Synergistic oxidation - filtration process analysis of catalytic CuFe2O4 - Tailored ceramic membrane filtration via peroxymonosulfate activation for humic acid treatment

Water Research 171 (2020) 115387

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Synergistic oxidation - filtration process analysis of catalytic CuFe2O4 Tailored ceramic membrane filtration via peroxymonosulfate activation for humic acid treatment Yumeng Zhao, Dongwei Lu**, Chengbiao Xu, Jinying Zhong, Mansheng Chen, Shu Xu, Ying Cao, Qi Zhao, Mo Yang, Jun Ma* State Key Laboratory of Urban Water Resource and Environment, Harbin Institute of Technology, Harbin, 150090, China

a r t i c l e i n f o

a b s t r a c t

Article history: Received 5 September 2019 Received in revised form 7 December 2019 Accepted 9 December 2019 Available online 16 December 2019

This work synthesized catalytic CuFe2O4 tailored ceramic membrane (CuFeCM), and systematically investigated the intercorrelated oxidation - filtration mechanism of peroxymonosulfate (PMS)/CuFeCM catalytic filtration for treating humic acid (HA). PMS/CuFeCM filtration exhibited enhanced HA removal efficiency while reduced the irreversible fouling resistance as compared with the conventional CM filtration. Results from HA characterizations showed that PMS/CuFeCM catalytic filtration oxidized HA into conjugated structures of smaller molecular weight. The unsaturated bonds further caused the reagglomeration of HA, hence enhancing the size exclusion of CuFeCM. Meanwhile, oxidized HA particles with changing physicochemical properties reduced the total attractive interaction energy between CuFeCM and HA, mainly attributed to the reduced acid-base interaction energy according to the Extended Derjaguin-Landau-Verwey-Overbeek (XDLVO) analysis. The changing of HA properties and HACuFeCM physicochemical interactions rendered more re-agglomerated HA particles retained above membrane with less attachment, which induced decreasing irreversible fouling resistance and facilitated easier external fouling removal by hydraulic cleaning. Overall, the PMS/CuFeCM configuration demonstrated in this study could provide a new insight into the synergistic oxidation - filtration interaction mechanism of hybrid catalytic ceramic membrane filtration process. © 2019 Elsevier Ltd. All rights reserved.

Keywords: Ceramic membrane Catalytic filtration Peroxymonosulfate oxidation Humic acid XDLVO theory

1. Introduction Natural organic matter (NOM) is chemically-complex macromolecular compound that can be ubiquitously found in natural water system (Adusei-Gyamfi et al., 2019; Jamil et al., 2019; Wang et al., 2013). NOM treatment is of significant environmental concerns mainly attributed to its active binding with organic contaminants, complexation of trace metals and influence on water pH and bioactivities (Deng et al., 2019; Hua et al., 2019). During the past decades, ultrafiltration (UF) membranes have been extensively applied in the removal of NOM. Yet the problematic NOM is considered as a significant source of fouling to UF membranes with a comparatively low removal efficiency (Cheng et al. 2016, 2017a; Park et al., 2012).

* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (D. Lu), [email protected] (J. Ma). https://doi.org/10.1016/j.watres.2019.115387 0043-1354/© 2019 Elsevier Ltd. All rights reserved.

As advanced oxidation processes (AOPs) enable reactive oxygen species generation and thus hold promise for contaminant degradation, UF membranes have been exploited by integrating with AOPs for achieving lower membrane fouling potential, higher foulant removal efficiency, and/or efficient membrane cleaning effect (Byun et al., 2011, De Angelis and Fidalgo de Cortalezzi, 2016; Park et al., 2012). Among numerous UF membrane materials, ceramic membranes (CM) with strong chemical and thermal resilience possess appealing features in enduring harsh operation environment and integrating with AOPs for synergistic enhancement of membrane performance (Lu et al. 2015, 2016; Zhao et al., 2018). Integrated AOP-CM filtrations for NOM treatment have been investigated in previous studies, which have shown improved NOM fouling control and removal efficiency (Harman et al., 2010; Kim and Van der Bruggen, 2010). Although foulant degradation mechanisms involving reactive oxygen species (ROS) analysis during AOP-CM filtration were broadly investigated in previous works (Bao et al., 2018; Guo et al., 2018; Shi et al., 2019), the analysis of the

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2. Experimental section Nomenclature 2.1. Materials CM Ceramic membrane CuFeCM CuFe2O4-tailored ceramic membrane NOM Natural organic matter HA Humic acid AOP Advanced oxidation process HAAOP Humic acid after catalytic oxidation PMS Peroxymonosulfate LW interaction Lifshitz-van der Waals interaction AB interaction Lewis acid-base interaction EL interaction Electrostatic interaction CA Contact angle

intercorrelation between catalytic oxidation and membrane filtration in integrated AOP-CM filtration process was lacking, furthermore their consequent mutual influence on NOM removal and fouling were rarely studied in-depth. Meanwhile, among the various integrated AOPs with CM, sulfate radical (SO4
¡)-based AOP is considered as an attractive alternative to the conventional hydroxyl radical (
OH)-based AOP, as SO4
¡ was found to be highly reactive and possessed a longer half-life compared to
OH allowing better diffusion of SO4
¡ for oxidation reaction in the bulk solution (Ding et al., 2013; Guan et al., 2013; Oh et al., 2015; Zhang et al., 2013). SO4
¡ can be generated by environmental-friendly peroxymonosulfate (PMS) via irradiation, ultraviolet, heat or transition metal catalysis, among which transition metal is considered most economical mainly attributed to its minimum energy supplement (Li et al., 2018; Ling et al., 2017; Matafonova and Batoev, 2018). Cobalt-based catalysts were reported to be the most efficient for PMS activation, yet due to the high toxicity of cobalt, substitute metals need to be investigated (Ling et al., 2018; Wei et al., 2019). In replacement, copper-based catalysts were less toxic and more economic to synthesize, among which CuFe2O4 spinel was able to efficiently activate PMS while remained structurally stable (i.e., less leaching of Cu and Fe metal ions into the bulk solution and able to maintain crystalline form) during the catalytic oxidation process (Guan et al., 2013; Zhang et al., 2013). Therefore, integration of CuFe2O4 with CM would be of potential significance in improving the filtration performance and membrane cleaning effect of hybrid catalytic ceramic membrane filtration. However, to the best of our knowledge, little work has been conducted in this field (Hosseini et al., 2019; Luan et al., 2019; Shi et al., 2019; Wang et al., 2019b). This study was to obtain a thorough understanding in terms of the synergistic influence of catalytic oxidation and membrane filtration on NOM treatment. Specifically, CuFe2O4-tailored CM (CuFeCM) via one-step synthesis was presented. Humic acid (HA), a typical NOM foulant, was selected as feed solution. The fouling and retention performances by PMS/CuFeCM catalytic filtration in treating HA solution were thoroughly evaluated. The HA property changes by PMS/CuFeCM oxidation and PMS/CuFeCM catalytic filtration were investigated in detail. Furthermore, the physicochemical interactions between HA and CuFeCM were analyzed according to the Extended DerjaguineLandaueVerweyeOverbeek (XDLVO) theory. The results were expected to explain the fouling and retention behaviors of PMS/CuFeCM catalytic filtration indepth. Additionally, this study demonstrated the intercorrelated oxidation-filtration mechanism in catalytic ceramic membrane performance, further highlighting the potential of integrated AOPCM system with tailored membrane surface functionalities for facilitating the treatment of NOM.

Potassium peroxymonosulfate (PMS), citric acid, sodium hyposulfite (Na2S2O3), formamide and diiodomethane were purchased from Sigma-Aldrich. Humic acid (HA) was also purchased from Sigma-Aldrich with its product code of No.53680. Ferric nitrate, cupric nitrate and glycol were purchased from Sinopharm Chemical Reagent Co., Ltd. All the applied chemicals were of analytical grade. Milli-Q water with resistivity of 18.2 MU cm was used in the experiment. Base membrane (flat-sheet ceramic membrane, molecular weight cut-off ¼ 300 kDa, diameter ¼ 47 mm, thickness ¼ 2.5 mm) was purchased from TAMI, which was mainly made of TiO2 and ZrO2 (Cheng et al. 2017a, 2017b; Lu et al. 2015, 2016). HA feed solution was set as 20 mg/L, and its UV absorbance per TOC (total organic carbon) was 7.93 L mg1 m1. HA feed solution was prepared by first dissolving HA in NaOH solution (pH ¼ 10) under continuous stirring for 24 h, then adjusting to pH ¼ 7 by the addition of HCl solution. 2.2. Preparation of CuFeCM CuFe2O4 decorated ceramic membrane (CuFeCM) was prepared via one-step sol-gel combustion method, as shown in Fig. 1(b). Cupric nitrate, ferric nitrate, citric acid and glycol were dissolved into 100 mL Milli-Q water with a molar ratio of 1:2:3.6:7.2. The mixed solution was stirred at 90  C for 5 h to form gel-like precursor. 0.7 mL of the precursor was placed on the top surface of the base ceramic membrane which was then spin-coated by a spin coater (CHEMAT TECHNOLOGY, KW-4A) at 300 rpm for 12 s. The membrane was further coated for another two rounds with the same amount of the added precursor and spin coating parameters. The coated membrane was then calcinated via temperature programming to 500  C under the heating rate of 3  C/min. Specifically, the membrane was heated at 100  C for 1.5 h followed by sintering at 300  C for 1.5 h, then at last calcinated at 500  C for 1.5 h. The coating dose of CuFe2O4 on ceramic membrane was approximately 0.6 g. 2.3. Characterization The Total Organic Carbon (TOC) contents of HA feed and permeate solutions were measured by Carbon/Nitrogen analyzer equipment (Analytikjena-Total Organic Carbon and Total Nitrogen Analyzer-multi N/C 3100, Germany). Zeta potentials of membranes were analyzed by Electrokinetic Analyzer for Solid Samples (SurPASS, Anton Paar). Zeta potentials and particle sizes of HA foulants were measured by Zetasizer instrument (Zetasizer Nano series Nano-ZS90, Malvern Panalytical, British & Holland). Contact angles of HA foulant and membranes were characterized by contact angle goniometer (Contact Angle Meter QSPJ-360, Jinshengxin, China). Preparation details of the contact angle measurement were provided in Text S1, SI. The probe liquids for contact angle measurement were ultra-pure water, formamide and diiodomethane. Molecular weight distribution of HA solution was analyzed by highperformance size exclusion chromatography with UV detection (HPSEC-UV) equipped with high-performance liquid chromatograph (LC-10A, Shimadzu, Japan) and gel column (ultrahydrogel™ linear 7.8  300 mm, Waters, Japan). HPSEC mobile phase was phosphate buffer solution, pH ¼ 6.85. The pH values of all solutions were measured by pH meter (PHSe3C, China). X-ray diffraction (XRD) patterns were analyzed by a Bruker D8 Advance X-ray diffractometer (Cu Ka radiation, 40 kV, 40 mA). X-ray photoelectron spectrometer (XPS-ESCALAB 250Xi) equipped with AXIS ULTRA

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Fig. 1. Fabrication and characterization of the as-prepared CuFeCM. (a): Optical images of CuFeCM and CM. (b): Schematic diagram of CuFeCM preparation. (c)e(f): SEM images of the surfaces and cross-sections of CuFeCM and CM.

DLD spectrometer (Shimadzu, Japan) with monochromatic Al Ka radiation source was applied to determine element states of CuFeCM. Scanning electron microscopy (SEM) along with X-ray energy dispersive spectroscopy (EDS) (ZEISS SUPRA 55 SAPPHIRE, Germany) was applied to characterize the morphology of the asprepared CuFeCM and the pristine CM substrate. Pore size distribution was measured by the Helium-Hexane Permporosimetry (Andersson and Hedlund, 2008; Zhao et al., 2018). Fouriertransform infrared (FT-IR) spectra was characterized by Spectrum One instrument (PerkinElmer, USA). Fluorescence excitationemission matrices (FEEMs) were measured by Fluorescence spectrophotometer (F7000, Hitachi, Japan). 2.4. Hybrid PMS/CuFeCM system 2.4.1. Batch experiment of PMS/CuFeCM catalytic oxidation To investigate the catalytic oxidation effect of CuFeCM, batch experiment was conducted. CuFeCM and 20 mg/L HA solution (200 mL) were placed in a beaker, and desired amount of PMS

solution (the effect of PMS concentration on HA degradation see Fig. S1 of the Supporting Information (SI)) was added into the beaker to start the reaction. Mechanical stirring (70 rpm) was employed during the reaction. Samples were taken at different time intervals, i.e. 0 min, 5 min, 10 min, 30 min and 60 min, and were filtered with 0.22 mm acetate-fiber syringe filters. 4 g/L Na2S2O3 was applied as reaction quench agent. According to Figs. S1 and SI, CuFeCM with 0.4 g/L PMS possessed the optimal degradation efficiency of HA. 2.4.2. Hybrid PMS/CuFeCM filtration Filtration was carried out in dead-end mode and driven by gravity force at the operation height of 80 cm. The configuration of gravity-driven filtration (GDF) was employed because of its low energy consumption and simple maintenance (Ding et al., 2017; Wang et al., 2017a; Wu et al., 2017). More importantly, thorough contact between foulant and catalytic reactive sites was enabled by the relatively low water flux of GDF system (Wang et al., 2017b). The filtration procedures were elucidated as follows.

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Clean membrane was immersed in Milli-Q water for 12 h prior to filtration. Milli-Q water was first filtered to obtain pure water flux. HA feed solution (20 mg/L) was then filtrated at a fixed volume of 200 mL with 0.4 g/L of PMS added to the feed solution prior to filtration. Hydraulic cleaning was conducted after the first filtration cycle. Fouled membrane was backwashed under 0.1 MPa with 100 mL Milli-Q water, and membrane surface was further cleaned by cross-flow (cross-flow velocity: 1.2 L/min) with 100 mL Milli-Q water. Pure water flux at 80 cm was measured after hydraulic cleaning, and the second cycle of HA filtration was then conducted. The permeate flux was measured periodically by the electronic balance.

2.5. Theoretical analysis 2.5.1. Fouling analysis Fouling resistances were calculated using Resistance-in-series Model, as shown in Eq (1) - Eq (4) (Liu et al., 2018; Shen et al., 2010). Total resistance (Rt) was calculated by Eq (1). △P (Pa) denotes the transmembrane pressure, m is the dynamic viscosity (Pa.s) and J1 (L.m2.h1) denotes the final permeate flux after filtrating feed solution.

Rt ¼

DP mJ1

(1)

Reversible fouling resistance (Rr) was calculated by Eq (2). J2 (L.m2.h1) is the pure water flux of the fouled membrane after hydraulic cleaning.

Rr ¼

DP DP  mJ1 mJ2

(2)

Intrinsic membrane resistance (Rm) was calculated by Eq (3). J0 (L.m2.h1) denotes the permeate flux of pure water of the clean membrane.

Rm ¼

DP mJ0

(3)

Irreversible fouling resistance (Ri) was calculated by Eq (4).

Ri ¼ Rt  Rm  Rr ¼

DP DP  mJ2 mJ0

(4)

In addition, four classic fouling models were applied to investigate the HA fouling of PMS/CuFeCM catalytic filtration, which included complete blocking, intermediate blocking, cake fouling and standard blocking. The equations of fouling models were listed in Tables S1 and SI (Cheng et al., 2017a; Liu et al., 2018).

2.5.2. XDLVO theory Physicochemical interaction between HA foulant and membrane was analyzed by Extended Derjaguin-Landau-VerweyOverbeek (XDLVO) theory, which consisted of van der Waals (LW) interaction, acid-base (AB) interaction and electrostatic (EL) interaction (Kang et al., 2004; Lin et al., 2014; Liu et al., 2017; Subramani and Hoek, 2008; Wang et al. 2005, 2011,). LW, AB, EL and total free energies were calculated by Eq (5) - Eq (8).

qffiffiffiffiffiffiffiffiffi

DGLW ¼ 2ð gLW  l

qffiffiffiffiffiffiffiffiffi qffiffiffiffiffiffiffiffiffi qffiffiffiffiffiffiffiffiffi LW gLW gLW Þ m Þð gs  l

(5)

pffiffiffiffiffiffiffiffi pffiffiffiffiffiffiffiffiffiffi

pffiffiffiffiffiffiffiffi

DGAB ¼ 2 gl þ ð gm  þ gs   þ2

pffiffiffiffiffiffi 

gl Þ qffiffiffiffiffiffiffi qffiffiffiffiffiffi qffiffiffiffiffiffi qffiffiffiffiffiffiffiffiffiffiffiffiffi qffiffiffiffiffiffiffiffiffiffiffiffiffi þ  Þ  2ð gl ð gþ gþ gþ gþ g mþ s  m gs þ m gs Þ l

pffiffiffiffiffiffi 

.

DGEL ¼ ε0 εr  xs xm lEL

(7)

DGTot ¼ DGAB þ DGLW þ DGEL LW

AB

DGEL

(6)

(8)

DGTot

△G , △G , and are LW, AB, EL and total free energies. gLW, gþ, g-are the Lifshitz-van der Waals, electron acceptor and electron donor components of surface tension. Subscript: s, l and m represent foulant, solvent and membrane, respectively. lEL are the characteristic decay length for EL interaction. ε0εr is the dielectric permittivity of water. zs and zm are the zeta potentials of the foulant and membrane surface. The extended Young equation was applied to calculate the surface tension parameters (i.e., gLW, gþ, g-) of membrane or colloid, as shown in Eq (9).

qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi qffiffiffiffiffiffiffiffiffiffiffiffi qffiffiffiffiffiffiffiffiffiffiffiffi qffiffiffiffiffiffiffiffiffiffiffiffi þ   LW LW  þ ðgP LW þ2 gþ P gP Þð1þcos qÞ¼2ð gT gP þ gT gP þ gT gP Þ (9)

q is the contact angle of the probe liquid on the tested sample. gT and gP are the surface tensions of the tested sample (i.e., membrane or colloid) and the probe liquid, respectively. gLW, gþ, g-of the tested sample are calculated by measuring the three q values of the three probe liquids (i.e., water, formamide, and diiodomethane) on the tested sample. 3. Results and discussions 3.1. Characterization of CuFeCM A discernable color transformation of the as-prepared CuFeCM was observed (Fig. 1(a)) compared to that of the pristine CM after the fabrication procedure (Fig. 1(b)). The SEM images of CuFeCM and the pristine CM exhibited resembling morphologies (Fig. 1(c)e(f)). Meanwhile, according to the SEM-EDS mapping results, Cu and Fe elements were evenly scattered on membrane surface, suggesting the uniform distribution of CuFe2O4 on membrane surface (Fig. 2(A)). Additionally, EDS signals of Cu and Fe elements were found within membrane pore structures (Fig. 2(B)), indicating that CuFe2O4 was impregnated into the CM substrate (Bao et al., 2018). To further verify the molecular structures and elemental states of the functionalized surface of CuFeCM, XRD and XPS characterizations were conducted, as shown in Fig. 3. XRD patterns (Fig. 3(a)) of CuFeCM surface showed that the diffraction peaks at 34.7, 35.9 , 41.8 and 43.8 were indexed to (103), (211), (004) and (220) planes of the CuFe2O4 spinel structure (JCPDS No. 34e0425) (Ren et al., 2015; Zhang et al., 2013), which verified the structure of CuFe2O4 on membrane surface. The other XRD diffraction peaks corresponded to the components of the CM substrate, i.e., rutile (JCPDS No. 73e1232) and zirconium oxide (JCPDS No. 49e1642). Additionally, the valence states of Cu and Fe were further validated by XPS analysis. The main peak of Cu 2p3/2 at ~933.6 eV validated the presence of Cu2þ cation (Fig. 3(c)), while the main peak of Fe 2p3/2 at ~711.3 eV and shakeup satellite peak at ~ 719.6 eV verified the existence of Fe3þ cation (Fig. 3(d)) (Ding et al., 2013; Wang et al., 2014). The electrostatic properties of CuFeCM and the pristine CM were further compared (Figs. S2(a) and SI). Zeta potentials of both membranes decreased along with the growth of pH, with CuFeCM more positively charged than the pristine CM. In addition, the pore size distribution of CuFeCM was

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Fig. 2. SEM-EDS mapping of CuFeCM surface and cross-section. The CM substrate was made of TiO2 and ZrO2 according to the manufacturer (Lu et al. 2015, 2016; Zhao et al., 2018).

similar and slightly lower than that of the pristine CM (Figs. S2(b) and SI), which also corresponded with the minorly reduced water flux of CuFeCM under gravity-driven filtration with the operation height of 80 cm (Figs. S2(c) and SI).

3.2. PMS/CuFeCM catalytic filtration performance 3.2.1. Fouling tendency and reversibility analysis Flux performances of CuFeCM and CM were depicted in Fig. 4(a). Fluxes declined to 58% and 56% of the initial values by CuFeCM and CM in the first filtration cycle. For both CuFeCM and CM single filtrations (“single filtration” refers to conventional filtration without PMS), flux declined sharply initially and tended to decline more slowly and stably afterwards during the first filtration cycle (“fast-slow decline” pattern), while fluxes of both PMS hybridmembranes appeared to decline continuously with a flux reduction of 50% and 66% by PMS/CuFeCM hybrid filtration and PMS/CM hybrid filtration (“hybrid filtration” refers to filtration with PMS), respectively. However, flux recovery rates were notably improved by 25.9% and 11.9% in PMS/CuFeCM and PMS/CM hybrid filtrations after hydraulic cleaning, as compared with 5.7% and 5.6% in CuFeCM and CM single filtrations. Corresponding fouling resistances were analyzed as shown in Fig. 4(b). With the addition of PMS, irreversible resistance of PMS/ CuFeCM hybrid filtration (0.13  1012 m1) decreased compared to

that of CuFeCM single filtration (0.22  1012 m1), while its reversible resistance (0.27  1012 m1) was higher than that of CuFeCM single filtration (0.06  1012 m1). In addition, the total resistance and irreversible resistance of CuFeCM single filtration (0.29  1012 m1 and 0.22  1012 m1) were both slightly lower than those of CM single filtration (0.3  1012 m1 and 0.25  1012 m1). The corresponding mechanism analysis was discussed in the following subsection 3.3.

3.2.2. HA retention performance The fluorescence spectra of HA feed solution and permeate were analyzed in Fig. 5. From the FEEM spectra of HA (Fig. 5(a)), two characteristic fluorescence regions of HA were detected, i.e. Ex/ Em ¼ 280e300/445e480 nm and Ex/Em ¼ 360e380/465e480 nm, which were assigned to carboxylic-like fluorophores (marked as Region “1#” in Fig. 5(a)) and phenolic-like fluorophores (marked as Region “2#” in Fig. 5(a)) of HA, respectively (Li et al., 2011; McKnight et al., 2001; Ohno, 2002). Without PMS addition, CuFeCM removed 1# group and 2# group by 48.3% and 66.7% (Fig. 5(c)). When integrating PMS with filtration, the removal efficiencies of the two fluorescence groups both improved, as evidenced by ~66.6% removal of 1# group and complete removal of 2# group (Fig. 5(e)). The HA fluorescence spectra by single PMS/CuFeCM oxidation were also investigated. From Fig. S4(A1) and (B1), SI, Region 1# and Region 2# of HA were rarely degraded by PMS alone

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Fig. 3. XRD and XPS characterizations of CuFeCM and CM. (a): XRD spectra of CuFeCM and CM surfaces. (b): XPS survey spectra of CuFeCM and CM surfaces. (c) and (d): XPS spectra of Cu and Fe elements of CuFeCM surface.

Fig. 4. Flux performances of CuFeCM and CM with or without the addition of PMS treating 20 mg/L HA feed solution: (a) flux decline curves and (b) corresponding fouling resistances. PMS concentration was set as 0.4 g/L, as it possessed the optimal degradation efficiency of HA via CuFeCM catalysis according to Figs. S1 and SI. The flux tests were conducted three times and the corresponding error bar of each flux performance was shown in Figs. S3 and SI.

and were degraded by approximately ~62.1% and ~53.3% via PMS/ CuFeCM oxidation. The HA removal rates of PMS/CuFeCM oxidation and CuFeCM single filtration were both lower than that of PMS/ CuFeCM hybrid filtration, suggesting that the synergistic effect of catalytic oxidation and filtration process enhanced the HA removal

efficiency of PMS/CuFeCM hybrid filtration. In addition, virgin CM also showed increasing HA removal rate when combined with PMS (Fig. 5(b), (d)), which could be attributed to the certain catalytic effect of the virgin CM. Specifically, CM was formed by TiO2 and ZrO2, which were reported to be catalytic towards PMS oxidation

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Fig. 5. FEEM spectra and TOC analysis of HA feed and permeate solutions by different filtration processes. (a): FEEM spectra of HA feed solution. (b) and (c): FEEM spectra of HA permeate by CM and CuFeCM single filtrations. (d) and (e): FEEM spectra of HA permeate by PMS/CM and PMS/CuFeCM hybrid filtrations. (f): TOC removal rates of HA solutions by different filtration processes. Possible quenching effect of PMS to HA spectra was also explored in Figs. S5 and SI, indicating minor influence of PMS addition to the EEM spectra of HA.

(Ali et al., 2019; Rostami-Vartooni et al., 2019; Wan et al., 2019; Wang et al., 2019a). TOC removal efficiency was further evaluated to qualitatively assess the HA removal performance (Fig. 5(f)). The TOC removal rate by CuFeCM single filtration was 30.1%, while it greatly improved to 76.2% in PMS/CuFeCM hybrid filtration (The corresponding PMS cost for removing unit TOC was also estimated: 2.02 $/gTOC, with detailed parameters provided in Tables S2 and SI). In addition, PMS/CuFeCM oxidation removed 12.0% of TOC content within 60 min (Figs. S6 and SI), which indicated that the mineralization extent was negligible by catalytic oxidation process alone. Taken together, the synergistic oxidation - filtration process in PMS/ CuFeCM catalytic filtration achieved more thorough removal of HA compared to that in single oxidation or single filtration process. 3.3. Synergistic oxidation - filtration process analysis 3.3.1. Influence of PMS/CuFeCM oxidation on HA properties In Fig. 6(a), FT-IR spectra illustrated the change of HA functional groups before and after PMS/CuFeCM catalytic oxidation. The broad band at 3440 cm1 of HA sample represented eOH stretching of phenolic compound, and the adsorption peak at 1640 cm1 indicated eC¼O stretching of aromatic ring. Band at 1400 cm1 came from the deformation of eOH and CeO stretching of phenol eOH (Elkins and Nelson, 2001; Enev et al., 2018; Guo et al., 2019; Yang et al., 2019). With the prolongation of the catalytic oxidation, IR bands at 3440 cm1 and 1640 cm1 gradually decreased and the adsorption band at 1400 cm1 disappeared, indicating that complex structure, i.e. phenolic compound or aromatic ring, degraded to certain extent. Meanwhile, the mitigation of eOH ligand suggested that the HA particles after catalytic oxidation were more

hydrophobic, which was confirmed by the contact angle (CA) measurement (Table 1): CAwater of HA was 59 and increased to 92 after catalytic oxidation. Additionally, CeO stretching of aliphatic compound, e.g. esters, ethers and alcohols, at 1130 cm1 and 1000 cm1 became stronger after catalytic oxidation, which suggested that the rings of complex aromatic structures were opened and degraded to aliphatic compound via PMS/CuFeCM catalytic oxidation. Molecular weight (MW) distribution was further investigated via HP-SEC (Fig. 6(b1), (b2)) characterization. The original HA particles presented two peaks: 8181 Da and 1130 Da, and degraded to three peaks after PMS/CuFeCM catalytic oxidation, i.e., 6683Da, 2578 Da and 841 Da. Interestingly, HA after catalytic oxidation (short as “HAAOP”) showed extremely high UV254 absorbance (approximately 1 V of the dominant peak) compared to that of virgin HA (approximately 3644  106 V of the dominant peak). The significantly high UV254 adsorption of the new structures indicated the formation of K band, which was normally formed by P-P* transition of conjugated structure (Fukushima and Tatsumi, 2006; Yakimenko et al., 2018). In addition, with the prolongation of the batch experiment, main peak at 6683 Da increased during the reaction time at 5 mine30 min, then tended to be stable after 30 min. This suggested that PMS/CuFeCM catalytic oxidation degraded HA into conjugated structures with smaller MW but could not further degrade the HA structure completely. Due to the transformation of HA structure, UV254 absorbance comparison between virgin HA and HAAOP permeate from the hybrid filtrations could not be directly applied to evaluate HA removal efficiency. Instead, in replacement of virgin HA feed solution, HA from the PMS/CuFeCM oxidation batch experiment at 30min (HAAOP30) were employed to evaluate the removal efficiency (i.e., the removal

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Fig. 6. FT-IR and HP-SEC analysis of HA feed and permeate solutions by varying filtration processes. (a). FT-IR spectra of HA by PMS/CuFeCM catalytic oxidation at different reaction times, i.e., 5 min, 30 min and 60 min. (b). Molecular weight distributions of HA after catalytic oxidations, hybrid filtrations (b1) and single filtrations (b2). In graph (b1), “CuFeCM þ PMS batch 5 (/30/60) min” and “CM þ PMS batch 5 (/30/60) min” represented oxidized HA by PMS/CuFeCM and PMS/CM catalytic oxidations, and the duration of the oxidation was 5 (/30/60) min. “CuFeCM þ PMS permeate” and “CM þ PMS permeate” represented the permeate from PMS/CuFeCM and PMS/CM hybrid filtrations. In graph (b2), “CuFeCM permeate” and “CM permeate” represented the permeate solutions from CuFeCM and CM single filtrations.

Table 1 Contact angles and zeta potentials of HA foulants and membranes. Item

Contact angle (o) water

HA HAAOP CuFeCM CM

59 92 30 50

± ± ± ±

3.65 6.98 1.53 1.22

Zeta potential (mV) formamide 45 47 30 33

± ± ± ±

3.81 3.08 1.70 1.87

diiodomethane 39 34 22 25

± ± ± ±

4.03 2.68 1.05 1.78

pH ¼ 3

pH ¼ 7

/ 41.6 ± 3.20 14.9 6.2

55.1 ± 3.85 / 12.4 40.9

*HAAOP in this table refers to the oxidized HA particles from the PMS/CuFeCM batch experiment at 30 min (Other abbreviations are explained as follows: HA (Humic acid), CuFeCM (CuFe2O4 tailored ceramic membrane), CM (ceramic membrane). The pH of HA feed solution was 7, and the pH value changed to 3 after adding PMS solution into the HA feed solution.

rate ¼ UV254 absorbance of HAAOP permeate/UV254 absorbance of HA AOP30). Without the combination of PMS, both CuFeCM and virgin CM single filtrations showed weak rejection of HA, which corresponded to the FEEM result (Fig. 5). Specifically, CuFeCM and CM removed ~49.4% and ~24.5% of the HA main structure at 8181Da. When combining with PMS, the removal efficiency greatly improved, as evidenced by ~80.1% and ~67.2% removal of the HAAOP main structure at 6683Da by PMS/CuFeCM and PMS/CM hybrid filtration, respectively, suggesting that the high UV absorbing compounds of HAAOP were largely rejected above CuFeCM in hybrid filtration. This also consisted with the higher reversible fouling resistance of PMS/CuFeCM hybrid filtration compared to that of single filtration, as more rejected HAAOP foulant likely caused heavier external fouling and consequently increased the reversible fouling resistance (Chang et al., 2017; Lu et al., 2015). It is worth noted that the hydrodynamic diameter of HA increased from 243 nm at 0 min to 988 nm at 60 min (Fig. 7(a)) during the batch experiment in PMS/CuFeCM catalytic oxidation system. The growing hydrodynamic size of HAAOP could be explained by the newly formed conjugated structures as suggested by HP-SEC analysis (Fig. 6(b1), (b2)). Conjugated structures consisted of large amount of unsaturated bonds which tended to cause aggregation and thus formed larger flocs (Scheme 1(a)) (Hertkorn et al., 2013; Zhong et al., 2017, 2018). Additionally, Fig. 7(b) showed the filtration and deposition of different HA foulants (sampled from different time intervals of PMS/CuFeCM oxidation process) on 100 kDa PES membrane. With the prolongation of

oxidation, given amount of HAAOP foulant sampled from the longer time interval of the oxidation system was rejected more above 100 kDa PES membrane, which consisted with the growing size of HA in PMS/CuFeCM catalytic oxidation process. This result indicated that HA particles were more prone to be retained above membrane due to the enhanced size exclusion effect, and consequently caused more external fouling in PMS/CuFeCM hybrid filtration than those in single filtration. Additionally, the larger flocs of the aggregated HAAOP likely prevented individual small pieces of HAAOP from directly penetrating through the membrane, thus assisting CuFeCM to remove the HA foulant more thoroughly, and consequently enhancing the TOC removal rate (Fig. 5(f)). 3.3.2. Membrane fouling analysis by classic fouling model fittings The four classic fouling models, including standard blocking, intermediate blocking, complete blocking and cake fouling, were employed to explain the fouling patterns of PMS/CuFeCM hybrid filtration. Correlation coefficient values (R2) of the regression analysis were shown in Fig. 8(a). R2 values of cake fouling and standard blocking in CuFeCM and CM single filtrations were the highest, indicating that cake fouling and standard blocking mainly occurred. This could be due to the much smaller molecular weight of HA than the molecular weight cut-off of CM (300 kDa). Smaller HA foulants thus likely penetrated membrane surface causing pore blocking (i.e., standard blocking) and piled up on membrane surface forming cake layer (i.e., cake fouling). When adding PMS into the filtration, R2 values of cake fouling and standard blocking were

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Fig. 7. Size analysis of HA particles after PMS/CuFeCM catalytic filtration process. (a) Change of HA hydrodynamic diameter during PMS/CuFeCM catalytic oxidation process and (b) filtration of HA sampled from different time intervals (i.e., 0 min, 5 min, 30 min and 60 min) of PMS/CuFeCM oxidation process (HA initial concentration: 20 mg/L, PMS ¼ 0.4 g/L) on 100 kDa PES ultrafiltration membrane (MSC76100, Shanghai Mosu Science Equipment, China). Filtration volume of each HA foulant solution was 50 mL. The PES ultrafiltration membrane area was 3.14 cm2.

still the highest, yet R2 values of intermediate blocking and complete blocking increased in hybrid filtrations (Fig. 8(b)). This could be ascribed to the growing size of HAAOP particles during catalytic oxidation (Fig. 7). Instead of penetrating membrane surface, sticking inside membrane pore and causing standard blocking, bigger HAAOP particles could not squeeze into membrane pore, thus were more prone to be retained above membrane and blocked membrane pore (i.e., complete blocking) or aggregated with other HA particles to block pore (i.e., intermediate blocking) on the membrane surface. SEM characterizations of the fouled CuFeCM further corroborated with the modeling analysis. From Figs. S7 and SI, agglomerated HA particles were accumulated and stacked on membrane surface after PMS/CuFeCM catalytic filtration (Fig. S7-a1, SI), while smaller HA particles were observed on membrane surface after single CuFeCM filtration (Fig. S7-b1, SI). Additionally, HA fouling layer mainly deposited above CuFeCM surface after PMS/ CuFeCM catalytic filtration (Fig. S7-a2, SI), yet HA foulants penetrated deeply into the membrane pores after single CuFeCM filtration (Fig. S7-b2, SI). The analysis of the four classic fouling models also consisted with the higher reversible fouling resistance and lower irreversible fouling resistance of PMS/CuFeCM hybrid filtration, as compared with those of CuFeCM single filtration. In previous studies, the reversible fouling resistance was reported to be caused mostly by external fouling due to the accumulation of foulant above membrane (i.e., intermediate blocking and complete blocking) and the formation of cake/gel layer (i.e., cake fouling), thus could be easily removed by hydraulic cleaning (Chang et al., 2017, De Angelis and Fidalgo de Cortalezzi, 2016; Lu et al., 2016; Lu et al., 2015). Similarly, in PMS/CuFeCM hybrid filtration, the HAAOP particles that aggregated more on CuFeCM surface likely caused more external fouling, which resulted in higher reversible fouling resistance and more efficient hydraulic cleaning compared to those in single filtrations.

3.3.3. Physicochemical interaction analysis of CuFeCM - HA According to the physicochemical properties of HA and membranes (Table 1), the physicochemical interactions between HA foulant and membranes were calculated and shown in Table 2. AB and LW free energies were all negative, which indicated attractive AB and LW interactions between HA and membranes. EL free energy between CuFeCM and HAAOP was negative and thus indicated attractive interaction, meanwhile EL free energies in other three scenarios were positive suggesting repulsive interactions. In Table 2, the trend of AB free energy (△GAB of CuFeCM-HAAOP <△GAB of CM-HAAOP<△GAB of CuFeCM-HA <△GAB of CM-HA) was in reverse order with the trend of LW free energy (△GLW of CuFeCM-HAAOP >△GLW of CM-HAAOP >△GLW of CuFeCM-HA >△GLW of CM-HA). The less AB free energy and higher LW free energy of CuFeCM-HAAOP compared to those in other three filtrations indicated that the polar interaction occurred less while apolar interaction occurred more between CuFeCM and HAAOP in PMS/ CuFeCM hybrid filtration (Anadao et al., 2018; Chu et al., 2017; Shen et al., 2017) as compared with those in other three scenarios. Meanwhile, the trend of total attractive interaction (△Gtot of CuFeCM-HAAOP <△Gtot of CM-HAAOP <△Gtot of CuFeCM-HA <△Gtot of CM-HA) was in accordance with the trend of AB attractive interaction. This was due to that AB free energy was the highest among three free energies (i.e., AB, LW and EL free energies) in each foulant-membrane interaction, which suggested the dominant influence of AB free energy on total attractive interaction energy in this case. Compared to the AB interaction of CM-HA, the reduced AB attractive interaction of CuFeCM-HA suggested less attraction and attachment of HA foulant on CuFeCM, thus likely causing the better flux performance of CuFeCM than that of CM (Fig. 4(a)). Similarly, due to the lowest AB free energy, the lowest total attractive interaction between HAAOP and CuFeCM in PMS/ CuFeCM hybrid filtration indicated least attraction of HAAOP foulant towards membrane and further least penetration, thus assisted

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Scheme 1. Schematics depicting the proposed mechanism regarding HA fouling and retention during PMS/CuFeCM catalytic filtration: (a) HA transformation by PMS/CuFeCM oxidation. The generation of
OH and SO4
 were confirmed by radical quenching experiment in Fig. S4(A2), (B2) and Text S2, SI. (b1) HA fouling and retention behaviors along with (b2) the state of CuFeCM after hydraulic cleaning in PMS/CuFeCM catalytic filtration. (c1) and (c2) are the corresponding performance of conventional CM filtration for comparison.

improving the retention performance (Scheme 1(b1), (c1)). This also corresponded to the notably reduced irreversible fouling resistance and highest flux recovery of PMS/CuFeCM hybrid filtration, as less attraction between membrane and foulant suggested easier removal of the foulant above membrane by hydraulic cleaning (Scheme 1(b2), (c2)). However, it is worth noted that albeit the least attractive interaction of CuFeCM-HAAOP, PMS/CuFeCM hybrid filtration presented continuous flux decline and higher reversible fouling resistance than single filtration. This could be due to that size exclusion, besides physicochemical factors (i.e., AB, LW and EL factors), also exerted critical influence on the filtration pattern of hybrid filtration. As shown in Fig. 7, the size of HA particles continually increased with the prolongation of PMS/CuFeCM oxidation, therefore it was likely that the growing particle size of HA continually caused membrane fouling along with the hybrid filtration, hence leading to the “continuous flux decline” rather than the pattern of “fast-slow” flux decline as shown in the single

filtrations. 4. Conclusion We demonstrated the fabrication and characterization of CuFe2O4-tailored ceramic membrane (CuFeCM), and further evaluated its catalytic filtration performance and intercorrelated oxidation-filtration mechanism in treating HA via PMS activation. PMS/CuFeCM hybrid filtration exhibited substantially enhanced HA removal efficiency with a reduced irreversible fouling resistance compared to conventional CM filtration, which could be ascribed to the changing properties of HAAOP and further decreased AB attractive interaction between HAAOP and CuFeCM. Specifically, PMS/CuFeCM catalytic oxidation degraded HA particles into lowermolecular weight and relatively stable structures with unsaturated bonds, which assisted aggregation of HAAOP and thus facilitated the size exclusion of CuFeCM filtration. Additionally, the less polar

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Fig. 8. Four classic fouling model analysis of PMS/CuFeCM catalytic filtration: (a). R2 values of four classic fouling model fittings in different filtration types (R2 value was calculated from the first cycle of each filtration). (b). Schematic illustrating the four fouling situations.

Table 2 Surface energy parameters between HA foulants and membranes. Item

Surface energy parameter with CuFeCM (mJ/m2) △G

HA HAAOP

AB

16.422 10.659

LW

EL

Surface energy parameter with CM (mJ/m2) tot

△G

△G

△G

△GAB

△GLW

△GEL

△Gtot

7.319 8.134

0.050 0.045

23.691 18.837

22.101 13.255

7.071 7.857

0.163 0.019

29.008 21.094

*HAAOP in this table refers to the oxidized HA particles from the PMS/CuFeCM batch experiment at 30 min (Other abbreviations are explained as follows: HA (Humic acid), △GAB (Lewis acid-base component of free energy), △GLW (Lifshitz-van der Waals component of free energy), △GEL (electrostatic component of free energy).

interaction between HAAOP and CuFeCM caused less total attraction of HAAOP on CuFeCM, suggesting easier HAAOP removal by hydraulic cleaning. The synergistic effect of PMS/CuFeCM catalytic filtration increased the overall HA rejection while decreased the irreversible fouling resistance and notably enhanced the hydraulic cleaning efficiency, which showed potential promising application in practical low-pressure cross-flow filtration for the NOM removal. It is worth noted that compared to other catalytic filtration processes where pollutants were generally oxidized and decomposed into smaller structures, PMS/CuFeCM configuration enabled the reagglomeration of the oxidized HA, which further improved TOC removal efficiency (due to membrane steric effect). This phenomenon could broaden the current understanding towards the features and functionalities of the integrated catalytic membrane filtration processes. Additionally, it should be mentioned that influencing factors, i.e., varied NOM sources with different properties, coexistence of the general divalent ions, etc., possess important impact on membrane performance. Meanwhile, the estimated cost of PMS/CuFeCM configuration for NOM removal is still comparatively higher as compared with those of current technologies. Therefore, future works regarding the analysis of relevant influencing factors and economic factors could be conducted to gain more comprehensive understanding and promote practicality of the PMS/CuFeCM filtration configuration.

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.

Acknowledgements This work was jointly supported by the National Key Research and Development Program of China (No.2017YFA0207203), China Postdoctoral Science Special Foundation (No.2018T110304), China Postdoctoral Science Foundation (No. 2017M621288), National Natural Science Foundation of China (No.51908162), Major National Science and Technology Projects and Key Research and Development Projects of China (GX18A024) and Applied Technology Research and Development Plan of Heilongjiang Province, China (GY2019YF0129). Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.watres.2019.115387. References Adusei-Gyamfi, J., Ouddane, B., Rietveld, L., Cornard, J.P., Criquet, J., 2019. Natural organic matter-cations complexation and its impact on water treatment: a critical review. Water Res. 160, 130e147. Ali, T.T., Narasimharao, K., Basahel, S.N., Mokhtar, M., Alsharaeh, E.H., Mahmoud, H.A., 2019. Template Assisted microwave synthesis of rGO-ZrO2 composites: efficient photocatalysts under visible light. J. Nanosci. Nanotechnol. 19 (8), 5177e5188. Anadao, P., Sato, L.F., Wiebeck, H., 2018. Study of the influence of graphite content on polysulfone-graphite composite membrane properties. J. Therm. Anal. Calorim. 134 (3), 1647e1656. Andersson, C., Hedlund, J., 2008. Effects of exposure to water and ethanol on silicalite-1 membranes. J. Membr. Sci. 313 (1e2), 120e126. Bao, Y., Lim, T.-T., Wang, R., Webster, R.D., Hu, X., 2018. Urea-assisted one-step synthesis of cobalt ferrite impregnated ceramic membrane for sulfamethoxazole degradation via peroxymonosulfate activation. Chem. Eng. J. 343, 737e747. Byun, S., Davies, S.H., Alpatova, A.L., Corneal, L.M., Baumann, M.J., Tarabara, V.V.,

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