Solid phase extraction of platinum group metal

Abstract

A method of recovering platinum group metal from a process stream comprising platinum group metal, the method comprising: contacting the process stream with a solid phase extractant to adsorb platinum group metal from the process stream onto the solid phase extractant; and recovering the adsorbed platinum group metal from the solid phase extractant, wherein the solid phase extractant is a porous carbonaceous material comprising a reduced sulfur surface chemistry, and wherein the reduced sulfur surface chemistry of the porous carbonaceous material is provided by adding an oxidized form of sulfur to the porous carbonaceous material and pyrolysing the porous carbonaceous material in the presence of the oxidized form of sulfur which is reduced by pyrolysis.

Description

[0001] SOLID PHASE EXTRACTION OF PLATINUM GROUP METAL

[0002] Field

[0003] The present specification relates to methods of extracting and recovering platinum group metals using solid phase extractants. The methodology is particularly useful for recovery of platinum group metals from low-grade waste streams.

[0004] Background

[0005] From automotive emissions control to pharmaceutical synthesis, modern society is increasingly dependent on the platinum group metals (PGMs) across a wide range of applications. However, at their current extraction rates, natural reserves of the PGMs Pt and Pd are expected to last only 50 and 100 years respectively. As a result, there is significant incentive to find more efficient and cost-effective technologies for the recovery of Pt and Pd from secondary sources and, in particular, from increasingly low-grade waste streams. In addition to concerns about their criticality, the recovery of Pt and Pd from secondary sources presents an opportunity to dramatically reduce their environmental impact. A 2022 report by the International Platinum Group Metals Association estimated the global warming potential of mining Pt and Pd from primary ores to be 33.3 and 23.7 kg of CO2equivalent (per gram of metal mined) respectively, while the impact of recovery from secondary sources was in both cases only ~3 % of these values (0.64 and 0.73 kg CO2equiv. g1). The high demand for both Pt and Pd, in addition to the other platinum group metals, along with the high value of the platinum group metals, is incentivising companies to turn to increasingly low-grade sources of PGMs. For example, remining, the recovery of PGMs from mining by-products, previously considered waste, has seen increased interest despite the poor efficiency with which precious metals can currently be recovered from such sources. As a result, there is both social and economic incentives to develop more efficient and low-cost technologies for recovering PGMs from ultra-low concentration feeds. The development of such technologies, could also allow for the utilisation of many more unconventional sources such as road dust, whereby Pt and Pd are deposited from degradation of automotive catalytic converters, and wastewaters from a range of industries known to contain Pt and Pd.

[0006] From a range of viable approaches, the potential for selectivity and the inherent sustainability of solid phase adsorption makes it an excellent candidate for extracting compounds at especially low concentrations. For the extraction of Pt(ll) and Pd(ll), the most effective materials reported have utilised either nitrogen or sulfur based ligands, ora combination of both. To this end, a range of materials including polymeric resins, silica, alumina and metal organic frameworks have been successfully used to support PGM-ligating groups. While amine modified supports are seen to exhibit excellent capacities and selectivities under certain conditions, they suffer from limitations common to all ion exchange media in that adsorption is dependent on the close control of solution conditions. Both pH and chloride concentration are important in ensuring the protonation of the sorbent and the desired PGM speciation. Alternatively, reduced sulfur functional groups such as thiols, sulfides and thiophenes are known to form very strong coordinate bonds with soft metal centers. As a result, sulfur-containing ligands can be supported to facilitate extremely selective uptake of Pd(ll) and Pt(ll) under a wide range of conditions. This strong metal-ligand interaction can be both an advantage, as far as selectivity is concerned, and a disadvantage when it comes to recovery of the metal. Unfortunately, sorbents with the highest PGMaffinity tend to result in effectively irreversible adsorption, at which point the only methods to recover the metals require destruction of the material. In applications where removal efficiency is essential, for example the recovery of residual PGM from pharmaceutical products, this trade-off is seen to prohibit the use of many materials either due to cost or inefficiency.

[0007] With the potential to be a more sustainable and cost-effective solution, porous carbon materials have been widely explored for application to the recovery of precious metals. While they have proven to be very effective for, and are now widely applied to, the recovery of gold from cyanide leach solution, carbon materials are yet to find widespread use in the refining of either platinum or palladium. Although carbon materials are seen to exhibit selective adsorption of both Pd(ll) and Pt(ll) via n-bonding with unsaturated groups on the carbon surface, the size and strength of this interaction with an unmodified carbon surface is insufficient to effect useful separation from more dilute media. Many common methods for carbon surface modification, including oxidation by gaseous oxygen, oxidation by a range of chemical oxidants, reduction using hydrogen and sulfonation have been found to impart no significant improvement to PGM uptake over the unmodified carbon. Probably the most successful attempts to enhance the adsorptive properties of carbon materials have been by the incorporation of amines to form strongly basic anion exchange media. To this end, chitosan, pyridine and dimethylglyoxime have all been shown to substantially increase the capacity of carbon materials for Pd(ll). While amine modified carbon materials have been shown to be very effective under certain conditions, they suffer from the same limitations as other anion exchange media previously described.

[0008] Accordingly, there is a need for improved solid phase extraction methods for platinum group metals, particularly from process streams which have a low concentration of platinum group metals.

[0009] Summary of Invention

[0010] The present specification provides a method of recovering platinum group metal from a process stream comprising platinum group metal, the method comprising:

[0011] contacting the process stream with a solid phase extractant to adsorb platinum group metal from the process stream onto the solid phase extractant; and

[0012] recovering the adsorbed platinum group metal from the solid phase extractant,

[0013] wherein the solid phase extractant is a porous carbonaceous material comprising a reduced sulfur surface chemistry, and

[0014] wherein the reduced sulfur surface chemistry of the porous carbonaceous material is provided by adding an oxidized form of sulfur to the porous carbonaceous material and pyrolysing the porous carbonaceous material in the presence of the oxidized form of sulfur which is reduced by pyrolysis. It has been found that porous carbonaceous materials comprising a reduced sulfur surface chemistry have excellent functionality for extracting platinum group metals, particularly platinum and / or palladium, even from process streams having a low platinum group metal concentration. Furthermore, it has been found that porous carbonaceous materials comprising a reduced sulfur surface chemistry can selectively extract such platinum group metals from mixed metal process streams and that the adsorption is reversible such that the platinum group metal can readily be recovered from the solid phase extractant. Further still, ithas been found that these extractants have excellent functionality in process stream conditions found in typical platinum group metal refining processes (e.g. HCI solutions at low pH).

[0015] The porous carbonaceous material is preferably one or more of: a mesoporous carbon material; a non-templated mesoporous carbon material; a bio-based porous carbonaceous material; a carbonaceous material produced from a polysaccharide feedstock; or sawdust. Such materials have been exemplified and found to have excellent functionality. Furthermore, such materials can provide a sustainable and accessible source of both sulfur and carbon.

[0016] The reduced sulfur surface chemistry of the porous carbonaceous material may comprise one or more of thiols, sulfides, disulfides and thiophenes and is provided by added an oxidized form of sulfur (e.g., a sulfate) to the porous carbonaceous material and pyrolysing to reduce the oxidized form of sulfur. The reduced sulfur surface chemistry is formed by treating the porous carbonaceous material to pyrolysis at, for example, a temperature of at least 800°C (optionally less than 1500°C, 1200°C or 1000°C). It has been found that such a pyrolysis treatment can increase the quantity of suitable sulfur species which are bonded to the porous carbonaceous material in a form which is not washed from the material (e.g., covalently bonded). The porous carbonaceous material can be washed after pyrolysis, e.g., to remove any sulfur species which are not bonded to the carbonaceous material. Advantageously, it has been found that if the porous carbonaceous material is washed in an acidic solution after pyrolysis, optionally HCI, then the amount of sulfur bonded to the porous carboneaous material can actually be increased. Preferably, the porous carbonaceous material comprises a sulfur content of at least 1 wt%, 3 wt%, 5 wt%, or 10 wt% (optionally, less than 30 wt%, 20 wt%, or 15 wt%).

[0017] Another advantage of incorporating sulfur into the porous carbonaceous material is that this can increase the BET surface area and / or BJH mesopore volume of the material. This additional surface area / porosity may be attributed to the corrosive effect of reduction of the oxidized form of sulfur (e.g., sulfate reduction) at elevated temperatures which has been shown to result in the oxidation of the carbon support and evolution of CO2. For example, the porous carbonaceous material may have a BET surface area of at least 500 m2.g-1, 700 m2.g-1, 900 m2.g-1, or 1200 m2.g-1(optionally, no more than 3000 m2.g-1, 2500 m2.g-1, or 2000 m2.g-1) and / or a BJH mesopore volume of at least 0.4 cm3.g-1, 0.6 cm3.g-1, 0.8 cm3.g-1, 1.0 cm3.g-1, 1.2 cm3.g-1, 1.4 cm3.g-1, or 1.6 cm3.g-1(optionally, no more than 3.0 cm3.g-1, 2.5 cm3.g-1, or 2.0 cm3.g-1).

[0018] The PGM process streams to which the present methodology is applied may comprise platinum and / or palladium and may comprise a HCI solution of PGMs at a pH of no more than 3, 2, or 1. The adsorbed platinum group metal can then be recovered from the solid phase extractant by stripping the platinum group metal from the solid phase extractant using a stripping reagent (e.g., acidified thiourea).

[0019] The present specification also provides a method of manufacturing solid phase extractants. The method comprises:

[0020] adding an oxidized form of sulfur to a porous carbonaceous material; and

[0021] pyrolysing the porous carbonaceous material in the presence of the oxidized form of sulfur which is reduced by pyrolysis to form a porous carbonaceous material comprising a reduced sulfur surface chemistry.The method thus involves thermally treating a carbonaceous material in the presence of an oxidized form of sulfur (e.g. a sulfate) which is added to the carbonaceous material and can be followed by washing to form a porous carbonaceous material comprising a reduced sulfur surface chemistry. Features of the method of manufacture are as described above in terms of composition and conditions for the thermal treatment and washing steps.

[0022] Brief Description of the Drawings

[0023] Figure 1: Simultaneous thermal analysis of a sulfur containing Starbon material (C000) pyrolysed under air to 400 °C: (a) Thermogravimetric (upper line) and differential scanning calorimetry (lower line); and (b) quantitative transmission FT-IR analysis of gases evolved during thermal analysis.

[0024] Figure 2: pXRD of the sulfur containing Starbon material (C000) pyrolysed to a range of temperatures under nitrogen, prior to washing.

[0025] Figure 3: pXRD of the sulfur containing Starbon material (C000) pyrolysed to a range of temperatures under nitrogen, after washing.

[0026] Figure 4: SEM-EDX analysis of Starbon material produced at 800°C (C800) before (a to g) and after (h to n) aqueous wash including SEM micrographs (a, h) and EDX elemental mapping for carbon (b, i), oxygen (f, k), sulfur (e, j), potassium (c, n), sodium (g, m) and chlorine (d, I).

[0027] Figure 5: SEM-EDX analysis of C450 before (a) and after (b) aqueous wash, and C600 before (c) and after (d) aqueous wash. EDX elemental map overlay (i) and for chlorine (ii), carbon (iii), potassium (iv), sulfur (v), oxygen (vi) and sodium (vii).

[0028] Figure 6: X-ray photoelectron spectroscopy of S (2p) chemistry that remains after washing of Starbon materials produced at 450°C (C450), 600°C (C600) and 800°C (C800) materials.

[0029] Figure 7: Schematic representation of the evolution of sulfur chemistry during the pyrolysis of Starbon material (C000).

[0030] Figure 8: Transmission FTIR of gases evolved during thermal analysis of C300RP-800N2 at 250-300 ° C. Figure 9: BJH adsorption differential pore size distribution of Starbon materials (C300RP, C300N2, C300RP-800N2, C800RPand C800N2).

[0031] Figure 10: Frost diagram of sulfur species under acidic and basic conditions.

[0032] Figure 11: X-ray photoelectron spectroscopy of S (2p) chemistry of Starbon materials (C300RP-800N2-water, C300RP-800N2-HCI, C800RP-water and C800RP-HCI).

[0033] Figure 12: Isotherms for the adsorption of Pd(ll) by activated carbon (Darco) and various Starbon materials (C300RP-800N2, CNE800N2, C300RP*-800N2, A800 and S-A800) from 2M NaCI at pH 1.

[0034] Figure 13: Adsorption of Ir(lll), Pd(ll), Pt(ll), Rh(lll) and Ru(lll) from a mixed metal solution of 2M NaCI at pH 1 by Starbon materials (C800 and P800): (a) in the absence of base metals; (b) in the presence of a large excess of the base metals Cu(ll), Ni(ll) and Co(ll); and (c) shows the kinetics of PGM uptake from a mixed metal solution in the presence of excess base metals.Figure 14: Elution of Pd(ll) from Starbon material (C300RP-800N2) by HCI and then thiourea.

[0035] Figure 15. Kinetics of successive Pd(ll) adsorption cycles by S-A800.

[0036] Figure 16: Powder X-ray diffractograms of bare Starbon material (C300RP-800N2) and Pd-loaded Starbon material (C300RP-800N2).

[0037] Figure 17: (a) Normalised X-ray absorption spectrum and c — x(k) spectrum (k weighting = 2) of Pd k-edge for Pd-loaded Starbon material (C300RP-800N2); (b) 1stderivative of Pd k-edge; and (c) Fourier transformed X(R) spectra of Pd-loaded Starbon material (C300RP-800N2), PdO, metallic Pd, Pd black, solid PdCI2and an aqueous PdCI2solution.

[0038] Figure 18: Pd 3d x-ray photoelectron spectroscopy of palladium loaded Starbon material (C800) and the sulfur-free Starbon P800 (pectin derived).

[0039] Figure 19 shows thermogravimetry of the pyrolysis process for carbon precursor material with added sulfate. Carbonisation occurs between 200 and 400°C and sulfate reduction / activation occurs between 800 and 1000°C.

[0040] Figure 20 shows thermogravimetric profile of the pyrolysis (under N2) of sawdust in the presence and absence of K2SO4. Carbonisation can be seen to occur between 200 and 400°C in both cases, while sulfate reduction / activation occurs between 800 and 1000 °C in the K2SO4-doped case. The thermogravimetric profile of the pyrolysis (under N2) of K2SO4-doped DARCO® (a pre-carbonised material) is also included in Figure 20. Sulfate reduction is seen to occur at 850-950 °C.

[0041] Figure 21 shows Pd(ll) uptake results for different carbon materials (sawdust / rice straw / DARCO®) pyrolysed with different sulfate loadings, types of sulfate, and / or pyrolysis temperatures. The results shown that sulfate can be used as both an activating agent and source of sulfur to yield porous carbonaceous materials comprising a reduced sulfur surface chemistry with high Pd(ll) capacity.

[0042] Detailed Description

[0043] Despite the large volume of research towards the modification of carbon materials for precious metal uptake, no one has previously investigated the potential for carbon materials containing reduced sulfur chemistry to be used for the adsorption and recovery of platinum group metal species such as Pd(ll) and Pt(ll) (i.e., porous carbonaceous materials having a reduced sulfur surface chemistry formed by reduction of an oxidized form of sulfur, such as a sulfate, which is added to the carbon material and pyrolysed to reduce the oxidized form of sulfur and form the reduced sulfur surface chemistry). The present specification is concerned with incorporating such reduced sulfur functionalities into a porous carbon material in order to dramatically improve adsorption selectivity and capacity towards these platinum group metals. To this end, there is precedent for the production of sulfur-doped pyrolytic carbon materials from a range of sources of both sulfur and carbon. Furthermore, in many cases the inherent reducing power of the carbon support is seen to result in the formation of reduced sulfur chemistry without the need for chemical reducing agents. The present specification is concerned with applying such reduced sulfur chemistry to platinum group metal recovery. Furthermore, it has also been noted that there are currently very few reports of the preparation of sulfur-containing carbon materials from entirely biobased feedstocks despite the advantages this would offer regarding cost and sustainability.The Starbon process, first published in 2006, provides a route to producing non-templated mesoporous carbon materials sustainably from a range of polysaccharide feedstocks. Utilising the inherent porosity of polysaccharide hydrogels, the Starbon process does not rely on pore activation during pyrolysis, providing access to a range of surface chemistries dependent on the temperature of choice. This has been shown to result in the versatility to 'fine tune' material properties towards a range of applications including heterogeneous catalysis, organic and inorganic separations, gas capture and electrochemistry.

[0044] Two approaches to the production of sulfur-containing Starbon materials have been identified. The first approach utilizes the inherent sulfur content of the marine polysaccharide K-carrageenan, a naturally abundant linear sulfated copolymer of repeating 0-D-galactopyranose and 3,6-anhydro-a-D-galactopyranose units. Fast growing and already widely cultivated throughout the world's tropical coastal regions, carrageenan containing seaweeds provide a sustainable and accessible source of both sulfur and carbon. Additionally, K-carrageenan is known to readily form rigid hydrogels and was anticipated to be a good candidate for substitution into the Starbon process previously described for both pectin and alginic acid. As an alternative approach, the incorporation of sulfur from an inorganic source, during the production of materials from the sulfur-free polysaccharide, alginic acid, was explored. With similar potential to be sustainably derived from aquaculture, alginic acid offers the additional advantage that a range of highly porous alginate-derived Starbon materials have already been reported. Using the Starbon process, it is possible to fine tune the chemistry of sulfur incorporated by both approaches for the adsorption of Pd(ll) and Pt(ll). Herein we report the production of the first K-carrageenan-derived Starbon materials as well as the first sulfur-containing alginate-derived Starbon materials, the characterization of their chemical and textural properties, and their adsorption of Pd(ll) and Pt(ll).

[0045] Synthesis of S-Starbons

[0046] Development of sulfur chemistry during pyrolysis

[0047] Known to readily form rigid hydrogels, the ester-sulfate containing polysaccharide K-carrageenan was expected to be a good candidate as a new feedstock for Starbon synthesis and was initially investigated by substitution into the Starbon process previously reported for pectin and alginic acid. The procedure developed by Borisova et al. follows three principal steps:

[0048] 1. Expansion / gelation of the polysaccharide precursor in water

[0049] 2. Removal of the solvent by freeze drying to yield a mesoporous aerogel

[0050] 3. Pyrolysis / carbonisation of the resulting aerogel to between 300-800°C

[0051] The gelation of K-carrageenan is induced thermally, whereby heating in water to 95°C results in dissolution, followed by cooling to result in the organisation of carrageenan coils into helices that assemble in a way that imparts macroscopic order. After gelation, 30 wt% t-butanol was added to form a eutectic mixture with water that was subsequently removed by freeze drying to preserve the mesoporous gel structure. The resulting aerogel (C000) was pyrolysed to a range of temperatures for characterisation so that the evolution of porous structure and surface chemistry could be explored.

[0052] The initial stages of C000 pyrolysis were analysed by simultaneous thermogravimetric, calorimetric and infra-red analysis and are reported in Figure 1. The loss of H2O, CO2and CO are seen to mirror the decomposition events previously reported in the literature covering the pyrolysis of polysaccharides. From the thermogravimetric trace in Figure 1(a), decomposition is seen to begin at 200°C with a pronounced mass loss between 200-230°C. The simultaneous FT-IR of the evolved gases in Figure 1(b) identifies the rapid loss of water over this temperature range due to intermolecular dehydration, resultingin the crosslinking of polysaccharide chains, while intramolecular dehydration results in increasing unsaturation. From 275°C, further dehydration is accompanied by decarboxylation and decarbonylation which is expected to result in the increasingly condensed unsaturated, and eventually, aromatic structures typical of activated carbons. More pertinent to the development of sulfur chemistry, at ~300°C an event occurs that results in the evolution of SO2 over a narrow temperature range, indicating the decomposition of some or all of the organic ester sulfate in the carrageenan structure.

[0053] CHNS combustion analysis was used to monitor change in the elemental composition during the course of pyrolysis and to determine the extent to which sulfur had been incorporated. The carrageenan feedstock is understood to be predominantly the potassium ester sulfate form, and as a result, was expected to contain a mixture of loosely bound potassium salts after pyrolysis. To remove this inorganic content, materials were subjected to an aqueous wash after pyrolysis and the resulting materials were characterised by CHNS combustion analysis. In all cases energy dispersive x-ray spectroscopy elemental analysis revealed no significant inorganic content remained after washing. As such, the sulfur content that remains after washing was assumed to be covalently incorporated into the carbon matrix.

[0054] The table below shows CHNS elemental analysis for the C000 aerogel pyrolysed to 450, 600 and 800 °C before and after aqueous washing.

[0055]

[0056] The table below gives SEM-EDX elemental analysis for C000 pyrolysed to a range of temperatures both as prepared and after aqueous wash to remove inorganic content. All values averaged over 5 sites.

[0057]

[0058] Despite the apparent loss of SO2at 300 °C, upon pyrolysis to 450 °C, elemental analysis reveals that 6.9 wt% sulfur remains. However, washing this material is seen to result in the complete removal of sulfur from the carbon matrix, suggesting the decomposition of ester sulfate groups to some water-soluble inorganic salt. It is hypothesised that this occurs at 300 °C, concurrent with the evolution of SO2. Upon pyrolysis to temperatures above 450 °C, sulfur incorporation is seen to increase, and in particular, between 600-800 °C, there is an increase from 1.4 wt% to 11.5 wt% sulfur. This is consistent with the idea that the carbon surface will be more strongly reducing at higher temperatures up to the temperature at which graphitisation begins.

[0059] Powder x-ray diffractograms of the S-Starbons produced by pyrolysis of C000 to a range of temperatures, were obtained before and after washing. Figure 2 shows Powder x-ray diffractograms of C000 pyrolysed to a range of temperatures under nitrogen, prior to washing. Figure 3 shows Powder x-ray diffractograms of C000 pyrolysed to a range of temperatures under nitrogen, after washing. The diffraction patterns of the materials produced at 450°C (C450), 600°C (C600) and 800°C (C800) after washing reveal only broad (amorphous) peaks attributed to graphitic crystallites, confirming the removal of inorganic content. Upon pyrolysis to 450 °C, and prior to washing, a single well-defined crystal phase arises that was assigned to potassium sulfate. This is consistent with CH NS elemental analysis and TG-IR which suggest decomposition of ester sulfate groups at 300 °C to K2SO4and an equivalent of SO2, followed by the complete removal of sulfur when the material is washed. Between 450-700 °C, limited change to the diffraction pattern is seen, consistent with elemental analysis which suggests only 1.4 wt% sulfur is incorporated over this temperature range. However, upon pyrolysis to 800 °C, concurrent with the incorporation of a significant amount of sulfur, changes to the diffraction pattern reveals the formation of an additional crystalline phase, attributed to potassium carbonate monohydrate. It is suggested that upon reaching sufficiently high temperatures, sulfate is reduced by the carbon surface to yield CO2, organic sulfur in lower oxidation states and potassium oxide. The TGA of potassium sulfate in the absence of the carbon matrix reveals no decomposition below 1000 °C, suggesting that it is reaction with the carbon matrix which results in the decomposition of sulfate. CO2evolved by concurrent oxidation of the carbon surface is expected to react readily with potassium oxide to form potassium carbonate, while any potassium oxide remaining after pyrolysis will readily react with atmospheric CO2.

[0060] EDX elemental mapping of C800 before (Figures 4a-g) and after (Figures 4h-n) washing was used to analyse the distribution of elements over the material surface and to confirm that aqueous washing had successfully removed inorganic content. EDX of C800 prior to washing reveals, in addition to the expected potassium sulfate, a minor sodium component and a small number of potassium chloride crystallites, while the remaining potassium, sodium, oxygen and sulfur are seen to be evenly distributed over the carbon surface. Upon washing, this is seen to result in the uniform distribution of sulfur over the material surface. SEM-EDX of C450 and C600 (Figure 5) yield a similar result whereby potassium, sulfur and oxygen are evenly distributed over the material prior to washing, but in both of these cases, washing is seen to remove the vast majority of sulfur in addition to potassium and sodium.

[0061] After pyrolysis to 450, 600 and 800 °C and aqueous washing to remove inorganic content, x-ray photoelectron spectroscopy was used to characterise the state of the sulfur that remained, and was thus assumed to be covalently incorporated. Figure 6 shows the x-ray photoelectron spectroscopy of S (2p) chemistry that remains after washing of C450, C600 and C800 materials. Analysis of the S 2p XPS signal allows for clear differentiation of contributions by different functionalities, where sulfur is known to exhibit XPS signals over a very wide range of binding energies determined largely by the sulfur oxidation state. The S 2p signals of the materials prepared at all three temperatures was found to be well describedby combinations of the same three sulfur environments centred at 163.9, 167.8 and 168.9 eV. A well-defined doublet at lower binding energy, 163.9 eV, could be attributed to reduced sulfur in a range of functionalities such as thiols, sulfides, disulfides and thiophenes. In similar sulfur-containing carbon materials, a signal at ~164 eV is often attributed to sulfide groups. This confirms the incorporation of a significant quantity of reduced sulfur chemistry, considered most promising for soft PGM adsorption, by 800 °C.

[0062] The two species at higher binding energy (167.8 and 168.9 eV) can be attributed to sulfur in higher oxidation states. The signal at 168.9 eV is likely due to the formation of sulfonate groups which are known to form upon the reaction of sulfuric acid with carbonaceous materials, a common method for sulfonation. The signal at 167.8 eV was attributed to other oxidised sulfur functionalities such as sulfones and sulfoxides, with the more difficult resolution of the doublets at higher binding energy being indicative of sulfur in a wider range of environments. XPS of incorporated sulfur reveals that in addition to the significant increase in reduced sulfur content upon pyrolyzing to 800 °C, a range of higher oxidation states are also incorporated and the changing ratio of reduced to oxidised sulfur follows no clear trend. The increase in sulfur incorporated from just 0.3 wt% in C450 to 11.5 wt% in C800 is indicative of the substantial increase in the reducing power of the carbon surface at higher temperatures. However, even at these higher temperatures the heterogeneity of the material surface is seen to result in sites with a wide range of reduction potentials, and as a result, sulfur is incorporated as the range of oxidation states illustrated in Figure 7. In addition to XPS, TG-FTIR analysis of C800 (Figure 8) identified a sulfur containing species which decomposed at ~200 °C to liberate gaseous sulfur dioxide, confirming the presence of oxidised sulfur species that were not found to participate in PGM adsorption.

[0063] Organic ester sulfate groups present in the carrageenan starting material are seen to decompose during pyrolysis before the eventual reduction and incorporation of sulfur via inorganic sulfate salts. On this basis we expected that it would be possible to produce similar materials by artificially doping potassium sulfate into a range of polysaccharide hydrogels with differing porous structures. This was seen as an attractive way of increasing the final sulfur content and demonstrating the wider scope of our approach to producing sustainably-derived sulfur-containing mesoporous carbon materials. To this end, S-A800 was produced by the addition of K2SO4to an alginic acid hydrogel at 7.5 wt% sulfur, before drying and pyrolysis under the same conditions used to produce S-Starbons from K-carrageenan. Characterisation of the porous structure and surface chemistry of S-A800 are reported in the following sections.

[0064] The role of process conditions on the development of porous structure

[0065] The N2-sorption porosimetry for sulfur-containing Starbon materials prepared under a range of conditions are summarised in the table below, while differences in mesopore size distributions are illustrated in Figure 9. The atmosphere under which carbon materials are pyrolysed is understood to have a significant impact on subsequent surface chemistry and in the preparation of Starbon materials, pyrolysis conditions have been shown to be important in determining porous properties. To this end, pyrolysis under reduced pressure and under a flow of N2gas were compared for the preparation of carrageenan-derived Starbon materials. Upon pyrolysis of the C000 aerogel to 300 °C, porosity is only retained when initial pyrolysis is conducted under reduced pressure (C300RP), while the N2pyrolysis to 300 °C is seen to result in the non-porous C300N2. This is consistent with previous work that found that the material porosity is highly dependent on this initial pyrolysis, in particular, the rate at which the material is heated between 150-300 °C. Marriott et al. found that increasing the rate of pyrolysis negatively impacted pore volume and surface area and it was suggested that this is largely due to the insufficient removal of volatile organiccompounds released within this temperature range. Similarly, we would suggest that the limited flow of nitrogen sustained during N2pyrolysis was not able to sufficiently remove volatile organic compounds and water liberated during the initial stages of pyrolysis, resulting in the collapse of the porous structure. As illustrated in Figure 9, upon further pyrolysis of C300RPto 800 °C, a significant volume of intermediatesized mesopore (10-40 nm), characteristic of Starbon materials, is seen to develop. However, beyond 300 °C, no significant difference in porous structure was observed between the materials pyrolysed under reduced pressure and N2. Upon further pyrolysis of C300N2to 800 °C, some porosity begins to develop due to thermal activation, however, this is largely limited to micropore (<2 nm). Likewise, when non-expanded K-carrageenan powder was pyrolysed to 800 °C, under reduced pressure to 300 °C and then under N2, the material develops extensive surface area (624 m2.g-1), but this is attributed almost entirely to micropore and small mesopore (<4nm in width).

[0066] The table below shows a summary of N2-sorption porosimetry for Starbons produced under a range of conditions. In the table: (a) refers to whether the materials were prepared either via the direct pyrolysis of the precursor (as received) or via expansion in water followed by freeze drying and pyrolysis; (b) refers to whether the materials were prepared by pyrolysis either under a flow of N2or under reduced pressure (RP); (c) is surface area estimated using the BET equation; and (d) is total pore volume defined as the sum of the t-plot micropore volume and the BJH (adsorption) mesopore volume.

[0067] Material Starting ExpandedeFinal Pyrolysis conditions Nz-sorption porosimetry material temperature

[0068] 0-300 °C 300-800 °C Surface area Pore volume %

[0069] ore

[0070]

[0071] A800 Alginic acid Yes 800 RP N2504 0.76 18.4 S-A800 Alginic acid Yes 800 RP N21377 1.70 8.8

[0072] While the porosity of alginate-derived Starbons have previously been reported several times, comparison of S-A800 to the sulfur-free A800, reveals the impact of sulfate-doping on porous structure. Sulfate doping of S-A800 was found to result in a substantial increase in both surface area and pore volume (BET surface area = 1377 m2.g-1, BJH mesopore volume = 1.70 cm3.g-1) compared to the S-free A800 (BET surface area = 504 m2.g-1, BJH mesopore volume = 0.76 cm3.g-1). This additional porosity may be attributed to the corrosive effect of sulfate reduction at elevated temperatures which has been shown to result in the oxidation of the carbon support and evolution of CO2.

[0073] The influence of process conditions on surface chemistry

[0074] Given that pyrolysis between 300-800 °C could be conducted under either reduced pressure or a flow of nitrogen to yield materials with similar textural properties, the surface chemistry resulting from each wasexplored. In addition, the pH of the solution used to wash the materials after pyrolysis was varied to determine the effect on surface chemistry.

[0075] Elemental analysis of S-Starbons produced under a range of conditions are summarised in the table below in which: (a) refers to whether the materials were prepared either via the direct pyrolysis of the precursor (as received) or via expansion in water followed by freeze drying and pyrolysis; (b) refers to whether the materials were prepared by pyrolysis either under a flow of N2 or under reduced pressure (RP); and (c) refers to whether the materials were washed in either 2M HCI or water after pyrolysis.

[0076]

[0077] A comparison of the elemental compositions of C300RP-800N2 materials, washed with HCI and water respectively, reveals the impact that the wash solution has on the degree that sulfur is incorporated. It was found that washing using 2M HCI resulted in 3-fold increase in the final sulfur content as compared to washing in water alone.

[0078] The results of pXRD suggest that upon pyrolysis of C300 to 800 °C, a small portion of the total K2SO4content is reduced in situ to yield reduced organic sulfur. In addition, pyrolysis of carbon materials under a nitrogen atmosphere is known to result in a highly reactive carbon surface that, if not quenched in-situ, will go on to react readily with atmospheric oxygen yielding a range of surface oxides. However, we have found that when these materials are washed very soon after pyrolysis there is the potential for further sulfur incorporation by reaction of solubilised SC2-with the reactive carbon surface.

[0079] The differences in reactivity between the acidified and basic solutions are easily rationalised in terms of the relative stability of SO42, and the successively reduced sulfur species, under both acidic and basic conditions. Figure 10 shows a Frost diagram of sulfur species under acidic and basic conditions.

[0080] Comparison of the sulfur content of the materials pyrolysed to 800 °C under reduced pressure (C800RP) and under N2 (C300RP-800N2), 5.8 wt% and 10.6 wt% respectively (after an acidic wash), reveals a substantial increase in sulfur content when pyrolysed under N2. While the acid wash of C300RP-800N2 resulted in a significant increase in sulfur content over the base washed material, comparison of C800RP-water (5.2 wt% S) and C800RP-HCI (5.8 wt% S), suggests limited sulfur incorporation during the wash step. This would suggest that, despite the similar textural properties of C300RP-800N2 and C800RP, N2pyrolysis results in a substantially more reactive surface that facilitates additional sulfur incorporation. S 2p XPS characterisation of C300RP-800N2-water, C300RP-800N2-HCI, C800RP-water and C800RP-HCI (Figure 11) reveals no significant differences in the ratio of incorporated sulfur oxidation states.In addition to pyrolysis conditions, material porous structure is seen to have an impact on sulfur incorporation. Despite being pyrolysed under the same conditions as C300RP-800N2, the microporous CNE300RP-800N2 produced from non-expanded carrageenan was found to retain only 6.0 wt% sulfur after acidic wash. If final sulfur incorporation is ultimately dependent on the reduction of SO42-by the reactive carbon matrix during the washing step, then it follows that the non-expanded CNE300RP-800N2, possessing no significant mesoporosity, would suffer from a lack of reactive surface area accessible to SO42-ions during washing.

[0081] In addition to increased porosity, the sulfate doped alginate-derived Starbon (S-A800) resulted in a significant quantity of sulfur being retained by the carbon matrix. Despite sulfur initially being doped to the same 7.5 wt% found in K-carrageenan, the sulfur content of S-A800 was found to be substantially higher than C800. This is attributed to the decomposition of the organic ester sulfate in carrageenan at <300 °C to yield ca. 0.5 equivalents of SO2. By comparison the potassium sulfate incorporated into the alginate hydrogel is expected to be thermally stable until eventual reduction by the carbon matrix at <700 °C or during the washing step.

[0082] Platinum group metal adsorption

[0083] In all PGM adsorption experiments, PGM salts were dissolved in aqueous 2M NaCI acidified to pH 1 using HCI. These conditions were chosen to explore the adsorptive properties of S-Starbons under conditions relevant to low grade PGM refinery streams and to avoid the complications of PGM hydroxide formation that occur at particularly low chloride concentration or high pH. In order to compare the Pd(ll) sorption properties of a range of S-Starbon materials, adsorption isotherms have been plotted whereby the capacity for Pd(ll) was measured as a function of Pd(ll) concentration. Figure 12 shows isotherms for the adsorption of Pd(ll) by Darco, C300RP-800N2, CNE800N2, C300RP*-800N2, A800 and S-A800 from 2M NaCI at pH 1.

[0084] It was found that in all cases, the adsorption isotherms were well described by the Freundlich isotherm, which has allowed for both the determination of Freundlich parameters and a comparison of estimated capacities at 1 and 100 ppm, as summarised in the table below (Freundlich parameters and calculated adsorption capacities at equilibrium Pd concentrations of 1 and 100 mg.L1).

[0085]

[0086] To determine the contribution of the carbon matrix in the absence of sulfur, C300RP*-800N2 was prepared as a sulfur-free analogue of C300RP-800N2. After pyrolysis to 300 °C under reduced pressure, the resulting C300 was acid washed to remove inorganic sulfates without altering the porous carbon matrix. The sulfur free C300 was then pyrolyzed to 800°C under N2, resulting in a material with very similar porous structure to C300RP-800N2 as determined by N2-sorption porosimetry. Comparison of the Pd(ll) adsorption capacitiesof C300RP*-800N2 and C300RP-800N2 at 1 ppm (33.3 and 5.45 mg.g1respectively) reveals a 6-fold increase upon the incorporation of reduced sulfur chemistry. This confirms that the carbon matrix alone possesses some capacity for Pd(ll) adsorption, as has been previously reported. This property of carbon materials has been convincingly attributed to the n-interaction between the soft Pd(ll) center and unspecified unsaturated groups on the carbon surface and is known to be selective for Pd(ll) and Pt(ll).

[0087] The increase in capacity, particularly at low Pd concentrations, is expected to be a result of the strong specific interaction between incorporated sulfur and Pd(ll).

[0088] Of a similar magnitude to C300RP*-800N2, the capacity of the commercial activated carbon Darco of 3.83 mg.g1at 1 ppm is representative of the limited potential of unmodified carbon materials for Pd / Pt separations and recovery.

[0089] Similarly, a comparison of A800 and S-A800 highlights the contribution of reduced sulfur chemistry to the Pd(ll) capacity of S-Starbons. In particular, at 1 ppm Pd, A800 and S-A800 are seen to exhibit capacities of 1.87 mg.g1and 89.0 mg.g1respectively, representing a 48-fold increase upon inclusion of sulfur. However, in this instance, the presence of sulfate during pyrolysis is seen to result in a significant increase in the porosity of S-A800 which may facilitate PGM uptake in addition to sulfur chemistry. The massive capacity of S-A800 also demonstrates the potential for even greater sulfur incorporation using alternative sources of sulfur in the Starbon process, and the direct impact this has on the eventual capacity for Pd(ll).

[0090] The role of S-Starbon porosity on Pd(ll) uptake may be inferred by the limited capacity of the microporous CNE300RP-800N2 (5.67 mg.g1at 1 ppm), produced from non-expanded carrageenan. Despite the substantial retention of sulfur by CNE300RP-800N2, in the absence of mesoporosity, Pd(ll) uptake is seen to be significantly reduced. Similar conclusions have been previously reported about the importance of Starbon mesoporosity on both dye adsorption and electrochemical properties, where capacity and charge kinetics were far more dependent on mesopore volume than specific surface area. However, it is important to note that the limited capacity of CNE300RP-800N2 could also be, in part, due to the limited development of sulfur chemistry in the material with reduced porosity.

[0091] Adsorption selectivity and kinetics

[0092] Having established the potential to incorporate large quantities of reduced sulfur chemistry into S-Starbon materials and the profound impact this has on the uptake of Pd(ll), we next wanted to explore the selectivity of these materials in the presence of a wide range of metals commonly found alongside Pd and Pt in low grade refinery streams.

[0093] To this end, the adsorption of Pd, Pt, Ir, Ru and Rh from a mixed metal solution was determined both in the absence and in the presence of a large (50x) excess of the base metals Cu(ll), Ni(ll) and Co(ll).

[0094] Figure 13 shows adsorption of Ir(lll), Pd(ll), Pt(ll), Rh(lll) and Ru(lll) from a mixed metal solution of 2M NaCI at pH 1 by C800 and P800: (a) in the absence of base metals; (b) in the presence of a large excess of the base metals Cu(ll), Ni(ll) and Co(ll); and (c) shows the kinetics of PGM uptake from a mixed metal solution in the presence of excess base metals.

[0095] Changes in PGM concentration are reported in Figure 13, while in all cases, no meaningful change in the concentration of base metals (Cu, Ni and Co) was observed. All three materials, including Darco, were found to selectively adsorb Pd(ll) and Pt(ll) irrespective of the presence of excess base metals confirming that competition with the first row transition metals had no impact on PGM adsorption. However, theselectivity of the sulfur-containing Starbons was found to be substantially greater than Darco, with C800 removing 99.5 % Pd and 96.0% Pt and S-A800, proving more selective still, removing 99.8 % Pd and 99.2 % Pt, in the presence of excess Cu(ll), Ni(ll) and Co(ll). The minor preferential adsorption of Pd(ll) over Pt(ll) is characteristic of the interaction with soft donor atoms and is seen for other PGM scavenging materials. The selective removal of Pd and Pt in the presence of a range of other metals highlights the excellent potential for S-Starbons to be used towards the recovery of these PGMs from low grade waste streams. In particular, the removal of Pd(ll) and Pt(ll) by S-A800 was highly efficient, resulting in residual concentrations of <0.1 ppm Pd and <0.8 ppm Pt.

[0096] To better understand the mechanisms of PGM uptake, the rates of Pd, Pt, Ir, Ru and Rh adsorption from a mixed metal solution by S-A800 were determined (Figure 13c). Following very fast initial adsorption, whereby ca. ~30% of both Pd and Pt are adsorbed within the first 30 seconds, the remaining Pd and Pt is seen to be adsorbed gradually over the course of 24 hours, while Rh, Ru and Ir adsorption appears to be complete within the first 50 minutes, with Ru and Rh then beginning to desorb over the subsequent 24 hours. Contrary to the relative ligand exchange kinetics for Pd(ll) and Pt(ll) chlorides, which predicts the uptake of Pt(ll) over several orders of magnitude more slowly than Pd(ll), both are seen to adsorb at similar rates, suggesting an alternative limiting process in this case, such as diffusion.

[0097] Desorption

[0098] Having established excellent selectivity and capacity for Pd (I I) and Pt(ll), the feasibility of recovering the metals by elution, thus regenerating the S-Starbon material for further use, was investigated. By stripping first using HCI and then an acidified thiourea solution, it was possible to determine both the maximum reversible capacity and then the capacity of sites of intermediate strength, most useful for selective PGM recovery. It is assumed that during adsorption from a concentrated Pd / Pt solution, all adsorption sites, including the strongest and the weakest will become occupied. As a mild stripping agent, aqueous HCI is expected to elute Pt / Pd adsorbed by the weakest sites only. The subsequent elution with acidified thiourea, a ligand with a much higher affinity for Pd / Pt(ll), is expected to strip Pt / Pd from sites of intermediate strength. The capacity that can be stripped using thiourea is considered to be the most useful. While these sites are sufficiently strong to adsorb Pt / Pd selectively and to very low residual concentrations, they are not so strong as to irreversibly bind the metals. The Pt / Pd that remains after elution by thiourea is adsorbed by the strongest sites and cannot be recovered easily by stripping.

[0099] Figure 14 shows elution of Pd(ll) from C300RP-800N2 by HCI and then thiourea. Following column adsorption of either Pd or Pt, S-A800 was initially eluted using 50 mL of 2M HCI, whereby the most weakly bound 0.55 mmol.g1Pd and 0.42 mmol.g1Pt were desorbed. Next, 60 mL of acidified thiourea was used to elute Pd / Pt bound to sites of intermediate strength. The capacity of these sites, 0.82 mmol.g1Pd and 0.53 mmol.g1Pt, represents the useful (reusable) capacity of S-A800 and is the capacity attributed to soft sulfur-containing donor groups. While the quantity of Pt adsorbed by weak and intermediate strength sites was very similar to that of Pd, the size of irreversible adsorption was seen to differ significantly, where just 0.09 mmol.g1Pd remained after elution with thiourea while 0.31 mmol.g1Pt remained. This may be rationalised in terms of the reduction potentials of the Pd(ll / 0) and Pt(ll / O) chloride couples, 0.59 and 0.76 V vs SHE respectively. Given that Pt(IV) and Pt(ll) are known to be readily reduced by carbon materials, we anticipate a component of irreversible reductive sorption, whereby Pt(O) is not easily resolubilised without the use of an oxidant.

[0100] Following desorption by acidified thiourea, S-A800 was washed using HCI and then water before being reused to determine the efficiency of subsequent adsorption cycles. The table below summarisesadsorption efficiency for successive reuses of S-A800 for Pd(ll) uptake. Pd stripped using acidified thiourea between each use. Over 48 hours, S-A800 was found to remove 96.3 % Pd during the first adsorption. After stripping, the removal efficiency of the second adsorption was found to increase to 99.9 %. This is attributed to the improved kinetics after conditioning S-A800 by the first adsorption / desorption cycle, whereby the second adsorption was seen to be substantially faster than the first (Figure 15). After again stripping with thiourea, the efficiency of the third adsorption (99.3 %) was seen to remain high. While this doesn't account for the long-term reusability of S-Starbons, it provides evidence for the reversible nature of the Pd(ll) uptake mechanism and suggests the excellent potential for recovery of the metals after extraction.

[0101]

[0102] Characterisation of Pd-C800

[0103] To probe the interaction responsible for Pd adsorption by S-Starbons, Pd-loaded C300RP-800N2 (5 wt.%) was characterised by pXRD, XAS and XPS. While high selectivity for Pd (I I) and Pt(ll), and the reversibility of sorption strongly suggests coordination to soft Lewis basic sulfur on the Starbon surface, physical characterisation of the state of adsorbed Pd(ll) was required to confirm this and to exclude alternative mechanisms.

[0104] pXRD of C300RP-800N2 before and after Pd(ll) sorption (Figure 16) revealed minimal changes, where only a single crystalline phase, matched to NaCI, was identified, suggesting that precipitation of Pd-containing species did not contribute significantly to adsorption.

[0105] Figure 17 shows: (a) Normalised X-ray absorption spectrum and c - x(k) spectrum (k weighting = 2) of Pd k-edge for Pd-loaded Starbon material (C300RP-800N2); (b) 1stderivative of Pd k-edge; and (c) Fourier transformed x(R) spectra of Pd-loaded Starbon material (C300RP-800N2), PdO, metallic Pd, Pd black, solid PdCI2and an aqueous PdCI2solution. Resulting from the excitation of an electron from a Is orbital, XAS analysis of the Pd k-edge energy allows for qualitative comparison of the formal oxidation state of the Pd centre. To more easily allow for this comparison to be made, the first derivative of the Pd k-edge has been plotted in Figure 17b. The Pd-C800 k-edge energy is seen to very closely coincide with the PdCI2and PdO standard samples in the +11 oxidation state at 24353 eV, while the edge energy of metallic Pd and Pd black occurs at the lower energy of 24349 eV. This strongly suggests that Pd remains in the +11 oxidation state upon adsorption and that very little if any Pd is adsorbed reductively. Upon transforming the Pd EXAFS data into / (R) space, a qualitative analysis of the scattering environment around Pd can be made. Comparison of Pd-C800 to the range of standard samples reveals a similarity to both PdCI2standards, while the occurrence of any significant quantity of PdO or metallic Pd can be ruled out. A quick first shell fit of the Pd-C800 / (R) spectrum was sufficient to confirm that palladium remains coordinated to four atoms of similar size to chlorine, but a comparison of the fits by chlorine and sulfur revealed no meaningful difference.

[0106] Being more sensitive to changes in electronic environment, XPS analysis of the Pd 3d orbital was obtained for both Pd loaded C300RP-800N2and Pd adsorbed by the S-free Starbon, P800 and are displayed in Figure 18. Pd bound to both C800 and P800 was found to be in predominantly a single chemical environmentwith peaks at 337.2 and 337.6 eV respectively. The binding energy of Pd bound to S-free Starbon (P800) was found to correspond most closely to the reference value for Na2PdCI4(337.9 eV), with only a small shift to lower binding energy that has been previously attributed to the electron rich n-bonding responsible for Pd (I I) adsorption. By comparison, the XPS signal for Pd bound to C800 was shifted to even lower binding energy. This is to be expected for coordination to strongly o-donating sulfur atoms, as illustrated by the binding energy of PdS (336.6 eV), and supports the Pd-S binding mechanism.

[0107] The aforementioned approach of recovering platinum group metal using Starbon carbonaceous materials with reduced sulfur surface chemistry has proved successful. Further work has also shown that it is possible to incorporate reduced sulfur surface chemistry into alternative sources of carbon in a similar manner for use in recovering platinum group metal. For example, inorganic sulfates can be used as a source of sulfur and as an activating agent to allow for preparation of porous S-doped carbon materials from cheaper sources of biomass. Examples using sawdust and rice straw are described below. To demonstrate the fact that the activation and sulfur incorporation is not unique to potassium sulfate, the further example of a caesium sulfate-activated sawdust-derived material has been included. Additionally, it is possible to apply the same method for sulfur incorporation to a pre-carbonised material, as has been demonstrated below for the activated carbon DARCO®.

[0108] Activation / S incorporation of biomass

[0109] Sawdust and rice straw were chosen as examples of cheap, readily available biomass precursors which are already fairly porous (which can aid impregnation with sulfate).

[0110] The basic procedure for fabricating the porous carbonaceous material with reduced sulfur surface chemistry was as follows:

[0111] - Impregnate the precursor with an aqueous solution of sulfate.

[0112] - Dry the impregnated precursor.

[0113] - Pyrolyse the impregnated precursor under N2at a sufficient temperature to both carbonize the precursor and also reduce the sulfate (sulfate reduction occurs at 850-950 °C).

[0114] - Acid wash the pyrolysed material.

[0115] - Dry the acid washed material.

[0116] Figure 19 shows thermogravimetry of the pyrolysis process. Carbonisation occurs between 200 and 400°C and sulfate reduction / activation occurs between 800 and 1000°C.

[0117] To determine the optimum loading of K2SO4, sawdust-derived materials were prepared at a range of K2SO4loadings (S-sawdust char (x:y)), where the ratio in brackets indicates the ratio of masses of sawdust : K2SO4, before pyrolysis to 950 °C. Results of N2-sorption porosimetry for materials fabricated at different K2SO4loadings (as well as sawdust pyrolysed in the absence of K2SO4) are shown in the table below. The results show that surface area of the carbonaceous material is significantly increased by pyrolysing the sawdust in the presence of sulfate to a temperature sufficient to reduce the sulfate. Highest surface areas were achieved with a mass ratio of carbon material to sulfate of 2:1.

[0118]

[0119]

[0120] At the optimum sulfate loading identified for ICSC -doped sawdust (2:1), a material was prepared by doping K2SO4 into rice straw (S-rice straw char (2:1)) and an additional material prepared by doping CS2SO4 into sawdust (S-sawdust char (Cs)). In the case of the Cs2SO4-doped material, the mass of Cs2SO4was adjusted to match the molar equivalents of K2SO4used in the material produced using a 2:1 ratio. The materials produced using rice straw and Cs2SO4were also found to possess extensive porosity, due to activation by K2SO4reduction.

[0121] Figure 20 shows thermogravimetric profile of the pyrolysis (under N2) of sawdust in the presence and absence of K2SO4. Carbonisation can be seen to occur between 200 and 400°C in both cases, while sulfate reduction / activation occurs between 800 and 1000 °C in the K2SO4-doped case.

[0122] The composition of the materials was analysed using CHNS combustion analysis. Results are shown in the table below.

[0123]

[0124] The results confirm that a significant amount of sulfur is incorporated into the carbonaceous materials, with sulfur content increasing with sulfate loading.

[0125] The materials prepared at a range of K2SO4loadings were tested for Pd(ll) uptake. Results are shown in Figure 21. The results show that K2SO4can be used as both an activating agent and source of meta I loph i 11 ic sulfur to yield materials with very high Pd(ll) capacity. The best results were achieved with a 2:1 mass ratio of sawdust : K2SO4. As such, preferably a mass ratio of porous carbonaceous material to oxidized form of sulfur is: at least 1:1; 1.5:1; or 2:1; no more than 4:1; 3.5:1; or 3:1; or within a range defined by any combination of the aforementioned lower and upper limits.

[0126] The rice straw-derived material was found to be activated at the same 2:1 ratio of rice straw : K2SO4 and uptake of Pd(ll) was found to be similarly high, demonstrating the possibility for the use of alternative sources of carbon (e.g., coconut husk, waste plastic, etc.).Likewise, the material prepared from sawdust using Cs2SO4was found to exhibit high Pd(ll) capacity, demonstrating the possibility of the use of a range of sulfate salts for activation / sulfur incorporation (e.g., Na, Rb, Mg or Ca).

[0127] S incorporation into pre-carbonised materials

[0128] The aforementioned method for activation / sulfur incorporation was found to apply to the incorporation of meta llophi llic-su Ifur chemistry into pre-carbonised materials. The example below describes the incorporation of sulfur into the commercially available activated carbon DARCO® by the following procedure:

[0129] - Impregnate DARCO® with an aqueous solution of sulfate to achieve a ratio of 1:1 (mass of DARCO® : K2SO4)

[0130] - Dry the impregnated DARCO®.

[0131] - Pyrolyse the impregnated DARCO® under N2at a sufficient temperature to reduce the sulfate (sulfate reduction occurs at 850-950 °C).

[0132] - Acid wash the pyrolysed material.

[0133] - Dry the acid washed material.

[0134] The thermogravimetric profile of the pyrolysis (under N2) of K2SO4-doped DARCO® is included in Figure 20. Sulfate reduction is seen to occur at 850-950 °C.

[0135] K2SO4-doped DARCO® was pyrolysed to a range of temperatures to yield 'S-DARCO® (x °C)’, where the temperature in the brackets indicates the maximum pyrolysis temperature. Results of N2-sorption porosimetry for S-DARCO® materials prepared at a range of temperatures (as well as DARCO® in the absence of K2SO4activation) are shown in the table below. Upon reaching temperatures sufficient for sulfate reduction, additional porosity is seen to be introduced into DARCO®.

[0136]

[0137] The compositions of the materials were analysed using CHNS combustion analysis. Results are shown in the table below.

[0138]

[0139]

[0140] These results show that maximum sulfur incorporation occurs at 1000 °C.

[0141] Pd(ll)-sorption isotherms for S-DARCO® prepared at a range of temperatures (and for DARCO® in the absence of sulfate activation) are included in Figure 21. Pd(ll) capacity is shown to increase with increasing activation temperature up to 1000 °C. It stands to reason that the Pd(ll) capacity of materials prepared at higher temperatures may be the same or larger. Sulfate doping and pyrolysis of DARCO® is seen to result in high Pd(ll) adsorption capacities, comparable to the capacities of the sulfate-doped sawdust derived materials and the sulfur-doped Starbon materials.

[0142] While this invention has been particularly shown and described with reference to certain examples, it will be understood to those skilled in the art that various changes in form and detail may be made without departing from the scope of the invention as defined by the appended claims.

Claims

Claims1. A method of recovering platinum group metal from a process stream comprising platinum group metal, the method comprising:contacting the process stream with a solid phase extractant to adsorb platinum group metal from the process stream onto the solid phase extractant; andrecovering the adsorbed platinum group metal from the solid phase extractant,wherein the solid phase extractant is a porous carbonaceous material comprising a reduced sulfur surface chemistry, andwherein the reduced sulfur surface chemistry of the porous carbonaceous material is provided by adding an oxidized form of sulfur to the porous carbonaceous material and pyrolysing the porous carbonaceous material in the presence of the oxidized form of sulfur which is reduced by pyrolysis.

2. A method according to claim 1,wherein the porous carbonaceous material is a mesoporous carbon material, optionally a non-templated mesoporous carbon material.

3. A method according to claim 1 or 2,wherein the porous carbonaceous material is a bio-based porous carbonaceous material.

4. A method according to any preceding claim,wherein the porous carbonaceous material is a carbonaceous material produced from a polysaccharide feedstock.

5. A method according to claim 1,wherein the porous carbonaceous material is sawdust.

6. A method according to any preceding claim,wherein the oxidized form of sulfur is an inorganic source of sulfur added to the porous carbonaceous material.

7. A method according to any preceding claim,wherein the reduced sulfur surface chemistry of the porous carbonaceous material comprises one or more of thiols, sulfides, disulfides and thiophenes.

8. A method according to any preceding claim,wherein the porous carbonaceous material comprises a sulfur content of at least 1 wt%, 3 wt%, 5 wt%, or 10 wt%.

9. A method according to any preceding claim,wherein the porous carbonaceous material has a BET surface area of at least 500 m2.g-1, 700 m2.g_1, 900 m2.g-1, or 1200 m2.g-1.

10. A method according to any preceding claim,wherein the porous carbonaceous material has a BJH mesopore volume of at least 0.4 cm3.g-1, 0.6 cm3.g-1, 0.8 cm3.g-1, 1.0 cm3.g-1, 1.2 cm3.g-1, 1.4 cm3.g-1, or 1.6 cm3.g-1.

11. A method according to any preceding claim,wherein the pyrolysis of the porous carbonaceous material in the presence of the oxidized form of sulfur is performed at a temperature of at least 800°C in order to reduce the oxidized form of sulfur and form the reduced sulfur surface chemistry.

12. A method according to any preceding claim,wherein the porous carbonaceous material is subjected to pyrolysis under nitrogen.

13. A method according to any preceding claim,wherein the porous carbonaceous material is washed after pyrolysis and prior to contacting with the process stream.

14. A method according to claim 13,wherein the porous carbonaceous material is washed in an acidic solution, optionally HCI.

15. A method according to any preceding claim,wherein the process stream comprises a HCI solution at a pH of no more than 3, 2, or 1.

16. A method according to any preceding claim,wherein the process stream comprises platinum and / or palladium.

17. A method according to any preceding claim,wherein the adsorbed platinum group metal is recovered from the solid phase extractant by stripping the platinum group metal from the solid phase extractant using a stripping reagent.

18. A method according to any preceding claim,wherein the oxidized form of sulfur is a sulfate.

19. A method according to any preceding claim,wherein a mass ratio of porous carbonaceous material to oxidized form of sulfur is: at least 1:1; 1.5:1; or 2:1; no more than 4:1; 3.5:1; or 3:1; or within a range defined by any combination of the aforementioned lower and upper limits.

20. A method of manufacturing a solid phase extractant for use in the method according to any preceding claim, the method comprising:adding an oxidized form of sulfur to a porous carbonaceous material; andpyrolysing the porous carbonaceous material in the presence of the oxidized form of sulfur which is reduced by pyrolysis to form a porous carbonaceous material comprising a reduced sulfur surface chemistry.

Images