Metal Imprinted polymers for selective pick up of Cobalt

Technical field of the invention

The present invention relates to an imprinted polymer selective for cobalt ions in the presence of iron ions, a process for preparation of said imprinted polymer, and use of said imprinted polymer.

Technical background

A variety of structural materials such as carbon steel, monel, Incoloy, stainless steel and stellite are employed in the cooling water systems of power plants. These materials interact with the coolant at high temperature and form deposits of metal oxides. This problem becomes severe in case of nuclear power plants as the metal oxide deposits may have trapped radioactive metal ions in the oxide lattice which leads to the build up of radioactivity, thereby necessitating regular clean up campaigns. During the clean up operation, which is generally done by continuous circulation of complexing chemical formulations, the metal ions are brought out into the solution and trapped by the ion-exchange resin beds that form a part of the clean up circuit. As the ion-exchange resins do not have high selectivity towards particular metal radioactive ions of interest the resin bed traps all the metal ions. For example in nuclear reactor decontaminations (R1 , R2), large quantity of ferrous ion which is non-radioactive, is released along with small quantity of radioactive cobaltous ions. Since the ion exchange resins currently used do not have high selectivity towards the radioactive cobaltous ions, they are coabsorbed with other ions, resulting in the generation of large amounts of radioactive waste which require costly and elaborate disposal procedures.. The amount of radioactive waste can be substantially reduced if a resin, which is highly selective towards cobaltous ions in presence of ferrous ions can be developed. This can translate into huge savings on cost and better management of compatibity to environment. This kind of highly selective resins can also be of great use during the decommissioning of existing nuclear power plants in selectively concentrating many radioactive metal ions present in small amount. Hence a material which can specifically pick up the particular metal ion of concern can reduce the amount of toxic waste. The present invention deals with providing such a material wherein the metal of interest which has to be trapped is cobaltous ions in presence of ferrous ions from a solution containing complexing agents as well.

Achieving this kind of selectivity for a particular metal ion against a close competitor through conventional ion-exchange resins may not be feasible unless there are highly specific ligands available for the particular metal ion. The technique of molecular imprinting (R3) has been widely accepted to be a convenient and powerful technique for preparing polymeric materials with pre-determined selectivity for various substances. The method as described in

R3 has been demonstrated to be effective for metal ions, saccharides, amino acid derivatives and other organic compounds of low molecular weight. The method usually involves pre-organisation of functional hosts around a "template" molecule in an organic solution containing a large excess of cross-linking monomers, followed by polymerization to form a cavity and stabilize the three-dimensional distribution of binding functionalities in the polymer matrix that is complementary to the template. The resulting polymer exhibits high selectivity for rebinding of the template molecule or ion which was used to prepare the polymer. Using metal ions as templates and suitable ligand groups as functional hosts, ion- selective imprinted polymers can be synthesized. Hence this approach is chosen for the synthesis of the material of interest.

In real system application this method has another advantage. The radioactive cobaltous ion which is picked up by the MIP can be easily stripped by change of the pH of the medium to extract the radioactive Cobalt in a smaller volume to process the liquid waste. The MIP can be reused. There is no need to handle the radioactive organic polymer waste.

US 7,001 ,963 discloses a process for removal of cobalt. The removal of cobalt disclosed in the application is not specific. As disclosed in the examples of US 7,001 ,963 iron is also bound to the imprinted polymer. Thus materials according to the aforementioned invention would not be useful for the above mentioned industrial application.

Consequently, the present invention relates to an improved imprinted polymer for specific removal of cobalt ions in presence of iron ions.

Summary of the invention

The present invention relates to an imprinted polymer capable of specific binding to cobalt ions in the presence of iron ions, thus solving the problems relating to additional binding of iron and/or iron ions.

The selective removal of cobalt ions in the presence of iron ions is achieved by an imprinted polymer having the general Formula I;

wherein 0<x, y<1 ;

D is a crosslinker bearing at least one vinylgroup;

A is a cobalt complex having the following general Formula II:

M _n Co _a (L) _b CH ₂ O (II),

wherein a is 1 ; b is 1-6; c is 0-8; n is 0-2

Co is cobalt in +2 oxidation state; M is any alkali or alkaline earth metal ion or a cation derived from organic bases, such as triethylamine, diisopropylamine or quartemary ammonium salts such as tetrabutyl ammonium hydroxide; and L is a polymerizable ligand. In one aspect the polymerizable ligand is N-(R)-iminodiacetic acid, where R preferably is 4-vinylbenzyl. Other possible polymerizable ligands (L) are any imino acid bearing at least one vinyl group; a Schiff base of the type 2, 6- pyridinedicarboxaldehydebis(p-vinylphenylimine) or other polymerizable Schiff bases derived from 2,6-pyridinedicarboxaldehyde; a Schiffs base prepared from a salicylaldehyde bearing at least one vinyl group; and a cyclohexane bearing at least one amino group. Furthermore the present invention also relates to a process for preparation of an imprinted polymer wherein an imprinted polymer having the general Formula I;

wherein 0<x, y<1 ;

D is a crosslinker bearing at least one vinylgroup;

A is a cobalt complex having the following general Formula II:

M _n Co _a (L) _b CH ₂ O (II),

wherein a is 1 ; b is 1 or 2; c is 0-8; n is 0-2 Co is cobalt in +2 oxidation state; M is any alkali or alkaline earth metal ion or a cation derived from organic bases, such as triethylamine and diisopropylamine or quartemary ammonium salts such as tetrabutyl ammonium hydroxide; and L is a polymerizable ligand such as N-(R)-iminodiacetic acid, where R preferably is 4-vinylbenzyl, is obtained by preparing the imprinted polymer by polymerization of at least one cobalt complex and at least one crosslinker in presence of a free radical initiator, and a porogen, and removing said cobalt from the cobalt imprinted polymer to obtain an imprinted polymer. Other possible polymerizable ligands (L) are any imino acid bearing at least one vinyl group; a Schiff base of the type 2, 6-pyridinedicarboxaldehydebis(p-vinylphenylimine) or other polymerizable Schiff bases derived from 2,6-pyridinedicarboxaldehyde; a Schiffs base prepared from a salicylaldehyde bearing at least one vinyl group and a cyclohexane bearing at least one amino group;. The porogen may be any solvent but is preferably a lower aliphatic alcohol such as methanol, ethanol or propanol.

Consequently, the invention provides an imprinted polymeric material which picks up only cobaltous ions and not ferrous ions from a solution containing both.

The invention also provides a method for the preparation of the above polymeric composition.

Furthermore, the present invention also relates to the use of the polymer according to the invention for specific removal of cobaltous ions and radioactive cobaltous ions from a complexing or a non-complexing solution containing cobaltous and ferrous ions.

Detailed description of the present invention

Compared to prior art the present invention as described above relates to an imprinted polymer having total selectivity for cobalt ions in the presence of iron ions. The above mentioned patent disclose a process for preparation of a composition which contains three different types of ligands, whereas the present invention provides a composition comprising at least one imprinted polymer which has only one type of ligand. Furthermore, the present invention provides a process wherein, the metal complex is prepared, separated and then incorporated into the polymeric network. One embodiment of the present invention relates to a process for preparation of an imprinted polymer, wherein said imprinted polymer is prepared in-situ in presence of an organic base. The use of organic lipophilic cations such as quartemary ammonium ions, as the counterions M, will result in an enhanced solubility of complex Il in organic solvents of lower polarity. This in turn will stabilize the complex which thus will be incorporated into the polymer with all Lewis basic sites of the ligands coordinated to the metal ion.

In terms of selectivity, which is an objective of the present invention, the polymeric composition provided in the present invention, as shown in the examples given below, surprisingly exhibits total selectivity (specificity) towards cobalt ions in the presence of ferrous ions, as shown in Example 2. The enhanced performance is due to a much improved material (imprinted polymer) compared to the material disclosed in prior art (US 7,001 ,963).

The material described in US 7,001 ,963 is not specific for cobalt, as shown in the Examples where they also exhibit a high capacity for ferrous ions.

In one embodiment the present invention retains the same specificity even in presence of a strong complexing agent selected from the group consisting of Nitrilotriacetic acid,

Ethylenedimaine tetraacetic acid, Ethyleneglycol-O, O'-bis(2-aminoethyl)-N, N, N', N'- tetraacetic acid and N-(2-Hydroxyethyl)ethylenediamine-N, N', N'-triacetic acid, as disclosed in Example 3. As per the examples given in US 7,001 ,963 one of the examples shows a selective pick up of cobalt (7.2mg/g) over iron (1.55mg/g) and the rest indicate a higher capacity for iron over cobalt. The present invention provides a polymeric composition which shows total specificity towards cobaltous ions over iron (ferrous) ions.

In the present invention, the wording 'template' means the cobaltous ion around which the crosslinked polymeric matrix is formed; the term "MIP" means a molecularly imprinted

polymer synthesized in presence of the template ion, and the term "NIP" means non- imprinted polymer which is synthesized in the absence of the template ion according to the definitions found in reference R3.

In the present invention the wording and terms "MIP", "imprinted polymer", and "molecularly imprinted polymer" are used interchangeably. Furthermore the terms "selectivity", "specificity" and "specific" are used interchangeably. In the present invention wherever an interval is present it is intended to mean each individual number within the interval, as well as each possible subinterval within the interval, for example the interval from 0 to 50 comprises the subintervals from 2 to 10, from 25.1 to 25.5 and from 5 to 40 etc.

Alkaline metals may be any of the monovalent metals of group I of the periodic table (lithium or sodium or potassium or rubidium or cesium or francium).

Alkaline earth metals may be any of a group of metallic elements, especially calcium, strontium, magnesium, and barium, also including beryllium and radium.

The expression "comprising" as used herein should be understood to include, but not be limited to, the stated items.

The present invention relates to the preparation of resins with predetermined selectivity through the process of metal imprinting. The present invention provides an imprinted polymer (MIP) which is highly specific for cobalt against ferrous ions. Imprinting process had long been used for inducing/enhancing selectivity for a particular metal or molecule. Molecular/metal imprinting involves pre-organisation of functional hosts around a template molecule/metal ion in a solvent containing a large excess of cross-linking monomers, followed by polymerization to provide a three-dimensional distribution of binding functionalities in the polymer matrix that is complementary to the template. After removal of the template, the resulting polymer exhibits high selectivity for rebinding of the template molecule or ion which was used to prepare the polymer. The present invention relates to metal imprinting with an objective to generate specificity towards cobalt ions.

One embodiment of the present invention relates to metal imprinting with an objective to generate a MIP (molecularly imprinted polymer) being selective towards cobalt ions.

Preferably the MIP has a selectivity of between 90-100%, more preferably between 95- 100%, more preferably between 98-100 %, and most preferred selectivity between 99-100%.

One embodiment of the present invention relates to metal imprinting with an objective to generate a MIP (molecularly imprinted polymer) being selective towards cobalt ions in the presence of iron ions, has a selectivity of between 90-100%, more preferably between 95- 100%, more preferably between 98-100 %, and most preferred a selectivity between 99- 100%.

In the present invention selective (specific) binding of cobalt ions is achieved by incorporating a cobalt complex within a crosslinked polymeric network followed by removal of the metal ions from the synthesized polymer. This results in a polymer having sites which are designed to take up cobalt ions preferentially over its close competitor ferrous ions from a solution containing both.

The selectivity of a metal imprinted polymer depends on many factors such as disclosed in R4: a) the specificity of the interaction of the ligand with the cation b) the coordination geometry and coordination number of the cations c) the charge on the cation and d) to some extent, the size of the cation.

In the specific case of Co ²⁺ vs Fe ²⁺ , the last two factors (charge and the size) may not be important deciding factors as both the ions have same charge and almost same size. The coordination number is in most of cases the same for both the metal ions. The present inventors have discovered that the most important factor that has to be looked at for this specific problem is the specificity of the interaction of the ligand(s) with the metal ion as expressed by the corresponding stability constant values for complex formation. Based on the above described factor, iminodiacetic acid was selected, which has a favourable stability constant towards cobalt over ferrous ions (R5). Hence, in one example, a (polymerisable) vinyl derivative of iminodiacetic acid namely, [N-(4-vinylbenzyl)imino]diacetic acid was synthesised and used as complexing monomer in the synthesis of cobalt imprinted polymer.

While the invention has been described in relation to certain disclosed embodiments, the skilled person may foresee other embodiments, variations, or combinations which are not specifically mentioned but are nonetheless within the scope of the appended claims.

All references cited herein are hereby incorporated by reference in their entirety.

Examples

Example 1 : A procedure for the synthesis of Imprinted and non-imprinted polymers: a) Synthesis of Cobalt Complex:

[N-(4-vinylbenzyl)imino]diacetic acid (2 mmol) synthesised according to a literature procedure (R6) was suspended in about 5 ml water and dissolved by raising the pH to 9.0 using 1 M NaOH. To this Co(NO ₃ ) ₂ 6H2O (1 mmol) solution made in about 15 ml water was added very slowly with stirring. Till the addition was over the pH was maintained between 8.0 and 9.0 using 1 M NaOH. The solution was stirred for 1 hr after the addition was over, filtered and the filtrate evaporated to dryness under vacuum. The solid was dissolved in 15 ml dry methanol and filtered to remove the insoluble portion. The filtrate was evaporated to dryness and again dissolved in dry methanol and the solvent removed. This procedure was repeated again and the solid obtained was used as the cobalt complex for the synthesis of the imprinted polymer. b) Synthesis of Imprinted and Non-imprinted polymers

0.3 mmol of the complex as synthesised above, 6.1 mmol of the crosslinking agent ethylene glycol dimethacrylate (EGDMA) and 1 wt% of initiator azo-bisisobutyronitile (AIBN) were dissolved in 5 ml of dry methanol, subjected to three cycles of freez-thaw and then heated at 62 ⁰ C for 24 hrs followed by 24 hrs at 75 ⁰ C for completion of polymerisation. Non- imprinted polymer was prepared by taking uncomplexed ligand in place of the cobalt complex. Uncomplexed ligand had to be dissolved by sonication and warming to 50 ⁰ C.

The polymers synthesised were first stirred in methanol to remove un-reacted reactants. NIP was washed with alkaline solution to remove any unreacted free ligands, washed with water and dried. The MIP was subjected to extraction of the metal template (Cobalt) by treatment with 1 :1 MeOH/HCI (0.1 N) mixture for 24 hrs, followed by repeated extractions with 0.1 N HCI till there was no more cobalt coming out of the polymer. The polymer was then washed water and methanol dried and used for evaluation.

The extracted solutions were analysed by Atomic Absorption Spectroscopy (AAS) for the amount of cobalt. The analysis showed that 41 % of the cobalt used in making the MIP was removed and the theoretical capacity was calculated to be 94.6 μmols/g.

Example 2: Evaluation of the MIP and NIP in a complexinq buffer

Rebinding studies were done by shaking 25 mg of the polymers synthesised with 2.5 ml of metal ion solutions (for used metal ions and concentrations see Table 1) in 0.1 M citrate buffer at pH 4.8 for 24 hrs at 30 ⁰ C. At the end of 24 hrs, the supernatant solutions were analysed for the metal ion concentration using Atomic Absorption Spectrometer. The results are tabulated in Table 1 below:

Table 1 :

Example 3: Evaluation of MIP and NIP in presence of strong complexinq agent: To check the ability of the polymers to pickup metal ions in presence of strong complexing agents, the tests were carried out in presence of Nitrilotriacetic acid. The tests were carried out the same way as explained in example 2, but the solution contained nitrilotriacetic acid along with metal ions and citrate buffer.

Table 2:

Example 4: In-situ synthesis of Cobalt imprinted polymer using an organic base: 0.32 mols of the ligand N-(4-vinylbenzyliminodiacetic acid), and 0.58 mmol of tetrabutylammonium hydroxide were taken in 2.5 ml of dry methanol or dry toluene. To this solution, 0.16 mmol of Co(NO ₃ ) ₂ x 6 H2O were added slowly and dissolved. To this 6.1 mmol of crosslinking agent Ethylene Glycol dimethacrylate and 1 wt% of initiator AIBN were added and the solution polymerised at 62 ⁰ C for 24 hrs followed by curing at 75 ⁰ C for 24 hrs. The polymer was crushed, washed with methanol and the cobalt ions removed by extraction with dilute HCI.

Example 5: A typical use of the polymer in a nuclear plant clean up process: The imprinted polymer is packed into a column and put before the normal ion exchange column during the clean up operation. This ensures that all the radioactive cobalt is picked up by the imprinted polymer and the other normal ion exchange resin would pick up only the other ions such as ferrous ions which are in excess. This results in concentration of radioactive waste within a small volume thereby reducing the amount of radioactive waste.

References:

R1. A.L.Rufus, V.S.Sathyaseelan, S.Velmurugan and S.V.Narasimhan, (2004) "NTA Based Formulation for the Chemical Decontamination of Nuclear Power Plants", Nucl. Energy, 43(1 ), 47-53.

R2. S.Velmurugan, A.L.Rufus, V.S.Sathyaseelan, T.V.Padmakumari, S.V.Narasimhan and P.K.Mathur, (1995) "Corrosion of PHWR PHT System Structural Materials by Dilute Chemical Decontamination Formulations Containing Ascorbic Acid, Nucl. Energy, 34(2), 103 -116,

R3. "Molecularly Imprinted Polymers: Man made mimics of antibodies and their applications in analytical chemistry", edited by Borje Sellergren, Elsevier Science, 245, 2001

R4. Gunter WuIf, Angew.Chem.lnt.Ed. Engl. 34,1812, 1995

R5.NIST data base on critically selected stability constants of metal complexes, vers. 8.

R6. Morris, L. R., Mock, R. A., Marshall, C. A. and Howe, J. H. (1959). Synthesis of some amino acid derivatives of styrene. J. Am.Chem. Soc. 81 , 377-382.