PROCESS TO OBTAIN GOLD PARTICLES FROM A SOLUTION

TECHNICAL FIELD

[0001] The present disclosure relates to a process to obtain metallic gold (Au(0)) nanoparticles (NPs) from solutions containing gold(lll), comprising the use of hydroalcoholic extracts of red fruit bush leaves, preferably leaves from Rubus idaeus L. (raspberry bush).

BACKGROUND

[0002] Precious metals, such as gold (Au), have been subject of several studies, since they are economically important, not only historically as currency but also as investment goods. Thus, and due to the limited availability of the precious metals, their recovery from aqueous solutions, namely solutions resulting from the leaching of wastes (secondary sources) is economically attractive (Das, 2010). Moreover, their recovery as nanoparticles (NPs) is particularly interesting and advantageous, given their extensive applications resulting from specific properties determined by the nanometric size.

[0003] The gold NPs have wide application, particularly in DNA recognition, in hereditary medicine, in hyperthermia of tumor, optical coatings, scanning tunneling microscopes as conductive tips, as well as in catalysis (Lee et al., 2022; Cai et al., 2011; Shankar et aL, 2005). The conventional physical and chemical methods used in the synthesis of gold NPs are frequently toxic to the environment due to the use of toxic chemicals and reagents, or expensive due to the use of extreme temperature and pressure conditions, which makes the processes more complex and energy dependent. On the other hand, the biosynthesis of metal NPs is usually high selective and can operate at room temperature and pressure, without requiring the use of synthetic chemical reagents addition (Cai et al., 2011). Indeed, processes relying on plant-based materials may not require the addition of stabilizing agents, since the phytochemicals can act both as reducing and stabilizing/capping agents. [0004] Therefore, the interest in developing "green" technologies for the synthesis of gold NPs has increased and the use of waste, namely the use of biological waste for this purpose has a particular importance since it benefits the desired circular economy. Thus, many biological systems have been investigated aiming the biosynthesis of gold NPs, including systems based on the use of fungi (Bhainsa and Souza, 2006; Senapati et al., 2005; Ahmad et al., 2003), algae (Singaravelu et aL, 2007; Mata et al., 2009), bacteria (He et al., 2007; Shahverdi et al., 2007) and plants (Sathishkumar et aL, 2009a; Sathishkumar et al., 2009b; Shankar et al., 2003), as well as plant residues (Ahmed et al., 2016).

[0005] The utilization of different parts of plants, such as fruits, flowers, leaves, barks, pods, shells, husks, roots, stalks has been reported. Demirbas et al. ,2019 reported the use of fruit extracts on the synthesis of gold NPs. The anthocyanin-rich berry extracts, prepared from red fruits such as Rubus idaeus (red raspberry), Fragaria ananassa (strawberry), Rubus fruticosus (blackberry), were used on the synthesis of gold nanoparticles since the anthocyanins present in the extracts preferentially made stable complexes with Au ³⁺ ions and resulted in the formation of gold NPs. However, of main environmental interest and economic attractivity is the possibility of applying nonused waste (Ahmed et al., 2016), thus allowing the development of low-cost processes and contributing for the circular economy.

[0006] The process described in the present disclosure allows to overcome the disadvantages of the chemical processes of synthesis and it is a viable alternative to obtain gold NPs with low cost, room temperature and atmospheric pressure, by means of benefiting from the use of a biologic waste.

[0007] These facts are disclosed in order to illustrate the technical problem addressed by the present disclosure.

GENERAL DESCRIPTION

[0008] The present disclosure relates to a process for obtaining metallic gold particles without the use of synthetic chemical reagents and/or extreme temperature and pressure conditions. [0009] The present disclosure demonstrates the capacity of an aqueous extract obtained from leaves of raspberry plants, strawberry plants, or blueberry plants, preferably leaves of Rubus idaeus L., to recover gold from solutions with different complexity (unimetallic and multimetallic), reducing it and allowing its recovery as Au(0) particles, in particular NPs.

[0010] In an embodiment, this process, using a leaves extract of Rubus idaeus L., can replace chemical processes that use reagents, such as NaBF , citrate and ascorbate for the same purpose. Moreover, considering that the process is based on the utilization of plant residue, which by this way is used and valorized, the process will certainly be more advantageous from environmental and economic points of view.

[0011] In an embodiment, metallic gold nanoparticles are obtained after treatment of a gold-containing solution with a hydro-alcoholic extract of leaves from Rubus idaeus L. plant (raspberry bush).

[0012] In an embodiment, the biological recovery process described in the present disclosure is suitable to reduce Au(lll) present in an aqueous solution, and recover it as Au(O)particles, preferably NPs, for subsequent applications.

[0013] In an embodiment, the extract was added to a chloride solution containing Au(lll) as a single metal ion (unimetallic solution) in a concentration of at least 1 mg/mL, preferably ranging from 1 mg/L to 200 mg/L, more preferably 30 mg/L to 100 mg/L, even more preferably 100 mg/L. In another embodiment, the extract was added to a more complex multimetallic solution, such as, for example, a solution resulting from the leaching of Printed Circuit Boards (PCBs), in batch, allowing to obtain nanoparticles of Au(0) with a yield higher than 95%.

[0014] In an embodiment, the hydroalcoholic extract from the leaves of the plant Rubus idaeus L. (raspberry bush), which constitute a residue resulting from the production of its fruit (raspberry), is a solution with anti-oxidant activity, that can be used for the reduction of gold(lll) present in aqueous solutions, thus allowing the recovery of Au(0) in the form of particles, preferably nanoparticles (NPs). In an embodiment, the extract can be used for the reduction of gold from unimetallic gold(lll) solutions or from multimetallic solutions, such as a solution resulting from the leaching of PCBs.

[0015] In an embodiment, the plant extract has the ability to reduce gold(lll) from aqueous solutions, with gold recovery yields greater than 95% from solutions containing a concentration of gold(lll) ranging from 1 mg/L to 200 mg/L, preferably 100 mg/L. Thus, it is possible to recover gold from an aqueous solution, in the form of Au(0) particles, in particular NPs.

[0016] In an embodiment, the extracts result from the use of a biological residue, with reduced cost, thus contributing to the low cost of the overall process of gold(lll) recovery from aqueous solutions. Furthermore, it is possible to recover gold in the form of Au(0) NPs, for which there are several applications described in the literature, making this an attractive process from an economic point of view.

[0017] In an embodiment, the process for the production of metallic gold particles, in particular nanoparticles, comprises the following steps: i) obtaining the residues of leaves of Rubus idaeus L. ii) preparing a hydroalcoholic extract from the leaves; iii) mixing the hydroalcoholic extract with a solution containing gold, in particular gold(lll); iv) collecting the Au(0) particles, preferably Au(0) nanoparticles.

[0018] In an embodiment, the hydroalcoholic extract is added to unimetallic aqueous solutions containing 100 mg/L Au(lll) by means of drop-by-drop addition, in a ratio of volume of extract/volume of Au(lll) ranging from 1:10 to 1:1.

[0019] The present disclosure relates to a process for obtaining metallic gold particles from an aqueous solution comprising gold, wherein the process comprises a step of mixing the aqueous solution comprising gold with an aqueous extract obtained from leaves, wherein the leaves are selected from plant selected from a list consisting of: raspberry plant, strawberry plant, blueberry plant, and combinations thereof.

[0020] In an embodiment, the aqueous extract is obtained from leaves of raspberry plant, preferably red raspberry (Rubus idaeus L.). [0021] In an embodiment, the aqueous extract obtained from leaves is an hydroalcoholic extract.

[0022] In an embodiment, the aqueous extract obtained from leaves is prepared by mixing dried and grinded leaves with an aqueous ethanol solution in a solid/liquid ratio of 1:10 (mass/volume), for 30 minutes at a temperature of 60 °C.

[0023] In an embodiment, the gold present in the aqueous solution comprising gold is gold(lll).

[0024] In a further embodiment, the process comprises the following steps: obtaining the aqueous extract from leaves of a raspberry plant, a strawberry plant, or a blueberry plant, preferably a raspberry plant; adding the aqueous extract from leaves to the aqueous solution comprising gold, to form a mixture; leaving the mixture until gold particles are formed, preferably for at least 24h, more preferably for 24 to 48h; centrifuging the mixture to obtain a pellet with gold particles and a supernatant, preferably centrifuging at 2500g for 30 min; separating the pellet from the supernatant, preferably by decanting; washing and disintegrating the pellet into gold particles with a solution of ethanol and acetone, preferably a 1:1 (volume:volume) solution of ethanol and acetone; drying the washed gold particles.

[0025] In an embodiment, the aqueous solution comprises at least lmg/L of gold.

[0026] In another embodiment, the aqueous solution comprises 1 mg/L to 200 mg/L of gold.

[0027] In yet another embodiment, the aqueous solution comprises 30 mg/L to 100 mg/L of gold. In a preferred embodiment the aqueous solution comprises 100 mg/L of gold.

[0028] In an embodiment the aqueous solution comprising gold is a multimetallic solution comprising gold, silver, palladium, lead, copper, zinc, iron, aluminium, and nickel.

[0029] In an embodiment the aqueous solution comprising gold is a leachable from an electronic equipment or electronic component. [0030] In an embodiment the process further comprises a step of removing lead from the precipitate (pellet) with gold particles.

[0031] In an embodiment the aqueous solution comprising gold and the aqueous extract are mixed in a ratio of volume of extract/volume of gold ranging from 1:10 to 1:1, preferably 1:10.

[0032] In an embodiment the particles are nanoparticles.

[0033] In an embodiment the nanoparticles size ranges from 30 to 90 nm, preferably 41 to 70 nm, more preferably 51 to 60 nm; this is at least one dimension of the nanoparticles. The size of the nanoparticles may be measured by several methods, in the present disclosure the nanoparticle size was measured by TEM.

[0034] In an embodiment at least 95% of the gold present in the aqueous solution comprising gold is recovered in the form of particles, preferably in the form of nanoparticles.

BRI EF DESCRIPTION OF THE DRAWINGS

[0035] The following figures provide preferred embodiments for illustrating the disclosure and should not be seen as limiting the scope of invention.

[0036] Figure 1: Embodiment of a process to obtain gold nanoparticles (NPs) in batch using an extract of leaves of Rubus idaeus L. (raspberry leaf extract).

[0037] Figure 2: Embodiment of the percentage of Au recovery after 48h from an unimetallic aqueous solution (in batch) by the addition drop by drop and under stirring of the Rubus idaeus L. (raspberry) leaves extract to the Au(lll) solution at different volume to volume ratios (1/1; 1/5 e 1/10) of both solutions.

[0038] Figure 3: diffractogram of X-Ray Diffraction (XRD) by the particles obtained in batch from the unimetallic solution. The diffraction peaks are consistent with the presence of Au(0) with cubic structure (JCPDS #01-071-4615).

[0039] Figure 4: Transmission Electron Microscopy (TEM) images (A) of the particles obtained in the batch assays from the unimetallic solution. Figure B shows the diffraction pattern that indicates the crystallinity structure of the particles, while in (C) the size distribution of the particles is shown.

[0040] Figure 5: spectrum resulting from the Energy Dispersive X-Ray (EDX) system associated to TEM, of the particles obtained in batch from the unimetallic solution. These particles are those that were observed by TEM in Figure 4.

[0041] Figure 6: percentage of Au recovery (in batch) after 48 h from a multimetallic aqueous solution resulting from PCBs leaching by the addition drop by drop and under stirring of the Rubus idaeus L. (raspberry) leaves extract to the Au(lll) solution at different volume to volume ratios (1/1; 1/5 e 1/10) of both solutions.

[0042] Figure 7: diffractogram of X-Ray Diffraction (XRD) by the particles obtained in batch from the multimetallic solution. The diffraction peaks are consistent with the presence of Au(0) with cubic structure (JCPDS #01-071-4614).

[0043] Figure 8: Transmission Electron Microscopy (TEM) images (A) of the particles obtained in the batch assays from the multimetallic solution. Figure B shows the diffraction pattern that indicates the crystallinity structure of the particles.

[0044] Figure 9: results obtained by Scanning Transmission Electron Microscopy coupled to Energy Dispersive X-Ray system (STEM-EDX) of the particles obtained in batch from the multimetallic solution. On the left figure 9A shows a STEM micrograph of a cluster of particles while on the right an EDX elemental mapping of those particles is displayed, identifying gold. Figure 9B shows the STEM-EDX spectrum, identifying gold as the main element of the particles.

DETAILED DESCRI PTION

[0045] The present disclosure relates to a process for obtaining metallic gold particles from an aqueous solution comprising gold, wherein the process comprises a step of mixing the aqueous solution comprising gold with an aqueous extract obtained from leaves, wherein the leaves are from raspberry plants, strawberry plants, or blueberry plants. [0046] In an embodiment, the disclosed process is for obtaining Au(0) particles, in particular nanoparticles (NPs), from an aqueous solution, using an extract of leaves of Rubus idaeus L. (raspberry bush) plant.

[0047] The extract of leaves of Rubus idaeus L. (raspberry bush) has no or low economic value. Thus, its application as described in the present disclosure allows the use of this residue, valorising it without requiring great intervention or adjustments before its use in a process that does not require extreme conditions and toxic or expensive chemicals.

[0048] Surprisingly, the contact of the Rubus idaeus L. leaves extract with a solution comprising gold(lll) results in the reduction of gold and thus, in the production of Au(0) particles, in particular NPs.

[0049] In an embodiment, the Au(lll) concentration in the aqueous solution is at least 1 mg/L. In another embodiment, the Au(lll) concentration in the aqueous solution ranges from 1 mg/L to 200 mg/L, preferably from 30 mg/L to 100 mg/L; even more preferably, the Au(lll) concentration in the aqueous solution is 100 mg/L.

[0050] In an embodiment, the solution containing gold(lll) was added to the extract. In another embodiment, the Rubus idaeus L. extract was added to the gold(lll)-containing solution. The addition is carried out drop by drop, under stirring. As the extract comes into contact with the solution comprising gold(lll), the gold(lll) is reduced and consequently precipitates, forming a brownish to purple precipitate.

[0051] Figure 1 shows an embodiment of gold removal and recovery from an aqueous solution using the leaves extract of Rubus idaeus L. performed in batch.

[0052] The present disclosure is more particularly described in the following examples that are intended as illustrative only since numerous modifications and variations are possible and will be apparent to those skilled in the art. [0053] Examples:

Example 1: Production of Au(0) NPs from an unimetallic solution of Au(lll) using an hydro-alcoholic extract of Rubus idaeus L leaves

[0054] In an embodiment, to obtain a hydroalcoholic extract, the leaves of Rubus idaeus L. are washed with water to remove impurities; preferably the leaves are washed three times to remove impurities. Then, the washed leaves are dried, preferably at 40 °C, in an oven until stabilization of the leave's weight. The obtained dry leaves are grinded to obtain a fine powder, which is then mixed with an aqueous ethanol solution, preferably a 50% (v/v) solution of ethanol in water, in a solid/liquid ratio of 1:10 (mass/volume), for 30 minutes at a temperature of 60 °C. The mixture is stirred, centrifuged, decanted to reject any remaining solid, and filtered, preferably with a Whatmann n^ 1 filter or equivalent, to ensure the separation and removal of possible solid particles entrained in the extract that may decrease the purity of the obtained particles. The obtained filtered extract is stored at 4°C until further use.

[0055] In an embodiment, the unimetallic aqueous solution is prepared by the dilution of a commercial standard solution of HAuC in HCI. In a further embodiment, the unimetallic aqueous solution with 100 mg/L Au(lll) is prepared by a 1:10 dilution of a commercial standard solution of 1000 mg/L HAuCI n 2 M HCI, in 0.02 M HCI.

[0056] In an embodiment, 5 mL of the extract from Rubus idaeus L. leaves was added, drop by drop under stirring using a syringe, to a flask of 100 mL containing 50 mL of a 100 mg/L Au(lll) solution. The final volume ratio between the extract solution and the Au(lll) solution was 1/10. The mixture remained in batch for 48 h, and the resulting pellet was collected.

[0057] In an embodiment, after 24 to 48 h, preferably 48 h, of contact between the hydroalcoholic extract and the gold-containing solution, the whole solution is centrifuged and the pellet, which contains the Au(0) particles, is collected. In a further embodiment, the whole solution is centrifuged at 2500g for 30 min. In a yet further embodiment, after the centrifugation the pellet is separated from the supernatant by decanting. [0058] In an embodiment, the pellet containing the Au(0) particles is washed with a 1:1 (volume:voliime) mixture of ethanol and acetone, to remove the impurities/compounds related to the biological material (raspberry plant leaves) and to disintegrate the pellet. Then, the ethanol/acetone solution is removed, and the particles are air dried. A gold recovery yield in the form of nanoparticles of 95.0 ± 0.3% was calculated by mass balance, from the initial and final gold concentration in the liquid phase.

[0059] In the state of the art, particles, in particular NPs, may be characterized by different methods and regarding different aspects, such as morphology, size and chemical composition. These properties are important to determine the potential functionalities of the NPs, and several techniques are available in the state of the art to perform such characterization.

[0060] In an embodiment, the dried pellet was subsequently analysed by X-Ray Diffraction (XRD) and by Transmission Electron Microscopy coupled to Energy Dispersive X-Ray Spectroscopy (TEM-EDX).

[0061] In an embodiment, the XRD analysis was performed using a PANalytical X'Pert Pro powder diffractometer operating at 45 kV and 35 mA with Cu Ka radiation filtered by Ni with a X'Celerator detector with a step size (20) of 0.03° and a time per step of 1000 s. Peak analysis and the identification of crystalline phases were based on comparison using High-Score Plus software with the ICDD PDF-2 database. Peaks were indexed to JCPDS reference number 01-071-4615. From the obtained diffractogram (Figure 3) it is possible to conclude that the obtained peaks are consistent with the presence of Au(0), with cubic structure (JCPDS #01-071-4615).

[0062] In an embodiment, the results obtained by TEM analysis, using a Hitachi H8100 with a LaB6 filament, of particles applied to copper grids (Figure 4) show particles with different sizes and morphologies, consisting mainly of spherical NPs (Figure 4A), with a size distribution between 50 and 70 nm (4C), although other forms and some agglomeration or aggregation (4A) were visualized. The diffraction pattern, shown in Figure 4B, confirms the particles crystallinity. [0063] In an embodiment, the analysis of the particles by EDX, using a ThermoNoran detector coupled with TEM, (Figure 5) allows to identify gold, which is consistent with the existence of Au(0). Copper was also identified, due to the sample support material used to perform the analysis. No other element was detected, showing that no contamination resulting from the use of the leaves extract occurs.

[0064] This assay demonstrates the ability of the Rubus idaeus L. leave extract to precipitate gold as Au(0) NPs from an Au(lll) solution, in a batch process.

Example 2: Production of Au(0) (nano)particles from a multimetallic solution from PCBs leaching using hydro-alcoholic leaves extract of Rubus idaeus L.

[0065] In another embodiment, the process can also be applied to multimetallic solutions containing dissolved Au, such as solutions resulting from the leaching of electronic material, namely printed circuits (PCBs) with aqua regia.

[0066] In an embodiment, the hydroalcoholic extract from Rubus idaeus L. (raspberry) leaves is added dropwise, with stirring, to a multimetal solution of Au, resulting from a 1:6 (v:v) dilution of a PCB leach solution in distilled water. In a further embodiment, the Rubus idaeus L. leaves extract was added, drop by drop under stirring using a syringe, to a multimetallic solution using a volume/volume ratio of hydroalcoholic extract to multimetallic solution ranging from 1:1 to 1:10, preferably 1:1, 1:5 and 1:10. The multimetallic solution used in the mixture results from leaching of PCBs with aqua regia (3 HCI:1 HNO3), after a first leaching step with 2M sulfuric acid and 0.2M hydrogen peroxide to remove copper (present in the PCBs in relatively high quantity), making the gold parts more exposed. The composition of the obtained solution, after dilution of the aqua regia leachate with distilled water by a dilution factor of 1:6, is described in Table 1. Table 1: Typical composition of the m u Iti meta I lie solution obtained from PCBs leaching with aqua regia, after a first leaching step with 2M sulfuric acid and 0.2M hydrogen peroxide to remove copper and expose gold parts.

Concentration

Element

(mg/L)

Au 232

Ag 26

Pd 11

Cu 151

Pb 15030

Zn 4

Fe 9

Al 57

Ni 110

[0067] In an embodiment, the extract and the multimetallic solution remains in batch for 48 h, and the particles formed during the contact time are recovered as described for unimetallic solutions.

[0068] As depicted in Figure 6, the percentage of gold recovery from the multimetallic solution is higher than 95 %, for any of the mixture ratios used, showing the capacity of the Rubus idaeus L. leaves extract to recover Au from a real and complex solution (multimetallic). However, the recovery of gold particles is contaminated by lead due to a high percentage (> 80%) of simultaneous precipitation of this metal (which is highly concentrated in the multimetallic leachate). These particles, after separation from the multimetallic solution treated with the extract by centrifugation (2500g for 30 minutes), washing with a mixture 1:1 volume/volume of acetone and ethanol and air dry, reveal by XRD analysis the presence of Gold and Lead Sulphate.

[0069] In an embodiment, to remove the lead present in the obtained pellet of particles at this stage, the particles were washed with 1 M HCI using a solid/liquid ratio of 1:1000 (mass/volume). After this lead removal step, the final particles are separated by centrifugation (2500g for 30 minutes), the pellet is dried, washed with a mixture 1:1 volume/volume of acetone and ethanol, and dried again.

[0070] In an embodiment, after being separate and dried, the resulting particles were analysed by XRD and TEM-EDX. The peaks of the diffractogram displayed in Figure 7, are consistent with the presence of only Au(0), of cubic structure (JCPDS #01-071- 4614), suggesting the removal of lead by the washing step, since this element, contra rily to that one observed before washing, was not anymore detected.

[0071] The results obtained by TEM, Figure 8, show micrographs of the Au(0) particles (A), showing individual particles and likely agglomerates or aggregates. The individual particles show what looks like a cubic morphology with sizes lower than 50 nm. The diffraction pattern that is shown in Figure 8B confirms the particles crystallinity.

[0072] The analysis of the particles by STEM-EDX, figures 9A (STEM micrograph on the left and EDX mapping on the right) and 9B (EDX spectrum), allows to identify gold, which is consistent with the existence of Au(0). The presence of some contaminants cannot be excluded by the analysis of Figure 9B; however, the much larger sizes of gold peaks compared to the contaminants' peaks indicate a low level of contamination. The use of external capping/stabilizing agents and deagglomeration and/or disaggregation processes such as ultrasounds may be used for the deagglomeration and/or disaggregation of clustered particles.

[0073] The term "comprising" whenever used in this document is intended to indicate the presence of stated features, integers, steps, components, but not to preclude the presence or addition of one or more other features, integers, steps, components or groups thereof.

[0074] The disclosure should not be seen in any way restricted to the embodiments described and a person with ordinary skill in the art will foresee many possibilities to modifications thereof. The above described embodiments are combinable.

[0075] The following claims further set out particular embodiments of the disclosure. References

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