GB2487760A - Composite adsorbent material - Google Patents

Abstract

A composite adsorbent material comprises a porous carbon carrier matrix and an adsorbent species which is precipitated within the pores of the carrier matrix. The adsorbent species may be capable of influencing the ionic composition of the surrounding aqueous phase and the resultant composite material produces an internal chemical environment which is different to that outside the material. The concentration of the carrier matrix in the carrier material may be between 10-99% (w/w) of the total weight of the carrier material. The carrier matrix may comprise, or be derived from a cellulosic precursor material, preferably a lignocellulosic material. The carrier material may be substantially macroporous and have pores in the range of 50-500 nm. The adsorbent species may be basic.

Description

COMPOSITE ADSORBENT MATERIAL

The present invention relates to composite adsorbent materials, and in particular, to highly porous carbon-based composite materials for the adsorption and stabilisation of inorganic substances. The invention extends to various uses of such adsorbent materials, for example in water purification, recovery of metals from waste streams and remediation applications, and where the absorbent material is amended into soil, waste etc. for the purpose of breaking pollutant-receptor linkages.

Heavy metals that are present in soil, effluents and sewage sludge can pose major environmental problems if there is a pathway by which they can reach receptors such as ground and surface waters, humans or ecosystems. For example, arsenic is a common contaminant in mining areas and is currently a significant problem in South America and South East Asia, in places where contaminated ground water is extracted for drinking. Effective, cheap and sustainable methods that allow adsorption of harmful metals and arsenic would therefore be very useful as a means for decontaminating substances that contain potentially harmful quantities of heavy metals and arsenic species. Similarly, compositions are required for the effective capture of radionucides from contaminated water.

Known compositions that can he used for adsorbing toxic compounds include particles of porous adsorbent species, such as silicate materials, or activated carbon particles which have been coated on their outer surface with adsorbent species (e.g. silicates).

However, a problem with these known adsorbent matetials is that because the adsorbent species are only present on the outer surface of the particles, in use, they are exposed directly to the surrounding fluids containing the toxic compound, and so their stability and thus activity under adverse conditions is significantly compromised. A further problem is that, because the adsorbent species are present on the surface of the particles, they are susceptible to abrasion, such that the adsorbent is removed, resulting in a further loss in efficacy. Jo

Conditioning or "amendment" of soils with carbonates, silicates and hydroxides are well-known methods to reduce metal toxicity in heavy metal contaminated soils. The addition of calcium carbonate or calcium hydroxide to soil is known as liming', and the addition of silicates in the form of Portland cement is known as cement stabilisation', in which contaminated soil is solidified by transforming it into concrete. The mode of action for these soil amendments is thought to be two-fold. Firstly, the soil amendments themselves raise the pH of the soil resulting in the formation of insoluble metal hydroxides. Secondly, since the amendments are normaliy in the form of a calcium salt, the calcium ion is displaced by a heavy metal ion with the resulting heavy metal salt being more stable than the calcium salt.

However, one problem with these soil conditioning methods using cement-like products is that they form aggregates that are damaging to the soil's structure, even at small concentrations. Another problem is that the amendments are very unstable at low pH. For example, for carbonates, a pH of about only 7.0 results in the disintegration of the carbonate into carbon dioxide and the release of heavy metal ions. In addition, the more acidic the soil is, the quicker this reaction is. In acid-generating soils, these methods therefore only give temporary relief. Addition of larger particles, in the form of limestone chips has the disadvantage that only a small surface area is reactive, and the metal carbonates that form on the surfaces of these chips prevent further reactions from taking place, limiting their adsorbent capability on a weight for weight basis.

Furthermore, the crust of metal carbonates that forms on the surface of such a chip is liable to erosion and subsequent rapid disintegration. Thus, methods that would significantly stabilise metal carbonates, silicates and/or oxides without compromising the reactive surface area of the adsorbent matetial would be extremely useful because this would result in a much longer treatment effect (proportional to the stability gain) even under acidic conditions.

The adsorbent properties of silicates and hydrotalcites are well-known, and have been used to remove heavy metals and arsenic species from waste streams, remove radionucides, and treat soils that are contaminated with heavy metals. However, a io major problem is that, in their pure form, these adsorbent matetials present themselves as a fine powder with thixotropic properties when wetted. In the case of hydrotalcites for example, when used in a filter, the adsorbent particles clog together severely impeding water flow. Alternatively, when mixed with water, they produce a very fine suspension that settles out very slowly, and is almost impossible to remove by filtration.

The presence of metal carbonates and oxides within charcoals produced from biomass is also known. However, these materials rely on minerals that are already present in the plant material, and suffer from the problem that they are unstable and of limited use for the removal of certain pollutants, such as heavy metals ions or arsenic species, from a contaminated source.

There is therefore a need for improved adsorbent materials, which can be used to adsorb heavy metal pollutants, for example in soil, effluents and sewage.

In a first aspect, the invention provides a composite adsorbent material comprising a porous carbon carrier matrix and an adsorbent species, wherein the adsorbent species is precipitated within the pores of the carrier matrix.

The absorbent material of the invention requires the precipitation of specific adsorbent species that are added to the carrier matrix, and precipitated within its pores. Thus, the adsorbent species were not part of the carbon carrier matrix originally. Advantageously:, the inventors have demonstrated that the composite materials of the invention may be used in a wide variety of applications, such as for addressing environmental pollution, for cleaning drinking water, or treatment of industrial and agricultural effluent, removal of heavy metals (such as arsenic) from landfill leachate, groundwater, sewage sludge, as well as in various soil applications. For example, as illustrated in Figure 5, the composite matetial can be used to efficiently amend a polluted soil, or a highly acidic soil, without destroying soil productivity and quality, thereby allowing plants to grow, which would not otherwise be possible. Furthermore, compared to known adsorbent compounds, the composite material of the invention can he designed with optimised pore and particle size characteristics in order to provide improved stability and io reactivity. Ability to manipulate size and reaction strength of the material by choosing the most appropriate precursors also ensures that composite materials can be created that can be used in scenatios where certain flow rates need to be maintained, e.g. water filters.

As discussed below, the composite material may be used to control and modify the dynamics of adsorption processes. It will be appreciated that adsorption involves the binding of a molecule (i.e. the adsorbate) to a site on a surface which has an affinity for that molecule (i.e. the sorhent species). Adsorption processes generally consist of two types, i.e. either physisorption (also called physical adsorption) or chemisorption (also called chemical adsorption). Physisorption describes binding which occurs as a result of weak Van der Waals forces, while chemisorption relies on the formation of chemical bonds. Chemisorption processes are heavily dependent on environmental conditions.

For example, for inorganic reactions, pH is one of the most critical variables for adsorption to occur.

As described in the Examples, calcium silicate will undergo a displacement reaction with divalent copper ions resulting in the formation of copper silicate, resulting in the removal of the copper ions from the solution. This process is efficient at a pH of 7, but hardly occurs at all at a pH of 5 or less. Calcium silicate is sparingly soluble, and raises the pH of any aqueous sy stem into which it is introduced. Thus, if calcium silicate is confined within a diffusion limited micro-environment, such as the composite material of the first aspect, it will raise the pH of that environment far higher than the equilibrium pH that would othenvise be achieved by the free chemical present in an aqueous solution.

Advantageously, this phenomenon enhances the chemisorption process between the sorbent species and the adsorbate (i.e. the copper ions) within the porous matrix.

Hence, the porous carbon not only acts as an efficacious support matrix for the adsorbent species that maximises the reactive surface of the adsorbent, it also serves to modify the chemical interactions between the adsorbate and the adsorbent species. The porous structure of the matrix acts to restrict diffusion allowing sparingly soluble io alkaline sorbent species to raise the internal pH of the fluid within the pores, while maintaining sufficient contact with the external environment to allow access by the adsorbate.

Therefore, in one embodiment the adsorbent species may be capable of influencing the ionic composition of the surrounding aqueous phase, wherein the resultant composite material produces an internal chemical environment which is different to that outside the material. Advantageously, this allows certain adsorption reactions to take place in conditions which would not normally be favourable to such a reaction. By way of example, heavy metals can he adsorbed from an acidic environment by creating an alkaline environment inside the composite material.

Due to the diffusion limitations created by the internal pore structure of the carrier matrix, dissolution of a sparingly soluble adsorbent species would require a very long period of time compared to the time that it would take to dissolve the same adsorbent in the absence of the mattix. Advantageously, therefore, during the extended period for which the composite material of the first aspect is stable, adsorption may take place unhindered even in environments that are not normally conducive to the adsorption process in question. Indeed, as described in the Examples, in field trials, silicate-containing carbons have been shown to retain heavy metals even when the pH of the soil approaches pH 2. Although the inventors do not wish to be bound by theory, they hypothesise that the primary condition for this mechanism may be that the adsorbent species is significantly less soluble than the adsorbate, and as a result, there will tend to be an accumulation of adsorbate within the confines of the porous structure of the matrix.

Other reactions that would stabilise compounds within a composite matetial are those that resuk in the production of a gas. For example, reaction of a carbonate with an acid will result in the formation of carbon dioxide. If the carbonate is in a free form (i.e. not incorporated within a porous matrix), then this carbon -dioxide would rapidly diffuse.

However, advantageously, within a porous structure, the carbon dioxide gas would form gas pockets, thus creating an effective barrier preventing further diffusion of the io adsorbed molecules, thereby effectively trapping them inside the composite material.

The composite material comprises an existing material @.e. die porous structure of carbon) as a matrix to create a particle with a large reactive surface area. The reactive properties of the composite material are determined by the presence of the adsorbent species that is precipitated (i.e. encapsulated) within the pores of the matrix. The maximum capacity of the composite material created to adsorb adsorbate ions is determined by the nature and quantity of the adsorbent species that is precipitated within the pore structure of the carrier matrix, provided that porosity is maintained.

Accordingly, if the pores in the matrix are blocked because of over-impregnation with the adsorbent species, the composite material may not be able to reach its maximum adsorption capacity. The reactivity of the composite material is determined by the particle size of the carbon matrix and the size of its pores. Source materials used as the matrix may therefore he chosen or modified to give specific properties in terms of reactivity and stability of the resultant material.

The composite material of the invention is distinguished from known adsorbent materials, such as porous silicate particles or activated carbon particles coated with silicate, because, in the composite material of the invention, the adsorbent species is incorporated inside or within the carbon mattix itself, whereas, in known materials, only the outer surface of the particles are coated with adsorbents, such as silicates. Thus, the composite matetial of the first aspect is far more stable, and, unlike the known materials, does not lose tile adsorbent species through abrasion. Furthermore, precipitation in the pores of tile matrix allows a very high surface area to be maintained ailowing maximum adsorption.

The composite material of the first aspect may take the form of a highly porous carbon mattix where the pore structure of the mattix itself is used to contain a chemically distinct adsorbent species. Precipitating the adsorbent species within the pores of the carbon mattix alters the kinetics of any reaction between chemical species dissolved in fluids in which the composite matetial is immersed and the adsorbent species within the carbon mattix. Thus, by varying the pore size distribution within the carbon material io and the percentage loading of the adsorbent species, one may optimise the behaviour of the adsorbent for a specific purpose or environment. Hence, increasing the percentage loading of adsorbent-by-mass changes the rate of adsorption/desorption due to diffusion ilmitation within the pores in the carbon matrix.

For example, raising the percentage loading of adsorbent species within the matrix increases total adsorption capacity for the composite material. Alternatively, reducing the percentage loading of adsorbent species within the matrix increases the available reaction surface at the expense of sorption capacity. Thus, it is possible to produce a composite material having a modest overall capacity but with a fast rate of reaction, or produce a composite material which has a slower reaction rate, but with a very high capacity.

The concentration of the carrier matrix in the composite material may be between 10- 99% w/w or between 30-95% w/w of the total weight of the composite material.

Preferably, the concentration of the cartier mattix in the composite material may be between 50-90% w/w of the total weight of the composite material. The cartier mattix may comptise or be derived from a cellulosic precursor matetial, preferably a ligno-cellulosic precursor matetial. For example, the carrier mattix may comprise, or be detived from, plant material, and preferably woody plant matetial. The cartier mattix may comptise or be detived from any hardwood species of plant. Alternatively, the carrier mattix may comprise or he detived from a softwood species, for example a conifer. Other source materials that are suitable as a carbon carrier precursor are those detived from bamboo. In the Examples, sweet chestnut wood has been used as the precursor matetial.

The cartier matrix may comptise activated or non-activated carbon. However, it is preferred that the cartier mattix comprises non-activated carbon.

In embodiments where the cartier mattix is an activated carbon, it may be microporous or mesoporous. Pores in an adsorbent material are called "micropores" if their pore io size is less than 2nm in diameter, and pores are called "mesopores" if their pore size is in the range of 2 to SOnm in diameter.

Preferably, however, the carrier matrix may be substantially macroporous. Pores in an adsorbent material are called "macropores" if their pore size is greater than 50nm in diameter. It is envisaged that macropores having diameters greater than 500nm do not usually contribute significantly to adsorbency of porous materials. Therefore, for practical purposes, pores having diameters in the range of 50nm to SOOnm, more typically 50 to 300nm, or 50 to 200nm, may be classified as macropores. Non-activated carbons have normally a pore structure that is dominated by macropores.

The concentration of the adsorbent species in the composite material may be between l-90% w/w or between 10-75% (w/w) of the total weight of the composite material.

Preferably, the concentration of the adsorbent species in the composite material may be between 2O-SO% of the total weight of the composite material.

The adsorbent species may.: be precipitated within the pores of the carrier matrix using is precipitation methods that will be commonly known to the skilled technician, examples of which are metathesis reactions or displacement reactions where a more reactive metal ion displaces a less reactive metal ion within a salt (It. H. Grubbs (Ed.), Handbook of Metathesis, Wiley-VCH, Weinheim, 2003).

The adsorbent species may he basic, and sparingly: soluble. For example, the adsorbent species may comprise a metal silicate, a metal hydrotalcite, a metal phosphate, a metal oxide, metal hydroxide, metal sulfide and/or a metal carbonate.

The reactivity seties of metals is as follows: K>Na>Li>Ca>Mg>Al>Mn>Zn>Cr>Fe>Co>Ni>Sn>Pb>H>Cu>Ag>Hg>Au>Pt.

Hence, elements higher up displace those that come before them. Therefore, suitable adsorbents may be constructed from anything that is lower down the reactivity series to capture elements that are higher up.

io For example, the metal carbonate may he a suitable alkali or alkaline earth metal carbonate. For example, the metal carbonate may be calcium carbonate or magnesium carbonate. It will be appreciated that calcium and magnesium are just two examples of carbonates that could be used. The metal carbonate may comprise a suitable group 3 metal carbonate, such as aluminium carbonate.

The metal phosphate may be a suitable alkah or alkaline earth metal phosphate. For example, the metal phosphate may be calcium phosphate or magnesium phosphate.

The metal oxide may be a suitable alkali or alkaline earth metal oxide. For example, the metal oxide may be calcium oxide or magnesium oxide. Again, it will be appreciated that calcium and magnesium are just two examples of oxides that could be used.

Aluminium oxide and even iron oxides may be used to remove specific metals that come higher up the reactivity series.

The metal silicate may he a suitable alkali or alkaline earth metal silicate. For example, the metal silicate may he calcium silicate, magnesium silicate, aluminium silicate, 2inc silicate or iron silicate. However, the higher up the reactivity series the metal within the adsorbent, the less its reactivity. The preferred metal silicate is therefore calcium silicate which is insoluble at neutral pH and is displaced by the maximum number of different elements.

The metal hydroxide may he a suitable alkali or alkaline earth metal hydroxide. For example, the metal hydroxide may be calcium hydroxide or magnesium hydroxide. The metal hydroxide may comprise a suitable group 3 metal hydroxide, such as aluminium hydroxide.

The metal sulfide may be a suitable alkali or alkaline earth metal sulfide. For example, the metal sulfide may be calcium sulfide or magnesium sulfide. The metal sulfide may comprise a suitable group 3 metal sulfide, such as aluminium sulfide.

The metal hydrotalcite may he a suitable alkali or alkaline earth metal hydrotalcite. For io example, the metal hydrotalcite may he calcium hydrotalcite or magnesium hydrotalcite.

It will be appreciated that a hydrotalcite is a layered double hydroxide of general formula: Mg6Al2(CO3)(OH)i6.4(H2O). Hydrotalcites are effective at binding anionic -10 -metal species such as arsenite, arsenate, phosphates and iodine ions via anion exchange.

Thus, positioning the hydrotalcite adsorbent species within the pores of the carbon matrix provides a composite material exhibiting both stability and reactivity that can be further manipulated to make products that can be easily removed from liquid media, for example using a sieve.

As described in the examples, in some embodiments of the invention, non-activated charcoal may be used as the matrix into which is precipitated a silicate salt or a layered-double hydroxide, i.e. a hydrotalcite The incorporation of silicates or hydrotalcites into a macro-porous carbon matrix allows the production of a friable, and easy to handle material that can be used in filters, mixed with water to adsorb pollutants, or can be amended to soil without negatively affecting soil properties. Silicate salts, such as magnesium silicate and calcium silicate, are exceptionally effective at adsorbing heavy metal cations that are placed higher up in the reactivity seties than calcium or magnesium via a displacement reaction to form metal-silicates.

As discussed previously, when in pure form, silicates and hydrotalcites form a fine powder having tltixotropic properties when wetted. Thus, when hydrotalcites are used, for example in a fiher, the powder will clog up the filter, thereby impeding water flow.

Further, even at small concentrations in soil amendment applications, silicates form aggregates that are damaging to the soil's structure. However, in the composite material of the invention, the pores of the carbon matrix are coated with a thin layer of silicates (or hydrotalcites), which allows free flow of water, which does not form a fine suspension. Hence, the composite matetial does not react with soil particles to form concrete', and still maintains its ability to immobilise specific ions.

By using a porous cartier mattix, such as wood charcoal, it is also possible to alter the properties of the composite matetials by precipitating different chemical species having desired properties into its pore structure. For example, in one embodiment, iron oxide io or iron hydroxide may be introduced into the pores of the composite matetial. It will be appreciated that the resultant matetial will exhibit magnetic properties allowing it to he removed effectively from slurries and liquid media using magnets.

-II -

The surface area of the composite material used in accordance with the invention is closely determined by the proportion of adsorbent species and the matrix formed during the precipitation step from its precursor. The surface area of the pores (preferably macro-pores) of the material may be at least 0.5 m2 g'. However, it is preferred that the compo site material has a pore surface area of at least 2 m2 g1, more preferably at least 3 m2 g', even more preferably at least 4 m g', and most preferably at least 5 m2 g1. In embodiments where the matrix is an activated carbon, combined meso-and micro-pores are commonly between 200 and 2000 m2 g'. The surface area can he measured by the (Brunauer, Emmett, and Teller) "BET method" as described by Kantro, D. L., Brunauer, S., and Copeland, L. E. in "BET Surface Areas: Methods and Interpretations" in The Solid-Gas Interface, Vol.1 E. A. Flood, Ed.), Marcel Dekker, New York, 1967.

Preferably, the composite material has a macro-pore volume which is greater than 0.5 cm3 mE, typically ranging from 0.6 to 1 cm3 ml1, and preferably about 0.7 to 0.9 cm3 ml-'. The porosity may be measured by mercury porosimetry, as described in Sol-Gel Materials: Chemistry and Applications (John Dalton Wright, Nico A.J.M., Matia Sommerdijk (Ed.), P.74, CRC Press 2001).

Preferably, the composite material has pores that have an average diameter that is greater than I Onm, more preferably greater than 2Onm, even more preferably greater than SOnm, and most preferably greater than I OOnm or more.

It will be appreciated that, once prepared, the sorbent composition may be used in any configuration, shape or size. For example, the composite adsorbent material may be formed as, or into, particles. Thus, the material may be employed in particulate form, or combined with an inert solid (monolithic substrate to produce what is referred to in the art as a monolithic structure. Jo

Thus, in a second aspect, there is provided a particle comprising the composite adsorbent material of the first aspect.

-12 -The particulate form of composite material may he desirable in embodiments of the invention where large volumes of adsorbent material are needed, and for use in circumstances in which frequent replacement of the material may be required. The composite material may comprise finely divided particles, which may be contacted with a polluted fluid to be cleaned.

The mean particle size of the composite material may be between about 0.1mm and 50mm, or between about 0.1mm and 25mm, or between about 0.2mm and 10mm, or bigger. In some embodiments, the mean particle size of the composite material may be between about 0.1mm and 10mm, or between about 0.2mm and 7mm, or between about 0.25mm and 5mm. The mean particle size may he between about 0.2mm and 1mm, or between about 0.5mm and 3mm, or between about 1mm and 5mm. However) for very slow reacting applications that require high stability in acid conditions, particles may be between 10mm and 50mm, or even larger. Large particles (for example, lumps of charcoal, charred blocks of wood etc.) that are impregnated with an adsorbent species may have exceptionally slow reaction speeds, but, as a result, could be very useful in vatious challenging applications.

The size of the particles may he modified to suit a specific application. For example, by: increasing the size of the particle, \vater flow through a filter may: be increased, hut reactivity speed' may he decreased. A similar effect may be obtained by using a carbon matrix having a smaller pore size. Reducing reactivity speed can be important where a pollutant is immobilised by: competing ions that are present in the environment (such as hydrogen ions). For example, as described in the Examples, copper in copper silicate is stable when exposed to a solution with a pH greater that 5.5. Below this pH, an increasing proportion of the copper ions are displaced with decreasing pH. Because both calcium and magnesium silicate act as an alkali, an environment is created within the particle that has a high pH and which resists, to a large extent, the influx of io hydrogen ions from the environment while stabilising the metal silicates that have already been formed.

-13 -This is believed to be important if the carrier matrix is impregnated with silicates, and is ingested by a bird or mammal for example. For example, free' metal silicates (i.e. not precipitated in the matrix) wouM dissolve releasing the metal ions when contacted with the acidic stomach juices. However, when embedded in the carrier matrix, a high pH will be maintained within the particles, preventing the release of heavy metals into the stomach juices, thus protecting human health, in cases where the material is accidentally ingested.

In some embodiments where a particulate form of adsorbent material is required, the material may be a loose powder. In other embodiments, the composite material may.: be formed into any shape, for example by moulding and/or the application of pressure thereto. For example, the composite material may he formed into a tablet, pellet, granule, ring, or sphere, etc. In a third aspect, there is provided use of the composite material of the first aspect or the particle of the second aspect for the adsorption of inorganic substances.

The composite matetial of the invention may have numerous applications, for example in the clean-up of environmental pollution, for cleaning drinking water, or treatment of industtial and agricultural effluent, for the removal of heavy metals from landfill leachate, groundwater or sewage sludge, as well as in various soil applications where soil or sediment is contaminated \vith heavy metals or heavy metal-containing compounds, phosphate or any other ionic species.

The term "heavy metal" can mean any of the higher atomic weight elements, which have the properties of a metallic substance at STP. For example, the heavy metal or heavy metal-containing compound that may be adsorbed by the adsorbent composite material may be selected from a group of heavy metals consisting of arsenic, beryllium, lead, cadmium, chromium, nickel, zinc, mercury, and barium. The inventors have clearly demonstrated, in the examples, that arsenic may be adsorbed by the materials of Jo the invention. -14-

It will be appreciated that an important use of the materials of the invention is for removing pollutants from fluids, such as water.

Hence, according to a fourth aspect of the invention, there is provided a method of removing a pollutant from a fluid, the method comprising contacting a fluid comprising a pollutant with the composite adsorbent material of the first aspect or the particle of the second aspect under conditions suitable to remove the pollutant from the fluid.

In aqueous systems, it is envisaged that the composite material may be supported on a support, for example in a cartridge or is placed inside a porous bag or a fiher, or is fixed onto a solid support over \vhich the polluted fluid may be passed. Advantageously, this \vould combine fast reaction times with the pollutant, hut only slow or no release of the pollutant as the environment in the bag or filter is buffered against environmental changes, as well as creating a double stabilising effect, i.e. inside the composite as well as within the bag.

The method of the fourth aspect may comprise feeding the fluid to be treated to the adsorbent material, or vice versa, and allowing the material to remove the pollutant from the fluid. The term fluid is intended to cover viscous fluids, such as a sludge or a slurry.

The method may comprise a step of separating at least some of the composite material from the fluid following sorption of the pollutant. For example, the separation step may comprise the use of a filter. Alternatively, in embodiments \vhere the composite material is magnetic, for example due to the incorporation of iron oxides/hydroxides, a magnet may be used to extract the suspended product.

Thus, in one embodiment, the method may comptise contacting a fluid comptising a pollutant with the composite adsorbent material according to the invention, which io adsorbent material comprises iron oxide and/or iron hydroxide, or the particle according to the invention, and allowing the adsorbent material or the particle to he removed from the fluid using a magnet.

-15 -The method may comprise a step of recovering the adsorbed pollutant from the spent adsorbent material, as it may be valuable, for example, as in the case of nickel, zinc or copper. The recovery step may comprise contacting the spent adsorbent material with an acid. By placing the spent composite in an acidic solution, metal ions may be released and they can then be subsequently recovered using displacement with a more reactive pure metal, for example using electro-kinetics where the metal cations accumulate at the cathode, or by using reduction reactions that lead to the formation of non-valent metal. In embodiments where the adsorbent removes ions via ion-exchange, the recovery step may comprise contacting the spent adsorbent material with a concentrated salt solution such as NaC1 or CaCl2, thereby regenerating the sorhent composition for treatment of fresh fluid.

In a fifth aspect, there is provided a soil amendment composition comprising the composite adsorbent material of the first aspect.

The term "soil amendment composition" can mean a material used for altering the pH of a soil. Figure 5 shows the effects of using the material of the first aspect as a soil amendment composition. Thus, the soil amendment composition may used for changing the pH of soil.

According to a sixth aspect, there is provided a method of prepating a composite adsorbent matetial, the method comprising the steps of: providing a porous carbon cartier matrix; and @i) precipitating an adsorbent species within the pores of the cartier mattix, to thereby form a composite adsorbent matetial.

The resultant composite matetial may be as defined in relation to the first aspect. Thus, the cartier mattix may comprise, or be detived from, a cellulosic precursor matetial, io preferably a llgno-cellulosic precursor material. For example, the precursor matetial may comptise wood. The precursor material may he heated (or charred) to form charcoal.

For example, the material may be heated to at least 300°C, 400°C, 450°C, 500°C, -16 - 600°C, 800°C, 1000°C or higher, preferably in the absence of oxygen. Thus, the carrier matrix may comprise charcoal. The precursor may he calcined.

The particle size of the carrier matrix may be reduced, for example by breaking larger particles into smaller ones. The method may comprise contacting the matrix with a solution comprising a metal silicate, a metal hydrotalcite, a metal phosphate, a metal oxide and/or a metal carbonate, for sufficient time to allow impregnation into the pores of the matrix. The metal silicate, hydrotalcite, phosphate, oxide and/or carbonate may he any suitable alkali or alkaline earth metal silicate, hydrotalcite, phosphate, oxide and/or carbonate, or any group 3 metal silicate, hydrotalcite, phosphate, oxide and/or carbonate. Preferably, the carrier matrix is contacted with a solution of potassium silicate or sodium silicate.

The method may then comprise contacting the treated cartier matrix with a soluble metal chloride, metal bromide, metal fluoride, metal hydroxide, metal nitrate or metal sulphate for sufficient time to allow precipitation of the adsorbent species within the pores of the cartier mattix. The metal chlotide, metal bromide, metal fluotide, metal hydroxide, metal nitrate or metal sulphate, may be any.: suitable alkali or alkaline earth metal chloride or hydroxide. For example, calcium or magnesium salts may be used. It will be appreciated that tile potassium will be displaced by the calcium forming calcium silicate adsorbent species. This process allows exceptional heavy loading of the pores witil reactive chemicals.

In one embodiment of the method, the silicate may be contacted with the mattix first.

However, in another embodiment, the method may comptise impregnating the matrix with a soluble metal salt first, followed by exposure to a soluble silicate. The result should be the same, i.e. precipitation of an insoluble metal silicate within the pores of the carrier matrix.

io An added benefit of the process is that during displacement, a new salt is formed that can be collected and used for alternative applications. For example, tile use of potassium silicate and calcium nitrate will result in the formation of insoluble calcium -17 -silicate in the matrix pore structure and a soluble potassium nitrate that can he collected and used as a plant fertiliser, for example.

The method may comprise washing the composite material to remove the displaced salt, e.g. potassium chloride. The method may comprise drying the material to remove moisture before use.

All of the features described herein (including any accompanying claims, abstract and drawings), and/or all of the steps of any method or process so disclosed, may be combined with any of the above aspects in any combination, except combinations where at least some of such features and/or steps are mutually exclusive.

For a better understanding of the invention, and to show how embodiments of the same may be carried into effect, reference will now be made, by way of example, to the accompanying diagrammatic drawings, in which:-Figure 1 is a graph showing the p1-I change with charcoal particles of 0.25-0.5mm; Figure 2 is a graph showing the pH change with charcoal particles of 1-2mm; Figure 3 is a graph showing the pH change with charcoal particles of 2-4mm; Figure 4 is a bar chart showing leachable copper ions 0, 1, 37 and 77 days after treatment of heavy metal contaminated acidic soil pH 2.5). Control soil was untreated and top soil received a layer of around 2cm of non-contaminated garden soil. N5; Figure 5 are photographs showing the establishment of Rye grass on non-amended soil (Figure 5a) and soil amended with 4% (w/w) silicate charcoal (Figure Sb). Soils were originally acidic pH 2.5) and contained high levels of a range of heavy metals; Figure 6 is a graph showing the removal of As3 and As3 ions using Al/Mg hydrotalcites in a pure form (Al/MT and cAl/I-IT) and charcoal products where Al/Mg hydrotalcites were precipitated within the charcoal pore structure. Table 4 summarises the abbreviations used. 23 mg product was added to 25 ml water containing 10 mg/I As. Adsorption was not adjusted for the amount of hydrotalcite in each product. N n3; Figure 7 is a bar chart showing the removal of As3 and As5 using Fe/Mg hydrotalcites io in a pure form (Fe/HT and cFel/HT) and charcoal products where Fe/Mg hydrotalcites were precipitated within the charcoal pore structure. 25 mg product was added to 25 ml water containing 10 mg As/i. Adsorption was not adjusted for the amount of hydrotaicite in each product N3; Figure 8is a bar chart showing the percentage removal of As34 and As54 using Al/Mg hydrotalcites precipitated in charcoaL Products were derived from pre-loaded pine wood that was subsequently charred at 350,450 or 550°C. 15 mg product was added to ml water containing 10 mg As/L N3; Figure 9 is a bar chart showing the percentage removal of As34 and As4 using Fe/Mg hydrotalcites precipitated in charcoaL Products were derived from pre-loaded pine wood that was subsequently charred at 350,450 or 550°C. 15 mg product was added to l5nilwatercontaininglOmgAs/L N=3; Figure 10 is a bar chart showing the percentage removal of As34 using Al/Mg hydrotalcites at different pH (3,7 and 11). Hydrotalcites were either used on their own (Al/IC and cAl/UT) or derived mm pre-loaded pine wood that was subsequently charred at 550°C, or from charcoals where the hydrotalcite was precipitated within existing charcoal (Al/UT/Charcoal and cAl/HT/Charcoal) The latter was caldned at 550°C. 15 mg product was added to 15 ml water containing 10 mg As/I. N=3; Figure 11 is a bar chart showing the percentage removal of As54 using Al/Mg hydrotalcites at different pH (3,7 and 11). Hydrotalcites were either used on their own (Al/IC and cAl/Hi) or derived from pre-loaded pine wood that was subsequently charred at 550°C, or from charcoals where the hydrotalcite was precipitated within existing charcoal (Al/HT/Charcoal and cAl/HT/Charcoal). The latter was calcined at 550°C. 15 mg product was added to 15 ml water containing 10 mgAs/1. N=3; Figure 12 is a bar chart showing the percentage removal of As34 using Fe/Mg hydrotalcites at different pH (3,7 and 11). Hydrotalcites were either used on their own (Fe/IC and cFe/HT) or derived from pre-loaded pine wood that was subsequently charred at 550°C, or from charcoals where the hydrotalcite was precipitated within existing charcoal (Fe/HT/Charcoal and cFe/HT/Charcoal). The latter was caldned at 550°C. 15 mgproduct was added to 15 ml water containing 10 mgAs/L N=3; and Figure 13 is a bar chart showing the percentage removal of As54 using Fe/Mg hydrotalcites at different pH (3,7 and 11). Hydrotalcites were either used on their own (Fe/IC and cFe/HT) or derived from pre-loaded pine wood that was subsequently charred at 550°C, or from charcoals where the hydrotalcite was precipitated within -19 -existing charcoal (Fe/HT/Charcoal and cFe/HT/Charcoal). The latter was calcined at 5500C. 15 mg product was added to 15 ml water containing 10 mg As/l. N3.

Examples

Example I -Stabilising effect of charcoal on copper silicate at low pH

Introduction

As discussed above, amendment of soils with carbonates, silicates and hydroxides are well-known methods to reduce metal toxicity in heavy metal contaminated soils.

However, unfortunately, these methods are unstable at low pH' (e.g. for heavy metal carbonates, a pH of around 7 results in the disintegration of the carbonate into carbon dioxide, water and the release of heavy metal ions), and the more acidic the soil, the quicker the reaction, and so in acid-generating soils, these methods only give temporary relief.

Methods that would significantly stabilise metal carbonates, silicates and/or oxides would be extremely useful as this would, even under acidic conditions, result in a much longer treatment effect (proportional to the stability gain). It was hypothesised that wood charcoal, being of a porous nature, would allow calcium silicates embedded into the charcoal structure via a displacement reaction to react with copper ions in the environment. It was hypothesised that the resulting copper silicates inside the charcoal would be more stable at low pH because the charcoal particle would create a relatively stable micro-environment where the pH would he higher than in the surrounding solution therefore reducing the rate of dissolution of the metal salt inside the charcoal.

To some extent, it was expected that larger particles would have a greater stabilising effect than smaller particles because of relative smaller effects'.

Matetials and methods Stability of CuSiO3 in solution To test this hypothesis, charcoal particles of different sizes were prepared from sweet io chestnut wood. Sweet chestnut wood was charred at 4500C, broken up in small pieces which were passed over a set of sieves to create charcoal particles with sizes ranging from 0.25 -0.5 mm, 1.0 -2.0 mm and 2.0 -4.0 mm. The charcoal was subsequently -20 -impregnated with liquid Potassium silicate (SO% K2S03 by weight to obtain charcoal containing 1 O% K2503 by weight.. Subsequently, this impregnated charcoal was soaked in calcium chloride to allow precipitation of calcium silicate within the charcoal.

Once the potassium was displaced by calcium, the charcoal was washed thoroughly to remove the formed potassium chloride from the solution.

Thus, treated charcoal (termed silicate charcoal') was dried at 70°C to remove most of the moisture and the silicate charcoal was stored in plastic bottles at room temperature.

To create powdered silicate charcoal (< 0.01 mm), the charcoal from the 0.25-0.5 size class was ground using a pestle and mortar.

To allow the calcium silicate to be converted into copper silicate, 18 bottles, each containing 1.07 g Cu504.5H2O per litre RO water was prepared. Subsequently, three bottles for each treatment were amended with 5 g silicate charcoal and three controls were prepared by adding 0.5 g CaSiO3 powder (Sigma, UK). The bottles were left for> 1 week to allow equilibrium between the CaSiO3 and the Cu ions in solution. Three bottles were not amended to allow determination of the actual concentration of Cu ions in the solution. In theory, sufficient CaSiO3 was present to remove all the copper from the solution. To check how much Cu was actually removed from the solution samples were taken from each bottle and the copper concentration was determined using atomic adsorption (FAAS).

Table 1: Removal of Copper ions from a solution of CuSO4.5H20 (1.07 g 1') using an estimated 0.5 g calcium silicate in free form or deposited in the pore structure of charcoal particles of different size classes (<0.01mm, 0.25-0.5mm. 1.0-2.0mm and 2.0- 4.0mm) . N3. Different letters indicate significant differences between means at P<0.05.

Treatment Cu concentration (mg H) in % removal solution ± SE Control 246.9 ± 9.5 (a) 0 CaSiO3 170.4 ± 9.9 (b) 31 -21 - <0.01 mm 185.8 ± 3.9 (b) 25 0.25-0.5 mm 175.8 ± 12.9 (b) 29 1.0-2.0 mm 181.3 ± 7.3 (b) 27 2.0-4.0 mm 184.0 ± 5.2 (b) 25 Significance P < 0.001 ____________ From Table 1, it is clear that the silicate only removed between 31 and 25% of all the available copper from the solution. The copper in solution was in excess of the approximated adsorption capacity of the silicate component of the composite. This was to ensure adequate copper ions were present to determine maximum sorption capacity.

The tests were carried out at a relatively low pH of 5 to demonstrate the functioning of the system under sub-optimal conditions. For comparison, a liming process would only immobilise copper cations at significantly higher pH. It also suggests that the amount of silicate in all the treatments was about equal. To check if the latter was the case, 5 g of each of the silicate charcoals was ashed at 6 00°C and the mineral content weighted.

A non-silicate charcoal was used as a control. Results in Table 2 suggest that the amount of silicate in each treatment was comparable (around 10% difference).

Table 2: Ash and silicate content in silicated and non-silicated charcoal with different particle si2es. 5 g charcoal was used for each assessment.

Treatment Ash content Silicate % silicate (g) content (g) Control 0.45 0 0 0.25-0.5 mm 0.88 0.43 8.6 1.0-2.0 mm 1.04 0.59 11.8 2.0-4.0 mm 0.93 0.48 9.6 After 3 months, the remaining copper sulphate solution was separated from the solid fraction either by pouring of the liquid leaving a layer of fine powder stuck to the bottom of the flask (control and finely ground charcoal) or by passing the suspension over a fine sieve, followed by a quick rinse of the charcoal with RO water. All the treatments remained saturated. -22 -

To test the stability of the silicate in the different treatments, each of the materials recovered from each flask was mixed with 100 ml HCL with a pH of 2. Since there was excess silicate each of the treatments, there was ample silicate to react with the acid and reach equilibrium at a pH of 5.2 according to the following reaction: CaSiO3 + 2H j* F125i03 + Cu2 + Ca2 or CuSiO3 + 2H -÷ H25i03 + Cu2 The speed with which the silicates react with the acid is reflected in the speed by which the pH of the solution changes. Using constant stirring, the pH of each solution was measured with a pH meter, by.: measuring the time it took for the suspension to reach a pH of 4.5 and then 5.0. Also, pH readings were taken every minute until the solution reached a pH > 4.5. To reach a pH of 5.0, some treatments took many hours and solutions were measured hourly the next day till a pH of 5.0 was reached.

Field experiment Parys Mountain

In this expetiment, the silicate charcoal was prepared using oak charcoal fines with sizes between 0.5 and 2 cm. The charcoal was treated first with sodium silicate and subsequently with calcium chloride to obtain around 20% calcium silicate by weight inside the charcoal.

The soil at Parys mountain was extremely acidic (pH 2.5) and contained a range of heavy metals (Arsenic (> 770 ppm), copper (> 1,100 ppm), zinc (> 2,400 ppm), lead (> 2,600 ppm) and iron (> 300,000ppm).

Three different treatments were compared: Control (no amendment), top soil (2 cm) coveting the contaminated soil and silicate charcoal at a rate of 4% by weight. For each treatment a plot measuring 2 by 2 meters was established. To monitor phyto-toxicity io each plot was sown in with rye grass Loliumpere;i;ie and germination and plant growth was monitored over the following 77 days. Also leachable metals were monitored using the British Standards Method (11351 2002) immediately after treatment (t0), I day after -23 -treatment, 37 days after treatment and 77 days after treatment. Five samples were taken from each plot and analysed separately using ICP analysis.

Results Stability of silicate in solution Control: pH change of solution of HC1 with pH of 2 when amended with an equivalent quantity (0.5g' lOOml') of silicate was on average 10.7 ± 0.7 pH units per minute n3).

Referring to Figures 1-3, there are shown the change of p1-T of solution of 100 ml HCI with a pH of 2 when amended with Sg of silicate charcoal with a particle size of 0.25- 0.5 mm (Figures 1), 1-2 mm (Figure 2) or 2-4 mm (Figure 3). Charcoal contained around lO% (0.5 g) CuSiO3 by weight. n2 or 3. From these three graphs, it can be seen that at low pH there is a steady reaction of the silicate trapped within the charcoal.

Even charcoal particles with a size between 0.25 and 0.5 mm slow the rate at which silicate reacts with acids down by around 50 fold. Larger particles (2-4) mm have a more stabilising effect and compared with free silicate' are more than 100 times more stable at a pH below 4.

Figures 1-3 suggest that the reaction of silicate occurs at a low pH (i.e. between 3 and 4. The relation between pH and particle size is more or less linear, but since pH is on a log scale, the release of ions is in fact log linear decreasing exponentially if the pH rises. The inventors have found that the reaction stops completely at pH 5.2, meaning that no copper appears to be released from the charcoal at pH > 5.2.

Whereas the release of ions from charcoal is log linear at low pH, the charcoal itself increases the stability of the bound metal even further when the pH increases relative to the control (see Table 3) below.

Table 3. Reactivity of silicate embedded in charcoal particles of different sizes compared with free copper silicate (controli at increasing pH. Figure in brackets denotes stability increase compared with control. N3: different letters denote significant (p<0.05) differences between treatments Treatment Reactivity of silicate Time to pH 4.5 Time to pH 5.0 from pH 4.5 Control 14 seconds (1) a 36 seconds (1) b Finely ground (< 45 seconds (3) b 10 minutes (17) c 0.01mm) Charcoal 0.25-0.5 mm 10 minutes (42) c 2 hours (200) e Charcoal 1.0 -2.0 mm 12 minutes (51) c 16 hours (1600) f Charcoal 2.0 -4.0mm 41 minutes (176) d >18 hours (>1800) f Table 3 shows that silicate embedded in charcoal reacts progressively less when (a) the particle size increases (P<0.001) and (1) when the pH nears equilibrium (P <0.001).

This means that silicate embedded in charcoal with a particle size of> 1 mm is >1500 times more stable at pH between 4.5 and 5.0 than free silicates exposed to the same pH range. Even silicates embedded in charcoal particles with a size between 0.25 and 0.5 mm, were at this pH around 200 times more stable than free silicates'. Surprisingly, vety finely ground silicate charcoal derived from the 0.25-0.5 mm silicate charcoal was also 17 times more stable than free silicates', suggesting an intimate connection between the charcoal, and the silicate that provides a significant degree of stabilisation to the silicate.

Field experiment Parys Mountain

Referring to Figure 4, there is shown the results of leachable copper ions 0, 1, 37 and 77 days after treatment of heavy metal contaminated acidic soil pH 2.5). Figure 4 shows that amendment of the acidic soil contaminated with a range of heavy metals, silicate charcoal provides a significant reduction in copper leaching. In fact, after I and 37 days copper leaching was reduced to below detectable levels, compared to the control soil where after 37 days, leachable copper was on average 13 mg Cu per kg soil. -25 -

After 77 days, soils amended with silicate charcoal leached less than 0.1 mg Cu per kg soil compared with the control where the level of copper leaching was around 11 mg per kg soil. Similar levels of leaching were found to occur in contaminated soil covered with top soil after 77 days.

Referring to Figure 5a and Sb, there is shown the establishment of Rye grass on non-amended soil and soil amended with 4% (w/w) silicate charcoal. Soils were originally acidic (pH 2.5) and contained high levels of a range of heavy metals. As can be seen, in Figure 5a, for non-amended soil, Rye grass was unable to become established.

However, the inventors were pleased to see that Rye grass did establish in the amended soil plot, as shown in Figure Sb.

Conclusions

In summary, the inventors have demonstrated that at a low pH @.e. between about 2 and 4.5), charcoal particles with a size between about 0.25 and 2 mm stabilises silicates by more than 50 fold. In addition, at low pH (i.e. between 2 and 4.5), larger charcoal particles provided surprisingly more stability than smaller ones. Furthermore, surprisingly, at a pH between 4.5 and 5.0, copper silicates in charcoal particles with a class size of between 1 and 2 mm are around 1600 times more stable than free silicates'. Silicates embedded in charcoal particles between 2 and 4 mm are more than 1800 times more stable than free silicates'. Silicate charcoals reduce metal leaching significantly in acidic soils that are heavily contaminated \vith heavy metals. Finally, the inventors have shown that amendment of silicate charcoal to acidic heavy metal contaminated soil restores plant growth.

Example 2 -Effectiveness of charcoals into which hydrotalcites are precipitated for the removal of arsenic species from water Two layered double hydroxides LDHs)/hydrotalcite materials precipitated into charcoal were investigated for their efficacy in removing arsenic species As3 and As5) io from water. Al-Mg based and Fe-Mg based hydrotalcites were prepared by co-precipitation of Mg and Al/Fe salts with sodium hydroxide solution at pH>12 into either wood or charcoal. Both \vere made with Cl-as the interlayer anion with a ratio of -26 -M2:M3 of 2.1 5:1 in the initial solutions (Giliman, 2006, Science o/the Total Environment 366:926-31). Materials were exposed to air, and solutions were therefore not guaranteed carbonate free resulting in the likely presence of some carbonate ions in the interlayer structure. Calcination was done at 550°C.

Two methods of loading hydrotalcites onto charcoal particles were used. Firstly, precipitation directly into charcoal derived from Scotch Pine wood charred at 5 50°C and secondly precipitation directly into wood pine wood shavings followed by charring at 550°C. Three different concentrations of hydrotalcite were used using this method that resulted in charcoals with approximately 20, 40 and 60% w/w hydrotalcite.

Matetials prepared by precipitation directly into the charcoal were also calcined at 550°C. Charcoal particle sizes used throughout were 0.5-4mm. Sorption experiments were carried out in triplicate.

In a further experiment, die effect of charting temperature on product performance was assessed using Al/Mg hydrotalcite and Fe/Mg hydrotalcite. Pine shavings were soaked in the different solutions to obtain a final concentration of hydrotalcite of 4O% by weight. The loaded wood was charred at 350, 450 and 550°C for 1 hour. Arsenic adsorption was assessed by placing 15 mg product in 15 ml arsenic solution containing 10 mg As/l. Solutions were shaken for 24 hours before remaining arsenic in the solution was assessed.

Subsequently an experiment was set up to determine the efficacy of charcoals containing Al/Mg hydrotalcites to adsorb arsenic from water with pH of 3, 7 and 11.

As in the previous expetiment, 15mg material was added to 15 ml arsenic solution containing 10 mg As/I. Solutions were shaken for 24 hours before remaining arsenic in the solution was assessed.

To determine the amount of arsenic adsorbed by the different materials, 25mg material io was shaken for 24 h at 20°C in 25m1, I Omg/l arsenic solution. Arsenic concentrations were determined using molybdenum blue colorimetric method BS1728-i2:i96i), which has a minimum detection limit of 20ppb arsenic. In btief, a sample containing -27 -the arsenic is mixed with an acid solution of Mon, for example ammonium molybdate, to produce AsMoi2O4o3, which has an oLKeggin structure. This anion is then reduced by, for example, asorbic acid, to form the blue coloured 3-keggin ion, PMo12O4o7. The amount of the blue coloured ion produced is proportional to the amount of phosphate present and the absorption can be measured using a colorimeter to determine the amount of arsenic.

Table 4: List of abbreviations used Abbreviation Material (M Al or Fe) M/HT Hydrotalcite cM/HT Calcined hydrotalcite at 5500C M/HT/woodl Hydrotalcite loaded onto wood then charred (±20% UT by weight in charcoal) M/HT/wood2 Hydrotalcite loaded onto wood then charred. Initial solution concentration 2x that used in M/HT/woodl (±40?4 HT by __________________ weight in charcoal) M/HT/wood3 Hydrotalcite loaded onto wood then charred. Initial solution concentration 3x that used in M/HT/woodl (±6004 UT by __________________ weight in charcoal) M/HT/charcoal Hydrotalcites loaded onto charcoal particles. Initial solution concentrations were the same as M/HT/wood2 (±40% HT by __________________ weight in charcoal) cM/UT/charcoal M/HT/charcoal calcined at 550°C (±40% UT by weight in ____________________ charcoal) Results: As3 and As5 sorption of hydrotalcites directly precipitated into charcoal or loaded onto wood first before charting Table 5: Estimated removal of As3 and As by Al/Mg hydrotalcites in a pure form (Al/UT and cAl/UT) and Al/Mg hydrotalcites precipitated in the pore structure of charcoal detived from pine wood. Amounts adsorbed are expressed as mg As removed by 1 g hydrotalcite. N3 As3 As5 Removal removal (mg/g) std error (mg/g) std error -28 -A1/HT 3.784 0.071 9.991 0.007 cAl/HT 7.659 0.279 9.734 0.187 A1/HT/charcoal 4.050 0.515 16.258 0.780 cAl/HT/charcoal 7.963 0.283 7.093 0.208 A1/HT/woodl 15.470 0.430 9.595 0.330 A1/HT/wood2 12.655 0.218 13.158 0.355 AI/HTwood3 10.085 0.332 10.640 0.070 Referring to Figure 6 and Table 5, it can be seen that calcination increases uptake of As3 AI/HT vs cAI/HT, Al/HT/charcoal vs cAI/HT/charcoal. However, calcination may decrease As5 sorption Al/HT/charcoal vs cAI/HT/charcoal). Precipitation of hydrotalcites directly into charcoal may have little effect on the removal of arsenic expressed as mg arsenic removed by 1 g hydrotalcite.

In relation to wood loaded materials, there is an increase in the sorption capacity with increasing concentration of loading solutions and this suggests an increased loading of charcoal with hydrotalcite. The sorption of As3 and As5 are similar, much like that of calcined material, possibly because they were charred at 55 0°C. The inventors believe that hydrotalcites precipitated in wood before charring may be more efficient at removing arsenic from water.

Table 6: Estimated removal of As3 and As by Fe/Mg hydrotalcites in a pure form (Fe/HT and cFe/HT and Al/Mg hydrotalcites precipitated in the pore structure of charcoal derived from pine wood. Amounts adsorbed are expressed as mg As removed by 1 g hydrotalcite. N n3* As3 As5 Removal Removal ______________________ (mg/g) std error (IMg/g) std error Fe/HT 8.567 0.076 9.791 0.068 cFe/HT 8.578 0.337 7.477 1.105 Fe/HT/charcoal 5.295 0.050 14.663 0.213 cFe/HT/charcoal 10.908 0.093 7.308 0.150 Fe/HT/woodl 5.130 0.255 6.780 0.240 Fe/HT/wood2 3.025 0.310 2.258 0.490 Fe/HT/wood3 3.612 0.135 3.402 0.450 -29 -Referring to Figure 7 and Table 6, calcination may, in some cases, decrease AsSt sorption (Fe/HT vs cFe/HT, Fe/HT/charcoal vs cFe/HT/charcoal but in other cases increases As3t sorption (Fe/HT/charcoal vs cFe/HT/charcoal). Adsorption capacity of Fe/Mg hydrotalcites was not markedly affected by precipitating them into charcoal.

Loading wood before charring with Fe/Mg hydrotalcites in general seemed to reduce the efficacy of the hydrotalcites, possibly suggesting a chemical change as a resuh of the charring process itself. . Although the inventors do not wished to be bound by theory, they believe that this may he due to the charcoal acting as a reducing agent. However, the result showed that there was a small increase in arsenic sorption with increasing amounts of hydrotalcite precipitated into the charcoal matrix.

Example 3 -Effect of charring temperature on arsenic uptake by hydrotalcites in charcoals derived from wood loaded materials Table 7: Estimated removal of As3t and As5t by Al/Mg hydrotalcites in charcoal.

Products were detived from pre-loaded pine wood that was subseguenily charred at 350. 450 or 550°C. Amounts adsorbed are expressed as mg As removed by 1 g hydrotalcite. N3.

As3t As3 Adsorption Adsorption ________________ (mg/g) std error g/g std error A1/HT 350 4.893 0.108 9.830 0.245 Al/HT 450 10.878 0.076 13.718 0.268 A1/HT 550 12.748 0.808 14.978 0.356 Referring to Figure 8 and Table 7, increased sorption of arsenic was achieved by increasing the charting temperature of wood pre-loaded with hydrotalcites. The inventors believe that there are two possible reasons for this. Firstly, at higher temperatures charcoals are more carbonised and generally have a higher surface area.

Secondly, as temperature increases, hydrotalcites become increasingly calcined as water and interlayer anions are lost.

-30 -Table 8: Estimated removal of As3 and As by Fe/Mg hydrotalcites in charcoal.

Products were derived from pre-loaded pine wood that was subseguentiv charred at 350. 450 or 550°C. Amounts adsorbed are expressed as mg As removed by 1 g hydrotalcite. N3.

As3 As5 _____________ Sorption (mg/g) std error pfption (mg/g) std error Fe/HT 350 6.568 0.228 6.140 0.323 Fe/HT 450 6.588 0.193 6.580 0.220 Fe/HT 550 5.035 0.320 6.083 0.188 Referring to Figure 9 and Table 8, the inventors noted that charring wood loaded with Fe/Mg hydrotalcites at higher temperatures (550°C) decreased capacity of the resulting product to adsorb arsenic compared to products that were charred at lower temperatures (350 and 450°C). In general, charring temperature seemed to have little effect on arsenic adsorption of chars that were derived from woods loaded with Fe/Mg hydrotalcites.

Arsenic sorption from solutions with different pH's Table 9: Adsorption estimates of As3 and As5 using /Mg or Fe/Mg hydrotalcites at different pH (3, 7 and ii). Hydrotalcites were either used on their own (Metal/HT and cMetal/HT or derived from from pre-loaded pine wood that was subseguentiv charred at 550°C. or from charcoals where the hydrotalcite was precipitated within existing charcoal (Metal/HT/Charcoal and cMetal/HT/Charcoal) The latter was calcined at 550°C. 15 mg product was added to 15 ml water containing 10 mg/l As. Adsorption is expressed as mg As/ g hydrotalcite. N3.

At pH3 -pH7 -pHil ____ Adsorption Std -Adsorption Std -Adsorption Std __________________ (mg/g) error -(mg/g) error -(mg/g) error A1/HT 2.702 0.150 -6.250 0.144 1.794 0.087 cAl/HT 7.941 0.056 -8.620 0.056 -7.741 0.358 Al/HT/wood2 18.843 0.178 -17.443 0.343 15.92 0.345 Al/HT/charcoal 1.495 0.333 -1.203 0.783 -2.108 0.055 cAl/HT/charcoal 7.108 0.428 -5.380 0.118 -3.565 0.293 Fe/HT 8.900 0.273 7.318 0.361 6.003 0.204 cFe/HT 9.560 0.225 -9.155 0.310 -9.550 0.058 Fe/HT/wood2 5.243 0.275 -2.600 0.250 -3.468 0.295 Fe/HT/charcoal 9.203 0.305 -3.748 0.390 -2.070 0.483 -31 -cFe/HT/charcoal 8.445 0.273 8.925 0.258 12.660 0.328 pH3 ____ -pH7 ____ -pHil ____ Adsorption Std Adsorption Std Adsorption Std __________________ mg/g error -mg/g error -mg/g error Al/HT 9.280 0.003 -9.158 0.708 -9.583 0.080 cAl/HT 10.000 0.000 -10.00 0.000 8.577 0.771 Al/HT/wood2 22.135 0.463 -21.01 0.370 16.355 0.638 Al/HT/charcoal 14.915 0.163 -5.515 0.950 -4.740 0.450 cAl/HT/charcoal 14.600 0.403 -18.255 0.228 -14.858 0.120 Fe/HT 9.393 0.350 8.710 0.151 5.763 0.249 cFe/HT 9.529 0.471 -9.917 0.070 -9.121 0.508 Fe/HT/wood2 3.268 0.173 -3.575 0.148 -5.520 0.368 Fe/HT/charcoal 11.605 0.198 -4.360 0.120 -4.080 0.303 cFe/HT/charcoal 10.848 0.670 -17.153 0.350 -17.373 0.655 Referring to Figures 10 and ii and Table 9, Al/Mg Hydrotalcites incorporated into charcoal using hydrotalcite loaded wood as the precursor material produced better products compared to charcoals where the hydrotalcites were precipitated directly into the charcoal or compared with free' hydrotalcites. As shown in Figure 10, calcining Al/Mg Hydrotalcites resulted in a 4-fold increase in arsenic adsorption capacity. As shown in Figures 12 and 13, Fe/Mg Hydrotalcites incorporated into charcoal using hyrotalcite loaded wood as the precursor material provided products were in general not as effective at adsorbing arsenic from solutions compared to charcoals \vhere the hydrotalcites were precipitated directly into the charcoal.

Claims (17)

1. -32 -Claims 1. A composite adsorbent material comprising a porous carbon carrier matrix and an adsorbent species, wherein the adsorbent species is precipitated within the pores of the carrier matrix.
2. 2. The adsorbent material according to claim I, wherein the adsorbent species is capable of influencing the ionic composition of the surrounding aqueous phase, and wherein the resultant composite material produces an internal chemical environment which is different to that outside the material.
3. 3. The adsorbent material according to either claim I or claim 2, wherein the adsorbent species is incorporated inside or within the carbon matrix.
4. 4. The adsorbent material according to any preceding claim, wherein the concentration of the carrier matrix in the composite material is between 10-99% w/w) or between 30-95% (w/w) of the total weight of the composite material.
5. 5. The adsorbent material according to any preceding claim, wherein the concentration of the carrier matrix in the composite material is between 50-90% w/w of the total weight of the composite material.
6. 6. The adsorbent material according to any preceding claim, wherein the carrier matrix comprises, or is derived from, a cellulosic precursor material, preferably a hgno-cellulosic precursor material.
7. 7. The adsorbent material according to any preceding claim, wherein the carrier matrix comprises or is derived from plant material, and preferably woody plant material.io
8. 8. The adsorbent material according to any preceding claim, wherein the carrier matrix comprises or is derived from any hardwood or softwood species of plant.-33 -
9. 9. The adsorbent material according to any preceding claim, wherein the carrier matrix comprises non-activated carbon.
10. 10. The adsorbent material according to any preceding claim, wherein the carrier matrix is substantially macroporous.
11. 11. The adsorbent material according to any preceding claim, wherein the macro-pores have diameters in the range of SOnm to SOOnm, or 50 to 300nm, or 50 to 200nm.
12. 12. The adsorbent material according to any preceding claim, wherein the concentration of the adsorbent species in the composite material is between 1 -9O% (w/w) or between 1 O-75% (w/w) of the total weight of the composite material.
13. 13. The adsorbent material according to any preceding claim, wherein the concentration of the adsorbent species in the composite material is between 2O-SO% of the total weight of the composite material.
14. 14. The adsorbent material according to any preceding claim, wherein the adsorbent species is precipitated within the pores of the carrier matrix using a metathesis reaction or a displacement reaction.
15. 15. An adsorbent material according to any preceding claim, wherein the adsorbent species is basic.
16. 16. The adsorbent material according to any preceding claim, wherein the adsorbent species comprises a metal silicate, a metal hydrotalcite, a metal phosphate, a metal oxide, metal hydroxide, metal sulfide and/or a metal carbonate.
17. 17. The adsorbent material according to any preceding claim, wherein the adsorbent io species comprises an alkali or alkaline earth metal silicate, hydrotalcite, phosphate, oxide, hydroxide, sulfide and/or carbonate. -34-18. The adsorbent material according to any preceding claim, wherein the adsorbent species comprises calcium, magnesium or aluminium silicate, hydrotalcite, phosphate, oxide, hydroxide, sulfide and/or carbonate.19. The adsorbent material according to any preceding claim, wherein the surface area of the pores of the material is at least 0.5 m2 g-'.20. The adsorbent material according to any preceding claim, wherein the composite material has a pore surface area of at least 2 m2 g1, 3 m g', 4 m2 g-' or at least 5 m2 g'.21. The adsorbent material according to any preceding claim, wherein the composite material has a macropore volume which is greater than 0.5 cm3 ml-'.22. The adsorbent material according to any preceding claim, wherein the composite material has a macropore volume between 0.6 and 1 cm3 mU, or between 0.7 and 0.9 cm3 mU.23. The adsorbent material according to any preceding claim, wherein the composite material has pores that have an average diameter that is greater than I Onm, 2Onm, SOnm or iOOnm.24. A particle comprising the composite adsorbent material according to any one of claims 1 to 23.25. The particle according to claim 24, wherein the mean particle size is between about 0.1mm and 50mm, or between about 0.1mm and 25mm, or between about 0.25mm and 50mm, or bigger.io 26. Use of the composite material according to any one of claims 1 to 23, or the particle according to either claim 24 or 25, for the adsorption of inorganic substances.-35 - 27. Use according to claim 26, wherein the composite material or the particle is used in the clean-up of environmental pollution; for cleaning drinking water, or treatment of industrial and agricultural effluent; for removal of heavy metals or heavy-metal containing compounds from landfill leachate, groundwater, sewage sludge; or in soil amendments where soil or sediment is contaminated with heavy metals or heavy metal-containing compounds, phosphate etc. 28. A method of removing a pollutant from a fluid, the method comprising contacting a fluid comprising a pollutant with the composite adsorbent material according to any one of claims I to 23, or the particle according to either claim 24 or 25, under conditions suitable to remove the pollutant from the fluid.29. The method according to claim 28, wherein the composite matetial is supported on a support, for example in a carttidge or is placed inside a porous bag or filter, or is fixed onto a solid support, over which the polluted fluid is passed.30. The method according to either claim 28 or claim 29, wherein the method comptises feeding the fluid to be treated to the adsorbent material, or vice versa, and allowing the material to remove the pollutant from the fluid.31. The method according to any one of claims 28-30, wherein the fluid is a sludge or a slurry.32. The method according to any one of claims 28-31, \vherein the method comptises a step of separating at least some of the composite matetial from the fluid following sorption of the pollutant.33. The method according to claim 32, wherein the separation step comptises the use of a filter.-36 - 34. The method according to claim 32, wherein the separation step comprises the use of a magnet, preferably when the adsorbent material comprises iron oxide and/or iron hydroxide.35. The method according to any one of claims 28-34, wherein the method comprises a step of recovering the adsorbed poll utant from the spent adsorbent material.36. The method according to claim 35, wherein the recovery step comprises contacting the spent adsorbent material with an acid to release the adsorbed pollutant therefrom.37. The method according to claim 35, wherein the recovery step comprises contacting the spent adsorbent material with a salt solution to release the adsorbed pollutant therefrom.38. A soil amendment composition comprising the composite adsorbent matetial according to any one of claims I to 23.39. The soil amendment composition according to claim 38, for use in changing the pH of soil.40. A method of prepating a composite adsorbent matetial, the method comprising the steps of: providing a porous carbon cartier matrix; and (ii) precipitating an adsorbent species within the pores of the cartier mattix, to thereby form a composite adsorbent matetial.41. The method according to claim 40, wherein the composite adsorbent material is io defined as in any one of claims I to 23.42. The method according to either claim 40 or claim 41, wherein the carbon -37 -matrix is heated to at least 300°C, 400°C, 450°C, 500°C, 600°C, 800°C or 1000°C prior to the precipitation step.