Synergistic effect of ferrate (VI)-ozone integrated pretreatment on the improvement of water quality and fouling alleviation of ceramic UF membrane in reclaimed water treatment

Author’s Accepted Manuscript Synergistic effect of ferrate (VI)-ozone integrated pretreatment on the improvement of water quality and fouling alleviation of ceramic UF membrane in reclaimed water treatment Jing Liu, Zhenghua Zhang, Qianhua Chen, Xihui Zhang www.elsevier.com/locate/memsci

PII: DOI: Reference:

S0376-7388(18)31485-6 https://doi.org/10.1016/j.memsci.2018.09.029 MEMSCI16474

To appear in: Journal of Membrane Science Received date: 30 May 2018 Revised date: 23 August 2018 Accepted date: 6 September 2018 Cite this article as: Jing Liu, Zhenghua Zhang, Qianhua Chen and Xihui Zhang, Synergistic effect of ferrate (VI)-ozone integrated pretreatment on the improvement of water quality and fouling alleviation of ceramic UF membrane in reclaimed water treatment, Journal of Membrane Science, https://doi.org/10.1016/j.memsci.2018.09.029 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Synergistic effect of ferrate (VI)-ozone integrated pretreatment on the improvement of water quality and fouling alleviation of ceramic UF membrane in reclaimed water treatment Jing Liu1,2,3, Zhenghua Zhang1,2,3*, Qianhua Chen1,2,3, Xihui Zhang1,2,3,4

1

Research Institute of Environmental Engineering & Nano-Technology, Graduate

School at Shenzhen, Tsinghua University, Shenzhen 518055, Guangdong, China 2

Guangdong Provincial Engineering Research Center for Urban Water Recycling and

Environmental Safety, Graduate School at Shenzhen, Tsinghua University, Shenzhen 518055, Guangdong, China 3

School of Environment, Tsinghua University, Beijing 100084, China

4

Tsinghua-Berkeley Shenzhen Institute, Tsinghua University, Shenzhen 518055,

Guangdong, China

Journal of Membrane Science Revised version Submitted August, 2018

*Corresponding author: [email protected] (Z.H. Zhang)

1

Abstract A novel ferrate (VI)-ozone integrated pretreatment process was proposed and applied herein for the first time in reclaimed water treatment. We demonstrated the synergistic effect of the ferrate (VI)-ozone integrated process in terms of •OH generation and its double-win effects on the improvement of water quality and fouling alleviation of ceramic ultrafiltration (UF) membrane. The concentration of •OH in the ferrate (VI) (0.15 mM)-ozone (10 mg/L) integrated case was almost 3-fold higher compared to the ferrate (VI) (0.15 mM) case, while no obvious •OH was detected in the pre-ozonation case (10 mg/L) in Milli-Q water. The reclaimed water quality was remarkably enhanced with the maximum removal efficiencies of 70.5% (DOC), 80.5% (UV254), 25.6% (protein) and 65.5% (polysaccharide) in influent achieved by the integrated pretreatment of ozone (10 mg/L) and ferrate (0.15 mM). Biopolymers (molecular weight (MW) of 20-2000 kDa) were the main removed organics with the best removal efficiency of 92.6%. Additionally, membrane fouling was effectively mitigated and the normalized trans-membrane pressure (TMP) was maximally decreased by 89.5% obtained after the integrated pretreatment of ozone (10 mg/L) and ferrate (0.15 mM). The statistical analysis results demonstrated that the increased particle size and the effective reduction of organic matters with ferrate-ozone integrated pretreatment led to a porous fouling layer with high permeability and thus a remarkable reduction of TMP and resistances, which was also in agreement with the modeling results that cake blocking was significantly alleviated.

Keywords: Membrane fouling mitigation; ozonation; hydroxyl radical; ferrate (VI); reclaimed water treatment; 2

1. Introduction Wastewater reclamation has become a promising strategy to address the critical water scarcity issue especially for farmland irrigation, landscaping and industrial applications such as cooling water [1-5]. Membrane technology particularly UF membrane has been extensively adopted in wastewater reclamation owing to the small footprint of membrane and good water quality of permeate after membrane filtration, however, membrane fouling caused by deposition and adsorption of soluble organic matters and suspended solid still remains a significant obstacle as fouling results in the decreased flux and lifespan of membrane and increased cost of membrane maintenance [6-8]. Effluent organic matters (EfOM) in reclaimed water including natural organic matters (NOM), soluble microbial products (SMP) and other organic mixtures generated after the biological secondary treatment [9,10] mainly contribute to the irreversible membrane fouling and decline of membrane flux [11,12]. Ozonation has been identified as an effective strategy to alleviate organicsmediated membrane fouling [13]. In our most recent studies, the hybrid treatment of ozonation and ceramic membrane successfully mitigated the protein-based and polysaccharide-based ceramic membrane fouling by breaking down the high MW organic matters into the low MW organic matters, resulting in the reduction of membrane interception of high MW organic matters [14,15]. Meanwhile, we also extended the applications of the combined treatment of ozonation (2-10 mg/L) and ceramic UF membrane in the treatment of wastewater [16,17], algal-rich water [18,19] and drinking water [20,21] with the excellent performance of membrane fouling alleviation. Ferrate (VI), a unique and “green” chemical with multifunctions, has been of particular interest in water and wastewater treatment for simultaneous oxidation, 3

disinfection and coagulation [22]. Ferrate (VI) (E0=2.20 V at acidic pHs; E0=0.72 V at basic pHs) [23] is a powerful oxidant for the mitigation and degradation of many contaminants in water and wastewater treatment without any disinfection by-products, such as natural organic matters (NOM) [24,25], phenol [26], chlorobenzene compounds [27], pharmaceuticals and personal care products (PPCPs) [28] and heavy metals [29,30]. Meanwhile, ferric hydroxide can be in-situ formed during the decomposition process of ferrate (VI), which could further facilitate the removal of organic matters with oxidation and coagulation occurring simultaneously [31]. Moreover, the commercial price of ferrate (VI) is still decreasing due to the recent synthesis progress with the current price of around $3, 000/ton in China, which makes ferrate (VI) economically available for water and wastewater treatment. The ferrate (VI)-ozone integrated process (in the same unit) in water and wastewater treatment would be more applicable and preferred for NOM removal and subsequent membrane fouling control in comparison to ferrate (VI) or ozone alone as the integrated process can not only reduce the ferrate dosage and related cost, but also enhance the oxidation performance given that the resultant particles of Fe(VI) such as Fe2O3 and FeOOH [29,30] would catalyze ozone decomposition and facilitate the generation of •OH [32,33]. Moreover, compared with the conventional advanced oxidation processes (AOPs), the ferrate (VI)-ozone integrated process would have simultaneous oxidation, disinfection and coagulation in the same unit [22]. However, extremely limited study about the ferrate (VI)-ozone integrated process has been reported and no study has been reported for membrane fouling control by using the ferrate (VI)-ozone integrated process. Up to now, there is only one paper reporting the application of ferrate (VI)-ozone integrated treatment for bromate inhibition [34], in

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which the synergistic effect of ferrate (VI)-ozone in terms of •OH generation was not investigated. In this study, we herein applied the ferrate (VI)-ozone hybrid treatment in reclaimed water treatment for the first time and investigated its synergistic effect on the improvement of water quality and fouling alleviation of ceramic UF membrane by assessing 1) characterization of influent and effluent; 2) •OH generation by electron paramagnetic resonance analysis; 3) membrane fouling control performance; 4) membrane fouling modeling analysis; 5) redundancy analysis (RDA) and clustering correlation analysis for membrane fouling. Our results first reported the synergistic effect of the ferrate (VI)-ozone integrated process in terms of •OH generation and demonstrated its double-win effects on the improvement of water quality and fouling alleviation of ceramic UF membrane in reclaimed water treatment.

2. Materials and methods 2.1 Experimental preparation and apparatus The secondary effluent after ultraviolet (UV) disinfection (Xili Wastewater Treatment Plant, Shenzhen) was sampled as the reclaimed water feed with the water quality shown in Table 1. The schematic of the ceramic membrane reactor accompanied by the hybrid process of ferrate (VI)-ozone was shown in Fig.1. The reactor was comprised of two units, stirring unit (5 L) (200 rpm) and filtration unit (10 L) in which ceramic membrane was applied (area of 0.0425 m2 and average pore size of 100 nm, Meidensha Corporation, Japan). TMP was automatically recorded every ten seconds by a data logger and normalized TMP (also named as fouling TMP (△TMP) = total TMP - pristine membrane TMP) was used to assess the performance

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of membrane fouling. The specific dimensions of the reactor, ceramic UF membrane module and instrumental models are the same as in our most recent paper [35]. Table 1. Characteristics of the feed water quality COD (mg/L) DOC (mg/L) UV254 (cm-1) TP (mg/L) Protein (mg/L) Polysaccharide (mg/L) TN (mg/L) Turbidity (NTU) pH Mean particle size (nm) Zeta potential (mV)

20.5 ± 4 7.5 ± 1 0.158 ± 0 .04 0.35 ± 0.1 4.3 ± 0.5 5.82 ± 1.2 12 ± 3 3.65 ± 1 7.4 ± 0.3 246.5 ± 25 -12.5 ± 1.5

2.2 Pretreatment and UF filtration tests Our most recent paper identified that 0.15 mM ferrate (VI) pretreatment was the optimized dosage in view of membrane fouling alleviation in reclaimed water treatment with the ceramic membrane filtration flux of 90 L/(m2•h) [35]. As such, preozonation alone (0, 2, 5, 10 mg/L) and the integrated pretreatment of ozone (0, 2, 5, 10 mg/L) and ferrate (0.15 mM, one-off dosing) (Potassium ferrate (K2FeO4), 90.4%, analytical grade) were conducted to the reclaimed water feed in the pretreated tank for 30 min with the flowrate of gaseous ozone of 100 mL/min. More detailed information about ozonation procedures and concentration calculation can be found in our previous studies [16-19]. The pHs of the pretreated feed were shown in Fig. S1 with the values of 7.5-7.7 (pre-ozonation) and 6.3-6.4 (ferrate-ozone integrated treatment). After pretreatment, the pretreated feed was subsequently flown into the ceramic membrane filtration unit using the peristaltic pump followed by eight filtration experiments (pre-ozonation (0, 2, 5, 10 mg/L) and the integrated pretreatment of ozone (0, 2, 5, 10 mg/L) and ferrate (0.15 mM, one-off dosing)) under the identically 6

operational circumstances. An intermittent mode with on/off ratio of 9 min: 1 min was adopted by the peristaltic pump to withdraw the effluent from the membrane module under a constant flux of 90 L/(m2•h). The filtration experiments with a constant flux of 90 L/(m2•h) lasted for 5.0 h and were repeated twice. The fouled membrane was subsequently cleaned by Milli-Q water (100 mL, cake layer cleaning), NaClO solution (500 ml 500 ppm available chlorine for 24 h, gel layer cleaning for organics) and citric acid solution (500 mL 500 ppm for 24 h, gel layer cleaning for inorganics). The different resistances were analyzed based on Darcy’s Law [36]: Rtotal 

TMP J

(1)

Rtotal  Rcake  Rgel  Rpore

(2)

where μ is the water viscosity at 25°C (0.8949×10-3N·s/m2); J is the permeate flux (90 L/(m2•h)). The overall membrane fouling resistance was represented by Rtotal. The cake layer resistance (the changed resistance before and after Milli-Q water rinsing) was represented by Rcake. The gel layer resistance (the changed resistance before and after chemical cleaning) was represented by Rgel. The pore blocking resistance (the changed resistance between the intrinsic resistance and the resistance after chemical cleaning ) was represented by Rpore.

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Fig. 1. Experimental apparatus diagram of filtration test coupled with the ferrate-ozone hybrid process. Noted that the feed flowing into the pretreated tank was sampled from a local WWTP.

2.3 Analytical methods 2.3.1 Conventional measurement The measurement methods of dissolved organic carbon (DOC) (≤ 0.45 μm), UV254, protein, polysaccharide, zeta potential, particle size, fluorescence excitation-emission matrix (EEM) and MW distribution were identical as in our previous studies [16-19]. 2.3.2 EPR analysis The electron paramagnetic resonance (EPR) spectrum of •OH was determined by an EMXplus spectrometer (Bruker, Germany) at room temperature with dimethyl pyridine N-oxide (DMPO) (CDMPO = 0.22 M) as the scavenger of •OH and the instrument parameters of field sweep; center field of 3516.25 G; receiver gain of 30 dB; conversion time of 15 ms; sweep width of 200 G; sweep time of 30 s; modulation amplitude of 1 G; microwave attenuation of 20 dB and microwave power of 2.129 mW. Meanwhile, the concentration of •OH and the number of spins were further analyzed by spin counting using Xenon software installed in EMXplus spectrometer. 8

2.3.3 SEM Scanning electron microscopy (SEM) (SU8010, HITACHI, Japan) was adopted to investigate the surface morphology of the pristine and fouled membranes. Besides, the porosity of fouled membrane surface was calculated by Image-Pro plus software using the magnified surface SEM micrographs (x20, 000) of the pristine and fouled membranes [37]. All the raw images were first converted to 8-bit gray-scale images followed by image binarization (0 for black color of fouled membrane surface and 255 for blue color of background). These images were then partitioned into only black and blue pixels after identifying the pores on the membrane surface and changing the color of pores from blue to black with the porous pixel area fraction representing the porosity of fouled membrane surface. 2.3.4 Membrane fouling modeling The experimental data were fitted by the fouling models (Table S1) to deeply understand the membrane fouling mechanism [38,39] and more detailed information can be found in our previous studies [16-19]. 2.3.5 RDA analysis and heat map analysis for membrane fouling The correlation between resistances (overall resistance, cake layer resistance, gel layer resistance and pore blocking resistance) and influent properties (MW, turbidity, mean particle size, polysaccharide, protein, UV254, EEM, DOC, MW, turbidity, zeta potential and mean particle size) was quantitatively analyzed by heat map analysis and redundancy analysis (RDA) with software Language R and software CANOCO version 4.5, respectively. More details of RDA analysis and heat map analysis can be found in our most recent study [35].

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3. Results 3.1 Effect of ferrate-ozone integrated pretreatment on water quality 3.1.1 Conventional properties of influent and effluent Fig. 2 shows the effect of the hybrid pretreatment of ozone (0-10 mg/L) and ferrate (0.15 mM) with membrane filtration on water quality of membrane influent and effluent. The concentrations of DOC, UV254, polysaccharide and protein in influent were effectively decreased by pre-ozonation alone. The combined pretreatment of ferrate and ozone further decreased the concentrations of organics in influent (Figs. 2a-2d). The maximum removal efficiencies of 70.5% (DOC), 80.5% (UV254), 25.6% (protein) and 65.5% (polysaccharide) in influent were achieved by the integrated pretreatment of ozone (10 mg/L) and ferrate (0.15 mM). Moreover, ceramic membrane filtration also further promoted the removal efficiency of organics with the further reduction of DOC, UV254, polysaccharide and protein in effluent. Meanwhile, the influent zeta potential augmented with ozonation, however, it decreased dramatically with ferrate dosing (Fig. 2e). Generally, ozonation changes the zeta potential of particles through damaging the organics adsorbed on the surface of particles with the formation of relatively polar ozonides, ketones, aldehydes, organic acids, and other functional groups [40,41], which could enhance the negative surface charge of particles and thus affect particle aggregation performance [42]. Herein, the zeta potential of reclaimed water became more negative, increasing from -12.5 mV to -16.8 mV with increasing ozone dosage up to 10 mg-O3/L (Fig. 2e). Instead, it effectively decreased from -12.5 mV to -7.85 mV with ferrate pretreatment (Fig. 2e). The increased electrostatic repulsion after ozonation would decrease the particle size of suspended solids in influent, while ferrate dosing remarkably decreased the electrostatic repulsion of suspended solids possibly leading to the significantly 10

increased particle size [43,44]. This was in accordance with the consequences of mean particle size (Fig. 2f) and turbidity (Fig. S1). After ceramic membrane filtration, however, turbidity was below 0.4 NTU in effluent (Fig. S2).

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Fig. 2. Effect of the integrated pretreatment of ozone (0-10 mg/L) and ferrate (0.15 mM) and ceramic membrane filtration on DOC (a), UV254 (b), protein (c), polysaccharide (d), zeta potential– Influent (e) and mean particle size– Influent (f).

3.1.2 Fluorescence EEM spectra 11

Fluorescence EEM spectra was adopted to evaluate the effect of the integrated pretreatment of ozone (0-10 mg/L) and ferrate (0.15 mM) on influent water quality. Five regions of EEM spectra were demarcated and the specific partition of five regions could be found in previous studies [45,46]. Fig. 3a shows that the fluorescent SMP-like substances (region IV) and humic-like substances (region V) rather than aromatic protein-like substances (regions I and II) and fulvic acid-like substances (region III) were the major components with strong fluorescent peak intensities in influent. For example, the EEM spectrum volume of regions IV and V accounted for 50.2% and 24.9% of the total EEM spectrum volume (3.08 ×107) of the control sample, respectively (Fig. S3). With the integrated pretreatment of ozone (0-10 mg/L) and ferrate (0.15 mM), the peak intensities of regions IV and V were effectively reduced especially for region IV, however, slight reduction happened to the peak intensities of regions I, II and III (Fig. 3). Pre-ozonation (2, 5 and 10 mg/L) resulted in the reduction of the overall EEM spectrum volume by 10.2%, 17.1% and 18.5%, respectively. The integrated pretreatment of ozone (0-10 mg/L) and ferrate (0.15 mM) further increased the removal efficiency of the overall EEM spectrum volume to 11.5% (2 mg/L), 20.4% (5 mg/L) and 24.9 (10 mg/L), respectively (Fig. S2).

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Fig. 3. Effect of the integrated pretreatment of ozone (0-10 mg/L) and ferrate (0.15 mM) on fluorescence EEM spectra of influent. (a) 0 mg/L ozone without ferrate; (b) 2 mg/L ozone without ferrate; (c) 5 mg/L ozone without ferrate; (d) 10 mg/L ozone without ferrate; (e) 0 mg/L ozone with ferrate; (f) 2 mg/L ozone with ferrate; (g) 5 mg/L ozone with ferrate; (h) 10 mg/L ozone with ferrate..

3.1.3 MW distribution

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The effect of the integrated pretreatment of ozone (0-10 mg/L) and ferrate (0.15 mM) on MW distribution of DOC in influent was shown in Fig. 4. The MW distribution of DOC mainly comprised of three fractions, biopolymers (20-2000 kDa), humic-like substances (0.35-20 kDa) and low MW substances (0.001-0.35 kDa) [18,19]. For the control sample, biopolymers, humic-like substances and low MW substances accounted for 55.2%, 31.7% and 13.1% of DOC, respectively. Preozonation alone effectively decreased the organic matters in influent, especially for the fraction of biopolymers (Fig. 4a). The amount of biopolymers was reduced by 23.1% (2 mg/L), 45.1% (5 mg/L) and 55.4% (10 mg/L), respectively (Fig. 4b). With the integrated pretreatment of ozone (0-10 mg/L) and ferrate (0.15 mM), the removal efficiencies of low MW organic matters, humic-like substances and biopolymers were further increased. The best removal efficiencies of 72.9% (low MW organic matters), 43.1% (humic-like substances) and 92.6% (biopolymers) were achieved after the integrated pretreatment of ozone (10 mg/L) and ferrate (0.15 mM) (Fig. 4b). High MW substances were generally degraded into low MW substances after oxidation and herein the decrease of low MW organics was possibly because part of the low MW organics in the reclaimed water were mineralized especially in the presence of •OH significantly generated by the ferrate (VI)-ozone hybrid process, which was discussed in the following part.

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DOC intensity (×105 AU)

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3.1.4 Discussion of the improved water quality The ferrate (VI)-ozone integrated process achieved better water quality in comparison to ferrate (VI) or ozone alone as shown in Fig. 2. Note that the investigation of disinfection byproducts (DBPs) is beyond the scope of this study as herein we mainly explore the synergistic effect of ferrate (VI)-ozone hybrid process on the removal of NOM and the subsequent membrane fouling control performance. The removal efficiencies of low MW organics, humic-like substances and 15

biopolymers after 0.15 mM ferrate (VI) pretreatment were 40.6%, 32.7% and 51.5%, respectively (Fig. 4b). Ferrate (VI) can induce simultaneous oxidation and coagulation for organics removal [22]. The intermediates of Fe (VI), such as Fe (V) and Fe (IV) generated through 1-e- and 2-e- transfer processes of Fe (VI) with much stronger oxidation capacity compared with Fe (VI), would facilitate the removal of organics [22]. Meanwhile, reactive oxygen species such as •OH would be produced during the self-decomposition of Fe (VI), which also improve the removal of organics [22]. Moreover, the in-situ formed ferric hydroxide would further remove organics. After the integrated pretreatment of ferrate-ozone, the removal efficiencies of low MW organic matters, humic-like substances and biopolymers were further increased with the highest removal efficiencies of 72.9% (low MW organic matters), 43.1% (humic-like substances) and 92.6% (biopolymers) achieved at ozone dose of 10 mg/L and ferrate dose of 0.15 mM (Fig. 4b). High MW substances were generally degraded into low MW substances after oxidation and herein the decrease of organics especially the low MW organics after the integrated pretreatment of ferrate-ozone might be because of mineralization of organics. This is possibly related to the synergistic effect of the ferrate (VI)-ozone integrated process with the enhanced generation of •OH [32,33], which is borne out by the following EPR results. DMPO was utilized to spin-trap the generation of •OH and the stable DMPO-OH adduct was monitored by EPR with a 1:2:2:1 quartet peak [47]. As shown in Fig. 5a, the quartet peak of DMPO-OH was detected in the ferrate (VI) case [48] and the quartet peak of DMPO-OH became significantly stronger with increase of ozone dosage in the ferrate (VI)-ozone integrated case. The total concentration of •OH and the total number of spins in the ferrate (VI) (0.15 mM)-ozone (10 mg/L) integrated case were almost 3fold higher compared to the ferrate (VI) (0.15 mM) case (Fig. 5b). In contrast, no 16

obvious quartet peak of DMPO-OH was detected in the pre-ozonation case (0-10 mg/L) in Milli-Q water (pH=6.0) (Fig. S4). This confirmed the synergistic effect of the ferrate (VI)-ozone integrated process with more •OH generated, which thus effectively mineralized and removed organic matters in influent (Fig. 4). Note that powder activated carbon (PAC) absorption might be suggested after the integrated process of ferrate-ozone in order to achieve better removal performance of organics given that the low and medium MW organics after oxidation can be more easily absorbed by PAC. The generation of •OH in the ferrate (VI)-ozone integrated process can be elucidated by the following mechanisms: i) •OH was generated by the direct ozone decomposition (O3 + OH- → O2•- + HO2• → •OH); ii) Ferrate (VI) produced •OH in its decomposition process (Fe(VI) → Fe(V) → Fe(III) + H2O2; Fe(VI) → Fe(IV) → Fe(II); Fe(II) + H2O2 → •OH + Fe(III)) [22,48,49]; iii) The resultant particles of Fe(VI) such as Fe2O3 and FeOOH [29,30] with abundant surface hydroxyl groups would interact with ozone by electrostatic force and/or hydrogen bonding and then catalyze ozone decomposition and facilitate the generation of •OH [32,33] (O3 + >OH- → O2•+ HO2• → •OH) (Fig. 5b).

17

a Intensity

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3465

3480

3495

3510

3525

3540

6

6

b

Ozonation + 0.15 mM Fe (VI) 5

5 4 4 3 3 2 2 1 1 0

5

2

10

0

Total number of spins (× 1013)

Concentration (×10-5 M)

Magnetic value (G)

Ozone dosage (mg/L) Fig. 5. EPR spectra of DMPO-OH adduct achieved from the ferrate (VI)-ozone integrated process in Milli-Q water (pH=6.0) (a); and the concentration of •OH and number of spins (b).

3.2 Effect of ferrate-ozone integrated pretreatment on membane fouling 3.2.1 TMP and resistance As shown in Fig. 6, the integrated pretreatment of ozone (0-10 mg/L) and ferrate (0.15 mM) had great influence on the performance of membrane fouling control. For the control sample (without pretreatment), △TMP ascended to 30.5 kPa with the fastest fouling rate (dTMP/dt) after 5.0 h filtration. With pre-ozonation alone, 18

membrane fouling was effectively alleviated and the maximum △TMP decrease of 37.1% was obtained at ozone dosage of 10 mg/L. Compared with pre-ozonation, ferrate dosing (0.15 mM) alone exhibited a better fouling alleviation effect with the △TMP reduction of 68.8%. With the integrated pretreatment of ozone (2, 5 and 10 mg/L) and ferrate (0.15 mM), △TMP after 5.0 h filtration further decreased to 8.3, 5.0, 3.2 kPa, respectively. As shown in Fig. 6b, the overall membrane fouling resistance especially the cake layer resistance was remarkably decreased with pre-ozonation and ferrate dosing. With pre-ozonation alone, the cake layer resistance was remarkably reduced from 7.9 × 1011 m-1 (0 mg/L) to 6.2 × 1011 (2 mg/L), 3.9 × 1011 (5 mg/L) and 2.7 × 1011 m-1 (10 mg/L), respectively. However, the gel layer resistance was slightly decreased with pre-ozonation alone. With the integrated pretreatment of ozone (0-10 mg/L) and ferrate (0.15 mM), the resistances of cake layer and gel layer were both dramatically diminished. For instance, with the integrated pretreatment of ozone (10 mg/L) and ferrate (0.15 mM), the resistances of cake layer and gel layer were maximally reduced to 0.5 × 1011 m-1 and 0.4 × 1011 m-1, respectively.

Normalised TMP (kPa)

35

a Ozone (0 mg/L) Ozone (2 mg/L) Ozone (5 mg/L) Ozone (10 mg/L) Ozone (0 mg/L) + ferrate Ozone (2 mg/L) + ferrate Ozone (5 mg/L) + ferrate Ozone (10 mg/L) + ferrate

30 25 20 15 10 5 0 0

1

2

3

Time (h)

19

4

5

Fouling resistance (1012m-1)

2.0 1.8

b

Rtotal Rgel

1.6

Rcake Rpore

1.4

Ferrate

1.2 1.0 0.8 0.6 0.4 0.2 0.0 0

2

5

10

0

2

5

10

Ozone dosage (mg/L) Fig. 6. Effect of ferrate (0.15 mM)-ozone (0-10 mg/L) integrated pretreatment on fouling TMP (△TMP) (a) and resistance (b) in reclaimed water treatment.

3.2.2 Morphological analysis of ceramic membrane surface The SEM micrographs of the membrane surface after reclaimed water filtration experiments with ferrate-ozone integrated pretreatment are shown in Fig. 7. It can be observed that there was a fouling layer deposited on the membrane surface for the fouled membranes in comparison to the pristine membrane. The porosity of the fouled membrane surface (without treatment) (Fig. 7c) dramatically descended to 1.6% compared to that of the pristine membrane (Fig. 7a) (44.4%) (Table 2), resulting in the severe deterioration of membrane filterability (Fig. 6). Nevertheless, preozonation alone effectively led to the porous membrane surface and the porosity of the fouled membranes augmented from 1.6% (0 mg/L) to 12.2% (10 mg/L) (Table 2). Moreover, the integrated pretreatment of ferrate-ozone further increased the porosity with the largest porosity of 25.7% (Table 2), which was in line with the minimum △TMP of 3.2 kPa obtained at 0.15 mM ferrate and 10 mg/L ozone (Fig. 2).

20

5 μm

2 μm

1 μm

2 μm

5 μm

5 μm

2 μm

2 μm

5 μm

5 μm

2 μm

2 μm

5 μm

5 μm

2 μm

2 μm

5 μm

5 μm

Fig. 7. SEM micrographs of the ceramic membrane surface with ferrate-ozone integrated pretreatment. (a) pristine membrane; (b) magnified pristine membrane; (c) fouled membrane 21

without any pretreatment; (d) fouled membrane with 2 mg/L ozone; (e) fouled membrane with 5 mg/L ozone; (f) fouled membrane with 10 mg/L ozone; (g) fouled membrane with 0.15 mM ferrate; (h) fouled membrane with 2 mg/L ozone and 0.15 mM ferrate; (i) fouled membrane with 5 mg/L ozone and 0.15 mM ferrate; (j) fouled membrane with 10 mg/L ozone and 0.15 mM ferrate.

Table 2. Ceramic membrane surface porosity with ferrate-ozone integrated pretreatment. Conditions

Porosity

Pristine membrane

44.37%

0 mg/L Ozone

1.62%

2 mg/L Ozone

4.69%

5 mg/L Ozone

9.62%

10 mg/L Ozone

12.23%

0.15 mM Fe (VI)

16.54%

2 mg/L Ozone + 0.15 mM Fe (VI)

18.67%

5 mg/L Ozone + 0.15 mM Fe (VI)

22.54%

10 mg/L Ozone + 0.15 mM Fe (VI)

25.71%

3.2.3 Properties of membrane fouling layers 3.2.3.1 DOC, protein and polysaccharide As shown in Fig. 8, with the integrated pretreatment of ozone (0-10 mg/L) and ferrate (0.15 mM), the concentrations of DOC, polysaccharide and protein of cake layer and gel layer were all distinctly reduced. For instance, the DOC concentration in fouling layers especially in cake layer was effectively decreased by pre-ozonation alone (Fig. 8a). The DOC removal efficiency in cake layer was 26.2% (2 mg/L), 64.7% (5 mg/L) and 75.9% (10 mg/L), respectively. In addition, the DOC removal efficiencies in both gel layer and cake layer were further increased especially in gel layer with the integrated pretreatment of ozone (0-10 mg/L) and ferrate (0.15 mM). 22

The maximum DOC removal efficiencies of 68.9% (gel layer) and 78.7% (cake layer) were achieved with the integrated pretreatment of ozone (10 mg/L) and ferrate (0.15 mM). Meanwhile, the integrated pretreatment of ozone (0-10 mg/L) and ferrate (0.15 mM) displayed the better polysaccharide and protein removal efficiencies in cake layer than pre-ozonation alone. The maximum removal efficiencies of 69.2% (protein) and 79.3% (polysaccharide) were obtained at 0.15 mM ferrate and 10 mg/L ozone (Fig. 8b), which was in accordance with the porosity results (Table 1).

a

Cake layer Gel layer

50 40

Ferrate

30 20 10 0 0

2

5

16

Concentration (mg/L)

DOC concentration (mg/L)

60

b

Protein Polysaccharide

12

Ferrate 8

4

0

10

0

2

5

10

0

Ozone dosage (mg/L)

2

5

10

0

2

5

10

Ozone dosage (mg/L)

Fig. 8. Effect of the integrated pretreatment of ozone (0-10 mg/L) and ferrate (0.15 mM) on DOC of cake layer and gel layer (a), and protein and polysaccharide concentrations of cake layer (b).

3.2.3.2 Fluorescence spectra As shown in Fig. 9, it was suggested that the SMP-like substances (region IV) were the principal foulants in the cake layer. For the control sample, the EEM spectra volume of region IV accounted for 52.9% of the total EEM spectrum volume (2.66 × 107) (Fig. S4). With pre-ozonation alone (0-10 mg/L) and the integrated pretreatment of ozone (0-10 mg/L) and ferrate (0.15 mM), the peak intensity of region IV exhibited a distinct decrease (Fig. 9). The overall EEM spectrum volume was decreased by 4.8% (2 mg/L), 11.4% (5 mg/L) and 13.2% (10 mg/L), respectively. The integrated

23

pretreatment of ozone (10 mg/L) and ferrate (0.15 mM) resulted in the maximum removal efficiency of the overall EEM spectrum volume (16.2%) (Fig. S5).

Excitation (nm)

380

420

a

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Ozone: 0 mg/L No ferrate

200.0

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Excitation (nm)

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200.0 400.0 600.0 800.0

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Ⅴ

Ⅳ

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e

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Ozone: 0 mg/L With ferrate

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Ⅴ

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400.0 600.0 800.0

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g

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Ozone: 5 mg/L With ferrate

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200.0 400.0 600.0 800.0

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Excitation (nm)

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Emission (nm)

24

480

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Fig. 9. Effect of the integrated pretreatment of ozone (0-10 mg/L) and ferrate (0.15 mM) on fluorescence EEM spectra of the cleaning solutions of cake layer. (a) 0 mg/L ozone without ferrate; (b) 2 mg/L ozone without ferrate; (c) 5 mg/L ozone without ferrate; (d) 10 mg/L ozone without ferrate; (e) 0 mg/L ozone with ferrate; (f) 2 mg/L ozone with ferrate; (g) 5 mg/L ozone with ferrate; (h) 10 mg/L ozone with ferrate.

3.2.2.3 MW distribution As shown in Fig. 10, the DOC MW distribution of cake layer was tested. Biopolymers (20-2000 kDa), humic-like substances (0.35-20 kDa) and low MW organic matters (0.001-0.35 kDa) contributed to the cake layer formation (Fig. 10a) [18,19]. For the control sample, biopolymers (20-2000 kDa), humic-like substances (0.35-20 kDa) and low MW organic matters (0.001-0.35 kDa) accounted for 90.1%, 8.8% and 1.1%, respectively (Fig. 10b). It was manifested that the principal foulants of cake layer were biopolymers (20-2000 kDa), consistent with the results of our previous papers that biopolymers were mostly intercepted by ceramic membrane (the same membrane used herein) [16,18]. Pre-ozonation alone (0-10 mg/L) and the integrated pretreatment of ozone (0-10 mg/L) and ferrate (0.15 mM) both effectively decreased the amount of DOC particularly biopolymers (20-2000 kDa) in the cake layer. For pre-ozonation alone, the amount of biopolymers was greatly decreased by 49.4% (2 mg/L), 89.5% (5 mg/L) and 93.4% (10 mg/L), respectively (Fig. 10b). The integrated pretreatment of ozone (0-10 mg/L) and ferrate (0.15 mM) further boosted the removal efficiency of biopolymers (20-2000 kDa) with 91.8% (2 mg/L), 96.9% (5 mg/L) and 98.6% (10 mg/L), respectively.

25

a

DOC intensity (×105 AU)

3.75

Ozone (0 mg/L) Biopolymer Ozone (2 mg/L) Ozone (5 mg/L) Ozone (10 mg/L) Ozone (0 mg/L) + ferrate Ozone (2 mg/L) + ferrate Ozone (5 mg/L) + ferrate Ozone (10 mg/L) + ferrate

3.50 3.25 3.00 2.75

Low MW organic matter

2.50 1

10

100

Humic like substance

1000

10000 100000 1000000

Molecular weight (Da)

Integral area (×106 AU)

20 16 12 8

b Ozone (0 mg/L) Ozone (2 mg/L) Ozone (5 mg/L) Ozone (10 mg/L) Ozone (0 mg/L) + ferrate Ozone (2 mg/L) + ferrate Ozone (5 mg/L) + ferrate Ozone (10 mg/L) + ferrate

4 0.2 0.0

0.005-0.2

0.2-20

20-2000

Molecular weight (kDa) Fig. 10. Effect of the integrated pretreatment of ozone (0-10 mg/L) and ferrate (0.15 mM) on DOC MW distribution of cake layer (a); (b) Integral area of three constituents.

3.2.4 Membrane fouling modeling The experimental data were fitted by nine fouling models (Table S1) as shown in Fig. 11. Cake-intermediate model best fitted the experimental data (SSE < 0.1) (Table S2) with pre-ozonation alone or ferrate-ozone integrated pretreatment (Fig. 11a). The contributions of cake and intermediate blockings could be assessed according to the magnitude of the fitted parameters [38]. With pre-ozonation alone (from 0 to 10 26

mg/L), the ratio of Kc*J/Ki diminished distinctly (Fig. 11b). Meanwhile, the ratio of Kc*J/Ki further decreased with ferrate-ozone integrated pretreatment and the lowest ratio was obtained at 0.15 mM ferrate and 10 mg/L ozone, indicating that the cake layer fouling was remarkably reduced. This was consistent with the outcomes of TMP and resistances (Fig. 6) and the porosity of the fouled membrane surface (Fig. 7, Table 1). 7 6

P/P0

5

a

Experimental data 0 mg/L

0 mg/L + ferrate

2 mg/L

2 mg/L + ferrate

5 mg/L

5 mg/L + ferrate

10 mg/L

10 mg/L + ferrate

Model fit

4 3

0 mg/L

0 mg/L + ferrate

2 mg/L

2 mg/L + ferrate

5 mg/L

5 mg/L + ferrate

10 mg/L

10 mg/L + ferrate

2 1 0

1

2

3

4

5

Time (h) 0.0

b

Kc*J/Ki

Kc*J/Ki

-0.1 Ferrate

-0.2

-0.3 0

2

5

10

0

2

5

10

Ozone dosage (mg/L) Fig. 11. TMP (P/P0) vs. time with the fit of cake-intermediate model (a) and the ratio of fitting parameters (b).

3.2.5 Discussion of membrane fouling alleviation 27

Membrane fouling alleviation was effectively enhanced with ferrate-ozone integrated pretreatment as shown in Fig. 6. Compared to the control sample, the best fouling control performance was obtained after the integrated pretreatment of ozone (10 mg/L) and ferrate (0.15 mM) with △TMP maximally decreased by 89.5%. In order to explicitly elucidate the quantitative correlation between influent characteristics and membrane fouling resistances, redundancy analysis (RDA) (Fig. 12a) and heat map analysis (Fig. 12b) were further executed. The total variance of RDA was interpreted by the first axis (72.2%) and the second axis (23.8%), respectively (Fig. 12a). Membrane fouling resistances exhibited a distinct positive correlation with six constitutes in influent containing MW, protein, DOC, UV254, polysaccharide, EEM (Fig. 12b). Polysaccharide was the most important positive factor to Rt and Rg with the correlation coefficients above 0.95, and MW exhibited the most important positive correlation with Rc (Table S3). In contrast, other components (mean particle size, turbidity, zeta potential) displayed a completely negative correlation with membrane fouling resistances (Fig. 12b). Hence, the statistical analysis outcome (Fig. 12) indicated that the increased particle size (Figs. 2f and S1) and the effective reduction of organic matters (Figs. 2, 3 and 4) principally contributed to the remarkable decrease of TMP and resistances with ferrate-ozone integrated pretreatment (Fig. 6). After the integrated pretreatment of ferrate (VI)-ozone, the enlarged suspended solids in influent (mean particle size > 296 nm) (Fig. 2f) would be more likely intercepted by the ceramic membrane (average pore size of 100 nm), instead of blocking membrane pores (Fig. 7). Moreover, porous fouling layer with high permeability could be formed by the particles with large size and low fractal dimension [50], which was indeed the case herein as evident from the increased 28

porosity (Table 2) and reduced TMP (Fig. 6) after the integrated pretreatment of ferrate (VI)-ozone. Meanwhile, organics especially biopolymers would absorb on the surface of particles and block the void spaces of cake layer with the formation of less permeable fouling layer. The significantly decreased organics especially the biopolymers in influent (Figs. 2, 3 and 4) after the integrated pretreatment of ferrate (VI)-ozone would remarkably alleviate organics-mediated membrane fouling with the formation of more porous fouling layer, which was indeed the case as demonstrated in Figs. 6, 7 and 8. Therefore, the increased particle size (Figs. 2f and S1) and the effective reduction of organic matters (Figs. 2, 3 and 4) with ferrate-ozone integrated pretreatment led to a more porous fouling layer with higher permeability (Fig. 7, Table 1) and thus a remarkable reduction of TMP and resistances (Fig. 6). This was also in agreement with the modeling results (Fig. 11) that cake blocking was significantly alleviated. Note that the effect of ferrate-ozone integrated pretreatment on the improvement of ceramic UF membrane fouling alleviation in a long-term operation will be executed in the future work, even though the excellent fouling control performance has been demonstrated by ferrate pretreatment in the long-term filtration experiments in our most recent study [35].

29

a

b

Fig. 12. The statistical correlation of influent characteristics and membrane fouling resistances. Redundancy analysis (a) and heat map analysis (b). 30

4. Conclusions The synergistic effect of ferrate (VI)-ozone integrated pretreatment on the improvement of water quality and ceramic membrane fouling alleviation in reclaimed water treatment was evaluated in this study. The conclusions were shown as below: 1. The synergistic effect of ferrate (VI)-ozone in terms of •OH generation was confirmed. The concentration of •OH in the ferrate (VI) (0.15 mM)-ozone (10 mg/L) integrated case was almost 3-fold higher compared to the ferrate (VI) (0.15 mM) case, while no obvious •OH was detected in the pre-ozonation case (10 mg/L) in Milli-Q water. 2. The reclaimed water quality was remarkably enhanced with the maximum removal efficiencies of 70.5% (DOC), 80.5% (UV254), 25.6% (protein) and 65.5% (polysaccharide) in influent achieved by the integrated pretreatment of ozone (10 mg/L) and ferrate (0.15 mM). The removed organic matters were principally ascribed to biopolymers (MW of 20-2000 kDa) with the best removal efficiency of 92.6%. 3. The best fouling control performance was obtained after the integrated pretreatment of ozone (10 mg/L) and ferrate (0.15 mM) and the normalized TMP (△TMP) was maximally decreased by 89.5%. The integrated pretreatment of ferrate (0.15 mM) and ozone (10 mg/L) resulted in a more porous fouling layer (porosity of 25.7%) on ceramic membrane surface. 4. The modeling results showed that cake blocking was significantly alleviated and cake-intermediate fouling dominated constantly with ferrate-ozone integrated pretreatment. 5. The statistical analysis results indicated that the increased particle size and the effective reduction of organics in influent principally contributed to the 31

remarkable decrease of TMP and resistances with ferrate-ozone integrated pretreatment.

Acknowledgements Research funding supported by the National Natural Science Foundation of China (51708325), the Committee of Science and Technology Innovation of Shenzhen (JCYJ20160331185156860; JCYJ20170817161942307; JCYJ20180301160236875) and the Development and Reform Commission of Shenzhen Municipality (urban water recycling and environment safety program) is kindly acknowledged.

References [1] W. Wang, Y. Wang, Z. Li, C. Wei, J. Zhao, L. Sun, Effect of a strengthened ecological floating bed on the purification of urban landscape water supplied with reclaimed water, Sci. Total Environ. 622-623 (2018) 1630-1639. [2] Y. Shang, P. Hei, S. Lu, L. Shang, X. Li, Y. Wei, D. Jia, D. Jiang, Y. Ye, J. Gong, X. Lei, M. Hao, Y. Qiu, J. Liu, H. Wang, China's energy-water nexus: Assessing water conservation synergies of the total coal consumption cap strategy until 2050, Appl. Energ. 210 (2018) 643-660. [3] M. Xu, X. Bai, L. Pei, H. Pan, A research on application of water treatment technology for reclaimed water irrigation, Int. J. Hydrogen Energ. 41 (2016) 15930-15937. [4] W. Chen, S. Lu, N. Pan, Y. Wang, L. Wu, Impact of reclaimed water irrigation on soil health in urban green areas, Chemosphere 119 (2015) 654-661. [5] H. Zhao, Y. Wang, L. Yang, L. Yuan, D. Peng, Relationship between phytoplankton and environmental factors in landscape water supplemented with reclaimed water, Ecol. Indic. 58 (2015) 113-121. [6] D.J. Barker, D.C. Stuckey, A review of soluble microbial products (SMP) in wastewater treatment systems, Water Res. 33 (1999) 3063-3082. [7] M.M.M. Tin, G. Anioke, O. Nakagoe, S. Tanabe, H. Kodamatani, L.D. Nghiem, T. Fujioka, Membrane fouling, chemical cleaning and separation performance assessment of a chlorineresistant nanofiltration membrane for water recycling applications, Sep. Purif. Technol. 189 (2017) 170-175. [8] C. Jarusutthirak, G. Amy, Understanding soluble microbial products (SMP) as a component of effluent organic matter (EfOM), Water Res. 41 (2007) 2787-2793. [9] H. Kim, B.A. Dempsey, Effects of wastewater effluent organic materials on fouling in ultrafiltration, Water Res. 42 (2008) 3379-3384. [10] F. Qu, H. Liang, J. Zhou, J. Nan, S. Shao, J. Zhang, G. Li, Ultrafiltration membrane fouling caused by extracellular organic matter (EOM) from Microcystis aeruginosa: Effects of membrane pore size and surface hydrophobicity, J. Membr. Sci. 449 (2014) 58-66. [11] C. Jarusutthirak, G. Amy, Role of soluble microbial products (SMP) in membrane fouling and flux decline, Environ. Sci. Technol. 40 (2006) 969-974. [12] C. Jarusutthirak, S. Mattaraj, R. Jiraratananon, Factors affecting nanofiltration performances in natural organic matter rejection and flux decline, Sep. Purif. Technol. 58 (2007) 68-75. [13] V. Camel, A. Bermond, The use of ozone and associated oxidation processes in drinking water treatment, Water Res. 32 (1998) 3208-3222. [14] J. Song, Z. Zhang, X. Zhang, A comparative study of pre-ozonation and in-situ ozonation on mitigation of ceramic UF membrane fouling caused by alginate, J. Membr. Sci. 538 (2017) 50-57. [15] J. Song, Z. Zhang, S. Tang, Y. Tan, X. Zhang, Does pre-ozonation or in-situ ozonation really 32

[16]

[17]

[18] [19]

[20] [21]

[22] [23] [24] [25] [26]

[27]

[28]

[29]

[30]

[31] [32] [33]

[34] [35]

[36]

[37]

mitigate the protein-based ceramic membrane fouling in the integrated process of ozonation coupled with ceramic membrane filtration? J. Membr. Sci. 548 (2018) 254-262. S. Tang, Z. Zhang, J. Liu, X. Zhang, Double-win effects of in-situ ozonation on improved filterability of mixed liquor and ceramic UF membrane fouling mitigation in wastewater treatment? J. Membr. Sci. 533 (2017) 112-120. S. Tang, Z. Zhang, X. Zhang, New insight into the effect of mixed liquor properties changed by pre-ozonation on ceramic UF membrane fouling in wastewater treatment, Chem. Eng. J. 314 (2017) 670-680. D. Wei, Y. Tao, Z. Zhang, L.Liu, X. Zhang, Effect of in-situ ozonation on ceramic UF membrane fouling mitigation in algal-rich water treatment, J. Membr. Sci. 498 (2016) 116-124. D. Wei, Y. Tao, Z. Zhang, X. Zhang, Effect of pre-ozonation on mitigation of ceramic UF membrane fouling caused by algal extracellular organic matters, Chem. Eng. J. 294 (2016) 157166. X. Zhang, J. Guo, L. Wang, J. Hu, J. Zhu, In situ ozonation to control ceramic membrane fouling in drinking water treatment, Desalination 328 (2013) 1-7. X. Fan, Y. Tao, L. Wang, X. Zhang, Y. Lei, Z. Wang, H. Noguchi, Performance of an integrated process combining ozonation with ceramic membrane ultra-filtration for advanced treatment of drinking water, Desalination 335 (2014) 47-54. V.K. Sharma, R. Zboril, R.S. Varma, Ferrates: Greener oxidants with multimodal action in water treatment technologies, Accounts Chem. Res. 48 (2015) 182-191. R.H. Wood, The heat, free energy and entropy of the ferrate (VI) ion, J. Am. Chem. Soc. 80 (1958) 2038-2041. Y. Song, Y. Deng, C. Jung, Mitigation and degradation of natural organic matters (NOMs) during ferrate(VI) application for drinking water treatment, Chemosphere 146 (2016) 145-153. Y. Jiang, J.E. Goodwill, J.E. Tobiason, D.A. Reckhow, Impacts of ferrate oxidation on natural organic matter and disinfection byproduct precursors, Water Res. 96 (2016) 114-125. X. Sun, Q. Zhang, H. Liang, L. Ying, M. Xiangxu, V.K. Sharma, Ferrate(VI) as a greener oxidant: Electrochemical generation and treatment of phenol, J. Hazard. Mater. 319 (2016) 130136. P. Dobosy, C.É. Vizsolyi, I. Varga, J. Varga, G. Láng, G. Záray, Comparative study of ferrate and thermally activated persulfate treatments for removal of mono- and dichlorobenzenes from groundwater, Microchem. J. 136 (2018) 61-66. M. Feng, L. Cizmas, Z. Wang, V.K. Sharma, Synergistic effect of aqueous removal of fluoroquinolones by a combined use of peroxymonosulfate and ferrate (VI), Chemosphere 177 (2017) 144-148. R. Prucek, J. Tuček, J. Kolařík, J. Filip, Z. Marušák, V.K. Sharma, R. Zbořil, Ferrate(VI)induced arsenite and arsenate removal by in situ structural incorporation into magnetic iron(III) oxide nanoparticles, Environ. Sci. Technol. 47 (2013) 3283-3292. R. Prucek, J. Tucek, J. Kolar ík, I. Hus ková, J. Filip, R.S. Varma, V.K. Sharma, R. Zbor il, Ferrate (VI)-prompted removal of metals in aqueous media: mechanistic delineation of enhanced efficiency via metal entrenchment in magnetic oxides, Environ. Sci. Technol. 49 (2015) 23192327. P. Xie, Y. Chen, J. Ma, X. Zhang, J. Zou, Z. Wang, A mini review of preoxidation to improve coagulation, Chemosphere 155 (2016) 550-563. C. Wu, C. Kuo, C. Chang, Homogeneous catalytic ozonation of C.I. Reactive Red 2 by metallic ions in a bubble column reactor, J. Hazard. Mater. 154 (2008) 748-755. T. Zhang, J. Lu, J. Ma, Z. Qiang, Comparative study of ozonation and synthetic goethitecatalyzed ozonation of individual NOM fractions isolated and fractionated from a filtered river water, Water Res. 42 (2008) 1563-1570. Q. Han, H. Wang, W. Dong, T. Liu, Y. Yin, Formation and inhibition of bromate during ferrate(VI)-ozone oxidation process, Sep. Purif. Technol. 118 (2013) 653-658. J. Liu, Z. Zhang, Z. Liu, X. Zhang, Integration of ferrate (VI) pretreatment and ceramic membrane reactor for membrane fouling mitigation in reclaimed water treatment, J. Membr. Sci. 552 (2018) 315-325. Z. Zhang, M.W. Bligh, Y. Wang, G.L. Leslie, H. Bustamante, T.D. Waite, Cleaning strategies for iron-fouled membranes from submerged membrane bioreactor treatment of wastewaters, J. Membr. Sci. 475 (2015) 9-21. P. Park, C. Lee, S. Lee, Determination of cake porosity using image analysis in a coagulation– microfiltration system, J. Membr. Sci. 293 (2007) 66-72. 33

[38]

G. Bolton, D. LaCasse, R. Kuriyel, Combined models of membrane fouling: Development and application to microfiltration and ultrafiltration of biological fluids, J. Membr. Sci. 277 (2006) 75-84. Q. Liu, S. Kim, Evaluation of membrane fouling models based on bench-scale experiments: A comparison between constant flowrate blocking laws and artificial neural network (ANNs) model, J. Membr. Sci. 310 (2008) 393-401. T. Li, X. Yan, D. Wang, F. Wang, Impact of preozonation on the performance of coagulated flocs, Chemosphere 75 (2009) 187-192. J. Chen, H. Yeh, I. Tseng, Effect of ozone and permanganate on algae coagulation removalPilot and bench scale tests, Chemosphere 74 (2009) 840-846. D. Im, N. Nakada, Y. Fukuma, Y. Kato, H. Tanaka, Performance of combined ozonation, coagulation and ceramic membrane process for water reclamation: Effects and mechanism of ozonation on virus coagulation, Sep. Purif. Technol. 192 (2018) 429-434. L. Zheng, Y. Deng, Settleability and characteristics of ferrate(VI)-induced particles in advanced wastewater treatment, Water Res. 93 (2016) 172-178. J.E. Goodwill, Y. Jiang, D.A. Reckhow, J. Gikonyo, J.E. Tobiason, Characterization of particles from ferrate preoxidation, Environ. Sci. Technol. 49 (2015) 4955-4962. W. Chen, P. Westerhoff, J.A. Leenheer, K. Booksh, Fluorescence excitation-emission matrix regional integration to quantify spectra for dissolved organic matter, Environ. Sci. Technol. 37 (2003) 5701-5710. Z. Wang, Z. Wu, S.Tang, Characterization of dissolved organic matter in a submerged membrane bioreactor by using three-dimensional excitation and emission matrix fluorescence spectroscopy, Water Res. 43 (2009) 1533-1540. J. Jung, M. Lee, EPR investigation on the efficiency of hydroxyl radical production of gammairradiated anatase and bentonite, Water Res. 36 (2002) 3359-3363. C. Wang, N. Klamerth, R. Huang, H. Elnakar, M. Gamal El-Din, Oxidation of oil sands processaffected water by potassium ferrate (VI), Environ. Sci. Technol. 50 (2016) 4238-4247. Y. Lee, R. Kissner, U. von Gunten, Reaction of ferrate (VI) with ABTS and self-decay of ferrate (VI): Kinetics and mechanisms, Environ. Sci. Technol. 48 (2014) 5154-5162. T.D. Waite, A.I. Schäfer, A.G. Fane, A. Heuer, Colloidal fouling of ultrafiltration membranes: Impact of aggregate structure and size, J. Colloid Interf. Sci. 212 (1999) 264-274.

[39]

[40] [41] [42]

[43] [44] [45]

[46]

[47] [48] [49] [50]

Highlights



The synergistic effect of ferrate-ozone with more •OH generated is confirmed.



Ferrate-ozone significantly improves the water quality of reclaimed water.



Ferrate-ozone pretreatment remarkably alleviates ceramic membrane fouling.



A more porous fouling layer is formed with ferrate-ozone pretreatment.



Cake-intermediate fouling is the main fouling mechanism.

34

3465

Intensity

2 mg/L Ozone 5 mg/L Ozone 10 mg/L Ozone 0 mg/L Ozone + 0.15 mM Fe (VI) 2 mg/L Ozone + 0.15 mM Fe (VI) 5 mg/L Ozone + 0.15 mM Fe (VI)

3510

3525

3540

10 mg/L Ozone + 0.15 mM Fe (VI)

3495

35 30 25 20 15 10 5 0 0

2

Ozone (0 mg/L) Ozone (2 mg/L) Ozone (5 mg/L) Ozone (10 mg/L) Ozone (0 mg/L) + ferrate Ozone (2 mg/L) + ferrate Ozone (5 mg/L) + ferrate Ozone (10 mg/L) + ferrate

1

Time (h)

3

4

Synergistic effect of the ferrate (VI)-ozone integrated process

3480

Magnetic value (G)

Normalised TMP (kPa)

5