A composite material for extracting gold

Abstract

There is a composite material for extracting gold, the composite material comprising a 2-dimensional material and a 1- dimensional macromolecule, wherein the composite material comprises a cross-dimensional structure for adsorption of gold. The cross-dimensional structure of the composite material may be formed by self-assembly of the 2D material and the 1 D macromolecule into self-assembled nanolayers with nanoconfinements. In a particular embodiment, the 2D material is a graphene-based material and the 1 D macromolecule is chitosan. There is also a method of forming the composite material.

Description

[0001] A composite material for extracting gold

[0002] Technical Field

[0003] The present invention relates to a composite material for extracting metals from waste, particularly gold from electronic waste (e-waste).

[0004] Electronic waste (e-waste) contains substantial quantities of valuable precious metals, particularly gold (Au). However, inefficient metal recovery leads to these precious metals being discarded in landfills, causing significant water and environmental contamination. Large efforts have been applied for the recovery of Au from e-waste using complex processes which include the dissolution of Au, its adsorption in an ionic state and succeeding reduction to metallic Au. These processes themselves being complex and utilizing harsh chemicals contribute to the environmental impact of e-waste. The adsorption of Au3+ions is typically followed by the reduction process, involving such chemical agents as hydrogen gas, citrate ions, sodium borohydride, and ascorbic acid. Extraction of Au+ions (which are typically less common in e-waste but can be found in specific electronic components) necessitates a more complex series of chemical reactions. Recovering Au in its Au+oxidation state typically demands adsorbents with higher capacitance and selectivity. Simultaneously, the reduction of Au+ions demands more potent reducing agents and the addition of stabilizing agents to counter the stability of the Au+complex, facilitating the reduction process.

[0005] A common method for recovering of Au+involves its adsorption onto specialized ion exchange resins (such as activated carbon resin impregnated with thiol groups), followed by the reduction using sodium metabisulfite and stabilization with polyethylene glycol. Such multistage procedure for the extraction and reduction of both Au3+and Au+leads to significant environmental impact and increased cost.

[0006] There is therefore a need for an improved method of recovering gold, particularly from e-waste.

[0007] Summary of the invention

[0008] The present invention seeks to address these problems, and / or to provide an improved manner to recover gold ions and eventually gold, particularly from e-waste.According to a first aspect, there is provided a composite material for extraction of gold, the composite material comprising:

[0009] a 2-dimensional (2D) material; and

[0010] a 1 -dimensional (1 D) macromolecule,

[0011] wherein the composite material comprises a cross-dimensional structure for adsorption of gold.

[0012] The composite material may comprise any suitable 2D material. For example, the 2D material is a graphene-based material. In particular, the 2D material may comprise, but is not limited to, graphene oxide (GO).

[0013] The composite material may comprise any suitable 1 D macromolecule. For example, the 1 D macromolecule may comprise polymers with amino groups. In particular, the 1 D macromolecule may be, but is not limited to, chitosan, polyethyleneimine, polyallylamine, or co-polymers thereof.

[0014] According to a particular aspect, the extraction of gold may comprise extracting gold ions and reducing the gold ions into gold. In particular, the composite material may have an extraction efficiency of > 90 wt. %.

[0015] The cross-dimensional structure of the composite material may be formed by selfassembly of the 2D material and the 1 D macromolecule into self-assembled nanolayers with nanoconfinements.

[0016] The composite material may be in any suitable form. For example, the composite material may be in the form of a sponge or membrane.

[0017] According to a particular aspect, the composite material may be in the form of a sponge. The sponge may comprise pores of a suitable size. In particular, the average pore size of the sponge may be 1 -200 pm.

[0018] According to a particular aspect, the composite material may be in the form of a membrane, wherein the membrane may be a multi-layer composite membrane. For example, the interlayer distance between each layer comprised in the multi-layer composite may be 9-13 A.In particular, the membrane may be comprised in a 2D electrode to facilitate extraction of gold.

[0019] According to a particular aspect, the composite material may be reusable for multiple times.

[0020] According to a second aspect, there is provided a method of forming the composite material according to the first aspect, the method comprising:

[0021] forming a dispersion of 1 D macromolecules in acid; and

[0022] mixing the dispersion of 1 D macromolecules with an aqueous dispersion of 2D material in a pre-determined ratio to form the composite material.

[0023] According to a particular aspect, the forming may comprise mixing the 1 D macromolecules and the acid under suitable conditions. For example, the mixing may be with magnetic stirring for a pre-determined period of time at room temperature.

[0024] The mixing may comprise mixing the dispersion of 1 D macromolecules with an aqueous dispersion of 2D material under suitable conditions. For example, the mixing may be in a shaker.

[0025] According to a particular aspect, the method may further comprise freeze-drying the composite material formed following the mixing to form a sponge.

[0026] According to another particular aspect, the method may further comprise vacuum filtration of the composite material formed following the mixing through a membrane filter to form a composite membrane.

[0027] The method may further comprise drying the composite membrane. The drying may be under suitable conditions.

[0028] Brief Description of the Drawings

[0029] In order that the invention may be fully understood and readily put into practical effect there shall now be described by way of non-limitative example only exemplary embodiments, the description being with reference to the accompanying illustrative drawings. In the drawings:

[0030] Figure 1a shows the geometry of a CS / GO sponge complex after self-assembly;Figure 1b shows the schematic diagram of a CS / GO sponge for adsorption of gold; Figure 1c shows the SEM image of a GO / CSs sponge;

[0031] Figure 2a shows the FT-IR spectra of GO / CS membrane, with pure chitosan powder and GO membrane as reference;

[0032] Figure 2b shows X-ray diffraction (XRD) spectra of GO and GO / CSs;

[0033] Figure 2c shows the TGA curves of GO, OS, and GO / CS composite membranes, with a heating rate of 10 °C min in nitrogen atmosphere;

[0034] Figure 2d shows the stress-strain curves of pristine GO membrane, pristine chitosan membrane and GO / CS composite membranes;

[0035] Figure 3a shows the gold extraction capacity as a function of CS:GO mass ratio after 1 h immersion in 6800 ppm Au3+;

[0036] Figure 3b shows the gold extraction capacity of GO / CS10 prepared at different pHs after 1 h immersion in 6800 ppm Au3+;

[0037] Figure 3c shows the results of a reusability test showing the gold extraction capacity of GO / CS sponges after 5 cyclic adsorption-desorption procedure;

[0038] Figure 3d shows the extraction thermodynamics study through the concentration equilibrium constant (Kc) dependence on temperature for GO / CS sponge immersed in 200 ppm Au3+for 1 h;

[0039] Figure 3e shows the extraction capacity of GO / CS sponge prepared at pH 5 after 1 h immersion in aqueous solution of Au3+with concentrations ranging from 2 ppm to 100 ppm, fitted by Hill model;

[0040] Figure 3f shows the extraction capacity of GO / CSw composite membrane calculated from QCM measurement, 2 ppm of Au3+is used as feed solution;

[0041] Figure 4 shows the QCM curves of the mass change of GO / CSw in 2 ppm of Au3+vs. time;Figure 5a shows the kinetic fitting results of pseudo-first order of reaction for GO / CS sponge;

[0042] Figure 5b shows the kinetic fitting results of pseudo-second order of reaction for GO / CS sponge;

[0043] Figure 5c shows the kinetic fitting results of pseudo-first order of reaction for GO / CS membrane;

[0044] Figure 5d shows the kinetic fitting results of pseudo-second order of reaction for GO / CS membrane;

[0045] Figures 5e and 5f show the Langmuir model fitted for the GO / CS10 membrane and sponge respectively;

[0046] Figures 5g and 5h show the Freundlich model fitted for the GO / CS membrane and sponge respectively;

[0047] Figures 5i and 5j show the Temkin model fitted for the GO / CS10 membrane and sponge respectively;

[0048] Figure 6a shows the cyclic voltammetry data for the Au3+detection in solution with the presence of GO (0.12 mg / mL), CS (1.4 mg / mL), and GO / CS mixture after 1 hour of exposure;

[0049] Figure 6b shows the cyclic voltammograms for the aqueous solutions of pure Au3+solution and with the presence of GO (0.12 mg / mL) and CS (1.4 mg / mL) after 1 hour of exposure for 50 ppm;

[0050] Figure 6c shows the cyclic voltammograms for the aqueous solutions of pure Au3+solution and with the presence of GO (0.12 mg / mL) and CS (1.4 mg / mL) after 1 hour of exposure for 100 ppm;

[0051] Figure 6d shows the cyclic voltammograms for the aqueous solutions of pure Au3+solution and with the presence of GO (0.12 mg / mL) and CS (1.4 mg / mL) after 1 hour of exposure for 200 ppm;

[0052] Figure 7a shows the 4f XPS spectrum of gold nanoparticles on pristine GO, CS and GO / CS sponges after Au3+adsorption;Figure 7b shows the 4f XPS spectrum of gold nanoparticles on pristine GO, CS and GO / CS sponges after Au+(b) adsorption;

[0053] Figure 7c shows the geometries of CS2+Au3+GO2-(left) and CS2+Au+GO2-(right) complexes;

[0054] Figure 7d shows the comparison of gold extraction performance in the aspect of Au3+(spheres) and Au+(tetrahedrons) concentration, equilibrium time of extraction, and extraction capacity;

[0055] Figure 8a shows the schematic diagram of GO / CSs membrane using as a gold selective layer for the formation of gold selective electrode for electro-extraction and reduction of gold, 5 = 1 or 3;

[0056] Figure 8b shows the cross-section SEM image of a free-standing GO / CS5 membrane; Figure 8c shows the cyclic voltammetry curves of bare glassy carbon electrode (GCE), GO / CS5 and GO / CS10 membrane electrodes at a scan rate of 10 mV / s in 0.1 M HCI solution with Ar purged;

[0057] Figure 8d shows the l-t curves when applying different negative potential on GO / CS10 membrane immersed in 2 ppm of Au3+in 0.1 M HCI for 1 h;

[0058] Figure 8e shows the gold extraction capacity as a function of applied potential;

[0059] Figure 8f shows the 4 / XPS spectrum of gold nanoparticles on pristine GO, CS and GO / CS membranes after electro-extraction of Au3+ions;

[0060] Figure 9a shows the amplitude spectroscopic parameter as a function of wavelength measured at the incident angle of 74° for GO / CS membranes subject to different concentrations of gold;

[0061] Figure 9b shows the extracted optical constants of a pristine GO / CS membrane;

[0062] Figure 9c shows the extracted optical constants of a pristine GO / CS membrane subject to 0.01 M of Au; and

[0063] Figure 9d shows the extracted optical constants of a pristine GO / CS membrane subject to 0.034M of Au.Detailed Description

[0064] As explained above, there is a need for an improved manner of extracting precious metals from waste, particularly gold from waste. The waste may be e-waste.

[0065] In general terms, the present invention provides a method for recovering gold ions, particularly from e-waste. There is also provided a composite material for the recovery of gold (Au) ions from the e-waste. The composite material combines high ion adsorption capacitance, fast adsorption kinetics, as well as regeneration capability.

[0066] According to a first aspect, there is provided a composite material for extraction of gold, the composite material comprising:

[0067] a 2-dimensional (2D) material; and

[0068] a 1 -dimensional (1 D) macromolecule,

[0069] wherein the composite material comprises a cross-dimensional structure for adsorption of gold.

[0070] For the purposes of the present application, a cross-dimensional structure is defined as being a structure in which at least one 1 D, 2D and / or 3D structures are combined. In particular, the composite material is suitable for chemisorption of gold ions and optionally, chemical reduction of the ions into gold nanoparticles. For example, the gold ions may comprise, but is not limited to, Au3+, Au+, or a mixture thereof.

[0071] The composite material may comprise any suitable 2D material. For example, the 2D material is a graphene-based material. In particular, the 2D material may comprise, but is not limited to, graphene oxide (GO).

[0072] The composite material may comprise any suitable 1 D macromolecule. For example, the 1 D macromolecule may comprise, but is not limited to, amino groups. According to a particular aspect, the 1 D macromolecule may comprise a polymer comprising an amino group. In particular, the 1D macromolecule may be, but not limited to, chitosan, polyethylene imine, polyallylamine, or co-polymers thereof.

[0073] The composite material, comprising both a 2D material and a 1 D macromolecule, comprises a fractional dimensionality, thereby amplifying ion extraction and subsequent reduction of the ions into nanoparticles. In particular, the fractional dimensionality facilitates ion transport and adsorption, as well as promotes efficient electron transferacross interfaces. Even more in particular, the composite material comprises oppositely charged functional groups and hydrophobic domains due to the choices of the 2D material and the 1D macromolecule. This brings about interfacial compatibility and stability of the composite material. The multiple interactions between the 2D material and the 1 D macromolecule enable the composite material to have synergistic effect and enhanced properties as compared to the individual components of the composite material.

[0074] According to a particular aspect, the extraction of gold may comprise extracting gold ions and reducing the gold ions into gold. In particular, the composite material may have an extraction efficiency of > 90 wt. %. For example, the extraction efficiency may be 90-99 wt. %, 91-98 wt. %, 92-97 wt. %, 93-96 wt. %, 94-95 wt. %. Even more in particular, the extraction may be about 95-99 wt. %, more particularly about 95 wt. %. This is much higher than currently achieved extraction efficiencies of about 75 wt. %.

[0075] The extraction efficiency of the composite material may be achieved in a short duration of time. For example, the extraction efficiency may be achieved within 8-30 minutes, 1 fl-25 minutes, 15-20 minutes. This is much shorter compared to common extraction methods used in the art which can span up to or even beyond 24 hours.

[0076] The extraction efficiency may be achieved at room temperature and / or without applying any voltage. However, the extraction efficiency may be further enhanced upon application of voltage. In particular, application of voltage may accelerate electrodiffusion of Au ions into the composite material followed by electro-reduction.

[0077] The enhanced extraction efficiency may be achieved due to the increased in the number of functional interfaces available for ion adsorption in the composite material. In particular, the electrostatic attraction between negatively charged sites on the 2D material and the positively charged 1 D macromolecule may be maximised, thereby enhancing the overall binding capacity for gold ions such as Au3+. Further, the negatively charged groups of the 2D material comprised in the composite material interact with the protonated groups of the 1 D macromolecule and / or directly with the metal ions, providing multiple binding sites that are chemically diverse and spatially optimized for interaction with Au3+.

[0078] According to a particular embodiment, the composite material may be formed from GO and chitosan. Accordingly, in use, the composite material can achieve the high extractionefficiency when the composite material is exposed to a waste mixture comprising Au, and wherein the waste mixture is adjusted to a suitable pH. The pH is determined as the optimal pH for maintaining a significant degree of protonation of the chitosan’s amino groups while avoiding excessive charge repulsion. At this pH, the balance maximizes electrostatic attractions between negatively charged sites on the GO and the positively charged chitosan to enhance the overall binding capacity for Au3+. On the other hand, the negatively charged carboxyl groups on GO edges interact with the protonated amino groups of chitosan and / or directly with the metal ions, providing multiple binding sites that are chemically diverse and spatially optimized for interaction with Au3+.

[0079] Even more in particular, the process in which the metal ions are adsorbed and reduced by the composite material may be summarised as a three-stage process, in which at stage one, the carboxylic groups (-COOH) on GO act as anchoring points for metal ions, such as Au ions, while the lone electron paid on the nitrogen of the protonated chitosan amino (-NH3+) groups may be available for chelating with cations; at stage 2, the reduction of metal ions, such as Au ions to metallic gold via the transfer of electrons from the reducing agent (amine) to gold ions may occur; and at stage 3, oxidation of aminium radical (-NH2+) and formation of imino groups (-NH).

[0080] Graphene n-electron clouds formed by sp2hybridized carbon atoms tend to maintain their structure without bonding with Au. Chitosan chains on the contrary have a significant ability to interact with Au ions. O and N atoms are preferable sites for Au ions binding as they keep the negative partial charge (Mulliken charge). This interaction may be driven by Coulombic forces, resulting in the adsorption of Au ions by the chitosan chains. The strength of the interaction may depend on the charge state of Au ions. For example, for Au+, interactions may primarily occur with the amino group, while for Au3+, the interaction may extend to include nearby oxygens from ether and hydroxyl groups. The cross-dimensional structure of the composite material may be formed by selfassembly of the 2D material and the 1 D macromolecule into self-assembled nanolayers with nanoconfinements. According to a particular aspect, the GO and chitosan may selfassemble into GO / chitosan building blocks and assemble into well-defined nanolayers with chemically active nanoconfinements.

[0081] The composite material may be in any suitable form. For example, the composite material may be in the form of a sponge or membrane.According to a particular aspect, the composite material may be in the form of a sponge. The sponge may comprise pores of a suitable size. For example, the average pore size of the sponge may be 1-200 pm. In particular, the average pore size may be 5-180 pm, 10-150 pm, 20-125 pm, 25-100 pm, 35-85 pm, 40-80 pm, 45-75 pm, 50-60 pm. Even more in particular, the average pore size may be 10-50 pm. The sponge may be used directly as a gold adsorbent, for example, as a filter to extract Au ions from solutions passing through the sponge.

[0082] The composite material in the form of a sponge may have a general formula GO / CSXwherein x represents the mass ratio of 2D material, such as graphene oxide, to 1 D macromolecule, such as chitosan. According to a particular aspect, x may be 5-15. In particular, x may be 5-12 or 7-10. Even more in particular, x may be 10.

[0083] The sponge may have a suitable adsorption capacity at room temperature. For example, the sponge may have an adsorption capacity of 5-20 g / g. In particular, the adsorption capacity of the sponge may be 6-18 g / g, 10-17 g / g, 15-16 g / g. Even more in particular, the adsorption capacity may be 15-17 g / g for Au3+and / or 5-7 g / g for Au+, more particularly 16.8 g / g for Au3+and / or 6.2 g / g for Au+.

[0084] According to a particular aspect, the composite material may be in the form of a membrane, wherein the membrane may be a multi-layer composite membrane. The membrane may comprise an increased number of functional interfaces available for ion adsorption. For example, the interlayer distance between each layer comprised in the multi-layer composite may be 9-13 A. In particular, the interlayer distance may be 10-12 A, 11-11.5 A.

[0085] According to a particular embodiment, when the composite material comprises GO as the 2D material and chitosan as the 1 D macromolecule to form a GO / chitosan membrane, GO sheets and chitosan macromolecules may be perfectly aligned to form a layer-by-layer structure. In particular, the interlayer distance within the layers may vary depending on the region within the membrane. For example, the interlayer distance of chitosan encapsulated between GO may be slightly more as compared to the region without chitosan.

[0086] The composite material in the form of a membrane may have a general formula GO / CSXwherein x represents the mass ratio of 2D material, such as graphene oxide, to 1 Dmacromolecule, such as chitosan. According to a particular aspect, x may be 5-15. In particular, x may be 5-12 or 7-10. Even more in particular, x may be 10.

[0087] The membrane may be a stable and mechanically strong membrane. For example, the membrane may have a Young’s modulus of 0.1-10 GPa. In particular, the Young’s modulus may be 0.5-8 GPa, 1-6 GPa, 2-5 GPa, 3-4 GPa, and even more in particular, the Young’s modulus may be about 6 GPa.

[0088] In particular, the membrane may be comprised in a 2D electrode to facilitate extraction of gold.

[0089] For example, in use, the composite material in the form of a membrane may be an Au-selective layer on an electrode surface for electrochemical ion recovery and reduction. The Au-selective electrode may be capable of extracting ions from a mixed ions solution under applied voltage, while also simultaneously reducing them to form metallic forms. The membrane may have a suitable adsorption rate constant. For example, the rate constant may be 0.1-0.35 g / g.min. In particular, the rate constant may be 0.15-0.3 g / g.min, 0.2-0.3 g / g.min. Even more in particular, the rate constant may be about 0.29 g / g.min. The rapid and efficient kinetics of Au ion extraction of the composite material in the form of membrane may be due to the capillary action in the nanochannels with heterogeneous surface chemistry, featuring hydrophobic sp2carbon domains and acetyl groups (-COCH3) as well as multiple hydrophilic groups.

[0090] According to a particular embodiment, when the membrane is use as an Au-selective layer on an electrode surface, the colour of the surface of the membrane may change to gold after a period of time. This shows that a substantial amount of gold may be successfully electro-extracted and reduced from a waste solution comprising gold, which is then embedded in the membrane. For example, the membrane may comprise reduced gold nanoparticles. The nanoparticles may have a suitable size. In particular, the nanoparticles may have an average size of < 20 nm. Accordingly, the membrane may serve as a template for the formation of gold nanoparticles.

[0091] According to another aspect, the membrane comprising the reduced gold nanoparticles may therefore be further used as a ready-to-use optical sensor. In this way, the membrane may be used directly for subsequent applications following the extraction of gold from waste, thereby saving time and cost and improving post-processing efficiency.According to a particular aspect, the composite material may be reusable for multiple times. The composite material may be regenerated by simple washing steps and reused without significant decrease in the adsorption capacity. In this way, the composite material provides an efficient tool for extracting gold from waste.

[0092] According to a second aspect, there is provided a method of forming the composite material according to the first aspect, the method comprising:

[0093] forming a dispersion of 1 D macromolecules in acid; and

[0094] mixing the dispersion of 1 D macromolecules with an aqueous dispersion of 2D material in a pre-determined ratio to form the composite material.

[0095] According to a particular aspect, the forming may comprise mixing the 1 D macromolecules and the acid under suitable conditions. The acid may be any suitable acid. For example, the acid may be a weak acid. In particular, the acid may be, but not limited to, acetic acid, or a mixture thereof.

[0096] For example, the mixing may be with magnetic stirring for a pre-determined period of time at room temperature. The pre-determined period of time may be 12-50 hours. For example, the pre-determined time may be 15-48 hours, 18-42 hours, 20-36 hours, 24-30 hours. In particular, the pre-determined time may be 40-50 hours, more particularly about 48 hours.

[0097] The mixing may comprise mixing the dispersion of 1 D macromolecules with an aqueous dispersion of 2D material under suitable conditions. For example, the mixing may be in a shaker. The mixing may comprise mixing a suitable amount of the dispersion of 1 D macromolecules and a suitable amount of the dispersion of 2D material, depending on the final composition of the composite material intended to be obtained.

[0098] According to a particular aspect, the composite material to be formed may have a general formula GO / CSXwherein x represents the mass ratio of 2D material, such as graphene oxide, to 1 D macromolecule, such as chitosan. According to a particular aspect, x may be 5-15. In particular, x may be 5-12 or 7-10. Even more in particular, x may be 10. The mixing may result in the formation of composite nanosheets. In particular, during the mixing, the 1D macromolecules may self-assemble on the surface of the 2D material to form accessible ion-binding sites.Prior to the mixing, the method may further comprise forming a dispersion of 2D material. For example, the 2D material may be in a suitable form and treated to form a stable dispersion. The treatment may comprise any suitable treatment. According to a particular aspect, the treatment may comprise, but is not limited to, sonicating the 2D material for a pre-determined period of time.

[0099] The composite material formed may be further treated to obtain the composite material in a specific form. According to a particular aspect, the method may further comprise freeze-drying the composite material formed following the mixing to form a sponge. The freeze-drying may be for a suitable period of time. For example, the freeze-drying may be for a period of 12-36 hours. In particular, the freeze-drying may be for 18-30 hours, 24-28 hours. Even more in particular, the freeze-drying may be for about 24 hours. According to another particular aspect, the method may further comprise vacuum filtration of the composite material formed following the mixing through a membrane filter to form a composite membrane. The vacuum filtration may be for a suitable period of time. For example, the vacuum filtration may be for a period of 12-36 hours. In particular, the vacuum filtration may be for 18-30 hours, 24-28 hours. Even more in particular, the vacuum filtration may be for about 24 hours.

[0100] The method may further comprise drying the composite membrane. The drying may be under suitable conditions. According to a particular aspect, the drying may be at room temperature. The drying may be for a suitable period of time.

[0101] Having now generally described the composite material and the method of forming it, the same will be more readily understood through reference to the following examples which are provided by way of illustration, and are not intended to be limiting.

[0102] Materials

[0103] The materials used include aqueous graphene oxide dispersion (GO, 4 mg mL1, monolayer content > 95%, Graphenea Inc., USA), Chitosan (CS, powder, from shrimp shells, approx. Mw = 190-375 kDa, Sigma-Aldrich). Hydrochloric acid (HCI, ACS reagent, 37%, Sigma-Aldrich). Sodium hydroxide solution (NaOH, 50% in H2O, Sigma-Aldrich).Gold (III) chloride (AuCI3, powder, catalyst reagent type, 99%, Sigma-Aldrich). Copper (II) Chloride (CuCI2, powder, 99%, Sigma-Aldrich). Polyethersulfon membrane filter (PES, 0.03 pm, 47 mm, Sterlitech Corporation, USA), Anodise 47™ filter (pore size -0.02 pm, diameter 47 mm, Whatman, USA), Acetic acid (HOAc, glacial, ReagentPlus®, >99%, Sigma-Aldrich). All materials were received and used without further purification. Preparation of GO / CS composites

[0104] Method 1 : CS was dissolved in 1 %v / v acetic acid (HOAc) and magnetically stirred at room temperature for 48 h to form a stable CS / HOAc dispersion. 2 mg mL1GO dispersion was sonicated for 30 min before use to form dispersed GO flakes. The second step involves the mixing of the CS dispersion with the GO dispersion in a certain mass ratio to form composite nanosheets. Upon mixing, the CS molecules self-assemble on the surface of GO flakes to form accessible ion-binding sites, as illustrated in Figure 1a.

[0105] Method 2: CS / HOAc dispersion (5 mg mL-1) was obtained by dissolving chitosan (2 g) in HOAc (1 vol% / vol, 400 mL) upon magnetic stirring for 24 h at room temperature. The original aqueous graphene oxide dispersion (4 mg mL-1, 20 mL) was added into deionized water (380 mL) to obtain diluted GO dispersion (0.2 mg mL-1). Then, CS / HOAc (5 mL, 5 mg mL-1) dispersion was mixed with GO dispersion (25 mL, 0.2 mg mL-1), and the colloids were then mixed for 10 minutes by a shaker (rotation speed -500 rpm, Vortex Mixer, USA).

[0106] The GO / CS flakes were organised in two readily-to-use macroscopic forms: membranes and sponges. Vacuum-assisted method was used to align GO / CS flakes to form membranes. GO / CS composite membranes were prepared by vacuum filtration of the aforesaid mixture through two types of membrane filters: AnodiscTM 47 and polyethersulfone membrane. Vacuum filtration was maintained for 24 h, and the obtained membrane was then dried overnight in a dry cabinet at room temperature. For comparison, a pristine GO membrane can be easily prepared by vacuum filtrating 25 mL GO dispersion (0.2 mg mL-1 ) through the filter. The thickness of such membranes varies from tens of nanometres to microns depending in the amount of used GO / CS. The GO / CS membrane serves as a renewable Au-scavenger coating on the electrode surface, contributing to sustainability in the traditional electrowinning process.The GO / CS sponge, as illustrated in Figures 1b and 1c, is designed as a composite adsorbent that can chemisorb Au ions with high capacitance and high extraction rate. To obtain GO / CS sponges, a lyophilization technique was employed on the GO / CS suspensions. CS / HOAc (0.8 mL, 5 mg mL-1) dispersion was mixed with GO dispersion (2 mL, 0.2 mg mL-1), and the colloids were then mixed for 10 minutes by a shaker (rotation speed - 500 rpm, Vortex Mixer, USA). GO / CS sponge was prepared by freeze-drying the aforesaid mixture in a 24-well culture plate for 24 h. For comparison, a pristine GO or CS sponge can be easily prepared by freeze-drying 2 mL GO dispersion (0.2 mg mL-1) or 0.8 mL CS / HOAc dispersion (5 mg mL-1) in the well-plate. A sponge-like morphology featuring a network of interconnected voids ranging from 10 to 50 pm enables direct use as a gold absorbent, enhancing mass flow. The distinctive characteristics allow the sponge to be directly applied as a filter to extract Au ions from solutions.

[0107] The ratio of CS to GO in GO / CS membrane or sponge was controlled at 5, 10 and 20 by introducing a different volume of CS dispersion, and the resulting GO / CS composite was named GO / CS5, GO / CS10 and GO / CS20.

[0108] Set up for gold adsorption test

[0109] Experiments were performed using a set of side-by-side diffusion cells (Yuyan Instruments, Shanghai) with a pristine GO membrane or GO / CS membrane (with 5 mg of GO loading) fixed between two cell compartments, for example, the feed compartment and the drain compartment. The feed compartment was filled with AuCh solution (20 mL), while the drain compartment was sucrose (20 mL, 2.5 mol L-1) to induce the osmotic pressure between two cell compartments. After 1 hour, the gold concentration inside the membrane was evaluated using Inductively Coupled Plasma-Optical Emission Spectroscopy (ICP-OES). Before elemental analysis, the solid membrane samples were digested with HNO3 / HCI (3:1) in a microwave at 240 °C for 15 minutes and topped up to 10 mL with H2O. Note that a clear solution was observed before analysis.

[0110] Preparation of GO / CS thin film

[0111] GO / CS thin film was prepared for QCM measurements. CS / HOAc (40 pL, 5 mg mL-1) dispersion was mixed with GO dispersion (0.1 mL, 0.2 mg mL-1) and 5 mL of Dl-water, and the colloids were then mixed for 10 minutes by a shaker (rotation speed - 500 rpm,Vortex Mixer, USA). GO / CS thin film were prepared by vacuum filtration of the aforesaid mixture through AnodiscTM 47 membrane filter. Vacuum filtration was maintained for 2 hours, and the obtained film was then dried overnight in a dry cabinet at room temperature. Next, the prepared membrane was peeled off on the water’s surface and transferred onto the surface of a 5 MHz Au / Ti electrode. QCM measurements were performed by the QSense Explorer System (QE 401 Electronic Unit, QCP 101 Chamber Platform, QFM 401 Flow Module). Dl-water or 2 ppm AuCI3 solution was pumped into the chamber at the speed of 50 uL min-1.

[0112] Optimisation of GO / CS composite

[0113] As illustrated in Figure 2a, Fourier Transform Infrared Spectroscopy (FT-IR) has shown that the composite material exhibits characteristic peaks of both GO and CS. The formation of the nanoscale cross-dimensional composite was further confirmed by X-ray Diffraction (XRD), as illustrated in Figure 2b, which revealed two characteristic peaks indicative of distinct structural elements. The XRD graph in Figure 2b also revealed 12.9 A in CS-containing nanoconfinements and 8.6 A in CS-free regions, as observed in the spectra of GO / CS5. Based on the composition, we assigned the membranes and sponges as GO / CSz, with x representing the mass ratio of GO to CS.

[0114] Chemical stability tests were carried out and it was found that the presence of CS in the GO / CS composites not only enhances the adsorption capacity and selectivity for Au3+ions but also contributes to improved stability. It was observed that the pure GO sponges, without the presence of CS, exhibit instability when exposed to the Au3+solution. The instability can be attributed to the inherent properties of GO, such as its hydrophilicity and high surface area, which make it prone to swelling and disintegration in aqueous environments. The hydrophilic nature of GO leads to swelling and disintegration of the material. Electrostatic cross-linking between GO and CS stabilizes the composites preventing them from swelling and disintegrating. At pH < 7, the GO / CS sponge retains a consistently stable structural configuration following the process of lyophilization, however, its structural integrity rapidly deteriorates and disintegrates within a few seconds when the sponge is prepared at pH>7. It was also observed that the adsorption capacity varied significantly with changes in pH. The threshold pH value is 5. At lower pH values, corresponding to acidic conditions, the adsorption capacity of the sponges was relatively low. At pH > 7, fully uncharged CS becomes insoluble in water and not miscible with aqueous GO dispersion.To determine the optimal mass ratio of GO to OS, mechanical tests and thermogravimetric analysis (TGA) were further conducted on the composite materials with varying GO to CS mass ratios, and the results are illustrated in Figures 1d and 1c respectively. The results revealed that GO / CS10 has an optimal set of thermal, mechanical, and chemical stability. The Young’s modulus of the GO / CS10 reaches 6 GPa and exceeds those for both the pure GO and CS.

[0115] Focusing on the recovery of Au (III), optimal conditions for maximum efficiency of the composite material in the treatment of e-waste can be determined by systematically varying and analyzing various parameters such as the material’s composition, pH, temperature, contact time, and ion concentration, thereby obtaining the optimal physicochemical parameters that allows for the highest Au ions recovery efficiency.

[0116] An aqueous solution containing Au(lll) ions at a fixed concentration of 6800 ppm was utilized to reveal the optimal GO / CSXcomposition and pH for Au extraction. 2.5 mg sponges were immersed for 1 h into a feed solution. The feed and supernatant solution concentrations are determined using Inductively Coupled Plasma - Optical Emission Spectroscopy (ICP-OES).

[0117] The extraction capacity is subsequently calculated from the ICP-OES results. As illustrated in Figure 3a, the sponge composed of GO / CS10 demonstrates the highest capability to chemisorb Au(lll) ions, achieving an adsorption capacity of 16.8±2.3 g / g at room temperature. As illustrated in Figure 3b, the optimal pH range for ion adsorption falls within the vicinity of the pKa values of GO (pKa 4.6) and CS (pKa 6.5). Thus, at pH 5-7, a diversity of surface chemistry of both GO and CS outcomes in increasing the ability of the material for Au extraction. At pH 5-7, -COOH groups electrostatically attract cations and -NHz groups coordinate Au ions and stabilize the complexes with Au ions. The reusability of sponges was tested in DI water. Following each adsorption phase, the sponge underwent a thorough washing process with Dl-water, involving 20 min of sonication at 20°C, repeated three times. As illustrated in Figure 3c, the results demonstrate that GO / CS sponges maintain their excellent adsorption capacity, ensuring the reversibility and reusability of the sponge throughout the testing period.Evaluating gold extraction capacity using ICP-OES

[0118] Gold extraction capacity was evaluated using Inductively Coupled Plasma-Optical Emission Spectroscopy (ICP-OES). A singular GO / CS sponge, weighing 2.5 mg, was immersed in 10 mL of varied aqueous AuCh solution concentrations, ranging from 2 ppm to 6800 ppm, for a duration of 1 h. The initial and post-adsorption concentrations of gold solutions were measured by ICP-OES. In the case of AuCh concentrations ranging from 2 ppm to 100 ppm, the solutions were diluted tenfold to achieve a 10 mL diluted dispersion for subsequent ICP-OES analysis. For AuCh concentrations spanning from 200 ppm to 6800 ppm, a hundredfold dilution was implemented to obtain a 10 mL diluted dispersion for ICP-OES testing. Standard Au calibration solutions procured from Sigma-Aldrich were utilized to calibrate ICP within the test range of 0.01 ppm to 100 ppm during testing. 2% of HCI / HNO3 is used for washing the tubing of ICP-OES equipment between each sample. After determining the AuCh concentrations before and after adsorption, the extraction capacity was calculated based on the ICP-OES results.

[0119] Thermodynamics of the adsorption process

[0120] The study of thermodynamics allows the estimation of the overall feasibility of such intensive chemisorption. To study the thermodynamics of the adsorption process, a single GO / CS10 sponge (prepared at pH 5), weighing 2.5 mg, was immersed in 10 mL of 200 ppm AU3+solution at various temperatures, ranging from 5SC to 60SC, for a duration of 1 h. Samples for 5SC test is conducted in a refrigerator, while the other temperatures are controlled by a water bath. The initial and post-adsorption concentrations of gold solutions were measured by the ICP-OES. After determining the Au3+concentrations before and after adsorption, the extraction capacity and the concentration equilibrium constant (Kc) were calculated based on the ICP-OES results.

[0121] The thermodynamic parameters for the adsorption of Au(lll) ions using GO / CS10 were tested at different temperatures: 5°C, 25°C, 30°C, 40°C, 50°C, and 60°C. The thermodynamics parameters enthalpy (AH°) and entropy (AS0) were calculated using established thermodynamic relationships, including the equilibrium constant expression, the van’t Hoff equation, and the Gibbs free energy equation.As illustrated in Figure 3d, the thermodynamics of the overall energy changes during Au ions chemisorption reveals the exothermic spontaneous chemisorption process (AG"ads - 1.87 kJ / mol), with a calculated AH"ads value of -0.97 kJ / mol. The positive value of AS ads (3 J / mol K) suggests an increased randomness at the heterogeneous solid-solution interface during the chemisorption of Au(lll) ions onto the binding sites of the CO / CS composite.

[0122] Kinetics of the adsorption process

[0123] The study of kinetics allows for the evaluation of the process rate. To study the kinetics of the adsorption process, a single GO / CSio sponge (prepared at pH 5), weighing 2.5 mg, was immersed in 10 mL of 200 ppm Au3+solution. The concentrations of gold solutions were measured by the ICP-OES at different immersing durations (0 s, 30 s, 1 min, 3 min, 5 min, 10 min, 20 min, 30 min, 40 min, 50 min, and 1 h). Specifically, 0.1 mL of the AU3+solution at different immersing durations is diluted 10 times by adding 9.9 mL of Dl-water before the ICP-OES test. After determining the Au3+concentrations, the extraction capacity as a function of immersing time was calculated based on the ICP-OES results.

[0124] As illustrated in Figure 4, the adsorption kinetics of GO / CSio composite membrane was calculated from QCM measurement, with 2 ppm of Au3+used as a feed solution. To prepare a GO / CSio membrane for QCM test, CS / HOAc (40 pL, 5 mg mL-1) dispersion was mixed with GO dispersion (0.1 mL, 0.2 mg mL-1) and 5 mL of Dl-water, and the colloids were then mixed for 10 minutes by a shaker (rotation speed - 500 rpm, Vortex Mixer, USA). GO / CSio membrane were prepared by vacuum filtration of the aforesaid mixture through AnodiscTM 47 membrane filter. Vacuum filtration was maintained for 2 h, and the obtained film was then dried overnight in a dry cabinet at room temperature. Next, the prepared membrane was peeled off on the water’s surface and transferred onto the surface of a 5 MHz Au / Ti electrode. QCM measurements were performed by the QSense Explorer System (QE 401 Electronic Unit, QCP 101 Chamber Platform, QFM 401 Flow Module). Dl-water or 2 ppm Au3+solution was pumped into the chamber at the speed of 50 uL min-1, for a duration of 1 h. Extraction capacity is calculated based on the frequency and mass change of the membrane.The data obtained from the kinetic studies were further used to investigate the details of the Au (III) ions adsorption process onto the GO / CSio using two known kinetics models, namely the pseudo-first order model and the pseudo-second order model, as illustrated in Figures 5a to 5d. The pseudo-first order model assumes that the adsorption rate varies with the number of non-adsorbed sites on the surface of adsorbent and the pseudosecond order kinetic model is based on chemisorption and is affected by the mass balance equation and the second-order rate derivative.

[0125] The theoretical equilibrium membrane adsorption capacity (12.19 g / g) calculated by the pseudo-second order kinetic model is close to the actual equilibrium adsorption capacity (12.25 g / g). In addition, as shown in Table 1 , the highest values of R2 were observed with the pseudo-second order model. Therefore, the pseudo-second order kinetic model described the adsorption process and the whole process was chemisorption. Furthermore, rate constants for the chemisorption on the GO / CS sponge and membrane are calculated to be 0.14 g / g min and 0.29 g / g min, respectively. This shows that membranes exhibit a faster rate of chemisorption compared to the sponges, suggesting that the process is diffusion-limited.

[0126] Table 1. Adsorption kinetic parameters from the fitted data.

[0127]

[0128] Mechanism of the adsorption process

[0129] To study the mechanism of the adsorption process, a single GO / CSio sponge (prepared at pH 5), weighing 2.5 mg, was immersed in 10 mL of varied aqueous Au3+ solution concentrations, ranging from 2 ppm to 6800 ppm, for a duration of 1 h. The initial and post-adsorption concentrations of gold solutions were measured by the ICP-OES. In the case of AU3+concentrations ranging from 2 ppm to 100 ppm, the solutions were diluted tenfold to achieve a 10 mL diluted dispersion for subsequent ICP-OES analysis. For Au3+concentrations spanning from 200 ppm to 6800 ppm, a hundredfold dilution wasimplemented to obtain a 10 mL diluted dispersion for ICP-OES testing. Standard Au3+calibration solutions procured from Sigma-Aldrich were utilized to calibrate ICP within the test range of 0.01 ppm to 100 ppm during testing. 2% of HCI / HNCh is used for washing the tubing of ICP-OES equipment between each sample. After determining the Au3+concentration before and after adsorption, the extraction capacity was calculated based on the ICP-OES results.

[0130] Extraction capacity of GO / CSio composite membrane calculated from QCM measurement, 2 ppm of Au3+is used as feed solution. To prepare a GO / CSio membrane for QCM test, CS / HOAc (40 pL, 5 mg mL-1) dispersion was mixed with GO dispersion (0.1 mL, 0.2 mg mL-1) and 5 mL of Dl-water, and the colloids were then mixed for 10 minutes by a shaker (rotation speed - 500 rpm, Vortex Mixer, USA). GO / CSio membrane were prepared by vacuum filtration of the aforesaid mixture through AnodiscTM 47 membrane filter. Vacuum filtration was maintained for 2 h, and the obtained film was then dried overnight in a dry cabinet at room temperature. Next, the prepared membrane was peeled off on the water’s surface and transferred onto the surface of a 5 MHz Au / Ti electrode. QCM measurements were performed by the QSense Explorer System (QE 401 Electronic Unit, QCP 101 Chamber Platform, QFM 401 Flow Module). Dl-water or 2 ppm AU3+solution was pumped into the chamber at the speed of 50 uL min-1, for a duration of 1 h. Extraction capacity is calculated based on the frequency and mass change of the membrane.

[0131] To study the adsorption mechanism, four isothermal models were used to describe the adsorption process, including Langmuir, Freundlich, Temkin and Hill, as shown in Figures 5e to 5j. The adsorption constant values for the fitted isotherm models and correlation coefficient value (R2) of GO / CSio can be found in Table 2.Table 2. Adsorption constant values for the fitted isotherm models of GO / CS10.

[0132]

[0133] K - adsorption constant

[0134] ads

[0135] qmax- maximal theoretical adsorption capacity

[0136] Kb- Temkin equilibrium binding constant

[0137] n - Freundlich equilibrium constant (intensity of adsorption)

[0138] As shown in Table 2, the highest R2corresponds to the Hill model. In contrast, poor correlation coefficients were obtained for the Freundlich and Temkin models. This evidence that Au ions adsorb at all adsorption centers simultaneously. The GO / CS10 composite is a soft matter-based material demonstrating the absence of heterogeneous surface adsorption with the Freundlich mechanism. The Langmuir adsorption model also demonstrates high approximation reliability for adsorption on GO / CS10 sponge and on GO / CS membrane. Langmuir's model proves that the main mechanism is chemisorption. The high Hill Kads for the membrane depicts strong ion-ligand interactions and, as a result, fast establishment of adsorption equilibrium. Nevertheless, Hill Kads for the membrane is two times lower than Langmuir Kads. Such difference for Kads results from the highly ordered membrane structure, leading to the spatial limitations in bulk material. The Langmuir qmaxvalues are close to the practical qmax values both for spongeand membrane. The Hill model considers the chemisorption process more accurately, considering the ion-ligand interactions. The high Hill Kadsfor the membrane depicts strong ion-ligand interactions and, as a result, fast establishment of adsorption equilibrium. The Hill Kads for the sponge was lower than for the membrane (2.4-104) and closer to the Langmuir Kads. The reason for such an interrelationship is the diffusion limitations for the sponge due to the fast first stage - chemisorption from the immediate environment and slow diffusion of Au ions from the bulk solution. For the membrane study, the convection by peristaltic pump leads to establishing the true value of Kadsfor the GO / CS composite.

[0139] As illustrated in Figures 3e and 3f, the Hill model demonstrated a high correlation coefficient. The Hill equation results in atypical sigmoidal curve when Au ions extraction capacity is plotted against Au ions feed concentration:

[0140]

[0141] where qeis the equilibrium adsorption capacity, qmax - maximal theoretical adsorption capacitance, Crest - rest concentration after adsorption, 1 / K is the adsorption constant (Kads), n - is the stochiometric coefficient for the number of ligands that bind with molecules.

[0142] This sigmoidal shape is characteristic of cooperative binding. The Hill coefficient (n) in equation (1) represents the degree of cooperativity. If n is greater than 1 , it indicates positive cooperativity (binding of one ligand enhances the binding of subsequent ligands). From Hill isotherm model for GO / CS sponge and membrane we found that the n value is 1.28 and 1.49, respectively. Thus, the study of thermodynamic and kinetics reveals an ions chemisorption phenomenon that is thermodynamically driven and suggests a typical for biological macromolecules cooperativity, where the binding of an ion at one site affects the binding of ions at other sites on the same macromolecule. Furthermore, the QCM allows us to measure the adsorption kinetics in higher concentration ranges. Fitting such curves by Hill model reveals an absorption capacity, reaching up to 95 wt.% of Au3+adsorption within 10 minutes at room temperature (Fig.

[0143] 2f and Supplementary Fig. 3). This adsorption rate is much greater than other graphenebased adsorbents, which may take several days to reach an equilibrium Au extraction.The rapid and efficient kinetics of Au ion extraction observed in our material arise due to the capillary action in the nanoconfinements with heterogeneous surface chemistry, featuring hydrophobic sp2carbon domains and acetyl groups (-COCH3) as well as multiple hydrophilic groups.

[0144] Both GO and OS have been used in Au extraction and reduction, although their efficiency is comparatively lower when used individually. A pure OS sponge has a significantly lower ability to extract Au compared to the GO / CS composite. OS’ maximum adsorption for Au3+ is measured to be 2.8±0.3 g / g, as shown in Figure 3a. On the other hand, it was reported that the extraction capacity of reduced graphene oxide (rGO) for Au(lll) ions exceeded a value of 1.85 g / g at 10 ppm feed Au concentration. However, when the concentration of Au ions was increased above 10 ppm, there was no significant increase in the extraction capacity of rGO. It was observed that the pure GO sponges, without the presence of OS, disintegrate when exposed to the concentrated feed Au3+ solutions. The high osmotic pressure leads to structural instability, resulting in layer separation and material expansion. Electrostatic cross-linking between GO and CS stabilizes the composites preventing the composite from swelling and disintegrating.

[0145] Electrochemical interaction ofAu3^ with GO and CS molecules

[0146] To prove the mechanism of Au3+adsorption on GO / CS10 composite, the relation between the adsorbed and reduced gold ions was studied. For this purpose, the electrochemical detection method of Au3+was used. The aqueous mixture of Au3+(50-200 ppm), GO (0.12 mg / mL), and CS (1.4 mg / mL) was prepared. The reaction time was estimated to be 1 hour, based on the adsorption saturation time for the GO / CS10 composite. A three-electrode setup was used for the Au3+detection. Glassy carbon electrode (3 mm diameter), Pt wire, and Ag / AgCI electrode were used as working, counter, and reference electrodes, respectively. The 0.01 M HCI serves as the background electrolyte. The volume of the electrochemical cell is 20 mL. The analytical signal received with the first cycle scan was from +1.5 V to -0.2 V, as shown in Figure 6a. The area of gold reduction peak at around +0.30 V is proportional to the concentration of Au3+. As shown in Figure 6b, the cyclic voltammograms for the calibration demonstrate the increase of peak area with an increase of Au3+concentration in solution. The calibration curve shown in Figure 6c demonstrates semilogarithmic behaviour that caused the Au3+electrodepositionkinetics on the glassy carbon electrode. The linear regression equation for the Au3+calibration curve:

[0147] IgSpeak = -8.18 + 0.015 ■ CAU3+

[0148] where Speak is the reduction peak area and CAu3+is the concentration of Au3+in solution. The R2correlation coefficient for the linear approximation is 0.9994.

[0149] The cyclic voltammograms of the GO, CS, and GO / CS mixtures exposed on Au3+demonstrate the partial chemical reduction of Au3+to Au°. The rest Au3+ions are complexed with GO and CS. The evidence of complexation is the reduction peak potential shifting from +0.30 V for the pure Au3+solution to +0.28 V for the GO, +0.36 V for the CS, and +0.38 V for the GO / CS mixture. The shifting of reduction potential is the evidence of capture with GO and CS nonreduced Au3+ions. The peak height for the GO does not change in comparison with pure Au3+solution, which proves the absence of chemical reduction by GO. The reduction peak for the CS and GO / CS shifts to the positive direction that prove the formation of the CS-Au3+complex with the facilitating of electrochemical reduction.

[0150] The initial amount of Au3+ions was 4 mg. The amount of Au3+reduced by GO / CS is 1.57-105mol (3.1 mg). The amount of chitosan is 1.47-105mol (22.4 mg). The molar relation between gold and chitosan is 1 :0.9. Such proportion corresponds to the ability to achieve high extraction capacity for the GO / CSw composite.

[0151] A detailed study of gold reduction by GO / CSw in solution was carried out at a fixed CS (22.4 mg) and GO (2.24 mg), with the Au3+content in situ increasing from 0 to 20 mg. The results, as illustrated in Figures 6e and 6f, show that the constant amount of gold is reduced within an hour until 15 mg of gold is achieved. The reduction process is delayed for 15mg and increases to 20 mg of Au3+content. Nevertheless, the gold reduction process still exists for the next 72 hours but at a lower rate. The inflection point in the graph of Figure 6f corresponds to the achievement of an equilibrium state between the Au° and Au3+complexed with GO / CSw. The amount of Au° is 3.8 1019atoms (12.5 mg). The amount of chitosan is 22.4 mg, and based on nitrogen content in chitosan (7 wt%), the number of N atoms is 6.74-1019(1.568 mg). Thus, the relation N to Au is 1.76 to 1 , which means the around 2 aminogroups of chitosan are able to reduce 1 Au3+ion. Suchrelation corresponds to the founded extraction capacity values and proves the ability to achieve high extraction capacity for the GO / CS composite.

[0152] Analyzing the results

[0153] Through the utilization of SEM / EDX, the surface of GO / CS sponges after the chemisorption process was analysed. It was observed that, along with chemisorbed Au ions, a significant and uniform distribution of solid Au forms such as Au clusters and Au nanoparticles (AuNPs) on the surface of the GO / CS sponges extracted either from Au(l) or Au(lll) solutions. As illustrated in Figures 7a and 7b, X-ray photoelectron spectroscopy (XPS) analysis was used to elucidate the contribution of functional groups of GO and OS to the adsorption and reduction of Au(l) and Au(lll) ions. The XPS peaks at 84 and 88 eV confirm the reduction of both Au3+(Fig. 3a) and Au+(Fig. 3b) to metallic Au°. However, in the spectrum of Au+, in addition to the peaks at 84 and 88 eV that confirm the reduction of Au+, we observe the presence of a peak at 90 eV. The presence of a peak at 90 eV in the XPS spectrum of pure OS and GO / CS sponges suggests the formation of a complex between Au+and CS. This peak can be attributed to the characteristic binding energy of the Au+-CS interaction. Density functional theory (DFT) computation was provided to get insight about Au cations interaction with graphene plane and CS chains of composites. Computations demonstrate repulsive interaction between positively charged Au ions and graphene lattice. Graphene TT-electron clouds formed by sp2hybridized carbon atoms tend to maintain their structure without bonding with Au. CS chains in contrary have significant ability to interact with Au ions. O and N atoms are preferable sites for Au ions binding as they keep the negative partial charge (Mulliken charge). This interaction is driven by Coulombic forces, resulting in the adsorption of Au ions by the CS chains. Notably, the strength of this interaction depends on the charge state of Au ions. For Au+, interactions primarily occur with the amino group, while for Au3+, this interaction extend to include nearby oxygens from ether and hydroxyl groups, as shown in Figure 7a. The adsorption of Au ions on these sites was associated with a significantly negative Gibbs free energy change.

[0154] The presence of multiple binding sites ensures the high efficiency of both the chemisorption and chemical reduction processes. The whole process can be described as a three-stage reaction: stage 1 - the carboxylic groups (-COOH) on GO act as anchoring points for Au ions, while the lone electron pair on the nitrogen of the CS amino(-NH2) groups is available for chelating with cations; stage 2 - the reduction of Au ions to metallic gold via the transfer of electrons from the reducing agent (amine) to gold ions; stage 3 - proton transfer reaction between protonated amino groups (-NH3+) and deprotonated carboxylic (-COO ) groups of GO to regenerate catalytic amino groups (-NH2):

[0155] > >

[0156] >

[0157]

[0158] Following the reduction of Au ions by amino groups, the proton transfer reaction with carboxylic groups of GO, CS ammonium groups deprotonates back to their initial state (amino groups), ensuring the cooperative catalytic function of the functional groups of GO and CS in our system. The cyclic voltammetry of the GO, CS, and GO / CS aqueous mixtures exposed on Au3+allow to demonstrate the partial chemical reduction of Au3+to Au°, as shown in Figure 6. The reduction peak height for the GO does not change in comparison with pure Au3+solution, which proves the absence of chemical reduction by GO. The reduction peak for the CS and GO / CS shifts to the positive direction that prove the formation of the CS-Au3+complex with the facilitating of electrochemical reduction. The molar relation between gold and chitosan is 1:0.9 (refer to SI). A detailed study of gold reduction by GO / CSw in solution was carried out at a fixed CS (22.4 mg) and GO (2.24 mg), with the Au3+content in situ increasing from 0 to 20 mg. The amount of Au° is 3.8 1019atoms (12.5 mg) accord to equilibrium state between the Au° and Au3+complexed with GO / CSw- Thus, the relation N to Au is 1.76 to 1 , which means the around 2 amino-groups of chitosan are able to reduce 1 Au3+ion. Such proportion corresponds to the partial amount of gold reduction process by chitosan and proves the ability to achieve high extraction capacity for the GO / CSw composite.

[0159] Comparing the composites to existing adsorbers, a significant improvement in ion recovery behavior for both Au(l) and Au(lll) ions was observed. While previous adsorbents demonstrated Au(l) extraction capacities of around 0.3 g / g from Au(CN)2“and Au(lll) extraction capacities of around 2 g / g from [AuCk]", our material can extract 16.7 g / g for Au(lll) and 6.2 g / g for Au(l) from technologically relevant AuCh and AuCI salt solutions within just 10 minutes and simultaneously reduce the both ions.

[0160] Next, we assessed the chemisorption capacity of our materials for 15 ml of simulated e-waste mixtures containing 1 mM concentrations of 12 representative heavy metals. In simulated e-waste mixtures our materials exhibited the insufficient selectivity for Au ions. We observed comparable adsorption efficiencies for Au3+and Cu2+. Nevertheless, EDX mappings of Au and Cu chemisorbed from single-ion solutions unveil distinct patterns. The EDX analysis indicates a homogeneous distribution of Au, while the distribution of Cu appears highly inhomogeneous. This disparity in spatial distribution still suggests the unique chemisorption mechanisms for Au3+.

[0161] In the context of a multi-component system with various ions and relatively low concentration of Au ions, the extraction efficiency of GO / CS can be achieved in combination with electrowinning. For gold recovery from Cu-rich solutions electrowinning is most affordable due to the electrodeposition of Au on cathode and Cu on anode. By electrowinning our GO / CS membrane can assist as an exchangeable coating on cathode to promote sustainable and cost-effective use of renewable electrodes by allowing their reuse in subsequent processes.

[0162] The electrochemical Au ions extraction experiments were performed in a three-electrode system. As illustrated in Figure 8a, a carbon cloth (CC) with a GO / CS membrane serves as the working electrode, and pure CC and Ag / AgCI (in 3 M KCI electrolyte solution) are used as counter and reference electrodes, respectively.

[0163] The cyclic voltammogram (CV) profiles of GO / CS membrane and bare glassy carbon electrode (GCE) were recorded at a constant voltage of -1.2 V (relative to Ag / AgCI) in an acidic mixture (25 mL) containing 20 ppm or 200 ppm of Au3+ / Cu2+(Fig. 4c). The specific capacitance value of the GO / CS membrane electrode is enhanced in comparison with bare GCE. As expected, GO / CS10 is more effective for Au(lll) extraction compared to GO / CS5, which is consistent with the results on the Au(lll) extraction properties of GO / CS sponges. The CV curves revealed nonsignificant shifting of oxidation peaks of Au° and reduction peaks of Au3+. Therefore, the Au redox equilibrium potential changed from the 0.70 V, for the bare GCE, to the 0.73 V for the GO / CS10. The current density valuesdrastically increased for the GO / CSw in comparison with bare GCE. Such negligible change of the equilibrium potential and increased current density proves that the insolating nature of the GO / CS membrane does not exert a limitation on the charge transfer process. As expected and shown in Figures 8d and 8e, the applied voltage influences the extraction capacity of the membrane. With increasing the applied voltage, the extraction capacity is increased linearly, caused by the accelerated electro-diffusion of Au ions into the membrane with subsequent electro reduction.

[0164] The GO / CS membranes can be effectively utilized as exchangeable coatings for electrodes in the electrowinning process. Due to the deposition of Au, the GO / CS membrane changed from black to gold, indicating that a substantial amount of Au° was successfully electro-extracted from the simulated e-waste solution and embedded in the membrane. At a higher concentration of e-waste (200 ppm), monocrystalline AuNPs are observed on GO / CS membrane. The TEM image reveals the lattice structure and grain boundaries of AuNPs and the appearance of the peaks of Au° in Figure 8f in the XPS spectra further support the electro-reduction of Au(lll) ions in e-waste to AuNPs with a size smaller than 20 nm. As a result, our membrane exhibits selective extraction of Au ions under an applied voltage and can also serve as a template for the formation of AuNPs. In addition to the electrochemical extraction of gold from e-waste, as shown in Figure 9, the GO / CS membrane embedded metallic gold is ready-to-use optical sensor applications. Variable angle spectroscopic ellipsometry measurements on GO / CS membranes exposed to varying gold concentrations revealed either the localized surface plasmon resonance peak or a "Drude response".

[0165] SEM / EDX Characterization method

[0166] X-ray diffraction (XRD) was carried out using Broker D8 ADVANCE with a Cu Ka tube radiation source (1.5418 A). Thermogravimetric analysis (TGA) was performed by TA Instrument Discovery TGA1-0247 under nitrogen at a heating rate of 10°C min1. The mechanical properties were measured using dynamic mechanical analyser (DMA 850, TA Instruments). SEM images were obtained by a ZEISS Sigma 300 FE SEM system with EDX equipped. The sponge samples were sputtered with 5 nm gold or carbon before observation. Transmission electron microscopy (TEM) is conducted by a JEOL JEM-2200FS electron microscope (JEOL Ltd., Tokyo, Japan) at 200 kV, equipped with a Direct Electron DE-16 camera (Direct Electron, LP, San Diego, CA, USA.) Theconcentration of metals permeate through the compressed scaffold and the concentration of metals adsorbed by scaffold was measured by a Perkin Elmer Avio 500 Inductively Coupled Plasma-Optical Emission Spectrometer (ICP-OES). Before elemental analysis, scaffold was digested with HNO3 / HCI (3:1) in microwave at 240°C for 15 minutes and topped up to 10 mL with H2O. Note that a clear solution was observed before analysis. Quartz-Crystal Microbalance (QCM) measurements were performed by the QSense Explorer System (QE 401 Electronic Unit, QCP 101 Chamber Platform, QFM 401 Flow Module). A 5 MHz Au electrode was used with the pump speed of 30 uL min1. Thin films for QCM measurements have the same composition ratio as GO / CS sponge but only include 0.1 mg GO to achieve tight attachment on the Au electrode.

[0167] DFT computation method

[0168] The calculations were carried out with the Gaussian 16 software package with the tight self-consistent field procedure and ultrafine integration grid. All calculations were produced via B3LYP method and Def2SVP basis set. The geometry optimizations were performed via the Berny algorithm with 1 *105root mean square force criterion (tight Opt). Freq command was used for the computation of force constants and vibrational frequencies to perform the correction for thermochemistry parameters.

[0169] Spectroscopic ellipsometry of GO / CS membranes

[0170] To investigate distribution of gold inclusions extracted by GO / CS membranes, variable angle spectroscopic ellipsometry measurements on samples that were subject to different gold concentrations was performed. Spectroscopic ellipsometry records optical spectra with higher accuracy than absorption spectroscopy and allows one to extract optical constants of the studied structures. The ellipsometric parameters T (ellipsometric reflection) and A (ellipsometric phase) were measured using a J. A. Woollam ellipsometer in the 300-1600 nm wavelength range. The ellipsometric parameters1and A are related to the sample amplitude reflections as tan('P) exp(iA) = rp / rs, where rpand rsare the amplitude reflection coefficients for p- and s-polarized light, respectively. Both functions (^ and A) strongly depend on the optical properties of investigated samples and can be used to extract optical constants of investigated layers. To determine the complex refractive index N = n + ikof the studied GO / CS membranes (withor without extracted gold), ellipsometric functions

[0171]

[0172] and A were experimentally measured and fitted using a Fresnel model.

[0173] Representative ellipsometric spectra of GO / CS samples subject to the solutions of different gold concentrations are shown in Figure 9. Combined with the ellipsometric phase A, these spectra allowed us to retrieve the optical constants of the samples. Figure 9b shows the extracted optical constants (n and k) for a GO / CS matrix. We see that the spectra of the optical constants are reasonably flat in the studied range of 300-1600 nm for pristine GO / CS samples. After being subject to modest Au concentration of 0.01 M, a GO / GS sponge extracted gold in the form of Au nanoparticles of different shapes and sizes. As a result, the localized surface plasmon resonance (LSPR) of these Au nanoparticles can be clearly seen as a peak in the absorption coefficient k in the extracted optical constants of GO / GS samples subject to low concertation of Au, as shown in Figure 9c. This LSPR peak is quite wide and shifted to red wavelengths as compared to pristine gold. Since both shape and size of the Au nanoparticles (as well as the matrix environment!) can affect the plasmon resonance, it is difficult to assign the widening of the LSPR peak to either shape or size of Au nanoparticles. From inset of Figure 9c we can assume that both factors are important. At higher Au concentrations, Au nanoparticles extracted with the help of a GO / GS membrane could also produce a connected network which normally results in the Drude behaviour associated with the metal response at infrared wavelengths. Hence, for a GO / GS sample subject to 0.034M of Au concentration, we see both the LSPR at around 600 nm wavelengthsand the Drude response at the infrared range (n and k rising with increasing wavelengths), as shown in Figure 9d.

[0174] To conclude, the approach focuses on nanoscale cross-dimensional composite material designed to chemisorb Au ions and accelerate catalytic reduction within nanocompartments. Two-dimensional graphene oxide and one-dimensional chitosan are combined due to their selective ionic transport, catalytic properties, and complementary functionalities. The self-organized composites exhibit heterogeneous surface chemistry, enabling the chemical reduction of Au ions and proton transfer reactions to return catalytic amino groups to their initial state. The study of the thermodynamics and kinetics of chemisorption reveals a spontaneous exothermic process with cooperative contributions from multiple functional groups. As a result, the ionic transport and catalytic properties of the composite surpass those of individual GO and CS components. Thematerials demonstrate exceptional affinity for Au(l) and Au(lll) ions, surpassing existing technologies. They are capable of chemisorbing 16.8 g / g of Au(lll) and 6.2 g / g of Au(l) within 10 minutes. This work provides a sustainable solution for gold recovery from e-waste, contributing to both environmental preservation and resource utilization.

[0175] Whilst the foregoing description has described exemplary embodiments, it will be understood by those skilled in the technology concerned that many variations may be made without departing from the present invention.

Claims

Claims1 . A composite material for extracting gold, the composite material comprising:- a 2-dimensional (2D) material; and- a 1 -dimensional (1D) macromolecule,wherein the composite material comprises a cross-dimensional structure for adsorption of gold.

2. The composite material according to claim 1 , wherein the cross-dimensional structure is formed by self-assembly of the 2D material and the 1D macromolecule into self-assembled nanolayers with nanoconfinements.

3. The composite material according to claim 1 or 2, wherein the 2D material is a graphene-based material.

4. The composite material according to claim 3, wherein the graphene-based material is graphene oxide (GO).

5. The composite material according to any preceding claim, wherein the 1 D macromolecule is an amine-group containing polymer.

6. The composite material according to claim 5, wherein the 1 D macromolecule is chitosan.

7. The composite material according to any preceding claim, wherein the extracting gold comprises extracting gold ions and reducing the gold ions into gold.

8. The composite material according to any preceding claim, wherein the composite material has an extraction efficiency of > 90 wt. %.

9. The composite material according to any preceding claim, wherein the composite material is in the form of a sponge or membrane.

10. The composite material according to claim 9, wherein the composite material is in the form of a sponge, the sponge comprising pores.

11. The composite material according to claim 10, wherein the pores have an average pore size of 1 -200 pm.

12. The composite material according to claim 9, wherein the composite material is in the form of a membrane, the membrane being a multi-layer composite membrane.

13. The composite material according to claim 12, wherein the interlayer distance between each layer comprised in the multi-layer composite is 9-13 A.

14. The composite material according to claim 12 or 13, wherein the membrane is comprised in a 2D electrode.

15. The composite material according to any preceding claim, wherein the composite material is reusable.

16. A method of forming the composite material according to any preceding claim, the method comprising:- forming a dispersion of 1 D macromolecule in acid; and- mixing the dispersion of 1 D macromolecule with an aqueous dispersion of 2D material in a pre-determined ratio to form the composite material.

17. The method according to claim 16, wherein the forming comprises mixing the 1 D macromolecule and the acid under magnetic stirring for a pre-determined period of time at room temperature.

18. The method according to claim 16 or 17, wherein the mixing comprises mixing the dispersion of 1D macromolecule with an aqueous dispersion of 2D material in a shaker.

19. The method according to any of claims 16 to 18, wherein the method further comprises freeze-drying the composite material formed following the mixing to form a sponge.

20. The method according to any of claims 16 to 18, wherein the method further comprises vacuum filtration of the composite material formed following the mixing through a membrane filter to form a composite membrane.

21. The method according to claim 20, wherein the method further comprises drying the composite membrane.

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