenerating the ion exchange column using the standard strong salt solution, a regenerant is prepared which comprises a salt solution having a modified pH prior to performing the regeneration. The pH of the salt solution is increased by the addition of an alkali metal hydroxide or alkali earth metal hydroxide, such as any of sodium hydroxide (NaOH), potassium hydroxide (KOH), lithium hydroxide (LiOH), magnesium hydroxide (Mg(OH)2), and calcium hydroxide (Ca(OH)2), or a mixture of the above. Other alkaline substances could alternatively be introduced to increase the pH, such as a buffer solution. Preferably, the pH is modified so as to be in a range of 9.1 to 9.3, although higher pHs may also achieve a similar result. Use of modified pH to be greater than 10 is not recommended since this will likely lead to degradation of the SB A exchange resin over time.

Typically, pH adjustment is achieved by dosing in a solution of a metal hydroxide to achieve the desired pH. For example, if a 3M solution of sodium hydroxide is used, around 200mL of 3M sodium hydroxide solution would be added to 1000L of 2M brine solution. The exact amount will, of course, be dependent upon the water quality due to the buffering effect, for example, based on the concentration of bicarbonate ions present in the water used to produce the 2M brine solution. A typical ion exchange resin used is A600/5149, supplied by Purolite Ltd. Flow rates during the regeneration with the modified pH brine solution would typically be of the order of lBV/hour, to allow the relatively slow reaction kinetics to occur.

Under normal circumstances, the pH of raw water typically ranges from 7 to 8, with the oxidation potential (Eh) being around 400 to 500mV relative to the standard hydrogen electrode. This is close to the equilibrium change where trivalent chromium hydroxide is the stable species, rather than hexavalent chromium. Local conditions around functional groups on the ion exchange resin will likely result in a proportion of the hexavalent chromium being converted to trivalent chromium hydroxide. The trivalent chromium hydroxide is insoluble, blinding the strong base anion resin to the hexavalent chromium, thereby reducing the efficiency of the regeneration process.

The modification of the pH of the salt solution solubilises the trivalent hexavalent chromium hydroxide which is blocking the functional groups of the ion exchange column, converting it back to hexavalent chromium which can be extracted in the ion exchange effluent during the regeneration process.

This effect can be seen in Figure 1. The chromium concentration in the treated water is illustrated with respect to a total cumulative flow across an ion exchange column, in mega-gallons, across many regeneration cycles.

When using a regenerant comprising pH-unmodified salt solution, as was the case for 5 the first 330 Mgals, there is a distinct upward trend for the residual level of chromium in the treated water. However, after the pH of the regenerant is modified to 9.3, the upward trend is reversed, and the level of residual chromium in the treated water has been declining consistently pH-modification was introduced, at or after 340 Mgals of regenerant had passed through the ion exchange column. The trial period of pH modification was ended on or around the passage of 340 Mgals through the ion exchange, and a clear upswing in the residual level of chromium in the treated water can once again be seen.

The initial data suggests that whereas using pH-unmodified salt solution has a regeneration efficiency of between 80% and 90% of releasing chromium trapped in the ion exchange material, using pH-modified salt solution results in a regeneration efficiency of at least 95%, and seemingly close to 100% regeneration efficiency, which is a significant improvement over the state of the art. As such, the need to replace the ion exchange material over time is reduced, and potentially eliminated.

Example 2

An ion exchange column is loaded with a strong base anion exchange resin for hexavalent chromium removal. Regeneration of the strong base anion exchange resin uses approximately 3 bed volumes of a strong salt solution, typically 7.4 to 12% solution by weight of sodium chloride or potassium chloride.

In this instance, the regenerant is divided into at least two fractions: a first fraction or a plurality of first fractions which comprise a strong salt solution which is not pH-treated; and at least one second fraction which comprises a strong salt solution which is pHtreated, preferably so as to be in the range 9.1 to 9.3, and most preferably 9.2.

An ion exchange bed regeneration system is shown in Figure 2, indicated globally at 100. The ion exchange bed regeneration system 100 comprises an ion exchange bed 102, here formed as an ion exchange column, within which is contained the strong base anion exchange resin.

The ion exchange bed 102 is fed by a plurality of regenerant tanks 104 which are in fluid communication with the ion exchange column 102. The ion exchange column 102 is then connected to one or more outlet tanks 106, into which effluent solution can drain.

Each regenerant tank 104 may preferably, but not necessarily exclusively, be arranged so as to contain half a bed volume of a regenerant solution, and each regenerant tank 104 may be activatable in sequence. Regenerant tanks 104 having said volume are used in this embodiment, but it will evidently be possible to provide tanks of different volumes. Six regenerant tanks 104 are shown, but any number of regenerant tanks 104 may be provided in fluid communication with the ion exchange column 102. The six regenerant tanks 104 may be provided with a regenerant which is suitable for chromium elution, but more regenerant tanks may be provided for other tasks, such as regenerant tanks comprising weak brine solution for the removal of, for example, sulphate and bicarbonate ions from the ion exchange column 102.

In a conventional regeneration, such as that described in the first example, all of the regenerant would be passed through the ion exchange column having been pH-treated. However, in the present example, only one of the regenerant tanks 104 contains a pHmodified or otherwise modified regenerant solution.

Firstly, one or more first fractions of regenerant may be passed through the ion exchange column 102; each first fraction is here an untreated brine solution. Subsequently, at least one second fraction of regenerant which comprises pH-treated brine solution may be passed through the ion exchange column 102. Whilst it may be preferable in some circumstances to utilise untreated brine solution before the treated brine solution, it is also possible to pass the regenerant through the ion exchange column 102 such that treated brine passes through first, thereby converting the trivalent chromium hydroxide to soluble hexavalent chromium hydroxide in advance of the passing of untreated brine through the ion exchange column 102.

Figure 3 shows a graph of the concentration of chromium in the ion exchange effluent against the bed volume passed through for a standard brine solution, shown using triangular points and the dashed line, and for a regenerant comprising the passage of two first unmodified fractions of brine solution, one bed volume each, followed by one bed volume of a second brine solution comprising pH-modified salt solution having a pH of 9.3, indicated by the circular points.

The chromium removed in the initial bed volumes, that is less than 2.5 bed volumes, is as would be expected for an untreated brine solution, since this will be what passes through the ion exchange column 102 in the first instance. However, as can be particularly seen in Figure 4, there is a secondary chromium elution peak centred on or around 2.6 bed volumes which is only present for the pH adjusted salt solution which is passed through in the second fraction. This strongly implies that the unmodified regenerant is not capable of eluting all of the chromium in the ion exchange material, which is leading to the drop in ion exchange efficiency over time.

The approximately 3 to 3.5 bed volumes of regenerant which is passed through the ion exchange column is formed from the or each first fraction and the or each second fraction. At least one first fraction is applied to the ion exchange column prior to the application of the or each second fraction, and is designed to elute the majority of the hexavalent chromium which is present and bound as hexavalent chromium in the strong base anion exchange resin.

Once a majority of the hexavalent chromium, and preferably at least 90% of the hexavalent chromium, has been removed from the ion exchange resin, then a second fraction can be applied to the ion exchange material.

This demonstrates how a sequential regeneration of the ion exchange material may be performed. There are, however, several types of sequential regeneration processes which could be performed. A standard maintenance regeneration might comprise a plurality of bed volumes of untreated brine solution to be passed through the ion exchange column 102, with a single bed volume or less of pH-modified brine solution being passed through the ion exchange column 102, preferably as a final step so as to remove the trivalent chromium hydroxide by conversion to soluble hexavalent chromium ions. This might be the more common regeneration process.

It may also be preferable to provide a mechanism of fully recovering the ion exchange material by increasing the proportion of the regenerant which comprises the second fraction, that is, comprising the pH-modified brine solution. The proportion of pHmodified solution may be increased from anywhere between one bed volume to the total volume of regenerant passed through the ion exchange column 102. This could be considered to be a full recovery stage of regeneration, and may be performed less frequently than the maintenance regeneration.

It is noted that whilst the terms first and second fraction are used to refer to the untreated and treated strong brine solutions respectively, it may be relatively common for the sequential regeneration to utilise other fractions of regenerant for the removal of other contaminants in the water. One or more third fractions could therefore be provided, typically comprising a weaker brine solution than that used for chromium elution, for the removal of other ions in the water. Typically, the or each third fraction would be passed through the ion exchange column 102 before the first and second fractions used for chromium elution.

Example 3

In this instance, the regenerant comprises at least one fraction which includes a strong salt solution which is treated with an oxidising agent so as to increase the oxidation potential (Eh) of the regenerant.

The modification of the Eh of the salt solution solubilises the trivalent hexavalent chromium hydroxide which is blocking the functional groups of the ion exchange column, converting it back to hexavalent chromium which can be extracted in the ion exchange effluent during the regeneration process.

In order to convert precipitated chromium hydroxide to soluble hexavalent chromium, it would seem prudent to raise the oxidation potential by 50 to lOOmV only. Increasing Eh by more than this amount would seem likely to lead to irreversible damage to the ion exchange resin.

It will be apparent to the skilled person that it may be possible to simultaneously alter both the pH and the oxidation potential of the regenerant in order to arrive at the desired invention.

It is therefore possible to provide a mechanism by which an ion exchange material, such as a strong base anion ion exchange resin, can be regenerated following the removal of chromium ions from a water supply. The mechanism of regeneration is such that the efficiency of the ion exchange material does not depreciate over time. This is achieved by modifying the characteristics of the regenerant applied, preferably by altering a pH, but possibly by altering a oxidation potential of the regenerant so as to shift a balance between trivalent and hexavalent chromium in the ion exchange material in favour of the hexavalent chromium, which is soluble in brine. The method is theorised to effectively solubilise trapped trivalent chromium present on the ion exchange material which is otherwise blocking functional sites of the ion exchange material.

The words ‘comprises/comprising’ and the words ‘having/including’ when used herein with reference to the present invention are used to specify the presence of stated features, integers, steps or components, but do not preclude the presence or addition of one or more other features, integers, steps, components or groups thereof.

It is appreciated that certain features of the invention, which are, for clarity, described in the context of separate embodiments, may also be provided in combination in a single embodiment. Conversely, various features of the invention which are, for brevity, described in the context of a single embodiment, may also be provided separately or in any suitable sub-combination.

The embodiments described above are provided by way of examples only, and various other modifications will be apparent to persons skilled in the field without departing from the scope of the invention as defined herein.

Claims

1. A method of regenerating an ion exchange material for an ion exchange bed, the method comprising the steps of:

a] preparing a regenerant which comprises a salt solution;

5 b] modifying the regenerant by increasing the pH and/or increasing the oxidation potential thereof to promote conversion of trivalent chromium hydroxide to soluble hexavalent chromium ions; and

c] passing the regenerant through the ion exchange bed to convert trivalent chromium hydroxide to hexavalent chromium ions and remove the hexavalent

10 chromium ions from the ion exchange material.

2. A method as claimed in claim 1, wherein, during step a], the salt solution comprises sodium chloride or potassium chloride.

3. A method as claimed in claim 2, wherein the salt solution comprises from 7.

4 to 12.0 % w/v of sodium chloride or potassium chloride.

15 4. A method as claimed in any one of claims 1 to 3, wherein, during step b] the regenerant is modified to increase the pH of the salt solution.

5. A method as claimed in claim 4, wherein the pH is increased by adding a metal hydroxide.

6. A method as claimed in claim 5, wherein the metal hydroxide added is an alkali 20 metal hydroxide or an alkali earth metal hydroxide.

7. A method as claimed in claim 6, wherein the metal hydroxide is any one of: sodium hydroxide; potassium hydroxide; lithium hydroxide; magnesium hydroxide; and/or calcium hydroxide; or a mixture thereof.

8. A method as claimed in any one of claims 4 to 7, wherein the pH is increased to 25 between 9.1 and 9.3.

9. A method as claimed in any one of the preceding claims, wherein, during step b], the regenerant is modified by addition of an oxidising agent to increase the oxidation potential of the regenerant.

10. A method as claimed in claim 9, wherein the oxidising agent is hydrogen 5 peroxide, ozone or hypochlorite.

11. A method as claimed in any one of the preceding claims, wherein the ion exchange material is a strong base anion (SBA) exchange resin.

12. A method as claimed in any one of the preceding claims, wherein during step a], the salt solution of the regenerant has a concentration of at least 2M.

10

13. A method as claimed in any one of the preceding claims, wherein, during step c], at least 2.5 bed volumes of the regenerant is passed through the ion exchange bed.

14. A method as claimed in claim 13, wherein, during step c], at least 3 to 3.5 bed volumes of the regenerant is passed through the ion exchange bed.

15. A method as claimed in any one of the preceding claims, wherein the regenerant 15 comprises at least one first fraction of an untreated salt solution and at least one second fraction of the said salt solution, and wherein, during step c], the at least one first and second fractions of the regenerant are passed through the ion exchange bed sequentially.

16. A method as claimed in claim 15, wherein the at least one second fraction is passed through the ion exchange bed after the or each first fraction has been passed

20 through the ion exchange bed.

17. A method as claimed in claim 15, wherein the at least one second fraction is passed through the ion exchange bed before the or each first fraction has been passed through the ion exchange bed.

18. A method as claimed in any one of claims 15 to 17, wherein the at least one 25 second fraction comprises no more than 1 bed volume of regenerant.

19. A method as claimed in any one of claims 15 to 17, wherein the at least one second fraction comprises between 1 bed volume and the total volume of regenerant.

20. A method as claimed in any one of claims 1 to 14, wherein the entire volume of regenerant comprises the said salt solution which is treated.

21. A use of a salt solution to remove trivalent chromium hydroxide ions from an 5 ion exchange material loaded with chromate ions, wherein the salt solution has a pH of between 9.1 and 9.3.

22. A use as claimed in claim 21, wherein the ion exchange material is a strong base anion exchange resin.

23. A use as claimed in claim 21 or claim 22, wherein the salt solution comprises 10 sodium chloride or potassium chloride.

24. A use as claimed in claim 23, wherein the salt solution comprises from 7.4 to 12.0 % w/v of sodium chloride or potassium chloride.

25. A use as claimed any one of claims 21 to 24, wherein the pH is increased by adding a metal hydroxide.

15

26. A use as claimed in claim 25, wherein the metal hydroxide added is an alkali metal hydroxide or an alkali earth metal hydroxide.

27. A use as claimed in claim 26, wherein the metal hydroxide is any one of: sodium hydroxide; potassium hydroxide; lithium hydroxide; magnesium hydroxide; and/or calcium hydroxide; or a mixture thereof.

Intellectual

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Application No: GB1700039.9 Examiner: Claire Jenkins