Effect of Iron Oxide Promoted Sulfated Zirconia on the Oxidative

Article pubs.acs.org/IECR

Effect of Iron Oxide Promoted Sulfated Zirconia on the Oxidative Efficiency of H2O2/O3 for Acetic Acid Degradation in Strong Acidic Water Qiangwei Wang, Tongdong Shen, and Shaoping Tong* College of Chemical Engineering, State Key Laboratory Breeding Base of Green Chemistry-Synthesis Technology, Zhejiang University of Technology, Hangzhou, Zhejiang 310014, China ABSTRACT: Acetic acid (HAc), a hydroxyl radical probe in ozonation, was chosen as a target contaminant to be degraded in strong acidic solution by different ozone-based processes. The highest removal rate of HAc was obtained by SZF/H2O2/ O3. The indirect method of dimethyl sulfoxide (DMSO) capture showed that hydroxyl radical was effectively produced in this process. A mechanism study indicated that the S− Fe(III) peroxo complex intermediate played an important role in the process. The effects of different parameters such as pH value, amount of catalyst, concentration of H2O2, and rate of ozone input were investigated. The experimental results showed that the presence of SO2−4/ZrO2−Fe2O3, abbreviated as SZF, could greatly improve the oxidative efficiency of H2O2/ O3 in the pH range from 0 to 5.0. The result is very significant to the application of H2O2/O3 in strong acid wastewater pretreatment.

1. INTRODUCTION

In the preliminary studies, we have proposed a Ti(IV)catalyzed H2O2/O3 (Ti(IV)/H2O2/O3) process which still maintains a high oxidative efficiency for acetic acid degradation even when the pH value is 2.8.11 However, this AOP-O3 has a drawback of titanium ion loss, and the degradation efficiency is too low when the pH value is less than 2.0. To solve this problem, immobilizing Ti(IV) in silicon molecular sieves were carried out, and it was found that the result was not very good (for example, relative instability of the catalyst, very low efficiency at pH less than 1.0). Therefore, we are devoting ourselves to seeking a more effective and controllable method to improve the oxidative efficiency of H2O2/O3 for the pretreatment of refractory acid wastewater. The work will be very significant for sustainable development of the chemical or pharmaceutical industry because wastewaters from these enterprises are always acidic. Sulfated zirconia (SO2−4/ZrO2, abbreviated as SZ) as well as sulfated iron oxide (SO2−4/Fe2O3, abbreviated as SF) are kinds of solid superacids (higher than that of 100% sulfuric acid) which have been extensively applied in skeletal isomerization,12 alkylation,13 acylation,14 dehydration,15 and esterification16 because of their exceptional acidic properties. Furthermore, to increase the stability and the activity of these catalysts, doping different metal cations or oxides to the parent oxide has

Ozone, due to its strong oxidation and disinfection potential, has been widely used as an effective oxidant in wastewater treatment.1,2 However, ozonation alone can not obtain the complete oxidation of organic pollutants. Therefore, various ozone-based advanced oxidation processes (AOPs-O3), such as H2O2/O3, UV/O3, and catalytic ozonation, have been developed to enhance the efficiencies of degradation of ozone-inert compounds.3−5 The AOPs-O3 are well-known for the generation of highly reactive species in water, mainly hydroxyl radicals. Hydroxyl radical can attack most organic molecules with rate constants ranging from 106 to 109 mol·L−1· s−1, leading to high oxidation efficiency of AOPs-O3 in the degradation of organic contaminants.6,7 Consequently, it is essential to ensure a certain concentration of hydroxyl radical in the reaction process. The combined process of hydrogen peroxide with ozone (H2O2/O3) is considered as the most promising process for degradation of different contaminants owing to its convenience and high efficiency. The H2O2/O3 process can significantly promote the decomposition of dissolved ozone to generate hydroxyl radicals through the initiation reaction of HO2− (the conjugate base of H2O2).8,9 However, when the pH of the solution is less than 5, the efficiency of H2O2/O3 becomes very low because the deprotonation of H2O2 can hardly occur.10 Therefore, H2O2/O3 is also ineffective for the pretreatment of acid toxic wastewater (always being discharged from chemical or pharmaceutical factories). © XXXX American Chemical Society

Received: June 29, 2016 Revised: September 23, 2016 Accepted: September 23, 2016

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DOI: 10.1021/acs.iecr.6b02483 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Industrial & Engineering Chemistry Research

10−80°. A scanning electron microscope (SEM, Hitachi S-570, Japan) and transmission electron microscope (TEM, Tecnai F30, Holland) were used to analyze the surface morphology of SZF and SZ. Surface area and pore size distribution were measured from Nitrogen adsorption−desorption isotherms at 77 K using a Micromeritics 3flex. The specific surface area was calculated by the Brunauer−Emmett−Teller (BET) equation. The pore diameter distribution was determined by the Barrett− Joyner−Halenda (BJH) method based on the Kelvin equation. The X-ray photoelectron spectroscopy (XPS) measurements were performed on Kratos AXIS Ultra DLD system with Al Kα radiation, and binding energies were calibrated versus the carbon signal at 284.60 eV. FTIR spectroscopies of pyridine adsorption (Py-IR) were recorded on a Fourier transform infrared spectrometer (Bruker, German) at 4 cm−1 resolution. 2.3. Catalytic Ozonation Procedure. All experiments were performed in a semicontinuous process. Ozonation contactor was a 1.0 L cylindrical glass reactor (Figure 1).

attracted much attention. For example, iron oxide promoted sulfated zirconia (SO2−4/ZrO2−Fe2O3, abbreviated as SZF) has been proved to have larger surface area, stronger Lewis and/or Brønsted acidity, better reusability, and superior catalytic properties in many reactions.17−19 Wu et al. have prepared SO2−4/Fe2−xZrxO3 bimetallic oxide which can oxidize X-3B in the catalytic photo-Fenton process effectively. The determining factor is the formation of an important S−Fe(III) peroxo complex (S−[FeIIIOOH]2+) through the interaction between the catalyst and H2O2. The peroxo complex (S−[FeIIIOOH]2+) under the light can break up to produce some active species, such as ·OH, ·OOH, and [FeVO]3+ oxidant; thus, the target contaminants can be effectively degraded.20,21 They also confirmed that there exist a large number of Lewis and Brønsted acid sites on the surface of the solids which can play a significant role in the catalytic reactions. According to the most popular views in the catalytic ozonation literature, the acidic centers (Brønsted or Lewis) on metal oxides are often considered responsible for ozone decomposition. Simultaneously, the oxygen-containing chemical (S−[FeIIIOOH]2+) is a HO2− like species, which might be considered to be a potential initiator to decompose ozone to generate hydroxyl radicals.22 Therefore, the iron oxide promoted sulfated zirconia catalyst incorporated with H2O2 may exhibit high efficiencies when being introduced to the ozone-based advanced oxidation process in strong acidic wastewater (pH < 2). In this paper, we chose an ozone inert compound acetic acid (HAc) as the contaminant to be degraded because HAc (a final product of chemical oxidation) is always regarded as the probe molecule for detecting hydroxyl radicals in ozonation.7,23 The efficiency of the SZF catalytic H2O2/O3 process in strong acidic medium (pH = 0−2) was investigated, and different factors such as pH value, amount of catalyst, concentration of H2O2, and ozone dosage were discussed. The yield of hydroxyl radicals was also measured through an indirect method of the reaction of dimethyl sulfoxide (DMSO) with ·OH.24,25 The experimental results will be of great significance for further broadening the working pH range of the peroxone process in the pretreatment of (strong) acid wastewater, especially for the biorefractory wastewater with high concentrations of ozone inert compounds. Furthermore, the exploitation of a suitable AOP-O3 for the pretreatment of refractory acid wastewater can automatically avoid the negative effect of CO2−3/HCO3−, which is really important during the application of ozonation.

Figure 1. Diagram of the experimental setup: (1) ozone generator; (2) flow-meter; (3) ozonation reactor; (4) outlet of circulation water; (5) outlet of material; (6) inlet of circulation water; (7) sampling; (8) inlet of material; (9) ozone destructor.

Before the experimental operation, the reactor was preozonated for 3 min and then was washed for several times with doubledistilled water to exclude possible side effects of the impurities. Ozone was produced by a corona ozone generator (CFS-1A, Ozonia, Switzerland). Oxygen gas was dried and purified with silica gel prior to entering the ozone generator. Ozone concentration was regulated by varying the voltage and oxygen flow rate. The rate of gas flow was monitored by a rotor flowmeter. The flow rate of ozonized oxygen was always 0.67 L· min−1, and the rate of ozone input was 47.6 mg·min−1 without any special instructions. Excess ozone was passed into two gas absorption bottles containing 2% KI solution before being emitted. In a typical experiment of HAc degradation, the reactor was filled with 500 mL of acetic acid aqueous solution with the required amounts of SZF and hydrogen peroxide, and the temperature of the reactor was kept at room temperature (21 ± 0.5)°C. The ozonized oxygen passed from the ozone generator into the ozone reactor through the sintered inlet that enabled small bubbles to be produced at the bottom of the reactor. The sample was withdrawn for analysis at certain regular intervals, and dissolved ozone was removed by immediately bubbling nitrogen for 3 min after sampling to terminate oxidative reaction. 2.4. Analytical Procedures. An ion chromatograph (Dionex DX 1500, USA) was used to measure the

2. EXPERIMENTAL SECTION 2.1. Chemicals and Reagents. All the reagents, such as acetic acid (HAc), hydrogen peroxide (H2O2, 30 wt %), sodium hydroxide (NaOH), sulfuric acid (H2SO4), dimethyl sulfoxide (DMSO), perchloric acid (HClO4), hydrochloric acid (HCl, 36−38 wt %), xylenol orange, ascorbic acid, zirconyl chloride octahydrate (ZrOCl2·8H2O), ammonium iron(II) sulfate hexahydrate ((NH4)2Fe(SO4)2·6H2O), hydroxylammonium chloride (NH2OH·HCl), ammonium acetate, and 1,10phenanthroline were of analytical grade and used without any further purification. The SZF catalyst and SZ catalysts were purchased from Zibo Haoye Industry and Trade Co., Ltd. All solutions in the experiments were prepared with doubledistilled water. 2.2. Characterization of Catalysts. X-ray powder diffraction patterns were detected by a PNAlytical X’Pert Pro X-ray diffractometer with Cu Kα radiation in the 2θ ranges of B


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Industrial & Engineering Chemistry Research concentration of methanesulfonic acid (eluent: 18 mmol·L−1 potassium hydroxide; the flow rate was 1.0 mL·min−1). A high performance liquid chromatograph (ThermoFisher Dionex Ultimate3000, USA) was used to measure the concentration of acetic acid. Hypersil Gold C18 column (250 × 4.6 mm, particle size of 5 μm) was used as analytical column. The mobile phase was a mixture of 0.0134 M phosphate buffer (pH = 3) and methanol (950:50, v/v). The flow rate was 1.2 mL· min−1, and UV detector was set at 210 nm. A PerkinElmer ELAN DRC-e ICP-MS was used to detect the leaching iron ion in solution. The amount of hydroxyl radical (·OH) generation was indirectly determined by the concentration of methanesulfonic acid from the reaction of dimethyl sulfoxide (DMSO) with hydroxyl radicals.24−27 The concentration of ozone in gas was determined by the iodometric titration method.28 The concentration of H2O2 was determined by an ultraviolet spectrophotometer using the potassium titanium(IV) oxalate method.29 The comparison tests showed that the presence of H2O2 did not interfere with the determination of the concentration of ozone or acetic acid. The ultraviolet spectrophotometer was also used to determine the content of Fe and Zr in SZF catalyst through the 1,10-phenanthroline method and xylenol orange method, respectively. Both methods can analyze the corresponding metal element by measuring the concentration of the complexes generated through the interaction of the specific indicator and the metal elements. The content of O in SZF was determined by an element analyzer (Elementar Vario Micro Cube, Germany). The pH value of the solution was recorded by a pH analyzer (PHS-3C, General Instrument Co., Ltd., Shanghai, China) and was controlled by adding H2SO4 or NaOH solution. It was found that the pH of the solution was almost unchanged (initial pH ± 0.1) throughout the experiment of HAc degradation under the experimental conditions. Each experiment in this work was repeated three times, and the result was the average of duplicate experiments.

was suppressed by the incorporation of Fe2O3,17,18 and the amorphous phase of SZF suggests that these two kinds of metal oxides were sufficiently homogeneously mixed with each other.18,19 N2 adsorption/desorption isotherms of SZ and SZF (presented in Figure 3) are of IV type isotherm for mesoporous

Figure 3. Adsorption/desorption isotherms and pore size distribution for two samples. Solid square: SZ; open square: SZF.

materials. Both of the two catalysts have wide and irregular pore distribution. SZF exhibits larger specific surface area and pore volume (as shown in Table 1), which may be beneficial for mass transfer in the pore of the catalyst. Table 1. N2 Physisorption Data for Different Samples

3. RESULTS AND DISCUSSION 3.1. Characteristics of SZF Catalyst. Figure 2 shows the XRD patterns of SZ and SZF samples. It was found that

sample

surface area BET (m2·g−1)

pore volume (cm3·g−1)

SZ SZF

118 144

0.17 0.26

The morphologies of SZ and SZF catalysts were examined by SEM and TEM (as shown in Figure 4). SEM images of the two catalysts indicate that there are large amounts of surface agglomerates on both of the two catalysts. TEM figures further show that SZ has a relative regular shape, smaller particle diameters, and smoother surface compared to SZF. Massive iron oxide particles adhere loosely to the surface of SZF and form plenty of irregular voids, thus increasing the particle size and the specific surface area of the catalyst30,31 (as shown in Table 1). EDS spectra of SZF revealed the presence of the S, O, Fe, and Zr elements, and the quantitative analysis of the four elements are illustrated in Figure 4g. The 1,10-phenanthroline method and xylenol orange method showed that the content of Fe and Zr in SZF were 3.29% and 22.49%, respectively. The element analyzer showed that the content of O in SZF was 66.18%. In order to further ascertain the surface chemical states of the catalysts, XPS analysis of SZ and SZF has been conducted. As shown in Figure 5A, the binding energy of Fe 2p 3/2 photoelectrons can be fitted into five contributions at 709.2, 710.9, 712.4, 713.79, and 715.6 eV. The major peak located at 712.4 eV can be assigned to FeIII−O species, which accounts for 26.86% of total Fe species. The emission peaks at around 710.9 and 713.79 eV ascribe to (O)Fe−OH (22.82%) and FeII− SO2−4 (24.53%), respectively.21 The contribution at 709.2 eV suggests that FeII−O species (12.44%) are also presented on

Figure 2. XRD patterns of SZ and SZF.

zirconia in the SZ sample existed mainly in the monoclinic phase, and only a very small amount of tetragonal zirconia was detected. However, in the Fe2O3 promoted sulfated zirconia sample, no crystalline peaks of ZrO2 were observed because the transformation of ZrO2 from amorphous to crystallized form C


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Figure 4. SEM micrographs of SZ ((a) and (b)) and SZF ((c) and (d)); TEM micrographs of SZ (e) and SZF (f); EDS spectra of SZF (g).

the surface of SZF. The last peak at 715.6 eV is due to the satellite signal of ferrous species (13.35%).32 Figure 5B reveals

that both of the two catalysts’ Zr 3d spectra show double peaks, suggesting the existence of two kinds of Zr components on the D


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Figure 6. FTIR spectra of pyridine adsorption on SZ (A) and SZF (B) samples evacuated at 30, 200, and 350 °C.

Figure 5. Fine XPS spectra of (A) Fe 2p for SZF and (B) Zr 3d for SZ and SZF.

catalysts’ surface. One component presented at 181.52 and 181.7 eV corresponds to ZrO2.21 Another component located around 183.8 eV is assigned to Zr-SO42− species, of which Zr is electron deficient. According to relevant literature,33 Zr 3d5/2 binding energy of SZ located at 181.52 eV is closer to that of monoclinic ZrO2 than tetragonal ZrO2, indicating that ZrO2 in SZ is mainly in monoclinic phase, which is in accordance with the result of XRD. Considering sulfated zirconia is a typical solid superacid, the surface acidities of the two catalysts were also compared through the FTIR-pyridine spectra. The samples were measured after evacuation at 30, 200, and 350 °C corresponding to the amount of weak, midstrong, and strong acidity. The absorption bands at 1439, 1607 cm−1 and 1541, 1639 cm−1 ascribe to the chemisorption of pyridine on Lewis acid sites and Brønsted acid sites, respectively.21 As shown in Figure 6A, large amounts of both Lewis and Brønsted acid sites exist on the surface of SZ catalyst. Figure 6B indicates that only a small amount of Lewis acid sites are present on the surface of SZF, and the peaks disappear at 200 °C, suggesting that the acidity of SZF is much weaker than that of SZ. 3.2. Ozonation of Acetic Acid. At the initial pH 1.5, HAc degradation was carried out by several AOPs-O3 including H2O2/O3, TS-1/H2O2/O3, SZ/H2O2/O3, SZF/H2O2/O3, and Ti(IV)/H2O2/O3 (as shown in Figure 7). It was found that the removal rate of HAc by H2O2/O3 was very low (only 4.0% in 30 min) because the deprotonation reaction of H2O2 is difficult to proceed at pH 1.5. The efficiency of the TS-1 catalytic H2O2/O3 process was also low at pH 1.5, which is in accordance with our previous work.34 The removal rate of HAc

Figure 7. Degradation of HAc by H2O2/O3, TS-1/H2O2/O3, SZ/ H2O2/O3, SZF/H2O2/O3, and Ti(IV)/H2O2/O3. Reaction conditions: initial pH, 1.5; concentration of H2O2, 100 mg·L−1; concentration of HAc, 100 mg·L−1; concentration of TS-1, SZ, and SZF, 0.2 g·L−1 respectively; concentration of Ti(IV), 6 mg·L−1; flow rate of O3/O2, 0.67 L·min−1; rate of ozone input, 47.6 mg·min−1.

reached 10.9% in the SZ/H2O2/O3 process. Due to the strong hydrolysis performance of Ti(IV), the addition of Ti(IV) can favor the deprotonation reaction of H2O2 in acid solutions; thus, the removal rate of HAc was improved by the Ti(IV)/ H2O2/O3 process (29.6% in 30 min). However, the highest degradation efficiency was achieved by the SZF catalytic H2O2/ O3 process, and the removal rate of HAc was greatly enhanced (41.1% in 30 min). In terms of the trend of the curve, one can deduce that the acetic acid will be further degraded under the suitable conditions. Figure 7 also shows that SZF is much more E


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the dissolved ozone to generate hydroxyl radicals.20−22,36 On the basis of the above conjecture, it can be concluded that the coexistence of SZF, H2O2, and O3 is of great significance to ensure the high efficiency for the degradation of acetic acid in the (strong) acidic solutions. 3.3. Effect of Concentration of H2O2. As discussed in Section 3.2, H2O2 is a necessary reactant in the process of SZF/ H2O2/O3 for it can interact with SZF to produce active peroxo species. Thus, the concentration of H2O2 will greatly affect the oxidative efficiency of SZF/H2O2/O3. Figure 9 shows the effect

efficient than SZ in the catalytic H2O2/O3 process, which is due to its special structure and larger surface area. However, FTIRpyridine spectra show that the acidity of SZF is much weaker than that of SZ, indicating that the activity of the catalyst in this system has no positive correlation with its acidity (as discussed in Section 3.1). Thus, it can be deduced that the presence of Fe element is essential in the degradation process, because it is the origin of the formation of the important S−Fe(III) peroxo complex (S−[FeIIIOOH]2+). Degradation of HAc by H2O2/O3, SZF/O3, SZF/H2O2/O3, SZF/O2, and SZF/H2O2/O2 was also investigated with the purpose of further identifying the function of SZF in ozonation (as shown in Figure 8). A removal rate of 3.4% was obtained

Figure 9. Effect of concentration of H2O2 on the removal rate of HAc. Reaction conditions: initial pH, 1.0; concentration of HAc, 100 mg· L−1; concentration of SZF, 0.4 g·L−1; flow rate of O3/O2, 0.67 L· min−1; rate of ozone input, 47.6 mg·min−1. Figure 8. Degradation of HAc by H2O2/O3, SZF/O3, SZF/H2O2/O3, SZF/O2, SZF/H2O2/O2, and Fe3+/H2O2/O3. Reaction conditions: initial pH, 1.0; concentration of H2O2, 100 mg·L−1; concentration of HAc, 100 mg·L−1; concentration of SZF, 0.4 g·L−1; concentration of Fe3+, 6 mg·L−1; flow rate of O3/O2, 0.67 L·min−1; rate of ozone input, 47.6 mg·min−1.

of the concentration of H2O2 on the efficiency of SZF/H2O2/ O3. When the concentrations of H2O2 changed from 50 to 100, 200, 300, and 500 mg·L−1, the removal rates of HAc were 39.9%, 53.2%, 65.5%, 69.7%, and 73.3%, respectively. The results indicated that, in a certain range of concentration (50 to 200 mg·L−1), increasing the amount of H2O2 could greatly improve the degradation rate of acetic acid. However, this improvement seemed to slow down in the range of high concentration (higher than 200 mg·L−1). The removal rates of HAc were almost similar when the concentrations of H2O2 were 200 and 300 mg·L−1 in 30 min but higher than that at 500 mg·L−1. This result can be explained because the excessive H2O2 will consume some of the ·OH, thus leading to low utilization of H2O2 for the degradation of target pollutant.37,38 The remaining rates of H2O2 at its different initial concentrations are shown in Figure 10. The majority of H2O2 decomposed in 30 min when its initial concentration was either 50 or 100 mg·L−1, and then, the corresponding degradation rates of HAc also slowed down (as shown in Figure 9) due to the shortage of H2O2. However, there still existed certain concentrations of H2O2 after 30 min where the initial concentrations of H2O2 were higher than 200 mg·L−1. Therefore, the SZF/H2O2/O3 maintained relative rapid degradation rates of HAc after 30 min in such cases (Figure 9). The concentrations of H2O2 all completely decomposed in 60 min under the experimental conditions, thus leading to the end of HAc degradation. Therefore, adding H2O2 in batch will not only improve the oxidative efficiency of SZF/H2O2/O3 but also reduce the amount of H2O2 consumed invalidly. The results in Figures 9 and 10 further illustrate that the presence of a certain concentration of H2O2 is the guarantee of effective degradation of HAc by SZF/H2O2/O3, meaning that the

within 30 min by SZF/O2 owing to the adsorption of HAc on the catalyst. The concentration of HAc was almost unchanged in the SZF/O3 process, indicating that there existed competitive adsorption between O3 and HAc on the active sites of SZF. The removal rate of HAc was 6.4% by the SZF/H2O2/O2 process in similar conditions. Considering the fact that the H2O2 molecule also can be attracted to the metal oxide catalyst35,36 and the HAc adsorption amount, some HAc might also be degraded by some active intermediates in this process.21 However, when SZF was used to catalyze the peroxone reaction, the removal rate of HAc reached 32.3% in 30 min, much higher than the sum (11.5%) of those by H2O2/O3, SZF/O3, and SZF/H2O2/ O2, revealing the existence of the synergistic effect in SZF/ H2O2/O3. It is the common knowledge that certain metal ions can leach from their oxides in acidic solution.20,21 The concentration of leaching iron ions was detected to be 6 mg· L−1 in the experimental conditions. It was found that iron ions of 6 mg·L−1 almost had no influence on the efficiency of H2O2/ O3 for the HAc degradation (Figure 8). Considering HAc is a probe compound for hydroxyl radical in ozonation, one can infer that some hydroxyl radicals might be generated in SZF/H2O2/O3. According to the relevant literature, the induction effect of SO enables the metal cation (Fe−O) in SZF to show more capabilities of attracting the O− O of H2O2, thus forming more S−Fe(III) peroxo complexes (S−[FeIIIOOH]2+).20,21,36 Combining the above experimental results with the literature, it is reasonable to deduce that S− Fe(III) peroxo complex (S−[FeIIIOOH]2+) might interact with F


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of ozone input on the efficiency is shown in Figure 12. It was found that the removal rate of HAc increased with the

Figure 10. Evolution of H2O2 concentration at different initial concentrations. Reaction conditions: initial pH, 1.0; concentration of HAc, 100 mg·L−1; concentration of SZF, 0.4 g·L−1; flow rate of O3/O2, 0.67 L·min−1; rate of ozone input, 47.6 mg·min−1.

Figure 12. Effect of the rate of ozone input on the removal rate of HAc. Reaction conditions: initial pH, 1.0; concentration of H2O2, 100 mg·L−1; concentration of HAc, 100 mg·L−1; concentration of SZF, 0.2 g·L−1; flow rate of O3/O2, 0.67 L·min−1.

removal of HAc can be controlled by adjusting the dosage of H2O2 in this process. 3.4. Effect of Concentration of SZF. As shown in Figures 7 and 8, addition of SZF can greatly improve the efficiency of H2O2/O3 for HAc degradation, and the S−Fe(III) peroxo complexes (S−[FeIIIOOH]2+) might be an initiator to decompose ozone to produce hydroxyl radicals.20−22,35,36 Therefore, the effect of concentration of SZF on the removal rate of HAc was investigated (Figure 11). It was found that

increasing rate of ozone input in the range of 6.9−57.9 mg· min−1, manifesting that the degradation of HAc was controlled by the mass transfer of ozone. However, the improvement in the removal rate of HAc was negligible when the rate of ozone input was higher than 57.9 mg·min−1, indicating that the ratedetermining step for degradation of HAc might change to chemical reaction in the solution under the experimental conditions. 3.6. Effect of Aqueous pH. The aqueous pH can affect the decomposition rate of dissolved ozone, existing state of a dissociable organic compound, the deprotonation reaction of H2O2, and the surface property of catalyst, consequently leading to a change in ozonation efficiency. As shown in Figure 13, the removal rates of HAc by SZF/H2O2/O3 were enhanced

Figure 11. Effect of concentration of SZF on the removal rate of HAc. Reaction conditions: initial pH, 1.0; concentration of H2O2, 100 mg· L−1; concentration of HAc, 100 mg·L−1; flow rate of O3/O2, 0.67 L· min−1; rate of ozone input, 47.6 mg·min−1; reaction time, 30 min.

there was a positive correlation between the concentration of SZF and the removal rate of HAc when the dosage of SZF was less than 0.4 g·L−1, while the correlation became negative when the dosage of SZF was higher than 0.4 g·L−1. The experimental results indicate that higher dosage of SZF in a certain range could provide a larger amount of active sites to activate more H2O2, thereby generating more ·OH to degrade HAc. Nonetheless, too much ·OH generated in a short time will lead to its invalid consumption by H2O2 and other anions in solution. Therefore, a suitable concentration of SZF should be optimized when there is an application of SZF/H2O2/O3. 3.5. Effect of Rate of Ozone Input. Besides SZF and H2O2, ozone is also a decisive factor in the SZF/H2O2/O3 process because it is the source of hydroxyl radical. The effect

Figure 13. Effect of pH and solution acidity on the removal rate of HAc. Reaction conditions: concentration of H2O2, 100 mg·L−1; concentration of HAc, 100 mg·L−1; concentration of SZF, 0.4 g·L−1; flow rate of O3/O2, 0.67 L·min−1; rate of ozone input, 47.6 mg·min−1.

with the increase of pH value in the range of 0 to 1.5 and the kinetics of HAc degradation followed a pseudo-first-order kinetic model. The results are in accordance with the common property of ozonation, which might be explained by the fact that the deprotonation reaction of H2O2 occurs more easily and more ·OH could be generated with the increasing pH of the G


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Industrial & Engineering Chemistry Research solution, thus leading to higher efficiency of ozonation. However, SZF/H2O2/O3 was not so efficient (4.2% in 30 min) for HAc degradation when the acidity of the solution was 10% (mass fraction of H2SO4), indicating that SZF/H2O2/O3 is not suitable to treat the waste acid which has an acidity of more than 10%. Figure 7 proves that the removal rate of HAc was greatly enhanced in SZF/H2O2/O3, which was much higher than that of TS-1/H2O2/O3 for treating a certain acidity of wastewater (pH < 2). In the previous work, we have found that TS-1/ H2O2/O3 has an effective oxidative efficiency for HAc degradation in the pH range from 3.0 to 5.0. Therefore, the oxidative efficiency of SZF/H2O2/O3 in this pH range was also investigated. The removal rates of HAc at pH values of 3, 4, and 5 were 39.4%, 51.6%, and 83.1%, respectively (shown in Figure 14). Under the same conditions, the removal rates of HAc by

Figure 15. Amount of hydroxyl radicals generated in different processes. Reaction conditions: initial pH, 1.0; concentration of H 2 O 2 , 100 mg·L −1 ; concentration of DMSO, 500 mg·L −1 ; concentrations of TS-1 and SZF, both 0.4 g·L−1; flow rate of O3/ O2, 0.67 L·min−1; rate of ozone input, 47.6 mg·min−1.

rate by SZF/H2O2/O3. In addition, Figure 16 displays that the amount of hydroxyl radicals is being raised as the pH is increased, which is also supported by the corresponding decomposition of H2O2 illustrated in Figure 17.

Figure 14. Effect of pH on the removal rate of HAc. Reaction conditions: concentration of H2O2, 100 mg·L−1; concentration of HAc, 100 mg·L−1; concentration of SZF, 0.4 g·L−1; flow rate of O3/O2, 0.67 L·min−1; rate of ozone input, 47.6 mg·min−1.

TS-1/H2O2/O3 were 27.8% and 38% at pH 3 and 4.34 It can be seen that SZF/H2O2/O3 is still more efficient than TS-1/ H2O2/O3 in the pH range from 2 to 5. Known as scavengers of hydroxyl radicals, CO32− and HCO3− existing in water can reduce the degradation efficiency of AOPs-O3. However, under the experimental conditions (all pH less than 5.0), CO32−/ HCO3− will transfer to H2O and CO2 (the pK1 and pK2 of H2CO3 are 6.35 and 10.33, respectively) and CO2 can be stripped by O3/O2; thus, SZF/H2O2/O3 can automatically avoid the negative effect of CO32−/HCO3−. The experimental results well indicate that the presence of SZF can widen the pH range of application of H2O2/O3 in water treatment. It is very important to effectively pretreat refractory acid wastewater. 3.7. Quantitative Determination of the Amount of Hydroxyl Radicals. The concentration of hydroxyl radical is very difficult to directly measure because of its high activity. Therefore, an indirect method was used to determine the concentration of hydroxyl radical in this study. It has been reported that dimethyl sulfoxide (DMSO) can react with hydroxyl radical to produce the only product of methanesulfinic acid. Because methanesulfinic acid is an ozone inert compound, the amount of hydroxyl radicals generated in the AOPs-O3 can be directly determined by the amount of methanesulfinic acid.24−26 As shown in Figure 15, more hydroxyl radicals could be generated in SZF/H2O2/O3 than in H2O2/O3 or TS-1/ H2O2/O3. The results are in accordance with that in Figure 7 and further verify the remarkable enhancement in HAc removal

Figure 16. Amount of hydroxyl radicals generated in SZF/H2O2/O3 under different pH values. Reaction conditions: concentration of H 2 O 2 , 100 mg·L −1 ; concentration of DMSO, 500 mg·L −1 ; concentration of SZF, 0.4 g·L−1; flow rate of O3/O2, 0.67 L·min−1; rate of ozone input, 47.6 mg·min−1.

3.8. Mechanism Study. According to the knowledge of ozone decomposition in water, the possible property of S− [FeIIIOOH]2+, and the relevant literature,20−22 being favored by the experimental results and elaborations above, a tentative mechanism is proposed as follows for the degradation of HAc by SZF/H2O2/O3 (as shown in Figure 18): First and foremost, owing to the induction effect of SO, it becomes much easier for the O−O of H2O2 to be attracted by the metal cation (Fe− O) in SZF, which is conducive to formation of the S−Fe(III) peroxo complex intermediates (S−[FeIIIOOH]2+) on the surface of the catalyst. Then, the peroxo complex intermediates react with adsorbed ozone to effectively produce hydroxyl radicals. In this process, some of the ·OH could be scavenged by excessive H2O2 and other similar chemicals. Moreover, the higher the pH value, the larger is the amount of ·OH that will be produced. Last but not the least, HAc transfers to the surface of the catalyst and will be degraded by the generated highly H


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Industrial & Engineering Chemistry Research Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (Grant No. 21176225).

Figure 17. Remaining rate of H2O2 in SZF/H2O2/O3 at different pH values. Reaction conditions: concentration of H2O2, 100 mg·L−1; concentration of DMSO, 500 mg·L−1; concentration of SZF, 0.4 g·L−1; flow rate of O3/O2, 0.67 L·min−1; rate of ozone input, 47.6 mg·min−1.

Figure 18. Proposed mechanism of HAc degradation by SZF/H2O2/ O3.

active hydroxyl radicals. As indicated by the analysis results of HPLC, HAc was first degraded to form oxalic acid and formic acid and finally carbon dioxide and water. The degradation of HAc almost stopped when H2O2 was completely consumed.

4. CONCLUSIONS In this study, SZF was used to catalyze the peroxone reaction to degrade HAc in acidic solution. The results showed that the efficiency of SZF/H2O2/O3 in the pH range from 0 to 5 was still high, meaning that SZF could greatly broaden the working pH range of the H2O2/O3 process. The S−Fe(III) peroxo complex intermediate (S−[FeIIIOOH]2+) produced from the interaction of SZF with H2O2 might be an effective initiator for ozone decomposition to produce hydroxyl radicals. The amount of hydroxyl radicals could be controlled by the concentration of H2O2, aqueous pH, and concentration of SZF. The experimental results indicated that the concentration of H2O2 or SZF should be in a certain range to avoid useless consumption of hydroxyl radicals. Furthermore, it is also of significance that SZF/H2O2/O3 can automatically avoid the negative effect of CO32−/HCO3− (known as scavengers of hydroxyl radicals) during ozonation of acidic wastewater. On the basis of the analysis above, the SZF/H2O2/O3 established in this study will be of great significance for effective pretreatment of strong acidic wastewater from chemical or pharmaceutical factories which always contain some toxic refractory compounds.

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Effect of Iron Oxide Promoted Sulfated Zirconia on the Oxidative

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