WO2008025977A1

WO2008025977A1 - Mercury scavenging

Info

Publication number: WO2008025977A1
Application number: PCT/GB2007/003259
Authority: WO
Inventors: James Durrant; Emilio Palomares; Li Xiaoe
Original assignee: Imperial Innovations Limited; Institute Of Chemical Research Of Catalonia (Iciq)
Priority date: 2006-08-31
Filing date: 2007-08-29
Publication date: 2008-03-06
Also published as: GB0617223D0

Abstract

The present invention provides a method of moving mercury from the sample using a mercury scavenging compound comprising a metal organic chromophore which comprises a sulphur containing ligand.

Description

MERCURY SCAVENGING

The present invention provides a method for removing mercury from a sample by the reversible binding of mercury to a molecular scavenging species.

Over the past half a century, global mercury emissions have increased substantially due mainly to natural processes, emissions from coal-burning power plants and gold mining. Mercury is a volatile metal, which has a relatively long atmospheric residence time, resulting in long-range transport and homogenization on hemispherical scale. In aqueous solution, bacteria can transform mercury into methylmercury, a potent neurotoxin, which accumulates in seafood entering the food-chain. When ingested by humans, methylmercury triggers several serious disorders including sensory, motor and neurological damage. For example, when ingested by pregnant woman, methylmercury readily crosses the placenta and targets the developing fetal brain and central nervous system, which can cause developmental delays in children.

These environmental and health problems have prompted the development of methods for the detection and quantification of mercury in aqueous solutions, including biological fluids. Examples of chromogenic and luminescent chemical sensors, electrochemical devices, bio-sensors and sensors based on mass changes have been reported over the past few years. However, these systems have important limitations in their sensitivity and selectivity and typically require expensive and sophisticated equipment. For example, devices based on thin-film gold layers have been used to detect mercury but operate at relatively high temperatures (150 - 300°C) and require, for substantial sensitivity, complicated electronic circuits. Most sensors based on simple organic luminophores can usually only function in organic solvents, and often need long equilibration times for quantitative detection. Bio-sensing of Hg²⁺ has also its limitations requiring the use of buffering solutions and long equilibration times before the reading can be carried out.

It will be appreciated that methods of detecting the presence of mercury in for example drinking water provide information important for public safety. Various methods of mercury scavenging known in the art include chemical bonding by functionalized monolayers on mesoporous supports (FMMS), absorption by hydrous manganese oxides, dietary fibre from wheat bran, sulphur-impregnated activated carbons, and modified cellulose absorbent containing vicinal sulphur groups, photocatalysis by ARG-modified TiO₂, solar irradiation with hydrogen peroxide, and the use of scavenger leeches which accumulate mercury from fish carcasses. However, these methods do not provide an effective solution to the problem of mercury contamination. There is therefore a need in the art for a method of removing mercury from environmental samples such as water or air.

The present invention provides a method of removing mercury from a sample comprising a) contacting a sample comprising mercury with a scavenging compound; b) allowing the formation of a mercury-scavenging compound complex between the scavenging compound and the mercury in the sample; wherein said scavenging compound comprises a metal organic chromophore which comprises a sulphur containing ligand.

The mercury-scavenging compound complex can then be removed from contact with the sample, thereby removing mercury from the sample. The method of the invention is particularly favoured as the mercury can be removed from the scavenging compound, allowing the scavenging compound to be regenerated and subsequently reused.

The method of the invention therefore further comprises regenerating the scavenging compound from the mercury-scavenging compound complex produced in step b) set out above. Regeneration of the scavenging compound can be carried out by immersion of the mercury-scavenging compound complex in solution or by electrochemical or photochemical regeneration. In particular, the scavenging complex can be regenerated by contacting the mercury-scavenging compound complex with an aqueous iodide solution. Mercury released from the complex can be used, stored or disposed of.

It will be appreciated that once the scavenging compound has been regenerated by the removal of the mercury, the scavenging compound can be re-used in the method of the invention. The invention therefore particularly relates to a method of removing mercury from a sample comprising a) contacting a sample comprising mercury with a scavenging compound; b) allowing the formation of a mercury-scavenging compound complex between the scavenging compound and the mercury in the sample; c) regenerating the scavenging compound from the mercury- scavenging compound complex produced in step b); and d) repeating steps a) to c) as required.

The method can be repeated until the mercury has been removed from the sample or until the level of mercury in the sample has been reduced to an acceptable or pre-determined level. The scavenging complex of the present invention binds to mercury in a sample, thereby allowing its removal from the sample. As indicated above, the binding of mercury to the scavenging complex is preferably reversible. It is a particular feature of the invention that the scavenging complex selectively binds to mercury in a sample. The method therefore utilises a scavenging complex having a selectivity for mercury up to two times greater than for other contaminant metals such as Ca²⁺, Mg²⁺, Mn²⁺, Fe²⁺, Cd²⁺, Co²⁺, Cu²⁺, Pd²⁺, Ni²⁺, Pb²⁺ and Zn²⁺. Preferably, the scavenging compound has a selectivity which is three times greater, more preferably four times greater for mercury.

The method therefore provides the selective removal of mercury from a sample comprising mercury and one or more metals selected from Ca , Mg , Mn , re , Cd , Co , Cu , Pd , Ni , Pb , Zn .

The sample comprising mercury can be a liquid or gaseous sample, such as an organic or aqueous liquid. Alternatively, the sample can be air. In particular, the invention provides a method for removal of mercury from aqueous solutions such as river water, sea water, ground water, drinking water, and the like. The method of the first aspect can further be provided to remove mercury from a biological or medical sample, such as a biological or medical fluid or tissue, or suspension thereof, including blood products and urine.

The scavenging compound can be added to the sample and the complex removed via conventional techniques known in the art such as filtration or chromatography such as ion exchange chromatography, size exclusion chromatography. Alternatively, the scavenging compound can be supported on a substrate. It will be appreciated that providing the scavenging compound on a substrate allows easier handling, use, regeneration, storage and disposal of the scavenging compound. For the purposes of this invention, the substrate can be provided as a film or a colloidal suspension. When the substrate is provided as a film, the film can be free standing or the film can be attached to a support, such as a glass slide or a plastic film. The substrate is preferably a polymer, an organic matrix, an inorganic matrix, an ordered aluminosilicate, an amorphous aluminosilicate or a mixture of two or more thereof.

In particular, the support (as a film or as a colloidal suspension) may a metal oxide for example metal oxide particles, such as TiO₂, ZnO₂, ZrO₂, SiO₂, SnO₂,

Nb₂θ₅, WO₃, SiTiO₃ or a mixture of two or more thereof. In a preferred feature of the invention, the substrate is a metal oxide film, said metal oxide film preferably comprising a mesoporous nanocrystalline metal oxide. In a particularly preferred feature of the invention, the metal oxide film is TiO₂ bis(2,2' -bipyridy 1 -4,4' -dicarboxylato)ruthenium(II) bis-tetrabuytlammonium bis-thiocyanate.

As set out above, the scavenging compound comprises a metal organic chromophore which comprises a sulphur comprising ligand. In particular, the sulphur comprising ligand can be -N=C=S, -S-H or -NH-C(S)-NHR, wherein R is an alkyl group or an aryl group. For the purposes of this invention, the group R can be an alkyl group having 1 to 10 carbon atoms (that is 1, 2, 3, 4, 5, 6, 7, 8, 9, or 10 carbon atoms). The alkyl group can be branched or straight chain. The R group can be an aryl group, i.e. a 3 to 12 membered aromatic ring optionally comprising one or more heteroatoms selected from N, S or O, preferably phenyl, or naphthyl.

The scavenging compound of the present invention is preferably provided to remove mercury from aqueous solutions such as water. In order to improve the solubility of the scavenging compound in aqueous solution, the scavenging compound may further comprise one or more solubilising ligands, such as a hydrophilic sugar, a protein, a polymer or a mixture of two or more thereof. In particular, the one or more solubilising ligands can be selected from the group consisting of albumin, dextran, sepharose, polyribose, polyxylose, polyvinyl alcohol, ethylene glycol, propylene glycol or a mixture of two or more thereof.

Preferably, the scavenging compound is a compound of formula (I):

or a compound of formula (II):

(H) wherein each R is independently hydrogen or a solubilizing ligand selected from the group consisting of a hydrophilic sugar, a protein and a polymer.

The scavenging compounds are preferably [bis(2,2'-bipyridyl-4,4'- dicarboxylate)-ruthenium(II) bis-tetrabutylammonium bis-thiocyanate] and/or [2,2':6',2"-terpyridine-4,4',4^M-tricai-boxylate)-ruthenium(II) tris- tetrabutylammonium tris(isothiocyanate)]

The method of the invention allows removal of mercury from a sample by binding of mercury to a scavenging compound to form a mercury-scavenging compound complex. The formation of this complex can result in a change in the optical or electrochemical properties of the scavenging compound. These changes can be utilized to allow the binding of the mercury to the scavenging complex to be detected. In particular, the formation of the mercury-scavenging compound complex can be indicated by a change in the colour of the scavenging compound.

The method of the invention therefore allows the detection of mercury from a sample, by detection of a change in the optical or electrochemical properties of the scavenging complex. The scavenging complex further allows removal of mercury from the sample. The presence of mercury in a sample can be detected at a level of less than lOOOppb, preferably less than lOOppb.

It will be appreciated that when the formation of the mercury-scavenging compound complex is indicated by a change in the colour of the scavenging compound, it is possible for the presence and removal of mercury in sample to be monitored by the naked eye. In addition, the regeneration of the scavenging compound can also be monitored by a change in the colour of the scavenging compound.

The mercury is preferably provided in the sample as a free ion (Hg²⁺), a complex, such as a salt or as a metal organic complex, preferably methylmercury.

All preferred features of each of the aspects of the invention apply to all other aspects mutatis mutandis.

The invention may be put into practice in various ways and a number of specific embodiments will be described by way of example to illustrate the invention with reference to the accompanying drawings, in which:

Figure 1 illustrates the chemical structures of the molecular probes N719 and N749.

Figure 2 illustrates changes on the UV-vis absorption spectra of a 2 mL solution from a stock 20 μM of N719 upon addition of increasing amounts of mercury (II). The inset shows the absorption maximum shift in the visible region.

Figure 3 illustrates the molecular structure of the N719-HgCl₂ complex showing (a) the asymmetric unit which contains one mercury atom, crystallographically disordered in two positions at half occupancy, one [Ru(N₂C₁₂O₄Hs)₂(NCS)₂] complex and two CF anions (disordered atoms from the carboxylic units and H atoms have been omitted for clarity); and (b) a representation of the structure of the [Ru(N₂C₁₂O₄H₈)₂(NCS)₂HgCl]_n ⁿ⁺ chains parallel to the axis. Figure 4 illustrates the addition of metals added in same equimolar amounts (approximately 13 ppm) from aqueous solutions of their respective salts to 1 mL of N749 dissolved in ethanol. From left to right the solutions contain no metal, Hg²⁺, Cd²⁺, Pb²⁺, Fe²⁺, Cu²⁺ and Zn²⁺.

Figure 5 illustrates the titration of the change in the absorption of a N749 in ethanol measured at a wavelength of 600 nm versus the concentration of added Hg²⁺ ions. The inset shows the ampliation of the graph for the low limit colorimetric response for mercury. The mercury was added from a stock HgCl₂ aqueous solution.

Figure 6 illustrates mesoporous nanocrystalline TiO₂ sensitized with N749 before (left) and after immersion in a 9 ppm solution for 1 hour (centre) and 300 ppm solution for 5 minutes (right) of HgCl₂ in distilled water.

Figure 7 illustrates a plot showing the changes in absorbance of a TiO₂ film sensitized with N719 upon sequential exposure of the dye to an aqueous solution of HgCl₂ (wavelength = 480 nm) followed by dipping into a KI solution (wavelength = 530 nm).

Figure 8 illustrates colour changes on the UV-Vis absorption spectra at different mercury concentration. Mercury concentration is in range of 0, 0.15, 0.3, 0.5, 0.8, 1.5, 3.0, 5.0 ppm.

Figure 9 illustrates color changes observed for M⁺ ions aqueous solution. Colour change observed after the N719/EtOH solution was added into 12.5 μM M⁺ ions aqueous solution (for Hg²⁺ 2.5 ppm). (a) from left to right: no metal ions, +Hg²⁺, +Ca²⁺, +Mg²⁺, +Mn²⁺, +Fe²⁺, +Cd²⁺, +Cu²⁺, +Co²⁺, +Ni²⁺, +Pb²⁺, +Zn²⁺, +Li⁺.

(b) From left to right: no metal ions, +Hg²⁺, + all EPA metals.

Figure 10 illustrates the calibration graph for calculate mercury concentration using the ratio of absorbance at 508 nm to 438 nm vs. the concentration of Hg²⁺ aqueous solution.

Figure 11 illustrates changes on the mercury scavenging efficiency as TiO₂/N719 solid film (a) and P25 free standing film (b) dipped into metals aqueous solution. The mercury scavenging efficiency η was defined as

C - C

V = - (D

where C₀ and C are the mercury concentration before and after scavenging, in ppm.

Figure 12 illustrates reduction of mercury concentration to TiO₂/N719 single film dipping times.

Figure 13 illustrates relationship between mercury scavenging efficiency and numbers of repeated dips of TiO₂/N719 films.

Examples

The invention will now be illustrated by reference to one or more of the following non-limiting examples. Example 1 - Investigations of the mercury-N719 dye interaction: solution and X-ray studies.

The origin of the observed color change of the TiO₂-supported [bis(2,2'- bipyridyl-4,4'-dicarboxylate)-ruthenium(II) bis-tetrabutylammonium bis- thiocyanate] (N719) dye in the presence of mercury(II) ions was investigated. When different amounts of Hg²⁺ were added to an aqueous solution of N719 the color change of the solution (from dark red-purple to orange) could be seen by naked eye inspection (down to 1.5 ppm). The aqueous solution of N719 was titrated with a solution of HgCl₂ and the Uv-Vis absorption spectra recorded. As is shown in Figure 1, upon addition of the metal salt to the dye, the absorption at 530 nm decreases while that one at 480 nm increases. The limit of sensitivity in solution (~ 20 ppb) is even lower than that one observed when the dye is supported onto the TiO₂ nanoporous film.

It was observed by IR spectroscopy that upon addition of HgCl₂ to an aqueous solution of N719, the peak for the C≡N stretching frequency corresponding to the NCS group was reduced in intensity and shifted from 2112 cm^"1 to 2150 cm^"1 suggesting the existence of an interaction between the mercury(II) ions and this group.

Crystallographic studies. Crystals of [Ru(N₂C₁₂O₄Hs)₂(NCS)₂HgCl₂] -5H₂O (M_w = 1067.22) were obtained by slow diffusion of a water solution (15 mL) of HgCl₂ (100 mg, 0.300 mmol) layered with an acetonitrile solution (10 mL) of N719 (20 mg, 0.017 mmol). Prismatic well-shaped orange crystals grew after several days in the dark. These crystals are sensitive to solvent loss, and out of the mother liquor become brittle and finally break up after a few hours. An orange prismatic crystal (0.3 x 0.1 x 0.1 mm) of the N719-HgCl₂ complex was picked from its mother liquor, secured on a glass fiber tip and mounted on a Kappa CCD Smart X-ray diffractometer equipped with a graphite- monochromated Mo-K_α radiation (λ = 0.71073 A). All frames were collected at 180 K to avoid solvent loss, integrated and data corrected for absorption using the KappaCCD software package (MAXUS). The structure was solved by direct methods using the SIR97 program, and refined on F² with the SHELXL-97 program. A total of 9359 unique reflections were collected. The crystal system was determined to be triclinic P-I, with unit cell parameters: a = 8.8369(3), b = 15.2570(7), c = 17.4370(7) A and α = 79.554(2), β = 86.526(1), γ = 80.798(2) °, V = 2280.8(2) A³; for Z = 2, p_calc = 1.554 g/cm³. All non- hydrogen atoms were located after successive Fourier difference maps, and also some of the H atoms from the carboxylic groups. The rest were located in its calculated positions, and in all H atoms their thermal parameters were fixed to be 50% larger than those of the atoms to which they are bound. According to the data, the positions corresponding to the Hg centers were refined at half occupancy. The carboxylic groups also showed positional disorder over two possible positions, and this was best modeled assigning half occupancy to each position. The solvent areas were modeled to disordered oxygen atoms from water molecules. All non-hydrogen atoms were refined anisotropically, except the disordered carboxylic groups and solvent molecules. Final refinement for 6330 reflections [I > 2σ(I)] and 521 parameters gave Rl = 0.0624 and R² _W = 0.1428 (Goof = 1.004). CCDC-259514 contains the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/conts/retrieving.html (or from the Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 IEZ, UK; fax: +44-1223-336-033; e-mail: deposit@ccdc.cam.ac.uk^'). However, we would like to notice that, depending on the relative concentration of species in solution, other compounds can be formed. In particular, we found that the first solid precipitate that appears at the limit of low Hg concentration/precipitate formation, showed a Ru/Hg ratio of 3:2, different to the 1 : 1 found in the crystalline solid isolated. This microanalysis was performed with a JEOL 6300 microscope equipped with an EDAX analysis system.

Spectroscopic studies in solution. The N719 and N749 molecular probes were dissolved in ethanol (HPLC grade) to obtain a 20μM stock solution. The mercury(II) sensitivity experiments were carried out on a quartz cuvette with a total volume of 3 mL. The mercury(II) aliquots were injected into the quartz cuvette from a ImM HgCl₂ aqueous stock solution. The changes in the Uv- Vis absorption spectra were monitored at room temperature using a double beam U v- Vis spectrophotometer Shimazdu Uv- 1601.

Single crystals suitable for an X-ray crystallographic study were obtained by layering an aqueous solution of HgCl₂ with a solution of N719 in acetonitrile. The structural characterization showed that the mercury(II) ions coordinate to the sulfur of the NCS groups from different N719 molecules (see Figure 2a). The asymmetric unit contains one mercury atom, crystallographically disordered in two positions at half occupancy, one Ru(N₂C₁₂O₄Hs)₂(NCS)₂ complex and two CF anions. One of the mercury positions (HgI) is tetrahedrally coordinated to three sulfur atoms from three different adjacent ruthenium complexes, and one Cl^" anion, with mercury-to-ligand distances between 2.463(2) A for the Hgl-Cll and 2.578(2) A for the longest HgI-Sl coordination bond. The angles are close to regular tetrahedral, except for two of the Cl-Hg-S angles, that deviate to 131.2(1)° (Cll-Hgl-S2) and 96.41(7)° (CIl-HgI-Sl). On the other hand, Hg2 presents a linear coordination, bound to one sulfur and one CF anion, with much shorter bond distances (Hg2-S2 = 2.289(2) A and Hg2-Cll = 2.268(2) A). The two mercury positions are just about 1 A apart, what clearly indicates that both positions cannot be occupied at the same time, while the CIl atom, with occupancy of one, coordinates both positions, whichever mercury atom is present locally. All thiocyanate ligands from the ruthenium complexes coordinate also to a mercury atom, with Sl coordinating only HgI positions, and with S2 coordinating to either HgI or Hg2, depending on a given local distribution. The growth of the asymmetric unit gives rise to chains than run parallel to the a axis (Figure 2b).

Between chains there are several short contacts between carboxylate groups that correspond to strong hydrogen bonding. The O-O distances, as short as 2.5 A, are clear evidence that the carboxylate groups are protonated. All protons from these groups were located (see the experimental section and supplementary information), even when the carboxylate groups show some disorder over two adjacent positions that deviate slightly, below or above, from the planar bipyridine rings.

These cationic chains of formula [Ru(N₂C ₁₂O₄H₈)₂(NCS)₂HgCl]_n ⁿ⁺ leave large empty channels running also along the a axis, that constitute over 35% of the unit cell volume. These channels are occupied by free Cl^~ anions required for electroneutrality and disordered solvent molecules, assigned to water. This explains the instability of the crystals, since the crystallization solvent molecules rapidly leave the structure at ambient pressure provoking a structural collapse.

The formulation show by X-ray crystallography of the N719-mercury(II) complex is consistent with the elemental analyses of the bulk sample. This crystalline solid is however an insoluble coordination polymer and hence is not likely to be the complex directly responsible for the changes observed in solution. The crystallographic evidence however strongly suggests that at low concentrations of mercury(II) and N719 dye, a molecular complex (involving NCS-mercury interactions) must be formed. The co-ordination polymer forms only at higher concentration and after long periods of time. Example 2 - Studies of the interaction between mercury and the ruthenium dye N749.

The interaction between mercury(II) salts and the complex (2,2':6',2"- terpyridine-4,4',4"-tricarboxylate)-ruthenium(II), tris-tetrabutylammonium tris(isothiocyanate)) (N749) was investigated. This was first investigated in solution by adding different cations (namely Hg²⁺, Cd²⁺, Pb²⁺, Fe²⁺, Cu²⁺ and Zn²⁺) to a solution of the N749 dye. As is shown in Figure 3, the ruthenium complex undergoes a dramatic color change from green-dark to pink upon addition of HgCl₂ salts but not in the presence of any other of the tested metal cations.

The solutions of the green-black ruthenium complex were titrated with HgCl₂, showing that upon addition of increasing amounts of the mercury(II) salt the charge transfer band centered at λ= 625 nm decreased while a new band appeared at λ = 560 nm. As can be seen in Figure 4, the dye has a linear response to increasing amounts of mercury(II) (between 0.5 and 60 nanomols). From this plot it is also possible to establish that the system is sensitive down to -100 ppb.

Preparation of sensitized nanoporous TiO₂ films. The mesoporous, nanocrystalline TiO₂ films comprising 15 nm sized anatase TiO₂ particles were prepared as follows: 20 ml of titanium isopropoxide were injected into 4.5 g. of glacial acetic acid under argon atmosphere and stirred for 10 minutes. The mixture was then injected into 120 ml of 0.1 M nitric acid under argon atmosphere at room temperature in a conical flask and stirred vigorously. After the nitric acid addition, the flask was left uncovered and heated at 80⁰C for 8 hours. After cooling, the solution was filtered using a 0.45 μm syringe filter, diluted to 5% wt. TiO₂ by the addition of H₂O and then autoclaved at 220⁰C for 12 hours. The colloids were re-dispersed with a 60 seconds cycle burst from a LDU Soniprobe horn as reported before. The solution was then concentrated to 12.5% on a rotary evaporator using a membrane vacuum pump at 40⁰C. Carbowax 20,000 (6.2% by weight) was added and the resulting paste was stirred slowly overnight to ensure homogeneity. The suspension was spread on the substrates by a glass rod, using 3M adhesive tapes as spacers. After the films were dried in air, they were sintered at 450⁰C for 20 minutes in air. The thickness of the TiO₂ films was controlled resulting in film thickness of 4 μm.

Spectroscopic measurement of N749 supported onto mesoporous nanocrystalline TiO₂ films. The N749 dye was adsorbed onto a 4 μm thick nanoporous, optical transparent TiO₂ film by soaking the film on 1 mM solution of the dye in a 1:1 mixture of acetonitrile/tert-buthanol at room temperature overnight, followed by rinsing in ethanol to remove unadsorbed dye, resulting in a strong green-black coloration of the film (as illustrated in Figure 3). The dye loading, determined from the optical density of N749 observed for the mesoporous nanocrystalline TiO₂ film at the N749 absorption maximum (600 nm), was estimated -95% of monolayer coverage.

Mercury(II) sensitivity experiments were carried out on a quartz cuvette with a total volume of 3 mL. The N749-sensitized TiO₂ film was immersed on the aqueous solution and the cuvette fixed to the Uv- Vis spectrophotometer holder. The UV- Vis absorption spectra were recorded after addition of the corresponding cation with an equilibration time before the measurement of 2 minutes. Experiments were conducted in unbuffered aqueous solutions, unless otherwise stated. Studies as a function of solution pH employed an aqueous solution starting at pH 3.0 and increasing the amount of base until pH 8.0 was reached. pH measurements in non-aqueous media (acetonitrile) where carried out using a HI 8424 microcomputer pH meter glass electrode. The calibration was performed according to the literature method.

The reversibility experiments were performed dipping the N719-sensitized TiO₂ film (~1 cm area) in a solution containing 1 x 10^" M cation concentration. After dipping the UV- Vis absorption spectra was measured and finally, the sample was dipped on a vial containing 1OmM KI in distilled water and dried at 40⁰C. The same sample was used in repeating cycles without appreciable dye desorption. However, it is noteworthy that the samples show less efficient reversibility after 6 cycles.

The ruthenium complex (N749) was adsorbed onto a 4 μm thick mesoporous TiO₂ film by soaking the film in 1 mM solution of the dye in a 1:1 mixture of acetonitrile/tert-buthanol at room temperature overnight. The cation sensing experiments with the TiO₂ / N719 films were carried out in distilled water (~ pH 5) by exposing the films to micromolar solutions of the metal cations under study (i.e. Hg²⁺, Cd²⁺, Pb²⁺, Fe²⁺, Cu²⁺ and Zn²⁺). As observed in the solution studies, the supported N749 dye only changed color when dipped into the mercury(II) solution (see Figure 5).

Mesoporous nanocrystalline TiO₂ film sensitized with N719 was first exposed to a 1 mM solution of HgCl₂ in water; the resulting orange film (due to the interaction of the mercury(II) cations with the NCS groups of the ruthenium complex) was subsequently dipped into a 10 mM solution of KI. This generated an immediate colour change of the film back to the red colour characteristic of the TiO₂-N719 film before exposing it to mercury(II). Subsequent dipping of this film into the HgCl₂ solution resulted in a change in the colour of the film to orange once again. This procedure can be repeated several times demonstrating that the supported dye can be re-utilized by dipping it into an iodide solution (see Figure 6).

Example 3 - Determination of the calibration for mercury scavenging

N719 in ethanol solution (cone. 2 mM) was added to different concentration of HgCl₂ solution which is in range of 0, 0.15, 0.3, 0.5, 0.8, 1.5, 3, and 5 ppm respectively. All experiments were performed at room temperature. 10 mM HEPES buffered H₂O (pH~5.65) chosen for blank experiment. The reduction of the ratio of absorbance measured at 508 nm to 438 nm was plotted vs. the concentration of Hg²⁺ ions in ppm.

Example 4 - Determination of the scavenged mercury

The mercury scavenging experiment was performed by dipping the TiO₂/N719 film (1 cm² area) in 12.5 μM Hg²⁺ solution for 10 min to scavenge mercury(II). After dipping, 2 mM N719 in ethanol solution was added to sense the residue mercury in solution and Uv-Vis absorption spectrum of mercury solution was measured. The mercury concentration was calculated using the calibration graph (figure 3), and the decrease in mercury concentration was plotted against TiO₂/N719 film dipping times (as shown in figure 5).

Example 5 - Determination of scavenging films' selectivity

The capability of the scavenging film for removal of mercury from solutions containing other ions was investigated using ICP-AES (Varian Vista-Pro) measurements of metal uptake from solutions of mixed metal ions. A solution containing 20 μM mercuric, ferric, cadmium, magnesium, lead, cupper, zinc, cobalt, lithium, magnesium, Manganese and Nickel ions was prepared from the nitrate salts of the respective metals.

Table 1 Mercury Uptake in Presence of Other Metal Ions*

TiO₂/N719 film dipped into mixed solution for 10 min.

Example 6 - Determination of scavenging films' reversibility

The mesoporous nanocrystalline TiO₂ film (1 cm² area) immoblized with N719 was first exposed to a 5 μM HgCl₂ aqueous solution; the resulting orange film (due to the interaction of the mercury(II) cations with the NCS groups of the ruthenium complex) was subsequently dipped into a 10 mM solution of HEPES buffered KI (pH~5.65). This generated an immediate color change of the film back to the red color characteristic of the TiO₂/N719 film before exposing it to mercury(II). If this film is subsequently dipped again into the HgC12 solution, the color of the film changes to orange once again. This procedure can be repeated several times demonstrating that the supported dye can be re-utilized by dipping it into an iodide solution.

Claims

1. A method of removing mercury from a sample comprising a) contacting a sample comprising mercury with a scavenging compound; b) allowing the formation of a mercury-scavenging compound complex between the scavenging compound and the mercury in the sample; wherein said scavenging compound comprises a metal organic chromophore which comprises a sulphur comprising ligand.

2. Method as claimed in claim 1 wherein the mercury-scavenging compound complex is removed from contact with the sample after formation of the mercury-scavenging compound-mercury complex.

3. Method as claimed in claim 1 or claim 2 wherein the scavenging compound is regenerated from the mercury-scavenging compound complex produced in step b) of claim 1.

4. Method as claimed in claim 3 wherein the mercury-scavenging compound is regenerated from the mercury-scavenging compound complex by immersion of the mercury-scavenging compound complex in solution or by electrochemical or photochemical regeneration.

5. Method as claimed in claim 3 or claim 4, wherein the mercury- scavenging compound complex is regenerated by contacting the complex with an aqueous iodide solution.

6. Method as claimed in any one of claims 3 to 5 further comprising the steps of c) regenerating the scavenging compound from the mercury- scavenging compound complex produced in step b); and d) repeating steps a) to c) as required.

7. Method as claimed in any one of claims 1 to 6 wherein the scavenging compound has a selectivity for mercury up to two times greater than for Ca²⁺,

Mg²⁺, Mn²⁺, Fe²⁺, Cd²⁺, Co²⁺, Cu²⁺, Pd²⁺, Ni²⁺, Pb²⁺ or Zn²⁺

8. Method as claimed in any one of claims 1 to 7 wherein said method provides the selective removal of mercury from a sample comprising mercury and one or more metals selected from Ca²⁺, Mg²⁺, Mn²⁺, Fe²⁺, Cd²⁺, Co²⁺, Cu²⁺, Pd²⁺, Ni²⁺, Pb²⁺, Zn²⁺.

9. Method as claimed in any one or claims 1 to 8 wherein the sample is an liquid or gaseous sample

10. Method as claimed in any one of claims 1 to 9 wherein the sample is an organic or aqueous liquid.

11. Method as claimed in claim 10, wherein the aqueous solution is drinking water.

12. Method as claimed in claim 10, wherein the aqueous solution is a biological or medical fluid or tissue, or suspension thereof.

13. Method as claimed in any one of claims 1 to 9 wherein the sample is air.

14. Method of any one of claims 1 to 13, wherein the scavenging compound is supported on a substrate.

15. Method as claimed in claim 14, wherein the substrate is a film or a colloidal suspension.

16. Method as claimed in claim 15 wherein the film is free standing or wherein the film is attached to a support.

17. Method as claimed in claim 16 wherein the film is attached to a glass slide or a plastic film.

18. Method as claimed in any one of claims 14 to 17 wherein the substrate is a polymer, an organic matrix, an inorganic matrix, an ordered aluminosilicate, an amorphous aluminosilicate or a mixture of two or more thereof.

19. Method as claimed in claim 15 wherein the colloidal suspension comprises metal oxide particles.

20. Method as claimed in any one of claims 15 to 17 wherein the substrate is a metal oxide film.

21. Method as claimed in claim 20, wherein the metal oxide film comprises a mesoporous nanocrystalline metal oxide.

22. Method as claimed in any one of claims 19 to 21, wherein the metal oxide is TiO₂, ZnO₂, ZrO₂, SiO₂, SnO₂, Nb₂O₅, WO₃, SiTiO₃ or a mixture of two or more thereof.

23. The method of any one of claims 20 to 22, wherein the metal oxide film is TiO₂ bis(2,2^>-bipyridyl-4,4'-dicarboxylato)ruthenium(II) bis- tetrabuytlammonium bis-thiocyanate.

24. Method as claimed in any one of claims 1 to 23, wherein the sulphur comprising ligand is -N=C=S, -S-H or -NH-C(S)-NHR, wherein R is an alkyl group or an aryl group.

25. Method as claimed in any one of claims 1 to 24, wherein the scavenging compound further comprises one or more solubilizing ligands.

26. Method as claimed in claim 25, wherein the one or more solubilizing ligands are a hydrophilic sugar, a protein, a polymer or a mixture of two or more thereof.

27. Method as claimed in claim 26, wherein the one or more solubilizing ligands is selected from the group consisting of albumin, dextran, sepharose, polyribose, polyxylose, polyvinyl alcohol, ethylene glycol, propylene glycol or a mixture of two or more thereof.

28. Method of any one of claims 1 to 27, wherein the scavenging compound is a compound of formula (I):

or a compound of formula (II):

wherein each R is independently hydrogen or a solubilizing ligand selected from the group consisting of a hydrophilic sugar, a protein and a polymer.

29. Method as claimed in any one of claims 1 to 28 wherein the formation of the mercury-scavenging compound complex results in change in the optical or electrochemical properties of the mercury-scavenging compound.

30. Method of any one of claims 1 to 29, wherein the mercury is in the form of a metal organic complex.

31. Method of any one of claims 1 to 30 wherein the formation of the mercury-scavenging compound complex is indicated by a change in the colour of the scavenging compound.

32. Method as substantially described herein with reference to one or more of the examples and/or figures.