GB2292943A

GB2292943A - Metal ion ligating materials

Info

Publication number: GB2292943A
Application number: GB9416072A
Authority: GB
Inventors: Namor Angela F D De; Jamie D C Garcia; Mohamed Amer; David A P Tanaka
Original assignee: University of Surrey
Current assignee: University of Surrey
Priority date: 1994-08-09
Filing date: 1994-08-09
Publication date: 1996-03-13
Also published as: GB9416072D0

Abstract

The invention relates to synthetic materials comprising molecular units, each unit comprising at least one polymeric chain and comprising a group for ligating from solution the ions of one or more selected metals, and also to methods for the production and use of such materials.

Description

METAL ION LIGATING MATERIALS The invention relates to materials for the removal of selected metal ions from aqueous solution or from solution in a water containing mixture of liquids.

The removal of selected metal ions from solution can be desirable in such diverse fields as polution control and medicine.

For example, the importance of the removal of pollutant metal ions from industrial waste liquids prior to release of the liquids into the environment is now widely accepted. The recovery of metal ions in such situations can be achieved by using extraction techniques that utilise organic solvents. However, the use of large amounts of organic solvents and the associated release of these compounds into the environment is itself becoming increasingly less environmentally acceptable.

Accordingly, there is a demand for environmentally safer methods of recovering pollutant metal ions.

Numerous metal ions are highly toxic to man, and in many cases excretion of these toxic ions from the body is extremely inefficient. Extracorporeal treatment of blood to remove toxic metal ions could potentially be of great benefit in cases of metal ion toxicity. However, blood contains many essential metal ions for which the maintenance of correct concentrations is vital for cell function. Accordingly, any treatment for the removal of toxic metal ions would require removal means specific for the toxic ions.

Many water soluble, relatively low molecular weight compounds are known which have the ability to selectively ligate the ions of some metals in aqueous solution. For example, ethylenediamine tetraacetic acid (EDTA) which has a molecular weight of approximately 290 will ligate iron and copper ions as well as the ions of some other d-block metals and some p-block metals but is much less effective at ligating alkaline or alkaline earth metals.

However, small, soluble ligand molecules such as EDTA are generally of little use for the removal of metal ions from solution as the ligand-metal ion complexes formed on ligation of metal ions by such ligands are themselves mostly soluble and relatively small and not readily removed from solution.

According to a first aspect of the invention there is provided a synthetic material comprising molecular units, each unit comprising at least one polymeric chain and comprising a group for ligating from solution the ions of one or more selected metals.

The term molecular unit as used throughout the specification refers to a contiguous unit of covalently joined atoms, however large, and its use in relation to a polymeric or substantially polymeric material does not imply that the molecular units of the material are identical in molecular size or formula.

According to a second aspect of the invention there is provided a method of preparing a polymer-ligand adduct material for ligating from solution the ions of one or more selected metals, comprising reacting a metal ion ligand molecule with a polymeric reactant, said reactant comprising molecular units, each unit comprising at least one polymeric chain, whereby to form a covalent bond between the ligand and at least one of the molecular units.

Many such ligand molecules are known, for example: ethylenediamine tetraacetic acid, ethylenediamine tetraacetanilide and diethylenetriaminepentaacetic acid.

Similarly many polymeric reactants are available to which such ligand molecules may be covalently linked. The polymeric reactant may be homopolymeric, i.e. composed of polymeric chains with a single repeating moiety or heteropolymeric, i.e. composed of polymeric chains having two or more types of repeating moiety.

The choice of the polymeric reactant used to prepare the polymer-ligand adduct is influenced by several factors.

It will generally be desirable to use a polymeric reactant with molecular units considerably larger than the ligand molecules such that the polymer-ligand adduct has molecular units of sufficient size to allow their removal from a liquid by filtration or other separation means based on molecular size.

Preferably the polymer-ligand adduct is in the form of an expanded, particulate cross-linked network. More preferably the particles have a diameter greater than 20 mm. Accordingly, polymeric reactants having such a form are good potential candidates for preparing the polymer-ligand adduct.

Additionally a suitable polymeric reactant preferably has molecular units with a functional group to which the ligand molecule may be covalently linked. The functional group is preferably provided on all or some of the subunits of the polymeric chains but not only on the end subunits of the polymeric chains.

An example of a suitable polymeric reactant is Merrifield's peptide resin. Merrifield's resin is an insoluble polymer network composed of polystyrene chains cross-linked with divinylbenzene, in which some of the benzene rings of the polystyrene chains are chloromethylated. The chloromethyl groups can be used for covalent attachment of a suitable ligand molecule.

The ligand molecule used for preparing the polymer-ligand adduct will be selected from a group of ligand compounds having the desired specificity for metal ions under the conditions in which the use of the polymer-ligand adduct to ligate metal ions is anticipated.

Preferably the ligand molecule is such that, when linked to the polymeric reactant to form the adduct, the adduct may be exposed to predetermined conditions which cause metal ions ligated to the ligand of the adduct to be released without causing irreversible changes to the adduct. Typically this will be achieved by acidification.

Additionally the ligand molecule preferably has a functional group, not essential for its metal ion ligating properties, which is suitable for linking the ligand molecule to the polymeric reactant.

Thus the functional group of the ligand molecule can be suitable for reaction with the functional group of the polymeric reactant to form a covalent bond therebetween.

Alternatively, the functional group of the ligand molecule can be suitable for covalent attachment to a linker molecule which itself can be covalently attached to the functional group of the polymeric reactant.

According to a third aspect of the invention there is provided a method of preparing a polymeric material for ligating the ions of one or more selected metals from solution comprising polymerising a metal ion ligand.

It will be appreciated that the third aspect of the invention can be embodied such that the polymeric material is homopolymeric or heteropolymeric. In the latter case one or more of the polymeric subunits may have metal ion ligating properties.

According to a fourth aspect of the invention, there is provided a method for the removal of the ions of one or more selected metals from a liquid comprising bringing material of the first aspect of the invention or a material produced via a method of the second or the third aspect of the invention into contact with the liquid, the liquid and material subsequently being separated.

The following is a more detailed description of some examples of the invention, by way of example, reference being made to: Figure 1 which shows the infrared spectrum of the polymeric product of Example I.

In examples I and II below EDTA derivatives are reacted with Merrifield's peptide resin by refluxing in the presence of a base and a phase transfer catalyst.

Examples of phase transfer catalysts are quaternary ammonium salts and crown ethers. These compounds have the property of being soluble in aqueous solution and also of being soluble, as an ion pair (with an anion), in organic solvents and can be used to "carry* a hydroxide ion into the organic phase of an aqueous-organic biphasic mixture for the hydroxide ion to react therein. Dimethylsulfoxide may be used in place of a quaternary ammonium salt or a crown ether. Benzene and dioxane are examples of suitable solvents in which the ref fluxing may be carried out.

EXAMPLE I A polymer-ligand adduct material was obtained by covalently linking a ligand, in this case, ethylenediamine tetraacetanilide to Merrifield's peptide resin.

Synthesis of Ethylenediamine Tetraacetanilide Ethylenediamine Tetraacetanilide was synthesised by two alternative methods.

Method 1 - The tetramethyl ester of EDTA was synthesised by the method of R.W. Hay and K.B. Nolan (J. Chem.Soc., Dalton Trans; 1975, 1348) in which a methanolic suspension of EDTA is allowed to react with thionyl chloride.

Ethylenediamine tetraacetanilide was synthesised by aminolysis of the tetramethyl ester of EDTA so produced, as described below. Freshly distilled dimethylsulfoxide (Fisons, 99%, 50 ml), sodium hydride (Aldrich, 60% dispersion in mineral oil, 3.lg, 78mmol) and freshly distilled aniline (BDH, ANALAR, 88mmol) were placed in a three-necked flask under anhydrous conditions and the mixture was cooled in an ice bath. A solution of the tetramethyl ester of EDTA (5.3g, Smmol) in dry dimethylsulfoxide (30 ml) was added dropwise to the mixture in the flask. After 19 hours at room temperature a brown solution was obtained. The brown solution was poured into cracked ice while stirring vigorously. A yellow mixture was obtained which was filtered under reduced pressure giving a solid residue.The residue was washed with a small amount of hexane and then several times with ethanol and finally with water to give a white solid which was dried and finally recrystallised twice by dissolving in hot hexane and then adding ethanol until a precipitate of ethylenediamine tetraacetanilide appeared.

The identity of the recrystallised product was confirmed by microanalysis and proton NMR.

Method 2 - EDTA dianhydride (Aldrich 2.05g, 8mmol), dimethylaminopyridine (Aldrich. 0.20g), anhydrous dichloromethane (Fisons, A.R. grade, distilled from calcium hydride, 80 ml) and aniline (BDH, Analar, 3ml,33mmol) were placed in a three necked flask and the mixture was cooled in an ice bath. After 10 minutes dicyclohexylcarbodiimide (DCC, Lancaster, 3.35g, 16mmol) was added. After 48 hours a white precipitate had formed which was recovered by filtration and washed with dichloromethane. The dichloromethane was evaporated and the residue was collected with dichloromethane and was then washed twice with hydrochloric acid (0.5 M), and twice with a saturated solution of sodium bicarbonate.

The dichloromethane was removed and the residue was recrystallised twice by dissolving in hot hexane and then adding ethanol until a precipitate appeared. The precipitate was ethylenediamine tetraacetanilide.

Synthesis of Polymer-Lipand Adduct A mixture of ethylenediamine tetraacetanilide (1.24g), potassium carbonate (1.302g), powdered sodium hydroxide (1.404g), tetrabutylammonium sulphate (0.797g) and Merrifield's peptide resin (9.455g, 1 mol C1 g 1 resin, 1% cross-linked, Aldrich (Cat.No.22,148-1)) was suspended in dry benzene (300ml) and ref fluxed at between 800C-900 with continuous, vigorous stirring for six days. The mixture was then allowed to cool and was subsequently filtered under vacuo. The solid compound--so produced was thoroughly washed with, in turn, water, ethanol, dimethyl- fonnamide, dioxane and finally ethanol. A pale yellow product was obtained and dried under vacuo at 80 C for 96 hours.The new material was characterized by infrared spectroscopy and microanalysis.

The microanalysis data were C:88.33%, H:7.41% and N:1.71%. The infrared spectrum of the new material is shown in Figure 1. These analyses are consistent with the product having the structure shown below:

where P represents a continuation of the chloromethylated polystyrene chain of Merrifield's resin and where each gram of polymer-ligand adduct contains about 0.20mmol of the ligand. The polymer-ligand adduct was not substantially soluble in water.

EXAMPLE II A polymer-ligand adduct material was obtained by covalently linking a ligand, in this case, N,N'-bis(acetic)-N,N'-bis(acetanilide) ethylenediamine to Merrifield's peptide resin.

Synthesis of N,N'-bis(acetic)-N,N'-bis(acetanilide) ethylenediamine A mixture prepared from EDTA dianydride (2g), aniline (1.45g) and dried dioxane (30 ml) was placed in a three necked flask equipped with a mechanical stirrer and was ref fluxed at 1100C. A few drops of ethanoic acid were added to the mixture 1 hour after starting to reflux. The reaction mixture, which was initially white, became viscous and yellow after 56 hours. The reaction was followed by thin layer chromatography (tlc) on Silica gel F 254 TLC Aluminium using a dichloromethane: methanol mixture (1:1) as eluent. After 56 hours, when no starting materials could be detected by tlc, the reaction was stopped and the mixture was allowed to cool at room temperature. A precipitate was separated from the mixture by filtration and this was washed several times with distilled water, ethanol and finally with diethylether.

The washed precipitate was then dried overnight at 1000C using calcium chloride and potassium hydroxide as drying agents. The dried precipitate was recrystallised from butan-l-ol and dried under vacuum at 1000C for 48 hours to give a final product (1.2g, yield 46%).

The final product had a melting point of 102 C and upon microanalysis was found to contain: C,59.37%, H,5.73%, and N,12.61%. This is consistent with the empirical formula C22 26 6 (requiring: C,59.72; H,5.92%; and N,12.67%). The final product is believed to be N,N'-bis(acetic)-N,N'-bis(acetanilide) ethylenediamine, the formula of which is shown below as formula 1, and will be referred to as such below.

Synthesis of Polvmer-Liaand Adduct A mixture comprising N,N'-bis(acetic)-N,N'-bis(acetanilide) ethylenediamine (1.09), NaOH (1.0g), tetrabutylammonium sulphate (0.79g), Merrifield's peptide resin (4.5g, lmol C1 g 1 resin, 1% cross-linked, Aldrich (Cat.No. 22, 148-1) and benzene (200ml) was placed in a three necked flask equipped with a mechanical stirrer. The contents were ref fluxed for five days at 900C. The mixture was then cooled and was subsequently filtered.

The solid was washed, in turn, with distilled water, dimethylformamide, dimethylsulfoxide and finally with ethanol. It was then left to dry under reduced pressure at 1000C for 48 hours. The final product was characterized by IR spectroscopy and microanalysis. The microanalysis data were C:81.76%, H:6.94% and N:1.30.

The results are consistent with the product having the structure shown below.

Where()P represents a continuation of the chloromethylated polystyrene chain of Merrifield's resin and where each gram of polymer-ligand adduct contains about 0.23mmol of the ligand. The polymer-ligand adduct was not substantially soluble in water.

The polymer-ligand adducts prepared above in accordance with Example I and Example II were tested for their utility in extracting metal ions.

The adduct of Example I (0.3g) was mixed with 20 ml of an aqueous solution of the metal cation (0.lmol dm whose extraction was being tested and stirred for 24 hours at 25 C. The concentration of metal ion in solution was determined before adding the adduct and after the 24 hours incubation by EDTA complexometric titration. From these measurements it was calculated that the adduct of Example I extracted 0.32mmoles of Hg per gram of resin from aqueous solution. The adduct of Example 1 did not extract significant amounts of lead or cadmium ions.

Similar incubations were carried out with the adduct of Example II. In this case the adduct was incubated, for each cation tested, with a series of aqueous solutions containing different concentrations of the test cation. A weighed aliquot (0.050g) of the adduct was mixed with 10 ml of each solution and the mixture was left overnight at 250C. Solutions containing between 0-330 ppm Cd2+, 2+ ppm 2+ 0-85 ppm Pb or 0-1580 ppm Hg were used and the pH values of the solutions were adjusted to 4.0 with perchloric acid at the beginning of the incubation.

Additionally, the ionic strengths of the test solutions were standardised by the addition of sodium perchlorate.

The concentration of the test cation in solution was measured before addition of the adduct and following incubation, by atomic absorption spectroscopy and by EDTA complexometric titration. From these measurements it was determined that the maximum amounts of cation extracted by the adduct were: Hg 2+ ,l.43mmo1/g adduct; pb2+ 0.057mmol/g adduct; and Cd2+, 0.39mmol/g adduct. The adduct of Example II was found not to extract alkali or alkaline earth metal cations.

It was found that the adduct of Example II can be easily recycled, i.e. ligated cations removed, by washing with aqueous solutions of HC1 (0.lmol dm-3).

It will be appreciated that Merrifield's peptide resins having chlorine contents and degrees of cross-linking distinct from those of the resins used in Examples I and II can be used to prepare polymer-ligand adducts.

It will also be appreciated that polymer-ligand adducts suitable for the removal of selected metal ions from solution can be prepared from ligand molecules other than the EDTA derivatives described above and from polymeric materials other than Merrifield's peptide resin.

For example the EDTA derivatives having the respective formulae shown as 2 and 3 below can be covalently linked to Merrifield's peptide resin by processes similar to those disclosed above in Examples I and II.

The formulae of two other ligands which can be used for preparing polymer-ligand adducts are shown below as formulae 4 and 5

The method described below can be used to prepare both of these ligands.

EDTA dianhydride (Aldrich, 2.05g, 8mmol), dimethylaminopyridine (Aldrich 0.20g), anhydrous dichloromethane (Fisons, A.R. grade, distilled from calcium hydride, 80ml) and thiophenol or benzylmercaptan (Aldrich 33mmol) were placed in a three necked flask and the mixture was cooled in an ice bath. After 10 minutes Dicyclohexylcarbodiimide (DDC, Lancaster, 3.35g, 16mmol) was added. After 48 hours a white precipitate was separated from the mixture by filtration and washed with dichloromethane. The dichloromethane was evaporated and the residue was collected with dichloromethane and was then washed twice with hydrochloric acid 0.5 M and twice with a saturated solution of sodium bicarbonate.The dichloromethane was removed and the solid product was recrystallised several times from methanol/dichloromethane.

Additionally it will be appreciated that polymeric materials suitable for ligating selected metal ions can be prepared by polymerising subunits which when polymerised have themselves a group suitable for ligating metal ions.

the preparation of such a material is examplified below in Example III.

Example III A polymeric material was synthesised by polymerising the ligand N,N'-bis(acetic)-N,N'-bis(acetanilide) ethylenediamine by reacting this ligand with formaldehyde in the presence of oxalic or formic acid.

N,N'-bis(acetic)-N,N'-bis(acetanilide) ethylenediamine (0.516g) was mixed with oxalic acid (0.07g) and formaldehyde (15ml) in a flask and was refluxed at 800C for 5 days. A solid yellow product was recovered by filtration and this was washed several times with distilled water and dried under reduced pressure. The product was characterised by IR spectroscopy and microanalysis.

The product formed is believed to have the formula (formula 6) shown below.

Ethylenediamine tetraacetanilide can also be polymerised in a similar manner to that described above for N,N'-bis(acetic)-N,N'-bis(acetanilide) ethylenediamine.

The polymeric products of Examples I to III can be used in several ways. Most simply the product is added to the liquid containing the metal ions to be removed and the mixture is mixed while ligation occurs. The product and metal ions ligated thereto can be recovered by filtration using a filtration medium with a pore size sufficiently small to retain the product. A product with larger molecular units will, of course, be removable from a liquid by filtration means with larger pore sizes and thus the filtration process will generally be faster.

Alternatively, in the case of particulate, cross-linked polymeric networks where the product particles are relatively large and have a density distinct from that of the liquid, separation of the product from the liquid may occur under gravity or may be hastened by centrifugation.

Additionally, the product can be conveniently used to remove metal ions from liquids by the use of a column or similar device in which the product is retained by a porous plug. The liquid containing the metal ions is passed through the column wherein the metal ions ligate to the product and the ion depleted liquid emerges from the column through the pores of the plug.

Products which are particulate, cross-linked polymeric networks are most suitable for use with a column. The size of the particles determines to a large extent the flow rate of the liquid through the column - larger particles allowing greater flow rates. Particle size is, of course, also a major determinant of the surface area to mass ratio of the product - a smaller particle size allowing the liquid to come into contact with more linked ligand molecules per a given mass of the product.

Additionally it will be appreciated that the scope of the invention includes the ligand compounds having the formulae 1 to 5 and the methods disclosed herein for their preparation.

Claims

1. A synthetic material comprising molecular units, each unit comprising at least one polymeric chain and comprising a group for ligating from solution the ions of one or more selected metals.

2. A synthetic material according to claim 1, wherein each molecular unit comprises a plurality of polymeric chains, each chain being covalently cross-linked to at least one other polymeric chain of the unit, and wherein the material is water insoluble.

3. A synthetic material according to claim 1 or claim 2, wherein the group is provided on a metal ion ligand molecule which has been covalently linked to a polymeric chain.

4. A synthetic material according to claim 3, wherein the ligand molecule is such that it may be covalently linked to two or more polymeric chains thereby cross-linking said polymeric chains.

5. A synthetic material as claimed in claim 3 or claim 4, wherein the polymeric chain or the polymeric chains are polystyrene in which some of the benzene rings have been chloromethylated and wherein the ligand molecule has been covalently linked to at least one said polymeric chain via reaction with at least one chloromethylated benzene ring.

6. A synthetic material according to any preceding claim, wherein the group is such that by subjecting the material to pH values lower than a predetermined value ions ligated to the group are released, the material not undergoing any changes which are not reversible by raising the pH.

7. A synthetic material according to any preceding claim, wherein one or more of the metals lead, cadmium and mercury are selected metals and the group is such that ions of alkali metals and alkaline earth metals will not substantially ligate to the group.

8. A synthetic material as claimed in any preceding claim, wherein the group iso N-CH2-CH2-N .

9. A synthetic material according to claim 3 or any claim dependent thereon, wherein the ligand molecule is an EDTA derivative.

10. A synthetic material according to claim 9, wherein the ligand is N,N'-bis(acetic)-N,N'-bis(acetanilide) ethylenediamine.

11. A synthetic material according to claim 9, wherein the ligand is ethylenediamine tetraacetanilide.

12. A synthetic material according to claim 10 or claim 11, when claim 9 is dependent on claim 5, wherein a covalent bond extends between a nitrogen atom of one of the acetanilide groups of the ligand and a methyl group derived from said at least one chloromethylated benzene ring.

13. A synthetic material according to claim 1, wherein the group is provided on a repeating moiety comprised within the at least one polymeric chain.

14. A synthetic material according to claim 13, wherein the group is - N-CH2-CH2-N .

15. A synthetic material according to claim 13 or claim 14, wherein the material is formed by polymerizing an EDTA derivative.

16. A synthetic material according to claim 15, wherein the EDTA derivative is ethylenediamine tetraacetanilide.

17. A synthetic material according to claim 15, wherein the EDTA derivative is N,N'-bis(acetic)-N,N'-bis (acetanilide) ethylenediamine.

18. A synthetic material according to any preceding claim for ligating from aqueous solution the ions of one or more selected metals.

19. A method of preparing a polymer-ligand adduct material for ligating from solution the ions of one or more selected metals, comprising reacting a metal ion ligand molecule with a polymeric reactant, said reactant comprising molecular units, each unit comprising at least one polymeric chain, whereby to form a covalent bond between the ligand and at least one of the molecular units.

20. A method according to claim 19, wherein each molecular unit of the reactant comprises a plurality of polymeric chains each chain being covalently cross-linked to at least one other polymeric chain of said each molecular unit, and wherein the material is water insoluble.

21. A method according to claim 19 or claim 20, wherein the polymeric chain or polymeric chains are polystyrene in which some of the benzene rings have been chloromethylated and wherein the said reaction includes reaction between the ligand and at least one of the chloromethylated benzene rings whereby to form said covalent bond.

22. A method according to claim 21, wherein the method includes incubation of the ligand with the reactant in the presence of a phase transfer catalyst and a base.

23. A method according to any one of claims 19 to 22, wherein the ligand is an EDTA derivative.

24. A method according to claim 23, when claim 23 is dependent on claim 22, wherein the ligand is either ethylenediamine tetraacetanilide or N,N'-bis(acetic)-N, N'-bis(acetanilide) ethylenediamine.

25. A method according to claim 24, wherein the phase transfer catalyst is a quaternary ammonium salt.

26. A method according to claim 24 or claim 25, wherein the ligand is ref fluxed with the reactant, the phase transfer catalyst and the base.

27. A method according to claim 26, wherein said reflux is carried out in benzene or dioxane.

28. A method of preparing a polymeric material for ligating the ions of one or more selected metals from solution comprising polymerising a metal ion ligand.

29. A method according to claim 28, wherein the metal ion ligand is an EDTA derivative.

30. A method according to claim 29, wherein the EDTA derivative is ethylenediamine tetraacetanilide or N,N'-bis(acetic)-N, N'-bis(acetanilide) ethylenediamine.

31. A method according to claim 29 or claim 30, wherein the method includes reaction of the ligand with formaldehyde in the presence of an acid.

32. A method according to claim 31, wherein the acid is formic or oxalic acid.

33. A method according to any one of claims 19-32, wherein said material is for ligating selected metal ions from aqueous solution.

34. A material for ligating the ions of one or more selected metals from solution formed by the method of any one of claims 19 to 33.

35. A method for the removal of the ions of one or more selected metals from a liquid, comprising bringing the material of any one of claims 1 to 18 or claim 34 into contact with the liquid, the liquid and material subsequently being separated.

36. A method according to claim 35, wherein the material is retained by a containment means and the liquid is passed through the containment means.

37. A method according to claim 35, wherein the subsequent separation involves centrifugation or filtration.

38. A method according to any one of claims 35 to 37, wherein the liquid is water or contains water.

39. A compound having the formula:

40. A compound having the formula:

41. A compound having the formula:

42. A synthetic material comprising molecular units, each unit comprising an at least one polymeric chain and comprising a group for ligating from aqueous solution the ions of one or more selected metals, said material being substantially as hereinbefore described in any one of Examples I to III.

43. A method for preparing a synthetic material comprising molecular units, each unit comprising an at least one polymeric chain and comprising a group for ligating from solution the ions of one or more selected metals, said method being substantially as hereinbefore described in any one of Examples I to III.