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Sonophotocatalytic treatment of Naphthol Blue Black dye and real textile wastewater using synthesized Fe doped TiO2
Chemical Engineering and Processing 99 (2016) 10–18
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Chemical Engineering and Processing: Process Intensification journal homepage: www.elsevier.com/locate/cep
Sonophotocatalytic treatment of Naphthol Blue Black dye and real textile wastewater using synthesized Fe doped TiO2 D. Rahul Reddya , G. Kumaravel Dinesha , Sambandam Anandanb , Thirugnanasambandam Sivasankara,* a b
Department of Chemical Engineering, National Institute of Technology Tiruchirappalli, Tamilnadu, India Department of Chemistry, National Institute of Technology Tiruchirappalli, Tamilnadu, India
A R T I C L E I N F O
A B S T R A C T
Article history: Received 27 August 2015 Received in revised form 26 October 2015 Accepted 27 October 2015 Available online 1 November 2015
This study examines the treatment of Naphthol-Blue–Black (NBB) dye and real textile wastewater through sonophotocatalytic technique. Fe, TiO2 and Fe-TiO2 were synthesized and characterized by SEM, XRD and DRS which revealed its high purity, good doping, nanosize and higher light absorption capacity. The effect of pH, gas content (Ar, O2, air and N2), H2O2 concentration and catalyst loading were examined. The maximum color removal was observed for initial pH 11, argon gas bubble, 44.1 mmol/L H2O2, 3 g/L Fe, 4 g/L TiO2 and 2.2 g/L Fe-TiO2. However, 96% was achieved by sonophotocatalytic process for 2.2 g/L 1:7 Fe:TiO2. Similar methodology for real textile wastewater with Fe-TiO2 (1:7) could remove 91% of TOC. ã 2015 Elsevier B.V. All rights reserved.
Keywords: Naphthol Blue Black dye Sonophotocatalysis Fe doped TiO2 Textile wastewater TOC removal
1. Introduction Advanced oxidation processes (AOPs) have been used as emerging wastewater treatment technologies for various hazardous organic pollutants. Sonophotocatalysis is gaining attention among advanced oxidation process by which the electron hole pairs are generated at high temperature and pressure by the integration of sonolysis, photolysis and semiconductor catalysts resulting in higher hydroxyl radical production [1–4]. Among many semiconductor catalysts, TiO2 has been identified to possess high degradation efficiency and self-regeneration and therefore, TiO2 has been used widely as a catalyst in various processes like water purification and also in many applications [5–7]. TiO2 supported catalysts are found to be more effective as they are inert both chemically and biologically and are photo-stable with high band energy (Ebg = 3.0–3.2 eV) owing to its activity only in the ultraviolet region. The catalytic activity of TiO2 can be enhanced by tailoring it with some noble metals or metal oxides. The advantages of such modified catalyst mainly depends on the inhibition of the recombination of generated radicals, induction of photoreactions even in solar visible region and thus the metal doped catalyst can be used for any type of degradation studies [8–12]. The degradation capability of doped TiO2 is based on the catalytic activity which
* Corresponding author. Fax: +91 431 2500133. E-mail address: [email protected] (T. Sivasankar). http://dx.doi.org/10.1016/j.cep.2015.10.019 0255-2701/ ã 2015 Elsevier B.V. All rights reserved.
occur while both sono and photo reactions taking place: (i) Sonolysis of reactants in the presence of catalyst produces more H and OH radicals (ii) Photolysis reaction in which the photo irradiation of the catalyst (absorption of photons by the catalyst tends to produce electron hole pairs where the loosely bound electrons at valence band and conduction band gets excited by charge transfer among the available electrons) [13,14]. The degradation of organic pollutants by sonophotocatalysis takes place at the aqueous phase where lot of electrons and holes are available for excitation by the process of cavitation and photooxidation. The catalytic activity is limited by the adsorption of the organic compounds (the complex and/or recalcitrant azo dyes with nitrogen double bond) on the catalytic surface which are difficult to break down into simpler compounds. Other technologies used for dye degradation accomplishes the transfer of these compounds from one form to another form for instance, from azo dyes to amines which are again toxic and carcinogenic to the environment. Most of the primary methods like electrocoagulation, chlorination, ozonation and adsorption methods help in only color removal as they require advanced treatment methods to break this non-destructive dye for achieving complete removal of toxicity from these compounds. This triggers to utilize the synergistic efficiency of the sonophotocatalytic process by making use of all the radicals, electron hole pairs which are formed during the process. The rapid generation of more radicals can be enabled by introducing a new metal ion to TiO2. The mechanism of this sonophotocatalysis effect
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consists of the following process (a) activation of the photocatalyst surface (b) enhancement of the mass transport of organic compounds and (c) breakage of aggregation [15–17]. The degradation of azo dye in an aqueous solution using doped catalyst portrays a new type of hybrid technology for environmental remediation [18]. The mineralization and recovery of catalysts employed are more desirable than mere decolorisation in view of environmental safety. The various processes individually have failed to remove the total organic carbon (TOC) [19]. Researchers have found that the combined effect of sonophotocatalysis assisted with green synthesized doped catalyst has been successful in removing the TOC much better than the other process. In this study, synthesized TiO2, green synthesized Fe and Fe doped TiO2 have been used as catalysts for the sonophotocatalytic degradation of the Naphthol Blue Black (NBB) dye. Initially, a series of experiments were carried out by varying initial solution pH, gas bubbling (argon, nitrogen, oxygen and air) and H2O2 concentration to understand the mechanism of degradation of the NBB dye using sonophotolytic technique. Then, the treatment efficiency of the NBB dye with synthesized catalyst such as Fe, TiO2 and Fe doped TiO2 with varying ratios was studied. The catalysts were characterized by SEM, XRD and DRS to understand the morphology, purity and light absorption capacity. Finally, based on the results of NBB dye treatment, the sonophotocatalytic technique was applied to real textile wastewater and was reported in terms of total organic carbon variation. 2. Materials and methods 2.1. Materials Naphthol Blue Black (Sigma–Aldrich), concentrated H2SO4 (Merck), NaOH (Merck), hydrogen peroxide (30%, Merck), titanium tetra-iso-propoxide (Merck), ethanol (Merck), FeSO4.7H2O (Merck), argon gas (99.99%), oxygen gas (99.99%), nitrogen gas (99.99%) (Priyam gas agency, Trichy, India), air (compressed air) and green tea leaves (purchased from local market) were used for this study. Double distilled water was used for preparing all the solutions.
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operating power of 200 W and has the provision for water circulation. The water level in the ultrasound bath, the reactor position and the reactor immersion height in the ultrasound bath were maintained same for all the studies. All the experiments were carried out in the setup as schematically represented in Fig 1. 2.3. Methods A stock solution of 1000 mg/L concentration of the NBB dye solution was prepared using 0.1 g of NBB dissolved in 100 mL distilled water. The mixture was then stirred for 10 min with the help of magnetic stirrer and stored in dark for further solution preparation of required concentration. The parametric studies were carried out with initial pH, cavitation bubble content and hydrogen peroxide concentration variations. In order to study the effect of initial pH on sonophotolysis of the NBB dye solution, the pH of the dye solution was adjusted either using 0.1N concentrated H2SO4 or 0.1N NaOH and was monitored with full featured multiparameter instrument (YSI Inc., Professional Plus, USA). The gas content in the cavitation bubble was varied by purging the required gas such as nitrogen, oxygen, argon and air at a flow rate of 1 LPM for 5 min for the saturation of the gas in the NBB dye solution just prior to sonophotolysis. In the case of H2O2 addition study, the H2O2 of required quantity was added to the NBB dye solution just before the start the of the sonophotolysis experiment in order to avoid any pre-oxidation reaction. All the color removal studies were carried out for 1 hour unless otherwise mentioned and 400 mL of the reaction solution was used. The variation in dye color change at different time intervals (15, 30, 45 and 60 min) was monitored by taking intermittent dye solution and measuring its absorbance value (at 618 nm the lmax for NBB dye) in a UV–vis spectrophotometer (Merck Spectroquant Pharo 100). The percent dye color removal was calculated based on the absorbance value of the dye using Eq. (1). % Colour removal ¼
Ci Cf 100 Ci
ð1Þ
where, Ci—initial concentration and Cf —final concentration after treatment.
2.2. Experimental setup 2.4. Synthesis of catalysts The experimental setup for sonophotocatalysis consists of Phillips UV-C (Philips, 256 nm) lamp of 4W placed over the reactor of 500 mL volume made of borosilicate glass. The reactor was placed in the ultrasound bath (Dakshin, Mumbai) filled with double distilled water which operates at 37 kHz frequency with an
For the synthesis of Fe [20], 2 g of green tea leaves was taken and brewed it for 10 min. in 100 mL of Millipore water at 80 C. The solution was then cooled down and filtered through Whatman filter paper no.1. Then, 13.9 g of FeSO4 was added to the filtered
Fig. 1. Schematic of sonophotolytic experimental setup.
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solution and sonicated for a period of 30 min using an ultrasonic probe (SONICS VCX 500) that operates at 20 kHz frequency with a variable power up to 500 W and with a tip diameter of 13 mm was dipped in a jacketed glass reactor of 100 mL volume. Black color Fe precipitate forms and it was then filtered and dried in a hot air oven at 105 C for 24 h. The particle thus obtained was ground well with a pestle and mortar to get a fine powder. The use of green tea extract (polyphenols) are advantageous such as, it avoids the use of stabilizers, non-toxic, reduces metals easily and the presence of alcoholic functional groups make it good reducing agent [20]. For the synthesis of TiO2, 20 mL of Millipore water was taken and 3 mL of ethanol was added to it. Then, 2 mL of titanium tetra-isopropoxide (TTIP) was added drop by drop with constant stirring in a magnetic stirrer. The particles, thus obtained were filtered through Whatman filter paper no.1. The filter paper was then dried in hot air oven at 105 C for 24 h. The particles obtained were then scraped off and grinded finely in a mortar and pestle. The chemical reaction takes place as follows [21]: ð2Þ Ti OCHðCH3 Þ2 4 þ 6C2 H5 OH ÞÞÞUSÞÞÞ TiO2 þ8ðCH3 Þ2 CHOH For the synthesis of Fe doped TiO2, 20 mL Millipore water 1 mL ethanol was added as like the TiO2 synthesis process. As it was intended to produce different doping ratios of Fe and TiO2, the amount of Fe to be doped, 0.1, 0.2, 0.3 and 0.4 g of Fe was added to this solution, respectively. Then, 1 mL of TTIP was added drop by drop while the solution was sonicated for a period of 30 min and the rest of the procedures were followed as described in the Fe synthesis process. The synthesized samples (Fe, TiO2, Fe doped TiO2) were analyzed for SEM (Carl Zeiss, ULTRA plus, Germany), XRD (Cu Ka radiation, D8 advance Bruker, Germany) and DRS (Specord (Analytikjena) S-600 UV–vis spectrophotometer, Germany).
3. Results and discussion Initially, individual sonolysis, photolysis and sonophotolysis experiments were performed to understand the trend in color removal of NBB dye under these conditions. All the results reported were for a NBB dye concentration of 10 mg/L with the solution pH being 6.8. It was found from Fig 2 that sonophotolysis treatment proved superior exhibiting better color removal of 3% when compared to simple sonolysis (2%) and photolysis (1%). Hence, it was proved that the synergistic effect of sonolysis and photolysis was more effective than individual processes. Although, direct comparison of sonolysis and photolysis cannot be made as both processes exhibit different mechanism for the production of hydroxyl radicals as discussed earlier in the manuscript. This study was performed to show that the hybrid (sonophotolysis) system treats the dye in a more efficient manner than the individual system. Moreover, sonolysis process utilizes the higher ultrasound power (200 W) than the photolysis (4 W) process which induces higher production of hydroxyl radicals. Another important factor that accounts for the lower color rate of NBB dye would be its higher solubility (>100 g/L at 20 C) and lower vapor pressure (4.8E1029 mm Hg at 25 C) characteristics. The major sonochemical degradation reaction for those pollutants with higher solubility and lower vapor pressure would be in the bulk liquid medium through hydroxylation reaction [23]. Since, the produced OH radicals (from H2O) out of sonophotolysis process are highly unstable and reactive [24], there was a higher possibility of recombination with H radicals to H2O without reacting with NBB dye molecule which remains in the bulk liquid medium due to its hydrophilic (deprotonated) nature [25]. As sonophotolytic process had provided higher color removal, the rest of the experimental studies were focused on improving this process using different methodologies to achieve better color removal.
2.5. Naphthol Blue Black dye color removal using synthesized catalyst 3.1. Effect of different initial solution pH in sonophotolysis process The removal of color from NBB dye was observed by performing experiments of sonophotocatalysis in the presence of catalysts of varying dosages for TiO2 (0.2, 1, 2, 3, 4, 5 g/L), Fe (0.2, 1, 2, 3, 4 g/L) and Fe doped TiO2 (2 g/L with different doping compositions of Fe and TiO2). The intermittent sampling and color removal efficiency of the treated NBB dye solution was done as reported for parametric studies. The catalyst was separated from the treated NBB dye solution by centrifuging at 2500 RPM and the supernatant was taken for analysis. 2.6. Real textile wastewater treatment using synthesized catalyst
The pH of the aqueous solution has a greater influence on the treatment of pollutants by sonolysis and photolysis. Fig 3 shows the color removal of NBB dye by sonophotolysis at different initial solution pH (2, 3, 4, 5, 6, 7, 8, 9, 10) and the results points out that the efficiency of NBB dye was highly dependent on pH. The NBB dye had exhibited highest color removal under extreme acidic (pH—2) condition and lowest at extreme basic (pH—10) basic conditions accounting to 41 and 2% respectively. It was a well known fact that at acidic pH most of the organic pollutants would tend to attain hydrophobic (protonated) form or ionic to molecular form [26,27]. Due to this hydrophobic nature, the NBB dye molecule
The real textile wastewater was collected at the outlet of the equalization tank from a textile processing industry in Periyasemur village, Erode District, Tamilnadu, India. The wastewater was stored at 4 C in the refrigerator and brought to room temperature before the sonophotocatalytic study. Immediately after the wastewater collection, its physical and chemical characteristics were examined in the laboratory using standard methods procedure [22]. Similar experimental setup and methodology as followed for sonophotocatalytic treatment of NBB dye was used for textile wastewater as well using the synthesized catalyst, Fe, TiO2 and Fe-TiO2, of varying dosages. Unlike the NBB dye solution, for textile wastewater the extent of treatment was followed with Total Organic Carbon (TOC) content variation in different time intervals. The TOC analysis was performed in a Total Organic Carbon analyzer (Shimadzu, TOC-LCPN, Japan). All these studies were performed in triplicate and the average was presented. Fig. 2. Treatment of NBB dye with simple sonolysis, photolysis and sonophotolysis (NBB: 10 mg/L, pH: 6.8, insert picture: molecular structure of NBB dye).
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time of bubble collapse than those with gases of low specific heat ratio. Although, the amount of OH radicals produced are nearly same for air and nitrogen cavitation bubble, nitrogen presence scavenges the radicals produced forming nitrogen oxides [28]. The solubility of the gas in the liquid medium also influences the radical production, gas of high solubility enriches the solution with more number of cavitation nuclei and thus, the OH radicals produced also enriches. Again, argon has a higher solubility nature [29] than the rest of the gases which supports the experimental results.
45 40 % Colour Removal
13
35 30 25 20
15 10 5 0 2
3
4
5
6 pH
7
8
9
10
Fig. 3. Effect of initial solution pH on sonophotolytic treatment of NBB dye (NBB: 10 mg/L, reaction time: 60 min).
moves toward the cavitation bubble or near to bubble-bulk interface or near to the site of OH radical production which could readily react with the highly reactive OH radicals formed out of sonolysis and photolysis coupled process. The initial solution pH variation study infers that even in extreme pH condition or changing the nature of the NBB dye (i.e. hydrophobic) had resulted in only 41% color removal. 3.2. Effect of type of cavitation bubble content on sonophotolysis One way of improving the hydroxylation reaction or production of OH radicals or oxidation reaction in a sonophotolysis process would be varying the cavitation bubble content by purging the NBB dye solution with respective gases to saturation before being subjected to sonophotolysis treatment. Fig 4 shows the effect of cavitation bubble content on the NBB dye treatment. Among the gases, argon cavitation bubble had resulted in the highest color removal of 17% as it has a high specific heat ratio of 1.66 while nitrogen cavitation bubble had resulted in the lowest color removal of 6% as it has a low specific heat ratio of 1.41. While, oxygen and air cavitation bubble exhibited 12 and 8 % color removal respectively. The trend in color removal rate was in line with our earlier study on OH radical production of these gases by cavitation bubble dynamics [23]. The findings showed that the amount of OH radicals produced out of the argon cavitation bubble was higher followed by oxygen and it was nearly the same for air and nitrogen cavitation bubble but lower than argon and oxygen bubble. The inert nature and high specific heat ratio of argon leads to higher temperature and pressure conditions at the
Fig. 4. Effect of gas content on sonophotolytic treatment of NBB dye (NBB: 10 mg/L, pH: 6.8).
3.3. Effect of variation in concentration of H2O2 in sonophotolysis The addition of H2O2 into the aqueous solution in a sonophotolysis process would disintegrate them into OH radicals [30] and this increases the availability of OH radicals in the bulk liquid medium, a favorable condition for oxidation of hydrophilic NBB dye molecules. The degradation of dye increases with increase in concentration of H2O2 of 8.82, 17.64, 26.46, 35.28, 44.1 mmol/L and it decreases further with the increase in H2O2 concentration of 52.92 mmol/L as shown in Fig 5. The color removal of NBB dye showed a drastic increase with H2O2 addition indicating higher OH radicals production in the bulk liquid medium enabling better interaction with NBB dye molecules. The addition of 44.1 mmol/L of H2O2 could remove 94% color. Further increase in H2O2 concentration (i.e. 52.92 or >44.1 mmol/L) causes the produced OH radicals to form H2O2 again [31,32] thus reducing the availability of OH radicals in the bulk liquid medium for the NBB dye to undergo oxidation. From the above parametric studies, the inferences that could be made are (i) initial pH variation or hydrophobic nature of NBB dye would increase the hydroxylation reaction to a certain extent (i.e. hydroxylation reaction in the bubble-bulk interface), (ii) cavitation bubble contents such as argon and oxygen had enhanced the hydroxylation reaction but only to a little extent and (iii) addition of H2O2 which readily produces OH radicals increased the hydroxylation reaction to a very large extent providing 94% color removal the highest of all the parameters. Although H2O2 addition removed 94% color removal, the quantity utilized was large and becomes practically difficult considering its hazard nature and storage issues [33]. Hence, a better method as equivalent to the efficiency of H2O2 also both economically and practically feasible method was required. The following section, i.e. the synthesis of a new nanocatalyst (Fe doped TiO2), characterization and its
Fig. 5. Effect of H2O2 concentration (mmol/L) on sonophotolytic treatment of NBB dye (NBB: 10 mg/L, pH: 6.8).
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application on the treatment of NBB dye, examines the possibility of satisfying these requirements. 3.4. Catalyst characterization The SEM images of Fe, TiO2, Fe doped TiO2 are shown in Fig 6A– C. The SEM image of Fe clustered aggregates representing their crystalline cubic indices as visualized (Fig 6A). The TiO2 particles were spherical shaped and distributed uniformly (Fig 6B). The doping of Fe on the TiO2 crystal matrix has been clearly identified by the homogenous distribution of TiO2 with Fe (Fig 6C). The agglomeration observed in both the images (Fe doped TiO2 and Fe) were due to the Vanderwaals forces among the magnetic Fe
particles and higher band gap energy of TiO2 particles. The high rate of Fe doping was evident as the doping reaction takes place efficiently in a solution rather than in a solid state of Fe. XRD spectra of Fe, TiO2 and Fe doped TiO2 (1:7 wt%) as shown in Fig 6D represents that the doping of Fe with TiO2 has changed its crystalline behavior. It was also found that the mixing of TiO2 has caused the increase in particle size of Fe. The peaks of Fe representing the Miller indices of (11 0) and (2 11) are characteristics of Fe which are found at the 2u angle of 28.75 and 45.76 respectively. However, TiO2 doping has been clearly confirmed by the reduced Fe characteristic peaks and also by high intensity anatase peaks (2u = 25.36 , 37.72 , 48.13 , 54.24 , 55.03 , 62.20 , 69.24 , 70.58 , 75.43 as per JCPDS No.21-1272) with lesser rutile
Fig. 6. Characterization of catalyst, (A) SEM of Fe, (B) SEM of TiO2, (C) SEM of Fe-TiO2, (D) XRD spectrum, (E) DRS spectrum.
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peaks (2u = 27.5 , 41.62 as per JCPDS No. 21-1276). The pure TiO2 constitutes the most active anatase and also some of the other crystalline rutile phases. The replacement of Fe in the crystal framework of TiO2 has been attributed to the reduction of the rutile phases thereby suggesting the active doping [34–36]. It is also due to the reduction in the oxygen vacancies on the TiO2 surface that inhibits the crystallization of other phases. The influence of doping on the particle size has been identified by XRD spectra and it was in good agreement with the SEM images. The UV–vis absorption spectrum wave measured under the diffuse reflectance mode in the range of 200–800 nm using a UV– vis spectrophotometer with an integrating sphere accessory. This sample exhibit significant increase in the photo absorption at wavelength greater than 375 nm. DRS was a powerful technique to provide direct information on the light absorption nature of different species. The Fe, TiO2 and Fe doped TiO2 sample show the variation in the absorption of light as illustrated in Fig 6E. However, the extent of absorption was higher on Fe-TiO2 than pure TiO2 and Fe. The pure TiO2 was characterized by lowest light absorption and when doped with Fe the light absorption had increased significantly indicating the suitability of this catalyst for photolysis reactions [37,38]. 3.5. Effect of catalyst dosage on sonophotolysis 3.5.1. TiO2 and Fe catalysts Effect of catalyst dosage (0.2, 1, 2, 3, 4 and 5 g/L) on sonophotolytic treatment of 10 mg/L NBB dye was studied by comparing the color removal efficiency of synthesized catalysts such as TiO2 and Fe. Fig 7 shows that by varying the dosage of catalyst the percentage of color removal increases up to a certain level and further increase had resulted in a decrease in the color removal percentage. With the TiO2 and Fe catalyst the maximum color removal observed was 58% for 4 g/L dosage and 55% for 3 g/L dosage respectively. Overloading of the catalyst reduced the color removal efficiency. This may be attributed due to the following reasons: (i) higher concentration of the catalyst results in increased turbidity that hinders the UV light penetration, (ii) high dosage of catalyst increases the OH radical concentration in the solution which can also act as a OH scavenger and (iii) the intensity of ultrasound decreases with increased quantity of particles [39]. Higher color removal with TiO2 catalyst than Fe catalyst could be explained by the fact that the particle size of TiO2 was relatively lower than Fe (from SEM images), TiO2 has a good UV light absorption capacity than Fe which allows them to produce
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higher OH radical than Fe and the TiO2 particles possess uniform spherical shape indicating higher surface active sites than Fe for OH radical production. This process utilizes the UV light and cavitation process effectively. In the case of Fe, the reaction could be initiated only with the strong oxidant H2O2 through the combination of two OH radicals released out of the transient cavitation bubble collapse. Fenton like reaction occurs when the Fe ions transform to Fe2+ ion which then reacts with H2O2 to form OH radicals and subsequently to Fe3+. In this process, the limiting step would be the cavitation reaction and only little contribution by the UV light. The blank experiments were performed with 4 g/L of TiO2 and 3 g/L of Fe to the 10 mg/L of NBB dye solution under dark with mild stirring so as to avoid settling of particles. The color removal accounts to adsorption and it was 3% for TiO2 and 2% for Fe. This implies that the contribution by the adsorption of NBB dye on the catalyst was minimal compared to the treatment efficiency of sonophotocatalysis. 3.5.2. Fe doped TiO2 Doping was made by varying the amount of Fe catalyst on TiO2. The different doping ratios studied are 7:1, 2:1, 1:1, 1:2 and 1:7 (Fe: TiO2). Fig 8 shows that maximum color removal was observed with 1:7 (Fe:TiO2) ratio which was 73%. For the comparison among the doped catalyst a concentration of 2 g/L was used. This shows that higher TiO2 percentage with lower Fe content had resulted higher color removal indicating that TiO2 content was the dominating factor in the color removal of NBB dye under the conditions studied. The following synergistic mechanisms would have assisted in achieving this highest color removal, (i) OH radical production from simple sonolysis process, (ii) semiconductor TiO2 catalyst’s UV light absorption and its electron hole excites to form OH radicals in reaction with water molecules [40], (iii) Fenton reaction due to oxidation of Fe particles to Fe2+ reacting with H2O2 (formed out of unreacted OH radicals) to form OH radical and Fe3+ ion and there was a possibility of regeneration of Fe2+ ion through the reaction of Fe3+ ion with H2O in the presence of UV light which again forms OH radical [41], (iv) OH radical formation by photolysis of H2O2 [42], (v) continuous regeneration of catalyst surface by the cavitation process [43], (vi) reduction of band energy of TiO2 particles due to Fe doping that enhances its activity upon UV light absorption [44–46], (vii) simple adsorption of the dye molecule over the catalyst surface. All the catalyst dosage studies were done without altering the initial solution pH (i.e. 6.8). Alternately, the catalyst with higher Fe and lower TiO2 also showed
80
60 70 60
% Colour Removal
% Colour rem oval
50
40
30
20
50 40 30 20
TiO2
10
10
Fe
0
0 0
1
2
3
4
5
Catalyst dosage (g/L) Fig. 7. Effect of Fe and TiO2 catalyst on sonophotocatalytic treatment of NBB dye (NBB: 10 mg/L, pH: 6.8, reaction time: 60 min).
7:1
2:1
1:1
2:1
1:7
TiO2:Fe Fig. 8. Effect of Fe doped TiO2 with various doping ratio on sonophotocatalytic treatment of NBB dye (NBB: 10 mg/L, Fe-TiO2: 2 g/L, pH: 6.8, reaction time: 60 min).
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Table 1 Physical–chemical characteristics of textile wastewater.
3.6. Sonophotocatalytic treatment of textile wastewater
SI. No.
Characteristics
Value
1. 2. 3. 4. 5. 6. 7. 8.
pH Turbidity (NTU) Total dissolved solids (mg/L) Total suspended solids (mg/L) Conductivity (Micromho’s/cm) BOD5 at 20 C (mg/L) COD (mg/L) TOC (mg/L)
8.05 20.4 21832 60 36400 118 2760 423
an increased color removal with an increase in Fe content, however, there exists a measurable difference in the color removal efficiency with higher TiO2 and lower Fe catalyst being at higher side. For example, 1:7 (Fe:TiO2) catalyst showed 73% color removal whereas it was 63% for 7:1 (Fe:TiO2) catalyst. As like the individual catalyst, blank (dark) experiments for doped catalyst (2 g/L of 1:7 (Fe:TiO2)) was also performed and the adsorption contributes to 6% under dark for the 10 mg/L NBB dye solution. Clearly, the doped catalyst has an upper edge over the individual catalyst and the similar kind of effect was observed by many researchers [47,48]. In order to achieve better efficiency with the doped catalyst of 1:7 (Fe:TiO2) ratio, the concentration variation (2.1, 2.2, 2.3, 2.4 and 2.5 g/L) study was performed under the same experimental conditions as done earlier. The maximum color removal of 96% was obtained with the catalyst concentration of 2.2 g/L and the color removal rate had fallen with further increase (>2.2 g/L) and the reasons were stated earlier. The inferences obtained from these studies were among the parametric variations and catalysts, the individual catalyst (1:7 Fe doped TiO2 catalyst) treatment of sonophotolysis proved effective deducing a maximum of 96 % color removal for 10 NBB dye solution. Although, higher color removal was observed for H2O2 addition in comparison with doped catalyst, doped catalyst was superior over H2O2 as doped catalysts are easily separable, easy to handle and safe whereas H2O2 was toxic and difficult to handle. Also, the cost comparison between the process, sonolysis—28 USD/ m3/hr, photolysis—0.4 USD/m3/hr, sonophotolysis—28.4 USD/m3/ hr, sonophotolysis +44.1 mmol/L H2O2—528.4 USD/m3/hr and sonophotolysis +2.2 g/L Fe doped TiO2—416.4 USD/m3/hr, adds further justification to the conclusion that sonophotolysis in the presence of Fe doped TiO2 catalyst has cost advantage (i.e. 20% lower cost) too over H2O2 assisted sonophotolysis.
The practical applicability of the sonophotocatalytic technique as studied with NBB dye was investigated for real textile wastewater. Table 1 shows that the textile wastewater was characterized by dark blue color, slight basic nature, higher total dissolved solids (TDS), chemical oxygen demand (COD) and conductivity, moderate biochemical oxygen demand (BOD5) and higher total organic carbon (TOC). Based on the results of NBB dye, the treatment of textile wastewater was restricted to catalytic study (TiO2, Fe and Fe-TiO2) only and the sonophotocatalytic treatment efficiency was reported in terms of TOC variation as shown in Fig 9. A similar kind of trend as observed with NBB dye was seen with wastewater in the presence of a catalyst. The doped catalyst (Fe-TiO2) had pronounced effect on the TOC removal when compared with Fe and TiO2. The TOC removal had attained a maximum of 92% with 3 g/L of Fe-TiO2 whereas 85% with 4 g/L of Fe and 88% with 5 g/L of TiO2 respectively. Beyond these catalyst dosage no significant change in the TOC was observed for either of the catalyst and dosage. The reason behind the higher TOC reduction for doped catalyst with lower dosage than individual catalyst was due to its synergistic effects and was stated in the previous section. The dark experiments with the optimized catalyst concentration had resulted in a different phenomena for the textile wastewater compared to NBB dye. The adsorption had accounted for 42, 28 and 35% TOC removal for 4 g/L of Fe, 5 g/L of TiO2 and 3 g/L of Fe-TiO2, respectively. 4. Conclusions The combined sonolysis and photolysis process (3% color removal) was more beneficial than individual sonolysis (2% color removal) and photolysis (1% color removal) from the experimental results of NBB dye. From the parametric studies, it was understood that the production of OH radicals and its effective interaction with the NBB dye molecules was essential. Acidic initial solution pH of 2 (41%), argon bubble gas content (17%) and the addition of 44.1 mmol/L H2O2 (94%) showed better color removal rates under sonophotolysis of NBB dye indicating the significance for the dye molecule to move near the OH radical production site, higher OH radical production in the bubble-bulk interface and higher OH radical production in the bulk liquid medium for the effective interaction with the dye molecules. The sonochemically synthesized of nanoparticles such as Fe, TiO2 and Fe doped TiO2 were characterized by spherical shape, nanosize, uniform in shape and
Fig. 9. Sonophotocatalytic treatment of textile wastewater with Fe, TiO2 and Fe doped TiO2 catalyst (pH: 8.05, reaction time: 60 min).
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high purity based on SEM, XRD and DRS analysis. As evident the doped catalyst had exhibited higher color removal of 96% with lower catalyst concentration (2.2 g/L) compared to Fe and TiO2 catalyst. Cost analysis among the processes studied revealed that sonophotolysis of NBB dye in the presence of Fe doped TiO2 was found economical with respect to the color removal achieved. The application of the sonophotocatalytic methodology to real textile wastewater showed positive results in terms of TOC removal. Again, Fe doped TiO2 catalyst with lower dosage showed higher TOC of 91% than the Fe and TiO2 catalyst. Hence, the studied methodology proved effective both for the synthetic NBB dye wastewater and real textile wastewater. Conflict of interest The authors declare that they have no conflict of interest. Acknowledgements The authors were grateful to the Ministry of Environment and Forest (MoEF), Government of India for the financial funding of the project. The authors also thank the Environmental Engineering Laboratory, Department of Civil Engineering, National Insitute of Technology Tiruchirappalli for tendering their help in textile wastewater characterization and TOC analysis. We thank the anonymous reviewers for their valuable comments and suggestions for the improvement of this manuscript. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.cep.2015.10.019. References [1] R.M. Nimlos, W.A. Jacoby, D.M. Blake, T.A. Milne, Gas-phase photocatalytic oxidation of trichloroethylene over-products and mechanisms, Environ. Sci. Technol. 27 (1993) 732–740. [2] M.R. Hoffmann, S.T. Martin, W. Choi, D.W. Bahnemann, Environmental applications of semiconductor photocatalysis, Chem. Rev. 95 (1995) 69–96. [3] P.R. Gogate, A.B. Pandit, A review of imperative technologies for wastewater treatment II: hybrid methods, Adv. Environ. Res. 8 (2004) 553–597. [4] P.R. Gogate, A.B. Pandit, Sonophotocatalytic reactors for wastewater streatment: a critical review, AIChE J. 50 (2004) 1051–1079. [5] A. Kudo, Y. Miseki, Heterogeneous photocatalyst materials for water splitting, Chem. Soc. Rev. 38 (2009) 253–278. [6] K. Rajeshwar, C.R. Chenthamarakshan, S. Goeringer, M. Djukic, Titania-based heterogeneous photocatalysis. Materials mechanistic issues, and implications for environmental remediation, Pure Appl. Chem. 73 (2001) 1849–1860. [7] T. Aarthi, P. Narahari, G. Madras, Photocatalytic degradation of Azure and Sudan dyes using nano TiO2, J. Hazard. Mater. 149 (2007) 725–734. [8] A. Fujishima, T.N. Rao, D.A. Tryk, Titanium dioxide photocatalysis, J. Photochem. Photobiol. C 1 (2000) 1–21. [9] O. Carp, C.L. Huisman, A. Reller, Photoinduced reactivity of titanium dioxide, Prog. Solid State Chem. 32 (2004) 33–177. [10] S.F. Chen, G.Y. Cao, Photocatalytic oxidation of nitrite by sunlight using TiO2 supported on hollow glass microbeads, Sol. Energy 73 (2002) 15–21. [11] R. Asahi, T. Morikawa, T. Ohwaki, K. Aoki, Y. Taga, Visible-light photocatalysis in nitrogen-doped titanium oxides, Science 293 (2001) 269–271. [12] C. Wang, J.C. Zhao, X. Wang, B.X. Mai, G.Y. Sheng, P.A. Peng, J.M. Fu, Preparation, characterization and photocatalytic activity of nano-sized ZnO/SnO2 coupled photocatalysts, Appl. Catal. B 39 (2002) 269–279. [13] Y.G. Adewuyi, Sonochemistry: environmental science and engineering applications, Ind. Eng. Chem. Res. 40 (2001) 4681–4715. [14] Z. Eren, Ultrasound as a basic and auxiliary process for dye remediation: a review, J. Environ. Manage. 104 (2012) 127–141. [15] J.M. Herrmann, J. Disdier, P. Pichat, Photoassisted platinum deposition on TiO2 powder using various platinum complexes, J. Phys. Chem. 90 (1986) 6028–6034. [16] Y. Cho, W. Choi, Visible light-induced reactions of humic acids on TiO2, J. Photochem. Photobiol. A 148 (2002) 129–135. [17] X.Z. Li, F.B. Li, Study of Au/Au3+–TiO2 photocatalysts toward visible photooxidation for water and wastewater treatment, Environ. Sci. Technol. 35 (2001) 2381–2387.
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Contents lists available at ScienceDirect
Chemical Engineering and Processing: Process Intensification journal homepage: www.elsevier.com/locate/cep
Sonophotocatalytic treatment of Naphthol Blue Black dye and real textile wastewater using synthesized Fe doped TiO2 D. Rahul Reddya , G. Kumaravel Dinesha , Sambandam Anandanb , Thirugnanasambandam Sivasankara,* a b
Department of Chemical Engineering, National Institute of Technology Tiruchirappalli, Tamilnadu, India Department of Chemistry, National Institute of Technology Tiruchirappalli, Tamilnadu, India
A R T I C L E I N F O
A B S T R A C T
Article history: Received 27 August 2015 Received in revised form 26 October 2015 Accepted 27 October 2015 Available online 1 November 2015
This study examines the treatment of Naphthol-Blue–Black (NBB) dye and real textile wastewater through sonophotocatalytic technique. Fe, TiO2 and Fe-TiO2 were synthesized and characterized by SEM, XRD and DRS which revealed its high purity, good doping, nanosize and higher light absorption capacity. The effect of pH, gas content (Ar, O2, air and N2), H2O2 concentration and catalyst loading were examined. The maximum color removal was observed for initial pH 11, argon gas bubble, 44.1 mmol/L H2O2, 3 g/L Fe, 4 g/L TiO2 and 2.2 g/L Fe-TiO2. However, 96% was achieved by sonophotocatalytic process for 2.2 g/L 1:7 Fe:TiO2. Similar methodology for real textile wastewater with Fe-TiO2 (1:7) could remove 91% of TOC. ã 2015 Elsevier B.V. All rights reserved.
Keywords: Naphthol Blue Black dye Sonophotocatalysis Fe doped TiO2 Textile wastewater TOC removal
1. Introduction Advanced oxidation processes (AOPs) have been used as emerging wastewater treatment technologies for various hazardous organic pollutants. Sonophotocatalysis is gaining attention among advanced oxidation process by which the electron hole pairs are generated at high temperature and pressure by the integration of sonolysis, photolysis and semiconductor catalysts resulting in higher hydroxyl radical production [1–4]. Among many semiconductor catalysts, TiO2 has been identified to possess high degradation efficiency and self-regeneration and therefore, TiO2 has been used widely as a catalyst in various processes like water purification and also in many applications [5–7]. TiO2 supported catalysts are found to be more effective as they are inert both chemically and biologically and are photo-stable with high band energy (Ebg = 3.0–3.2 eV) owing to its activity only in the ultraviolet region. The catalytic activity of TiO2 can be enhanced by tailoring it with some noble metals or metal oxides. The advantages of such modified catalyst mainly depends on the inhibition of the recombination of generated radicals, induction of photoreactions even in solar visible region and thus the metal doped catalyst can be used for any type of degradation studies [8–12]. The degradation capability of doped TiO2 is based on the catalytic activity which
* Corresponding author. Fax: +91 431 2500133. E-mail address: [email protected] (T. Sivasankar). http://dx.doi.org/10.1016/j.cep.2015.10.019 0255-2701/ ã 2015 Elsevier B.V. All rights reserved.
occur while both sono and photo reactions taking place: (i) Sonolysis of reactants in the presence of catalyst produces more H and OH radicals (ii) Photolysis reaction in which the photo irradiation of the catalyst (absorption of photons by the catalyst tends to produce electron hole pairs where the loosely bound electrons at valence band and conduction band gets excited by charge transfer among the available electrons) [13,14]. The degradation of organic pollutants by sonophotocatalysis takes place at the aqueous phase where lot of electrons and holes are available for excitation by the process of cavitation and photooxidation. The catalytic activity is limited by the adsorption of the organic compounds (the complex and/or recalcitrant azo dyes with nitrogen double bond) on the catalytic surface which are difficult to break down into simpler compounds. Other technologies used for dye degradation accomplishes the transfer of these compounds from one form to another form for instance, from azo dyes to amines which are again toxic and carcinogenic to the environment. Most of the primary methods like electrocoagulation, chlorination, ozonation and adsorption methods help in only color removal as they require advanced treatment methods to break this non-destructive dye for achieving complete removal of toxicity from these compounds. This triggers to utilize the synergistic efficiency of the sonophotocatalytic process by making use of all the radicals, electron hole pairs which are formed during the process. The rapid generation of more radicals can be enabled by introducing a new metal ion to TiO2. The mechanism of this sonophotocatalysis effect
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consists of the following process (a) activation of the photocatalyst surface (b) enhancement of the mass transport of organic compounds and (c) breakage of aggregation [15–17]. The degradation of azo dye in an aqueous solution using doped catalyst portrays a new type of hybrid technology for environmental remediation [18]. The mineralization and recovery of catalysts employed are more desirable than mere decolorisation in view of environmental safety. The various processes individually have failed to remove the total organic carbon (TOC) [19]. Researchers have found that the combined effect of sonophotocatalysis assisted with green synthesized doped catalyst has been successful in removing the TOC much better than the other process. In this study, synthesized TiO2, green synthesized Fe and Fe doped TiO2 have been used as catalysts for the sonophotocatalytic degradation of the Naphthol Blue Black (NBB) dye. Initially, a series of experiments were carried out by varying initial solution pH, gas bubbling (argon, nitrogen, oxygen and air) and H2O2 concentration to understand the mechanism of degradation of the NBB dye using sonophotolytic technique. Then, the treatment efficiency of the NBB dye with synthesized catalyst such as Fe, TiO2 and Fe doped TiO2 with varying ratios was studied. The catalysts were characterized by SEM, XRD and DRS to understand the morphology, purity and light absorption capacity. Finally, based on the results of NBB dye treatment, the sonophotocatalytic technique was applied to real textile wastewater and was reported in terms of total organic carbon variation. 2. Materials and methods 2.1. Materials Naphthol Blue Black (Sigma–Aldrich), concentrated H2SO4 (Merck), NaOH (Merck), hydrogen peroxide (30%, Merck), titanium tetra-iso-propoxide (Merck), ethanol (Merck), FeSO4.7H2O (Merck), argon gas (99.99%), oxygen gas (99.99%), nitrogen gas (99.99%) (Priyam gas agency, Trichy, India), air (compressed air) and green tea leaves (purchased from local market) were used for this study. Double distilled water was used for preparing all the solutions.
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operating power of 200 W and has the provision for water circulation. The water level in the ultrasound bath, the reactor position and the reactor immersion height in the ultrasound bath were maintained same for all the studies. All the experiments were carried out in the setup as schematically represented in Fig 1. 2.3. Methods A stock solution of 1000 mg/L concentration of the NBB dye solution was prepared using 0.1 g of NBB dissolved in 100 mL distilled water. The mixture was then stirred for 10 min with the help of magnetic stirrer and stored in dark for further solution preparation of required concentration. The parametric studies were carried out with initial pH, cavitation bubble content and hydrogen peroxide concentration variations. In order to study the effect of initial pH on sonophotolysis of the NBB dye solution, the pH of the dye solution was adjusted either using 0.1N concentrated H2SO4 or 0.1N NaOH and was monitored with full featured multiparameter instrument (YSI Inc., Professional Plus, USA). The gas content in the cavitation bubble was varied by purging the required gas such as nitrogen, oxygen, argon and air at a flow rate of 1 LPM for 5 min for the saturation of the gas in the NBB dye solution just prior to sonophotolysis. In the case of H2O2 addition study, the H2O2 of required quantity was added to the NBB dye solution just before the start the of the sonophotolysis experiment in order to avoid any pre-oxidation reaction. All the color removal studies were carried out for 1 hour unless otherwise mentioned and 400 mL of the reaction solution was used. The variation in dye color change at different time intervals (15, 30, 45 and 60 min) was monitored by taking intermittent dye solution and measuring its absorbance value (at 618 nm the lmax for NBB dye) in a UV–vis spectrophotometer (Merck Spectroquant Pharo 100). The percent dye color removal was calculated based on the absorbance value of the dye using Eq. (1). % Colour removal ¼
Ci Cf 100 Ci
ð1Þ
where, Ci—initial concentration and Cf —final concentration after treatment.
2.2. Experimental setup 2.4. Synthesis of catalysts The experimental setup for sonophotocatalysis consists of Phillips UV-C (Philips, 256 nm) lamp of 4W placed over the reactor of 500 mL volume made of borosilicate glass. The reactor was placed in the ultrasound bath (Dakshin, Mumbai) filled with double distilled water which operates at 37 kHz frequency with an
For the synthesis of Fe [20], 2 g of green tea leaves was taken and brewed it for 10 min. in 100 mL of Millipore water at 80 C. The solution was then cooled down and filtered through Whatman filter paper no.1. Then, 13.9 g of FeSO4 was added to the filtered
Fig. 1. Schematic of sonophotolytic experimental setup.
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solution and sonicated for a period of 30 min using an ultrasonic probe (SONICS VCX 500) that operates at 20 kHz frequency with a variable power up to 500 W and with a tip diameter of 13 mm was dipped in a jacketed glass reactor of 100 mL volume. Black color Fe precipitate forms and it was then filtered and dried in a hot air oven at 105 C for 24 h. The particle thus obtained was ground well with a pestle and mortar to get a fine powder. The use of green tea extract (polyphenols) are advantageous such as, it avoids the use of stabilizers, non-toxic, reduces metals easily and the presence of alcoholic functional groups make it good reducing agent [20]. For the synthesis of TiO2, 20 mL of Millipore water was taken and 3 mL of ethanol was added to it. Then, 2 mL of titanium tetra-isopropoxide (TTIP) was added drop by drop with constant stirring in a magnetic stirrer. The particles, thus obtained were filtered through Whatman filter paper no.1. The filter paper was then dried in hot air oven at 105 C for 24 h. The particles obtained were then scraped off and grinded finely in a mortar and pestle. The chemical reaction takes place as follows [21]: ð2Þ Ti OCHðCH3 Þ2 4 þ 6C2 H5 OH ÞÞÞUSÞÞÞ TiO2 þ8ðCH3 Þ2 CHOH For the synthesis of Fe doped TiO2, 20 mL Millipore water 1 mL ethanol was added as like the TiO2 synthesis process. As it was intended to produce different doping ratios of Fe and TiO2, the amount of Fe to be doped, 0.1, 0.2, 0.3 and 0.4 g of Fe was added to this solution, respectively. Then, 1 mL of TTIP was added drop by drop while the solution was sonicated for a period of 30 min and the rest of the procedures were followed as described in the Fe synthesis process. The synthesized samples (Fe, TiO2, Fe doped TiO2) were analyzed for SEM (Carl Zeiss, ULTRA plus, Germany), XRD (Cu Ka radiation, D8 advance Bruker, Germany) and DRS (Specord (Analytikjena) S-600 UV–vis spectrophotometer, Germany).
3. Results and discussion Initially, individual sonolysis, photolysis and sonophotolysis experiments were performed to understand the trend in color removal of NBB dye under these conditions. All the results reported were for a NBB dye concentration of 10 mg/L with the solution pH being 6.8. It was found from Fig 2 that sonophotolysis treatment proved superior exhibiting better color removal of 3% when compared to simple sonolysis (2%) and photolysis (1%). Hence, it was proved that the synergistic effect of sonolysis and photolysis was more effective than individual processes. Although, direct comparison of sonolysis and photolysis cannot be made as both processes exhibit different mechanism for the production of hydroxyl radicals as discussed earlier in the manuscript. This study was performed to show that the hybrid (sonophotolysis) system treats the dye in a more efficient manner than the individual system. Moreover, sonolysis process utilizes the higher ultrasound power (200 W) than the photolysis (4 W) process which induces higher production of hydroxyl radicals. Another important factor that accounts for the lower color rate of NBB dye would be its higher solubility (>100 g/L at 20 C) and lower vapor pressure (4.8E1029 mm Hg at 25 C) characteristics. The major sonochemical degradation reaction for those pollutants with higher solubility and lower vapor pressure would be in the bulk liquid medium through hydroxylation reaction [23]. Since, the produced OH radicals (from H2O) out of sonophotolysis process are highly unstable and reactive [24], there was a higher possibility of recombination with H radicals to H2O without reacting with NBB dye molecule which remains in the bulk liquid medium due to its hydrophilic (deprotonated) nature [25]. As sonophotolytic process had provided higher color removal, the rest of the experimental studies were focused on improving this process using different methodologies to achieve better color removal.
2.5. Naphthol Blue Black dye color removal using synthesized catalyst 3.1. Effect of different initial solution pH in sonophotolysis process The removal of color from NBB dye was observed by performing experiments of sonophotocatalysis in the presence of catalysts of varying dosages for TiO2 (0.2, 1, 2, 3, 4, 5 g/L), Fe (0.2, 1, 2, 3, 4 g/L) and Fe doped TiO2 (2 g/L with different doping compositions of Fe and TiO2). The intermittent sampling and color removal efficiency of the treated NBB dye solution was done as reported for parametric studies. The catalyst was separated from the treated NBB dye solution by centrifuging at 2500 RPM and the supernatant was taken for analysis. 2.6. Real textile wastewater treatment using synthesized catalyst
The pH of the aqueous solution has a greater influence on the treatment of pollutants by sonolysis and photolysis. Fig 3 shows the color removal of NBB dye by sonophotolysis at different initial solution pH (2, 3, 4, 5, 6, 7, 8, 9, 10) and the results points out that the efficiency of NBB dye was highly dependent on pH. The NBB dye had exhibited highest color removal under extreme acidic (pH—2) condition and lowest at extreme basic (pH—10) basic conditions accounting to 41 and 2% respectively. It was a well known fact that at acidic pH most of the organic pollutants would tend to attain hydrophobic (protonated) form or ionic to molecular form [26,27]. Due to this hydrophobic nature, the NBB dye molecule
The real textile wastewater was collected at the outlet of the equalization tank from a textile processing industry in Periyasemur village, Erode District, Tamilnadu, India. The wastewater was stored at 4 C in the refrigerator and brought to room temperature before the sonophotocatalytic study. Immediately after the wastewater collection, its physical and chemical characteristics were examined in the laboratory using standard methods procedure [22]. Similar experimental setup and methodology as followed for sonophotocatalytic treatment of NBB dye was used for textile wastewater as well using the synthesized catalyst, Fe, TiO2 and Fe-TiO2, of varying dosages. Unlike the NBB dye solution, for textile wastewater the extent of treatment was followed with Total Organic Carbon (TOC) content variation in different time intervals. The TOC analysis was performed in a Total Organic Carbon analyzer (Shimadzu, TOC-LCPN, Japan). All these studies were performed in triplicate and the average was presented. Fig. 2. Treatment of NBB dye with simple sonolysis, photolysis and sonophotolysis (NBB: 10 mg/L, pH: 6.8, insert picture: molecular structure of NBB dye).
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time of bubble collapse than those with gases of low specific heat ratio. Although, the amount of OH radicals produced are nearly same for air and nitrogen cavitation bubble, nitrogen presence scavenges the radicals produced forming nitrogen oxides [28]. The solubility of the gas in the liquid medium also influences the radical production, gas of high solubility enriches the solution with more number of cavitation nuclei and thus, the OH radicals produced also enriches. Again, argon has a higher solubility nature [29] than the rest of the gases which supports the experimental results.
45 40 % Colour Removal
13
35 30 25 20
15 10 5 0 2
3
4
5
6 pH
7
8
9
10
Fig. 3. Effect of initial solution pH on sonophotolytic treatment of NBB dye (NBB: 10 mg/L, reaction time: 60 min).
moves toward the cavitation bubble or near to bubble-bulk interface or near to the site of OH radical production which could readily react with the highly reactive OH radicals formed out of sonolysis and photolysis coupled process. The initial solution pH variation study infers that even in extreme pH condition or changing the nature of the NBB dye (i.e. hydrophobic) had resulted in only 41% color removal. 3.2. Effect of type of cavitation bubble content on sonophotolysis One way of improving the hydroxylation reaction or production of OH radicals or oxidation reaction in a sonophotolysis process would be varying the cavitation bubble content by purging the NBB dye solution with respective gases to saturation before being subjected to sonophotolysis treatment. Fig 4 shows the effect of cavitation bubble content on the NBB dye treatment. Among the gases, argon cavitation bubble had resulted in the highest color removal of 17% as it has a high specific heat ratio of 1.66 while nitrogen cavitation bubble had resulted in the lowest color removal of 6% as it has a low specific heat ratio of 1.41. While, oxygen and air cavitation bubble exhibited 12 and 8 % color removal respectively. The trend in color removal rate was in line with our earlier study on OH radical production of these gases by cavitation bubble dynamics [23]. The findings showed that the amount of OH radicals produced out of the argon cavitation bubble was higher followed by oxygen and it was nearly the same for air and nitrogen cavitation bubble but lower than argon and oxygen bubble. The inert nature and high specific heat ratio of argon leads to higher temperature and pressure conditions at the
Fig. 4. Effect of gas content on sonophotolytic treatment of NBB dye (NBB: 10 mg/L, pH: 6.8).
3.3. Effect of variation in concentration of H2O2 in sonophotolysis The addition of H2O2 into the aqueous solution in a sonophotolysis process would disintegrate them into OH radicals [30] and this increases the availability of OH radicals in the bulk liquid medium, a favorable condition for oxidation of hydrophilic NBB dye molecules. The degradation of dye increases with increase in concentration of H2O2 of 8.82, 17.64, 26.46, 35.28, 44.1 mmol/L and it decreases further with the increase in H2O2 concentration of 52.92 mmol/L as shown in Fig 5. The color removal of NBB dye showed a drastic increase with H2O2 addition indicating higher OH radicals production in the bulk liquid medium enabling better interaction with NBB dye molecules. The addition of 44.1 mmol/L of H2O2 could remove 94% color. Further increase in H2O2 concentration (i.e. 52.92 or >44.1 mmol/L) causes the produced OH radicals to form H2O2 again [31,32] thus reducing the availability of OH radicals in the bulk liquid medium for the NBB dye to undergo oxidation. From the above parametric studies, the inferences that could be made are (i) initial pH variation or hydrophobic nature of NBB dye would increase the hydroxylation reaction to a certain extent (i.e. hydroxylation reaction in the bubble-bulk interface), (ii) cavitation bubble contents such as argon and oxygen had enhanced the hydroxylation reaction but only to a little extent and (iii) addition of H2O2 which readily produces OH radicals increased the hydroxylation reaction to a very large extent providing 94% color removal the highest of all the parameters. Although H2O2 addition removed 94% color removal, the quantity utilized was large and becomes practically difficult considering its hazard nature and storage issues [33]. Hence, a better method as equivalent to the efficiency of H2O2 also both economically and practically feasible method was required. The following section, i.e. the synthesis of a new nanocatalyst (Fe doped TiO2), characterization and its
Fig. 5. Effect of H2O2 concentration (mmol/L) on sonophotolytic treatment of NBB dye (NBB: 10 mg/L, pH: 6.8).
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application on the treatment of NBB dye, examines the possibility of satisfying these requirements. 3.4. Catalyst characterization The SEM images of Fe, TiO2, Fe doped TiO2 are shown in Fig 6A– C. The SEM image of Fe clustered aggregates representing their crystalline cubic indices as visualized (Fig 6A). The TiO2 particles were spherical shaped and distributed uniformly (Fig 6B). The doping of Fe on the TiO2 crystal matrix has been clearly identified by the homogenous distribution of TiO2 with Fe (Fig 6C). The agglomeration observed in both the images (Fe doped TiO2 and Fe) were due to the Vanderwaals forces among the magnetic Fe
particles and higher band gap energy of TiO2 particles. The high rate of Fe doping was evident as the doping reaction takes place efficiently in a solution rather than in a solid state of Fe. XRD spectra of Fe, TiO2 and Fe doped TiO2 (1:7 wt%) as shown in Fig 6D represents that the doping of Fe with TiO2 has changed its crystalline behavior. It was also found that the mixing of TiO2 has caused the increase in particle size of Fe. The peaks of Fe representing the Miller indices of (11 0) and (2 11) are characteristics of Fe which are found at the 2u angle of 28.75 and 45.76 respectively. However, TiO2 doping has been clearly confirmed by the reduced Fe characteristic peaks and also by high intensity anatase peaks (2u = 25.36 , 37.72 , 48.13 , 54.24 , 55.03 , 62.20 , 69.24 , 70.58 , 75.43 as per JCPDS No.21-1272) with lesser rutile
Fig. 6. Characterization of catalyst, (A) SEM of Fe, (B) SEM of TiO2, (C) SEM of Fe-TiO2, (D) XRD spectrum, (E) DRS spectrum.
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peaks (2u = 27.5 , 41.62 as per JCPDS No. 21-1276). The pure TiO2 constitutes the most active anatase and also some of the other crystalline rutile phases. The replacement of Fe in the crystal framework of TiO2 has been attributed to the reduction of the rutile phases thereby suggesting the active doping [34–36]. It is also due to the reduction in the oxygen vacancies on the TiO2 surface that inhibits the crystallization of other phases. The influence of doping on the particle size has been identified by XRD spectra and it was in good agreement with the SEM images. The UV–vis absorption spectrum wave measured under the diffuse reflectance mode in the range of 200–800 nm using a UV– vis spectrophotometer with an integrating sphere accessory. This sample exhibit significant increase in the photo absorption at wavelength greater than 375 nm. DRS was a powerful technique to provide direct information on the light absorption nature of different species. The Fe, TiO2 and Fe doped TiO2 sample show the variation in the absorption of light as illustrated in Fig 6E. However, the extent of absorption was higher on Fe-TiO2 than pure TiO2 and Fe. The pure TiO2 was characterized by lowest light absorption and when doped with Fe the light absorption had increased significantly indicating the suitability of this catalyst for photolysis reactions [37,38]. 3.5. Effect of catalyst dosage on sonophotolysis 3.5.1. TiO2 and Fe catalysts Effect of catalyst dosage (0.2, 1, 2, 3, 4 and 5 g/L) on sonophotolytic treatment of 10 mg/L NBB dye was studied by comparing the color removal efficiency of synthesized catalysts such as TiO2 and Fe. Fig 7 shows that by varying the dosage of catalyst the percentage of color removal increases up to a certain level and further increase had resulted in a decrease in the color removal percentage. With the TiO2 and Fe catalyst the maximum color removal observed was 58% for 4 g/L dosage and 55% for 3 g/L dosage respectively. Overloading of the catalyst reduced the color removal efficiency. This may be attributed due to the following reasons: (i) higher concentration of the catalyst results in increased turbidity that hinders the UV light penetration, (ii) high dosage of catalyst increases the OH radical concentration in the solution which can also act as a OH scavenger and (iii) the intensity of ultrasound decreases with increased quantity of particles [39]. Higher color removal with TiO2 catalyst than Fe catalyst could be explained by the fact that the particle size of TiO2 was relatively lower than Fe (from SEM images), TiO2 has a good UV light absorption capacity than Fe which allows them to produce
15
higher OH radical than Fe and the TiO2 particles possess uniform spherical shape indicating higher surface active sites than Fe for OH radical production. This process utilizes the UV light and cavitation process effectively. In the case of Fe, the reaction could be initiated only with the strong oxidant H2O2 through the combination of two OH radicals released out of the transient cavitation bubble collapse. Fenton like reaction occurs when the Fe ions transform to Fe2+ ion which then reacts with H2O2 to form OH radicals and subsequently to Fe3+. In this process, the limiting step would be the cavitation reaction and only little contribution by the UV light. The blank experiments were performed with 4 g/L of TiO2 and 3 g/L of Fe to the 10 mg/L of NBB dye solution under dark with mild stirring so as to avoid settling of particles. The color removal accounts to adsorption and it was 3% for TiO2 and 2% for Fe. This implies that the contribution by the adsorption of NBB dye on the catalyst was minimal compared to the treatment efficiency of sonophotocatalysis. 3.5.2. Fe doped TiO2 Doping was made by varying the amount of Fe catalyst on TiO2. The different doping ratios studied are 7:1, 2:1, 1:1, 1:2 and 1:7 (Fe: TiO2). Fig 8 shows that maximum color removal was observed with 1:7 (Fe:TiO2) ratio which was 73%. For the comparison among the doped catalyst a concentration of 2 g/L was used. This shows that higher TiO2 percentage with lower Fe content had resulted higher color removal indicating that TiO2 content was the dominating factor in the color removal of NBB dye under the conditions studied. The following synergistic mechanisms would have assisted in achieving this highest color removal, (i) OH radical production from simple sonolysis process, (ii) semiconductor TiO2 catalyst’s UV light absorption and its electron hole excites to form OH radicals in reaction with water molecules [40], (iii) Fenton reaction due to oxidation of Fe particles to Fe2+ reacting with H2O2 (formed out of unreacted OH radicals) to form OH radical and Fe3+ ion and there was a possibility of regeneration of Fe2+ ion through the reaction of Fe3+ ion with H2O in the presence of UV light which again forms OH radical [41], (iv) OH radical formation by photolysis of H2O2 [42], (v) continuous regeneration of catalyst surface by the cavitation process [43], (vi) reduction of band energy of TiO2 particles due to Fe doping that enhances its activity upon UV light absorption [44–46], (vii) simple adsorption of the dye molecule over the catalyst surface. All the catalyst dosage studies were done without altering the initial solution pH (i.e. 6.8). Alternately, the catalyst with higher Fe and lower TiO2 also showed
80
60 70 60
% Colour Removal
% Colour rem oval
50
40
30
20
50 40 30 20
TiO2
10
10
Fe
0
0 0
1
2
3
4
5
Catalyst dosage (g/L) Fig. 7. Effect of Fe and TiO2 catalyst on sonophotocatalytic treatment of NBB dye (NBB: 10 mg/L, pH: 6.8, reaction time: 60 min).
7:1
2:1
1:1
2:1
1:7
TiO2:Fe Fig. 8. Effect of Fe doped TiO2 with various doping ratio on sonophotocatalytic treatment of NBB dye (NBB: 10 mg/L, Fe-TiO2: 2 g/L, pH: 6.8, reaction time: 60 min).
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Table 1 Physical–chemical characteristics of textile wastewater.
3.6. Sonophotocatalytic treatment of textile wastewater
SI. No.
Characteristics
Value
1. 2. 3. 4. 5. 6. 7. 8.
pH Turbidity (NTU) Total dissolved solids (mg/L) Total suspended solids (mg/L) Conductivity (Micromho’s/cm) BOD5 at 20 C (mg/L) COD (mg/L) TOC (mg/L)
8.05 20.4 21832 60 36400 118 2760 423
an increased color removal with an increase in Fe content, however, there exists a measurable difference in the color removal efficiency with higher TiO2 and lower Fe catalyst being at higher side. For example, 1:7 (Fe:TiO2) catalyst showed 73% color removal whereas it was 63% for 7:1 (Fe:TiO2) catalyst. As like the individual catalyst, blank (dark) experiments for doped catalyst (2 g/L of 1:7 (Fe:TiO2)) was also performed and the adsorption contributes to 6% under dark for the 10 mg/L NBB dye solution. Clearly, the doped catalyst has an upper edge over the individual catalyst and the similar kind of effect was observed by many researchers [47,48]. In order to achieve better efficiency with the doped catalyst of 1:7 (Fe:TiO2) ratio, the concentration variation (2.1, 2.2, 2.3, 2.4 and 2.5 g/L) study was performed under the same experimental conditions as done earlier. The maximum color removal of 96% was obtained with the catalyst concentration of 2.2 g/L and the color removal rate had fallen with further increase (>2.2 g/L) and the reasons were stated earlier. The inferences obtained from these studies were among the parametric variations and catalysts, the individual catalyst (1:7 Fe doped TiO2 catalyst) treatment of sonophotolysis proved effective deducing a maximum of 96 % color removal for 10 NBB dye solution. Although, higher color removal was observed for H2O2 addition in comparison with doped catalyst, doped catalyst was superior over H2O2 as doped catalysts are easily separable, easy to handle and safe whereas H2O2 was toxic and difficult to handle. Also, the cost comparison between the process, sonolysis—28 USD/ m3/hr, photolysis—0.4 USD/m3/hr, sonophotolysis—28.4 USD/m3/ hr, sonophotolysis +44.1 mmol/L H2O2—528.4 USD/m3/hr and sonophotolysis +2.2 g/L Fe doped TiO2—416.4 USD/m3/hr, adds further justification to the conclusion that sonophotolysis in the presence of Fe doped TiO2 catalyst has cost advantage (i.e. 20% lower cost) too over H2O2 assisted sonophotolysis.
The practical applicability of the sonophotocatalytic technique as studied with NBB dye was investigated for real textile wastewater. Table 1 shows that the textile wastewater was characterized by dark blue color, slight basic nature, higher total dissolved solids (TDS), chemical oxygen demand (COD) and conductivity, moderate biochemical oxygen demand (BOD5) and higher total organic carbon (TOC). Based on the results of NBB dye, the treatment of textile wastewater was restricted to catalytic study (TiO2, Fe and Fe-TiO2) only and the sonophotocatalytic treatment efficiency was reported in terms of TOC variation as shown in Fig 9. A similar kind of trend as observed with NBB dye was seen with wastewater in the presence of a catalyst. The doped catalyst (Fe-TiO2) had pronounced effect on the TOC removal when compared with Fe and TiO2. The TOC removal had attained a maximum of 92% with 3 g/L of Fe-TiO2 whereas 85% with 4 g/L of Fe and 88% with 5 g/L of TiO2 respectively. Beyond these catalyst dosage no significant change in the TOC was observed for either of the catalyst and dosage. The reason behind the higher TOC reduction for doped catalyst with lower dosage than individual catalyst was due to its synergistic effects and was stated in the previous section. The dark experiments with the optimized catalyst concentration had resulted in a different phenomena for the textile wastewater compared to NBB dye. The adsorption had accounted for 42, 28 and 35% TOC removal for 4 g/L of Fe, 5 g/L of TiO2 and 3 g/L of Fe-TiO2, respectively. 4. Conclusions The combined sonolysis and photolysis process (3% color removal) was more beneficial than individual sonolysis (2% color removal) and photolysis (1% color removal) from the experimental results of NBB dye. From the parametric studies, it was understood that the production of OH radicals and its effective interaction with the NBB dye molecules was essential. Acidic initial solution pH of 2 (41%), argon bubble gas content (17%) and the addition of 44.1 mmol/L H2O2 (94%) showed better color removal rates under sonophotolysis of NBB dye indicating the significance for the dye molecule to move near the OH radical production site, higher OH radical production in the bubble-bulk interface and higher OH radical production in the bulk liquid medium for the effective interaction with the dye molecules. The sonochemically synthesized of nanoparticles such as Fe, TiO2 and Fe doped TiO2 were characterized by spherical shape, nanosize, uniform in shape and
Fig. 9. Sonophotocatalytic treatment of textile wastewater with Fe, TiO2 and Fe doped TiO2 catalyst (pH: 8.05, reaction time: 60 min).
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high purity based on SEM, XRD and DRS analysis. As evident the doped catalyst had exhibited higher color removal of 96% with lower catalyst concentration (2.2 g/L) compared to Fe and TiO2 catalyst. Cost analysis among the processes studied revealed that sonophotolysis of NBB dye in the presence of Fe doped TiO2 was found economical with respect to the color removal achieved. The application of the sonophotocatalytic methodology to real textile wastewater showed positive results in terms of TOC removal. Again, Fe doped TiO2 catalyst with lower dosage showed higher TOC of 91% than the Fe and TiO2 catalyst. Hence, the studied methodology proved effective both for the synthetic NBB dye wastewater and real textile wastewater. Conflict of interest The authors declare that they have no conflict of interest. Acknowledgements The authors were grateful to the Ministry of Environment and Forest (MoEF), Government of India for the financial funding of the project. The authors also thank the Environmental Engineering Laboratory, Department of Civil Engineering, National Insitute of Technology Tiruchirappalli for tendering their help in textile wastewater characterization and TOC analysis. We thank the anonymous reviewers for their valuable comments and suggestions for the improvement of this manuscript. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.cep.2015.10.019. References [1] R.M. Nimlos, W.A. Jacoby, D.M. Blake, T.A. Milne, Gas-phase photocatalytic oxidation of trichloroethylene over-products and mechanisms, Environ. Sci. Technol. 27 (1993) 732–740. [2] M.R. Hoffmann, S.T. Martin, W. Choi, D.W. Bahnemann, Environmental applications of semiconductor photocatalysis, Chem. Rev. 95 (1995) 69–96. [3] P.R. Gogate, A.B. Pandit, A review of imperative technologies for wastewater treatment II: hybrid methods, Adv. Environ. Res. 8 (2004) 553–597. [4] P.R. Gogate, A.B. Pandit, Sonophotocatalytic reactors for wastewater streatment: a critical review, AIChE J. 50 (2004) 1051–1079. [5] A. Kudo, Y. Miseki, Heterogeneous photocatalyst materials for water splitting, Chem. Soc. Rev. 38 (2009) 253–278. [6] K. Rajeshwar, C.R. Chenthamarakshan, S. Goeringer, M. Djukic, Titania-based heterogeneous photocatalysis. Materials mechanistic issues, and implications for environmental remediation, Pure Appl. Chem. 73 (2001) 1849–1860. [7] T. Aarthi, P. Narahari, G. Madras, Photocatalytic degradation of Azure and Sudan dyes using nano TiO2, J. Hazard. Mater. 149 (2007) 725–734. [8] A. Fujishima, T.N. Rao, D.A. Tryk, Titanium dioxide photocatalysis, J. Photochem. Photobiol. C 1 (2000) 1–21. [9] O. Carp, C.L. Huisman, A. Reller, Photoinduced reactivity of titanium dioxide, Prog. Solid State Chem. 32 (2004) 33–177. [10] S.F. Chen, G.Y. Cao, Photocatalytic oxidation of nitrite by sunlight using TiO2 supported on hollow glass microbeads, Sol. Energy 73 (2002) 15–21. [11] R. Asahi, T. Morikawa, T. Ohwaki, K. Aoki, Y. Taga, Visible-light photocatalysis in nitrogen-doped titanium oxides, Science 293 (2001) 269–271. [12] C. Wang, J.C. Zhao, X. Wang, B.X. Mai, G.Y. Sheng, P.A. Peng, J.M. Fu, Preparation, characterization and photocatalytic activity of nano-sized ZnO/SnO2 coupled photocatalysts, Appl. Catal. B 39 (2002) 269–279. [13] Y.G. Adewuyi, Sonochemistry: environmental science and engineering applications, Ind. Eng. Chem. Res. 40 (2001) 4681–4715. [14] Z. Eren, Ultrasound as a basic and auxiliary process for dye remediation: a review, J. Environ. Manage. 104 (2012) 127–141. [15] J.M. Herrmann, J. Disdier, P. Pichat, Photoassisted platinum deposition on TiO2 powder using various platinum complexes, J. Phys. Chem. 90 (1986) 6028–6034. [16] Y. Cho, W. Choi, Visible light-induced reactions of humic acids on TiO2, J. Photochem. Photobiol. A 148 (2002) 129–135. [17] X.Z. Li, F.B. Li, Study of Au/Au3+–TiO2 photocatalysts toward visible photooxidation for water and wastewater treatment, Environ. Sci. Technol. 35 (2001) 2381–2387.
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