Regular ArticleNOM fouling resistance in response to electric field during electro-ultrafiltration: Significance of molecular polarity and weight
Graphical abstract
Introduction
Membrane fouling greatly limits the application of membrane technology in water treatment. Membrane pore blocking by foulants seriously reduces water flux and increases operating costs. Thus, anti-fouling is a key process for effective membrane separation. The application of an electric field (EF) is an effective approach for resisting membrane fouling [1] as electrophoresis and electrostatic repulsion induced by an EF can effectively inhibit foulant deposition on a membrane surface. Furthermore, coupling membrane filtration with EF application can significantly mitigate organic [2], [3], [4] and bacterial fouling [5], [6], and enhance the separation of particles [2], [7], [8], [9], natural organic matter (NOM) [7], [10], [11], [12], dyes [13], [14], and salt ions [15], [16].
NOM is a well-known category of foulant in the ultrafiltration (UF) of natural water [17], [18]. It can be divided into different components based on molecular polarity, including into hydrophobic and hydrophilic substances [19]. Prior research has indicated that hydrophilic substances are the most important factors in membrane fouling [20], [21]. Several studies have concluded that higher molecular weight (MW) hydrophobic compounds are the main factors causing membrane fouling [22]. Thus, the extent of fouling depends on membrane material and NOM characteristics. Previous reports indicated that electro-microfiltration of NOM produced higher water flux and organic removal compared to traditional microfiltration without applied voltage due to the electro-repulsion effect under an EF [11], [23]. The fouling resistance of electro-ultrafiltration with polyethersulfone membranes is more pronounced for NOM with hydrophobic substances and low MW.[12], [24] In addition, higher flux and lower protein transmission were observed in electro-ultrafiltration process, compared to conventional UF process [25], [26].
NOM such as humic acid (HA) generally presents a negative charge in water due to the deprotonation of carboxyl, hydroxyl, and amine groups in molecules. These functional groups have larger electric dipole moments and are prone to polarization under an EF [27], resulting in greater charges concentrated on both ends of the HA molecules along the EF direction. This could facilitate the connection of HA molecules to each other and thus their attachment to membrane surfaces. Our recent work demonstrated that the cake layer formed in an electrochemical UF membrane reactor became hydrophilic and porous due to the molecular polarization of HA, leading to higher water flux in electro-ultrafiltration than in traditional UF [28], [29].
Although membrane fouling by NOM in electro-ultrafiltration has been studied previously, no study examined the polarization of NOM molecules during electro-ultrafiltration and the corresponding influence on fouling resistance for polyvinylidene fluoride (PVDF) membranes. Moreover, the NOM molecular with various polarities and MW would present different polarization extent under EF, which could determine the anti-fouling performance during the NOM electro-ultrafiltration. In the present study, membrane fouling from NOM with different molecular polarity and MW was respectively examined during electro-ultrafiltration with a PVDF membrane. We focused on NOM polarization in response to EF strength, and emphasized cake layer formation and structure during the electro-ultrafiltration of NOM. We also investigated NOM fouling resistance with an EF under various pH conditions and ionic strengths. This study will provide new insight for the future development and application of electro-ultrafiltration.
Section snippets
Chemicals and synthetic feed water
Humic acid (Sigma-Aldrich, USA) stock solution was prepared in deionized water (DI, Millipore Milli-Q, USA) at a concentration of 815 ±12 mg/L after filtration through a mixed cellulose ester (MCE) membrane (0.45 μm), and then stored at 4 °C in the dark until use. Flat sheet UF membranes (PVDF) with MW cut-offs of 5, 10, and 30 kDa were applied to separate the HA solution into three parts using a Millipore stirred UF cell (Amicon 8400, USA). The three parts had MWs of 5–10 kDa, 10–30 kDa, and
electro-ultrafiltration of NOM with different polarity
The MW of HA, FA and HI were 15.72 kDa, 14.31 kDa and 10.69 kDa as measured by GPC. Generally, the polarity of NOM components varies in the order HI > FA > HA [32], Furthermore, HI is more hydrophilic than HA and FA because HI molecules possess more polarized functional groups, such as carboxyl, hydroxyl, and amine groups [33], which have higher electric dipole moments and are more readily polarized under an EF. Here, membrane fouling from HA, FA, and HI was investigated in the presence of an
Conclusions
Applying a voltage in UF was an effective method to mitigate membrane fouling. Membrane flux was significantly enhanced by the application of an EF and was more pronounced for the electro-ultrafiltration of NOM with higher hydrophilicity and MW. The membrane flux of HI was enhanced by up to 20% after EF application, whereas the increase in flux for HA electro-ultrafiltration was 2% at maximum. With increasing EF strength in the electro-ultrafiltration of HA, the membrane flux increased at first
Acknowledgments
he authors are grateful for financial support from the National Natural Science Foundation of China (51678556 and 51438011) and the Major Science and Technology Program for Water Pollution Control and Treatment (2015ZX07402003-3).
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2022, ChemosphereCitation Excerpt :First, the electrostatic repulsion force between the membrane surface and organic particles was not sufficient to resist the drag force of the filtrate flow (Altmann and Ripperger, 1997), causing the organic compounds to still adhere to the membrane surface. Secondly, the polarized organic particles in the electric field collided with each other and aggregated to form large particles, which increased the proportion of large particles (shown in Fig. 2(d)) (Hu et al., 2019). However, the affinity of organic matter to the membrane has not changed.