IMPROVEMENTS IN AND RELATING TO COMPLEXANTS

Abstract: The invention provides a complexant for complexing an anion, particularly, but not exclusively, a pertechnetate anion. The complexant has at least one cation binding site and, when said at least one cation binding site is occupied by a cation, at least one anion binding site. The invention also relates to the extraction of an anion from a liquid medium using the complexant.

Description: WO 00/54881 PCTtGBOO/00962 Improvements in and relating to complexants The present invention relates to a complexant for compiexing an anion, particularly, but not exclusively, a pertechnetate anion. The invention also relates to the extraction of an anion from a liquid medium, particularly, but not exclusively, it relates to the extraction of a pertechnetate anion. The efficient removal of undesirable chemical species from waste streams is a persistent problem faced by industry, e.g. the nuclear industry. Anionic species, particularly tetrahedral oxoanions, such as pertechnetate, Tc0 4 N are particularly problematic to remove. Current methods for pertechnetate extraction include electrodeposition and tetraphenylphosphonium salt extraction. However, these techniques have a number of drawbacks including operational complexity, high running cost and larger than desirable waste volumes . It is an aim of the present invention to overcome the aforementioned drawbacks of the current technology relating to anion extraction. According to a first aspect of the invention there is provided a complexant for forming a complex with an anion, the complexant having at least one cation binding site and, when said at least one cation binding site is occupied by a cation, at least one anion binding site. Advantageously, the complexant may be selective for a particular anion, i.e. the complexant may preferentially complex the particular anion from a liquid medium containing a number of different anions. The precise structure of the WO 00/54881 PCTtGBOO/00962 2 complexant can be designed to be specific for a particular anion . The complexant is effective for compiexing tetrahedral oxoanions for example, chro ate, Cr(-V ~ , pertechnetate, Tc0 4 " , perrhenate, ReCN and perchlorate. The complexant is particularly effective for compiexing chromate, CrO.-N " and pertechnetate, Tc0 4 " . The invention thus enables the selective complexation of pertechnetate and/or chromate. Preferably, the complexant has a plurality of cation binding sites and, when said cation binding sites are occupied by cations, at least one anion binding site. Where the complexant has a plurality of cation binding sites, in use, preferably all cation binding sites are occupied by cations. Where the complexant has a plurality of cation binding sites, typically the complexant has, when said cation binding sites are occupied by cations, one anion binding site. Advantageously, the cation may come from the same liquid medium as the anion, thus making use of cations which may be co-present in the liquid medium for compiexing and/or extracting the anion. The complexant is preferably multipodal, each leg providing a cation binding site. Herein leg means an elongate group capable of ligating the anion. A preferred complexant contains the tripodal "tren" structure, where tren=tris(2- amino ethyl) amine, shown in Figures 1 and 2, which is a tripodal or three legged complexant. The complexant is preferably tripodal, i.e. comprising three legs . The complex is preferably multidentate, each leg proving a coordination site for the anion. The cation binding site preferably is located towards the outer end of the leg, the outer end being that end opposite to the end where the legs are joined. Most preferably the site is located at the outer end of the leg. The multipodal structure typically defines a space or cavity between the legs and in the complex the anion may occupy the space or cavity defined, as shown schematically in Figure 3. Preferably each leg further comprises a group which assists formation of the complex. Preferred such groups include amino, ammonium, urea, thio urea, and guanidinium moieties. Where the amino group has an N-H bond, the hydrogen may participate advantageously in hydrogen bonding to the anion, thus enhancing formation of the complex. Reference herein to an amino group includes reference to a quaternary ammonium ion group. A preferred complexant comprises a compound having the general structure shown in Figure 1, where Rj , R 2 and R 3 independently comprise (CH 2 ) X where x = 1 to 4 or aryl and at least one of Yi, Y 2 and Y 3 comprise a crown ether group including crown ether derivatives. Preferably, x is 2. Preferably, all three groups Yi, Y 2 and Y 3 each comprise a crown ether group. A more preferred complexant comprises C ? H 4 for R l f R 2 and R 3 and amidebenzo-15-crown-5 for Yi, Y 2 and Y 3 as shown in Figure 2. The complexants shown in Figures 1 and 2 are especially effective selective for chromate and pertechnetate. The cation may comprise sodium, potassium or caesium for example. The cation binding site preferably comprises a crown ether. Reference herein to crown ether includes crown ether derivatives, for example, amidebenzo crown ethers. 15-crown-5 is preferred for binding sodium and 21-crown-7 is preferred for caesium. Thus the invention may provide a single method for extracting both TcCN and Cs + simultaneously from a waste stream. The complexant is soluble in a wide range of organic solvents. The solubility of the complexant in solvents such as odourless kerosene (OK) may be increased by the addition of hydrocarbon substituents to the complexant, for example to the crown ether groups . According to a further aspect of the invention there is provided a use of the complexant according to the first aspect of the invention for the extraction of an anion from a liquid medium. According to another aspect of the invention there is provided a method of extracting an anion from a liquid medium, the method comprising: providing a complexant according to the first aspect of the invention; causing at least one cation to occupy said at least one cation binding site; forming a complex between the resultant cation-bound complexant and the anion; and separating the complex from the liquid medium. It will appreciated from the description of the complexant that the method of extraction is effective for the extraction of tetrahedral oxoanions, for example chromate, Cr0 4 2" , pertechnetate, TcCN , perrhenate, ReCN and perchlorate. The method is particularly effective for extracting chromate, Cr0 4 2" and pertechnetate, Tc0 4 " . The cation may comprise sodium, potassium or caesium for example . Advantageously, the cation may come from the same liquid medium, thus making use of cations which may be co-present in the liquid medium for extracting the anion. Thus the invention may provide, for example, a single method for extracting both Tc0 4 " and Cs + simultaneously from a waste stream. The invention will now be described in detail by way of the following examples with reference to the accompanying drawings in which: Figure 1 shows a preferred complexant according to the present invention; Figure 2 shows a more preferred complexant according to the present invention; Figure 3 shows a schematic mechanism for anion extraction using the present invention; Figure 4 shows a reaction scheme for synthesising a complexant; Figure 5 shows a graph for technetium extraction using different concentrations of complexant; and Figure 6 shows a schematic set-up for a liquid membrane experiment . Example 1 Preferring to Figure 4, there is shown a reaction scheme for preparing a preferred complexant tris (amιdebenzo-15-crown- 5)tren, where tren = tris (2-ammoethyl) amine . In the reaction, the compound tren is reacted with the carboxybenzo- crown compound 1 in the presence of S0C1 2 to form the complexant . Example 2 - Liquid-liquid Extraction of Pertechnetate An aqueous simulated waste consisting of 140ppm NH 4 TcO^ and 200gl "1 NaNOj was prepared and adjusted to pHll with NaOH and concentrated nitric acid. An organic phase consisting of L (varying between 1.5xl0exp-3 to 1.5xl0exp-2M) m dichloromethane was also prepared, where L=tris (amidebenzo-15-crown-5) tren; tren=tris (2- aminoethyl ) amine . The amount of L in the organic phase varied between 1 and 10 equivalents w.r.t. pertechnetate. 2ml of each phase were shaken together at 20 deg C at 800rpm. 0.1ml samples of the aqueous phase were removed and made up to 1ml with water at lh, 2h and 24h. The final organic phase was separated from the aqueous layer and studied using 99Tc nmr. The results are shown in Figure 5. With a 1:1 equivalent of complexant to pertechnetate, 20% pertechnetate was extracted. This value rose to 70% when 10:1 complexant to pertechnetate was used. Similar results were obtained at neutral pH. Overall the quantity of pertechnetate extracted was found to be approximately linearly dependant on the concentration of L. Example 3 - Liquid membrane study An apparatus was set up as shown schematically in Figure 6. An acceptor phase consisting of H0 was separated from a donor phase consisting of simulated waste as described in Example 2 by a glass divider. Communication of the two phases was through a liquid membrane of lxlOexp-3 M L in dichloromethane , where L is as defined above. 5ml of each of the aqueous phases and 12ml of the bulk liquid membrane stirred at lOOrpm. Samples of both the acceptor and donor phases were removed at 5min, lOmin, 30min, 60min, 5h and 24h. The final phases were analysed by 99Tc nmr and ICP-MS. The results are shown in Table 1 below. For comparison, a similar experiment was performed, but using a liquid membrane of lxl0exp-3 M benzo-15-crown-5 ("Benzo") in dichloromethane. The results are shown in Table 1. Also, a further "blank" experiment was performed using no complexant. In the blank experiment, little or no pertechnetate was transported. TABLE 1 Both L ("Trenbenzo") and benzo-15-crown-5 ("Benzo") were shown to mediate the transport of pertechnetate through the dichloromethane layer. L was shown to transport pertechnetate approximately 4 times more efficiently than benzo-15-crown-5.

Claims: CLAIMS 1. A complexant for forming a complex with an anion, the complexant having at least one cation binding site and, when said at least one cation binding site is occupied by a cation, at least one anion binding site. 2. A complexant as in claim 1 wherein the complexant has a plurality of cation binding sites and, when said cation binding sites are occupied by cations, at least one anion binding site. 3. A complexant as in claim 2 wherein when said cation binding sites are occupied by cations, the complexant has one anion binding site. . A complexant as in any one preceding claim wherein the complexant has a multipodal structure, each leg providing a cation binding site. 5. A complexant as in claim 4 wherein the complexant has a tripodal structure. 6. A complexant as in claim 4 or 5 wherein the complex is multidentate, each leg providing a co-ordination site for the anion. 7. A complexant as in any one of claims 4 to 6 wherein each leg further comprises a group for assisting complexation by binding to the anion. 8. A complexant as in claim 7 wherein the group comprises an amino group. 9. A complexant as in claims 4 to 8 wherein the multipodal complexant comprises a tris (amino alkyl) amine group, each amino alkyl group forming part of a leg. 10. A complexant as in claim 9 wherein the tris (amino alkyl) amine group comprises a tris (2-amino ethyl) amine moiety. 11. A complexant as in any one preceding claim wherein the at least one cation binding site comprises a crown ether. 12. A complexant as in claim 11 wherein the crown ether comprises 15-crown-5 or 21-crown-7. 13. A complexant as in any preceding claim wherein the anion binding site is a binding site for a tetrahedral oxoanion. 14. A complexant as in claim 13 wherein the tetrahedral oxoanion comprises pertechnetate or chromate. 15. A complexant having the following formula: wherein Ri, R 2 and R 3 independently comprise (CH 2 ) X where x = 1 to 4 and Yi, Y 2 and Y 3 independently comprise crown ether groups including crown ether derivatives. 16. A complexant as in claim 15 wherein x is 2. 17. A complexant as in either claim 15 or 16 wherein Y λ r Y 2 and Y 3 comprise amidebenzo-15-crown-5. 18. Use of a complexant as in any one of claims 1 to 17 in the extraction of an anion from a liquid medium. 19. A method of extracting an anion from a liquid medium, the method comprising: providing a complexant according to any one of claims 1 to 17; causing at least one cation to occupy said at least one cation binding site; forming a complex between the resultant cation-bound complexant and the anion; and separating the complex from the liquid medium. 20. A method as in claim 19 wherein the complexant is contacted with a solution containing both the cation and the anion. 21. A method as in either claim 19 or 20 wherein the anion comprises a tetrahedral oxoanion. 22. A method as in claim 21 wherein the tetrahedral oxoanion comprises pertechnetate or chromate.

Inventors: BEER PAUL, HOPKINS PETER, MCKINNEY JAMES DARCY, BEER, PAUL, HOPKINS, PETER, MCKINNEY, JAMES, DARCY
Applicants: BEER PAUL, BRITISH NUCLEAR FUELS PLC, HOPKINS PETER, MCKINNEY JAMES DARCY, BEER, PAUL, HOPKINS, PETER, MCKINNEY, JAMES, DARCY
Application year: 2000
Main classification: B01J45
IPC codes: B01D11, B01J45, C02F1, C07C211, C07D323, G21F9