Environ Sci Pollut Res DOI 10.1007/s11356-017-8619-z

RESEARCH ARTICLE

Advanced treatment of municipal secondary effluent by catalytic ozonation using Fe3O4-CeO2/MWCNTs as efficient catalyst Zhiyong Bai 1,2 & Jianlong Wang 1,3

&

Qi Yang 2

Received: 12 October 2016 / Accepted: 8 February 2017 # Springer-Verlag Berlin Heidelberg 2017

Abstract The advanced treatment of municipal secondary effluent was performed by catalytic ozonation using Fe3O4-CeO2/ MWCNTs as catalyst. The experimental results showed that in catalytic ozonation system, the removal efficiency of soluble COD was more than 46% after 30 min reaction, and about 36% of effluent organic matters (EfOMs) were mineralized, which was four times higher than that in single ozonation system. Moreover, proteins, humic acids, and UV254 decreased obviously after 30 min reaction, but polysaccharides did not significantly decrease. In catalytic ozonation system, the ozone utilization increased, which is favorable for the degradation of EfOM. The organic compounds and alkalinity were the main hydroxyl radical consumers in municipal secondary effluent. The catalytic ozonation process was also effective for the degradation of two target micropollutants (sulfamethazine and carbamazepine). The catalyst could be stable after five-time reuse for catalytic ozonation of effluent.

Keywords Catalytic ozonation . Secondary effluent . Ozone utilization . Advanced treatment Responsible editor: Philippe Garrigues * Jianlong Wang [email protected]

1

Collaborative Innovation Center for Advanced Nuclear Energy Technology, INET, Tsinghua University, Beijing 100084, People’s Republic of China

2

School of Water Resources and Environment, China University of Geosciences, Beijing 100083, China

3

Beijing Key Laboratory of Radioactive Waste Treatment, INET, Tsinghua University, Energy Science Building, Beijing 100084, People’s Republic of China

Introduction The reclamation and reuse of the municipal secondary effluent (MSEF) is very important for the sustainable water resource management, which has received increasing attention in recent years. However, the presence of effluent organic matter (EfOM) in the secondary effluent, such as natural organic matter (NOM), soluble microbial products (SMPs), pharmaceutical and pesticide residues, could pose a threat to ecological system and human health, thus limiting the application of the reclaimed wastewater (Jin et al. 2013; Luo et al. 2014). Several studies reported the potential toxic, mutagenic, and endocrine effects on fish and invertebrates (Fatima et al. 2007; Pomati et al. 2007; Garcia-Segura et al. 2015). The advanced treatment of the secondary effluent to remove the effluent organic matters, especially the toxic micropollutants, is crucial to guarantee the safety of the reclaimed water (Giannakis et al. 2015; Cai and Lin 2016). Various technologies have been studied to remove the EfOM, such as adsorption (Kalkan et al. 2011; Yu et al. 2012), ion exchange (Ebrahimi and Roberts 2013; Wang et al. 2014; Yang et al. 2015), membrane filtration (Jiang et al. 2013, Jiang and Choo 2016; Tang et al. 2016), and advanced oxidation (Garcia-Segura et al. 2015; Giannakis et al. 2015; Puspita et al. 2015; Yuan et al. 2015). In particular, advanced oxidation is an attractive option because the generation of hydroxyl radicals (∙OH), which is highly reactive and non-selective for the destruction of pollutants (Wang and Xu 2012). Giannakis et al. (2015) presented an overview of various advanced oxidation process (UV, UV/H2O2, solar irradiation, Fenton, solar photo-Fenton) on the treatment of micropollutants contained in municipal wastewater, and three kinds of wastewater were collected from different secondary treatment facilities. They found that UV/H2O2 showed the highest oxidation efficiency, but the retention time should be further reduced. Moreover, the energy consumption of UV is also a

Environ Sci Pollut Res

challenge. Consequently, more efficient and low-cost techniques are required. Ozonation was used for the removal of organic contaminants from drinking water (Glaz 1987), due to ozone may attack organic compounds through either direct oxidation with molecular ozone or indirect oxidation with highly reactive hydroxyl radicals (∙OH) (Nawrocki and Kasprzyk-Hordern 2010). With the development of ozonizer technology, the cost of ozonation is becoming lower. The ozonation has recently been considered as a competitive technique for advanced treatment of municipal secondary effluents. However, there are still some problems for the application of ozonation. Firstly, ozone is a highly selective oxidant, so single ozonation cannot effectively remove the O3-refractory compounds (Huber et al. 2003; Nawrocki and Kasprzyk-Hordern 2010; Hübner et al. 2015). Secondly, ozonation may generate potentially carcinogenic bromate from bromide present in secondary effluents (Gerrity and Snyder 2011; Pisarenko et al. 2012; Yao et al. 2016). Moreover, the utilization ratio of ozone is quite low, especially for the ozonation of low organic content wastewater, which is caused by the weakly solubility of ozone in neutral water (Zhao et al. 2009; Dai et al. 2014; Marce et al. 2016). Heterogeneous catalytic ozonation is a modified process of ozonation, which can provide fast degradation of organic pollutants and improve utilization ratio of ozone by the addition of solid catalyst. Heterogeneous catalytic ozonation have proved its high efficiency for the removal of refractory compounds (Bing et al. 2015; Goncalves et al. 2015; Huang et al. 2015; Ikhlaq et al. 2015; Bai et al. 2016a), natural organic matter (Park et al. 2004; Park et al. 2012), and tertiary treatment of industrial wastewater, such as bio-treated dyeing and finishing wastewater (Wu et al. 2016), heavy oil refining wastewater (Chen et al. 2014), and Lurgi coal gasification wastewater (Zhuang et al. 2014). However, a few studies focused on the catalytic ozonation of municipal secondary effluents. Our previous study (Bai et al. 2016b) showed that the addition of Fe3O4/ multi-walled carbon nanotubes (MWCNTs) can significantly increase the degradation and mineralization of phydroxybenzoic acid (p-HBA), and the catalyst showed a good stability and reusability. Moreover, ceria-containing catalysts are attractive due to their high oxygen storage capacity and high catalytic ability based on Ce4+/Ce3+ redox cycles. For example, Fe3O4/CeO2 exhibited a good performance for catalytic Fenton-like system (Xu and Wang 2012a), Ce0.1Fe0.9OOH showed high catalytic efficiency for sulfamethazine mineralization in catalytic ozonation system (Bai et al. 2016a). Therefore, Fe3O4-CeO2/ MWCNTs can be used as ozonation catalyst for the advanced treatment of municipal secondary effluents. The objective of this study was to investigate the catalytic ozonation of municipal secondary effluents using Fe3O4-CeO2/ MWCNTs as catalyst. The catalytic activity was also evaluated

by the degradation of sulfamethazine (SMT) and carbamazepine (CBZ). We focused on the removal efficiency of EfOM in municipal secondary effluents and the utilization efficiency of ozone in catalytic ozonation system.

Materials and methods Chemicals and wastewater Chemicals used in this study, including p-hydroxybenzoic acid (p-HBA), FeSO4·7H2O, Fe2(SO4)3, Ce(NO3)3, and NaOH, were purchased from Sinopharm Chemical Reagent Co. Ltd. (China), which were of analytical grade. SMT and CBZ were obtained from Alfa Aesar Company with purity >99%. MWCNTs were purchased from Chengdu Organic Chemical Co. Ltd., Chinese Academy of Sciences (Chengdu, China). Additionally, their specifications were described previously (Bai et al. 2016b). The secondary effluent samples were collected from a Municipal Wastewater Treatment Plant in Beijing, China. The particulate matter present in the secondary effluent was removed by filtering 0.45-μm filter and then stored at 4 °C. The physicochemical characteristics of the effluent samples were analyzed and listed as follows: pH, 7.5 ± 0.3; UV254, 0.12 cm−1; chemical oxygen demand (COD), 47.6 ± 2.1 mg L−1; soluble chemical oxygen demand (SCOD), 40.4 ± 2.2 mg L −1 ; DOC, 6.2 ± 0.2 mg L−1; TN, 22.4 ± 1.1 mg L−1; and total suspended solids, 2.8 ± 0.3 mg L−1. Catalyst synthesis and characterization Fe 3 O 4 -CeO 2 /MWCNTs were prepared through the co-precipitation method. The mass ratio of Fe3O4, CeO2, and MWCNTs was 1:1:1. The preparation process was modified from references (Bai et al. 2016a, b, c; Xu and Wang 2012a, b). Cerium oxide particle was firstly precipitated, then mixing with NaOH solution in a four-necked flask. A certain ratio of FeSO4·7H2O, Fe2(SO4)3, H2SO4, and MWCNTs was dissolved or dispersed in water under ultrasound for 10 min, and then the mixed solution was added dropwise into fournecked flask 80 °C under Ar gas protection. The collected catalyst was washed with oxygen-free deionized water for two times and dried in a vacuum freeze dryer overnight. The catalyst before and after use were characterized by XRD diffractometer (D8-Advance, Bruker) with Cu Kα radiation at 40 kV and 40 mA. And the morphologies were observed by SEM (Hitachi SU-8000) at 10 kV. Ozonation experiments Ozone used in these experiments was generated by an ozonizer (3S-A3 Tonglin Technology, China). Ozone gas flow was controlled at 0.4 L min−1 by gas flow meter.

Environ Sci Pollut Res

Results and discussion EfOM removal efficiency SCOD and DOC The removal efficiency of SCOD and DOC in catalytic or single ozonation processes are shown in Fig. 1. As shown in Fig. 1a, only 34% of SCOD was removed by single ozonation in 90 min. The addition of the catalyst significantly improved the SCOD removal efficiency, which was 51% in catalytic ozonation system. It is noteworthy that a rapid degradation stage (0–30 min) and a steady period (30–90 min) were observed in catalytic ozonation system, which is very different with single ozonation. This process is consistent with catalytic ozonation of erythromycin (Gonçalves et al. 2014). The reason may be due to the adsorption of organic matters by Fe3O4/ CeO2/MWCNTs, which facilitated the reaction between ozone and organic matters. Moreover, the generated hydroxyl

a 1.0

Catalytic ozonation Ozonation

0.9

SCOD/SCOD0

Two ozone analyzers (BMT 963 Germany) were used to detect the concentration of ozone in the inlet and outlet, and the inlet concentration of ozone is 15 mg L−1. A 1.2-L cylindrical glass container was used as reactor. In a typical experiment, 1 L secondary effluent and 0.3 g catalyst were added into the reactor, and then ozone was continuously added to the solution through a porous diffuser device. At a given time, samples were collected and filtered through a PTFE filter (pore size 0.22 mm) for analysis. An aliquot of Na2S2O3 (20 μL) was subsequently added to remove the residual ozone. In the reuse experiments, the catalyst and wastewater were separated by a strong magnet. Considering the possibility of practical application, the catalyst was directly used for the next reaction without washing. The experiment of hydroxyl radical consumer in MSEF was tested by the degradation efficiency of p-HBA in different reaction systems. p-HBA (10 mg L−1) was dissolved in deionized water (DW), MSEF, and MSEF with low alkalinity, respectively. MSEF with low alkalinity was carried out by adjusting air bubbling of water for 20 min at acidic pH (pH = 3, adjusted by 0.1 M HCl), and then pH was adjusted to 7.0 by NaOH (0.1 M) solution. With the same conditions described above, samples were collected and the concentration of p-HBA was detected. In addition, in order to evaluate the efficiency of this system for the treatment of micropollutants, two target micropollutants were added into the MSEF. The usage of SMT and CBZ in the experiments is 20 and 10 mg L−1 under the same reaction conditions, respectively.

0.8 0.7 0.6

Analytical methods

0.5

The concentration of DO was measured with a DO meter (OXI 3310, WTW); SCOD, TP, and NH3-N were measured according to the National Standard Methods of China. The concentration of p-HBA, SMT, and CBZ was measured using high-performance liquid chromatography (HPLC, Agilent 1200) (Bai et al. 2016b, c, Wang and Wang 2017). Dissolved organic carbon (DOC) was analyzed by a Multi-TOC/TN Analyzer (2100, Analytik Jena AG Corporation). Metal ions released into solution were quantified by flame atomic absorption spectrophotometer (ZA3000, Hitachi). The concentrations of humic acid and protein were measured by modified Folin-Lowry method. The 3DEEM was determined using a fluorometer (Horiba Jobin Yvon FluoroMax-4, France). The organic pollutants were analyzed using GC-MS by Agilent 5975C (60.0 m × 0.25 mm × 0.25 μm, HP5MS); GC temperature program was 40 °C for 2 min, linearly ramped to 300 °C at 5 °C min−1, and held at 300 °C for 3 min.

0.4 0

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Time (min)

b 1.0

Catalytic ozonation Ozonation

DOC/DOC0

0.9

0.8

0.7

0.6

0.5 0

20

40

60

80

100

Time (min)

Fig. 1 Removal efficiency of SCOD and DOC in single and catalytic ozonation processes

Environ Sci Pollut Res

radicals have higher oxidation ability, which can also accelerate the reaction rate. However, along with the extension of reaction time, the accumulation of refractory compounds and the production of small molecule acids inhibited the reaction. The initial DOC value was about 6.2 mg L−1, which is similar to the secondary effluents used by previous study (Yao et al. 2016). The removal of DOC was also evaluated, and the results are shown in Fig. 1b. Even though the tendency was consistent with SCOD, the removal efficiency was lower in catalytic and single ozonation systems, which was 12.9 and 36.6%, respectively. However, this result was higher than that of previous studies using UV, UV/H2O2, solar irradiation, Fenton, and solar photo-Fenton (Giannakis et al. 2015), suggesting that the catalytic ozonation is an effective way for the advanced treatment of municipal secondary effluent, but there are still some organics difficult to be further mineralized.

0.88 mg L−1. In the catalytic ozonation system, about 68.6% of protein was removed after 30 min reaction, suggesting that most of the protein-like substances can be removed by ozone molecules. The polysaccharide concentration decreased weakly; their concentration was 5.56 and 5.27 mg L−1 after ozonation and catalytic ozonation, respectively; and the removal efficiency was only 5.64 and 9.97%. The low removal efficiency of polysaccharides was similar with the reported results (Wu et al. 2016). Therefore, the removal of polysaccharides should be further studied in order to reduce SCOD. The concentration of humic acids in the secondary effluent was 15.12 mg L−1, which was higher than SMPs. In the catalytic ozonation system, the humic acid concentration decreased to 1.59, with a removal efficiency of 89.49%, indicating that humic acids can be effectively removed in the catalytic ozonation system.

Humic acids and SMPs

UV254

The soluble microbial products (SMPs) are always formed during biological treatment process (Aquino and Stuckey 2004), and humic acids are one kind of natural organic matters (NOMs) (Yuan et al. 2013). They constitute the majority of the effluent organic matters in secondary effluents. In order to deep understand the degradation process of municipal secondary effluent, the concentration change of humic acids and SMPs (including proteins and polysaccharides) were determined and shown in Fig. 2. The concentration of proteins, polysaccharides, and humic acids in the secondary effluent was 2.16, 5.83, and 15.12 mg L−1, respectively. According to the relationship between SMPs and COD, the stoichiometric conversion factor for proteins and polysaccharides is 1.5 and 1.2, respectively (Aquino and Stuckey 2004). Hence, SMPs accounted for 25% of the SCOD. In single ozonation system, the protein removal efficiency was 60.3%, and the concentration decreased to

The parameter of UV254 is directly related to the content of unsaturated compounds, especially aromatic substances dissolved in the water (Marce et al. 2016). The initial value of UV254 of this study is located in the intermediate level among references (Yang et al. 2015; Jiang and Choo 2016; Marce et al. 2016; Yao et al. 2016). The removal of UV254 is represented in Fig. 3. Similar with that observed for SCOD degradation, two oxidation stages were also observed in the removal of the normalized UV254. But a difference is that the first stage was more quickly than SCOD removal. In catalytic ozonation system, only after 5 min, the value of UV254 decreased to 0.055; the corresponding removal efficiency was 53.04%. After 30 min reaction, the removal efficiency increased to 73.14%. In single ozonation system, the removal efficiency was a little lower; it was 68.86% after 30 min reaction. The fast and high removal efficiency was also obtained in the research of (Marce

16

Ozonation Catalytic ozonation

Ozonation Catalytic ozonation

0.10

0.08

10

80

-1

60

8 6

0.06 40 0.04

4

20

0.02

2 0.00

0

0

MSEF

Ozonation

Catalytic ozonation

Fig. 2 Variation of humic acids and SMPs in different processes

5

10

15

20

25

30

Time (min)

Fig. 3 Level and removal efficiency of UV254 in different processes

UV254 remove efficiency (%)

12

UV254(cm )

-1

Concentration (mg L )

0.12

Proteins Polysaccharides Humic acids

14

Environ Sci Pollut Res

In the 3DEEM fluorescence of municipal secondary effluents, as shown in Fig. 4a, there are two conspicuous peaks. Peak I is located at EX/EM = 275/360 nm, which is associated with soluble microbial product-like substances; peak II is classified as humic acid-like substance, situated at EX/EM = 240/425 nm (Chen et al. 2003). In the reported studies (Wu et al. 2016, Yang et al. 2015), fulvic acid-like and protein-like extracellular organic matters were also found in the secondary effluent of industry wastewater, but they were not found in this study. The 3DEEM fluorescence after 30 min reaction in single ozonation and catalytic ozonation system are shown in Fig. 4b, c. Compared with Fig. 4a, the signal strength became weakly after single or catalytic ozonation. It should be noteworthy that there were same peaks (and III) in Fig. 4, which belongs to SMP-like substances. But the peak position is moved to left and located at EX/EM = 275/300 nm, compared with the original wastewater, which caused by the changes of SMPs types. Combining the results of secondary effluent, we inferred that the residual signal strength belongs to polysaccharides. Moreover, the peak II only appeared after single ozonation, which also represent SMP-like substances, which proved the higher removal efficiency of catalytic ozonation system.

Variation of organic pollutants The main organic pollutants were detected by GC-MS; there were 53 organic pollutant species with relative peak areas >0.1% in the municipal secondary effluent. After catalytic ozonation, the number of detected organic species decreased from 53 to 42 (Table 1). It can be seen that some macromolecular organic matters, such as anthracene and pyrene, were detected before ozonation, but they cannot be detected after catalytic ozonation. Meanwhile, alkanes were generated after ozonation, possibly due to the decomposition of macromolecular organic matters by ozone and ∙OH. Moreover, dimethyl phthalate increased obviously, maybe due to the incomplete degradation of phthalate acid esters. Alkanes generated by ozonation were difficult to mineralize. These results can explain the low mineralization rate after 30 min reaction.

450

Ex wavelength (nm)

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Ċ

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Em wavelength(nm)

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-50.00

Ex wavelength (nm)

The 3DEEM fluorescence

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ĉ

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Em wavelength (nm)

c 500 450

Ex wavelength (nm)

et al. 2016). As we all know, ozone is a very selective oxidant (Wang and Xu 2012), due to its special structure; ozone prefers to oxidize most unsaturated organic compounds (Kasprzyk-Hordern et al. 2003). Therefore, the unsaturated organic compounds can be oxidized and the UV254 value obviously decreased.

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ĉ

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Em wavelength (nm)

Fig. 4 Variations in 3DEEM for different effluents. a Initial. b Ozonation. c Catalytic ozonation

The utilization efficiency of ozone In catalytic ozonation system, the added catalyst can be used as adsorbent; both pollutants and ozone molecules can be adsorbed onto it. Therefore, the addition of catalyst can improve the mass-transfer efficiency and increase the utilization

Environ Sci Pollut Res Table 1

Main organic pollutants detected by GC-MS before and after catalytic ozonation

Organic compounds

Full name

Before catalytic

After catalytic

ozonation

ozonation

Relative peak

Relative peak

area (%)

area (%)

0.46

ND

0.71

ND

0.93

ND

3.45

ND

11.07

ND

7.93

ND

Chemical structure

#CAS

Totally removed Formic acid, 1-

625-

methylethyl ester

55-8 79-06-

O

O

O

2-Propenamide 1 1-ethynyl-

931-

Cyclohexene

49-7

NH2

120Anthracene 12-7

O

Butyl octyl

84-78-

phthalate

6

O O

O

129Pyrene 00-0

Environ Sci Pollut Res Table 1 (continued)

Organic compounds

Full name

Before catalytic

After catalytic

ozonation

ozonation

Relative peak

Relative peak

area (%)

area (%)

5.69

ND

0.67

0.18

16.63

21.16

11.1

12.39

ND

0.98

ND

0.80

Chemical structure

#CAS

206Fluoranthene 44-0

Partially removed 1,1-

78-99-

Dichloropropane

9

Cl

Cl

Increased O O

131-

O

Dimethyl phthalate 11-3

O

1043-Phenyl-2-propenal 55-2

O

Newly generated

2,6,11-

31295-

Trimethyldodecane

56-4

O

77-93-

OH

O

Ethyl citrate

OH

0

HO

O

O

Environ Sci Pollut Res Table 1 (continued)

Organic compounds

Full name

Before catalytic

After catalytic

ozonation

ozonation

Relative peak

Relative peak

area (%)

area (%)

ND

0.95

ND

0.95

ND

0.88

ND

0.49

Chemical structure

#CAS

Undecane,4,6-

17312-

dimethyl-

82-2

HO

39131-Octanol, 2-butyl02-8

3-Methyl-5-

31081-

propylnonane

18-2

O

0485Ninhydrin

O

47-2 O

of ozone. In order to quantify these process, the inlet and outlet ozone concentration were monitored and recorded; the results are shown in Fig. 5. The inlet ozone concentration was controlled by the ozonizer and kept at a stable level in single ozonation and catalytic ozonation process. Figure 5 clearly indicates the difference of outlet ozone concentration, which was lower in catalytic ozonation than single ozonation. The concentration of total applied ozone and end gas ozone can be calculated by following expressions

1200

½O3  ¼ ∫0

½O3  dt ¼ ∑1200 ½O3 n ⋅Δt ðΔt ¼ 10sÞ 0

½O3 C ¼ ½O3 T −½O3 E . Ru ¼ ½O3 C ½O3 T

ð1Þ ð2Þ ð3Þ

where [O3]T represents the concentration of total applied ozone, [O3]E is the concentration of end gas ozone, [O3]C is the concentration of consumed ozone, and Ru is ozone utilization.

Environ Sci Pollut Res 16

1.0

DW-ozonation DW-catalytic ozonation MSEF-ozonation MSEF-catalytic ozonation MSEF (La)-catalytic ozonation

0.8

12 10

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8

Inlet (Catalytic ozonation) Outlet (Catalytic ozonation) Inlet (Ozonation) Out (Ozonation)

6 4 2 0

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Ozone concentration (mg L )

14

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5

Fig. 5 The variation of ozone inlet and outlet concentration during the reaction

During the reaction process, we recorded the [O3]T and [O3]E every 10 s. Based on above equations, the Ru in single and catalytic ozonation was 22.19 and 30.89%, respectively. The improvement of the ozone utilization was proved to be one of the catalytic mechanisms. The utilization of ozone in this study was a little lower than that in the catalytic ozonation of acetylsalicylic acid (Dai et al. 2014). However, the concentration of catalyst was not given in their study. In another study (Marce et al. 2016), urban primary and bio-treated wastewaters were treated by ozonation; although they did not calculate the ozone utilization, a slight difference of ozone concentration between outlet and inlet can be seen. The complex and low concentration of the organic matters in effluent may cause relatively low ozone utilization. Moreover, the dosage of the catalyst in catalytic ozonation system had obvious influence on the ozone utilization (Zhao et al. 2009). Hydroxyl radical consumers in municipal secondary effluents p-HBA is a hydroxyl radical scavenger, because it has a higher reaction rate with HO than ozone. In order to assess the hydroxyl radical consumers in municipal secondary effluent, 10 mg L −1 p-HBA was used in different treatment systems. Usually, the inorganic molecules and EfOM can act as hydroxyl radical consumers in water. HCO 3− and CO 32− were main inorganic molecules to scavenge hydroxyl radicals (Souza et al. 2014). So, the secondary effluent with low alkalinity was prepared by firstly air bubbling at acidic pH for 20 min, and then pH was adjusted to 7.6 by NaOH solution. Figure 6 shows that p-HBA decreased quickly during ozonation process; moreover, the presence of catalyst greatly decreased the reaction time. When the secondary effluent was

10

15

20

Time (min)

Time (s)

Fig. 6 Degradation of p-HBA in different conditions

used, the degradation of p-HBA was inhibited. In order to better compare the reaction rates under different experimental conditions, the first-order kinetic constants were calculated and are given in Table 2. From the k values, we can see that p-HBA removal rate was influenced by organic compounds and alkalinity presenting in aqueous solutions. The following equations were used to evaluate the effect of inorganic molecules and organic matters. . E t ¼ ðk DW −k MSEF Þ k DW

ð4Þ

 . E a ¼ k MSEFðlaÞ −k MSEF k MSEFðlaÞ

ð5Þ

where Et and Ea represent the effect of all components in MSEF and the effect of alkalinity, and kDW, kMSEF, and kMSEF(la) represent first-order kinetic constants for p-HBA removal in deionized water, MSEF, and MSEF with low alkalinity, respectively. By calculating, the components in MSEF inhibited about 35.4% for p-HBA removal, and 18.5% of them was caused by the presence of HCO3− and CO32− ions. Therefore, the competition for hydroxyl radicals between organic compounds and alkalinity decreased the ozonation efficiency.

Table 2

First-order kinetic constants for p-HBA removal

Method

Matrix

k (min−1)

R2

Catalytic ozonation

DW MSEF MSEF low alkalinity DW MSEF

0.4481 0.2893 0.3548 0.3898 0.2239

0.9573 0.9632 0.9546 0.9682 0.9745

Ozonation

Environ Sci Pollut Res

Degradation efficiency of micropollutants in municipal secondary effluents The decomposition profiles of target pollutants (SMT and CBZ) in the single and catalytic ozonation process were examined using the municipal secondary effluents (Fig. 7). It can be seen that both SMT and CBZ can be degraded by ozone. However, the addition of Fe3O4-CeO2/MWCNTs can increase the degradation rate. After 4 min reaction, the residual SMT in aqueous solution during single and catalytic ozonation process was 57.8 and 49.7%, respectively. And the degradation efficiency of CBZ in single and catalytic ozonation process was 50.3 and 64.1% after 4 min reaction. This result implied that the production of hydroxyl radicals accelerated the removal rate with addition of catalyst. Our previous research showed that SMT can be attacked by ozone in pure water, but the intermediates are difficult to further react with ozone, and with addition of catalyst, the mineralization of SMT could be

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enhanced significantly through the production of hydroxyl radicals (Bai et al. 2016a, c). Similar results were also obtained in the catalytic ozonation of CBZ using titanium dioxide (Rosal et al. 2008), CBZ disappeared during the first few minutes by single or catalytic ozonation system, and the mineralization rate obviously increased with addition of titanium dioxide. The results were consistent with the decreasing tendency of SCOD and TOC removal; as discussed above, the organic pollutants in effluent can be oxidized by ozone, but hydroxyl radical is more powerful for their mineralization. Therefore, catalytic ozonation is effective for degrading micropollutants in the municipal secondary effluents by using Fe3O4-CeO2/MWCNTs as efficient catalyst.

The stability and reusability of catalyst The stability and reusability of the catalysts in catalytic ozonation were examined and are shown in Fig. 8. The removal efficiency of SCOD decreased from 49.8 to 43.2% after five times run. However, DOC removal efficiency got a little higher decrease, from 39.6 to 25.3%. After five times use, the iron leaching was not detected by atomic absorption spectrophotometer (detection limits of iron is 0.01 mg L−1). Moreover, the catalyst can be easily separated from the solution by magnet. These results indicated that the catalyst was favorable to reuse. There was a little inactivation, similar with the reported catalysts (Bai et al. 2016a, b, c, Wu et al. 2016). In order to explore the change of catalyst after five times use, the newly and used catalysts were characterized by SEM and XRD and the results are shown in Figs. 9 and 10. Based on the XRD results, there were three characteristic reflections in both newly and used catalyst. Diffraction peak at 2θ = 26° should be assigned to MWCNTs (Saleh et al. 2011).

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Time (min)

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60

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C/C0 (CBZ)

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SCOD DOC

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Fig. 7 Degradation of SMT and CBZ in different conditions

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Fig. 8 Catalytic ozonation performance of the catalyst in successive runs

(440)

(220)

(220)





MWCNTs Fe3O4



CeO2

(511)

(422)

(400)

(311)

(111)

Intensity (a.u.)

b



(311) (511)



(400)

(111)

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(200) (311)

(002) (111)

Environ Sci Pollut Res

a

0

10

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that the newly prepared catalyst was very uniform, large amount of particles attached on MWCNT surface. According to EDS analysis and XRD result, Fe3O4 and CeO2 were uniformly distributed on MWCNT surface. These uniform size and crystallization sites could be explained by the surface charge of functionalized MWCNTs, its interaction with metal ions, and specificities of synthetic solvent (Seo et al. 2014). But after five-time use, this uniform distribution was changed. Fe3O4 or CeO2 was dropped from MWCNTs. In accordance with the observation results from XRD, the corrosion of the catalyst decreased particle surface and decreased the catalytic activity during the catalytic ozonation process.

2θ

Fig. 9 XRD patterns of a new catalyst and b used catalyst

Conclusions The XRD patterns of CeO2 with a cubic fluorite structure (JCPDS 65-5923) were dominant, and Fe3O4 also appeared with a cubic spinel structure corresponding to the standard card of magnetite (JCPDS 19-0629). Compared to the newly and used catalyst, the peak intensity of Fe3O4 became weak; peak (220) and peak (422) disappeared implying that Fe3O4 involved in the reaction and corroded during the catalytic ozonation process. Figure 10 illustrates the size and morphology of the newly and used Fe3O4-CeO2/MWCNTs. It can be seen

Fig. 10 SEM micrographs of a, b new catalyst and c, d used catalyst

Municipal secondary effluent was further treated by catalytic ozonation system using Fe3O4-CeO2/MWCNTs as catalyst. The catalytic ozonation system exhibited high degradation efficiency for organic pollutants in the effluent. The addition of catalyst increased ozone utilization obviously. Matrix compounds in the effluent, including organic compounds and alkalinity, could consume hydroxyl radicals. The catalyst showed high stability and excellent reusability, but the corrosion of catalyst should be further studied.

Environ Sci Pollut Res Acknowledgements The research was supported by the National Natural Science Foundation of China (51338005) and the Program for Changjiang Scholars and Innovative Research Team in University (IRT13026).

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