CHAPTER 2 LITERATURE REVIEW AND SCOPE OF THE PRESENT STUDY

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1 19 CHAPTER 2 LITERATURE REVIEW AND SCOPE OF THE PRESENT STUDY 2.1 LITERATURE REVIEW AOPs techniques are widely used to remove dyes from textile wastewaters. The AOPs, which include H 2 O 2 /Fe 2+, UV/H 2 O 2 /Fe 2+, O 3, UV/O 3, UV/H 2 O 2, O 3 /UV/H 2 O 2, O 3 /H 2 O 2, Ultrasound, Electrochemical oxidation, Electro Fenton and combination of AOPs have been used for the removal of dyes in wastewater. The following sections describe the detailed literature survey of different AOPS and their efficiency in the removal of synthetic dyes. 2.2 ADVANCED OXIDATION PROCESSES Fenton s Process Sureyya et al (2003) studied the degradation of Reactive Black 5 from synthetic wastewater using Fenton s oxidation (FO) process. The study was performed in a systematic approach searching optimum values of FeSO 4 and H 2 O 2 concentrations, ph and temperature. Optimum ph and temperature for 100 mg l -1 of Reactive Black 5 were observed as 3.0 and 40 C, respectively, using 100 mg l -1 of FeSO 4 and 400 mg l -1 of H 2 O 2 resulted in 71 % Chemical Oxygen Demand (COD) and 99 % color removal. For 200 mg l -1 of Reactive Black 5, 84 % COD removal and more than 99 % color

2 20 removal were obtained using 225 mg l -1 of FeSO 4 and 1000 mg l -1 of H 2 O 2 yielding 0.05 molar ratios at ph 3.0 and 40 C. Xu et al (2004) performed the degradation of 20 different dyes in aqueous solutions by the Fenton process. These dyes include 6 types: acidic, reactive, direct, cationic, disperse and vat dyes. The former four types of dyes were decolorized and their TOC values were decreased greatly, while the color and TOC removals of the latter two types were lower. The catalytic activities of four metal ions on the degradation efficiencies of Vat Blue, which was chosen as a model dye because of its lowest color and TOC removals, were compared in the dark and under the ultraviolet light irradiation. The catalytic ability of different metals was Fe 2+ > Cu 2+ > Mn 2+ > Ag + in the dark, and the same sequence was obtained under irradiation condition with greater degradation efficiency. Furthermore, the efficiencies of three oxidation processes, including H 2 O 2 /UV, Fe 2+ /H 2 O 2 and Fe 2+ /H 2 O 2 /UV were compared. The results showed that the oxidation by Fe 2+ /H 2 O 2 /UV was the strongest, and even greater than the arithmetic sum of the other two processes, which suggests the synergistic effect of ultraviolet and ferrous ions on the degradation reaction. Sureyya et al (2005) used Fenton's oxidation (FO) to decolorize and degrade some reactive dyes (Remazol Black 5, Remazol Red, Remazol Blue, Remazol Yellow) and raw textile finishing industry effluents (S1, S2, S3) containing mainly reactive dyes. The operational conditions for ph varied between 2.5 and 4.0 while temperature ranged from 30 C to 50 C. The concentrations of FeSO 4 and H 2 O 2 varied to a wide range ( mg l -1 of FeSO 4, mg l -1 of H 2 O 2 ) depending on the type of the dyes and their mixture and textile additives used in the process. FO was highly effective for color removal (>99 %) for reactive dyes and (87-94 %) for textile finishing wastewater.

3 21 Yu et al (2005) investigated the removal of color, dye and dissolved organic carbon by Fenton using the synthetic dye wastewaters containing various dyes (Reactive blue 19, Eriochrome Black T and Fast Green FCF). The results indicated that discoloration of dyes was very rapid but mineralization of dyes was insignificant based on the removal of dissolved organic carbon. The rates of color, dye and dissolved organic carbon removal were in the order of reactive blue 19>Fast Green FCF>Eriochrome Black T. Zeynep et al (2006) investigated to efficiently operate Fenton's reagent oxidation to degrade a reactive dye, CI Reactive Yellow 15, which is non-biodegradable and has high chemical oxygen demand. The results obtained show that the best ph value for decolorization was ph 3, with an average decolorization of 98.7 % and average removal of chemical oxygen demand 93.3 % at 15 C for a molar ratio of Fenton's reagent. An increase in temperature resulted in higher removal rates. Nilesh et al (2006) investigated a two stage sequential Fenton s oxidation followed by aerobic biological treatment to achieve decolorization and to enhance mineralization of azo dyes, viz. Reactive Black 5, Reactive Blue 13, and Acid Orange 7. Results reveal that ph 3 was optimum ph for achieving decolorization and dearomatization of dyes by Fenton s process. More than 95 % of color was removed with Fenton s oxidation process in all dyes. In overall treatment train 81.95, 85.57, and % of COD reduction was achieved in Reactive Black 5, Reactive Blue 13, and Acid Orange 7 dyes, respectively. In the Fenton s oxidation process 56, 24.5 and 80 % reduction in naphthalene group was observed in Reactive Black 5, Reactive Blue 13 and Acid Orange 7, respectively, which further increased to 81.34, and 92 % after aerobic treatment. Fenton s oxidation process followed by aerobic SBRs treatment sequence seems to be viable method for achieving significant degradation of azo dye.

4 22 Jian et al (2007) investigated the degradation of an azo dye Amido black 10 B in aqueous solution by Fenton oxidation process. The optimal reacting conditions were experimentally determined and it was found to be initial ph = 3.50, H 2 O 2 = 0.50 mm, Fe 2+ = mm for dye = 50 mg l -1 at temperature = 25 C. Under optimal conditions, % degradation efficiency of dye in aqueous solution was achieved after 60 min of reaction. The experimental results showed that the Fenton oxidation process was an effective process for the degradation of azo dye Amido black 10 B at low H 2 O 2 and Fe 2+ concentrations. Arslan and Teksoy (2007) studied the synthetic acid dye bath effluent (SADB) bearing two azo and one anthraquinone dye together with two dye auxiliaries subjected to pretreatment with Fenton's reagent. Obtained results indicated that 30 % COD and practically complete color removal (99 %) could be achieved at T = 50 C. The kinetic studies revealed that a strong correlation existed between COD removal and H 2 O 2 utilization rates. In the final part of the study, the acute toxicity of raw (untreated) and pretreated SADB on heterotrophic biomass was investigated employing a modified (COD-balanced), activated sludge inhibition test. The toxicity experiments demonstrated that the inhibitory effect of SADB towards sewage sludge could be completely eliminated when the effluent was pretreated with Fenton's reagent. Gutowska et al (2007) used Fenton process for the degradation of Reactive Orange 113, an azo derivative in the majority of synthetic dyes currently used in the textile industry. With this method, it was possible to obtain a decolorization higher than 80 % after 2 hours of oxidation. The highest removal efficiency was obtained with a concentration of Fe 2+ ions ranging from 5 to 10 mg l -1. Furthermore, an increase in the H 2 O 2 concentration of up to 0.4 mg per 1 mg of dye led to a 100 % total

5 23 decolorization. The results of this study showed that the optimal conditions for the removal of this dye were a concentration of Fe 2+ ions of 10 mg l -1 ; 0.4 mg of H 2 O 2 per mg of dye; and a ph value of 3. Under these conditions after 5 h of oxidation, the reduction of the COD initial value was 70 %, and after 20 h, it was higher than 90 %. Riga et al (2007) also used a homogeneous oxidation process to degrade another type of commercial textile dye. The optimal decolorization rate was obtained with an initial dye concentration of 75 mg l -1 ; a FeSO 4 initial concentration of w/w; and an H 2 O 2 initial concentration range of % w/w. At a higher FeSO 4 initial concentration of w/w, decolorization occurred even more swiftly, especially at H 2 O 2 initial concentrations lower than 0.1 % w/w. The decolorization efficiency in this case was %. Furthermore, it was found that the addition of CO 3, HCO 3 and SO 4, among others, to the dye solutions had an adverse effect on the decolorization rates. Filiz et al (2008) investigated advanced oxidation of Direct Red 28 in aqueous solution by Fenton's reagent using FeSO 4 as source of Fe (II). Complete color removal (100 %) was achieved in 5 minutes. However, mineralization of the dyestuff took 15 minutes and required higher doses of H 2 O 2. Percent color removal was always higher than TOC removal indicating formation of colorless organic intermediates. Optimal H 2 O 2 /Fe (II)/ dyestuff ratio resulting in the maximum TOC (99.2 %) and color (100 %) removals was found to be 1450/78/235 (mg l -1 ). Rodrigues et al (2009) studied the Chemical oxidation by Fenton's reagent of a reactive azo dye (Procion Deep Red H-EXL gran) solution and optimized making use of the experimental design methodology. The variables considered for the oxidative process optimization were the temperature and the initial concentrations of hydrogen peroxide and ferrous ion, for a dye

6 24 concentration of 100 mg l -1 at ph 3.5, the latter being fixed after some preliminary runs A quadratic model with good adherence to the experimental data in the domain analyzed was developed, which was used to plot the response surface curves and to perform process optimization. It was concluded that temperature and ferrous ion concentration are the only variables that affect TOC removal, and due to the cross-interactions, the effect of each variable depends on the value of the other one, thus affecting positively or negatively the process response Photo-Fenton Process Arslan et al (1999) used UV/Fenton, near-uv-visible/fenton, dark Fenton, and H 2 O 2 /UV processes to treat simulated dye house effluents representing wastewater from the textile dyeing and rinsing process. Experiments were carried out in a lab-scale photochemical reactor using concentrations of mm H 2 O 2, mm Fe 2+ ion and different dilutions of textile wastewater. To assess the extent of mineralization, decolorization kinetics and the effect of different light sources on treatment efficiency, DOC, optical density at 254 nm and 600 nm wavelength and residual H 2 O 2 concentrations were measured during the course of the advanced oxidation reactions. Comparative evaluation of the obtained results revealed that the decolorization rate increased with applied H 2 O 2 and Fe 2+ -ion dose as well as the strength of the synthetic textile wastewater. The best results were obtained by the near UV/visible/Fenton process with a decolorization rate constant of 1.57 min 1, a UV 254nm reduction of 97 % and a DOC removal of 41 % at relatively low doses of the H 2 O 2 oxidant and Fe 2+ ion catalyst within 60 min treatment time. Xu et al (2001) studied the decolorization of Reactive N=N brilliant red X-3B by UV/Fenton process. Stable decoloration (Dye = M) within 20 min with H 2 O 2 = M, Fe 2+ = M, 75 W UV

7 25 ( < 320 nm) lamp. Use of Fe 2+ is preferable to Fe 3+ because of faster reaction rate with H 2 O 2 and evolution of HO. instead of HO 2. Aparna et al (2003) carried out the photo degradation of Direct yellow 12 using UV/H 2 O 2 /Fe 2+ has been carried out in a photochemical reactor. It was observed that simultaneous utilization of UV irradiation with Fenton s reagent increases the degradation rate of Direct yellow 12. The dye quickly losses its color and there is an appreciable decrease in COD value, indicating that the dissolved organic have been oxidized. The kinetics of degradation of the dye in dilute aqueous solutions follows pseudo-first order kinetics. Results indicate that dye degradation is dependent upon ph, UV-intensity, concentration of Fenton s reagent and dye. Acidic ph has been found to be more suitable in comparison to neutral and alkaline. The optimum concentration of Fenton s reagent (H 2 O 2 /Fe 2+ ) was found as 1500/500 mg l -1 for 50 mg l -1 DY12 dye in water at ph 4. The results indicate that the treatment of DY12 dye wastewater with UV/Fe 2+ /H 2 O 2 system is efficient. Park and Choi (2003) demonstrated a new photochemical remediation method for dye-polluted waters. The photo degradation of Acid Orange 7 (AO7) was successfully achieved in the presence of Fe (III) ions only under visible light ( 420 nm). Upon adding Fe (III) to AO7 solution, ferric ions formed complexes with AO7 mainly through the azo chromophoric group. The AO7-Fe (III) complex formation was highly ph sensitive and maximized around ph 3.7. The visible light induced degradation of AO7 was effective only when the complex formation was favored. The proposed mechanism of the dye degradation is the visible light induced electron transfer from the azo chromophoric group to the iron center in the complex. Therefore, when the formation of AO7-Fe (III) complex was inhibited in the presence of excess interfering anions such as sulfites and sulfates, the photo degradation of the dye was also prevented. Since this process does not require the addition

8 26 of hydrogen peroxide, it might be developed into an economically viable method to pretreat or decolorize azo-dye wastewaters using sunlight. Feng et al (2003) developed a novel nanocomposite of iron oxide and silicate, prepared through a reaction between a solution of iron salt and a dispersion of laponite clay for the photo assisted Fenton degradation of azo-dye Orange. The optimal H 2 O 2 molar concentration was about 4.8 mm for 0.1 mm Orange, and the optimal ph of the reaction solution was 3.0. The catalyst exhibits high catalytic activity only under the irradiation of UV light, and the UV light with a short wavelength is much more effective than the light with a long wavelength. An increase in the UV light power also accelerates the degradation of Orange. As the catalyst loading increases, the degradation of Orange also increases until a saturated catalyst loading is achieved. The saturated catalyst loading was about 1.0 g of Fe nanocomposite catalyst/l. It was found that the discoloration of Orange undergoes a faster kinetic than the mineralization of Orange and 75 % total organic carbon (TOC) of 0.1 mm Orange can be eliminated after 90 min in the presence of 0.1 g of Fe nanocomposite/l, 4.8 mm H 2 O 2 and 8W UV. Muruganandham and Swaminathan (2004) carried out the photochemical decolorization of chlorotriazine reactive azo dye Reactive Orange 4 by Fenton and photo-fenton processes. The effects of solution ph, applied H 2 O 2, Fe 2+ dose, UV light intensity have been studied. The increase of initial dye concentration decreases the removal rate. Under optimum conditions the photo-fenton process is found to be more efficient than Fenton process. About 2 % of color resurgent was observed at the end of the reaction. Marco and Jose (2006) investigated the oxidative decolorization of Reactive Black 5 (RB5) in aqueous solution using Fenton (H 2 O 2 /Fe 2+ ) and Photo-Fenton (H 2 O 2 /Fe 2+ /UV) processes. The optimal conditions found were a ratio [H 2 O 2 ] 0 /[RB5] 0 of 4.9:1, a ratio [H 2 O 2 ] 0 /[Fe 2+ ] 0 of 9.6:1 and ph = 3.0.

9 27 The decolorization experiments indicate that RB5 can be effectively decolorized using Fenton and photo-fenton processes with a little difference between the two processes, 97.5 % and 98.1 %, respectively, for optimal conditions. This small difference in dye decolorization is not similar to TOC removal: with photo-fenton process there was a significant increment (46.4 % TOC removal) relatively to Fenton process (only 21.6 % TOC removal). This fact indicates that although UV low-pressure mercury lamp has little effect on dye decolorization it is particularly important in dye mineralization. Kusic et al (2006) investigated the application of Fenton and photo- Fenton type processes for dye wastewater treatment. In the first stage of the study, Fenton type processes were optimized regarding the iron catalyst concentration and the iron catalyst/h 2 O 2 ratio. In the next step, Fe 2+ /H 2 O 2 and Fe 0 /H 2 O 2 at optimal process parameters were combined with UV radiation in order to enhance dye degradation. All studied processes showed high efficiency in the bleaching of the studied dye model solution with complete decolorization in all cases. Mineralization extents depended on the type and the dosage of added iron catalysts for Fenton type reaction. The highest mineralization extent was achieved by UV/Fe 0 /H 2 O 2, % of TOC removal. Liu et al (2007) investigated the degradation and decolorization of direct dye (Everdirect supra turquoise blue FBL), acidic dye (Isolan orange S-RL) and vat dye (Indanthrene red FBB) by Fenton and UV/Fenton processes. Fenton process is highly efficient for color removal for three dyes tested and for TOC removal of FBB and FBL. UV/Fenton showed slighter increase in treatment efficiency than that of Fenton process for both FBB and FBL dye solutions. S-RL improved much more TOC removal % by UV-irradiation. Sludge productions were compared for FeCl 3, FeSO 4

10 28 coagulation and Fenton reaction in order to prove the dual role of FeSO 4 in Fenton process. Patricia et al (2007) investigated the oxidation of C.I. Reactive Blue 4 by photo-fenton process mediated by ferrioxalate under artificial and solar irradiation. The Reactive Blue 4 degradation in acidic medium (ph 2.5) was evaluated by the decrease in Total Organic Carbon (TOC) content and color, measured by the decrease in chromophore absorption band (600 nm). The influence of ferrioxalate and H 2 O 2 concentrations on the dye degradation was studied and best results were obtained using 1.0 mm ferrioxalate and 10 mm of hydrogen peroxide. Under these experimental conditions, 80 % of TOC and 100 % of color removal were obtained for a 0.1 mm Reactive Blue 4 dye in 35 min of solar irradiation. Peternel et al (2007) conducted a study to discover if the photo- Fenton method could effectively degrade this Reactive Red 45. It was observed that an aqueous solution treated with an initial Reactive Red 45 dye concentration of 80 mg l -1 decolorized completely after 1 h of treatment at a range of Fe 2+ /H 2 O 2, oscillating between 1:20 and 1:100. Furthermore, when the H 2 O 2 dose was increased from 10 to 30 mm, TOC removal also increased from 53.1 % to 74.2 %. However, further increase in the H 2 O 2 dose made the mineralization process less effective. Hence, the ratio of Fe 2+ /H 2 O 2 = 1:60 was found to be optimal at ph 3, a concentration of (Fe 2+ ) at 0.5 mm, and an incident UV-light flux at Einstein/s. Modirshahla et al (2007) studied the degradation of azo dye C.I. Acid Yellow 23 by Fenton and photo-fenton processes. The degradation rate is strongly dependent on the ph, initial concentrations of the dye, Fe 2+, H 2 O 2 and UV light intensity. The effect of these parameters has been studied and the optimum operational conditions of these two processes were found. The optimum conditions were obtained at ph = 3 for the H 2 O 2 /Fe 2+ and

11 29 UV/H 2 O 2 /Fe 2+ systems. The photo-fenton process proved to be the most efficient and occurs at a much higher oxidation rate than Fenton process and allows achieving 90 % degradation of aromatic content of Acid Yellow 23 in about 10 min of reaction time. Zheng et al (2007) investigated the oxidation of acidic dye Eosin Y with Fenton process and photo-fenton process (solar light or artificial light source). With UV/Fenton process and Fenton, 42.5 % and 21.3 % of dye could be removed from the water, respectively. However, 94.1 % of dye was removed in solar-fenton in 90 min. Based on solar-fenton process, the effect of ph value and the concentration of dye, Fe 2+, H 2 O 2 as well as oxalic acid concentration on Eosin Y degradation efficiency were investigated. In 60 min, 96 % of Eosin Y was degraded when the ph value was 3.5 and the concentration of Fe 2+, H 2 O 2 and oxalic acid were 10 mol/l, 600 mg l -1 and 300 mg l -1, respectively. The Eosin Y degradation was dependent on the dye concentration. That is higher Eosin Y concentration resulted in lower degradation efficiency Bahadir et al (2008) studied the photolytic degradation of Basic Red 2 via UV radiation in the presence of H 2 O 2 and optimized using Response Surface Methodology (RSM). Under the optimized conditions of 20 M BR2, 1.67 mm H 2 O 2 and ph 7.6, the ANOVA results indicated that the proposed model can be used to navigate the design space. It was found that the response of Basic Red 2degradation is very sensitive to the independent factors of Basic Red 2concentration and H 2 O 2 concentration. In the optimization, R 2 and correlation coefficients for the model were evaluated as 0.89 and 0.80, respectively. Sanja and Dinko (2008) investigated the oxidative decolorization and mineralization of three reactive dyes, C.I. Reactive Yellow 3, C.I. Reactive

12 30 Blue 2 and C.I. Reactive Violet 2 by using homogeneous and heterogeneous Fenton and UV/Fenton processes. The effects of H 2 O 2, Fe 2+ and Fe 0 concentrations, Fe 2+ /H 2 O 2 and Fe 0 /H 2 O 2 molar ratios at ph 3 and T = 23 ± 1 C was studied. Optimal operational conditions for the efficient degradation of all three dye solutions (100 mg l -1 ) were found to be Fe 2+ /H 2 O 2 = 0.5 mm/20 mm and Fe 0 /H 2 O 2 = 2 mm/1 mm. The experimental results showed that the homogeneous Fenton process employing UV irradiation was the most effective. By using this process, the high levels of mineralization (78 84 %) and decolorization ( %) were achieved. Pseudo-first-order degradation rate constants were obtained from the batch experimental data. Taha et al (2009) investigated the decolorization of the Mordant red 73 azo dye in water using UV/H 2 O 2 and Photo-Fenton treatments. Photo degradation experiments were carried out in a stirred batch photo reactor equipped with a low-pressure mercury lamp as UV source at 254 nm. The effect of operating parameters such as ph, H 2 O 2, dye and the presence of inorganic salts (NaNO 3, NaCl and Na 2 CO 3 ) were also investigated. The results indicated that complete dye decolorization was obtained in less than 60 min under optimum conditions. Furthermore, results showed that dye degradation was dependent upon ph, H 2 O 2 and initial dye concentration. The presence of chloride ion led to large decreases in the photo degradation rate of MR73 while both nitrate and carbonate ions have a slight effect. The photo- Fenton treatment, in the presence of Fe powder as a source of Fe 2+ ions, was highly efficient and resulted in 99 % decolorization of the dye in 15 min. Mineralization of Mordant red 73 dye was investigated by determining Chemical Oxygen Demand (COD). In a 3 h photo period 65 % of the dye was mineralized by the H 2 O 2 /UV process, while the photo-fenton treatment was more efficient producing 85 % mineralization over the same 3 h period.

13 31 Arslan et al (2009) investigated the treatability of synthetic azo dye production wastewaters from Acid Blue 193 and Reactive Black 39 production and real Reactive Black 39 production effluent via Photo-Fentonlike process was investigated. Response surface methodology was employed to assess individual and interactive effects of critical process parameters (Fe 3+, H 2 O 2 concentrations; initial Chemical Oxygen Demand (COD) and reaction time) on treatment performance in terms of color, COD and Total Organic Carbon (TOC) removal efficiencies. Optimized reaction conditions for synthetic AB 193 production wastewater were established as Fe 3+ = 1.5 mm; H 2 O 2 = 35 mm for CODs 200 mg l -1 and a reaction time of 45 min. Under these conditions, 98 % color, 78 % COD and 59 % TOC removals were experimentally obtained and fitted the model predictions well. The same model also described the treatment of synthetic Reactive Black 39 production wastewater satisfactorily. Experimentally achieved removals were considerably lower than model predictions for real Reactive Black 39 production effluent due to its high chloride content. Filiz et al (2009) investigated the advanced oxidation of an azo-dye, Direct Red 28 by photo-fenton treatment in batch experiments using Box Behnken statistical experiment design and the response surface analysis. Both H 2 O 2 and Fe (II) concentration had profound effects on decolorization. Percent color removal was higher than TOC removal indicating formation of colorless organic intermediates. Complete color removal was achieved within 5 min while complete mineralization took nearly 15 min. The optimal reagent doses varied depending on the initial dyestuff dose. For the highest dyestuff concentration tested, the optimal H 2 O 2 /Fe (II)/dyestuff ratio resulting in the maximum color removal (100 %) was predicted to be 715/71/250 (mg L 1 ), while this ratio was 1550/96.5/250 for maximum mineralization (97.5 %).

14 Sono-Fenton Process Vinodgopal et al (1998) studied a new method for degradation of reactive Black dye in oxygen-saturated aqueous solution under high frequency ultrasonic generator. Sonochemical degradation method is relatively new and involves exposing aqueous solutions containing the organic pollutant to ultrasound. The OH radical initiated oxidative degradation of the dye results in 65 % mineralization was measured by the decrease in the total organic content. The advantage of using ultrasound rests with the simplicity of its use. Fung et al (1999) studied the decolorization of Cuprophenyle yellow RL C=C by Sonolysis/UV/H 2 O 2 process. Sonication (320 khz) dramatically enhanced oxidation efficiency of UV (11 W) and H 2 O 2 (0.1 ml/l) system (ph = 11) by improving oxygen uptake and transfer, the combined process achieving 94 % dye (0.1 g/l) removal in 60 min following pseudo first-order kinetics. Joseph et al (2000) studied the degradation of Acid N=N Methyl orange by Sonolysis/Fenton reaction. Addition of FeSO 4 (Fe 2+ = mm) resulted in Fenton s reaction with H 2 O 2 evolving from simultaneous sonification (500 khz, 50 W) and achieved 3-fold increase in decoloration (15 min, 10 M dye) and TOC removal (50 %, 20 min) as compared to sonification only. Lorimer et al (2000) found that Acid N=N sandolan Yellow Electro-oxidation of dye (50 mg l -1 ) in saline solution (0.01 mol/l NaCl) involving in situ generation of hypochlorite ion was enhanced using ultrasound (20 khz, 22 W) when carried out in a semi sealed cell, which minimized the effects of ultrasonic degassing.

15 33 Fung et al (2001) studied the degradation of one of these dyes, namely, C.I. Reactive Red 120. The ultrasound frequency used was 320 khz. The decolorization process was carried out at a 4.5 L/h wastewater flow rate with a 1.8 ml l -1 H 2 O 2 feeding rate. When H 2 O 2 /US was used, the color removal efficiency was only 18 %. However, by adding ultrasounds to the H 2 O 2 /UV treatment, the color removal improved significantly with values higher than 80 %. Taicheng et al (2003) investigated the decolorization and COD removal from synthetic wastewater containing Reactive Brilliant Orange K-R dye using sonophotocatalytic technology. Experimental results showed that this hybrid technology could efficiently remove the color and reduce COD from the synthetic dye-containing wastewater, and that both processes followed pseudo first-order kinetics. At the condition of 0.1 m 3 /h air flow, 0.75 g dm -3 titanium dioxide and 0.5 mmol/dm3 Reactive Brilliant Orange K-R solution, the rate constants of decolorization and COD removal were and min -1, respectively for the photocatalytic process and and min -1, respectively for the sonochemical process. The rate constants of sonophotocatalysis were greater than that the both the photocatalytic and sonochemical process either in isolation or as a sum of the individual process, indicating an apparent synergetic effect between the photo-and sono-processes. Wang et al (2008) investigated the degradation of reactive brilliant red K-BP in aqueous solution by means of ultrasonic cavitation for a variety of operating conditions. It was found that the degradation of reactive brilliant red K-BP in aqueous solution follows pseudo-first-order reaction kinetics and the degradation rate is dependent on the initial concentration of reactive brilliant red K-BP, the temperature and acidity of the aqueous medium. The effects of Fe 2+, Fenton reagent and NaCl addition on the sonochemical

16 34 degradation of reactive brilliant red K-BP were also investigated. The results obtained here indicate that the degradation rate of brilliant red K-BP in aqueous solution was substantially accelerated by Fe 2+, NaCl or Fenton reagent addition. Lin et al (2008) investigated the degradation of azo dye C.I. Acid Red 14 (AR14) using cast iron in the absence and presence of low frequency ultrasound (59 khz). The effects of ph, amount of cast iron ([Fe] 0) and initial concentration of AR14 on the degradation of AR14 by cast iron combined with low frequency ultrasound had been assessed. The degradation followed the first-order kinetics model. The first-order rate constant of AR14 degradation by cast iron was min 1 while that by US cast iron was min 1. A 3.4-fold increase in the reaction rate was observed in the presence of ultrasound compared with that of absence of ultrasound. This kinetic effect is quantitatively accounted for a simple kinetic model based on the reaction of Fe (II) from cast iron in aqueous solution with sonochemically produced H 2 O 2 (Fenton's reaction). This latter effect illustrates a simple way of achieving a substantial improvement in the efficiency of sonochemical degradation reactions. It was found that for azo dye AR14, the rate of color decay was the first order with respect to the visible absorption of the dye. The destruction of the naphthalene rings in azo dyes was slower than that of color. A significant mineralization of AR14 was observed. Oturan et al (2008) investigated the results obtained by applying a novel hybrid advanced oxidation technique, sonoelectro-fenton process, for the degradation of organic pollutants in aqueous medium. An undivided electrolytic cell with a Pt anode and a three-dimensional carbon-felt cathode was used to carry out the very effective electro-fenton process at constant current, which allows the production of great amounts of hydroxyl radicals

17 35 ( OH). The synergistic action of sonication in the sonoelectro-fenton process was studied at low and high frequency within a range of output power. It was observed that the destruction of the herbicides 4,6-dinitro-o-cresol and 2,4- dichlorophenoxyacetic acid (2,4-D) is significantly accelerated, whereas no improvement is observed for the degradation of the dye azobenzene, suggesting that the nature of the organic structure plays an important role. A pseudo-first order kinetics is found in all cases. Similar results were found for sono-ef at low and high frequency, but the lowest ultrasounds power (i.e., 20 W) is shown to be much better Electro-Fenton Process Dyes are also susceptible to degradation by means of electro- Fenton processes. Various studies have achieved positive results in this respect. Wang et al (2004) studied the comparison of Activated Carbon Fiber (ACF) felt and graphite cathode and suggested that H 2 O 2 might effectively be electrogenerated from O 2 reduction on the large surface area ACF felt cathode, this was more adaptive for electro-fenton process. The removal of color and Total Organic Carbon (TOC) from simulated dye wastewater containing Acid Red 14 (AR14) was experimentally investigated using electro-fenton reaction with ACF cathode. After 360 min of electrolysis and under the operation conditions of 0.36 A current, 1 mm Fe 2+ at ph 3, 70 % TOC was removed as well as complete decolorization occurred. Peralta et al (2006) studied the design and construction of an annular tube reactor for the electrochemical and photo-electrochemical in situ generation of H 2 O 2. By cathodic reduction of dissolved oxygen and the coupled oxidation of water at a UV-illuminated nanocrystalline-tio 2

18 36 semiconductor anode, it was found that the electrochemically generated H 2 O 2 can be employed to readily oxidize the model compound Direct Yellow-52 in dilute acidic solution at high rates in the presence of small quantities of dissolved iron (II). Although, the model organic compound is chemically stable under UV radiation, its electrochemical oxidation rate increases substantially when the semiconductor anode is illuminated as compared to the same processes carried out in the dark. Lopez and Gutierrez (2006) investigated the optimization of the electrochemical decolorization of textile effluents containing reactive dyes. Colored waters were treated in continuous at low current density, to reduce the electrical consumption. Ti/PtO x electrodes were used to oxidize simulated dyebaths prepared with an azo/dichlorotriazine reactive dye (C.I. Reactive Orange 4). The decolorization yield was dependent on the dyeing electrolyte (NaCl or Na 2 SO 4 ). Dyeing effluents which contained from 0.5 to 20 g l 1 of NaCl reached a high decolorization yield, depending on the current density, immediately after the electrochemical process. These results were improved when the effluents were stored for several hours under solar light. After the electrochemical treatment the effluents were stored in a tank and exposed under different lighting conditions: UV light, solar light and darkness. The time of storage was significantly reduced by the application of UV light. Zhou et al (2007) studied the electro-fenton oxidation of a model azo dye, methyl red. To promote treatment efficiency, an efficient undivided electrochemical system using a graphite-polytetrafluoroethylene (PTFE) cathode with high production rate of hydrogen peroxide and current efficiency under wider ph ranges was applied to the electro-fenton oxidation of a model azo dye, methyl red. The influences of operating parameters including electrolyte Na 2 SO 4 concentration, cathodic potential, ph, Fe 2+ concentration and initial methyl red concentration were investigated. At the conditions of

19 37 cathodic potential of 0.55 V versus SCE, ph 3, Fe 2+ concentration of 0.2 M, methyl red of initial concentration of 100 mg l -1 could be removed 80 % in 20 min. Due to the consumption of ferrous ion and formation of hard-to-treat intermediates during treatment, the removal of methyl red was found degraded at two different stages, and the second stage was much slower than the first one. Samiha et al (2007) studied the degradation of an azo dye, the direct orange 61 by electro-fenton process using a Pt anode and a carbon felt cathode. The great oxidation ability of this process is due to the large production of hydroxyl radicals ( OH) in the medium from electrochemically assisted Fenton s reaction which takes place between electro generated H 2 O 2 and Fe 2+ formed by cathodic reduction of O 2 and Fe 3+, respectively. These radicals are able to oxidize any organics until their mineralization. A factorial experimental design was used for determining the influent parameters on the DO 61 degradation in aqueous medium by electrochemically generated OH radicals. The results obtained show that the current intensity and the dye initial concentration were the main influent parameters on the degradation rate. Thus, under the obtained conditions, kinetic analysis of DO 61 degradation showed a pseudo-first order reaction. The optimal experimental parameters for DO 61 mineralization have been investigated by the use of Doehlert matrix. It has been demonstrated that under the optimal conditions determined by this method, electro-fenton process can lead to a complete mineralization of the dye solution after 6 h (98 % of TOC removal) giving carboxylic acids and inorganic ions as final end-products before mineralization. The results show the efficiency of the electro-fenton process to remove organic pollutants from aqueous medium. Peralta et al (2008) managed to eliminate the color from aqueous solutions containing 50 ppm Orange II in 0.05 M Na 2 SO 4 at ph 3 and with

20 38 two different iron (II) concentrations (0.2 and 0.5 mm). The solution was decolorized in 5 min and destroyed 80 % of the TOC in 60 min. The comparable TOC removal efficiency was 63 % for the electro-fenton system without UV radiation. The cathode current density was 300 ma cm -2. At levels of ferrous sulfate, 0.2 and 0.5 mm, and with an H 2 O 2 concentration of 50 ppm, the orange peak in the visible region of the spectrum disappeared almost completely within the first 5 min of treatment. The decrease in TOC was nearly linear with time, and the removal efficiency after 60 min was 63 %. Consequently, the TOC level was still important after the first hour of treatment. However, when the treatment was combined with UV radiation ( =365 nm and P=75 mw/cm 2 ), the color also disappeared in the first 5 min of treatment. Furthermore, with a lower concentration of Fe 2+ (0.2 nm), 80 % of the TOC was eliminated in the first hour of treatment. Zhang et al (2008) studied the degradation of Amaranth azo dye by electro-fenton method using an undivided cell containing the polypyrrole (PPy)/anthraquinonedisulphonate (AQDS) composite film modified graphite cathode and Pt anode. In acidic media, the PPy/AQDS composite film exhibits the characteristic of gas diffusion cathode and is highly efficient for hydrogen peroxide electro generation with high generation rate and current efficiency. This new electro-fenton system can degrade amaranth azo dye efficiently in various acidic solutions. The amaranth decay and Total Organic Carbon (TOC) removal were determined as a function of ph, cathode potential, Fe 2+ and doping AQDS concentrations. Total dye decay and 80.3 % mineralization were achieved at the optimum conditions (ph 3.0, E cath = 0.65 V vs. SCE, 2.0 mm Fe 2+ concentration). Gao et al (2008) used a voltage of 600 V and achieved the removal of alizarin red S solution (30 mg l -1 ) in the first 10 min in the presence of Fe 2+ ions by glow discharge electrolysis. The smaller the distance between the

21 39 cathode and anode, the greater the intensity of the glow discharge. However, since distance less than 10 mm caused the destruction of the anode, 10 mm was established as the threshold distance. When the ph was increased from 3 to 12, the decolorization rate increased. In contrast, when the ph value was greater than 12, decolorization was reduced, probably because of the formation of ferric hydroxide. The reduction observed in absorbance at 255 nm indicates that the anthracene ring was gradually destroyed as a result of the process. When Fe 2+ ( mol l -1 ) ions were added to the contaminated sample, the TOC value decreased significantly, and the decolorization rate rose to 80 % in the first 10 min of treatment. Yao et al (2008) studied the comparison between photo-fenton and a novel electro-fenton called Fered Fenton to study the mineralization of 10,000 mg l -1 of dye-reactive Black B aqueous solution, which was chosen as the model dye contaminant. Results indicate that the traditional Fenton process only yields 70 % mineralization. This result can be improved by using Fered Fenton to yield 93 % mineralization resulting from the action of ferrous ion regenerated on the cathode. Furthermore, photo-fenton allows a fast and more complete destruction of dye solutions and as a result of the action of ferrous ion regenerated by UV irradiation yields more than 98 % mineralization. In all treatments, the Reactive Black B is rapidly decayed to some carboxylic acid intermediates. The major intermediates found are formic acid and oxalic acid. This study finds that formic acid can be completely mineralized by photo-fenton, but its destruction is problematic using the Fenton method. Oxalic acid is much more difficult to treat than other organic acids. It could get further mineralization with the use of the Fered Fenton process. Rosales et al (2009) focused on the application of Electro-Fenton technique for the remediation of wastewater contaminated with synthetic dyes.

22 40 A bubble reactor was designed to develop this treatment operating in continuous mode. A first-order kinetic model was used to simulate the experimental results operating at different ph, and iron concentration of 150 mg L 1. This kinetic model for Lissamine Green, Methyl Orange and Reactive Black 5 was successfully used in the progression of the process from batch to continuous mode. About 80 % color removal was achieved for Lissamine Green and Methyl Orange with a residence time of 21 h. The decoloration for Reactive Black 5 was lower, reached a value around 60 % at the same residence time Sono-Electro-Fenton Process Li et al (2007) studied that at its natural ph (6.95), the decolorization of Reactive red 24 in ultrasound, ultrasound/h 2 O 2, exfoliated graphite, ultrasound/exfoliated graphite, exfoliated graphite/h 2 O 2 and ultrasound/exfoliated graphite/h 2 O 2 systems. An enhancement was observed for the decolorization in ultrasound/exfoliated graphite/h 2 O 2 system. The effect of solution ph, H 2 O 2 and exfoliated graphite dosages, and temperature on the decolorization of Reactive red 24 was investigated. The sonochemical treatment in combination with exfoliated graphite/h 2 O 2 showed a synergistic effect for the decolorization of Reactive red 24. The results indicated that under proper conditions, there was a possibility to remove Reactive red 24 very efficient from aqueous solution. References and further reading may be available for this article. To view references and further reading you must purchase this article.

23 41 Zhang et al (2009) investigated the effect of ultrasonic power density, goethite addition, hydrogen peroxide concentration, initial ph, hydroxyl radical scavenger, and initial dye concentration on the decolorization of C.I. Acid Orange 7 by ultrasound/goethite/h 2 O 2 process was investigated. The results showed that the decolorization rate increased with power density, goethite addition, and hydrogen peroxide concentration, but decreased with the increase of initial dye concentration. The ultrasonic power density, goethite addition, and initial dye concentration have little effect on decolorization efficiency after 30 min reaction, while the increase of hydrogen peroxide concentration results in the increase of decolorization efficiency. There existed an optimal initial ph to achieve the highest decololrization rate and decolorization efficiency. The presence of hydroxyl radical scavenger would inhibit the decolorization reaction. Only less than half of Total Organic Carbon (TOC) was removed after 90 min reaction, indicating more aggressive conditions are required to achieve the complete mineralization than those employed to simply break the chromophore group Photo-Electro-Fenton Process Wang et al (2008) studied the mineralization of an azo dye Acid Red 14 (AR14) by the Photo-Electron-Fenton (PEF) process in an undivided electrochemical reactor with a RuO 2 /Ti anode and an Activated Carbon Fiber (ACF) cathode able to electrochemically generate H 2 O 2. Anodic oxidation and UV irradiation of AR14 were also examined as comparative experiments. Results indicate that the electro-fenton process yielded about % mineralization of AR14, while the photo-electro-fenton could mineralize AR14 more effectively (more than 94 % Total Organic Carbon (TOC) removal) even at low current densities assisted with UV irradiation after 6 h electrolysis. The Mineralization Current Efficiency (MCE) of the PEF process increased with the increasing AR14 concentrations. In addition, the initial

24 42 solution ph ranging from 1.49 to 6.72 had little influence on the TOC removal probably due to the formation of organic carboxylic acids which balanced the ph increase caused by the cathodic generation of hydrogen gas. The ACF cathode showed a long-term stability during multiple experimental runs for degradation of AR14, indicating its good potential for practical application in treating refractory organic pollutants in aqueous solutions. Kusruvan et al (2004) studied the degradation of Reactive N=N Red 120 by UV/electro-Fenton process. TOC removal [180 min]: 30 %; Decoloration [30 min]: % for mg l -1 concentration; Low efficiency due to radical scavenging by the graphite cathode Electrochemical Oxidation Canizares et al (2007) carried out an electrochemical oxidation process on various dyes, solvents, and surfactants. Boron-doped diamond was used as an anode and stainless steel as a cathode. For all the compounds tested, this type of oxidation achieved the virtually complete removal of COD from the waste with very high current efficiency. The efficiency obtained in each case seemed to depend on the current density as well as on the nature of the anions container in the wastewater. It was also observed that the use of chloride media favored the treatment of dyes. In contrast, the use of sulfates or phosphates improved the removal of the aliphatic compounds studied (solvents). Fan et al (2008) found that the degradation of another dye (amaranth) was possible by means of electrochemical oxidation with an Activated Carbon Fiber (ACF) electrode. In this study, in which electro oxidation potential used was 1,000 mv, color removal was 95.4 %; COD removal was 35 % and TOC removal was 30 %.

25 H 2 O 2 /UV Process Oxidization of the textile wastewater with hydrogen peroxide alone has been found ineffective at acid and alkali values (Olcay et al 1996), while under UV irradiation, H 2 O 2 are photolyzed to form two hydroxyl radicals (2OH ) that react with organic contaminants (Crittenden et al 1999). Application of UV to synthetic textile wastewater for one hour with addition 2 ml/l of H 2 O 2 decreased the inhibitory of microbial growth during subsequent biodegradation of textile wastewater form 47 to 26 % (Stanislaw and Monika 1999; Stanislaw et al 2001). Galindo and Kalt (1998) documented H 2 O 2 /UV process was more effective in an acid medium (ph 3-4) in term of discoloration. No efficient color removal at alkaline ph is contributed to the fact that hydrogen peroxide undergoes decomposition leading to dioxygen and water, rather than producing hydroxyl radicals under UV radiation. Therefore the instantaneous concentration in HO is lower than expected. Furthermore, the H 2 O 2 /UV process is more sensitive to the scavenging effect of carbonate at higher ph values. Others documented that removal of textile dye by H 2 O 2 /UV increased as doses of effective hydrogen peroxide increased up to a critical value (Galindo and Kalt 1998; Arslan et al 1999; Nilsun 1999). High concentration of the H 2 O 2 acts as a radical scavenger, while; low concentration of H 2 O 2 generates not enough of hydroxyl radicals (OH ) that consumed by dye and this leads slow rate of oxidation. Thus a trade-off between them will result in an optimum H 2 O 2 dose, which still needs to be verified experimentally.

26 44 Arslan and Isil (2001, 2002) also documented that treatment of textile wastewater was not effective via H 2 O 2 /UV-C oxidation unless a preliminary ozonation period was introduced to produce sufficient OH to observe a significant color and COD reduction. Previous researchers documented even at 50 mm of H 2 O 2 concentration, raw textile wastewater did not suffer any significant oxidation and COD removal was negligible but COD removal efficiencies of the biotreated effluent were significant (Arslan and Isil 2001, 2002). Results presented by researchers revealed that sufficient reaction time for dye removal using H 2 O 2 /UV process was found to be subjective. Complete destruction of reactive dyes and azo dye have been recorded in min by previous researches (Georgiou et al 2002; Mariana et al 2002; Rosario et al 2002; Tanja et al 2003), Shyh et al (1999) documented that the longer reaction time the more advantageous for color removal where color removal at 10 min was only 9 % of removal at 120 min. Arslan et al (2002) found that TOC removal for simulated reactive dye bath effluent of H 2 O 2 /UVC was only 30.4 % and 13.9 % using 680 mg l -1 of H 2 O 2 at ph 3 and 7 respectively. Although, in some cases it has been reported that organic substances undergo photochemical reactions as a consequence of light absorption, rarely these transformations contribute to the TOC reduction. In this sense, the light absorbed by organic molecules can be generally considered as wasted light. Reduced efficiencies can also result during photolytic treatment of wastewaters containing suspended material, since a fraction of irradiated energy is scattered by these particles. Due to the fact that the inhibitory effect on treatment efficiency for the UV/ H 2 O 2 system is related directly to the decreasing light transmittance of the less diluted wastewater matrix, it can be inferred that for more concentrated effluents exhibiting higher optical density, light assisted AOPs are less suitable.

27 45 The relationship between UV light intensity and dye decomposition in UV/H 2 O 2 process was investigated by (Shen and Wang 2002). The decomposition rate of dye was found to increase with increasing UV light intensity. They documented that more than 90 % of the dye was decomposed at 82 W m -2. But for the UV light intensity higher than 102 W m -2, further increase of UV energy only slightly improved the decomposition efficiency of dye indicating the photons provided was excessive. Cisneros et al (2002) studied the degradation of Hispamin Black CA (Direct N=N Black 22) by UV/H 2 O 2 process. UV (125 W) - H 2 O 2 (565.8 mg l -1, 16.6 mm). Results showed complete decoloration (35 min) and 82 % TOC removal (60 min) for 40 mg l -1 dye at natural ph (7.5). Neamtu et al (2002) studied the decolorization of Reactive-red 120N=N / black 5 N=N / yellow 84 N=N by UV/H 2 O 2 process. UV (15 W), H 2 O 2 (optimum dose 24.5 mmol l -1 ). 65 % color and % COD were removed in 15 minutes. The decolorization was more than 99 % in 60 minutes. Aleboyeh et al (2003) studied the decolorization of Acid dye (orange 8 N=N/blue 74 C=C, Methyl orange N=N) by UV/H 2 O 2 process. Removal by only UV (15 W, nm, incident photon flux = Einstein s 1, 4.54 ph 5.5) and only H 2 O 2 in absence of UV was negligible. By combining UV/ H 2 O 2 the decolorization rate rises by increasing the initial dosage of H 2 O 2 up to a critical value ([H 2 O 2 / dye] = 50 70) beyond which it is inhibited. Olya et al (2008) studied the mineralization of C.I. Acid Orange 7 azo dye by UV/H 2 O 2 Advanced Oxidation Process in a flux through annular photoreactor. An experimental design based on Response Surface Methodology (RSM) was applied to evaluate the simple and combined effects of influencing independent parameters on mineralization efficiency and