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Degradation of acid red 88 by the combination of sonolysis and photocatalysis
Separation and Purification Technology 74 (2010) 336–341
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Separation and Purification Technology journal homepage: www.elsevier.com/locate/seppur
Degradation of acid red 88 by the combination of sonolysis and photocatalysis Jagannathan Madhavan a , Panneer Selvam Sathish Kumar a,b , Sambandam Anandan b , Franz Grieser a , Muthupandian Ashokkumar a,∗ a b
Particulate Fluids Processing Centre, School of Chemistry, University of Melbourne, Vic 3010, Australia Nanomaterials & Solar Energy Conversion Lab, Department of Chemistry, National Institute of Technology, Trichy, Tamil Nadu 620015, India
a r t i c l e
i n f o
Article history: Received 26 February 2010 Received in revised form 1 July 2010 Accepted 1 July 2010 Keywords: Sonolysis Photocatalysis Sono-Fenton Sonophotocatalysis Acid red 88
a b s t r a c t Acid red 88 (AR88) is a mono-azo textile dye and a widely used colorant in the textile and food industries. The sonolytic, photocatalytic and sonophotocatalytic degradation of AR88 in the presence of homogeneous (Fe3+ ) and heterogeneous (TiO2 ) photocatalysts were studied. The effects of initial dye concentration and ultrasound power on the sonochemical degradation were also investigated. The degradation by sonolysis and photocatalysis followed first-order like kinetics with respect to the concentration of AR88. The sonophotocatalytic degradation rates using TiO2 or Fe3+ were found to be higher than that observed with sonolysis or photocatalysis. The synergy index calculated for TiO2 sonophotocatalysis was 1.3, suggesting that the combination of sonolysis and TiO2 photocatalysis resulted in a small synergetic effect. Also, the sonophotocatalysis in the presence of Fe3+ was synergistic with a synergy index of 2.3. Total organic carbon (TOC) analysis was performed to study the extent of mineralization. TOC data showed that the mineralization using TiO2 sonophotocatalysis was additive whereas a synergistic mineralization was observed for the sonophotocatalysis in the presence of Fe3+ . The simultaneous operation of sonoFenton and photo-Fenton reactions is likely to be the underlying reason for the observed synergy in the presence of Fe3+ assisted sonophotocatalysis. Electrospray mass spectrometry (ESMS) was employed for the identification of the degradation intermediates. The sonication of AR88 led to the formation of its mono and di-hydroxylated products as the primary intermediates. A possible degradation pathway has been proposed. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Azo dyes are among the notorious environmental pollutants associated with textile, food, cosmetic, printing and pharmaceutical industries. It has been estimated that more than 1–20% of the dye is released from the production sites into the aqueous environment and presents a major threat to the ecosystem. Several advanced oxidation processes (AOPs) have been proposed and widely applied for the degradation of many non-biodegradable, toxic, mutagenic and carcinogenic pollutants [1]. Many researchers have used AOPs such as photocatalysis, sonolysis, Fenton and modified Fenton reactions, ozonation and UV/H2 O2 for the degradation of azo dyes [2–6]. In recent years, much attention has been given to the sonochemical degradation of organic contaminants in aqueous environments [7–10]. Sonochemistry generally involves the chemical effects of ultrasound that arise when a sound wave is passed through an aqueous medium. Often, the generation of highly reactive radical species such as, • H and • OH are observed as a consequence
∗ Corresponding author. Tel.: +61 3 93475180; fax: +61 3 83447090. E-mail address: [email protected] (M. Ashokkumar). 1383-5866/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.seppur.2010.07.001
of the homolytic cleavage of water within the cavitation bubbles. Once these active radicals are formed, they non-selectively attack any organic pollutant present in the water phase and convert them into a variety of products. Sometimes the degradation products are more toxic than the parent compound itself. So the complete mineralization (conversion of organic pollutant into harmless products such as CO2 , H2 O and mineral acids) is essential for any advanced oxidation process to be industrially applicable. The sonochemical degradation of azo dyes has been studied by many researchers world wide [9,11–14]. However, this technique often fails to achieve complete mineralization over short irradiation times due to several reasons, such as the formation of hydrophilic intermediate products, volatility of the pollutants, and reaction volume [15,16]. Hence, the combination of sonolysis with other advanced oxidation processes has been generally recognized as a suitable tactic in order to overcome the limitations of the sonochemical degradation process alone [17–19]. Peller et al. [15] reported that the formation of hydrophilic intermediates derived from the sonolytic degradation of hydrophobic substrates tends to slow down the mineralization rate. Therefore, the combination of sonolysis with photocatalysis (sonophotocatalysis) is often considered to be a suitable adaption because sonolysis is
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able to degrade the hydrophobic products and photocatalysis can degrade the hydrophilic products. The most commonly used heterogeneous photocatalyst is titanium dioxide (Degussa P25). The general mechanism of action of TiO2 mediated degradation of organic pollutants has been widely reported in the literature [4]. Similarly, homogeneous photocatalysis using Fenton (Fe2+ /H2 O2 ) and modified Fenton reagents (Fe3+ /H2 O2 , ferrioxalate, Fe3+ /oxone, etc.) has also been reported as a useful method for the degradation of aqueous organic pollutants [20]. The Fenton oxidation process operates by producing hydroxyl radicals through the splitting of hydrogen peroxide by iron salts. The ultrasound assisted water splitting to produce hydrogen peroxide (Reactions (1) and (2)) is considered to be the main advantage of combining sonolysis with Fenton (sono-Fenton) since the external addition of hydrogen peroxide is not required.
(1) 2• OH
→ H2 O2
(2)
The combination of two individual processes may show a negative or an additive or a synergetic effect. The synergistic degradation and mineralization of organic solutes by sonophotocatalysis has been reported in the literature [15,16,21–25]. For example, Theron et al. [23] observed a synergy between photocatalysis and sonolysis at 30 kHz for the degradation of phenyltrifluromethylketone. Kaur and Singh [24] studied the sonophotocatalytic degradation of reactive red 198 using dye sensitized TiO2 and reported that the combined process yielded synergistic degradation with a synergy index of 2.5 when compared to the individual sonolysis and visible light assisted photocatalysis. An et al. [25] reported that the sonophotocatalytic degradation rate of reactive brilliant orange KR using a 47 kHz sonicator was about 3.7 times higher than that of the individual processes. Madhavan et al. [8] recently showed that the synergy index of the sonophotocatalytic degradation of orange G in the presence of Fe3+ ions was about 1.4. The photocatalytic degradation and mineralization of acid red 88 (AR88) in the presence of doped/undoped TiO2 using either UV or visible radiation has already been reported [26–28]. To the best of our knowledge, the sonochemical and sonophotocatalytic degradation of AR88 using Degussa P25 TiO2 has not been studied. The present investigation examines the sonolytic, photocatalytic and sonophotocatalytic degradation of acid red 88 using TiO2 and Fe3+ in order to expand the knowledge base in the area of sonophotocatalysis. 2. Experimental details 2.1. Experimental conditions All experiments involving ultrasound, except power variation, were performed at an ultrasound (US) frequency of 213 kHz in a continuous wave mode with a power output of 55 mW mL−1 . The actual power experienced by the aqueous dye solution was calculated by the calorimetric method [29]. A commercially available ELAC LVG-60 RF generator coupled with an ELAC Allied Signal transducer with a plate diameter of 54.5 mm used as the source of ultrasound. Unless otherwise mentioned, all chemicals used were of analytical grade and were used without further purification. The desired concentration of acid red 88 (C.I. 15620, Sigma–Aldrich) was prepared from the stock solution using Milli-Q water. Titanium dioxide (TiO2 Degussa P25; surface area 55 m2 /g) was used as the heterogeneous photocatalyst. Hydrated iron (III) nitrate (FeNO3 ·9H2 O) purchased from Sigma–Aldrich was used for carrying out the Fenton-like degradation reactions. The temperature (25 ± 2 ◦ C) around the reaction cell was maintained by a constant
337
temperature water bath. A Xenon arc lamp (450W, Oriel, USA) was used for photocatalytic reactions and a 320 nm cut-off filter was fitted at the exit of the light set-up to ensure that only the radiations with wavelength above 320 nm reach the reactor. All sonolytic, photocatalytic and sonophotocatalytic experiments were performed in the same experimental setup and the volume of the solutions taken was 250 mL. A desired concentration of AR88 was prepared by dissolving the appropriate amount in 250 mL water and then 1 g L−1 of TiO2 was added to the dye solution. The pH was adjusted to 2.7 using HCl (1 M). In our previous study [30], a maximum photocatalytic degradation rate was observed at 1 g L−1 and therefore experiments at lower or higher catalyst loadings were not carried out. The aqueous suspensions were mixed continuously using a mechanical stirrer in the dark for 45 min to allow the equilibrium adsorption of AR88 onto the photocatalyst surface. The concentration of the dye in solution obtained after equilibrium adsorption was used for further kinetic analysis [31]. During irradiation, 3 mL aliquots were withdrawn at appropriate time intervals and the photocatalyst was filtered immediately through a 0.45 m syringe filter (Pall Corporation). 2.2. Analytical determinations The degradation products were analyzed using electrospray mass spectrometry (ESMS). The mass spectrometer used was a Micromass QUATTRO 11 coupled to a Hewlett Packard series 1100 degasser. The instrument was calibrated using the automatic tuning procedure with respect to the parent compound as the standard. The mass range scanned was m/z 50–1000 and several spectra were obtained across each chromatographic peak. The analysis was carried out in negative electrospray ionisation mode at cone voltages of 30 V, 50 V and 80 V. The mobile phase consisted of 50/50 (acetonitrile/water). The flow rate of the solvent was 0.03 mL/min and the capillary voltage was set at 3.5 kV. The concentration of AR88 was determined by a high-performance liquid chromatograph, Shimadzu LC-10 AT VP system with a Shimadzu SPD-M10 A VP photo diode array detector with a Phenomenex reversed phase column (Kromasil, C18, 250 mm × 4.6 mm inner diameter, 5 m beads). The separation was carried out using a binary gradient elution (Solvent A = 0.1 M ammonium acetate and Solvent B = 100% acetonitrile). The gradient used was 25% B reaching 50% in 20 min and the flow rate was maintained at 1.0 mL/min. The peak for AR88 appeared at the retention time of 18.5 min in the chromatogram. TOC was determined using a TOC-VCSH (Shimadzu) instrument which operates through oxidative combustion followed by infrared detection. The instrument was calibrated before each use with standard solutions in the range 1–100 mg L−1 . A Cary Varian 50 Bio UV–visible spectrophotometer was used to record the absorption spectrum of the AR88 solution. The samples were placed in a quartz cell and the spectra were recorded in the wavelength range 200–600 nm. 3. Results and discussion 3.1. Effect of [AR88] The effect of concentration of AR88 on the sonolytic degradation was studied by varying its concentration in the range 0.025–0.09 mM. It was observed that the sonolytic degradation followed first-order-like kinetics with respect to the concentration of AR88. Plots of ln[AR] vs. time were linear (data not shown). The slope of the plot yielded first-order rate constants. It was observed that the first-order rate constants decreased with increasing initial concentration of AR88. Hence, the sonolytic degradation of AR88 cannot be strictly considered as a first-order reaction. The observed
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Fig. 1. Effect of AR88 concentration on its sonolytic degradation rate. Acoustic power = 35 mW mL−1 .
decrease in the rate constant values is likely due to a decrease in the amount of OH radicals generated with an increase in the [AR88] as discussed in our earlier work on the sonochemical degradation of benzoic acid [32]. The OH radical yield is affected by the changes in bubble temperatures during the degradation process as a consequence of volatile products that are formed and enter the cavitation bubbles, and a detailed discussion on this issue is provided elsewhere [32]. While acid red 88 is non-volatile, the initial degradation products are likely to be volatile which may evaporate into the bubble resulting in a decrease in the bubble temperature. We use these kinetic data to allow a comparison of the rate of degradation under various selected experimental conditions. The reason for the observed increase in the rate (calculated by multiplying rate constants with the initial concentrations) with an increase in the [AR88] (Fig. 1) may be due to an increase in the reaction between AR88 and hydroxyl radicals as the [AR88] concentration was increased [18]. 3.2. Effect of power The effect of the applied ultrasonic power on the degradation of AR88 is depicted in Table 1. It can be clearly seen from Table 1 that the degradation rate increased with an increase in the ultrasound power. For example, a higher degradation rate 36.9 × 10−7 M min−1 was obtained at 55 mW mL−1 whereas only about 0.77 × 10−7 M min−1 of dye degradation was observed at 16 mW mL−1 . The reason for this trend may be attributed to an increase in the number of active bubbles with increasing power delivered leading to an increase in the amount of hydroxyl radicals produced [33]. 3.3. Sonocatalytic, photocatalytic and sonophotocatalytic degradation The degradation of AR88 during sonolysis (US + TiO2 ), sonocatalysis (US + TiO2 ) photocatalysis (UV + TiO2 ) and sonophotocatalysis (US + UV + TiO2 ) was studied with an initial dye concentration of 0.09 mM and a fixed catalyst amount of 1 g L−1 . The degradation Table 1 Effect of ultrasound power on the degradation of AR88 (0.09 mM). Power (mW mL−1 )
Rate/10−7 M min−1
16 35 55 64
0.77 19.5 36.9 47.7
Fig. 2. (A) Comparison of degradation rates of AR88 (0.09 mM) in the presence of TiO2 (1 g L−1 ) using different processes. (B) Comparison of degradation rates of AR88 (0.09 mM) in the presence of Fe3+ (0.05 mM) using different processes.
rates obtained for the sonolytic, sonocatalytic, photocatalytic and sonophotocatalytic processes are presented in Fig. 2A. From Fig. 2A, it can be observed that the sonocatalytic (US + TiO2 ) degradation rate was about 2.7 times higher than that of the sonochemical degradation (US). However, HPLC analysis of the filtered solution showed no presence of AR88, as the dye was retained onto the photocatalyst surface. In order to check the reliability of above trend, an experiment was performed to measure the amount of dye adsorbed onto TiO2 surface after equilibrium adsorption. It was found that the dark equilibrium adsorption of the dye onto the photocatalyst under our conditions was about 50%. In order to check whether or not the dye could be released from the surface, another experiment was also carried out by continuous stirring of the filtered dye adsorbed TiO2 samples in water. It was noted that the dye was not released completely from the catalyst surface even on continuous stirring for 60 min showing that the dye adsorption was essentially irreversible under the conditions used, which was observed in a previous study [26]. Therefore, the apparent enhancement is really an artifact of solute adsorption onto the TiO2 surface. From the data shown in Fig. 2A, it can be seen that the degradation of the dye using TiO2 (UV + TiO2 ) is 40.2 × 10−7 M min−1 , due to photo redox reactions [27]. However, when compared with photocatalysis, an approximately threefold increment in the degradation rate was observed on combining sonolysis and photocatalysis. This likely to be due to the increased formation of hydroxyl radicals by the sonochemical splitting of water (Reactions (1) and (2)) as well as due to the sono-cleaning of the photocatalyst surface enabling the redox reactions to occur at its surface more efficiently. A quantitative way of evaluating any synergistic effect when multiple processes are used is by reference to a synergy index [18], namely, Synergy index =
R(US+UV+TiO2 ) R(US+TiO2 ) + R(UV+TiO2 )
(3)
Synergy index values >1 indicate the combined process exceeds the sum of the separate processes. In order to check the effectiveness of combining two processes, the synergy index was calculated from the degradation rates of sonocatalysis R(US+TiO2 ) ,
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photocatalysis R(UV+TiO2 ) and sonophotocatalysis R(US+UV+TiO2 ) (Fig. 2A). Using this data a synergy index of 1.3 was obtained indicating that there is only a slight synergy in combining the two separate processes. Recently, we studied the sonophotocatalytic degradation of orange G using TiO2 and found that the combined processes also offered only a slight synergetic effect [18]. 3.4. Effect of Fe3+ on the sonophotocatalytic degradation In order to study the effect of Fe3+ on the sonolytic, photocatalytic and sonophotocatalytic degradation, a few experiments were performed under the following conditions; [AR88] = 0.09 mM, [Fe3+ ] = 0.05 mM, with pH 2.7 maintained in all cases. It is observed in Fig. 2B under the conditions used that the degradation rates of sonolysis and photolysis in the presence of Fe3+ were about the same. Comparing the data in Fig. 2A with Fig. 2B it can be seen that there is a decrease in the sonolytic (US) degradation rate in the presence of Fe3+ ions (US + Fe3+ ). This may be due to the formation of a complex between Fe3+ and the dye ([AR88–Fe3+ ]) as noted by Madhavan et al. [34] who studied the photo-Fenton-like degradation of AR88 using Fe3+ /oxone and Cu2+ /oxone systems. They reported that a complex was formed between the Fe3+ and AR88. In order to confirm this, control experiments were carried out with AR88 and Fe3+ ions, and it revealed that the absorption maximum (506 nm) was blue shifted, indicating the formation of a metal–ligand complex. A greater extent of degradation was observed for sonophotocatalysis (US + UV + Fe3+ ) compared with the individual processes (Fig. 2B). The photolytic degradation of AR88 in the presence of Fe3+ (UV + Fe3+ ) may be attributed to the formation of a [AR88–Fe3+ ] complex which absorbs the light and effectively produces AR88•+ , Fe3+ and organics [33]. The Fe3+ (exists as an aqua complex [Fe(OH)(H2 O)5 ]2+ ) converts to Fe2+ (Reaction (4)) which drives the Fenton reaction for the dye degradation. The regeneration of the catalyst with the formation of an additional hydroxyl radical is achieved by continuous irradiation of the Fe3+ solution. However, the reaction is stopped once all the H2 O2 is consumed by Fe2+ . Further, it is interesting to note from Fig. 2B that the combination of these two processes resulted in the synergistic enhancement in the degradation rate compared with that of the individual processes. A synergy value of 2.3 was obtained for the sonophotocatalytic degradation of AR88 in the presence of Fe3+ indicating that the combination of photolysis and sonolysis favours dye degradation. The synergistic enhancement in the sonophotocatalytic degradation may be attributed to the additional generation of hydroxyl radicals through sono-Fenton and photo-Fenton reactions. That is, the sonochemically formed H2 O2 are involved in the Fenton reaction with Fe2+ produced via Reaction (5) to form Fe3+ , which on photoirradiation again forms Fe2+ and the cycle continues for the dye degradation. This continuity in the production of hydroxyl radicals was not possible in the absence of light. Madhavan et al. [8] studied the sonophoto-Fenton degradation of orange G and reported that the intermittent addition of hydrogen peroxide was essential for its complete degradation. Light activation of the sono-Fenton reaction appears to be responsible for the observed synergistic enhancement. Fe3+ + H2 O + h → Fe2+ + HO• + H+ 2+
Fe
+ H2 O2 → Fe
3+
+ HO•
+ H2 O
Fig. 3. Change in TOC as a function of time for the degradation of AR88 (0.09 mM) under different processes using TiO2 (1 g L−1 ).
sonolysis (US) was found to be slower than the corresponding degradation rate. Even though 67% degradation was observed after 50 min, the extent of mineralization was only marginal. It showed that the complete sonolytic degradation process was slow and the degradation products still exist as water-soluble intermediates in the treated solution. Also, it was seen that photocatalysis offered about 93% TOC removal in 4 h. Sonophotocatalysis achieves higher TOC removal than the individual processes. It is worth noting that even though sonophotocatalysis showed a slight enhancement in the mineralization in the first 2 h, it showed similar AR88 results as that of the photocatalysis after 2 h of treatment. Also, it is clearly evident that, similar to AR88 decomposition, the mineralization process shows an additive effect during sonophotocatalysis in the presence of TiO2 . Similar mineralization studies were also carried out for the sonolytic, photocatalytic and sonophotocatalytic degradation using Fe3+ . It is revealed in Fig. 4 that there is no significant TOC removal in the presence of Fe3+ during the sonolysis and photolysis processes. Also, there was no significant TOC removal (7%) for the sonophotocatalysis until 2 h of irradiation. After 2 h, a synergistic enhancement (37%) in the mineralization for sonophotocatalysis in the presence of Fe3+ was obtained (Fig. 4). The observed synergistic mineralization may be due to simultaneous operation of sono-Fenton and photo-Fenton reactions (Reactions (4) and (5)). Apart from the above mentioned reasons, it is anticipated that the
(4) (5)
3.5. Mineralization studies In order to determine the extent of mineralization of AR88 (0.09 mM) into CO2 and water, total organic carbon (TOC) analysis was performed. The obtained results for the sonolytic, photocatalytic and sonophotocatalytic mineralization using TiO2 (1 g L−1 ) photocatalyst are shown in Fig. 3. The TOC removal rate for
Fig. 4. Change in TOC as a function of time for the degradation of AR88 (0.09 mM). under different processes using Fe3+ (0.05 mM).
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Scheme 1. Degradation products of AR88 using sonolysis, photocatalysis and sonophotocatalysis.
formation of carboxylated products which are capable of forming a photoactive complexes with Fe3+ might also led to the formation of higher amounts of hydroxyl radicals and undergo decarboxylation reactions.
3.6. Degradation pathway The samples collected after 0–2 h treatment by sonolytic, photocatalytic and sonophotocatalytic processes were injected into a mass spectrometer (ESMS). The ESMS analysis showed peaks at m/z values such as 175, 223, 239, 255, 378, 394, 396, 412, 428 and the possible structures to match the above mass values are presented in Scheme 1. It was observed that the base peak of AR88 (1) with m/z 378 decreased in intensity with an increase in sonication time and at the same time as the formation of new peaks from the generation of degradation products. The hydroxyl addition onto the azo bond of AR88 produced 2 (m/z 396) which on further hydroxylation led to the formation of product 3. The titanium dioxide mediated photocatalytic degradation of AR88 showed the formation of hydroxyamino naphthol (4) and 4-aminonaphthalene sulphonic acid (5) as intermediate products. Saquib and Muneer [27] studied the photocatalytic degradation of AR88 under UV irradiation and also observed the formation of 4-aminonaphthalene
sulphonic acid. They suggested that the formation of five might occur by the transfer of an electron from the azo bond to form the radical cation, which upon the addition of a hydroxyl radical followed by cleavage would form 4-aminonaphthalene sulphonic acid (Scheme 1). The hydroxyl radical attack of 4-aminonaphthalene sulphonic acid (5) on different positions forms products with m/z values 239 (5a) and 255 (5b). Since sonolysis involved the reaction of the substrate with the hydroxyl radicals, the formation of hydroxylated products of AR88 were also expected. As anticipated, mono and di-hydroxy substituted products of AR88 (6a and 6b) were identified at shorter irradiation times showing that the attack of the hydroxyl radical as the major pathway for the sonochemical degradation of the dye. The formation of the hydroxylated products due to the sonication of orange G was recently reported by us [18]. The 4-aminonaphthalene sulphonic acid was also formed during the sonophotocatalytic degradation of acid red 88. 4. Summary The sonolytic, photocatalytic and sonophotocatalytic degradation of AR88 in the presence of the photocatalysts, TiO2 and Fe3+ were carried out.
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1. The sonolytic degradation followed first-order-like kinetics with respect to AR88 and the degradation rate increased with an increase in the ultrasound power. 2. A comparison of the degradation rates of photocatalysis with that of sonophotocatalysis revealed a threefold enhancement in the degradation rate for the latter process. 3. A slight synergetic effect was found for the TiO2 mediated sonophotocatalytic degradation, whereas mineralization of AR88 was additive. 4. The sonophotocatalytic degradation and mineralization of AR88 in the presence of Fe3+ was synergistic, most likely due to the involvement of sono-Fenton and photo-Fenton reactions. 5. The formation of mono and di-hydroxylated products were identified for the sonolytic, photocatalytic and sonophotocatalytic degradation of AR88. Acknowledgements The authors thank DIISR, Australia and DST, New Delhi for the financial support from an India–Australian Strategic Research Fund (INT/AUS/P-1/07 dated 19 September 2007). References [1] R.O.A. De Lima, A.P. Bazo, D.M. Faveri Sakvadori, C.M. Rech, D. De Palma Oliveira, G. De Aragao Umbuzeiro, Genet. Toxicol. Environ. Mutagen. 626 (2007) 53–60. [2] B. Neppolian, H.C. Choi, S. Sakthivel, B. Arabindoo, V. Murugesan, J. Hazard. Mater. 89 (2002) 303–317. [3] V. Augugliaro, M. Litter, L. Palmisano, J. Soria, J. Photochem. Photobiol. C: Photochem. Rev. 7 (2003) 127–144. [4] M.R. Hoffmann, S.T. Martin, W. Choi, Chem. Rev. 95 (1995) 69–96. [5] Z. He, L. Lin, S. Song, M. Xia, L. Xu, H. Ying, J. Chen, Separ. Purif. Technol. 62 (2008) 376–381. [6] S.H. Kim, H.H. Ngo, H.K. Shon, S. Vigneswaran, Separ. Purif. Technol. 58 (2008) 335–342.
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Separation and Purification Technology journal homepage: www.elsevier.com/locate/seppur
Degradation of acid red 88 by the combination of sonolysis and photocatalysis Jagannathan Madhavan a , Panneer Selvam Sathish Kumar a,b , Sambandam Anandan b , Franz Grieser a , Muthupandian Ashokkumar a,∗ a b
Particulate Fluids Processing Centre, School of Chemistry, University of Melbourne, Vic 3010, Australia Nanomaterials & Solar Energy Conversion Lab, Department of Chemistry, National Institute of Technology, Trichy, Tamil Nadu 620015, India
a r t i c l e
i n f o
Article history: Received 26 February 2010 Received in revised form 1 July 2010 Accepted 1 July 2010 Keywords: Sonolysis Photocatalysis Sono-Fenton Sonophotocatalysis Acid red 88
a b s t r a c t Acid red 88 (AR88) is a mono-azo textile dye and a widely used colorant in the textile and food industries. The sonolytic, photocatalytic and sonophotocatalytic degradation of AR88 in the presence of homogeneous (Fe3+ ) and heterogeneous (TiO2 ) photocatalysts were studied. The effects of initial dye concentration and ultrasound power on the sonochemical degradation were also investigated. The degradation by sonolysis and photocatalysis followed first-order like kinetics with respect to the concentration of AR88. The sonophotocatalytic degradation rates using TiO2 or Fe3+ were found to be higher than that observed with sonolysis or photocatalysis. The synergy index calculated for TiO2 sonophotocatalysis was 1.3, suggesting that the combination of sonolysis and TiO2 photocatalysis resulted in a small synergetic effect. Also, the sonophotocatalysis in the presence of Fe3+ was synergistic with a synergy index of 2.3. Total organic carbon (TOC) analysis was performed to study the extent of mineralization. TOC data showed that the mineralization using TiO2 sonophotocatalysis was additive whereas a synergistic mineralization was observed for the sonophotocatalysis in the presence of Fe3+ . The simultaneous operation of sonoFenton and photo-Fenton reactions is likely to be the underlying reason for the observed synergy in the presence of Fe3+ assisted sonophotocatalysis. Electrospray mass spectrometry (ESMS) was employed for the identification of the degradation intermediates. The sonication of AR88 led to the formation of its mono and di-hydroxylated products as the primary intermediates. A possible degradation pathway has been proposed. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Azo dyes are among the notorious environmental pollutants associated with textile, food, cosmetic, printing and pharmaceutical industries. It has been estimated that more than 1–20% of the dye is released from the production sites into the aqueous environment and presents a major threat to the ecosystem. Several advanced oxidation processes (AOPs) have been proposed and widely applied for the degradation of many non-biodegradable, toxic, mutagenic and carcinogenic pollutants [1]. Many researchers have used AOPs such as photocatalysis, sonolysis, Fenton and modified Fenton reactions, ozonation and UV/H2 O2 for the degradation of azo dyes [2–6]. In recent years, much attention has been given to the sonochemical degradation of organic contaminants in aqueous environments [7–10]. Sonochemistry generally involves the chemical effects of ultrasound that arise when a sound wave is passed through an aqueous medium. Often, the generation of highly reactive radical species such as, • H and • OH are observed as a consequence
∗ Corresponding author. Tel.: +61 3 93475180; fax: +61 3 83447090. E-mail address: [email protected] (M. Ashokkumar). 1383-5866/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.seppur.2010.07.001
of the homolytic cleavage of water within the cavitation bubbles. Once these active radicals are formed, they non-selectively attack any organic pollutant present in the water phase and convert them into a variety of products. Sometimes the degradation products are more toxic than the parent compound itself. So the complete mineralization (conversion of organic pollutant into harmless products such as CO2 , H2 O and mineral acids) is essential for any advanced oxidation process to be industrially applicable. The sonochemical degradation of azo dyes has been studied by many researchers world wide [9,11–14]. However, this technique often fails to achieve complete mineralization over short irradiation times due to several reasons, such as the formation of hydrophilic intermediate products, volatility of the pollutants, and reaction volume [15,16]. Hence, the combination of sonolysis with other advanced oxidation processes has been generally recognized as a suitable tactic in order to overcome the limitations of the sonochemical degradation process alone [17–19]. Peller et al. [15] reported that the formation of hydrophilic intermediates derived from the sonolytic degradation of hydrophobic substrates tends to slow down the mineralization rate. Therefore, the combination of sonolysis with photocatalysis (sonophotocatalysis) is often considered to be a suitable adaption because sonolysis is
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able to degrade the hydrophobic products and photocatalysis can degrade the hydrophilic products. The most commonly used heterogeneous photocatalyst is titanium dioxide (Degussa P25). The general mechanism of action of TiO2 mediated degradation of organic pollutants has been widely reported in the literature [4]. Similarly, homogeneous photocatalysis using Fenton (Fe2+ /H2 O2 ) and modified Fenton reagents (Fe3+ /H2 O2 , ferrioxalate, Fe3+ /oxone, etc.) has also been reported as a useful method for the degradation of aqueous organic pollutants [20]. The Fenton oxidation process operates by producing hydroxyl radicals through the splitting of hydrogen peroxide by iron salts. The ultrasound assisted water splitting to produce hydrogen peroxide (Reactions (1) and (2)) is considered to be the main advantage of combining sonolysis with Fenton (sono-Fenton) since the external addition of hydrogen peroxide is not required.
(1) 2• OH
→ H2 O2
(2)
The combination of two individual processes may show a negative or an additive or a synergetic effect. The synergistic degradation and mineralization of organic solutes by sonophotocatalysis has been reported in the literature [15,16,21–25]. For example, Theron et al. [23] observed a synergy between photocatalysis and sonolysis at 30 kHz for the degradation of phenyltrifluromethylketone. Kaur and Singh [24] studied the sonophotocatalytic degradation of reactive red 198 using dye sensitized TiO2 and reported that the combined process yielded synergistic degradation with a synergy index of 2.5 when compared to the individual sonolysis and visible light assisted photocatalysis. An et al. [25] reported that the sonophotocatalytic degradation rate of reactive brilliant orange KR using a 47 kHz sonicator was about 3.7 times higher than that of the individual processes. Madhavan et al. [8] recently showed that the synergy index of the sonophotocatalytic degradation of orange G in the presence of Fe3+ ions was about 1.4. The photocatalytic degradation and mineralization of acid red 88 (AR88) in the presence of doped/undoped TiO2 using either UV or visible radiation has already been reported [26–28]. To the best of our knowledge, the sonochemical and sonophotocatalytic degradation of AR88 using Degussa P25 TiO2 has not been studied. The present investigation examines the sonolytic, photocatalytic and sonophotocatalytic degradation of acid red 88 using TiO2 and Fe3+ in order to expand the knowledge base in the area of sonophotocatalysis. 2. Experimental details 2.1. Experimental conditions All experiments involving ultrasound, except power variation, were performed at an ultrasound (US) frequency of 213 kHz in a continuous wave mode with a power output of 55 mW mL−1 . The actual power experienced by the aqueous dye solution was calculated by the calorimetric method [29]. A commercially available ELAC LVG-60 RF generator coupled with an ELAC Allied Signal transducer with a plate diameter of 54.5 mm used as the source of ultrasound. Unless otherwise mentioned, all chemicals used were of analytical grade and were used without further purification. The desired concentration of acid red 88 (C.I. 15620, Sigma–Aldrich) was prepared from the stock solution using Milli-Q water. Titanium dioxide (TiO2 Degussa P25; surface area 55 m2 /g) was used as the heterogeneous photocatalyst. Hydrated iron (III) nitrate (FeNO3 ·9H2 O) purchased from Sigma–Aldrich was used for carrying out the Fenton-like degradation reactions. The temperature (25 ± 2 ◦ C) around the reaction cell was maintained by a constant
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temperature water bath. A Xenon arc lamp (450W, Oriel, USA) was used for photocatalytic reactions and a 320 nm cut-off filter was fitted at the exit of the light set-up to ensure that only the radiations with wavelength above 320 nm reach the reactor. All sonolytic, photocatalytic and sonophotocatalytic experiments were performed in the same experimental setup and the volume of the solutions taken was 250 mL. A desired concentration of AR88 was prepared by dissolving the appropriate amount in 250 mL water and then 1 g L−1 of TiO2 was added to the dye solution. The pH was adjusted to 2.7 using HCl (1 M). In our previous study [30], a maximum photocatalytic degradation rate was observed at 1 g L−1 and therefore experiments at lower or higher catalyst loadings were not carried out. The aqueous suspensions were mixed continuously using a mechanical stirrer in the dark for 45 min to allow the equilibrium adsorption of AR88 onto the photocatalyst surface. The concentration of the dye in solution obtained after equilibrium adsorption was used for further kinetic analysis [31]. During irradiation, 3 mL aliquots were withdrawn at appropriate time intervals and the photocatalyst was filtered immediately through a 0.45 m syringe filter (Pall Corporation). 2.2. Analytical determinations The degradation products were analyzed using electrospray mass spectrometry (ESMS). The mass spectrometer used was a Micromass QUATTRO 11 coupled to a Hewlett Packard series 1100 degasser. The instrument was calibrated using the automatic tuning procedure with respect to the parent compound as the standard. The mass range scanned was m/z 50–1000 and several spectra were obtained across each chromatographic peak. The analysis was carried out in negative electrospray ionisation mode at cone voltages of 30 V, 50 V and 80 V. The mobile phase consisted of 50/50 (acetonitrile/water). The flow rate of the solvent was 0.03 mL/min and the capillary voltage was set at 3.5 kV. The concentration of AR88 was determined by a high-performance liquid chromatograph, Shimadzu LC-10 AT VP system with a Shimadzu SPD-M10 A VP photo diode array detector with a Phenomenex reversed phase column (Kromasil, C18, 250 mm × 4.6 mm inner diameter, 5 m beads). The separation was carried out using a binary gradient elution (Solvent A = 0.1 M ammonium acetate and Solvent B = 100% acetonitrile). The gradient used was 25% B reaching 50% in 20 min and the flow rate was maintained at 1.0 mL/min. The peak for AR88 appeared at the retention time of 18.5 min in the chromatogram. TOC was determined using a TOC-VCSH (Shimadzu) instrument which operates through oxidative combustion followed by infrared detection. The instrument was calibrated before each use with standard solutions in the range 1–100 mg L−1 . A Cary Varian 50 Bio UV–visible spectrophotometer was used to record the absorption spectrum of the AR88 solution. The samples were placed in a quartz cell and the spectra were recorded in the wavelength range 200–600 nm. 3. Results and discussion 3.1. Effect of [AR88] The effect of concentration of AR88 on the sonolytic degradation was studied by varying its concentration in the range 0.025–0.09 mM. It was observed that the sonolytic degradation followed first-order-like kinetics with respect to the concentration of AR88. Plots of ln[AR] vs. time were linear (data not shown). The slope of the plot yielded first-order rate constants. It was observed that the first-order rate constants decreased with increasing initial concentration of AR88. Hence, the sonolytic degradation of AR88 cannot be strictly considered as a first-order reaction. The observed
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Fig. 1. Effect of AR88 concentration on its sonolytic degradation rate. Acoustic power = 35 mW mL−1 .
decrease in the rate constant values is likely due to a decrease in the amount of OH radicals generated with an increase in the [AR88] as discussed in our earlier work on the sonochemical degradation of benzoic acid [32]. The OH radical yield is affected by the changes in bubble temperatures during the degradation process as a consequence of volatile products that are formed and enter the cavitation bubbles, and a detailed discussion on this issue is provided elsewhere [32]. While acid red 88 is non-volatile, the initial degradation products are likely to be volatile which may evaporate into the bubble resulting in a decrease in the bubble temperature. We use these kinetic data to allow a comparison of the rate of degradation under various selected experimental conditions. The reason for the observed increase in the rate (calculated by multiplying rate constants with the initial concentrations) with an increase in the [AR88] (Fig. 1) may be due to an increase in the reaction between AR88 and hydroxyl radicals as the [AR88] concentration was increased [18]. 3.2. Effect of power The effect of the applied ultrasonic power on the degradation of AR88 is depicted in Table 1. It can be clearly seen from Table 1 that the degradation rate increased with an increase in the ultrasound power. For example, a higher degradation rate 36.9 × 10−7 M min−1 was obtained at 55 mW mL−1 whereas only about 0.77 × 10−7 M min−1 of dye degradation was observed at 16 mW mL−1 . The reason for this trend may be attributed to an increase in the number of active bubbles with increasing power delivered leading to an increase in the amount of hydroxyl radicals produced [33]. 3.3. Sonocatalytic, photocatalytic and sonophotocatalytic degradation The degradation of AR88 during sonolysis (US + TiO2 ), sonocatalysis (US + TiO2 ) photocatalysis (UV + TiO2 ) and sonophotocatalysis (US + UV + TiO2 ) was studied with an initial dye concentration of 0.09 mM and a fixed catalyst amount of 1 g L−1 . The degradation Table 1 Effect of ultrasound power on the degradation of AR88 (0.09 mM). Power (mW mL−1 )
Rate/10−7 M min−1
16 35 55 64
0.77 19.5 36.9 47.7
Fig. 2. (A) Comparison of degradation rates of AR88 (0.09 mM) in the presence of TiO2 (1 g L−1 ) using different processes. (B) Comparison of degradation rates of AR88 (0.09 mM) in the presence of Fe3+ (0.05 mM) using different processes.
rates obtained for the sonolytic, sonocatalytic, photocatalytic and sonophotocatalytic processes are presented in Fig. 2A. From Fig. 2A, it can be observed that the sonocatalytic (US + TiO2 ) degradation rate was about 2.7 times higher than that of the sonochemical degradation (US). However, HPLC analysis of the filtered solution showed no presence of AR88, as the dye was retained onto the photocatalyst surface. In order to check the reliability of above trend, an experiment was performed to measure the amount of dye adsorbed onto TiO2 surface after equilibrium adsorption. It was found that the dark equilibrium adsorption of the dye onto the photocatalyst under our conditions was about 50%. In order to check whether or not the dye could be released from the surface, another experiment was also carried out by continuous stirring of the filtered dye adsorbed TiO2 samples in water. It was noted that the dye was not released completely from the catalyst surface even on continuous stirring for 60 min showing that the dye adsorption was essentially irreversible under the conditions used, which was observed in a previous study [26]. Therefore, the apparent enhancement is really an artifact of solute adsorption onto the TiO2 surface. From the data shown in Fig. 2A, it can be seen that the degradation of the dye using TiO2 (UV + TiO2 ) is 40.2 × 10−7 M min−1 , due to photo redox reactions [27]. However, when compared with photocatalysis, an approximately threefold increment in the degradation rate was observed on combining sonolysis and photocatalysis. This likely to be due to the increased formation of hydroxyl radicals by the sonochemical splitting of water (Reactions (1) and (2)) as well as due to the sono-cleaning of the photocatalyst surface enabling the redox reactions to occur at its surface more efficiently. A quantitative way of evaluating any synergistic effect when multiple processes are used is by reference to a synergy index [18], namely, Synergy index =
R(US+UV+TiO2 ) R(US+TiO2 ) + R(UV+TiO2 )
(3)
Synergy index values >1 indicate the combined process exceeds the sum of the separate processes. In order to check the effectiveness of combining two processes, the synergy index was calculated from the degradation rates of sonocatalysis R(US+TiO2 ) ,
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photocatalysis R(UV+TiO2 ) and sonophotocatalysis R(US+UV+TiO2 ) (Fig. 2A). Using this data a synergy index of 1.3 was obtained indicating that there is only a slight synergy in combining the two separate processes. Recently, we studied the sonophotocatalytic degradation of orange G using TiO2 and found that the combined processes also offered only a slight synergetic effect [18]. 3.4. Effect of Fe3+ on the sonophotocatalytic degradation In order to study the effect of Fe3+ on the sonolytic, photocatalytic and sonophotocatalytic degradation, a few experiments were performed under the following conditions; [AR88] = 0.09 mM, [Fe3+ ] = 0.05 mM, with pH 2.7 maintained in all cases. It is observed in Fig. 2B under the conditions used that the degradation rates of sonolysis and photolysis in the presence of Fe3+ were about the same. Comparing the data in Fig. 2A with Fig. 2B it can be seen that there is a decrease in the sonolytic (US) degradation rate in the presence of Fe3+ ions (US + Fe3+ ). This may be due to the formation of a complex between Fe3+ and the dye ([AR88–Fe3+ ]) as noted by Madhavan et al. [34] who studied the photo-Fenton-like degradation of AR88 using Fe3+ /oxone and Cu2+ /oxone systems. They reported that a complex was formed between the Fe3+ and AR88. In order to confirm this, control experiments were carried out with AR88 and Fe3+ ions, and it revealed that the absorption maximum (506 nm) was blue shifted, indicating the formation of a metal–ligand complex. A greater extent of degradation was observed for sonophotocatalysis (US + UV + Fe3+ ) compared with the individual processes (Fig. 2B). The photolytic degradation of AR88 in the presence of Fe3+ (UV + Fe3+ ) may be attributed to the formation of a [AR88–Fe3+ ] complex which absorbs the light and effectively produces AR88•+ , Fe3+ and organics [33]. The Fe3+ (exists as an aqua complex [Fe(OH)(H2 O)5 ]2+ ) converts to Fe2+ (Reaction (4)) which drives the Fenton reaction for the dye degradation. The regeneration of the catalyst with the formation of an additional hydroxyl radical is achieved by continuous irradiation of the Fe3+ solution. However, the reaction is stopped once all the H2 O2 is consumed by Fe2+ . Further, it is interesting to note from Fig. 2B that the combination of these two processes resulted in the synergistic enhancement in the degradation rate compared with that of the individual processes. A synergy value of 2.3 was obtained for the sonophotocatalytic degradation of AR88 in the presence of Fe3+ indicating that the combination of photolysis and sonolysis favours dye degradation. The synergistic enhancement in the sonophotocatalytic degradation may be attributed to the additional generation of hydroxyl radicals through sono-Fenton and photo-Fenton reactions. That is, the sonochemically formed H2 O2 are involved in the Fenton reaction with Fe2+ produced via Reaction (5) to form Fe3+ , which on photoirradiation again forms Fe2+ and the cycle continues for the dye degradation. This continuity in the production of hydroxyl radicals was not possible in the absence of light. Madhavan et al. [8] studied the sonophoto-Fenton degradation of orange G and reported that the intermittent addition of hydrogen peroxide was essential for its complete degradation. Light activation of the sono-Fenton reaction appears to be responsible for the observed synergistic enhancement. Fe3+ + H2 O + h → Fe2+ + HO• + H+ 2+
Fe
+ H2 O2 → Fe
3+
+ HO•
+ H2 O
Fig. 3. Change in TOC as a function of time for the degradation of AR88 (0.09 mM) under different processes using TiO2 (1 g L−1 ).
sonolysis (US) was found to be slower than the corresponding degradation rate. Even though 67% degradation was observed after 50 min, the extent of mineralization was only marginal. It showed that the complete sonolytic degradation process was slow and the degradation products still exist as water-soluble intermediates in the treated solution. Also, it was seen that photocatalysis offered about 93% TOC removal in 4 h. Sonophotocatalysis achieves higher TOC removal than the individual processes. It is worth noting that even though sonophotocatalysis showed a slight enhancement in the mineralization in the first 2 h, it showed similar AR88 results as that of the photocatalysis after 2 h of treatment. Also, it is clearly evident that, similar to AR88 decomposition, the mineralization process shows an additive effect during sonophotocatalysis in the presence of TiO2 . Similar mineralization studies were also carried out for the sonolytic, photocatalytic and sonophotocatalytic degradation using Fe3+ . It is revealed in Fig. 4 that there is no significant TOC removal in the presence of Fe3+ during the sonolysis and photolysis processes. Also, there was no significant TOC removal (7%) for the sonophotocatalysis until 2 h of irradiation. After 2 h, a synergistic enhancement (37%) in the mineralization for sonophotocatalysis in the presence of Fe3+ was obtained (Fig. 4). The observed synergistic mineralization may be due to simultaneous operation of sono-Fenton and photo-Fenton reactions (Reactions (4) and (5)). Apart from the above mentioned reasons, it is anticipated that the
(4) (5)
3.5. Mineralization studies In order to determine the extent of mineralization of AR88 (0.09 mM) into CO2 and water, total organic carbon (TOC) analysis was performed. The obtained results for the sonolytic, photocatalytic and sonophotocatalytic mineralization using TiO2 (1 g L−1 ) photocatalyst are shown in Fig. 3. The TOC removal rate for
Fig. 4. Change in TOC as a function of time for the degradation of AR88 (0.09 mM). under different processes using Fe3+ (0.05 mM).
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Scheme 1. Degradation products of AR88 using sonolysis, photocatalysis and sonophotocatalysis.
formation of carboxylated products which are capable of forming a photoactive complexes with Fe3+ might also led to the formation of higher amounts of hydroxyl radicals and undergo decarboxylation reactions.
3.6. Degradation pathway The samples collected after 0–2 h treatment by sonolytic, photocatalytic and sonophotocatalytic processes were injected into a mass spectrometer (ESMS). The ESMS analysis showed peaks at m/z values such as 175, 223, 239, 255, 378, 394, 396, 412, 428 and the possible structures to match the above mass values are presented in Scheme 1. It was observed that the base peak of AR88 (1) with m/z 378 decreased in intensity with an increase in sonication time and at the same time as the formation of new peaks from the generation of degradation products. The hydroxyl addition onto the azo bond of AR88 produced 2 (m/z 396) which on further hydroxylation led to the formation of product 3. The titanium dioxide mediated photocatalytic degradation of AR88 showed the formation of hydroxyamino naphthol (4) and 4-aminonaphthalene sulphonic acid (5) as intermediate products. Saquib and Muneer [27] studied the photocatalytic degradation of AR88 under UV irradiation and also observed the formation of 4-aminonaphthalene
sulphonic acid. They suggested that the formation of five might occur by the transfer of an electron from the azo bond to form the radical cation, which upon the addition of a hydroxyl radical followed by cleavage would form 4-aminonaphthalene sulphonic acid (Scheme 1). The hydroxyl radical attack of 4-aminonaphthalene sulphonic acid (5) on different positions forms products with m/z values 239 (5a) and 255 (5b). Since sonolysis involved the reaction of the substrate with the hydroxyl radicals, the formation of hydroxylated products of AR88 were also expected. As anticipated, mono and di-hydroxy substituted products of AR88 (6a and 6b) were identified at shorter irradiation times showing that the attack of the hydroxyl radical as the major pathway for the sonochemical degradation of the dye. The formation of the hydroxylated products due to the sonication of orange G was recently reported by us [18]. The 4-aminonaphthalene sulphonic acid was also formed during the sonophotocatalytic degradation of acid red 88. 4. Summary The sonolytic, photocatalytic and sonophotocatalytic degradation of AR88 in the presence of the photocatalysts, TiO2 and Fe3+ were carried out.
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1. The sonolytic degradation followed first-order-like kinetics with respect to AR88 and the degradation rate increased with an increase in the ultrasound power. 2. A comparison of the degradation rates of photocatalysis with that of sonophotocatalysis revealed a threefold enhancement in the degradation rate for the latter process. 3. A slight synergetic effect was found for the TiO2 mediated sonophotocatalytic degradation, whereas mineralization of AR88 was additive. 4. The sonophotocatalytic degradation and mineralization of AR88 in the presence of Fe3+ was synergistic, most likely due to the involvement of sono-Fenton and photo-Fenton reactions. 5. The formation of mono and di-hydroxylated products were identified for the sonolytic, photocatalytic and sonophotocatalytic degradation of AR88. Acknowledgements The authors thank DIISR, Australia and DST, New Delhi for the financial support from an India–Australian Strategic Research Fund (INT/AUS/P-1/07 dated 19 September 2007). References [1] R.O.A. De Lima, A.P. Bazo, D.M. Faveri Sakvadori, C.M. Rech, D. De Palma Oliveira, G. De Aragao Umbuzeiro, Genet. Toxicol. Environ. Mutagen. 626 (2007) 53–60. [2] B. Neppolian, H.C. Choi, S. Sakthivel, B. Arabindoo, V. Murugesan, J. Hazard. Mater. 89 (2002) 303–317. [3] V. Augugliaro, M. Litter, L. Palmisano, J. Soria, J. Photochem. Photobiol. C: Photochem. Rev. 7 (2003) 127–144. [4] M.R. Hoffmann, S.T. Martin, W. Choi, Chem. Rev. 95 (1995) 69–96. [5] Z. He, L. Lin, S. Song, M. Xia, L. Xu, H. Ying, J. Chen, Separ. Purif. Technol. 62 (2008) 376–381. [6] S.H. Kim, H.H. Ngo, H.K. Shon, S. Vigneswaran, Separ. Purif. Technol. 58 (2008) 335–342.
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