Pre-deposition layers for alleviating ultrafiltration membrane fouling by organic matter: Role of hexagonally and cubically ordered mesoporous carbons

Pre-deposition layers for alleviating ultrafiltration membrane fouling by organic matter: Role of hexagonally and cubically ordered mesoporous carbons

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Journal Pre-proofs Pre-deposition layers for alleviating ultrafiltration membrane fouling by organic matter: Role of hexagonally and cubically ordered mesoporous carbons Xiaoxiang Cheng, Weiwei Zhou, Daoji Wu, Congwei Luo, Ruibao Jia, Peijie Li, Lu Zheng, Xuewu Zhu, Heng Liang PII: DOI: Reference:

S1383-5866(19)35015-4 https://doi.org/10.1016/j.seppur.2020.116599 SEPPUR 116599

To appear in:

Separation and Purification Technology

Received Date: Revised Date: Accepted Date:

2 November 2019 19 January 2020 19 January 2020

Please cite this article as: X. Cheng, W. Zhou, D. Wu, C. Luo, R. Jia, P. Li, L. Zheng, X. Zhu, H. Liang, Predeposition layers for alleviating ultrafiltration membrane fouling by organic matter: Role of hexagonally and cubically ordered mesoporous carbons, Separation and Purification Technology (2020), doi: https://doi.org/ 10.1016/j.seppur.2020.116599

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© 2020 Published by Elsevier B.V.

Revised Manuscript for Separation and Purification Technology Date: 2020-01-19

Pre-deposition layers for alleviating ultrafiltration membrane fouling by organic matter: Role of hexagonally and cubically ordered mesoporous carbons Xiaoxiang Chenga,*, Weiwei Zhoua,b, Daoji Wua, Congwei Luo a,*, Ruibao Jiac, Peijie Lia, Lu Zhenga, Xuewu Zhud, Heng Liangd a

School of Municipal and Environmental Engineering, Shandong Jianzhu University, Jinan, 250101, P.R. China b

c

Shandong Urban Construction Vocational College, Jinan, 250103, P. R. China

Shandong (Jinan) Water &Waste Water Monitoring Center, Jinan, 250101, P. R. China d

State Key Laboratory of Urban Water Resource and Environment, School of Environment, Harbin Institute of Technology, Harbin, 150090, P. R. China

*Corresponding author. E-mail address: [email protected] (Xiaoxiang Cheng); [email protected] (Congwei Luo).

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ABSTRACT In this study, ordered mesoporous carbons (OMCs), i.e., CMK-3 and CMK-8 were pre-deposited on ultrafiltration membrane surface to alleviate membrane fouling by organic matter in the water. CMK-3 shows a typical 2D hexagonal symmetrical structure with long straight parallel channels, while CMK-8 exhibits a 3D porous structure with cubic channels. The pre-deposited CMK-3/CMK-8 layers exhibited significantly superior performance for both flux enhancement and fouling mitigation, whereas powdered activated carbon (PAC) showed limited influence. The fouling mechanism was apparently altered by the OMCs pre-deposition layers, and pore blocking always dominated throughout the filtration process without the formation of cake filtration. In comparison with PAC, CMK-3 and CMK-8 were more effective for the rejection of tryptophan-like protein substances, which were the major foulants causing membrane fouling. The effects of pollutant fraction and deposited amount (i.e., 6, 12, 25 and 50 g/m2) were systematically explored, and the results indicated that this technology was more efficient for alleviating the fouling induced by humic acid and bovine serum albumin, whereas ineffective for sodium alginate fouling, and the optimal deposited amount varied with the pollutant fraction. CMK-3 exhibited significantly superior performance than CMK-8, likely due to its long straight parallel channels with lower internal resistance for pollutants adsorption. Both adsorption and size exclusion of the pre-deposited layers contributed to fouling mitigation. These results demonstrate that pre-depositing with OMCs as alternatives to conventional PAC shows promising 2

potential for mitigating membrane fouling in water treatment. Key words: Membrane fouling, organic matter, ordered mesoporous carbons (OMCs), pre-deposition layers, surface water treatment.

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1. Introduction Recently, ultrafiltration (UF) membrane has been increasingly applied for potable water supply and wastewater treatment [1-3]. While effective and beneficial, membrane fouling is still an obstacle limiting its further widespread application [4, 5]. During numerous pollutants, natural organic matter (NOM) was a problematic membrane foulant causing severe fouling in water treatment, thus leading to increased operational costs, as well as decreased water productivity and purification efficiency [6]. Feed water pretreatments with coagulation, adsorption, filtration or oxidation were the most commonly used methods for fouling mitigation [7-10], and have made a solid contribution to the development of UF membrane. However, in many cases, pretreatments exhibited an adverse effect on membrane fouling control. For example, although powdered activated carbon (PAC) was extensively used coupled with UF and demonstrated to be effective for mitigating membrane fouling [11], several studies still indicated that the presence of PAC could exacerbate the fouling condition [12, 13]. The discrepancy may be associated with the operating conditions, feed quality, as well as other influence factors. Therefore, how to efficiently mitigate membrane fouling during drinking water treatment is still a challenge, and needs to be further studied. From the perspective of membrane material, membrane modification is another choice for fouling control [14-17]. The commonly used modification methods include blending, bulk modification, and surface coating [18-20], among which surface coating is more convenient for membrane surface modification. It is a method that a functional 4

layer is directly deposited on the membrane surface as a selective coating layer. This layer could avoid a direct contact between foulants in the feed water and membrane, thus showing a great potential for fouling control. Ajmani et al. [21] utilized carbon nanotubes (CNTs) as coating layers to modify low-pressure membranes with the method of surface coating, and founded that the CNTs layers could reduce the pollutants reaching the underlying membrane, hence improving the antifouling performance during water treatment, especially for larger diameter CNTs. Liu et al. [22] employed carbon nanofiber as a coating layer on UF membrane surface. Due to the effects of both surface adsorption and physical separation, the antifouling performance of the modified membrane was substantially improved during treatment of sodium alginate (NaAlg) and bovine serum albumin (BSA). More recently, mesoporous materials have drawn more and more attention on membrane fouling control [23]. It was noteworthy that traditional PAC is generally a typical microporous adsorbent (<2 nm), which might be inaccessible for high molecular weight (MW) organics [24]. By contrast, mesoporous materials have much larger pore sizes (2-50 nm) [25], and exhibit great potential for the adsorption of high MW organic matter, which is a major component for membrane foulants. Li at al. [23] observed that mesoporous adsorbent resin was significantly superior to PAC for alleviating UF membrane fouling. Among various mesoporous materials, ordered mesoporous carbons (OMCs) have aroused great research interest due to the properties of well-ordered pore structure, high surface areas, tunable pore sizes, high porosity, and good mechanical 5

stability [26, 27]. Ryoo et al. [28] firstly synthesized OMC utilizing MCM-48 (cubic mesoporous silica) as the template and sucrose as the carbon source, which was referred to as CMK-1. Subsequently, OMCs with different structures were synthesized using the similar method. The synthesis of a hexagonal OMC material denoted as CMK-3 was reported by Jun et al. [29] using SBA-15 as the hard template instead of MCM-48. Similarly, a cubic OMC designated as CMK-8 was synthesized using cubic Ia3d silica (KIT-6) as the template [30]. OMCs can be used as efficient adsorbents for NOM in the water, hence alleviating NOM-related membrane fouling. They also have the potential to modify UF membrane as surface coating materials during drinking water treatment. It was noted that significant structural differences existed between hexagonal CMK-3 and cubic CMK-8, which may influence their performance for fouling mitigation. However, as far as we know, OMCs as pre-deposition layers on membrane surface for fouling control has not been reported to date. Therefore, further systematic studies are needed to investigate its potential application during drinking water treatment. In the present study, CMK-3 and CMK-8 were pre-deposited on UF membrane surface as functional layers for fouling mitigation in surface water treatment. The fouling control performance and rejection of organic pollutants were systematically studied using membrane permeate flux, fouling resistance distributions, fouling mechanism analysis, as well as fluorescence excitation-emission matrices (EEMs). The effects of pollutant fraction (i.e., humic acid (HA), BSA and NaAlg) and deposited amount of CMK-3/CMK-8 on the fouling control performance were further explored, 6

and the fouling mitigation mechanisms were preliminary revealed.

2. Materials and methods 2.1 Feed water The feed water was collected from Lake Yingxue, a typical surface water located in Jinan, Shandong Province. To remove particulate matter, the water samples were pre-filtered through 0.45 μm filters prior to experiment, and stored at 4℃ in laboratory for further study. The main characteristics of the feed water are presented as below: ultraviolet absorbance at 254 nm (UV254) = 0.031-0.035 cm-1, dissolved organic carbon (DOC) = 2.08-2.36 mg/L, pH = 7.4-7.6. To simulate various organic pollutant fractions in natural water, synthetic feed solutions were prepared using HA, BSA and NaAlg obtained from Aladdin Chemicals (Shanghai, China), representing humic substances, proteinaceous matter, and polysaccharides, respectively. The DOC concentration of each synthetic solution was 3.0 mg/L. To simulate the ionic strength and pH in natural water sources, NaCl, CaCl2 and NaHCO3 with concentrations of 6.0, 1.0 and 1.0 mmol/L were spiked into the synthetic solutions, and the pH value was controlled at 7.0±0.1 by the addition of dilute HCl or NaOH. 2.2 Preparation of CMK-3/CMK-8 pre-deposition layers CMK-3 with hexagonal structure and cubically ordered CMK-8 were obtained from XFNANO Materials Tech Co., Ltd (Nanjing, China), and the material information is summarized in Table 1. PAC particles (>200 mesh) obtained from Aladdin Chemicals (Shanghai, China) were also employed as a contrast. Polyethersulfone (PES) UF 7

membranes (molecular weight cutoff of 150 kDa) supplied by Microdyn-Nadir (UP150, Germany) were utilized as supports. The OMCs layers were pre-deposited on membrane surfaces via the surface coating method. Specifically, the PES membrane support was washed by immersing in ethanol solution (50%) for 15-30 min, subsequently in ultrapure water for 24 h. Simultaneously, the dispersed suspensions of CMK-3/CMK-8 particles were prepared by sonication for 30 min. Then, the predeposited layers were deposited on the PES support by filtrating the suspensions using a pre-coating device (Fig. S1), which was mainly composed of a membrane filtration cell (Mosu, China) and a nitrogen pressurizer. To investigate the effect of deposited amount on membrane performance, various amounts of CMK-3 and CMK-8 (i.e., 6, 12, 25 and 50 g/m2) were utilized according to preliminary tests and previous relevant studies using carbon nanotube layers [21, 31]. Table 1 2.3 Membrane fouling evaluation 2.3.1 Membrane permeate flux A dead-end membrane filtration system (Fig. S2) under a constant transmembrane pressure was used to evaluate the variation of membrane permeate flux. The experiment was performed in the filtration cell with an effective filtration area of 37.4 cm2 at ambient temperature (25±1°C). The variation of permeate flux was determined through an electronic balance (HZY-B2200, HUAZHI, USA) integrated with a data collection system. The permeate flux (J) was calculated according to Eq. (1): 8

J = V/(A·∆t)

(1)

where ∆t is the collection interval (h), V and A are the filtration volume (L) and area (m2), respectively. 2.3.2 Fouling resistances distribution The fouling resistances were calculated by classical resistance-in-series model [32, 33], as presented in Eq. (2): Rt = Rm + Rr + Rir = TMP/(μ·J)

(2)

where Rt is the total resistance (m-1), Rm is intrinsic resistance of membrane (m-1), Rr and Rir are reversible and irreversible fouling resistances (m-1), respectively, which were divided by hydraulic backwash, rather than chemical cleaning. TMP is the transmembrane pressure (Pa), μ is the dynamic viscosity (Pa·s), and J is the permeate flux (m/s). The detailed calculation methods can be founded in our previous publications [34, 35]. 2.3.3 Filtration model analysis It was noteworthy that the fouling mechanisms may vary with the increase of filtration volume, thus individual fouling models could not well explain the fouling behavior of membrane. To this end, a combined pore blockage-cake filtration model [36, 37] was utilized to better understand the fouling mechanism. The detail analytical methods are presented in Text S1. 2.4 Analytical methods The concentrations of DOC and UV254 were analyzed through a total organic 9

carbon analyzer (TOC-L CPH CN200, Shimadzu, Japan) and an UV/vis spectrophotometer (UV-9000, Metash, China), respectively. The EEM spectra of NOM in the surface water were detected using a fluorescence spectrophotometer (F7000, Hitachi, Japan). The spectra were scanned over emission (Em) wavelengths of 210-550 nm and excitation (Ex) wavelengths of 210-450 nm. The microstructure of OMCs particles and the surface microtopography of membrane were observed using a scanning electron microscope (SEM, JSM-7610F, JEOL, Japan). Prior to SEM observation, the samples were dried in a desiccator (45°C) for 72 h, then fixed on copper sheets and coated with gold using a precision etching coating system (JEC-3000FC, JEOL, Japan). The particle size distribution (PSD) of CMK-3/CMK-8 was detected using a laser diffraction particle size analyzer (Mastersizer 2000, Malvern Instruments, UK).

3. Results and discussion 3.1 Characterization of UF membranes pre-deposited with OMCs layers Characterization of OMCs and the resultant membranes were conducted by SEM, PSD and pure water flux measurement, and the results are illustrated in Fig. 1. As can be observed clearly, these two OMC materials exhibited significant different microstructures. To be specific, CMK-3 shows a typical 2D hexagonal symmetrical structure with long straight parallel channels (Fig. 1(a)), while CMK-8 exhibits a 3D porous structure with cubic channels (Fig. 1(b)). The mesoporous structures could provide channels for pollutants adsorption, which may reduce the pollutants directly 10

contacting with the underlying membrane. It was noteworthy that the structure difference between CMK-3 and CMK-8 may influence the role and performance for fouling control, which will be discussed in the following sections. PSD analysis was carried out to determine the morphologies of OMC particles with the results illustrated in Fig. 1(c). Both CMK-3 and CMK-8 particles exhibited a uniform distribution of particle size with mean diameters of 8.1 and 15.2 μm, respectively, indicating great aqueous dispersibility without significant agglomerates. The discrepancy of PSD was insignificant, and the slightly higher diameter of CMK-8 was mainly attributed to the size of single particle with a cubic structure. Fig. 1(d) shows the pure water flux of the membranes pre-deposited with CMK-3 and CMK-8 layers. The pre-deposition layers decreased the permeate flux of virgin membrane by 6.3-23.2%. It seemed that CMK-3 showed a greater influence on flux decline than CMK-8, and both the flux decreased with the increase of deposited amount. Bai et al. [38] reported similar results when utilizing cellulose nanocrystals and cellulose nanofibers as membrane surface coating materials. This was likely ascribed to the blockage of membrane pores and constriction of water channels resulting from the pre-deposited layers. However, the flux decline was not serious, likely due to the much larger size of CMK-3/CMK-8 than membrane pores. Fig. 1 3.2 Fouling control performance The fouling control performance of pre-deposited CMK-3 and CMK-8 layers were 11

investigated, and compared with conventional PAC under the same deposited amount of 12 g/m2. As shown in Fig. 2(a), a serious flux decline was observed for the virgin membrane, and the final membrane flux was reduced to 25.8% of the initial flux. It seemed that the pre-deposited PAC layer on membrane surface exhibited little influence, and the flux even slightly decreased at the end of filtration. By contrast, the CMK-3 and CMK-8 layers significantly increased the terminal normalized flux to 0.43 and 0.51, respectively, after filtration of 300 mL feed water. During the entire filtration process, the flux curve of CMK-3 pre-deposited membrane was always higher than that of CMK-8, suggesting that CMK-3 outperformed CMK-8 in membrane flux improvement. Fig. 2 Fig. 2(b) illustrates the fouling resistances of different membranes. As seen, the irreversible and reversible fouling resistances were 0.29 and 0.93×1012 m-1, respectively. Considering the PAC pre-deposited membrane, both the total and reversible fouling resistances were higher than the virgin membrane, while the irreversible fouling slightly decreased. Similar results have been reported in previous studies [12, 39, 40] that the interaction between PAC and NOM in the feed water could form a more complex and denser cake layer on membrane surface, which was detrimental for fouling mitigation. The slight decrease of irreversible fouling was probably attributed to the adsorption of low MW compounds, hence reducing the foulants attached to membrane pores. Due to the mesoporous structure, CMK-3 and CMK-8 layers were efficient for alleviating both reversible and irreversible membrane fouling with reduction rates of 55.2-63.9% and 12

65.4-87.8%, respectively. To this end, the feasibility of pre-deposited OMCs layers for fouling mitigation in surface water treatment was demonstrated by the above results, among which CMK-3 exhibited better performance. 3.3 Fouling mechanism analysis The pore blockage-cake filtration model was utilized to investigate the fouling mechanism during various filtration processes, and the results showed that all data fitted well to the combined model (Fig. S3), revealing the feasibility of this model. The filtration and model fitting data were replotted as d2t/dV2 versus dt/dV according to Text S1, as well as V versus dt/dV, and the results are illustrated in Fig. 3. It was noted that the variation of n reflected the change of dominated fouling mechanism, and the required filtration volume for fouling mechanism transition was determined graphically. Considering the virgin membrane (Fig. 3(a)), the fouling was governed by standard blocking (n = 1.424) in the initial stage, then subsequently a negative slope appeared because of the transition to cake filtration [37, 41]. Finally, cake filtration dominated (n = 0) in the last filtering stage at the volume of 65-300 mL. Similar results were obtained for PAC pre-deposited membrane with the deposited amount of 12 g/m2 (Fig. 3(b)), implying that the PAC layer exhibited little influence on the fouling mechanism. Fig. 3(c) shows that the fouling mechanism of CMK-3 pre-deposited membrane (12 g/m2) was always governed by complete blocking with n = 2.0. In the presence of CMK-8 layer (12 g/m2), intermediate blocking played a dominated role prior to 125 mL (n = 0.813), followed by a transition stage from intermediate blocking to cake filtration. It 13

was indicated that the pre-deposited CMK-3 and CMK-8 layers apparently altered the fouling mechanism, and pore blocking always dominated throughout the filtration process without the formation of cake filtration. Fig. 3 3.4 Rejection of organic pollutants To better understand the role of pre-deposition layers for fouling mitigation, the rejections of DOC, UV254 and each fluorescent component by the membranes predeposited with 12 g/m2 of CMK-3/CMK-8 were comprehensively studied. As depicted in Fig. 4(a), the virgin membrane exhibited limited effect for DOC and UV254 removal with reduction efficiencies of 13.0% and 11.8%, respectively. The pre-deposited PAC layer apparently contributed to pollutants removal, and the rejection rates of DOC and UV254 increased to 34.6% and 29.4%, respectively. While beneficial, CMK-8 exhibited relatively lower efficiency in comparison with PAC for DOC and UV254 removal. By contrast, the pre-deposited CMK-3 layer was most effective with DOC and UV254 removal efficiencies of 66.2% and 52.9%, respectively. In general, the efficiency of pre-deposited layers for DOC and UV254 removal followed the order of CMK-3 > PAC > CMK-8. Fig. 4 Fig. 4(b) illustrates that four significant fluorescence peaks (i.e., peaks A, C, T1, and T2) were presented in the raw water, representing different fluorescent components. To be specific, peaks A and C were attributed to humic substances derived from the 14

breakdown of plants, while T1 and T2 were related to tryptophan-like protein substances [42, 43]. Table 2 shows the positions and intensities of fluorescence peaks extracted from the EEM spectra, reflecting the discrepancies in composition and concentration of each sample [44]. Filtration with virgin membrane significantly decreased the intensities of peaks T1 and T2, whereas the reductions of peaks A and C were limited (Fig. 4(c)). The results indicated that tryptophan-like protein substances were the major fluorescent component rejected by virgin membrane, while humic substances could pass through the membrane. It was inferred that the main NOM fraction causing membrane fouling was tryptophan-like protein substances, rather than humic substances. A significant reduction in peaks A and C was observed in Fig. 4(d), revealing that the PAC layer pre-deposited on membrane surface could improve the rejection of humic substances, rather than tryptophan-like protein substances, which was the major membrane foulant. Therefore, the pre-deposited PAC layer was ineffective for membrane fouling mitigation, which was consistent with the fouling control results (Fig. 2). Interestingly, CMK-8 showed an exactly opposite effect for pollutant removal, and tryptophan-like protein substances were preferentially removed with reduction rates of 35.3% and 36.7% for peaks T1 and T2, respectively. As seen in Fig. 4(e), only peaks T1 and T2 with significantly lower fluorescence intensities can be observed, while peaks A and C nearly disappeared, suggesting that both tryptophanlike protein and humic substances were effectively rejected by the CMK-3 predeposited membrane. In summary, the PAC layer preferentially removed humic 15

substances, CMK-8 devoted to the rejection of tryptophan-like protein substances, while CMK-3 enhanced the removal of both tryptophan-like protein and humic substances. Table 2 3.5 Effects of pollutant fraction and deposited amount 3.5.1 Flux decline The pollutant fraction and deposited amount of CMK-3/CMK-8 are two key factors influencing the fouling control performance, which will be systematically investigated in the following section. Fig. 5 shows the variation of membrane flux. Considering HA, both CMK-3 and CMK-8 layers at the deposited amount of 6 g/m2 were effective for the enhancement of membrane flux with the final normalized flux increased from 0.45 to 0.61 and 0.55, respectively. However, further increase of deposited amount to 12-50 g/m2 was detrimental for the further improvement of membrane flux, and both CMK-3 and CMK-8 exhibited similar regularities. In this case, as was reported between PAC and HA [13, 45], the synergistic fouling effect between excess CMK-3/CMK-8 and HA molecules resulted in a more compact and denser deposited layer on membrane surface, which was negative for the further improvement of permeate flux. As observed in Fig. 5(b), the pre-deposited CMK-3 and CMK-8 layers greatly improved the flux decline caused by BSA, with the final normalized flux increased from 0.42 to 0.93 and 0.75, respectively, at the deposited amount of 50 g/m2. Different from HA, the permeate flux increased with the increase of deposited amount 16

in the filtration of BSA. Fig. 5(c) illustrates that the most severe flux decline was induced by the NaAlg foulant, and the terminal permeate flux decreased to 3.1% of the initial. Both CMK-3 and CMK-8 layers showed limited effect for flux enhancement, regardless of the deposited amount. Due to the high hydrophilicity of NaAlg molecules, the hydrophobic interaction between CMK-3/CMK-8 and NaAlg was extremely weak [46, 47], still resulting in pollutant penetration through pre-deposited layers and severe flux decline. To this end, it can be deduced that both pollutant fraction and deposited amount of CMK-3/CMK-8 exhibited certain influence on the flux of underlying membrane. Fig. 5 3.5.2 Fouling resistances Fig. 6 shows the distributions of fouling resistance. As can be observed in Fig. 6(a), both CMK-3 and CMK-8 layers efficiently reduced the irreversible fouling by HA, and the deposited amount showed little influence. By contrast, the reduction of reversible fouling was apparently affected by the deposited amount. To be specific, the minimum amount of 6 g/m2 achieved the best performance, whereas the reversible fouling resistance increased with the increase of deposited amount, which was caused by the excess deposition of CMK-3/CMK-8 integrated with HA. Fig. 6(b) shows that CMK-3 was very efficient for the reduction of both reversible and irreversible fouling by BSA, and the increase of deposited amount was beneficial for performance improvement. By contrast, CMK-8 was more effective for alleviating reversible fouling. 17

As seen in Fig. 6(c), in the presence of CMK-3 and CMK-8 pre-deposited layers, the reversible fouling caused by NaAlg was slightly decreased, while the irreversible fouling resistance changed little. The above results indicated that the fouling control performance varied with the pollutant fraction, and generally followed the order of BSA >HA > NaAlg. It was suggested to select an appropriate deposited amount according to the pollutant fraction in the feed water. Fig. 6 The dominated fouling mechanisms in the filtration of various pollutant fractions were investigated, and the results are shown in Figs. S5 and S6. For the virgin membrane, the dominated mechanisms converted to cake filtration from pore blocking at the filtration volumes of 150 and 80 mL for HA and BSA, respectively. It seemed that the presence of CMK-3 and CMK-8 layers apparently avoided the development of cake filtration. With respect to NaAlg fouling, pore blocking played a major role at the initial stage, and then changed to cake filtration at 70 mL, which was earlier than HA and BSA. CMK-3 and CMK-8 slightly increased the transition volumes to 80 and 75 mL, respectively, suggesting the delay of cake formation. Overall, the transition of fouling mechanism from pore blocking to cake filtration was delayed to varying degrees for different pollutants with the pre-deposited CMK-3 and CMK-8 layers on membrane surface. 3.5.3 Pollutants removal The DOC removal of HA, BSA, and NaAlg by UF membranes with various OMCs 18

deposited amounts is shown in Fig. 7. The presence of OMCs layers apparently increased the removal rate of HA from 46.4% to 61.3-84.0% and 48.7-59.7%, respectively (Fig. 7(a)). The removal efficiency increased with the increase of deposited amount, indicating that more HA pollutants were adsorbed or retained by the deposited layers. Fig. S4 shows that the removal rule of UV254 was consistent with that of DOC for HA. As seen in Fig. 7(b), similar phenomenon was observed for BSA, and the rejection rate was greatly improved from 15.3% to 42.3-83.5% and 38.0-48.8% for CMK-3 and CMK-8, respectively. CMK-3 exhibited significantly superior performance than CMK-8, likely due to its long straight parallel channels with lower internal resistance for pollutants adsorption. In comparison with HA and BSA, the improvement for NaAlg removal was relatively limited, irrespective of the deposited amount. In general, the pre-deposited CMK-3/CMK-8 layers on membrane surface were more effective for the removal of HA and BSA, and the performance of CMK-3 was superior than CMK-8. Fig. 7 3.6 Mechanisms of fouling control with pre-deposited OMCs layers The aforementioned observations indicated that OMCs are promising alternatives to conventional PAC, and pre-depositing of OMCs on membrane surface might be a feasible technology for both fouling control and pollutants removal in water treatment. Fig. 8 further illustrates the mechanism of membrane fouling control with pre-deposited OMCs layers. As seen in Fig. 8, adsorption by the pre-deposited layers could reduce 19

the pollutants directly contacting with underlying membranes, which occupied an important role in fouling mitigation. The adsorption of various NOM fractions onto CMK-3 and CMK-8 were investigated, and the adsorption data were fitted using the Langmuir and Freundlich isotherm models (Fig. S7 and Table S1). It can be seen that CMK-3 exhibited significantly superior adsorption capacity of HA than CMK-8. Specifically, the adsorbed amount of HA onto CMK-3 reached 89.46 mg/g at the equilibrium concentration (Ce) of 10.53 mg/L, while that onto CMK-8 was 39.51 mg/g at the Ce of 13.02 mg/L. With respect to BSA, the adsorption efficiency of CMK-3 was still better than CMK-8, but the gap had narrowed. Due to the weak hydrophobic interaction between CMK-3/CMK-8 and NaAlg, the adsorption abilities of CMK-3 and CMK-8 for NaAlg were quite limited. Table S1 shows that the adsorption of various NOM fractions by CMK-3 and CMK-8 fitted well to the Langmuir and Freundlich isotherm models. For HA and NaAlg, the adsorption processes could be better explained by the Freundlich model, while for BSA the Langmuir model dominated. It was noteworthy that the adsorption results were also consistent with the fouling control performance for varying NOM fractions. Adsorption by CMK-3/CMK-8 layers could reduce the pollutants reaching the underlying membrane, thus contributing to membrane fouling control. From the perspective of material structure, PAC has a microporous structure (< 2 nm), which may inaccessible for high MW pollutants in the feed water, as confirmed by EEM analysis (Fig. 3(d)) that humic substances rather than tryptophan-like protein 20

were preferentially rejected by the PAC layer. However, high MW protein substances were regarded as the major foulant for membrane fouling [48], thus the mitigation effect of PAC was limited. By contrast, the aperture sizes of mesoporous CMK-3 and CMK8 are much larger (Table 1), and the rejection of high MW protein substances was dramatically improved (Fig. 3(e)-(f)). CMK-3 exhibited superior performance than CMK-8 for both pollutants removal and fouling control, likely due to their structure difference [49, 50]. To be specific, CMK-3 shows a 2D hexagonal structure with long straight channels (Figs. 1 and 8), resulting in a lower internal resistance and larger number of pathways. It was noteworthy that membrane pollutants could transport into the mesopores of CMK-3 from both axial (along the straight channels) and radial (the other side into the channels) directions during the filtration process (Fig. 8). However, CMK-8 exhibits a 3D cubic structure with closely entangled and packed channels, leading to increased internal resistance and less pathways than CMK-3. In addition, the relatively higher total pore volume and specific surface area (Table 1) also contributed to the superior performance of CMK-3. The SEM images of fouled membranes by various pollutants (Fig. S8) illustrate that the NOM foulants were deposited on the surfaces of pre-deposited layers, and bound with CMK-3/CMK-8 particles. In this case, the very high MW organic pollutants could be rejected through size exclusion, thus protecting the underlying membrane from rapid fouling. To sum up, the CMK-3/CMK8 pre-deposition layers significantly alleviated the organic fouling of underlying membranes in surface water treatment. 21

Fig. 8

4. Conclusions In this study, OMCs, i.e., hexagonal CMK-3 and cubic CMK-8 were proposed as pre-deposition layers for alleviating UF membrane fouling. The feasibility of this technology was verified in surface water treatment, and the mechanisms were preliminary revealed. The following insights can be obtained: (1) The pre-deposited OMCs layers exhibited significantly better performance than PAC for both flux enhancement and fouling mitigation, among which CMK-3 outperformed CMK-8. OMCs layers apparently altered the fouling mechanism, and pore blocking always dominated throughout the filtration process without the formation of cake filtration. (2) PAC preferentially removed humic substances, CMK-8 devoted to the rejection of tryptophan-like protein substances, while CMK-3 enhanced the removal of both tryptophan-like protein and humic substances. (3) The pre-deposited CMK-3/CMK-8 layers were more efficient for mitigating membrane fouling induced by HA and BSA, whereas ineffective for NaAlg fouling. The optimal deposited amount varied with the pollutant fraction in the feed water. (4) Both adsorption and size exclusion of the pre-deposited layers contributed to fouling mitigation. Due to its long straight parallel channels with lower internal resistance, CMK-3 exhibited significantly superior performance than CMK-8 with closely entangled and packed channels. 22

Acknowledgments This research was jointly supported by National Natural Science Foundation of China (51908334, 51908335), China Postdoctoral Science Foundation (2019M652427), Shandong Provincial Natural Science Foundation (ZR2019BEE058), Shandong Postdoctoral Innovation Project (201902031), Open Project of State Key Laboratory of Urban Water Resource and Environment, Harbin Institute of Technology (QA201937), Doctoral Research Fund of Shandong Jianzhu University (XNBS1806), National Key Research and Development Program of China (2017YFF0209903), and Youth Innovation Technology Project of Higher School in Shandong Province (2019KJD003). Appendix A. Supplementary information Supplementary data associated with this article can be found in the online version.

23

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31

Table 1 Characteristics of CMK-3 and CMK-8. Components

CMK-3

CMK-8

Average length (μm)

1.0

-

Total Pore volume (cm3/g)

1.2-1.5

0.75-1.1

Specific surface area (m2/g)

≥900

>500

Pore diameter (nm)

3.8-4.0

3.2-6.6

Purity

>99.6%

>99.6%

Table 2 The positions and intensities of fluorescence peaks for each water sample. Water

Peak T1

Peak T2

Peak A

Peak C

samples

Ex/Em

Int.

Ex/Em

Int.

Ex/Em

Int.

Ex/Em

Int.

Raw water

275/310

1305

230/310

796

250/418

380

325/418

171

Control

275/3120

1030

225/311

589

250/420

369

325/421

170

PAC

275/309

1265

225/310

745

250/417

158

325/421

68

CMK-3

275/310

829

225/310

469

245/420

58

325/421

27

CMK-8

275/310

844

225/310

504

245/423

226

325/421

112

32

Figure Captions Fig. 1 SEM images of CMK-3 (a) and CMK-8 (b); particle size distributions of CMK3 and CMK-8 (c); pure water flux of PES membranes pre-deposited with OMCs layers (d). Fig. 2 Membrane fouling control with pre-deposited PAC, CMK-3 and CMK-8: (a) normalized flux decline, (b) membrane fouling resistances. Fig. 3 d2t/dV2 versus dt/dV curves for the virgin membrane (a), membranes predeposited with PAC (b), CMK-3 (c) and CMK-8 (d) layers. Fig. 4 Removal of DOC and UV254 by various UF membranes (a); EEM fluorescence spectra of raw water (b), permeate of virgin membrane (c), and membranes predeposited with PAC (d), CMK-3 (e), and CMK-8 (f). Fig. 5 The variation of membrane flux with pre-deposited CMK-3 and CMK-8 layers: (a) HA, (b) BSA, (c) NaAlg. Fig. 6 The variation of membrane fouling resistances with pre-deposited CMK-3 and CMK-8 layers: (a) HA, (b) BSA, (c) NaAlg. Fig. 7 Removal of DOC by UF membranes pre-deposited with CMK-3 and CMK-8 layers: (a) HA, (b) BSA, and (c) NaAlg. Fig. 8 Illustration of the mechanism for membrane fouling control with pre-deposited CMK-3 and CMK-8 layers.

33

Fig. 1 SEM images of CMK-3 (a) and CMK-8 (b) (100,000× magnification); particle size distributions of CMK-3 and CMK-8 (c); pure water flux of PES membranes predeposited with OMCs layers (d). b

a

6 5

Frequency(%)

d

CMK-3 CMK-8

4

Virgin membrane CMK-3 CMK-8

d(0.5)=15.2 μm

1500

Pure water flux (L/m2·h)

c

d(0.5)=8.1 μm

1000

3 2 1 0 0.1

1

100 10 Particle size (μm)

500

0

1000

34

0

6

12

25

Deposited amount (g/m2)

50

Fig. 2 Membrane fouling control with pre-deposited PAC, CMK-3 and CMK-8: (a) normalized flux decline, (b) membrane fouling resistances. Conditions: deposited amount of PAC = CMK-3 = CMK-8 = 12 g/m2. a

Virgin membrane PAC CMK-3 CMK-8

Normalized flux (J/J0)

1.0 0.8 0.6 0.4 0.2 0

50

100 150 200 Permeate volume (mL)

Fouling resistance (1012 m-1)

b 1.2

250

300

Reversible fouling Irreversible fouling

1.0 0.8 0.6 0.4 0.2 0.0 Virgin membrane PAC

35

CMK-3

CMK-8

Fig. 3 d2t/dV2 versus dt/dV curves for the virgin membrane (a), membranes predeposited with PAC (b), CMK-3 (c) and CMK-8 (d) layers. Conditions: deposited amount of PAC = CMK-3 = CMK-8 = 12 g/m2.

.424

100

65 mL 5.0x106

1.0x107

Data

79

6.0x10

10

200

n= 1.6

8.0x10

10

150 100 50

50

4.0x1010

10

V (mL)

150

c 4.0x10

250

dt2/dV2 (s/m6)

6.0x10

10

200

4.0x1010

2.0x107

dt/dV (s/m3)

2.5x107

5.0x106

Filtration volume

Model

300

0

70 mL

0

1.5x107

d 2.6x10

1.0x107

1.5x107

dt/dV (s/m3)

-50 2.5x107

Filtration volume

Model

Data

2.0x107

300 250

100

n

1.0x1010

.0 =2

50

00

6x106

7x106

8x106

9x106

dt/dV (s/m3)

1x107

100 50

10

125 mL

-50 5x106

200 150

2.2x1010

2.0x10

0

0. 8

13

150

2.4x1010

n=

2.0x10

10

200

V (mL) dt2/dV2 (s/m6)

dt2/dV2 (s/m6)

3.0x10

1.8x10

1x107

0

10

-50 5x106

36

350

10

250 10

350 300

1.0x1011

250

n= 1

dt2/dV2 (s/m6)

8.0x10

Filtration volume

Model

b

300

1.0x1011

10

Data

350

V (mL)

Filtration volume

Model

6x106

7x106

8x106

9x106

dt/dV (s/m3)

1x107

1x107

1x107

V (mL)

Data

a

Fig. 4 Removal of DOC and UV254 by various UF membranes (a); EEM fluorescence spectra of raw water (b), permeate of virgin membrane (c), and membranes predeposited with PAC (d), CMK-3 (e), and CMK-8 (f). Conditions: deposited amount of PAC = CMK-3 = CMK-8 = 12 g/m2. b 450

3

1 0

UV254 (cm-1)

0.04 0.03 0.02 0.01 0.00

1000 900.0

2

Excitation wavelength (nm)

DOC (mg/L)

a

400

800.0 700.0

350

600.0 500.0

C

400.0

300

300.0

T1

200.0

A

250

100.0

T2

Raw water

Control

PAC

CMK-3

CMK-8

250

300

0.000

350

400

450

500

550

Emission wavelength (nm)

c

d 450

450 1000

1000

400

800.0 700.0

350

600.0 500.0

C

400.0

300

300.0

T1

200.0

250

A

100.0

T2 250

300

900.0

Excitation wavelength (nm)

Excitation wavelength (nm)

900.0

400

800.0 700.0 600.0

350

400.0

300

300.0

T1

400

450

500

A

100.0

T2 250

550

300

0.000

350

400

450

500

550

Emission wavelength (nm)

Emission wavelength (nm)

e

200.0

250

0.000

350

500.0

C

450 1000

f

450 1000 900.0

400

800.0 700.0 600.0

350

500.0

C

400.0

300

300.0

T1

200.0

250

A

100.0

T2 250

300

Excitation wavelength (nm)

Excitation wavelength (nm)

900.0

400

800.0 700.0 600.0

350

400

450

500

300.0

T1

200.0

250

A

100.0

T2 250

550

400.0

300

0.000

350

500.0

C

300

0.000

350

400

450

Emission wavelength (nm)

Emission wavelength (nm)

37

500

550

Fig. 5 The variation of membrane flux with pre-deposited CMK-3 and CMK-8 layers: (a) HA, (b) BSA, (c) NaAlg.

a 1.0

CMK-3

CMK-8

0.9 Normalized flux(J/J0)

0.8 0.7 0.6 0.5 0.4

Virgin membrane 6 g/m2 12 g/m2 25 g/m2 50 g/m2

0.3 0.2 0.1 0.0

0

50 100 150 200 250 300 0 50 100 150 200 250 300 Permeate volume (mL)

b 1.0 0.9 Normalized flux(J/J0)

0.8 0.7 0.6 0.5 CMK-3

0.4 0.3 0.2 0.1 0.0

CMK-8

Virgin membrane 6 g/m2 12 g/m2 25 g/m2 50 g/m2

0

50 100 150 200 250 300 0 50 100 150 200 250 300 Permeate volume (mL)

c 1.0

Virgin membrane 6 g/m2 12 g/m2 25 g/m2 50 g/m2

0.9 Normalized flux(J/J0)

0.8 0.7

CMK-3

0.6

CMK-8

0.5 0.4 0.3 0.2 0.1 0.0

0

50 100 150 200 250 300 0 50 100 150 200 250 300 Permeate volume (mL)

38

Fig. 6 The variation of membrane fouling resistances with pre-deposited CMK-3 and CMK-8 layers: (a) HA, (b) BSA, (c) NaAlg.

Fouling resistance (1011 m-1)

a

5

Fouling resistance (1011 m-1)

Fouling resistance (1012 m-1)

CMK-8

12

12

3 2 1

0

6

5

25 50 0 6 Deposited amount (g/m2)

Reversible fouling

CMK-3

25

50

Irreversible fouling

CMK-8

4 3 2 1 0

c

Irreversible fouling

4

0

b

Reversible fouling

CMK-3

0

6

12 11 10 9 8 7

12

25 50 0 6 12 Deposited amount (g/m2)

Reversible fouling

CMK-3

25

50

Irreversible fouling

CMK-8

0.10 0.05 0.00

0

6

12

25 50 0 6 12 Deposited amount (g/m2)

39

25

50

Fig. 7 Removal of DOC by UF membranes pre-deposited with CMK-3 and CMK-8 layers: (a) HA, (b) BSA, and (c) NaAlg. a 120

Virgin membrane CMK-3 CMK-8

DOC removal rate (%)

100 80 60 40 20 0

0

b 120

12

25

50

12

25

50

12

25

50

Deposited amount (g/m2)

Virgin membrane CMK-3 CMK-8

100

DOC removal rate (%)

6

80 60 40 20 0

0

c 120

Deposited amount (g/m2)

Virgin membrane CMK-3 CMK-8

100

DOC removal rate (%)

6

80 60 40 20 0

0

6

Deposited amount (g/m2)

40

Fig. 8 Illustration of the mechanism for membrane fouling control with pre-deposited CMK-3 and CMK-8 layers.

41

Highlights CMK-3 and CMK-8 were pre-deposited on UF membrane surface for fouling control. OMCs exhibited superior performance as alternatives to PAC in water treatment. OMCs layers altered the fouling mechanism without the formation of cake filtration. Pollutant fraction and deposited amount affected the fouling control performance. CMK-3 outperformed CMK-8 due to its long straight channels with lower resistance.

Graphical Abstract

42

Declaration of interests ☒ 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. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:

43

CRediT author statement Xiaoxiang

Cheng:

Conceptualization,

Methodology,

Writing-Original

draft

preparation, Data curation, Funding acquisition. Weiwei Zhou: Investigation, Resources, Data Curation, Writing - Review & Editing. Daoji Wu: Resources, Supervision, Data Curation, Funding acquisition. Congwei Luo: Conceptualization, Resources, Writing - Review & Editing. Ruibao Jia: Data Curation, Funding acquisition. Peijie Li: Methodology, Investigation, Data Curation. Lu Zheng: Investigation, Data Curation, Formal analysis. Xuewu Zhu: Investigation, Writing - Review & Editing. Heng Liang: Conceptualization, Writing - Review & Editing.

44