Mineralization of CI Reactive Yellow 145 in Aqueous Solution by

At an initial pH of 8.0, bulk temperature of 30 °C, ozone feed of 4.8 g/h, and initial ... and photographic industries may have serious detrimental e...
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Mineralization of CI Reactive Yellow 145 in Aqueous Solution by Ultraviolet-Enhanced Ozonation Shuang Song,* Xing Xu, Lejin Xu, Zhiqiao He, Haiping Ying, and Jianmeng Chen College of Biological and EnVironmental Engineering, Zhejiang UniVersity of Technology, Hangzhou 310032, People’s Republic of China

Bing Yan* Department of Chemistry, Tongji UniVersity, Shanghai 200092, People’s Republic of China

We investigated the mineralization of CI Reactive Yellow 145 (RY145) in aqueous solution in a UV-enhanced ozonation (UV/O3) reactor. The effects of several operational parameters, including solution pH, initial dye concentration, ozone feed, and bulk temperature, on total organic carbon (TOC) removal were investigated. The experimental results illustrated that ozonation combined with UV radiation for removal of TOC was more efficient than ozonation alone or UV irradiation alone. At an initial pH of 8.0, bulk temperature of 30 °C, ozone feed of 4.8 g/h, and initial dye concentration of 500 mg/L, the TOC removal efficiency of RY145 reached ∼80% after 150 min under 175 W UV irradiation. Intermediates formed during RY145 mineralization were detected by gas chromatography coupled with mass spectrometry (GC/MS) and ion chromatography (IC). On the basis of our results, we propose a tentative degradation pathway. Introduction Dyestuffs from the textile and photographic industries may have serious detrimental effects on aquatic habitats. Thus, the presence of dyes in effluent requires effective treatment techniques. Chemical coagulation, activated carbon adsorption, photocatalysis, chlorination, and ozonation have been studied to determine their effectiveness at removing dyes from textile wastewater.1,2 Chemical coagulation and adsorption are nondestructive; they transfer organic compounds from water to another phase and cause secondary pollution. These techniques use chemicals, adsorbents, or catalysts, which may themselves require treatment, thereby resulting in high operating costs. Recent developments in the chemical treatment of dyes in effluents by dyeing companies led to improvement of the oxidative degradation of organic compounds in aqueous solution, called advanced oxidation processes (AOPs). In AOPs, the UV/ H2O2, UV/O3, photo-Fenton, and UV/photocatalyst procedures employ UV as the initiator to generate hydroxyl radical (•OH), which has a very high oxidation potential and is nonselective.3,4 These processes potentially can mineralize most of the organic contaminants into carbon dioxide and water. However, the UV/ photocatalyst procedure requires additional treatment, such as catalyst separation and recovery.5 The photo-Fenton process requires large amounts of H2O2 and FeSO4, and one of the end products is an Fe(OH)3 precipitate.6,7 The UV-enhanced ozonation process is effective in the treatment of bleaching water, organic refractory compounds, and insecticides.9,10 When compared with ozonation alone, UV/ O3 can produce more hydroxyl radicals and results in a net enhancement to degrade organic pollutant11 via the following processes: * Corresponding authors. (S.S.) Tel: 86-571-88320726. Fax: 86571-88320276. E-mail: [email protected]. (B.Y.) Tel: 86-21-65984663. Fax: 86-21-65982287. E-mail: [email protected].

O3 + hν f O2 + O(1D)

(1)

O(1D) + H2O f •OH + •OH

(2)

O(1D) + H2O f H2O2

(3)

H2O2 + hν f 2•OH

(4)

Reactive dyes, which link to fibers through chemical combining, now are widely used in the textile industry. Some contain typical azo-based chromophores combined with types of reactive groups, such as vinyl sulfone, chlorotriazine, trichloropyrimidine, and difluorochloropyrimidine.12 This property makes these dyes suitable dyestuffs, but it also makes them hard to degrade in wastewater.13 Reactive azo dyes are of special environmental concern because they are resistant to aerobic degradation and their degradation products under anaerobic conditions, such as aromatic amines, are considered highly carcinogenic.14,15 In this paper, we chose CI Reactive Yellow 145 (RY145), a widely used dye in textile industries, as a model substrate for our mineralization studies. To date, various techniques, such as adsorption onto wheat bran or chitosan, photocatalysis with TiO2, and purification by combined membrane filtration, have been reported to remove RY145.16-19 However, decolorization is not the correct end point to evaluate in treating wastewater. Wastewater could still be hazardous even if decolorization occurs. Consequently, destruction of the dye should be evaluated as an overall degradation process by monitoring the reduction of total organic carbon (TOC). The objectives of this study were to (1) examine the effects of important variables, such as pH, bulk temperature, ozone feed, and initial dye concentration on the TOC reduction of RY145 by UV/O3; (2) analyze the concentrations of related ions produced during the mineralization process; and (3) propose a possible degradation pathway for the reactive azo dye based on the intermediates identified.

10.1021/ie0711628 CCC: $40.75 © 2008 American Chemical Society Published on Web 02/09/2008

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Materials and Methods Chemicals Used. RY145 (molecular mass 1026.20 g/mol) was purchased from Wujiang Taoyuan Dyestuffs Plant (Wujiang, China) and used without further purification. The solubility of RY145 is 130 g/L in water at 298 K. The TOC of 500 mg/L RY145 in aqueous solution is 128 ( 9 mg/L. UV/O3 Oxidation. The reactor was made from Pyrex glass with a volume of 1000 mL (inside diameter 12 cm, height 15 cm). A mercury lamp of 175 W (Beijing Electric Light Sources Research Institute, Beijing, China) equipped with a quartz tube was located at the center of the reactor vertically and immersed in the dye solution. The UV lamp was attached to a power supply (Philips HID, Beijing, China) and the irradiation intensity (at 365 nm) was approximately 24 mW/cm2 on the outer surface of the quartz tube, as measured by a UV radiometer (UV-A, Photoelectric Instrument Factory of Beijing Normal University, Beijing, China). The output of the mercury lamp peaks at 365 nm and is significant in the range 250-450 nm. We used an O3 supply system (CHYF-3A, Hangzhou Rongxin Electronic Equipment Co. Ltd., Hangzhou, China) to generate O3 with dried pure oxygen. The ozone diffuser, placed at the bottom of the reactor, was a cylindrical shaped unit with coarse porosity, and the flow rate was set with a rotameter incorporated into the ozone generator. Surplus ozone was trapped by gas absorption bottles containing 2% KI solution. A port at the top of the reactor was used for measuring temperature and withdrawing samples. During operation, the solution was vigorously mixed with a magnetic stirrer. A thermostat (THD-2015, Tianheng Instrument Factory, Ningbo, China) was used to cool the water-jacketed reactor to maintain a constant solution temperature. The batch experiments were carried out with 1000 mL of RY145 solutions prepared in appropriate concentrations and with various pH values. The initial solution pH was adjusted to different values (4.0, 6.0, 8.0, and 11.0) by adding an appropriate amount of phosphate buffer. Phosphate buffers were prepared in deionized water by mixing calculated amounts of sodium hydroxide solution and phosphoric acid solution to yield an ionic strength of 0.0667 mol/L. Measurements taken before and after the experiments showed that the solution pH remained almost constant. Initial dye concentrations of 50, 100, 300, and 500 mg/L were selected to investigate the effect of the initial concentration on the mineralization of RY145. We used different ozone feeds (0.45, 1.8, 3.3, and 4.8 g/h) and various bulk temperatures (20, 25, 30, and 35 °C) as well. Samples were withdrawn at predetermined timed intervals, diluted at specific times with distilled water, and then analyzed for TOC, organic intermediates, and inorganic ions. The mineralization experiments were repeated three times, and error bars represent the standard deviation of the mean. Instrumental Analysis. The solution pH was measured with a pHs-25 instrument (Rex Analytical Instrument Co. Ltd., Shanghai, China). The ozone concentration was determined by an iodometric method. The TOC was determined by use of a TOC-VCPH total organic carbon analyzer (Shimadzu, Kyoto, Japan). The concentration of ammonium ions was measured by the ammonia-Nessler’s reagent colorimetric method by use of a UV-vis spectrophotometer (T6, Beijing Purkinje General Instrument Co. Ltd., Beijing, China) equipped with a photoelectric detector at 420 nm.20 Carboxylic acids and inorganic anions were identified with an ion chromatograph (IC). A Dionex model ICS 2000 ion chromatograph equipped with a dual-piston (in series) pump, a Dionex IonPac AS19 analytical column (4 mm × 250 mm), and an IonPac AG19 guard column (4 mm × 250 mm) was

Figure 1. Comparison of UV, O3, and UV/O3 for TOC removal: pH 8.0; bulk temperature 30 °C; ozone feed 4.8 g/h; C0 500 mg/L; UV power 175 W.

used throughout. Detection was performed with a Dionex DS6 conductivity detector. Suppression of the eluent was achieved with a Dionex anion ASRS electrolytic suppressor (4 mm) in the autosuppression external water mode. To detect the intermediate products of RY145 mineralization, we used gas chromatography/mass spectrometry (GC/MS; Varian cp3800 gas chromatograph and Varian Saturn 2000 mass spectrometer). A WCOT fused silica series column (30 m × 0.25 mm, 0.25 µm film thickness) was employed and the temperature was programmed at 80 °C for 1 min, ramped up to 250 °C at a rate of 12 °C min-1, and held at 250 °C for 5 min. The other experimental conditions were as follows: electron impact ionization 70 eV; helium as the carrier gas; injection temperature 280 °C; and source temperature 100 °C. Results and Discussion Enhanced Mineralization. We measured the mineralization of 500 mg/L RY145 at a pH of 8.0, an ozone feed of 4.8 g/h, and a bulk temperature of 30 °C via different treatment processes: (1) UV photolysis, (2) ozonation, and (3) the combined UV/O3 system. Figure 1 shows the corresponding reduction in TOC, in which TOC0 is defined as the initial TOC of the RY145 aqueous solution before experiments and the reduction of TOC/TOC0 indicates the degree of mineralization. Direct UV photolysis of RY145 was less effective than the ozonation and UV/O3 processes; that the change in TOC for UV photolysis alone was negligible might be because UV irradiation is highly absorbed by dye and might not be powerful enough to induce the generation of •OH.21 The total mineralization at 150 min in the UV, O3, and UV/ O3 schemes was 4%, 63%, and 80%, respectively. Consequently, the combination of UV and ozonation resulted in an enhanced increase in the overall rate of mineralization. Generally, oxidation of organic compounds by ozonation takes place in aqueous solution by molecular ozone and/or by •OH.22 When UV irradiation is performed during the ozonation process, three components produce •OH and oxidize organic compounds for subsequent reactions: UV irradiation, ozone, and hydrogen peroxide.23 Thus, the UV/O3 system generates additional •OH (eqs 1-4), which consequently enhances the ozonation process. Effect of pH. Solution pH is an important parameter in the UV/O3 process, so first we determined the optimal pH for TOC removal. Due to the generation of free radicals, an increased ozonation rate usually appears at elevated pH levels.24 However,

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Figure 2. Effect of process variables on the degradation of RY 145. (a) Effect of initial pH: bulk temperature 30 °C; ozone feed 4.8 g/h; C0 500 mg/L; UV power 175 W. (b) Effect of bulk temperature: pH 8.0; ozone feed 4.8 g/h; C0 500 mg/L; UV power 175 W. (c) Effect of ozone feed: pH 8.0; bulk temperature 30 °C; C0 500 mg/L; UV power 175 W. (d) Effect of initial concentration: pH 8.0; bulk temperature 30 °C; ozone feed 4.8 g/h; UV power 175 W.

the mineralization rate of RY145 did not exactly follow this expected trend. As shown in Figure 2a, the mineralization efficiency increased with the pH increase from 4.0 to 8.0, then decreased substantially when the solution pH increased to 11.0. This phenomenon might be explained by the dissociation of •OH into the oxygen anion radical and scavenging of •OH by HCO3-/CO32-, which are present in the reaction solution at alkaline pH.25,26 Moreover, phosphate that was used to adjust the initial pH in our experiments occurs in different species depending on the pH of the solution, and the reaction rate of •OH with various phosphate species would be lowest in the pH range 2.8-4.5.27 Therefore, the optimal pH for mineralization of RY145 was 8.0. Effect of Bulk Temperature. Oxidation is a temperaturedependent process, and the Henry’s law constants at different temperatures will also affect the gas/liquid equilibrium.28 The effect of bulk temperature on the mineralization of RY145 by UV/O3 treatment was investigated at four different temperatures (Figure 2b). TOC removal clearly was temperature-dependent. The mineralization of RY145 increased as the temperature increased from 20 to 30 °C and then gradually dropped when the temperature reached 35 °C. Both negative and positive effects on TOC removal rate occur with respect to temperature change. At higher temperatures, reactions of the hydroxyl radicals and other oxidative species in the bulk solution are facilitated by the increasing temperature due to the higher mass transfer of different species, and this leads to an enhancement of the reaction rate of the radicals with pollutant. However, high temperature also decreases the partial pressure of dissolved ozone in aqueous condition.29 In other words, when temperature increases, the solubility of ozone decreases. From the data obtained, it seems that in the bulk temperature range 20-30 °C, the mineralization rate increases caused by the temperature increase could compensate for the rate decrease caused by lower

ozone solubility, and hence the optimal bulk temperature is 30 °C for the UV/O3 system. Effect of Ozone Feed. Ozone feed is one of the most important factors that determines the efficacy of the UV/O3 process. In this experiment, ozone feed was varied to study the effect on TOC removal. Obviously, increasing the ozone feed can raise the TOC removal efficiency at a given time (Figure 2c). This finding indicates that dye degradation depends on the transfer of ozone into the reaction solution, which affects the dissolution of ozone and determines the amount of •OH generation. It also indicates that the reaction is mass-transferlimited in the range investigated. In our experiments, an increase in O3 feed improved the mass transfer of O3 by applying constant UV energy density to the ozonation system. Although other studies have confirmed a continuous increase in the rates of degradation with an increase in the ozone feed, the effect cannot be extrapolated to extremely high loadings of ozone.29 Therefore, it was not necessary to try to increase the concentration of O3 to a maximum value when the corresponding increase of the energy consumption and the amount of exhaust O3 gas was considered. Effect of Initial Dye Concentration. Figure 2d shows the time course of RY145 mineralization by the UV/O3 process for different initial dye concentrations. These data show that higher mineralization efficiencies can be reached with lower initial dye concentrations at a given time. For photochemical reactions, a high concentration causes high light absorption, and a significant quantity of UV light may be absorbed by the dye molecules rather than by the O3 molecules, thus reducing the TOC removal efficiency; in this case, the dye has a UV-screening effect.30 Hence, the RY145 and intermediates competing with O3 for UV light reduces the formation of •OH radicals. In addition, more free radical scavengers, such as CO32- and SO42-, might be generated when the initial dye concentration is higher. The

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Figure 3. Effect of light intensity and gas flow rate on the mineralization of RY 145. (Trace a) pH 8.0; bulk temperature 30 °C; ozone feed 4.8 g/h; C0 500 mg/L; UV power 175 W; gas flow rate 60 L/h. (Trace b) pH 8.0; bulk temperature 30 °C; ozone feed 4.8 g/ h; C0 500 mg/L; UV power 175 W; gas flow rate 120 L/h. (Trace c) pH 8.0; bulk temperature 30 °C; ozone feed 4.8 g/h; C0 500 mg/L; UV power 250 W; gas flow rate 60 L/h.

competition by free radical scavengers for •OH would become intense because of the nonselective reactivity of •OH. Accordingly, the mineralization rate decreases when the initial concentration of RY145 increases. Preliminary Analysis of Process Dynamics. UV/O3 degradation is a dynamic process due to the interrelated processes that occur simultaneously during UV/O3 treatment. The efficiency of UV/O3 oxidation depends on the mass transfer rate of ozone from the gas phase to the liquid phase, the rate of ozone decomposition, and the rate of the reaction between ozone and contaminants. According to the film theory, only the resistance to ozone transfer in the liquid phase needs to be considered, as ozone is sparingly soluble in water.31 The rate of mass transfer can be increased by thinning the liquid film and/or increasing the contact surface areas, which can be accomplished by increasing the flow rate of the feed gas.32 In our experiments, we increased the gas flow rate by use of N2 (purity 99.99%) as the carrier gas. When the flow rate was increased from 60 to 120 L/h, the TOC removal efficiency increased from 80% to 84% after 150 min of reaction, regardless of the decreased concentration of ozone in the feed gas, indicating that the effect of ozone mass transfer on TOC reduction could not be neglected (Figure 3). If it is assumed that mass transfer is the unique rate-determining step, there is almost no transfer of molecular ozone into the water.33-35 However, a higher level of TOC removal efficiency was observed when the light intensity was increased from 175 to 250 W (Figure 3). This is because the UV/O3 process was not controlled only by ozone mass transfer in our experiments. In other words, the TOC reduction may be controlled by both chemical reaction and mass transfer when the initial concentration of the dye was 500 mg/L. In addition, the results given in Figure 2d show that the rate of TOC reduction was very slow at very low concentrations of TOC (approximately 2 mg/L), which can be attributed to the rate-determining chemical kinetics. As the UV/O3 treatment proceeded, UV/O3 oxidation was controlled at first by both mass transfer and the chemical reaction. The process was controlled only by chemical kinetics when the concentration of TOC was less than 2 mg/L. Further work is needed for accurate determination of the mass transfer

Figure 4. Variation of the concentration of related ions with time: pH 8.0; bulk temperature 30 °C; ozone feed 4.8 g/h; C0 500 mg/L; UV power 175 W.

Figure 5. Probable degradation pathway of RY145: pH 8.0; bulk temperature 30 °C; ozone feed 4.8 g/h; C0 500 mg/L; UV power 175 W.

regime and the kinetics regime based on the Hatta number and the enhancement factor.36 Variation in the Concentrations of Related Ions. Figure 4 shows the concentration variations of ions (oxalate, acetate, formate, NO3-, NO2-, SO42-, Cl-, and NH4+) detected during the mineralization of RY145 with UV/O3 treatment. The concentration of SO42- increased during the treatment process. After a reaction time of 150 min, at least 93% of the element S in RY145 was transformed to SO42-. Therefore, most of the sulfur existed in the form of SO42- in aqueous solution after UV/O3 treatment. The concentration of Cl- analyzed by IC was 65% higher than the maximum theoretical value, which may be due to chloric impurities in RY145 dye. Approximately 24% of the measured TOC was acetate, which accounted for the majority of the residual TOC in solution, and 9% and 14% of the TOC was formate and oxalate, respectively. The remainder of the TOC after partial mineralization might consist of butene diacid and other undetected byproducts. In our study, the concentration of oxalate first increased and then decreased gradually during the degradation process. Previous

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Table 1. Intermediate Compounds Identified by GC/MSa

a

C0 500 mg/L; ozone feed 4.8 g/h; pH 8.0; bulk temperature 30 °C; UV power 175 W.

research has shown that oxalate is a relatively stable product that is not efficiently oxidized via ozonation alone.37 In comparison, the UV/O3 system is more effective than ozonation alone during the later stages of oxalate oxidization and the TOC degradation. Therefore, we recommend the combination of ozonation and UV radiation for deep mineralization of RY145 to CO2 and H2O. We also detected nitrite ions, nitrate ions, and ammonium as degradation products. NO2- was detected in the initial stages and then disappeared after 90 min, possibly because the nitrite ions were oxidized to become nitrate ions. In addition, the total nitrogen content of NH4+ and NO3- was lower than that of the initial RY145 in the solution. This might be because some of the nitrogen was degassed in the form of NH3 at high pHs or transformed to N2, NO, and NO2, and some might exist in undetected byproducts.38-40 A Possible Reaction Mechanism. To analyze the reaction mechanism of RY145, we identified intermediate compounds by GC/MS (Table 1). In Figure 5, we propose a possible reaction mechanism based on the GC/MS and IC data. The C(1)-S, C(3)-N(7), N(7)-C(8), C(10)-N(14), N(14)-C(15), C(18)N(21), N(22)-C(23), C(26)-S, C(27)-S, and C(32)-S bonds are cleaved easily by UV/O3 oxidation. After rearrangement, rings A and C could be transformed into aniline (D1). Because urea was not detected by GC/MS and because the UV/O3 system has strong oxidation characteristics, the intermediate formed by the cleavage of ring B could be oxidized directly to form NH4+ or NO3-. After the cleavage of the naphthalene ring, ring D could open to form phthalic acid (D5), which can be further oxidized to yield butene diacid (D7) by O3 and/or •OH. NH3 was detected in aqueous solution by the ammonia-Nessler’s reagent colorimetric method. NH3 could be initiated by UV photolysis to generate the amido radical (‚NH2), which can react with aniline (D1) via substitution to yield cyclohexa-2,5-diene1,4-diimine (D2). In the UV/O3 system, compound D2 could be oxidized to 3,6-dinitrosocyclohexa-1,4-diene (D4), which could be further transformed into butene diacid (D7) by ring cleavage. Similarly, under the oxidation of free radicals such as •OH, aniline (D1) was transformed to phenol (D3), which was oxidized to form benzo-1,4-quinone (D6), and the aromatic ring in D6

was then cleaved to form butene diacid (D7). Butene diacid (D7) was further oxidized into smaller molecular organic acids, such as oxalic acid (D8) and acetic acid (D9). Conclusion This study shows that ozonation in combination with UV irradiation has an enhanced effect on the mineralization of RY145 in aqueous solution compared to their separate effects. Increasing ozone feed had a positive effect on the mineralization of RY145, but the mineralization rate decreased with an increase of the initial dye concentration. The optimal pH was slightly alkaline (about 8.0), and both lower and higher pH decreased the mineralization rate. The preferred bulk temperature was 30 °C based on the influence of temperature on reaction rate and ozone solubility. The UV-enhanced ozonation in this study was controlled by both mass transfer and chemical reaction until the concentration of TOC was less than 2 mg/L. RY145 mainly converted to aniline, cyclohexa-2,5-diene-1,4-diimine, phenol, 3,6-dinitrosocyclohexa-1,4-diene, phthalic acid, benzo-1,4quinone, and inorganic anions (e.g., Cl-, SO42-, NH4+). The organic intermediates then degraded into smaller molecular organic acids such as oxalic acid, acetic acid, and formic acid, which finally converted into CO2 and H2O. Mineralization results suggest that the UV/O3 system is an effective technology for the degradation of RY145 under appropriate reaction conditions. The clear synergy achieved by the use of combined techniques makes the UV/O3 system worthy of further application in the degradation of other organic contaminants in wastewater. Acknowledgment We are grateful for the financial support provided by the National High Technology Research and Development program of China (Grant 2007AA06Z328) and the Science and Technology Project of Zhejiang Province, People’s Republic of China (Grant 2007C23054). Literature Cited (1) Chu, W.; Ma, C. W. Reaction Kinetics of UV-decolourization for Dye Materials. Chemosphere 1998, 37, 961.

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ReceiVed for reView August 27, 2007 ReVised manuscript receiVed December 11, 2007 Accepted December 24, 2007 IE0711628