Adsorption and Extraction of Gold by Using Polymer Inclusion Membrane: Effect of Membrane Materials on Performance
Article information
Abstract
Precious metals, such as gold, are widely used in the manufacturing of various products, including electronic components and jewelry, due to their unique physical and chemical properties. Recycling these metals is economically advantageous, as the concentration of valuable metals in discarded waste is often higher than in natural resources. In this study, polymer inclusion membranes (PIMs) with various compositions were fabricated and characterized. The optimal pH for each membrane was determined to be 1, based on pH experiments. Temperature and contact time conditions varied for different membrane materials: membranes containing polyvinyl chloride (PVC), polyvinylidene fluoride (PVDF), and cellulose triacetate (CTA) exhibited optimal performance at 25°C for 30 minutes, 35°C for 60 minutes, and 45°C for 360 minutes, respectively. Among the tested membranes, the best-performing compositions for each base polymer were identified as CTA-2, PVDF-5, and PVC-4. For gold extraction studies, membranes incorporating PVC, trioctylamine (TOA), and methyltrioctylammonium chloride (Aliquat 336) were developed based on adsorption results. Experimental findings indicated that 0.25M hydrochloric acid (HCl) was the optimal concentration for the process. Extraction analysis determined that the gold concentration in the feed phase should be 10 parts per million (ppm). The most effective membrane for extraction was composed of 40% PVC, 40% A336, and 20% TOA. Under these optimized conditions, the gold recovery factor after 6 hours was recorded as 37.93%.
1. Introduction
In recent years, the use of membrane technologies has increased instead of traditional separation methods to reduce the difficulties and problems arising especially from water and resource scarcity [1]. Membrane processes are more advantageous because they consume less energy and are more selectively permeable than traditional methods [2]. Various methods have been used to extract and recover precious metals from solutions containing many metals, especially from wastewater. These methods include chemical precipitation, resin adsorption, ion exchange, and solvent extraction. Of these methods, solvent extraction is widely used for the selective recovery and separation of precious metals from complex mixtures. Although solvent extraction has been successful in recovering non-essential precious metals, it faces challenges such as slow extraction, difficult separation phases, and low efficiency [3]. For more than 40 years, the idea of a liquid membrane made of polymers has been discussed as a potential replacement for solvent extraction [4,5].
Polymer inclusion membranes (PIMs) are new extraction and separation methods for both inorganic and organic compounds [6,7]. They are employed in the treatment of wastewater to remove metal ions, small molecules, inorganic anions, etc. from aqueous solutions. It is a system employed in processes of separation, particularly in the separation of several species [8]. These membranes can be referred to be ecologically friendly because they are employed in the process of removing valuable metals from aqueous solutions [2,9]. Due to the properties of PIMs to separate and recover metal ions from aqueous solutions, their use among researchers has increased significantly. Due to these advantages, it has proven to be a suitable alternative to solvent extraction. The advantages of PIMs over solvent extraction (SX) and other extraction methods are their flexibility, affordability, ability to control their permeability, and the fact that extraction and striping are performed simultaneously. These advantages allow PIMs to be used for a variety of elements and combinations. On the other hand, compared to solvent extraction, PIMs have disadvantages such as low extraction and striping speeds, low mechanical stability, carrier leakage, and membrane degradation at high temperatures [10].
Base polymer, plasticizer, and carrier materials are used in the production of polymer inclusion membranes. Polymer selection is made based on the physical and chemical structure of the polymer, as well as its flexibility and durability. Additionally, the selected polymer must be compatible with the selected carrier. In addition, plasticizers are used to make the membrane more flexible and softer. Finally, the carrier is very important for transporting the material from the feed phase to the stripping phase.
Precious metals such as gold are used in the manufacture of various products such as electrical & electronic products and jewelers, due to their characteristic physical and chemical properties such as good electrical conductivity and corrosion resistance [11]. Their use is very common, especially in the electronic field, and the demand for these metals is increasing rapidly with the development of new technologies [12]. Recycling of metals is economically beneficial since the metal ratio in the wastes discarded as a result of this increase is higher than natural resources [12]. According to the literature, Ghiasi et al. found that when 50% PVC, 40% dibutyl carbitol (DBC), and 10% di (2-ethylhexyl) phosphate (D2EHP) were used, extraction efficiency and stripping efficiency were found to be 99.1% 40.9%, respectively. Permeability efficiency was calculated as 27*10-3m/h, and initialflux was calculated as 82.1*10-7mol/m2.s[10]. In addition, when the gold recovery was examined using the membrane consisting of 50% PVDF, 40% Aliquat - thiocyanate ion (A336-SCN), and 10% 2-nitrophenyl octyl ether (NPOE), 98.6% extraction efficiency and 98.2% stripping efficiency were observed, and the permeability was calculated as 1.02*10-5 m/s[13]. Furthermore, in the research article conducted by Liu et al., it was found that the extraction efficiency was 99.5% and the stripping efficiency was 99.1% when 70% PVDF, 25% deep eutectic solvent (DES) and 5% NPOE were used. Permeability efficiency was calculated as 3.52*10-5 m/s and extraction capacity as 32.1mg/g[14].
The increasing use of precious metals such as gold and the depletion of natural resources necessitates the development of sustainable recovery methods. The gold content in electronic waste and industrial by-products is often higher than that in natural ores, making recycling processes both economically and environmentally attractive. However, conventional gold recovery methods typically involve high energy consumption, excessive chemical usage, and waste generation, posing significant environmental and economic challenges. At this point, polymer inclusion membranes (PIMs) emerge as an innovative and sustainable alternative. PIMs are membrane systems that operate without the need for solvents, enable selective ion transport and function with low energy consumption. Their high selectivity allows for the efficient separation of target metal ions, significantly improving the effectiveness of recovery processes. Additionally, PIMs offer excellent mechanical and chemical stability, ensuring long-term usability while providing both environmentally friendly and cost-effective solutions. This study investigates the performance of PIMs in gold recovery, aiming to introduce an innovative approach to existing techniques.
This study investigated the impact of polymers on the retention and recovery of gold utilizing various polymers. The purpose of this project was to provide resource recovery. For the production of membranes, several carrier and plasticizer materials, such as Aliquat 336, NPOE, and TOA, are added to PVC, CTA, and PVDF polymers as a base material. Following characterization tests on these membranes, the membranes' adsorption operations were carried out using a 10-ppm gold feed solution that was generated from gold standard solution. Therefore, by varying the pH, contact time, solution temperature, and membrane types, the adsorption of gold metal was studied. Then, the membrane that gave the optimum result as a result of adsorption was selected and extraction experiments were carried out. First of all, different concentrations ranging from 0.1M HCl to 5M HCl were tried to select the stripping phase, and the HCl that gave the maximum result was selected. Then, to examine the effect of the feed phase, experiments were made with Au solutions with different concentrations between 2.5 ppm Gold (Au) and 15 ppm Au, and the best results were selected. Finally, the membrane was produced by keeping the selected polymer concentration constant and changing the carrier and plasticizer concentrations. The final result has been achieved and the membrane has been optimized.
2. Materials and Methodology
2.1. Chemicals
Cellulose triacetate (CTA), polyvinylidene fluoride (PVDF), and polyvinyl chloride (PVC) (MW: 80000 g/mol) were used as base polymers. Also, 3 different solvents including tetrahydrofuran (THF), dimethylacetamide (DMAc), and chloroform were used. Trioctylamine (TOA), Methyltrioctylammonium chloride (Aliquat336-A336), and 2-Nitrophenyl octyl ether (NPOE) were used as carriers and plasticizers. For the preparation of polymer inclusion membranes, Sigma-Aldrich was the source of all chemicals. To comprehend chemical stability, nitric acid (HNO3) (65%) and sodium hydroxide (NaOH) were utilized. These substances were bought from TEKKİM and Merck. The gold ions in the 1000 mg/L metal ions solutions were bought from CPA Chem. The Seven Compact pH meter was used to calibrate the pH of aqueous solutions.
2.2. Membrane Preparation
Aliquat336, TOA, NPOE, and 3 distinct base polymer types (PVC, CTA, and PVDF) were combined to create 18 different membrane types. A minimal amount (10 mL) of solvents were used to dissolve the 300 mg of components. Following the basic polymer's dissolution in the solvent, plasticizers and carriers were added to the mixture. At 25°C, the solution was shaken for 2 hours. The mixture was then poured onto a 9.0 cm-diameter glass ring that was set on a flat glass plate. A clear and flexible circular membrane was left behind when the solvent was allowed to slowly evaporate for 24 hours by covering the mixture with filter paper and a watch glass. To make it easier to remove, the glass plate was submerged in the water bath. The membranes were dried before analysis. All membranes were shown in Table 1.
The composition of polymer inclusion membranes.
2.3. Characterization
The morphology and surface characteristics of PIMs were analyzed using various techniques. Chemical functional groups were identified via the Attenuated Total Reflection-Fourier transforms infrared spectroscopy (ATR-FTIR, Bruker Alpha II, Germany). Spectral analysis was performed in the wavenumber range of 4000-400 cm-1 with a resolution of 4 cm-1, and an average of 32 scans were recorded for each sample. The thickness of the membranes was measured with a Mitutoyo Digital Micrometer (±0.001 mm precision, Japan) to ensure uniformity and reproducibility in membrane fabrication. Measurements were taken at five different points on each membrane, and the average thickness was recorded. The hydrophilic or hydrophobic nature of the membranes was assessed using a Theta Model Contact Angle Goniometer (KSV Attention, Biolin Scientific, Finland). The static water contact angle was measured using the sessile drop method with a droplet volume of 7μL, and an average of three measurements at different positions was taken to ensure accuracy. Also, the roughness was determined by a Zygo NewView 7100 Optical Profilometer (Zygo Corporation, USA). The root mean square (RMS) roughness and average roughness (Ra) were determined from 3D surface scans at five randomly selected regions on each membrane. Membrane surface morphology was examined with scanning electron microscopy (SEM) (FEI - Quanta FEG 250, USA). Before imaging, membrane samples were coated with a thin layer of gold (Au) using a sputter coater (Leica EM ACE200, Germany) to enhance conductivity. SEM images were obtained at various magnifications (500x, 1000x, 5000x) to observe membrane pore structure and surface features.
Furthermore, thermal stability was evaluated through thermal gravimetry analysis (TGA, PerkinElmer TGA 8000, USA). Samples were heated from 25°C to 800°C at a ramp rate of 10°C/min under a nitrogen atmosphere (flow rate: 50 mL/min) to determine degradation temperatures and weight loss patterns. Additionally, inductively coupled plasma-optical emission spectrometry (ICP-OES) (7000 DV, Perkin Elmer, USA) was employed to determine the concentration of precious metals and key elements in the samples. Sample digestion was performed in aqua regia (HCl: HNO3, 3:1 ratio) at 90°C for 2 hours, and the solutions were filtered before analysis. Calibration curves were generated using certified gold standards to ensure accurate quantification.
2.4. Adsorption Experiments
The pH of the solution is one of the critical factors influencing membrane function [15]. The pH value range was altered to range from 1.00 to 5.00 to clarify this. After 240 minutes, a 10 mL sample of the solution was obtained and the experiment was run. The solution was mixed at a steady temperature at 400 rpm.
Using a polymer inclusion membrane in a flask with 100 mL of the solution at 4 different temperatures 25°C, 35°C, 45°C, and 55°C and with agitation at 400 rpm, the impact of contact time and temperature on the adsorption of gold metal was investigated between 0-420 min. 10 milliliters of the solution were removed at various times to gauge its residual metal ion concentration.
The effect of membrane type was examined. For this experiment, 15 mL of the solution was contacted with the polymer inclusion membrane for a selected contact time at a selected temperature and pH. The adsorption capacity was calculated using the following equation, which took into consideration the solution's concentration difference at the start nd equilibrium [16].
where Co and Ct (mg/L) were the metal ion concentrations before and after adsorption, V was the volume of solution and m was the area of the membrane, respectively.
2.5. Extraction Device Set-Up
Transport of precious metal ions across the PIM follows a facilitated diffusion mechanism involving a carrier. The carrier plays a key role in the removal process by selectively binding metal ions from aqueous solution and facilitating their transfer across the membrane structure. The efficiency of metal removal depends on the chemical structure of the carrier, its binding affinity, and its stability within the membrane matrix. When the metal ion-carrier complex diffuses across the membrane, ions are released into the acceptor phase via a stripping reaction, allowing the carrier to regenerate and continue the transport cycle [10].
The selection of an appropriate stripping solution is critical because it affects both the metal ion release rate and the overall membrane performance.
The impact of the stripping phase, feed phase, and membrane contents on gold extraction was observed using the membrane's extraction mechanism. This system is divided into 2 parts, the feed, and the receiving phase. 250 mL of a 10-ppm gold standard solution was made for the gold extraction tests in the feed phase. Additionally, 250 mL of a 37% HCl solution was added to the receiving phase. To guarantee that the gold ions moved equally from the feed phase to the receiving phase, they were kept in identical volumes (250 mL) and mixed at the same speed (600 rpm) in both phases. Samples from the 2 phases were taken at 30-minute intervals for 6 hours at the start of the experiment. Figure 1 shows the extraction device set up for gold extraction. Also, Figure 2 shows the diagram of the abstract for the production and characterization of PIM, adsorption, and extraction of gold.
Extraction device set-up. (Feed solution on the left, stripping phase on the right, PIM in the middle)
Production, characterization, and experimental summary diagram.
2.6. Transport Experiment
Membrane performance is influenced by 3 key factors: the stripping phase, feed phase, and membrane contents. To systematically evaluate these factors, a series of experiments were conducted under controlled conditions. First, the effect of HCl molarity in the stripping phase was investigated. HCl concentrations of 0.1M, 0.25M, 0.5M, 1M, 2M, and 5M were tested, and for each concentration, a 10 mL sample was drawn from both the feed and receiving phases at regular intervals while stirring at 400 rpm at a constant temperature. Second, the effect of gold concentration in the feed phase was examined by preparing solutions containing 2.5 ppm, 5 ppm, 7.5 ppm, 10 ppm, and 15 ppm of gold. The optimal HCl molarity and gold concentration were determined based on the highest transport efficiency and recovery factor. Finally, experiments were performed using membranes with varying compositions and thicknesses to identify the most effective membrane formulation for gold transport.
To describe the kinetics of gold ion transport through the PIM, the following mathematical models were applied:
Transport measurements were performed in the experimental rig described and presented in detail elsewhere [7,17]. The following equation can be used to predict the kinetics of gold ion transport across the PIM [18].
where k is the rate constant (s-1), t is the time of transport (s), c is the metal ion concentration (M) in the source phase at a specific time, and ci is the initial metal ion concentration in the source phase. The plot of ln(C/Ci) vs. time was used to determine the value of the rate constant (k). Following that, the permeability coefficient (P) was determined using equation (3) [18].
where A is the membrane's effective area and V is the volume of the aqueous source phase. The initial flux value (Ji) was subsequently calculated using
Equation (5) was used to obtain the recovery factor percentage (RF%) [19]:
These equations provide a comprehensive understanding of the gold transport kinetics through PIMs and help identify optimal conditions for maximizing recovery efficiency.
3. Results and Discussion
3.1. Characterization of PIM
FTIR Spectroscopy was employed to characterize the chemical structures of all polymer inclusion membranes.
Functional groups and molecular structure of polymer inclusion membranes were characterized using FTIR Spectroscopy as shown in Table 2 and Figure S1.
The FTIR results for all membranes.
According to FTIR analysis, the characteristic features of the CTA-based polymer include O-H bands between 3380 and 3400 cm-1, C-H bonding between 2850 and 2930 cm-1, and C=O stretching between 1740 and 1750 cm-1. Additionally, the typical characteristics of the PVDF-based polymer are attributed to C-H bands between 3300 and 2700 cm-1, C=C bands between 1680 and 1600 cm-1, and C-N bands between 1230 and 1020 cm-1. Moreover, the characteristic features of the PVC-based polymer are associated with C-Cl bands between 610 and 692 cm-1, C-H bonding at 1425 cm-1, and C-H stretching at 2915 cm-1. The presence of an O-H bond was observed in the TOA-added samples between 3700 and 3550 cm-1. The addition of Aliquat 336 revealed ammonium group bonds between 1467 and 1378 cm-1 and –CH3 group bonds between 2926 and 2857 cm-1. Furthermore, the strong peaks around 1550 and 1500 cm-1 observed upon NPOE addition were attributed to N-O groups.
The contact angle was used to assess the hydrophilicity of the membrane at different plasticizers and carriers. Depending on how hydrophilic a membrane is, it may either dissolve in water and release the carrier or it will have poor extraction efficiency and be susceptible to biofouling. The PIM needs to be both hydrophobic and hydrophilic to both stop the carrier from seeping away and to extend the time the membrane surface is in contact with the feed solution [26].
Table 3 shows the average contact angle values for different PIMs. The kind of polymer matrix and the additives (carrier, plasticizer) introduced into the membrane will determine the membrane’s water contact angle [27]. It is evident that membrane surface topography, in addition to surface polarity, can influence the water contact angle. The addition of carrier molecules to the CTA polymer base can change the resulting PIMs’ water contact angle, which is a measure of the membrane’s wettability [28].
Characterization of polymer inclusion membranes.
The contact angles were reduced when the carrier concentrations were raised and the base polymer concentrations were lowered. Additionally, the addition of NPOE caused CTA-3’s contact angle to be greater than CTA-2’s. This indicates that the CTA film’s surface hydrophobicity is decreased by the presence of NPOE. Additionally, the inclusion of NPOE made PVDF-2 less hydrophilic than PVDF-3. Additionally, the inclusion of NPOE made PVC-2 less hydrophilic than PVC-3. This indicates that the PVDF and PVC films' surface hydrophobicity is increased by the presence of NPOE.
According to Table 3, the Aliquat 336 was more hydrophilic than NPOE. Aliquat 336 has hydrophilic ammonium groups [29]. Number 5’s contact angles were all greater than those of number 3’s. CTA-5, PVDF-5, and PVC-5 all had greater contact angles than CTA-3, PVDF-3, and PVC-3, respectively. This finding may be explained by the potential greater polarity of TOA in contrast to other PIM members [19]. The long-chain alkyl hydrophobicity of NPOE causes an increase in the water contact angle of CTA membranes when it is used as a plasticizer. Lower water contact angles after the addition of several carrier molecules (extractants) to the membrane show that the inclusion of Aliquat 336, NPOE, or TOA and modifiers enhanced the wettability of the membrane surface [30].
Figure 3 and Tablo S1 depicted the cross-sectional SEM images of the membranes. To determine the membranes' surface morphology, SEM studies were conducted. Every membrane was dense [31]. According to the study, the membrane morphology has been described here to be highly reliant on the preparation technique, and dense membranes should be seen when the solvent evaporation approach is applied [26,32].
Scanning electron microscope of polymer inclusion membrane (after production 500x).
It is essential to make certain that the materials are thermodynamically stable and do not degrade at different temperatures. TGA analysis was therefore used to characterize every membrane. Following the analysis, the following circumstances were noted. For CTA, degradation happened in 2 phases. The temperature at which dehydrochlorination occurs is 320°C, while the temperature at which conversion to carbon dioxide and volatile hydrocarbons occurs is 380°C. At 400°C, the final portion of the weight loss was noticed, and at 600°C, carbonization to ash was noticed. The dissociation of the HCl molecule from the chain causes the first deterioration of the PVC membrane at 200°C. The entire breakdown and charring of the polymer chain is the cause of the second thermal breakdown, which occurs at 400°C. 3 stages of degradation were seen in the PVDF membrane. The material underwent 3 stages of decomposition: the first occurred between 20 and 420°C, the second occurred between 420 and 450°C and resulted in the largest mass loss of the material and the third mass loss step began at 450°C and persisted even though the temperature reached 600°C. The carrier and plasticizer were added, and as a result, each weight loss curve was distinct into 3 sections. The evaporation of the solvent accounts for the first portion of the weight loss below 180°C. The subsequent procedures are referred to as weight loss because of the additional plasticizer and carrier. Furthermore, it was noted that Aliquat 336 has superior thermal stability to the others.
The membrane roughness was investigated using optical profilometry (RMS). The RMS measurements indicated that the addition of TOA and Aliquat336 had a minor impact on the membranes' surface roughness. Generally speaking, an increase in average membrane surface roughness causes a corresponding increase in membrane flow, creating a very efficient filtration area. However, as membrane roughness increases, the efficacy of the membranes to prevent deposits declines because of the retention of pollutants in the high-roughness membrane surface valleys. As a result of adsorption experiments, 4 membranes were produced with different ratios of the best base polymer, carrier, and plasticizer selected, and extraction studies were carried out using these membranes. Surface roughness results of the produced membranes are also given below (Figure 4).
The surface morphologies of membranes a) PIM containing 60% TOA, b) PIM containing 20% A336 and 40% TOA, c) PIM containing 40% A336 and 20% TOA and d) PIM containing 60% A336.
3.2. Adsorption performance
3.2.1. The effect of solution pH
Experiments were conducted at various pHs to understand the polymer inclusion membranes' ability to absorb gold. Since the gold standard solutions' initial pH was 0.21. By adding NaOH and HCl, the solutions' pH levels were changed. Table 4. illustrates how the gold adsorption capacity changes as the pH of the solution rises. This demonstrates that the production of the more stable ion-pair complex is encouraged by the equilibrium at pH 1. Hydrolysis might occur if an adsorption solution with a higher pH is utilized. This decreases the efficacy of adsorption by indirectly promoting competing equilibria through the creation of an ion-pair complex [33].
The pH effect of the solution on membrane adsorption.
At low pH levels, excess H⁺ ions compete with gold ions for membrane binding sites, reducing adsorption. Gold is primarily found in the AuCl4⁻ complex, which is stable and easily adsorbed at pH 1. However, as pH increases (pH 2-5), hydrolysis may occur, destabilizing gold ions and decreasing adsorption. Different polymers interact with gold ions uniquely due to their chemical properties, affecting adsorption capacity. Additionally, high Cl⁻ concentrations at low pH can stabilize or inhibit gold adsorption. Overall, ion competition, complex stability, and polymer interactions influence the adsorption trend.
The ideal pH for all following studies using CTA, PVC, and PVDF base polymers resulted in gold adsorption capacities of roughly 36.10, 44.20, and 38.00 mg/m2, respectively.
3.2.2. The effect of temperature and contact time
In the range of 0 to 420 min and 25°C to 55°C, the impact of contact time and temperature on gold adsorption was studied. All of the figures show that the gold adsorption (i.e., the adsorption capacity increased with increasing time) increased positively with increasing contact time, reaching equilibrium after 240 min.
The adsorption capacities of gold as a function of temperature are presented in Figure 5. Examining the findings of the temperatures and contact time for the adsorption of gold revealed that the adsorption capacities increased with temperature. Additionally, it was found that lengthening the contact times enhanced the membranes' capacity for adsorption, but after a while, the adsorption % remained the same.
The contact time and temperature effect of solution (a) for CTA base polymer, (b) for PVC base polymer, and (c) for PVDF base polymer.
For all polymers, the adsorption capacity generally increases with time initially and reaches a peak before decreasing at longer times. Higher temperatures (45°C and 55°C) generally increase adsorption, probably due to increased molecular interactions and diffusion rates. However, prolonged exposure at higher temperatures sometimes leads to decreased adsorption, probably due to desorption effects.
In addition, CTA-based membranes (Figure 5(a)) show the highest adsorption within the first 30 min, followed by a gradual decrease, indicating rapid initial binding followed by possible desorption or saturation effects. For CTA membranes, the adsorption at 55°C is highest at early time points but decreases significantly after 120 min, probably due to the weaker retention of gold ions at higher temperatures. CTA adsorption is more sensitive to temperature with rapid adsorption at longer times and also early desorption. Also, the highest adsorption capacity in CTA membranes was observed when the temperature was 25°C and the contact time was 30 minutes.
PVC-based membranes (Figure 5(b)) show a slower increase in adsorption, reaching a peak at 60 min before decreasing. This trend indicates a slower but more stable adsorption process compared to CTA. PVC membranes show optimum adsorption at moderate temperatures (35-45°C), indicating that extreme temperatures may not be suitable for stable adsorption. PVC membranes show a moderate and stable adsorption behavior, indicating that both enthalpic and entropic factors play a role. The highest adsorption capacity in PVC membranes was observed when the temperature was 35°C and the contact time was 60 minutes.
Finally, PVDF-based membranes (Figure 5(c)) show a more consistent adsorption trend, with a broad plateau observed between 45–300 min, indicating a relatively stable interaction between gold ions and the polymer matrix. PVDF membranes show the most stable adsorption trend at all temperatures, with the highest capacity at long contact times of 55°C. PVDF membranes exhibit thermodynamically optimum adsorption over a wide contact time and temperature range, making them a promising material for gold recovery. In PVDF membranes, the highest adsorption capacity was observed when the temperature was 45°C and the contact time was 360 minutes. Therefore, these temperatures and times were chosen for all membranes in subsequent gold experiments.
3.2.3. The effect of the type of membrane
In the study in this section, pH, temperature, and contact time were determined as those selected in the previous sections. It was found that the adsorption of precious metal increased with the change in content in Table 5’s analysis of the impact of membrane content on gold adsorption.
The effect of membrane type on adsorption.
PVC shows the highest adsorption capacity in all experiments and the values range from 530.9 mg/m2 to 808.0 mg/m2. This indicates that PVC has a strong affinity for gold ions due to its hydrophobic structure and specific interactions with gold complexes. PVDF shows moderate adsorption with capacities ranging from 223.2 mg/m2 to 634.0 mg/m2. Fluorine content in PVDF may affect the gold adsorption behavior, leading to intermediate performance between CTA and PVC. CTA has the lowest adsorption capacity with values ranging from 70.2mg/m2 to 691.1mg/m2. The lower adsorption performance can be attributed to its hydrophilic nature, which may hinder efficient gold ion binding. CTA shows the most inconsistent adsorption performance, with a sharp increase in adsorption (up to 691.1 mg/m2 in one case) but significantly lower values in others. This suggests that the adsorption efficiency of CTA is more sensitive to environmental conditions. Furthermore, PVC may contain active sites that enhance complexation with gold ions, while PVDF and CTA may have fewer available sites for strong binding. In addition, differences in membrane morphology, such as porosity and surface area, may also affect adsorption efficiency. The higher surface area generally leads to better adsorption performance.
The results show that PVC is the most effective membrane for gold adsorption, followed by PVDF, and CTA is the least effective. The observed differences highlight the importance of polymer chemistry in designing effective adsorption membranes for metal recovery applications.
This is due to both the inclusion of various carriers and the result of the compatibility between the carriers and plasticizers. Additionally, it was shown that adding Aliquat336 enhanced the adsorption capacity and resulted in the adsorption of more valuable metals than other added materials. Figure 6 shows the surface SEM images of the membranes after the adsorption of gold by using PIM.
Scanning electron microscope of polymer inclusion membrane (after adsorption).
3.3. Filtration performance
In this section, gold extraction performance was investigated by selecting the membrane with the best results as a result of adsorption experiments, changing the feed phase, receiving phase, and membrane contents. In this study, membranes containing PVC base polymer and containing carrier and plasticizer were produced. Experiments were carried out using gold as a precious metal using the membranes.
3.3.1. Effect of stripping phase (HCl) concentration
When the literature is examined, HCl was used in this study because both gold and acid contents are high in industrial wastewater and the most effective environment for the separation of gold group metals is the chloride environment [34]. In the study, the stripping phase was changed by keeping the membrane content and the gold concentration in the feed phase constant. To examine the effects of HCl concentration in the extraction solution on the transport of gold(I), extraction experiments were carried out by adjusting the HCl concentration at different concentrations from 0.1M to 2M.
In the study, the stripping phase was changed by keeping the membrane content and gold concentration in the feed phase constant. To examine the effects of HCl concentration in the extraction solution on the transport of gold (I), extraction experiments were performed by adjusting the HCl concentration at different concentrations from 0.1M to 2M.
Figure 7 illustrates that gold in solution is extracted within the 0.1-2 M HCl concentration range and that the percentage of gold extracted increases continuously as the HCl concentration rises. At any HCl concentration, however, gold extraction is minimal.
Effect of stripping phase on extraction efficiency of gold.
When oxidizing chemicals are present in hydrometallurgical processes, gold in welds is dissolved using a concentrated HCl solution. Since the combination of gold and extractant reacts with the proton in the recovery solution, it can be expected that the gold ion can be recovered using any concentration of hydrochloric acid.
Membranes containing PVC, Aliquat 336, and TOA were fabricated for the extraction research based on the adsorption results. The primary goal is to analyze the gold removal performance. To do this, the impact of the membrane contents, feed phase gold concentration, and receiving phase were all investigated. It was looked into how the receiving phase affected things. The receiving phase’s molarity was adjusted from 0.1M to 2M HCl during the research phase. The results of this experiment indicated that 0.25M HCl was the ideal concentration. Following the decision on the receiving phase, the concentrations of gold were ascertained. The extraction analysis determined that the feed phase’s gold concentration should be 10 parts per million. The topic of the final step is membrane contents. During the extraction experiments, the concentrations of the plasticizer, carrier, and base polymer were adjusted. The recovery rates were investigated because the results of the 6-hour removal efficiency of all membranes containing Aliquat336 were nearly identical. This led to the selection of the membrane with 40% PVC, 40% A336, and 20% TOA.
The impact of the stripping phase’s HCl concentration on rate constants, permeability coefficients, and recovery factors is displayed in Table 6. Using 0.25M HCl as the stripping phase produced the highest rate constant of the transport process. At 0.5 M HCl, the gold recovery factor peaked. However, when the recoveries were examined, no significant differences were observed, so the stripping phase was chosen as 0.25M HCl for future experiments. In this phase, both the removal efficiency is high and the permeability coefficient is higher than the others. Figure S2 shows the surface SEM images of the membranes after the extraction of gold by changing the HCl concentration.
Effect of HCl concentration on the kinetic parameters of gold.
3.3.2. Effect of gold concentration in the feed phase
The feed phase’s gold concentration ranged from 2.5 to 15 parts per million. The permeability stabilized because only mononuclear species were present in the feed phase and the metal concentration in this concentration range did not influence gold extraction. Except for 10 ppm gold, more than 55% gold was delivered in every instance (Figure 8). As a result, the feed solution’s gold concentration was set at 10 ppm, and tests were conducted using these solutions to alter the membrane content.
Effect of gold concentration in feed phase on PIM performance.
The impact of feed phase gold concentration on rate constants, permeability coefficients, and recovery factors is displayed in Table 7. The highest rate constant of the transport process was obtained when 10 ppm gold was used as the feed phase. The recovery factor of gold was highest (over 20%) at 10 ppm. The surface SEM images of the membranes following gold extraction by varying the concentration of gold ppm are displayed in Figure S3.
Effect of gold concentration on the kinetic parameters of gold.
3.3.3. Effect of membrane type
The concentration of an extractant has a huge impact on the efficiency of target metal ions extraction. Moreover, in the study of theoretical phenomena, the concentration of extractant is one of the most significant factors that have an effect on the extraction process [34]. The effects of TOA and Aliquat 336 on the kinetic characteristics of gold ions were examined in this section.
Figure 9 shows that gold in the solution was extracted by using PIMs. Membrane performance as a result of changing membrane content is shown in this figure. The performance of membranes without Aliquat 336 is lower than other membranes. Because Aliquat 336 acts as a carrier and TOA acts as a plasticizer. Membranes without carriers do not give good results.
Effect of membrane contents on extraction of gold.
Reduced metal ion flux was the outcome of further increases in carrier concentration in the membrane phase, most likely as a result of the carrier’s limited solubility. The metal ion transporter complex’s membrane saturation is another explanation for this effect [35]. However, once the concentration of Aliquat 336 was reached, the rate of separation fell. The increasing viscosity of the liquid membrane phase explains this [36,37].
The impact of membrane content on the initial flux (Ji) value and other kinetic parameters, including rate constants, permeability coefficients, and recovery factors, is displayed in Table 8. When 20% A336 and 40% TOA membrane were utilized, the transport process’s highest rate constant was achieved. The recovery factor of gold was the highest (above 37.9%) at 0.25M HCl by using 40% PVC-40% A336 membrane. Following the extraction of gold by altering the membrane composition, Figure S4 displays the surface SEM images of the membranes.
Effect of membrane content on the kinetic parameters of gold.
The viscosities of the plasticizer and carrier determine the variations that arise from the alteration of membrane contents. Viscosity variations in mixed membranes led to either an increase or a decrease in kinetic parameters. However, an increase in extractant concentration also encourages permeation swelling, which dilutes the aqueous receiving phase and reduces process efficiency.
The transport rates were impacted by the increase in membrane thickness brought on by the rise in carrier ion concentration in the membrane phase. The permeability coefficient rose with the thickness of the resulting membranes, which varied in thickness. The impact of membranes made with varying concentrations of Aliquat 336 on the movement of gold ions has been studied in the literature. Consequently, it was found that as the carrier concentration rose, so did the permeability coefficient.
4. Conclusion
PIMs with various contents were fabricated and characterized in this study. The analysis was conducted to determine how the generated membranes affected the gold adsorbent. pH’s were investigated first in the research section. The ideal pH for every membrane was found to be 1 based on pH experiments. Temperature and contact time were established after the pH was chosen. The membranes containing PVC, PVDF, and CTA had contact times and temperatures of 25°C-30 min, 35°C-60 min, and 45°C-360 min, respectively. The parameters that were established led to an investigation of each membrane s gold adsorption capabilities. The study’s best findings for each base polymer were CTA-2, PVDF-4, and PVC-4.
Membranes containing PVC, Aliquat 336, and TOA were fabricated for the extraction research based on the adsorption results. The primary goal is to analyze the gold removal performance. To do this, the impact of the membrane contents, feed phase gold concentration, and receiving phase were all investigated. It was looked into how the receiving phase affected things. The receiving phase’s molarity was adjusted from 0.1M to 2M HCl during the research phase. The results of this experiment indicated that 0.25M HCl was the ideal concentration. Following the decision on the receiving phase, the concentrations of gold were ascertained. The extraction analysis determined that the feed phase s gold concentration should be 10 parts per million. The topic of the final step is membrane contents. During the extraction experiments, the concentrations of the plasticizer, carrier, and base polymer were adjusted. The recovery rates were investigated because the results of the 6-hour removal efficiency of all membranes containing Aliquat336 were nearly identical. This led to the selection of the membrane with 40% PVC, 40% A336, and 20% TOA.
Supplementary Materials
Notes
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
