# Selective Chelating Resin for Copper Removal and Recovery in Aqueous Acidic Solution Generated from Synthetic Copper-Citrate Complexes from Bioleaching of E-waste

Authors: Kowit Suwannahong; Jiyapa Sripirom; Chadrudee Sirilamduan; Vanlop Thathong; Torpong Kreetachart; Piyapat Panmuang; Anat Deepatana; Suphot Punbut; Surachai Wongcharee
Journal: Adsorption Science & Technology
 (2022)

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## Abstract

This research focused on batch experiment using a new generation of chelating resins via an ion exchange process to describe the metabolic adsorption and desorption capacity onto iminodiacetic acid/Chelex 100, bis-pyridylmethyl amine/Dowex m4195, and aminomethyl phosphonic/Lewatit TP260 functional groups in bioleaching. The results showed that Dowex m4195 had the highest performance of adsorption capacity for copper removal in both H+-form and Na+-form. Results for Lewatit TP260 and Chelex 100 revealed lower adsorption performance than results for Dowex m4195. The investigation of desorption from chelating resins was carried out, and it was found that 2 M ammonium hydroxide concentration provided the best desorption capacity of about 64.86% for the H+-form Dowex m4195 followed by 52.55% with 2 M sulfuric acid. Lewatit with 2 M hydrochloric acid gave the best desorption performance in Na+-form while Chelex 100 using hydrochloric at 1 M and 2 M provided similar results in terms of the H+-form and Na+-form. As aspects of the selective chelating resins for copper (II) ions in aqueous acidic solution generated from synthetic copper-citrate complexes from bioleaching of e-waste were considered, H+-form Dowex m4195 was a good performer in adsorption using ammonium hydroxide for the desorption. However, chelating resins used were subsequently reused for more than five cycles with an acidic and basic solution. It can be concluded from these results that selective chelating resins could be used as an alternative for the treatment of copper (II) ions contained in e-waste or application to other divalent metals in wastewater for sustainable water and adsorbent reuse as circular economy.


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## Body

## 1. Introduction

Copper (Cu) is a metal with significant usages forming an essential part of materials and products for human well-being. Sulfide ores and Cu oxides are the primary sources of Cu [1]. Cu is widely used in building and construction, power generation, industrial machinery and equipment, transport equipment, and general consumer items [2], especially electrical and electronic devices. Consequently, copper production has been increased from over 600 thousand tons in 2010 to over 850 million metric tons in 2021, an annual average growth rate of over 4 percent [3]. The major copper-producing countries are Chile, Peru, and China [3]. Development of new technologies using Cu has increased over the time and is associated with economic growth linked to electronic devices or components [4] as waste printed circuit boards (WPCBs) [5]. With the rapid advancement of electronic products, massive qualities of electronic waste (e-waste) have led to growing environmental concerns as there is no adequate management technology for more than 50 million metric tons of e-waste generated by the end of 2018 [6]. Therefore, the concept of reusing or recycling metals from e-waste plays an important role in allocating resources, and it is widely accepted that the recycling of Cu is an essential and beneficial supplement to primary metal production.Currently, electronic waste is becoming valuable because it is known as waste electric and electronic equipment (WEEE) or e-waste and is a secondary source of Cu [7] and also contains a mixture of more than a thousand toxic substances [8]. Literature reported that 1 ton of e-waste contains up to 0.2 tons of Cu, of which Cu-rich waste was composed of 44.8% nonmetals by mass and 55.2% of metals (34% Cu, 11% Zn, 5.2% Al, 2.8% Fe, 1.3% Sn, 0.13% Mg, and 0.75% Pb) [9, 10]. Bachofen and Brandl [11] reported that metals contain 851.3 g/kg of Cu, 1.3 g/kg of Al, 0.6 g/kg of Mg, and 0.3 g/kg of Zn.From 2017 to the present time, about 33% of e-waste (over 72 million metrics tons) will be generated worldwide. In Southeast Asia, the total of e-waste produced is around 16 metric tons giving rise to recent concerns for removal and recovery of Cu from e-waste [12] for sustainable development. Europe has also focused on Cu recycling from e-waste [4] using various recovery methods such as electrolysis, ion exchange electrochemical extraction [7], hydrometallurgy [13, 14], chemical precipitation, biotechnology [15, 16], solvent extraction, mechanical methods [17], or even adsorption [7, 18–21]. Therefore, its recovery and reusability from e-waste are important features of waste management/treatment, and appropriate technology must be developed for sustainable metal reuse.Alternative technologies to extract a metal have been studied in the last few decades. Bioleaching is one such technology used in the extraction of metal from ores and e-waste, which is popular because it is environmentally friendly and economical [15, 22]. Numerous leaching solutions have been projected for leaching of valuable Cu from e-waste such as organic acids (tartaric acid, oxalic acid, and citric acid) [23], inorganic acid (H2SO4 with H2O2 and H2SO4 with CuSO4 and NaCl) [24], microbes [25], and fungi [26]. Bioleaching processes are simple and effective and are like natural biological cycles based on the use of microorganisms. It has been successfully used to leach metal from ores, but a few studies have been applied in the leaching of metal from e-waste material. Currently, there are a few studies of using microorganisms from different e-wastes such as Aspergillus niger for leaching metals (Al, Cu, Pb, Ni, Sn, and Zn) [27] from e-waste, and there is a lack of recovery or reusability data from bioleaching solutions. Based on the sorption ability and selectivity, adsorption processes by ion exchange are more effective for the recovery of metal ions from bioleaching solutions, but again, the literature is limited and a detailed knowledge of adsorption mechanism is very important [17]. As reported by earlier studies, numerous materials have been used as adsorbents, and chelating ion exchange resins are extremely effective. A chelating resin sorbent basically comprises of two components, the chelate forming the functional group and a polymeric matrix that can be applied or used for the adsorption of metal ions in aqueous solutions, wastewater, and metal-organic complexes that originate from bioleaching processing [28–30].In the bioleaching process, microorganisms such asA. niger excreted a metabolic acid during the leaching of metal. A. niger excreted 0.003 M oxalic acid and 0.18 M citric acid, P. simplicissimum produced 0.005 M oxalic acid, and heterotrophic fungi produced 0.02 nitric acid, 0.13-1.4 M citric acid, 0.00047-0.56 M malic, and 0.60-3.30 M lactic acids [31]. Metals are complexed with metabolic products such as metal-citrate, metal-oxalate, metal-malate, and metal-lactate. It can be seen that when leaching with fungi, citric acid is considered the most effective of the leaching metals [10, 32]. After the leaching process, the adsorption process is used as a key to reclamation of valuable metal ions from acidic solutions. The literature reveals that the adsorption mechanism of metal ions from metal complexes in acidic conditions is rare because most adsorbents can only be used on media with moderate pH values. Therefore, there is much research that has pointed out that most adsorbents have poor adsorption and selectivity for metals from strong acid media [33]. There is no in-depth understanding of metal-complex adsorption.In recent years, researchers had studied the adsorption of Cu using a new novel chelating resin called Chelex 100 and found that the chelating iminodiacetic acid functional group is able to select Cu and divalent metal from wastewater produced from the manufacture of semiconductors and printed circuit boards (PCB contained Cu (II) ions, Ni (II) ions, and Cu-citrate complexes) [34–36]. In addition, the pH of the solution originating from bioleaching had been studied, and it was found that Chelex also had potential for Cu (II) ions and Zn (II) ions at pH 1-5 [37]. In addition, alternative choices of chelating resin such as the bispicolylamine functional group (Dowex m4195) have been effectively used for the adsorption of Cu from strong acid media with pH lower than 2 [33, 38, 39]. One other option of chelating resin is the methyl phosphonic acid functional group (Lewatit) which is selective for the removal and recovery of Cu from bioleaching solutions from sulfide and Cu oxide ores at acidic pH [40, 41]. It can be concluded that Chelex 100, Dowex m4195, and Lewatit that is an ionic form of Na ion (Na+) were achieved for Cu adsorption [42], while the ionic form of H ion (H+) has not been investigated yet for the removal and recovery of Cu-organic complexes from bioleaching of e-waste that should be addressed.There is an essential need for evaluation of data of Cu adsorption and recovery from Cu-organic complexes generated from bioleaching of e-waste using chelating resins. This work presents a selective chelating resin in both Na+ and H+ forms for Cu adsorption and recovery from Cu-organic complexes from bioleaching of e-waste. The potential of various commercial chelating resins with particular consideration of updated ion exchangers such as Chelex 100, Dowex m4195, and Lewatit TP260 for metabolic adsorption capacity onto iminodiacetic acid, bis-pyridylmethyl amine, and aminomethyl phosphonic functional groups and recovery of Cu-citrate complexes in a batch experimental system was investigated for sustainable water and adsorbent reuse.


## 2. Materials and Methods

### 2.1. Chelating Resin Preparation

To achieve its large-scale application in removing Cu (II) ions in aqueous acidic solutions generated from bioleaching of e-waste, modification of selective chelating resin had been studied with various types of functional groups, specifically, iminodiacetic acid, bispicolylamine, and aminomethyl phosphonic.Three commercial macroporous chelating resins in Na+-form were used in this study: Chelex 100 (styrene-divinylbenzene matrix, iminodiacetic acid, Sigma-Aldrich Company), Dowex m4195 (polystyrene-divinylbenzene matrix, bispicolamine, Dow Chemical Company, supplied by Supelco), and Lewatit TP260 (polystyrene-divinylbenzene matrix, aminomethyl phosphonic, Sigma-Aldrich Company), which are shown in Table1. Modified H+-form chelating resins were prepared and used as described in the literature [41, 43–47]. 100 g of the Na+-form of Chelex 100 was washed with 2 M of nitric acid (HNO3, AR grade, Merck, Germany), 100 g of Dowex m4195 was washed with 1 M of sulfuric acid (H2SO4, AR grade, Ajax Finechem), and 100 g of Lewatit TP260 was washed with 1 M of hydrochloric acid (HCl, AR grade, Merck (Supelco)) at room temperature for 24 h with magnetic stirring at 60 rpm as described in the references. After that, the obtained wet H+-form chelating resins were subsequently decanted several times with deionized (DI) water to remove excess acid used at a neutral pH and dried in an oven at 60°C for 24 h. The dried H+-form chelating resins were kept at room temperature in a glass bottle for further batch sorption analysis.Table 1
Characteristics and properties of Chelex 100, Dowex m4195, and Lewatit TP260 chelating resins.
PropertiesChelating resinChelex 100Dowex m4195Lewatit TP260MatrixStyrene-divinylbenzenePolystyrene-divinylbenzenePolystyrene-divinylbenzeneFunctional groupIminodiacetic acidBispicolylamineAminomethyl phosphonicStructureMacroporousMacroporousMacroporousIonic formNa+Na+Na+Particle size (μm)150-300300-850400-1250pH range0-140-70-14Temperature (°C)75°C max temp.60°C max temp.85°C max temp.


### 2.2. Synthetic Copper-Citrate Complex Preparation

Aqueous acidic solutions from synthetic copper-citrate complexes (Cu-citrate complexes) generated from bioleaching of e-waste were prepared by dissolving about 690 mg/L of copper nitrate salts (Cu(NO3)2·3H2O, AR grade, Qrec, New Zealand) in 0.01 M of an aqueous solution of citric acid monohydrate (C6H8O7·H2O, AR grade, Loba Chemie). 0.01 M citric acid concentration is typical of that produced by the microorganisms (Bacillus megaterium, Pseudomonas putida, Acidithiobacillus sp., and Aspergillus niger) used in bioleaching metal from e-waste [27]. Table 2 shows that the citric acid can metabolize and/or extract the maximum Cu (II) ions from e-waste using microorganisms (Aspergillus Niger or A. Niger) which are about 86% maximum of metal mobilization (metal solubilization in acidophilic microorganisms) from the initial Cu concentration (690 mg/L) [10, 31].Table 2
Electronic scrap concentration and metal mobilization of e-waste leachates at different concentrations usingA. niger for the biotechnology treatment of Cu (II) ions contained in solid waste [27, 31].
ElementElectronic scarp concentration (g/L)1 (g/L)10 (g/L)50 (g/L)100 (g/L)Metal mobilization (%)Cu2+ (copper (II) ions)85 (0.07 g/L)86 (0.69 g/L)70 (2.80 g/L)8 (0.60 g/L)


### 2.3. Sorption Studies

Batch sorption and/or adsorption were performed in batch experiments at an initial pH (2.30) of Cu-citrate complexes (mixed solution), using 250 mL Erlenmeyer flasks containing about 100 mL of synthetic Cu-citrate with 1 g of the unmodified and modified chelating resins (Chelex 100, Dowex m4195, and Lewatit TP260) added. The concentration and modified chelating resin mass were examined after evaluating literature data and characteristic discharge concentration and performing several screening experiments. The samples were then shaken at 120 rpm in a shaker (WiseShake SHO-2D model) at room temperature for 24 h in separate runs to ensure that equilibrium was reached without adjusting pH. At the end of the preset time, each modified chelating resin was removed by passing aliquots through a Whatman 0.45μm GF/C filter for measuring the residual copper concentration. Eutech pH 700 was used to measure pH for all experiments.Inductively Coupled Plasma Atomic Emission Spectroscopy (ICP-AES) model Optima 8000 (Perkin Elmer Inc. USA) was used to measure the initial and the final concentrations of copper in the solutions with the analytical wavelength of about 327.393 nm. The adsorption capacity per unit mass (mg copper/g resin) was determined at equilibrium from(1)qe=Ci−Cem⋅V,where Ci is the initial concentration (mg/L) of Cu, Ce is the equilibrium Cu concentration (mg/L), m is the dry weight of the chelating resins (g), V is the volume of the solution (L), and qe is mg of Cu (II) ion adsorbed per gram of chelating resin.The percent sorption efficiency at equilibrium was determined following(2)S%=C0−CiC0⋅100,where S is the percent sorption efficiency (%) at the equilibrium of Cu (II) ions.


### 2.4. Desorption Studies

Desorption of loaded copper (II) ions onto unmodified and modified chelating resins was examined by rinsing with deionized water to remove any unadsorbed copper (II) ions. The adsorbents were then dried at room temperature for 24 h and kept in a desiccator. Afterward, adsorbents (1 g) were desorbed with 100 mL of the eluents including hydrochloric acid (HCl), sulfuric acid (H2SO4), and ammonium hydroxide (NH4OH, AR grade, Qrec, New Zealand) of 0.5 M, 1 M, and 2 M solution, respectively [48–50], and were mildly sonicated for 5 min under similar conditions to that of bath adsorption. The supernatants were filtered, and then, the desorbed copper concentration was analyzed using inductively coupled plasma atomic emission spectroscopy. The processes of the desorption batch experiment were repeated until negligible desorbed copper (II) ions were found in the supernatant. The percentage of desorption (%D) was calculated following
(3)D%=qdesorbedqadsorbed⋅100,where D is the percent desorption efficiency (%), qdesorbed is mg of Cu (II) ions desorbed per gram of chelating resin at negligible desorbed Cu which was found in the supernatant, and qadsorbed is mg of Cu ion adsorbed per gram of chelating resin at equilibrium. The completion of all experiments was carried out in triplicate runs, and only mean values were presented.


## 2.1. Chelating Resin Preparation

To achieve its large-scale application in removing Cu (II) ions in aqueous acidic solutions generated from bioleaching of e-waste, modification of selective chelating resin had been studied with various types of functional groups, specifically, iminodiacetic acid, bispicolylamine, and aminomethyl phosphonic.Three commercial macroporous chelating resins in Na+-form were used in this study: Chelex 100 (styrene-divinylbenzene matrix, iminodiacetic acid, Sigma-Aldrich Company), Dowex m4195 (polystyrene-divinylbenzene matrix, bispicolamine, Dow Chemical Company, supplied by Supelco), and Lewatit TP260 (polystyrene-divinylbenzene matrix, aminomethyl phosphonic, Sigma-Aldrich Company), which are shown in Table1. Modified H+-form chelating resins were prepared and used as described in the literature [41, 43–47]. 100 g of the Na+-form of Chelex 100 was washed with 2 M of nitric acid (HNO3, AR grade, Merck, Germany), 100 g of Dowex m4195 was washed with 1 M of sulfuric acid (H2SO4, AR grade, Ajax Finechem), and 100 g of Lewatit TP260 was washed with 1 M of hydrochloric acid (HCl, AR grade, Merck (Supelco)) at room temperature for 24 h with magnetic stirring at 60 rpm as described in the references. After that, the obtained wet H+-form chelating resins were subsequently decanted several times with deionized (DI) water to remove excess acid used at a neutral pH and dried in an oven at 60°C for 24 h. The dried H+-form chelating resins were kept at room temperature in a glass bottle for further batch sorption analysis.Table 1
Characteristics and properties of Chelex 100, Dowex m4195, and Lewatit TP260 chelating resins.
PropertiesChelating resinChelex 100Dowex m4195Lewatit TP260MatrixStyrene-divinylbenzenePolystyrene-divinylbenzenePolystyrene-divinylbenzeneFunctional groupIminodiacetic acidBispicolylamineAminomethyl phosphonicStructureMacroporousMacroporousMacroporousIonic formNa+Na+Na+Particle size (μm)150-300300-850400-1250pH range0-140-70-14Temperature (°C)75°C max temp.60°C max temp.85°C max temp.


## 2.2. Synthetic Copper-Citrate Complex Preparation

Aqueous acidic solutions from synthetic copper-citrate complexes (Cu-citrate complexes) generated from bioleaching of e-waste were prepared by dissolving about 690 mg/L of copper nitrate salts (Cu(NO3)2·3H2O, AR grade, Qrec, New Zealand) in 0.01 M of an aqueous solution of citric acid monohydrate (C6H8O7·H2O, AR grade, Loba Chemie). 0.01 M citric acid concentration is typical of that produced by the microorganisms (Bacillus megaterium, Pseudomonas putida, Acidithiobacillus sp., and Aspergillus niger) used in bioleaching metal from e-waste [27]. Table 2 shows that the citric acid can metabolize and/or extract the maximum Cu (II) ions from e-waste using microorganisms (Aspergillus Niger or A. Niger) which are about 86% maximum of metal mobilization (metal solubilization in acidophilic microorganisms) from the initial Cu concentration (690 mg/L) [10, 31].Table 2
Electronic scrap concentration and metal mobilization of e-waste leachates at different concentrations usingA. niger for the biotechnology treatment of Cu (II) ions contained in solid waste [27, 31].
ElementElectronic scarp concentration (g/L)1 (g/L)10 (g/L)50 (g/L)100 (g/L)Metal mobilization (%)Cu2+ (copper (II) ions)85 (0.07 g/L)86 (0.69 g/L)70 (2.80 g/L)8 (0.60 g/L)


## 2.3. Sorption Studies

Batch sorption and/or adsorption were performed in batch experiments at an initial pH (2.30) of Cu-citrate complexes (mixed solution), using 250 mL Erlenmeyer flasks containing about 100 mL of synthetic Cu-citrate with 1 g of the unmodified and modified chelating resins (Chelex 100, Dowex m4195, and Lewatit TP260) added. The concentration and modified chelating resin mass were examined after evaluating literature data and characteristic discharge concentration and performing several screening experiments. The samples were then shaken at 120 rpm in a shaker (WiseShake SHO-2D model) at room temperature for 24 h in separate runs to ensure that equilibrium was reached without adjusting pH. At the end of the preset time, each modified chelating resin was removed by passing aliquots through a Whatman 0.45μm GF/C filter for measuring the residual copper concentration. Eutech pH 700 was used to measure pH for all experiments.Inductively Coupled Plasma Atomic Emission Spectroscopy (ICP-AES) model Optima 8000 (Perkin Elmer Inc. USA) was used to measure the initial and the final concentrations of copper in the solutions with the analytical wavelength of about 327.393 nm. The adsorption capacity per unit mass (mg copper/g resin) was determined at equilibrium from(1)qe=Ci−Cem⋅V,where Ci is the initial concentration (mg/L) of Cu, Ce is the equilibrium Cu concentration (mg/L), m is the dry weight of the chelating resins (g), V is the volume of the solution (L), and qe is mg of Cu (II) ion adsorbed per gram of chelating resin.The percent sorption efficiency at equilibrium was determined following(2)S%=C0−CiC0⋅100,where S is the percent sorption efficiency (%) at the equilibrium of Cu (II) ions.


## 2.4. Desorption Studies

Desorption of loaded copper (II) ions onto unmodified and modified chelating resins was examined by rinsing with deionized water to remove any unadsorbed copper (II) ions. The adsorbents were then dried at room temperature for 24 h and kept in a desiccator. Afterward, adsorbents (1 g) were desorbed with 100 mL of the eluents including hydrochloric acid (HCl), sulfuric acid (H2SO4), and ammonium hydroxide (NH4OH, AR grade, Qrec, New Zealand) of 0.5 M, 1 M, and 2 M solution, respectively [48–50], and were mildly sonicated for 5 min under similar conditions to that of bath adsorption. The supernatants were filtered, and then, the desorbed copper concentration was analyzed using inductively coupled plasma atomic emission spectroscopy. The processes of the desorption batch experiment were repeated until negligible desorbed copper (II) ions were found in the supernatant. The percentage of desorption (%D) was calculated following
(3)D%=qdesorbedqadsorbed⋅100,where D is the percent desorption efficiency (%), qdesorbed is mg of Cu (II) ions desorbed per gram of chelating resin at negligible desorbed Cu which was found in the supernatant, and qadsorbed is mg of Cu ion adsorbed per gram of chelating resin at equilibrium. The completion of all experiments was carried out in triplicate runs, and only mean values were presented.


## 3. Results and Discussion

### 3.1. pH Behavior after the Adsorption Process in Acidic Solution

In adsorption and desorption processes, pH behavior and/or value of solutions have effects on the coordination reactions and electrostatic interactions in physical and chemical mechanisms, due to iminodiacetic acid, bispicolylamine, and aminomethyl functional groups in unmodified and modified chelating resins affecting the pH value of its solution by changing the initial pH: Na+-form and H+-form chelating resins of Chelex 100, Dowex m4195, and Lewatit TP260. The influence of pH change after adsorption process characteristics is presented in Table3. The initial pH of the synthetic Cu-citrate complex solution used in the experiment was about 2.30. As shown in Table 3, the H+-form chelating resins showed different changes after the sorption processes for Chelex 100 H+-form (pH 2.03), Dowex m4195 H+-form (pH 2.01), and Lewatit TP260 H+-form (pH 2.02) as compared to the initial pH. Hence, copper (II) ions preferred acidolysis at low pH, and lack of coprecipitation with H+-form chelating resin plays a supplementary role to an ion exchange mechanism in this case study. It was obviously seen that an affinity of H+-form chelating resin for copper (II) ions was sensitive to pH value attributable to competition of adsorption among the copper (II) ions and H+ ions on the same active site (nitrogen atoms).Table 3
Percentage copper (II) ions adsorbed onto Na+-form and H+-form chelating resins and changed pH of the acidic solutions.
Chelating resinNa+-formH+-formPercent adsorption (S%)Final pHPercent adsorption (S%)Final pHChelex 10041.472.4839.212.03Dowex m419556.972.3586.202.01Lewatit TP26052.912.4346.842.02The Na+-form resins had contributed to the rise of pH value in the order of Dowex m4195 Na+-form (pH 2.35), Lewatit TP260 Na+-form (pH 2.43), and Chelex 100 Na+-form (pH 2.48), respectively. This indicated that when the resins in their Na+-form interacted with water, the hydrolysis of the resin resulted in the formation of a three-component ion exchange system of H3O+/Na+/M2+ as explained in the literature [32]. Divalent copper (II) ions will react with nitrogen atoms to form a very stable charge balance (strong ligand bonding).It can be concluded that metal adsorption started when the pH reached a high enough range from which most acidic ion exchange sites started to exchange the H3O+ (hydronium ions) and reached their maximum uptake capacity. Likewise, pH promoted the ion exchange sites that contributed to the reaction and the functional group to shape the chelate rings with the metallic cations of copper divalent.Comparison of three chelating resins in H+-form showed that the highest percentage of selective adsorption capacity of about 86% as represented in Table3 occurred with a chelating resin ion exchange with the bispicolylamine or bis(2-pyridylmethyl) functional groups (uncharged tridentate ligand) containing two nitrogen atoms (donor atoms) of two chela (chaws) [51]. Nitrogen atoms can possess an electron pair to form charged complexes with divalent copper that shows very strong chelating properties at low pH (a weak base ion exchanger, Lewis acids) [52]. In terms of the Na+-form, it was also confirmed that Dowex m4195 demonstrated the highest percentage of selective adsorption capacity as compared to the other Na+-form chelating resins of Lewatit TP260 (52.91%) and Chelex 100 (41.47%), respectively. It is suggested that Dowex m4195 was appropriate for copper (II) ions in acidic solutions generated from bioleaching of e-waste; in other words, the cation species influence was greater than that of the anion species for these adsorbent chelating resins. However, isotherm and kinetic need a further study that will be impacted and vital in process design with a straightforward adsorption or desorption process.Figure1 presents the sorption process reaction formation of iminodiacetic acid, bispicolylamine, and aminomethyl groups in the acidic solutions, where nitrogen atoms will be protonated and may occur or combine.Figure 1
Possible sorption process reaction of copper (II) ion removal onto iminodiacetic acid (a), bispicolylamine (b), and aminomethyl (c) groups in the acidic solutions.
(a)
Iminodiacetic acid group(b)
Bispicolylamine group(c)
Aminomethyl group


### 3.2. Metabolic Adsorption Capacity onto Iminodiacetic Acid (Chelex 100) Functional Group Chelating Resins

Figure2 demonstrates chelating resins of copper (II) ions adsorbed at different uptake levels using an initial copper concentration of about 690 mg/L with 0.01 M citric acid as the original model substrate for studying the adsorption capacity onto Chelex 100, Dowex m4195, and Lewatit TP260. Chelex 100 demonstrated a small adsorption capacity in both H+-form and Na+-form as compared with other chelating resins of Dowex m4915 and Lewatit TP260 for both H+-form and Na+-form. It could be indicated that iminodiacetic acid chelating resin (Chelex 100) had low capacity in acidic solution conditions (at the low pH) as compared to sulfate and tartrate complexes for bioleaching of e-waste that were predominant at low pH [36]. According to the literature, it was found that copper (II) ion adsorption capacity was not very well as compared with moderate pH for the iminodiacetic acid functional group [38]. In addition, it can be seen that iminodiacetate chelating resin in terms of H+-form and Na+-form has not shown much difference of about 5.45% for the adsorption capacity.Figure 2
Adsorption capacity of copper (II) ions from Cu-citrate complexes prepared using 0.01 M citric acid by Lewatit TP260, Dowex m4195, and Chelex 100 chelating resins.In contrast to literature reports, iminodiacetate resin was not able to adsorb copper (II) ions in 0.05 M citrate acid atpH<2 [53]; however, it was successful at the lower concentration of citric acid at 0.005 M and 0.01 M. It might be pointed out that the functional group dissociates and since cation exchange reactions cannot be removed by neutral acids, the adsorption performance of iminodiacetate chelating resin with cations decreased due to the presence of acidic conditions. Furthermore, the pKa value of iminodiacetic acid was around 2, and at pH≤2, the uptake of negatively charged anions was promoted whereas the surface charge of chelating resin and metal complexes was similar to low pH. It is unlikely to capture the metal complexes of two carboxylic groups that are protonated when the Chelex 100/chelating resin with its functional groups of iminodiacetic acid is behaving as a poorly/weakly basic anion exchanger as shown in Figure 3 [39, 54]. As a result of the weakly acidic nature of iminodiacetic acid, the exchanger strongly favored hydronium ions and the competitiveness of hydrogen ions with metal ions to active sites on the resin surface in the medium. In summary, the adsorption ability of the iminodiacetic acid functional group of Chelex 100 was weak in highly acidic solutions.Figure 3
Structure of the iminodiacetic acid functional group at pH less than 2 and pH 3.99.


### 3.3. Metabolic Adsorption Capacity onto Bis-pyridylmethyl Amine (Dowex m4195) Functional Group Chelating Resin

Dowex m4195 showed a higher adsorption capacity compared to Lewatit TP260 and Chelex 100 chelating resins as presented in Figure2. Dowex m4195 is a special adsorbent for metal recovery from strongly acidic media [55, 56]. It was found that the Cu-loaded Dowex m4195 chelating resin in H+-form gave the highest adsorption capacity of about 59.49 mg copper/g resin, which was virtually the same as in earlier reports [57], whereas Dowex m4195 was the best selective sorption agent for divalent metal of Ni (II) ions and Co (II) ions from acidic solution at low pH of about 1. In other words, Dowex m4195 was favored at a high citric acid concentration and low pH; therefore, sorption of Cu-loaded chelating resin was better than that of both iminodiacetic acid and aminomethyl phosphonic chelating resin functional groups of Chelex 100 and Lewatit TP260 for both H+-form and Na+-form, respectively. Literature reports [33, 53] found that Cu-loaded Dowex m4195 and iminodiacetic resin of Amberlite 748 and Lewatit TP270 had favorable properties in acidic solution that could be associated with the unique chemical characters of the bis-pyridylmethyl amine functional group or Dowex m4195, which is appropriate in the elimination of Cu (II) ions from very strong acid (pH lower than 2) solution. Dowex m4915 also forms complexes with divalent metal ions through their free electron pair-bearing nitrogen atom of the bis-pyridylmethyl amine functional group. In comparison to the H+-form and Na+-form of Dowex m4195, it was found that the H+-form was more effective than Na+-form. This might be because the H+-form helps to split complexes and take on the sorption of Cu loaded onto chelating resin [53]. Additionally, it can be explained that the bis-pyridylmethyl amine functional group ligand contains three nitrogen donor atoms (see Figure 4) of electron pairs and at low pH. It can be seen that three nitrogen donor atoms of chelating resin are protonated [58, 59] and formed a chelate ring with copper (II) ions as shown in Figure 1(b). It can be concluded that Dowex m4195 H+-form has superior adsorption performance than Lewatit TP260 and Chelex 100 chelating resins at the low pH for both forms. Figure 5 demonstrates the discussed before and after adsorption of Dowex m41954 for the visual confirmations of the results. Observation of the image confirms that H+-form shows complete adsorption onto the surface of the resin.Figure 4
Structure of the bis-pyridylmethyl amine functional group.Figure 5
Microscope image (a) before adsorption and (b) after adsorption of copper (II) ion removal onto Dowex m4195.
(a)(b)


### 3.4. Metabolic Adsorption Capacity onto Aminomethyl Phosphonic (Lewatit TP260) Functional Group Chelating Resin

Lewatit TP260 has an aminomethyl phosphonic as a functional group in nature (Figure6), and the results indicated an intermediate adsorption level as compared to Dowex m4195 and with similar results to Chelex 100. It can be seen that Na+-form of the chelating resin of Lewatit TP260 (36.51 mg/g) was superior to the H+-form of Chelex 100 (27.05 mg/g) based on the adsorption capacity for Cu. Due to the ability of Lewatit TP260 to function at low pH, the adsorption capacity also correlated with those suggested in the literature. It is owing to the effect of a competition binding among divalent copper metal ions and hydrogen ions over the surface of the chelating resins. It can be noticed that at low pH, an overabundance of hydrogen ions can achieve successfully with metal ions for binding sites, resulting in a lower level of metal adsorption on aminomethyl phosphonic/methylphosphonic chelating resin. Additionally, the pKa values of phosphonic acid were about 2 and 5; thus, at increasing pH, more negative sites become accessible for the adsorption, leading to an increase in uptake for the adsorption capacity. Therefore, the adsorption of metal complexes on methylphosphonic acid resin was beyond hindered by the strong affinity of the resin for hydrogen ions in accordance with the acidic solution conditions [32]. It can be concluded that the aminomethyl phosphonate functional group is affected by pH. Due to the fact that the aminomethyl phosphonate functional group is a tridentate ligand, it has two binding sites at a phosphonic acid group and one coordination site at the secondary nitrogen atom as shown in Figure 6 [41]. On the other hand, the adsorption capability of chelating ion exchangers was also influenced by the structure of functional groups, their position in relation to each other, and the characteristics of the matrix (skeleton). The possible schematic mechanism for ion exchange adsorption and desorption of copper (II) ions is shown in Figure 7. Based on adsorption capacity, Lewatit TP260 was not satisfactory for Cu (II) adsorption ability as compared to Dowex m4195. These results confirmed that Dowex m4195 had the capability to capture Cu (II) ions generated from synthetic copper-citrate complexes from bioleaching of e-waste at a low pH value.Figure 6
Aminomethyl phosphonic/methyl phosphonic functional group.Figure 7
Possible schematic mechanism for adsorption and desorption of copper (II) ions using Lewatit TP260 chelating resin as an adsorbent.


### 3.5. Copper Desorption

Subsequent to the adsorption step, copper (II) ions loaded onto chelating resins were recovered/desorbed using 0.5 M, 1 M, and 2 M of HCl, H2SO4, and NH4OH, respectively, in the liquid phase and batch mode experiments that were examined. Prior to each run of the desorption experiment, an adsorption step was performed as detailed in Section 2.4 for the three chelating resins used in both H+-form and Na+-form. Desorption efficiencies of copper (II) ions are shown in Table 4 (desorption capacity) and Figures 8–10 (desorption efficiency), and these can help to clarify the mechanism of adsorption processes and the reusability of the chelating resin used. If the copper (II) ions are adsorbed onto the chelating resins that can be desorbed easily by water/deionized water (useful for physical adsorption), arguably the attachment of the copper (II) ions onto the chelating resins involves relatively weak bonds (electric forces)/or van der Waals forces. However, this research comprised only preliminary experiments of desorption using water and found that copper (II) ions persisted on the surface of chelating reins used as a complex structure of ion exchanges. Therefore, strong acid and base were used to study the capability of copper (II) ion desorption that confirmed the attachment of copper (II) ions onto the chelating resin as an ion exchange mechanism process. As shown in Table 4, the desorption capacity increased with increasing eluent concentration substances in the order of 0.5 M, 1 M, and 2 M for HCl, H2SO4, and NH4OH, respectively. Overall, H+-form chelating resins showed the best desorption as compared to Na+-form chelating resins whereas Chelex 100 had good selectivity with HCl 1 M (11.56 mg/g), Dowex m4195 has good selectivity with NH4OH (38.57 mg/g), and Lewatit TP260 has good selectivity with HCl 1 M (7.55 mg/g), H2SO4 0.5 M (7.63), and H2SO4 1 M (7.69 mg/g). However, Lewatit TP260 (HCl 2 M) was not shown to have a different desorption capacity with using H2SO4 0.5 M and H2SO4 1 M of the eluent; therefore, it depends on the user which is selective; desorbed substances are available in terms of cost, being environmentally friendly, and recovery or reusability.Table 4
Desorption capacity of copper (II) ions onto three chelating resins with various eluent concentrations.
Eluentqe,desorption: H+-form(mg desorbed/g resin)qe,desorption: Na+-form (mg desorbed/g resin)Chelex 100Dowex m4195Lewatit TP260Chelex 100Dowex m4195Lewatit TP260HCl 0.5 M2.410.190.327.160.150.91HCl 1 M11.550.320.7311.351.3012.06HCl 2 M11.560.247.5512.451.6616.90H2SO4 0.5 M0.223.337.632.871.387.19H2SO4 1 M9.767.297.691.553.3813.26H2SO4 2 M10.5031.251.4111.3010.3915.26NH4OH 0.5 M0.024.420.020.031.460.03NH4OH 1 M0.038.790.030.046.120.03NH4OH 2 M0.0338.570.030.049.710.03Figure 8
Desorption efficiency of copper (II) ions loaded onto Chelex 100 chelating resin at the differences of concentrations used of desorbed substances.Figure 9
Desorption efficiency of copper (II) ions using Dowex m4195 chelating resin as an adsorbent with 0.5 M, 1 M, and 2 M concentrations of HCl, H2SO4, and NH4OH desorbing substances, respectively.Figure 10
Desorption efficiency of copper (II) ions using Lewatit TP260 chelating resin as an adsorbent with 0.5 M, 1 M, and 2 M concentrations of HCl, H2SO4, and NH4OH desorbing substances, respectively.Figure8 illustrates the desorption efficiency of copper (II) ions loaded onto Chelex 100 chelating resin for both H+-form and Na+-form using 0.5, 1, and 2 M of HCl, H2SO4, and NH4OH desorbed substances.It was revealed that HCl gave the highest desorption capacity of about 11.56 mg desorbed/g resin (42.74%) and was very similar to 1 M HCl (11.55 mg desorbed/g resin, 42.71%) of 0.07% difference. Therefore, it can be suggested that 1 M HCl is suited to the H+-form of Chelex 100 if the user is concerned for cost and environment. With the Na+-form of Chelex 100, HCl showed the best percent desorption of about 43.52%. It can be seen that the percentage desorption significantly increased with increasing concentrations of the HCl. Sulfuric acid showed that at 2 M concentration, desorption capacity was indicated predominantly as demonstrated in Figure8. In the case of NH4OH, it can be concluded that desorption capacity was very low and similar to that reported in the literature [57] due to it unsuccessfully eluting Cu, Ni, Co, Fe, and Zn from the functional group of iminodiacetic-Amberlite chelating resin, which has weak acidic functional groups. In other words, the functional group of iminodiacetic acid chelating resin was not selective towards OH− ions. In summary, HCl showed a high ability for the desorption of copper (II) ions loaded onto Chelex 100.Figure9 demonstrates the desorption efficiency of copper (II) ions loaded onto Dowex m4195 Chelex chelating resin for both H+-form and Na+-form using 0.5 M, 1 M, and 2 M HCl, H2SO4, and NH4OH eluents, respectively.In the case of the H+-form Dowex m4195, 2 M NH4OH provided the highest desorption capacity (38.57 mg desorbed/g resin, 65.34%) followed by 2 M H2SO4 (31.25 mg desorbed/g resin, 52.55%) and 1 M NH4OH (8.79 mg desorbed/g resin, 14.84%). With Dowex m4195 Na+-form chelating resin, the elution by using 2 M NH4OH was 9.71 mg desorbed/g resin (24.59%) and 8.50 mg desorbed/g resin (21.52%) by using 2 M H2SO4 and 6.12 mg desorbed/g resin by using 1 M NH4OH (see Table 4 and Figure 9). Vice versa, all sample elution with HCl had the lowest desorption capacity for both the H+-form and Na+-form of Dowex m4195 chelating resin. These results were related to the previous report [38], which found that elution of copper (II) ions from Dowex m4195 with H2SO4 and with HCl was not very efficient at low concentration and at low to high concentration, respectively, but it had potential with 2 M NH4OH and 2 M H2SO4 eluent with a yield of copper (II) ion desorption.Figure10 reveals the desorption efficiency of copper (II) ions loaded onto Lewatit TP260 chelating resin for both H+-form and Na+-form using 0.5 M, 1 M, and 2 M of HCl, H2SO4, and NH4OH desorbing substances, respectively.Desorption efficiency results of copper (II) ion elution from Lewatit TP260 chelating resin indicated that 2 M HCl concentration represented the highest desorption efficiency of about 59.07 percent (15.26 mg desorbed/g resin) for Na+-form chelating resin. For H+-form Lewatit chelating resin, using 0.5 M and 1 M H2SO4 concentrations for desorbing copper (II) ions loaded has similar desorption capacity of about 28.21% (7.63 mg desorbed/g resin) and 28.43% (7.69 mg desorbed/g resin), respectively. It can be concluded that high acid concentration provided higher copper desorption from the methyl phosphonic functional group [40].There were two main aspects of the selective chelating resins for copper (II) ions in aqueous acidic solution generated from synthetic copper-citrate complexes from bioleaching of e-waste which were considered: the adsorption ability and the possible recovery of copper (II) ions by desorption from loaded chelating resins. In comparison with other chelating resins used, the H+-form of Dowex m4195 was a good performer in adsorption in acidic solution and in desorption by ammonium hydroxide 2 M concentration used for copper (II) ions generated from bioleaching of e-waste. Thus, Dowex m4195 H+-form was the best selective chelating resin for copper removal and was selected for future investigation, but that data is not shown here.


## 3.1. pH Behavior after the Adsorption Process in Acidic Solution

In adsorption and desorption processes, pH behavior and/or value of solutions have effects on the coordination reactions and electrostatic interactions in physical and chemical mechanisms, due to iminodiacetic acid, bispicolylamine, and aminomethyl functional groups in unmodified and modified chelating resins affecting the pH value of its solution by changing the initial pH: Na+-form and H+-form chelating resins of Chelex 100, Dowex m4195, and Lewatit TP260. The influence of pH change after adsorption process characteristics is presented in Table3. The initial pH of the synthetic Cu-citrate complex solution used in the experiment was about 2.30. As shown in Table 3, the H+-form chelating resins showed different changes after the sorption processes for Chelex 100 H+-form (pH 2.03), Dowex m4195 H+-form (pH 2.01), and Lewatit TP260 H+-form (pH 2.02) as compared to the initial pH. Hence, copper (II) ions preferred acidolysis at low pH, and lack of coprecipitation with H+-form chelating resin plays a supplementary role to an ion exchange mechanism in this case study. It was obviously seen that an affinity of H+-form chelating resin for copper (II) ions was sensitive to pH value attributable to competition of adsorption among the copper (II) ions and H+ ions on the same active site (nitrogen atoms).Table 3
Percentage copper (II) ions adsorbed onto Na+-form and H+-form chelating resins and changed pH of the acidic solutions.
Chelating resinNa+-formH+-formPercent adsorption (S%)Final pHPercent adsorption (S%)Final pHChelex 10041.472.4839.212.03Dowex m419556.972.3586.202.01Lewatit TP26052.912.4346.842.02The Na+-form resins had contributed to the rise of pH value in the order of Dowex m4195 Na+-form (pH 2.35), Lewatit TP260 Na+-form (pH 2.43), and Chelex 100 Na+-form (pH 2.48), respectively. This indicated that when the resins in their Na+-form interacted with water, the hydrolysis of the resin resulted in the formation of a three-component ion exchange system of H3O+/Na+/M2+ as explained in the literature [32]. Divalent copper (II) ions will react with nitrogen atoms to form a very stable charge balance (strong ligand bonding).It can be concluded that metal adsorption started when the pH reached a high enough range from which most acidic ion exchange sites started to exchange the H3O+ (hydronium ions) and reached their maximum uptake capacity. Likewise, pH promoted the ion exchange sites that contributed to the reaction and the functional group to shape the chelate rings with the metallic cations of copper divalent.Comparison of three chelating resins in H+-form showed that the highest percentage of selective adsorption capacity of about 86% as represented in Table3 occurred with a chelating resin ion exchange with the bispicolylamine or bis(2-pyridylmethyl) functional groups (uncharged tridentate ligand) containing two nitrogen atoms (donor atoms) of two chela (chaws) [51]. Nitrogen atoms can possess an electron pair to form charged complexes with divalent copper that shows very strong chelating properties at low pH (a weak base ion exchanger, Lewis acids) [52]. In terms of the Na+-form, it was also confirmed that Dowex m4195 demonstrated the highest percentage of selective adsorption capacity as compared to the other Na+-form chelating resins of Lewatit TP260 (52.91%) and Chelex 100 (41.47%), respectively. It is suggested that Dowex m4195 was appropriate for copper (II) ions in acidic solutions generated from bioleaching of e-waste; in other words, the cation species influence was greater than that of the anion species for these adsorbent chelating resins. However, isotherm and kinetic need a further study that will be impacted and vital in process design with a straightforward adsorption or desorption process.Figure1 presents the sorption process reaction formation of iminodiacetic acid, bispicolylamine, and aminomethyl groups in the acidic solutions, where nitrogen atoms will be protonated and may occur or combine.Figure 1
Possible sorption process reaction of copper (II) ion removal onto iminodiacetic acid (a), bispicolylamine (b), and aminomethyl (c) groups in the acidic solutions.
(a)
Iminodiacetic acid group(b)
Bispicolylamine group(c)
Aminomethyl group


## 3.2. Metabolic Adsorption Capacity onto Iminodiacetic Acid (Chelex 100) Functional Group Chelating Resins

Figure2 demonstrates chelating resins of copper (II) ions adsorbed at different uptake levels using an initial copper concentration of about 690 mg/L with 0.01 M citric acid as the original model substrate for studying the adsorption capacity onto Chelex 100, Dowex m4195, and Lewatit TP260. Chelex 100 demonstrated a small adsorption capacity in both H+-form and Na+-form as compared with other chelating resins of Dowex m4915 and Lewatit TP260 for both H+-form and Na+-form. It could be indicated that iminodiacetic acid chelating resin (Chelex 100) had low capacity in acidic solution conditions (at the low pH) as compared to sulfate and tartrate complexes for bioleaching of e-waste that were predominant at low pH [36]. According to the literature, it was found that copper (II) ion adsorption capacity was not very well as compared with moderate pH for the iminodiacetic acid functional group [38]. In addition, it can be seen that iminodiacetate chelating resin in terms of H+-form and Na+-form has not shown much difference of about 5.45% for the adsorption capacity.Figure 2
Adsorption capacity of copper (II) ions from Cu-citrate complexes prepared using 0.01 M citric acid by Lewatit TP260, Dowex m4195, and Chelex 100 chelating resins.In contrast to literature reports, iminodiacetate resin was not able to adsorb copper (II) ions in 0.05 M citrate acid atpH<2 [53]; however, it was successful at the lower concentration of citric acid at 0.005 M and 0.01 M. It might be pointed out that the functional group dissociates and since cation exchange reactions cannot be removed by neutral acids, the adsorption performance of iminodiacetate chelating resin with cations decreased due to the presence of acidic conditions. Furthermore, the pKa value of iminodiacetic acid was around 2, and at pH≤2, the uptake of negatively charged anions was promoted whereas the surface charge of chelating resin and metal complexes was similar to low pH. It is unlikely to capture the metal complexes of two carboxylic groups that are protonated when the Chelex 100/chelating resin with its functional groups of iminodiacetic acid is behaving as a poorly/weakly basic anion exchanger as shown in Figure 3 [39, 54]. As a result of the weakly acidic nature of iminodiacetic acid, the exchanger strongly favored hydronium ions and the competitiveness of hydrogen ions with metal ions to active sites on the resin surface in the medium. In summary, the adsorption ability of the iminodiacetic acid functional group of Chelex 100 was weak in highly acidic solutions.Figure 3
Structure of the iminodiacetic acid functional group at pH less than 2 and pH 3.99.


## 3.3. Metabolic Adsorption Capacity onto Bis-pyridylmethyl Amine (Dowex m4195) Functional Group Chelating Resin

Dowex m4195 showed a higher adsorption capacity compared to Lewatit TP260 and Chelex 100 chelating resins as presented in Figure2. Dowex m4195 is a special adsorbent for metal recovery from strongly acidic media [55, 56]. It was found that the Cu-loaded Dowex m4195 chelating resin in H+-form gave the highest adsorption capacity of about 59.49 mg copper/g resin, which was virtually the same as in earlier reports [57], whereas Dowex m4195 was the best selective sorption agent for divalent metal of Ni (II) ions and Co (II) ions from acidic solution at low pH of about 1. In other words, Dowex m4195 was favored at a high citric acid concentration and low pH; therefore, sorption of Cu-loaded chelating resin was better than that of both iminodiacetic acid and aminomethyl phosphonic chelating resin functional groups of Chelex 100 and Lewatit TP260 for both H+-form and Na+-form, respectively. Literature reports [33, 53] found that Cu-loaded Dowex m4195 and iminodiacetic resin of Amberlite 748 and Lewatit TP270 had favorable properties in acidic solution that could be associated with the unique chemical characters of the bis-pyridylmethyl amine functional group or Dowex m4195, which is appropriate in the elimination of Cu (II) ions from very strong acid (pH lower than 2) solution. Dowex m4915 also forms complexes with divalent metal ions through their free electron pair-bearing nitrogen atom of the bis-pyridylmethyl amine functional group. In comparison to the H+-form and Na+-form of Dowex m4195, it was found that the H+-form was more effective than Na+-form. This might be because the H+-form helps to split complexes and take on the sorption of Cu loaded onto chelating resin [53]. Additionally, it can be explained that the bis-pyridylmethyl amine functional group ligand contains three nitrogen donor atoms (see Figure 4) of electron pairs and at low pH. It can be seen that three nitrogen donor atoms of chelating resin are protonated [58, 59] and formed a chelate ring with copper (II) ions as shown in Figure 1(b). It can be concluded that Dowex m4195 H+-form has superior adsorption performance than Lewatit TP260 and Chelex 100 chelating resins at the low pH for both forms. Figure 5 demonstrates the discussed before and after adsorption of Dowex m41954 for the visual confirmations of the results. Observation of the image confirms that H+-form shows complete adsorption onto the surface of the resin.Figure 4
Structure of the bis-pyridylmethyl amine functional group.Figure 5
Microscope image (a) before adsorption and (b) after adsorption of copper (II) ion removal onto Dowex m4195.
(a)(b)


## 3.4. Metabolic Adsorption Capacity onto Aminomethyl Phosphonic (Lewatit TP260) Functional Group Chelating Resin

Lewatit TP260 has an aminomethyl phosphonic as a functional group in nature (Figure6), and the results indicated an intermediate adsorption level as compared to Dowex m4195 and with similar results to Chelex 100. It can be seen that Na+-form of the chelating resin of Lewatit TP260 (36.51 mg/g) was superior to the H+-form of Chelex 100 (27.05 mg/g) based on the adsorption capacity for Cu. Due to the ability of Lewatit TP260 to function at low pH, the adsorption capacity also correlated with those suggested in the literature. It is owing to the effect of a competition binding among divalent copper metal ions and hydrogen ions over the surface of the chelating resins. It can be noticed that at low pH, an overabundance of hydrogen ions can achieve successfully with metal ions for binding sites, resulting in a lower level of metal adsorption on aminomethyl phosphonic/methylphosphonic chelating resin. Additionally, the pKa values of phosphonic acid were about 2 and 5; thus, at increasing pH, more negative sites become accessible for the adsorption, leading to an increase in uptake for the adsorption capacity. Therefore, the adsorption of metal complexes on methylphosphonic acid resin was beyond hindered by the strong affinity of the resin for hydrogen ions in accordance with the acidic solution conditions [32]. It can be concluded that the aminomethyl phosphonate functional group is affected by pH. Due to the fact that the aminomethyl phosphonate functional group is a tridentate ligand, it has two binding sites at a phosphonic acid group and one coordination site at the secondary nitrogen atom as shown in Figure 6 [41]. On the other hand, the adsorption capability of chelating ion exchangers was also influenced by the structure of functional groups, their position in relation to each other, and the characteristics of the matrix (skeleton). The possible schematic mechanism for ion exchange adsorption and desorption of copper (II) ions is shown in Figure 7. Based on adsorption capacity, Lewatit TP260 was not satisfactory for Cu (II) adsorption ability as compared to Dowex m4195. These results confirmed that Dowex m4195 had the capability to capture Cu (II) ions generated from synthetic copper-citrate complexes from bioleaching of e-waste at a low pH value.Figure 6
Aminomethyl phosphonic/methyl phosphonic functional group.Figure 7
Possible schematic mechanism for adsorption and desorption of copper (II) ions using Lewatit TP260 chelating resin as an adsorbent.


## 3.5. Copper Desorption

Subsequent to the adsorption step, copper (II) ions loaded onto chelating resins were recovered/desorbed using 0.5 M, 1 M, and 2 M of HCl, H2SO4, and NH4OH, respectively, in the liquid phase and batch mode experiments that were examined. Prior to each run of the desorption experiment, an adsorption step was performed as detailed in Section 2.4 for the three chelating resins used in both H+-form and Na+-form. Desorption efficiencies of copper (II) ions are shown in Table 4 (desorption capacity) and Figures 8–10 (desorption efficiency), and these can help to clarify the mechanism of adsorption processes and the reusability of the chelating resin used. If the copper (II) ions are adsorbed onto the chelating resins that can be desorbed easily by water/deionized water (useful for physical adsorption), arguably the attachment of the copper (II) ions onto the chelating resins involves relatively weak bonds (electric forces)/or van der Waals forces. However, this research comprised only preliminary experiments of desorption using water and found that copper (II) ions persisted on the surface of chelating reins used as a complex structure of ion exchanges. Therefore, strong acid and base were used to study the capability of copper (II) ion desorption that confirmed the attachment of copper (II) ions onto the chelating resin as an ion exchange mechanism process. As shown in Table 4, the desorption capacity increased with increasing eluent concentration substances in the order of 0.5 M, 1 M, and 2 M for HCl, H2SO4, and NH4OH, respectively. Overall, H+-form chelating resins showed the best desorption as compared to Na+-form chelating resins whereas Chelex 100 had good selectivity with HCl 1 M (11.56 mg/g), Dowex m4195 has good selectivity with NH4OH (38.57 mg/g), and Lewatit TP260 has good selectivity with HCl 1 M (7.55 mg/g), H2SO4 0.5 M (7.63), and H2SO4 1 M (7.69 mg/g). However, Lewatit TP260 (HCl 2 M) was not shown to have a different desorption capacity with using H2SO4 0.5 M and H2SO4 1 M of the eluent; therefore, it depends on the user which is selective; desorbed substances are available in terms of cost, being environmentally friendly, and recovery or reusability.Table 4
Desorption capacity of copper (II) ions onto three chelating resins with various eluent concentrations.
Eluentqe,desorption: H+-form(mg desorbed/g resin)qe,desorption: Na+-form (mg desorbed/g resin)Chelex 100Dowex m4195Lewatit TP260Chelex 100Dowex m4195Lewatit TP260HCl 0.5 M2.410.190.327.160.150.91HCl 1 M11.550.320.7311.351.3012.06HCl 2 M11.560.247.5512.451.6616.90H2SO4 0.5 M0.223.337.632.871.387.19H2SO4 1 M9.767.297.691.553.3813.26H2SO4 2 M10.5031.251.4111.3010.3915.26NH4OH 0.5 M0.024.420.020.031.460.03NH4OH 1 M0.038.790.030.046.120.03NH4OH 2 M0.0338.570.030.049.710.03Figure 8
Desorption efficiency of copper (II) ions loaded onto Chelex 100 chelating resin at the differences of concentrations used of desorbed substances.Figure 9
Desorption efficiency of copper (II) ions using Dowex m4195 chelating resin as an adsorbent with 0.5 M, 1 M, and 2 M concentrations of HCl, H2SO4, and NH4OH desorbing substances, respectively.Figure 10
Desorption efficiency of copper (II) ions using Lewatit TP260 chelating resin as an adsorbent with 0.5 M, 1 M, and 2 M concentrations of HCl, H2SO4, and NH4OH desorbing substances, respectively.Figure8 illustrates the desorption efficiency of copper (II) ions loaded onto Chelex 100 chelating resin for both H+-form and Na+-form using 0.5, 1, and 2 M of HCl, H2SO4, and NH4OH desorbed substances.It was revealed that HCl gave the highest desorption capacity of about 11.56 mg desorbed/g resin (42.74%) and was very similar to 1 M HCl (11.55 mg desorbed/g resin, 42.71%) of 0.07% difference. Therefore, it can be suggested that 1 M HCl is suited to the H+-form of Chelex 100 if the user is concerned for cost and environment. With the Na+-form of Chelex 100, HCl showed the best percent desorption of about 43.52%. It can be seen that the percentage desorption significantly increased with increasing concentrations of the HCl. Sulfuric acid showed that at 2 M concentration, desorption capacity was indicated predominantly as demonstrated in Figure8. In the case of NH4OH, it can be concluded that desorption capacity was very low and similar to that reported in the literature [57] due to it unsuccessfully eluting Cu, Ni, Co, Fe, and Zn from the functional group of iminodiacetic-Amberlite chelating resin, which has weak acidic functional groups. In other words, the functional group of iminodiacetic acid chelating resin was not selective towards OH− ions. In summary, HCl showed a high ability for the desorption of copper (II) ions loaded onto Chelex 100.Figure9 demonstrates the desorption efficiency of copper (II) ions loaded onto Dowex m4195 Chelex chelating resin for both H+-form and Na+-form using 0.5 M, 1 M, and 2 M HCl, H2SO4, and NH4OH eluents, respectively.In the case of the H+-form Dowex m4195, 2 M NH4OH provided the highest desorption capacity (38.57 mg desorbed/g resin, 65.34%) followed by 2 M H2SO4 (31.25 mg desorbed/g resin, 52.55%) and 1 M NH4OH (8.79 mg desorbed/g resin, 14.84%). With Dowex m4195 Na+-form chelating resin, the elution by using 2 M NH4OH was 9.71 mg desorbed/g resin (24.59%) and 8.50 mg desorbed/g resin (21.52%) by using 2 M H2SO4 and 6.12 mg desorbed/g resin by using 1 M NH4OH (see Table 4 and Figure 9). Vice versa, all sample elution with HCl had the lowest desorption capacity for both the H+-form and Na+-form of Dowex m4195 chelating resin. These results were related to the previous report [38], which found that elution of copper (II) ions from Dowex m4195 with H2SO4 and with HCl was not very efficient at low concentration and at low to high concentration, respectively, but it had potential with 2 M NH4OH and 2 M H2SO4 eluent with a yield of copper (II) ion desorption.Figure10 reveals the desorption efficiency of copper (II) ions loaded onto Lewatit TP260 chelating resin for both H+-form and Na+-form using 0.5 M, 1 M, and 2 M of HCl, H2SO4, and NH4OH desorbing substances, respectively.Desorption efficiency results of copper (II) ion elution from Lewatit TP260 chelating resin indicated that 2 M HCl concentration represented the highest desorption efficiency of about 59.07 percent (15.26 mg desorbed/g resin) for Na+-form chelating resin. For H+-form Lewatit chelating resin, using 0.5 M and 1 M H2SO4 concentrations for desorbing copper (II) ions loaded has similar desorption capacity of about 28.21% (7.63 mg desorbed/g resin) and 28.43% (7.69 mg desorbed/g resin), respectively. It can be concluded that high acid concentration provided higher copper desorption from the methyl phosphonic functional group [40].There were two main aspects of the selective chelating resins for copper (II) ions in aqueous acidic solution generated from synthetic copper-citrate complexes from bioleaching of e-waste which were considered: the adsorption ability and the possible recovery of copper (II) ions by desorption from loaded chelating resins. In comparison with other chelating resins used, the H+-form of Dowex m4195 was a good performer in adsorption in acidic solution and in desorption by ammonium hydroxide 2 M concentration used for copper (II) ions generated from bioleaching of e-waste. Thus, Dowex m4195 H+-form was the best selective chelating resin for copper removal and was selected for future investigation, but that data is not shown here.


## 4. Conclusion

Copper (II) ion adsorption and recovery from e-waste are becoming an increasingly popular issue of scientific studies due to it being an essential metal for human life and the demand for urban planning and the development of crucial new technology to improve the quality of wastewater containing heavy metals. E-waste is a discarded product resulting from poor management that may have an environmental impact and cause loss of valuable metal especially copper. In this study, three chelating resins were used for adsorption and desorption of copper in an aqueous acidic solution generated from synthetic copper-citrate complexes from bioleaching of e-waste. In summing up these preparatory studies, it can conclude that bis-pyridylmethyl amine/Dowex m4195 was suited to the adsorption of copper in acidic solution atpH<2 with a maximum adsorption capacity of about 86.20% (H+-form) and 56.97% (Na+-form) chelating resins. Absorption on Dowex m4195 H+-form was mainly dependent on a functional group of the resin and was affected by the pH of the solution. To select the most satisfactory recovery of Cu loaded onto chelating resin, desorption ability was investigated by the use of various concentrations of HCl, H2SO4, and NH4OH. Desorption with 2 M NH4OH for the elution of Cu loaded onto Dowex m4195 (H+-form) had the highest percentage recovery of about more than 65%. Therefore, it is confirmed that the bispicolylamine functional group chelating resin reduces the amount of e-waste and hazardous materials in the environment and decreases the scarcity of laterite ores for sustainable metal and water and adsorbent reuse.



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