Journal of Water Process Engineering 7 (2015) 314–327
Contents lists available at ScienceDirect
Journal of Water Process E...
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high power densities in the processing...
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Fig. 1. Characteristics of RY 145 dye....
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Fig. 3. Kinetic study of the degradati...
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Table 2
Effect of operating volume on ...
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Fig. 5. Effect of the reaction volume ...
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Fig. 7. Effect of operating pH on the ...
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Table 5
The extent of RY 145 degradati...
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Fig. 9. Effect of conventional heating...
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Fig. 10. Comparative Ea plots between ...
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Fig. 12. FTIR spectra of RY 145 and RY...
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∴ Calorimetric energy efficiency = 270....
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D.2. Frequency factor calculations for...
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peroxide, J. Hazard. Mater. 179 (2010)...
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  • 1. Journal of Water Process Engineering 7 (2015) 314–327 Contents lists available at ScienceDirect Journal of Water Process Engineering journal homepage: Degradation of Reactive Yellow 145 dye by persulfate using microwave and conventional heating Namata N. Patil, Sanjeev R. Shukla∗ Department of Fibres and Textile Processing Technology, Institute of Chemical Technology (University under the Section-3 of UGC act 1956) Nathalal Parekh Marg, Matunga, Mumbai 400019, India a r t i c l e i n f o Article history: Received 3 May 2015 Received in revised form 4 August 2015 Accepted 12 August 2015 Keywords: Reactive Yellow 145 Degradation Potassium persulfate Microwave heating Conventional heating a b s t r a c t Degradation of C. I. Reactive Yellow (RY 145), a bi-functional dye containing azo chromophore has been attempted in the presence of potassium persulfate (KPS) as an initiator in the presence of microwave irradiation. The conditions of degradation were optimized and its kinetics was studied. Complete dye degradation was observed in 3600 and 280 s, respectively, using conventional and microwave heating at the original pH (5.8) of the dye solution with COD removal of 60 and 66.5%. Under optimized conditions, efficiency of H2O2 was also compared for degrading the dye. Pseudo-first-order kinetics with higher kinetic rate constant (0.0165 × 102 s−1 ) and maximum energy efficiency (67.6%) was achieved through microwave assisted KPS system. Additional benefit of microwave approach was accrued by the extensive reduction in electrical energy consumption by 10638.32 kW h/m3 . The effect of addition of SO4 2− and CO3 2− anions to the dye solution has indicated that the former enhances the degradation, whereas the latter reduces it. The temperature study using Arrhenius equation indicated decrease in the activation energy for the microwave system along with 2.6 times higher frequency factor than that for conventional heating. Thus, the microwave radiation assisted KPS system has been found to be the most cost effective and environmentally benign for the dye degradation process. © 2015 Published by Elsevier Ltd. 1. Introduction Out of about 0.7 million tons of worldwide consumption of dyes, the azo chromophore based dyes represent more than half the share in textile coloration. However, up to 30% of theses dyes remain unfixed, washed away and therefore discharged into wastewater [1]. The presence of even very small amounts of dyes in water cre- ates aesthetic problems, hinders the photosynthesis, badly affects the aquatic life thereby posing significant risk to the food chain [2,3]. Hence, the remediation of dye wastewater has been consid- ered to be one of the global aim in controlling the environmental pollution [4]. Well known physico-chemical methods and the bio- logical treatments suffer from inherent disadvantages related to secondary pollution, poor efficiency and/or longer reaction time [5,6]. These limitations are overcome by advanced oxidation pro- Abbreviations: RY 145, C. I. Reactive Yellow 145; KPS, potassium persulfate; MW, Microwave; k, kinetic rate constant; R2 , correlation coefficient; EE0, electrical energy per order; COD, chemical oxygen demand; Ea, activation energy. ∗ Corresponding author. Fax: +91 22 33611020. E-mail addresses: (N.N. Patil), (S.R. Shukla). cesses (AOPs), which are based on non-selective and powerful oxidising species [7,8] including sulfate radicals (SO4 •−) [9,10]. Several reports have mentioned the activation of persulfate ion (S2O8 2−) through various heating sources for SO4 •− generation. The previously reported work enlists the persulfate activation by con- ventional heating [11,12], by using UV-radiations [13–15], electron beam [16] and ultrasonication [17]. Kusic et al. [18] have applied transition metal catalysis for persulfate activation in which low valent metal ions act as electron donors. Yang et al. [10] achieved the microwave assisted decolorization of an azo chromophore based Acid Orange 7 dye within 5–7 min at 800 W power using potassium persulfate (KPS). The parent ion S2O8 2− has several advantages like non- selectivity in attack, relatively stable nature, cheap [19] and easy availability in the form of ammonium, sodium, or potassium salt. Under neutral pH, KPS gives better results in photo-oxidative removal of some organic materials than corresponding ammonium salts [20]. Microwave (MW) irradiation is a well-known approach for the process intensification via selective absorption of the ther- mal energy by polar molecules resulting in uniform heating of the reaction liquid. The important advantages of microwave based pro- cessing over the conventional systems are the development of very 2214-7144/© 2015 Published by Elsevier Ltd.
  • 2. N.N. Patil, S.R. Shukla / Journal of Water Process Engineering 7 (2015) 314–327 315 high power densities in the processing zone, less reaction time, less energy consumption, homogeneous and selective heating as well as cleaner and greener process [21]. Reactive Yellow 145 (RY 145), an anionic hetero bi-functional reactive dye containing azo chromophore, is commonly used in dyeing of cotton, rayon, polyester blended fabric, printing and Moroccan tannery factories [22]. It reacts with the cellulosic hydroxyl group in dual way either by nucleophilic addition to an activated double bond (sulphatoethylsulphone group) or by nucle- ophilic substitution of reactive chlorine atom (monochlorotriazine group) [23]. The presence of these two reactive species enhance the light fastness property [24]. However, one should not neglect their mutagenic and carcinogenic effects on human and aquatic life [25]. Therefore, this dye has been selected in present study as a rep- resentative refractory pollutant due to its fairly high consumption (approx. 680 metric tonnes in India) for dyeing. Some researchers have reported its decolorization by biological treatment (Enterococ- cus faecalis strain YZ66) [26] and also its adsorption onto wheat bran [27]. There are also reports on the decolorization of this dye, which are based on the multiple combinations of AOPs such as ozona- tion and UV-C/H2O2 treatment [28], ozonation involving granular activated carbon [23], photocatalytic systems with TiO2 coated on synthetic non-woven fibres [29], UV irradiation coupled with per- sulfate [30] and UV/TiO2 followed by biological treatment (using Pseudomonas fluorescens) [22]. However, there are no reports on use of KPS in the presence of microwave irradiation for degradation of this dye. The present work, therefore investigates the efficiency of microwave irradiation for the degradation of Reactive Yellow 145 using KPS. The results have been compared with those obtained by conventional heating. The kinetics and the effect of presence of Na2SO4 and Na2CO3, which are essentially present in reactive dye bath has been investigated. Further, the power dissipation, percent energy efficiency and electrical energy consumption have also been analyzed. A temperature sensitive Arrhenius equation has been studied in details that yielded activation energy and frequency factor. 2. Materials and methods 2.1. Chemicals Potassium persulfate (K2S2O8), Na2SO4, Na2CO3, HCl, H2O2 (50% v/v) etc. were of analytical reagent grade obtained from S.D. Fine Chem., Pvt., Ltd., Mumbai, India. The commercial dye RY 145 (Fig. 1) was received from Atul Ltd., India and was used without any further purification. HCl and Na2CO3 were used for adjustment of pH. All the chemicals were used as received from the supplier and their solutions were prepared in distilled water. 2.2. Microwave setup Experiments under microwave were performed in a stoppered Erlenmeyer flask with a capacity of 200 mL by modifying a domestic microwave oven (Morphy Richards, Model-MWO 20 MBG) with rated output power of 800 W as a source of microwave irradiation which has been described elsewhere [31]. The setup was fitted with a programmer for temperature setting, time and rate of heating and a Teflon stirrer driven by a motor so as to ensure slow agitation of the dye solution during the microwave heating. The power function could be varied up to 100% and the experiments were conducted at 50% power output i.e., 400 W. The solutions were exposed to microwave irradiation in a batch process till 280 s. The calorimetric measurement was carried out to calculate actual power dissipation in the microwave system and was observed to be 270.4 W for output power of 400 W, which indicates an energy efficiency of around 67.6% (Appendix A). 2.3. Experimental methodology For all the experiments, the reaction volume (50 mL) and the dye concentration (50 mg/L) were kept constant, except for studies on operating volume and dye concentration. KPS was dissolved in dye solution and exposed to microwave irradiation. The flask was placed at the centre of the rotating plate directly below the mag- netron source. The effect of temperature by conventional heating was also investigated and compared by maintaining the constant temperature of dye solution at 40, 50, 70 and 90 ◦C, using a ther- mostatically controlled water bath. After each experiment, 5 mL solution was withdrawn, cooled immediately to 25 ◦C and analyzed for the extent of decolorization using UV–vis spectrophotometer. 2.4. Analytical procedures The extent of degradation of dye as a function of microwave irra- diation time was analyzed using UV–vis Spectrophotometer (Model 8500, TECHCOMP, Hong Kong). The absorbance at = 419 nm was measured and the dye degradation was calculated using the fol- lowing equation (Eq. (1)): Degradation (%) = (Ai − At) (Ai) × 100 (1) where Ai and At are the absorbance values before and after the treatment. The samples were also analyzed for COD removal by using closed reflux micro method and measured on Hach Colorimeter (model DR/850) at a filter value 610 nm [15] and percentage COD removal was calculated using the following equation (Eq. (2)): COD removal (%) = (CODi − CODt) CODi × 100 (2) where CODi and CODt are the COD (mg/L) of the dye solution before and after the treatment. The changes in the functional groups were investigated using FTIR spectrum 2000 PerkinElmer spectrophotometer in the IR region of 750–4000 cm−1. The reproducibility of the results was checked by repeating all the experiments twice and observed to be reproducible within the limits of ±2%. Error bars have also been revealed to represent the variation which was within 2% of the reported average value. 2.5. Kinetics of RY 145 degradation process Kinetics of the degradation process was evaluated and has been observed to follow pseudo-first-order model (using Eq. (3)): ln At A0 = −kt (3) The reaction rate constant (k) was obtained by plotting the values of ln [At/A0] against time, using experimental data of dye degradation. 3. Results and discussion 3.1. Effect of microwave power dissipation and temperature study at each power The effect of power dissipation on the extent of RY 145 dye degradation has been studied and the corresponding temperature for each microwave power output at the end of 280 s has been depicted in Fig. 2. Nominal degradation (7.3%) at power dissipa- tion of 80 W has been observed due to the kinetically insignificant
  • 3. 316 N.N. Patil, S.R. Shukla / Journal of Water Process Engineering 7 (2015) 314–327 Fig. 1. Characteristics of RY 145 dye. Fig. 2. Effect of microwave power dissipation and temperature on the extent of dye degradation. thermal decomposition of KPS (3.7 mM) at temperatures lower than 50 ◦C. It has been noticed that the extent of degradation at power dissipation of 240 W at the end of 80 s reached 35%, which was less than half as against 85.4% at 400 W under otherwise similar condi- tions. This has been attributed to much insufficient production of the sulfate radicals (SO4 •−) at lower temperature (48 ◦C at 240 W) as compared to that at 68 ◦C and 400 W, which is actually essential for the radical formation. It can also be observed from Fig. 2 that the degradation of the dye increased with the time of microwave irradiation and also with relative increase in the temperature of microwave system as the power was raised from 80 to 800 W. This observation is in concurrence with several reports where efficiency of microwave in terms of pollutant degradation has been enhanced either by increasing the irradiation time [32,33] or by increasing the microwave power [34,35]. The observed results may be attributed to the fact that the higher microwave energy provokes electronic vibrations and raises the temperature of the system [34] thereby producing more SO4 •− radicals from the dissolved persul- fate ions. Fig. 3 explains the kinetic analysis of the degradation process (using Eq. (3)) and it has been observed that the kinetic rate constants obtained for varying power dissipation levels at 240, 400, 640 and 800 W were 0.008, 0.045, 0.049 and 0.054 s−1, respec- tively. Complete degradation of the dye was achieved within 5 min of the microwave irradiation for power dissipation of 400 W and above. The rate constant for dye degradation at 800 W (0.054 s−1) was only marginally higher as compared to that obtained at power dissipation of 640 W (0.049 s−1) and 400 W (0.045 s−1), indicating that 400 W power was the optimum. Jou [35] has reported similar mechanism for degradation of the pentachlorophenol solution using zero-valent Fe under microwave irradiation. It has been reported that the higher microwave power increases the reaction temperature leading to faster conversion of persulfate ions to sulphate radicals [10]. Thus, it is essential to opti- mize the threshold microwave power (operating temperature) and irradiation time for the effectiveness of the reaction so as to avoid unnecessarily excessive consumption of energy. 3.2. Effect of initial dye concentration The effect of RY 145 concentration in the range 50–200 mg/L on the extent of degradation has been studied at microwave output power of 400 W, at (original dye solution) pH 5.8 and reaction time up to 280 s keeping the reaction volume (50 mL) constant. These results are presented in Fig. 4. The extent of dye degradation gradually increased after about 120 s, whereas with increasing dye concentration it decreased.
  • 4. N.N. Patil, S.R. Shukla / Journal of Water Process Engineering 7 (2015) 314–327 317 Fig. 3. Kinetic study of the degradation process at different microwave power. Fig. 4. Effect of initial concentration of RY 145 on the extent of degradation. Table 1 Effect of initial dye concentration on the extent of degradation. Concentration (mg/L) Degradation (%) at 120 s Pseudo-first- order rate constant, k × 10−2 (s−1 ) R2 50 93.6 2.2 0.988 75 91.8 2.1 0.988 100 90.0 1.7 0.977 150 83.6 1.3 0.972 200 77.0 1.0 0.963 The pseudo-first-order reaction kinetics fitted well for this study. Table 1 indicates that with 120 s irradiation time, the extent of degradation at 50 mg/L dye concentration was 93.6%, which sig- nificantly reduced to 77% at 200 mg/L. The pseudo-first-order rate constant (k) also decreased to less than half in its value from 2.2 × 10−2 s−1 to 1 × 10−2 s−1. The reduced degradation at higher concentration of RY 145 may be due to the insufficiency of amount of KPS, which was kept constant at 3.7 mM. As a result, SO4 •− radicals generated in the solution were not sufficient enough for reaction with the dye pollutant. At the end of 280 s of microwave irradiation, the 50 mg/L dye solution showed 99.7% degradation and at 200 mg/L concentration it was little less as 94.6%. The reaction kinetics was observed to be faster at 50 mg/L. Since the highest rate constant for maximum degradation was at 50 mg/L and also the fact that the wastewater after the secondary biological treatment mostly contains less than 50 mg/L dye concentration, rest of the experiments were performed at 50 mg/L initial concentration of RY 145. Similar degradation
  • 5. 318 N.N. Patil, S.R. Shukla / Journal of Water Process Engineering 7 (2015) 314–327 Table 2 Effect of operating volume on the extent of degradation. Volume (mL) Degradation (%) at 120 s Pseudo-first- order rate constant, k × 10−2 (s−1 ) R2 50 94.3 2.1 0.989 75 60.6 0.9 0.981 100 28.7 0.4 0.982 trend was observed by Zhang et al. [36] for microwave assisted pho- tocatalytic degradation of Reactive Brilliant Red X-3B dye where the net degradation was found to increase till 60% with an increase in the initial concentration up to 1000 ppm and decreased thereafter. Dafale et al. [37] have observed that the first-order kinetic con- stant decreased from 2.277 × 10−1 to 0.906 × 10−1 h−1 as the dye concentration increased from 25 to 100 ppm, during the biological degradation of azo chromophore based Remazol Black-B dye. 3.3. Effect of operating volume of dye solution Studies related to operating volume were performed at constant initial concentration of 50 mg/L of RY 145 at original solution pH of 5.8 by varying the volume (50, 75 and 100 mL). The effect of the reaction volume on the extent of degradation has been depicted in Fig. 5, which indicates that the extent of percentage degrada- tion decreased with an increase in the reaction volume at fixed microwave power dissipation of 400 W. Table 2 gives the data related to kinetic rate constants for the dye degradation at various operating volumes. In 120 s of irra- diation time, the kinetic rate constant for 50 mL reaction volume was 2.1 × 10−2 s−1 (94.3% degradation), which was much higher as compared to that of 0.4 × 10−2 s−1 at 100 mL with 28.7% degrada- tion. These results indicate that the increase in reaction volume correspondingly decreases the power dissipation per unit volume, thereby causing decreased power density of the system. Remya and Lin [38] have mentioned the comparative decrease in the degrada- tion of ammonia as its concentration and volume was increased in microwave application. 3.4. Effect of KPS concentration Effect of KPS concentration (0.7–4.4 mM) on dye degradation at fixed power dissipation of 400 W and constant volume of 50 mL has been shown in Fig. 6. It may be observed that the dye degradation increased continuously from 0.7 to 3.7 mM KPS, the reason being that more sulfate and hydroxyl radicals will be available to attack the azo bond of the dye molecule thereby enhancing the degra- dation reaction. However, further increase in KPS concentration to 4.4 mM showed decreased dye degradation. S2O2− 8 + SO •− 4 → SO2− 4 + S2O •− 8 (4) This may be attributed to the presence of S2O8 2− ion, which itself is a scavenger of SO4 •− radicals as shown in Eq. (4), mainly under acidic conditions. Similar results were observed by Hori et al. [39] for decomposition of perfluorocarboxylic acids. Highly linear (R2 > 0.95) correlation coefficient values for all KPS concentrations, given in Table 3, confirm a pseudo-first-order kinetics pattern for this parameter. It may be observed from the calculated rate con- stants that the k value at KPS concentration of 2.2 mM was 4 times as high as that at 0.7 mM. Also, dye degradation at 120 s for 2.2 mM KPS was significant with respect to 1.5 mM, the maximum being 93.2% at 3.7 mM KPS concentration. Based on these findings, the KPS concentration was fixed at 2.2 mM. The experiments have shown that KPS alone could give only 12.3% dye degradation at a con- centration of 2.2 mM, whereas microwave alone had no effect at Table 3 Pseudo-first-order kinetics details for various KPS concentrations. KPS (mM) Degradation (%) at 120 s Pseudo-first- order rate constant, k × 10−2 (s−1 ) R2 0.7 43.6 0.4 0.964 1.5 46.3 0.6 0.965 2.2 81.8 1.6 0.976 3.0 91.2 1.9 0.978 3.7 93.2 2.3 0.996 4.4 80.9 1.3 0.990 all. Microwave irradiation is known to change the thermodynamic function and reduce the activation energy of the reaction system [38]. The temperature effect of microwave plays role in fast sul- fate radical generation in less reaction time. The reason behind this is microwave irradiation can produce efficient internal heating by direct coupling with the molecules of solvents via rapid rotation and selective thermal energy absorption of polar molecules in the reaction [40]. Only on combination of the two, dye degradation was achieved at 68 ◦C. 3.5. Synergistic index The synergistic index (f) of the combined microwave assisted KPS system was calculated using the Eq. (5), in order to estimate its effectiveness over the individual systems [41]. f = k(PS+MW) k(PS) + k(MW) = (1.6 × 10−2 ) (0.03 × 10−2 ) + (0.01 × 10−2 ) = 40 (5) where kPS and kMW are the kinetic rate constants in the presence of KPS (2.2 mM) and microwave for individual reactions, respectively. These values were obtained from the reaction kinetics. Synergistic index calculated for microwave process was found to be 40, which was 15 in case of conventional heating, hence both processes con- firmed the synergistic effect. Higher the value of the synergistic index, greater is the effectiveness of the assisted process. Signifi- cantly higher value of f in case of microwave process indicates the interdependence of KPS and microwave on each other, as enhanced formation of the SO4 •− radicals took place under the microwave activation. 3.6. Effect of operating pH on degradation as a function of relative concentration of persulfate ions and hydroxyl radicals The rate of degradation of organic pollutants is usually influ- enced by the pH of reaction. Microwave assisted degradation of RY 145 (50 mg/L) was investigated at different pH between 3 and 10.5 using 2.2 mM KPS under microwave irradiation of 280 s. The decolorized solutions were iodometrically titrated to a colorless endpoint so as to measure residual persulfate concentration [42] followed by a qualitative ‘starch iodide paper’ test to indirectly study OH radical generation at each pH. It may be observed from Fig. 7 that the degradation at 120 s of irradiation gradually increased from 71 to 89% as the pH of the solution decreased from 10.5 to 3, maximum being at pH 3. Table 4 gives the corresponding values of the rate constants and correlation coefficients at different initial pH. The kinetic rate constant increased from 1 × 10−2 to 2 × 10−2 s−1 with this decrease in pH. However, the pH 5 was chosen with little compromise and used in further experiments, since it was nearer to the original RY 145 dye solution (pH 5.8), and no addition of HCl required to adjust the pH. Iodometric titration resulted in 0.03, 0.05, 0.07, 0.10 and 0.13 mM residual persulfate for solution pH 3, 5, 7, 9 and 10.5, respectively. It could be seen in Fig. 8 that starch iodide
  • 6. N.N. Patil, S.R. Shukla / Journal of Water Process Engineering 7 (2015) 314–327 319 Fig. 5. Effect of the reaction volume on the extent of degradation. Fig. 6. Effect of initial KPS concentration on the extent of degradation. Table 4 Pseudo-first-order kinetics details for operating pH. pH Degradation (%) at 120 s Pseudo-first- order rate constant, k × 10−2 (s−1 ) R2 Change in pH after 280 s 3 89.0 2.0 0.985 2.1 5 85.7 1.7 0.980 2.2 7 82.8 1.5 0.980 2.4 9 79.1 1.0 0.975 5.4 10.5 71.0 1.0 0.980 9.6 paper turned darker (blackish blue) as the solution pH increased to 10.5. Enhanced degradation of RY 145 under acidic conditions could be due to improved generation of SO4 •− radicals that can also be correlated to above two tests i.e., the least residual persulfate ion concentration being 0.03 mM at pH 3 and no color change in starch iodide paper. This confirms almost complete conversion of persul- fate ion into SO4 •− radical [19], as shown in Eqs. (6) and (7). S2O2− 8 + H+ → HS2O− 8 (at acidic pH) (6) HS2O− 8 → SO •− 4 + SO2− 4 + H+ (7) Higher concentration of residual persulfate ions as the pH increases might possibly be due to scavenging action of OH− ions on SO4 •− radical resulting in the formation of OH• as a dominant radical species along with SO4 2− ions [43] as shown in Eq. (8) which could visibly be confirmed by turning of starch iodide paper more dark. SO •− 4 + OH− → SO2− 4 + OH • (at alkaline pH) (8) Based on the obtained persulfate ion and OH• concentrations, predominant radical species at different pH levels can be confirmed
  • 7. 320 N.N. Patil, S.R. Shukla / Journal of Water Process Engineering 7 (2015) 314–327 Fig. 7. Effect of operating pH on the extent of degradation. (i.e., SO4 •− at pH 3; SO4 •− and OH• at pH 7; OH• at pH 10.5). These observations are in agreement with the ESR results reported by Norman et al. [44] who stated that, as the pH increases, the conver- sion of SO4 •− to HO• becomes significant. As far as degradation is concerned, similar results were observed in earlier findings where decolorization efficiency of the dye Direct Red 16 was highest at its original solution pH of 6 as compared to alkaline pH [45]. Lee et al. [43] investigated effect of initial pH on the extent of perfluo- rooctanoic acid (PFOA) degradation using microwave-induced KPS activation over the pH range 2–11.5 and reported that the pseudo- first-order rate constant enhanced 7.4 times at pH 2 compared to pH 11.5. In 0.5 h of reaction time, the PFOA decomposition efficiency increased from 0% (pH 11.5) to 34.5% (pH 2). However, no significant difference in the decomposition efficiency was observed when the solution pH was between 3.6 (k = 0.33 h−1) and 8.8 (k = 0.32 h−1). In our study, initial pH value of RY 145 solution was 5.8 which remained unchanged after the addition of KPS. However, at the end of 280 s of microwave irradiation, the final pH value decreased to 2.5, possibly due to the protons released in the reaction when SO4 •− radicals react with water [46]. Hence KPS treated effluent (which is toward acidic range) shows probable commercial utilization in the neutralization of cotton textile industry wastewater, which is alka- line in nature. Similarly, optimum pH for specific pollutant under question can be established using laboratory scale characterization studies for its probable reuse. 3.7. Effect of carbonate and sulphate anions at various loadings The textile effluent generated from reactive dyeing is rich in inorganic salts like sodium sulphate (SO4 2− anion) and sodium car- bonate (CO3 2− anion). Sodium sulfate is used for enhanced pickup of dye on cotton, whereas sodium carbonate provides alkaline pH for covalent fixation of the dye on cotton [9]. There is no detailed study reported on addition of SO4 2− and CO3 2− anions to the dye solution over a broad range of their concentrations. These condi- tions were simulated in the present work by separately adding these salts over the concentration range of 1–100 mM/L each to the 50 mg/L dye solution. The data on dye degradation is given in Table 5 along with corresponding change in the pH for each case. Under optimized operating conditions, the control sample resulted Fig. 8. OH radical confirmation by starch iodide paper as a function of solution pH (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.). in 82.0% degradation after 120 s of microwave irradiation. In the case of SO4 2− anions the degradation enhanced to 88.1% at 1 mM/L but decreased to 83.5% with further increase in concentration to 100 mM/L. The pH decreased to 2.7 from initial 5.8. This could be due to the fact that the added SO4 2− anions are converted to stronger SO4 •− radicals either by abstraction of proton from the solution (as shown in Eq. (9)) or due to interaction of SO4 2− anions with hydroxyl radical (as shown in Eq. (10)). SO2− 4 + H+ → SO •− 4 (9) SO2− 4 + OH • → SO •− 4 + OH− (10) On the other hand, the presence of CO3 2− anions decreased slightly the extent of degradation from 78.5% (1 mM/L) to 76.1% (100 mM/L) and increased the pH of the dye solution. The hin- dered degradation can be explained in terms of the change in the solution pH as observed earlier for the effect of pH on dye degra- dation in section 3.6. The addition of Na2CO3 (100 mM/L) gave the solution pH 11.5. The solution will have CO3 2− anions at the solu- tion pH > 7.5, which could initiate decomposition of SO4 •− radicals
  • 8. N.N. Patil, S.R. Shukla / Journal of Water Process Engineering 7 (2015) 314–327 321 Table 5 The extent of RY 145 degradation in the presence of SO4 2− and CO3 2− anions. Solution Concentration (mM/L) Degradation (%) Initial pH Change in pH after 120 s of MW irradiation RY 145 – 82.0 5.8 3.1 SO4 2− ions 1 88.1 5.5 2.7 10 85.6 6.1 3.4 100 83.5 6.3 4.4 CO3 2− ions 1 78.5 9.6 8.4 10 77.2 11 10.7 100 76.1 11.7 11.6 Table 6 Comparison of dye degradation efficiencies of KPS and H2O2 under microwave. Time (s) Dye degradation (%) KPS H2O2 2.2 mM 150 mM 300 mM 0 12.3 0.0 0.0 40 20.1 0.4 1.6 80 55.6 1.2 2.5 120 81.8 1.8 3.8 160 91.3 2.2 4.8 200 94.7 2.7 5.9 240 96.9 3.3 6.7 280 99.1 3.8 8.0 thereby rapidly transforming them into weaker SO4 2− [36] anions as shown in the Eq. (11). SO •− 4 + CO2− 3 → SO2− 4 + CO •− 3 (11) Thus, as the solution pH approaches to 11.5, CO3 2− anions become more dominant and act as a scavenger for SO4 •− radicals hindering the dye degradation. Same was the observation by Zhang et al. [36] for the presence of CO3 2− anions, which caused a sig- nificant inhibition effect on photocatalytic degradation of the dye Reactive Brilliant Red X-3B followed by the kinetic k drop from 0.0203 to 0.0062 min−1. 3.8. Comparative study of microwave assisted H2O2 process 50 mL of 50 mg/L RY 145 dye concentration was treated with hydrogen peroxide (150 and 300 mM) under microwave power of 400 W for 280 s at original solution pH and the results are given in Table 6. Only 4 and 8% dye degradation was observed with these dosages of H2O2. Thus, peroxide was found inefficient in degrading RY 145. The probable reason for poor efficiency of peroxide has been explained by Saien et al. [30] by relating decomposition of peroxide at higher temperature simply into water and oxygen. Yang et al. [46] have also reported similar inactivity of peroxide in degrading Acid Orange 7 azo dye at higher temperature, which has been attributed to extremely large O O bond energy of H2O2 (213.3 kJ/mol) with respect to persulfate (140 kJ/mol). 3.9. Comparative performance of conventional and microwave heating for KPS activation Exact reasons behind microwave enhanced degradation of RY 145 among thermal and other non-thermal effects of microwave were the main cause of the debate. In order to investigate the same, the conventional heating was used for KPS activation. Heating the RY 145 dye solution alone (in absence of KPS) even up to 90 ◦C had no degradative effect indicating that the dye does not decompose on its own even at elevated temperature, which was the obser- vation with microwave system also. The optimized 2.2 mM KPS was dissolved in 50 mL of 50 mg/L aqueous solution of RY 145 and heated at constant temperature of 70 ◦C, on a conventional ther- mostatically controlled water bath for the period up to 60 min. The selection of temperature 70 ◦C was corresponding to the tempera- ture 68 ◦C achieved by the optimized microwave power of 400 W. The calorimetric measurement was carried out to calculate actual power dissipation in the conventional heating and was observed to be as low as 217.6 W for power of 1000 W, which indicates an energy efficiency of 21.8% (Appendix A). The extent of RY 145 degra- dation using conventional heating in the presence of 2.2 mM KPS is shown in Fig. 9 from which it may be observed that after 60 min (3600 s) reaction at 40 ◦C, the extent of degradation was as poor as 17.3%, which enhanced to 61.2% as the temperature was raised to 50 ◦C. The rate of RY 145 degradation was faster at 90 ◦C as com- pared to 70 ◦C, which can be correlated to the temperatures of the microwave system at the power output of 800 and 400 W, respec- tively. In terms of the reaction time, conventional system needed 50 min (at 70 ◦C) and 15 min (at 90 ◦C) to reach 90% degradation. The extent of COD removal for microwave system was 66.5%, which was relatively higher with respect to 60% COD removal for conventional heating process (at the end of 60 min for temperature of 70 ◦C). Since electric energy represents a major portion of the operating costs, simple figures-of-merit based on electric energy consump- tion could be very useful to compare electrical efficiency of the two processes. The appropriate figure-of-merit is the electrical energy per order (EE0), defined as the number of kW h of the electrical energy required for reducing the concentration of a pollutant by 1 order of magnitude in 1 m3 of the contaminated water [15]. EE0 (kW h/m3) can be calculated from the following equation (Eq. (12)): EEO = P × t × 1000 V × 60 × log (Ci/Cf ) (12) where P is the power (kW) used in the process, t is the irradi- ation time (min), V is the volume (L) of the RY 145 solution in the reactor along with Ci and Cf as the initial and final concentra- tions. The electric energy (EEO, kW h/m3) required for degradation of 50 mg/L of RY 145 in microwave and conventional processes at same (≈70 ◦C) temperature was 305.7 and 10989 kW h/m3, respec- tively (Appendix B). EEO can be correlated to the energy costs. Considering the cost of electricity and the time taken for the pro- cess, the microwave treatment will prove highly cost effective, which save up to 10683.32 kW h/m3 of electrical energy (Appendix B). The comparative analysis of the processes in terms of the first-order kinetic rate constant, percentage degradation and time required for degradation, presented in Table 7, indicates that microwave irradiation was much more efficient compared to the conventional heating.
  • 9. 322 N.N. Patil, S.R. Shukla / Journal of Water Process Engineering 7 (2015) 314–327 Fig. 9. Effect of conventional heating on the extent of dye degradation. Table 7 Kinetic rate constant, percentage degradation and time requirement for RY 145 degradation at 50 mg/L concentration. Process k × 10−2 (s−1 ) Degradation (%) Reaction time R2 Microwave 1.65 99.1 280 0.9769 Conventionalheating 0.3 98.5 3600 0.9921 3.10. Activation energy and frequency factor Activation energy (Ea) is an important parameter associated with the changes in kinetics and thermodynamics of the two heat- ing processes, microwave and conventional, (Arrhenius equation (Eq. (13)). Ea is the threshold energy that the reactant(s) must acquire before reaching the transition state. Ea values for both the processes were calculated by plotting ln (k) versus 1/T and the slope Ea/T was obtained by the linear regression. This ratio was multiplied by the universal gas constant (R = 8314 −1 kJ/mol−1) to obtain the Ea value. k = Aexp − Ea RT (13) The comparative Ea plots for microwave and conventional pro- cess have been given in Fig. 10. The high degree of linearity (R2 > 0.90) between the two variables yields reliable estimations of the activation energy (Ea). It is well known that a reaction with large activation energy needs much more energy to reach the tran- sition state. The obtained activation energy values were 41.4 and 44.4 kJ/mol for microwave and conventional process, respectively (Appendix C). It can be realized that the higher temperature was favorable to enhance the degradation and the activation energy was reduced by using the microwave irradiation. The microwave irradiation decreases the threshold energy needed for the oxidation of the pollutants but does not alter the nature of the reaction. The term, e−Ea/RT, in the Arrhenius equation is known as the frequency factor and it represents the molecules with energy equal to or greater than the activation energy. Fre- quency factor (4.5 × 10−7) for the microwave assisted degradation was calculated (Appendix D) and found to be 2.6 times higher than that for conventional heating (1.7 × 10−7). This confirms that the microwaves facilitate the reaction kinetics since nearly 2.6 times more number of molecules get involved in microwave process whose energy is equal to or greater than Ea. This proves that the non-thermal effects of microwave are responsible to increase the reactant mobility and diffusion exchange rate between the pollu- tant and sulfate radicals. This study demonstrated that microwave assisted system can significantly lower the energy barrier with 2.6 times increase in the reaction rate. The weakening of the chromophoric bond of the RY 145 dye by the combination of the polarization effect and the thermal effect (that enhanced production of SO4 •− radicals) improved the overall rate of degradation. The present work has, thus, established that microwave greatly raises the overall process efficiency as an alter- native energy source of KPS activation, which drastically shortened the reaction time (to 4 min) by almost 15 times. 3.11. Elucidation of probable dye degradation mechanism using FTIR analysis Probable degradation mechanism for RY 145 is given in Fig. 11. The dye RY 145 and its KPS-microwave assisted degradation prod- ucts were analyzed by FTIR (Fig. 12). It can be seen from Fig. 12 that the spectrum for RY 145 has a sharp characteristic peak at 1655 cm−1 confirming presence of N N chromophore [47], which completely disappeared in the spectrum of degraded sam- ple, confirming oxidative cleavage of the chromophoric group by attack of SO4 •− radicals. Similarly, the peak at 1256 cm−1 for C O stretching is absent in treated sample as a result of deprotona- tion of SO2CH2CH2OSO3Na group in the dye molecule. Besides, the dye spectrum shows a hump in the region of 3330–3550 cm−1 indicating the presence of primary, secondary and tertiary amines (aliphatic as well as aromatic) in the structure, which diminished in spectra of KPS-microwave treated dye indicating further degra- dation. The intensity of peak at 1050.62 cm−1, for meta/ortho substi- tuted aromatic SO3 bending vibration in the dye decreased after the degradation, indicating desulfonation of the dye by the attack of SO4 •− radicals [48]. The peak at 1496.99 cm−1 in the dye spec-
  • 10. N.N. Patil, S.R. Shukla / Journal of Water Process Engineering 7 (2015) 314–327 323 Fig. 10. Comparative Ea plots between microwave and conventional process. trum corresponds to conjugated cyclic system of C N for stretching vibration, which disappeared on treatment. This indicates attack on polar C N bonds linked to the benzene ring, confirming the cleav- age of recalcitrant triazine ring. Similarly, a peak between 720 and 600 cm−1 in the dye molecule is observed for the C S bond of sul- fonate group linked to the benzene ring that was absent in treated spectrum as a result of C S bond cleavage to form organic acids with or without hydroxyl group, sulfate, ammonium and nitrate ions [18]. Fig. 11. Probable degradation mechanism of RY 145 using FTIR.
  • 11. 324 N.N. Patil, S.R. Shukla / Journal of Water Process Engineering 7 (2015) 314–327 Fig. 12. FTIR spectra of RY 145 and RY 145 subjected to microwave degradation. According to the literature [49] and our study elaborated in section 3.6, decrease in pH of treated sample represents the forma- tion of acid as one of dye degradation by-product, which usually is further oxidized to carboxylic acids and CO2. The results clearly establish the fragmentation of the RY 145 dye into smaller compo- nents by KPS oxidation. 4. Conclusions Two different KPS activation approaches based on microwave and conventional heating have been observed to follow pseudo- first-order kinetics for degradation of azo dye RY 145. The microwave approach was much faster at 400 W (68 ◦C, 4 min) with respect to conventional heating (70 ◦C, 60 min), which has been attributed to additional non-thermal effects of microwave respon- sible for polarization of dye molecules that enhance the kinetics of the system by keeping the related thermodynamics unchanged. Significance of this non-thermal effect was further proved by high synergistic factor of 40 as against 15 for conventional heating. There was extensive reduction in activation energy of the system and the electrical energy consumption. Effect of addition of SO4 2− ions has enhanced the degradation whereas CO3 2− ions hindered the KPS activity. Probable degradation mechanism of RY 145 has been proposed which was confirmed by FTIR analysis and supported by enhanced COD removal. The present lab-scale study leads to a path- way for use of KPS activation by designing a pilot-scale reactor for degradation of various harmful pollutants. Acknowledgements Authors would like to express their gratitude to University Grants Commission, New Delhi, India, for funding of this project under Major Research Project grant. Namata Patil is also thankful for fellowship under UGC-SAP. Appendix A. Calorimetric energy efficiency calculations Actual power dissipation (W) = mCp dT/dt (A.1) where Cp is the heat capacity of the solvent (J kg−1 K−1), m is the mass of solvent (kg), dT is the temperature difference between the initial temperature and the final temperature after a specific reaction time (K), and dt is the time (s). Calorimetric energy efficiency can then be calculated as follows (using Eq. (A.2)): Calorimetric energy efficiency = Actual power dissipation (from Eq. (A.1)) Electric power supplied to the system (A.2) A1. Calorimetric energy efficiency calculations for microwave system Basis: 50 mL (0.050 L) of aqueous RY 145 solution. Power of microwave used = 400W Initial temperature of the system = 31◦ C Final temperature of the microwave system after240s = 68◦ C Temperature difference (dT) = (68 − 31) = 37◦ C = 310K dT/dt = 310/240 = 1.292 Actual power dissipation (W) = mCp dT/dt ∴ Actual power dissipation (W) = (0.050 × 4185 × 1.292) Actual power dissipation (W) = 270.35W
  • 12. N.N. Patil, S.R. Shukla / Journal of Water Process Engineering 7 (2015) 314–327 325 ∴ Calorimetric energy efficiency = 270.35/400 Calorimetric energy efficiency = 67.6%for microwave system A.2. Calorimetric energy efficiency calculations for conventional heating Basis: 50 mL (0.050 L) of aqueous RY 145 solution Power of conventional heating used = 1000W Initial temperature of the system = 31◦ C Final temperature of the microwave system after300s = 70◦ C Temperature difference(dT) = (70 − 31) = 39◦ C = 312K dT/dt = 312/300 = 1.04 Actual power dissipation (W) = mCp dT/dt ∴ Actual power dissipation (W) = (0.050 × 4185 × 1.04) Actual power dissipation (W) = 217.62W ∴ Calorimetric energy efficiency = 217.62/1000 Calorimetric energy efficiency = 21.8% for conventional heating Appendix B. Electrical energy per order (EEO) calculations Sample calculation for the electrical energy per order (EEO), for complete degradation using microwave system is as follows (using Eq. (A.3)): EE0 = P × t × 1000 V × 60 × log(Ci/Cf) (A.3) B.1. EEO calculations for microwave system Basis: 50 mL (0.050 L) of aqueous RY 145 solution Power of microwave used = 400W = 0.4kW Time requuired for degradation = 4.7 min = 280s Initial concentration ofRY145 = Ci = 50mg/L Final concentration ofRY145 = Cf = 0.45mg/L ∴ log(50/0.45) = 2.05 EE0 = 0.4 × 4.7 × 1000 0.050 × 60 × 2.05 EE0 = 305.69kW h/m3 ) = (A) Similar EEO calculation has been performed for the conventional heating. B.2. Electrical energy calculations for conventional heating Basis: 50 mL (0.050 L) of aqueous RY 145 solution Power of water bath used = 500W = 0.5kW Time required for degradation = 600 min = 3600s Initial concentration of RY145 = Ci = 50mg/L Final concentration of RY145 = Cf = 0.75mg/L ∴ log(50/0.75) = 1.82 EE0 = 1 × 60 × 1000 0.050 × 60 × 1.82 EE0 = 10989.01(kW h/m3 ) = (B) B.3. Net electrical energy saved (using Eq. (A.4)) Net electrical energy saved = (A) − (B) (A.4) where A corresponds to Electrical energy needed for conven- tional heating.where B corresponds to Electrical energy needed for microwave system. Net electrical energy saved = (10989.01 − 305.69)kW h/m3 Net electrical energy saved = 10683.32kW h/m3 Appendix C. Activation energy calculations Activation energy can be calculated using Eq. (A.5). The slope values for calculating C.1 and C.2 are applied from Fig. 9 in the manuscript. Activation energy = slope × universal gas constant(R) = slope × 8314kJ−1 mol−1 (A.5) C.1. Activation energy calculations for microwave system Activation energy = 4983 × 8314 ∴ Activation energy of the microwaves ystem = 41400J/mol Appendix D. Frequency factor calculations Frequency factor: it can be calculated using the equation given below (Eq. (A.6)): k = Ae− Ea RT (A.6) D.1. Frequency factor calculations for microwave system Activation energy for microwave system = 41.400 J/mol Temperature at 400 W power at the end of 280 s = 68 ◦C = 341 K e− Ea RT = e − 41400 (8.31×341) ∴ Frequency factor for microwave system = 4.5 × 10−7
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