Thin-film composition membranes are extensively used in seawater desalination plant to solve water shortage problems. The major challenge faced with TFC membranes is significant fouling on the surface, which restricts the overall permeation performance. Many attempts have been made to increase membrane fouling resistance by surface modification. Here we review a highly antifouling TFC polyamide membrane constructed by grafting a zwitterionic polymer brush with controlled architecture via atom-transfer radical-polymerization (ATRP). The selection of ATRP provides an environmentally benign solution for grafting the zwitterionic brush layer from the membrane surface without changing the transport properties. The excellent fouling resistance imparted by the zwitterionic brush layer was also manifested comprehensively from membrane surface characterization, interfacial adhesion force test, adsorption of proteins and bacteria test, and dynamic fouling experiments. Considering future applications, we also discuss the deficiencies of this paper. More detailed explanations are required for zeta potential and optimum layer thickness. The application of a long term test may show the lifetime of zwitterionic brush layer and its usability in practice. Overall, the controlled architecture of the zwitterionic polymer brush via ATRP has the potential for a facile antifouling modification of a wide range of water treatment membranes without compromising intrinsic transport properties.
This article focuses on the study of a highly antifouling thin-film composite (TFC) polyamide membrane by grafting a zwitterionic polymer brush layer with controlled architecture via atom-transfer radical-polymerization (ATRP). The reason why we choose this topic is that membrane-based desalination is widely-used to augment fresh water supply. The detrimental effects of membrane fouling make it necessary to develop efficient antifouling surface membrane.
Water scarcity is one of the most serious global challenges of our time and seawater desalination offers a seemingly unlimited, steady supply of high-quality water, without impairing natural freshwater ecosystems (Elimelech & Phillip, 2011). Thin-film composite (TFC) membranes are the most widely-used technology for desalination. But a major limitation in using membrane-based separation processes is the loss of performance due to membrane fouling (Nady, 2016). To achieve higher efficiencies, antifouling membranes should be developed.
Researchers (Rana & Matsuura, 2010) agree that an increase in hydrophilicity offers better fouling resistance because protein and many other foulants are hydrophobic in nature. Many attempts have been made to increase the membrane surface hydrophilicity by surface modification. The purpose of surface modification of membranes is either to minimize unwanted interactions which reduce the performance (membrane fouling), or to introduce additional interactions for improving the selectivity or creating novel separation function (Nady, 2016). Grafting functionalized polymer with the aid of UV, ozone, or plasmas is considered to be relatively stable since novel coating would not easily be removed after a longer period use (Ma, Rahaman, & Therien-Aubin, 2015). A lot of hydrophilic materials using for grafting today have limitations and may even deteriorate the fouling under specific conditions. Among these, zwitterion-based polymers have drawn attention as antifouling materials due to their high hydrophilicity, long-term durability, and environmental stability.
Summary of Paper
In this paper, the authors produced the target antifouling TFC polyamide membrane via membrane surface modification. And then they used a series of equipment and methods to find the changes in the surface characteristics, as well as using chemical force microscopy to quantify the fouling resistance. Besides, they also applied static adsorption tests and dynamic forward osmosis fouling experiments to demonstrate the fouling resistance of the modified membrane.
The first step of surface modification was the coupling of initiator α-bromoisobutyryl bromide (BiBB) and dopamine hydrochloride to produce BiBB-DA. Then, initiators were immobilized on the TFC membrane surface via polymerization of BiBB-DA to prepare TFC-PDA membrane. At last, zwitterionic polymer brushes were grafted on the membrane surface via ATRP to produce the target TFC-PSBMA membrane, where PSBMA is poly (sulfobetaine methacrylate).
During the membrane characterization experiment, from SEM and AFM images, it could be found that a smooth film was formed on the membrane surface, and surface roughness decreased significantly after grafting of the PSBMA brush layer. As for membrane surface chemistry, ATR-FTIR spectroscopy was utilized. Two additional peaks were observed at 1726 and 1039 cm−1, which represent carbonyl group and sulfonate group so that verified the successful grafting of PSBMA brushes. Next, by measuring contact angle and zeta potential of PSBMA, it was observed that modified membrane has an increased surface hydrophilicity and decreased surface charge, since the contact angle reduced noticeably and the zeta potential became less negative. Based on 4-step method from authors’ previous publication, it was detected that structural parameter (S) remained almost unchanged, water permeability coefficient (A) decreased slightly while salt permeability coefficient (B) increased after the modification of the active layer. Additionally, by measuring adhesion forces, Chemical force microscopy demonstrated that modified membrane has a smaller membrane-foulant interaction forces.
Membrane antifouling properties were then assessed via adsorption tests, where bovine serum albumin (BSA) and Escherichia coli (E. coli) represented respectively proteins and bacteria as common foulants. The results that no fluorescence was shown in the micrograph and nearly 90% CFU was reduced compared to the pristine TFC membrane indicated that TFC- PSBMA membrane has excellent fouling resistance on protein and bacteria. FO fouling experiments, with a mixture of model organic foulants which simulate a more realistic environment, were conducted in a cross-flow test unit to further evaluate the antifouling property of the modified membranes for long-term applications. PSBMA modified membrane has a significantly decrease in the rate of flux decline which demonstrated its excellent organic fouling resistance.
The authors also summarized the antifouling mechanisms of zwitterionic polymer modified membranes. The complete and dense coverage of the polymer brushes on the membrane surface reduce surface roughness so that surface area for membrane-foulant interaction reduces. Besides, the dense film shields surface carboxylic groups, which effectively prevents calcium-ion induced fouling. Increased surface hydrophilicity is due to the strong hydration capacity of zwitterionic polymers so that a dense hydration layer can be formed, which imposes an energetic barrier for foulant adsorption.
- ATRP is an environmentally benign process
Atom transfer radical polymerization (ATRP), which has been applied to develop a variety of well-defined functional polymeric materials, is a controlled and metal complex-catalyzed method (Tsarevsky & Matyjaszewski, 2007). ATRP is generally considered as an environment friendly chemical process for three main reasons: (1) efficient strategies on catalyst removal and potential recycling have been employed, (2) active catalysts which can be used at low concentration have been developed, and (3) ATRP can be successfully conducted in environmentally friendly media, such as water, supercritical carbon dioxide, and ionic liquids (Tsarevsky & Matyjaszewski, 2006). In this paper, authors used copper (II) chloride as catalyst. From previous papers, it is known that copper formed catalyst could be removed by several approaches (Tsarevsky & Matyjaszewski, 2006). Besides, the authors also mentioned that it is more desirable to use benign and aqueous media for membrane surface modification and selected water, a common and non-toxic solvent, as media. Thus, the selection of ATRP as grafting method provides an environmental friendly solution for membrane surface modification process.
- Membrane properties can be controlled by ATRP grafting method
As mentioned in the paper, the narrow polydispersity and relatively slow polymerization enable the control of the thickness of the polymeric surface coating, thereby maintaining the membrane transport properties. ATRP is one of the most powerful and versatile controlled radical polymerization (CRP) processes used for the synthesis of functional copolymers with well-defined architectures, controlled molecular weights, and tunable sequences. It enables precise control over molecular weight, molecular weight distribution, and functionality (Król & Chmielarz, 2014). ATRP has been successfully used to prepare various (co)-polymers with essentially any desired complex macromolecular architecture. ATRP itself is controlled by an equilibrium between propagating radicals and dormant species, predominantly in the form of initiating alkyl halides/macromolecular species (PnX) (Matyjaszewski, 2012).
Narrow block copolymers (controlled) are more useful for coating application rather than polydisperse random copolymers. ATRP could synthesize narrow molecular weight distribution block copolymers based on their ability to form well-defined nanostructures with different morphologies of tunable periodicity or size. And it is possible to design the entire molecular weight distribution by adding initiator stepwise, terminating some chains, using a mixture of mono- and multifunctional initiators, etc (Król & Chmielarz, 2014; Matyjaszewski, 2012). In addition, polymer properties can be modified by adding the functional groups. It is possible to place functionality at the ends of the chains or at a specific site of a macromolecule by using ATRP. And ATRP is tolerant of many functionalities and can also be used to successfully incorporate functionalities after polymerization (Król & Chmielarz, 2014). By using ATRP, the zwitterionic brush layer can be precisely grafted from the membrane surface as the functional groups and increase the antifouling properties.
Thus, by using ATRP “graft from” method, the narrow molecular weight distribution polymers can be synthesized and the functional groups can be placed to the membrane surface precisely. Hence the thickness of the brush layer could be controlled and the membrane transport properties could be maintained.
- Four aspects showing the antifouling properties
This paper shows the antifouling properties of zwitterionic brush layer covered membranes from four aspects: membrane surface characterization, interfacial adhesion force test, adsorption of proteins and bacteria test, and dynamic fouling experiments. And they have their own unique considerations for each experiment design.
(1) During membrane surface characterization, they also measured the surface roughness in addition to contact angle and surface charge. Fouling propensity is lower for membranes with lower surface roughness because of the reduced surface area for membrane-foulant interaction (Vrijenhoek, Hong, & Elimelech, 2001). The reduce of surface roughness after grafting the PSBMA brush layer shows less possibility of foulings. Membranes with larger surface roughness, such as MPD-based thin-film composite (TFC) polyamide membranes, have higher fouling propensity, because of the increased surface area in contact with foulants (Zhang et al., 2016). This was attributed to the enhancement of interactions between colloidal particles with an increase in surface roughness; that is, colloidal particles preferentially accumulate at the valleys of the rough membrane surface. As a result, valleys become blocked and fouling becomes more severe for the rougher membrane surface (Rana & Matsuura, 2010). The grafting of zwitterionic brush layer reduces the surface roughness, hence increase the antifouling properties.
(2) They also conducted the interfacial adhesion force test to show the interactions between foulant and membranes. The adhesion strategies for different types of foulants can vary widely, therefore it is important to combine polymer brushes that contain different functionalities for membrane fouling control (Rahaman et al., 2014). As a result, they use a carboxylated latex particle attached to an AFM cantilever serving as a model organic foulant. Most organic foulants (e.g., natural organic matter, polysaccharides, and proteins) possess carboxylic groups, which aggravate fouling by complex formation with carboxylic groups on polyamide-based TFC membrane surfaces in the presence of calcium ions. Therefore, interaction forces between the membrane surface and the carboxylated particle can provide a quantitative measure of membrane fouling propensity. As a result, the reduction of adhesion forces with much narrow distributions shows the enhance of fouling resistance after grafting the zwitterionic polymers layer.
(3) In addition, they also conducted the proteins and bacteria adsorption test with BSA and E. coli to show the bio-fouling resistance of modified membrane. Protein adsorption on the membrane surfaces provides a conditioning layer for microbial colonization and subsequent biofilm formation. And the attachment of bacteria to a surface leads to subsequent colonization resulting in the formation of a biofilm as well. Biofilm formation on biological implants leads to infection (Banerjee, Pangule, & Kane, 2011). Therefore, it is crucial for antifouling modification of TFC membranes to demonstrate the protein fouling and bacterial adsorption resistance. The test results demonstrate the excellent protein fouling resistance and bacterial adsorption reduction of the PSBMA modified membrane, hence show the resistance of bio-foulings.
(4) To further assess the antifouling property of the modified membranes in a more realistic situation and for long-term applications, dynamic fouling experiments in Forward Osmosis membrane with model organic foulants were conducted. According to the paper, their study is the first to systematically investigate the organic fouling behavior of zwitterionic polymer coated TFC membranes using a mixture of model organic foulants (i.e.,alginate, proteins, and natural organic matter), which simulate realistic fouling environments. After 500 mL of permeate collected, the pristine TFC membrane showed a 25% decline in water flux, while the TFC-PSBMA membrane exhibited a remarkably lower flux decline of 15%. Again, the dynamic fouling experiments clearly demonstrate the excellent antifouling functionality of the zwitterionic polymer brushes.
Deficiency and Suggestions
- Lifetime and Applications
This paper focuses on antifouling properties of TFC membranes, but only the FO membranes were tested. In addition, no reason was explained about why FO membrane was chosen. There are doubts about whether the performances of zwitterionic polymer brush layer on reverse osmosis (RO) and nanofiltration membrane (NF) are similar to the performance on FO membranes. RO and NF are membrane filtration technologies in which pressure is applied to a liquid stream, driving it through a semipermeable membrane in order to remove dissolved solids. Forward osmosis is the process of spontaneous water diffusion across a semipermeable forward osmosis membrane in response to a difference in solute concentrations on either side of the semi-permeable membrane. As no hydraulic pressure is involved, the fouling in FO processes have been found more reversible. And fouling control and membrane cleaning in FO are much more feasible since FO fouling is reversible to simple physical cleaning (Lee, Boo, Elimelech, & Hong, 2010; Wang, Cui, Ge, Tew, & Chung, 2015). In addition, due to the difference in operation conditions (with or without pressure), the grafting methods and supporting layer materials may be varies from different TFC membranes (Lee et al., 2010; Mi et al., 2017). Although the zwitterionic brush layer is the same for all kinds of TFC membranes, it is still reasonable to question that if the antifouling properties will be affected by the membrane operation condition, grafting method and supporting layer properties. To be much clearly for the audience, the reason why FO was chosen should be stated. In addition, a short explanation is also necessary to prove that the success of FO is representative for all kinds of TFC membranes.
In addition, the fouling mechanism may changes with time. At the early stage of fouling, the fouling mechanism is membrane-foulant interaction, and the zwitterionic layer coating membrane shows more reduction in flux decreasing in dynamic fouling experiments. But, as time goes by, once foulants adsorb to the membrane surface, the fouling mechanism changes to foulant-foulant interaction. And the rates of flux decline for both pristine and modified membranes were eventually comparable. This paper shows that the zwitterionic brush layer does have fouling resistance at the early stage of fouling but no conclusion made for long term performance. Another question comes out: how long does the zwitterionic brush layer works? Since this topic is only analysis based and no real application is available now. Thus, not enough information is available on long term performance of zwitterionic brush layer. Our suggestion is that long term test should be conducted to show how long the brush layer could exist on the membrane surface and how long the antifouling properties still be outstanding comparing to the non-modified membranes.
- Zeta potential
In addition to hydrophilicity and surface roughness, surface charge also influence membrane fouling, so that determination of the zeta potential of the membrane surface is important for membrane fouling research (Childress & Elimelech, 1996; Montemor, 2015). Generally, when zeta potential is less negative, the electrostatic repulsive force between the charged membrane surface and foulant decrease, which cause the increase in membrane fouling (Cai et al., 2016). In this paper, the zeta potential of modified membrane became less negative but the fouling resistance of the membrane increase, which is inconsistent with the general thinking. Authors showed that the reason of less negative in zeta potential was due to the coverage of net-zero charged zwitterionic polymer brushes on the membrane surface shielding the functional groups on the membrane, and this reason also can explain why fouling resistance increase. However, authors did not provide a direct explanation about the inconsistency. Our suggestion is to give a more direct explanation. For example, it should state that when polymer layer shields surface carboxylic groups, the enhancement of membrane fouling resistance due to preventing calcium-ion induced foulings is more significant than the reduction of resistance due to the decrease in electrostatic repulsive force.
- Optimum Thickness
While improving antifouling properties, the coating and grafting add another layer on the pristine membrane and thus affect the membrane transport property, such as water permeability and salt permeability (Ni, Meng, Geise, Zhang, & Zhou, 2015; Shahkaramipour, Tran, Ramanan, & Lin, 2017). In this paper, authors gave the results of the changes on these two properties and then mentioned that coating thickness could be further optimized via controlling the growth of PSBMA layer to least affect membrane transport properties while keep antifouling performance at highly effective level. Besides, authors also mentioned doing optimization to enhance the antifouling property while maintain the transport properties. However, we do not know how the thickness will affect the transport properties and the antifouling property. For example, whether the thicker the coating layer is, the more the fouling resistance could be improved while less the water permeability could be obtained. If so, is there a trade-off between increase in fouling resistance and decrease in transport properties? Besides, author just mentioned about the brush thickness optimization, but we do not know whether the grafting density(Ran, Wu, Zhang, & Xu, 2014) and distribution mentioned by other papers need to be taken into consideration or the thickness could be regarded as the most important factor. Therefore, we suggest that authors should provide a simple explanation about the degree of importance and the effect of thickness on transport properties and antifouling property, and might add an optimization experiment to show the optimal thickness.
In conclusion, this paper is well-written and well-organized with a logical order that is membrane modification, characteristic testing, anti-fouling performance verification and mechanism description. Authors did good on selection of membrane surface modification method as ATRP is a controlled and environmentally friendly grafting process, which also can maintain the membrane transport properties by controlling the thickness of the layer. Additionally, the article is very convincing in successfully developing the desired anti-fouling membrane since anti-fouling properties of modified membrane are demonstrated from four aspects. Nevertheless, this article also reveals some small problems. Considering future application, a long term test should be conducted to show the lifetime of the brush layer. And more researches could be conducted on the anti-fouling performance of different TFC membranes. We also suggest that more straightforward description is needed to explain the inconsistent result on fouling resistance with the change of zeta potential. Besides, in order to have more clear understanding on thickness, more explanation about thickness is desirable and optimum thickness of the layer should be addressed.
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"The terms offer and acceptance." freeessays.club, 17 May 2016
"The terms offer and acceptance." freeessays.club, 17 May 2016