Selective Recovery of Gold from Electronic Waste by New Efficient Type of Sorbent

Modular connectors are applied by computer users, and they can be metallic secondary sources containing metals such as gold and copper. Because gold is a micro-component, the solution obtained after the pin digestion contains a low concentration of gold(III) ions, and efficient and selective sorbent should be used for gold(III) ion recovery. The selective removal of small amounts of gold(III) from 0.001–6 M hydrochloric acid solutions using pure and solvent-impregnated macroporous polystyrene crosslinked with divinylbenzene sorbents (Purolite MN 202 and Cyanex 272) is presented. Gold(III) ions were recovered effectively from the chloride solution after the digestion of the modular connector RJ 45 (8P8C) using Purolite MN 202 after the impregnation process. The dependence of the recovery percentage (R%) of gold(III) on the contact time was determined. The highest value of gold(III) ion sorption capacity (259.45 mg·g−1) was obtained in 0.001 M HCl for Purolite MN202 after the Cyanex 272 impregnation. The results can be applied to gold recovery from e-waste. The presented method of gold recovery does not generate nitrogen oxides and does not require the use of cyanides.


Introduction
In addition to being prized for millennia, recently precious metals have been employed in a great number of technical applications due to their unique physical and chemical properties. The main applications of platinum and palladium are in electronic and catalytic processes in the chemical industry, and the number of chemical processes based on the palladium or platinum-catalysed reactions is still growing. These include hydrogen synthesis and catalytic conversion. Gold has been used for a long time as currency as well as in the electronic, medical, and jewellery industries [1].
In recent years, scraps containing gold are more sought after than ever before as a source of income. They can originate from different sources such as jewellery and spent catalysts as well as waste electrical and electronic equipment (WEEE), called "E-waste", which is considered to be one of the fastest growing waste streams in Europe [2][3][4]. It is estimated that the amount of WEEE generated in 2023 will reach 61.3 million tonnes [5].
Prices for noble metals fluctuate drastically. The price of gold in December 2022 was USD 1750 per troy ounce, and in 2023 it may rise to USD 3000. High prices as well as scarcity contribute to the search for new methods of precious metal recovery, as traditional methods of recovery require aggressive reagents that exert a negative impact on the natural environment [6]. Numerous techniques have been described for gold recovery from e-waste: pyrometallurgical processing, hydrometallurgical processing, or bio-hydrometallurgical processing [2,7].
Gold containing e-waste should be digested to extract gold into a solution. Both ionexchange and solvent extraction (SX) methods can be used for recovery and purification H2O2 and acetone from Stanlab, Poland, which were all reagent grade. Sodium hydroxide produced by Chempur, Poland, was analytically pure. Demineralized water was prepared by the Polwater system DL2-150, Poland. The gold(III) 1000 μg·cm −3 reference solution was produced by ROMIL. Modular connectors RJ 45 (8P8C) were obtained from spent computer connectors.

Preparation of Pure Resin
The Purolite MN 202 resin was washed three times with acetone to remove any impurities then rinsed thoroughly with the deionized water and air-dried at 298 K.

Preparation of Pure Resin
The Purolite MN 202 resin was washed three times with acetone to remove any impurities then rinsed thoroughly with the deionized water and air-dried at 298 K.

Preparation of the Impregnated Resin
A 15 g sample of resin prepared according to Section 2.4.1 was mixed with 6 g of Cyanex 272 (ratio Cyanex 272 g: Purolite MN 202 g-0.4:1) and stirred for 4 h. The mixture of impregnated resin and solution was separated by filtration and the solid was washed with distilled water to remove the impregnation mixture and then dried at 298 K.

Preparation of Pure Resin
The Purolite MN 202 resin was washed three times with acetone to remove any impurities then rinsed thoroughly with the deionized water and air-dried at 298 K.

Preparation of the Impregnated Resin
A 15 g sample of resin prepared according to Section 2.4.1 was mixed with 6 g of Cyanex 272 (ratio Cyanex 272 g: Purolite MN 202 g-0.4:1) and stirred for 4 h. The mixture of impregnated resin and solution was separated by filtration and the solid was washed with distilled water to remove the impregnation mixture and then dried at 298 K.

Surface Area
The samples were placed in a Micromeritics Accelerated Surface Area and Porosimetry ASAP 2405 instrument and degassed at 293 K. The surface area, total pore volume, and average pore diameter were determined for both Purolite MN 202 and Purolite MN 202 after the Cyanex 272 impregnation process.

Preparation of Metal Solutions
Hydrochloric acid solutions (0.001-6 M HCl) containing 100 µg·cm −3 gold(III) ions were prepared before the experiment. The real solution was prepared by leaching pins in the hydrochloric acid-hydrogen peroxide system.

Adsorption Studies
A 0.25 g portion of dry pure resin was placed in a 100 cm 3 Erlenmeyer flask closed with a ground-glass stopper. A 25 cm 3 aliquot of metal ion was added, and the flask was shaken for 1-360 min at 293 K using type 385 S laboratory shaker (produced in Poland). The solution was filtered and the gold(III) ion concentrations were determined using atomic absorption spectrometry (Varian 240). The same procedure was employed for the impregnated resin. The gold(III) ion concentrations in the solution after leaching pins were determined using the standard addition method to eliminate matrix interferences.

Desorption Studies
For the desorption studies, 0.1 g of sorbent (before and after the impregnation) was loaded with gold(III) using 10 cm 3 of 100 mg·dm −3 gold(III) solution in 3 M HCl, and an agitation period of 24 h was applied. Five percent thiourea in the 0.1 M HCl solution was used as the desorption agent. The gold(III) ion concentrations in the solutions were determined by the AAS method.

SEM
The scanning electron microscopy (SEM) measurements were conducted by means of the Quanta 3D FEG scanning electron microscope produced by the FEI Company (Lincoln, NE, USA).

Sorption Studies of Au(III) on Purolite MN 202
The recovery percentage (R%) of gold(III) was calculated using [21]: in which C is the concentration of adsorbed gold(III) ions calculated from the difference in the solution concentration before and after the sorption process, and C 0 is the initial concentration of gold(III) ions. The influence of the hydrochloric acid concentration on the ion sorption on Purolite MN 202 is illustrated in Figure 3.

Extraction Studies of Au(III) Using Cyanex 272 in Toluene
The extraction percentage (E%) for gold(III) were calculated from: in which C is the gold(III) ion concentration calculated from the difference in concentration before and after the extraction in the aqueous phase, and C0 is the initial concentration of gold(III) ions. The influence of HCl concentration on the gold(III) extraction using Cyanex 272 is presented in Figure 4.

Extraction Studies of Au(III) Using Cyanex 272 in Toluene
The extraction percentage (E%) for gold(III) were calculated from: in which C is the gold(III) ion concentration calculated from the difference in concentration before and after the extraction in the aqueous phase, and C 0 is the initial concentration of gold(III) ions. The influence of HCl concentration on the gold(III) extraction using Cyanex 272 is presented in Figure 4.

Extraction Studies of Au(III) Using Cyanex 272 in Toluene
The extraction percentage (E%) for gold(III) were calculated from: in which C is the gold(III) ion concentration calculated from the difference in concentration before and after the extraction in the aqueous phase, and C0 is the initial concentration of gold(III) ions. The influence of HCl concentration on the gold(III) extraction using Cyanex 272 is presented in Figure 4.   molecule is a donor of electron pair, whereas the gold(III) ion as a transition metal is able to accept the electron.

Determination of Cyanex 272 Concentration in Purolite MN 202
The titration experiment was conducted to estimate the amount of Cyanex 272 in Purolite MN 202. The titration endpoint occurred after the addition of 3.54 cm 3   The broad band at 3458 cm −1 is related to OH from water. The alkenyl C-H band occurs at 3024 cm −1 . The aromatic ring C=C stretching vibrations occur at 1604 cm −1 . The bands at 902 cm −1 and 762 cm −1 correspond to the aromatic =C-H deformation vibrations for the substituted benzene ring.

Surface Area
To identify the changes for Purolite MN 202 before and after the impregnation, the sorption studies employing nitrogen were carried out. The surface area of Purolite MN 202 was initially 818 m 2 ·g −1 , decreasing to 566 m 2 ·g −1 after the impregnation. The total volume of pores smaller than 968.8 Å was 0.547 cm 3 ·g −1 , decreasing to 0.428 cm 3 ·g −1 after the impregnation. The surface area and the total pore volume decreased because of the Cyanex 272 location in the sorbent pores.
The average pore size of the sorbent before the impregnation was 21.25 Å . This increased to 23.98 Å after the impregnation by Cyanex 272, indicating that the molecules of Cyanex 272 were located in the sorbent micropores.

Adsorption Investigations
The influence of hydrochloric acid concentration on the gold(III) ion sorption on Purolite MN 202 impregnated with Cyanex 272 is depicted in Figure 6. The impregnated The broad band at 3458 cm −1 is related to OH from water. The alkenyl C-H band occurs at 3024 cm −1 . The aromatic ring C=C stretching vibrations occur at 1604 cm −1 . The bands at 902 cm −1 and 762 cm −1 correspond to the aromatic =C-H deformation vibrations for the substituted benzene ring.

Surface Area
To identify the changes for Purolite MN 202 before and after the impregnation, the sorption studies employing nitrogen were carried out. The surface area of Purolite MN 202 was initially 818 m 2 ·g −1 , decreasing to 566 m 2 ·g −1 after the impregnation. The total volume of pores smaller than 968.8 Å was 0.547 cm 3 ·g −1 , decreasing to 0.428 cm 3 ·g −1 after the impregnation. The surface area and the total pore volume decreased because of the Cyanex 272 location in the sorbent pores.
The average pore size of the sorbent before the impregnation was 21.25 Å. This increased to 23.98 Å after the impregnation by Cyanex 272, indicating that the molecules of Cyanex 272 were located in the sorbent micropores.

Adsorption Investigations
The influence of hydrochloric acid concentration on the gold(III) ion sorption on Purolite MN 202 impregnated with Cyanex 272 is depicted in Figure 6. The impregnated sorbent was characterized by good sorption of Au(III) ions, which was dependent on the HCl concentration. The values of R% increased slightly with increasing HCl concentration. The highest sorption of gold(III) ions was observed in the HCl concentration range 3-6 M (R%-94-95%). The maximal removal of gold(III) ions in the hydrochloric acid concentration range 3-6 M is similar to the results obtained in the extraction process. After the impregnation, the presence of Cyanex 272 on the Purolite MN 202 surface affects the mechanism of gold(III) ion sorption. The high sorption values of R% in the 3-6 M HCl concentration range are related to the coordination process of gold(III) ions by the donor atoms such as oxygen in the Cyanex 272 structure. sorbent was characterized by good sorption of Au(III) ions, which was dependent on the HCl concentration. The values of R% increased slightly with increasing HCl concentration. The highest sorption of gold(III) ions was observed in the HCl concentration range 3-6 M (R%-94-95%). The maximal removal of gold(III) ions in the hydrochloric acid concentration range 3-6 M is similar to the results obtained in the extraction process. After the impregnation, the presence of Cyanex 272 on the Purolite MN 202 surface affects the mechanism of gold(III) ion sorption. The high sorption values of R% in the 3-6 M HCl concentration range are related to the coordination process of gold(III) ions by the donor atoms such as oxygen in the Cyanex 272 structure.

Kinetic Parameters
The sorption mechanism of Au(III) ions on pure and Cyanex 272-impregnated Purolite MN 202 was modelled assuming the pseudo-second-order kinetics of the form [22][23][24]: in which q2 is the amount of Au(III) ions sorbed at the equilibrium in mg·g −1 , qt is the amount of metal ions sorbed at the time t in mg·g −1 , and k2 is the pseudo-second-order equilibrium rate constant (g·(mg·min) −1 ). Integrating Equation (3) with the boundary conditions qt = 0 at t = 0 and qt = qt when t = t yields: Equation (4) can be written in the linear form as Equation (5) and the initial sorption rate h (mg·(g·min) −1 ) is described by: The kinetic parameters were calculated using the Microsoft Excel 2010 spreadsheet program.
The pseudo-second-order kinetic parameters obtained using Purolite MN 202 before and after the impregnation with Cyanex 272 for various HCl concentrations are presented in Table 1.

Kinetic Parameters
The sorption mechanism of Au(III) ions on pure and Cyanex 272-impregnated Purolite MN 202 was modelled assuming the pseudo-second-order kinetics of the form [22][23][24]: in which q 2 is the amount of Au(III) ions sorbed at the equilibrium in mg·g −1 , q t is the amount of metal ions sorbed at the time t in mg·g −1 , and k 2 is the pseudo-second-order equilibrium rate constant (g·(mg·min) −1 ). Integrating Equation (3) with the boundary conditions q t = 0 at t = 0 and q t = q t when t = t yields: Equation (4) can be written in the linear form as Equation (5): and the initial sorption rate h (mg·(g·min) −1 ) is described by: The kinetic parameters were calculated using the Microsoft Excel 2010 spreadsheet program.
The pseudo-second-order kinetic parameters obtained using Purolite MN 202 before and after the impregnation with Cyanex 272 for various HCl concentrations are presented in Table 1.
After the impregnation, the values for k 2 and h are smaller for gold(III) ions than before the impregnation process, possibly because the presence of Cyanex 272 exerts changes on the physicochemical properties of Purolite MN 202. In addition to changing the physical sorbent properties such as surface area, pore volume, and volume diameter, the presence of Cyanex 272 affects chemical properties, as well. The changes in the kinetic parameters after the sorbent impregnation by the extractant indicate that the sorption mechanism is different from that before the impregnation process. The molecules of Cyanex 272 present in the polymer can react with gold(III) ions by the coordination reaction.

Adsorption Isotherms Models
The Langmuir and Freundlich models are two of the most widely applied adsorption models [25]. Both were employed for characterization of gold(III) ion adsorption on Purolite MN 202 before and after the impregnation process. Linear regression was used to determine the most fitted isotherm. The linearized form of the Langmuir model can be written as: where q e is the amount of gold(III) ions in the adsorbent (mg·g −1 ); C e is the equilibrium concentration of gold(III) ions (mg·dm −3 ); b is the Langmuir isotherm constant (dm 3 ·g −1 ); and Q 0 is the maximum monolayer coverage capacity (mg·g −1 ). Further analysis of the Langmuir equation can be made based on the dimensionless equilibrium parameter, R L , also known as the separation factor.
where C 0 is the initial concentration of gold(III) ions solution (mg·dm −3 ). The Freundlich isotherm was calculated according to Equation (7): where q e is the amount of gold(III) ions in the adsorbent (mg·g −1 ); K F is the characteristic constant related to the adsorption capacity (dm 3 ·g −1 ); n is the adsorption intensity; and C e is the equilibrium concentration of gold(III) ions (mg·dm −3 ). The results of the calculated parameters using the Freundlich and Langmuir models are given in Table 2 for both impregnated and non-impregnated resins. The adsorption capacity for the gold(III) ions is higher for the impregnated resin than for the non-impregnated one. The high adsorption capacity is achieved by the impregnation process. The presence of bis (2,4,4-trimethylpentyl) This better fit of the equilibrium data to the Langmuir isotherm suggests monolayer coverage of Cyanex 272 on Purolite MN 202. Similar results were obtained in the other papers, where the experimental data of the adsorption equilibrium are obtained for Au(III) sorbed on the chemically modified cellulose [26] and N-carboxy methyl chitosan [27] correlated well with the Langmuir isotherm equation.

SEM Studies
The SEM images of Purolite MN 202 before and after the impregnation process are given in Figure 7a The SEM image of Au on Purolite MN 202 is given in Figure 7c. The SEM image of gold shows that the surface of Purolite MN 202 was covered with a thin layer of metallic gold. The SEM image of Au on Purolite MN 202 impregnated with Cyanex 272 is given in Figure 7d. In this case, gold creates small particles on the surface of the impregnated sorbent. The size of particles is about 500 nm. The comparison of two forms of reduced gold on the surface of impregnated and non-impregnated sorbents indicates that the impregnation process affects the form of gold. Formation of gold particles is due to the presence of Cyanex 272 on Purolite MN 202.

Recovery of Gold from the Modular Connector RJ 45 (8P8C)
The pins were recycled from the modular connector RJ 45 (8P8C) (an eight-wire connector used commonly to connect computers (LAN), especially Ethernets) ( Figure  8a,b)).  The SEM image of Au on Purolite MN 202 is given in Figure 7c. The SEM image of gold shows that the surface of Purolite MN 202 was covered with a thin layer of metallic gold. The SEM image of Au on Purolite MN 202 impregnated with Cyanex 272 is given in Figure 7d. In this case, gold creates small particles on the surface of the impregnated sorbent. The size of particles is about 500 nm. The comparison of two forms of reduced gold on the surface of impregnated and non-impregnated sorbents indicates that the impregnation process affects the form of gold. Formation of gold particles is due to the presence of Cyanex 272 on Purolite MN 202.

Recovery of Gold from the Modular Connector RJ 45 (8P8C)
The pins were recycled from the modular connector RJ 45 (8P8C) (an eight-wire connector used commonly to connect computers (LAN), especially Ethernets) (Figure 8a,b)). The SEM image of Au on Purolite MN 202 is given in Figure 7c. The SEM image of gold shows that the surface of Purolite MN 202 was covered with a thin layer of metallic gold. The SEM image of Au on Purolite MN 202 impregnated with Cyanex 272 is given in Figure 7d. In this case, gold creates small particles on the surface of the impregnated sorbent. The size of particles is about 500 nm. The comparison of two forms of reduced gold on the surface of impregnated and non-impregnated sorbents indicates that the impregnation process affects the form of gold. Formation of gold particles is due to the presence of Cyanex 272 on Purolite MN 202.

Recovery of Gold from the Modular Connector RJ 45 (8P8C)
The pins were recycled from the modular connector RJ 45 (8P8C) (an eight-wire connector used commonly to connect computers (LAN), especially Ethernets) ( Figure  8a,b)).   Table 3 presents the composition of the solution after the pin leaching process. The main components of the solution were copper(II) and nickel(II) ions. The content of gold(III) ions was 16.53 mg·dm −3 . The colour of the solution was dark green without residues after the digestion process. The hydrochloric acid-hydrogen peroxide system is efficient and does not need the use of nitric(V) acid. There is no emission of toxic nitrogen oxides. Moreover, the solution after leaching does not contain nitrate(V) ions. The sorption of gold(III) ions from the solution after the pin leaching was performed on Purolite MN 202 before and after the impregnation process. The results of the sorption process are presented in Figure 9.  Table 3 presents the composition of the solution after the pin leaching process. The main components of the solution were copper(II) and nickel(II) ions. The content of gold(III) ions was 16.53 mg·dm −3 . The colour of the solution was dark green without residues after the digestion process. The hydrochloric acid-hydrogen peroxide system is efficient and does not need the use of nitric(V) acid. There is no emission of toxic nitrogen oxides. Moreover, the solution after leaching does not contain nitrate(V) ions. The sorption of gold(III) ions from the solution after the pin leaching was performed on Purolite MN 202 before and after the impregnation process. The results of the sorption process are presented in Figure 9. As follows from the results, the concentration of gold(III) after the sorption process was lower for Purolite MN 202 impregnated with Cyanex 272 than for non-impregnated Purolite MN 202. This result confirms that the impregnation process is efficient.

Limitation of Purolite MM 202 Impregnated with Cyanex 272
The impregnated sorbent used in this paper shows good adsorption capacity for gold(III) ions. In acidic solutions after the digestion of connectors, the sorbent is selective and allows for the separation of gold(III) ions. One disadvantage resulting from the presence of Cyanex 272 can be the loss of selectivity in neutral and alkaline solutions. Cyanex 272 can extract copper(II) ions under these conditions.

Desorption Studies
The results of the desorption process are presented in Figure 10. As follows from the results, the concentration of gold(III) after the sorption process was lower for Purolite MN 202 impregnated with Cyanex 272 than for non-impregnated Purolite MN 202. This result confirms that the impregnation process is efficient.

Limitation of Purolite MM 202 Impregnated with Cyanex 272
The impregnated sorbent used in this paper shows good adsorption capacity for gold(III) ions. In acidic solutions after the digestion of connectors, the sorbent is selective and allows for the separation of gold(III) ions. One disadvantage resulting from the presence of Cyanex 272 can be the loss of selectivity in neutral and alkaline solutions. Cyanex 272 can extract copper(II) ions under these conditions.

Desorption Studies
The results of the desorption process are presented in Figure 10. As follows from the results, the desorption process was more efficient for Purolite MN 202 impregnated with Cyanex 272 (94.19%) than for non-impregnated Purolite MN 202 (86.51%). These results demonstrate the effectiveness of using the impregnated sorbent for the sorption and desorption of gold(III) ions. Given the good desorption efficiency of gold(III) ions and the high sorption capacity, it is possible to reuse the impregnated sorbent. However, industrial application can be possible after more extensive studies, including, e.g., 100 sorption-desorption cycles. As follows from the results, the desorption process was more efficient for Purolite MN 202 impregnated with Cyanex 272 (94.19%) than for non-impregnated Purolite MN 202 (86.51%). These results demonstrate the effectiveness of using the impregnated sorbent for the sorption and desorption of gold(III) ions. Given the good desorption efficiency of gold(III) ions and the high sorption capacity, it is possible to reuse the impregnated sorbent. However, industrial application can be possible after more extensive studies, including, e.g., 100 sorption-desorption cycles.

Conclusions
Currently, the amount of e-waste is constantly increasing; therefore, it is a source of valuable metals such as gold.
The results demonstrated that Purolite MN 202 impregnated with Cyanex 272 removes gold(III) ions effectively from modular connectors RJ 45 (8P8C) compared with non-impregnated Purolite MN 202. The results can be applied to gold recovery from e-waste.
The presented method of gold recovery overcomes the problem of emission of nitrogen oxides that can form during the dissolution of e-waste using the aqua regia.
In addition to acidic solutions, cyanides are also used for gold leaching [28]. Cyanide is considered to be a hazardous compound because of its toxicity. The method of gold dissolution and recovery presented in this paper does not require the use of toxic cyanides.

Conclusions
Currently, the amount of e-waste is constantly increasing; therefore, it is a source of valuable metals such as gold.
The results demonstrated that Purolite MN 202 impregnated with Cyanex 272 removes gold(III) ions effectively from modular connectors RJ 45 (8P8C) compared with non-impregnated Purolite MN 202. The results can be applied to gold recovery from e-waste.
The presented method of gold recovery overcomes the problem of emission of nitrogen oxides that can form during the dissolution of e-waste using the aqua regia.
In addition to acidic solutions, cyanides are also used for gold leaching [28]. Cyanide is considered to be a hazardous compound because of its toxicity. The method of gold dissolution and recovery presented in this paper does not require the use of toxic cyanides.