Interface Mechanisms of Catalytic Ozonation with Amorphous Iron

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Interface mechanisms of catalytic ozonation with amorphous iron silicate for removal of 4-chloronitrobenzene in aqueous solution Lei Yuan, Jimin Shen, Pengwei Yan, and Zhonglin Chen Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.7b04875 • Publication Date (Web): 09 Jan 2018 Downloaded from http://pubs.acs.org on January 9, 2018

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Environmental Science & Technology

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Interface mechanisms of catalytic ozonation with amorphous iron

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silicate for removal of 4-chloronitrobenzene in aqueous solution

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Lei Yuan, ‡,* Jimin Shen,† Pengwei Yan, † and Zhonglin Chen,†,* *

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†

State Key Laboratory of Urban Water Resource and Environment, School of Municipal and

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Environmental Engineering, Harbin Institute of Technology, Harbin, 150090, People’s Republic

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of China

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‡

National and Provincial Joint Engineering Laboratory of Wetland Ecological Conservation, Heilongjiang Academy of Science, Harbin, 150040, People’s Republic of China

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*Address correspondence to either author:

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Tel.: +86 451 86053935 (L. Yuan); +86 451 86287000 (Z.L. Chen).

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Fax: +86 451 86664613 (L. Yuan); +86 451 86283028 (Z.L. Chen).

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E-mail address: [email protected] (L. Yuan); [email protected] (Z.L. Chen).

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ACS Paragon Plus Environment


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Abstract: Iron silicate was synthesized and characterized as an efficient ozonation catalyst.

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Results indicated that iron silicate is a microporous material with poor crystallinity. Fe–O–Si and

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Fe–O bonds were observed on its surface. The Fe–O bonds belonged to α-Fe2O3. Heterogeneous

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catalytic ozonation test was performed in batch reaction mode, and 4-chloronitrobenzene was used

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as model organic compounds. Amorphous iron silicate exhibited high catalytic activity, ozone

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utilization efficiency, and stability in catalytic ozonation. Hydroxyl radical was the dominant oxide

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species in this process. The reaction mechanism at the solid–water interface indicates that Fe–Si

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binary oxides on iron silicate surface inhibited ozone futile decomposition. This behavior resulted

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in enhanced probability of the reaction between ozone and α-Fe2O3 on the iron silicate surface to

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generate hydroxyl radicals which promoted 4-chloronitrobenzene removal in aqueous solution.

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Keywords: catalytic ozonation, iron silicate, hydroxyl radical, interface mechanisms

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1. Introduction

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Heterogeneous catalytic ozonation is interesting and effective technique for the removal of

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refractory organic compounds in water 1. However, practical applications of this technique are still

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limited in the field of water treatment 2. The ambiguous reaction mechanism at the solid–water

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interface is a key factor for this limitation 3. The interface mechanisms include ozone mass

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transformation and conversion on different catalyst surfaces, route of oxide species generation,

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and interface synergistic effect between catalyst and ozone. Thus, the interface mechanism should

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be elucidated to extensively employ this technique.

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Production of hydroxyl radicals (•OH) 4, which is the primary secondary oxide species from

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ozone decomposition in water, was deemed as an important characteristic in this process. Hydroxyl

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radical has high oxidation rate constants with almost all organic compounds in water. In the past

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decade, many studies have focused on promoting hydroxyl radicals production and ozone mass

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transfer in the reaction system. Some transition metal oxides, namely, Fe2O3, MnO2, TiO2, and

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CuO, are widely used ozonation catalysts for their high catalytic capability 5-9. However, several

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studies have recently found that not all heterogeneous catalytic ozonation processes follow the

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route of hydroxyl radicals oxidation 10-16. Among these processes, the application of ozone-loaded

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silicon materials had received much attention because they can adsorb ozone, and adsorbed ozone

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further reacts with organic compounds on their surfaces.

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Some silicates, such as natural zeolite, volcanic sand, bauxite, pumice, cordierite, red mud

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and ceramic honeycomb, have been used in catalytic ozonation because of their high mechanical

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strength, chemical stability, and low costs 17–23. In this study, iron silicate was synthesized for the

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catalytic ozonation of refractory organic compounds in aqueous solution. The adsorption property

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of ozone and the ozone decomposition property at the solid–water interface were remarkably

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significant in the design of this catalyst.

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Synthetic 4-chloronitrobenzene (4-CNB) is a refractory organic compound that is usually

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used as chemical material for dyes, pesticides, and other industries. It corresponds to probe

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organic compounds for active radicals in catalytic ozonation because of its low reaction rate

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constant with ozone molecules (k = 1.6 M−1·s−1) in aqueous solution 24.

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The present test investigates the catalytic effectiveness of iron silicate, the dominant oxides species and its generation route, ozone mass transformation and conversion, and the reaction

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mechanisms at the solid–water interface.

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2. Methodology

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2.1. Chemicals and reagents

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Synthetic 4-CNB was supplied by Chem Service (USA). Synthetic 5,

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5-dimethyl-1-pyrroline-N-oxide (DMPO) was supplied by Sigma (USA). The water used in the

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experiment was supplied by an ultrapure water system. Other chemicals and reagents were of

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analytical grade and used without further purification.

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2.2. Synthesis of catalysts

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The precursor solution consisted of Na2SiO3 and Fe(NO3)3 with different Si/Fe molar ratios. The

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solution was slowly titrated to pH 12 with NaOH at 25 °C and then aged further at 60 °C for 24 h.

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The solid was washed repeatedly with ultrapure water and baked at 350 °C for 2 h. The dry solid

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catalyst was crushed and stored in a vacuum desiccator. This catalyst (iron silicate, Si/Fe molar

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ratio = 0.5) exhibited the highest activity and was used in the experiments. As references, α-Fe2O3

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was synthesized via the same procedure without Na2SiO3.

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2.3. Experimental procedure

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The tests were conducted in batch reaction mode. The reactor was a modified 1L glass flask

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(Figure S1), and ozone was bubbled in ultrapure water within this reactor to achieve the desired

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concentration via ozone generator (COM-AD-01, Germany). Then, the catalyst and 4-CNB were

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quickly added to the stock ozone solution. A magnetic stirrer was used to mix the suspension

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continuously. The reaction temperature was controlled at 25 °C using a thermostatic bath. The

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suspensions collected from the reactor at different time intervals were immediately quenched with

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Na2S2O3 solution. The quenched suspension was passed through 0.45 µm filter before the analysis

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of the residual 4-CNB concentration in the solution.

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2.4. Analytical method

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X-ray diffraction (XRD, D8 Advance diffractometer, Germany) was performed to analyze the

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crystalline structure of the catalyst. Accelerated surface area and porosimetry instrument (ASAP

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2020, USA) was used to analyze the BET surface area and pore diameter of the catalyst.

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Scanning electron microscopy (SEM, Quanta 200, Netherlands) was conducted to analyze the

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surface morphology of the catalyst. Energy dispersive spectrometry (EDAX, Genesis, USA) was

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performed to analyze the elemental concentration of the catalyst. Fourier transform infrared

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spectroscopy (FT-IR, Spectrum One, USA) and X-ray photoelectron spectroscopy (XPS, PHI 5700,

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USA) was also conducted to analyze surface chemical composition of the catalyst.

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Electron spin resonance (ESR, EMX-8/2.7, Germany) was employed to analyze the intensity

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of hydroxyl radicals generated in the system. High-performance liquid chromatograph (LC-1200,

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USA) was used to analyze the concentration of 4-CNB in aqueous solution via the Extend-C18

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(4.6 mm × 250 mm) column through UV detection at 265 nm. The mobile phase comprised 80%

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CH3OH and 20% H2O. The concentration of residual ozone in aqueous solution was determined

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using the indigo method 25. Total organic carbon analyzer (TOC-VCPH, Japan) was used to analyze

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the concentration of TOC in aqueous solution. The acid–base titration method was used to analyze

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the point of zero charge (pHPZC) of the catalyst 26. Inductively coupled plasma spectrometer

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(Optima 5300DV, USA) was used to analyze Fe ions in the solution.

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3. Results and discussion

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3.1. Characterization

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The crystalline structure of the two catalysts was assessed using the XRD patterns in Figure S2.

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The crystallinity of iron silicate was poor, because its XRD patterns indicated two wide peaks at

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34.4° and 62.1°. The peaks were excessively wide because of the lack of a relationship between

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crystallinity and a specific Miller index. Therefore, iron silicate was believed to be amorphous,

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and high-purity α-Fe2O3 was obtained.

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The physisorption isotherms of catalysts are shown in Figure S3. The reversible isotherm of

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iron silicate was a type I(b), indicating the presence of microporous materials 27. α-Fe2O3 exhibited

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typical type IV(a) isotherm of mesoporous materials with a hysteretic loop in the range from 0.9

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P/P0 to 1.0 P/P0 27. The pore size distribution was calculated according to the SF method and BJH

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model in Figure S4. The results indicate that iron silicate displayed a significantly narrower pore

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size distribution than α-Fe2O3. Doped Si increased the BET surface area from 55.2 m2/g of

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α-Fe2O3 to 203.4 m2/g of iron silicate. Then, the average pore diameter decreased from 30.5 nm of

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α-Fe2O3 to 0.9 nm of iron silicate.

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The morphology of catalysts is shown in Figure S5. Iron silicate consisted of a flat surface

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and agglomerated particles. The morphology of α-Fe2O3 was represented by aglobular particles.

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As shown in Table S1, the EDAX results indicated an uneven elemental concentration on the iron

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silicate surface.

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3.2. Activity and stability of iron silicate

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Control tests were performed according to three processes, namely, sole ozonation, catalytic

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ozonation, and adsorption on catalysts. The curve of 4-CNB removal versus reaction time is

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illustrated in Figure 1.

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Minimal 4-CNB (less than 3.2%) was adsorbed on the surfaces of the tested catalysts after

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30 min, suggesting that self-adsorption slightly contributes to the removal of 4-CNB during

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catalytic ozonation. Only 49.2% of 4-CNB was removed after 30 min during sole ozonation,

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indicating that 4-CNB cannot be effectively removed by this process.

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4-CNB removal was significantly more effective in catalytic ozonation when two catalysts

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were introduced than in sole ozonation. The best result was obtained with iron silicate, in which

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82.8% of 4-CNB was removed. Analysis results showed 4-CNB removal followed

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pseudo-first-order kinetics model in ozonation processes. The rate constants were determined to be

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1.86×10−2 min−1 for sole ozonation. The rate constants were obviously accelerated in the presence

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of α-Fe2O3 and iron silicate, with the removal rate increasing 1.4- and 2.5-fold, respectively.

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Meanwhile, the presence of two catalysts increased TOC removal compared to sole ozonation.

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Catalytic ozonation with α-Fe2O3 removed 45.1% of TOC compared to 27.2% by sole ozonation.

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The enhancement of TOC removal is even more pronounced in the presence of iron silicate,

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68.7% of TOC was removed after 30 min. This finding indicates that the presence of a

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heterogeneous catalytic surface initiates an accelerated effect with the ozone molecules during the

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removal and minerlization of 4-CNB in aqueous solution. After reaction, Fe ion concentration in

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treatment water was below the detection limit. Furthermore, iron silicate maintains its catalytic

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activity after five cycles because the removal of 4-CNB and TOC were 82.2% and 67.4%,

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respectively (Figure S6). Thus, the amorphous iron silicate exhibited high catalytic activity and

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fine stability in catalytic ozonation.

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3.3. Ozone utilization efficiency

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The evaluated ozone utilization efficiency is an important factor in catalyst performance. The

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ozone utilization efficiency is expressed as the 4-CNB removal (total removal subtracted by the

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removal in adsorption) per concentration of ozone used.

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Figure 2 shows that this ratio considerably increased relative to sole ozonation when catalysts

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were introduced. Calculation results indicated that the lowest ratio was 0.054 in sole ozonation

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after 30 min. The ratio for catalytic ozonation with iron silicate was 0.091, which was 1.2 times

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higher than that for ozonation with α-Fe2O3 after 30 min. Ozone reacts with organic compounds in

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aqueous solution by either the direct oxidation of ozone molecules or the indirect oxidation of

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active radicals derived from ozone decomposition 28. Therefore, the obtained enhancement of

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ozone utilization efficiency in the catalytic ozonation with iron silicate is apparently caused by the

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active radical production in the solution.

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3.4. Effect of tert-Butanol

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Because of its high oxidative capability and unselective reaction with organic compounds,

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hydroxyl radicals are the most important active radical in catalytic ozonation system. tert-Butanol

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(TBA) is an effective hydroxyl radicals scavenger which is often used to determine hydroxyl

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radicals in catalytic ozonation 29.

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Tests were performed to determine 4-CNB removal with TBA to identify hydroxyl radicals in

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catalytic ozonation with iron silicate. Figure 3 shows that introducing different TBA

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concentrations strongly inhibited 4-CNB removal, even at a very low concentration (1.0 mg/L).

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High TBA concentration also negatively affected 4-CNB removal. The removed 4-CNB after

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30 min of reaction without TBA reached 82.8%, whereas only 19.7% was removed with TBA

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(20 mg/L). This result suggests that TBA competitively captured and rapidly consumed hydroxyl

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radicals in the solution. Therefore, 4-CNB removal follows the route of hydroxyl radicals

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oxidation in catalytic ozonation with iron silicate. The inhibitory effect in catalytic ozonation was

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greater than that in sole ozonation, indicating that more hydroxyl radicals were generated in

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catalytic ozonation with iron silicate.

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3.5. Effect of solution pH

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The solution pH strongly affects hydroxyl radicals generation because of ozone reactions with OH

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to hydroxyl radicals 30, and solution pH adjusts to catalyst surface charge

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S − OH + H + ↔ S − OH +2

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S − OH + OH − ↔ S − O − + H 2 O

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, as follows:

The catalyst surface becomes neutral, protonated, or deprotonated when the solution pH is

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near, below, or above the pHPZC. The effect of pH on 4-CNB removal was investigated in different

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processes, and the results are shown in Figure 4. The results indicated that iron silicate (pHPZC =

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9.1) showed catalytic activity relative to sole ozonation in solution pH levels ranging from 3 to 11,

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particularly at solution pH 7. Previous studies reported that neutral-charged catalyst surface was

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beneficial for ozone decomposition to hydroxyl radicals 32, 33. This phenomenon was due to the

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following factors. First, the neutral charge on iron silicate surface decreased in solution pH levels

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ranging from 7 to 3. In addition, hydroxyl radicals were difficult to generate from ozone

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decomposition in aqueous solution under acidic condition. Second, OH− competitively

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decomposed the ozone to hydroxyl radicals with the surface active site of iron silicate when the

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solution pH levels ranged from 7 to 11, although increasing neutral charge on iron silicate surface

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was also considered. Consequently, the catalytic activity of iron silicate was inhibited when

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solution pH either decreased to 3 or increased to 11.

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3.6. Effect of Si/Fe molar ratio

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The effect of the Si/Fe molar ratio of iron silicate on 4-CNB removal was investigated in catalytic

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ozonation.

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Figure 5 show that the five catalysts significantly enhanced 4-CNB removal relative to sole

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ozonation. However, iron silicate with different Si/Fe molar ratios exhibited different catalytic

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activities in catalytic ozonation. 4-CNB removal increased as molar ratio of Si/Fe increased from

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0.1 to 0.5. However, further increase in molar ratio of Si/Fe to 2.0 resulted in a negative effect on

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4-CNB removal in aqueous solution. These results indicated that optimal critical molar ratio of

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Si/Fe was 0.5 under the present experimental conditions. Variation in the molar ratio of Si/Fe may

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affect the surface properties of iron silicate which cause different concentrations of hydroxyl

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radicals to be generated in aqueous solution.

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3.7. Interface mechanisms

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The FT-IR spectra of iron silicate are shown in Figure 6a. The surface hydroxyl group peaked at

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3368 and 1622 cm−1 34. The Si–O–Fe bonds peaked at 1012 and 684 cm−1 35. The peaks at 532 and

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451 cm−1 represented the Fe–O bonds. Furthermore, XPS analysis is carried out to study the

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surface chemistry of iron silicate. It can be clearly seen that the spectrum contains Fe, O, Si, Na,

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and C (Figure S7a). The peaks for Na and C can be attributed to contamination caused by sample

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preparation and testing instrument. As shown in Figure S7b, the binding energy of Fe 2p3/2 is

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711.2 eV for iron silicate. The wide Fe 2p3/2 signals obtain for iron silicate could be fitted

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satisfactorily to two principal peaks after deconvolution. The peak at 710.5 eV belongs to Fe3+ of

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α-Fe2O3 36. Meanwhile, another peak appears at 712.1 eV. The XPS spectra of Si 2p for iron

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silicate showed a narrow, symmetrical peak (Figure S7c). The binding energy of Si 2p is found to

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be 101.9 eV, which is higher than that of pure SiO2 (100.2 eV) 36. The analyses confirmed that the

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core-level shifts of both Fe and Si were due to the formation of Si–O–Fe bonds. The asymmetric

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O 1s peaks could be fit into four peaks with binding energies (Figure S7d). They corresponded to

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α-Fe2O3 (529.9 eV), -OH (531.9 eV), Fe–Si binary oxides (532.1 eV), and adsorbed water (535.8

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eV), respectively 36. Therefore, iron silicate denotes a mixture of Fe–Si binary oxides and α-Fe2O3.

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The ESR spin-trap technique is suitable for investigating hydroxyl radicals generation. The

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test results shown in Figure 6b indicated that hydroxyl radicals production was observed in sole

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ozonation and catalytic ozonation. The presence of α-Fe2O3 and iron silicate significantly

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enhanced the relative intensity of the DMPO-OH adduct signal compared with the intensity in sole

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ozonation, especially iron silicate. Therefore, iron silicate enhanced ozone decomposition to

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generate the highest amounts of hydroxyl radicals in the three reaction processes. Comparison of

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Figure 6a with Figure 6b shows that high catalytic activity of iron silicate was possible because of

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two hypothesises: (1) Fe–Si binary oxides on its surface exhibited high catalytic capability, or (2)

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Fe–Si binary oxides enhanced catalytic capability of α-Fe2O3 on its surface.

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Ozone concentration in aqueous solution decreased with reaction time in sole ozonation

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because of self-decomposition (Figure 6c). Introduction of α-Fe2O3 promoted ozone

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decomposition to hydroxyl radicals relative to that obtained from sole ozonation (Figures 6b and

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6c). Meanwhile, the ozone concentration in aqueous solution decreased significantly in the

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presence of iron silicate for 5 min. However, the reaction time was also extended by 5–20 min.

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This extension exerted an obvious positive effect on the ozone concentration in the solution. This

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phenomenon indicates that ozone is adsorbed on Fe–Si binary oxide surfaces because of

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desorption in aqueous solution. Figure 5 also indicated that Si/Fe molar ratio of iron silicate had

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no positive correlation with its catalytic activity. Therefore, these results suggest that Fe–Si binary

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oxides have an assisted effect with α-Fe2O3 on the iron silicate surface in promoting the generation

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of hydroxyl radicals in aqueous solution.

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According to the test results and the aforementioned theory, the reaction mechanism at the solid –water interface is proposed and illustrated. First, Fe–Si binary oxides on iron silicate surface could adsorb ozone and adsorbed ozone

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was desorbed, which inhibits ozone futile decomposition in aqueous solution. Second, adsorption

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of ozone significantly increased the reaction probability between ozone in aqueous solution and

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α-Fe2O3 on iron silicate surface that accelerate hydroxyl radicals generation from ozone

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decomposition and promote 4-CNB removal. Finally, no decomposition adsorbed ozone on Fe–Si

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binary oxides was desorbed into aqueous solution to begin the next cycle of reaction. It was

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generally accepted that the reaction of ozone with catalyst surface involves two mechanisms: (1)

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ozone was decomposed into active species to oxidize organic compounds, (2) adsorption of ozone

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and organic compounds before interacting with each other 2. The findings would provide the new

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ideas for the development of an efficient ozonation catalyst in the field of water treatment.

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Supporting Information Available

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A detailed discussion of the experimental apparatus schematic, catalyst characterization and

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stability test. This material is available free of charge via the Internet at http://pubs.acs.org.

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Acknowledgment

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The support from the National Key R&D Program of China (Grant No. 2017YFA0207203), the

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National Natural Science Foundation of China (Grant No. 51208186), the Open Project of State

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Key Laboratory of Urban Water Resource and Environment (Grant No. HCK201711), and the

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HAS Fund for Distinguished Young Scholars (Grant No. CXJQ2018ZR02) are greatly

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appreciated.

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26. Mullet, M.; Fievet, P.; Szymczyk, A.; Foissy, A.; Reggiani, J.C.; Pagetti, J. Simple and accurate determination of the point of zero charge of ceramic membranes. Desalination 1999, 121, 41–48. 27. Thommes, M.; Kaneko, K.; Neimark, A. V.; Olivier, J. P.; Rodriguez-Reinoso, F.; Rouquerol, J.; Sing K. S. W. Physisorption of gases, with special reference to the evaluation of surface area and pore size distribution. Pure Appl. Chem. 2015, 87, 1051–1069. 28. Legube, B.; Karpel, V.L.N. Catalytic ozonation: A promising advanced oxidation technology for water treatment. Catal. Today 1999, 53, 61–72. 29. Guan, Y.H.; Ma, J.; Ren, Y.M.; Liu, Y.L.; Xiao, J.Y.; Lin, L.Q.; Zhang, C. Efficient degradation of atrazine by magnetic porous copper ferrite catalyzed peroxymonosulfate, oxidation via the formation of hydroxyl and sulfate radicals. Water Res. 2013, 47, 5431–5438. 30. von Gunten, U. Ozonation of drinking water: part I. Oxidation kinetics and product formation. Water Res. 2003, 37, 1443–1467. 31. Stumm, W. Chemistry of the Solid–Water Interface, John Wiley & Sons, New York, 1992, PP. 15–20. 32. Yuan, L.; Shen, J.M.; Chen, Z.L.; Guan, X.H. Role of Fe/pumice composition and structure in promoting ozonation reactions. Appl. Catal., B 2016, 180, 707–714. 33. Zhang, T.; Li, C.J.; Ma, J.; Tian, H.; Qiang, Z.M. Surface hydroxyl groups of synthetic α-FeOOH in promoting •OH generation from aqueous ozone: property and activity relationship. Appl. Catal., B 2008, 82, 131–137. 34. Richmond, G.L. Molecular bonding and interactions at aqueous surfaces as probed by

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Figures

Figure 1. Removal of 4-CNB in the ozonation process. Conditions: T = 25 °C, 1 mmol/L phosphate buffered pH = 7.0, 4-CNB dosage = 0.1 mg/L, ozone dosage = 0.9 mg/L, catalyst dosage = 100 mg/L

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Figure 2. Effective ozone use ratio in the ozonation process. Conditions: T = 25 °C, 1 mmol/L phosphate buffered pH = 7.0, 4-CNB dosage = 0.1 mg/L, ozone dosage = 0.9 mg/L, catalyst dosage = 100 mg/L

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Figure 3. Effect of radical scavenger on 4-CNB removal. Conditions: T = 25 °C, 1 mmol/L phosphate buffered pH = 7.0, 4-CNB dosage = 0.1 mg/L, ozone dosage = 0.9 mg/L, catalyst dosage = 100 mg/L

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Figure 4. Effect of pH on 4-CNB removal. Conditions: T = 25 °C, 1 mmol/L phosphate buffered pH = 3, 7, 11, 4-CNB dosage = 0.1 mg/L, ozone dosage = 0.9 mg/L, catalyst dosage = 100 mg/L

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Figure 5. Effect of Si/Fe molar ratio on 4-CNB removal. Conditions: T = 25 °C, 1 mmol/L phosphate buffered pH = 7.0, 4-CNB dosage = 0.1 mg/L, ozone dosage = 0.9 mg/L, catalyst dosage = 100 mg/L, reaction time = 30 min

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Figure 6 (a) FT-IR spectra of iron silicate, (b) spectra of DMPO–OH signals and (c) ozone concentration in the solution. Conditions: T = 25 °C, initial pH of the ozone solution was adjusted to pH = 7.0 with NaOH, ozone dosage = 0.9 mg/L, catalyst dosage = 100 mg/L, DMPO = 100 mmol/L 24


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Graphical Abstract

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Interface Mechanisms of Catalytic Ozonation with Amorphous Iron

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