3.2.1. NF Tests to Evaluate Membrane Performance
Pure water permeability is an intrinsic membrane characteristic mainly governed by the active layer pore size distribution. The hydraulic permeability for the polymeric membrane was determined to be 1.81 ± 0.28 Lh−1bar−1m−2, at 22 ± 2 °C. The fabricant of the polymeric membrane, Koch, lists a hydraulic permeability from 1.5 to 2.0 Lh−1bar−1m−2, which is in line with the obtained experimental value.
Membrane performance was addressed after aging for 6 and 12 weeks with spent caustic (effluent No. 1 in
Table 1). The values of
LP/
LP0 (hydraulic permeability at a given time normalized by the hydraulic permeability of the pristine membrane) obtained for 6 weeks and 12 weeks aged membranes were, respectively, 4.46 and 4.62. A considerable increase in the hydraulic permeability of the membrane after soaking in the spent caustic effluent can be observed. It is interesting that the increase in the permeability mainly occurs within the initial period of 6 weeks since similar permeabilities were attained after 6 and 12 weeks of aging (8.07 ± 0.31 Lh
−1bar
−1m
−2 vs. 8.36 ± 0.34 Lh
−1bar
−1m
−2 after 6 and 12 weeks, respectively).
Figure 2 shows the rejection of contaminants during membrane separation tests. COD removals were very high for the pristine membrane (99.6%); however, a continuous decrease is observed with the aging time (95.8% for 6 weeks and 90.0% for 12 weeks aged membranes (
Figure 2A)).
Considering the first 6 weeks of aging, COD removal decreased on average 0.63% per week, but in the second period of 6 weeks, COD removal decreased on average 0.97% per week. For the membranes aged for 6 and 12 weeks, phenolic compound removals reached 91.3% and 78.9%, respectively, which are much lower than the pristine membrane performance (98.1%) (
Figure 2B). It should be noted that phenolic compounds are the target compound class with the lowest MW of all (180–220 Da) [
42]; therefore, it may be deduced that the more significant loss of retention is for phenolic compounds. The decrease in their rejection is aligned with the decrease in membrane retentive properties and increase in membrane permeability, which may be related to swelling effects [
37]. Sulfide compounds are not presented in
Figure 2 since their rejection is 100% in all samples, which may be related to the natural degradation of sulfides into the atmosphere (NF tests occurred during summertime in the open air). Total O&G removal reached a maximum of 99.5% for the pristine membrane. Considering aged membranes, the performance in O&G removal decreased also (96.6% removal for 6 weeks aged membrane and 88.3% removal for 12 weeks aged membrane) (
Figure 2C). For aged membranes, polar O&G removals reached 98.4% for 6 weeks and 95.4% for 12 weeks (0.15% average removal loss per week during the first 6 weeks of aging and 0.51% average removal loss per week during the second 6 weeks of aging) (
Figure 2D). These results indicate a loss of rejection of polar O&G contaminants on aged membranes, especially for the 12 weeks aged membrane. This loss of retention capacity was more relevant during the second 6 weeks of aging.
As mentioned previously, the major increase in permeability occurred during the first 6 weeks of aging, while the highest decreases in the rejection capacities were observed during the second 6 weeks of aging. The small increase in permeability in the second 6 weeks of aging seems not to be able to justify the increase in the loss of retention capacity in the same period. It could be related to molecular exclusion mechanisms, as it seems that the membrane pore integrity may have been compromised during the aging period, as reported elsewhere [
11,
12,
13,
15,
16,
18,
19,
20,
22].
One possible reason to change membrane pore integrity in such a way that compromises permeability and retention capacity could be the very high pH of the spent caustic. Data from FT−IR observations (described hereinbelow) suggest that the active layer is not likely to have been modified since the differences observed are related to the adsorption of organic components that are present in spent caustic. However, permeability is a property largely governed by the narrowest pore (that of the active layer), which leads to the conclusion that, to some extent, permeability data variations observed with aging are related to the possible deterioration of the active layer, although not observable. On the other hand, the support layer seems to have been considerably modified, according to the alterations in retention properties.
Although the membrane is to some extent pH-resistant, it could happen that the extreme pH conditions negatively affect the membrane structure. In order to test this, spent caustic pH was decreased from 14 to 10 and 3. The values of
LP/
LP0 obtained for pH of 14, 10 and 3 were, respectively, 3.04, 2.37 and 2.07. Hydraulic permeability seems to suffer more the higher the pH of the solution, particularly between pH 10 and 14.
Figure 2 shows the impact of pH on the retention capacity of the polymeric membrane. Retention capacity of the pristine membrane is nearly independent of the pH value, with all contaminant rejections being similar in all experiments. Therefore, it can be concluded that pH manipulation will only cause an alteration in the permeability and is not responsible for retention capacity alterations.
Santos et al. [
24] attempted to treat naphthenic spent caustic with a Koch NF polymeric SeIRO spiral-wound membrane module. It was suggested that the contaminants present in the spent caustic do not significantly affect the membrane performance. The results herein obtained corroborate the study of Santos et al. [
24] for a pristine membrane; however, a great loss of performance was observed for aged membranes: a great loss of performance in permeability during the first 6 weeks of ageing and a great loss of performance in retention capacity in the second 6 weeks of ageing.
3.2.3. SEM-EDS
Active and support layers as well as cross-sections of pristine and aged membranes (for 6 and 12 weeks) were analyzed by scanning electron microscopy (SEM).
Figure 4 shows the results obtained.
In terms of physical modifications of the membrane structure, it appears that the intermediate structure (right after the active layer in the cross-section view) becomes slightly looser with aging (
Figure 4B,E,H), although it is not completely clear. Pore coalescence may be the cause for this observation. The support layer (
Figure 4A,D,G) seems unchanged. No conclusions can be drawn regarding the active layer since the structure is very compact and SEM maximum magnification does not enable comparisons among the membranes with the required level of detail. On the other hand, variations in permeability and thickness were observed with time, thus making it impossible to draw a clear conclusion of a connection between membrane structure and function (
LP, and rejection capacity) based on SEM results alone. To further study membrane structure, FT−IR profiles were collected and are discussed in the next section. Chemical surface composition (EDS) was also studied to determine elementary composition (
Table 2).
It is interesting to observe a continuous increase in the carbon content with aging in the active layer while other elements (S and O) are decreasing, which could mean a possible degradation of this layer. On the support, an alteration occurs between 6 and 12 weeks of aging and is related to a loss of sulfur. Although the membrane composition is not available, SEM-EDS analysis suggests that its composition essentially includes carbon, oxygen and sulfur in the active layer and mostly carbon in the support. Especially for the active layer, which has the selectivity property, several polymers used in NF polymeric membranes contain such composition, such as polyethersulfone (PES) and polysulfone (PS) [
43].
Pore density analyses were performed on the pristine membrane, as well as for the 6 and 12 weeks aged membranes, prior to NF tests. An SEM cross-section image (×300 image magnification) of each membrane was processed using the ImageJ software. For that, the total cross-section area was calculated over SEM images that were also binarized (
Figure S1 in the Supplementary Material section), and the values obtained are summarized in
Table 3, comprising only the pore features in the membrane support (due to the very low MWCO of the active layer, SEM could not show any pores). As such, the discussion of the impact of membrane aging may only be performed considering the main porous parameters for the membrane cross-section. However, a cross-sectional analysis also has some limitations since it only indicates possible distortion of the pores due to membrane aging.
The cross-section area obtained, calculated pore density (number of pores divided by area of membrane), porosity (area of pores divided by area of membrane), circularity (Equation (3)) and Feret’s diameter (the longest distance between any two points along the selection boundary) [
44] are presented in
Table 3.
From the data presented in
Table 3, is clear that the pores in the support are of micro scale, even in the pristine membrane. However, the membrane support aged 6 and 12 weeks showed an increase in the porosity, number of pores and pore density. Extremely high Feret’s diameter values were detected, indicating the highly irregular shape of pores, particularly for the 6 weeks aged membrane. The estimated average circularity value was around 0.8 in all measurements with estimated values obtained that varied between 0.6 and 1 (perfect circles) but with a high standard deviation (
Figure S2). Therefore, a considerable variation in the average Feret’s diameter values was also obtained, indicating a highly irregular shape of pores in the support layer, particularly finger-like for this part of the membrane support. Moreover, the increase in the maximum Feret’s diameter obtained for the 6 weeks aged membrane indicates a possible distortion of the porous structure (clearly shown for the wider porous section area) after aging. On the other hand, the decrease in this value with the aging time indicates a possible compaction at the end of 12 weeks.
3.2.4. FT−IR
The objective of these studies was to determine the possible structure alterations in the membranes that occurred during aging experiments and after NF tests. The tested membranes are divided as the pristine membrane without any treatment (“Pristine”) and with an NF test (“Pristine + NF test”), the duplicate aged membranes after immersion for 12 weeks (“Membrane 1 + 12 weeks” and “Membrane 2 + 12 weeks”) and the membranes aged for 6 and 12 weeks followed by an NF test (“Membrane 6 weeks + NF test” and “Membrane 12 weeks + NF test”). It is important to mention that both the side in contact directly with the effluent in the NF tests (active layer) and the opposite side (support layer) were tested.
The FT−IR spectra obtained for the active layers for two membranes immersed under the same conditions in the effluent were analyzed and compared with the membrane that was not immersed; data are shown in
Figure 5.
The results obtained by FT−IR for the active layer of 12 weeks aged membranes after immersion (duplicate: Membrane 1 and Membrane 2) depicted in
Figure 5 and amplified in
Figure S3 were nearly identical but showed some differences compared to the pristine membrane. Thus, among similarities, bands associated with the C=C bond at around 3100 cm
−1, 871 cm
−1 and 833 cm
−1 (
Figure 5 and
Figure S3) were identified in all spectra. The pristine and both the 12 weeks aged membranes showed bands at around 3370, ~3100, 2950–2924 and 2860 cm
−1 (
Figure S3A), characteristic of –OH, =C-H, Cl-C-H and H-C-H bonds, respectively [
45]. The peak at 1142 cm
−1 was associated with C-O st and was also present in all spectra, as were peaks in the range of 800–600 cm
−1, also assigned to C=CH stretching vibration [
45] and occasionally C-Cl [
46,
47]. A peak at around 1576 cm
−1 was also observed in all spectra, probably associated with the stretching mode of -COO st group. Peaks at 1665 cm
−1 and 1435 cm
−1 were linked to asymmetric and symmetric vibration of -COO- groups, respectively [
48,
49], and peaks at 1070 and 1031 cm
−1 associated with S=O bonds were also present, seeming to be in accordance with the presence of those atoms in SEM-EDS results (
Table 2).
Among the differences observed, the pristine membrane spectrum presented a characteristic band at 1746 cm
−1 due to the stretching vibration of the C=O group, while for the 12 weeks aged membranes this peak disappeared (
Figure 5 and
Figure S3B). This fact could be attributed to the aminolysis reaction with the urethane groups with ammonia or alkanolamines, present in the effluent and giving as subproducts alcohols and amides, as previously reported [
50]. Moreover, this is in line with the increase in intensity observed in the spectra for the band associated with these groups (OH bond), around 3370 cm
−1 (
Figure 5) and 1410 cm
−1, as well as the differences in the C=O band in the amide region (around 1690–1630 cm
−1) (
Figure S3B). The increase in the intensity for bands at 2950 cm
−1 and 2924 cm
−1 in the spectra of 12 weeks aged membranes could be attributed to the presence of compounds from the effluent (
Figure S3A). A broad band appearing at around 1520 cm
−1 is associated with the presence of ammonia (NH
4+) and salt derivates (-NH
3+) and could be also related to the effluent. Bands at around 1550–1550 cm
−1 associated with N–O asymmetric stretch; 1377 cm
−1, symmetric stretch; and 773 cm
−1, out-of-plane stretch modes for deformation of nitro compounds (nitrates (NO
3−) and nitrites (NO
2−)) [
45], which are present as residues in the aged membranes, are also often found in the spectra of other oily effluents [
51]. New peaks at 809 cm
−1 and at 685 cm
−1 are also associated with the effluent residue.
Thus, after analyzing and comparing the spectra, we can conclude that the immersion of the membranes does not have an impact on the main bands associated with the membrane initial polymeric structure (-OH, C-H, C-N, C-O and Cl-C bonds) and the changes observed in the spectra are likely due to the adsorption of components from the effluent as well as chemical reactions due to the presence of potential additives employed during membrane production.
The impact of filtration in the chemical structure of the membrane was also checked by comparing the pristine membrane before and after the NF test as well as two membranes immersed for 6 and 12 weeks prior to an NF test (
Figure 6).
Spectra depicted in
Figure 6, for all membranes tested by filtration, did not show significant differences apart from the new signals detected in the region of 2090–1952 cm
−1, which could be assigned to isothiocyanate bonds (N=C=S st) [
45,
52], and a new peak at 1819 cm
−1 (C=O st) associated with acid halide or anhydride, which is an indicator of chemical modifications in the urethane structure. The intensity of the peaks at around 3413 cm
−1, around 2960–2854 cm
−1 and around 1737 cm
−1 (
Figure 6) increases as the contact time with the effluent increases. Thus, similarly to the analysis shown in
Figure 5, the presence of the bands at 1410 cm
−1 and 1377 cm
−1 is probably related to the presence of alcohols and effluent components (
Figure S4).
The support layers of the membranes were also analyzed and compared by FT−IR (
Figure 7). First, the chemical structure for pristine and aged duplicate membranes (named Membrane 1 and Membrane 2), immersed in caustic effluent for 12 weeks (
Figure S5), was analyzed. Similarly to the analysis conducted for the active layer (
Figure 6), FT−IR characterization was conducted for the support layer in aged membranes immersed for 6 and 12 weeks in caustic effluent and subjected to NF tests (
Figure 7), and the results were compared to NF-tested and untested pristine membrane support.
The spectra for the support layer of membranes immersed for 12 weeks in spent caustic effluent were identical to those of the pristine membrane and showed peaks associated with C-H bonds (2985–2800 cm−1, 1457 cm−1 and 1383 cm−1) that remained in the main structure. However, an evident deacetylation with the loss of -OH (region around 3400 cm−1) and COO- groups (around 1660 cm−1) and an important change in the region for C-O bonds (1165–1015 cm−1) groups indicates that the polymeric structure of the support layers might have been considerably damaged after exposure to the spent caustic effluent.
As expected, these changes in the chemical structure were similar to those found in submerged membranes subjected to filtration (
Figure 7). The peak at 809 cm
−1 present in both spectra (
Figure S5 and
Figure 7) could be also related to the detected effluent components (
Figure S4).
As a conclusion, the data discussed above suggest that the active layer is not likely to have been modified since the differences observed are related to the adsorption of organic components that are present in spent caustic, affecting its retentive properties, as shown above. On the other hand, the support layer seems to have been considerably altered, in line with the changes observed in the permeability of the membranes, porosity properties and EDS results.