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polyvinyl chloride crosslinked fiber for Pd(II) recovery from acidic solutions
Journal of Environmental Management 204 (2017) 200e206
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Journal of Environmental Management journal homepage: www.elsevier.com/locate/jenvman
Research article
A reusable adsorbent polyethylenimine/polyvinyl chloride crosslinked fiber for Pd(II) recovery from acidic solutions Han Ah Choi a, Ha Neul Park b, Sung Wook Won a, b, * a
Department of Marine Environmental Engineering and Institute of Marine Industry, Gyeongsang National University, 38 Cheondaegukchi-gil, Tongyeong, Gyeongnam 53064, Republic of Korea b Department of Ocean System Engineering, Gyeongsang National University, 38 Cheondaegukchi-gil, Tongyeong, Gyeongnam 53064, Republic of Korea
a r t i c l e i n f o
a b s t r a c t
Article history: Received 24 May 2017 Received in revised form 19 August 2017 Accepted 25 August 2017
In this study, a mixture of polyethylenimine (PEI) and polyvinyl chloride (PVC) was reacted at 80 C for 6 h to synthesize crosslinked PEI/PVC polymer solution, which was injected to produce the PEI/PVCcrosslinked fiber (PEI/PVC-CF). PEI/PVC-CF was investigated as an adsorbent to remove and recover Pd(II) from acidic solutions. In order to examine the adsorption characteristics and usability of PEI/PVCCF for Pd(II) recycling, several experiments such as isotherm, kinetics, desorption and reuse were conducted. The adsorption isotherms were fitted using the Langmuir and Freundlich models, respectively. The maximum adsorption capacity was estimated as 146.03 mg/g according to the Langmuir model. The kinetic experiments demonstrated that adsorbent reaches adsorption equilibrium within 60 min for initial Pd(II) concentrations of 25e100 mg/L. After adsorption, Pd(II) on PEI/PVC-CF was easily desorbed using acidified thiourea solution, and the desorption efficiency increased with the thiourea concentration. It was also demonstrated that PEI/PVC-CF can be used repeatedly for at least five cycles without reduction in adsorption capacity. © 2017 Elsevier Ltd. All rights reserved.
Keywords: Palladium Recovery Adsorbent Reusability
1. Introduction Precious metal plating is frequently applied in areas such as connectors or sensors of electronic devices to achieve desired chemical, physical, and electrical properties (Kato et al., 2016). The electric power industry previously used gold plating to achieve high resistance to decay and increase electrical conductivity (Khazaeli et al., 2013). However, now palladium (Pd) plating is used instead on the surface of electrical parts for cost reduction (Rouya et al., 2010; Huang et al., 2014). Pd is only produced in a handful of countries including Russia, South Africa, Canada, and the United States. Therefore, there is a great imbalance between the limited Pd supply and the high demand for it, leading to precarious price changes (Morcali and Zeytuncu, 2015). Industries in Korea are highly dependent on imported raw Pd, and use a coagulation-sedimentation method, which is superior
* Corresponding author. Department of Marine Environmental Engineering and Institute of Marine Industry, Gyeongsang National University, 38 Cheondaegukchigil, Tongyeong, Gyeongnam 53064, Republic of Korea. E-mail address: [email protected] (S.W. Won). http://dx.doi.org/10.1016/j.jenvman.2017.08.047 0301-4797/© 2017 Elsevier Ltd. All rights reserved.
in cost and treatment efficiency, among various methods for plating wastewater treatment. However, this method often produces large amounts of sludge using excess chemicals in each process. It is estimated that Korea discharges 76,000 tons of plating sludge per year and in Japan it emits about 50,000 tons per year (Hosseini et al., 2016; Oh et al., 2004). Therefore, it is absolutely necessary to develop technologies for effective Pd recycling from plating wastewaters. In addition, Pd recovery is crucial in terms of resource recycling and sustainable practice in chemical industries. In general, such plating wastewaters are acidic at the temperature of 20e50 C, and contain up to 100 mg/L in Pd (Hosseini et al., 2016). The conventional methods to retrieve valuable metals such as Pd include cementation, precipitation (Lee, 2013), electrowinndez-Tapia et al., 2013), solid phase extraction ning (Herna (Khazaeli et al., 2013), and solvent extraction (Herce-Sesa et al., 2017). However, these methods entail not only high cost but also long periods of treatment. In contrast, adsorption is known to be a more economical and promising method, as it can effectively eliminate target substances from wastewater in a simple process. Also, this method could be effective in reducing sludge incidence by
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applying directly to wastewaters without the use of chemicals neutralizing acidic plating wastewater in existing processes. However, the treatment efficiency is highly dependent on the capacity of the adsorbent. Therefore, it is highly desirable to develop effective adsorbents for Pd from acidic wastewaters, such as that from Pd plating, as well as easy and economical methods to recover the adsorbed Pd(II). Recent literature has included a variety of studies related to the recovery of Pd from acidic solutions or wastewaters, and many researchers are interested in developing biosorbents using natural resources. The reason may be that it contains biocompatibility, biodegradability and excellent reactivity and hydroxyl or amino functional groups effective for adsorption. For example, natural resin (Yi et al., 2016), b-cyclodextrin grafted chitosan (Sharma and Rajesh, 2017) and 2-Mercaptobenzothiazole impregnated cellulose (Sharma and Rajesh, 2014) have been developed and evaluated for Pd recovery. However, they may be vulnerable to acidity and have some defects such as swelling and degradation of strength due to absorption of water (Kim et al., 2016; Won et al., 2013). Therefore, it is required to develop a suitable adsorbent having high acid resistance in order to improve practical applicability. In this study, we developed a new material that is effective for adsorbing Pd(II) in acidic solutions, by modifying our previously developed method (Kim et al., 2015). Polyvinyl chloride (PVC) featuring high mechanical strength and acid resistance was used as the main body of the adsorbent, and polyethylenimine (PEI) containing ample amines as the coupler. The surface of the adsorbent and the crosslinking between PEI and PVC were analyzed through Fourier transform infrared spectroscopy (FTIR) and scanning electron microscopy (SEM). The adsorption performance of PEI/PVC-CF was analyzed through isotherm and kinetic experiments. An acidified thiourea solution was used for desorption of the adsorbed Pd(II), and the optimal concentration of thiourea was examined. Finally, the reusability of PEI/PVC-CF was investigated through repeated adsorption-desorption experiments.
2.2. Preparation of PEI/PVC-crosslinked fiber PEI (3.75 g) and PVC (3 g) were each dissolved in a bottle containing 15 mL DMF solution at 40 C for 24 h. The completely dissolved PEI solution was added to the PVC solution, and the mixture was vigorously stirred at 80 C for 6 h to produce the crosslinked PEI/PVC polymer solution. The reaction scheme of PEI and PVC is demonstrated in Fig. 1. The color of the mixture gradually changed from transparent to yellow. After 6 h, the crosslinked polymer solution was quickly cooled down to room temperature to stop the reaction between the two polymers. The PEI/PVC crosslinked polymer solution was extruded into distilled water using a needle (0.2 mm in the inner diameter) to produce the PEI/PVC-crosslinked fiber (PEI/PVC-CF). The produced PEI/PVC-CF was washed with distilled water multiple times and cut relatively to a length of about 1 cm. The PEI/PVC-CFs were freeze-dried for 24 h and stored in a desiccator before use in the experiments. 2.3. Batch adsorption experiments In all batch experiments, a Pd(II) solution (30 mL) and PEI/PVCCF (0.02 g) were added to 50 mL polypropylene conical tubes. The tubes were stirred at 160 rpm and 25 C. To determine the time required to reach adsorption equilibrium, kinetic experiments were performed with Pd(II) solutions of 25, 50, and 100 mg/L. During the process, samples were obtained at regular intervals, and the amount of adsorbed Pd(II) was measured. In the case of isotherm experiments, the initial concentration of Pd(II) was varied between 0 and 500 mg/L, and the mixtures were stirred for 24 h to ensure that the adsorption equilibrium was reached. Afterwards, each sample was centrifuged at 9000 rpm for 5 min. The residual Pd(II) concentration in the supernatant was measured after adequate dilution, using an inductively coupled plasma-atomic emission spectrometer (ICP-AES, ICPS-7500, Shimadzu, Japan). The Pd(II) uptake (q) by PEI/PVC-CF was calculated using the following mass balance equation.
2. Materials and methods
q¼
2.1. Materials PVC (Mw ¼ 80000) was purchased from Sigma-Aldrich Korea Ltd. (Yongin, Korea). And N,N-dimethylformamide (DMF, 99.5%) was obtained from Daejung chemicals & metals co., Ltd. (Siheung, Korea). PEI of 50% purity with Mw ¼ 70000 was purchased from Habjung Moolsan Co., Ltd. (Seoul, Korea). Since the water in PEI tends to harden the PVC solution, the PEI was dried at 60 C for 24 h in a drying oven before the experiment. 99.0% PdCl2 (Kojima Chemicals, Saitama, Japan) was used as the adsorbate. All other reagents were of analytical grade.
201
Ci Cf V m
(1)
Ci and Cf here represent the initial and final Pd(II) concentrations (mg/L) respectively, and V is the volume (L) of the solution. m represents the amount of adsorbent (g) used in the experiment. 2.4. Desorption and reuse studies In order to investigate the desorption efficiency and reusability of the adsorbent, 0.02 g of PEI/PVC-CF was added to 30 mL Pd(II) solutions (50 or 100 mg/L), and the adsorption was carried out for
Fig. 1. Alkylation reaction between amine groups in PEI and alkyl halides in PVC.
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24 h. The suspension was then centrifuged at 9000 rpm, and the concentration of Pd(II) in the supernatant was analyzed using ICPAES. Pd(II) ions on the adsorbent surface were removed by washing with 0.01 M HCl solution. Acidified thiourea (0e10 mM thiourea and 0.01 M HCl) was used as the eluent to desorb Pd(II) from PEI/ PVC-CF. Changes in desorption efficiency were observed with different concentrations of thiourea. Afterwards, the adsorptiondesorption experiments were repeatedly performed up to 5 times to confirm the reusability of PEI/PVC-CF for Pd(II). The amount of desorbed Pd(II) was analyzed with ICP-AES, and the desorption efficiency was calculated according to equation (2).
Desorbed Pd weight ðmgÞ Desorption efficiency ð%Þ ¼ Initially adsorbed Pd weight ðmgÞ 100 (2) 2.5. FTIR and SEM analyses FTIR spectroscopy (FT/IR-300E, Jasco, Japan) was used to verify the successful production of PEI/PVC-CF by identifying the major functional groups of PEI and PVC. The sample was produced in the form of a KBr disc, and the spectrum was recorded in the range of 4000e400 cm1. The surface of PEI/PVC-CF was observed using SEM (JSM-6400, Jeol, Japan) at 300 and 2000 magnifications. In addition, Pd(II)-adsorbed, and Pd(II)-desorbed PEI/PVC-CF were also imaged by SEM at 2000 magnification. 3. Results and discussion 3.1. Adsorption isotherms Isothermal curves provide important information about the adsorption process. They demonstrate the adsorption equilibrium relationship between the adsorbent and adsorbate, through which the maximum adsorption capacity can be calculated. The isotherm experiments were performed in acidic condition at 25 C, similar to the state of Pd plating wastewater. In the obtained adsorption isotherm (Fig. 2), the Pd(II) adsorption rapidly increased at low concentrations (Ci < 90 mg/L), and the isothermal adsorption curve became saturated at relatively high concentrations (Ci > 160 mg/L). Additionally, the steep incline at the initial section of the curve indicates that PEI/PVC-CF has high affinity for Pd(II).
While empirical models such as the Langmuir and Freundlich models cannot verify the adsorption mechanisms, they are useful for estimating the maximum uptake, which is difficult to assess experimentally. Thus, they are often used to assess the capacity of adsorbents. In these models, the isothermal adsorption curves are described as follows:
Langmuir model: qe ¼
qmax bL Ce 1 þ bL C e 1=n
Freundlich model: qe ¼ KF Ce
(3)
(4)
where, qe (mg/g) indicates the amount of adsorbed Pd(II) at equilibrium, and Ce (mg/L) is the concentration of remaining Pd(II) at equilibrium. qmax (mg/g) indicates the maximum uptake, and bL (L/ g) is the Langmuir constant which means the affinity between the adsorbent and adsorbate. KF (L/g) and n are Freundlich constants describing the adsorptive capacity and strength of the adsorbent, respectively. These parameters were calculated through non-linear regression analysis using the SigmaPlot 10.0 software and shown in Table 1. The R2 value (coefficient of determination) of the Langmuir model was 0.946, higher than that of the Freundlich model (0.922). In addition, the qmax value suggested by the Langmuir model was close to the actual experimental value. Therefore, the Langmuir model is more suitable for describing the adsorption of Pd(II) on PEI/PVC-CF. We compared the adsorptive performance of PEI/PVC-CF for Pd(II) with that of various adsorbents reported in recent literature (Table 2). In the Freundlich model, the constant of 1/n represents the adsorption strength, and adsorption control is known to be effective in the range of 0 < 1/n < 1 (Tan et al., 2008). When 1/n is close to 0, the adsorption strength increases more, while 1/n > 1 indicates cooperative adsorption (Fytianos et al., 2000). The other Freundlich constant KF is an indicator of adsorptivity, and a higher value means better adsorptivity. According to Table 2, the 1/n value of PEI/PVC-CF (0.07) is very close to 0. The corresponding KF value of 101.10 L/g was much higher than the following ion exchange resins: Amberlite IRA 400 (73.11 L/g), Amberlite IRA 411 (60.52 L/g), and Amberlite XAD 7HP (32.22 L/g) (Gandhi et al., 2015). In addition, the KF values of other sorbents, such as chitosan grafted persimmon tannin (Zhou et al., 2015), PEI-coated polysulfone/E. coli biomass composite fiber (Cho et al., 2016), 1-lysine modified crosslinked chitosan (Fujiwara et al., 2007), and calcium alginate gel bead (Cataldo et al., 2016) were 99.5, 62.0, 47.47, and 54 L/g, respectively, which were lower than that of the PEI/PVC-CF. Therefore, based on the 1/n and KF values, it is thought that PEI/PVC-CF demonstrated higher adsorptive strength and excellent adsorptivity compared to other adsorbents reported in recent literature. In general, high maximum adsorption capacity (qmax) and affinity (bL) are regarded as good qualities for an adsorbent (Barka et al., 2013). In Table 2, the qmax value of PEI/PVC-CF is 146.03 mg/g, which is between the values of Amberlite IRA 400 (296.74 mg/g) and Amberlite XAD 7HP (129.87 mg/g). However, in
Table 1 Isotherm parameters of the Langmuir and Freundlich isotherm models for Pd(II) adsorption onto the PEI/PVC-CF. Isotherm model
Parameter
Langmuir
qmax (mg/g) bL (L/mg) R2 KF (L/g) n R2
Freundlich Fig. 2. Pd(II) adsorption isotherm on the PEI/PVC-CF. The curves were predicted by the Langmuir (solid line) and Freundlich (dotted line) isotherm models.
146.03 1.698 0.946 101.10 14.75 0.922
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Table 2 Comparison of Pd(II) adsorption performance of PEI/PVC-CF Pd(II) with that of various sorbents. Sorbents
KF (L/g)
1/n
qmax (mg/g)
bL (L/mg)
HCl-controlled solution
Temp. (oC)
Refs.
Amberlite IRA 400 Amberlite IRA 411 Amberlite XAD 7HP Chitosan grafted persimmon tannin PEI-coated polysulfone/E. coli biomass composite fiber L-Lysine modified crosslinked chitosan resin Calcium alginate gel beads PEI/PVC-CF
73.11 60.52 32.22 99.5 62.00 47.47 54 101.1
0.23 0.25 0.19 0.26 0.23 0.26 0.38 0.07
296.74 289.02 129.87 295.00 216.9 109.47 119 146.03
0.029 0.021 0.008 0.530 0.108 1.579 1.2 1.698
pH 4.0 pH 4.0 pH 4.0 pH 5.0 0.1 M HCl pH 2.0 pH 2.0 0.1 M HCl
29.85 29.85 29.85 29.85 25 30 25 25
Gandhi et al., 2015 Gandhi et al., 2015 Gandhi et al., 2015 Zhou et al., 2015 Cho et al, 2016 Fujiwara et al., 2007 Cataldo et al., 2016 This work
terms of affinity for Pd(II), the bL value of PEI/PVC-CF (1.698) is considerably higher than most other reported adsorbents, including Amberlite IRA 400, which showed relatively higher maximum adsorption capacity. 3.2. Adsorption kinetics The adsorption rate is another important property to consider when assessing the adsorption performance, investigating the adsorption mechanism, and designing adsorbents. The time courses of Pd(II) adsorption by PEI/PVC-CF are plotted in Fig. 3. Pd(II) adsorption rapidly increased in the initial period for all concentrations (25, 50 and 100 mg/L), and reached the adsorption equilibrium very quickly (20, 30 and 60 min, respectively). The amount of Pd(II) adsorbed on PEI/PVC-CF also increased with the initial concentration. The adsorption equilibrium is dependent on the initial Pd(II) concentration. At low concentrations, there are more surface Pd(II) binding sites (amines) than Pd(II) ions, thus the latter are quickly adsorbed. However, at higher initial Pd(II) concentrations, some of the Pd(II) ions need to bind to sites inside the PEI/ PVC-CF, thereby increasing the time needed to reach the adsorption equilibrium. To understand the adsorption dynamics, the experimental data were described using pseudo-first-order and pseudo-second-order kinetic models. Each model is expressed as a non-linear form according to Eqs. (5) and (6):
Pseudo first order kinetic model: qt ¼ q1 ð1 expð k1 tÞÞ (5)
Pseudo second order kinetic model: qt ¼
q22 k2 t 1 þ q2 k2 t
(6)
where, q1 and q2 (mg/g) both represent the adsorption capacity in equilibrium, and qt (mg/g) is the adsorption capacity at time t. k1 (L/ min) and k2 (g/mg min) are the pseudo-first-order and pseudosecond-order rate constants, respectively. Another characteristic parameter h (mg/g min) is the initial adsorption rate defined by the following equation when t / 0.
h ¼ k2 q22
(7)
The values of these variables, h, and the calculated coefficients of determination (R2) for the two models are shown in Table 3. The R2 values of the pseudo-first-order model for initial Pd(II) concentrations of 25 and 50 mg/L were 0.998 and 0.996, respectively. These values are higher than those of the pseudo-second-order model (0.976 and 0.984, respectively). The value of q1 calculated from the pseudo-first-order model was close to the experimental qexp value, whereas q2 obtained through the pseudo-second-order model was very different from qexp. On the contrary, at the initial Pd(II) concentration of 100 mg/L, R2 of the pseudo-second-order model (0.999) was higher than that of the pseudo-first-order model (0.980), and q2 matches the qexp value better than q1. Therefore, it is more suitable to apply the pseudo-first-order model for initial Pd(II) concentrations below 50 mg/L, and the pseudo-second-order model for above 100 mg/L. The pseudo-first-order rate constant k1 was calculated as 0.2039, 0.2483, and 0.1742 L/min for initial Pd(II) concentrations of 25, 50, and 100 mg/L, respectively (Table 3). k1 first increased from 25 to 50 mg/L, and then decreased at the relatively higher concentration of 100 mg/L. The pseudo-second-order rate constant k2 was 0.0074, 0.0055, and 0.0019 g/mg min for Pd(II) concentrations of 25, 50, and 100 mg/L, respectively, decreasing gradually as the initial Pd(II) concentration was increased. The value of h increased to about 2.7 times (from 11.81 to 32.09 mg/g min) as the initial Pd(II) concentration increased from 25 to 50 mg/L, then maintained to 32.09 mg/g min at 100 mg/L. Regardless of the variations, these
Table 3 The parameters of kinetic models for Pd(II) adsorption on PEI/PVC-CF at different initial Pd(II) concentrations. Kinetic model
Parameter
Initial Pd(II) concentration (mg/ L) 25
50
100
Pseudo-first-order
q1 (mg/g) k1 (L/min) R2 q2 (mg/g) k2 (g/mg min) R2 h
37.05 0.2039 0.998 39.95 0.0074 0.976 11.81
72.12 0.2483 0.996 76.39 0.0055 0.984 32.09
120.02 0.1742 0.980 129.95 0.0019 0.999 32.09
Pseudo-second-order Fig. 3. PEI/PVC-CFs Pd(II) adsorption kinetic on the PEI/PVC-CF at different initial concentrations. Curves were fitted by the pseudo-first-order (solid lines) and pseudosecond-order (dotted lines) kinetic models.
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values are much higher than those of reported biosorbent (PEIcoated polysulfone/E. coli biomass composite fiber) (Cho et al., 2016) and ion exchange resins (Amberlite IRA 400 and Amberlite XAD 7HP) (Gandhi et al., 2015) when assessed under similar experimental conditions. Additionally, the time needed for PEI/ PVC-CF to reach adsorption equilibrium was also much shorter than for the other adsorbents. 3.3. Desorption Effective desorption is necessary to recover the Pd(II) adsorbed from palladium plating wastewater. Acidified thiourea solution was known to desorb Pd from adsorbents with high efficiency. For example, Awual et al. (2015) has used a mixed solution of 0.1 M HCl and 0.10 M thiourea to recover Pd(II) adsorbed on MBHB ligand immobilized meso-adsorbent with >90% efficiency and without damaging the material. Therefore, this study has used acidified thiourea as the eluent, and investigated the relationship between thiourea concentration and Pd(II) desorption efficiency. For desorption experiments, Pd(II)-sorbed PEI/PVC-CFs were prepared using solutions with the initial Pd(II) concentrations of 50 and 100 mg/L. We measured the amount of Pd(II) desorbed with mixed solutions of 0.01 M HCl and various amounts of thiourea (0e10 mM). In Fig. 4, the desorption efficiency dramatically increased as the thiourea concentration went up from 0 to 5 mM, and continued to increase with thiourea to finally reach 100%. Adsorbents treated with 50 and 100 mg/L Pd(II) required 5 and 10 mM thiourea to reach 100% desorption efficiency, respectively. Therefore, in the following reusability assessment, the mixed solution of 0.01 M HCl and 10 mM thiourea was selected as the desorption solution. 3.4. Characterization of Pd(II)-sorbed and edesorbed PEI/PVC-CFs The SEM images of the free, Pd(II)-sorbed and Pd(II)-desorbed PEI/PVC-CFs are shown under 300 and 2000 magnifications in Fig. 5. As shown in Fig. 5a, the surface of free PEI/PVC-CF was significantly rough and generally porous, and its thickness was nearly 210 mm. At higher magnification (Fig. 5a’), the fiber was clearly observed that the surface of the adsorbent was more porous. Fig. 5b and b' showed a change in surface morphology of Pd(II)sorbed PEI/PVC-CF compared to that of free PEI/PVC-CF. After adsorption, the pores on the surface of the adsorbent were lost by
Pd (II) ions and the thickness of adsorbent was also increased by about 425 mm. In contrast, the thickness of the Pd(II)-desorbed PEI/ PVC-CF returned to about 215 mm, which is equal to the thickness of free adsorbent (Fig. 5c). This result revealed that Pd(II) adsorption and desorption could be assumed to occur on the surface of the adsorbent. Fig. 6 shows the FTIR spectrum of PEI/PVC-CF with characteristic peaks of PVC and PEI. The major peaks of PVC were observed at 1245 (CeC stretching), 1334 (CH2 deformation), 2836 and 620 cm1 (both CeCl stretching) (Luo et al, 2015; Wang et al, 2015). The characteristic peaks of PEI were observed at 1435 (eNH bending and deformation of eNH2), 1650 (eNH2 for amine I or eNH for amide II), and 3460 cm1 (NeH stretching of amines) (Kim et al., 2015; Wang et al., 2013; Moradian et al., 2014). In addition, the strong peak at 620 cm1 (C-Cl stretching) decreased significantly, providing the indirect evidence of alkylation reaction between the amine of PEI and chlorine of PVC (data not shown). In the sorption stage, the N-H stretching peak shifted from 3460 cm1 to 3483 cm1, while the shifted peak returned to its original state after Pd(II) desorption. Therefore, based on the interesting findings of SEM and FTIR, PEI/PVC-CF has a reversible property for the adsorption and desorption of Pd(II), which may be due to the electrostatic attraction between the positively charged amine groups in the sorbent and the negatively charged Pd(II) anions. 3.5. Reusability If an adsorbent cannot be reused, then the depleted adsorbent would have to be deposed through a secondary process such as incineration. This increases the maintenance cost of the adsorption process, and the secondary treatment can also cause environmental pollution. Therefore, reusable adsorbents or desorption processes are desirable for environmental and economic reasons. Furthermore, the desorption should be easy, and the adsorption capacity should not decline with repeated use. We used the Pd(II) solution of 50 mg/L and the mixed eluent of 0.01 M HCl and 10 mM thiourea, in order to evaluate the repeated usability of PEI/PVC-CF. The adsorption-desorption experiment was performed consecutively for 5 times. The results (Fig. 7) show that the amount of Pd(II) adsorbed on PEI/PVC-CF changed very little with repeated adsorption and desorption, and Pd(II) was completely desorbed at the end of each cycle. This indicates that our PEI/PVC-CF absorbent is highly stable in this adsorption-desorption process and can therefore be repeatedly used. 4. Conclusions In this study, we developed a new adsorbent, PEI/PVC-CF, which can efficiently remove and recover Pd(II) from acidic solution, and proposed a desorption process to allow easy recovery of the adsorbed Pd(II). The developed adsorbent and its recovery process could be also applied for Pd(II) recovery from palladium plating wastewater. The major conclusions gained through this study are as follows.
Fig. 4. Pd(II) desorption using different thiourea concentrations in 0.1 M HCl solutions.
C The results of SEM and FTIR showed that the PEI/PVCcrosslinked fiber was successfully produced and was effective to adsorb Pd(II) anions by electrostatic attraction. C The isotherm data were better described by the Langmuir model than the Freundlich model, and the maximum Pd(II) uptake was 146.03 mg/g. C The adsorption equilibrium was reached faster with lower initial Pd(II) concentration. In 100 mg/L Pd(II) solution, it took about 60 min for the material to reach adsorption equilibrium.
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Fig. 5. SEM images of free (a and a’), Pd(II)-sorbed (b and b’), and Pd(II)-desorbed (c and c’).
Fig. 7. Repeated reuse of PEI/PVC-CF through adsorption-desorption cycles. Fig. 6. FTIR spectra of Pd(II)-sorbed (a) and Pd(II)-desorbed (b) PEI/PVC-CFs.
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C A mixture of 0.01 M HCl and 10 mM thiourea was the best eluent to induce 100% of Pd(II) adsorbed on PEI/PVC-CF. C PEI/PVC-CF was used repeatedly for adsorption-desorption for at least 5 cycles while maintaining high performance. C In conclusion, PEI/PVC-CF has been demonstrated to possess high potential as an adsorbent to efficiently remove and recover Pd(II) in acidic solution. Acknowledgements This work was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT & Future Planning (NRF2017R1A1A1A05000741). References Awual, M.R., Khaleque, M.A., Ratna, Y., Znad, H., 2015. Simultaneous ultra-trace palladium (II) detection and recovery from wastewater using new class mesoadsorbent. J. Ind. Eng. Chem. 21, 405e413. Barka, N., Ouzaouit, K., Abdennouri, M., Makhfouk, M.E., 2013. Dried prickly pear cactus (Opuntia ficus indica) cladodes as a low-cost and eco-friendly biosorbent for dyes removal from aqueous solutions. J. Taiwan Inst. Chem. Eng. 44, 52e60. Cataldo, S., Gianguzza, A., Pettignano, A., 2016. Sorption of Pd(II) ion by calcium alginate gel beads at different chloride concentrations and pH. A kinetic and equilibrium study. Arab. J. Chem. 9, 656e667. Cho, C.-W., Kang, S.B., Kim, S., Yun, Y.-S., Won, S.W., 2016. Reusable polyethylenimine-coated polysulfone/bacterial biomass composite fiber biosorbent for recovery of Pd (II) from acidic solutions. Chem. Eng. J. 302, 545e551. Fujiwara, K., Ramesh, A., Maki, T., Hasegawa, H., Ueda, K., 2007. Adsorption of platinum (IV), palladium (II) and gold (III) from aqueous solutions onto l-lysine modified crosslinked chitosan resin. J. Hazrd. Meter. 146, 39e50. Fytianos, K., Voudrias, E., Kokkalis, E., 2000. Sorption-desorption behaviour of 2,4dichlorophenol by marine sediments. Chemosphere 40, 3e6. Gandhi, M.R., Yamada, M., Kondo, Y., Shibayama, A., Hamada, F., 2015. p-Sulfonatothiacalix [6] arene-impregnated resins for the sorption of platinum group metals and effective separation of palladium from automotive catalyst residue. J. Ind. Eng. Chem. 30, 20e28. pez-Lo pez, J.A., Pinto, J.J., Moreno, C., 2017. Ionic liquid based Herce-Sesa, B., Lo solvent micro-extraction of Ag and Cd from saline and hyper-saline waters. Chem. Eng. J. 308, 649e655. ndez-Tapia, J.R., Vazquez-Arenas, J., Gonza lez, I., 2013. A kinetic model to Herna describe the nickel electro-recovery from industrial plating effluents under variable electrolyte conductivity. Electrochim. Acta 103, 266e274. Hosseini, S.S., Bringas, E., Tan, N.R., Ortiz, I., Ghahramani, M., Shahmirzadi, M.A.A., 2016. Recent progress in development of high performance polymeric membranes and materials for metal plating wastewater treatment: a review. J. Water Process Eng. 9, 78e110. Huang, C.-C., Chen, H.-I., Liu, I-P., Chen, C.-C., Chou, P.-C., Liou, J.-K., Liu, W.-C., 2014. Comprehensive study of hydrogen sensing phenomena of an electroless plating (EP)-based Pd/AlGaN/GaN heterostructure field-effect transistor (HFET). Actuator B Chem. 190, 913e921.
Kato, T., Igarashi, S., Ohno, O., Saito, S., Ando, R., 2016. Homogeneous liquid-liquid extraction (HoLLE) of palladium in real plating wastewater for recovery. J. Environ. Prot. 7, 277e286. Khazaeli, S., Nezamabadi, N., Rabani, M., Panahi, H.A., 2013. A new functionalized resin and its application in flame atomic absorption spectrophotometric determination of trace amounts of heavy metal ions after solid phase extraction in water samples. Microchem. J. 106, 147e153. Kim, M.H., Hwang, C.H., Kang, S.B., Kim, S., Park, S.W., Yun, Y.-S., Won, S.W., 2015. Removal of hydrolyzed Reactive Black 5 from aqueous solution using a polyethylenimine-polyvinyl chloride composite fiber. Chem. Eng. J. 280, 18e25. Kim, S., Choi, Y.-E., Yun, Y.-S., 2016. Ruthenium recovery from acetic acid industrial effluent using chemically stable and high-performance polyethyleniminecoated polysulforne-Escherichia coli biomass composite fibers. J. Hazard. Mater. 313, 29e36. Lee, H.Y., 2013. Separation and recovery of nickel from spent electroless nickelplating solutions with hydrometallurgical processes. Sep. Sci. Technol. 48, 1602e1608. Luo, Q., Wang, L., Wang, D., Yin, R., Li, X., An, J., Yang, X., 2015. Preparation, characterization and visible-light photocatalytic performances of composite films prepared from polyvinyl chloride and SnO2 nanoparticles. J. Environ. Chem. Eng. 3, 622e629. Moradian, H., Fasehee, H., Keshvari, H., Faghihi, S., 2014. Poly (ethyleneimine) functionalized carbon nanotubes as efficient nano-vector for transfecting mesenchymal stem cells. Colloid Surf. B Biointerfaces 122, 115e125. Morcali, M.H., Zeytuncu, B., 2015. Investigation of adsorption parameters for platinum and palladium onto a modified polyacrylonitrile-based sorbent. J. Miner. Process. 137, 52e58. Oh, J.H., Kim, M.S., Shin, H.D., 2004. On the generation and processing of the sludge containing heavy metals in Korea. J. Korean Ins. Res. Recycl. 13.5, 3e16. Rouya, E., Stafford, G.R., Bertocci, U., Mallett, J.J., Schad, R., Begley, M.R., Kelly, R.G., Reed, M.L., Zangari, G., 2010. Electrodeposition of metastable AueNi alloys. J. Electrochem. Soc. 157, D396eD405. Sharma, S., Rajesh, N., 2017. Expnditious preparation of b-cyclodextrin grafted chitosan using microwave radiation for the enhanced palladium adsorption from aqueous waste and an industrial caralyst. J. Environ. Chem. Eng. 5, 1927e1935. Sharma, S., Rajesh, N., 2014. 2-Mercaptobenzothiazole impregnated cellulose prepared by ultrasonication for the effective adsorption of precious metal palladium. Chem. Eng. J. 241, 112e121. Tan, I.A.W., Ahmad, A.L., Hameed, B.H., 2008. Adsorption of basic dye on highsurface-area activated carbon prepared from coconut husk: Equilibrium, kinetic and thermodynamic studies. J. Hazard. Mater. 154, 337e346. Wang, C., Wang, H., Fu, J., Zhang, L., Luo, C., Liu, Y., 2015. Flotation separation of polyvinyl chloride and polyethylene terephthalate plastics combined with surface modification for recycling. Waste Manag. 45, 112e117. Wang, F., Liu, P., Nie, T., Wei, H., Cui, Z., 2013. Characterization of a polyamine microsphere and its adsorption for protein. Int. J. Mol. Sci. 14, 17e29. Won, S.W., Kim, S., Kotte, P., Lim, A., Yun, Y.-S., 2013. Cationic polymer-immobilized polysulfone-based fibers as high performance sorbents for Pt(IV) recovery from acidic solutions. J. Hazard. Mater. 263, 391e397. Yi, Q., Fan, R., Xie, F., Zhang, Q., Luo, Z., 2016. Recovery of palladium(II) from nitric acid medium using a natural resin prepared from persimmon dropped fruits residues. J. Taiwan Inst. Chem. Eng. 61, 299e305. Zhou, Z., Liu, F., Huang, Y., Wang, Z., Li, G., 2015. Biosorption of palladium(II) from aqueous solution by grafting chitosan on persimmon tannin extract. J. Boil. Macromol. 77, 336e343.
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Journal of Environmental Management journal homepage: www.elsevier.com/locate/jenvman
Research article
A reusable adsorbent polyethylenimine/polyvinyl chloride crosslinked fiber for Pd(II) recovery from acidic solutions Han Ah Choi a, Ha Neul Park b, Sung Wook Won a, b, * a
Department of Marine Environmental Engineering and Institute of Marine Industry, Gyeongsang National University, 38 Cheondaegukchi-gil, Tongyeong, Gyeongnam 53064, Republic of Korea b Department of Ocean System Engineering, Gyeongsang National University, 38 Cheondaegukchi-gil, Tongyeong, Gyeongnam 53064, Republic of Korea
a r t i c l e i n f o
a b s t r a c t
Article history: Received 24 May 2017 Received in revised form 19 August 2017 Accepted 25 August 2017
In this study, a mixture of polyethylenimine (PEI) and polyvinyl chloride (PVC) was reacted at 80 C for 6 h to synthesize crosslinked PEI/PVC polymer solution, which was injected to produce the PEI/PVCcrosslinked fiber (PEI/PVC-CF). PEI/PVC-CF was investigated as an adsorbent to remove and recover Pd(II) from acidic solutions. In order to examine the adsorption characteristics and usability of PEI/PVCCF for Pd(II) recycling, several experiments such as isotherm, kinetics, desorption and reuse were conducted. The adsorption isotherms were fitted using the Langmuir and Freundlich models, respectively. The maximum adsorption capacity was estimated as 146.03 mg/g according to the Langmuir model. The kinetic experiments demonstrated that adsorbent reaches adsorption equilibrium within 60 min for initial Pd(II) concentrations of 25e100 mg/L. After adsorption, Pd(II) on PEI/PVC-CF was easily desorbed using acidified thiourea solution, and the desorption efficiency increased with the thiourea concentration. It was also demonstrated that PEI/PVC-CF can be used repeatedly for at least five cycles without reduction in adsorption capacity. © 2017 Elsevier Ltd. All rights reserved.
Keywords: Palladium Recovery Adsorbent Reusability
1. Introduction Precious metal plating is frequently applied in areas such as connectors or sensors of electronic devices to achieve desired chemical, physical, and electrical properties (Kato et al., 2016). The electric power industry previously used gold plating to achieve high resistance to decay and increase electrical conductivity (Khazaeli et al., 2013). However, now palladium (Pd) plating is used instead on the surface of electrical parts for cost reduction (Rouya et al., 2010; Huang et al., 2014). Pd is only produced in a handful of countries including Russia, South Africa, Canada, and the United States. Therefore, there is a great imbalance between the limited Pd supply and the high demand for it, leading to precarious price changes (Morcali and Zeytuncu, 2015). Industries in Korea are highly dependent on imported raw Pd, and use a coagulation-sedimentation method, which is superior
* Corresponding author. Department of Marine Environmental Engineering and Institute of Marine Industry, Gyeongsang National University, 38 Cheondaegukchigil, Tongyeong, Gyeongnam 53064, Republic of Korea. E-mail address: [email protected] (S.W. Won). http://dx.doi.org/10.1016/j.jenvman.2017.08.047 0301-4797/© 2017 Elsevier Ltd. All rights reserved.
in cost and treatment efficiency, among various methods for plating wastewater treatment. However, this method often produces large amounts of sludge using excess chemicals in each process. It is estimated that Korea discharges 76,000 tons of plating sludge per year and in Japan it emits about 50,000 tons per year (Hosseini et al., 2016; Oh et al., 2004). Therefore, it is absolutely necessary to develop technologies for effective Pd recycling from plating wastewaters. In addition, Pd recovery is crucial in terms of resource recycling and sustainable practice in chemical industries. In general, such plating wastewaters are acidic at the temperature of 20e50 C, and contain up to 100 mg/L in Pd (Hosseini et al., 2016). The conventional methods to retrieve valuable metals such as Pd include cementation, precipitation (Lee, 2013), electrowinndez-Tapia et al., 2013), solid phase extraction ning (Herna (Khazaeli et al., 2013), and solvent extraction (Herce-Sesa et al., 2017). However, these methods entail not only high cost but also long periods of treatment. In contrast, adsorption is known to be a more economical and promising method, as it can effectively eliminate target substances from wastewater in a simple process. Also, this method could be effective in reducing sludge incidence by
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applying directly to wastewaters without the use of chemicals neutralizing acidic plating wastewater in existing processes. However, the treatment efficiency is highly dependent on the capacity of the adsorbent. Therefore, it is highly desirable to develop effective adsorbents for Pd from acidic wastewaters, such as that from Pd plating, as well as easy and economical methods to recover the adsorbed Pd(II). Recent literature has included a variety of studies related to the recovery of Pd from acidic solutions or wastewaters, and many researchers are interested in developing biosorbents using natural resources. The reason may be that it contains biocompatibility, biodegradability and excellent reactivity and hydroxyl or amino functional groups effective for adsorption. For example, natural resin (Yi et al., 2016), b-cyclodextrin grafted chitosan (Sharma and Rajesh, 2017) and 2-Mercaptobenzothiazole impregnated cellulose (Sharma and Rajesh, 2014) have been developed and evaluated for Pd recovery. However, they may be vulnerable to acidity and have some defects such as swelling and degradation of strength due to absorption of water (Kim et al., 2016; Won et al., 2013). Therefore, it is required to develop a suitable adsorbent having high acid resistance in order to improve practical applicability. In this study, we developed a new material that is effective for adsorbing Pd(II) in acidic solutions, by modifying our previously developed method (Kim et al., 2015). Polyvinyl chloride (PVC) featuring high mechanical strength and acid resistance was used as the main body of the adsorbent, and polyethylenimine (PEI) containing ample amines as the coupler. The surface of the adsorbent and the crosslinking between PEI and PVC were analyzed through Fourier transform infrared spectroscopy (FTIR) and scanning electron microscopy (SEM). The adsorption performance of PEI/PVC-CF was analyzed through isotherm and kinetic experiments. An acidified thiourea solution was used for desorption of the adsorbed Pd(II), and the optimal concentration of thiourea was examined. Finally, the reusability of PEI/PVC-CF was investigated through repeated adsorption-desorption experiments.
2.2. Preparation of PEI/PVC-crosslinked fiber PEI (3.75 g) and PVC (3 g) were each dissolved in a bottle containing 15 mL DMF solution at 40 C for 24 h. The completely dissolved PEI solution was added to the PVC solution, and the mixture was vigorously stirred at 80 C for 6 h to produce the crosslinked PEI/PVC polymer solution. The reaction scheme of PEI and PVC is demonstrated in Fig. 1. The color of the mixture gradually changed from transparent to yellow. After 6 h, the crosslinked polymer solution was quickly cooled down to room temperature to stop the reaction between the two polymers. The PEI/PVC crosslinked polymer solution was extruded into distilled water using a needle (0.2 mm in the inner diameter) to produce the PEI/PVC-crosslinked fiber (PEI/PVC-CF). The produced PEI/PVC-CF was washed with distilled water multiple times and cut relatively to a length of about 1 cm. The PEI/PVC-CFs were freeze-dried for 24 h and stored in a desiccator before use in the experiments. 2.3. Batch adsorption experiments In all batch experiments, a Pd(II) solution (30 mL) and PEI/PVCCF (0.02 g) were added to 50 mL polypropylene conical tubes. The tubes were stirred at 160 rpm and 25 C. To determine the time required to reach adsorption equilibrium, kinetic experiments were performed with Pd(II) solutions of 25, 50, and 100 mg/L. During the process, samples were obtained at regular intervals, and the amount of adsorbed Pd(II) was measured. In the case of isotherm experiments, the initial concentration of Pd(II) was varied between 0 and 500 mg/L, and the mixtures were stirred for 24 h to ensure that the adsorption equilibrium was reached. Afterwards, each sample was centrifuged at 9000 rpm for 5 min. The residual Pd(II) concentration in the supernatant was measured after adequate dilution, using an inductively coupled plasma-atomic emission spectrometer (ICP-AES, ICPS-7500, Shimadzu, Japan). The Pd(II) uptake (q) by PEI/PVC-CF was calculated using the following mass balance equation.
2. Materials and methods
q¼
2.1. Materials PVC (Mw ¼ 80000) was purchased from Sigma-Aldrich Korea Ltd. (Yongin, Korea). And N,N-dimethylformamide (DMF, 99.5%) was obtained from Daejung chemicals & metals co., Ltd. (Siheung, Korea). PEI of 50% purity with Mw ¼ 70000 was purchased from Habjung Moolsan Co., Ltd. (Seoul, Korea). Since the water in PEI tends to harden the PVC solution, the PEI was dried at 60 C for 24 h in a drying oven before the experiment. 99.0% PdCl2 (Kojima Chemicals, Saitama, Japan) was used as the adsorbate. All other reagents were of analytical grade.
201
Ci Cf V m
(1)
Ci and Cf here represent the initial and final Pd(II) concentrations (mg/L) respectively, and V is the volume (L) of the solution. m represents the amount of adsorbent (g) used in the experiment. 2.4. Desorption and reuse studies In order to investigate the desorption efficiency and reusability of the adsorbent, 0.02 g of PEI/PVC-CF was added to 30 mL Pd(II) solutions (50 or 100 mg/L), and the adsorption was carried out for
Fig. 1. Alkylation reaction between amine groups in PEI and alkyl halides in PVC.
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24 h. The suspension was then centrifuged at 9000 rpm, and the concentration of Pd(II) in the supernatant was analyzed using ICPAES. Pd(II) ions on the adsorbent surface were removed by washing with 0.01 M HCl solution. Acidified thiourea (0e10 mM thiourea and 0.01 M HCl) was used as the eluent to desorb Pd(II) from PEI/ PVC-CF. Changes in desorption efficiency were observed with different concentrations of thiourea. Afterwards, the adsorptiondesorption experiments were repeatedly performed up to 5 times to confirm the reusability of PEI/PVC-CF for Pd(II). The amount of desorbed Pd(II) was analyzed with ICP-AES, and the desorption efficiency was calculated according to equation (2).
Desorbed Pd weight ðmgÞ Desorption efficiency ð%Þ ¼ Initially adsorbed Pd weight ðmgÞ 100 (2) 2.5. FTIR and SEM analyses FTIR spectroscopy (FT/IR-300E, Jasco, Japan) was used to verify the successful production of PEI/PVC-CF by identifying the major functional groups of PEI and PVC. The sample was produced in the form of a KBr disc, and the spectrum was recorded in the range of 4000e400 cm1. The surface of PEI/PVC-CF was observed using SEM (JSM-6400, Jeol, Japan) at 300 and 2000 magnifications. In addition, Pd(II)-adsorbed, and Pd(II)-desorbed PEI/PVC-CF were also imaged by SEM at 2000 magnification. 3. Results and discussion 3.1. Adsorption isotherms Isothermal curves provide important information about the adsorption process. They demonstrate the adsorption equilibrium relationship between the adsorbent and adsorbate, through which the maximum adsorption capacity can be calculated. The isotherm experiments were performed in acidic condition at 25 C, similar to the state of Pd plating wastewater. In the obtained adsorption isotherm (Fig. 2), the Pd(II) adsorption rapidly increased at low concentrations (Ci < 90 mg/L), and the isothermal adsorption curve became saturated at relatively high concentrations (Ci > 160 mg/L). Additionally, the steep incline at the initial section of the curve indicates that PEI/PVC-CF has high affinity for Pd(II).
While empirical models such as the Langmuir and Freundlich models cannot verify the adsorption mechanisms, they are useful for estimating the maximum uptake, which is difficult to assess experimentally. Thus, they are often used to assess the capacity of adsorbents. In these models, the isothermal adsorption curves are described as follows:
Langmuir model: qe ¼
qmax bL Ce 1 þ bL C e 1=n
Freundlich model: qe ¼ KF Ce
(3)
(4)
where, qe (mg/g) indicates the amount of adsorbed Pd(II) at equilibrium, and Ce (mg/L) is the concentration of remaining Pd(II) at equilibrium. qmax (mg/g) indicates the maximum uptake, and bL (L/ g) is the Langmuir constant which means the affinity between the adsorbent and adsorbate. KF (L/g) and n are Freundlich constants describing the adsorptive capacity and strength of the adsorbent, respectively. These parameters were calculated through non-linear regression analysis using the SigmaPlot 10.0 software and shown in Table 1. The R2 value (coefficient of determination) of the Langmuir model was 0.946, higher than that of the Freundlich model (0.922). In addition, the qmax value suggested by the Langmuir model was close to the actual experimental value. Therefore, the Langmuir model is more suitable for describing the adsorption of Pd(II) on PEI/PVC-CF. We compared the adsorptive performance of PEI/PVC-CF for Pd(II) with that of various adsorbents reported in recent literature (Table 2). In the Freundlich model, the constant of 1/n represents the adsorption strength, and adsorption control is known to be effective in the range of 0 < 1/n < 1 (Tan et al., 2008). When 1/n is close to 0, the adsorption strength increases more, while 1/n > 1 indicates cooperative adsorption (Fytianos et al., 2000). The other Freundlich constant KF is an indicator of adsorptivity, and a higher value means better adsorptivity. According to Table 2, the 1/n value of PEI/PVC-CF (0.07) is very close to 0. The corresponding KF value of 101.10 L/g was much higher than the following ion exchange resins: Amberlite IRA 400 (73.11 L/g), Amberlite IRA 411 (60.52 L/g), and Amberlite XAD 7HP (32.22 L/g) (Gandhi et al., 2015). In addition, the KF values of other sorbents, such as chitosan grafted persimmon tannin (Zhou et al., 2015), PEI-coated polysulfone/E. coli biomass composite fiber (Cho et al., 2016), 1-lysine modified crosslinked chitosan (Fujiwara et al., 2007), and calcium alginate gel bead (Cataldo et al., 2016) were 99.5, 62.0, 47.47, and 54 L/g, respectively, which were lower than that of the PEI/PVC-CF. Therefore, based on the 1/n and KF values, it is thought that PEI/PVC-CF demonstrated higher adsorptive strength and excellent adsorptivity compared to other adsorbents reported in recent literature. In general, high maximum adsorption capacity (qmax) and affinity (bL) are regarded as good qualities for an adsorbent (Barka et al., 2013). In Table 2, the qmax value of PEI/PVC-CF is 146.03 mg/g, which is between the values of Amberlite IRA 400 (296.74 mg/g) and Amberlite XAD 7HP (129.87 mg/g). However, in
Table 1 Isotherm parameters of the Langmuir and Freundlich isotherm models for Pd(II) adsorption onto the PEI/PVC-CF. Isotherm model
Parameter
Langmuir
qmax (mg/g) bL (L/mg) R2 KF (L/g) n R2
Freundlich Fig. 2. Pd(II) adsorption isotherm on the PEI/PVC-CF. The curves were predicted by the Langmuir (solid line) and Freundlich (dotted line) isotherm models.
146.03 1.698 0.946 101.10 14.75 0.922
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203
Table 2 Comparison of Pd(II) adsorption performance of PEI/PVC-CF Pd(II) with that of various sorbents. Sorbents
KF (L/g)
1/n
qmax (mg/g)
bL (L/mg)
HCl-controlled solution
Temp. (oC)
Refs.
Amberlite IRA 400 Amberlite IRA 411 Amberlite XAD 7HP Chitosan grafted persimmon tannin PEI-coated polysulfone/E. coli biomass composite fiber L-Lysine modified crosslinked chitosan resin Calcium alginate gel beads PEI/PVC-CF
73.11 60.52 32.22 99.5 62.00 47.47 54 101.1
0.23 0.25 0.19 0.26 0.23 0.26 0.38 0.07
296.74 289.02 129.87 295.00 216.9 109.47 119 146.03
0.029 0.021 0.008 0.530 0.108 1.579 1.2 1.698
pH 4.0 pH 4.0 pH 4.0 pH 5.0 0.1 M HCl pH 2.0 pH 2.0 0.1 M HCl
29.85 29.85 29.85 29.85 25 30 25 25
Gandhi et al., 2015 Gandhi et al., 2015 Gandhi et al., 2015 Zhou et al., 2015 Cho et al, 2016 Fujiwara et al., 2007 Cataldo et al., 2016 This work
terms of affinity for Pd(II), the bL value of PEI/PVC-CF (1.698) is considerably higher than most other reported adsorbents, including Amberlite IRA 400, which showed relatively higher maximum adsorption capacity. 3.2. Adsorption kinetics The adsorption rate is another important property to consider when assessing the adsorption performance, investigating the adsorption mechanism, and designing adsorbents. The time courses of Pd(II) adsorption by PEI/PVC-CF are plotted in Fig. 3. Pd(II) adsorption rapidly increased in the initial period for all concentrations (25, 50 and 100 mg/L), and reached the adsorption equilibrium very quickly (20, 30 and 60 min, respectively). The amount of Pd(II) adsorbed on PEI/PVC-CF also increased with the initial concentration. The adsorption equilibrium is dependent on the initial Pd(II) concentration. At low concentrations, there are more surface Pd(II) binding sites (amines) than Pd(II) ions, thus the latter are quickly adsorbed. However, at higher initial Pd(II) concentrations, some of the Pd(II) ions need to bind to sites inside the PEI/ PVC-CF, thereby increasing the time needed to reach the adsorption equilibrium. To understand the adsorption dynamics, the experimental data were described using pseudo-first-order and pseudo-second-order kinetic models. Each model is expressed as a non-linear form according to Eqs. (5) and (6):
Pseudo first order kinetic model: qt ¼ q1 ð1 expð k1 tÞÞ (5)
Pseudo second order kinetic model: qt ¼
q22 k2 t 1 þ q2 k2 t
(6)
where, q1 and q2 (mg/g) both represent the adsorption capacity in equilibrium, and qt (mg/g) is the adsorption capacity at time t. k1 (L/ min) and k2 (g/mg min) are the pseudo-first-order and pseudosecond-order rate constants, respectively. Another characteristic parameter h (mg/g min) is the initial adsorption rate defined by the following equation when t / 0.
h ¼ k2 q22
(7)
The values of these variables, h, and the calculated coefficients of determination (R2) for the two models are shown in Table 3. The R2 values of the pseudo-first-order model for initial Pd(II) concentrations of 25 and 50 mg/L were 0.998 and 0.996, respectively. These values are higher than those of the pseudo-second-order model (0.976 and 0.984, respectively). The value of q1 calculated from the pseudo-first-order model was close to the experimental qexp value, whereas q2 obtained through the pseudo-second-order model was very different from qexp. On the contrary, at the initial Pd(II) concentration of 100 mg/L, R2 of the pseudo-second-order model (0.999) was higher than that of the pseudo-first-order model (0.980), and q2 matches the qexp value better than q1. Therefore, it is more suitable to apply the pseudo-first-order model for initial Pd(II) concentrations below 50 mg/L, and the pseudo-second-order model for above 100 mg/L. The pseudo-first-order rate constant k1 was calculated as 0.2039, 0.2483, and 0.1742 L/min for initial Pd(II) concentrations of 25, 50, and 100 mg/L, respectively (Table 3). k1 first increased from 25 to 50 mg/L, and then decreased at the relatively higher concentration of 100 mg/L. The pseudo-second-order rate constant k2 was 0.0074, 0.0055, and 0.0019 g/mg min for Pd(II) concentrations of 25, 50, and 100 mg/L, respectively, decreasing gradually as the initial Pd(II) concentration was increased. The value of h increased to about 2.7 times (from 11.81 to 32.09 mg/g min) as the initial Pd(II) concentration increased from 25 to 50 mg/L, then maintained to 32.09 mg/g min at 100 mg/L. Regardless of the variations, these
Table 3 The parameters of kinetic models for Pd(II) adsorption on PEI/PVC-CF at different initial Pd(II) concentrations. Kinetic model
Parameter
Initial Pd(II) concentration (mg/ L) 25
50
100
Pseudo-first-order
q1 (mg/g) k1 (L/min) R2 q2 (mg/g) k2 (g/mg min) R2 h
37.05 0.2039 0.998 39.95 0.0074 0.976 11.81
72.12 0.2483 0.996 76.39 0.0055 0.984 32.09
120.02 0.1742 0.980 129.95 0.0019 0.999 32.09
Pseudo-second-order Fig. 3. PEI/PVC-CFs Pd(II) adsorption kinetic on the PEI/PVC-CF at different initial concentrations. Curves were fitted by the pseudo-first-order (solid lines) and pseudosecond-order (dotted lines) kinetic models.
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values are much higher than those of reported biosorbent (PEIcoated polysulfone/E. coli biomass composite fiber) (Cho et al., 2016) and ion exchange resins (Amberlite IRA 400 and Amberlite XAD 7HP) (Gandhi et al., 2015) when assessed under similar experimental conditions. Additionally, the time needed for PEI/ PVC-CF to reach adsorption equilibrium was also much shorter than for the other adsorbents. 3.3. Desorption Effective desorption is necessary to recover the Pd(II) adsorbed from palladium plating wastewater. Acidified thiourea solution was known to desorb Pd from adsorbents with high efficiency. For example, Awual et al. (2015) has used a mixed solution of 0.1 M HCl and 0.10 M thiourea to recover Pd(II) adsorbed on MBHB ligand immobilized meso-adsorbent with >90% efficiency and without damaging the material. Therefore, this study has used acidified thiourea as the eluent, and investigated the relationship between thiourea concentration and Pd(II) desorption efficiency. For desorption experiments, Pd(II)-sorbed PEI/PVC-CFs were prepared using solutions with the initial Pd(II) concentrations of 50 and 100 mg/L. We measured the amount of Pd(II) desorbed with mixed solutions of 0.01 M HCl and various amounts of thiourea (0e10 mM). In Fig. 4, the desorption efficiency dramatically increased as the thiourea concentration went up from 0 to 5 mM, and continued to increase with thiourea to finally reach 100%. Adsorbents treated with 50 and 100 mg/L Pd(II) required 5 and 10 mM thiourea to reach 100% desorption efficiency, respectively. Therefore, in the following reusability assessment, the mixed solution of 0.01 M HCl and 10 mM thiourea was selected as the desorption solution. 3.4. Characterization of Pd(II)-sorbed and edesorbed PEI/PVC-CFs The SEM images of the free, Pd(II)-sorbed and Pd(II)-desorbed PEI/PVC-CFs are shown under 300 and 2000 magnifications in Fig. 5. As shown in Fig. 5a, the surface of free PEI/PVC-CF was significantly rough and generally porous, and its thickness was nearly 210 mm. At higher magnification (Fig. 5a’), the fiber was clearly observed that the surface of the adsorbent was more porous. Fig. 5b and b' showed a change in surface morphology of Pd(II)sorbed PEI/PVC-CF compared to that of free PEI/PVC-CF. After adsorption, the pores on the surface of the adsorbent were lost by
Pd (II) ions and the thickness of adsorbent was also increased by about 425 mm. In contrast, the thickness of the Pd(II)-desorbed PEI/ PVC-CF returned to about 215 mm, which is equal to the thickness of free adsorbent (Fig. 5c). This result revealed that Pd(II) adsorption and desorption could be assumed to occur on the surface of the adsorbent. Fig. 6 shows the FTIR spectrum of PEI/PVC-CF with characteristic peaks of PVC and PEI. The major peaks of PVC were observed at 1245 (CeC stretching), 1334 (CH2 deformation), 2836 and 620 cm1 (both CeCl stretching) (Luo et al, 2015; Wang et al, 2015). The characteristic peaks of PEI were observed at 1435 (eNH bending and deformation of eNH2), 1650 (eNH2 for amine I or eNH for amide II), and 3460 cm1 (NeH stretching of amines) (Kim et al., 2015; Wang et al., 2013; Moradian et al., 2014). In addition, the strong peak at 620 cm1 (C-Cl stretching) decreased significantly, providing the indirect evidence of alkylation reaction between the amine of PEI and chlorine of PVC (data not shown). In the sorption stage, the N-H stretching peak shifted from 3460 cm1 to 3483 cm1, while the shifted peak returned to its original state after Pd(II) desorption. Therefore, based on the interesting findings of SEM and FTIR, PEI/PVC-CF has a reversible property for the adsorption and desorption of Pd(II), which may be due to the electrostatic attraction between the positively charged amine groups in the sorbent and the negatively charged Pd(II) anions. 3.5. Reusability If an adsorbent cannot be reused, then the depleted adsorbent would have to be deposed through a secondary process such as incineration. This increases the maintenance cost of the adsorption process, and the secondary treatment can also cause environmental pollution. Therefore, reusable adsorbents or desorption processes are desirable for environmental and economic reasons. Furthermore, the desorption should be easy, and the adsorption capacity should not decline with repeated use. We used the Pd(II) solution of 50 mg/L and the mixed eluent of 0.01 M HCl and 10 mM thiourea, in order to evaluate the repeated usability of PEI/PVC-CF. The adsorption-desorption experiment was performed consecutively for 5 times. The results (Fig. 7) show that the amount of Pd(II) adsorbed on PEI/PVC-CF changed very little with repeated adsorption and desorption, and Pd(II) was completely desorbed at the end of each cycle. This indicates that our PEI/PVC-CF absorbent is highly stable in this adsorption-desorption process and can therefore be repeatedly used. 4. Conclusions In this study, we developed a new adsorbent, PEI/PVC-CF, which can efficiently remove and recover Pd(II) from acidic solution, and proposed a desorption process to allow easy recovery of the adsorbed Pd(II). The developed adsorbent and its recovery process could be also applied for Pd(II) recovery from palladium plating wastewater. The major conclusions gained through this study are as follows.
Fig. 4. Pd(II) desorption using different thiourea concentrations in 0.1 M HCl solutions.
C The results of SEM and FTIR showed that the PEI/PVCcrosslinked fiber was successfully produced and was effective to adsorb Pd(II) anions by electrostatic attraction. C The isotherm data were better described by the Langmuir model than the Freundlich model, and the maximum Pd(II) uptake was 146.03 mg/g. C The adsorption equilibrium was reached faster with lower initial Pd(II) concentration. In 100 mg/L Pd(II) solution, it took about 60 min for the material to reach adsorption equilibrium.
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Fig. 5. SEM images of free (a and a’), Pd(II)-sorbed (b and b’), and Pd(II)-desorbed (c and c’).
Fig. 7. Repeated reuse of PEI/PVC-CF through adsorption-desorption cycles. Fig. 6. FTIR spectra of Pd(II)-sorbed (a) and Pd(II)-desorbed (b) PEI/PVC-CFs.
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C A mixture of 0.01 M HCl and 10 mM thiourea was the best eluent to induce 100% of Pd(II) adsorbed on PEI/PVC-CF. C PEI/PVC-CF was used repeatedly for adsorption-desorption for at least 5 cycles while maintaining high performance. C In conclusion, PEI/PVC-CF has been demonstrated to possess high potential as an adsorbent to efficiently remove and recover Pd(II) in acidic solution. Acknowledgements This work was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT & Future Planning (NRF2017R1A1A1A05000741). References Awual, M.R., Khaleque, M.A., Ratna, Y., Znad, H., 2015. Simultaneous ultra-trace palladium (II) detection and recovery from wastewater using new class mesoadsorbent. J. Ind. Eng. Chem. 21, 405e413. Barka, N., Ouzaouit, K., Abdennouri, M., Makhfouk, M.E., 2013. Dried prickly pear cactus (Opuntia ficus indica) cladodes as a low-cost and eco-friendly biosorbent for dyes removal from aqueous solutions. J. Taiwan Inst. Chem. Eng. 44, 52e60. Cataldo, S., Gianguzza, A., Pettignano, A., 2016. Sorption of Pd(II) ion by calcium alginate gel beads at different chloride concentrations and pH. A kinetic and equilibrium study. Arab. J. Chem. 9, 656e667. Cho, C.-W., Kang, S.B., Kim, S., Yun, Y.-S., Won, S.W., 2016. Reusable polyethylenimine-coated polysulfone/bacterial biomass composite fiber biosorbent for recovery of Pd (II) from acidic solutions. Chem. Eng. J. 302, 545e551. Fujiwara, K., Ramesh, A., Maki, T., Hasegawa, H., Ueda, K., 2007. Adsorption of platinum (IV), palladium (II) and gold (III) from aqueous solutions onto l-lysine modified crosslinked chitosan resin. J. Hazrd. Meter. 146, 39e50. Fytianos, K., Voudrias, E., Kokkalis, E., 2000. Sorption-desorption behaviour of 2,4dichlorophenol by marine sediments. Chemosphere 40, 3e6. Gandhi, M.R., Yamada, M., Kondo, Y., Shibayama, A., Hamada, F., 2015. p-Sulfonatothiacalix [6] arene-impregnated resins for the sorption of platinum group metals and effective separation of palladium from automotive catalyst residue. J. Ind. Eng. Chem. 30, 20e28. pez-Lo pez, J.A., Pinto, J.J., Moreno, C., 2017. Ionic liquid based Herce-Sesa, B., Lo solvent micro-extraction of Ag and Cd from saline and hyper-saline waters. Chem. Eng. J. 308, 649e655. ndez-Tapia, J.R., Vazquez-Arenas, J., Gonza lez, I., 2013. A kinetic model to Herna describe the nickel electro-recovery from industrial plating effluents under variable electrolyte conductivity. Electrochim. Acta 103, 266e274. Hosseini, S.S., Bringas, E., Tan, N.R., Ortiz, I., Ghahramani, M., Shahmirzadi, M.A.A., 2016. Recent progress in development of high performance polymeric membranes and materials for metal plating wastewater treatment: a review. J. Water Process Eng. 9, 78e110. Huang, C.-C., Chen, H.-I., Liu, I-P., Chen, C.-C., Chou, P.-C., Liou, J.-K., Liu, W.-C., 2014. Comprehensive study of hydrogen sensing phenomena of an electroless plating (EP)-based Pd/AlGaN/GaN heterostructure field-effect transistor (HFET). Actuator B Chem. 190, 913e921.
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