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, TcO-, 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 complexant can be designed to be specific for a particular anion.

The complexant is effective for complexing tetrahedral oxoanions for example, chromate, CrOq, pertechnetate, Tc04-, perrhenate, ReO4 and perchlorate. The complexant is particularly effective for complexing chromate, CrO, 2- and pertechnetate, Tc04. 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 complexing 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 co- ordination 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, Rz and R3 independently comprise (CH2) x where x = 1 to 4 or aryl and at least one of Yl, Y2 and Y3 comprise a crown ether group including crown ether derivatives. Preferably, x is 2. Preferably, all three groups Y1, Y2 and Y3 each comprise a crown ether group.

A more preferred complexant comprises C2il4 for R1, R2 and R3 and amidebenzo-15-crown-5 for Yl, Y2 and Y3 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 TcO,-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, Cru42~, pertechnetate, Toc04, perrhenate, ReO'and perchlorate. The method is particularly effective for extracting chromate, CrO42~ and pertechnetate, Tc04-.

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 Tc04-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 (amidebenzo-15-crown- 5) tren, where tren = tris (2-aminoethyl) amine. In the reaction, the compound tren is reacted with the carboxybenzo- crown compound 1 in the presence of SOC12 to form the complexant.

Example 2-Liquid-liquid Extraction of Pertechnetate An aqueous simulated waste consisting of 140ppm NH4TcOa and 200gl~1 NaN03 was prepared and adjusted to pHll with NaOH and concentrated nitric acid.

An organic phase consisting of L (varying between 1.5xlOexp-3 to 1.5xlOexp-2M) in 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. lml samples of the aqueous phase were removed and made up to lml 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 <BR> <BR> <BR> 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 H2O 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 100rpm. Samples of both the acceptor and donor phases were removed at 5min, 10min, 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 lxlOexp-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 [TcO4-] in receiving phase Flux, Carrier after 24 hrs (ppm) J. mol hr~' ICP-MS 99Tc nmr Trenbenzo 30 38 6. 3 x 10-" Benzo 1. 0 x 10-U 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.