3.2. Membrane Characteristics
The membranes obtained in the experiment differed in color depending on the amount of rGO addition to the PAN matrix (Figure 2
). Membranes from pure PAN (0) were white. On the other hand rGO/PAN (A–H) membranes were characterized by different shades of grey. The more rGO added in the composite membrane, the darker the color. Black is dominant in the H membrane, which contains the highest addition of rGO (29.4% w
The external appearance of the membranes (presented in Figure 2
) does not say much about the morphology of their structure, so scanning electron microscopy (SEM) pictures were taken. The use of SEM allowed to examine the surface and cross sections morphology of produced composite rGO/PAN membranes and the pure PAN membranes.
(1) confirms the asymmetric construction of all membranes. For membrane 0 and A to G a dense skin layer with a thickness of 3.5–6.0 mm is clearly visible. The layer is hardly visible only in the case of membrane H, which contains almost 30% w
of the rGO addition. The cross-sectional appearance is similar for most obtained membranes. Looking from the skin side, the membranes are covered with small capillaries, which become larger and thicker as they move away from the skin [27
]. On the other hand, the support layer, is made of large, porous chambers and makes up the majority of the membrane surface. Analyzing in more detail the cross-sectional images of membranes D and E, it can be observed that the walls of the support layer show numerous lumps with a size of several µm. On the other hand, the SEM images of the cross-sections of membranes F, G, and H show randomly scattered flakes—perhaps rGO and its agglomerates, as indicated by their dimensions above 20 µm.
Analyzing the images of the surface of membranes from the skin layer (Figure 3
(2)) one can observe a smooth and even surface morphology, which changes for individual membranes. Membranes A and B show small but visible bright spots, which may indicate fragments of rGO protruding from the membrane, whose surface is not flat, but folded [28
]. SEM images of membranes D, E, F reveal inclusions that transform into microcracks for subsequent membranes (G and H). The structure of the support layer (Figure 3
(3)) of the membranes obtained in the experiment is even more interesting to observe. There are random holes and pores on it, and, at higher magnifications, a very interesting and rich multidimensional structure is revealed.
The paper analyzes the way in which the amount of rGO additive added to the PAN matrix influences the physicochemical properties of rGO/PAN composite membranes. Tests of physicochemical properties were carried out, which allowed to characterize membranes from pure PAN as well as rGO/PAN membranes. The obtained results are summarized in Table 2
Analysing the results of thickness measurements (Table 2
), it can be noticed that the thickness of the membrane 0 is ~190 µm. The introduction of a small amount of rGO into the PAN matrix caused the thickness of membrane A to increase by less than 9 µm, which can also be seen in the scanning microscope images (Figure 3
). It can therefore be concluded that an addition of 0.11% w
of rGO slightly slows down the membrane coagulation process, which increases its thickness. For other membranes, a slow decrease in their thickness is observed with an increase in the amount of rGO in the direction from membrane B to H. Thus, the introduction of reduced graphene oxide to the PAN matrix in an amount above 0.11% w
results in a slight acceleration of the coagulation process of composite rGO/PAN membranes in water.
The results of mass per unit area measurements (Table 2
) indicate a decrease in the mass of composite membranes in the direction from membrane A to E, compared to membrane 0. The addition of reduced graphene oxide facilitates coagulation of composite membranes. At the same time, it facilitates water penetration, which ultimately leads to a decrease in the mass per unit area of rGO/PAN membranes. However, for membranes that contain high amounts of rGO, namely: 7.7% w
(membrane F), 14.3% w
(membrane G), 29.4% w
(membrane H) an increase in mass per unit area is observed. The obtained results may prove that a large part of the mass of these composite membranes is rGO itself.
Analyzing the apparent density values for all membranes obtained (Table 2
), it can be seen that the pure PAN membrane has a density of 0.170 g/cm3
. In the case of rGO/PAN membranes, it can be noticed that with the addition of rGO to the polymer matrix (membranes: A, B, C, D), the density of membranes drops to 0.140 g/cm3
for membrane D. This is due to their low mass per unit area and high porosity. The density of membranes E, F, G, and H, on the other hand, increases with the increase in rGO content in membranes.
Water sorption tests (Table 2
) indicate that the membranes obtained in the experiment were characterized by water sorption values at the level of 209–330%. For membrane 0, water sorption is ~250% and porosity is 74.5%. Analyzing the results obtained for membranes A, B, C, D, and E, an increase in water sorption is observed, up to a value of 329.8% for membrane E. Moreover, as the rGO content in rGO/PAN membranes increases, their porosity also increases in the following order: 74.6%, 75.1%, 76.9%, 77.0%, and 79.4% for membranes: A, B, C, D, and E, respectively. However, the introduction of rGO addition above 4% w
results in a decrease in sorption properties of composite membranes F, G, and H. The water sorption determined for the H membrane is ~209%, which is 40% less compared to a membrane obtained from pure PAN. At the same time, the porosity of these membranes is reduced to 75.3%. The observed decrease in water sorption, accompanied by a decrease in the porosity, of membranes F, G, and H may be the result of accelerating the coagulation process of these membranes in water.
Analyzing the contact angle values (Table 2
) of membrane 0 (52°) and composite membranes A–H (49.5–40.7°), it can be concluded that the addition of rGO reduces these values, regardless of its amount in the PAN matrix. Consequently, rGO slightly improves the hydrophilic properties of membranes [29
The comparison of the results of testing the physicochemical properties of composite membranes leads to a conclusion that a small addition of rGO to the PAN matrix (up to 4% w/w) reduces membrane thickness, mass per surface area, apparent density and contact angle, while increasing water porosity and sorption. On the other hand, the introduction of subsequent portions of rGO (above 4% w/w) into the PAN matrix results in an increase in their density and contact angle, while the porosity and sorption properties of rGO/PAN composite membranes decrease.
3.3. Transport and Separation Properties of the Membranes
An important parameter determining the transport properties of membranes is the volumetric permeate flux (Figure 4
). Pure PAN membranes (membrane 0) obtained in the experiment were characterized by permeate flux with subsequent values: 51.5, 87.7, and 127.0 L/m2
h for operating pressures of distilled water: 0.1, 0.15, and 0.2 MPa. On the other hand, composite membranes A and B, containing a small amount of rGO as an additive to the PAN matrix, were characterized by lower values of the volumetric permeate flux. The obtained results may indicate the formation of connections between the functional groups of the polymer and oxygen groups on the edges of the flakes of reduced graphene, which results in a more compact structure of the skin layer of the obtained rGO/PAN membranes, as also confirmed by SEM images (Figure 3
A-2 and B-2). Further increase in the rGO concentration in composite rGO/PAN membranes causes an increase in the specific permeate flux. The highest values of the volumetric permeate flux were found in membranes D, E, F which contained 0.83%, 4.9%, 7.7% of rGO additive, respectively. The values were 323.6, 391.4, 361.3 L/m2
h, respectively for an operating pressure of 0.2 MPa. Such a large increase in flow through the membranes may be caused by the high content of rGO in the PAN matrix, which results in agglomeration of nanoparticles in the skin layer, which can also be seen in SEM images (Figure 3
D-2, E-2, F-2). This also results in a decrease in the contact angle values for membranes D, E and F (Table 2
). For membranes that contain high amounts of rGO (membranes G and H), the volumetric permeate flux drops dramatically to 41.3 and 37.6 L/m2
h for a pressure of 0.2 MPa. The observed phenomenon may be the result of reduced porosity, and thus water sorption (Table 2
), which are the result of a change in the internal structure of the membranes, visible in SEM images (Figure 3
Study of transport properties was extended to include tests of separation properties against BSA (Figure 5
). The obtained results indicate that under the influence of BSA there is a decrease in the volumetric permeate flux for all tested membranes. The largest decrease was ~84% and it was recorded for membrane 0. For this membrane, the rejection is also high and amounts to 94%, which is consistent with the lack of resistance of pure PAN to fouling [20
]. Very good rejection results were also determined for membranes A and B, which contained 0.11% and 0.22% w
, respectively, of rGO in the PAN matrix. They were 85 and 81%, with the volumetric permeate flux decreasing only by ~26% for membrane A and by ~25% for membrane B. Membranes D, E and F, which were characterized by the highest values of volumetric permeate flux for distilled water, under the influence of BSA reduce flux values by ~36% (for membrane D); ~25% (for membrane E); ~ 35% (for membrane F). For these membranes, at the same time, an exceptionally low rejection values (in the range of 33–39%) were recorded, which indicates that membranes D, F, and G have pores in the skin layer large enough to allow free penetration by BSA. The obtained results confirm the physicochemical properties, which indicated that the membranes are relatively thin (170–183 µm), with low apparent density (140–149 g/dm3
) and high water sorption (325–330%) (Table 2
The obtained results of the separation properties of rGO/PAN composite membranes have shown that a small addition of rGO results in good separation of protein molecules without significantly reducing the liquid flow through the membrane. rGO nanoparticles can form links with PAN functional groups, lowering the contact angle (Table 2
), resulting in repelling BSA particles and preventing fouling. A feature of a protein dissolved in water is the ability to form tightly packed structures with unpolished core and polar functional groups on the outside [30
], which in our case prevents clogging of membranes A and B. On the other hand, the introduction of rGO in an amount of 0.45% w
and more (D, E, F, G, and H) adversely reduces the BSA rejection value on the composite membranes, which prevents their use with the protein-like chemical compounds.
3.4. Biocidal Properties of rGO Solutions and Biostatic Properties of rGO/PAN Membranes
The literature reports that thermally reduced graphene oxide has bacteriostatic and bactericidal properties [3
]. The destruction of bacterial cells occurs as a result of physical and chemical interactions with rGO. One of the reasons for bacteria destruction may be breaking of the thin cell membrane, resulting in a leakage of cytoplasm, described in the literature [14
]. In another case, bacteria death occurs as a result of wrapping and/or trapping the bacteria, resulting in membrane stress and/or oxidative stress, as well as loss of cell viability and DNA fragmentation [15
]. Therefore, we conducted tests with the use of thermally reduced graphene oxide with two different nanoparticle sizes. The research indicated that both rGO20 and rGO150 nanoparticles show biocidal properties and inhibit the multiplication of tested bacteria (Figure 6
). In the case of Gram-negative bacteria, it was observed that as the concentration of rGO increases, so does the area of inhibition, which for rGO20 is 1 mm, while for rGO150 it is 0.8 mm. However, for Gram-positive bacteria it is observed that for small rGO flakes the inhibition zone is 0.6 mm (for rGO20 concentration of 0.1 g/L), and for large rGO flakes it decreases to this value as the rGO150 concentration increases. In addition, the difference in rGO biocidal properties is closely related to the structure of bacterial cells [31
]. E. coli
are rod-shaped, with dimensions 2 × 0.8 µm and a thin cell membrane. S. aureus,
on the other hand, have a spherical structure, with dimensions of 1 × 0.8 µm, surrounded by a thick cell membrane. A slight difference in the biocidal properties of rGO may be related to the fact that at the same weight concentration of nanoparticles, the probability of E. coli
bacteria meeting rGO150 flakes is lower than for rGO20 flakes. In addition, agglomeration of large rGO particles is likely in solutions with a higher concentration (0.1 g/L), which in the case of bacteria of a spherical shape (S. aureus
) hinders its destruction, which is manifested by the inhibition zone reduced to 0.6 mm.
The aim of our research was to obtain composite membranes with biocidal properties that could be used in water disinfection processes. In the literature, there are many examples of polymer composites containing an rGO addition that have biocidal properties [14
]. Therefore, we conducted microbiological tests on rGO/PAN composite membranes obtained in the experiment. The following microorganisms were selected: S. aureus
, E. coli
, and C. albicans
, and the obtained results are summarized in Figure 7
. These studies show that membranes "0" show biostatic properties, which has already been described in the literature [6
]. Pure PAN causes the occurrence of narrow inhibition zones with a width of 0.12–0.14 mm (Figure 7
). However, for composite membranes rGO/PAN it is observed that the addition of rGO of up to 0.83% w
does not increase the E. coli
growth inhibition zone. Only larger amounts of nanoparticles in the polymer matrix effectively interact with thin Gram-negative bacteria membranes causing their death within 3 mm of the membrane H sample. A similar nature of the changes was observed for C. albicans
fungi. The fungal colonies growth inhibition zones slowly increased from a width of 0.13 mm (membrane D) to 0.18 mm (membrane H). On the other hand, when analyzing the results of biological activity of rGO/PAN membranes (Figure 7
) against S. aureus
bacteria, it can be observed that only a small addition of nanoparticles (0.11–0.22% w
of rGO) to the PAN matrix causes an increase in the bacterial growth inhibition zone. Comparing the results obtained in the research of Zou et al. [15
] it can be concluded that the surface of the membrane from the skin side is covered with sharp fragments of rGO, which easily destroy E. coli
, rupturing their cell membrane. Such structure of rGO/PAN membranes also prevents the growth of S. aureus
, but to a lesser extent. Therefore, it can be concluded from the research that the rGO/PAN membranes obtained in the experiment show biostatic properties, which, combined with the separation properties, create the potential for their use in water disinfection processes.