Surface modification of PA layer of TFC membranes: Does it effective for performance Improvement?

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Abstract

Separation process based on polyamide (PA) thin film composite (TFC) membrane is the dominant desalination technology in the 21st century to produce potable water to meet the growing global freshwater demand. Despite offering good filtration performance and exhibiting stable chemical and mechanical properties, TFC membrane still encounters other technical challenges such as surface fouling and trade-off feature between water permeability and solute selectivity which prompt continuous research efforts. There has been growing interest in recent years among researchers to alter the surface properties of PA layer via various modification methods to overcome the drawbacks of typical selective layer. But the question that remains to be answered – Does the modified PA layer effective for performance improvement? It is the aim of this article to provide a comprehensive review on the modification techniques (i.e., surface coating, surface grafting, plasma treatment and layer-by-layer assembly) and the modifying materials (both organic and inorganic materials) that have been commonly employed to establish a functionalized layer atop PA layer to improve its characteristics for desalination process. We hope this review that covers the relevant articles published over the past 10 years could provide insights to scientists in modifying selective layer of TFC membranes for better performance in water treatment processes.

Introduction

The rapid population growth and industrialization coupled with the impacts due to climate change have exacerbated water scarcity to human. According to the World Health Organization (WHO), half of the global nations will live in water-stressed zone and at least 2 billion people will consume water sources contaminated with faeces in 2025 [1]. Based on the data from the World Resources Institute, the Middle East countries such as United Arab Emirates, Israel, Saudi Arabia and Palestine are expected to experience high level of water stress by 2040 [2]. In order to solve the high demand of fresh water for domestic and industrial utilization, brackish water/seawater desalination as well as wastewater reclamation could be the best strategies to produce sufficient water.

Fig. 1 shows that the total desalination plants and desalination capacity were significantly increased over the last two decades [3]. Of the various desalination technologies, membrane technology based on reverse osmosis (RO) accounted for more than two-thirds of the total desalination capacity in 2019. Its simplicity during operation, easily to scale up as well as relatively low energy consumption compared to the thermal-based desalination such as multi-stage flash (MSF) and multi-effect distillation (MED) make it a main choice of technology [4], [5]. Currently, membrane technologies are growing significantly in countries such as Spain, Australia, China, USA, India and Israel [3], [6]. An analyst report indicated that the global market for major components of RO water treatment systems was US$11.7 billion in 2020 and is expected to be sustained over the next 5 years with a compound annual growth rate (CAGR) of ~10% [7].

Thin film composite (TFC) membrane is the dominant technology used in the industrial RO and nanofiltration (NF) processes due to its promising water permeability and salt removal rate coupled with relatively good chemical and mechanical stabilities. Despite the fact that the TFC membrane could produce fresh water without experiencing major technical limitations, this membrane is still prone to different types of surface fouling and is highly vulnerable to free chlorine attack, even at extremely low concentration of chlorine (i.e., parts per billion, ppb). Membrane fouling is one major constraint that causes a reduction in water flux and selectivity as well as diminishes the membrane durability during filtration operation. The presence of natural organic matters (NOMs), e.g., proteins, amino acids, fulvic acid and humic acid in seawater can adsorb on the membrane surface, forming cake or gel layer [8], [9]. Although the fouled membranes can be cleaned by specialized chemicals to reinstate its performance, the use of harsh chemicals (e.g., ethers and concentrated acid) often creates serious problems such as membrane (material) degradation and chemical waste disposal [10].

Besides organic fouling, membrane scaling and salt precipitation are also among the fouling issues of seawater desalination. Membrane scaling is triggered by high concentrations of salts, e.g., calcium carbonate (CaCO3), barium sulfate (BaSO4), silica (SiO2) and calcium sulphate (CaSO4). The deposition of insoluble salt causes severe flux decline and limits the efficiency and water recovery of membrane desalination process [11]. Current strategy of scaling control in industrial settings is highly dependent on the use of antiscalants (e.g., sodium hexametaphosphate and 1-diphosphonic acid) to inhibit scale precipitation formation by interrupting crystallization stages [10]. Musale et al. [12] from Nalco Company, Naperville demonstrated that CaCO3 scaling can be mitigated by adding 0.01–30 ppm of effective copolymer antiscalant comprised of acrylic acid-2-methylpropyl sulfonic acid (AA-AMPS) in the feed stream of RO membrane process. Results showed that the RO membrane with antiscalant treatment was able to achieve higher permeate flux (~0.039 gallons per minute, gpm) and salt rejection (~81.8%) compared to the membrane without having antiscalant treatment (~0.036 gpm, ~80.2%) after 24-h operation. However, the utilization of antiscalants in membrane desalination process is associated with higher operating cost and might promote biological and organic fouling [13], [14]. This, as a result, requires the plant operators to carry out chemical cleaning process to alleviate the negative impacts caused by different types of fouling [15]. Typically, to remove organic and inorganic fouling of the RO, acid and alkali solutions are used in a RO plant. Maqsood and Stephen [16] from Genesys International Ltd. utilized 2 wt% sodium hydroxide (NaOH) aqueous solution to clean the biofouled membrane in a RO plant for up to 2 h at 15 bar and reported that the permeate flux of fouled membrane could be increased from 31.4 to 41.3 L/m2.h without affecting the sodium chloride (NaCl) rejection (~98.4%). Separately, Fremont et al. [17] reported that the water flux of fouled membrane could be effectively enhanced from 53.8 to 310.5 gallons/ft.day after 38% HCl aqueous solution was used as cleaning agent. Alkaline cleaning is typically used for the removal of organic foulants while acid washing is used for the removal of inorganic fouling/scaling.

Many attempts have been carried out over the years to improve the surface characteristics of polyamide (PA) selective layer of TFC membranes to address the fouling issue [18], [19]. One of the strategies that has been widely evaluated in the recent years is to modify PA layer via incorporation of inorganic nanomaterials during interfacial polymerization (IP) process. However, the embedment of nanomaterials with perfect distribution within the thin selective layer is quite challenging. Reports always related the surface defects of PA layer to the poor dispersity/agglomeration of nanomaterials which led to reduced salt rejection, although membrane water permeability was ameliorated [20], [21], [22]. For instance, Huang et al. [23] reported that the NaCl rejection of TFC membrane was significantly dropped from 91.4% to 87.5% by increasing the loading of NaA zeolite from zero to 0.2 w/v% (dispersed in aqueous solution) in PA layer. The adverse impact on salt reduction is likely due to the aggregated pores or non-selective voids formed within the PA matrix upon the nanoparticles incorporation. These surface defects negatively affected PA integrity and caused the salt rejection to decrease, even though the membrane water flux was raised by about 30%. Emadzadeh et al. [24], on the other hand, found that the incorporation of 0.1 w/v% titania nanotubes (TNTs) in the PA layer could have adverse impacts on NaCl rejection, reducing it from 94.05% (pristine PA) to 85.9%. The researchers attributed the phenomenon to the severe TNTs aggregation that resulted in the formation of microvoids within polymer matrix.

To address the issues associated with the bulk modification of PA layer during IP process, surface modification on the existing PA layer is more practical as it allows desired functionality to be imparted without interrupting the intrinsic PA characteristics [25], particularly high rejection capability against dissolved ions. Furthermore, the presence of additional coating layer could act as a sacrificial/protective layer to the underlying PA layer for enhanced resistances against fouling and chlorine attack [26], [27]. Commercially, adhesive polyvinyl alcohol (PVA) is coated on the surface of TFC membranes to further improve membrane hydrophilicity and antifouling property [28]. Research findings also indicated that the presence of PVA coating layer on the membrane surface could offer good fouling and chlorine resistance [29], [30].

Although there is a significant amount of information regarding the positive impacts of surface modification of PA layer on the membrane performance reported in the scientific literature, the question yet to be answered is: Why are these modification techniques not being employed for the development of commercial TFC membranes? There are cases in which the researchers reported significant improvement in the membrane antifouling properties, but the membrane water flux was adversely affected [31], [32]. Some reports showed that the in-house synthesized TFC membranes could achieve much higher water flux than those of commercial membranes, but their rejection rate against NaCl was compromised with values as low as 93.5%–97.3% were reported [33], [34]. The decline in the salt rejection of the TFC RO membrane is a serious issue as the commercial standard for the seawater desalination is at least 99.8% NaCl rejection [35].

In the recent literature, growing interests have been focused on the plasma surface modification for TFC membrane due to its fast reaction time and solvent free process (eco-friendly) [36]. Most importantly, plasma technique offers advanced platform by functionalizing the membrane surface without compromising the integrity of membrane selective layer [37]. Surface coating meanwhile is another widely used process to enhance membrane performance and antifouling properties owing to its simplicity in the process that does not require complicated experimental setup [38]. Other two methods that are commonly adopted by researchers to alter the surface characteristics of TFC membrane are surface grafting [39] and layer-by-layer (LbL) assembly [40].

The aim of this article is to review the effectiveness of several main surface modification techniques that have been commonly employed by researchers to establish a functionalized layer atop PA layer to improve its characteristics for desalination process. Depending on the technique and the materials (either organic or inorganic materials) used, the TFC membranes with different properties could be produced. Emphasis will be placed on the importance of developing TFC membranes with reduced fouling propensity and chlorine sensitivity without significantly compromising existing filtration performance, in particular water permeability. At last, we conduct an in-depth analysis on the surface-modified TFC membranes developed at laboratory to examine if they are indeed effective to achieve better desalination performance.

Section snippets

Surface modification methods of TFC membranes

Membrane separation and fouling characteristics are commonly correlated with the PA surface properties such as pore size, surface roughness, hydrophilicity and surface charge [41]. Hence, modification of the PA layer of TFC membranes is an effective and feasible solution to achieve desired membrane performance [42]. There is a great deal of research devoted to activities mainly focusing on the TFC membrane surface modification, aiming to improve antifouling resistance, in addition to permeate

Analysis on the surface modification method: Does it effective for TFC membrane performance improvement?

Over the past decade, a large number of studies have been published that aim to improve top surface of TFC membranes via modification techniques such as surface coating, grafting, plasma treatment and LbL assembly. Fig. 13(a) presents the breakdown of the research articles contributed by each technique for the development of TFC membranes for the period between 2010 and 2020. As can be seen, grafting and surface coating are more popular to be employed, contributing 38% and 37%, respectively to

Flux/rejection trade-off effect

Fig. 14 shows the relationship between the salt rejection and the water permeability of different types of surface-modified TFC membranes. Compared to the commercial TFC NF and RO membranes, we can see that only a handful of modified membranes were found to be able to overcome the water flux/rejection trade-off effect of commercial membranes. Many relevant research studies reported reduced water flux of the membranes upon modification. This could be attributed to the formation of relatively

Conclusions and future perspective

For the past three decades, the TFC membrane made of interfacial polymerization technique is the most dominating membrane type for desalination application. Nevertheless, this kind of membrane is not without shortcomings. Its surface fouling caused by various foulants always remains as the main problem during operation. Besides affecting the stability of water productivity, the existence of fouling also adversely reduces salt rejection and membrane lifespan as a result of frequent chemical

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgement

The authors acknowledge the Ministry of Higher Education Malaysia for providing financial support under the AMTEC-HICoE Grant Scheme Phase II (R.J090301.7851.4J432).

References (179)

  • S. Alzahrani et al.

    J. Water Process Eng.

    (2014)
  • E. Jones et al.

    Sci. Total Environ.

    (2019)
  • W. Guo et al.

    J. Li Bioresour. Technol.

    (2012)
  • A. Matin et al.

    Desalination

    (2019)
  • A. Sweity et al.

    M. Herzberg J. Memb. Sci.

    (2015)
  • M. Turek et al.

    Desalination

    (2017)
  • J. Yin et al.

    B. Deng J. Memb. Sci.

    (2012)
  • H. Dong et al.

    Ho J. Memb. Sci.

    (2015)
  • D. Emadzadeh et al.

    Desalination

    (2015)
  • M. Liu et al.

    Desalination

    (2015)
  • Y.N. Kwon et al.

    T. Tak J. Memb. Sci.

    (2012)
  • H.Z. Shafi et al.

    Desalination

    (2015)
  • X. Wei et al.

    S. Wang J. Memb. Sci.

    (2010)
  • S. Davari et al.

    M. Abdollahi J. Memb. Sci.

    (2018)
  • W.-J. Lau et al.

    J. Ind. Eng. Chem.

    (2019)
  • R. Reis et al.

    J. Memb. Sci.

    (2017)
  • M. Asadollahi et al.

    Desalination

    (2017)
  • G.-R. Xu et al.

    J. Memb. Sci.

    (2015)
  • J. Wang et al.

    S. Wang J. Memb. Sci.

    (2015)
  • R. Ma et al.

    Desalination

    (2016)
  • F. Shao et al.

    Y. Chen J. Memb. Sci.

    (2017)
  • R. Rajakumaran et al.

    Desalination

    (2019)
  • F. Li et al.

    Desalination

    (2014)
  • W. Song et al.

    J. Memb. Sci.

    (2018)
  • S. Azari

    L. Zou J. Memb. Sci.

    (2012)
  • A. Akbari et al.

    J. Ind. Eng. Chem.

    (2015)
  • P. Wang et al.

    B. Cheng J. Memb. Sci.

    (2015)
  • Y. Zhang et al.

    Desalination

    (2017)
  • K. Wang et al.

    Appl. Surf. Sci.

    (2018)
  • J. Sun et al.

    Purif. Technol.

    (2016)
  • D. Emadzadeh et al.

    Chem. Eng. J.

    (2015)
  • H. Zhao et al.

    C. Gao J. Memb. Sci.

    (2014)
  • D. Li et al.

    Prog. Polym. Sci.

    (2016)
  • D. Li et al.

    Prog. Polym. Sci.

    (2016)
  • J.S. Louie et al.

    M. Reinhard J. Memb. Sci.

    (2011)
  • S. Yu et al.

    Sep. Purif. Technol.

    (2013)
  • L. Ni et al.

    Y. Zhang J. Memb. Sci.

    (2014)
  • A. Matin et al.

    Desalination

    (2016)
  • Y. Zhang et al.

    Appl. Surf. Sci.

    (2017)
  • H. Guo et al.

    J. Memb. Sci.

    (2018)
  • Y. Zhang et al.

    Appl. Surf. Sci.

    (2018)
  • H. Choi et al.

    Desalination

    (2012)
  • G. Kang et al.

    Desalination

    (2011)
  • A.E. Childress

    M. Elimelech J. Memb. Sci.

    (1996)
  • Y. Zhou et al.

    Sep. Purif. Technol.

    (2009)
  • J. Glater et al.

    Desalination

    (1994)
  • Y.-N. Kwon et al.

    T. Tak J. Memb. Sci.

    (2012)
  • G.D. Kang et al.

    Q. Yuan J. Memb. Sci.

    (2007)
  • Y. Wang et al.

    S. Wang J. Memb. Sci.

    (2018)
  • T. Cai et al.

    J. Memb. Sci.

    (2016)
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