Synergetic decolorization of azo dyes using ultrasounds,photocatalysis and photo-fenton reaction

In the present work, ultrasound irradiation, photocatalysis with TiO₂, Fenton/Photo-Fenton reaction, and the combination of those techniques were investigated for the decolorization of industrial dyes in order to study their synergy. Three azo dyes were selected from the weaving industry. Their degradation was examined via UV illumination, Fenton and Photo-Fenton reaction as well as ultrasound irradiation at low (20 kHz) and high frequencies (860 kHz). In these experiments, we investigated the simultaneous action of the ultrasound and UV irradiation by varying parameters like the duration of photocatalysis and ultrasound irradiation frequency. At the same time, US power, temperature, amount of TiO₂ photocatalyst and amount of Fenton reagent remained constant. Due to their diverse structure, each azo dye showed different degradation levels using different combinations of the above-mentioned Advanced Oxidation Processes (AOPs). The Photo-Fenton reagent is more effective with US 20 kHz and US 860 kHz for the azo dyes originated from the weaving industry at pH = 3 as compared to pH = 6.8. The combination of the Photo-Fenton reaction with 860 kHz ultrasound irradiation for the same dye gave an 80% conversion at the same time. Experiments have shown a high activity during the first two hours. After that threshold, the reaction rate is decreased. FT-IR and TOC measurements prove the decolorization due to the destruction of the chromophore groups but not complete mineralization of the dyes.     

Sonochemical degradation of azo dyes in aqueous solution: a new heterogeneous kinetics model taking into account the local concentration of OH radicals and azo dyes

The sonochemical decolorization and decomposition of azo dyes, such as C. I. Reactive Red 22 and methyl orange, were performed from the viewpoints of wastewater treatment and to determine the reaction kinetics. A low concentration of the azo dye solution was irradiated with a 200 kHz and 1.25 W/cm2 ultrasound in a homogeneous aqueous solution. The azo dye solutions were readily decolorized by the irradiation. The sonochemical decolorization was also depressed by the addition of the t-butyl alcohol radical scavenger. These results indicated that azo dye molecules were mainly decomposed by OH radicals formed from the water sonolysis. In this paper, we propose a new kinetics model taking into account the heterogeneous reaction kinetics similar to a Langmuir-Hinshelwood mechanism or an Eley-Rideal mechanism. The proposed kinetics model is based on the local reaction site at the interface region of the cavitation bubbles, where azo dye molecules are quickly decomposed because an extremely high concentration of OH radicals exists in this region. To confirm the proposed kinetics model, the effects of the initial concentration of azo dyes, irradiated atmosphere and pH on the decomposition rates were investigated. The obtained results were in good agreement with the proposed kinetics model.     

Ultrasound and heat enhanced persulfate oxidation activated with Fe0 aggregate for the decolorization of C.I. Direct Red 23

Effluents from the paper printing and textile industries are often heavily contaminated with azo dyes. Azo dyes are difficult to oxidize biologically. This work investigated the decolorization of an azo dye, C.I. Direct Red 23 (DR23), by persulfate (PS) activated with Fe⁰ aggregates (PS/Fe⁰). Ultrasound (US) and heat were used as enhancement tools in the PS oxidation system. Neither US-activated PS nor thermally activated PS was effective in oxidizing DR23. However, the decolorization was significantly enhanced by PS/Fe⁰ combined with US (PS/Fe⁰/US) or heat (PS/Fe⁰/55 °C). Approximately 95% decolorization of 1 × 10⁻⁴ M DR23 was achieved within 15 min in the PS/Fe⁰/US system at an initial pH of 6.0, PS of 5 × 10⁻³ M, Fe⁰ of 0.5 g/L and US irradiation of 106 W/cm² (60 kHz). Complete decolorization was achieved within 10 min in the Fe⁰/PS/55 °C system. The rate of decolorization doubled when US was introduced in the PS/Fe⁰ system during the treatment of different initial dye concentrations. The dependence of dye and true color (ADMI) depletion on PS concentration has been discussed. DR23 was completely degraded based on the disappearance of aromatic groups of UV–vis spectra and the variation of TOC mineralization. The observed pseudo-first-order decolorization rate was substantially enhanced by increasing temperature. The Arrhenius activation energy for the PS activated with Fe⁰ was estimated as 8.98 kcal/mol, implying that higher temperature is beneficial for the DR23 decolorization. The addition of US into the PS/Fe⁰ system did not incur a substantial increase in electricity, whereas the mineralization of DR23 occurred quickly. Thus, both PS/Fe⁰/US and heated PS/Fe⁰ systems are practically feasible for the effective degradation of the direct azo dye in textile wastewater.     

Modeling of ultrasonic degradation of non-volatile organic compounds by Langmuir-type kinetics

Sonochemical degradation of phenol (Ph), 4-isopropylphenol (4-IPP) and Rhodamine B (RhB) in aqueous solutions was investigated for a large range of initial concentrations in order to analyze the reaction kinetics. The initial rates of substrate degradation and H₂O₂ formation as a function of initial concentrations were determined. The obtained results show that the degradation rate increases with increasing initial substrate concentration up to a plateau and that the sonolytic destruction occurs mainly through reactions with hydroxyl radicals in the interfacial region of cavitation bubbles. The rate of H₂O₂ formation decreases with increasing substrate concentration and reaches a minimum, followed by almost constant production rate for higher substrate concentrations. Sonolytic degradation data were analyzed by the models of Okitsu et al. [K. Okitsu, K. Iwasaki, Y. Yobiko, H. Bandow, R. Nishimura, Y. Maeda, Sonochemical degradation of azo dyes in aqueous solution: a new heterogeneous kinetics model taking into account the local concentration OH radicals and azo dyes, Ultrason. Sonochem. 12 (2005) 255–262.] and Seprone et al. [N. Serpone, R. Terzian, H. Hidaka, E. Pelizzetti, Ultrasonic induced dehalogenation and oxidation of 2-, 3-, and 4-chlorophenol in air-equilibrated aqueous media. Similarities with irradiated semiconductor particulates, J. Phys. Chem. 98 (1994) 2634–2640.] developed on the basis of a Langmuir-type mechanism. The five linearized forms of the Okitsu et al.’s equation as well as the non-linear curve fitting analysis method were discussed. Results show that it is not appropriate to use the coefficient of determination of the linear regression method for comparing the best-fitting. Among the five linear expressions of the Okitsu et al.’s kinetic model, form-2 expression very well represent the degradation data for Ph and 4-IPP. Non-linear curve fitting analysis method was found to be the more appropriate method to determine the model parameters. An excellent representation of the experimental results of sonolytic destruction of RhB was obtained using the Serpone et al.’s model. The Serpone et al.’s model gives a worse fit for the sonolytic degradation data of Ph and 4-IPP. These results indicate that Ph and 4-IPP undergo degradation predominantly at the bubble/solution interface, whereas RhB undergoes degradation at both bubble/solution interface and in the bulk solution.     

Decolorization of azo dye Orange G by aluminum powder enhanced by ultrasonic irradiation

In this work, the decolorization of azo dye Orange G (OG) in aqueous solution by aluminum powder enhanced by ultrasonic irradiation (AlP-UI) was investigated. The effects of various operating operational parameters such as the initial pH, initial OG concentration, AlP dosage, ultrasound power and added hydrogen peroxide (H₂O₂) concentration were studied. The results showed that the decolorization rate was enhanced when the aqueous OG was irradiated simultaneously by ultrasound in the AlP-acid systems. The decolorization rate decreased with the increase of both initial pH values of 2.0–4.0 and OG initial concentrations of 10–80 mg/L, increased with the ultrasound power enhancing from 500 to 900 W. An optimum value was reached at 2.0 g/L of the AlP dosage in the range of 0.5–2.5 g/L. The decolorization rate enhanced significantly by the addition of hydrogen peroxide in the range of 10–100 mM to AlP-UI system reached an optimum value of 0.1491 min⁻¹. The decolorization of OG appears to involve primarily oxidative steps, the cleavage of NN bond, which were verificated by the intermediate products of OG under the optimal tested degradation system, aniline and 1-amino-2-naphthol-6,8-disulfonate detected by the LC–MS.     

Modification of nanosized natural montmorillonite for ultrasound-enhanced adsorption of Acid Red 17

This work aims to modify montmorillonite (MMT) via dodecyltrimethylammonium bromide (DTMA) and investigate its ability in ultrasound (US) assisted decolorization of a polluted solution. BET surface area of MMT was increased from 19.76 to 42.57 m²/g and basal spacing of MMT structural layers was increased from 1.13 to 1.69 nm by DTMA modification. The application of DTMA–modified MMT (DTMA–MMT) and US for the decolorization of Acid Red 17 (AR17) showed that US could improve the ability of DTMA–MMT on decolorization of AR17 solution due to simultaneous adsorption and sonocatalysis. The ability of US assisted DTMA–MMT was slightly decreased with pH, the initial dye concentration and the presence of inorganic anions.     

Decolorizing of azo dye Reactive red 24 aqueous solution using exfoliated graphite and H2O2 under ultrasound irradiation

At its natural pH (6.95), the decolorization of Reactive red 24 in ultrasound, ultrasound/H₂O₂, exfoliated graphite, ultrasound/exfoliated graphite, exfoliated graphite/H₂O₂ and ultrasound/exfoliated graphite/H₂O₂ systems were compared. An enhancement was observed for the decolorization in ultrasound/exfoliated graphite/H₂O₂ system. The effect of solution pH, H₂O₂ and exfoliated graphite dosages, and temperature on the decolorization of Reactive red 24 was investigated. The sonochemical treatment in combination with exfoliated graphite/H₂O₂ 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.

The decolorization of other azo dyes (Reactive red 2, Methyl orange, Acid red 1, Acid red 73, Acid red 249, Acid orange 7, Acid blue 113, Acid brown 75, Acid green 20, Acid yellow 42, Acid mordant brown 33, Acid mordant yellow 10 and Direct green 1) was also investigated, at their natural pH.     

The characteristic study and sonocatalytic performance of CdSe–graphene as catalyst in the degradation of azo dyes in aqueous solution under dark conditions

The sonocatalytic degradation of azo dyes; methyl orange (MO) and rhodamine B (RhB) were studied catalyzed by cadmium selenide (CdSe)-graphene in dark ambiance. The CdSe–graphene composites were prepared by simple hydrothermal method. The characterizations of composites were studied by X-ray diffraction (XRD), scanning electron microscope (SEM), transmission electron microscope (TEM), specific surface area (BET) and with energy dispersive X-ray (EDX). The UV-spectroscopic analysis of the dyes was done by measuring the change in absorbance. The degradation of the organic dyes was calculated based on the decrease in concentration of the dyes with respect to regular time intervals. The rate coefficients for the sonocatalytic process were successfully established and the reusability tests were done to test the stability of the used catalysts.     

Enhanced photocatalytic–electrolytic degradation of Reactive Brilliant Red X-3B in the presence of water jet cavitation

Photocatalysis, electrolysis, water jet cavitation (WJC), alone and in combinations were applied to degrade an azo dye, Reactive Brilliant Red X-3B (X-3B). Experiments were conducted in a 4.0 L aqueous solution with different initial dye concentrations, TiO₂ dose, and solution pH. WJC substantially increased the photocatalytic, electrolytic and photocatalytic–electrolytic rates of the dye removal. The observed first-order rate of X-3B decolorization in the process of combined photocatalysis and electrolysis coupled with WJC was 1.6–2.9 times of that in the process of combined photocatalysis and electrolysis coupled with mechanical stirring. The rate enhancements may be attributed primarily to the reduced diffusion layer thickness on the electrodes and the deagglomeration of photocatalyst particles due to the chemical and physical effects of WJC. Under the conditions of 80 mg/L X-3B solution, 100 mg/L TiO₂ dose and solution pH 6.3, 97% and 71% of color and chemical oxygen demand (COD_Cr) were removed, respectively, within 90-min photocatalytic–electrolytic treatment coupled with WJC. During this process, azo groups and naphthalene, benzene and triazine structures of the dye can be destroyed. Industrial textile effluent was also investigated, and a positive synergistic effect between photocatalytic–electrolytic system and WJC was observed considering color removal.     

Degradation of aryl-azo-naphthol dyes by ultrasound, ozone and their combination: effect of alpha-substituents

Lab-scale degradation of azo dyes with ultrasound (300 kHz), ozone and both was investigated using an aryl-azo-naphthol dye-C.I. Acid Orange 8. It was found that in all schemes color decay was faster than UV absorbance, and the rates followed pseudo-first-order kinetics except for the decay of UV-254 band by ozone. Sonication alone was sufficient for decolorization, but not for UV absorption abatement or mineralization. Ozonation was more effective than ultrasound in bleaching, but not as much for the mineralization of the dye. Combined operation of ultrasound and ozone improved the rate of bleaching and UV absorption decay and remarkably enhanced the mineralization of the dye. This was attributed to increased mass transfer of ozone in solution and its decomposition in the gas phase to yield hydroxyl radicals and other oxidative species. The effect of alpha-methyl substituent at the aryl carbon of the dye was found to decelerate the rate of degradation as a result of weakened intramolecular hydrogen bonding.     

Intensification of sonochemical decolorization of anthraquinonic dye Acid Blue 25 using carbon tetrachloride

In this work, the influence of CCl₄ on the sonochemical decolorization of anthraquinonic dye Acid Blue 25 (AB25) in aqueous medium was investigated using high frequency ultrasound (1700 kHz). This frequency, reputed ineffective, was tested in order to introduce the ultrasound waves with high frequency in the field of degradation or removal of dyes from wastewater, due to its limited use in this field, and to increase the application of high frequency ultrasound wave in the field of environmental protection. The effects of various parameters such as the concentration of CCl₄, frequency (22.5 and 1700 kHz), solution pH, temperature and tert-butyl alcohol adding on the decolorization rate of AB25 was studied. The obtained results clearly demonstrated the significant intensification of AB25 decolorization in the presence of CCl₄. The enhancement effect of CCl₄ increased by decreasing temperature and by increasing the CCl₄ concentration. The pH has a significant influence on the bleaching of dye both in the absence and presence of CCl₄. The three investigated dosimeter methods (KI oxidation, Fricke reaction and H₂O₂ production) well corroborate the improvement of the sonochemical effects in the presence of CCl₄. The best sonochemical decolorization rate of AB25 in aqueous solution both in the absence and presence of CCl₄ is observed to occur at 1700 kHz compared to 22.5 kHz. The sonochemical oxidation of CCl₄ generates oxidizing species in the liquid phase that are highly beneficial for oxidation of hydrophilic and non-volatile pollutant, such as dyes, because they are less susceptible to free radical attack due to lower stability of the generated free radicals.     

Decolorization of azo dyes Orange G using hydrodynamic cavitation coupled with heterogeneous Fenton process

The present work demonstrates the application of the combination of hydrodynamic cavitation (HC) and the heterogeneous Fenton process (HF, Fe⁰/H₂O₂) for the decolorization of azo dye Orange G (OG). The effects of main affecting operation conditions such as the inlet fluid pressure, initial concentration of OG, H₂O₂ and zero valent iron (ZVI), the fixed position of ZVI, and medium pH on decolorization efficiency were discussed with guidelines for selection of optimum parameters. The results revealed that the acidic conditions are preferred for OG decolorizaiton. The decolorization rate increased with increasing H₂O₂ and ZVI concentration and decreased with increasing OG initial concentration. Besides, the decolorization rate was strongly dependent on the fixed position of ZVI. The analysis results of degradation products using liquid chromatography–ESI–TOF mass spectrometry revealed that the degradation mechanism of OG proceeds mainly via reductive cleavage of the azo linkage due to the attack of hydroxyl radical. The present work has conclusively established that the combination of HC and HF can be more energy efficient and gives higher decolorization rate of OG as compared with HC and HF alone.     

Application of power ultrasound for azo dye degradation

Power ultrasound of 850 kHz at 60, 90 and 120 W was used for the degradation of industrial azo dyes Acid Orange 5 and 52, Direct Blue 71, Reactive Black 5 and Reactive Orange 16 and 107. The results show that power ultrasound is able to mineralize azo dyes to non-toxic end products, which was confirmed by respiratory inhibition test of Pseudomonas putida. All investigated dyes have been decolorized and degraded within 3-15 h at 90 W and within 1-4 h at 120 W, respectively. Mass spectrometric investigations show, that hydroxyl radicals attack azo dyes by simultaneous azo bond scission, oxidation of nitrogen atoms and hydroxylation of aromatic ring structures. A volumetric scale-up showed a correlation between the energy input and the absolute amount of degraded dye. Up to an energy input of about 90 W no enzymatic deactivation of laccase was observed which might be helpful for a simultaneous action of sonochemical and enzymatic treatments.     

Degradation of Rhodamine B in aqueous solution by laser cavitation

A novel method of laser cavitation (LC) was proposed for degrading organic dye wastewater. Rhodamine B (RhB) aqueous solution was employed as the simulated organic dye wastewater, and a LC system was designed to conduct the experiments of degrading RhB. The effects of laser energy, initial concentration and cavitation time on the degradation were investigated. Moreover, the degradation kinetics, degradation mechanism and energy efficiency were analyzed. The experimental results indicate that RhB aqueous solution can be degraded effectively by LC and the degradation follows the pseudo-first-order kinetics. The extent of degradation increases by 27.6% with the rise of laser energy (50–100 mJ) while it decreases by 7.8% with increasing the initial concentration from (20–40 mg/L), but RhB can not be degraded when exceeding 100 mg/L. The degradation extent of RhB at 100 mJ and 20 mg/L for 3 h is 81.11%, and the RhB solution is almost completely degraded at 150 mJ (98.4%). The degradation velocity of RhB rises firstly and then decreases as the cavitation time increases. The degradation of RhB by LC can be attributed to the N-de-ethylation and chromophore cleavage caused by oxidation of hydroxyl (OH) radical and thermal decomposition. LC has a higher energy efficiency compared with other methods and is more energy efficient at lower laser energy.     

An optimization study using response surface methods on the decolorization of Reactive Blue 19 from aqueous solution by ultrasound

The decolorization of reactive dye C.I. Reactive Blue 19 from aqueous solution was studied by using ultrasound, activated carbon and combined ultrasound/activated carbon. The combined effects of independent variables, such as ultrasound power, temperature, time, activated carbon concentration, dye concentration and initial pH were investigated on the decolorization by using the central composite design. The decolorization of RB 19 was modelled statistically and optimized by means of the Matlab computer software. The decolorization were accomplished at optimum conditions by using ultrasound, activated carbon and combined ultrasound/activated carbon as 36%, 91% and 99.9%, respectively. The application of ultrasonic irradiation was found to be beneficial for decolorization of RB 19 from aqueous solution by adsorption.     

Drastically enhanced ultrasonic decolorization of methyl orange by adding CCl4

Effects of CCl(4) were investigated on the ultrasonic decolorization of azo dye methyl orange (MO). The decolorization of MO was observed to behave as a pseudo-first reaction in kinetics under all the conditions tested in the present work. The apparent rate constant of the decolorization was demonstrated to be dependent on CCl(4) concentration, MO concentration and the solution pH value. Under appropriate conditions, the rate constant of the ultrasonic decolorization of MO was able to be increased more than 100 times by adding CCl(4) into the MO solution. A reaction mechanism was proposed to explain the promoting effect of CCl(4) on the ultrasonic decolorization of MO, which was attributed to the generation of highly-oxidizing such as (*)Cl radical and HClO species, and then their attack at the azo bond of MO.     

C.I. Reactive Black 5 decolorization by combined sonolysis and ozonation

Decolorization of the azo dye C.I. Reactive Black 5 (RB5) in solution by a combination of sonolysis and ozonation was investigated. The results showed that the optimum pH for the reaction was 11.0, and both lower and higher pH decreased the decolorization rate. Increasing the initial concentration of RB5 led to a decreasing decolorization rate. Under the experimental conditions, the decolorization rate increased with an increase in temperature. The decolorization of RB5 followed pseudo-first-order reaction kinetics. Based on the decolorization rate constants obtained at different temperatures within the range 287-338K and the Arrhenius equation, the apparent activation energy (E(a)) was calculated to be 11.2kJmol(-1). This indicated that the reaction has little dependence on temperature. The color decay was considerably faster than the decrease in total organic carbon (TOC), which was attributed to the ease of chromophore destruction. Hence the efficiency of decolorization was 84% compared with 4% of TOC removal after 5min reaction. Additionally, muconic acid, (2Z)-pent-2-enedioic acid and maleic acid were identified as main oxidation products by gas chromatography coupled with mass spectrometry (GC-MS) after 150min of reaction.     

SnO2-TiO2复合颗粒的形态结构及其光催化活性

在气溶胶反应器中，利用ＴｉＣｌ４高温氧化反应制备超细ＴｉＯ２，采用均匀沉淀法在ＴｉＯ２表面沉积ＳｎＯ２制备ＳｎＯ２－ＴｉＯ２复合颗粒，应用ＴＥＭ、ＥＥＤＳ、ＳＲＤ、ＢＥＴ比表面积测试等手段对粒子进行表征。以活性艳红Ｘ－３Ｂ复合颗粒，应用ＴＥＭ、ＥＤＳ、ＸＲＤ、ＢＥＴ比表面积测试等手段对粒子进行表征。以活性艳红Ｘ－３Ｂ溶液为处理对象考察复合颗粒的光催化活性。结果表明ＳｎＯ２－ＴｉＯ２复合颗粒的光催化     

Optimization of a heterogeneous catalytic hydrodynamic cavitation reactor performance in decolorization of Rhodamine B: Application of scrap iron sheets

A low pressure pilot scale hydrodynamic cavitation (HC) reactor with 30 L volume, using fixed scrap iron sheets, as the heterogeneous catalyst, with no external source of H₂O₂ was devised to investigate the effects of operating parameters of the HC reactor performance. In situ generation of Fenton reagents suggested an induced advanced Fenton process (IAFP) to explain the enhancing effect of the used catalyst in the HC process. The reactor optimization was done based upon the extent of decolorization (ED) of aqueous solution of Rhodamine B (RhB). To have a perfect study on the pertinent parameters of the heterogeneous catalyzed HC reactor, the following cases as, the effects of scrap iron sheets, inlet pressure (2.4–5.8 bar), the distance between orifice plates and catalyst sheets (submerged and inline located orifice plates), back-pressure (2–6 bar), orifice plates type (4 various orifice plates), pH (2–10) and initial RhB concentration (2–14 mg L^?1) have been investigated. The results showed that the highest cavitational yield can be obtained at pH 3 and initial dye concentration of 10 mg L^?1. Also, an increase in the inlet pressure would lead to an increase in the ED. In addition, it was found that using the deeper holes (thicker orifice plates) would lead to lower ED, and holes with larger diameter would lead to the higher ED in the same cross-sectional area, but in the same holes’ diameters, higher cross-sectional area leads to the lower ED. The submerged operation mode showed a greater cavitational effects rather than the inline mode. Also, for the inline mode, the optimum value of 3 bar was obtained for the back-pressure condition in the system. Moreover, according to the analysis of changes in the UV–Vis spectra of RhB, both degradation of RhB chromophore structure and N-deethylation were occurred during the catalyzed HC process.     

Ultrasonic-assisted ozone degradation of organic pollutants in industrial sulfuric acid

In this work, a combination of ozone (O₃) and ultrasound (US) has been firstly used to decolorize black concentrated sulfuric acid with high organic content. The effect of different reaction factors on the transparency, extent of decolorization, H₂SO₄ mass fraction, and organic pollutants removal is studied. In addition, the systematic interaction between ultrasound and ozone on the decolorization process is reviewed through comparative experiments of O₃, US and US/O₃. A sulfuric acid product that meets the requirements for first-class products in national standards, with an extent of decolorization of 74.07%, transparency of 70 mm, and a mass fraction of 98.04%, is obtained under the optimized conditions. Under the same conditions, it has been established that the treatment time can be saved by 25% using the US/O₃ process compared to using O₃. Further, the production of oxidative free radicals (•OH) in a concentrated sulfuric acid system is enhanced using the US/O₃ process compared with O₃. In addition, the degree of effectiveness of different oxidizing components on the decolorization process is revealed by adding different free radical shielding agents when the US/O₃ process is used.