No Catalyst Dye Photodegradation in a ... - ACS Publications

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No Catalyst Dye Photodegradation in a Microemulsion Template Adina Roxana Petcu,† Aurelia Meghea,† Elena Adina Rogozea,*,† Nicoleta Liliana Olteanu,† Cosmina Andreea Lazar,† Daniela Cadar,‡ Adrian Victor Crisciu,‡ and Maria Mihaly*,‡ †

Research Centre for Environmental Protection and Eco-friendly Technologies, University POLITEHNICA of Bucharest, Polizu 1, RO-011061, Bucharest, Romania ‡ Faculty of Applied Chemistry and Materials Science, Inorganic Chemistry, Physical Chemistry and Electrochemistry Department, University POLITEHNICA of Bucharest, Polizu 1, RO-011061, Bucharest, Romania S Supporting Information *

ABSTRACT: This paper proposes an innovative solution for the efficient dye degradation from water treatment plant effluent, using microemulsion assisted photodegradation without the addition of a solid catalyst. Total photodegradation of dyes was achieved using for the first time a process based on combined photocatalytic effects of the water/oil interface processes and the potential activity of ethyl acetate as a UV initiator. The catalytic effect of nanostructured fluids based on the carboxylic oil phase is an absolute scientific premiere, and microemulsion assisted photodegradation processes open a new direction to pollutants photodegradation approach, exceeding the performances of both homogeneous and heterogeneous catalysis. The phenomenological modeling, with and without initiator, on both homogeneous and heterogeneous systems showed that the photodegradation of cationic and anionic dyes in a water/nonionic surfactant/ethyl acetate microemulsion is a radical process based on a chain reaction scheme. The complete photodegradation of cationic (Crystal Violet) and anionic (Methyl Orange) dyes in the microemulsion system occurs in 25 and 20 min, respectively, after an induction period of 5 min. The surfactant was recovered and reused in four successive processes, and for the degradation of 3.23 mg dye, 1 g of surfactant was used. KEYWORDS: Microemulsion, Photodegradation, UV initiator, Dye, Kinetics modeling

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INTRODUCTION

By combining the two techniques, a fast (minutes), efficient, cost-effective, and environmentally friendly (no sludge generation, uses nontoxic chemicals) method is proposed. More specifically, in this paper a detailed study has been carried out to determine the optimal conditions for total dye photodegradation from textile wastewaters using a combined process of dye extraction in the organic phase followed by the complete removal of dyes by photodegradation in a microemulsion. The present study aims to elucidate the mechanism of a nanoheterogeneous photodegradation process of textile dyes from wastewaters without the addition of solid catalysts. In order to simulate the practical working conditions of a water treatment plant, the effect of several factors on removal efficiency was studied: irradiation source, pH, addition of carboxylic compounds. The recovery and reuse of surfactant was also investigated. To the best of our knowledge, no study has been reported on the UVC photodegradation of a mixed dye solution extracted in a microemulsion system, without the addition of solid catalysts. Instead, the catalyst role might be played by the energy excess created in surfactant self-assembling or in microemulsion systems.

One of the most difficult tasks confronted by wastewater treatment plants (WWTPs) from dyeing industries is the incomplete removal of color in effluent, mainly because dyes are designed to resist biodegradation and other classical treatment processes.1,2 An incomplete decolouration of dyes leads an excess of total nitrogen content in the effluent which is out of the limits imposed by the European regulations (Directive 2000/60/EC) as the treatment process is not efficient and ionic surfactants, color stabilizers, and other toxic compounds are found at the WWTP effluent. Most conventional wastewater treatment technologies have significant drawbacks like long processing time,2,3 inability to remove the entire quantity of pollutant,4−8 the persistence of some residual materials,9−14 etc. Choosing the proper technique for dye removal from wastewater depends on the matrix features (concentration, stabilizers, preservatives, detergents, etc.).14,15 In terms of efficiency liquid−liquid extraction is a technical solution for dye removal, but it is expensive and requires additional chemicals that generate secondary wastes.12,13 In recent years, advanced oxidation processes (AOPs) to remove organic compounds have become an emerging area of fundamental research.16−21 © 2017 American Chemical Society

Received: February 27, 2017 Revised: April 26, 2017 Published: May 15, 2017 5273

DOI: 10.1021/acssuschemeng.7b00616 ACS Sustainable Chem. Eng. 2017, 5, 5273−5283

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ACS Sustainable Chemistry & Engineering

Figure 1. Chemical structures of (a) Methyl Orange, (b) Brij 30, (c) Crystal Violet, and (d) Rhodamine B.

Figure 2. Degradation of (a) CV, (b) Rh B, and (c) MO monocomponents dyes extracted in mE and in aqueous solutions under UVC irradiation.

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The paper brings significant original contributions in treatment of wastewater polluted with cationic and anionic dyes using nanostructured systems, a new approach in the field of soft nanotechnology techniques. The main element of originality lies in an innovative conceptual approach on nanoheterogeneous photodegradation of complex organic mixtures. Data kinetic modeling has highlighted the role of surfactant and initiator (ethyl acetate) in nanoheterogeneous photodegradation of the three ionic dyes.

MATERIALS AND METHODS

Materials. Ethyl acetate (EtOAc) 99.8%, butyl acetate (BuOAc) 99%, Rhodamine B (Rh B) dye, hydrochloric acid (HCl), ammonium hydroxide solution 25%, 2-propanol, n-heptane and 2,2,4-trimetylpentane (iso-octane) were purchased from Sigma-Aldrich. Crystal Violet (CV) and Methyl Orange (MO) dyes were obtained from Riedel-de Haen. Polyoxyethylene (4) lauryl ether (Brij 30) was provided by Acros Organics. Potassium oxalate and sodium citrate were bought from Chimopar. Distilled water was used for the preparation of the microemulsion (mE) samples. All chemicals were used as received 5274


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ACS Sustainable Chemistry & Engineering without further purification. The chemical structures of dyes and surfactant used in this work are presented in Figure 1. Methods. Extraction and Photodegradation Procedure. The Winsor II (WII) composition used in this study (12.61% EtOAc, 8.01% Brij 30, and 79.38% water) has been chosen from the ternary phase diagrams published by the authors elsewhere.21 The detailed experimental extraction was described in a previous paper.19 A WII microemulsion was prepared using nonionic surfactant (Brij 30), organic phase (EtOAc), and a mixed dye solution (aqueous phase). The dye solution was obtained by mixing three ionic dye, Crystal Violet, Methyl Orange, and Rhodamine B aqueous, solutions in equal amounts and of similar concentration (10 mg/L). The extraction experiments were carried out at room temperature by separating the microemulsion phase with the extracted dyes and the clean aqueous phase, based on the density difference of the two phases. The photodegradation experiments were performed on the microemulsion phase containing the extracted dyes by irradiation under different light sources, from UV to VIS. A Luzchem photoreactor, LZC4X, equipped with eight lamps, each of 12 W (using UVC 254 nm, UVB 300 nm, UVA 350 nm, and VIS 580 nm), was used. The intensity per each lamp was 0.024 W/m2. The temperature was kept constant throughout the reaction time by the ventilation system in the main chamber. The influence of some physical−chemical parameters on the photodegradation efficiency has been investigated as follows: irradiation source, pH, addition of carboxylic groups, and the kinetic constants have been calculated. The recovery and reuse of surfactant has been also studied. Color Analysis. Taking into consideration that the aqueous phase is formed by a mixed dye solution, the individual concentrations are difficult to be calculated using the absorbance values. Therefore, the photodegradation kinetic studies were followed by monitoring the color difference (ΔE*ab) values.19 All ΔE*ab values were corrected by subtracting the background value provided by Tyndall effect caused by surfactant in similar microemulsion systems without dye. The changes in the absorption spectra of the microemulsion systems and the transmission spectra were recorded on a UV−VIS spectrophotometer type V-670, Jasco, and the color parameters were determined using appropriate software for color analysis. Modeling the Dye Photodegradation. Kinetic modeling of experimental data aims to identify a set of reactions that better explain the variation in time of the measured value for the extinction of the dye solution. In the absence of any information on the chemical nature of intermediaries (molecular and/or radical) formed in the photodegradation process, the modeling is purely phenomenological, based on the general structure of a chain reaction scheme. In all reaction schemes: A designates the principal reactant (dye), I is initiator (EtOAc), P is the reaction products (grouped in one category), and R is radical intermediaries (also represented by a single species). In the differential equations drawn under such schemes, ki and xi stand for the rate constant and the degree of extent per unit volume of the ith reaction, respectively. The concentrations and the volume degree of extent per unit volume are all expressed in value scales of molar extinction at optimal wavelength for the colorful species (chromophore).

Figure 3. Photodegradation efficiency of dyes mixture extracted in mE under UVC irradiation using different oil phase.

radicals in natural light, thus playing the potential initiators role for the photochemical degradation of dyes. These results were also confirmed by thin layer chromatography analysis (Supporting Information). Physical−Chemical Parameters Influencing the Dye Photodegradation. The influence of different oil phases (nheptane, iso-octane, butyl acetate) and ionic carboxylic groups (the above-mentioned organic salts) on dyes photodegradation was studied as well. Oil Phase Type. The role of ethyl acetate as potential initiator of photodegradation was investigated by changing the oil phase as it follows: (i) hydrocarbon oils (n-heptane, isooctane), when a significant decrease of the photodegradation efficiency (cca 20%) was noted; (ii) butyl acetate oil, when a similar photodegradation efficiency to that of using ethyl acetate was obtained, but over a longer time (Figure 3). Ionic Carboxylic Groups. To examine whether carboxylic groups accelerate or decelerate the dye photodegradation, the influence of the carboxylic salts, potassium oxalate (125.6 g/L) and sodium citrate (129.6 g/L), was investigated. The experiment was realized by separating the microemulsion phase with the extracted dyes after (i) 1 h of equilibrium time (Figures 4a, 5a, and 6a); (ii) 12 h of equilibrium time (Figures 4b, 5b, and 6b). After the separation, the samples were irradiated using UVC light until the no color was observed (absorbance value < 0.01 au). In the case of the system with one carboxylic group (EtOAc), the absorbance slowly decreased after 12 h equilibrium time compared to 1 h (Figure 4). For MO peak the maximum wavelength was constant (420 nm) in the presence of the carboxylic salts, while for CV and Rh B spectra a significant hypochromic effect is revealed, after 15 min UVC irradiation. The absorption peak of Rh B also presented an important hypsochromic shift, from 553 to 530 nm. As it was observed from the UV−vis spectra, dye degradation occurred in 11 h (the difference between the samples separated after 12 h (Figure 6b) compared to 1 h (Figure 6a)), without irradiation. The degradation of each dye in 12 h was calculated using the initial absorbance values presented in Figures 4−6, and the results are collected in Table 1. It can be seen that in the case of Rh B and CV the dye degradation efficiencies were above 50% when potassium oxalate or sodium citrate were added in the system. It is once again demonstrated that the carboxylic compounds generate free radicals in natural light able to initiate the photochemical degradation of dyes. From the ANOVA nonparametric tests (Supporting Information), the probability values, p = 0, show that the null hypothesis can be surely rejected. Furthermore, the distributions from the Dunn test are pairwise different; hence, the statistical representation from Figure 7 is relevant. It can be seen that the degradation occurs slower in the systems with added carboxylic salts than in those without, as the

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RESULTS AND DISCUSSION Microemulsion Assisted Photodegradation of Dyes. A comparative study on degradation of dyes aqueous solutions and dyes extracted in microemulsion by using UVC irradiation (Figure 2) has been performed. In the case of aqueous solutions a complete dye photolysis could not be achieved even after 300 min irradiation, while for the dyes extracted in microemulsion, the complete degradation was obtained after 15, 35, and 25 min for CV, Rh B, and MO, respectively. The high efficiency of photodegradation seems to be achieved both by the microemulsion nanostructure and ethyl acetate−oil phase, known as carboxylic compound able to generate free 5275


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Figure 4. Dye degradation in the presence of one carboxylic group after (a) 1 and (b) 12 h equilibrium time.

Figure 5. Dye degradation in the presence of organic salt with three carboxylic groups after (a) 1 and (b) 12 h of equilibrium time.

Figure 6. Dye degradation in the presence of organic salt with 4 carboxylic groups after (a) 1 and (b) 12 h of equilibrium time.

Table 1. Dye Degradation Efficiencies in 11 h without Irradiation carboxylic comp. EtOAc

EtOAc + oxalate

EtOAc + citrate

dye

n%

CV Rh B MO CV Rh B MO CV Rh B MO

48.47 23.15 no changes 71.81 55.96 no changes 79.80 51.82 no changes

half-life time is higher. The carboxylic salts adsorption at the surfactant monolayer lead to (i) the known salting-in effect and a significant enhancing of dyes extraction efficiency (Supporting Information), and (ii) the light screening phenomenon as the photodegradation becomes more difficult. Light Source. Since outdoor water conditions are highly variable, to determine how the light sources influence the dye photodegradation, UVA, UVB, UVC, and VIS irradiation sources

Figure 7. Influence of carboxylic groups onto the photodegradation process after 12 h equilibrium time: (1) system with EtOAc, (2) system with oxalate, and (3) system with citrate.

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Figure 8. Degradation of CV, Rh B, and MO monocomponent dyes under UVB irradiation.

have been used. Electronic spectra of CV, Rh B, and MO dye degradation under UVC irradiation are presented in the Supporting Information. The UVA and VIS radiations exhibited no significant influence on the dye degradation process, while UVB and UVC radiation led to high photodegradation efficiency. As can be seen from Figure 8 the three CV, Rh B, and MO dyes extracted in the microemulsion system were completely photodegraded under UVB irradiation in 70, 110, and 150 min, respectively. Results from this study show that almost any light source from 200 to 310 nm is suitable for dye photodegradation. However, since the UVC degradation of dyes is 20 times faster in the CV case than that using UVB, the former light source is used for further studies. pH. Besides dyes, the effluents coming from the textile wastewater contain very high concentrations of organic and inorganic nitrogen, sulfates, and chlorides. Thus, the effect of pH value in the 1−5 range on UVC degradation of mixed dye solution extracted in microemulsion was investigated. The pH of the system was adjusted by adding different amounts of HCl. The original pH of the microemulsion system with the mixed dyes extracted was around 5. From Figure 9, it can be observed that at strong acidic pH, the half-life time decreased (at pH = 1, t1/2 = 6.9 min), thus one can affirm that the photodegradation half-life time was influenced by the pH of the microemulsion. In strong acidic media the EtOAc hydrolysis takes place and acetic acid is formed. The acetic acid under UV radiation produces CH3COO● radicals, probably faster than EtOAc hydrolysis. From the statistical analysis (Supporting Information), the probability values (p) are zero which shows that the null hypothesis (different factors do not influence the half time) may be indubitably rejected. More, a Dunn test (Supporting Information) demonstrates that the distributions are pairwise different, so the variation in Figure 9 is statistically relevant. Surfactant Recovery and Reuse. The microemulsion phase obtained after the extraction process was photodegraded using UVC light until the color was removed and then it was thermally treated (120 °C) in order to evaporate the water and EtOAc. The recovered surfactant was reused in a new extraction process. The extraction produced an efficient color removal

Figure 9. Influence of pH on photodegradation time of the mixed dye solution in the EtOAc system.

Table 2. Surfactant Reuse

a b

extraction

surfactant recovery

dye mass, mg

1 2 3a 4b

87% 72% 68% 57%

1.59 1.38 1.15 1.07

The equilibrium time between aqueous and organic phase was 48 h. The equilibrium time between aqueous and organic phase was 72 h.

Table 3. Kinetic Parameters for Monocomponent Dye Degradation under UVC Irradiation dye

R2 ith-order model

R2 sigmoidal representation

t1/2, min

CV Rh B MO

0.8893 0.9344 0.8528

0.9830 0.9948 0.9935

10.77 14.85 11.49

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Figure 10. Kinetic evolution of monocomponent dyes in mE degradation under UVC irradiation.

Photodegradation Kinetics. Photodegradation Kinetics Experiments. Photodegradation kinetics experiments, under UVC irradiation source, were performed, on both heterogeneous S0 system: water + surfactant + EtOAc (mE) S1 system: water + surfactant (above the critical micellar concentration) and homogeneous systems S2 system: water + EtOAc (in the miscibility domain, water as major component) S3 system: EtOAc + water (in the miscibility domain, EtOAc as major component) S4 system: EtOAc + surfactant S5 system: surfactant S6 system: water The kinetic data obtained for S0 system, such as half-life times (t1/2) and determination coefficients (R2), for the ith-order model and sigmoidal representation, are collected in Table 3. As illustrated, the value of half-life time is smaller for CV than for MO and Rh B. The kinetic curves from Figure 10, and the values of the determination coefficients (R2 lin) for the first-order model, indicated that the pseudo-first-order model was not the best fit model for the present photocatalytic experiments due to the fact that the photodegradation in the microemulsion process is much more complex than the heterogeneous catalysis. By comparing the photodegradation efficiency in time for all the investigated systems (S0−S6) the best results were obtained in the presence of both EtOAc and surfactant, underlining, once again, the photoinitiator role of EtOAc (Figure 11). Since the

Figure 11. Photodegradation efficiency in time of monocomponent dyes in mE under UVC irradiation.

(around 96%), and the process was repeated 4 times. However, it should be mentioned that in the case of the third and fourth extraction, the equilibrium time between the organic and the aqueous phase was higher than for the first and second extraction. The surfactant recovery was calculated in every case, and Table 2 shows that after the fourth extraction process only 57% of surfactant was recovered. It can be seen that 1 g of surfactant extracted 3.23 mg dye in four extractions. Even though after four extractions the recovery of surfactant was above 50%, the extraction process was completed since the equilibrium time was too high and the photodegradation time of the microemulsion was 120 min compared with the first extraction when the photodegradation time was 20 min only. 5278


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ACS Sustainable Chemistry & Engineering Table 4. Mathematical Models for Photodegradation Kinetics model

linear propagation without initiator

reaction scheme

reaction steps

A→R

1 initiation

A+R→P+R

dx 2 = k 2(CA° − x1 − x 2)(x1 − x3) dt

2 propagation by chain transfer

R→P

dx 3 = k 3(x1 − x3) dt

3 interruption

A→R

dx1 = k1(CA° − x1 − x 2 − x3) dt

1 initiation

A+R→P+R

dx 2 = k 2(CA° − x1 − x 2 − x3)(x1 + x3 − x4) dt


A + R → P + 2R

dx 3 = k 3(CA° − x1 − x 2 − x3)(x1 + x3 − x4) dt

3 propagation by chain branching

R→P

dx4 = k4(x1 + x3 − x4) dt

4 interruption

I→R

dx 0 = k0 dt

0 initiation by initiator cleavage

A→R

dx1 = k1(CA° − x1 − x 2) dt

1 initiation by dye cleavage

A+R→P+R

dx 2 = k 2(CA° − x1 − x 2)(x0 + x1 − x3) dt


R→P

dx 3 = k 3(x0 + x1 − x3) dt

3 interruption

I→R

dx 0 = k0 dt

0 initiation by initiator cleavage

A→R

dx1 = k1(CA° − x1 − x 2 − x3) dt

1 initiation by dye cleavage

A+R→P+R

dx 2 = k 2(CA° − x1 − x 2 − x3)(x0 + x1 + x3 − x4) dt


A + R → P + 2R

dx 3 = k 3(CA° − x1 − x 2 − x3)(x0 + x1 + x3 − x4) dt

3 propagation by chain branching

R→P

dx4 = k4(x0 + x1 + x3 − x4) dt

4 interruption

nonlinear propagation without initiator

linear propagation with initiator

nonlinear propagation with initiator

equations

dx1 = k1(CA° − x1 − x 2) dt

tested dyes are hydrophilic, the microemulsion is an appropriate environment for their solubilization in an organic media (EtOAc). All the results obtained suggest that the high efficiency obtained by conducting the photodegradation in mE are based on both the adsorption of dyes at the surfactant monolayer selfassembled at the water−EtOAc interface and the photoinitiator role of EtOAc. These assumptions are further considered in the modeling of the photodegradation kinetics. Modeling of the Photodegradation Kinetics. The phenomenological modeling of CV and MO photodegradation in homogeneous systems (S1−S6) was carried out in order to better understand the degradation mechanism of dyes. For this purpose the extinction values of the solutions at the absorption maximum of the dye were used. For the modeling procedure four approaches have been taken into consideration by combining two mechanistic elements: first, a propagation step by simple chain transfer (linear propagation) or involving also branching (nonlinear propagation), and second, an initiation step involving the dye molecule only, or with a preliminary initiator species as well. These mathematical models are detailed in Table 4. In the case of models by linear and nonlinear propagation with initiator, the initiator (EtOAc) is present in the system at higher

concentration than the dye. Indeed, the minimum concentration of ethyl acetate is 2.5% (w/w), which corresponds to a molar concentration of 2.83 × 10−2 mol/L. On the other hand, the dye concentrations are, in each case, on the order of 10−5 mol/L; therefore ethyl acetate is present in excess by three magnitude orders, which justifies the above assumption. The optimality criteria for choosing the best kinetic model are presented in the Supporting Information. Correlations of experimental data with these kinetic models are illustrated in Figure 12 for CV dye and Figure 13 for MO dye, and corresponding rate constants are collected in Tables 5−8. Decreasing curves describe how well experimental data are fitting for reactant dye degradation, while the second curve simulates the evolution of the intermediary species. The interpretation of these concentration evolutions in tight connection with relative values of the kinetic constants provides relevant information to understand more deeply the mechanisms of these complex systems. Cationic Dye Photodegradation Kinetic in Homogeneous Systems. The CV photodegradation in hydrophilic media, water (S6) and aqueous micellar dispersion (S1), is slower (250 min) compared with dye dissolved directly into the surfactant (S5−30 min), as shown by the values, higher in the last case, for both 5279


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Figure 12. Experimental data correlation with kinetic models proposed for CV dye in S1−S6 and mE systems.

reaction rate constants of the initiation (k1) and propagation (k2) steps (Table 5). For the systems with EtOAc as initiator, a model in which the initiation stage consisting in two steps, the EtOAc cleavage (k0) and CV dye splitting (k1) was proposed (model 2, Table 4). In the S3 system, where EtOAc is the major component, the initiation rate is higher than in the case of the S2 system (Figure 13). For the S4 system, the radicals come only from the cleavage of the CV dye molecules since EtOAc breakage was prevented by the presence of surfactant in the system. The fastest decomposition of the dye occurred in the S4 system, where the rate constant of the propagation step was much higher than in the S2 and S3 systems. From the last diagram of Figure 13, one may observe that photodegradation of dye extracted in microemulsion is faster than in any homogeneous system. Completion of the dye photodegradation reaction is decisively determined by the mechanism and the interruption reaction rate constant (k3), with deactivation of the reactive intermediary species. This deactivation cannot be produced by direct interaction of two radicals, but by mechanisms that allow to take-over the energy excess of the radicals, either by side wall reactions or by bi-/ trimolecular collisions with inert solvent molecules, in this case water or surfactant.

In the case of the CV photodegradation in hydrophilic systems (S1, S5, S6), the highest interruption rate constant was calculated for S1 system, which means that the fast reactive species deactivation leads to a longer time for reactant decomposition. The lower value of k3 = 6.6 × 10−4 min−1 justifies the radical intermediaries accumulation which remain long time after the fast dye degradation. For the S6 system, the slower initiation and degradation reactions lead to an easy-going dye deactivation; the first reaction is the rate-limiting step of the entire chain reactions system. For the other three systems (S2, S3, S4, Table 6), the excessive accumulation of reactive species is obvious in the kinetic curves of CV photodegradation both in the hydrophilic (S2) and hydrophobic (S3) water−EtOAc systems and organic surfactant solution (S4), with the predominance of the species from EtOAc over those obtained directly from dye cleavage (k0 > k1), concomitantly with the lack of interruption reactions, in both cases k3 → 0. Anionic Dye Photodegradation Kinetic in Homogeneous Systems. The similar analysis performed for the MO dye based on best fitted kinetic model (Table 4 model 3) led to the conclusion that the propagation step takes place both through transfer and branching of the dye chain. 5280


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Figure 13. Experimental data correlation with kinetic models proposed for MO dye in S1−S6 and mE systems.

Table 5. Phenomenological Modeling for CV Dye in Systems without Initiator Contigue sequence test

rate constants·103 min−1

Spearman linear correlation coefficient

Wilcoxon nonparametric test (T)

system

k1

k2

k3

R2

U

P (probability)

T+

T−

no. of exp observations

Rs


S1 S5 S6

2.99 33.27 1.49

56.58 121.54 29.23

66.10 66.10 × 10−4 16.59 × 10−3

0.9997 0.9968 0.9961

6 8 6

0.786 0.992 0.786

27 21 23

18 24 22

9 9 9

−0.19 −0.66 −0.69

8 8 8

Table 6. Phenomenological Modeling for CV Dye in Systems with Initiator Contigue sequence test

rate constants·103 min−1 system S2 S3 S4

k0 35.75 169.57 0.26

k1 2.19 22.87 415.59

k2 26.81 15.57 5992.62

k3 −20

12.70 × 10 31.66 × 10−52 2904.92



R2

U

P

T+

T−


Rs


0.9966 0.9858 0.9533

6 5 2

0.643 0.357 0.0159

20 26 33

35 29 12

10 10 9

−0.116 0.083 0.619

9 9 8

From Figure 13 and Table 7 is observed that for MO anionic dye in aqueous media (S6) or aqueous micellar dispersion (S1), the photodegradation reaction is slow and incomplete, while the evolution curve of radical intermediaries is undetectable, due to negligible propagation and branching rate constants, along with

high values of the interruption step. In this respect, the decomposition reaction of MO appears as a simple first order reaction, without reaching the total dye degradation even after 250 min. Instead, the propagation phase rate is much faster in the system with surfactant only (S5), the propagation rate through 5281


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ACS Sustainable Chemistry & Engineering Table 7. Phenomenological Modeling for MO Dye in Systems without Initiator Contigue sequence test

−1

3

rate constants·10 min



system

k1

k2

k3

k4

R2

U

P

T+

T−


Rs


S1 S5 S6

2.56 18.27 × 10−3 2.39

1.71 × 10−20 6.34 57.06 × 10−3

2.37 × 10−23 430.55 26.03 × 10−6

828.01 745.56 997.23

0.9837 0.9898 0.992

3 5 5

0.114 0.629 0.629

8 19 14

28 17 22

8 8 8

0.6071 −0.3214 −0.4643

7 7 7

Table 8. Phenomenological Modeling for MO Dye in Systems with Initiator 3

rate constants·10 min

Contigue sequence test

−1



system

k0

k1

k2

k3

k4

R2

U

P

T+

T−


Rs


S2 S3 S4

11.99 0.10 0.88

6.73 57.18 9.03 × 10−3

90.56 37.25 363.45

0.11 56.59 710.06

13.59 519.15 65.31 × 10−3

0.9793 0.9783 0.9355

4 4 5

0.262 0.167 0.357

13 41 27

32 14 28

9 10 10

0.5 0.70 −0.15

8 9 9

chain branching being by two magnitude orders higher than that by chain transfer. In the interruption step, the rate constants (k4) have comparable values in all three systems without initiator. The difference between them is that in the surfactant system (S5) the formation of a large amount of radicals in the propagation and branching steps resulted in a total degradation of both the dye and intermediate species after 30 min (Figure 13). The fourth proposed model (Table 4 model 4), that relates to the EtOAc presence, sustained that MO dye degradation occurred due to both initiator (EtOAc) and radicals formed from the decomposition of dye molecules. As in the previous case, the propagation step occurred with transfer and branching chain. In Table 8 are highlighted the rate constants for each phase. It is noticed that in the surfactant and ethyl acetate system (S4), the radicals formed by cleavage of EtOAc are enough to degrade quickly the MO dye. The very low value of constant rate in the interruption step shows that not all radicals formed were consumed in the complete degradation of the dye. For the two systems (S2, S3) without surfactant, it has been shown that radicals responsible for degradation of the dye come from the cleavage of both EtOAc and dye molecules. Also, the high value of the rate constant of the interruption step for the S3 system leads to an incomplete degradation of the dye due to radical recombination. In the case of MO extracted in microemulsion, the photodegradation rate is considerably enhanced and the behavior is similar to CV photodegradation. In this context, the dye photodegradation in mE kinetic benefits of the synergistic effect of surfactant self-assembling at water/oil interface and radicals generated from EtOAc (photodegradation initiator). Extrapolation of Dye Photodegradation Kinetic in Heterogeneous Systems. Based on the kinetic considerations made in homogeneous systems, the dyes photodegradation behavior in microemulsion system can be interpreted by extrapolation, this taking place very quickly (about 20 min), after an induction period in the first 5 min. From composition point of view, the microemulsion approaches S4 and S3, as the EtOAc is the dispersant and water is the dispersed, and the surfactant self-assembles at their interface. Through these interfaces the dye molecules are exposed both to radicals generated by EtOAc and UV radiation,

making it susceptible to photodegradation by own radicals, such as in the case of CV, thus propagating the photodegradation. TLC analysis was used to confirm that the dyes extracted in microemulsion were totally degraded under UVC radiation without the addition of any catalyst. The CV and MO dye photodegradation in hydrophilic systems (S1, S5, and S6) are kinetically similar, as the rate constant of the initiation reaction decreased in the sequence kS5 > kS1 > kS6 due to the surface energy of the self-assembled surfactant which contribute to the activation energy necessary for the initiation step. Thus, the self-assembling effect in dye decomposition process, after a chain mechanism (Figures 12 and 13), is underlined. A similar self-assembling effect is also observed in the propagation phase, where k2 rate constant is by one magnitude order greater than k1. In the case of hydrophobic system (S4), the amplification of both initiation and propagation reaction rates is noticed, as a result of dye solubilization in EtOAc by self-assembling effect of surfactant, which leads to the shortest time for the photodegradation reaction. The rate constant value is also high for the interruption reaction, which confers to the reactive species a profile with maximum (even thought for rather low values), specific for the intermediaries of successive chains reaction, where the total consumption of both reactant and intermediary species is produced. The induction period in the microemulsion system may be interpreted by the formation of radical species from EtOAc (k0), which does not have direct access to CV dye located toward the interior of the droplet between polar head groups of surfactant molecules. The radical species once formed into the organic phase and by their transfer through the surfactant membrane coating, the dye degradation process is enhanced by the activation energy decreasing of decomposition reactions due to superficial energy uptake by bringing the reactant at the nanoscale. In conclusion, the microemulsion seems to be the most efficient and feasible system for dye photodegradation. Finally, the advantages of microemulsions used as photodegradation media reside in their capability to be used as extraction and concentration systems together with their function as UV nanoreactors. Therefore, the location of anionic or cationic dyes in reverse micelles imprints similar features on 5282


Research Article


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their photodegradation kinetics in microemulsion, the decisive factor being the catalytic effect caused by surfactant selfassembling in this nanoheterogeneous system.

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CONCLUSIONS The water/Brij 30/ethyl acetate microemulsions were highly efficient in dye photodegradation with 100% efficiency. Dyes were degraded using UVC radiation without the addition of a solid catalyst. The acidic pH significantly increased the dyes degradation rates in the studied microemulsion system. Kinetic modeling of experimental data demonstrated that the photodegradation of dyes in microemulsion phase is a radical process based on a chain reaction scheme. The catalytic effect of nanostructured fluids based on carboxylic oil phase is an absolute scientific premiere, and coupling of photoirradiation and microextraction processes opens a new direction to the pollutant degradation approach, exceeding the performances of both homogeneous and heterogeneous catalysis. The two-step process proposed in this paper can be easily implemented in a water treatment plant, as it is a fast (30 min), efficient, cheap, comfortable to use procedure with enhanced selectivity and no need to process at high temperature or pressure compared with existing treatment solutions that are time-consuming, require expensive equipment, and have significant energy requirements.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.7b00616. Selection of optimum kinetic model, electronic spectra of CV, Rh B, and MO dye degradation under UVC irradiation, ANOVA test for how pH and COO groups influence the dye photodegradation efficiency, photodegradation stage monitored by thin-layer chromatography (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*Tel/fax: 0040213154193. E-mail: [email protected] (M.M.). *E-mail: [email protected] (E.A.R.). ORCID

Maria Mihaly: 0000-0002-1518-3298 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We want to thank the project Clean Integrated Nanotechnology for Dyes Removal from Wastewaters (CLIENT-DR), PN3-P3301/02.05.2017, contract no. 12/2017 for providing financial support.

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REFERENCES

(1) Clay, J. W. World Agriculture and the Environment: a Commoditybycommodity Guide to Impacts and Practices; Island Press: Washington D.C., 2004. (2) Drioli, E.; Giorno, L. Comprehensive Membrane Science and Engineering, 1st ed.; Elsevier Science: Amsterdam, 2010. 5283


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