GB2314074A - Negatively charged carbon as chromatographic material and preparation by electrolysis - Google Patents

Abstract

A material is provided comprising negatively charged carbon particles with a particle diameter of from about 40 nm to about 2 mm which is suitable for use as a chromatographic reagent or as a lubricant. The particulate carbon material is produced by passing a DC current between two carbon electrodes disposed in a suitable electrolyte such as a mineral acid, an organic acid or deionised water.

Description

CHROMATOGRAPHIC MATERIALS AND PROCESSES FOR PREPARATION THEREOF The invention relates to a chromatographic material and in particular, to charged particulate carbon which is useful as an ion exchanger. The invention also relates to apparatus and methods for preparing charged particulate carbon, as well as uses thereof.

Activated charcoal has been known for many years as a sorbent which is used in a variety of chemical industrial production processes. However, the chemical nature of the surface groups which confer this sorbent property is still not fully understood.

Many attempts have been made to modify the surface chemistry of charcoal, so altering the sorbent properties. For example, chemical surface modification of charcoal is discussed in the following references: Vinke, P. et al (1994) Carbon 32 (4) 675; Giroir, A et al (1988), Heterogeneous Catalysis and Fine Chemicals, ed. Guisnet et al, Elsevier Science Publishers, Amsterdam P171; Schwegler M.A. et al (1992) Appl. Catal A. 80, 41; van Dam H.E. et al (1991) J. Catal 131 335; Boehm H.P. (1990) High Temp. - High Press 22, 275; Garten V.A. et al (1957) Rev. Pure Appl. Chem 7 69; Puri B.R. (1970) Chemistry and Physics, ed P.L.

Walker, Jr, 6, 191, Marcel Dekker, New York.

Particulate carbon, and especially colloidal carbon is a known form of carbon which may be used as a sorbent but which has other known uses such as, for example, a label in immunoassays or to abrogate antigenic competition in polyvalent vaccines.

The present inventors have developed a simple electrochemical method for producing a particulate carbon, in one embodiment a colloidal carbon, in which the surface chemistry of the carbon particles has been modified so as to be negatively charged.

These negatively charged particles have a cationic exchange capacity of up to about 7.0 milliequivalents per gram of carbon, which is higher than most known ion exchange resins. The colloidal particles are stable and retain their charge for more than two years. They are therefore very suitable for use as an ion exchanger.

Thus, in accordance with a first aspect of the invention there is provided a chromatographic material comprising negatively charged carbon particles with a particle diameter of from about 40 nm to about 2 mm.

In a second aspect the invention provides a method of producing particulate carbon, in particular negatively charged carbon particles, which method comprises passing a DC current between two carbon electrodes disposed in an electrolyte selected from a mineral acid, an organic acid or deionized water.

In a third aspect the invention provides an electrolytic cell or apparatus which comprises a container for the electrolyte and two carbon electrodes acting as anode and cathode respectively and adapted for connection to a source of direct current.

In carrying out the method of the invention the source of direct current is connected to the two carbon electrodes and switched on. Within a few hours a black colouration accumulates around the anode and remains in the vicinity of the anode indicating that it is negatively charged. If the current polarity is reversed this colouration immediately migrates to the other electrode.

Analysis of the material which collects around the anode has confirmed it to be colloidal carbon which is negatively charged and which has a particle diameter in the range from about 40 nm to about 900 nm. As will be demonstrated below, the amount of colloidal carbon generated and the time needed depends upon the voltage, the current density and the electrolyte used. In most of the electrolytic conditions tested, in addition to the colloidal carbon collecting around the anode, a particulate carbon slurry is produced which falls to the bottom of the electrolytic cell. This slurry material is also negatively charged and has a particle diameter of from about 0.1 mm to 2 mm.

Mineral acids, organic acids and even deionized water are suitable electrolytes for carrying out the method of the invention. Preferred acids are ethanoic acid or sulphuric acid at a concentration of from 0.01 to 1.0 M.

The carbon electrodes may be in the form of rods, sheets, felt or tow. For example, carbon rods taken from zinc/carbon batteries are particularly suitable.

Alternatively, spec pure carbon rods or reactor grade graphite sheets may be used.

In the electrolytic cell of the invention it is preferable if the container for the electrolyte has separate compartments for the anode and cathode while allowing adequate passage of current therebetween.

For example, a suitable electrolytic cell is a U-tube in which the carbon anode and cathode are disposed in each of the arms, respectively.

Depending on the type and amount of carbon product required, the direct current may need to be passed for a period of up to eight hours but preferably longer. For example, current may be passed for eight hours in any one twenty-four hour period for as long as three weeks. With some electrolytes, such as sulphuric acid, it is observed that the level of current which can be passed falls with time. However, the current flow may be restored to normal by swapping the electrodes from one electrode compartment to another without changing the polarity.

The DC voltage applied may be from 2 to 200 volts but is preferably from about 8 to about 33 volts. The current may be from about 1 to about 80 mA and preferably from about 3 to about 15 mA.

When the electrolysis is complete the current is switched off and the colloidal carbon and/or slurry can be recovered from the electrolyte. There are a number of ways in which this recovery may be facilitated. For example, the electrolyte may be evaporated from the particulate carbon in an oven.

Alternatively, the electrolyte/particle mixture may be passed through a filter of appropriate pore size.

Other alternatives for recovery of the particulate carbon include azeotropic distillation of water from the particulate carbon/electrolyte mixture with chloroform, concentrating the colloid in a Dreschle bottle for a period of between 1 to 3 days, centrifugation of the particle/electrolyte mixture or vacuum drying of the mixture. A particularly preferred method of recovering the colloidal carbon is to slurry it with ammonium carbonate, evaporate the water under vacuum and then sublime off the ammonium carbonate to leave a fine carbon powder.

Both the colloidal carbon and the carbon slurry of the invention have been analysed to determine the cationic exchange capacity, the BET surface area, the solvation state and the nature of the chemical modification on the surface.

The cationic exchange capacity of the slurry has been measured by passing ferric nitrate solution through a bed of a known weight of carbon particles produced in accordance with the method of the invention, washing with water and eluting the absorbed iron with 5 M hydrochloric acid. The eluant is then analysed for iron and the absorption of iron per gram calculated. The ferric nitrate method shows the slurry to have a cationic exchange capacity of from 0.4 to 2.0 milliequivalents per gram.

The surface area measurement of the colloidal particles is performed on dried material with a Gemini 2375 surface area analyser using the BET method with nitrogen absorption at 77 K (see Brunauer et al (1940) J.A. Chem Soc 62 1723) assuming a surface area of the nitrogen molecule of 0.162 nm2. The colloidal particles have been found to have a BET surface area in the range from about 14 m2g1 to about 80 m2gl.

Preferred particles have a BET surface area in the range from 50 m2g- to 80m2g-l Titration of colloidal carbon prepared in double deionized water as the electrolyte has identified three distinct solvation shells showing ratios of approximately 1.7 acid molecules: carbon atoms in the inner shell, approximately 16 acid molecules: carbon atoms in the second shell and approximately 20 acid molecules: carbon atoms in the outer shell and this method shows the colloidal carbon to have a cation exchange capacity of approximately 7.0 milliequivalents per gram. This is a comparatively high exchange capacity since commercial cation exchange resins have exchange capacities in the region of about 4.5 milliequivalents g-1.

The nature of the chemical modification of the particulate carbon to introduce the negative charge has been analysed using FTIR spectroscopy. This has confirmed the presence of ketonic carbonyl groups tautomeric with the enolic form and hydroxyl groups, both of which give rise to the ion exchange properties of the particles.

From the spectroscopic data and observation during electrolysis it appears that the negatively charged colloidal carbon arises by erosion of the anode. During electrolysis oxygen is generated at the anode which is evolved as bubbles, apparently coming from the interior of the anode. These bubbles cause the electrode to disintegrate but the rate of this disintegration and the size of the resulting particles depends upon the current, the voltage applied and the electrolyte chosen. For example, in experiments where the current density was less than 3 mAcm2 the anode slowly eroded to form both colloidal carbon and a carbon slurry. On the other hand, at a current density of 16 mAcm2 with 1.0 M sulphuric acid as the electrolyte, the anode broke up in a few minutes.

These observations suggest that a relatively low current density, for example, about 3mAcm2, is more effective at producing colloidal carbon than a high one. If the rate of oxygen evolution at the anode is too much higher than its rate of diffusion through the electrode there is a pressure build up within the electrode causing it to disintegrate. For production of charged colloidal carbon and carbon slurry the rate of oxygen evolution should be such that some pressure builds up within the electrode but it is able to diffuse out of the electrode before disintegration occurs, small pieces of carbon breaking off in the process to form the colloidal carbon.

It has been determined that a particularly preferred method for producing charged colloidal carbon in accordance with the invention requires 0.03 M sulphuric acid as electrolyte, a DC voltage of about 25 volts and a current density of about 15 mA.

Preferred conditions for producing the charged carbon slurry are 0.03 M sulphuric acid, about 20 volts and about 20 mA current. The rate of colloid production may be increased by connecting a number of electrolyte cells in accordance with the invention, in parallel.

During electrolysis hydrogen is generated at the cathode which is observed as very small bubbles coming from the surface of the carbon cathode. The evolution of the hydrogen is never such as to build up pressure and cause disintegration of the cathode.

On the basis of the titrimetric and spectral evidence, while not wishing to be bound by such, the applicants have proposed a mechanism for the processes occurring at the electrodes. The spectroscopy shows double bonds present in the carbon colloid which are needed to form a radical cation which can be hydrated as follows: Anode Reaction -e HzO H > c-C < - > C±c < - - -C(OH)-C < - -C(OH)-C(H) -C(O)-C(H)2- radical enol form keto form cation There is keto-enol tautomerism and since the reaction takes place in acid medium the reaction is acid catalysed.

H+ Fast -H4 Slow -CH2-C(O)- " -CHz-C(OlH)- u -CH-C(OH)- Slow H Fast Cathode Reaction If electrode polarity is reversed during electrolysis then the colloidal carbon or carbon slurry produced at the anode is brought into contact with nascent hydrogen. It is proposed that in these circumstances the carbonyl group of the ketonic oxygen is reduced to a secondary alcohol by hydrogen at the cathode. Similarly, with the double bond of the material in the enol form.

+e Hf H+ > C = 0 -4 > C - 0 - > C(OH) b > CH(OH) radical radical anion Therefore keto enol tautomerism also occurs in this material as well as reduction of the double bond by nascent hydrogen.

H -C(OH)=C < ) -CH(OH)-CH2 In practice it may sometimes be desirable during the electrolysis to reverse the polarity of the electrodes, preferably every 10 minutes or so, to introduce hydrogen into the colloidal carbon. To this end a voltage reversal unit may be included with the power pack for supplying direct current to the carbon electrodes.

As aforesaid, the colloidal carbon produced at the anode in accordance with the method of the invention has a cationic exchange capacity of up to 7.0 milliequivalents per gram. It can therefore serve as a cationic exchange material for ion exchange chromatography or be used as a catalyst support. The chromatographic material of the invention is capable of removing radioactive metal ions from solution. The particles themselves may be used as a chromatographic reagent or alternatively they may be derivatized or coated onto glass beads. The charged carbon particles of the invention may have attached thereto an affinity reagent, such as an antibody or another affinity ligand. Finally, irrespective of whether the particles are negatively charged, the particulate carbon produced by the method of the invention may be useful as a lubricant.

The invention will now be described in more detail with reference to the following examples and figures in which: Figure 1 shows the particle size distribution of colloidal carbon where (1) is carbon prepared in ethanoic acid (0.017M), (2) double deionised water and (3) sulphuric acid (0.03M); Figure 2 shows electrophoretic separation of colloidal carbon where (1) is carbon prepared in double deionized water and (2) is carbon prepared in ethanoic acid; Figure 3 shows sub-microtitrimetry of colloidal carbon prepared in double deionised water and titrated with standard hydrochloric acid; Figure 4 shows W-vis spectra of colloidal carbons where (1) is carbon prepared in ethanoic acid and (2) carbon prepared in double deionised water; and Figure 5 shows FTIR spectra of colloidal carbons where (1) is the original material, (2) is carbon prepared in double deionised water (3) is carbon prepared in ethanoic acid (0.17M), (4) is carbon prepared in sulphuric acid (0.03M) and (5) is carbon prepared in sulphuric acid (0.03M) in which the electrode polarity has been reversed.

Example 1 A glass U-tube of 1.5 cm diameter and 15 cm long was filled with 0.03 M sulphuric acid. Carbon rods of 0.5 cm diameter and 5 cm long from a zinc/carbon battery were washed with double deionised water and placed in each arm of the tube to a depth of 3 cm in the electrolyte, i.e. the surface area in contact with the electrolyte was about 5 cm2. Direct current of 15 mA was passed for eight hours a day at 25 volts using a 0-33 VDC power pack and a voltage reversal unit.

Since the current flow fell over time it was periodically restored to normal by changing the electrodes from one compartment to the other without reversing the polarity. The colloid produced at the anode and the slurry was collected once every three weeks. On removal the particulate material was filtered through a Whatman 541 filter paper to remove small pieces of carbon.

Example 2 A similar procedure was carried out as in Example 1 except that 200 volts DC was applied to carbon electrodes, the electrolyte was 0.17 methanoic acid the current flow was 25 mA. The evolution of oxygen at the anode caused the production of colloidal carbon plus some carbon slurry. This was filtered off to give a sol of colloidal at about 2 mg carbon/cm3.

Example 3 The procedure was the same as in Example 2 except that the voltage was reduced to 33 volts DC, the current passing at about 3 mA. There were no lumps of carbon produced but a small amount of carbon slurry was present at the bottom of the U tube. The resulting colloid was filtered through a Whatman 541 filter paper to give a carbon sol of about 2 mg carbon/cm3 in 60 cm3 of water. The particle size averaged 200 nm with a particle size range of 40-870 nm and a BET surface area of 50 m2/g.

Example 4 The procedure was the same as Example 3 except that the electrolyte was double deionised water and the current at 33 volts DC was 1 mA. There was no slurry formed and the particle size averaged 182 nm with a particle size range 42-630 nm. The BET surface area was 79 m2/g Example 5 The procedure was the same as Example 4 except that the electrolyte was 1 M sulphuric acid. The current at 33 volts was 80 mA. Colloidal carbon was produced at this current but the carbon anode broke up into lumps.

Example 6 The procedure was the same as Example 5 except that the electrolyte was 0.03 M sulphuric acid. The voltage applied was 8 volts DC, the current flow being 14 mA. There was some slurry formed and the particle size averaged 133 nm with a particle size range of 30630 nm. The BET surface area was 75 m2/g.

Example 7 The procedure was the same as Example 6 except that a number of colloid producing columns were wired up in parallel.

Example 8 The procedure was the same as Example 6 except that the polarity of the electrodes was reversed every 10 minutes. This had the effect of introducing hydrogen into the carbon.

Example 9 The procedure was the same as Example 6 except that the slurry was removed by centrifuging, washed with water until the washings were neutral and the slurry air dried at 1100C. It was then examined for ion exchange properties and had an ion exchange of 0.4 milliequivalents/g carbon.

Example 10 The procedure was the same as Example 6 except that the carbon battery electrodes were replaced with reactor grade graphite.

Example 11 The procedure was the same as Example 6 except that the carbon battery electrodes were replaced with spec pure carbon.

Table 1 below shows results of analysis of colloidal carbon prepared in Examples 1, 3, 4 and 5 above.

Table 1 Ex. 3 Ex. 5 Ex. 1 Ex.4

CH3COOH H2SOX H2SO4 Double Electrolyte (0.17 M) (1 M) (0.03 M) deionised water Potential V 33 33 25 33 of electrode Current mA 3 80 15 1 Surface area 4.9 4.9 4.9 4.9 cm-2 Current 0.61 16.33 3.06 0.2 density/ mA cm2 Observations Colloid Electrode Colloid Colloid present broke up present present plus plus within 5 plus carbon carbon carbon min to granules granules ca.

granules form ca. 1 mm 0.1 mm ca. 1 mm granules diameter diameter diameter ca. 3 mm diameter Charge on Negative - Negative Negative colloid Time/day 21 21 21 Polarity No Yes No reversal C-H Stretch No Yes No > C=O Present - Present Present Analysis of colloidal carbon produced in accordance with the invention Example 12 Particle Size Distribution The particle size distribution of the colloidal material was determined using a Malvern sub-micron particle analyser in relation to colloidal carbon prepared with 0.017 M ethanoic acid as the electrolyte (1), double deionised water as the electrolyte (2) and 0.03 M sulphuric acid as the electrolyte (3). The results are shown in Figure 1 and in Table 2. The expected particle size distribution for a polydispersed system corresponds to a simple Gaussian distribution curve. However, Figure 1 shows that the colloid had a broad trinodal particle size distribution curve irrespective of the electrolyte.

(D.L. Shaw, Introduction to Colloid and Surface Chemistry (1992) p 9, 4th Edition, Butterworth Heinemann Ltd.). The colloid prepared in ethanoic acid (0.17 M) has a significantly larger particle size range than that prepared in either sulphuric acid (0.03 M) or double deionised water. The trinodal distribution suggests that the colloid comprises three major components.

ExamPle 13 Surface Area Measurement Surface area measurements were carried out on a dried colloidal material. The dried material was prepared by precipitating the colloid with methyl alcohol, washing it with methyl alcohol and drying at 1100C.

The measurements were performed on the dry material using a Gemini 2375 surface area analyser, using the BET method with nitrogen adsorption at 77 K, assuming a surface area of the nitrogen molecule of 0.162 nm2. Five points in the BET curve were taken between 0.1-0.3 P/PO. The results are shown in Table 2 below.

Table 2

Surface area 50 75 79 m2g 1 Calc. Surface 14 39 17 area m2g- Particle size 40 - 870 30 - 630 40 - 630 nm.

Average 202 133 182 particle size nm.

Molecular 3.9x106-5X1010 l.4x106-l.3x1010 3.9X106-1.3X1010 weight (calc) The low surface areas (Table 2) show that there is little microporosity within the particles and the calculated surface areas, assuming spherical particles, are slightly lower than the observed values suggesting that the particles are irregularly shaped.

Example 14 Electrophoresis of Colloidal Carbon Electrophoresis was performed on the colloidal carbon using standard techniques (J. Sambrook, E.F.

Fritsch and T. Mamatis (1989). Molecular Cloning, a Laboratory Manual, 2nd Ed. Cold Spring Harbor, New York). The electrophoretic film was 0.7% agarose in TRIS buffer at pH 8.8 with a driving emf of 60 VDC.

The results are shown in Figure 2.

The expected pattern produced by electrophoresis was a continuum of carbon distributed along the length of the agarose strip. However, Figure 2 shows the separation of the carbon into three components (the last being carbon of the largest size) in agreement with the trinodal particle size distribution curve (Figure 1). The fact that there was a separation at all indicates that there was a different charge to mass ratio for each component i.e. the material at the front had a higher charge to mass ratio than later forms even though, by virtue of the method of preparation, it would have been expected that the charge to mass ratio would be constant. The low surface areas indicate a fairly dense material so that oxidation was confined to the surface of the particle.

The functional groups are, therefore, confined to the surface and probably the smaller the particle size the greater the charge to mass ratio.

Example 15 Carbon Analysis Known volumes of the concentrated colloidal carbon were evaporated in a dry weighed silica crucible. The colloid was heated to 900C in an oven and when dry, the temperature was raised to 1200C and held at temperature overnight. The crucibles were cooled in a dessiccator, reweighed and the carbon content calculated. They were then heated in air, over a bunsen burner, cooled and reweighed. The crucibles attained their original mass confirming that there were no non-volatile constituents present.

Using this concentrated colloidal carbon as the standard, W-vis spectra were obtained using a Kontron Uvicon 860 spectrophotometer. Aliquots of the standard carbon colloid were made up to different, but known, volumes with 0.17 M ethanoic acid. (Reference cell 0.17 M ethanoic acid). The spectrum of each was recorded and a plot of the absorption, at AMAX (228 nm), against the carbon concentration was linear, i.e.

the system obeys Beer and Lambert's Law up to a concentration of 10 Hg cm3, (A.J. Vogel Text Book of Quantitative Chemical Analysis, Fifth Edition, p 649).

A typical UV-vis spectrum of colloidal carbon prepared in ethanoic acid (0.17 M) is shown in Figure 4, spectrum 1, (reference 0.17 M ethanoic acid) showing AMAX to be at 228 nm. The spectrum of colloidal carbon, prepared in double deionised water is also shown in Figure 4, spectrum 2, and shows Ax now at 189 nm. The difference between these two spectra suggested that the ethanoic acid has protonated the functional groups on the surface of the carbon. Accordingly, a sample of carbon prepared by electrolysis in double deionised water was acidified with 0.17 M ethanoic acid and its spectrum recorded.

This also resulted in the suppression of the peak at 189 nm with the spectrum being identical to that obtained from colloidal carbon prepared in ethanoic acid (0.17 M) (spectrum 1 Figure 4). The experiment was repeated using both HC1 and H2S04 and similar results were obtained. As discussed, it can be postulated that the carbon was oxidised at the anode to form ketonic > C-O groups which then react with acids to form C-O-H. Therefore, in acid media, the colloid comprises carbon particles bound with surface > C-OH groups which could act as cation exchangers by replacement of the H atom.

Example 16 Equivalent Weight Determination of Colloidal Carbon Double deionised water (3 cm3) was pipetted into 1 cm silica spectrometer cells i.e. both the sample and the reference. Colloidal carbon (200 Zdm3 0.1056 M) prepared in double deionised water was pipetted into the same cell and water (200 pom3) pipetted into the reference cell. The spectrum was then recorded and the absorption noted at AMAX (189-228 nm). To each cell was then added equal volumes of standard acid and the spectrum was again recorded.

The process was repeated a number of times, with the absorption at AMAX being recorded each time. A plot of the absorption at AMAX against the volume of concentrated acid showed a number of points of discontinuity. The titrations were done with standard ethanoic, hydrochloric and sulphuric acids. Figure 3 shows the titration curve for concentrated hydrochloric acid. The end-points were calculated by solving the simultaneous equations formed by each line at the point of intersection. In order to emphasise the initial end-points, section A to D has been expanded and inserted in Figure 3. Knowing the mass of carbon present and the volume of standard acid required to titrate the carbon to point B, (Figure 3), then the equivalent weight can be calculated.

Sub-micro titration with acids was carried out in the carbon prepared using double deionised water as the electrolyte, the advantage being that there were no major species present other than colloidal carbon and water. There are four distinct regions shown in Figure 3, AB, BC, CD and DE. A similar effect was observed in the titration with both concentrated ethanoic and sulphuric acids, the results for which are given in Table 3 below.

The W-vis spectrum of the colloid prepared in double deionised water showed a double peak (Figure 4 spectrum 2). The peak, at the higher energy end of the spectrum was suppressed by the addition of standard acid. The amount of acid required to suppress this peak so that the spectrum is the same as that obtained from carbon prepared in 0.17 M ethanoic acid (Figure 4 spectrum 1) corresponded to the amount of acid required to titrate the carbon to the first point of discontinuity. (Point B, Figure 3). This value was approximately the same for titrations with standard ethanoic, hydrochloric and sulphuric acids (i.e.m ca 7 meqiv H/g carbon) (Table 3). This indicated that a quantitative protonation had taken place between the acid and the carbon. The approximate proton uptake was estimated as follows: Knowing the weight of carbon present, the concentration and volume of acid required to take the system up to point B (Figure 3) can then be estimated. Averaging the results (Table 4) gives 12 carbon atoms/atom of oxygen i.e. a proton exchange capacity of 7 meqiv. g.

Referring to Figure 4, spectrum 2 is of the colloid prepared in double deionised water and it can be attributed to the material in the keto form. On addition of acid it is converted into the enol form and, referring to Figure 3, the acid required is equal to that from point A to point B i.e. protonating the keto form to produce the enolic form. This value is the same for a number of acids (ethanoic, hydrochloric and sulphuric acids) and indicates a quantitative change in the system. The charge on both the carbonyl and enolic oxygens explains why the material is negatively charged and remains within the vicinity of the anode during electrolysis, the hydrogen in the enolic form giving it cation exchange properties.

Regions BC, CD and DE (Figure 3) are interpreted as the formation of envelopes of acid surrounding the carbon particles.

Table 3

Titrant CH3COOH HC1 H2SO4 (17.6 M) (10.56 M) (18.95 M) Proton m eqiv g-l 7.9 4.8 7.5 dry carbon Molecules of acid/ 1.72 0.92 1.76 carbon atom shell 1* Molecules of acid/ 16.0 8.93 20.34 carbon atom shell 2 Molecules of acid/ 20.07 18.25 77.38 carbon atom shell 3* *Estimated values from microtitrimetry Example 17 FTIR Spectroscopy FTIR spectra were recorded on a Perkin Elmer 1720-X FTIR spectrophotometer. The pellet was prepared by adding the colloid to solid KBr, evaporating to dryness at 110 , grinding and pressing into a 13 mm diameter pellet. A pellet of the original material was prepared by scraping the electrode to remove some carbon, grinding with KBr and pressing the pellet as before. The results are shown in Figure 5.

The FTIR spectrum of the original carbon (Figure 5, spectrum 1) showed no significant features, the absorption bands at 3422, 2361, 2330, 1634 and 676cm being present in the original KBr. The FTIR spectra of colloidal carbon prepared in either double deionised water (Figure 5, spectrum, 2) and ethanoic acid (Figure 5, spectrum 3) were identical both showing absorption bands at 3442 (-OH) 1727 ( > C-O) and 1645cm (C=C). Spectrum 4 (Figure 5) is that of the colloid prepared in 0.03 M H2S04 and is similar to spectra 1,2 and 3, however the bands below 1299cm1 are due to the presence of H2SO4 in the sample. Spectrum 5 is that of the colloidal carbon prepared in H2SO4 (0.03 M) where the electrode polarity was reversed every 10 minutes. Spectra 2 to 4 all show a trace of the doublet at 2907 and 2835cm due to the C-H stretch. Spectrum 5, however shows the doublet to be considerably enhanced. This is attributed to the reduction of double bonds, present in the carbon, by the nascent hydrogen produced at the cathode. Again bands below 1299cm1 are due to residual H2SO4.

The FTIR spectrum of carbon after electrolysis confirmed the presence of > C=O groups and since the surface area of the system is low (approximately 60m2g-1) most of the carbonyl groups are probably on the exterior surface of the particle. Since the electrolyte system consisted of carbon, water and oxygen, only the species Cn-C-0 could be present.

Spectrum 1 of Figure 5 indicates the absence of both > C=O and C-H stretch in the original material.

Spectrum 2 (prepared in double deionised water), spectrum 3 (prepared in 0.17 ethanoic acid) and spectrum 4 (prepared in 0.03 M His04) are identical and suggest that in the dry state the keto form is the most stable since all had been heated to 1100C.

ExamPle 18 Ion exchange properties of charged colloidal carbon produced in accordance with the invention The ion exchange properties of the colloidal carbon were examined by adding colloid to a solution containing a mixture of radioactive cations at trace levels.

Specifically, 5 cm3 of the colloidal carbon (ca lomg) was shaken with 50 cm3 of an aqueous solution of trace Xy active cations, the pH having been adjusted to 9.6 with 3 cm3 0.1 moldm3 NaOH. After equilibrating for various times the suspension was filtered through an 0.02 ssm syringe filter and ssy counted. The percentage cation removed with time is shown in Table 4.

The results shown in Table 4 below indicate the ion exchange potential of colloidal carbon produced in accordance with the method of the invention.

Table 4 Decontamination factors for some radioactive cations. (% cation removed with time)

Cation Contact time hours 1 2 4 6 24 Cs. 50 86 92.5 94.2 97 Co. 97.7 96.6 93.3 91.1 88.2 Ru. 78.3 81.1 82.8 81.1 84.4 Ag. 84.4 92.1 95.9 95.9 97.2 Mn. 96.9 89.9 82.8 79.2 75.0 Zn. 99.9 99.6 99.8 99.9 99.8 Fe. 99.8 99.8 99.8 99.5 99.5 Hg. 72.2 78.3 86.8 86.1 90.5 Cd. > 99 > 99 > 99 > 99 > 99 Cr(III) 99.7 99.8 99.9 99.9 99.7

Claims (42)

CLAIMS:

1. A chromatographic material comprising negatively charged carbon particles with a particle diameter of from about 40 nm to about 2 mm.

2. A material as claimed in claim 1 wherein said charged particles have a particle diameter of from about 40 nm to about 900 nm.

3. A material as claimed in claim 1 wherein said charged particles have a particle diameter of about 0.1 mm to about 2 mm.

4. A material as claimed in any preceding claim which has a cationic exchange capacity of from about 0.4 to about 7.0 milliequivalents/gram of carbon.

5. A material as claimed in claim 4 which has a cationic exchange capacity of from about 0.4 to about 2.0 milliequivalents/gram of carbon.

6. A material as claimed in claim 4 which has a cationic exchange capacity of about 7.0 milliequivalents/gram of carbon.

7. A material as claimed in any preceding claim wherein said charged carbon particles have a BET surface area in the range from about 14 m2g1 to about 80 m2g~1

8. A material as claimed in any preceding claim wherein said carbon particles comprise ketonic carbonyl groups tautomeric with the enolic form.

9. A material as claimed in any preceding claim wherein said carbon particles comprise hydroxyl groups.

10. A method of producing particulate carbon which comprises passing a DC current between two carbon electrodes disposed in an electrolyte.

11. A method as claimed in claim 10 wherein said electrolyte is selected from a mineral acid, an organic acid and deionized water.

12. A method as claimed in claim 11 wherein said organic acid is ethanoic acid.

13. A method as claimed in claim 12 wherein said ethanoic acid is at a concentration of from 0.01 to 1. OM.

14. A method as claimed in claim 11 wherein said mineral acid is sulphuric acid.

15. A method as claimed in claim 14 wherein said sulphuric acid is at a concentration of from 0.01 to 1. OM.

16. A method as claimed in any one of claims 10 to 15 wherein said carbon electrodes are in the form of rods, sheets, felt or tow.

17. A method as claimed in any one of claims 10 to 16 wherein the DC voltage applied ranges from about 2 to about 200 volts.

18. A method as claimed in claim 17 wherein the DC voltage applied ranges from about 8 to about 33 volts.

19. A method as claimed in any one of claims 10 to 18 wherein the DC current passed is from about 1 to about 80 mA.

20. A method as claimed in claim 19 wherein the DC current passed is from about 3 to about 15 mA.

21. A method as claimed in any one of claims 10 to 20 wherein said particulate carbon has a particle diameter from about 40 nm to about 2 mm.

22. A method as claimed in claim 21 wherein said particulate carbon has a particle diameter from about 40 nm to about 900 mm.

23. A method as claimed in claim 21 wherein said particulate carbon has a particle diameter of from about 0.1 mm to about 2.0 mm.

24. A method as claimed in claim 22 wherein the electrolyte is 0.03 M sulphuric acid, the DC voltage is about 25 volts and current is about 15 mA.

25. A method as claimed in claim 23 wherein the electrolyte is 0.03 M sulphuric acid, the DC voltage is about 12 volts and the current is about 20 mA.

26. A method as claimed in any one of claims 10 to 25 wherein said direct current is passed between said electrodes for a period of at least 8 hours.

27. A method as claimed in claim 26 wherein said direct current is passed between said electrodes for periods of up to 8 hours in any one 24 hour period for up to a total period of about three weeks.

28. A method as claimed in claim 26 or claim 27 wherein, following a period of passage of direct current, the electrode polarity is reversed.

29. A method as claimed in claim 27 wherein the polarity is reversed every 10 minutes.

30. A method as claimed in any one of claims 10 to 29 which comprises the additional step of recovery of said particulate carbon from said electrolyte.

31. A method as claimed in claim 30 wherein said particulate carbon is recovered by a method selected from: (a) evaporation of the electrolyte from said particulate carbon in an oven; (b) azeotropic distillation of water from said particulate carbon/electrolyte mixture with chloroform; (c) placement of said particulate carbon/electrolyte mixture in a Dreschle bottle and continuous passage of air for a period of between 1 and 3 days; (d) centrifugation of said particulate carbon/electrolyte mixture; (e) vacuum drying of said particulate carbon/electrolyte mixture; and (f) filtration of said particulate carbon/electrolyte mixture.

32. Particulate carbon when made by the method of any one or claims 10 to 31.

33. Colloidal carbon when made by the method of any one of claims 10 to 31.

34. A chromatographic material which comprises negatively charged particles as defined in any one of claims 1 to 9 or the particulate carbon made by the method of any one of claims 10 to 31 coupled to an affinity reagent.

35. A chromatographic material which comprises glass beads coated with the negatively charged carbon particles defined in any one of claims 1 to 9 or the particulate carbon made by the method of any one of claims 10 to 31.

36. A method for removing metal ions from a solution contaminated therewith which comprises exposing said solution to the chromatographic material as claimed in any one of claims 1 to 9 or claim 35 or the particulate carbon made by the method of any one of claims 10 to 31 and recovering said chromatographic material or particulate carbon from said solution.

37. Use of the particulate carbon made by the method of any one of claims 10 to 31 as a lubricant.

38. An electrolytic cell which comprises a container for electrolyte, an anode and a cathode adapted for connection to a source of direct current characterized in that both the anode and cathode are carbon.

39. An electrolytic apparatus which comprises a plurality of electrolytic cells as claimed in claim 37 connected in parallel.

40. A chromatographic material as claimed in claim 1 and substantially as described herein with reference to the accompanying drawings and examples.

41. A method of preparing a particulate carbon as claimed in claim 10 and substantially as described herein with reference to the accompanying examples.

42. An electrolytic cell as claimed in claim 38 and substantially as described herein with reference to the accompanying examples.