Composite adsorbent material

Abstract

The invention relates to composite adsorbent materials, and in particular, to highly porous carbon-based composite materials for the adsorption and stabilization of inorganic substances. The composite adsorbent material comprises a porous carbon carrier matrix and an adsorbent species, wherein the adsorbent species is precipitated within the pores of the carrier matrix. 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 adsorbant material is amended into soil, waste etc. for the purpose of breaking pollutant-receptor linkages.

Claims

1. A composite adsorbent material comprising a porous carbon carrier matrix and an adsorbent species, wherein: the carrier matrix comprises non-activated charcoal, and the adsorbent species is selected from one or more of a silicate or a hydrotalcite, precipitated within the pores of the carrier matrix.

2. The adsorbent material according to claim 1, wherein the adsorbent species is also disposed in void spaces formed between adjacent particles comprising the carrier matrix.

3. The adsorbent material according to claim 1, wherein the carrier matrix comprises or is derived from charred plant material, charred compost, a charred hardwood or a charred softwood species of plant.

4. A soil or waste amendment composition comprising the composite adsorbent material according to claim 1, for use in changing the pH of soil or waste.

5. The adsorbent material according to claim 1, wherein the carrier matrix is substantially macroporous, and wherein the macropores have average diameters in the range of 50 nm to 500 nm, or 50 to 300 nm, or 50 to 200 nm.

6. The adsorbent material according to claim 1, wherein the concentration of the adsorbent species in the composite material is between 1-90% (w/w), 10-75% (w/w) or 20-50% of the total weight of the composite material.

7. A particle comprising the composite adsorbent material according to claim 1, wherein the mean particle size is between about 0.1 mm and 50 mm, or between about 0.1 mm and 25 mm, or between about 0.25 mm and 50 mm.

8. The adsorbent material according to claim 1, wherein the adsorbent species is basic.

9. The adsorbent material according to claim 1, wherein the adsorbent species comprises a silicate.

10. The adsorbent material according to claim 1, wherein the adsorbent species comprises a metal silicate and/or hydrotalcite.

11. The adsorbent material according to claim 10, wherein the metal silicate and/or hydrotalcite comprises a reduced metal species of manganese, cobalt, copper, zinc, iron, nickel, bismuth or silver.

12. A method of preparing a composite adsorbent material according to claim 1, the method comprising the steps of: (i) providing a porous carbon carrier matrix comprising non-activated charcoal; and (ii) precipitating an adsorbent species selected from one or more of a silicate or a hydrotalcite within the pores of the carrier matrix, to thereby form a composite adsorbent material.

13. The method according to claim 12, wherein the carbon carrier matrix is heated to at least 300 C., 400 C., 450 C., 500 C., 600 C., 800 C., 1000 C. or more in an oxygen limited environment prior to the precipitation step, and wherein the carbon matrix is impregnated with a soluble salt prior to being heated resulting in the reduction and precipitation of at least one of the ions of the impregnating salt.

14. A composite adsorbent material comprising a porous carbon carrier matrix and an adsorbent species, wherein: the carrier matrix comprises non-activated carbon, and the adsorbent species is an alkaline earth metal oxide, precipitated within the pores of the carrier matrix.

15. The composite adsorbent material according to claim 14, wherein the alkaline earth metal oxide is calcium oxide or magnesium oxide.

16. A method of preparing a composite adsorbent material according to claim 14, the method comprising the steps of: (i) providing a porous carbon carrier matrix comprising non-activated carbon; and (ii) precipitating an adsorbent species comprising an alkaline earth metal oxide within the pores of the carrier matrix, to thereby form a composite adsorbent material.

17. A method of adsorbing inorganic substances, the method of comprising contacting the composite material of claim 1 with the inorganic substances.

18. A method according to claim 17, wherein the inorganic substances comprise an environmental contaminant or pollutant present in drinking water, or an industrial or agricultural effluent; the inorganic substances comprising heavy metals or heavy-metal containing compounds present in landfill leachate, groundwater, drilling waste, mine drainage, mine spoil, or sewage sludge; the inorganic substances comprising heavy metals or heavy metal-containing compounds, phosphates etc. and are present in soil amendments; or the inorganic substances comprise bromates, arsenates, selenium, antimony, strontium, cyanides, chlorinated compounds, nitrates, sulphates or arsenites.

19. 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 claim 1 under conditions suitable to remove the pollutant from the fluid, wherein the composite material is supported on a support, for example in a cartridge or is placed inside a porous bag or filter, or is fixed onto a solid support, over which the polluted fluid is passed.

Description

(1) 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:

(2) FIG. 1 is a graph showing the pH change with charcoal particles of 0.25-0.5 mm;

(3) FIG. 2 is a graph showing the pH change with charcoal particles of 1-2 mm;

(4) FIG. 3 is a graph showing the pH change with charcoal particles of 2-4 mm;

(5) FIG. 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 2 cm of non-contaminated garden soil. N=5;

(6) FIG. 5 are photographs showing the establishment of Rye grass on non-amended soil (FIG. 5a) and soil amended with 4% (w/w) silicate charcoal (FIG. 5b). Soils were originally acidic (pH 2.5) and contained high levels of a range of heavy metals;

(7) FIG. 6 is a graph showing the removal of As.sup.3+ and As.sup.5+ ions using Al/Mg hydrotalcites in a pure form (Al/HT and cAl/HT) and charcoal products where Al/Mg hydrotalcites were precipitated within the charcoal pore structure. Table 4 summarises the abbreviations used. 25 mg product was added to 25 ml water containing 10 mg/l As. Adsorption was not adjusted for the amount of hydrotalcite in each product. N=3;

(8) FIG. 7 is a bar chart showing the removal of As.sup.3+ and As.sup.5+ using Fe/Mg hydrotalcites 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/l. Adsorption was not adjusted for the amount of hydrotalcite in each product. N=3;

(9) FIG. 8 is a bar chart showing the percentage removal of As.sup.3+ and As.sup.5+ 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 15 ml water containing 10 mg As/l. N=3;

(10) FIG. 9 is a bar chart showing the percentage removal of As.sup.3+ and As.sup.5+ 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 15 ml water containing 10 mg As/l. N=3;

(11) FIG. 10 is a bar chart showing the percentage removal of As.sup.3+ using Al/Mg hydrotalcites at different pH (3, 7 and 11). Hydrotalcites were either used on their own (Al/HT and cAl/HT) or derived rom 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 mg As/l. N=3;

(12) FIG. 11 is a bar chart showing the percentage removal of As.sup.5+ using Al/Mg hydrotalcites at different pH (3, 7 and 11). Hydrotalcites were either used on their own (Al/HT and cAl/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 (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 mg As/l. N=3;

(13) FIG. 12 is a bar chart showing the percentage removal of As.sup.3+ using Fe/Mg hydrotalcites at different pH (3, 7 and 11). Hydrotalcites were either used on their own (Fe/HT 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 calcined at 550 C. 15 mg product was added to 15 ml water containing 10 mg As/l. N=3;

(14) FIG. 13 is a bar chart showing the percentage removal of As.sup.5+ using Fe/Mg hydrotalcites at different pH (3, 7 and 11). Hydrotalcites were either used on their own (Fe/HT 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 calcined at 550 C. 15 mg product was added to 15 ml water containing 10 mg As/l. N=3;

(15) FIG. 14 shows the removal of arsenate (40 mg l.sup.1) solutions over time after amendment with 2.5 g l.sup.1 iron char at different states of oxidation (fully reduced: FeCH; Partly oxidised: FePO; Fully oxidised; FeFO) or amended with 2.5 g l1 iron oxide (Fe.sub.3O.sub.4: IOB; Fe.sub.2O.sub.3: IOR). N=2;

(16) FIG. 15 shows pH of arsenate solutions over time after amendment with 2.5 g l.sup.1 iron char at different states of oxidation (fully reduced: FeCH; Partly oxidised: FePO; Fully oxidised: FeFO) or amended with 2.5 g l1 iron oxide (Fe.sub.3O.sub.4: IOB; Fe.sub.2O.sub.3: IOR). N=2;

(17) FIG. 16 shows adsorption of copper (mg kg.sup.1 composite) from 500 mg l.sup.1 solutions of Cu(NO.sub.3).sub.2 adjusted to pH values of 2, 3, 4, 5 and 6. Adsorption was measured after 1, 3, 6, 24 and 48 hours. (N=2);

(18) FIG. 17 shows adsorption of cadmium (mg kg.sup.1 composite) from 500 mg l.sup.1 solutions of Cd(NO.sub.3).sub.2 adjusted to pH values of 2, 3, 4, 5 and 6. Adsorption was measured after 1, 3, 6, 24 and 48 hours. (N=2); and

(19) FIG. 18 shows adsorption of zinc (mg kg.sup.1 composite) from 500 mg l.sup.1 solutions of Zn(NO.sub.3).sub.2 adjusted to pH values of 2, 3, 4, 5 and 6. Adsorption was measured after 1, 3, 6, 24 and 48 hours. (N=2).

EXAMPLES

Example 1Stabilising Effect of Charcoal on Copper Silicate at Low pH

(20) Introduction

(21) 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.

(22) 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 be 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 edge effects.

(23) Materials and Methods

(24) Stability of CuSiO.sub.3 in Solution

(25) To test this hypothesis, charcoal particles of different sizes were prepared from sweet chestnut wood. Sweet chestnut wood was charred at 450 C., 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 impregnated with liquid Potassium silicate (50% K.sub.2SO.sub.3 by weight) to obtain charcoal containing 10% K.sub.2SO.sub.3 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.

(26) 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.

(27) To allow the calcium silicate to be converted into copper silicate, 18 bottles, each containing 1.07 g CuSO.sub.4.5H.sub.2O per liter 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 CaSiO.sub.3 powder (Sigma, UK). The bottles were left for >1 week to allow equilibrium between the CaSiO.sub.3 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 CaSiO.sub.3 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).

(28) TABLE-US-00001 TABLE 1 Removal of Copper ions from a solution of CuSO.sub.45H.sub.2O (1.07 g 1.sup.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 (<0.01 mm, 0.25-0.5 mm, 1.0-2.0 mm and 2.0-4.0 mm). N = 3. Different letters indicate significant differences between means at P < 0.05. Cu concentration (mg 1.sup.1) in Treatment solution SE % removal Control 246.9 9.5 (a) 0 CaSiO.sub.3 170.4 9.9 (b) 31 <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

(29) 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 600 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).

(30) TABLE-US-00002 TABLE 2 Ash and silicate content in silicated and non-silicated charcoal with different particle sizes. 5 g charcoal was used for each assessment. Ash content Silicate Treatment (g) content (g) % silicate 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

(31) 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.

(32) 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:

(33) ##STR00001##

(34) 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.

(35) Field Experiment Parys Mountain

(36) In this experiment, 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.

(37) 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 (>300,000 ppm).

(38) Three different treatments were compared: Control (no amendment), top soil (2 cm) covering 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 each plot was sown in with rye grass (Lolium perenne) and germination and plant growth was monitored over the following 77 days. Also leachable metals were monitored using the British Standards Method (BSI 2002) immediately after treatment (t=0), 1 day after treatment, 37 days after treatment and 77 days after treatment. Five samples were taken from each plot and analysed separately using ICP analysis.

(39) Results

(40) Stability of Silicate in Solution

(41) Control: pH change of solution of HCl with pH of 2 when amended with an equivalent quantity (0.5 g.sup.1 100 ml.sup.1) of silicate was on average 10.70.7 pH units per minute (n=3).

(42) Referring to FIGS. 1-3, there are shown the change of pH of solution of 100 ml HCl with a pH of 2 when amended with 5 g of silicate charcoal with a particle size of 0.25-0.5 mm (FIG. 1), 1-2 mm (FIG. 2) or 2-4 mm (FIG. 3). Charcoal contained around 10% (0.5 g) CuSiO.sub.3 by weight. n=2 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.

(43) FIGS. 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.

(44) 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.

(45) TABLE-US-00003 TABLE 3 Reactivity of silicate embedded in charcoal particles of different sizes compared with free copper silicate (control) at increasing pH. Figure in brackets denotes stability increase compared with control. N = 3; different letters denote significant (p < 0.05) differences between treatments Reactivity of silicate Time to pH 5.0 from Treatment Time to pH 4.5 pH 4.5 Control 14 seconds (1) a 36 seconds (1) b Finely ground (<0.01 mm) 45 seconds (3) b 10 minutes (17) c 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 2.0-4.0 mm 41 minutes (176) d >18 hours (>1800) f

(46) Table 3 shows that silicate embedded in charcoal reacts progressively less when (a) the particle size increases (P<0.001) and (b) 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, very 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.

(47) Field Experiment Parys Mountain

(48) Referring to FIG. 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). FIG. 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 1 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. 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.

(49) Referring to FIGS. 5a and 5b, 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 FIG. 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 FIG. 5b.

(50) Conclusions

(51) In summary, the inventors have demonstrated that at a low pH (i.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 with heavy metals. Finally, the inventors have shown that amendment of silicate charcoal to acidic heavy metal contaminated soil restores plant growth.

Example 2Effectiveness of Charcoals into which Hydrotalcites are Precipitated for the Removal of Arsenic Species from Water

(52) Two layered double hydroxides (LDHs)/hydrotalcite materials precipitated into charcoal were investigated for their efficacy in removing arsenic species (As.sup.3+ and As.sup.5+) from water. AlMg based and FeMg 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 were made with Cl.sup. as the interlayer anion with a ratio of M.sup.2+:M.sup.3+ of 2.15:1 in the initial solutions (Gillman, 2006, Science of 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.

(53) Two methods of loading hydrotalcites onto charcoal particles were used. Firstly, precipitation directly into charcoal derived from Scotch Pine wood charred at 550 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. Materials prepared by precipitation directly into the charcoal were also calcined at 550 C. Charcoal particle sizes used throughout were 0.5-1 mm. Sorption experiments were carried out in triplicate.

(54) In a further experiment, the effect of charring 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 40% 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.

(55) 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 experiment, 15 mg material was added to 15 ml arsenic solution containing 10 mg As/l. Solutions were shaken for 24 hours before remaining arsenic in the solution was assessed.

(56) To determine the amount of arsenic adsorbed by the different materials, 25 mg material was shaken for 24 h at 20 C. in 25 ml, 10 mg/l arsenic solution. Arsenic concentrations were determined using molybdenum blue colorimetric method (BS1728-12:1961), which has a minimum detection limit of 20 ppb arsenic. In brief, a sample containing the arsenic is mixed with an acid solution of Mo.sup.VI, for example ammonium molybdate, to produce AsMo.sub.12O.sub.40.sup.3, which has an -Keggin structure. This anion is then reduced by, for example, asorbic acid, to form the blue coloured -keggin ion, PMo.sub.12O.sub.40.sup.7. 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.

(57) TABLE-US-00004 TABLE 4 List of abbreviations used Abbreviation (M = Al or Fe) Material M/HT Hydrotalcite cM/HT Calcined hydrotalcite at 550 C. M/HT/wood1 Hydrotalcite loaded onto wood then charred (20% HT by weight in charcoal) M/HT/wood2 Hydrotalcite loaded onto wood then charred. Initial solution concentration 2x that used in M/HT/wood1 (40% HT by weight in charcoal) M/HT/wood3 Hydrotalcite loaded onto wood then charred. Initial solution concentration 3x that used in M/HT/wood1 (60% HT 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/HT/charcoal M/HT/charcoal calcined at 550 C. (40% HT by weight in charcoal)
Results:
As.sup.3+ and As.sup.5+ Sorption of Hydrotalcites Directly Precipitated into Charcoal or Loaded onto Wood First Before Charring

(58) TABLE-US-00005 TABLE 5 Estimated removal of As.sup.3+ and As.sup.5+ by Al/Mg hydrotalcites in a pure form (Al/HT and cAl/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 = 3 As.sup.3+ As.sup.5+ Removal removal (mg/g) std error (mg/g) std error Al/HT 3.784 0.071 9.991 0.007 cAl/HT 7.659 0.279 9.734 0.187 Al/HT/charcoal 4.050 0.515 16.258 0.780 cAl/HT/charcoal 7.963 0.283 7.093 0.208 Al/HT/wood1 15.470 0.430 9.595 0.330 Al/HT/wood2 12.655 0.218 13.158 0.355 Al/HTwood3 10.085 0.332 10.640 0.070

(59) Referring to FIG. 6 and Table 5, it can be seen that calcination increases uptake of As.sup.3+ (Al/HT vs cAl/HT, Al/HT/charcoal vs cAl/HT/charcoal). However, calcination may decrease As.sup.5+ sorption (Al/HT/charcoal vs cAl/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.

(60) 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 As.sup.3+ and As.sup.5+ are similar, much like that of calcined material, possibly because they were charred at 550 C. The inventors believe that hydrotalcites precipitated in wood before charring may be more efficient at removing arsenic from water.

(61) TABLE-US-00006 TABLE 6 Estimated removal of As.sup.3+ and As.sup.5+ 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 = 3. As.sup.3+ As.sup.5+ Removal Removal (mg/g) std error (Mg/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/wood1 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

(62) Referring to FIG. 7 and Table 6, calcination may, in some cases, decrease As.sup.5+ sorption (Fe/HT vs cFe/HT, Fe/HT/charcoal vs cFe/HT/charcoal) but in other cases increases As.sup.3+ 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 result of the charring process itself. Although the inventors do not wished to be bound by theory, they believe that this may be 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 3Effect of Charring Temperature on Arsenic Uptake by Hydrotalcites in Charcoals Derived from Wood Loaded Materials

(63) TABLE-US-00007 TABLE 7 Estimated removal of As.sup.3+ and As.sup.5+ by Al/Mg hydrotalcites in charcoal. Products were derived from pre-loaded pine wood that was subsequently charred at 350, 450 or 550 C. Amounts adsorbed are expressed as mg As removed by 1 g hydrotalcite. N = 3. As.sup.3+ As.sup.5+ Adsorption Adsorption (mg/g) std error (mg/g) std error Al/HT 350 4.893 0.108 9.830 0.245 Al/HT 450 10.878 0.076 13.718 0.268 Al/HT 550 12.748 0.808 14.978 0.356

(64) Referring to FIG. 8 and Table 7, increased sorption of arsenic was achieved by increasing the charring 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.

(65) TABLE-US-00008 TABLE 8 Estimated removal of As.sup.3+ and As.sup.5+ by Fe/Mg hydrotalcites in charcoal. Products were derived from pre-loaded pine wood that was subsequently charred at 350, 450 or 550 C. Amounts adsorbed are expressed as mg As removed by 1 g hydrotalcite. N = 3. As.sup.3+ As.sup.5+ Sorption (mg/g) std error Sorption (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

(66) Referring to FIG. 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.

(67) Arsenic Sorption from Solutions with Different pH's

(68) TABLE-US-00009 TABLE 9 Adsorption estimates of As.sup.3+ and As.sup.5+ using Al/Mg or Fe/Mg hydrotalcites at different pH (3, 7 and 11). Hydrotalcites were either used on their own (Metal/HT and cMetal/HT) or derived from from pre-loaded pine wood that was subsequently 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. N = 3. As.sup.3+ pH 3 pH 7 pH 11 Adsorption Std Adsorption Std Adsorption Std (mg/g) error (mg/g) error (mg/g) error Al/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 cFe/HT/charcoal 8.445 0.273 8.925 0.258 12.660 0.328 As.sup.5+ pH 3 pH 7 pH 11 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

(69) Referring to FIGS. 10 and 11 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 FIG. 10, calcining Al/Mg Hydrotalcites resulted in a 4-fold increase in arsenic adsorption capacity. As shown in FIGS. 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 where the hydrotalcites were precipitated directly into the charcoal.

Example 4Use of Reduced Iron Char for the Removal of Bromate from Water

(70) Introduction

(71) Contamination of drinking water with bromate (BrO.sup.3) at levels ranging from 0.4 to 60 g L.sup.1 may be found following ozonation of water containing background bromide (Br.sup.). Based on rodent studies, bromate is classified as a possible human carcinogen, and drinking water standards of 10-25 g L.sup.1 are now implemented in many countries. Bromate is highly soluble, stable in water, and difficult to remove using conventional treatment technologies.

(72) Materials and Methods

(73) Production of iron char: 112 g Fe.sub.2(SO.sub.4).sub.3.7H.sub.2O was dissolved in 50 ml water till fully dissolved. The resulting solution was mixed with 100 g dried pine shavings and dried at 80 C. overnight till dry. The thus impregnated wood was charred at 450 C. for 1 hour. The metal char that was produced in this way was highly magnetic and contained an estimated 50% iron by weight.

(74) Experimental set up: a 5 mg L1 Bromate solution was produced by dissolving 5.88 mg NaBrO3 in one liter RO water. This was done by creating a stock solution containing 0.588 g NaBrO3 in 100 ml and transferring one ml of this stock solution to 1000 ml RO water.

(75) The following treatments were created: 1. Control, containing 5 mg l.sup.1 bromate 2. 0.1 g iron char in 500 ml, 5 mg l.sup.1 bromate 3. 0.2 g iron char in 500 ml, 5 mg l.sup.1 bromate 4. 0.3 g iron char in 500 ml, 5 mg l.sup.1 bromate 5. 0.4 g iron char in 500 ml, 5 mg l.sup.1 bromate 6. 0.5 g iron char in 500 ml, 5 mg l.sup.1 bromate

(76) Solutions were held in 500 ml brown plastic bottles to prevent exposure to light. two bottles per treatment were used. Bottles were strapped onto a head over heels shaker with a rotation speed of one turn per 2 seconds. To determine how fast the iron char removed (reduced) the bromate, each bottle was sampled after 1 hour, after 3 hours and after 24 hours. After sampling the 10 ml solution was passed through a 0.45 m syringe filter to remove any particulates and the filtered solutions were stored in the dark at 5 C. till analysis.

(77) Bromate analysis: Bromate concentrations were estimated using a protocol described by Brookman et al., (2011). In brief, to 7 ml filtered solution, 2 ml hydrochloride acidified glycine at pH 1 was added. To this mixture 1 ml of a 1M solution of Iodite (KIO.sub.3) was added. The solution was allowed to react for 5 minutes before absorbance was measured on a spectrophotometer at =352 nm. The measured absorbance was compared against a standard curve of absorbance against known (0, 0.01, 0.1, 1, 2 and 5 ppm) concentrations of bromate to determine reduction of bromate in the samples.

(78) Results

(79) TABLE-US-00010 TABLE 10 Effect of amendment rate and exposure time of iron-char on concentrations of bromate in water (n = 2). Data are presented as averages SEM. Different letters in sub- script indicate significant differences (P < 0.05) between values. Amendment Bromate rate Exposure Concentration Reduction Treatment (g 1.sup.1) (hours) (mg 1.sup.1) (%) Control 0 1 5 0 0 3 5 0 0 24 5 0 Fe-char 0.2 1 0.80 0.25.sup.a 84.0 5.0 0.2 3 0.56 0.03.sup.b 88.7 0.6 0.2 24 0.31 0.03.sup.c 93.7 0.5 0.4 1 0.76 0.07.sup.a 84.8 1.4 0.4 3 0.40 0.02.sup.b 91.9 0.4 0.4 24 0.11 0.00.sup.d 97.9 0.0 0.6 1 0.63 0.20.sup.a 87.5 4.0 0.6 3 0.35 0.03.sup.c 93.1 0.7 0.6 24 0.10 0.00.sup.d 98.0 0.03 0.8 1 0.62 0.21.sup.a 87.5 4.1 0.8 3 0.30 0.03.sup.c 93.9 0.5 0.8 24 0.10 0.00.sup.d 98.1 0.07 1.0 1 0.36 0.02.sup.c 93.2 0.48 1.0 3 0.29 0.01.sup.c 94.2 0.27 1.0 24 0.10 0.00.sup.d 98.1 0.03 significance P < 0.001 P < 0.001

(80) A relative small quantity of iron char (0.2 g l.sup.1) removed 85% of the bromate from water that was spiked with 5 mg l.sup.1 bromate. Increasing the exposure time to 24 hours increased removal rates to 94%, increasing the amendment rate had a slight effect with removal rates of the 0.4-1.0 g l.sup.1 amendment rate resulted in 93% removal of bromate after one hour and >98% removal after 24 hours. The levels of removal achieved were below the detection limit. There were no significant differences in the removal rates of bromate between amendment rates of 0.4, 0.6, 0.8 and 1.0 g iron char per liter.

(81) In a subsequent experiment where oxidised iron char was used to remove bromate no significant removal of bromate could be shown (1 g oxidised iron char l.sup.1 water removed 36% of the bromate (5 ppm) after 24 hours shaking). Oxidation was achieved by wetting the iron char and subsequently drying it at 80 C.

(82) Conclusions:

(83) Composites made from charcoal and iron were effective at removing bromate from water The most likely mechanism by which bromate is removed is via reduction of bromate to bromide Oxidised iron incorporated within the charcoal was ineffective at removing bromate, further confirming that the iron created a reducing environment within the char that allowed its removal from the water.

Example 5Ability of Iron Char to Remove Arsenic from Water

(84) Introduction

(85) An experiment was set up to assess the ability of char into which iron (both elementary iron and oxidised iron) was incorporated via a reduction reaction to remove arsenate from water. The treatment was compared with amendments of iron oxide (both red iron oxide (Fe.sub.2O.sub.3) and black iron oxide (Fe.sub.3O.sub.4)). Iron oxides are commonly used to remove arsenic from drinking water via a co-precipitation reaction with As(V).

(86) Materials and Methods

(87) Production of iron char: 112 g Fe.sub.2(SO.sub.4).sub.3.7H.sub.2O was dissolved in 50 ml water. The resulting solution was mixed with 100 g dried pine chips and dried at 80 C. overnight. The thus impregnated wood was charred at 450 C. for 1 hour. The metal char that was produced in this way was highly magnetic and contained an estimated 50% iron by weight (Product Code: FeCH). To test if oxidising the iron inside the char had an effect on arsenic adsorption, some of the iron-char was wetted and subsequently dried in the oven to create Fe-oxide/hydroxide (rust) inside the char. It was assumed that this treatment only partially oxidised the iron inside the char (Product Code: FePO). To complete the oxidation of the iron inside the char, the iron char was subjected to a controlled burn to create a product that was visually red (Product Code: FeFO). Experimental set up: The ability of the three charcoal products containing iron (FeCH) or iron oxides/hydroxides (FePO and FeFO) were compared with black iron oxide (Fe.sub.3O.sub.4) and red iron oxide (Fe.sub.2O.sub.3) for their ability to remove arsenic from water. One liter plastic bottles were filled with a one liter solutions of arsenate (AsO.sub.4.sup.3) containing 40 mg l.sup.1 arsenate.

(88) The following treatments were created in each bottle in duplicate 1. Control, no amendment 2. 2.5 g iron char (FeCH) 3. 2.5 g partially oxidised iron char (FePO) 4. 2.5 g fully oxidised iron char (FeFO) 5. 2.5 g black iron oxide iron (Fe.sub.3O.sub.4; IOB) 6. 2.5 g red iron oxide (Fe.sub.2O.sub.3; IOR)

(89) Bottles were strapped onto a head over heels shaker with a rotation speed of one turn per 2 seconds. To determine how fast the iron char removed the arsenate, each bottle was sampled at time 0 (before amendment), after 10 minutes. 1 hour, 3 hours, 7 hours and after 4 days (168 hours). After sampling the 10 ml solution was passed through a. 0.45 m syringe finer to remove any particulates and the filtered solutions were stored in the dark at 5 C. till analysis. Arsenic concentrations in each sample were determined using ICP-OES. Besides arsenic concentrations in each sample, pH was measured using a pH probe.

(90) Results

(91) Iron char was 4 time more effective at removing arsenic from the water than iron-oxide (see FIG. 14) removing the equivalent of 16 g arsenic per kg product. Based on the amount of adsorbent, which was half the weight of the iron char, the iron in the char removed 8 times more arsenic than the equivalent amount of iron-oxide. In general, arsenic is least soluble at pH between 3.5 and 6. The arsenic removal capacity of the different products did not seem to be affected by pH. In fact the iron-char (especially the reduced products) raised the pH significantly from pH 3 to pH 7 (see FIG. 15).

(92) Conclusions

(93) Charcoal composites containing either reduced iron or iron oxide were highly effective at adsorbing arsenic from water Charcoal composites were 8 times more effective at removing arsenic from water than the equivalent amount of iron oxide

Example 6Production of Charcoal-Silicate Composites for Removing Heavy Metals from Solution

(94) Introduction

(95) Impregnation of macro pores with a solution of potassium silicate followed by impregnation with a calcium salt to bring about precipitation of the silicate in the form of insoluble calcium silicate is technically challenging as a concentrated solution of potassium silicate is viscous and therefore does not easily soak into the macro-pores present in charcoal. To simplify the creation of a porous charcoal/calcium silicate granule a range of materials was tested for their ability to form a granule of sufficient hardness to act as a product for heavy metal removal.

(96) Materials and Methods

(97) For this purpose green waste compost (GWC) was charred at 450 C. for at least 1 hour. Charcoal produced from green waste compost contains around 50% carbon and 50% minerals, half of which are alkaline earth and alkali minerals (potash), the other half being insoluble minerals such as silica. It was hypothesised that the alkaline earth metal ions (calcium mainly) would react with the potassium silicate to form insoluble CaSiO.sub.3. An experiment was set up to determine how much Calcium ions (added in the form of Ca(NO.sub.3).sub.2.4H.sub.2O or Ca(OH).sub.2 needed to be added to the mix to create a granular material that could act as a product for metal adsorption. Ca(OH).sub.2 is mainly insoluble and therefore slow acting while Ca(NO.sub.3).sub.2.4H.sub.2O is highly soluble. The aim was to create a product with great hardness.

(98) The following mixes were created using charred green waste compost (GWC char). 1) 50 g GWC Char, 50 g 30% K.sub.2SiO.sub.3, 11.75 g Ca(NO.sub.3).sub.2.4H.sub.2O 2) 50 g GWC Char, 50 g 15% K.sub.2SiO.sub.3, 5.90 g Ca(NO.sub.3).sub.2.4H.sub.2O 3) 50 g GWC Char, 50 g 7.5% K.sub.2SiO.sub.3, 2.95 g Ca(NO.sub.3).sub.2.4H.sub.2O 4) 50 g GWC Char, 50 g 30% K.sub.2SiO.sub.3, 9.4 g Ca(NO.sub.3).sub.2.4H.sub.2O 5) 50 g GWC Char, 50 g 15% K.sub.2SiO.sub.3, 4.7 g Ca(NO.sub.3).sub.2.4H.sub.2O 6) 50 g GWC Char, 50 g 7.5% K.sub.2SiO.sub.3, 2.35 g Ca(NO.sub.3).sub.2.4H.sub.2O 7) 50 g GWC Char, 50 g 30% K.sub.2SiO.sub.3, 3 g Ca(OH).sub.2 8) 50 g GWC Char, 50 g 15% K.sub.2SiO.sub.3, 1.5 g Ca(OH).sub.2 9) 50 g GWC Char, 50 g 30% K.sub.2SiO.sub.3 10) 50 g GWC Char, 50 g 15% K.sub.2SiO.sub.3 11) 50 g GWC Char, 50 g 7.5% K.sub.2SiO.sub.3

(99) Ingredients were mixed thoroughly and the material was shaken in an aluminium tray to break the material up into small balls with a size of between 2-3 mm. The resulting aggregates were dried in an oven at 80 C. overnight and were subsequently assessed according to appearance, colour and hardness.

(100) The following hardness scale was used 1) Soft: Granules break when an object less than 200 g is placed on top 2) Fairly hard: Granules break when an object of between 200 and 5000 g is placed on top 3) Hard: Granules break when an object of 5000-10,000 g is placed on top 4) Very hard: Granules break when an object of >10,000 g is place on top
Results

(101) TABLE-US-00011 TABLE 11 Product characteristics using different mixtures of green waste compost char, potassium silicate, Ca(NO.sub.3).sub.24H.sub.2O Mixture Appearance Colour Hardness 1 granulated grey hard 2 granulated grey fairly hard 3 granulated grey soft 4 granulated grey hard 5 granulated grey fairly hard 6 granulated grey soft 7 granulated black soft 8 granulated black fairly hard 9 granulated black very hard 10 granulated black soft 11 granulated black Fairly hard

(102) The best product was produced by directly amending Green waste compost char with an equal weight of a 30% potassium silicate solution. The thus produced product formed a hard absorbent granulated product with the ability to resist pressures of between 50 and 70 kg. In theory the product should be able to bind 15% of its weight in heavy metals (150,000 mg metal ions kg.sup.1).

(103) Use of Charcoal-Silicate Composites for Removing Heavy Metals from Solution

(104) Introduction

(105) An experiment was set up to assess the capacity of composites made from green waste compost char and silicate to remove different heavy metals from solutions with different pH. It was hypothesised that if the composite was capable of stabilising its internal pH to a pH>6 it would be capable of removing heavy metals from solutions with an acidic pH.

(106) Materials and Methods

(107) Production of composite: 100 g of charred green waste compost (charred at 450 C. for 1 hour) was mixed with 100 g 30% K2SiO3. The mix was shaken in a tray to form aggregates with a size between 2 and 5 mm. The particles were then dried in an oven at 80 C. for 24 hours. In theory, the product should be able to bind 15% of its weight in heavy metals.

(108) Preparation of water with pH 2, 3, 4, 5 and 6: The pH of 10 litters of RO water was adjusted using concentrated nitric acid. Six 500 ml plastic bottles were filled with water of a specific pH.

(109) Preparation of heavy metal solutions: 5 ml stock solutions containing 50,000 mg of either cadmium (Cd), copper (Cu) or zinc (Zn) were pipetted into 495 ml RO water, resulting in a concentration of 500 mg heavy metal in the water. The metals were added as metal nitrates and actual concentrations were measured using ICP-OES subsequently.

(110) Experimental set up: The above preparations resulted in the following treatments:

(111) TABLE-US-00012 Heavy Metal pH Copper Cadmium Zinc 2 500 mg 1.sup.1 500 mg 1.sup.1 500 mg 1.sup.1 3 500 mg 1.sup.1 500 mg 1.sup.1 500 mg 1.sup.1 4 500 mg 1.sup.1 500 mg 1.sup.1 500 mg 1.sup.1 5 500 mg 1.sup.1 500 mg 1.sup.1 500 mg 1.sup.1 6 500 mg 1.sup.1 500 mg 1.sup.1 500 mg 1.sup.1

(112) Each treatment was set up in duplicate and to each bottle 0.5 g of product was added. Once the composite was added (which sank to the bottom of each bottle) treated bottles were left static on a bench at room temperature to allow the composite to adsorb the metal.

(113) Metal analysis: each bottle was sampled before amendment (t=0) to obtain the actual metal concentration in each bottle, after 1 hour, 3 hours, 6 hours, 24 hours and 48 hours. For this purpose 20 ml solution was withdrawn from the middle of each bottle with a 20 ml syringe. The sample was then passed through a 0.45 m filter to remove any particulates and the concentration of heavy metal was assessed using ICP-OES.

(114) Concentrations obtained for the different treatments were subtracted from the initial concentration of metal in each bottle to calculate metal uptake by the composite. pH: To ensure that the composite did not change the pH of the solutions significantly (a rise in pH above 7 could result in the formation of insoluble metal hydroxides), the pH of the solution at the bottom of each flask was measured after 24 hours.

(115) Results

(116) At pH 6 the silicate/char composite adsorbed more than 175,000 mg Cu kg.sup.1 composite (see FIG. 16). This equates to an adsorption of Cu ions above the theoretical maximum of 150,000 mg Cu kg.sup.1. Whereas most of the adsorption can be accounted for by the formation of Copper silicate, it is possible that part of the removal of Cu was due to precipitation of Cu in the form of Copper hydroxides. Even at pH 3 the composite adsorbed between 30,000 and 40,000 mg Cu kg.sup.1 (see FIG. 16). Table 12 shows that the pH of the solution was around 3.3 and therefore the only possibility is that the composite maintained an alkaline pH (>pH6) within the composite as allowing copper silicate to form.

(117) TABLE-US-00013 TABLE 12 pH changes after 20 hours near silicate/char composites in 500 ml bottles containing metal solutions (Cu, Zn and Cd) that had an original pH of 2, 3, 4, 5 or 6. Metal solution Original pH Cu Zn Cd Average SE 2.11 2.17 2.28 2.25 2.23 0.02 3.00 3.34 3.52 3.70 3.52 0.08 4.05 5.21 5.97 6.27 5.82 0.20 4.94 5.35 5.92 6.34 5.86 0.17 5.65 5.36 5.92 6.34 5.87 0.18

(118) A similar result was obtained with the removal of cadmium from solution. Best results were obtained at pH 6 with removal of 100,000 mg Cd kg.sup.1 composite (see FIG. 17).

(119) However at pH 3, still more than 60,000 mg of Cd was removed from a solution that measured a pH of 3.7 by the composite suggesting that the composite retained a pH well above this value to allow formation of Cadmium silicate within the composite.

(120) Zinc was adsorbed the least from the three heavy metals tested with maximum adsorption of 70,000 mg kg.sup.1 composite at pH 6 (see FIG. 18). However, at lower pH (4 and 3) still more than 40,000 mg kg-1 composite (see FIG. 18) was adsorbed suggesting that the composite maintained an alkaline pH within allowing Zinc silicate to form via a displacement reaction.

(121) The silicate composite changed the pH of the solutions near the particles slightly, with the greatest increase observed in the solutions containing Cadmium nitrate. In none of the treatments did the composite raise the pH of the solution above pH 6.34, while on average solutions remained below pH 5.9. This implies that the metals that were taken out of solution by the silicate composite did so because the internal pH of the composite was significantly higher than the surrounding liquid.

(122) Conclusions

(123) Composites made from green waste compost char and silicates were effective at adsorbing a range of heavy metals, including copper, cadmium and zinc from solutions with a pH as low as 3 The mechanism by which these composites remove metals is via maintenance of a high pH within their structure allowing formation of insoluble metal silicates

Example 7Use of Calcium Phosphate Modified Chars for the Adsorption of Heavy Metals from Anaerobic Muds

(124) Introduction

(125) Drill cutting muds are often heavily contaminated with heavy metals, including barium (weighting agent), aluminium (anti-frothing agent) and others that are part of the geology of the rock that is drilled. The pH of these materials is often highly alkaline (pH>10) and their redox is often negative (they become highly anaerobic when water-logged). Methods using alkaline adsorbents are inappropriate for these materials as they only raise the pH of the material further.

(126) Materials and Methods

(127) Production of a phosphate/char composite: An acidic metal adsorbent was produced using charcoal with a very high mineral content that was produced from the pyrolysis of straw at high temperatures. The material had an ash content of >30% most of which consisted of calcium oxides and hydroxides with a small proportion of Calcium carbonates. To produce a char that contained mainly Calcium phosphate, 50 g char was treated with 6 ml 85% H.sub.3PO.sub.4 dissolved in 330 ml water to allow the char to become saturated with the acid. The thus created material was left for two days at 20 C. to allow the phosphoric acid to react with the calcium oxides/hydroxides and carbonates inside the charcoal to form Calcium phosphates. After the reactions were complete, the material was dried in an oven at 40 C. The pH of the thus created material was between 3.2 and 4.0, suggesting that most of the alkaline earth metals inside the char had reacted with the acid and that oxides, hydroxides and carbonates were converted to water (and carbon dioxide in the case of carbonates).

(128) Experimental set up: Drilling muds containing a cocktail of leachable metals (including barium, aluminium, copper, lead chromium and nickel), a pH of 10.4 and a redox potential of 110 mV were amended with 1% and 2% phosphate char composites which were mixed thoroughly with a spatula. Controls received no amendment but were mixed as well. The materials were incubated for three days and remained water logged for the duration of the experiment. The experiments were set up in 100 ml porcelain containers that were closed with a porcelain lid (not air-tight). Each containing approximately 50 g of material. Each treatment was set up in duplicate.

(129) Metal analysis: After 3 days, approx. 3 g mud was taken from each container and suspended in a plastic sample bottle containing 20 ml of RO water. Samples were shaken vigorously for 2 minutes in order to bring leachable metals into suspension. Samples were left overnight and shaken again, before being centrifuged for 10 minutes at 3000 rpm. The supernatant was then passed through a 0.45 m filter and the filtrate was analysed for metals using ICP-EOS.

(130) To allow the amount of leachable metals to be expressed as metals per g dry weight soil, approx. 6 g of wet mud was weighted out on a pre-weighted piece of aluminium and dried in an oven at 100 C. overnight. Water content of the samples was subsequently calculated and used to convert sample wet weight into sample dry-weight.

(131) Results

(132) TABLE-US-00014 TABLE 13 Leachable metals present in non-treated drill cuttings (Control) and drill cuttings that were amended with either 1% or 2% (by weight) phosphate modified char. Data represent the means and standard error of two replicates. Leachable metal concentrations (g kg.sup.1) Treatment Al Cu Ba Pb Cr Ni Cd Control 261,400 1,372 13,200 2,024 358 357 12 SE 47,200 105 1,600 346 51 33 2 1% amendment 73,900 280 3,600 964 104 84 3 SE 14,800 36 900 69 23 10 0 2% amendment 60,200 186 2,100 810 87 61 3 SE 10,900 14 400 91 15 7 0 Significance P < 0.001 P < 0.001 P < 0.001 P < 0.001 P < 0.001 P < 0.001 P < 0.001 of differences

(133) Amendment with phosphate modified char resulted in a 75-90% reduction of leachable aluminium, copper, barium, nickel and cadmium. Leachable lead and chromium were reduced by around 60%. Increasing the amendment rate of the phosphate decreased the amount of leachable metal, but only slightly.

(134) Conclusions

(135) Phosphate modified chars were effective at reducing leachable concentrations of most heavy metals in alkaline and highly anaerobic drilling muds.