3.1. Characterizations
The characterizations of the RH and RHA materials are reported previously [42, 47]. The PABA-MCM-41 (RHA) modified mesoporous array exhibited a hexagonal arrangement with surface area value of the magnitude of 438 m2 g− 1, the N2 adsorption/desorption analysis also revealed a type IV isotherm with H1 hysteresis. Finally, the mesoporous structure showed an excellent thermal stability and total pore volume and pore diameter values of 0.41 cm3 g− 1 and 3.59 nm, respectively [43]. In addition, the PABA-MCM-41 (RHA) presented the same typical bands of the modified mesoporous array, as demonstrated previously [36].
Figure 1(i) shows the ATR-FTIR spectra obtained for the pristine PES membrane and the MMMs prepared with the PABA-MCM-41 (RHA) in percentages between 1.0–10.0 wt.%. It is possible to observe the main characteristic bands of the PES, which are centered in the range between 2962 − 2839 cm− 1 and those at around 831, 799, and 689 cm− 1 are assigned to the stretching and vibration of C–H bond of the backbone of the aromatic hydrocarbon of PES, respectively [48, 49]. The band centered at 1408 cm− 1 is assigned to the − CH3 bond of the CH3 − C−CH3 group [49, 50]. The band at 1231 cm− 1 and those between 872 − 856 cm− 1 are attributed to the asymmetric vibration of the ether group [48, 50].
The bands around 1321, 1296 and 1009 cm⁻1 can be assigned to the symmetrical and asymmetric O = S = O vibrations of the sulfone group, as well as the bands near 1145 and 1100 cm⁻1 are attributed to the S = O elongation. The bands around 1577 and 1483 cm⁻1 belonging to the C = C bond of the aromatic groups of PES [48, 51, 52].
The spectra of PES-based MMMs show an increase of the band intensity between 2962 − 2839 cm⁻1. These bands are attributed to the vibrations of the methylene groups (−(− CH2)3−) of the PABA-Si present into the PABA-MCM-41 functionalized mesoporous material [36, 43], as well as the appearance of a band at around 1725 cm⁻1 on the MMMs, which can be assigned to the C = O stretching of carboxylic acid present on the PABA-Si functional group from PABA-MCM-41 (RHA). Likewise, the new band close to 1535 cm⁻1 is assigned to the amide − CONH − vibrations of PABA-Si groups [36], confirming that there was a good dispersion of mesoparticles on the PES MMMs.
The presence of the main bands of PES on the MMMs confirms the compatibility between the mesoparticles and the polymer chains. In this way, a decrease in the intensity of the bands close to 1321, 1296, and 1009 cm⁻1 (O = S = O symmetric and asymmetric vibrations), 1145 and 1100 cm⁻1 (elongation of S = O bond), and the bands centered at around 1577 and 1483 cm⁻1 (C = C bond of aromatic groups of PES) with the increase of the PABA-MCM-41 amount. Therefore, this decrease is attributable to the interaction between these PES groups with the carboxylic acid groups (− CO2H) present within the PABAMCM-41 arrays. However, the characteristic bands of amorphous silica of the PABA-MCM-41 were not evidenced in the spectra of the MMMs because of the overlapping of these bands in the presence of PES.
Figure 1(ii) shows the X-ray diffractograms obtained of PABA-MCM-41 (RHA), pristine PES membrane, and MMMs. The PABA-MCM-41 and the PES membrane exhibited a single diffraction peak, which is attributed to the amorphous halo of the silica source used on the PABA-MCM-41 and the amorphous structure of PES, respectively. It is also possible to observe that the MMMs presented the same wide band observed for the pristine membrane. This behavior is due to the compatibility between PABA-MCM-41 mesoparticles and the PES matrix.
Figure 1(iii) shows the SAXS profiles obtained of pristine membrane and MMMs. The PABA-MCM-41 array exhibited a ratio of relative distances of 1:√3:2, derived from the peaks ratio (q100/q100, q110/q100, and q200/q100), respectively, confirming the mesoporous pattern of PABA-MCM-41 [43]. However, the pristine membrane did not present SAXS profile. Conversely, the PES-based MMMs presented three peaks at around the scattering vector q100, q110, and q200 related to the (100), (110), and (200) planes from the PABA-MCM-41 within the MMMs. Furthermore, there was an increase in peak intensity q100 with increase in the proportion of mesoparticles within the MMMs.
Figure 1(iv) shows the DSC curves of the second heating obtained of the pristine membrane and MMMs, while Table 1 shows the glass transition temperature (Tg) values found for same samples. There was only one thermal event for all samples, which is associated with the glass transition, where it is possible to observe a small decrease in Tg values of the PES-based MMMs compared to pristine membrane. The presence of the mesoparticles within the PES matrix seems to facilitate the flexibility of PES chain of PES-based MMMs in the interface PES/PABA-MCM-41.
Table 1
Glass transition temperature (Tg) values of pristine PES membrane and PES-based MMMs.
Polymer membrane
|
Tg
(°C)
|
Polymer
|
PABA-MCM-41 (RHA)
(wt.%)
|
PES
|
0.0
|
223.4
|
1.0
|
221.1
|
2.5
|
221.6
|
5.0
|
220.9
|
7.5
|
222.2
|
10.0
|
221.7
|
Figure 2 shows the SEM images obtained of the pristine membrane and PES-based MMMs. The pristine membrane (Fig. 2a) exhibits an asymmetrical structure with a dense layer at top and a sublayer in the form of elongated channels. In contrast, the MMMs show the appearance of interfacial voids surrounded by cavities at around mesoparticles incorporated.
The findings of the chemical compositions of the pristine membrane and PES-based MMMs obtained from the SEM-EDS analyzes are shown in Table 2. The C contents did not have significant changes for the MMMs compared to pristine membrane, the same behavior was seen for the impurity contents of Cr, Al, Fe, and Ca. On the other side, the Si and O contents increased with the increase in the percentage of PABA-MCM-41, but the same was not noticed for S and Cl contents.
Table 2
Chemical composition of pristine PES membrane and PES-based MMMs determined by SEM-EDS.
Polymer membrane
|
|
Analyte
(%)
|
Polymer
|
PABA-MCM-41 (RHA)
(wt.%)
|
|
C
|
O
|
Si
|
S
|
Cl
|
Cr
|
Al
|
Fe
|
Ca
|
|
0.0
|
|
61.07
|
10.30
|
0.15
|
18.92
|
9.11
|
0.02
|
0.01
|
0.35
|
0.12
|
|
1,0
|
|
59.28
|
20.57
|
1.14
|
13.59
|
3.34
|
0.11
|
0.01
|
1.03
|
0.99
|
|
2,5
|
|
55.93
|
23.30
|
3.47
|
13.74
|
3.09
|
0.04
|
0.02
|
0.26
|
0.16
|
PES
|
5,0
|
|
53.11
|
20.25
|
5.77
|
11.77
|
7.29
|
0.00
|
0.12
|
0.76
|
0.92
|
|
7,5
|
|
59.47
|
16.24
|
8.62
|
8.22
|
5.68
|
0.00
|
0.14
|
1.41
|
0.23
|
|
10,0
|
|
51.97
|
20.67
|
8.52
|
12.38
|
5.55
|
0.00
|
0.10
|
0.20
|
0.65
|
3.2. Absorption of MMMs
The WVT and P values for the pristine PES and MMMs are shown in Table 3. The WVT values for the MMMs were higher than the value obtained for the pristine membrane. The same behavior was also observed for the P values, except in MMM with 1.0 wt.% of mesoparticles. This phenomenon occurs due to the increase in the number of preferred paths for the water vapor transport with the incorporation of the mesoporous material within the MMMs, as seen in the SEM results.
Table 3
Water vapor transport (WVT) and permeation (P) values obtained for pristine PES membrane and PES-based MMMs
Polymer membrane
|
WVT
(g h− 1 m− 2)
|
P
x 10− 11 (g Pa− 1 s− 1 m− 1)
|
Polymer
|
PABA-MCM-41 (RHA)
(wt.%)
|
|
0.0
|
1.68
|
2.03
|
|
1.0
|
3.25
|
1.93
|
|
2.5
|
2.27
|
2.17
|
PES
|
5.0
|
2.27
|
2.45
|
|
7.5
|
2.04
|
2.22
|
|
10.0
|
2.83
|
2.70
|
Figure 3(a) shows the correlation between the P and Tg values of the MMMs. It is possible to observe an inverse relationship between P and Tg values, except for MMM with 1.0 wt.%. This behavior occurs because at low Tg values, there is a greater mobility of the polymer chains, thus facilitating the permeation process through these MMMs.
Table 4 presents the water static sorption values for the pure PES membrane and MMMs. There was a decrease in the swelling degree after incorporation of 1.0 wt.% of PABA-MCM-41 in the PES matrix. This behavior can be related to the occupation of the PES free volume by the well dispersed mesoparticles. However, for the 2.5, 7.5, and 10.0 wt.% contents, the swelling degree values were higher than those obtained for the other MMMs, and this can also be related to the occupation of the water molecules in the pores of the mesoparticles within the MMMs.
Table 4
Water static sorption values obtained for pristine PES membrane and PES-based MMMs.
Polymer membrane
|
Swelling degree
(%)
|
Polymer
|
PABA-MCM-41 (RHA)
(wt.%)
|
|
0.0
|
1.78
|
|
1.0
|
0.49
|
|
2.5
|
7.69
|
PES
|
5.0
|
1.60
|
|
7.5
|
3.46
|
|
10.0
|
2.51
|
Figure 3(b) shows the correlation of the swelling degree with the Tg values for the MMMs. There is a direct correlation between the mentioned values, so an increase in Tg causes an increase in the amount of water imprisoned in the free volume of the polymer matrix, as well as in the pores of PABA-MCM-41. In contrast, this behavior reflected in lower P values (Fig. (3a)), because of the strong imprisonment of the water molecules by the silanol and PABA-Si groups of the modified mesomaterial structure within the PES-based MMMs.
Figure 4(a-c) shows the results found from the PAHs concentration in feed solution against time for PES-based MMMs. The increase in the operation time caused a decrease in the PAHs concentrations since the equilibrium time found was at around 24 h for all MMMs. As with the final PAHs concentrations of the pristine membrane, whose values were lower than those obtained for the PES-based MMMs.
In contrast, Fig. 4(d-f) shows the PAHs concentration values of permeated solution through the MMMs. An increase in the permeate concentration was observed as the operating time was longer, given that the equilibrium time found was in the order of 24 h. The values of the final concentrations permeated through the pristine membrane, for all PAHs, were lower than those obtained for the PES-based MMMs. However, these values are close to those found for the MMM with a content of 7.5 wt.% mesoparticles. In view of these results, we can suggest that the occupation of the free volume of PES by the mesoparticles facilitated the permeation of PAHs through these MMMs, due to the appearance of interfacial voids within the MMMs, as observed in the SEM analyzes.
Figure 4(g-i) shows the results of adsorbed concentrations of PAHs mixture by the pure PES membrane and MMMs. It is possible to note for all PAHs, that the pure PES membrane presented values of maximum concentration higher than those found for MMMs. On the other hand, the MMMs also presented relevant adsorption values. Therefore, the equilibrium time was reached in approximately 24 h for all MMMs. In summary, the preparation of MMMs with PABA-MCM-41 as filler material caused little interference in the transport property of PAHs. Since there was a slight increase in permeation of PAHs from the occupation of the free volume of PES polymer chains by the incorporated mesoparticles, as well as a decrease in adsorption of PAHs.
The results of permeation (PR) and retention (RR) rates and the removal percentage of PAHs mixture by PES-based MMMs were compiled in Table 5. The PR values observed for the PAHs mixture followed the following increasing order: B[b]F < B[k]F < B[a]P, due to the hydrophobic effect of PAHs mixture in solution. Therefore, for all PAHs, the respective PR values for the pristine membrane were lower than the values obtained for the MMMs, suggesting that the incorporation of PABA-MCM-41 into the PES facilitated the permeation of PAHs through the PES MMMs. However, the achieved values for the PES MMM with 7.5 wt.% were not so much higher than those found for the pristine membrane, as well as for the other MMMs.
Table 5
Permeation and retention rates and removal percentage values of PAHs mixture for PES-based MMMs.
Polymer membrane
|
PAH
|
Rate permeation
(%)
|
Rate retention
(%)
|
Removal percentage
(%)
|
Polymer
|
PABA-MCM-41 (RHA)
(wt.%)
|
PES
|
0.0
|
B[b]F
|
1.670
|
98.330
|
83.004
|
B[k]F
|
4.666
|
95.334
|
79.676
|
B[a]P
|
5.574
|
94.426
|
79.190
|
PES
|
1.0%
|
B[b]F
|
2.190
|
97.810
|
44.628
|
B[k]F
|
5.028
|
94.972
|
47.095
|
B[a]P
|
6.016
|
93.984
|
51.103
|
PES
|
2.5%
|
B[b]F
|
2.121
|
97.879
|
53.781
|
B[k]F
|
4.893
|
95.107
|
55.317
|
B[a]P
|
5.949
|
94.051
|
59.680
|
PES
|
5.0%
|
B[b]F
|
2.544
|
97.456
|
61.746
|
B[k]F
|
4.939
|
95.061
|
55.833
|
B[a]P
|
6.162
|
93.838
|
60.160
|
PES
|
7.5%
|
B[b]F
|
1.783
|
98.217
|
44.604
|
B[k]F
|
4.668
|
95.332
|
48.597
|
B[a]P
|
5.606
|
94.394
|
65.988
|
PES
|
10.0%
|
B[b]F
|
2.364
|
97.636
|
43.333
|
B[k]F
|
5.023
|
94.977
|
46.050
|
B[a]P
|
6.322
|
93.678
|
40.696
|
In the RR values, which have an inverse relationship to the PR values, it was noted that the results found for the pristine PES membrane were higher than the results of the MMMs, except for MMM with 7.5 wt.%, whose RR values are close to the pristine membrane. On the other hand, the incorporation of PABA-MCM-41 within the MMMs caused a reduction of the removal percentage.
Figure 5(a-c) shows the correlation between the PR and Tg values of PES-based MMMs. It is possible to observe an inverse relation between these values, except for MMM with 2.5 wt.%, in which the increase of PR values occurred because of the decrease in Tg values. This comportment can be attributed to the increase in the mobility of the PES chains, promoting more easily the permeation process of PAHs. However, this phenomenon was less meaningful for the MMM with 7.5 wt.% of mesoparticles.
On the contrary (Fig. (5d-f)), a direct relationship between the PR and WVT values. On the one hand, the decrease in Tg affected in the growth in the PR values. This decrease also boosted an increase in the WVT values. This comportment can be attributed to the growth in the number of preferred paths in the PES-based MMMs with the filling of mesoparticles, in addition to the mobility of the PES chains.
Similarly, to PR values, the RR values were also influenced by the Tg values, as shown in Fig. 5(g-i). A direct relationship was observed between the RR and Tg values, less for the MMM with 2.5 wt.%. In this way the decrease of the RR occurred due to the decay of Tg values. However, the decrease in Tg was less evident for MMM with 7.5 wt.% of PABA-MCM-41. As a direct consequence, the decrease in retention rate (RR) was directly influenced by the increase of the permeation rate (PR) of PAHs mixture. In this way (Fig. 6(a-c)), it is possible to observe an inverse correlation between the RR and WVT values for PES MMMs.
Figure 6(d-f) and 6(g-i) present the correlations of the removal percentage of PAHs mixture with the Tg and WVT values, respectively. From Fig. 6(d-f), for some cases, that the removal percentage values do not have a defined relationship with the Tg values. It was expected larger removal percentages at higher Tg values, as well as lower removal percentage values at lower Tg values. This behavior is due, respectively, to a greater or lesser ease of permeation of PAHs through the polymer chains of the MMMs. In this sense, this tendency was noted in the B[b]F and B[k]F removal, in the MMMs with 1.0 and 10.0 wt.%, as well as in the B[a]P removal in the contents of 1.0, 7.5, and 10.0 wt.% MMMs. These results show, for these MMMs, that the polymer matrix has more contribution in the adsorption of PAHs than the mesoporous array. But, for the other PES-based MMMs, we can observe an inverse relation between the removal percentage and Tg values, suggesting that the mesoporous array has a greater contribution in the adsorption of these PAHs than the PES matrix. Finally, from the analysis of Fig. 6(g-i), the removal percentage values of PAHs correlate inversely with the WVT values, except for the adsorption of B[b]F and B[k]F by the MMM with 7.5 wt.% MMMs.