CeO2 composite as a heterogeneous Fenton-like catalyst

Accepted Manuscript Degradation of 2,4,6-trichlorophenol using magnetic nanoscaled Fe3O4/CeO2 composite as a heterogeneous Fenton-like catalyst Lejin Xu, Jianlong Wang PII: DOI: Reference:

S1383-5866(15)00300-7 http://dx.doi.org/10.1016/j.seppur.2015.05.011 SEPPUR 12340

To appear in:

Separation and Purification Technology

Received Date: Revised Date: Accepted Date:

11 February 2015 16 May 2015 20 May 2015

Please cite this article as: L. Xu, J. Wang, Degradation of 2,4,6-trichlorophenol using magnetic nanoscaled Fe3O4/ CeO2 composite as a heterogeneous Fenton-like catalyst, Separation and Purification Technology (2015), doi: http:// dx.doi.org/10.1016/j.seppur.2015.05.011

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Degradation of 2,4,6-trichlorophenol using magnetic nanoscaled Fe3O4/CeO2 composite as a heterogeneous Fenton-like catalyst

Lejin XU1, Jianlong WANG1,2 *

1

Collaborative Innovation Center for Advanced Nuclear Energy Technology,

INET, Tsinghua University, Beijing 100084, P.R. China 2

Beijing Key Laboratory of Radioactive Wastes Treatment, Tsinghua University,

Beijing 100084, P.R. China ∗ Corresponding author Full post address: Neng Ke Lou, Tsinghua University, Beijing 100084, P. R. China

Tel.: +86 10 62784843 Fax: +86 10 62771150 E-mail address: [email protected]

∗ Corresponding author. Tel.: +86 10 62784843; fax: +86 10 62771150. E-mail address: [email protected]. 1

Abstract The degradation of 2,4,6-trichlorophenol (TCP) was investigated by using magnetic nanoscaled Fe3O4/CeO2 composite as a heterogeneous Fenton-like catalyst. The individual and interactive effects of four process variables, i.e. solution pH, initial TCP concentration, Fe3O4/CeO2 dosage and H2O2 concentration, on TCP removal, mineralization and dechlorination were investigated by response surface methodology (RSM) using the central composite design (CCD). The optimal regions of degradative conditions were pH 2.0–2.1, TCP 20–100 mg/L, Fe3O4/CeO2 1.5–2.5 g/L, and H2O2 17–30 mM. The removal efficiency, mineralization and dechlorination rate of TCP was 99%, 65% and 95% after 90 min, respectively under the conditions of pH 2.0, TCP 100 mg/L, Fe3O4/CeO2 2.5 g/L and H2O2 30 mM, which agreed well with the modeling prediction. Fe3O4/CeO2 showed a high catalytic ability for the removal of TCP in comparison with other processes. The recyclability of Fe3O4/CeO2 was also examined. According to the results of iron leaching, the effects of radical scavengers and intermediates determination, a possible pathway of TCP degradation was proposed based on •OH mechanism (including free •OH in the bulk liquid and surface-bounded •OH).

Keywords: Chlorophenol; Advanced oxidation process; Fenton reaction; Nano-catalyst; Response surface methodology

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1. Introduction 2,4,6-Trichlorophenol (TCP), widely employed in the manufacturing of fungicides, herbicides, pesticides, insecticides, antiseptics, pharmaceuticals, dyes and plastics, has been listed as a priority pollutant by the US Environmental Protection Agency and the European Union [1,2]. Adverse effects on human health caused by TCP, such as respiratory effects from cough to serious pulmonary defects, gastrointestinal effects, and cardiovascular effects, have been reported [1]. Due to its xenobiotic characteristics, carcinogenic and mutagenic properties, high toxicity and bioaccumulation, TCP must be removed before being discharged into the aquatic environment.

The toxicity and removal efficiency of cholorophenols are largely dependent on the number and position of chlorine atoms in the aromatic ring, which can be attributed to steric, inductive, and resonance effects [3]. As a result of high toxicity, carcinogenic properties and structural stabilization, traditional processes such as biological treatment are not very effective for removal of cholorophenols. In recent years, various technologies using novel materials have been studied to remove cholorophenols from aqueous solutions, such as adsorption technology [1,4,5], reductive treatment with zero-valent iron [6], electrochemical oxidation [7], catalytic wet oxidation [8,9], radiation-induced degradation [10,11]. Among the advanced oxidation processes, Fenton technology has become a favored process because of its

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high performance, cost-effectiveness, simplicity of technology, and low toxicity of the reagents [12]. Many researchers have investigated the decomposition of TCP by Fenton’s reagent [13,14]. When combined with photolysis, the TCP degradation was faster than that by the Fenton process (dark reaction) [15,16]. Yuan and Lu [17] reported the removal of chlorophenols by electro-Fenton method, and they observed that the addition of small quantities of Fe2+ or Fe3+ significantly accelerated the degradation rate. However, the assistance of photolysis or electrochemical technology increases the energy requirements and the overall cost of treatment. Ferromagnetic nanoparticles have recently received more and more attention due to their large specific surface area and high surface reactivity as well as their stability and reusability [18–21]. In our previous study, a heterogeneous Fenton-like system using superparamagnetic nanoscaled Fe3O4/CeO2 composite was successfully developed to oxidize 4-chlorophenol [20]. The component CeO2 facilitated the dissolution of Fe3O4, and hydroxyl radicals (•OH) were generated by the reaction of Fe2+ and Ce3+ with H2O2, leading to the enhanced Fenton chemistry [20].

The efficiencies of heterogeneous Fenton-like systems depend on several operation parameters, mainly solution pH, initial pollutant concentration, catalyst dosage, H2O2 concentration, and reaction time. To optimize experimental conditions and to study the interactions of these variables, response surface methodology (RSM) has been found to be a very useful tool, as it is faster, more economical and effective. In the

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water treatment field, RSM has already been applied in many processes, such as adsorption [3], electrocoagulation [22], UV/H2O2 [23], O3/UV [24], O3/UV/H2O2 [24], Fenton and Fenton-like integrated process [25,26], and photo-Fenton process [27]. An experimental comparison between Box-Behnken design (BBD), central composite design (CCD), and Doehlert matrix design (DM) has been conducted by Zolgharnein et al. [28], which shows that CCD has a better prediction than the others. Nevertheless, few studies have been reported on the heterogeneous Fenton-like degradation of TCP, especially on the application of RSM to optimize the operating conditions.

The objective of this research was to investigate the treatability of wastewater containing TCP with the heterogeneous Fenton-like system using magnetic nanoscaled Fe3O4/CeO2 composite, and also to optimize the key operating conditions using RSM. A central composite design was selected to study simultaneously the individual and interactive effects of four variables (pH value, initial TCP concentration, Fe3O4/CeO2 dosage and H2O2 concentration) on the three responses (TCP removal, mineralization and dechlorination). Quadratic models were proposed to describe the relationships between these responses and variables. Furthermore, iron leaching, H2O2 decomposition, reactive oxidizing species mediated in the system, the mechanism of TCP degradation as well as the reusability of the catalyst were evaluated.

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2. Materials and methods 2.1. Chemicals and materials Ferrous sulfate (FeSO4·7H2O) and ferric sulfate (Fe2(SO4)3) were supplied by Shenyang Reagent Factory and Tianjin Yongda Chemical Reagent Co., Ltd., respectively. Cerium (III) nitrate hexahydrate (Ce(NO3)3·6H2O) and polyethylene glycol (PEG) 4000 were purchased from Sinopharm Chemical Reagent Co., Ltd. Ammonium carbonate ((NH4)2CO3) was purchased from Beijing Yili Fine Chemicals Co., Ltd. TCP was obtained from Alfa Aesar (USA). Analytical grade carboxylic acids, H2O2 (30%, v/v), sulphuric acid (H2SO4), n-butanol, and KI were supplied by the Beijing Chemical Factory. Double distilled water was used throughout this study.

The magnetic nanoparticles (MNPs) with the weight ratio of Fe3O4 and CeO2 1:1 used in this study were synthesized and characterized as described previously [20]. First, CeO2 NPs were precipitated from aqueous solution of 100 mL 0.1 M Ce(NO3)3·6H2O and 4 g/L PEG 4000 by the addition of 150 mL 0.1 M (NH4)2CO3 solution under violently stirring for 10 min at 40 °C. After filtrated, dispersed by ultrasonic wave (frequency 99 kHz), and washed repeatedly with distilled water and ethanol, the resulting precipitate was dried at room temperature under vacuum and finally calcined at 300 °C for 1 h in air. Second, Fe3O4/CeO2 MNPs were synthesized by impregnation method. A solution of 100 mL 0.2 M NaOH and 0.23 g CeO2 was deoxygenated by bubbling Ar gas. Then, 100 mL of 0.01 M FeSO4·7H2O and 0.02 M

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Fe2(SO4)3 was added dropwise under violently stirring in a four-necked flask equipped with constant temperature bath (80 °C) and Ar protection. After washed with distilled water and ethanol several times, nanoparticles were dried under vacuum at room temperature for instant usage. The particle size, BET surface area, pore size, pore volume, and saturation magnetization (Ms) of Fe3O4/CeO2 MNPs were 5 10 nm, 80.21 m2/g, 3.44 nm, 0.12 cm3/g, and 14.4 emu/g, respectively [20]. The superparamagnetic property of Fe3O4/CeO2 MNPs indicated that the composite could be easily separated and recycled from solution by an external magnetic field.

2.2. Experimental procedure The degradation of TCP was conducted in a conical flask (25 mL) placed in a water bath shaker (Gyromax 939, Amerex, USA) with an agitation of 150 rpm at 30 °C in the dark. With the addition of determined Fe3O4/CeO2, the reaction suspension containing 10 mL of TCP solution was prepared, and its initial pH was adjusted by H2SO4. The reaction was initiated by adding a known concentration of H2O2 to the solution. At designated time intervals, samples were collected using a 5 mL syringe, filtered through a 0.22 μm membrane filter, and subsequently quenched with excess n-butanol. To evaluate the recyclability of Fe3O4/CeO2 composite, the catalyst was gathered, washed and then dried under vacuum, which was used for another experiment under the same reaction conditions. All experiments were repeated at least two times, and the results are presented as average.

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2.3. Analytical methods The TCP concentration was analyzed by a high performance liquid chromatography (HPLC Agilent 1200 Series, USA) coupled with a diode array detector (DAD) and a C18 reversed-phase column (5 μm, 4.6 mm ×150 mm). The testing condition was methanol and water (70:30, v/v) as the mobile phase with a flow rate of 1.0 mL/min at the analytical wavelength of 284 nm. Total organic carbon (TOC) was measured using a Multi 2100 TOC/TN analyzer (Analytik Jena AG Corporation). Chloride ions (Cl-) and carboxylic acids were detected by an ion chromatography (DX-100, Dionex, Germany) equipped with a Dionex RFICTM IonPac® AS 14 analytical column (4 mm × 250 mm) and a Dionex RFICTM IonPac® AG 14 guard column (4 mm ×50 mm). An eluent solution of 3.5 mM Na2CO3 and 1.0 mM NaHCO3 was pumped at a flow rate of 1.0 mL/min, and the sample loop volume was 25 μL. The solution pH was determined with a Thermo Orion model 8103BN pH-meter.

Degradation intermediates of TCP were identified by an Agilent 7890A gas chromatograph and an Agilent 5975C mass spectrometer (GC/MS) equipped with a quartz capillary column (50 m ×0.25 mm, 0.25 μm film thicknesses). The temperature was programmed at 70 °C for 3 min, and then increased to 280 °C at a rate of 3 °C/min. The other experimental conditions were MS ion source temperature 200 °C, injection temperature 280 °C, EI impact ionization 70 eV with helium as the carrier gas.

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The concentrations of ferrous ion and total dissolved iron were detected according to the 1,10-phenanthroline method [19,20], using a UV/vis spectrophotometer (Lambda 25, PerkinElmer) at 510 nm. The evolution of hydrogen peroxide concentration in the solution was identified by the iodimetric titration [19,20].

2.4. Experimental design and statistical analysis RSM based on the central composite design, with the help of Design Expert 7.1.3, was applied to optimize the experimental conditions for TCP degradation. A second-order polynomial equation as shown in Eq. (1) was used to describe the relation between desired response and independent variables. n

n

n

n

i =1

i =1

i< j

j

Y = β 0 + ∑ βi X i + ∑ βii X i2 + ∑∑ βij X i X j + ε

(1)

where Y is the predicted response; Xi and Xj are the coded levels of the independent variables; n is the number of variables; ε is the random error; β0, βi, βii and βij are the constant, linear, quadratic and interaction coefficients, respectively. In the present work, a central composite design with four factors at five levels and six replicates at the center point for each independent variable, led to a total number of 30 experiments employed for response surface modeling, as shown in Table 1. Four important operating parameters, i.e. solution pH (A), initial TCP concentration (B), Fe3O4/CeO2 dosage (C), and H2O2 concentration (D), were chosen as the independent variables. Based on the preliminary assays, the ranges of four parameters chosen in this study 9

were 2.0–4.0 for pH, 20–100 mg/L for initial TCP concentration, 0.5–2.5 g/L for Fe3O4/CeO2 dosage, and 6–30 mM for H2O2 concentration (Table 2). Three responses analyzed were TCP removal (R1), mineralization (R2) and dechlorination (R3) after 90 min of reaction. The TCP removal and mineralization efficiencies were measured by the reduction of C/C0 and TOC/TOC0, respectively, while the degree of dechlorination was calculated by comparing the concentration of Cl- detected in the solution and the theoretical Cl- concentration of TCP. Eq. (1) can be written for four independent variables with three responses in their actual values as presented in the following form: R = β0 + β1A + β2B + β3C + β4D + β12AB + β13AC + β14AD + β23BC + β24BD + β34CD + β11A2 + β22B2 + β33C2 + β44D2 + ε

(2)

Analysis of variance (ANOVA) was performed to check the adequacy of the developed models and to identify the interactions between the variables and the responses. The model adequacies were checked from the coefficients of determination R2 and adjusted R2, and the statistical significance of regression coefficients was evaluated by the F-test in the same program. Model terms were selected or rejected according to the probability value with a 95% confidence level. Three-dimensional (3D) plots and their respective contour plots illustrated the individual and the interactive effects of the independent variables on TCP degradation.

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3. Results and discussion 3.1. Degradation of TCP by various processes To investigate the catalytic activity of Fe3O4/CeO2 to the H2O2 activation, control experiments were carried out to compare the removal efficiencies of TCP by various processes at pH 2.0 with initial TCP concentration 100 mg/L. As shown in Fig. 1, after 120 min of reaction, about 5% TCP was removed with only 30 mM H2O2, indicating that the oxidation ability of H2O2 is limited. Within 120 min reaction, only 13% removal of TCP was obtained in control reactions with 2.5 g/L Fe3O4/CeO2 composite without H2O2, and even throughout 24 h reaction about 15% removal was observed (data not shown). About 0.036 mM free chloride released from TCP was detected in the solution after 24 h, which suggested that the TCP removal was probably ascribed to surface adsorption and reductive dechlorination [19,20]. With CeO2 and H2O2, a slight removal of TCP was observed mainly due to the surface adsorption of CeO2 NPs. An induction period and a followed rapid degradation stage of TCP was investigated by the Fe3O4 H2O2 process, and TCP was almost completely removed after a 120 min reaction. With the simultaneous presence of Fe3O4/CeO2 composite and H2O2, almost complete removal of TCP was achieved within 90 min, indicating the high catalytic ability toward H2O2 activation for Fe3O4/CeO2.

Under acidic conditions, the reaction of Fe3O4/CeO2 composite with H2O2

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generates highly oxidative hydroxyl radicals (•OH) that can degrade organic pollutants rapidly [20]. The process involves the redox recycling of Fe2+/Fe3+ both at the surface of catalyst and in the bulk solution as seen in Eq. (3). Some Fe2+ species are reproduced through the reactions (4) and (5). The component CeO2 in the composite enhances the dissolution of Fe3O4, and Ce4+ can be reduced by the transfer of an electron from Fe2+ to Ce4+ (Eq. (6)). The formed Ce3+ can catalyze a Fenton-like reaction with H2O2 generating •OH (Eq. (7)), behaving similar to Fe2+ (Eq. (3)) [20,29]. Fe2+ + H2O2 + H+ → Fe3+ + •OH + H2O Fe3+ + H2O2

(3)

→ Fe2+ + HOO• + H+

(4)

Fe3+ + HOO• → Fe2+ + O2 + H+

(5)

Ce4+ + Fe2+ ↔ Ce3+ + Fe3+

(6)

Ce3+ + H2O2 + H+ → Ce4+ + •OH + H2O

(7)

3.2. Regression model and analysis of variance Three quadratic regression models were developed by the analysis of the observed responses (TCP removal, mineralization and dechlorination) given in Table 1 using Eq. (2), which are shown below as in terms of coded factors: R1 = 30.43 – 20.61A – 4.35B + 4.43C + 2.42D + 1.45AB – 5.34AC – 1.70AD – 0.36BC – 0.029BD + 1.32CD + 6.22A2 – 0.81B2 – 1.08C2 – 1.31D2 R2 = 16.83 – 13.02A – 1.44B + 2.49C + 0.13D – 0.37AB – 2.73AC – 0.34AD

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(8) (9)

– 1.34BC + 1.71BD + 0.98CD + 4.21A2 – 1.72B2 – 1.09C2 – 1.43D2 R3 = 28.39 – 22.11A – 5.48B + 4.13C + 2.72D + 0.68AB – 4.24AC – 0.88AD + 0.20BC + 0.70BD + 1.88CD + 5.59A2 – 0.76B2 – 1.36C2 – 1.35D2

(10)

Analysis of variance (ANOVA) was conducted to estimate the goodness of the fit of three equations for the experimental data, and the results are summarized in Table 3. Data listed in this table show that all the models were highly significant, as the F-values for the models were 15.48, 12.51 and 21.06, with the corresponding p-values of regression <0.0001. The “adequate precision” ratios of the models for TCP removal, mineralization and dechlorination were 16.316, 15.971 and 19.278, respectively, which were greater than 4 indicating an adequate signal for the models to be used to navigate the design space. The lack-of-fit F-values of 53.51, 40.60 and 72.31 obtained were statistically significant, as their p-values were less than 0.05. As similarly reported by others [30–32], a significant lack of fit suggests that there might be some systematic variation unaccounted for in the hypothesized model, owing to the exact replicate values of the independent variable in the model that provide an estimate of pure error. The correlation coefficients (R2) between the observed and predicted values were obtained as 0.9353, 0.9211 and 0.9516 for TCP removal, mineralization and dechlorination, respectively, which indicated that the equations are highly reliable. The corresponding adjusted R2 values were 0.8749, 0.8475 and 0.9064 for these three models, which ensured a satisfactory adjustment of the quadratic models to the

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experimental data. Therefore, the response surface models were accurately employed for predicting the relationships between these four parameters and the responses. Besides, the plots of the comparison of actual and predicted values for TCP removal, mineralization and dechlorination indicated an adequate agreement between real data and the ones obtained from the models (Fig. 2), which demonstrates again that the models are reliable to describe the heterogeneous Fenton-like degradation of TCP.

The significance of each independent variable was also evaluated according to its p-value (a p-value less than 0.05 indicates that the model term is significant, whereas the value greater than 0.10 is not significant). Among the test variables, the terms of pH value (A) produced the largest effect on TCP removal efficiency (R1). The coefficient of the quadratic effect of pH value (A2) was highly significant, the coefficients of the linear effects of initial TCP concentration (B) and Fe3O4/CeO2 dosage (C) as well as the interaction effect of pH value and Fe3O4/CeO2 dosage (AC) were slightly significant, and other factors were not significant (data not shown). For mineralization of TCP (R2), the linear effects of pH value (A) and Fe3O4/CeO2 dosage (C) were significant model terms, as well as the quadratic effect of pH value (A2). The linear terms (A, B and C), the interaction term (AC) and the quadratic term (A2) were statistically significant for dechlorination of TCP (R3).

3.3. Response surface plots and optimization conditions

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To assess the interactive relationships between the process variables and the studied responses for heterogeneous Fenton-like degradation of TCP, the response surface and contour plots were utilized. Almost the similar phenomena were observed for TCP removal, mineralization and dechlorination, except that the corresponding ranges of removal efficiency were 7–100%, 0–71% and 0–100%, respectively. In order to save space, only the most interesting and informative plots were illustrated in Fig. 3, which represents the effect of two variables varying within their studied ranges with the others kept at constant.

As seen in Fig.3, solution pH appears to be the most important variable governing TCP degradation, due to its role in controlling the dominant iron species, the stability of H2O2, and the activity of the oxidant and the substrate [18,19,33,34]. Within the pH ranged from 4.0 to 2.0, the TCP degradation was significantly improved, which can be attributed to the increased iron leaching from the catalyst, the less decomposition of H2O2 to H2O and O2, and the higher oxidation potential of •OH [19]. At a fixed Fe3O4/CeO2 dosage of 1.5 g/L and H2O2 concentration of 18 mM in Fig. 3a, initial TCP concentration had a negative effect on TCP degradation, which means that the TCP removal decreases as the initial TCP concentration increases. Increased amount of TCP molecules adsorbed on the catalyst surface may occupy a greater number of active sites, which results in less H2O2 interacting with catalyst surface, producing less •OH at the surface [35]. When Fe3O4/CeO2 dosage increased from 0.5 to 2.5 g/L,

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TCP removal was enhanced from 62% to 100% (Fig. 3b) at pH 2.0 with 60 mg/L TCP and 18 mM H2O2. The addition of H2O2 had an expected positive effect on TCP degradation as shown in Fig. 3c. Similarly, the plot of the effect of these two variables on TCP removal (Fig. 3d) at pH 3.0 with 60 mg/L TCP showed that higher TCP removal was achieved at higher Fe3O4/CeO2 dosage and H2O2 concentration, which was ascribed to the increasing amount of active sites on catalyst surface for H2O2 activation producing more oxidative species (mainly •OH) [19,20]. Due to the negative effects of pH and initial TCP concentration and positive effects of Fe3O4/CeO2 and H2O2 dosage observed on TCP degradation, it is desirable to run heterogeneous Fenton-like process at low pH, low initial TCP concentration, high catalyst dosage and high H2O2 concentration.

The optimization conditions where all parameters simultaneously meet the desirable criteria are searched by overlaying critical response contours on a contour plot. After defining the optimization criteria for the chosen responses (TCP removal ≥ 90%, mineralization ≥ 50%, and dechlorination ≥ 90%), the optimal regions were shaded as shown in Fig. 4, which corresponded to the areas where the initial pH was 2.0–2.1, the initial TCP concentration was 20–100 mg/L, Fe3O4/CeO2 dosage was 1.5–2.5 g/L, and H2O2 concentration was 17–30 mM.

While the results presented in this section offer insight into the effects of pH, initial

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TCP concentration, Fe3O4/CeO2 dosage and H2O2 concentration on the heterogeneous Fenton-like degradation of TCP, it should be noted that the obtained optimal reaction conditions are restricted to low pH values and relatively high concentrations of Fe3O4/CeO2 and H2O2, which are likely relevant to only a limited number of practical applications. It is hoped that our work could provide a basis for the investigation of more complex systems of relevance to treatment of wastewater containing TCP. More efforts are required to adopt chelating agents to keep iron in solution or to develop heterogeneous iron catalysts used under slightly acidic or near-neutral pH conditions in practical wastewater treatment applications.

3.4. Model validation and confirmation An additional experiment was carried out at pH 2.0 with 100 mg/L TCP, 2.5 g/L Fe3O4/CeO2 and 30 mM H2O2 (recommended by RSM as the optimum reaction conditions) to check the agreement of the results achieved from models and experiments. As shown in Fig. 5, experimental findings for all responses were in close agreement with the model prediction, with low errors of –0.95%, 0.61% and –5.06% and low standard deviations of ±0.67%, ±0.43% and ±3.58% for three responses (R1, R2 and R3), respectively.

3.5. Comparison with other methods The removal of TCP at the optimum conditions followed a pseudo-first-order

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kinetics with the rate constant (k) of 4.6 ×10-2 min-1 and R2 of 0.8302, which was compared with other studies as shown in Table 4 [8–10,13,36–43]. The rate constants obtained by Fenton-based reactions were in the range of 1.6 ×10-4 to 1.5 ×10-1 min-1 [9,13,36,37], while that by photolysis and photocatalysis were from 7.5 ×10-5 to 9.3 × 10-2 min-1 [9,10,37–41]. In comparison with these methods, the degradation of TCP by heterogeneous Fenton-like system using Fe3O4/CeO2 in this study seemed to have a relatively high removal rate. The rate constants of catalytic wet oxidation and adsorption were found to be 2.8 ×10-3 and 8.8 ×10-3 min-1, respectively [8,43], which were one order of magnitude smaller than that of this study. The result provides further support for the high catalytic ability of Fe3O4/CeO2 in heterogeneous Fenton-like oxidation of TCP.

3.6. Iron leaching and H2O2 decomposition Variation of the concentrations of dissolved iron, ferrous ion and H2O2 in the solution was investigated during TCP degradation under the optimum reaction conditions. It has been shown in Fig. 6 that the Fe2+ concentration increased and reached a peak value of 14.3 mg/L at 90 min that was close to the time of complete removal of TCP (Fig. 5), and then the concentration decreased to 8.5 mg/L at 120 min. As similarly reported by earlier studies [44,45], the ascending period of the dissolution of Fe2+ to solution arises from the gradual oxidation of the catalyst surface by H2O2, and the descending period is probably caused by the oxidation of the

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dissolved ferrous ions to ferric ions by remaining H2O2 and •OH. The total dissolved iron, ascribed to the continuous leaching of Fe2+ and Fe3+ from Fe3O4/CeO2, amounted to 60.6 mg/L after 120 min of reaction, equivalently about 4.8% of total iron of 2.5 g/L catalyst used. This indicates that at initial solution pH 2.0, the observed degradation of TCP was caused by the combination of homogeneous Fenton reaction owing to the continuous leaching of Fe into solution as a homogeneous catalyst and heterogeneous Fenton-like reaction due to the presence of Fe3O4/CeO2 as a heterogeneous catalyst.

Figure 6 depicts that the concentration of H2O2 decreased gradually as reaction time increased during the degradation of TCP. As expressed in Eq. (11), the mineralization of one mole TCP consumes 11 moles of H2O2 in stoichiometry. The H2O2 amount for TCP degradation was calculated by measuring the TOC removal after 120 min which was about 65% as seen in Fig. 5. The amount of the consumed H2O2 was about 6.2 mM during 120 min reaction (Fig. 6). According to the study of Luo et al. [46], the utilization efficiency of H2O2 is calculated by the ratio of H2O2 amount used for TCP degradation against the total amount of the consumed H2O2 in the reaction. Therefore, this value was calculated as 57% in our system. This means that H2O2 consumed was not completely used to oxidize TCP, which could also be used by scavenging reactions. Combined with the result of Fig. 3d, TCP was mainly degraded by the attack of •OH radicals rather than directly by H2O2.

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C6H3OCl3 + 11H2O2 → 6CO2 + 11H2O + 3HCl

(11)

3.7. Possible degradation mechanism of TCP Figure 7 shows the results obtained when TCP degradation was conducted in the presence of radical scavengers that were n-butanol and iodide ion to eliminate all the •OH formed in the system and surface-bounded •OH produced at the surface of Fe3O4/CeO2 [47,48]. With addition of 300 mM n-butanol, TCP removal was almost completely inhibited, suggesting that TCP was decomposed by the attack of •OH radicals generated both in the bulk liquid and at the surface of catalyst. After adding excess KI (10 mM), the removal of TCP decreased from 100% (in 120 min without KI) to 58%, which indicated that surface-bounded •OH played a considerable role in the degradation of TCP.

The intermediates of TCP degradation at pH 2.0 by heterogeneous Fenton-like process using Fe3O4/CeO2 were determined by HPLC, GC/MS and IC (Table 5), and a probable multistep pathway for oxidative destruction of TCP is proposed in Fig. 8. The addition of •OH to the para-position of the aromatic ring was preferred leading to the formation of 2,6-dichloro-hydroquinone (S3) because of the steric effect. If the addition was at ortho-position, TCP (D1) was transformed to 3,5-dichloro-catechol (S4) by •OH attacking and substituting the Cl-atom. Under the highly oxidizing environment, compounds S3 and S4 existed in equilibrium with

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2,6-dichloro-1,4-benzoquinone (D2) and 3,5-dichloro-1,2-benzoquinone (D3), as proposed by Basu and Wei [14]. The substitution of the electron-withdrawing Cl-atoms by hydroxyl groups progressed successively by converting compounds S3 and S4 to 2-chloro-1,4-benzoquinone (D4), and then to 2-hydroxycyclohexa-2,5-diene-1,4-dione (D5). As this reaction proceeded, the amount of chloride ions in the aqueous solution also increased at the same rate, which can be observed from Fig. 5. After dechlorination, compound D5 underwent further transformation through ring cleavage and subsequent degradation to simple carboxylic acids, such as succinaldehyde (D6), (E)-but-2-enoic acid (D7), maleic acid, and oxoacetic acid (D8). With further attack of •OH, these acids were degraded to smaller molecular organic acids like oxalic acid, acetic acid (D9), formic acid (D10), etc., as the final products remaining in solution.

The release of chlorines from the aromatic ring can lower the overall toxicity of TCP, because with increasing chlorine substitution the toxicity of chlorophenols increases [3]. About 65% TOC (Fig. 5) was removed, and TCP was not oxidized completely into CO2 and H2O that was decomposed to smaller molecular organic acids. However, almost complete dechlorination of TCP (Fig. 5) was obtained by heterogeneous Fenton-like process using Fe3O4/CeO2, indicating that this process could be used as a pretreatment stage to reduce toxicity and enhance biodegradability.

21

3.8. Recyclability of Fe3O4/CeO2 composite The potential to reuse Fe3O4/CeO2 composite was tested, and the result is seen in Fig. 9. The removal of TCP gradually decreased by repeated reuse of Fe3O4/CeO2 composite at conditions of pH 2.0, TCP 100 mg/L, Fe3O4/CeO2 2.5 g/L, and H2O2 30 mM, which could be explained by the deactivation of the composite and the leaching of iron from the catalyst surface detected in the section 3.6. The reactivation of the catalyst and the synthesis of more stable catalyst with higher catalytic activity that can be used in a wider pH range need to be developed in the future work.

4. Conclusions The degradation of TCP by heterogeneous Fenton-like system using magnetic nanoscaled Fe3O4/CeO2 composite has been optimized by RSM based on the central composite design. Three regression models were developed to describe the significant effects of four operating variables (solution pH, initial TCP concentration, Fe3O4/CeO2 dosage and H2O2 concentration) as well as their interactive effects on three responses (TCP removal, mineralization and dechlorination). ANOVA indicated that the models were highly reliable, and the model prediction was in agreement with the experimental results obtained at conditions of pH 2.0, 100 mg/L TCP, 2.5 g/L Fe3O4/CeO2 and 30 mM H2O2. Compared with other methods, our study showed a high removal rate in the degradation of TCP. Based on the aromatic intermediates, carboxylic acids and chloride ion detected by HPLC, GC/MS and IC, a probable

22

degradation pathway of TCP by the attack of •OH generated both in the bulk liquid and at the surface of catalyst was proposed. Although the catalytic and magnetic properties of Fe3O4/CeO2 show a potential availability in wastewater treatment, further work is needed to determine the possible toxicity of nano-catalyst and to diminish the leaching of active components from the catalyst.

Acknowledgements The research was supported by Program for Changjiang Scholars and Innovative Research Team in University (IRT-13026). The authors are also grateful for the financial support provided by the National Natural Science Foundation of China (Grant No. 51338005).

23

References [1] J.L. Fan, J.A. Zhang, C.L. Zhang, L.A. Ren, Q.Q. Shi, Adsorption of 2,4,6-trichlorophenol from aqueous solution onto activated carbon derived from loosestrife, Desalination, 267 (2011) 139–146. [2] S. Han, X. Li, , X. Li, Y. Wang, S.N. Chen, Graphene oxide-based fluorescence molecularly imprinted composite for recognition and separation of 2,4,6-trichlorophenol, RSC Adv., 5 (2015) 2129–2136. [3] L.J. Xu, J.L. Wang, Degradation of chlorophenols using a novel Fe0/CeO2 composite, Appl. Catal. B: Environ., 142–143 (2013) 396–405. [4] B.H. Hameed, I.A.W. Tan, A.L. Ahmad, Preparation of oil palm empty fruit bunch-based activated carbon for removal of 2,4,6-trichlorophenol: Optimization using response surface methodology, J. Hazard. Mater., 164 (2009) 1316–1324. [5] J.L. Wang, Y. Qian, N. Horan, E. Stentiford, Bioadsorption of pentachlorophenol (PCP) from aqueous solution by activated sludge biomass, Bioresouc. Technol., 75(2000)157-161. [6] R. Cheng, J.L. Wang, W.X. Zhang, Comparison of reductive dechlorination of p-chlorophenol using Fe(0) and nanosized Fe(0), J. Hazard. Mater., 144(2007)334-339. [7] H. Wang, J.L. Wang, Electrochemical degradation of 4-chlorophenol using a novel Pd/C gas-diffusion electrode, Appl. Catal. B: Environ., 77(2007)58-65. [8] S. Chaliha, K.G. Bhattacharyya, Catalytic wet oxidation of 2-chlorophenol,

24

2,4-dichlorophenol and 2,4,6-trichlorophenol in water with Mn(II)-MCM41, Chem. Eng. J., 139 (2008) 575–588. [9] P. Saritha, D.S.S. Raj, C. Aparna, P.N.V. Laxmi, V. Himabindu, Y. Anjaneyulu, Degradative oxidation of 2,4,6 trichlorophenol using advanced oxidation processes A comparative study, Water Air Soil Poll., 200 (2009) 169–179. [10] C.G. Joseph, G.L. Puma, A. Bono, Y.H. Taufiq-Yap, D. Krishnaiah, Operating parameters and synergistic effects of combining ultrasound and ultraviolet irradiation in the degradation of 2,4,6-trichlorophenol, Desalination, 276 (2011) 303–309. [11] J. Hu, J.L. Wang, Degradation of chlorophenols in aqueous solution by gamma radiation, Radiat. Phy. Chem., 76(2007) 1489-1492. [12] J.L. Wang, L.J. Xu, Advanced oxidation processes for wastewater treatment: Formation of hydroxyl radical and application, Crit. Rev. Environ. Sci. Technol., 42 (2012) 251–325. [13] F.J. Benitez, J. Beltran-Heredia, J.L. Acero, F.J. Rubio, Chemical decomposition of 2,4,6-trichlorophenol by ozone, Fenton's reagent, and UV radiation, Ind. Eng. Chem. Res., 38 (1999) 1341–1349. [14] S. Basu, I.W. Wei, Mechanism and kinetics of oxidation of 2,4,6-trichlorophenol by Fenton's reagent, Environ. Eng. Sci., 17 (2000) 279–290. [15] C.Y. Kuo, S.L. Lo, M.T. Chan, Oxidation of aqueous chlorophenols with photo-Fenton process, J. Environ. Sci. Health. B, 33 (1998) 723–747. [16] M. Vinita, R.P.J. Dorathi, K. Palanivelu, Degradation of 2,4,6-trichlorophenol by

25

photo Fenton's like method using nano heterogeneous catalytic ferric ion, Sol. Energy, 84 (2010) 1613–1618. [17] S.H. Yuan, X.H. Lu, Comparison treatment of various chlorophenols by electro-Fenton method: relationship between chlorine content and degradation, J. Hazard. Mater., 118 (2005) 85–92. [18] L.J. Xu, J.L. Wang, A heterogeneous Fenton-like system with nanoparticulate zero-valent iron for oxidation of 4-chloro-3-methyl phenol, J. Hazard. Mater., 186 (2011) 256–264. [19] L.J. Xu, J.L. Wang, Fenton-like degradation of 2,4-dichlorophenol using Fe3O4 magnetic nanoparticles, Appl. Catal. B: Environ., 123 (2012) 117–126. [20] L.J. Xu, J.L. Wang, Magnetic nanoscaled Fe3O4/CeO2 composite as an efficient Fenton-like heterogeneous catalyst for degradation of 4-chlorophenol, Environ. Sci. Technol., 46 (2012) 10145–10153. [21] Y.S. Zhao, C. Sun, J.Q. Sun, R. Zhou, Kinetic modeling and efficiency of sulfate radical-based oxidation to remove p-nitroaniline from wastewater by persulfate/Fe3O4 nanoparticles process, Sep. Purif. Technol., 142 (2015) 182–188. [22] A.R. Amani-Ghadim, S. Aber, A. Olad, H. Ashassi-Sorkhabi, Optimization of electrocoagulation process for removal of an azo dye using response surface methodology and investigation on the occurrence of destructive side reactions, Chem. Eng. Process., 64 (2013) 68–78. [23] H. Kusic, N. Koprivanac, S. Papic, A.L. Bozic, Influence of substituent type and

26

position on photooxidation of phenolic compounds: Response surface methodology approach, J. Photochem. Photobiol. A: Chem., 242 (2012) 1–12. [24] A. Arslan, S. Veli, D. Bingöl, Use of response surface methodology for pretreatment of hospital wastewater by O3/UV and O3/UV/H2O2 processes, Sep. Purif. Technol., 132 (2014) 561–567. [25] X.B. Zhu, J.P. Tian, R. Liu, L.J. Chen, Optimization of Fenton and electro-Fenton oxidation of bioligically treated coking wastewater using response surface methodology, Sep. Purif. Technol., 81 (2011) 444–450. [26] J.R. Domínguez, T. González, P. Palo, E.M. Cuerda-Correa, Fenton plus Fenton-like integrated process for carbamazepine degradation: Optimizing the system, Ind. Eng. Chem. Res., 51 (2012) 2531–2538. [27] M. Vedrenne, R. Vasquez-Medrano, D. Prato-Garcia, B.A. Frontana-Uribe, M. Hernandez-Esparza, J.M. de Andrés, A ferrous oxalate mediated photo-Fenton system: Toward an increased biodegradability of indigo dyed wastewaters, J. Hazard. Mater., 243 (2012) 292–301. [28] J. Zolgharnein, A. Shahmoradi, J.B. Ghasemi, Comparative study of Box-Behnken, central composite, and Doehlert matrix for multivariate optimization of Pb (II) adsorption onto Robinia tree leaves, J. Chemom., 27 (2013) 12–20. [29] E.G. Heckert, S. Seal, W.T. Self, Fenton-like reaction catalyzed by the rare earth inner transition metal cerium, Environ. Sci. Technol., 42 (2008) 5014–5019. [30] M.H. Muhamad, A.S.R. S., A. Mohamad, R.R. A., K.A.A. H., Application of

27

response surface methodology (RSM) for optimisation of COD, NH3-N and 2,4-DCP removal from recycled paper wastewater in a pilot-scale granular activated carbon sequencing batch biofilm reactor (GAC-SBBR), J. Environ. Manage., 121 (2013) 179–190. [31] X. Liu, X.M. Li, Q. Yang, X. Yue, T.T. Shen, W. Zheng, K. Luo, Y.H. Sun, G.M. Zeng, Landfill leachate pretreatment by coagulation-flocculation process using iron-based coagulants: Optimization by response surface methodology, Chem. Eng. J., 200 (2012) 39–51. [32] Y.J. Wen, Y.H. Wu, Optimizing adsorption of Co(II) and Ni(II) by 13x molecular sieves using response surface methodology, Water Air Soil Poll., 223 (2012) 6095–6107. [33] S.X. Zhang, X.L. Zhao, H.Y. Niu, Y.L. Shi, Y.Q. Cai, G.B. Jiang, Superparamagnetic Fe3O4 nanoparticles as catalysts for the catalytic oxidation of phenolic and aniline compounds, J. Hazard. Mater., 167 (2009) 560–566. [34] D. Hermosilla, N. Merayo, R. Ordóñez, A. Blanco, Optimization of conventional Fenton and ultraviolet-assisted oxidation processes for the treatment of reverse osmosis retentate from a paper mill, Waste Manage., 32 (2012) 1236–1243. [35] X.F. Xue, K. Hanna, M. Abdelmoula, N.S. Deng, Adsorption and oxidation of PCP on the surface of magnetite: Kinetic experiments and spectroscopic investigations, Appl. Catal. B: Environ., 89 (2009) 432–440. [36] W.Z. Tang, C.P. Huang, Effect of chlorine content of chlorinated phenols on

28

their oxidation kinetics by Fenton's reagent, Chemosphere, 33 (1996) 1621–1635. [37] F.J. Benitez, J. Beltran-Heredia, J.L. Acero, F.J. Rubio, Contribution of free radicals to chlorophenols decomposition by several advanced oxidation processes, Chemosphere, 41 (2000) 1271–1277. [38] S. Rengaraj, X.Z. Li, Enhanced photocatalytic activity of TiO2 by doping with Ag for degradation of 2,4,6-trichlorophenol in aqueous suspension, J. Mol. Catal. A: Chem., 243 (2006) 60–67. [39] J. Bandara, J.A. Mielczarski, A. Lopez, J. Kiwi, 2. Sensitized degradation of chlorophenols on iron oxides induced by visible light - Comparison with titanium oxide, Appl. Catal. B: Environ., 34 (2001) 321–333. [40] E. Androulaki, A. Hiskia, D. Dimotikali, C. Minero, P. Calza, E. Pelizzetti, E. Papaconstantinou, Light induced elimination of mono- and polychlorinated phenols from aqueous solutions by PW12O403-. The case of 2,4,6-trichlorophenol, Environ. Sci. Technol., 34 (2000) 2024–2028. [41] H.H. Ji, F. Chang, X.F. Hu, W. Qin, J.W. Shen, Photocatalytic degradation of 2,4,6-trichlorophenol over g-C3N4 under visible light irradiation, Chem. Eng. J., 218 (2013) 183–190. [42] W.J. Huang, G.C. Fang, C.C. Wang, A nanometer-ZnO catalyst to enhance the ozonation of 2,4,6-trichlorophenol in water, Colloids Surf. Physicochem. Eng. Aspects, 260 (2005) 45–51. [43] L. Ren, J. Zhang, Y. Li, C.L. Zhang, Preparation and evaluation of cattail

29

fiber-based activated carbon for 2,4-dichlorophenol and 2,4,6-trichlorophenol removal, Chem. Eng. J., 168 (2011) 553–561. [44] J.Y. Feng, X.J. Hu, P.L. Yue, Discoloration and mineralization of orange II using different heterogeneous catalysts containing Fe: A comparative study, Environ. Sci. Technol., 38 (2004) 5773–5778. [45] M.L. Luo, D. Bowden, P. Brimblecombe, Catalytic property of Fe-Al pillared clay for Fenton oxidation of phenol by H2O2, Appl. Catal. B: Environ., 85 (2009) 201–206. [46] W. Luo, L.H. Zhu, N. Wang, H.Q. Tang, M.J. Cao, Y.B. She, Efficient removal of organic pollutants with magnetic nanoscaled BiFeO3 as a reusable heterogeneous Fenton-like catalyst, Environ. Sci. Technol., 44 (2010) 1786–1791. [47] Z.Q. He, S. Song, H.P. Ying, L.J. Xu, J.M. Chen, p-aminophenol degradation by ozonation combined with sonolysis: Operating conditions influence and mechanism, Ultrason. Sonochem., 14 (2007) 568–574. [48] S.T. Martin, A.T. Lee, M.R. Hoffmann, Chemical mechanism of inorganic oxidants in the TiO2/UV process: Increased rates of degradation of chlorinated hydrocarbons, Environ. Sci. Technol., 29 (1995) 2567–2573.

30

Figure captions Figure 1. Comparison of TCP removal by various processes at pH 2.0 with initial TCP concentration 100 mg/L. Figure 2. Actual and predicted values of TCP removal, mineralization and dechlorination. Figure 3. Response surface and contour plots for TCP removal: (a) effects of solution pH and initial TCP concentration; (b) effects of solution pH and Fe3O4/CeO2 dosage; (c) effects of solution pH and H2O2 concentration; (d) effects of Fe3O4/CeO2 dosage and H2O2 concentration. Figure 4. Overlay plots for optimal region. Figure 5. Verification experiments at pH 2.0 with 100 mg/L TCP, 2.5 g/L Fe3O4/CeO2, and 30 mM H2O2. Figure 6. H2O2 decomposition and iron dissolution during TCP degradation under the optimum conditions. Figure 7. Effect of radical scavengers on the degradation of TCP under the optimum conditions. Figure 8. Proposed reaction pathway for the degradation of TCP at pH 2.0 by heterogeneous Fenton-like process using Fe3O4/CeO2. Figure 9. Reuse of Fe3O4/CeO2 composite at pH 2.0 with 100 mg/L TCP, 2.5 g/L 31

Fe3O4/CeO2, and 30 mM H2O2.

32

Figure 1

2.5 g/L Fe3O4/CeO2 + 30 mM H2O2

100

1.5 g/L Fe3O4 + 30 mM H2O2 1.5 g/L CeO2 + 30 mM H2O2 2.5 g/L Fe3O4/CeO2

TCP removal (%)

80

30 mM H2O2 60

40

20

0 0

20

40

60

Time (min)

33

80

100

120

Figure 2 100

TCP removal Mineralization Dechlorination

Predicted (%)

80

60

40

20

0 0

20

40

60

Actual (%)

34

80

100

Figure 3 d value

d value

100 100

100 100

TCP removal (%)

(b)d value

TCP removal (%)

(a)d value

75 75

50 50 25 25

00 100 100 80 80 60 60

B: TCP (mg/L)

23 40 57 74 91 40 40 20 2.0 2.0 20

75 75

50 50 25 25

00 18 2.5 2.5

4.0

4.0 3.5 3.5 3.0 3.0 2.5 2.5

81

18

60

2.0 2.0

39 3.0 3.0

1.0 1.0 0.5 2.0 2.0 C: Fe3O4/CeO2 (g/L) 0.5

A: pH

d value

100 100

100 100

TCP removal (%)

(c)d value

75 75

50 50 25 25

00 71

55

24 24

39

4.0

23

18 18

D: H2O2 (mM)

4.0

3.5 3.5

2.5 2.5 2.0 66 2.0

25 25

00 30 30

16 24 24

35

35 21 2.0 2.0

26

D: H2O2 (mM)

A: pH

A: pH

50 50

18 18

3.0 3.0 12 12

2.5 2.5

75 75

30

87

30 30

4.0 4.0 3.5 3.5

1.5 1.5

d value (d)d value

TCP removal (%)

100

12 12

21 16 0.5 66 0.5

1.0 1.0

2.5 2.5

1.5 1.5 C: Fe3O4/CeO2 (g/L)

Figure 4 Overlay Plot

(a) 100.00 100

Fe3O4/CeO2: 1.8 g/L 80

B: TCP (mg/L)

80.00

H2O2: 28 mM R R1: 1:9090 R2: R250 : 50 R3: 90 R3: 90

60

60.00

40

40.00

20

20.00 2.00 2.0

2.50

3.00

2.5

3.0

3.50 3.5

4.00 4.0

A: pH

C: Fe3O4/CeO2 (g/L)

(b)

Overlay Plot

2.50 2.5

TCP: 87 mg/L

R R3: 3:9090 R1: 90 R 1: 90

2.00 2.0

H2O2: 28 mM

R2: :5050 R 2

1.50 1.5

1.00 1.0

0.50 0.5 2.00 2.0

2.50

3.00

2.5

3.0

3.50 3.5

4.00 4.0

A: pH (c)

30

Overlay Plot

30.00

R3: 90 R3: 90

TCP: 87 mg/L 24

D: H2O2 (mM)

24.00

R1: R190 : 90

Fe3O4/CeO2: 1.8 g/L

R2: R250 : 50

18

18.00

12

12.00

6

6.00 2.00 2.0

2.50

3.00

2.5

3.0

A: pH

36

3.50 3.5

4.00 4.0

Figure 5 100

TCP Mineralization Dechlorination

Removal efficiency (%)

80

60

40

20

Response

R1

R2

R3

Actual (%)

99.05

64.88

94.94

Predicted (%)

100

64.27

100

Error (%)

−0.95

0.61

−5.06

STDEV (± %)

0.67

0.43

3.58

0 0

20

40

60

Time (min)

37

80

100

120

Figure 6

50

40 26

Dissolved iron Ferrous ion H2O2

24

30

20

2+

H2O2 (mM)

28

Fe and dissolved iron (mg/L)

60

30

22

10

20 0

20

40

60

Time (min)

38

80

100

0 120

Figure 7

1.0

0.8

C/C0

0.6

0.4

None 300 mM n-Butanol 10 mM KI

0.2

0.0 0

20

40

60

Time (min)

39

80

100

120

Figure 8 OH Cl

OH

OH Cl

Cl

• OH

Cl (D1)

Cl OH

+

• Cl

OH

Cl

Cl

-Cl

-

Cl

OH (S3)

H 2C

(D4)

H 2C

• OH

Cl (S4)

O (D2)

CHO COOH CHO (D6) CH3 COOH (D7)

S: tentative states D: intermediates determined by GC/MS and IC

40

COOH

• OH

CH3COOH (D9)

COOH

HCOOH (D10)

COOH CHO

CO2, H2O, etc

COOH (D8)

O

+

- Cl

-

O (D5)

O Cl

Cl

-

Cl

O

Cl

• OH

• OH OH

OH

+

(S2)

O

- Cl

O

OH Cl

Cl

•

OH (S1)

O

Cl

Cl (D3)

Figure 9

100

First run Second run Third run

TCP removal (%)

80

60

40

20

0 0

20

40

60

Time (min)

41

80

100

120

Table 1 Central composite design and experimental results of heterogeneous Fenton-like degradation of TCP. Run

Variables

Responses

C: R1: TCP R2 : R3: D: H2O2 A: pH B: Initial TCP concentration Fe3O4/CeO2 concentration removal Mineralization Dechlorination (mg/L) (mM) (%) (%) (%) dosage (g/L) 1

3.0

60

1.5

18

32.5

16.62

27.25

2

3.0

60

1.5

18

29.66

16.5

27.18

3

2.5

80

2.0

24

69.23

37.91

64.63

4

3.0

60

1.5

18

30.24

17.39

29.45

5

3.0

60

1.5

18

29.18

15.36

29.38

6

3.5

40

1.0

12

18.5

11.33

16.8

7

2.5

80

1.0

12

41.01

26.76

41.12

8

3.0

60

1.5

30

24.74

9.01

23.48

9

3.5

80

2.0

24

17.65

7.37

15.79

10

3.5

40

2.0

24

20.13

8.71

16.21

11

2.5

40

2.0

12

72.89

43.3

69.02

12

3.5

40

2.0

12

18.38

9.4

16.07

13

3.5

40

1.0

24

18.34

3.7

14.32

14

2.5

80

2.0

12

57.2

29.66

54.28

15

4.0

60

1.5

18

7.38

0.08

2.86

16

3.0

60

1.5

18

31.48

18.5

28.23

17

3.5

80

2.0

12

12.32

6.66

1.95

18

2.0

60

1.5

18

87.8

59

85.76

19

2.5

40

1.0

12

47.5

24.35

48.87

20

3.5

80

1.0

24

10.04

4.52

3.26

21

2.5

80

1.0

24

43.96

26.58

41.06

22

3.0

60

1.5

6

10.25

4.94

9.6

23

3.5

80

1.0

12

16.07

2.12

3.07

42

24

3.0

60

2.5

18

20.85

7.09

18.04

25

3.0

60

1.5

18

29.51

16.61

28.82

26

3.0

60

0.5

18

15.96

9.59

14.99

27

3.0

100

1.5

18

8.8

1.94

8.42

28

2.5

40

1.0

24

54.73

19.43

54.87

29

3.0

20

1.5

18

30.15

9.7

29.41

30

2.5

40

2.0

24

78.81

40.44

78.47

43

Table 2 Range of different parameters investigated with CCD design. Actual values for the coded levels Factors

Variables –2

–1

0

1

2

X1 (A)

pH

2.0

2.5

3.0

3.5

4.0

X2 (B)

Initial TCP concentration (mg/L)

20

40

60

80

100

X3 (C)

Fe3O4/CeO2 dosage (g/L)

0.5

1.0

1.5

2.0

2.5

X4 (D)

H2O2 concentration (mM)

6

12

18

24

30

44

Table 3 Analysis of variance (ANOVA) for the regression models. Response

Source

Sum of squares

Degrees of freedom

F-value

p-Value

R1

Model

13150.53

14

15.48

<0.0001

Residual

910.15

15

Lack of fit

901.72

10

53.51

0.0002

Pure error

8.43

5

Total

14060.68

29

R2 = 0.9353

Adj R2 = 0.8749

Model

5246.17

14

12.51

<0.0001

Residual

449.20

15

Lack of fit

443.73

10

40.60

0.0004

Pure error

5.46

5

Total

5695.36

29

R2 = 0.9211

Adj R2 = 0.8475

Model

14534.45

14

21.06

<0.0001

Residual

739.37

15

Lack of fit

734.29

10

72.31

<0.0001

Pure error

5.08

5

Total

15273.82

29

R2

R3

2

Adj R2 = 0.9064

R = 0.9516

45

Table 4 Pseudo first-order kinetic constants for TCP degradation by various methods. T (°C)

k (min-1)

Reference

2.0

30

4.6×10-2

this study

5.0

2.0

30

5.2×10-3

[13]

0.50

5.0

3.5

Room

1.5×10-1

[36]

0.1 mM Fe2+

0.30

7.5

2.0

25

9.8×10-2

[37]

Fenton

0.09 mM Fe2+

0.51

2.9

3.0

Room

1.6×10-4

[9]

Photo-Fenton

0.09 mM Fe2+ with 270 nm UV

0.51

2.9

3.0

Room

1.1×10-3

[9]

Photo-Fenton

0.01 mM Fe2+ with 185–436 nm UV

0.30

0.5

2.0

25

5.2×10-2

[37]

Photolysis

185–436 nm UV

0.30

–

2.0

25

2.6×10-2

[37]

Photolysis

270 nm UV

0.51

–

3.0

Room

1.1×10-4

[9]

Photolysis

365 nm UV

0.26

–

–

30

6.9×10-4

[10]

UV/H2O2

185–436 nm UV

0.30

0.5

2.0

25

3.3×10-2

[37]

UV/H2O2

270 nm UV

0.51

2.9

3.0

Room

2.5×10-4

[9]

UV/O3

185–436 nm UV with O3

0.30

–

2.0

25

6.8×10-2

[37]

Photocatalysis 0.1 g/L TiO2 with 270 nm UV

0.51

–

3.0

Room

5.9×10-4

[9]

Photocatalysis 1.0 g/L Ag-TiO2 with 365 nm UV

0.10

–

5.15 25

9.3×10-2

[38]

Photocatalysis 1.5 g/L α-Fe2O3 with suntest light irradiation

0.51

–

6.0

32–33

3.3×10-4

[39]

Photocatalysis 0.7 mM PW12O403with Oriel 1000 W Xe arc lamp

1.00

–

1.0

20

7.5×10-5

[40]

Photocatalysis 1.0 g/L g-C3N4 with 0.5 420 nm visible light

–

–

Room

2.4×10-3

[41]

Sonolysis

–

–

30

1.0×10-3

[10]

Methods

Conditions

Fenton-like

2.5 g/L Fe3O4/CeO2 0.51

30

Fenton

0.1 mM Fe2+

0.50

Fenton

0.2 mM Fe2+

Fenton

20 kHz US

[TCP]0 [H2O2]0 pH (mM) (mM)

0.26

46

Sonophotolysis 20 kHz US with 365 nm UV

0.26

–

–

30

1.7×10-3

[10]

Ozonation

O3

0.30

–

2.0

25

4.4×10-2

[37]

Catalytic ozonation

0.01 g/L ZnO with 0.15 mM O3

0.0025 –

7.5

25

2.4×10-1

[42]

Catalytic wet oxidtion

2.0 g/L Mn(II)-MCM41

2.00

2.0

5.5

80

2.8×10-3

[8]

Adsorption

1.0 g/L cattail fiber-based activated carbon

0.51

–

–

30

8.8×10-3

[43]

47

Table 5 Intermediates identified by HPLC, GC/MS and IC during the degradation of TCP under the optimum conditions. Symbol

Compounds

Structural formula

Sample time (min) 10

30

45

60

√

√

√

√

√

√

√

√

√

√

√

√

√

√

√

√

√

√

OH Cl

D1

Cl

2,4,6-Trichlorophenol Cl O

D2

2,6-Dichloro-1,4-

Cl

Cl

benzoquinone O O

D3

3,5-Dichloro-1,2-

Cl

O

benzoquinone Cl O

D4

Cl

2-Chloro-1,4benzoquinone O O

D5

OH

2-Hydroxycyclohexa2,5-diene-1,4-dione O

48

90

120

CHO H2C

D6

Succinaldehyde

√

√

√

√

√

√

√

√

√

√

√

√

√

√

√

H2C CHO CH3 HC

D7

(E)-but-2-enoic acid HC COOH

CHO

D8

Oxoacetic acid COOH

D9

Acetic acid

CH3COOH

√

√

√

√

√

D10

Formic acid

HCOOH

√

√

√

√

√

49

Highlights

 Magnetic nanoscaled Fe3O4/CeO2 composite was used to degrade TCP.  •OH mechanism was determined to predominate in the process.  A possible degradation pathway of TCP was proposed.

50