Article pubs.acs.org/IECR
Magnetically Separable and Durable MnFe2O4 for Efficient Catalytic Ozonation of Organic Pollutants Jun Chen,† Weijie Wen,† Linjun Kong,† Shuanghong Tian,*,†,‡ Fuchuan Ding,§ and Ya Xiong*,†,‡ †
School of Environmental Science & Engineering, Sun Yat-sen University, Guangzhou 510275, P. R. China Guangdong Provincial Key Laboratory of Environmental Pollution Control and Remediation Technology, Guangzhou 510275, P. R. China § Department of Chemistry and Biochemistry, Texas State University, San Marcos, Texas 78666, United States ‡
ABSTRACT: Magnetically separable MnFe2O4 was prepared successfully by a sol−gel combustion method. The synthesized MnFe2O4 was characterized by X-ray diffraction (XRD), scanning electronic microscopy (SEM), Brunauer−Emmett−Teller (BET) surface area, zeta potential, and magnetic hysteresis loops. The catalytic activity of MnFe2O4 was evaluated by the ozonation of 4-chlorophenol (4-CP) at different solution pH value, ozone gas concentration, MnFe2O4 dosage, and initial 4-CP concentration. The ozone consumption was monitored and the ·OH exposure was calculated in different processes. It was found that MnFe2O4 was highly effective in catalyzing ozone decomposition and favored to generate more hydroxyl radicals. Moreover, the synergistically catalytic effect between Mn and Fe in MnFe2O4 was confirmed. Finally, lifetime of MnFe2O4 was measured in a novel integrated membrane-heterogeneous catalytic ozonation reactor designed for the first time. All the experimental results show that MnFe2O4 is a highly recoverable, efficient, and durable catalyst in the ozonation. activating O3 to generate more ·OH radicals in the catalytic ozonation. Spinel ferrite MnFe2O4 is a typical compound containing both manganese and iron. It is a good soft magnetic material with excellent structural stability. The magnetic property provides a convenient method to remove and recycle the nanoparticles from the treated water under an external magnetic field. Manganese ferrite (MnFe2O4) nanoparticles have proven to be useful in many magnetic applications, and they can provide a potential advantage for repeated magnetic separation purposes.19,20 Manganese ferrite also has excellent catalytic ability because the ferrite can be reduced to a cation excess composite MnFe2O4 and reoxidized to initial while keeping its spinel structure.21 Actually, manganese ferrite has been employed as catalysts in various reactions such as organic dehydrogenation, catalytic oxidation, Fenton reaction, and CO2 reduction.22−25 Therefore, manganese ferrite, as a magnetic bimetallic compound, may show a good combination of properties: high catalytic activity, good durability, and easy magnetic separation. However, manganese ferrite has not been reported to be used in catalytic ozonation. Therefore, the investigation of manganese ferrites as magnetic separable ozonation catalysts is an interesting issue. This study is mainly focused on five aspects: (I) preparation and characterization of MnFe2O4; (II) comparisons of ozonation alone and catalytic ozonation with MnFe2O4 by monitoring pollutant degradation, TOC removal rate, ozone consumption, and hydroxyl radicals evolution; (III) effects of operating variables in MnFe2O4/O3 process, including initial
1. INTRODUCTION Heterogeneous catalytic ozonation is an attractive and promising advanced oxidation process (AOP) due to its effective use of ozone and its improved treatability of refractory organic compounds through radical reactions.1 It combines ozone with solid catalyst to degrade and mineralize pollutants efficiently at ambient temperature, avoiding operation costs associated with reaction at high temperature and pressure.2 Usually, metal oxides (e.g., MnO2,3 Fe2O3,4 CeO2,5 TiO26), supported metal oxides (e.g., Pr/Al2O3,7 MnOx/ZrO2,8 Ni/ TiO2,9 CeO2/Al2O3 or SiO2 or TiO210) and some porous materials (e.g., activated carbon (AC),11 carbon nanotubes (CNTs),12 zeolite13) have been proposed as catalysts for ozonation. However, these catalysts, usually employed in the form of micro- and even nano-sized powders, are difficult to separate from treated water.14 Furthermore, the catalytic efficiency and the stability of catalyts need to be enhanced. These problems limit the application of heterogeneous catalytic ozonation in water treatment. Therefore, it is highly desirable to develop a novel ozonation catalyst with both good separation and remarkable catalytic activity. Iron oxides are nontoxic and comparatively inexpensive due to the abundance of this metal on Earth. They are fit for being used as catalysts in water treatment. In fact, iron type catalysts have already been used in ozonation processes to remove different compounds.4 Recently, we reported some durable Fecontaining catalysts, in which the iron ions and the anions could synergistically activate H2O2 to generate more ·OH radicals toward the oxidation of pollutants, such as FeVO4,15 Fe2V4O13,16 and Fe2(MoO4)3.17 Since manganese oxides are the most extensively studied and have been reported to have a higher ozone decomposition rate than Fe, Co, Ni, Cr, Ag, Cu, Ce, V or Mo oxides,18 the combination of manganese and iron may also produce a durable and effective catalyst, synergistically © 2014 American Chemical Society
Received: Revised: Accepted: Published: 6297
November 18, 2013 February 25, 2014 March 20, 2014 March 20, 2014 dx.doi.org/10.1021/ie403914r | Ind. Eng. Chem. Res. 2014, 53, 6297−6306
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ozonation experiment was conducted at room temperature (25 °C) in a continuous flow mode with a specially designed integrated membrane-heterogeneous catalytic ozonation reactor designed by our laboratory. As shown in Figure 1, the
pH value, ozone gas concentration, MnFe2O4 dosage, and initial 4-CP concentration; (IV) investigation of the catalytic mechanism of MnFe2O4; (V) lifetime measurement of MnFe2O4 in a continuous flow mode with a specially designed integrated membrane-heterogeneous catalytic ozonation reactor. In this paper, 4-chlorophenol (4-CP) is selected as a model compound since 4-CP is nonbiodegradable and used extensively in the textile industry.
2. MATERIALS AND METHODS 2.1. Materials. Manganese(II) nitrate hexa-hydrate, iron(III) nitrate nono-hydrate, citric acid, and ammonium hydroxide (25%, w/v) were purchased from Guangzhou Chemical Reagent Co. Ltd., China, and 4-chlorophenol was obtained from Aladdin Chemistry Co. Ltd., China. All reagents used in the experiment were of analytical reagent grade and used without further purification. 2.2. Preparation of Catalysts. MnFe2O4 was prepared by a sol−gel combustion process using citrate acid as fuel. The precursor was obtained in aqueous solution from metal nitrates and citric acid, at a molar ratio of Mn(II):Fe(III):citric acid = 1:2:3. Briefly, manganese hexa-hydrate and iron(III) nitrate nono-hydrate were dissolved in deionized water to form a mixed solution, which was slowly added dropwise at room temperature into a citric acid solution under vigorous agitation. After the addition is completed, aqueous ammonia was slowly introduced to adjust the solution pH to 5 and stabilize the nitrate−citrate sol. The resulting solution was continuously stirred at 60 °C for 2 h and then evaporated in a water bath at 85 °C to form brown sticky gel. Subsequently, the gel dried at 70 °C and was heated in a muffle furnace to 250 °C, at which temperature autocombustion occurred. The entire combustion lasted a few seconds and formed voluminous crumbly foam, which was transformed into powder at the slightest touch. The as-burned powder was then calcined at different temperatures in a muffle furnace for 2 h. 2.3. Instruments and Analytical Methods. The atomic ratio of MnFe2O4 and the concentrations of leached Fe and Mn in the solution were measured by inductively coupled plasma optical emission spectrometry (ICP-OES, optima 5300DV, Perkin-Elmer). The XRD patterns of the catalyst were obtained by using D/max 2200 vpc diffratometer (Rigaku Corporation, Japan) with the Cu Kα radiation at 40 kV and 30 mA. The specific BET surface area was determined with a micromeritics ASAP 2010 apparatus by nitrogen adsorption at 77 K. The SEM was performed on gold-coated samples using a JSM6330F-mode field emission scanning electron microscope (JEOL, Japan). The pH at the point of zero charge (PZC) was tested with zeta potential analyzer (Zetasizer Nano ZS, Marlven). The magnetic property was evaluated at room temperature with magnetic property measurement system (MPMS XL-7, Quantum Design, USA). The concentration of 4-CP was monitored using high performance liquid chromatography (HPLC, LC15, Shimadzu, Japan) with a C18 column (4.6 mm × 150 mm, GL Sciences, Inc.). The mobile phase was a filtered solution of 70/30 (v/v) methanol−water with a flow rate of 1 mL·min−1. The total organic carbon (TOC) was tested by a TOC analyzer (Shimadzu, Japan). The errors for TOC determination were less than 1%. The pH of the solution was checked with a PHS-3C pH meter (Rex Instrument Factory, Shanghai, China). 2.4. Experimental Procedures. In an attempt to investigate the lifetime of catalysts, the heterogeneous catalytic
Figure 1. Schematic diagram of an integrated membrane-heterogeneous catalytic ozonation reactor (MHOR): a, oxygen cylinder; b, ozone generator; c, ozone gas concentration detector; d, PC monitor; e, KI traps; f, membrane module; g, bubbles; h, catalyst particles; i, gas diffuser; j, control pumps (with a flowrate of 33.33 mL/min in the continuous mode and 0 mL/min in the semicontinuous mode); k, effluent tank; l, influent tank.
reactor consists of a reaction cylinder, a submerged ceramic membrane module, an ozone feed system, a wastewater feed system, an effluent collecting system, and ozone traps. The reaction cylinder with several input/output ports has a height of 820 mm, and an inside diameter of 80 mm. A ceramic membrane with the pore diameter of 3 μm was used as the submerged membrane module, which could retain the catalyst powder inside the reactor. The ozone feed system includes an ozone generator and an ozone gas concentration detector, which is connected through a RS-232 port to a personnel computer and used for storing and monitoring all data transmitted by the detector. Ozone gas is produced from cylinder oxygen a YE-TG-02PII ozone generator (made in China) and diffuses into the reactor through a ceramic sparger at a flow rate of 1.0 L·min−1 monitored by a flow meter on the generator. This flow rate is enough to hydraulically suspend the MnFe2O4 fine particles. The concentration of ozone was 5.0 mg/L monitored online by the computer. 4-Chlorophenol aqueous solution was pumped into the reactor continuously with a flowrate of 33.33 mL/min through a control pump. The treated water is collected at the same rate through another control pump to keep the solution volume constant. Thus, the hydraulic retention time (HRT) was controlled at 30 min. Simultaneously, MnFe2O4 powder was retained in the reactor by the membrane module to continuously activate ozone. The excess ozone in the outlet gas was trapped by KI solution. At a given time interval, 3 mL of sample was taken from the reactor to quantify adsorption/oxidation. An aliquot of 0.1 M Na2S2O3 was subsequently added to quench the aqueous ozone remaining in the reaction solution. Then samples were analyzed immediately after filtration through 0.22 μm Millipore 6298
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Figure 2. Characterization of MnFe2O4: (a) XRD patterns, MnOx and FeOx prepared by the same sol−gel method are also included and the symbol * stands for MnFe2O4; (b) SEM image; (c) zeta potentials as a function of solution pH; (d) magnetic hysteresis loops at room temperature, (inset) demonstration of magnetic separation of the catalyst from solution. The two photographs correspond to the appearance before and after applying an external magnetic field.
membrane filters to remove suspended particles. The reaction temperature was maintained at 25 °C. All the other degradation experiments were carried out in a semicontinuous flow mode using the above reactor, except that 1.0 L of 4-CP solution was added into the reaction tank and no influent and effluent was supplied. In the adsorption test, nitrogen but not ozone was bubbled into the reactor with other reaction conditions remaining identical to the catalytic process.
from the exothermic combustion reaction heats the system to high temperature. Thus, a low autoignition temperature of 250 °C was enough to form the final spinel structure. The peak intensity gradually increased with the increase in calcined temperature. This indicates that aggregation occurred during the calcination, which is also proved by the decrease in the specific BET surface area of the catalysts calcined at higher temperature. The BET surface area for the as-burned MnFe2O4 is 46.5 m2/g, while that of the sample calcined at 300, 400, and 500 °C decreased to 39.5, 34.3, and 28.7 m2/g, respectively. Considering that the less specific surface area diminishes the active sites of the quantitative catalyst and the as-burnt MnFe2O4 has already shown well-crystallized structure, no more calcination was needed and all the following catalysts were used as received from the combustion. The SEM image of the as-burned MnFe2O4 reveals that the product is a low-density, loose, and porous material (Figure 2b). It is worth mentioning that the synthesized microstructures generates a lot of edges, corners, step edges, and step corners on the surfaces of synthesized MnFe2O4 morphologies, which are recognized as active sites in heterogeneous catalysis.26 This kind of morphology gave high BET surface area of 46.5 m2/g and is favorable to catalytic application. In heterogeneous catalytic systems, pH can affect the surface properties of metal oxides, which are covered by surface hydroxyl groups in the presence of water. Under different pH conditions, proton transference may occur on the surface of metal oxides, affecting, therefore, an eventual adsorption step of the reaction pathways. Zeta potential indicates the surface
3. RESULTS AND DISCUSSION 3.1. Characterization of MnFe2O4. The atomic ratio of Mn to Fe in the as-burned brown powder is 1:2.08 determined by ICP-OES, in close proximity to the theoretical ratio in MnFe2O4. The XRD pattern of the as-burned brown powder calcined at different temperatures are displayed in Figure 2a. The as-burnt precursor shows narrow and well-defined diffraction lines expected for well-crystallized MnFe2 O4 structure instead of an amorphous structure. The considered diffraction peaks with 2θ values of 18.07°, 30.62°, 36.05°, 43.72°, 54.12°, 57.72°, and 63.32° correspond to the crystal planes (111), (220), (311), (400), (422), (511), and (440) of MnFe2O4, respectively. It can be indexed as cubic spinel structure belonging to the fcc system according to the standard JCPDS (card no. 10-0319). No peaks attributable to manganese oxide and/or iron oxide and unreacted precursor materials were detected. This suggests that the well-crystallized MnFe2O4 phase with a spinel structure has formed in the combustion process. The sol−gel process ensures that the Mn and Fe mixes thoroughly while a large amount of heat released 6299
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Figure 3. Comparison of 4-CP removal in ozonation alone and catalytic ozonation with MnFe2O4: (a) 4-CP removal, (b) ozone conc. in effluent gas, (c) Rct, (d) TOC removal. Conditions: ozone gas flow rate, 1.0 L/min; ozone gas concentration, 5.0 mg/L; initial concentration of 4-CP, 100.0 mg/ L; dose of MnFe2O4, 1.0 g/L; pH, 6.21.
solution, including ozonation alone, MnFe2O4/O3, and adsorption on MnFe2O4. Figure 3a shows the 4-CP removal against reaction time in different processes. It can be seen that the adsorption on MnFe2O4 results in only slight removal of 4CP, fluctuating between 0.5% and 2.3%, which is too small to make a significant contribution to the degradation efficiency of 4-CP and can therefore be neglected when compared to the oxidation. For the experiment results of ozonation alone and catalytic ozonation processes, the concentration of 4-CP decreased with increasing reaction time. The presence of ozone alone enhances the degradation efficiency of 4-CP greatly, while the best result is obtained for the MnFe2O4/O3 system. After 30 min of reaction time, ozonation alone caused 77.0% of 4-CP to decompose and the introduction of MnFe2O4 to ozonation caused 95.7% decomposition. In an attempt to examine the effect of MnFe2O4 on the decomposition of ozone, the concentrations of ozone in the influent and effluent gas were monitored during the reaction, as presented in Figure 3b. The concentration of ozone in the influent gas was controlled at 5.0 mg/L. Under these conditions, the ozone concentration in the effluent gas in ozonation alone increased gradually and, then, reached a steady state of 3.6 mg/L within 22 min. When MnFe2O4 was added as a heterogeneous catalyst, the ozone concentration in the effluent gas decreased. It took 25 min for the ozone concentration to reach a steady state of 3.5 mg/L in the presence of MnFe2O4. It is obvious that ozone was rapidly consumed in MnFe2O4/O3 system at first. This result indicates that MnFe 2 O 4 is highly effective in catalyzing ozone decomposition. However, some catalysts can promote ozone decomposition while they have low catalytic activity toward the
charge of the particles. Figure 2c displays zeta potentials as a function of solution pH for MnFe2O4. With an increase in solution pH, the zeta potential of MnFe2O4 decreased. At pH 7.21, the net charge of MnFe2O4 is zero, which is the point of zero charge (PZC) of MnFe2O4. Below pH 7.21, the surface of MnFe2O4 attracted protons and therefore was positively charged. On the contrary, by increasing the solution pH over to 7.21, MnFe2O4 particles released the protons into the solution and the surface became negatively charged. Correspondingly, the charged surface will affect the adsorption of organic pollutants due to the electrostatic attraction or repulsion, depending on the solution pH and the property of the organic pollutants. The magnetic property of MnFe2O4 was measured by a magnetic hysteresis loop at room temperature. As shown in Figure 2d, MnFe2O4 has a large saturation magnetization (Ms) of 61.5 emu/g, a low remnant magnetization (Mr) of 16.2 emu/g, and a low coercivity (Hc) of 67.8 Oe, indicating that MnFe2O4 is a good soft magnetic material. The good soft ferromagnetism ensures that MnFe2O4 particles can be simply recovered from the reaction solution by using an external magnetic field. This was experimentally proven by the fact that MnFe2O4 was rapidly attracted (7 s) by a conventional magnet placed close to the vessel as observed in the inset of Figure 2d, demonstrating the efficacy of magnetic separation. This is economically suitable to the industrial application from a practical point of view. 3.2. Comparison of Ozonation Alone and Catalytic Ozonation with MnFe2O4. Different experimental processes were performed to investigate the catalytic activity of the asburned MnFe2O4 for the degradation of 4-CP in aqueous 6300
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MnFe2O4 promoted the ozonation greatly at each pH value and therefore over 90% of 4-CP degraded in a wide pH range of 4.0−10.0. It can be obviously seen that the degradation efficiency of 4-CP in MnFe2O4/O3 process is slightly affected by the solution pH whereas the single ozonation is much more sensitive to the pH change. This fact can be explained by ozone decomposition, the dominant species of hydroxyl groups on the surface of MnFe2O4, the formation of the alkaline microenvironment near the surface of MnFe2O4 particles, as well as the dissociation rate of 4-CP at a certain solution pH. In catalytic ozonation process, pH value of aqueous solution can affect the decomposition rate of the dissolved ozone, the surface properties of catalyst, and the dissociation of organic molecules, which would finally determine the degradation of pollutants.29,30 First of all, water pH indicates the concentration of OH− in the solution, which is an initiator for aqueous ozone decomposition into hydroxyl radicals.31 When pH increased, more available OH− could initiate ozone decomposition to form more hydroxyl radicals, which had high reaction activity toward 4-CP (kHO·,4‑CP = 7.6 × 109 1/M·s).32 Second, the surface properties of MnFe2O4 in water vary with the increase of pH. It is accepted that MnFe2O4−OH, MnFe2O4−OH2+, and MnFe2O4−O− are the dominant species of surface hydroxyl groups in the different pH ranges. Since the pHpzc of the catalyst was determined to be 7.21, the species of the surface hydroxyl group under different pH conditions can be determined. As mentioned above, the catalyst surface is positively charged at pH values below the pHPZC and negatively charged at pH values above the pHPZC. Therefore, the MnFe2O4−OH group, of which net charge was zero, was the dominant species at water pH 7.21, MnFe2O4−OH2+ was the dominant specie below water pH 7.21, and MnFe2O4−O− was the dominant specie above water pH 7.21. With the increase of pH, the zeta potential decreased rapidly and the fraction of neutral MnFe 2 O 4 −OH species and even the negative MnFe2O4−O− increased, which favored the interaction of the ozone with the catalyst due to the electrophilic characteristics of ozone.27 This is an important property for enhancing transformation of ozone into hydroxyl radicals. Even some studies indicate that the deprotonated surface groups of the catalyst can act as initiators of radical reactions.33 Third, the dissociation of 4-CP in water affects the removal rate of 4-CP. 4-CP in aqueous solution is present in dissociated and undissociated form whose relative occurrence is regulated by the solution pH (pK4‑CP = 8.2). Deprotonated 4-CP dominates above pH 8.2 while molecular 4-CP prevailed below pH 8.2. Since the direct attack of ozone to aromatic substrates develops through an electrophilic mechanism, it is evident that the higher the concentration of deprotonated 4-CP, the higher the reactivity. Actually, it is proven that the reaction rate constant of the deprotonated 4-CP with ozone is 5 orders of magnitude higher than that of the molecular 4-CP.34 With the increase of pH, more 4-CP is deprotonated, which leads to the increase in the total reaction constant rate. These three reasons resulted in higher removal rate of 4-CP at higher pH conditions. In contrast, less OH− in the solution is obtained under acidic conditions; however, the PZC of MnFe2O4 is 7.21. When the solution pH is below 7.21, the surface of MnFe2O4 is positively charged and counterbalanced ions OH− are attracted, thus the alkaline microenvironment near the surface of MnFe2O4 particles forms, which facilitates the generation of hydroxyl radicals. That may explain the unusual high removal rate of 4CP at acidic conditions.
degradation of pollutants due to the low generation rate of hydroxyl radicals during ozonation.27 Elovitz and von Gunten28 proposed an interesting experimental approach to check the concentrations of the transient hydroxyl radicals and ozone. For this purpose, an Rct parameter was introduced, which is defined as the ratio of the ·OH and O3 concentrations at any given time in the reaction as follows: ⎛ [4 − CP]t ⎞ ln⎜ ⎟ = −(k O3 + k ·OH/4‐CPR ct) × ⎝ [4 − CP]0 ⎠
∫ [O3] dt (1)
In this study, the amount of the aqueous ozone during the reaction was estimated by subtracting the ozone off-gas concentrations in control experiments without catalysts and pollutants by the ozone off-gas concentrations in ozonation alone or catalytic ozonation process. This method is very useful in deducing a knowledge of the ·OH evolution in an ozonation process. According to eq 1 and the slope of the curves in Figure 3c, the Rct values of ozonation alone and catalytic ozonation with MnFe2O4 were calculated as 1.91 × 10−7 and 1.21 × 10−6. The experimental result demonstrates that O3/MnFe2O4 produce more ·OH species than O3 alone, in accordance with their corresponding degradation efficiencies. Herein it can be concluded that MnFe2O4 showed high catalytic activity toward the degradation of 4-CP based on Rct value. Additionally, the catalytic ozonation process is more effective than ozonation alone to remove TOC from aqueous 4-CP. As shown in Figure 3d, the TOC removal of 4-CP achieved by MnFe2O4/O3 process after 90 min reaction is 60.3%, compared to 26.8% by ozonation alone, the former is 2.25 times as much as the latter. Furthermore, it is found that the removal rate of TOC is much lower than the removal rate of 4-CP, indicating that 4-CP had been mineralized partly, and the byproducts were formed via the degradation of initial compound in the every selected processes. All these experiments indicate that MnFe2O4 was an effective catalyst and can be applied in the ozonation process well. 3.3. Effects of Operating Variables in MnFe2O4/O3 Process. Effect of pH. Generally, the degradation of pollutants is highly pH-dependent. As shown in Figure 4, the removal rate of 4-CP within 30 min increased with an increase in the initial pH value both in the single ozonation and MnFe2O4/O3 processes. It can be put another way that higher pH value brought out higher ozonation rate. Moreover, the presence of
Figure 4. Effect of pH on the removal of 4-CP in the single ozonation and MnFe2O4/O3 process. Conditions: ozone gas flow rate, 1.0 L/ min; ozone gas concentration, 5.0 mg/L; initial concentration of 4-CP, 100.0 mg/L; dose of MnFe2O4, 1.0 g/L. 6301
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Effect of Ozone Dosage. The purpose of catalytic ozonation is to accelerate the decomposition of ozone for generating more oxidation potential species, such as ·OH. Therefore, ozone gas concentration is an important factor which affects the amount and the rate of ·OH production when the ozone gas flow rate is fixed. According to Figure 5a, the removal rate of 4-CP
ozonation process, 77.4% of 4-CP removal can be observed at 30 min. However, when the ozone dosage increases to 5 mg/L, the degradation efficiency of 4-CP in catalytic ozonation is raised up to 95.7%, close to complete removal and that with the ozone dosage of 8 mg/L. Considering the operation cost, 5 mg/L of ozone dosage was applied. Effect of Catalyst Dosage. In the heterogeneous catalytic ozonation process, the catalyst is introduced to ozonation and enhances the oxidation process. It would be expected that more active sites are available at higher catalyst dosage. In order to make effective use of the catalyst in the practical application, it is necessary to explore the effect of catalyst dosage on the degradation efficiency and find out the optimum dosage. The effect of catalyst dosage on the removal rate of 4-CP has been investigated in MnFe2O4/O3 process by changing MnFe2O4 dosage from 0 to 2.0 g/L (Figure 5b). The degradation efficiency of 4-CP increased from 83.1% to 95.7% with increasing MnFe2O4 dosage from 0 to 1.0 g/L (Figure 5b). However, further increasing the MnFe2O4 dosage to 1.5 g/L does not increase the degradation rate appreciably, which even slightly decreases at 2.0 g/L. Although MnFe2O4 made it possible that the chain of radical reactions could be induced and propagated by the ozone introduced in the reactor, the excess MnFe2O4 particles easily aggregated and affected the use efficiency.35 Hence, 1.0 g/L of MnFe2O4 catalyst is found to be optimum dosage and used in all other experiments. Effect of Initial 4-CP Concentration. Concentration of pollutant, as an important parameter in wastewater treatment, has great influence on the required ozone quantity and the treatment time. Therefore, it is very important from a practical point of view to study how the initial 4-CP concentration affects the removal of pollutants in the MnFe2O4 catalytic ozonation process. The removal rate of 4-CP along the reaction time for the initial concentrations of 50, 100, 200, and 400 mg/ L, respectively, was displayed in Figure 5c. It is observed that the removal rate decreased significantly as the initial 4-CP concentration increased. However, high removal rate can be obtained when the initial 4-CP concentration is below 100 mg/ L. Further increasing the initial concentration to 200 or 400 mg/L resulted in the sharp deduction of 4-CP removal rate. Actually the absolute amount of 4-CP was degraded, much more than the cases of the initial concentration below 100 mg/ L. That can be explained by the fact that ozone is not enough to destroyed all the 4-CP molecules when initial concentration of 4-CP increased to above 200 mg/L. In addition, more intermediates are generated and would also consume the available ozone. Therefore, increased ozone should be supplied to obtain a more complete removal of 4-CP at high concentration of above 200 mg/L. 3.4. Exploration of the Catalytic Mechanism. Direct ozone reactions and indirect ·OH radical reactions are generally considered to work in the heterogeneous catalytic ozonation of organic contaminates.36 As is well-known, the tert-butanol (TBA) is a typical hydroxyl radical scavenger and reacts with · OH radicals with a high rate constant of 6 × 108 1/M·s, while it can hardly be oxidized by ozone directly under acidic or neutral pH.37 Therefore, TBA was widely used as probe molecules to indentify if the reactions involved hydroxyl radicals or just direct ozone reactions occurred. The effects of TBA on the degradation of 4-CP after 30 min reaction and the TOC removal after 90 min reaction were illustrated in Figure 6. In the presence of 0.2 g/L TBA, 96% removal of 4-CP was obtained. These variances were similar to
Figure 5. (a) Effect of ozone gas concentration on the removal of 4CP. Conditions: ozone gas flow rate, 1.0 L/min; initial concentration of 4-CP, 100.0 mg/L; dose of MnFe2O4, 1.0 g/L; pH, 6.21. (b) Effect of MnFe2O4 dose on the removal of 4-CP. Conditions: ozone gas flow rate, 1.0 L/min; ozone gas concentration, 5.0 mg/L; initial concentration of 4-CP, 100.0 mg/L; pH: 6.21. (c) Effect of 4-CP initial concentration on the removal of 4-CP. Conditions: ozone gas flow rate, 1.0 L/min; ozone gas concentration, 5.0 mg/L; dose of MnFe2O4, 1.0 g/L; pH, 6.21.
increased with increasing the ozone gas concentration from 2.0 to 8.0 mg/L. That can be explained by the formation of more oxidation species within a certain time. Furthermore, increasing ozone inlet enhanced the mass transportation from the gas phase to the liquid phase, which also favors the oxidation of 4CP in the water. When 2 mg/L ozone is applied in the catalytic 6302
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60.3% to 21.7% when adding 0.2 g/L of TBA into the catalytic ozonation system. This should be due to the scavenging effect on hydroxyl radicals generated from the self-decomposition of ozone, which led to the accumulation of some intermediates with poor degradability by ozone alone.40 Therefore, ·OH radical mechanism was involved in the MnFe2O4 catalytic ozonation system. As mentioned above, MnFe2O4 was prepared by a sol−gel combustion process using citrate acid as fuel. Similarly, manganese oxide and iron oxide were prepared with the same method using manganese hexa-hydrate and iron(III) nitrate nono-hydrate as the precursor, respectively. The XRD of the products, FeOx and MnOx, were measured and compared, as shown in Figure 2a. The manganese oxide was presented as hausmannite phase of Mn3O4 (card no. 24-0734) and iron oxide as hametite phase of Fe2O3 (card no. 33-0664) according to the standard JCPDS. To further characterize the valence states of Fe and Mn on the surface of these samples, the XPS analyses were carried out, as displayed in Figure 7a−c. The Fe 2p spectra observed from Fe2O3 show three peaks at 711.3, 719.3, and 725.0 eV, which represent the binding energies of Fe 2p3/2, shakeup satellite Fe 2p3/2 and Fe 2p1/2 of Fe3+, respectively.41 The Fe 2p from MnFe2O4 also show such a spectra feature, indicating the presence of Fe3+. However, all the peaks for Fe 2p from MnFe2O4 shifted to a slightly lower value compared to those from Fe2O3 because the Fe chemical environment changed. Since the oxidation state of Fe in MnFe2O4 is +3, it is reasonable to believe that the oxidation state of Mn in this sample is +2. The binding energy value of Mn 2p3/2 and Mn 2p1/2 are 640.5 and 652.2 eV, respectively,
Figure 6. Effect of tert-butanol (0.2 g/L) on 4-CP and TOC removal in ozonation and MnFe2O4 catalytic ozonation. Conditions: ozone gas flow rate, 1.0 L/min; ozone gas concentration, 5.0 mg/L; initial concentration of 4-CP, 100.0 mg/L; dose of MnFe2O4, 1.0 g/L; pH, 6.21.
the experimental results without scavenger. It is clear that adding TBA into the catalytic ozonation system did not have a significant influence on the degradation of 4-CP, which is in accordance with the result obtained by Pi.38 Sui also found that the presence of TBA has insignificant effect in MnOx-0.013/ MCM-41 catalytic ozonation processes, but ·OH radicals were detected by ESR spectra.39 Therefore, it cannot be concluded that the catalytic ozonation did not follow hydroxyl radicals reaction mechanism only based on the less inhibiting effect of the presence TBA. In an attempt to avoid drawing the cursory conclusion of nonhydroxyl radicals reaction mechanism, the TOC removal was further investigated, which decreases from
Figure 7. XPS spectra of (a) Fe 2p from Fe2O3 and MnFe2O4, (b) Mn 2p from MnFe2O4, (c) Mn 2p from Mn3O4. (d) Comparison of the catalytic activity of various compounds or mixtures. Conditions: ozone gas flow rate, 1.0 L/min; ozone gas concentration, 5.0 mg/L; initial concentration of 4CP, 100.0 mg/L; dose of MnFe2O4, 1.0 g/L; pH, 6.21. 6303
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which perfectly matches the previously reported values for Mn2+.42 Analyzing the Mn 2p3/2 in more detail would allow a fit to two peaks at 639.9 and 641.3 eV, which should be attributed to the difference between the tetragonal and octahedral sites for Mn2+.43For Mn3O4, the binding energy value of Mn 2p3/2 is 641.6 eV, and the spin orbit splitting between the Mn 2p3/2 and Mn 2p1/2 level is 11.7 eV, which agrees well with the value for hausmannite reported in the literature.43 It was accepted that Mn3O4 existed in the form of Mn2+[Mn2+Mn4+]O4 or Mn2+[Mn3+]2O4 structure.43,44 Therefore, the fitting of the data was performed using two different structures. By comparisons, the Mn2+[Mn2+Mn4+]O4 structure for Mn3O4 was accepted in this article. Due to two binding energies for Mn2+ in tetragonal and octahedral sites and one binding energy for Mn3+, three binding energies should be used to fit the data. Mn 2p3/2 peak was divided into three parts with the ratio Mn2+(in tetragonal sites): Mn2+(in octahedral sites):Mn3+ = 1:1:1. In order to clarify if Mn and Fe in MnFe2O4 independently activated O3 or there is any synergistically catalytic effect between them, we carried out comparable experiments using the above prepared ozonation catalysts including MnFe2O4, Mn3O4, and Fe2O3. As shown in Figure 7d, the catalytic activity of MnFe2O4 is much higher than that of 0.5Mn3O4, Fe2O3, and also their mixture (0.5Mn3O4 + Fe2O3) with identical Mn2+ and Fe3+ molar ratios. According to the above XPS results, one Mn3O4 molecule contains two Mn2+ and one Mn4+. Hence, 0.5Mn3O4 contains one Mn2+ and the molar ratio of Mn2+ to Fe3+ in the mixture (0.5Mn3O4 + Fe2O3) is the same as that in MnFe2O4. Obviously, MnFe2O4 showed the highest catalytic activity among the investigated compounds. This result suggests that there is a synergistically catalytic effect between Mn and Fe in MnFe2O4. As can be seen from Figure 7a−c, both Fe 2p3/2 and Mn 2p3/2 for MnFe2O4 shifts to lower value compare to that for Fe2O3 and Mn3O4, confirming a strong interaction between Mn and Fe in MnFe2O4. Moreover, more redox pairs of Fe3+/Fe2+, Mn3+/Mn2+, and Mn3+/Fe2+ exist in the oxidative and reductive reaction catalyzed by MnFe2O4. These may result in more active sites on the catalyst and contribute to the catalytic activity of MnFe2O4. 3.5. Lifetime Measurement of MnFe2O4. The lifetime of MnFe2O4 was measured for the first time by a continuous-flow catalytic experiment in a specially designed integrated membrane-heterogeneous catalytic ozonation reactor shown in Figure 1. The hydraulic residence time was set at 30 min. In the novel reactor, submerged porous ceramic membrane module acted as a simple barrier for the catalyst particles and kept them back in the reaction system to continuously catalyze O3 without additional separation process. The removal of 4-CP were monitored continuously for 12 h by measuring the residual concentration in the effluents. As shown in Figure 8, the removal of 4-CP increased rapidly and then reached 90.2% at 60 min. After that, a steady-state value were fluctuating between 90.2% and 93.3%, which is slightly lower than the removal rate attained for the semibatch reactor at 30 min. That is because the reactants were mixed better in the latter reactor. Most importantly, the removal rate of 4-CP in the continuous reactor was still very high (above 90.2%) and no considerable loss of catalytic activity occurred during 12 h lifetime measurement. Three samples at 1.5, 3, and 12 h reaction time were collected to track the TOC evoluation. The results showed that the TOC removal rate reached 52.3%, 57.2%, and 54.4%, correspondingly. The values are close to that in the
Figure 8. Lifetime measurement of MnFe2O4 by a continuous-flow catalytic experiment in a specially membrane-heterogeneous catalytic ozonation reactor. (inset) XRD patterns on the fresh and used catalyst.
semibatch reactor, and finally, the TOC decreased to 54.4%. The lower TOC removal rate may be due to the mixing status or the accumulation of the highly refractory products, such as formic acid, oxalic acid, and so on. To explore the catalyst leaching and further determine the stability of MnFe2O4 in aqueous solution, leached Fe and Mn ions in effluents were too low to be detected by inductively coupled plasma optical emission spectrometry during 12 h operation of the continuous flow reactor. It was also noticed that after reaction for 12 h, MnFe2O4 almost retained the catalytic activity as efficiently as the fresh one for a constant amount of MnFe2O4 catalyst. This observation suggests that Fe and Mn leaching, if it existed, did not decrease the native catalytic activity of MnFe2O4 for 12 h. In addition, the good chemical stability of MnFe2O4 was further confirmed by XRD measurements on the used and fresh catalysts (inset of Figure 8). No considerable changes in XRD spectra was observed after being used for 12 h, further indicating the durability of the catalyst. It is worth noting that the catalyst could be magnetically separated from the reaction medium and reused directly in the successive semibatch experiments.
4. CONCLUSIONS Pure and well-crystallized MnFe2O4 with a spinel structure was synthesized by a sol−gel combustion process without further calcinations. The magnetically separable MnFe2O4 nanoparticles with a high specific surface area of 46 m2/g showed high catalytic activity in the ozonation of organic pollutants. More 4-CP and TOC were removed in the catalytic ozonation with MnFe2O4 than ozonation alone. On the basis of comparison of ozone consumption and Rct value in different processes, it was found that MnFe2O4 was highly effective in catalyzing ozone decomposition and favored to generate more hydroxyl radicals. In MnFe2O4/O3 process, over 90% of 4-CP could be removed within 30 min under conditions of a pH ranged from 4.0 to 10.0, ozone gas concentration larger than 5.0 mg/L, and 1.0 g/L of MnFe2O4, as well as the 4-CP concentration lower than 100.0 mg/L. The effect of TBA on TOC removal in MnFe2O4/O3 process showed that the reaction involved hydroxyl radical. The higher catalytic activity of MnFe2O4 than that of 0.5Mn3O4, Fe2O3 and also their mixture (0.5Mn3O4 + Fe2O3) with identical Mn and Fe molar ratio indicated that there was a synergistically catalytic effect between Mn2+ and Fe3+ in MnFe2O4. A lifetime measurement for MnFe2O4 catalyst in a novel integrated heterogeneous catalytic ozonation-membrane separation reactor showed that a 6304
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steady-state removal rate of above 90% can be achieved during 12 h operation of the continuous flow reactor. No leaching of Fe and Mn was detected and no considerable changes in XRD spectra were observed after MnFe2O4 was used for 12 h. These experimental results imply that MnFe2 O 4 is a highly recoverable, durable, and efficient catalyst.
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AUTHOR INFORMATION
Corresponding Authors
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[email protected] (Y.X.). Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This research was supported by the Nature Science Foundations of China (20977117, 21107146), Nature foundations of Guangdong Province (92510027501000005), the project of Education Bureau of Guangdong Province (cgzhzd1001), the Fundamental Research Funds for the Central Universities (121pgy20), and Innovative Talents Training Funding of Doctoral Students of Sun Yat-sen University and Scholarship Award for Excellent Doctoral Student Granted by Minsitry of Education.
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REFERENCES
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