Jianqiu Zhang*a,
Tao Tiana,
Jinyang Chen*a,
Jianhua Zub and
Yangjun Wanga
aSchool of Environmental and Chemical Engineering, Shanghai University, Shanghai 200444, China. E-mail: chenjy@shu.edu.cn
bSchool of Nuclear Science and Engineering, Shanghai Jiao Tong University, Shanghai, 200240, China
First published on 1st December 2014
Recycling the nonmetal components of waste printed circuit boards (WPCBs), mainly thermosetting epoxy resins (TEPRs), is quite difficult because they are insoluble and inflexible. We report a new method to convert TEPRs into ion exchange resin by treatment with sulphuric acid and the equilibrium ion exchange capacity (IEC) of the produced sample is 1.63 meq g−1. FTIR indicated that TEPRs were modified by sulphuric acid and X-ray photoelectron spectroscopy (XPS) verified that sulfonic acid group (–SO3−) was introduced. The produced ion exchange resin was stable up to 200 °C by TG-DTG analysis. The maximum adsorption capacities for Cu(II) and Ca(II) were 23.30 and 56.27 mg g−1, respectively. The Langmuir model gave a better fit for adsorption than the Freundlich model. The activation energies (Ea) were 21.18 and 48.53 kJ mol−1 for Cu(II) and Ca(II), respectively, with pseudo-second-order kinetics.
Ion exchange resins based on polystyrene are capable of ion exchange mainly because of sulfonic acid groups (R–SO3H), R is the network structure form of the resin frame part. The chemical structure of TEPRs, including the main aromatic ring, is shown in Fig. 1. It is possible to introduce functionalities, such as sulfonic acid groups, by sulfonation.12 In the sulfonation of TEPRs, an –SO3H group is attached to the aromatic ring. Ion exchange resins consisting of sulfonated TEPRs (S-TEPRs) have a fixed negative charge (–SO3−) that allows mobile cations (H+) to migrate while excluding co-ions and S-TEPRs can be cation exchange resins.
Here we present the new process using sulfonation to recycle TEPRs into ion exchange resins (S-TEPRs). We measured several properties of the products: ion exchange capacity (IEC), swelling degree (SD) or water uptake (WU). Resins were characterized by Fourier transform infrared spectroscopy (FTIR), elemental analysis, thermal analysis (TG-DTG) and X-ray photoelectron spectroscopy (XPS). To investigate the ion exchange process, the product was used to remove Cu(II) and Ca(II) and ion exchange isotherms and kinetics were determined.
![]() | (1) |
WU and SD were calculated from eqn (2).14
![]() | (2) |
X-ray photoelectron spectroscopy (XPS) was performed on an ESCALAB 250Xi (Thermo Fisher Scientific China Ltd) and the data were analysed using Avantage software.
![]() | (3) |
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Fig. 2 Photographs of aTEPRs, bTEPRs and the sulfonated products of S-aTEPRs and S-bTEPRs. (a) aTEPRs, (b) S-aTEPRs, (c) bTEPRs, (d) S-bTEPRs. |
The most important parameter of ion exchange resins is IEC. Fig. 3 shows the IEC of 732 IER, S-aTEPRs and S-bTEPRs at ambient temperature. It shows that IEC increases rapidly in a relatively short time. However, the increase in IEC becomes slowly when exchange time is prolonged. The IEC reaches a maximum at 120 min and as for 732 IER, S-bTEPRs and S-aTEPRs the values are 2.21, 1.98, and 1.63 meq g−1, respectively. The IEC maintains a relatively stable value after 120 min, indicating that it reaches equilibrium.
Table 1 shows the adsorption properties of the samples. The equilibrium IEC of S-aTEPRs and S-bTEPRs are 73.76 and 89.29% for the value of 732 IER, respectively. The WU (SD) of S-aTEPRs is lower than that of S-bTEPRs and 732 IER, which may be due to glass fibres. It is obvious that S-aTEPRs has excellent ion exchange capacity and has a greatly potential application.
Sample | IEC (meq g−1) | WU or SD (%) |
---|---|---|
S-aTEPRs | 1.63 | 18.05 |
S-bTEPRs | 1.98 | 25.54 |
732 IER | 2.21 | 27.69 |
Comparing the FTIR spectra of bTERPs and S-bTEPRs, there are noticeable differences in the spectra. The presence of two new peaks at approximately and 1037 and 1096 cm−1 are assigned to the symmetric and asymmetric stretching vibrations of –SO3H group in S-bTEPRs. That the absorbance at approximately 3500 cm−1 representing the stretching vibration of O–H groups in S-bTEPRs is wider and stronger than in bTERPs should be due to the addition of –SO3H in S-bTEPRs, which cause the formation of intermolecular hydrogen bonding between –SO3H groups and absorbed water molecules.16 The result shows that the bTEPRs has been added –SO3H group after sulfonation.
The thermal analysis of ion exchange resin has two main objects. One is to study the thermal properties of the resin to determine the thermal stability and degradation temperature.17,18 The other important object is to characterize the resin.19 Fig. 5 shows the TG of 732 IER and S-aTEPRs and it is clear that there are three steps in the decomposition curves of both samples. The first step is almost a level line up to the temperatures of 185 for 732 IER and 192 °C for S-aTEPRs, indicating that they are stable less than 200 °C. The second step shows obvious weight loss starting at approximately 280 up to 487 for 732 IER and 465 °C for S-aTEPRs owing to the decomposing of the molecular structure. Then the third step starts continuous weight loss up to 800 °C till totally carbonization. Comparing with 732 IER, the more residual of S-aTEPRs (approximately 20%) may be due to glass fibres.
Fig. 6 and 7 show the TG-DTG of bTEPRs and S-bTEPRs, respectively. From room temperature to 250 °C, comparing the weight loss in Fig. 7 with Fig. 6, an obvious weight loss step can be observed from 25 to 200 °C in Fig. 7 which should be the loss of water combining with –SO3H group in S-bTERPs. Fig. 6 shows significant weight loss at 435 °C; however, significant weight loss from S-bTEPRs presents at 310, 346 and 428 °C. It indicates that the two processes of decomposition are completely different and the reason should be due to the decomposition of the addition of –SO3H group of S-bTEPRs.
The products of different weight loss steps for S-bTEPRs were analysed by FTIR, as shown in Fig. 8. Comparing the first step of no weight loss with the initial sample, there is no obvious change and the elemental composition indicates that the C and S is unchanged, while the O and H has some decrease, as shown in Table 2. This may be due to evaporation of bound moisture in –SO3H group resulting in weight loss of 5.27%.
TG steps | C area | H area | N area | O area | S area |
---|---|---|---|---|---|
Intial | 30![]() |
14![]() |
2803 | 21![]() |
1662 |
First step | 30![]() |
11![]() |
2707 | 15![]() |
1571 |
Second step | 9670 | 1529 | 1089 | 12![]() |
793 |
Third step | 3062 | 427 | 112 | 2303 | 217 |
As for the second step with weight loss, the characteristic C–S absorption peak at approximately 609 cm−1 decreases significantly, indicating the breakdown of partial C–S bonds between –SO3H groups and phenyl rings.20 Elemental analysis shows a significant weight loss of the S and O, indicating the decomposition of –SO3H group. Furthermore, as for C, there is also some weight loss which indicates that the carbon linkage molecular structure also decomposes. In the third step, the peaks of characteristic –CH2– and phenyl ring disappear, indicating that S-bTEPRs is decomposed completely. This is accorded with the corresponding significantly decrease of C, H, O and S.
To confirm that the –SO3H group is covalently bonded to the bTEPR after sulfonation, XPS measurement of bTEPRs and S-bTEPRs was carried out and the XPS wide scan spectra are shown in Fig. 9. In the bTEPRs and S-bTEPRs samples, a strong peak at approximately 285 eV (corresponding to C, 1s), a weak peak at approximately 400 eV (corresponding to N, 1s), and a strong peak at 533 eV (corresponding to O, 1s) are all observed. However, a new peak for sulphur (S, 2p, 168 eV) is observed originated from –SO3H groups in S-bTEPRs and it is shown obviously in Fig. 10, indicating successful sulfonation to produce S-bTEPRs. The binding energies and elemental compositions are summarized in Table 3 and it shows that the content of S in S-bTEPRs is 8.58%, indicating sulfonation of S-bTEPRs from bTEPRs.
Samples | C 1s (285 eV) | O 1s (533 eV) | N 1s (400 eV) | S 2p (168 eV) |
---|---|---|---|---|
bTEPRs | 84.23 | 10.62 | 5.51 | — |
S-bTEPRs | 52.18 | 29.47 | 9.77 | 8.58 |
The XPS narrow scan C 1s scan spectra and fit curves for bTEPRs, along with the XPS narrow scan C 1s and S 2p spectra and fit curves for S-bTEPRs, are shown in Fig. 11. Comparing the C 1s fit curves of (a) with (b), there is one new peak at approximately 285.5 eV and it is attributed to the C–S bond of the sulfonated skeleton. The most important information comes from S 2p spectrum and fit curves for S-bTEPRs in Fig. 11(c). The spectrum is deconvoluted into two peaks at 168.1 and 169.2 eV which be assigned to the 2p3/2 and 2p1/2 of sulphur in a high oxidation state, i.e., sulfonic acid groups (–SO3−).21
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Fig. 11 The XPS narrow scan spectra and fit curves. (a) C 1s of the S-bTEPRs, (b) C 1s of the bTEPRs, (c) S 2p of the S-bTEPRs. |
The Langmuir isotherm is expressed as:22,23
![]() | (4) |
The Freundlich isotherm model is expressed as:23,24
log![]() ![]() ![]() | (5) |
The Langmuir isotherm describes mono-layer adsorption on a homogenous surface and the Freundlich isotherm is a satisfactory empirical isotherm used for non-ideal adsorption on heterogeneous surface.24
Fig. 12 and 13 show the Langmuir and Freundlich adsorption of Cu(II) and Ca(II) and their adsorption constants evaluated from isotherms are given in Table 4. The correlation coefficients of Cu(II) and Ca(II) of the Langmuir are 0.9981 and 0.9983 which are more perfect than corresponding 0.7855 and 0.8476 of the Freundlich. The result indicates that the Langmuir isotherm model is a better fit to the equilibrium adsorption data.
Langmuir constants | Freundlich constants | |||||
---|---|---|---|---|---|---|
Q0 (mg g−1) | b (L mg−1) | R2 | kf | 1/n | R2 | |
Ca(II) | 23.30 | 0.13 | 0.9981 | 41.85 | 0.050 | 0.7855 |
Cu(II) | 56.27 | 0.16 | 0.9983 | 23.45 | 0.066 | 0.8476 |
As for the IER with the –SO3Na group which is the functional group grafted on the surface, the Cu(II) and Ca(II) will take place of Na+ during the ion exchange process. The ion exchange reactions occurring in the Ca2+ and Cu2+ solutions can be represented by the following reactions.
2RSO3Na + Ca2+ ↔ (RSO3)2Ca + 2Na+ |
2RSO3Na + Cu2+ ↔ (RSO3)2Cu + 2Na+ |
Obviously, the adsorption is chemical adsorption and thus the mono-layer Langmuir model is more suitable.
The pseudo-second-order equation is expressed as:
![]() | (6) |
Plots of t/qt against “t” at 298–338 K for the second-order adsorption are shown in Fig. 14. The result provides the adsorption rate constants k1 and qe values from the slope and intercept in Table 5. The calculated qe from the second-order kinetic model corresponded very well with the experimental values, with R2 > 0.99. It is obvious that the pseudo-second-order model provides a perfect fit of the experimental data and thus, the kinetics model is reliable and suitable.
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Fig. 14 The fit curves of the pseudo-second-order equation for adsorption of Ca(II) and Cu(II) in S-bTEPRs at different temperatures. |
Metal | T (K) | qe,expa (mg g−1) | k2 × 10−4 (g mg−1 min−1) | qe (mg g−1) | R2 |
---|---|---|---|---|---|
a Amount of metal adsorbed for 24 h which can be considered as the equilibrium absorption capacity because there is no change with more time. | |||||
Ca(II) | 298 | 35.49 | 7.9021 | 33.31 | 0.9968 |
318 | 36.06 | 9.4898 | 35.31 | 0.9974 | |
338 | 36.74 | 13.7276 | 36.46 | 0.9985 | |
Cu(II) | 298 | 43.76 | 6.6763 | 39.43 | 0.9946 |
318 | 44.86 | 7.3414 | 40.21 | 0.9955 | |
338 | 45.73 | 7.9477 | 40.87 | 0.9953 |
The rate constant k can be expressed in Arrhenius form of lnk = −(Ea/RT) + ln
A, where A is the pre-exponential factor, Ea is the activation energy (J mol−1), R is the universal gas constant (8.314 J mol−1 K−1), and T is the temperature (K). Then according to Arrhenius equation, the line fitting relation of ln
k and 1/T gives apparent adsorption activation energy Ea and Arrhenius pre-exponential factor A.
The relation of linearly fit of adsorption rate constant k1 and temperature is shown in Fig. 15. According to Arrhenius equation, the linear fitting relation of lnk1and 1000/T gives that the apparent adsorption activation energy (Ea) for the adsorption of Cu(II) and Ca(II) on S-bTEPRs are 21.18 and 48.53 kJ mol−1, respectively. Thus, the adsorption kinetic equations are shown as eqn (7) and (8).
![]() | (7) |
![]() | (8) |
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Fig. 15 Relationship between reaction rate constants and temperature for adsorption of Cu(II) and Ca(II) in S-bTEPRs. |
The positive values of Ea suggest that higher temperature favours adsorption and that the adsorption is endothermic.
The produced IER is an effective adsorbent for the removal of Cu(II) and Ca(II) and the Langmuir isotherm model is suitable for the adsorption. The adsorption capacity for Cu(II) and Ca(II) calculated from the Langmuir model are 56.27 and 23.30 mg g−1, respectively. The adsorption kinetics of Cu(II) and Ca(II) are suitable for pseudo-second-order rate model and the apparent Ea for Cu(II) and Ca(II) are 21.18 and 48.53 kJ mol−1, respectively.
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