Characterization of Waste Amidoxime Chelating Resin and Its Reutilization Performance in Adsorption of Pb(II), Cu(II), Cd(II) and Zn(II) Ions

The continuous expansion of the market demand and scale of commercial amidoxime chelating resins has caused large amounts of resin to be discarded around the world. In this study, the waste amidoxime chelating resin was reutilized as an adsorbent for the removal and recovery of Pb(II), Cu(II), Cd(II) and Zn(II) ions from aqueous solutions. The physical morphology and chemical composition of the waste amidoxime chelating resin (WAC-resin) from the factory was characterized by the elemental analyzer, X-ray photoelectron spectroscopy and Fourier-transform infrared spectroscopy. The influence of the initial metal ions concentration, contact time, temperature and the solution pH on the adsorption performance of the metal ions was explored by batch experiments. It was shown that the optimal pH was 4. Kinetic studies revealed that adsorption process corresponded with the pseudo-second-order kinetic model and the adsorption isotherm was consistent with the Langmuir model. At room temperature, the adsorption capacities of WAC-resin for Pb2+, Cu2+, Zn2+ and Cd2+ reached 114.6, 93.4, 24.4 and 20.7 mg/g, respectively.


Introduction
The rapid development of the metal mining and electronics industries has caused serious environmental pollution concerns, especially the discharge of wastewater containing heavy toxic metals [1] such as Pb, Zn, Cd, Ni, Cu, Cr, Co, etc. The amidoxime chelating resin containing both an amino (-NH 2 ) and an oximido (=N-OH) at the same carbon atom is of broad interest, due to the fact that it can coordinate with a wide range of metal ions [2,3]. It has been of extensive application for wastewater treatment and separation and enrichment of metals, such as the removal of metal cations [4][5][6], extraction of uranium from seawater [7,8], recovery of rare earth metals from Bayer solution [9,10], etc.
Global synthetic resin consumption was about 255 million tons in 2020. However, after service, due to the reduction of its specific surface area, the destruction of functional groups and the shrinkage of spherical particles, these resins are discarded as waste. If these waste resins are not effectively treated, they will take up a lot of space, and the harmful substances leached from the waste resin will also pollute the environment. At present, the main treatment method of these waste resins is to be incinerated in Solid Waste Treatment Company for 2000 RMB/t, which will produce CO 2 , CO, SO 2 , NO and NO 2 gases, leading to serious environmental impacts [11]. For example, in the Guangxi branch of Chalco, the amidoxime chelating resins for adsorption gallium from Bayer solution were discarded after being reused 50-60 times a month, and 225 tons of waste resin are produced and shelved each year [12]. This is extremely incompatible with the advocacy of recycling resources and carbon neutrality. Therefore, there is an urgent need to reutilizing such hard-to-degrade waste resins. Some studies have utilized other types of waste resin to prepare carbon adsorbents [13,14] via high-temperature pyrolysis for adsorbing organic substances (naphthalene, benzene, toluene and phenol) [15][16][17][18], heavy metal ions [19] and also for carbon capture and storage [20]. In addition, the high-yield and low-cost waste resins may also be precursor materials of composite adsorbents, such as the solvent impregnated resin for separating and purifying rare earth metals [21].
In this work, the physical structure and chemical functional groups of the waste amidoxime chelating resin (WAC-resin) were characterized through various technologies which will provide a research foundation for the synthesis of new composite materials based on waste resin in the future. Then, in order to extend the service life of commercial amidoxime chelating resins, another part of our research was to investigate the effectiveness of waste resin as an adsorbent to remove and recover various metals from a synthetic aqueous solution containing lead, copper, zinc and cadmium ions. The adsorption mechanism between existing functional groups of the WAC-resin and metal ions was investigated by instrumental and kinetic analyses. This study was considered as a vital step toward further studies on reusing the WAC-resin to synthesis composite materials and to investigate reusability performance of the WAC-resin in heavy-metals recovery from real wastewater.

Materials
The raw material used for the experiments was the WAC-resin which was provided from the Guangxi branch of Chalco, with a particle diameter range of 0.2-0.6 mm. The WACresin was successively rinsed with hydrochloric acid and ultrapure water to remove the organic and inorganic impurities of polymeric beads, followed by drying under vacuum at 323 K and sieving to 40 mesh. Reagents used in this research, such as lead nitrate (Pb(NO 3 ) 2 ), copper nitrate trihydrate (Cu(NO 3 ) 2 ·3H 2 O), cadmium nitrate tetrahydrate (Cd(NO 3 ) 2 ·4H 2 O) and zinc nitrate hexahydrate (Zn(NO 3 ) 2 ·6H 2 O) were of reagent grade and purchased from Aladdin. Deionized water was used in all experiments.

Equipment
The pristine clean amidoxime chelating resin (PAC-resin); the WAC-resin; and the WAC-resin loaded with lead, copper, zinc and cadmium ions (WAC-resin-MIs) were characterized by various techniques. In detail, the surface morphology and corresponding element distribution were analyzed by scanning electron microscopy with energy-dispersive spectroscopy (SEM-EDS, Phenom Pro 800-07334, Eindhoven, The Netherlands). The C, N, O and H content of the resin beads was also estimated by using an element analyzer (Vario EL cube, Hanau, Germany). The functional groups of samples were identified by employing Fourier-transform infrared spectroscopy (FTIR, THS-108, Thermo Scientific, Waltham, MA, USA). Furthermore, to analyze the adsorption mechanism of the WAC-resin, broken resin powders were characterized by X-ray photoelectron spectroscopy (XPS, ESCALAB 250XI+, Thermo Scientific, Waltham, MA, USA). Moreover, the zeta potential of the suspension (100 mg/L) containing nonmetallic particles of the WAC-resin (200 mesh) was determined by using a high-sensitivity zeta potential analyzer (NanoBrook Omni, New York, NY, USA) for exploring the surface charge as a function of pH.

Batch Adsorption
The influence of the initial metal ions concentration, contact time, temperature and the solution pH on the adsorption percentage of various metal ions was explored by batch experiments. The metal ions solution was prepared by dissolving four analytical-grade reagents in ultrapure water, namely Pb(NO 3 ) 2 , Cu(NO 3 ) 2 ·3H 2 O, Cd(NO 3 ) 2 ·4H 2 O and Zn(NO 3 ) 2 ·6H 2 O. Except that the concentration ranges of initial metal ions concentration and adsorption isotherm experiments are 20-200 and 50-7000 mg/L, respectively, the concentration of pH, temperature and kinetics experiments were all 100 mg/L. To study the effect of pH on the adsorption percentage of Pb(II), Cu(II), Cd(II) and Zn(II) ions and zeta potential of the WAC-resin, solutions with pH values ranging from 1.0 to 5.0 were prepared by adding diluted HNO 3 and NaOH solutions. The variation of the adsorption capacity with a contact time from 5 min to 24 h and temperature from 298 to 333 K was investigated. The WAC-resin (0.1 g) was dispersed to metal ions stock solution (25 mL), and the various adsorption experiments were conducted in a thermostatic water-bath oscillator, with a stirring speed of 140 rpm. After the adsorption reaction reached equilibration, the solution was separated from the WAC-resin, and the ion concentration was analyzed by inductively coupled plasma emission spectrometry (ICPS-7510, Kyoto, Japan). The adsorption efficiency, E (%), and adsorption capacity, Q (mg/g), were calculated by using Equations (1) and (2), respectively, as follows: where C 0 and C e (mg/mL) represent the initial and final concentrations of Pb 2+ , Cu 2+ , Cd 2+ and Zn 2+ in the aqueous media, respectively. V (mL) signifies the solution volume, and m (g) denotes the weight of the dry waste resin.

Elution
The hydrochloric acid solution was employed as the elution agent to avoid precipitation between the alkaline solution and lead and copper ions. After the adsorption experiment reached equilibrium under optimal conditions, the WAC-resin loaded with Pb(II) and Cu(II) was collected and washed by using water and then immersed in the 3% hydrochloric acid solution (liquid-solid ratio: 50 mL/g), with an elution duration from 5 to 60 min and temperature ranging from 298 to 323 K, respectively.

Physical Structure
The pictures and SEM images (×600) of the PAC-resin and WAC-resin beads are shown in Figure 1a,b. Both samples possessed smooth spherical morphologies with a diameter of approximately 0.4 mm. The difference was that, after magnifying them to the same higher magnification (×5.00 k), the surface of the white PAC-resin was extremely loose and porous, and there was no adhesion of other substances. However, due to the large number of holes being blocked after adsorption reaction occurred, the surface morphology of the WAC-resin presented obvious change, becoming tighter and showing many rough gullies.
In addition, the color of the WAC-resin-MIs is brown-green, and its SEM images (×600) exhibited regular spherical shape (Figure 2a). Under higher magnification (×3.00 k), the rougher surface of the WAC-resin-MIs and tiny particles attached may be ascribed to the adsorption action ( Figure 2b). The results of the energy-dispersive spectroscopy of the WAC-resin-MIs further confirmed that lead (Pb) and copper (Cu) were predominantly detected on the surface of the WAC-resin, including a small amount of cadmium (Cd) and zinc (Zn). This indicates that the WAC-resin can effectively adsorb Pb(II) and Cu(II) from aqueous solutions.

Chemical Composition Analysis
Firstly, the relative contents of carbon, nitrogen, oxygen and hydrogen in the PACresin and the WAC-resin according to the element analyzer are presented in Table 1. The results showed that relative oxygen content increased from 57.86% to 88.29% after the

Chemical Composition Analysis
Firstly, the relative contents of carbon, nitrogen, oxygen and hydrogen in the PACresin and the WAC-resin according to the element analyzer are presented in Table 1. The results showed that relative oxygen content increased from 57.86% to 88.29% after the

Chemical Composition Analysis
Firstly, the relative contents of carbon, nitrogen, oxygen and hydrogen in the PAC-resin and the WAC-resin according to the element analyzer are presented in Table 1. The results showed that relative oxygen content increased from 57.86% to 88.29% after the PAC-resin converting to the WAC-resin, while the relative nitrogen content was decreased from 23.96% to a very low value of 8.24%. Previous works in the literature have reported that, at the same time as the amidoxime functionalized active fiber adsorbing Au(III), its amidoxime groups were gradually oxidized into the carboxyl groups [22,23]. Therefore, the changes in the relative contents of nitrogen and oxygen of the PAC-resin and WAC-resin may also be due to that the coordination between the amidoxime groups of the PAC-resin and gallium element accompanied by oxidation-reduction reaction. This may lead to the WAC-resin being rich in the oxygen-containing functional groups. To further evaluate the chemical composition of the WAC-resin, we measured the X-ray photoelectron spectroscopy of the PAC-resin and WAC-resin, as shown in Figure 3a. The XPS spectra provide the principal information concerning the chemical environment, including the composition and binding state of the outermost atomic layers on the solid surface [24,25]. In addition, the FTIR spectra can identify the remaining functional groups and the organic components' conversion of the materials. The FTIR spectra are recorded and presented in Figure 3b. PAC-resin converting to the WAC-resin, while the relative nitrogen content was decreased from 23.96% to a very low value of 8.24%. Previous works in the literature have reported that, at the same time as the amidoxime functionalized active fiber adsorbing Au(III), its amidoxime groups were gradually oxidized into the carboxyl groups [22,23]. Therefore, the changes in the relative contents of nitrogen and oxygen of the PAC-resin and WAC-resin may also be due to that the coordination between the amidoxime groups of the PAC-resin and gallium element accompanied by oxidation-reduction reaction. This may lead to the WAC-resin being rich in the oxygen-containing functional groups. To further evaluate the chemical composition of the WAC-resin, we measured the Xray photoelectron spectroscopy of the PAC-resin and WAC-resin, as shown in Figure 3a. The XPS spectra provide the principal information concerning the chemical environment, including the composition and binding state of the outermost atomic layers on the solid surface [24,25]. In addition, the FTIR spectra can identify the remaining functional groups and the organic components' conversion of the materials. The FTIR spectra are recorded and presented in Figure 3b.  The major peaks of C 1s, N 1s and O 1s were noted to appear in the survey XPS spectra. Compared with that of the PAC-resin (Figure 3a(a1)), the N 1s peak in the XPS spectrum of the WAC-resin (Figure 3a(a2)) was significantly weakened, whereas the O 1s peak was strengthened, which was consistent with the low nitrogen and high oxygen levels of Table 2. The FTIR spectra can also validate the above deduction about the oxidation of the amidoxime groups into carboxyl groups by the observed shifting of peaks. As can be observed, the characteristic adsorption peaks in the FTIR spectrum of PAC-resin (Figure 3b(b1)) at 1650, 1109 and 935 cm −1 were assigned to the stretching vibrations of the C=N, C-N and N-O bonds [22], respectively. However, in that of the WAC-resin (Figure 3b(b2)), these bands shifted to a higher wavenumber and were noted to weaken significantly. Simultaneously, two new bands appeared at 1697 and 1195 cm −1 corresponding to the stretching vibrations of the C=O and C-O bonds of the carboxyl groups [26,27], respectively. The major peaks of C 1s, N 1s and O 1s were noted to appear in the survey XPS spectra. Compared with that of the PAC-resin (Figure 3a(a 1 )), the N 1s peak in the XPS spectrum of the WAC-resin (Figure 3a(a 2 )) was significantly weakened, whereas the O 1s peak was strengthened, which was consistent with the low nitrogen and high oxygen levels of Table 2. The FTIR spectra can also validate the above deduction about the oxidation of the amidoxime groups into carboxyl groups by the observed shifting of peaks. As can be observed, the characteristic adsorption peaks in the FTIR spectrum of PAC-resin (Figure 3b(b 1 )) at 1650, 1109 and 935 cm −1 were assigned to the stretching vibrations of the C=N, C-N and N-O bonds [22], respectively. However, in that of the WAC-resin (Figure 3b(b 2 )), these bands shifted to a higher wavenumber and were noted to weaken significantly. Simultaneously, two new bands appeared at 1697 and 1195 cm −1 corresponding to the stretching vibrations of the C=O and C-O bonds of the carboxyl groups [26,27], respectively. Overall, it can be concluded that the WAC-resin is rich in oxygen-containing functional groups due to the oxidation of the oxime groups into the carboxyl groups [22]. Studies have shown that there is a strong chemical interaction between carboxyl groups and metals ions of solution, such as lead, copper, cadmium and zinc ions [25,[28][29][30], so the WAC-resin bearing carboxyl group is expected to adsorb these metal ions from aqueous solution.

Effect of pH
As the overall charge on the surface of adsorbents usually varies with the pH, the effect of the initial pH value on the adsorption properties and zeta potential [31] was studied in the range 1.0-5.0 (to prevent the formation of the hydroxide precipitates) (Figure 4).
When the pH value was controlled between 1.0 and 4.0, it was noticed that the removal efficiency of all the metal ions increased (pH from 1.5 to 3.0) and then stabilized (pH from 3.0 to 4.0) (Figure 4a). As observed from Figure 4b, the zeta potential of the WAC-resin decreased from about 6 to −36 mV, and the point of zero charge pH was close to 2 (Figure 4b). In the range of pH 1.0-2.4, the value of the zeta potential was positive, and the protonation of the active sites inhibited the positive metal cations, thereby declining the adsorption effect [32]. As the pH value exceeded 3.0, the surface of the WAC-resin possessed an overall negative, which made it attract the positively charged metal ions entering the pores by electrostatic interaction [33]. As expected, in the pH range of 3-4, the adsorption increased to reach an optimal extent, so further experiments were carried out at a pH value of 4. In contrast, the adsorption percentage of lead and copper ions was above 80% and 90%, respectively, which was higher than that of zinc and cadmium. Compared with Cd 2+ and Zn 2+ , the Cu 2+ with a smaller radius usually corresponds to a higher charge density and can compete with H + or H 3 O + in solution for negatively charged binding sites of the adsorbent [34]. For Pb 2+ with high electronegativity, its competitive advantage is not only related to the electrostatic interaction, but also the coordination with groups [35].

Effect of Temperature
The adsorption analysis was conducted at a pH value of 4 in the temperature range 298-333 K, and the results are presented in Figure 4c. A small increment in the removal percentage of zinc and cadmium was observed with elevated temperature, which demonstrated that the adsorption process of the zinc and cadmium ions was endothermic. This was because the resin was swelled under high-temperature conditions, which promoted the entry of the metal ions, thereby enhancing the adsorption efficiency. However, it is noted that the high adsorption percentage of lead and copper ions remained unchanged with increasing temperature. Next, 298 K was chosen as the optimal adsorption temperature.

Effect of Contact Time on Adsorption Kinetics
To determine the adsorption speed of the solute molecules on the adsorbent, the batch-adsorption experiments were conducted as a function of the reaction duration (5 min to 24 h) (Figure 4d). The adsorption capacity of all the metal ions increased with the extension of the reaction time, and the adsorption equilibrium was reached within 300 min.
The pseudo-first-order-kinetic model [36] and pseudo-second-order-kinetic model [37] were applied to the further analysis given as Equations (3) and (4), respectively: where k1 (min −1 ) and k2 (g/mg/min) are the kinetic rate constants. Qt and Qe (mg/g) represent the extent of metal ions adsorbed per unit adsorbent at time t and equilibrium state respectively. The detailed parameters for the two models are depicted in Table 3. Compared to that of pseudo-first-order model, the value of correlation coefficients (R 2 ) obtained from the pseudo-second-order model is larger and closer to 1. In addition, the cal-

Effect of Contact Time on Adsorption Kinetics
To determine the adsorption speed of the solute molecules on the adsorbent, the batchadsorption experiments were conducted as a function of the reaction duration (5 min to 24 h) (Figure 4d). The adsorption capacity of all the metal ions increased with the extension of the reaction time, and the adsorption equilibrium was reached within 300 min.
The pseudo-first-order-kinetic model [36] and pseudo-second-order-kinetic model [37] were applied to the further analysis given as Equations (3) and (4), respectively: where k 1 (min −1 ) and k 2 (g/mg/min) are the kinetic rate constants. Q t and Q e (mg/g) represent the extent of metal ions adsorbed per unit adsorbent at time t and equilibrium state respectively. The detailed parameters for the two models are depicted in Table 3. Compared to that of pseudo-first-order model, the value of correlation coefficients (R 2 ) obtained from the pseudo-second-order model is larger and closer to 1. In addition, the calculated Q e value is also in better agreement with the experimental results. It can be concluded that the adsorption of Pb 2+ , Cu 2+ , Zn 2+ and Cd 2+ onto the WAC-resin obeys the pseudo-second-order kinetic model. Table 3. Fitting parameters of the pseudo-first-order and pseudo-second-order kinetic models.

Effect of Initial Concentration on Adsorption Isotherm
The initial metal ions concentration is a vital factor affecting the adsorption process. It can be seen from Figure 5 that the equilibrium adsorption capacity of lead, copper, zinc and cadmium ions increased with the initial concentration of metal ions, improving from 20 to 200 mg/L. culated Qe value is also in better agreement with the experimental results. It can be concluded that the adsorption of Pb 2+ , Cu 2+ , Zn 2+ and Cd 2+ onto the WAC-resin obeys the pseudo-second-order kinetic model. Table 3. Fitting parameters of the pseudo-first-order and pseudo-second-order kinetic models.

Effect of Initial Concentration on Adsorption Isotherm
The initial metal ions concentration is a vital factor affecting the adsorption process. It can be seen from Figure 5 that the equilibrium adsorption capacity of lead, copper, zinc and cadmium ions increased with the initial concentration of metal ions, improving from 20 to 200 mg/L. This is because the driving force for metal ions entering from the solution to the adsorbent will increase correspondingly with the increase of initial concentration. The ad-  This is because the driving force for metal ions entering from the solution to the adsorbent will increase correspondingly with the increase of initial concentration. The adsorption amount of lead and copper was 36.5 and 21.8 mg/g, respectively, under the condition of 200 mg/L and 120 min, which showed that the adsorption speed of the WACresin towards lead was faster than that of copper under high concentration conditions. However, for cadmium and zinc, under the condition of high initial concentration of metal ions, the adsorption capacity was still relatively low, remaining at about 12mg/g in Figure 5c,d.
The results of adsorption isotherm experiments ( Figure 6) at room temperature and pH 4 demonstrated that 1 g of WAC-resin could adsorb up to 114.6, 93.4, 20.7 and 24.4 mg Pb(II), Cu(II), Cd(II) and Zn(II), respectively. The WAC-resin exhibited an excellent adsorption affinity towards lead (II) and copper (II), with the complete sequence observed to be Pb(II) > Cu(II) > Zn(II) > Cd(II). sorption amount of lead and copper was 36.5 and 21.8 mg/g, respectively, under the condition of 200 mg/L and 120 min, which showed that the adsorption speed of the WACresin towards lead was faster than that of copper under high concentration conditions. However, for cadmium and zinc, under the condition of high initial concentration of metal ions, the adsorption capacity was still relatively low, remaining at about 12mg/g in Figure  5c,d.
The results of adsorption isotherm experiments ( Figure 6) at room temperature and pH 4 demonstrated that 1 g of WAC-resin could adsorb up to 114.6, 93.4, 20.7 and 24.4 mg Pb(II), Cu(II), Cd(II) and Zn(II), respectively. The WAC-resin exhibited an excellent adsorption affinity towards lead (II) and copper (II), with the complete sequence observed to be Pb(II)> Cu(II) >Zn(II)> Cd(II). The adsorption isotherms can be fitted by the commonly used Langmuir [38] and Freundlich [39] isotherm models expressed as Equations (5) and (6), respectively: lnQ e = lnK f + ln C e ⁄ where Qe and Qm represent the equilibrium and maximum adsorption capacities (mg/g), respectively. Ce is the equilibrium metal ions concentration (mg/L), KL and Kf are the Langmuir and Freundlich constants separately, and 1/n is the Freundlich heterogeneity factor. The Langmuir model is noted to fit the adsorption isotherm data more effectively than the Freundlich model, based on the calculations of the R 2 values (Table 4). This suggests that the metal-ions adsorption on the WAC-resin surface is an entirely homogeneous process and follows a monolayer adsorption mechanism. In addition, the theoretical maximum adsorption capacities of Pb(II), Cu(II), Cd(II) and Zn(II) were estimated to be 115.47, 93.80, 20.63 and 26.55 mg/g, respectively, which were close to the actual values. In this study, the low-cost WAC-resin exhibits better adsorption properties for Pb 2+ and Cu 2+ in terms of the adsorption capacity compared with that of the adsorbents reported previously (Table  5). This reveals that the easily available and cheap WAC-resin may be an excellent adsorbent for treating Pb 2+ and Cu 2+ from the real wastewater samples. The adsorption isotherms can be fitted by the commonly used Langmuir [38] and Freundlich [39] isotherm models expressed as Equations (5) and (6), respectively: ln Q e = ln K f + lnC e /n (6) where Q e and Q m represent the equilibrium and maximum adsorption capacities (mg/g), respectively. C e is the equilibrium metal ions concentration (mg/L), K L and K f are the Langmuir and Freundlich constants separately, and 1/n is the Freundlich heterogeneity factor. The Langmuir model is noted to fit the adsorption isotherm data more effectively than the Freundlich model, based on the calculations of the R 2 values (Table 4). This suggests that the metal-ions adsorption on the WAC-resin surface is an entirely homogeneous process and follows a monolayer adsorption mechanism. In addition, the theoretical maximum adsorption capacities of Pb(II), Cu(II), Cd(II) and Zn(II) were estimated to be 115. 47, 93.80, 20.63 and 26.55 mg/g, respectively, which were close to the actual values. In this study, the low-cost WAC-resin exhibits better adsorption properties for Pb 2+ and Cu 2+ in terms of the adsorption capacity compared with that of the adsorbents reported previously (Table 5). This reveals that the easily available and cheap WAC-resin may be an excellent adsorbent for treating Pb 2+ and Cu 2+ from the real wastewater samples.

Adsorption Mechanism
First, compared with the XPS spectrum of the WAC-resin (Figure 7a(a 1 )), two new peaks corresponding to Pb 4f and Cu 2p could be detected in that of the WAC-resin-MIs (Figure 7a(a 2 )). Meanwhile, the intensity and binding energy of the O 1s peak decreased due to the interaction between the oxygen atoms and metal ions. Further, in the FTIR spectrum of the WAC-resin-MIs (Figure 7b(b 2 )), the tensile vibration band of the C=O group shifted from 1697 to 1716 cm −1 and the C-O stretching vibrations shifted from 1195 to 1219 cm −1 . At the same time, the peak band corresponding to the O-H stretching in the range of 3000-3600 cm −1 was broadened [39]. In summary, the changes observed above are led by the chemical coordination between the -COOH groups of the WAC-resin and metal ions [30].
Furthermore, to further demonstrate the adsorption mechanism, the high-resolution XPS of the O 1s region is presented in Figure 7c. The O 1s spectrum of the WAC-resin contained two components with the binding energies at 531.75 eV for C=O and 533.3 eV corresponding to the C-OH groups [26]. After the adsorption of the metal ions, the binding energy shifted to 531.58 and 532.86 eV, respectively, which was attributed to the high electronegativity of the oxygen atoms. In summary, based on the all change of these parameters of EDS, XPS and FTIR, the oxygen-containing bands of the carboxyl groups in the WAC-resin make the main contribution in interacting strongly with the lead and copper ions. Finally, in the Pb 4f and Cu 2p3/2 photoelectrons (Figure 7d), Pb 2+ and Cu 2+ ions adsorbed on the WAC-resin also were displayed. The conversion process from the PAC-resin into the WAC-resin and adsorption behavior of the metal ions on the WAC-resin is clearly depicted in Figure 7e.

Effect of Temperature and Time on Elution
The elution experiment was performed with the WAC-resin treated in the solution containing lead and copper ions as the elution object. The effect of time and temperature on the elution behavior of Pb(II) and Cu(II) is presented in Figure 8a,b, respectively, based on the optimal adsorption conditions (pH of 4, at room temperature and 7 h). It could be observed that the elution duration had a significant effect on the elution percentage of lead and copper and that both reach more than 90% when it is completely eluted after 1 h. With the increase of temperature, the elution percentage of lead and copper could maintain about 90%, indicating that the temperature had an insignificant effect on elution.

Effect of Temperature and Time on Elution
The elution experiment was performed with the WAC-resin treated in the solution containing lead and copper ions as the elution object. The effect of time and temperature on the elution behavior of Pb(II) and Cu(II) is presented in Figure 8a,b, respectively, based on the optimal adsorption conditions (pH of 4, at room temperature and 7 h). It could be observed that the elution duration had a significant effect on the elution percentage of lead and copper and that both reach more than 90% when it is completely eluted after 1 h. With the increase of temperature, the elution percentage of lead and copper could maintain about 90%, indicating that the temperature had an insignificant effect on elution.

Conclusions
The extremely cheap and readily available waste-amidoxime chelating resin was characterized through various instrumental techniques, and we found that its reutilization performance in removing and recovering Pb 2+ and Cu 2+ from aqueous solutions at pH 4 and room temperature is effective. The theoretical analyses confirmed that a majority of the amidoxime groups of the pristine amidoxime chelating resin were destroyed and oxidized to the carboxyl groups. For the adsorption mechanism of metal ions onto the WAC-resin, the -COOH groups played a leading role in the chemical coordination with the Pb 2+ and Cu 2+ ions during adsorption. Finally, the adsorbed Pb 2+ and Cu 2+ ions could be eluted and recover by the hydrochloric acid solution. Therefore, this research can provide a research foundation and a direction for the WAC-resin treating real wastewater, extend the service life of commercial WAC-resin and recover the metal resources.