Fast and Selective Adsorption of Au (III) from the Waste Printed Circuit Boards Using a Low-Cost Adsorbent: Optimization by Central Composite Design Based on Response Surface Methodology

Zandi-Darehgharibi, Farideh; Haddadi, Hedayat; Asfaram, Arash

doi:https://doi.org/10.1155/2023/7465722

Journal of Chemistry

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Abstract Introduction Analysis Results and Discussion Conclusion Data Availability Disclosure Conflicts of Interest Acknowledgments Supplementary Materials References Copyright Related Articles

Research Article | Open Access

Volume 2023 | Article ID 7465722 | https://doi.org/10.1155/2023/7465722

Fast and Selective Adsorption of Au (III) from the Waste Printed Circuit Boards Using a Low-Cost Adsorbent: Optimization by Central Composite Design Based on Response Surface Methodology

Farideh Zandi-Darehgharibi,¹Hedayat Haddadi,¹and Arash Asfaram²

Academic Editor: Doina Humelnicu

Received18 May 2023

Revised03 Aug 2023

Accepted22 Aug 2023

Published04 Sept 2023

Abstract

A low-cost new green adsorbent (GA) was synthesized by tannin-rich pomegranate peel powder and formaldehyde for the fast and selective recovery of Au (III). It was characterized by Fourier transform infrared spectroscopy (FT-IR), Brunauer–Emmett–Teller (BET), elemental analysis (CHN), field emission scanning electron microscopy (FE-SEM), and energy-dispersive X-ray spectroscopy (EDS) (FE-SEM-EDS-mapping). The optimal values of influential factors were defined using a central composite design based on the response surface methodology (CCD-RSM). Adsorption properties were investigated by the kinetic, isotherm, thermodynamic, and interference of coexisting metal ions at optimum conditions. The experimental adsorption percentage with three repetitions under the optimized conditions such as pH = 2, adsorbent mass = 23 mg, Au (III) concentration = 32 , and contact time = 30 min was 97% and the highest adsorption capacity of the GA was 315.450 . The adsorption isotherm and kinetic were clarified by the Freundlich (R² = 0.952) and pseudo-second-order (R² = 0.998) models. The thermodynamic study (∆S° > 0, ∆H° > 0, and ∆G° < 0) revealed that Au (III) adsorption via GA was a facile, endothermic, and spontaneous process. The adsorption mechanism could be an electrostatic interaction and reductive adsorption. A small amount of GA (23 mg) adsorbed gold selectively and rapidly (30 min) from coexisting metals present in the waste printed circuit board (PCB) such as Ag, Al, Si, Zn, Pb, Ba, Ni, Ca, Mo, Co, Cr, Mn, Cu, Mg, Fe, and W. These results confirm the use of low-cost and high-efficiency GA to fast and selectively recover gold from waste PCBs.

1. Introduction

Gold is used in the manufacture of various electronic devices such as cell phones, televisions, and computers due to its physical, chemical, and conductive properties. With the advancement of technology, new electronic devices are replacing older ones every day, thus producing electronic waste (e-waste). Printed circuit boards (PCBs) as one of the electronic waste contain 350 grams of gold per ton, while the gold ore contains 5 to 30 grams of this precious metal per ton. As this e-waste contains much larger amounts of gold than the gold ore, it may be considered a secondary source of gold [1–7]. Due to the limited natural resources of gold, it is of paramount importance to recover this precious metal from e-waste. Economically, the recovery process should be as follows: gold is isolated very selectively from the base metals such as iron, copper, and zinc that are often present in disproportionate amounts with this precious metal [8]. Therefore, there is a need to develop efficient technologies for the recovery of gold from a wide range of secondary sources.

At present, there are various processes such as the use of metal-organic frameworks, ion exchange resins, solvent extraction, and adsorption with biosorbents to recover gold. Among these processes, the adsorption process with natural adsorbents is simpler and more effective. There are many reports on the use of natural wastes such as persimmon peel, orange waste, banana peel, and lemon peel due to their phenolic compounds and tannins content to recover gold. In other words, phenolic compounds and tannins have a high affinity for gold due to their hydroxyl groups. One of the most important advantages of using biological waste for gold recovery is that they are easily available in large quantities at low cost. Environmental compatibility, degradability, and renewability are other advantages of them [8–18].

Factors such as solution pH, contact time, initial concentration, and adsorbent dose have been found to affect adsorbent performance. The correct understanding of the interaction of these factors is thus necessary to optimize the adsorption condition. Experimental design is a useful technique to optimize effective parameters. To investigate independent parameters and their interactions that are effective in the adsorption process, central composite design-response surface methodology (CCD-RSM) is used. CCD is a useful statistical technique with a small number of tests [19–28].

Considering that the process of recovering precious metals should be simple and cost-effective from the economic point of view, a low-cost and readily accessible natural adsorbent was synthesized from pomegranate peel tannins with a simple and facile process as an adsorbent for fast and selective recovery of gold. CCD-RSM was used to determine the optimal values of the parameters affecting the gold adsorption process to reach the maximum amount of adsorption. The synthesis of this adsorbent and its combination with the experimental design technique to optimize the effective parameters in gold recovery have been performed for the first time in this study. Moreover, the isotherm, kinetic, thermodynamic, and interference of coexisting metal ions were investigated under optimum conditions.

2. Methodology and Analysis

2.1. Materials and Chemicals

Tannin-rich pomegranate peel was obtained from the Sheyvand village of Izeh City, Iran. NaOH, HNO₃, HCl, HAuCl4·xH2O, Folin–Ciocalteu’s phenol reagent, formaldehyde, solvents, and other chemicals were purchased from Merck. Waste PCB was obtained from Shahrekord University.

2.2. Quantifying Total Phenolic Content (TPC) of Pomegranate Peel

The Folin–Ciocalteu methodology was employed to define the total phenol content of tannin-rich pomegranate peel [29]. Tannin-rich pomegranate peel(0.01 g) was dissolved in 60% methanol (10 mL). In the next step, 10% Folin–Ciocalteu reagent (0.5 mL) was added to a portion (2 mL) of the previous solution (10 mg/mL). The mixture was placed for 5 min at an ambient temperature and then mixed with the Na₂CO₃ solution (7.5% w/v) (0.4 mL). The sample was kept in the darkness for 30 min, and the UV-Vis Spectrophotometer defined the absorbance at 765 nm. Gallic acid was used as the standard, and the data were computed as milligrams of gallic acid equivalent per gram dry weight (mg GAE/g DW).

2.3. Preparation and Characterization of the Adsorbent

Tannin-rich pomegranate peel powder (30 g) was dissolved in 65 mL of NaOH fresh solution (0.25 M) at room temperature and 6 mL of 37% formaldehyde solution was then added to it. Then, the mixture was heated at 353 K. After 12 hours, the product was filtered and crushed and then rinsed several times with deionized water. The HNO₃ solution (0.05 M) was used to remove the unreacted materials. After that, the product was rinsed with distilled water. Finally, it was dried at 338 K for 24 hours [4]. This product was green adsorbent (GA) and used for adsorption tests. To characterize the product obtained before and after adsorption, FTIR, BET, CHN, and FE-SEM-EDS-mapping were performed.

2.4. Experimental Design

Design Expert software version 13 was used to optimize the factors. The optimal values of factors were determined using the CCD-RSM. CCD was run for each factor at five levels (, low (−1), center (0), high (+1), and +) and 21 experiments were performed in the small mode. Ranges of factors, including solution pH (1–5), adsorbent mass (7–35 mg), Au (III) concentration (20–100 ), and contact time (10–50 min) are shown in Table S1. Using the optimal values determined by the CCD, kinetic, thermodynamic, adsorption isotherm, and interference of coexisting metal ions were investigated.

2.5. Adsorption Experiments

To investigate factors affecting Au (III) adsorption by GA, 21 experiments designed by CCD were conducted in aqueous solutions with different pH values, adsorbent doses, contact times, and Au (III) concentrations using a thermostatic shaker at 200 rpm. The solution obtained by filtering the heterogeneous mixture was collected for inductively coupled plasma optical emission spectroscopy (ICP-OES) analysis. The percentage of Au (III) adsorption by GA was calculated using the initial and equilibrium concentrations through the following equation:where and are Au (III) initial and equilibrium concentrations in .

In addition, the experiments of kinetic, thermodynamic, isotherm, and coexisting metal ions were performed using the optimal values obtained from CCD for effective factors.

2.6. Aqua Regia Digestion of Real Sample (Waste PCB)

First, the waste PCB was placed in the furnace with a high temperature of 1023 K to burn plastic, etc., for 4 hours. The melted metals were then turned into uniform fine particles by the planetary mill. Five grams of powdered metals were digested by 56 mL of aqua regia reverse (14 mL of 37% w HCl with 42 mL of 69% w HNO₃). The mixture was heated to 473 K for 2 hours. After cooling, the mixture was filtered [30]. The concentration of metals in the solution at pH = 2 (Au = 30.12, Ag = 6, Al = 8423, Ba = 133, Ca = 1160, Co = 2, Cr = 59, Cu = 8196, Fe = 744, Mg = 71, Mn = 13.6, Mo = 1, Ni = 176, Pb = 16.3, Si = 7, W = 0.3, and Zn = 40 ) was measured using ICP-OES.

3. Results and Discussion

3.1. Adsorbent Characterization

3.1.1. TPC in Pomegranate Peel

The TPC of pomegranate peel was 168.00 ± 5.75 mg GAE/g DW. This measurement provides evidence on considerable quantities of phenolic compounds in the pomegranate peel that have two major components, carbon and oxygen, in their structure. The TPC of the two types of pomegranate peel was reported to be 102.9 ± 0.9 and 85.9 ± 1.3 mg GAE/g DW [31]. Several studies have found pomegranate peel to be rich in phenolic compounds [32–37].

3.1.2. FTIR Analysis

Figure 1 shows the FTIR spectra of tannin-rich pomegranate peel powder and GA. The broad peaks at 3600−3100 cm⁻¹ exhibit the stretching vibration of the O-H group of phenol [38]. The peaks at 1450 and 1446 cm⁻¹ correspond to the C-C of the ring [5, 39, 40]. The peaks in the narrow area of 1390−1310 cm⁻¹ are associated with O-H phenolic bending. The peaks at 1232 and 1033 cm⁻¹ display the stretching vibrations of the carbonyl group (C-O) [3, 4, 31]. These data confirm that the pomegranate peel is rich in phenolic compounds which have carbon and oxygen as their two main elements. The reaction between formaldehyde and pomegranate peel tannin leads to methylene bridges. The slight peaks at 2937 and 2931 cm⁻¹ are associated with the C-H stretching of phenolic rings of pomegranate peel tannin and methylene bridges [4, 41, 42]. The peaks at 910–740 cm⁻¹ display deformation vibrations of the C-H bond in phenolic rings and the intensity of this area has dwindled due to the cross-links between pomegranate peel tannin and formaldehyde (Figure 1(b)) [4].

3.1.3. FE-SEM, BET, CHN, and EDS-Mapping Analysis

FE-SEM examination was performed to investigate the morphology of the surface (Figure 2). The images of FE-SEM at diverse magnifications showed that GA has a rough, nonuniform, and porous surface. Moreover, the GA surface area was computed by BET and the outcomes displayed a surface area of 2.115 . In addition, the pore volume and the mean pore diameter of the GA were 0.0063 and 11.964 nm, respectively (Table 1). BET results implied that the GA can be mesoporous and this is in agreement with the reports [43, 44]. This mesoporous and free form may lead to the adsorption of Au (III) ions (Figures 2(a)–2(c)). After adsorption, diverse outcomes emerged from the GA composite, indicating that Au (III) ions are hooked onto the rough and porous surface of the GA to cover its surface (Figures 2(d)–2(f)). Furthermore, the agglomeration of Au onto the GA surface is also proved with EDS-mapping (Figures 3(c) and 3(d)).

(a)

(b)

(c)

(d)

(e)

(f)

(a)

(b)

(c)

(d)

CHN elemental analysis of GA indicates that carbon (47.49%), hydrogen (4.91%), and nitrogen (1.17%) are its composition (Table 2). Figure 3 shows the results of EDS-mapping of the GA before and after loading Au (III) on it. EDS-mapping elemental analysis showed the existence and percentage of the weight of C (67.0%) and O (33.0%) in the GA before adsorption (Figures 3(a) and 3(b)). Moreover, carbon (30.6%–51.40%) and oxygen (46.21%–61.7%) as the two main components of pomegranate peel were reported by researchers [43–48]. The existence and percentage of the weight of C (69.9%), O (29.7%), and Au (0.4%) after adsorption are shown in Figures 3(c) and 3(d). The EDS-mapping elemental analysis confirmed the adsorption of Au (III) ions via the GA surface.

3.2. Factors Optimization to Reach the Maximum Au (III) Adsorption

Optimization is obtaining the maximum effect of several independent variables and their simultaneous interactions on a process [21]. Therefore, to achieve maximum Au (III) adsorption on the GA, optimization was accomplished by CCD-RSM, and the optimal amount of factors was determined. The results of CCD are shown in Table S2. Under the conditions of pH 2, adsorbent mass 23 mg, Au (III) concentration 32 , and contact time 30 min, the adsorption percentage was predicted to be greater than 98% with the desirability of 1.0. The desirability is evaluated in a range of 0 to 1, with 0 indicating undesirable conditions, and 1 showing optimal desirable conditions. Three repetitions were conducted with the acquired optimal values by CCD, to confirm the adsorption percentage predicted by it. The experimental adsorption percentage was 97%, which was in good compromise with the anticipated amount.

3.3. Statistical Analysis

Table S3 shows the analysis of variance (ANOVA) results and model summary statistics. As shown in Table S3, the significance of the model is given by the F-value of 221.42. Due to the value of the model being less than 0.0500, the A, B, C, D, AC, AD, BD, C², and D² terms of the model are significant. The F-value of 1.81 reveals that the lack of fit is not significant relative to the pure error, and the nonsignificant lack of fit is good. The anticipated R² of 0.8926 is in good accord with the adjusted R² of 0.9936; i.e., the difference is less than 0.2. Adequate precision calculates the signal-to-noise proportion and a ratio of more than 4 is acceptable. The proportion of 52.347 shows an adequate signal.

3.4. Influence of Interaction of Variables on Au (III) Adsorption

The simultaneous effect of the influential variables including pH, adsorbent mass, Au (III) concentration, and contact time on Au (III) adsorption by GA was evaluated. Contour plots and three-dimensional response surfaces of Au (III) adsorption via GA are shown in Figure 4.

(a)

(b)

(c)

(d)

(e)

(f)

3.4.1. Effect of Interaction between pH and Adsorbent Mass

The percent of Au (III) adsorption decreased by increasing the pH up to 5, and it increased as the adsorbent amount increased from 7 to 35 mg (Figure 4(a)). Adsorption of the metal ions via the adsorbent surface depends on the solution pH which corresponds to the ionization form of the functional groups of adsorbent and the chemistry of metal in the solution, which influences the number of available active sites [49, 50]. An increase in the amount of adsorbent increases the active sites of the adsorbent surface; therefore, the number of ions that are adsorbed by the adsorbent increases [51, 52]. At low pH (<3), facile protonation of the adsorbent leads to its positive surface charge, and the dominant species of Au (III) anions is Au ; therefore, this condition is desired for the Au (III) anions adsorption [39, 53–58].

At pH = 1, with a GA amount greater than 20 mg, the percentage of Au (III) adsorption reached 100%. The electrostatic attraction between Au (III) ions (Au ) and the greater number of positive surface charges of the GA causes the Au (III) ions adsorbed by the GA to increase. At a pH above 1, with an increase in pH and a reduction in the GA amount, the percentage of adsorption reduces. At a high pH, some oxygenated functional groups are deprotonated, and Au is hydrolyzed [39, 59, 60]. Therefore, the electrostatic repulsion between Au (III) ions and the small number of functional groups in GA increases, consequently, the Au (III) ions are repulsed by the GA surface, and the adsorption percentage decreases. Indeed, the pH and adsorbent mass interaction indicates that an increase in the adsorbent mass, accompanied by a simultaneous decrease in pH, increases the percentage of Au (III) adsorption by GA.

3.4.2. Effect of Interaction between pH and Au (III) Concentration

As is observed in Figure 4(b), the adsorption percentage of Au (III) increased significantly with a simultaneous decline in pH and Au (III) concentration. At a pH of 2.4 and a concentration less than 50 , the Au (III) adsorption percentage reached 100%. The number of active sites on the surface of the GA becomes less than the number of Au (III) molecules with an increase in Au (III) concentration in the solution [51, 61]. In consequence, the available sites on the surface of the GA are saturated very quickly, and more sites are required for the complete adsorption of the desired concentration; therefore, the amount of adsorption decreases with the increase in concentration.

3.4.3. Effect of Interaction between pH and Contact Time

Figure 4(c) shows that a decline in pH accompanied by an increase in contact time (10–50 min) leads to a significant increase in Au (III) adsorption percentage. In this condition, a reduction in pH leads to the positive surface charge of the GA and the rise of Au species in the solution. Moreover, by increasing the shaking time, Au (III) anions have adequate time to be adsorbed on the surface of the GA. The probable speed of the Au (III) adsorption process of GA is determined using contact time. As a result, the optimal time for the complete adsorption of Au (III) ions by the adsorbent is determined. At a pH between 1 and 2, less than 50 min of contact time is required for the adsorption percentage to reach 100%, which indicates the fast adsorption of gold ions on the GA surface.

3.4.4. Effect of Interaction between Adsorbent Mass and Au (III) Concentration

As shown in Figure 4(d), the Au (III) adsorption percentage increased by simultaneously increasing the mass of the GA and decreasing the Au (III) concentration. As the number of active sites on the surface of GA increased with the adsorbent portion, a reduction in the Au (III) concentration in the solution led to a smaller number of Au (III) molecules, and the adsorption rate of Au (III) ions was thus increased [51, 52].

3.4.5. Effect of Interaction between Adsorbent Mass and Contact Time

Figure 4(e) shows that the amount of Au (III) adsorption increased significantly with a simultaneous increase in the amount of GA and contact time. Due to increasing the amount of the GA, the active sites on its surface were increased. Moreover, by increasing the contact time, the active sites of the GA surface had more time to adsorb Au (III) ions [19, 51]. Consequently, the adsorption rate was 90% in a contact time of 50 min with the adsorbent mass of 35 mg.

3.4.6. Effect of Interaction between Au (III) Concentration and Contact Time

The interaction between Au (III) concentration and contact time is shown in Figure 4(f). By simultaneously reducing the concentration and raising the contact time, the Au (III) adsorption percentage was increased. These results show that GA had sufficient time to adsorb a few Au (III) ions. This can be attributed to the prolonged shaking time and low concentration of Au (III) ions that led to an increase in the rate of Au (III) adsorption on the GA surface [19].

3.5. Mechanism of Au (III) Adsorption onto the GA Surface

Various mechanisms including electrostatic interaction and reduction have been suggested for gold adsorption by biosorbents containing hydroxyl groups [2, 9, 14]. Acidic medium leads to protonation of the surface of biosorbents with phenolic compounds and their positive charge. As aforementioned, the dominant form of Au (III) ions in the acidic medium is the chloro complex anion [39, 53–58].

Accordingly, the positive surface charge of GA attracts gold anionic species via electrostatic attraction. Hence, the electrostatic attraction mechanism is considered to be the main factor responsible for Au (III) adsorption by the GA surface.

Gold has a high reduction potential and acts as an oxidizing species. On the contrary, phenolic hydroxyl groups are very sensitive to oxidation [2, 9, 14, 62]. Thus, Au (III) anionic species adsorbed on the GA by electrostatic interaction are reduced to Au (0) by the hydroxyl phenolic groups of the GA. Therefore, it is reasonable to suggest the combination of electrostatic interactions and reduction as the mechanisms of Au (III) adsorption on the GA surface (Figure 5).

3.6. Adsorption Equilibrium Isotherms

The isotherm inquiry of Au (III) adsorbed on the GA was defined by Freundlich, Temkin, Dubinin–Radushkevich, and Langmuir isotherms. The sorption isotherm illustrates the interaction between the adsorbent and the adsorbed component and is used to define the adsorption capacity of the adsorbent and optimize the adsorption process [63–65]. The Langmuir isotherm illustrates a monolayer adsorption. In this model, all adsorption sites have the same energy and enthalpy, and there is no interaction between the adsorbed molecules and the absorbent surface [66, 67]. Freundlich isotherm exemplifies multilayer sorption and heterogeneous surface adsorption underneath diverse nonideal circumstances. The Temkin model expresses a situation in which the adsorption heat declines linearly with an increase in the capacity of adsorption. The mechanism of adsorption via a Gaussian distribution of energy on a heterogeneous surface is described by Dubinin–Radushkevich [38, 68–71]. Table S4 shows the parameters of isotherm models. Adsorption isotherms were performed using the optimum values of pH = 2, contact time = 30 min, and adsorbent amount = 23 mg to determine the GA adsorption capacity.

The most appropriate isotherm for adsorption studies was selected by comparing the correlation coefficients (R²) of the isotherm equations [64]. If R² for each model is closer to 1, then the model is more appropriate. As shown in Table S4, the Freundlich model is more suitable than other models, because its R² is closest to 1 and is greater than the R² values of other models. The maximum adsorption capacity of the GA for Au (III) adsorption was 315.450 . Table 3 shows the adsorption capacities of Au (III) for several adsorbents. It can be seen that GA has a high affinity with Au (III) ions due to the presence of a large number of phenolic hydroxyl groups on its porous surface.

The verification of experimental data with the Freundlich model shows the heterogeneity of the GA. In addition, the results exhibited that the gold molecules formed a multilayer coating on the GA surface. The value of less than 1 implies a desirable adsorption of Au (III) onto GA. The dimensionless parameter (R_L) indicates the nature of adsorption with R_L 0 indicating the irreversible adsorption process, R_L > 1 showing an undesirable adsorption process, 0 < R_L < 1 demonstrating a desirable adsorption process, and RL 1 representing a linear process [17, 55, 64, 71, 80, 81]. The obtained value for R_L (0.421–0.814) was less than one, indicating that the adsorption process was desirable.

3.7. Kinetic Studies and Activation Energy

Two important factors in the adsorption process are rate constants and order of reaction. The kinetic investigation of Au (III) adsorbed on the GA was determined by the pseudo-first-order (PFO), pseudo-second-order (PSO), Elovich, and intraparticle diffusion models.

In the pseudo-first-order model, diffusion happens in a single layer where the change in adsorption with time is commensurate with the absorbent surface of unoccupied sites. The pseudo-second-order model shows that chemical interactions are the rate-controlling stage in the adsorption process, where the rate of occupancy of adsorption sites is proportional to the number of occupied ones. The chemical interactions include forces via the sharing or swap of electrons between the adsorbed component and the adsorbent [66, 70]. Determining the effectual stages on the rate of the adsorption process and the rate control stage is accomplished by intraparticle diffusion [70]. Elovich’s model depicts chemisorption [64].

The kinetic model parameters are shown in Table S5. The most suitable model was selected according to R². The R² quantity of the pseudo-second-order model is closest to 1 and is larger than the R² quantity of other models. Therefore, the pseudo-second-order model is more appropriate to express the kinetic of the adsorption process of Au (III) on the GA surface. Furthermore, the values of q_e obtained (q_e (exp)) and calculated (q_e (calc)) by the pseudo-second-order model are very close to each other, indicating that this model is very suitable for representing the experimental data. These results show chemical interactions between gold molecules and the GA surface. Table 3 shows the time required to reach the equilibrium for the GA compared to several adsorbents. It can be seen that gold adsorption on the GA reaches the equilibrium state in a much shorter time than other adsorbents, signifying the fast adsorption of gold by the GA.

By using kinetic data at diverse temperatures, the activation energy (E_a) for the gold adsorption process was calculated. Activation energy is the least energy needed for reacting adsorbed molecules with functional groups of adsorbent. The negative E_a means that the adsorption process is carried out at a low temperature and its nature is exothermic. Contrarily, the positive E_a indicates that the adsorption process is accomplished at a high temperature and the process needs energy and its nature is endothermic [82–84]. E_a is measured using the following Arrhenius equation:where A is the Arrhenius factor, K is the rate constant, T is the temperature, and R is the universal gas constant. The Ln K versus 1/T plot provides a straight line and E_a is the slope of that line. E_a for the gold adsorption process was 30.178 kJ (Figure 6). The type of adsorption (physisorption or chemisorption) can be determined by the amount of activation energy. Physisorption needs an activation energy of less than 40 kJ. Physisorption is commonly reversible [84]. Accordingly, the gold adsorption process on the GA surface might be physisorption and reversible.

3.8. Thermodynamic Studies

The influence of temperature on the adsorption of Au (III) at different temperatures was studied and the thermodynamic factors entropy (), enthalpy (), and Gibbs free energy () were computed using the following equations [79, 85–88]:where is the adsorption distribution coefficient, is the amount of Au (III) adsorbed by the GA at equilibrium (mg ), is the amount of Au (III) residual in the solution at equilibrium (mg ), T () is the solution temperature, and R (8.314 J·) is the universal gas constant. The amount of and was measured from the slope and intercept of the Ln plot versus .

An increase in the temperature causes changes in the adsorption process by increasing the diffusion of the adsorbate molecules in the outer border layer and the internal pores of the absorbent particles and changing the adsorbent equilibrium capacity for a peculiar adsorbate [63]. As seen in Table S6, the percentage of Au (III) adsorption by the GA increases with rising temperature, and the > 0 verifies the endothermic nature of the adsorption process. The demonstrates that the adsorption process of Au (III) was spontaneous. > 0 depicts a rise in disorder at the solid-liquid interface upon adsorption. Table 3 depicts the temperature required to reach the equilibrium for the GA compared to several adsorbents. As can be seen, gold adsorption on the GA reaches the equilibrium at 303 K.

3.9. Effect of Competing Metals in the Real Sample (Waste PCB) on Gold Adsorption

There are several metals accompanying gold in PCBs. The presence of these competing metals may affect the adsorption of gold by the GA. Therefore, it is mandatory to examine the selectivity of the GA for the gold. GA application for gold recovery from the PCB matrix was investigated using competing metals with different concentrations (pH = 2, adsorbent mass = 23 mg, temperature = 303 K, and contact time = 30 min), and the outcomes are shown in Figure 7. As shown in Figure 7, GA has an excellent selectivity for gold adsorption as its positive surface attracts negatively charged gold anions by electrostatic attraction. GA’s affinity to adsorb iron was negligible and the adsorption of other metals on the GA surface was zero.

Figure 7

Effect of competing metals in the waste PCB on Au (III) adsorption. The initial concentration of metals in the solution at pH = 2 (Au = 30.12, Ag = 6, Al = 8423, Ba = 133, Ca = 1160, Co = 2, Cr = 59, Cu = 8196, Fe = 744, Mg = 71, Mn = 13.6, Mo = 1, Ni = 176, Pb = 16.3, Si = 7, W = 0.3, and Zn = 40 ), volume of solution = 5 mL, adsorbent mass = 23 mg, temperature = 303 K, and contact time = 30 min.

Despite the anionic form of gold, the base metal ions exist mainly in the cationic or neutral form in an acidic medium; and hence, they are not adsorbed on GA due to the electrostatic repulsion between them and the positive surface charge of GA [39, 59, 62]. Table 4 shows the adsorbent mass and the number of competing metals of several adsorbents for gold adsorption. Compared to other adsorbents, a small amount of the GA adsorbed gold from a larger number of metals present in the real sample.

3.10. Reusability of GA

Adsorbent regeneration is important because of economic and resource considerations. The adsorption efficiency of GA recycled in three successive cycles was evaluated, and experimental examinations were performed using 32 of Au (III) and 23 mg of GA at pH = 2, with a contact time of 30 min. Elution of gold loaded on the GA surface was accomplished using thiourea (0.5 M) in HCl (2 M) by shaking for 4 hours. The GA showed over 90% adsorption for the three cycles (Figure 8). Hence, the GA could be a reusable and cost-effective adsorbent. The rapid elution of gold loaded on the GA surface implies that gold adsorption on the GA surface can be physical and reversible.

4. Conclusion

In this research, the GA was utilized for the fast and selective recovery of Au (III) from waste PCBs. In optimum conditions of pH = 2, adsorbent mass = 23 mg, Au (III) concentration = 32 , and contact time = 30 min, experimental adsorption percentage was 97%. The highest capacity of the GA to adsorb Au (III) was 315. 450 . Freundlich (R² = 0.952) and pseudo-second-order (R² = 0.998) models expressed the isotherm and kinetic of Au (III) adsorption onto the GA. The thermodynamics of Au (III) adsorption by the GA was an endothermic (∆H° > 0), facile (∆S° > 0), and spontaneous (∆G° < 0) process. The adsorption mechanism could include several reactions of electrostatic interaction and reductive adsorption. GA adsorbed gold with the highest adsorption percentage (97%) along with an excellent selectivity from competing metals (Ag, Al, Si, Zn, Pb, Ba, Ni, Ca, Mo, Co, Cr, Mn, Cu, Mg, Fe, and W) in waste PCB. The outcomes of this study suggest the use of the GA with low cost, high effectiveness, and selectivity to recover gold from secondary sources such as waste PCBs.

Data Availability

Data supporting the results of this article are presented as supplementary materials.

Disclosure

This article was extracted from a Ph.D. thesis.

Conflicts of Interest

The authors declare that they have no conflicts of interest.

Acknowledgments

We would like to thank Dr. Abdolhadi Farrokhnia for helping us. Also, we thank everyone who helped us in this work. This work was supported by Shahrekord University.

Supplementary Materials

Tables S1–S3 show the CCD experimental design and Tables S4–S6 show the isotherm, kinetic, and thermodynamic parameters. (Supplementary Materials)

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