1. Introduction
As global water crisis is one of the eternal problems for mankind to solve, employing the osmosis as a technique to reproduce fresh water from saline water or other wastewater sources is constantly receiving a lot of attention. Here, what hinders the rise of these technologies the most is the fouling of the membranes used in a water filtration[1-12]. The formation of biofilms not only reduces the water flux but also the lifetime of the membrane. Therefore, there has been natural growing interests on figuring out the best solution to synthesize the antimicrobial membranes. This review article focused on two types of materials, the ones that use silver nanoparticles and the ones that utilizes non silver particles. For the former, in order for the improvement brought by the silver nanoparticles to show its full performance, the uniform dispersion of these silver nanoparticles with great attachment on the membrane surface is crucial. To do so, various attempts came up with different perspectives have arised, including partially hydrolyzing membranes, adding the hydrophilic material layer, applying arc plasma deposition, using polyelectrolyte layer-by-layer self-assembly or modifying the membranes with other materials such as cysteamine.
Yet, while adding silver nanoparticles does have a lot of advantages and potential as one of the most frequently mentioned solution, it also has some risks such as bringing the possibility of toxicity in some cases. In the continuous endeavor to find more optimal method, now there are also many studies for conjugating non silver particles to impart antibacterial properties. In these processes, silica particles are functionalized (or embedded) with copper nanoparticles, amine moieties, graphene oxides or zinc oxides. The summary of the article is represented in the form of schematic in Fig. 1.
2. Silver Nanoparticle
2.1. Polyamide TFC membrane
Park et al. reported a new technique of integrating silver nanoparticles onto polyamide (PA) thin film composite (TFC) reverse osmosis membranes through arc plasma deposition (APD)[13]. The advantages of facilitating arc plasma deposition (APD) is that it allows a direct deposition of silver nanoparticles under vacuum dry condition, which simultaneously solving the negative results caused by the conventional wet-chemical processes. Silver nanoparticles were successfully deposited throughout the polyamide (PA) selective layer, while partially some penetrated into the polyamide (PA) matrix. The amount of silver loading depends on the number of arc plasma deposition (APD) purse shots. The strong silver-polyamide interaction and partial portion that penetrated allowed the deposited silver nanoparticles to display good leaching stability. It turns out that the silver incorporated TFC (Ag-TFC) membranes have excellent and long-term antibacterial properties toward gram-negative and gram-positive bacteria. In terms of the water flux, there was approximately 40% of improvement without noticeable declining NaCl rejection. (Up to 60 shots, the NaCl rejection remained unchanged with the value of 98.9 ± 0.2% and only slightly decrease to 97.8 ± 0.8% after 100 shots.) These were provisionally ascribed to the partial destruction of the polyamide (PA) layer and the increased hydrophilicity of the membrane under the strong arc plasma deposition (APD) condition. The result proves that utilizing arc plasma deposition (APD) is a simple, effective and environment-friendly method that can enhance the overall performance of membranes and impart various new functionalities to them such as antibacterial properties. Same research group immobilized silver nanoparticles on polyamide thin film composite membranes to impart antibacterial properties to them[14]. In this method cysteamine is used as a linker. An excellent leaching stability of silver nanoparticles and AgNP@SiO2 were provided with the formation of multiple silver-sulfur chemical bonds between the rough surface of membranes and AgNP@SiO2. The membrane surface coverage by AgNP@SiO2 was controlled by the deposition time and the concentration of AgNP@SiO2 particles. Without severe aggregation witnessed, the AgNP@SiO2 were successfully distributed and dispersed over the entire surface of membrane. The morphology of synthesized particles and membranes were confirmed and analyzed by transmission electron microscopy (TEM) and scanning electron microscopy (SEM). These AgNP@SiO2 immobilized membranes displayed exquisite antibacterial properties against E. coli, P. aeruginosa and S. aureus (the reduction value of 92.7 ± 1.8%, 99.5 ± 0.3% and 73.3 ± 5.5% respectively), even with a relatively low particle coverage of 11%. This strength can be attributed to the effective distribution of SiO2 particles densely bonded with silver nanoparticles which not only allows an in crease on the contact between silver nanoparticles and bacteria but also enables the valid elution of silver cations. One another important strength of this method is that this particle immobilization did not negatively affect the separation performance of the water flux and salt rejection. This result addresses the usefulness of uniformly distributed AgNP@SiO2 since it can reinforce the antibacterial characteristics and bring only negligible impact to the hydraulic resistance.
2.2. Polyether sulfone-based membrane
Developing from the initial silver of the biogenic nanoparticle with high concentration, high stability and the average diameter (6 nm), this article introduces an upgraded method of fabricating a biogenic nanocomposite polyethersulfone (Bio-Ag(0)-6/PES) membranes by applying the dope solution to different amounts of biogenic nanoparticle silver[15]. The fabricated nanocomposite membranes were tested with pure water permeability, molecular weight cut-off (MWCO), contact angle. Also, their surface and morphological structures were characterized and confirmed to have well dispersed polythersulfone nanoparticles into PES membranes without visual aggregation through energy dispersive X-ray spectra (EDX), scanning electron microscopy (FESEM) and atomic force microscopy (AFM). Here, the addition of polythersulfone nanoparticles utilize the extension of micro voids to the sub-layer, enhancing the interconnectivity of pores that exist between the sub-layer and the bottom layer. It has been showed that these added polythersulfone successfully improved the pure water permeability of the membrane with slightly increasing the hydrophilicity while not lowering the selectivity of BSA. Due to the increased hydrophilicity, it also realized a huge decrease in the protein adsorption and improved smoothness to the membrane surface. The disk diffusion test displayed that these nanoparticles with polythersulfone has excellent antibacterial properties. In the comparison with Bio-Ag(0)-11/PES membranes, Bio- Ag(0)-6/PES membranes do show greater maintenance on the silver releasing rate within the safe range and hold less influence on the filtration performance, indicating better stability of immobilized silver in the membranes. Moreover, the biofouling experiments (of sludge immersion and the bacterial suspension filtration) revealed that the flux of Bio-Ag(0)-6/PES membranes has declined with relatively smaller amount of 43, 34, 14, and 8% for four distinct modified models (whereas the pure PES membranes it was 66% flux decline). This demonstrates that these modified membranes can inhibit not only the attachment of the bacteria but also the reproduction and growth of biofilms on the membrane surface. The article highlights the potential of using Bio-Ag(0)-6/PES membranes as an effective approach to solve the biofouling problems.
The existence of polydopamine (PDA) layer of adhesivity and reductivity on the membrane surface can bring not only more reduction of silver cations on the membrane surface without blocking the surface pore but also an improved attachment of silver nanoparticles with better strength and uniform distribution[16]. This is because that polydopamine (PDA) is a highly hydrophilic material with its chemical composition and has adhesive capability by being a bio-inspired polymer. Therefore, this article has aimed to fabricate AgNPs-PDA/PSf membrane through the surface modification of two steps starting with first applying dopamine solution on PSf membrane, which followed by adding Ag(NH3)2OH solution on PDA/PSf membrane formerly made. The analysis on surface chemical composition and the morphological characterization for the membrane fabricated was done through X-ray photoelectron spectroscopy (XPS), SEM and AFM. The results have confirmed that silver nanoparticles are well immobilized both on the surface and the top layer cross section of membrane. The pure water flux of AgNPs-PDA/PSf membrane was 336 L which is a value that increased about 35% compared to that of PSf membrane (248 L This tendency was continued during BSA (bovine serum albumin) filtration, since AgNPs-PDA/PSf both outshined PSf membrane on both permeation flux and antifouling performance. AgNPs-PDA/PSf has also showed excellent antibacterial performance against E. coli and B. subtilis with successfully providing high sterilization ratio and a firm inhibiting zone. Moreover, it has displayed a great stability on its performance for maintaining 80% of water flux after exposed to bacteria for 48 hours during filtrate operation and losing only little of silver after 12 days of statically immersed in water.
In this article, the strategy of using hydrogel thin layers to attach silver nanoparticles on the surface of membrane was chosen[17]. To provide PES membrane surface anchoring sites, double bonds were introduced at first step onto the PES membrane surface. This was then followed by the synthesis between the hydrogel layers and the membrane surfaces which was done through UV light-initiated crosslinking copolymerization. Hydrogel layers can absorb silver cations, leading the ions to be reduced to silver nanoparticles with sodium borohydride. This absorption of cations can be adjusted by controlling the mole ratios of carboxylate groups in the hydrogel thin layers. The attachment, form, and a typical three-dimensional porous structure of these layers and its great hemocompatibility was confirmed and analyzed via FESEM, attenuated total reflection-Fourier transform infrared (ATR-FTIR). Fabricated membrane not only did show the excellent ability of restricting the formation of biofilms and removing surrounding bacteria but also was proved its consistency on the strong antibacterial performance for more than 5 weeks. When the solution changed from the pure water to BSA solution, the water flux of both pristine provide PES membranes and provide PES with hydrogel layers dramatically falls because of the fouling caused by absorbed proteins in the membrane pores on the surfaces. As for the water flux recovery ratio, the PES membranes also showed better performance of 74.7, 68.4 and 59.3% for each trial, whereas due to the bovine serum albumin (BSA) molecule being absorbed onto the pores, the pure provide polyether sulfone (PES) showed the value of 0%. This was possible because the hydrogel thin layers can bring higher hydrophilicity of the surface, which leads to detaching the absorbed protein and raises the recovery ratio. Moreover, while there was no cytotoxicity witnessed since the amount of silver cations absorbed did not reach the threshold value, the membranes were found out to have intense anti-cell adhesion properties toward the cells and bacteria with being attached to super hydrophilic hydrogel layers, which were both indicated via cell culture tests (Fig. 2).
2.3. Polyacrylonitrile based membrane
Liu et al. reported uniform dispersion of silver nanoparticles by partially hydrolyzing the PAN (polyacrylonitrile) membrane to provide enough surface carboxylate groups that can bind with silver cations[18]. The reduction under a mild reducing condition (gaseous CO) for turning silver cations to silver nanoparticles took place before the membrane exposed to a silver precursor to localize the region for the synthesis and deposition of silver nanoparticles on the exterior surface. The formation and the surface of the membrane was confirmed with FESEM and XPS. Such method is turned out to bring the highest silver content on the exterior surface of the resultant membrane. In addition, the method introduced can be a successful solution for the cases including TFC membranes because it is free of causing the decrease in the water flux. TFC membranes fabricated by applying the method showed an excellent performance and stability in antibacterial resistance for a minimum 14 days under the conditions optimized for the growth of E. coli. The antibacterial adhesion features of membranes with silver nanoparticles were quantitatively evaluated, which lead to a conclusion that the membranes can provide more than 99.9% of bacterial removal rate, guaranteeing no visible bacterial growth practically (Figs. 3, 4). Zhang et al. grafted silver nanoparticles on the PI membrane surface was done through three-step process[19]. This process includes hydrolysis of adding KOH into pure PI membrane, the silver ion exchange reaction and post thermal treatment which causes the silver reduction at high temperature. The performance of membranes that went through this modification route has been evaluated by disk diffusion test, bacterial suspension immersion ex periment and using the bovine serum albumin (BSA) aqueous solution. Their structural characterization and the demonstrations of the reaction mechanism was done via FTIR, Thermogravimetric Analysis (TGA), SEM, and AFM. From the result of color change from yellow to grey to brown in surface, it is observed that the roughness of the surface was increased, and the hydrophilicity was decreased since the brown color can be achieved when there are silver particles with a size about 0.5 micrometer. In the fouling test, considering that all membrane inevitably go through the decline in flux drop after 60 minutes pass, all PI membranes with silver nanoparticles added showed superior performance, comparing with the virgin PI membrane thermally treated at 200°C. This result indicates that the silver modification does enhance the antibacterial and antifouling properties. Also, testing the silver stability of PI membrane, higher rate of silver ions turning into silver (0) particles was achieved by the increase of the temperature in the thermal treatment. While most of membranes with lower thermal temperatures showed decreased silver concentration in day 6 than day 2, the PI-Ag membrane of 200°C maintained almost same value of concentration of about 1.5 ppm. This can demonstrate that with increasing thermal treatment temperature, the silver release rate stability on the membrane surface can be improved significantly.
Mechanism of fabricating an antimicrobial surface by depositing biocidal silver nanoparticles on the membrane surface after modifying polyvinylidene fluoride (PVDF) membrane into thiol functionalized form was reported in this article[20]. PVDF membranes can be adjusted depending on the alkaline treatment and further surface engineering of functionalizing with a specific thiol compound. In other words, modifying PVDF membrane with TGA or PETMP will allow different surface assembly of silver nanoparticles to be tailored respectively. During the experiment, the silver nanoparticles were assembled with the PVDF membrane via esterification which was formed between thioglycolic acid (TGA) and alkaline treated polyvinylidene fluoride membrane (TGA-PVDF). Accessing thiol-ene reaction of pentaerythritol tetrakis (3-mercaptopropionate) (PETMP) and alkaline treated PVDF (PETMP-PVDF) facilitate a great dispersion of silver nanoparticles on the membrane surface. The surface morphology was analyzed with SEM and selective area EDS scans. The function and the control leaching of this biocidal silver nanocluster assembly on TGA-PVDF membranes was confirmed via inductively coupled plasma atomic emission spectroscopy (ICP) and XPS. This article proved that a specific thiol compound can direct the silver nanocluster assembly on the modified membranes. The performance of TGA-PVDF membrane distinctly suppressed that of PETMP-PVDF. One example of this can be the value of silver ions leaching of these two membranes where that of TGA-PVDF membranes (0.068 ppm) were approximately 5 times higher than that of PETMP-PVDF membranes (0.014 ppm). It was assayed that the silver nanocluster assembly were impeded from sustaining its form because of PETMP evoking a detachment, while silver cations released from the surface of the membrane playing a key role of keeping the surface antibacterial. Wu et al. introduces two new methods that not only allow easy control of total amount of silver loaded but also aim to achieve relatively slow and steady release of antimicrobial agents (silver)[21]. Two techniques of forming the silver zeolite (Ag-zeolite) based antimicrobial coating were one, using polyvinyl alcohol (PVA) and the other using polydopamine (PDA). Both methods successfully loaded a wide range of silver and is attractive since they can replenish silver upon depletion that leads to long-term biofouling resistivity. Through the experiment of silver release and bacterial exposure, it has been confirmed that the increase of silver-zeolite loading accelerates the release of silver particles, which can facilitate stronger antimicrobial activity. Interestingly, though, it cannot bring noticeable enhancement when it comes to extending the antimicrobial lifetime of the membrane. However, reduction of the silver cations can realize significant stabilization of silver, decrease of silver release rate and prolonging the antimicrobial efficacy against the growth of the bacteria on the surface of the membrane. Despite the bacterial growth in the suspension, all coatings showed strong inhibition of cell attachment and growth on the surface, which demonstrates that the surface coating can perform better compares to disinfection pretreatment of feed water. The surface characterization was done through SEM. Throughout the evaluation, PVA/Ag(0)-zeolite coating does exhibit slower and more stable result in silver release due to its polyvinyl alcohol (PVA) coating hindering the diffusion of silver. Nonetheless, it has also been noted that polyvinyl alcohol (PVA) coatings greatly elevates water transport resistance of membrane with displaying the permeability value of less than 1E-11 m/s-Pa for all three membranes (PVA, PVA-L and PVA-H), while polydopamine (PDA) coating only slightly affecting the water permeability. Therefore, this article is suggesting future studies to head on finding a new technology of polydopamine (PDA) coating with modification such as polymer coating, silver-zeolite nanoparticles, to both have good control of silver release rate while sustaining the water permeability of the membrane. Opening a potential of applying polymer/silver-zeolite nanocomposite coatings to the surface of membranes for their long lasting and regenerable antibiofouling properties, the article also did highlight that implementing slow-release mechanism in high silver-zeolite loading samples are essential to fully utilize the loaded silver.
2.4. Modified commercial TFC membrane
Rahaman et al., introduces the method of using polyelectrolyte layer-by-layer (LBL) self-assembly to fabricate the surface coatings on the membranes functionalized with silver nanoparticles and antifouling polymer brushes[22]. This new type of coating was prepared with polyelectrolyte layer-by-layer (LBL) films including polyacrylic acid (PAA) and polyethylene imine (PEI). Here, polyethylene imine (PEI) could be either a pristine type or Ag-PEI which is silver nanoparticles coated with PEI. These novel coatings then were functionalized by adding polymer brushes via using hydrophilic polysulfobetaine or low surface energy polydimethylsiloxane (PDMS). The elemental composition of membrane surface was confirmed by XPS and SEM. Layer-by-layer (LBL) films and sulfobetaine polymer brushes significantly elevated the hydrophilicity of the surface, whereas low surface energy polydimethylsiloxane (PDMS) caused a decrease in the membrane surface energy. The membranes with modified surfaces displayed strong and permanent reduction of bacteria. In microbial adhesion tests with E. coli, only 4 to 16% of normalized cell adhesion was observed in the case of modified membranes. Surfaces modified with adding silver nanoparticles showed great antimicrobial activity and especially membranes coated with layer-by-layer (LBL) films of polyacrylic acid (PAA) and Ag-PEI presented more than 95% inactivation of bacterial cells on the surface within 1 hours of contact time. Only negligible changes of insignificantly slight decrease and increase were observed about pure water permeability and salt rejection respectively. This new type of surface coatings introduced showed meaningful results in both antifouling and antimicrobial aspects, lighting the potential of being applied to the fouling of reverse osmosis (RO) membranes.
Liu et al. reported a facile approach of simultaneously generate in situ silver nanoparticles on both surfaces of thin film composite forward osmosis (TFC FO) membranes was introduced for the first time[23]. The article has demonstrated that silver nanoparticles could form on both sides upon the polydopamine (PDA) film synthesis. Scanning electron microscopy (SEM) was used to observe the formation and distribution of silver nanoparticles on the surface of the membranes and the cross-sectional surfaces. This was done in this study by formerly going through the polydopamine (PDA) coating on both surfaces of membrane with the Mussel-inspired dopamine chemistry then a simple dip coating in silver nitrate aqueous solution to generate silver nanoparticles. Results from SEM micrographs showed that this method opens the rechargeability for multiple times if depleted. The silver nanoparticles imparted the forward osmosis (FO) membrane showed an outstanding antibacterial property, since it displayed the decrease rate of 95.6 ± 2.4% and 4 to 5 log CFU reduction of the pristine membrane for E. coli and S. aureus respectively, while it was only 4.5 ± 3.6% and 37.5 ± 6.3% in the case of polydopamine (PDA) coating. This excellent efficacy of killing bacteria was sustained even after multiple cycles. The number of exposed E. coli was reduced by 94.4 ± 2.3% and 91.8 ± 4.3% for two and three cycles each. Deposited silver nanoparticles have showed sustainable antibacterial ability against both gram-negative and gram-positive bacteria and adjustable effect on membrane performances, mainly on the water flux and reverse salt flux in the forward osmosis (FO) test mode. These suggest a possibility of realizing a long-term prevention of membrane biofouling. Moreover, considering that polydopamine (PDA) coating can be fabricated on various substrates covering from metals and plastics to other materials, it is expected to generate silver nanoparticles surface-independently once the coating is done. This can bring a possibility of a large-scale membrane antibiofouling for various filtration membranes made with diverse surface materials (Figs. 5, 6). Zhu et al. immobilized silver on the surface of chitosan membrane[24]. The fabrication process was first started with the step where chitosan base membrane (CS) was immobilized with ionic silver to become CS_Ag+ (chitosan membrane with silver cation), which will then be followed by the surface treatment using reduced silver to create CS_ Ago (Chitosan with metallic silver). The interaction between silver and chitosan base membrane and the oxidation states of immobilized silver on both CS_Ag+ and CS_Ago were confirmed via X-ray photoelectron spectroscopy (XPS). The morphological structures were also characterized by Confocal laser scanning microscopy (CLSM) and SEM. Through a leaching evaluation test, it dem- onstrated not only that silver was successfully immobilized onto the membranes through the coordination on the surface between silver ions and the amino groups (nitrogen atoms) but also the silver on CS_Ago showed better stability of having less than 1% of leached out silver with a lower oxidation state than CS_Ag+ that kept 73% of the immobilized silver remained stable. On the other hand, through the antibacterial and antibiofouling test for pure chitosan base membrane (CS), CS_Ag+ and CS_Ago, both modified membranes displayed a lot stronger performance than the pure membrane, while between those two, CS_Ag+ slightly outshined the other in prevention against bacteria. In the long time (up to 10 days) immersed suspension antibiofouling experiments, however, between two modified types, CS_Ago showed more stable surface and better antibiofouling performance, even though both types were almost equally great for first 24 hours. This study was the first to assess the relative antibiofouling performance by immobilized silver of ionic and reduced states on a surface of membranes.
2.5. Non silver particles embedded nanocomposite membrane
2.5.1. Graphene oxide
Chae et al. reported chemically exfoliated graphene oxide (GO) embedded in the PA (polyamide) layer to enhance the water permeability, anti-biofouling property and chlorine resistance without the loss in salt rejection[ 25]. After going through the fractionation of size control, graphene oxide (GO) was dispersed in an aqueous solution of m-phenylenediamine (MPD) which was followed by the interfacial polymerization. The structure, surface and presence of fractionated graphene oxide (GO) and the results were all characterized and examined via scanning probe microscope (SPM), FE-SEM, TEM, XPS and Raman spectrometer. The fabricated membrane of GO-embedded TFC (GO-TFC) membrane displayed 80% and 98% better performance respectively in the water permeability and anti-biofouling property. These enhancements that GO-embedded TFC (GO-TFC) membrane outshined pure TFC membrane were dedicated by not only the increase of hydrophilicity and negative surface charge zeta potential but also the decrease of surface roughness and thickness of the polyamide (PA) layer, which were all possible because of the existence of graphene oxide (GO). While retaining all these plus effects, it successfully managed to maintain the high salt rejection of 48,000 ppm h chlorination. The improvement brought by the incorporation with graphene oxide (GO) can be adjusted and developed by controlling mainly the size and the concentration of graphene oxide (GO) ejected. In another method a azide-functionalized graphene oxide (AGO) embedded covalently onto commercial reverse osmosis (RO) membrane surface via utilizing the photochemistry of azide[26]. This simple process of surface modification was composed of largely two steps of first, coating the surface of reverse osmosis (RO) membrane with the aqueous dispersion of azide-functionalized graphene oxide (AGO) and second, allowing the UV exposure under ambient conditions. The surface composition and the results were observed and analyzed through X-ray photoelectron spectroscopy (XPS) and scanning electron microscope (SEM). Through this modification, the membrane can be fabricated into hydrophilic, smooth and antibacterial version without suffering huge reduction in water permeability and salt selectivity. These characteristics were shown in results of fabricated membrane - GO-RO membrane - showing 17-fold reduction in biofouling after 24 hours contact with E. coli. The GO-RO membrane also succeeded on displaying about 2 times lesser flux reduction in biofouling of 40% after a 1 week cross-flow test with yellow Bovine Serum Albumin (BSA) compared to that of the pure reverse osmosis (RO) membrane without any modification which was a total of 70% flux reduction. From the result, it was clear to say that grafting graphene oxide (GO) facilitate the enhancement of hydrophilicty and the smoothness of the surface, ultimately mitigating he formation of the long-term gel layer caused by the accumulation of foulant by effectively resisting the attachment of foulant molecules and providing less accessible sites for foulant deposition (Figs. 7, 8).
2.5.2. Copper nanoparticle
Zhu et al. reported two strategies of two-step deposition and co-deposition using mussel-inspired polydopamine (PDA) to firmly embed copper nanoparticles (CuNPs) onto a porous polymeric membrane surface was introduced, aiming to bridge the surface cavities from ultrafiltration (UF) to loose nanofiltration (NF) membranes[27]. The optimization of membrane properties was confirmed via SEM, EDX, AFM, water contact angle, and zeta potential measurements. The results collected showed that both strategies can bring the general enhancement on the performance of surface properties with a homogeneous nanoparticle dispersion, higher smoothness, favorable hydrophilicity and relatively neutral charge. However, comparing two strategies taken, co-deposition exhibits more convenient and time-saving process, with having the most optical modification parameters as 3 hours of PDA and CuNP, that accelerates a higher loading of copper nanoparticles than the two-step strategy. It has also turned out that integrating polyethyleneimine (PEI)-modified copper nanoparticles (CuNPs) with high density of positive charges can realize the fine-tuning of hydrophilicity and compatibility with PDA and strongly neutralizing the negative charge of PDA, which ultimately leads to expediting an excellent salt permeation of 82% and 98% NaCl. Moreover, superior nanofiltration performance was proved with the fact that the fabricated membranes displayed extremely high rejection of three types of textile dyes (600-800 DA, greater than 99%). Functionalized membranes also showed 93.7% reduction of E. coli bacteria. Selecting copper as an antimicrobial agent is relatively inexpensive and more importantly, the facile co-deposition strategy can offer possibilities of strong bindings of other functional nanoparticles such as silver, zinc oxide and graphene oxides that will bring better antimicrobial properties onto the membrane surface. In conclusion, this research is highlighting the effectiveness of a simple co-deposition strategy to assemble multifunctional coating onto an ultrafiltration (UF) and opens a wide potential for the fractionation of dye/salt mixtures
2.5.3. Zinc oxide
Jaydaneh et al., introduces the fabrication of a nanocomposite membrane with a positive surface charge through the modification using zinc oxide (ZnO) nanoparticles on a matrix of a polysulfone ultrafiltration membrane to enhance the filtration of biological macromolecules[ 28]. This research uses the phase inversion method to synthesize this new type of nanoparticles and investigated them in the filtration of BSA protein solution. Having the biofilm structure developed onto the surface, the membrane can experience the increased membrane flux, elevated rejection of protein and reinforced antibiofouling properties of the membrane. The characterization and the observation for determination of membranes’ properties were done via AFM, SEM, thermo gravimetric analysis (TGA), mechanical resistance analysis, dynamic light scattering (DLS), and FTIR. Here, at the pH levels above the isoelectric point of 4.7, the electrostatic absorption between the membrane and the protein grows between the positive and negative surface charge of synthesized membrane and of protein. Higher weight percentage of nanoparticles evokes greater level of surface charge and zeta potential, which leads to increase protein absorption. The collected data from AFM and FTIR also indicated that at the higher pH levels, that the biofilm formed has increased the roughness and the extent of biological macromolecules grafted on the surface of the membrane is greater. Therefore, by adjusting pH levels of the protein solution used and testing through various engineering conditions of nanocomposite fabrication, it is turned out that the best antifouling performance can be realized with the nanocomposite that contains 0.5 wt% of nanoparticles and at the pH level of 8.9. Antibacterial tests done with BSA also proved the antibacterial properties of the nanocomposites modified with zinc oxide (ZnO) nanoparticles, since even though the extent of activated sludge biofilm developed on the membrane surface increases with higher pH, it can bring greater flux by having greater porosity.
2.5.4. Santa Barbara Amorphous-15 (SBA-15)
Diez et al., suggested the method of incorporating mesostructured Santa Barbara amorphous-15 (SBA-15) silica particles that are functionalized with silver, copper and amine moieties. There were 3.6 wt% of the doping particles in the final membranes fabricated which was added via being included the casting solution[ 29]. For silica particles, a uniform dispersion of metals inside the mesoporous structure was found, while in case of the membranes doped with the hydrophilic fillers, a reduction of skin layer, increasing pore interconnectivity and large number of pores were shown. The surfaces of hybrid membranes were indicated as slightly less hydrophobic than the others. These were confirmed via the characterization of membranes and particles which were done through small angle X-ray scattering (SAXS), TEM, XRD and cross-sectional SEM. The performance of fabricated membrane showed improvement with the considerable enhancement of water permeation of more than 30% comparing to the pure membranes without degrading the membrane selectivity. It also displayed better ability of organic antifouling but not offsetting the membrane rejection properties. Metal-loaded silica particles included endue the great antibacterial activity to the preparing membranes, allowing fouling reduction up to 29% during protein filtration. Among various types of them, silver-loaded composites had high antimicrobial activity by showing a complete removal of bacterial colonies both on membrane surface of less than 1 CFU/ and in the liquid culture which was exposed or contacted to them with the value of less than 10 CFU/ mL. Copper-loaded materials also showed significant antimicrobial activity as silver-loaded composites, while the effect was lower than that of silver-loaded. The metal releasing ratio are controlled by metal speciation and insists a 0.1~0.6% of the total metal content of membranes. This antimicrobial action can be useful to the release of metals that was diffused from the loaded mesoporous silica and polyether sulfone (PES) matrix to the bulk.
3. Conclusions
Sharing one goal rooted on grappling with the most optimal solution for the problem of biofouling, lots of applications of grafting creative and simple perspectives were introduced and addressed in this article. For the attempts using silver nanoparticles, partial hydrolyzing the polyacrylonitrile membrane or polydopamine film synthesis showed stabilized and outstanding antibacterial resistance. Some cases in the category of using silver nanoparticles focuses on displaying great stability and recovery ratio on the water flux after time. Cases utilizing non silver particles also has its primary purpose on imparting the antibacterial properties to the membrane with stability and longevity. While the accomplishments do highlight the bright future on the potential of these enhanced membranes, universal challenges left to be solved which can be narrowed down into two. One is concreting the ability to be applied not only to the laboratory scale but also and more to the actual dynamic water process surroundings. The other is imparting the antibacterial properties that last long enough to be meaningful and not hinders the filtration performance, the original function of the membrane. Considering the degree of dependence of this global issue of water crisis on producing these membranes, there should and will be continuous researches in integrated multiple fields.
