Performance evaluation of amino-functionalized mesoporous/PES nanofiltration membrane in anionic dye removal from aqueous solutions

In this study, the FSM-16 and FSM-16 modified by metformin (FSM-16-met) additives were applied as modifiers for polyethersulfone-based membranes. The modified membranes were evaluated in terms of morphology, surface roughness, hydrophilicity, pure water flux, pore size and porosity, and resistance ability. The obtained experimental data indicated promising enhancement in permeated flux and hydrophilicity after introducing the FSM-16-met nanomaterials into the membrane texture. In the case assessment of the fabricated membranes, dye removal capability was performed using the direct red-16 and methyl orange (30 mg/L). The modified membranes showed significantly higher dye rejection ability (> 97% for direct red-16 and 95% for methyl orange) compared to the bare membrane (76% for direct red-16 and 74% for methyl orange). The antifouling evaluations were run for milk powder (1000 mg/L) filtration and the best performance was obtained for the modified membrane with 0.1wt.% FSM-16-met (FRR = 95.55 ± 1.91%, Rir = 4.45 ± 0.089%). Also, the effect of the feed pH and concentration, temperature, and membrane driving force was considered on the membrane behavior. High stability in long-term performance was also observed for the modified membrane after 25 h re-running.


Introduction
Increased demand for water along with water pollution resulted in an immediate urgent for water reuse (Moradi et al. 2019). Numerous wastewater treatment methods have been developed, such as biochemical, chemical, and physical processes (Yang et al. 2012). Various limitations include complex operating methods, production of harmful by-products, sludge, and environmentally resistant materials are ahead (Baker 2010). Membrane separation processes have emerged as promising technologies for wastewater treatment and water reuse (Moradi et al. 2018;Abdikheibari et al. 2018;Anand et al. 2018). They offer numerous benefits such as cost-effectiveness, being environmentally friendly, excellent efficiency and quality, and easy operation (Wang et al. 2018;Yao et al. 2016;Hegab et al. 2015). Polyethersulfone (PES) has received much attention due to its unique properties, such as mechanical and thermal stability (Bruggen 2009). The fouling phenomenon is one of the main problems of polymeric membranes (Xu et al. 2016). The hydrophobic nature of PES-based membranes is the vulnerability result to fouling, which is due to strong interactions between the membrane surface and foulants, leading to blockage of pores (inner deposition) and outer membrane deposition during the filtration process (Rajabi et al. 2015).
It has been reported in various studies to reduce the aforementioned problems such as plasma treatment, bonding hydrophilic monomers, and combination with hydrophilic additives (Seman et al. 2012;Moghimifar et al. 2015;Ng et al. 2013;Noeiaghaei et al. 2014). Lately, the combination of nanoparticles (NPs) with polymer texture has led to the preparation of mixed matrix membranes (MMM) with increased high selectivity and permeability flux and substantial anti-fouling properties related to increased hydrophilicity and altered membrane morphology (Bano et al. 2015;Kim and Bruggen 2010). Nano-shaped materials (e.g., MWCNT, GO, zeolite, boehmite, Fe 3 O 4 , TiO 2 , ZnO, SiO 2, Fe NPs, and Cu NPs) have been used as modifiers for PES-based membranes (Zinadini et al. 2017;Ghaemi et al. 2015;Vatanpour et al. 2012a;Vatanpour et al. 2012b;Vatanpour et al. 2012c;Qin et al. 2005;Rambabu and Velu 2014;Akar et al. 2013). Moghimifar and coworkers reported the coating of the PES membranes with zeolite and TiO 2 NPs, and the results indicated notable separation performance and anti-fouling behavior with no significant changes in the membrane texture (Moghimifar et al. 2015). Fouled silica mesoporous (FSM-16) are adsorbed, because of pore size distribution, large surface area, and porosity (Samari et al. 2021).
In our previous work, FSM-16 and metformin-modified FSM-16 (FSM-16-met) were used for the fabrication of PESbased ultrafiltration membranes for the separation of oil/ water mixture (Samari et al. 2021). The promising performance of the ultrafiltration of PES/FSM-16-met membranes has encouraged us to design nanofiltration membranes based on PES and FSM-16-met for eliminating dyes from wastewater. The fabricated membranes were characterized with SEM and AFM, and their performances were evaluated by measuring zeta potential, contact angle, porosity, and water flux. Organic dyes of direct red-16 (DR-16) and methyl orange (MO) were used in this study.

Modification of nanoparticles
FSM-16 was initially prepared using the reported method (Hashemi-Uderji et al. 2019) and subsequently modified with metformin according to our previous report (Samari et al. 2021). In brief, FSM-16 (1 g) was added to dried toluene (50 mL) containing 3-chloropropyltrimethoxysilane (CPTMS, 0.6 g) and refluxed for 24 h. The resulting powder (i.e., FSM-16-Cl) was separated by centrifugation, washed with dichloromethane and methanol several times, and dried overnight at 80 ºC. A quantity of FSM-16-Cl (1 g) was added to the mixture of metformin (1 g), NaOH (0.25 g), and KI (1 g) in acetonitrile (30 mL), and the mixture was refluxed for 12 h. The synthesized FSM-16-met was centrifuged, washed with ethanol and distilled water, and dried at 80 °C for 24 h.

Fabrication of the membranes
The PES-based membranes were fabricated via the phase inversion method. Table1 shows the constituents of the fabricated membranes. The prepared casting solutions were homogenized by keeping the mixture stirring (400 rpm) overnight and sonication for 20 min to remove all air bubbles. By applying a film applicator(150 µm thickness). membranes were cast on neat glassy plates. The plates were submerged in distilled water to form the flat-sheet membranes. To complete phase separation, after the formation of polymeric membrane, they were transferred to unused distilled water, overnight. Subsequently, the fabricated membranes were dried at room temperature overnight (Samari et al. 2020).

Characterization techniques
The membrane characteristics have been evaluated by various techniques. The scanning electron microscopy was applied to examine the surface morphology (SEM, model: Philips-XL30). The membrane surface roughness was evaluated by atomic force microscopy (AFM, Nanosurf® Mobile). The hydrophilicity of the fabricated membranes was measured by water contact angle (WCA).

Porosity measurements
The gravimetric method was used for determining the membrane porosity (ɛ). A random portion of the membrane was selected, cut (2 × 2 cm), and then weighed. The membrane was then submerged in distilled water and weighed again (after 24 h). The membrane porosity can be calculated by Eq. (1): where ω 1 and ω 2 are the membrane weight after and before immersing in water. A, L , and dW are surface diameter (m 2 ) , thickness, and water density (999 kg. m 3 ), respectively (Gholami et al. 2018).

Membrane setup and performing analysis
To investigate the membrane performance, a dead-end setup was applied which was made of stainless steel (150 mL, 12.56 cm 2 ) armed with an N 2 gas (Fig. 1). Three frequent experiments were consecutively examined (distilled water-feed-distilled water) (simulated dye feed (30 ppm, DR-16, and MO) and 1000 ppm milk powder solution as fouling agents were applied as feed). Firstly, distilled water was filtered (60 min), then without any further operation, milk powder (1000 ppm) or dye feed was passed (90 min) and the membrane was washed with distilled water, after this step, distilled water was finally filtered again (60 min). The PWF was calculated by Eq. (2): where J W.1 , M, A, Δ t, are PWF ( kg∕m 2 .h ), the weight of permeated volume (kg), membrane surface area ( m 2 ), and filtering time ( h ), respectively. (1) For evaluation of the membrane recovery, membrane flux recovery ratio (FRR) was determined using the experimental data of fouling solution filtration (milk powder) (Eq. 3): here J W.2 , and J W.1 are the permeated volume ( kg∕m 2 .h ) after and before the milk powder filtering. The membrane fouling resistance including irreversible resistance (R ir ), reversible resistance (R r ), and total resistance (R t ) were evaluated according to Eqs. 4, 5, and 6:

Rejection
A spectrophotometer was applied to evaluate the membrane ability for dye rejection of DR-16 (λ = 526 nm) and MO (λ = 464 nm) by the following equation: where C F and,C P are the dye concentrations of the feed and permeate solutions.
Moreover, the rejection performance of the optimally modified membranes (M1 and M4) was compared with that of the bare membrane at different pHs. The long-term durability of the fabricated membranes in the filtration process was also examined.

Physico-chemical properties and PWF of membranes
The applied membrane modifiers were synthesized and characterized according to our previously published report (Samari et al. 2021). The bare PES membrane exhibited a hydrophobic nature (Table 2, water contact angle = 98°).
Although the WCA of M1 and M4 membranes decreased by increasing the content of FSM-16 and FSM-16-met in the membrane matrix, which indicates the hydrophilic nature of FSM-16-met. In the best case, the WCA of M4 was close to 40.78°, which was notably lower than the other fabricated membranes in this study. The hydrophilicity can be improved by the introduction of active sites (-NH 2 and -OH) presenting in FSM-16-met, into the membrane matrix. Additionally, the effect of FSM-16-met on hydrophilicity enhancement can be related to the reduction in interfacial energy (Celik et al. 2011). However, further addition of FSM-16-met into the membrane matrix did not increase the membrane surface hydrophilicity, but rather reduces it to some extent. This could be related to two main reasons: (i) pore size reduction on the surface due to FSM-16-met aggregation in the membrane matrix, and (ii) the reduction of functional groups on the membrane surface. The FSM-16-met has a high tendency to accumulate in high concentrations, leading to a decrease in the effective loading of FSM-16-met into the polymeric matrix.
The PWF of the fabricated membranes with different contents of unmodified and modified additives are compared in Fig. 2. As anticipated, the PWF increment coincided and was well-matched with an increase in hydrophilicity. The bare PES showed the lowest amount of PWF compared to M1 and M4 membranes. It seems that the increase in porosity, hydrophilicity and the pores size of the M1 and M4 membrane has enhanced the PWF. By adding 0.1 wt.% of FSM-16-met (M4), the PWF was increased to 55.56 kg/ m 2 . The PWFs of 52.54 and 39.78 kg/m 2 were found for M5 and M6, respectively. The reduction in the PWF with increasing the nanofiller content (> 0.1 wt.%) was related to the increased viscosity of the casting solution, which caused slow phase inversion. In other words, the high loading of nanofiller blocks membrane pores and reduces their mean pore sizes (Zinadini et al. 2014a). The mean pore radius and total porosity of the membranes are given in Table 2. The mean radius and porosity of membranes increased with the addition of FSM-16 up to 0.1 wt.% and then reduced. Dadari  (Zinadini et al. 2014b). It can be the reason for increasing the pore channel size, porosity, (Table. 2), and PWF (Fig. 2) of the prepared membranes.
The obtained results were proved by SEM (Fig. 3). The modified membranes exhibited relatively smooth structures. The addition of a high amount of FSM-16-met into the polymeric matrix (> 0.1 wt.%), hurts the membrane properties. In addition, the effect of compaction was evaluated and the PWF was examined at different transmembrane pressures. Figure 2 shows that by increasing the transmembrane pressure, the PWF was increased and a linear relationship between PWF and pressure was observed. Furthermore, the stability of membranes at harsh thermal conditions (45, 60, 75, and 90 °C) was evaluated (Fig. 4). The results indicated that the thermal stability of the membranes can be enhanced by increasing the additive loading (Samari et al. 2020;Matsumoto et al. 2002).
To realize the effect of additives on the surface roughness of the membranes, the technique of AFM was utilized. The results of surface parameter and topography are presented in Table 3 and Fig. 5, respectively. It was found that the surface roughness of the modified membranes was significantly reduced by the addition of the additive. The bare membrane (M0) exhibited the highest roughness of 45.85 ± 0.97 nm, while the roughness of 1.63 ± 0.05 nm and 1.34 ± 0.04 nm were found for M1 and M4, respectively. This behavior can be related to the regular decoration of additives, in low

Antifouling evaluation
The anti-fouling properties of the membranes were examined by measuring irreversible (R ir ), reversible (R r ), total membrane resistance (R t ), and FRR (Fig. 6). The R t can be divided into R r and R ir . Higher FRR and lower R t indicate superior membrane anti-fouling properties. The deposited foulant can be also removed by hydraulic cleaning. The bare  The addition of additives (0.1 wt.%), the corresponding R ir , was reduced to 13.1% and 4.45% for M1 and M4, respectively. The best result was observed for the membrane with a modified additive, which could be promising compared to the reported data in the literature (Samari et al. 2020).
The durability of the membranes was evaluated by three filtration cycles using milk powder solution as a model foulant solution (Fig. 7). For the purification cycle, the FRR of 54.29% was found for M0 due to its hydrophobic nature, while the corresponding values of 86.18 and 95.55% were observed for M1 and M4, respectively. In fact, the membrane with FSM-16-met (M4) exhibited the best recovery performance and the foulants can be easily removed from the membrane surface by physical washing. However, by increasing the additive loading (0.3 and 0.5 wt.%), the corresponding FRR was reduced, which could be related to the enhanced roughness in higher loading. Fig. 6 The FRR, R r , and R ir of the membranes

Dye rejection
To further justify the widespread application of the fabricated nanofiltration membranes, dye rejection behavior was examined using MO and DR-16 solutions (30 ppm). Table 4 represents the results of dye rejection of various fabricated membranes. The results showed that the modified membranes exhibited higher dye rejection than the bare membrane. It was also found that the surface modification of the additive had a significant effect on the performance of the membranes with modified FSM (i.e., FSM-16-met) compared to using unmodified FSM. The highest dye rejection was found for membrane M4 with 97.5% and 95.5% for DR-16 and MO, respectively. The surface modification of FSM-16 with organic species possessing amine groups (-NH 2 and -OH) enhanced the surface hydrophilicity and generated more adsorption sites, which could be beneficial for dye rejection. The data of WCA (Table 2) and zeta potential ( Fig. 8) are proved changes in the surface hydrophilicity of membranes with the addition of the additives, particularly modified additives. According to the results of zetapotential, membrane M4 (PES/FSM-16-met) has negative surface charges at pH = 6 because of the presence of added functional groups located in the pores and membrane texture (Abdi et al. 2018). On the other hand, MO and DR-16 have negative charges in neutral pH due to the dissociation of sulfuric groups; therefore, electrostatic repulsions between the membrane surface and the dyes caused high rejection. In addition, the activated sites in DR-16 (e.g., -NH, -OH, aromatic rings) and/or MO (-SO 3 , aromatic rings) may form some hydrogen bonding interactions with available functional groups of FSM-16-met (-NH 2 and -OH). In fact, the organic dye cannot stabilize on the surface of the modified membrane, PES/FSM-16-met (Fig. 9).

The effect of concentration
In this part of the study, the bare (M0), and optimally modified membranes (M1, and M4) were examined by mixed solutions made of different dyes (DR-16 and MO) in various concentrations (Fig. 10). As can be seen from the data (a, b) by increasing the dye concentration, the membrane PWF was generally decreased although the dye rejection was kept stable.
According to the obtained data in Fig. 10 (a), dye concentration increment leads to a reduction in membrane rejection. The possible explanation of this phenomenon can be related to the aggregation of the dye molecules in the feed, which could be suppressed by the TDS. In this case, more dye molecules will adsorb in pores and on the membrane surface and form a fouling layer, which acts as an additional resistance layer gradually resulting in a reduction in the effective pore size and then the membrane permeability. When the concentration of dyes was increased from 10 mg/L to 50 mg/L, the rejection essentially changed, and the PWF was dropped (Tavangar et al. 2020;Vatanpour et al. 2020). Figure 11 illustrates that, by increasing the transmembrane pressure (TMP), the PWF of bare and optimally modified membranes was increased almost linearly. As can be obtained, the PWF of the optimally modified membrane (M4) was almost 27 kg/m 2 .h, at 7 bar pressure while the bare PES exhibits a flux of 18.2 kg/m 2 .h at the same pressure for MO dye filtration. The observed linear dependence of PWF on TMP illustrates the stable membrane structure due to the high network crosslinking of the PES membrane. As mentioned before the mean pore size of the modified membrane was increased, which plays a crucial role in PWF enhancement and dye rejection. This phenomenon is likely  Fig. 8 The zeta potential of the fabricated membranes following the synergy effect of reduced crosslinking density and loos of microstructure brought by membrane modification (Samari et al. 2021).

The effect of pH
The effect of feed pH on the dye rejection performance of the membranes was investigated at three different pHs (2,7,12). According to the presented results in Fig. 12a, the dye    At pH = 2, the dye molecules turn to acidic form and functional groups (-NH 2 ) of FSM-16-met protonate rapidly to ammonium (-NH 3 + ) groups. Therefore, the electrostatic repulsion between the membrane surface and the feeds enhanced the dye rejection (Moradi et al. 2020). In alkaline conditions (pH = 12), the dye rejection efficiencies were further increased compared to the corresponding in acidic conditions, which could be related to the role of hydrogen bonding in the process of dye rejection. Metformin in FSM-16-met is in bidentate forms in alkaline pH, which facilitates the availability of amine groups. The dye rejection behavior of the membranes is strongly supported by the result of PWF (Fig. 12b). For the M4 membrane, the PWF was higher for DR-16 (25.5 kg/m 2 ) at pH = 12, due to the fluid volume and enhanced membrane rejection. Similar results were also observed for MO. The best performance was observed for the M4 membrane with an optimal loading of 0.1 wt.% of the modified FSM-16.

Long-term performance
The long-term performance of the membranes was evaluated in nine frequent cycles of the dyes at 30 ppm (Fig. 13). The modified membranes (M1 and M4) exhibited considerable performance and stability within 1500 min compared to the bare membrane. M0 has lost its performance by a significant reduction in PWF (from 14.33 to 0 kg/m 2 ) after 420 min. This phenomenon displays the effective role of additives in manipulating the texture of polymeric membranes. Furthermore, the enhanced PWF might be related to the generated thin hydration layer on the membrane surface through hydrogen bonding interactions of H 2 O molecules with the membrane surface. In other words, by increasing the resistance against the formation of the cake layer on the membrane surface, this phenomenon is pronounced in longterm performance (Fig. 13) (Samari et al. 2020).
The performance of the M4 membrane (PES/FSM-16-met) in dye rejection is compared with other systems reported in the literature (Table 5). Our system exhibited a quite high flux and comparable rejection efficiency in comparison with the reported systems.

Conclusion
In this work, FSM-16 and FSM-16 modified with metformin (FSM-16-met) were utilized as additives to improve the performance of PES-based membrane in the separation of organic dyes from wastewater. PWF, antifouling behavior, and dye rejection efficiency of the membranes were evaluated for the model dyes of MO and DR-16. The results revealed membranes with modified additives showed higher performance than those with unmodified additives. Moreover, higher performances were observed for membranes with low additive loading (0.1 wt.%) compared to membranes with the loading of 0.3 and 0.5 wt.%. The best result was found for a membrane with 0.1 wt.% of FSM-16met: PWF (55.56 ± 1.11 kg/m 2 .h) and rejection efficiency (97.5% and 95.5% for DR-16 and MO, respectively). The antifouling results exhibited that by the introduction of the modified additives to the membrane, the FRR was significantly increased (from 54.29% to 95.55%) compared to the bare membrane, while the irreversible fouling was decreased (from 45.07% to 4.45%) due to enhanced hydrophilicity of the membrane in the presence of the modified additive. Moreover, the results of long-term performance revealed the high stability of the modified membrane. These results can be promising to extend the application of PES/FSM-16-met membrane for purification of industrial wastewater before discharging to the environment and separation of organic pollutants from urban wastewater.