TECHNICAL FIELD

The present invention relates to a method of desalinating a salt solution. BACKGROUND

Urbanisation and rapid utilisation of fossil fuels have led to an increase in demand for fresh water and energy. Around 1.2 billion people lack access to clean and safe drinking water currently with an expected even higher demand for clean and safe drinking water in the current century. To address this issue, various desalination technologies have been designed to improve global access to clean and safe drinking water. Common techniques for large scale desalination of sea water to form drinking water include distillation and reverse osmosis. Distillation and reverse osmosis are energy intensive processes.

Ion exchange (IEX) resins have been used for many years in various water treatment related practices. For example, mixed-bed ion exchange resins have been used to remove scale-forming ions such as Ca ²⁺ and Mg ²⁺ from feed water and to produce high quality water (i.e. comparable to distilled water) from tap water. Such resins can also be used for the desalination of fairly concentrated brackish water and even sea water, without the need for high pumping pressures, extensive pretreatment or high thermal energy input. However, utilization of ion-exchange resins on a large scale for desalination of water has been limited by the depletion of the resin and the need for large volumes of acid and base solutions to regenerate the spent resins, limiting the economic viability of the technique.

Ion-exchange resins are insoluble polymers that have bound ions which are able to be exchanged with other ions in solutions which come in contact with them. The resins comprise charged functional groups, with mobile counter-ions of the opposite charge associated with the functional groups. It is the mobile counter-ions which may exchange with other ions of similar charge in an "ion exchange" process. An ion- exchange resin may be referred to as "spent" when the majority of the mobile counter- ions associated with the charged functional groups have been replaced with the other ions of similar charge. During a typical desalination process using an ion-exchange resin, for example, a desalination process to remove NaCl from water, the water passes through (i.e. elutes through) both a cation-exchange resin, in which the mobile counter-ion is exchanged with the cation (e.g. Na ⁺ ) in the water, and an anion- exchange resin, in which the mobile counter-ion is exchanged with the anion (e.g. CI ^" ) in the water. For a typical desalination process for producing drinking water, the mobile counter-ion of the cation-exchange resin is typically H ⁺ and the mobile counter-ion of the anion-exchange resin is typically OH ^" . Typically, the cation- exchange resin and the anion-exchange resin are in the form of beads housed in an ion- exchange column. To regenerate the spent resin, the resin beads are firstly separated into the beads of the cation-exchange resin and the beads of the anion-exchange resin, and each component is then washed separately with a regenerating solution. A regenerating acid solution is used to wash and thereby remove the exchanged cation on the cation-exchange resin. A regenerating basic solution is used to wash and thereby remove the exchanged anion on the anion-exchange resin. Further washing steps (usually using the product water) are then subsequently used to rinse the regenerating solution away from the resin.

Some alternative methods have been investigated to regenerate IEX resins, such as thermal energy (e.g. the Sirotherm™ process), electrical energy (electrodialysis) or mechanical energy (piezodialysis). For example, in the Sirotherm™ process developed by CSIRO, resin beads containing both a weak acid component and a weak base component were formed (using either a physical mixture of a weakly acidic resin and a weakly basic resin, or a resin containing both weakly acidic and weakly basic components), having a substantially reduced ion adsorption capacity at higher temperatures, allowing the resins to be regenerated by heating, e.g. to 60° C to 80° C. This process has only been used to dilute brackish water and is currently not used on a large scale as it requires large energy investment during the heat treatment step.

Furthermore, repeated heating of the ion-exchange resin over numerous cycles was found to decompose the resin. It would be advantageous to provide an alternative method for desalinating salt solutions. It would also be advantageous if at least preferred embodiments of the present invention were to provide a desalination method using an ion-exchange resin in which the ion-exchange resin is able to be regenerated for further use without requiring separation of cation-exchange resin beads from anion-exchange resin beads, the use of separate acid and base regenerant solutions and/or heating of the resin.

SUMMARY OF THE INVENTION

In a first aspect, the present invention provides a method for desalinating a salt solution, the method comprising the steps of:

a) eluting an ion-exchange bed having both cation and anion exchangers with an eluent comprising ammonium bicarbonate to produce an ammonium

bicarbonate-loaded ion-exchange bed; and

b) eluting the ammonium bicarbonate-loaded ion-exchange bed with the salt

solution.

In one embodiment, the ion-exchange bed comprises one or more ion-exchange resins.

In one embodiment, the one or more ion-exchange resins is in the form of a mixed-bed resin.

In one embodiment, the mixed-bed resin is comprised of a mixture of a strongly acidic ion-exchange resin and a strongly basic ion-exchange resin.

In one embodiment, the mixed-bed resin is comprised of a mixture of Amberlite IR 120 and Amberlite IRA 402.

In one embodiment, the mixed-bed resin is comprised of a mixture of a weakly acidic ion-exchange resin and a weakly basic ion-exchange resin.

In one embodiment, the mixed-bed resin is comprised of a mixture of Amberlite IRC 86 and Amberlite IRA 67. In one embodiment, the ion-exchange bed comprises a first ion-exchange resin in a first zone and a second ion-exchange resin in a second zone, wherein mobile ions of the first ion-exchange resin are oppositely charged to mobile ions of the second ion-exchange 5 resin.

In one embodiment, the ion-exchange resin is a polymer comprising both an anion exchanger and a cation exchanger on the same polymer. The anion exchanger and cation exchanger of the polymer may be, for example, strongly acidic and strongly basic o exchangers. Alternatively, the anion exchanger and cation exchanger of the polymer may be, for example, weakly acidic and weakly basic exchangers.

In one embodiment, the eluent comprising ammonium bicarbonate is an aqueous solution of about 0.05 M to about 4 M ammonium bicarbonate. Typically, the eluent is5 at a temperature of less than approximately 40 °C, e.g. about 15 °C to about 40 °C, about 15 °C to about 30 °C, or at about room temperature (e.g. at about 20 °C or about 22 °C).

In one embodiment, the salt comprises a cation selected from Na ⁺ , K ⁺ , Ca ²⁺ , Sr ²⁺ and o Cs ⁺ . In one embodiment the salt is selected from the group consisting of NaCl, KC1,

CaCi ₂ , SrCl ₂ and CsCl.

Eluting the ammonium bicarbonate-loaded ion-exchange bed with the salt solution produces an ammonium bicarbonate solution as the eluate.

5

In an embodiment, the method comprises the further step c) of treating the ammonium bicarbonate solution produced by step b) to remove at least some of the ammonium bicarbonate. 0 In one embodiment, the ammonium bicarbonate solution produced by step b) is treated to remove at least some of the ammonium bicarbonate by heating the ammonium bicarbonate solution to produce NH ₃ gas and CO ₂ gas. In one embodiment, the ammonium bicarbonate solution is heated using a bubble column evaporator (BCE) to produce NH ₃ gas and CO ₂ gas.

In one embodiment, the NH ₃ gas and CO ₂ gas are used to form a subsequent ammonium bicarbonate solution. In one embodiment, the subsequent ammonium bicarbonate solution is used to elute the ion-exchange bed.

In a second aspect, the present invention provides a method for desalinating a salt solution, the method comprising the steps of:

a) eluting an ion-exchange bed having both cation and anion exchangers with ammonium bicarbonate solution to produce an ammonium bicarbonate -loaded ion-exchange bed;

b) eluting the ammonium bicarbonate-loaded ion-exchange bed with the salt solution to produce an ammonium bicarbonate solution and a salt-loaded ion- exchange bed; and

c) treating the ammonium bicarbonate solution produced by step b) to remove at least some of the ammonium bicarbonate.

In some embodiments, the method further comprises repeating steps a) to c).

In one embodiment, the ammonium bicarbonate solution produced by step b) is treated to remove at least some of the ammonium bicarbonate by heating the ammonium bicarbonate solution to produce NH ₃ gas and CO ₂ gas.

In one embodiment, the ammonium bicarbonate solution is heated using a bubble column evaporator (BCE) to produce NH ₃ gas and CO ₂ gas.

In one embodiment, the NH ₃ gas and CO ₂ gas are used to form a subsequent ammonium bicarbonate solution. In one embodiment, the subsequent ammonium bicarbonate solution is used to elute (and thus regenerate) the ion-exchange bed. In one embodiment, the salt solution is saline water. BRIEF DESCRIPTION OF THE FIGURES

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

Figure 1 is a schematic diagram of the regeneration of a mixed-bed system

(comprising Amberlite IR 120, Na ⁺ form and Amberlite IRA 402, CI ^" form) in a fixed column with a continuous flow of an ammonium bicarbonate (AB) solution as described in the Example.

Figure 2 is a schematic representation of the cation-exchange resin and the anion- exchange resin in the mixed-bed resin used in the Example showing the functional groups (SO ₃ ^" for the cation-exchange resin and NR ₃ ⁺ , where each R is an alkyl group, for the anion-exchange resin) in the exhausted, or salt-loaded, state (State 1), and in the regenerated, or ammonium bicarbonate -loaded, state (State 2).

Figure 3 is a graphical representation of the concentration (mol/1) of Na ⁺ , CI ^" and Nl¾ ⁺ in the eluate with the volume of 0.1M ammonium bicarbonate (ml) as the feed solution (eluent) to a NaCl saturated mixed-bed resin at 20°C.

Figure 4 is a graphical representation of the cumulative desorption (mmol) of Na ⁺ , CI ^" and NH ₄ ⁺ in the eluate with the volume of 0.1M ammonium bicarbonate (ml) as the feed solution (eluent) to a NaCl saturated mixed-bed resin at 20°C

Figure 5 is a graphical representation of the concentration (mol/1) of Na ⁺ and CI ^" in the eluate with the volume of 0.1M NaCl (ml) as the feed solution (eluent) to an ammonium bicarbonate-loaded mixed-bed resin at 20°C Figure 6 is a graphical representation of the cumulative desorption (mmol) of Na ⁺ and

CI ^" in the eluate with the volume of 0.1M NaCl (ml) as the feed solution (eluent) to an ammonium bicarbonate-loaded mixed-bed resin at 20°C Figure 7 is a schematic diagram for a complete desalination process using a strong acid/strong base 50:50 mixed-bed resin using ammonium bicarbonate regeneration. DW refers to drinking water. The bubble column evaporation (BCE) process is used to decompose the ammonium bicarbonate product solution.

DESCRIPTION OF THE INVENTION

In a first aspect, the present invention provides a method for treating a salt solution to remove at least some of the salt from the solution. The method comprises a first step of eluting an ion-exchange bed having both cation and anion exchangers with an eluent comprising ammonium bicarbonate to produce an ammonium bicarbonate-loaded ion- exchange bed. The method comprises a second step of eluting the ammonium bicarbonate-loaded ion-exchange bed with the salt solution. Eluting the ammonium bicarbonate-loaded ion-exchange bed with the salt solution produces a solution comprising ammonium bicarbonate as the eluate. The ammonium bicarbonate in the eluate can then be removed, for example, by heating the solution to form NH ₃ gas and C0 ₂ gas.

In order for an ion-exchange bed to desalinate a salt solution comprising a salt such as NaCl, the ion-exchange bed must be able to capture by ion exchange, and thus exchange, both the cation and the anion of the salt. The ion-exchange bed must therefore comprise a cation exchanger and an anion exchanger. The mobile counter cation bound to the cation exchanger exchanges with the cation species to be removed from the solution, and the mobile counter anion bound to the anion exchanger exchanges with the anion species to be removed from the solution. After use, the ion- exchange bed is "salt loaded" and must be regenerated for further use. A "salt loaded" ion-exchange bed may also be referred to as "spent" or "exhausted". Conventionally, a salt-loaded ion-exchange bed is regenerated by separating out the spent cation exchanger and anion exchanger. The spent cation exchanger is typically contacted with a concentrated acid solution to regenerate the cation exchanger. The spent anionic exchanger is typically contacted with a concentrated base to regenerate the anion exchanger. In contrast, the method of the present invention enables the ion-exchange bed to be regenerated for further use in desalinating a salt solution without the need for separating the ion-exchangers. Furthermore, the method of the present invention does not require the ammonium bicarbonate to be removed from the ion-exchange bed prior to the ammonium bicarbonate -loaded ion-exchange bed being used in the desalination of the salt solution. Eluting the ammonium bicarbonate-loaded ion-exchange bed with the salt solution produces a solution comprising ammonium bicarbonate.

Advantageously, the ammonium bicarbonate can be removed from the solution by simple, low cost processes, such as by heating the solution to form NH ₃ gas and CO ₂ gas which are then separated from the solution. As the ammonium bicarbonate can be removed from the solution, it is not necessary to remove the ammonium bicarbonate from the ion-exchange bed prior to the ammonium bicarbonate-loaded ion-exchange bed being used in the desalination of the salt solution. Furthermore, NH ₃ gas and CO ₂ gas recovered from the ammonium bicarbonate solution can be regenerated into an ammonium bicarbonate solution and the resultant ammonium bicarbonate solution used to again elute and regenerate the ion-exchange bed.

The ion-exchange bed used in step a) may be a salt-loaded ion-exchange bed previously used in a desalination process. The method of the present invention enables such a salt-loaded ion-exchange bed to be regenerated for further use in desalinating a salt solution. The ion-exchange bed used in step a) may alternatively be an ion- exchange bed as initially manufactured or supplied by a supplier which requires treatment to replace the mobile-ions of the bed as manufactured or supplied with mobile ions suitable for use in a desalination method.

Ion-exchange bed

As used herein, the term "ion exchange" broadly refers to an exchange of ions between two electrolytes or between an electrolyte solution and a solid or gel (e.g. an ion- exchange resin). The ion exchange process can be used to purify, separate, and decontaminate aqueous and other ion-containing solutions with "ion exchangers". As used herein, an "ion exchanger" is a substance capable of exchanging an ion with an ion in solution. Ion exchangers are either cation exchangers that exchange positively charged ions (cations) or anion exchangers that exchange negatively charged ions (anions).

An "ion-exchange bed" is a bed comprising one or more ion exchangers through which an eluent can pass and exchange ions with the ion exchanger. The bed may comprise the one or more ion exchangers in any manner that allows the eluent to pass through the bed and exchange ions with the one or more ion exchangers. The bed typically comprises particles of one or more ion exchangers, e.g. in the form of beads. The ion- exchange bed may be housed, for example, in an ion-exchange column. The ion- exchange bed used in the method of the present invention comprises both cation and anion exchangers, that is, the ion-exchange bed comprises at least one cation exchanger and at least one anion exchanger. The ion-exchange bed may, for example, comprise a uniform mixture of cation exchangers and anion exchangers or may comprise regions or zones comprising different proportions of cation exchangers and anion exchangers. In some embodiments, the ion-exchange bed comprises an amphoteric exchanger, that is, a single substance comprising both a cation exchanger and an anion exchanger.

The cation and anion exchangers may, for example, be ion-exchange resins, zeolites, porous ceramics or a clay, e.g. montmorillonite.

Typically, the cation and anion exchangers are ion-exchange resins. As used herein, the term "ion-exchange resin" broadly refers to an insoluble matrix of an organic polymeric material comprising a charged functional group, where the charged functional group has an exchangeable (or mobile) counter-ion associated with the charged functional group. The functional group may be weakly acidic or strongly acidic (e.g. sulphonates, -SO ₃ ^" ), or may be weakly basic or strongly basic (e.g. quaternary ammonium, -CH ₂ N ⁺ (CH ₃ ) ₃ ). A cation-exchange resin is able to exchange cations with cations in a solution. Such a resin comprises an acidic functional group with an associated cation. An anion- exchange resin is able to exchange anions with anions in a solution. Such a resin comprises a basic functional group with an associated anion. It will be appreciated to those skilled in the art that a reference to a "weakly basic" (or "weak base") ion- exchange resin refers to an ion-exchange resin with a weakly basic functional group, and a reference to a "weakly acidic" (or "weak acid") ion-exchange resin refers to an ion-exchange resin with a weakly acidic functional group. Similarly, it will be appreciated to those skilled in the art that a reference to a "strongly basic" (or "strong base") ion-exchange resin refers to an ion-exchange resin with a strongly basic functional group, and a reference to a "strongly acidic" (or "strong acid") ion-exchange resin refers to an ion-exchange resin with a strongly acidic functional group.

To preserve the electrical neutrality of an ion-exchange resin, a mobile counter-ion is associated with each charged functional group. The mobile counter-ion may exchange with other ions of similar charge in an "ion exchange" process. An ion-exchange resin may be referred to as "spent" (or "exhausted" or "salt loaded") when the majority (more than 50%, e.g. more than 70%, 80% or 90%) of the mobile counter-ions associated with the charged functional groups have been replaced with ions from the eluent.

Regeneration of a spent ion-exchange resin may be achieved by reversing the ion exchange reactions referred to above. Conventionally, a spent ion-exchange resin is regenerated by eluting the spent ion-exchange resin with a relatively concentrated solution of the original mobile counter-ions.

The ion-exchange bed may comprise one or more ion-exchange resins. In some embodiments, the one or more ion-exchange resins are in the form of a mixed-bed resin. A mixed-bed resin is a resin bed in which separate particles of a cation-exchange resin and an anion-exchange resin, usually in the form of beads, are mixed together. That is, particles of a cation-exchange resin are mixed with particles of an anion- exchange resin to form a mixed resin bed or mixed-bed resin. In some embodiments, the mixed-bed resin comprises a uniform mixture of the particles of the cation- exchange resin and anion-exchange resin. However, as one skilled in the art will appreciate, a mixed-bed resin may comprise a mixture of a cation-exchange resin and an anion exchange resin where the mixture comprises different relative proportions of the cation-exchange resin and the anion exchange resin in different regions of the mixed-bed resin. As one skilled in the art will appreciate, an ion-exchange bed comprising a cation-exchange resin and an anion-exchange resin may alternatively comprise the cation-exchange resin and the anion-exchange resin in configurations other than a mixture of the resins. For example, the cation-exchange resin and anion- exchange resin may be provided in series, e.g. a cationic resin zone or unit followed by an anionic resin zone or unit, and vice versa.

In some embodiments, the ion-exchange bed comprises a mixed bead resin. Mixed bead resins are beads of a resin which comprise both an acidic functional group and a basic functional group, with an associated cation and anion, respectively, on the same bead, that is, the bead comprises both a cation-exchanger and an anion-exchanger. Examples of mixed bead resin are given, for example, in Chandrasekara, N.P.G.N. and R.M. Pashley, Study of a new process for the efficient regeneration of ion-exchange resins. Desalination, 2015. 357(0): p. 131-139.

Typically, the ion exchange bed is in the form of a mixed-bed resin comprising particles of a cation-exchange resin and particles of an anion-exchange resin.

An ion-exchange resin typically comprises a partially cross linked aliphatic polymer, such as cross-linked polystyrene, comprising charged functional groups having an exchangeable counter-ion associated with the functional group. In some embodiments, the resin polymer comprises one or more additional copolymers. The copolymers may include, but are not limited to, butadiene, ethylene, propylene, acrylonitrile, styrene, acrylic, vinylidene chloride, vinyl chloride, and derivatives and mixtures thereof. The charged functional groups can be an integral part of the monomer or can be incorporated into the polymer after polymerisation.

The ion-exchange resin may be provided in any shape and size, including beads, rods, disks or combinations of more than one shape. The ion-exchange resin may also include a mixture of particle sizes, such as a mixture of large and small particles. In an ion-exchange bed, the ion-exchange resins are typically in the form of beads. Typically, the beads are small (0.2 to 1.5 mm diameter) porous beads with a high surface area.

In some embodiments, the cation-exchange resin is a strongly acidic cation-exchange resin comprising a polystyrene matrix and sulfonic acid functional groups. In other embodiments, the cation-exchange resin is a weakly acidic cation-exchange resin, such as a cross-linked acrylic acid with carboxylic acid functional group, a cross-linked methacrylic acid with carboxylic acid functional group, or mixture thereof.

Various commercially available strongly acidic cation-exchange resins are available, and include but are not limited to: Amberlite® IR 120, Dowex® Mathron C and Dowex® 650C (Dow Chemical Company).

Various commercially available weakly acidic cation-exchange resins are available, and include but are not limited to: Amberlite® IRC 86 and Dowex® MAC-3 (Dow Chemical Company).

In some embodiments, the anion-exchange resin is a strongly basic anion-exchange resin comprising quaternary amino groups, for example, trimethylammonium groups, e.g. poly AMPS. In other embodiments, the anion-exchange resin is a weakly basic anion-exchange resin comprising primary, secondary or tertiary amine groups, e.g. polyethylene amine.

Various commercially available strongly basic anion-exchange resins are available, and include but are not limited to: Amberlite® IRA 400, Amberlite® IRA 410, Amberlite® IRA 402 and Dowex® Marathon A (Dow Chemical Company).

Amberlite® IRA 402 is a styrene divinylbenzene copolymer, comprising the functional group trimethyl ammonium.

Various commercially available weakly basic anion-exchange resins are available, and include but are not limited to: Amberlite® IRA 67 and Dowex® 66 (Dow Chemical Company).

Examples of resins which could, for example, be used in the method of the invention are given in Table 1 below.

Table 1- Examples of ion-exchange resin which may be used in the method of the invention.

Each of the resins referred to in Table 1, other than Diaion WA30, is produced by Dow Chemical Company. Diaion WA30 is produced by Mitsubishi Chemical Corporation.

The "form" of the resin referred to in Table 1 is the form of the resin as supplied by the manufacturer. The resins may be used in the form supplied by the manufacturer or the resins may be associated with another counter ion.

Step (a) eluting with an eluent comprising ammonium bicarbonate

The method comprises a first step of eluting the ion-exchange bed with an eluent comprising ammonium bicarbonate (ammonium hydrogen carbonate) to produce an ammonium bicarbonate-loaded ion-exchange bed. The eluent comprising ammonium bicarbonate may be aqueous or non-aqueous. However, the eluent is typically an aqueous solution of ammonium bicarbonate. A reference herein to an "aqueous solution" refers to a solution in which water is the only solvent or is at least 50 % (e.g. at least 80%, at least 90 %, at least 95 %, or at least 99 %) by weight of the total solvents in the solution. Accordingly, a reference to an aqueous solution of ammonium bicarbonate refers to an ammonium bicarbonate solution in which water is the only solvent or is at least 50 % by weight of the total solvents in the solution. An aqueous ammonium bicarbonate solution may comprise a solution of ammonium bicarbonate in water and a water miscible co-solvent, such as methanol or ethanol, provided that water comprises at least 50 % by weight of the solvents present. Preferably, water comprises at least 90 %, at least 95 % or at least 99 %, by weight of the total solvents in the solution.

As the ion-exchange bed is eluted with the eluent comprising ammonium bicarbonate, the ions associated with the ion exchangers exchange with the ions of the ammonium bicarbonate (i.e. Nl¾ ⁺ and HCO ₃ ^" ) of similar charge in an "ion exchange" process. An ion-exchange bed may be referred to as "ammonium bicarbonate loaded" when the majority (more than 50%, e.g. more than 70%, 80% or 90%) of the mobile ions associated with the cation and anion exchangers have been replaced with Nl¾ ⁺ or HCO ₃ ^" ions.

Recent studies showed that certain ion-exchange mixed bead resins can be regenerated using an aqueous ammonium bicarbonate solution followed by thermal treatment. In this process, a concentrated ammonium bicarbonate solution was used as a regenerant for the mixed bead resin and the ammonium and bicarbonate ions were thermally decomposed, in situ, within the mixed bead resin, into ammonia and carbon dioxide gases [see, e.g. Chandrasekara, N.P.G.N. and R.M. Pashley, Study of a new process or the efficient regeneration of ion-exchange resins. Desalination, 2015, 357(0): p. 131-139] . In contrast to the process described in that document, the method of the present invention does not require heating of the ion exchangers. Further, to the best of the inventors' knowledge, ammonium bicarbonate has not previously been used to regenerate mixed-bed resins.

Preferably the eluent comprising ammonium bicarbonate is an aqueous solution comprising a concentration of ammonium bicarbonate of about 0.05 M or greater. The eluent comprising ammonium bicarbonate may, for example, be an aqueous solution comprising about 0.05 M to about 4.0 M ammonium bicarbonate. In some embodiments, the eluent comprising ammonium bicarbonate is an aqueous solution comprising about 0.05 to 2.0 M ammonium bicarbonate. In some embodiments, the eluent comprising ammonium bicarbonate is an aqueous solution comprising about 0.05 to 1.0 M ammonium bicarbonate. In some embodiments, the eluent comprising ammonium bicarbonate is an aqueous solution comprising about 0.05 to 0.5 M ammonium bicarbonate. Concentrations of ammonium bicarbonate above 0.5 M may cause bubbling of the ammonium bicarbonate solution, which in turn may produce channels inside a mixed-bed resin. This bubbling can, however, be reduced or completely prevented by application of a modest (e.g. < 1 bar) over-pressure acting on the concentrated solution. Concentrations of ammonium bicarbonate below 0.05 M may not be sufficient to effectively remove the ions associated with the ion- exchangers to form an ammonium bicarbonate-loaded ion-exchange bed.

Step (b) - eluting with the salt solution

As used herein the word "salt" is used to refer broadly to an ionic compound which is electrically neutral (i.e. without a net charge) in its solid form, but which dissolves into cations (positively charged ions) and anions (negative ions) in solution. A salt is generally formed from the neutralization reaction of an acid and a base. The component ions of a salt can be inorganic, such as chloride (CI ^" ), or organic, such as acetate (CH ₃ CO ₂ ^" ); and can be monatomic, such as fluoride (F ^" ), or polyatomic, such as sulfate (SO ₄ ^2" ). There are several varieties of salts. Salts that hydrolyze to produce hydroxide ions when dissolved in water are basic salts, whilst those that hydrolyze to produce hydronium ions in water are acidic salts. Neutral salts are those that are neither acid nor basic salts. An ion of the salt may be radioactive, for example, a salt comprising a radioactive isotope of Sr ²⁺ or Cs ⁺ .

Saline water is a general term for water that contains a significant concentration (i.e. >500 ppm) of dissolved salts (typically NaCl). The salt concentration is usually expressed in parts per thousand or parts per million (ppm). Saline water may include, but is not limited to, groundwater, brackish water, sea water, hypersaline water, brine, produced water or process water.

The salt may be any salt (other than ammonium bicarbonate). The salt may, for example, be selected from the group consisting of NaCl, KC1, CaCl ₂ , SrCl ₂ and CsCl.

The salt solution is typically an aqueous solution. Typically water is the only solvent in the salt solution. However, the process of the invention can also be used to desalinate solutions comprising salts dissolved in polar solvents other than water.

In some embodiments, the salt solution comprises one or more cations selected from Na ⁺ , K ⁺ , Ca ²⁺ , Sr ²⁺ and Cs ⁺ . In some embodiments, the salt solution comprises one or more anions selected from CI ^" , F ^" , Br ^" , CH ₃ C0 ₂ ^" , P0 ₄ ^3" and S0 ₄ ^2" .

In some embodiments, the salt solution is an aqueous solution comprising one or more dissolved salts selected from NaCl, KC1, CaCl ₂ , MgSO/ _t , A1 ₂ (SC>4)3, FeCl ₃ , ZnCl ₂ , SrCl ₂ and CsCl.

Eluting the ammonium bicarbonate ion-exchange bed with the salt solution produces an ammonium bicarbonate solution, that is, a solution comprising ammonium bicarbonate (and a salt-loaded ion-exchange bed). The ammonium bicarbonate solution comprises Nt¾ ⁺ and HCO ₃ ^" ions. The ammonium bicarbonate can readily be removed from the ammonium bicarbonate solution as described below to form a solution having a lower salt concentration than the salt solution. Step (c)-treatment of the ammonium bicarbonate solution

Advantageously, ammonium bicarbonate may readily be removed from the ammonium bicarbonate solution produced by step b) by heating the ammonium bicarbonate solution to produce NH ₃ gas and CO ₂ gas. For example the ammonium bicarbonate solution can be heated by heating the solution to about 60°C to produce NH ₃ gas and CO ₂ gas. The NH ₃ gas and CO ₂ gas, being in gaseous form, can then be easily separated from the solution.

In some embodiments, the ammonium bicarbonate solution produced by step b) is heated using a bubble column evaporator (BCE) to remove the ammonium bicarbonate by converting the ammonium bicarbonate into NH ₃ gas and CO ₂ gas. In a BCE, bubbles of a gas are passed through a solution. Heated gas may be used so that as the hot bubbles pass through the solution, they convert some of the ammonium bicarbonate into NH ₃ gas and CO ₂ gas and also capture and remove at least some of the formed NH ₃ gas and CC> ₂ gas from the solution. The BCE process can be used as a quick and efficient manner to convert the ammonium bicarbonate into NH ₃ gas and CO ₂ gas and remove the NH ₃ gas and C0 ₂ gas from the solution. In this process, ammonium bicarbonate is decomposed into NH ₃ and CO ₂ gases at temperatures of about 60 °C. The use of a BCE to convert ammonium bicarbonate in a solution into NH ₃ and CO ₂ and remove the NH ₃ and CO ₂ from the solution is described in M. Shahid, X. Xue, C. Fan, B. W. Ninham and R. M. Pashley, Study of a Novel Method for the Thermolysis of Solutes in Aqueous Solution Using a Low Temperature Bubble Column Evaporator. J. Phys. Chem. B. 1 19 (25), 8072-8079 (2015). BCEs have also been used to capture water vapor in a desalination process as described in Shahid, M.; Pashley, R. M., A Study of the Bubble Column Evaporator Method for Thermal Desalination. Desalination 2014, 351, 236- 242.

The ammonium bicarbonate solution may be heated in the presence of air or another gas. In such embodiments, the formed NH ₃ and CO ₂ gases are mixed with air or other gases. The resulting mixture of gases, comprising NH ₃ and CO ₂ , can be used to form a further ammonium solution without separation of the gases by, for example, passing the mixture of gases through water or an aqueous solution at a temperature of about room temperature or lower, resulting in the NH ₃ and CO ₂ dissolving in the water to form an ammonium bicarbonate solution. In some embodiments, the NH ₃ gas and CO ₂ gas may be separated from the mixture of gases, for example, in order to store the NH ₃ gas and CO ₂ gas for later use to form an ammonium bicarbonate solution. The NH ₃ and C0 ₂ gases may be separated from the mixture of gases using conventional gas separation techniques.

In some embodiments, a hollow-fibre membrane is used to separate the NH ₃ gas and CO ₂ gas from the ammonium bicarbonate solution. In such embodiments, the ammonium bicarbonate solution is heated just prior to entering a hollow-fibre membrane gas exchange unit, where a counter flow air current continuously collects the decomposed gases, NH ₃ gas and CO ₂ gas, on the other side of the hollow-fibre membrane. These gas exchange units are available commercially and use either dense membranes, which allow gases to diffuse through the permeable hollow-fibre membrane, or use porous but hydrophobic hollow-fibre membranes.

Advantageously, the NH ₃ gas and CO ₂ gas can readily be dissolved in water, for example water at about room temperature, e.g. at about 15°C to 25°C, to form an ammonium bicarbonate solution. Typically the NH ₃ gas and CO ₂ gas are dissolved in the water by contacting the NH ₃ gas and CO ₂ gas, optionally in a mixture with air or other gases, with the water, for example, by bubbling the gases through the water or passing the water through an atmosphere of the gases. An elevated pressure may be used to expedite the dissolution of the NH ₃ gas and CO ₂ gas. The ammonium bicarbonate solution can then be used to again regenerate the ion-exchange bed after it has been used in the desalination of the salt solution.

Accordingly, in one aspect, the present invention provides a method for desalinating a salt solution comprising the steps of:

a) eluting an ion-exchange bed with ammonium bicarbonate solution to produce an ammonium bicarbonate-loaded ion-exchange bed; b) eluting the ammonium bicarbonate-loaded ion-exchange bed with the salt solution to produce an ammonium bicarbonate solution and a salt-loaded ion-exchange bed;

c) treating the ammonium bicarbonate solution of step b) to produce NH ₃ gas and CO ₂ gas to thereby remove at least some of the ammonium bicarbonate from the solution; and, optionally,

d) using the NH ₃ gas and C0 ₂ gas of step c) to form an ammonium bicarbonate solution and repeating steps a) to c).

As used herein, the term "desalinating" is broadly used to refer to a process for removing at least some amount of a dissolved salt from a salt solution. Typically, the salt solution is an aqueous solution. Typically, the desired concentration of dissolved salts in desalinated water is at or below a threshold for potable municipal water, although it will be appreciated that this threshold may vary depending on the potential use for the water. For example, higher concentrations of dissolved salts may be tolerated for stock water, irrigation water, or water for use in industry or mining processing. The cyclic nature of the method comprising steps a) to d) referred to above has significant inherent advantages, in that large amounts of acid and base waste regenerants (and waste from subsequent washes) are not generated. Such a cyclic method would be beneficial in the treatment and removal of radioactive ions in solution, for example, in nuclear power waste water.

The methods of the present invention may be used to desalinate brackish water or sea water. The method can be used on a large or small scale. Advantageously, the method does not require the use of high pressures or temperatures, or the use of concentrated acid and base solutions. Reverse osmosis desalination processes can be noisy in practice, presenting a difficulty for use in some circumstances, e.g. on a ship or submarine. In contrast, the method of the present invention can be carried out in a relatively quiet manner.

The invention is further described below with reference to the following non-limiting Example. EXAMPLE

MATERIALS AND METHOD

1.1 Materials

A strong acid resin (Amberlite IR 120, H ⁺ form) and a strong base resin (Amberlite IRA 402, CI ^" form) were used for the mixed-bed system, purchased as the analytical grade from Sigma-Aldrich, Australia. Ammonium bicarbonate (99%) was obtained from May & Baker LTD, Dagenham, England and 99% sodium chloride was obtained from Sigma-Aldrich, Australia.

1.2 Analysis

The concentrations of Na ⁺ , NH _t ⁺ and CI ^" in solutions were analysed through their ionic activity using ion selective electrodes (ISE), HQ440d-Hach.

1.2.1 Ion Exchange studies

1.2.1.1 Resin regeneration

Amberlite IR 120, which was received initially in its H ⁺ form, was converted to the Na ⁺ form by exposing to brine solution, followed by washing with Mili-Q water to remove the excessive NaCl present at the resin. The strong acid resin (Amberlite IR 120, Na ⁺ form) and a strong base resin (Amberlite IRA 402, CI ^" form) samples were taken in roughly 4.2 g and 5.4 g samples, as their wet weight, respectively, to maintain roughly equal IEX capacity within the columns. The resin beads were then mixed and packed into a thin column without trapping any air bubbles, with Mili-Q water up to bed heights of about 14.5 cm.

Regeneration studies for the mixed-bed resins were performed with 0.1 M NH4HCO3 (AB) solutions (at pH 8.5) at room temperature 20°C, in a continuous flow system at a rate of about lOml/hr (see Figure 1). Here, the AB solution was passed through the column from the top, and each 10 ml of eluate was collected and tested for the level of Na ⁺ , NHt ⁺ and CI ^" until the values of Na ⁺ and CI ^" were reduced to levels near to the Mili-Q water, and close to that of the 0.1M NH4HCO3 feed solution. 1.2.1.2 Desalination

The mixed-bed systems regenerated by AB (refer Figure 2, State 2), was then exposed from the top to 0.1M NaCl solutions (at pH 5.6) at 20°C, in a continuous flow system at a rate of lOml/hr. The eluate was collected in 10 ml samples and tested for the level of Na ⁺ , NHt ⁺ and CI ^" until it reached the values of Na ⁺ and CI ^" close to drinking quality water, as per the guidelines of the World Health Organisations (WHO). All of the chemical analyses were performed at least twice to ensure the accuracy of the results.

RESULTS

2.1 Mixed-bed resins behaviour for salts

2.1.1 Ion exchange behaviour with NH4HCO3

The IEX behaviour of the mixed-bed, SA/SB IEX resins was studied with continuous flow in a fixed column. The eluted amount of Na ⁺ , CI ^" and NHt ⁺ in the first IEX cycle demonstrates the regeneration behaviour of these mixed-bed systems, as given in Figure 3 and Table 2.

The cumulative values of Na ⁺ and CI ^" desorbed from the resins showed that 100 ml of 0.1M AB was consumed during the regenerating process of the resin (see Figure 4). The volume of AB use for the regeneration can be reduced by increasing its concentration if, necessary. Concentrated AB solutions, even at room temperature, can form decomposition bubbles, especially when exposed to foreign substances and even ion exchange materials [Chandrasekara, N. and R. Pashley, Enhanced ion exchange capacity of polyampholytic resins. Sep. Purif. Technol., 2016. 158: p. 16-23]. This can create pore expansions if the AB is present inside the porous matrix of the material. Bubbling of concentrated AB solutions can produce channels inside the mixed-bed resin, thus, it is important to maintain the concentration of AB less than 0.5M at atmospheric pressure, although larger beds operated at higher pressures suppress these effects and so more concentrated AB solutions could then be used.

These results clearly demonstrate that AB has the ability to efficiently regenerate these mixed-bed resins, following exhaustion by Na and CI ^" , even with mild

concentrations, such as with 0.1M AB. Therefore, AB is a good alternative regenerant for the recovery of exhausted resins during the desalting processes.

Table 2- Amount of Na , CI ^" and NH4 in each 10 ml of eluant sample during the regeneration of exhausted mixed-bed resins and cumulative millimoles (Cum.mmol) eluted.

2.1.2 Ion exchange behaviour with NaCl

When the AB regenerated resin was used for the desalting of 0.1M NaCl solution, it typically recovered about 60 ml of drinking water product from about 9.6 grams of resin (See Figure 5 and Table 3). This basic process can be applied to desalination in a continuous process, to produce water containing AB (at a pH of about 8.2) which can be purified with heating, e.g. heating using the low-cost, low energy bubble column evaporator (BCE) method [Shahid, M., et al., Study of a Novel Method for the Thermolysis of Solutes in Aqueous Solution Using a Low Temperature Bubble Column Evaporator. The Journal of Physical Chemistry B, 2015. 119 (25): p. 8072-8079].

5

Also, the decomposition gases produced in the BCE process (or by other processes) can then be used to regenerate concentrated AB solutions for further re-use.

l o Table 3 - Amount of Na , CI ^" and NH4 in each 10ml of eluant sample during the

desalting process with regenerated AB mixed-bed resins.

* Indicates the maximum allowable level of sodium (Na ⁺ ) in the sample for drinking purpose 15 according to water quality guidelines.

2.2 Selectivity of ions for the mixed-bed SA/SB resins

IEX selectivity coefficients for various cations are quite diverse and Na ⁺ typically has higher selectivity compared to the NH ion. However, during the IEX process, the law of mass action plays a dominant role, especially for strong acid and strong base resins, thus replacing Na ⁺ by N¾ ⁺ is readily achieved and is completely reversible, as confirmed by the results obtained in this study. Further, the IEX selectivity coefficients for anions are different and CI ^" typically has stronger binding than HCO ₃ ^" [DOWEX™ Ion Exchange Resins: Using Ion Exchange Resin Selectivity Coefficients. Technical Information: 1-3]. However, again the mass action law supports the regeneration process of the strong acid/strong base mixed-bed resins, even though the relative ion selectivity coefficients facilitate the desalting process.

2.3 A complete desalting process

The properties shown by the AB regenerated strong acid/strong base mixed-bed resins demonstrate that continuous flow column processes could be used to desalt NaCl solutions, at relatively high concentrations (e.g. 0.1 M or higher), producing a product solution of ammonium bicarbonate (AB). This solution can be readily decomposed at modest temperatures to produce desalted water. The gases produced, ammonia and carbon dioxide, can then be recollected in cold water to re-form a concentrated AB solution. The method of the invention can therefore be used to provide a complete and continuous desalination process, which does not require heating of the resin, separation of the resin, or exposure of the resin to strong acids and bases during regeneration. An example of such a process is described schematically in Fig. 7. This process combines the purification of saline water to drinking water together with regeneration of exhausted resins, through a recycling process, with low chemical waste. In this process, the aqueous salt solution comprising dissolved NaCl (Vi NaCl feed (aq)) is eluted through the ammonium bicarbonate-load mixed-bed ion- exchange resin (AB Resin) to form an ammonium bicarbonate solution (Vi DW + AB (aq)) and a NaCl-loaded resin (NaCl Resin). The resultant ammonium bicarbonate solution may be heated, for example in a BCE, to form CO ₂ (gas) and NH ₃ (gas) which are separated from the remainder of the solution, thus resulting in desalted drinking water (Vi/2 DW product). The CO ₂ (gas) and NH ₃ (gas) can then be used to form a concentrated ammonium bicarbonate solution (Vi/2 Cone. AB (aq)), for example, by dissolving the CO ₂ (gas) and NH ₃ (gas) in water at a temperature of about 20°C or lower, optionally at above atmospheric pressure. The resultant concentrated ammonium bicarbonate solution may then be used to regenerate the NaCl-loaded resin forming a concentrated NaCl-waste solution and a regenerated ammonium bicarbonate-loaded resin.

The present invention therefore enables the removal of salts from saline water using commercial acid and base resins combined together in a single column. Ammonium bicarbonate solutions can be used as a regenerant solution throughout this process without the need for separating or heating of the resins, which offers an effective process for continuous desalination together with the production of drinking quality water. The complete desalting process could be widely used commercially because it does not depend on the costly consumption of acid and base solutions for resin regeneration. A further advantage is that the resins are never exposed to concentrated acids or bases, so increasing their operating life.

It is to be understood that, if any prior art publication is referred to herein, such reference does not constitute an admission that the publication forms a part of the common general knowledge in the art, in Australia or any other country.

In the claims which follow and in the preceding description of the invention, except where the context requires otherwise due to express language or necessary implication, the word "comprise" or variations such as "comprises" or "comprising" is used in an inclusive sense, i.e. to specify the presence of the stated features but not to preclude the presence or addition of further features in various embodiments of the invention.