Selective hydrometallurgical recovery of gold by using low-cost biomass waste: Kiwi peel

Abstract

Utilization of kiwi peel (KP) is proposed for gold recovery from electronic wastes (e-wastes). KP was simply modified by dehydration pretreatment with concentrated sulfuric acid. Among of the e-wastes metals, modified KP exhibited high selectivity on Au(III) adsorption. Ions dependencies on adsorption revealed that the increased chloride ion concentration has inhibition to Au(III) adsorption. After Au(III) adsorption, microscope images found gold particles on the adsorbents surface. X-ray diffraction analyses suggested that Au(III) reduction occurred along with the adsorption process, till the adsorption reached the equilibrium. Modified KP exhibited higher adsorption capacity (5.71 mmol g^–1) than original KP (lower than 0.5 mmol g^–1) or other natural adsorbents. Modification improved the adsorption capacity of KP. Gold was successfully recovered from the mixed precious metals through the adsorption and elution. KP waste is an applicable adsorbent with rich resource and high adsorption capacity.

Keywords

e-wastes metal recovery solid waste treatment technology biomass wastes gold adsorption bio-adsorbent

Introduction

Gold has an exalted position throughout the ages, its superior intrinsic qualities and rare resource make it unique. Gold is used in the manufacture of various electronic devices such as cell phones, televisions, and computers, due to its superior malleability, resistance to corrosion, and high conductivity of electricity (Farideh et al., 2023; Rabinovich et al., 2011). The conductivity of gold is only followed by silver (the highest electrically conductive element), it is more often used as a conductor, due to its corrosion resistance, such as exposure to water (Goodman, 2002). With the rapid technical innovation, older electronic devices are replacing by new ones, thus dramatically electronic wastes (e-wastes) are being produced. According to Global E-waste Monitor 2024 report (ITU Council Geneva, 2024), from 2010 to 2022, the amount of e-waste generated has increased from 34 billion kg to 62 billion kg. But, the amount of e-waste documented to be formally collected and recycled has maintained at about 20%. The giant amount of e-waste production with poor recovery scale has become an urgent issue for environment and economic development. Most of e-wastes come from printed circuit boards and integrated circuit chips, which contain many valuable recyclable elements, such as precious metals (Bizzo et al., 2014; Guo et al., 2009). E-waste contains a much larger amount of gold than gold ore, it may be comparable to 7% of the world gold reserves (World Energy Resources, 2016). Therefore, e-waste has been focused as a future “urban mining” resource. Apparently, gold recovery from e-wastes is the paramount important to sustainable development with a limited resource.

For recovering gold from e-wastes, hydrometallurgical technology has extensively developed and achieved commercial success. In advanced hydrometallurgical processes, methods involved precipitation, solvent extraction, solid adsorption, and electrolysis are used for metal gradual separation. Classical precipitation methods were extensively applied for rough separation, such as Ag removal (Quinet et al., 2005). Metal composition in e-waste is complicated, gold needs to be isolated very selectively from other precious metals and base metals such as iron, copper, and that is often present in disproportionate amounts (Biswas et al., 2021a). Therefore, an efficient technique for gold recovery from a wide range of secondary sources is necessary. Solvent extraction or solid adsorption is mainly employed for individual metal recovery, because the extractants or adsorbents have specific selectivity and high efficiency on metal separation (Fan et al., 2019; Huang et al., 2020). Solid adsorption, with less consumption of organic solvent as well as simple synthesis of adsorbents, has been focused and frequently applied. Lately, hundreds of researches on biomass waste for metal adsorption have been reported. Bio-adsorbents are recognized as the commercial and environmentally friendly adsorbents (Biswas et al., 2021b; Ghomi et al., 2020; Wang et al., 2023; Zhang et al., 2020). Bio-adsorbents sourced from biomass waste are rich in active components, such as polysaccharides, proteins, and polyphenols. These compounds are thought to have a high ability to gold coordination (Liu et al., 2023; Xiang et al., 2022).

In this work, kiwi peel (KP) was investigated on selective gold recovery from precious metals. KPs are an example of biomass waste with excellent bioactive properties together with generation in significant quantities. Many literature reports have studied various substances and components in KPs. KP is predominantly composed of carbohydrates (about 77%), fats (about 3.7%), proteins (about 4% to 12%), and ash content (about 6.5%). Furthermore, it is enriched with minerals such as potassium, calcium, sodium, and phosphorus. Additionally, it boasts an array of antioxidant components, including polyphenols, flavonoids, and tannins (Salama et al., 2018; Soquetta et al., 2016). The related studies have reported that KP has higher polyphenolic content (60 mg g^–1) than kiwi fruit flesh. It also has a stronger antioxidant capacity than its flesh (Fiorentino et al., 2009; Hunter et al., 2011; Liang et al., 2021). Moreover, Al-Qahtani has reported that KP powder had the ability to heavy metals adsorption (Al-Qahtani, 2016). Global kiwi fruit production amounted to approximately 4.54 million metric tons in 2022. The high yield of kiwi fruits indicates the abundant source of KP (accounts for 10% of kiwi fruit) (Dias et al., 2020; Shahbandeh, 2022). Therefore, KP is a potential adsorbent for gold recovery from e-waste metals. However, most of biomass wastes are inevitably facing perishable problems. The short shelf life has limited their application.

In the present work, to promote KP resistance to deterioration, KP was pretreated with concentrated sulfuric acid. The natural KP and modified KP were investigated on base and precious metals adsorption. To study the adsorption mechanism, the ions dependencies on Au(III) adsorption were investigated. In addition, structural characterization of KP and modified KP were analyzed by IR spectra. The Au(III) adsorption capacity of adsorbents was evaluated by Langmuir model. The state of Au(III) after adsorption was studied by X-ray diffractometer (XRD) pattern and digital microscope images as well. In addition, the gold recovery from precious metals by adsorption and elution was studied to reveal the possibility of KP application.

Materials and methods

Materials and chemicals

The metal solutions are prepared with the copper chloride (CuCl₂), zinc chloride (ZnCl₂), iron(II) chloride (FeCl₂), iron(III) chloride hexahydate (FeCl₃·6H₂O), lead chloride (PbCl₂), hydrogen tetrachloroaurate(III) tetrahydrate (HAuCl₄·4H₂O), hydrogen hexachloroplatinate(IV) hexahydrate (H₂PtCl₆·6H₂O), palladium(II) chloride (PdCl₂) salts, respectively. Analytical grade of HCl, HClO₄, concentrated H₂SO₄ (17.97 M, 96%) solvents and those metal salts were purchased from Shanghai Aladdin Biochemical Technology Co., Ltd. Green kiwi fruits were purchased from the supermarket.

Preparation of adsorbents

The KP was obtained by hand removing and dried in the oven (CarboliteGero, HTMA 6/28) at 40°C for two weeks. Cut the dried KP into 3-mm-long pieces. For modification, the cut KPs (15 g) were stirred (Magnetic stirrer, IKA, RET control-visc) in 50 cm³ concentrated sulfuric acid at 90 °C for 12 h. After materials cooling down, the materials were filtered and washed with deionized water (300 cm³) for several times until the eluted solution became neutral. The products were dried at 60 °C and ground to powder. Sieved them through 150 μm mesh to unify the adsorbents size. The yield ratio of modified KP particles was about 73%. The dried KP pieces and prepared modified KP particles are used for the adsorption experiment.

Measurement

For metal concentration measurement, the initial solutions were diluted with 0.10 M HCl solution and measured using an inductively coupled plasma mass spectrometer (ICP-MS, iCAP RQ, Thermo Fisher). After adsorption, the collected metal solutions were also diluted and measured with ICP-MS. To study the structure of adsorbents before and after adsorption, the dried test samples were covered on the center of sample holder and measured using the XRD (Cu Kɑ, scanning speed of 12° min⁻¹ from 10° to 80°, Bruker Co. Ltd, D8 Advance). Mixed test samples with KBr powder in a certain proportion, ground them and pressed into tablets. The tablets were measured with Fourier transform infrared spectrometer (FT-IR, Tensor 27, BruBruker Co. Ltd). The morphological characterization of adsorbents and Au-loaded adsorbents were covered on the sample holder and measured using a digital microscope (KEYENCE, VHX-1000).

Adsorption experiment

A batch mode of operation was conducted to test various metals adsorption. Comprehensively consider the experimental equipment, reactor size, and experimental economy, the parameters selected for the adsorption experiment was definite. In a represent experiment, 20 mg of adsorbents were shaken together with 10 cm³ of each metal solution (1.0 mM) at 150 r/min, 30 °C for 48 h in hydrochloric acid media (0.1 M). For the time course dependency, each of 20 mg adsorbent was added into Au(III) solutions (10.0 cm³) and shaken at different time intervals. After the adsorption, the adsorbents were dried and analyzed by XRD and digital microscope.

For hydrochloric acid concentration dependency test, 10.0 mg of adsorbents were added to 1.0 mM Au(III) solution (10.0 cm³) with different concentrations of hydrochloric acid (0.10 M-5.5 M). For proton concentration dependency, 1.0 mM Au(III) solution (10.0 cm³ of 0.01 M HCl) in different concentrations of HClO₄ (0.10 M-5.0 M) was adjusted. For chloride concentration dependency, the solvent of Au(III) solution (10.0 cm³) was the mixture of HCl (3.0 M) and HClO₄ (3.0 M), the different volume ratios of HCl and HClO₄ were adjusted to control the chloride ion concentration. For adsorption isotherm experiments test, 10 mg of adsorbents were added to 0.10 M HCl solutions (10.0 cm³) with different Au(III) concentrations (1.0 mM–23.0 mM). After the filtration, the solution was diluted and measured Au(III) concentration.

For gold recovery test, modified KP was added into 10 cm³ of 1.0 mM mixed precious metal solution (Au(III), Pd(II), and Pt(IV)) in 0.1 M HCl solvent media. After 48 h adsorption, the adsorbents were directly washed three times with 10.0 cm³ deionized water. Subsequently, eluted them with 5.0 cm³ of 1.5 M thiourea in different concentration of HCl (0.5 M to 6.0 M) media and shaken for 2 h.

The adsorption percentage was calculated by equation (1),

% Adsorption = \frac{C_{i} - C_{e}}{C_{i}} \times 100 %

(1)

The metal adsorption amount on the adsorbents, q (mmol/g), were calculated by equation (2),

q = \frac{C_{i} - C_{e}}{W} \times V_{M}

(2)

where C_i (mmol dm^–3) and C_e (mmol dm^–3) are the initial and equilibrium concentrations of metal ions, respectively. V_M is the volume (dm³) of sample solution and W is the weight of adsorbent (g).

The eluted amount of gold from Au-loaded modified KP was calculated by equation (3),

% Elution = \frac{η_{E}}{η_{A d}} \times 100 %

(3)

where ɳ_E and ɳ_Ad individually stand for the eluted and adsorbed molar quantities of gold (mmol).

Results and discussion

Metal adsorption on adsorbents

Selective metal adsorptions on modified kp

Generally, e-wastes contain large amounts of base metals but a small amount of precious metals (Dutta et al., 2023; Ding et al., 2019). Therefore, high selective adsorption of adsorbents is crucial to gold recovery from e-waste metals. To study the selective adsorptions of the metals, the modified KP was individually soaked into each precious and base metal solutions. Figure 1 shows each metal adsorption percentage on modified KP, Au(III) adsorption was closed to 100%, which was much higher than those adsorptions of other metals. Apparently, modified KP performed specific selectivity on Au(III) ions, among of those metals. The high adsorption performance reveals the potential application of modified KP to gold recovery.

Figure 1.

Metal adsorptions on modified KP ([metal], 1.0 mM; [HCl], 0.1 M; shaking time, 48 h; adsorbents weight, 20 mg).

Time course dependency

Since modified KP exhibited high selectivity to Au(III) adsorption, Au(III) adsorption at different time intervals was investigated. Figure 2 shows the time of Au(III) adsorption on pure and modified KP. The adsorption on pure KP seemed to reach a plateau at 60 h. Interestingly, the time to plateau of adsorption on modified KP became shorter (48 h). It means modification has improved the adsorption efficiency. The adsorption rates for modified KP at 30 °C were analyzed in terms of the pseudo-first-order and pseudo-second-order adsorption kinetic models. According to the above equations, the equation kinetics has obeyed pseudo-send-order, the linearized model indicated that the R² value is 0.990. The effect of time on Au(III) adsorption on KP and modified KP together with the pseudo-second-order kinetic adsorption model Au(III) on modified KP is shown in Supplemental Figure S1. With the calculation of the linearized equation, the kinetic determined to be 1.28 × 10⁻⁵. Similar trends in kinetic models have been previously reported by other studies, indicating that both adsorbate and adsorbent participate in the surface reaction, which exhibits chemisorption nature (Cai et al. 2015; Ghosh et al. 2014; Tang and Zhang 2016). Adsorption on KP was hard to fit any of the models. The equations of the model are described in supplementary material.

Figure 2.

Time course dependency of Au(III) adsorption on adsorbents (adsorbents weight, 20 mg; [Au(III)], 1.0 mM; [HCl], 0.1 M).

Surface characteristics of adsorbents before and after adsorption

Structural characterization

During the Au(III) adsorption process, the structural characterizations on modified KP were analyzed. Figure 3 shows the patterns of modified KP at different adsorption time. Before adsorption (adsorption time is 0), the modified KP showed an amorphous structure with the wide peak. While, a faint peak appeared at 37.5° after 1 h soaking into Au(III) solution. Moreover, peaks at 37.5°, 44°, 64°, and 77° were appeared and gradually became stronger with the increase of soaking time. Those four peaks correspond to metallic gold structure (Zhu et al., 2024). The redox potential of Au(0) is relatively high, which means that Au(III) ion has a strong oxidizing ability in solution and is easily reduced to gold (Shimura and Yoshida, 2011). It illustrates that Au(III) reduction occurred with Au(III) adsorption process till the adsorption reach the equilibrium. When adsorption reached the equilibrium, the reduced gold particles had largely covered the adsorbent surface, therefore, the wide peak belonging to modified KP became weak and disappeared.

Figure 3.

XRD patterns of modified KP at different soaking time ([Au(III)], 1.0 mM).

Morphology characterization

After Au(III) adsorption, the adsorbents morphology was recorded by microscope image to further illustrate. It is clearly shown in Figure 4 that visible gold particles were observed on the KP (Figure 4(a)) and modified KP (Figure 4(b)) surfaces. KP contains various antioxidant contents (Cairone et al., 2022; Mattioli et al., 2024), therefore the active groups possibly released in the acid media and caused reduction of Au(III). In addition, modification has changed KP morphology, soft KP became hard and black by the dehydration with concentrated H₂SO₄.

Figure 4.

Microscopic images of gold-loaded (a) KP and (b) modified KP.

FT-IR analysis of adsorbents

To study the active functional group to Au(III) adsorption, the adsorbents were analyzed by FT-IR spectra. Figure 5 shows spectra of KP, modified KP, and Au-laden modified KP. KP has various chemicals composition, such as polysaccharides, polyphenol, protein, and ascorbic acid (Dias et al., 2020), it contains more than hydroxyl, ethereal, and carbonyl groups. After modification with dehydration, some of the compounds could be activated and exhibited good ability to Au(III) adsorption, such as polysaccharides (Pangeni et al., 2012). Therefore, their spectra are complicated, there showed broad peak on adsorbents from 3683 cm^–1 to 3010 cm^–1. After modification, the broad peak at 3450–3300 cm^–1 was assigned to O-H stretching vibration and became slightly sharper. It is possibly caused by the dehydration of hydroxyl group. After Au(III) loading, the C-O-C peak (1168 cm^–1) was shifted. It may be caused by the Au(III) coordination with ether group. It has been reported that crosslinked polysaccharides has the ability to Au(III) adsorption and reduction (Pangeni et al., 2012). The reduction of Au(III) possibly occurred with the oxidation of hydroxyl groups on modified KP. Polysaccharides are the main component in KP. Therefore, the oxidation of hydroxyl groups on polysaccharides may cause the appearance of new peaks at 934 cm⁻¹ and 635 cm⁻¹, which belong to the bending vibration of the C-H bond on the terminal carbon of sugar rings and the skeletal vibration of sugar rings (Hong et al., 2021).

Figure 5.

FT-IR spectra of (a) KP, (b) modified KP, and (c) Au-loaded (0.042 mmol) modified KP.

Mechanism study of Au(III) adsorption on modified kp

Ions concentration dependencies of Au(III) adsorption

To study the mechanism of Au(III) adsorption on modified KP, nonmetallic ions dependencies on adsorption were investigated. Figure 6(a) shows HCl dependency on Au(III) adsorption with modified KP. The Au(III) adsorption was decreased as the increased concentration of HCl. It was revealed the certain relation of adsorption process to concentration of proton or chloride ions. To further study, the dependency of proton concentration on Au(III) adsorption was investigated. Figure 6(b) shows Au(III) adsorption in different concentrations of HClO₄ media. Proton seems to have a bare effect on adsorption, modified KP exhibited high Au(III) adsorption performance at different region of proton concentration. The dependency of chloride ion concentration was studied as well. Figure 6(c) shows Au(III) adsorption on modified KP in mixed media of HCl (3.0 M) and HClO₄ (3.0 M). Unlike adsorption with proton charge, Au(III) adsorption was dramatically suppressed with the increased chloride ions concentration, it exhibited a monotonous decrease. Based on the close relation of chloride ions to Au(III) adsorption, the adsorption was expressed as equation 4:

{AuCl}_{4}^{-} + R_{1} - O - R_{2} \to [{Cl}_{(4 - n)} - Au - O - R_{1} R_{2}]^{(n - 1) +} + n {Cl}^{-} \begin{matrix} (1 \leq n \leq 4) \end{matrix}

(4)

Figure 6.

(a) Hydrochloric acid concentration, (b) proton concentration([Cl^–], 0.014 M) and (c) chloride ions concentration([H⁺], 3.0 M) dependencies of Au(III) adsorption on modified KP; (d) chloride concentration on distribution ration of Au(III) on modified KP ([Au(III), 1.0 mM; shaking time, 65 h).

Equilibrium constant is calculated by using equation 5,

K_{e x} = \frac{a {[{Cl}^{-}]}^{n}}{b {[{AuCl}_{4}]}^{-}} = \frac{(C_{i} - C_{e}) {[{Cl}^{-}]}^{n}}{b C_{e}}

(5)

where K_ex represents the equilibrium constant, C_i and C_e express the initial and equilibrium concentrations of Au(III) (mmol dm^–3), a and b stand for the molar amount of [Cl_(4–n)-Au-O-R₂]^(n–1)+ and R-O-R (mmol). The ratio of metal concentrations between solid and aqueous phases is simply defined as the distribution ratio D and expressed in equation (6),

D = \frac{C_{i} - C_{e}}{C_{e}} = \frac{q}{C_{e}} \times \frac{m}{V}

(6)

where V, m, and q stand for the volume of Au(III) solution (dm³), the weight of adsorbents (g) and the adsorbed metal on the adsorbents (mmol g^–1). After those formula derivations, the distribution ratio was taken logarithm, and the obtained value of log D is expressed and derived as equations (7) and (8):

\log D = - n \log [{Cl}^{-}] + \log K_{e x} + \log b

(7)

\log (\frac{q}{C_{e}}) = - n \log [{Cl}^{-}] + \log K_{e x} + \log (\frac{V}{M})

(8)

where M stands for the molecular weight of adsorbents (g mol^–1). The relationship between log q/C_e and –log[Cl^–] was linear as shown in Figure 6(d). The plot lies on a straight line fitting with a certain slope of 0.93. The R² value of the fitted line was 0.993. The appropriate fitting line illustrated the ratio of adsorbed Au(III) ion with released chloride ion was 1. When Au(III) was contacted with the adsorbents surface, a hydrogen bond formed between chlorine and hydroxyl group to immobilize Au(III) onto modified KP surface and promote adsorption ability.

Proposed Au(III) adsorption process

Combining the IR spectra analysis and ions dependencies study, the possible Au(III) adsorption process is proposed. KPs contain various compounds such as polysaccharides, phenols, flavonoids, and tannins which can provide active hydroxyl groups as well as potential to react with Au(III) (Liu et al., 2020; Sjodin et al., 2006; Valdes et al., 2024; Xu et al., 2023; Zhang et al., 2021). Firstly, KP was activated by sulfuric acid modification, ethereal groups were generated by the dehydration of some phenolic compounds on KP. In the HCl media, Au(III) exists as [AuCl₄]⁻ species, while the adsorbents surface becomes positively charged due to the protonation of ethereal groups. This induced electrostatic interaction between the negatively charged [AuCl₄]⁻ and the positively charged adsorbents surface. The electrostatic interaction drove [AuCl₄]⁻ to contact on the adsorbents surface. Since the majority of base metals exist as cationic species even in the HCl media, these metals are difficult to contact with the adsorbents surface. Consequently, the adsorbents exhibited poor performance on base metals. When [AuCl₄]⁻ was contacted onto the modified KP surface, a hydrogen bond was probably formed between chlorine and hydroxyl groups (the groups have not been dehydrated) of modified KP to immobilize [AuCl₄]⁻ onto solid phase. Subsequently, Au(III) ion was adsorbed through the coordination with ether groups and released chloride ion into the aqueous phase. Due to the high redox potential of Au(0), Au(III) was easily reduced by the active groups of modified KP, the gold particles were generated and accumulated on the adsorbents surface during the adsorption process till the adsorption reached the equilibrium. Due to the lower reduction potential of Pd and Pt than that of Au, the adsorbents exhibited poor adsorption toward these metals, even though Pd(II) and Pt(IV) also exist as [PtCl₆]²⁻ and [PdCl₄]²⁻ species in HCl. The possible adsorption process was proposed in Figure 7.

Figure 7.

Proposed possible Au(III) adsorption process.

Adsorption isotherms of Au(III)

The adsorption isotherm of Au(III) on KP and modified KP in 0.1 M HCl media was studied. As shown in Figure 8(a), the amount of Au(III) adsorbed on each adsorbent increased in line with the equilibrium concentration of Au(III) and reached the plateau. The maximum adsorption capacity of KP was much lower than that adsorption of modified KP. The fitted curve of modified KP follows a typical Langmuir adsorption model (Swensin and Stadie, 2019) described as equation (9):

q_{e} = \frac{q_{m} b C_{e}}{1 + b C_{e}} = \frac{q_{m} C_{e}}{1 / b + C_{e}}

(9)

where q_e and q_m are the adsorption amounts at equilibrium (mmol g^–1) and the maximum adsorption capacity (mmol g^–1), respectively; the term “b” is the Langmuir constant related to the adsorption energy (dm³ mmol⁻¹). When the equilibrium concentration, C_e, is greater than the value of 1/b, the maximal adsorption capacity is obtained.

Figure 8.

(a) Adsorption isotherm of Au(III) on adsorbents, (b) the linear form of Langmuir equilibrium isotherm of Au(III) adsorption on modified KP.

The maximum adsorption capacity of modified KP reached around 5 mmol g^–1. While the capacity of KP was much less than 0.5 mm g^–1. It means the modification improved the Au(III) adsorption capacity of KP. To further confirm the valid maximum adsorption capacity, equation (9) was converted to equation (10):

\frac{C_{e}}{q_{e}} = \frac{1}{q_{m} b} + \frac{C_{e}}{q_{m}}

(10)

Equation (10) depicts a linear relationship between C_e q^–1 and C_e for Au(III) adsorption on modified KP. Figure 8(b) shows the converted Langmuir adsorption isotherm of Au(III) on modified KP. The Langmuir parameters and the maximum adsorption capacity were calculated from the slope and intercept using linear regression analysis. The regression coefficient (R²) value is 0.992. It indicates that the Langmuir model fits closely to the experimental data. A typical Langmuir model depicts the homogeneous monolayer adsorption (Swensin and Stadie, 2019). Therefore, Au(III) adsorption is possibly reacted on the surface of modified KP. The capacity of modified KP is calculated as 5.71 mmol g^–1_, which is comparable to some biomass adsorbents listed in Table 1 (Al-Saidi, 2016; Chen et al., 2011; Choudhary et al., 2018; Dwivedi et al., 2014; Ishikawa et al., 2002; Khosravi et al., 2017; Torres et al., 2005; Zhu and Huang, 2022). Apparently, modified KP is an excellent candidate for gold recovery.

Table 1.

Au(III) adsorption capacities of various adsorbents for gold.

Adsorbent	Adsorption capacity (mmol g^–1)	pH _i	References
Durio zibethinus husk	1.72	2	Choudhary et al., 2018
Polyethylenimine-modified Lagerstroemia speciosa leaves powder	1.45–1.59	1	Al-Saidi, 2016
Raw date pits	0.396	0.3	Torres et al., 2005
Calcium alginate	1.47	2	Chen et al., 2011
Silk sericin	1.0	2.4–2.55	Ishikawa et al., 2002
Modified cellulose	0.152	1.06–1.71	Dwivedi et al., 2014
Modified garlic peel	0.017	1	Zhu and Huang, 2022
Activated carbon	0.131	11	Khosravi et al., 2017
Modified KP	5.71	1	Present work

KP: kiwi peel.

Recovery of gold from precious metals

To further investigate the specific selectivity on Au(III) adsorption, Au(III) adsorption was studied in the mixed precious metals solution. Figure 9(a) shows Au(III), Pd(II), and Pt(IV) adsorptions on modified KP. Due to the increase of chloride ion concentration derived from platinum salt, Au(III) adsorption (92%) was lower than that of its adsorption (99%) in single metal solution. Compared to poor adsorption of Pd(II) and Pt(IV), modified KP still exhibited specific selectivity on Au(III) adsorption. Science thiourea is proven to provide fast initial gold leaching rates and lower toxicity, the gold recovery with thiourea was studied (Groenewald, 1976; Li and Miller, 2002). After the selective adsorption, gold-loaded adsorbents were washed with deionized water at few times and eluted with thiourea in HCl media. Figure 9(b) shows Au(III) elution at different concentrations of HCl medias. More than 75% of Au(III) was collected from modified KP in the wide range of HCl concentration. Due to the low adsorptions, Pd(II) and Pt(IV) were nearly undetectable in the eluate. The result shows the potential application of modified KP on gold recovery. The pure gold recovery from e-waste can be achieved with a simple stepwise process, the detail is shown in Figure 10. After physical disassemble work, the e-waste metals can be collected by hydrometallurgical method. With modified KP selective adsorption and acidic thiourea elution, most of Au(III) can be recovered from the e-wastes. The high specific selectivity and superior adsorption capacity indicate the modified KP is a promising bio-adsorbent for gold recovery from e-wastes.

Figure 9.

(a) Competitive adsorption of precious metals on modified KP (precious metal), 1.0 mM; [HCl], 0.1 M; shaking time, 48 h), (b) eluted gold from the Au-loaded adsorbents.

Figure 10.

Scheme of gold recovery by modified KP.

Conclusions

E-waste metals adsorption on modified KP was investigated. Modified KP exhibited specific selective adsorption on Au(III) than other metals (Cu(II), Pd(II), Pb(II), Zn(II), Fe(II), Fe(III), Pt(IV)). The microscopic images found gold particles on the adsorbent surface after adsorption. According to XRD analyses, Au(III) adsorption was involved in Au(III) reduction, the reduction occurred during the whole adsorption process till the adsorption reached the equilibrium. The nonmetallic ion dependencies on Au(III) adsorption study suggested that chloride anion concentration has a relation with Au(III) adsorption. When Au(III) adsorbed on the adsorbents, [Cl^–] ions was released from [AuCl₄^–]. Based on the Langmuir model calculation, the adsorption capacity of KP was lower than 0.5 mmol g^–1, the capacity of KP was 5.71 mmol g^–1. Concentrated sulfuric acid treatment greatly improved the adsorption capacity. The Au(III) adsorption capacity of modified KP is much higher than that of other natural or synthesized adsorbents. Through the selective adsorption and elution, Au(III) was successfully recovered from mixed precious metals. Those excellent performances in this fundamental research offer a reliable approach for the utilization of biomass waste in gold recovery. The advantages of KP, such as abundant resources, low cost, and environmental friendliness, show the possibility of the application of modified KP in gold recovery from e-wastes.

Supplemental Material

sj-docx-1-adt-10.1177_02636174251371754 - Supplemental material for Selective hydrometallurgical recovery of gold by using low-cost biomass waste: Kiwi peel

Supplemental material, sj-docx-1-adt-10.1177_02636174251371754 for Selective hydrometallurgical recovery of gold by using low-cost biomass waste: Kiwi peel by Dan Yu, Xiaomei Guo and Yang Liu in Adsorption Science & Technology

Footnotes

ORCID iD

Yang Liu

Funding

The authors disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: This work was supported by the Nanxun Scholars Program for Young Scholars of ZJWEU, National Natural Science Foundation of China (Grant Nos. RC2024021384 and 52376037).

Declaration of conflicting interests

The authors declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Availability of data and materials

The authors declare that the data supporting the finding of this study is available within the paper and its files. Should any raw data files be needed in another format they are available from the corresponding author upon reasonable request.

Supplemental material

Supplemental material for this article is available online.

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