Ozonation is very efficient in order to provide discoloration. Therefore, ozonation comprises an important process for treatment of several different colored wastewaters.
Based on the EPA’s toxic release inventory [10], approximately 2,200 ton of four hazardous dyes are discharged annually into publicity owned treatment works. The classical methods employed for treating textile wastewaters include various combinations of the biological (activated sludge), physical and chemical processes [61].
The textile effluent is an important case for AOP processes since the use of classical methods for discoloration does not furnish satisfactory results [92-102]. Dye molecules are highly structured polymers, which are very difficult to break down biologically and, therefore, they cannot be treated efficiently by simple classical methods. Besides, the main drawback of these processes is the generation of a large amount of sludge or solid waste, resulting in high operational costs for sludge treatment and disposal.
Different alternative technologies based on ozonation can be used in combination with biological process aiming to provide an efficient discoloration and a TOC/COD reduction of the textile wastewater [93-98].
Textile waste effluents are one of the wastewaters that are very difficult to treat satisfactorily because they are highly variable in composition and contain several different recalcitrant compounds [101,102]. Wastewaters that are generated at various stages of the dyeing process differ in compositions and temperature. The high pollution load is mainly caused by spent dyeing baths. Their constituents are unreacted dyeing compounds, dispersing agents (surfactants), salts and organics washed out of the material which undergoes dyeing [61].
For low strength dye waste effluents, ozonation alone is sufficient to totally eliminate the color and reduce the turbidity. However, for medium and high strength waste effluents, ozonation is found to be sufficient to reduce the color, but not enough to reduce the turbidity. Hence, coagulation of the textile effluent using aluminum sulphate (~ 60 mg dm-3 for a TOC value of ~600-1000 ppm) or especially designed polymers can be necessary [92-102].
As reported by Tzitzi et al. [48], ozonation of the wastewater after coagulation- precipitation process, under the same conditions as the raw wastewater ozonation, exhibited more efficient discoloration (> 90%) and COD reduction (> 30%), while the biodegradability was found to increase. After this initial step of degradation the residual organic carbon generated during partial mineralization can be further effectively degraded using the activated sludge process. Thus, the combination of ozonation with proper chemical coagulation and an activated sludge process is a promising alternative technology for dealing with textile industry effluent, which considerable reduces the sludge disposal.
According to Shu and Huang [95], who investigated the chemical oxidation of non- biodegradable azo dyes by ozonation and photo-oxidation process in a pilot scale using a photochemical UV/O3 reactor, application of the UV-light did not significantly enhance the
degradation ability of the ozonation reaction.
As previously discussed by Franco et al. [54], there is a distinction between total discoloration of the dye solution and the total degradation (mineralization) of the aqueous solution containing refractory dyes. The discoloration process via ozonation takes place when the chromophore bond(s) is(are) removed, while many colored byproducts of the parent dye molecule may remain stable in solution [52,54,55]. Therefore, discoloration may be the initial step in the degradation route of a dye molecule, which is not necessarily accompanied by quantitative carbon removal (considerable degree of mineralization), thus requiring a lower oxidant dose than mineralization.
Discoloration Kinetics
During the curse of the ozonation reaction is possible that the daughter products (colored byproducts) may compete with the parental dye molecules for the oxidant (ozone and/or hydroxyl radical). Besides, taking into account the complexity nature of the ozonation process of long-chain organic molecules, the overall pseudo-first order rate constant, kobs, contains the
effect of the intrinsic kinetics and may reflect more than one mass transfer-chemical regime [54,55].
In the light of these considerations, the kinetic of the discoloration process carried out in the semi-batch mode (pseudo-first order conditions) can be adequately described by the following rate law [54,55,103]:
Figure 8. Flow diagram representing the experimental set-up used for ozone generation and its application for discoloration/degradation of textile dyes (adapted from ref. [54]).
Figure 9. Chemical structures of the commercial textile dyes: (A) Reactive Blue 264 (RB 264 - CAS Number: 70528-89-1 - C31H18ClFN10Na4O13S4) and (B) Reactive Yellow 143 (RY 143 - CAS Number:
75268-65-4 - C24H19ClFN9Na2O9S2).
t
k
t
cro
A
A
=−
⋅
=−
⋅
⎟⎟
⎠
⎞
⎜⎜
⎝
⎛
• obs z 0 y - x EOP 0}
])
[HO
]
([
]
[HO
(
)
ν
(
α
{
ln
(12)where: A/A0 represents the normalized absorbance measured at a fixed wavelength; [cro]o is
hydroxyl radical concentration and [OH-] is the hydroxyl anion concentration. α, x, y and z are the empirical constants for the particular ozonation process.
Figure 8 presents a flow diagram representing the experimental set-up used for EOP and its application for discoloration/degradation of textile dyes under controlled pH conditions [54,55]. In this case ozone was generated using the electrochemical reactor OZONA-ZY60 developed by Da Silva and Jardim (Patent – EGT/FAPEMIG, Brazil).
Figures 9A and 9B show the molecular structures for the commercial textile dyes Reactive Yellow 143 (RY 143) and Reactive Blue 264 (RB 264).
Figure 10. UV-VIS spectra for textile dyes RY 143 (A) and RB 264 (B) before and after ozonation at pH 12. IDC = 100 mg dm-3. Ozone dose: 0.35 ± 0.02 g h−1 (adapted from ref. [54]).
Table 3. Dependence of td values on pH for RY 143 and RB 264. IDC = 100 mg dm-3, V = 1 dm3 (adapted from ref. [54])
pH RY143 RB 264 td/min td/min
2 ± 0.05 25 14 7 ± 0.05 23 12 12 ± 0.05 20 11
Figure 10 shows the UV-VIS spectra obtained for RY 143 and RB 264 before and after ozonation (t = 1 h and pH 12) using a low ozone dose of 0.35 ± 0.02 g h−1. This study was carried out as a function of the solution pH (2 – 12) using the Column Bubble Reactor A (see Fig. 8) [54]. Both dyes show a maximum absorbance in the visible region, corresponding to 421 and 619 nm for RY 143 and RB 264, respectively. Initial Dye Concentration (IDC) was 100 mg dm-3 and final discoloration was reported as absorbance reduction higher than 99 %
obtained for a given ozonation time defined as td.
Analysis of figure 10 reveals that ozonation leads to a quantitative removal of the absorbance in the visible region. Besides, analysis of the absorbance bands in the UV-region revealed the total amount of aromatics decreased about 15 and 40% for both RY 143 and RB 264, respectively [54]. Chu et al. [104] also reported the dependence of the UV absorption bands on the ozonation time in order to account for degradation of the Reactive Black 5 (RB5) and found decay of the absorbance at 311 nm as evidence of the degradation of aromatic groups in the dye molecules and their intermediates.
Table 3 shows the total discoloration time, td, values as functions of the pH for RY 143
and RB 264, where is found that total discoloration (≥ 99%) is slightly attained faster in alkaline solution as a result of the influence of the indirect oxidative pathway via hydroxyl radical on discoloration process. Discoloration also depends on the nature of the dye (td(RB264)
< td(RY143)), thus revealing that the destruction of the chromophore centers in RB 264 is more
pronounced.
Figure 11 shows representative kinetic profiles for discoloration, while Table 4 gathers
kobs as a function of the pH for RY 143 and RB 264. A rather good linear behavior (r > 0.998)
was verified in all cases using the pseudo-first order kinetic model described by equation 12. Two linear segments in the pseudo-first order profile were obtained in all cases. The presence of two kobs in the kinetic profile supports the existence of two corresponding half-life
time constants, τ1/2, for discoloration and formation of intermediate colors before total
discoloration is achieved (production of Transient Persistent Colored Byproducts - TPCBP) [54,55,104].
The existence of two linear segments in the kinetic profile was also verified by Hsu et al. [52], who proposed that slope changes in the kinetic profile, after a given threshold t-value, were due to changes in the chemical nature of primary substances in solution, forming TPCBP.
Ozonation of the RB5 solution carried out by Chu et al. [104] revealed that that dye solution gradually bleached from black, to brown, to yellow, and then to colorless. Besides, these findings revealed that the lower the IDC, the higher the discoloration rate. This
dependency of discoloration rate on the IDC was attributed to the production of more than one kind of intermediate during the ozonation processes [54,55].
Analysis of Table 4 clearly reveals the pH does not lead to a significant change in discoloration kinetics, thus indicating that the “direct oxidative process” comprises the main oxidative process leading to discoloration.
0
4
8
12
16
20
-5
-4
-3
-2
-1
0
0,0
0,2
0,4
0,6
0,8
1,0
(A)
kobs-2 = 0.076 min-1 kobs-1 = 0.371 min-10
2
4
6
8
10
12
14
-6
-5
-4
-3
-2
-1
0
ln(A/A
0)
0,0
0,2
0,4
0,6
0,8
1,0
(B)
kobs-2 = 0.478 min-1 kobs-1 = 0.260 min-1A/A
0t / min
Figure 11. Kinetic profiles representative of discoloration via ozonation. (A) RY 143, pH 2; (B) RB 264, pH 2. IDC = 100 mg dm-3. T = 24 oC. Ozone dose: 0.35 ± 0.02 g h−1. V = 1 dm3 (adapted from ref.
Table 4. Dependence of kobs on pH for RY 143 and RB 264. IDC = 100 mg dm-3 (adapted from ref. [54])
pH 2 ± 0.05 pH 7 ± 0.05 pH 12 ± 0.05
DYES kobs-1 / min-1 kobs-2 / min-1 kobs-1 / min-1 kobs-2 / min-1 kobs-1 / min-1 kobs-2 / min-1
RB 264 0.260 0.478 0.210 0.380 0.284 0.581 RY 143 0.371 0.076 0.360 0.070 0.202 0.099
The linear behavior in the pseudo-first order profiles supports that ozonation of parental dye molecule and TPCBP are both slow chemical processes. Considering that νEOP, [cro]o,
[OH-] and [OH•] (see eq. 12) are both constants in the present case, one can propose that changes in kobs as functions of the ozonation time and the dye nature, can be attributed to
modifications suffered in the intrinsic kinetics between the oxidant (O3 and/or HO•) and the
different chromophore centers present in the parental and non-parental (TPCBP) dye molecules.
The discoloration kinetics can be divided in three stages [54,55]: (i) primary attack: the discoloration process is governed by chemical reaction involving the oxidant (O3 and/or HO•)
and the more reactive chromophore centers of the parental molecule; (ii) secondary (transient) attack: color removal rate is influenced by changes suffered in the intrinsic nature of the oxidation process as a consequence of the competition between the new chromophore centers present at the TPCBP structure, which are formed after the primary attack, and (iii) tertiary attack (last stage of discoloration): color removal takes place via oxidation of the remaining chromophore centers present in TPCBP.
Therefore, changes in kobs as functions of the ozonation time can be attributed to
modifications in the [dye]/[TPCBP] ratio, which results in a considerable modification in the intrinsic kinetics between the oxidant (O3 and/or HO•) and the chromophore centers [54].
In order to account for influence of mass transfer on ozonation process, it was proposed an empirical linear relation for the discoloration process, which correlates, for a given temperature, the enhancement factor (E) with the initial dye concentration ([IDC]) and the ozone application rate (z(νEOP)). This relation was denoted as [105]:
E = x + y[IDC] + z(νEOP), (13)
where x, y and z are experimental parameters determined for each dye system.
Analysis of equation 13 reveals that E increases with [IDC] due to the chemical kinetics, since ozonation of dye is pseudo-first order with respect to O3 and the dye. Therefore, the E-
value should increase with the concentration of both the dye and the dissolved ozone. Besides, any increase in νEOP leads to a concomitant increase in turbulence at the gas/liquid
interface, thus enhancing the mass transfer at the reactive zone (see model for bubble/liquid interface presented in Fig. 6). From these considerations, one has that keeping constant the νEOP under semi-batch conditions the ozonation becomes pseudo-first order with respect to
It was found that kobs declines logarithmically with [IDC]. This behavior was empirically
described by the next relation [105]:
kobs = w[IDC]-m, (14)
where w and m are empirical constants. It was verified by Wu and Wang [105] that the linear log(kobs)-log([IDC]) relationship can be always applied for discoloration of azo dye solution
regardless the co-existence of other compounds.
Degradation of eight commercial reactive azo dyes with different structures containing different substituted groups was studied by ozonation individually and in mixture [106], where was proved that ozonation carried out in alkaline conditions (pH 10) using an appropriate ozone dose is indeed effective for discoloration and COD removal. The influence of the molecular structure was evidenced in this study; even for dye solutions decolorized up to 95-99%, it was verified that COD removal was different for each dye. Contrary to discoloration process, this behavior indicates the complexicity of the molecular structure plays a key role during COD removal [106].
Oxidation and cleavage of substituent groups were evidenced by the release of chloride, nitrate and sulfate during ozonation. Increase in biodegradability was observed after ozonation, as measured by the BOD5/COD ratio [106].