Factors and mass ratio analyses for Reactive Blue 19 dye decolorization using ozone: an experimental and analytical modelling approach Análises de atributos e taxa mássica da descolorização do corante Azul Reativo 19 usando ozônio: modelagem experimental e analítica

Dyes highly reduce sunlight penetration into the stream, and consequently affect photosynthesis and oxygen transfer into water bodies. An experimental and analytical modelling approach to Reactive Blue 19 (RB19) removal using ozone was carried out. For this purpose, factors and mass ratio analyses were assessed based on batch assays experiments. Removal efficiency increased from 64 to 94% when the dosage increased from 38.4 to 153.6 mg O 3 .L -1 . Results showed that RB19 is more efficiently removed when initial pH is 7. The rate of RB19 removal decreased as the initial dye concentration increased. Kinetic studies showed that the ozonation of RB19 was a pseudo first-order reaction with respect to the dye, and the apparent rate constant declined logarithmically with the initial dye concentration. Mass ratio studies showed that, for the empirical analysis, the power law equation was adequate to describe mass ratio over time and the analytical analysis suggests that the process is influenced by mass transfer in the liquid film as well as in the bulk fluid.


INTRODUCTION
Dyes are organic compounds that consist of chromophores and auxochrome structures, which are responsible for the color of the dye and the intensity of the color, respectively (CHRISTIE, 2015). They are classified according to their chemical structure (azo, anthraquinone, phthalocyanine, triarylmethane, among others), their dissociation in an aqueous solution (anionic, cationic and non-ionic) and the method of application onto fabric (acidic, basic, direct, reactive, disperse and metal-complex), wherein acidic, direct and reactive dyes are anionic, basic dyes are cationic, and disperse dyes are non-ionic (MOUSSAVI & MAHMOUDI, 2009;ASGHER, 2012).
Dyes are also highly visible in water, even at low concentrations (RAGHUVANSHI et al., 2005;GAO et al., 2014), damaging the aesthetic nature of water and reducing light penetration through the water's surface (TURHAN & OZTURKCAN, 2013;JESUS et al., 2014). Thereby, photosynthesis and the oxygen transfer into water bodies are affected (SIDDIQUE et al., 2009;TURHAN et al., 2012). In addition, they are chemically stable, poorly biodegradable (ASGHER, 2012) and are generally more toxic than acid or direct dyes (RAGHUVANSHI et al., 2005). It has been estimated that approximately 10-15% of dyes are wasted by dispersion into the environment upon completion of their use in the dyeing unit (MOUSSAVI & MAHMOUDI, 2009).
Synthetic dyes are extensively used in the cosmetic, food, leather, paper, pharmaceutical, plastic, printing, rubber and textile industries (MOUSSAVI & MAHMOUDI, 2009;GAO et al., 2014). Reactive dyes are widely used to color the cellulosic fibers of the fabric due to the presence of reactive groups in their molecules, which bind to the fibers through covalent bonds ASGHER, 2012). However, reactive dyes are water soluble and have low affinity for fabrics, being thus released into effluents (WEBER & STICKNEY, 1993;. Textile effluents are characterized by extremes and by the fluctuation of pH and temperature, high levels of color, high chemical oxygen demand, and high concentrations of suspended solids and dissolved organics (RAGHUVANSHI et al., 2005).
They can also be associated with chelating agents, surfactants and heavy metals (WU & WANG, 2001), which are recalcitrant and toxic.
Because dyes are very stable and resistant to conventional biological wastewater treatment, a range of technologies has been developed and used for the removal of dye contaminants from wastewater. These methods include adsorption, advanced oxidation, chemical precipitation, coagulation/flocculation, electrochemical degradation, ion exchange, membrane filtration, ozonation, photocatalytic degradation, etc. (ADAMS & GORG, 2002;SOUZA et al., 2013;GAO et al., 2014;JESUS et al., 2014). Among these treatment methods, ozonation has been found to be effective to decolorize dye-colored wastewater and to degrade several dyes (WU & WANG, 2001;DOAN;UPRETI, 2008;TURHAN et al., 2012;TURHAN & OZTURKCAN, 2013). Ozone is an excellent agent in the decomposition of aromatic covalent compounds, and hence it is good for the decolorization of the dyestuff wastewater (TURHAN et al., 2012). In addition to the potential to efficiently accomplish both color removal and organic reduction in one step, ozonation has the following advantages: the aerobic biodegradability of dyes is increased, there is no solid waste, less space is required, it is easily operated, and all residual ozone can easily be decomposed to oxygen and water (CHU & MA, 2000).
Reactive Blue 19 (RB19) is one of the most widely used textile dyes.
However, more research is needed to assess the role of kinetics and mass ratio relationship on dye decolorization. Adams and Gorg (2002) presented the results of a laboratory study on the effects of pH (2, 5 and 9) and gas-phase ozone concentration (1, 7 and 11 wt%) on the decolorization efficiency via ozonation for seven common textile dyes, including RB19; the authors concluded that higher gas-phase ozone concentrations resulted in higher decolorization rates, and that lower pH levels were often more efficient to decolorize dyes. Fanchiang and Tseng (2009a) concluded that ozonation is efficient to decolorize and to eliminate RB19, though complete mineralization does not occur. It has also been reported by Fanchiang and Tseng (2009b) that the degradation of RB19 by ozonation involves the destruction of dye chromophore components, anthraquinone rings and their functional groups. In addition, authors performed essays to find intermediate products, thus indicating that non-toxic products were generated after 7 minutes of oxidation.  Ozone production and the off-gas ozone concentration were measured by 2% KI solution using the titrimetric method described in Standard Methods for Examination of Water and Wastewater (APHA; AWWA; WEF, 2012). Thus, it is possible to establish a relationship between oxygen flow, nominal power and ozone production. The gas flow rate was controlled by a needle valve and was measured by a flow meter, which is able to measure from 30 to 4,000 mL.min -1 . Specific to the mass ratio assays, the ozone production was kept constant at 53 mg O 3 .h -1 .

Factors affecting the decolorization of RB19
To evaluate the effect of ozone dosage on RB19 removal, assays were carried out at dosages of 38.4, 57.5, 76.7, 115.1 and 153.6 mg O 3 .L -1 at a pH of 7, with initial dye concentration (C 0 ) of 300 mg.L -1 . Ozone doses were chosen considering operation limits of equipment and preliminary investigation in this study; and the initial dye concentration was equal to 300 mg.L -1 to increase the accuracy of the analytical method. In these conditions, ozone was not detected in any off-gas samples, showing that all ozone was indeed consumed via RB19 degradation. To detect the effect of the initial pH on the removal of RB19, dye samples of 161 mL (300 mg.L -1 ) were submitted to 38.4 mg O 3 .L -1 at different pH levels (2, 4, 6, 7, 8, 10 and 12).
Diluted sodium hydroxide (1 M) and hydrochloric acid (1 M) were used to adjust the different initial pH values, and the pH of samples were measured by an Orion 710 Aplus pH meter (Thermo Electron Corporation, Waltham, Massachusetts, USA). The combined electrode was calibrated using the following high purity standards at pH levels of 4.00 (4.00 ± 0.01 at 25ºC ± 0.2ºC) and 7.00 (7.00 ± 0.01 at 25ºC ± 0.2ºC). Both assays were initiated immediately after the admission of ozone gas, and oxidation lasted 7 minutes. In addition, the effect of different RB19 concentrations (150,200,250 and 300 mg.L -1 ) on the removal of RB19 was also tested at pH 7 and with an ozone dose of 38.4 mg O 3. L -1 . For spectrophotometric investigation, assays were carried out with an ozone dose of 38.4 mg O 3. L -1 at a pH of 7, with a C 0 equal to 300 mg L -1 , at different oxidation times (0, 9, 12 and 15 minutes). Then, the concentrations of RB19 solutions were determined indirectly by absorbance, which was measured using the variable wavelength Spectrophotometer DR 2800 (Hach Company, Loveland, Colorado, USA) at its maximum absorption wavelength (592 nm), regardless of the pH range. During all experiments, the temperature was fixed at 22 ± 1 °C. All experiments were performed in triplicate, and the results are presented as their average with error bars.
However, if the ozone concentration is constant, which is the case in this study, the reaction can be assumed to be pseudo first-order In which: k = the decolorization kinetic constant (min -1 ).

Mass ratio (z) over time
In this study, the mass ratio (z) over time was investigated using an empirical equation and an analytical formulation. The empirical relationship follows power law decay, while the analytical relationship was analyzed according to the liquid film theory. Thus, assuming that all ozone is consumed only via RB19 degradation, it is possible to suppose that ozone decay may be expressed by means of z (mg of ozone per mg of RB19) over time, which can be followed by an equation similar to the one presented by film theory. The stoichiometric ratio z of the reaction between ozone and RB19 was calculated using Equation (4) to obtain the ozone consumption per unit amount of RB19 reacted in the decolorization step. Though this model is not very realistic, it has the advantage of simplicity. Furthermore, predictions based on this model are often similar to those based on sophisticated models (CHARPENTIER, 1981); thus, it may assist designers and operators in both project and operational interventions.
The attempt to analytically describe the behavior of z over time was based on the film theory, whereas the reaction is slow and occurs mostly in the bulk. Therefore, decay can be described by Equation 5.

Spectrophotometric investigation
The decolorization of RB19 at different oxidation times is illustrated in Figure 3.
Absorption peak occurs at a wavelength (λ) of 592 nm. According to Fanchiang and Tseng (2009b), RB19 has two major characteristic absorption bands, one in the UV region of 310 nm, characterizing the chromophore components of the dye molecules (anthraquinone), and the other in the visible region of 592 nm, which corresponds to the color blue. It is clear from Figure 3 that absorption intensity decreased as reaction time increased at every wavelength investigated, indicating that ozone is able to degrade (by destroying the chromophore structure) and decolorize the dye. Additionally, the decrease in the absorption peak by over 80% in the visible region suggests a rapid decolorization of RB19 by ozone.

Effect of ozone dose on the removal of RB19
The effect of the ozone dose on the removal of RB19 is shown in Figure 4.

Effect of initial pH on the removal of RB19
The efficiency of dye removal as a pH function is shown in Figure 5.
Results show that RB19 is more efficiently removed when initial pH is 7, value within the operating pH range of WWTP. The pH affects the ozonation process by affecting the rate of ozone decomposition and ozonation kinetics (ELOVITZ & GUNTEN, 1999). In the case of neutral pH, reactions can be mediated by both molecular ozone and hydroxyl radicals. Chu and Ma (2000) also noticed that the increase of pH does not necessarily improve the decolorization of anthraquinone dyes. In addition, Chu and Ma (2000) and Adams and Gorg (2002) concluded that, with respect to the decolorization efficiency, ozonation at a low pH is more efficient than at higher pH levels, probably because selective direct ozone reactions target unsaturated chromophoric bonds in the dyes.

Effect of initial dye concentrations on the removal of RB19
The impact of various initial dye concentrations (150,200,250 and 300 mg.L -1 ) on the removal of RB19 at a pH of 7 and ozone dose of 38.4 mg O 3 .L -1 is illustrated in Figure 6.
It is evident that the rate of RB19 removal decreased as the initial dye concentration increased. The rate of dye removal was higher at the beginning and decreased over time, as previously mentioned by Turhan et al. (2012) and Turhan and Ozturkcan (2013).

Decolorization kinetics
Pseudo first-order trends of decolorization were observed in all the experimental runs. The same behavior was also reported in Chu and Ma (2000), Sevimli and Kinaci (2002), Demirev and Nenov (2005) The logarithmic relationship between k ap and C 0 derived in this study is in agreement with the relationships observed by other     Wu, Eiteman and Law (1998), Sevimli and Sarikaya (2002) and Wu, Doan and Upreti (2008) affirm that k ap decreases with an increase in C 0 because more intermediate compounds are generated.
These intermediates also consume ozone, implying lower concentration of ozone with higher C 0 . The linearity between ln (k) and ln (C 0 ) seems valid for the ozonation of azo and reactive dyes, regardless of the co-existence of other components (WU & WANG, 2001;DOAN;UPRETI, 2008).

Mass ratio analyses
The experimental results and the power law empirical trend are shown in Figure 9.
Results showed that the mass ratio over time is slightly dependent on C 0 values. This behavior can be described by exponential decay, with a good fit in the empirical equation. The result suggests that the desired removal of RB19 can be achieved by simply controlling the consumed ozone.
For the analytical analyses, the obtained values of each parameter for different C 0 are shown in Table 1.
It is possible to conclude that the parameters used in Equation 5 followed a linear relationship with C 0 , with low slope values and R 2 > 0.87 (Table 2), thus reinforcing that RB19 is slightly dependent on C 0 .    data around the line that describes the analytical behavior suggests that mass transfer occurs both in liquid film and in bulk fluid.

CONCLUSIONS
The removal of RB19 by ozone demands an understanding of the decomposition rate. The main conclusions obtained in this study were as follows: • the RB19 removal efficiency increased from 64 to 94% when the ozone dose increased from 38.4 to 153.6 mg O 3 .L -1 ; • RB19 decolorization is more efficient at pH 7; • the rate of RB19 removal decreased as the initial RB19 concentration increased; • the ozonation of RB19 was a pseudo first-order reaction with respect to the dye; • the apparent rate constant decreased logarithmically with the initial RB19 concentration; • the experimental mass ratio obtained is independent of the initial RB19 concentration; • power law decay is adequate to describe the mass ratio over time for the empirical analysis; • the decolorization reaction is influenced by mass transfer in the liquid film as well as in the bulk fluid.  Air andSoil Pollution, v. 225, p. 1946-1958. http:// dx.doi.org/10.1007/s11270-014-1946-5