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Selecting and improving activated homogeneous catalytic processes for pollutant removal. Kinetics, mineralization and optimization
Journal of Environmental Management 256 (2020) 109972
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Journal of Environmental Management journal homepage: http://www.elsevier.com/locate/jenvman
Research article
Selecting and improving activated homogeneous catalytic processes for pollutant removal. Kinetics, mineralization and optimization T. Gonz�alez a, *, J.R. Dominguez a, E.M. Cuerda-Correa b, S.E. Correia a, G. Donoso a a b
Dept. Chemical Engineering and Physical Chemistry, University of Extremadura, Avda. Elvas, 06006, Badajoz, Spain Dept. Organic and Inorganic Chemistry, University of Extremadura, Avda. Elvas, 06006, Badajoz, Spain
A R T I C L E I N F O
A B S T R A C T
Keywords: Activated homogeneous catalytic processes Fenton Photolysis Kinetic rate constants Mineralization Oxidation path-ways
The degradation of a model pollutant, tartrazine, very used in food industry and usually present in WWTPs effluents and surface waters, was investigated by nine activated homogeneous catalytic processes, namely, Fe3þ/ H2O2, Fe2þ/H2O2, UV/H2O2, UV/S2O28 , UV/Fe2þ/H2O2, UV/Fe3þ/H2O2, UV, VIS/Fe3þ/H2O2, and VIS/Fe3þ/ H2O2/C2O24 . In order to compare the mineralization and oxidation ability of each process, the removal of dye, chemical oxygen demand (COD) and total organic carbon (TOC) were analyzed, as well as the overall kinetic rate constant. Also, the different oxidation path-ways (direct photolysis and/or oxidation by free radicals) were estimated for each system. After the comparison, the Fenton process, which had the highest mineralization values, was tested in luminous and dark phases using designed experiments, and the influences of all operating variables were studied by RSM.
1. Introduction For decades, the scientific community had focused its efforts on the study of chemical pollutants whose presence in the environment is regulated by different legislations, mostly non-polar, toxic, persistent, and bioaccumulative substances, such as polycyclic aromatic hydro carbons (HPAs), biphenyls (PCBs) and dioxins. However, in recent years, the development of new and more effective detection methods has made it possible to detect the presence of other potentially dangerous con � and taminants, known globally as “emerging pollutants (EPs)” (Barcelo �pez de Alda, 2011; Huang et al., 2019; Moreau et al., 2019). Lo Among them, synthetic dyes represent a group of recalcitrant com pounds that are discharged in the aquatic environment in large quan tities primarily by manufacturing and industrial activities (Ngulube et al., 2017). Its accumulation in the environment constitutes a risk to flora and aquatic fauna reducing the penetration of light radiation, altering photosynthetic activity (Collivignarelli et al., 2019), and also, reducing oxygen transfer into waters (Tee et al., 2015). These com pounds are very difficult to degrade, mainly because they usually possess a very complex structure that makes them quite stable and, consequently, they are not readily biodegradable (Li et al., 2016). Therefore, the treatment of synthetic dyes in aqueous medium consti tutes one of the greatest challenges in the water treatment field due to
their visual impact and ability to noticeably increase organic and toxicity parameters. Currently, there are different methods to degrade these compounds, such as ozonation (El Hassani et al., 2019), electro chemical oxidation (Singh et al., 2016), sonochemical-oxidation (Tunc Dede et al., 2019), ultraviolet irradiation in presence or absence of catalysts (Pascariu et al., 2019; Zuorro et al., 2013), membrane-based treatment techniques (Dasgupta et al., 2015), and adsorption pro cesses (Islam et al., 2017). The azo dyes group is the most common group among the synthetic dyes (Abe et al., 2019; Ba^eta et al., 2013). Their production is relatively economical, and they can be used in a wide variety of sectors, including textile, food, paper and pharmaceutical sectors, among others. They are characterized by the presence of one or several azo groups chromophore – ¼N-). All azo dyes were obtained by chemical synthesis, and none (-N– of them exists in nature. The synthesis takes place through the diazoti zation of a primary aryl-amine in the presence of sodium nitrite in hy drochloric acid, obtaining a diazonium salt (Umape et al., 2013). Subsequently, it is reacted with an aromatic amine or an alcoholic compound. Tartrazine, Yellow 5, Acid Yellow 23, Food yellow 4 (FDA-USA), E102 (UE), CI 19140 is an azo sulfonate dye used in the food industry to give colour between yellow and bright orange to food products. It is also used in cosmetic and medicinal products (Tekin et al., 2018). It is
* Corresponding author. E-mail address: [email protected] (T. Gonz� alez). https://doi.org/10.1016/j.jenvman.2019.109972 Received 9 October 2019; Received in revised form 25 November 2019; Accepted 6 December 2019 Available online 13 December 2019 0301-4797/© 2019 Published by Elsevier Ltd.
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Journal of Environmental Management 256 (2020) 109972
marketed in the form of powder and is soluble in water. Its commercial potential has increased considerably in recent years because in addition to the yellow-orange tones when mixed with other dyes such as bright blue (E133) or green S (E142) various greenish shades are obtained. It has been widely used since 1916 in soft drinks, cereals, cake combina tions, soups, jam, sauces, ice cream, some rice, candy, chomping gum, marzipan, jelly, gelatins, mustard, marmalade, yogurt, noodles, chips, glycerin, lemon and honey products. Tartrazine was evaluated by the Joint FAO/WHO Expert Committee on Food Additives (JECFA) in 1966 and by the EU-SCF in 1975 and 1984. Both committees established an ADI of 0–7.5 mg/kg of body weight per day (EFSA, 2009). Many studies have been conducted to reevaluate tartrazine, including that carried out by Sasaki et al. (2002), which reported its effects on the migration of nuclear DNA in mice, and the study by McCann et al. (2007), which revealed that mixtures of colourants and/or preservatives including AY 23, induce an increase in hyperactivity in children (EFSA, 2008). Advanced oxidation processes (AOPs) are water purification tech nologies that have been widely used in the last 15 years due to their versatility and a broad spectrum of applicability. They are very efficient methods for water and wastewater treatments (Baiju et al., 2018; Sathya € et al., 2018). These are methods that use the et al., 2019; Sillanp€ aa extraordinary oxidizing ability of the radicals �OH. These AOPs are technologies that are compatible with the environment, and their viability depends on the efficacy of the generation of �OH, which is considered to have the highest oxidizing power after fluorine (Guinea et al., 2008). This radical acts as a non-selective oxidant on the organic contaminants. It is very effective in transforming them and, in many cases, it leads to their complete mineralization (Alcocer et al., 2018; Tarkwa et al., 2019; Wohlmuth da Silva et al., 2018). In this sense, AOPs can be classified into two large blocks, homogeneous and heterogeneous catalytic processes. This research is focused on the first group, activated homogeneous catalytic processes (AHCPs), namely, Fe3þ/H2O2, Fe2þ/H2O2, UV/H2O2, UV/S2O28 , UV/Fe2þ/H2O2, UV/Fe3þ/H2O2, UV, VIS/Fe3þ/H2O2, and VIS/Fe3þ/H2O2/C2O24
model FOC 225E, that allows it to operate in a temperature range of 3 to 50 � C. This procedure was carried out in the luminous phase (allowing the dye dissolution to be in contact with visible light) and in dark phase (the flask was covered with aluminium foil). Once each experiment was initiated, several samples were taken out periodically, and the remain ing tartrazine concentration was determined by absorbance measure ments at 428 nm. For chemical oxygen demand (COD) measurement, the Hanna In struments HI 83099 photometer and cuvettes required for this purpose (for a concentration range between 0 and 140 mg O2⋅L 1) were used. Total organic carbon (TOC) measurements were carried out by a Lange photometer, model DR-2800, and the corresponding commercial cu vettes, for a concentration range between 3 and 30 mg O2⋅L 1.
2. Materials and methods
εC ¼ 4
2.1. Chemicals
This parameter will give an idea of the mineralization degree reached for this dye. The maximum state of oxidation is equal to þ4, which corresponds to total mineralization to CO2. The initial values of the oxidation state ԑc of the organic carbon as well as the initial values of TOC and COD (for an initial dye concentration of 10 4 M) are equal to 0.4 (and corresponds to values of 15.15 mg L 1 and 45.4 mg L 1 respectively).
3. Results and discussion Removal percentages of compound, TOC, and COD, were used to compare the nine homogeneous catalytic processes applied to the aqueous solution of the dye. Also, the pseudo-first-order kinetic rate constant was determined for each oxidation system (Eq. (1)): ln
CA0 ¼ k⋅t CA
(1)
where k is the global pseudo-first-order kinetic rate constant of each process, which can be considered as the sum of two contributions that are part of the global oxidation process, and can be written as follows (Eq. (2)): (2)
k ¼ kUV þ kR
where kUV refers to the possible direct photolysis of the compound and kR refers to the oxidation contribution by free radicals (mainly �OH radicals) in the global process of oxidation. Also, it is of great importance to know that the state of oxidation of the remaining carbon in the solution ԑc is defined as shown in Eq. (3). COD0 32 TOC0 12 = =
Tartrazine, (4E)-5-oxo-1-(4-Sulfonatophenyl)-4-[(4-Sulfonato phenyl)-Hydrazino]-3-Pyrazole Carboxylate trisodium was obtained from Sigma-Aldrich. Hydrogen peroxide (30% w/v), sodium oxalate, iron(II) sulfate heptahydrate, iron(III) chloride hexahydrate, sodium perchlorate hydrate, perchloric acid and potassium persulfate were obtained from Merck.
4
(3)
3.1. Tartrazine removal and kinetic
2.2. Experimental procedure
Fig. 1 shows the removal of normalized dye concentration versus time for each homogeneous chemical oxidation process. As shown in Fig. 1, the highest dye removal percentages were ob tained for the UV-photocatalytic processes. The oxidation rates for UV, VIS/Fe3þ/H2O2 and Fe3þ/H2O2 processes were very low, not surpassing for any case, the removal of 10% after an hour of treatment. These results corroborate those achieved by Ali et al. (2018) and Chekir et al. (2017) demonstrating high stability of this azo dye for these processes. On the other hand, Guimaraes et al. (2012) and Kalikeri et al. (2018) studied the degradation of the azo dye RB-19 by UV obtaining a very slow removal rate. Also, it is demonstrated that the photo-Fenton process with VIS-light is much less effective than when UV-light is used. This disadvantage could be solved in part with the addition of oxalate. This is because ferrioxalate anion has a higher absorption co efficient at higher wavelengths due to the generation of hydroxyl radi cals with a high quantum yield (Domínguez et al., 2005). Despite this, the removal rate obtained by the UV-light system (UV/Fe3þ/H2O2) was not reached with the activated VIS-light process
Experiments were performed in a reactor in which the radiation source was located axially. The reactor was always charged with 350 cm3 aqueous tartrazine solution (10 4 M). pH was adjusted by adding sodium perchlorate hydrate and perchloric acid. The temperature in all cases remained constant at 20 � 0.5 � C. To achieve this, a Frigiterm ultra-thermostat was used. The UV radiation source was a Heraeus TNN-15W low pressure mercury vapour lamp, which emits 2.0⋅10 6 E s 1. The Vis-radiation source was a halogen lamp Philips Capsuleline Pro-50W (936 cd/sr). In the experiments with hydrogen peroxide, the calculated amount of this oxidant was added to the solution. In all the oxidation systems involving hydrogen peroxide, 20 μL of 0.1 M sodium thiosulfate was added to each sample to stop the reaction. When oxalate and/or persulfate compounds were used, they were dis solved at concentrations of 2⋅10 4 M and 5⋅10 4 M respectively. The oxidation experiments, using Fenton or Fenton-like reagents were car ried out in a 350 cm3 flask introduced into a Velp Scientifica chamber, 2
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Journal of Environmental Management 256 (2020) 109972
2003), which are reduced again by the dye to ferrous ions. Meanwhile, ferric ions can also react with H2O2, producing hydroperoxide radicals, causing a second oxidation process. (4)
Fe2þ þ H2 O2 → Fe3þ þ ⋅ OH þ OH
In Fenton-like reaction, ferric ions react with H2O2 to produce ferrous ions at a very slow rate (k ¼ 0.001–0.01 M 1 s 1) (Neyens and Baeyens, 2003); so only a few ferrous ions are generated and, conse quently, the Fenton reaction is very limited. Other studies have demonstrated that organic oxidation is faster with Fenton reagent than with Fenton-like due to the immediate formation of hydroxyl radicals in the first case (Wang, 2008). This fact is evident in the values of the ki netic rate constant, which is two hundred times higher in Fenton reac tion. Similar results were observed (see Table 1) by comparing the Photo Fenton (UV/Fe2þ/H2O2) and photo Fenton-like (UV/Fe3þ/H2O2) processes. Previous studies carried out with Tartrazine for the UV/H2O2 process (Oancea and Meltzer, 2014) show the dependence of the ratio [dye]: [H2O2] on the kinetic rate constant and the percentage of dye removal the ratio is 1:40 for the best values for both parameters after 180 min of treatment (k ¼ 4.746 ⋅ 10 2 min 1 and 97% of removal). In this work (1:5 ratio and 80 min of reaction time) we obtained results very close to those obtained by these authors using a 1:10 ratio (k ¼ 1.482 ⋅ 10 2 min 1 and 180 min). This result is relevant because it would reduce the operation costs by decreasing the H2O2 concentration. Therefore, the process that shows a higher reaction rate is the photoFenton (UV/Fe2þ/H2O2). The processes, ranked from lowest to highest reaction rate are as follows: Fe3þ/H2O2 � UV � VIS/Fe3þ/H2O2 < Vis/ Fe3þ/H2O2/C2O24 < UV/S2O28 < UV/H2O2 < UV/Fe3þ/H2O2 < Fe2þ/ H2O2 < UV/Fe2þ/H2O2.
Fig. 1. Removal of normalized dye concentration versus time for each homo geneous chemical oxidation process. [H2O2]o ¼ 5⋅10 4 M; [Tartrazine]o ¼ 1⋅10 4 M; [S2O48 ]o ¼ 5⋅10 4 M; [C2O24 ]o ¼ 2⋅10 4 M; [Fe2þ]o ¼ [Fe3þ]o ¼ 5⋅10 5 M. T ¼ 20 � C.
(UV/Fe3þ/H2O2/C2O24 ). Among the UV-light activated photocatalytic processes, the UV/S2O28 achieved the lowest efficiency, less than 60%. Table 1 shows the global pseudo-first-order kinetic rate constant (k) and its radical pathway constant (kR) for each process, calculated ac cording to Eqs. (1) and (2), as well as the removal achieved after 30 and 60 min of treatment. As shown, UV-photocatalytic treatments produced a much faster degradation than VIS-photocatalytic processes. A comparison of the Fenton type processes shows that the slower and less efficient process was the Fenton-like reagent (Fe3þ/H2O2). The use of VIS-light did not considerably improve the kinetics and removal percentage. Better re sults the dye elimination were obtained only when oxalate was added. This is due, as pointed out previously, to the generated oxalate-Fe3þ complex, which absorbs radiation in the VIS region (λ > 300 nm). This fact explains the formation of �OH radicals in this spectral region, and consequently, the possible applications of this system with solar radia tion. Therefore, the highest reaction rates were obtained by combining UV-light with Fenton-like processes. These advantages were previously demonstrated with different pollutants, showing that UV radiation ac celerates the classic Fenton reaction (Fe2þ/H2O2), obtaining higher pollutant degradation rates (Verma and Haritash, 2019) and even higher mineralization rates (De Oliveira et al., 2019). Comparison between Fenton (Fe2þ/H2O2) and Fenton-like (Fe3þ/H2O2) processes shows that there is hardly any compound degradation with Fenton-like. It is well known, that Fenton type processes generate very oxidizing active spe cies (hydroxyl and hydroperoxide radicals mainly). Hydroperoxide radicals have a lower oxidation potential than hy droxyl radicals (Malika and Saha, 2003). Thus, during the Fenton re action, ferrous ions can react quickly with H2O2 to produce hydroxyl radicals and ferric ions (Eq. (4)), k ¼ 70 M 1 s 1 (Neyens and Baeyens,
3.2. Mineralization efficiency The results obtained for the removal of TOC and COD expressed in percentage as well as the oxidation state of the remaining organic carbon in the solution are shown in Table 2 and Fig. 2. As shown in Fig. 2, the most efficient process (from a mineralization point of view) was the Fenton process, which obtains higher values than UV/Fe2þ/H2O2 and UV/H2O2 processes, which have notable differences with the Fe3þ systems. This is because, as demonstrated above, the processes involving Fe2þ are faster, which would imply greater miner alization of organic matter. Also, for these two processes that use Fe2þ, the COD removals are higher than 40%, and the change in the remaining organic carbon oxidation state (ranging from 0.7 to 1.25) is also noticeable. The Fe3þ/H2O2 and UV/S2O28 processes obtained mineral ization values (COT removal was less than 6% and COD removal was about 10%). Table 2 Mineralization obtained at 60 min [H2O2]o ¼ 5⋅10 4 M; [Tartrazine]o ¼ 1⋅10 4 M; [S2O48]o ¼ 5⋅10 4 M; [C2O24 ]o ¼ 2⋅10 4 M; [Fe2þ]o ¼ [Fe3þ]o ¼ 5⋅10 5 M. T ¼ 20 � C.
Table 1 Pseudo-first order kinetic rate constants and tartrazine removals at 30 and 60 min of reaction time. [H2O2]o ¼ 5⋅10 4 M; [Tartrazine]o ¼ 1⋅10 4 M; [S2O48 ]o ¼ 5⋅10 4 M; [C2O24 ]o ¼ 2⋅10 4 M; [Fe2þ]o ¼ [Fe3þ]o ¼ 5⋅10 5 M. T ¼ 20 � C. Process 2þ
UV/Fe /H2O2 Fe2þ/H2O2 UV/Fe3þ/H2O2 UV/H2O2 UV/S2O28 VIS/Fe3þ/H2O2/C2O4VIS/Fe3þ/H2O2 UV Fe3þ/H2O2
k⋅100 (min
1
)
7.1 � 0.1 3.7 � 0.2 2.12 � 0.09 1.27 � 0.07 0.84 � 0.02 0.33 � 0.02 0.116 � 0.004 0.081 � 0.002 0.017 � 0.002
kR⋅100 (min
1
)
7.0 � 0.1 3.7 � 0.2 2.04 � 0.09 1.19 � 0.07 0.76 � 0.02 0.33 � 0.02 0.116 � 0.004 0.017 � 0.002
X30 (%)
X60 (%)
78.1 51.5 43.2 22.1 19.5 4.2 1.9 1.6 0.5
87.2 68.4 92.3 50.4 34.1 11.9 4.0 3.6 0.9
3
Process
TOC removal (%)
COD removal (%)
ԑc
Fe2þ/H2O2 UV/H2O2 UV/Fe2þ/H2O2 UV/Fe3þ/H2O2 UV/S2O28 VIS/Fe3þ/H2O2 UV Fe3þ/H2O2 VIS/Fe3þ/H2O2/C2O24
24.5 22.3 22.3 7.8 5.8 5.8 4.5 3.9 3.2
52.7 34.8 42.3 41.2 10.4 20.3 2.4 12.3 19.8
1.25 0.3 0.7 0.8 0.2 0.3 0.5 0.01 0.4
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Journal of Environmental Management 256 (2020) 109972
Table 3 Design of Experiments. Operating levels region. [Tartrazine]o ¼ 1⋅10 20 � C.
4
M; T ¼
Parameter
Low value
High value
Center point
Step value
pH [H2O2]0 (M) [Fe2þ]0 (M) [Fe3þ]0 (M)
2.0 0.0 0.0 0.0
5 1.25⋅10 1.25⋅10 1.25⋅10
3.5 6.25⋅10 6.25⋅10 6.25⋅10
0.75 3.125⋅10 3.125⋅10 3.125⋅10
3 4 4
4 5 5
4 5 5
(32 runs) after 24 h of reaction time. It can be concluded the statistically significant factors with a probability of 95% that would modify the target variable with their variations are the initial concentration of H2O2 and its square. Likewise, the values of R2 and R2 adjusted were 98.58% and 97.64%, respectively, standard error ¼ 3.029 and medium error ¼ 1.633, which indicate the goodness of the adjusted equations. Once it has been determined by the ANOVA test that the model is adequate to represent effectively the influence of the variables, ac cording to the obtained p-value, it is possible to calculate the coefficients of the polynomial that adjusts the experimental values Eq. (5).
Fig. 2. TOC and COD removals and oxidation state of remaining organic car bon (εc) at 120 min of reaction. [H2O2]o ¼ 5⋅10 4 M; [Tartrazine]o ¼ 1⋅10 4 M; [S2O48 ]o ¼ 5⋅10 4 M; [C2O24 ]o ¼ 2⋅10 4 M; [Fe2þ]o ¼ [Fe3þ]o ¼ 5⋅10 5 M; T ¼ 20 � C; pH ¼ 3.5.
Removal (%) ¼ 87.89–0.430⋅pH þ 20.89⋅[H2O2] þ 0.454⋅[Fe2þ] – 0.035⋅ [Fe3þ] þ 0.990 pH2 þ 0.086⋅pH⋅[H2O2] – 0.317⋅pH⋅[Fe2þ] þ 0.247⋅pH⋅[Fe3þ] – 9.553⋅[H2O2]2–0.472⋅[H2O2]⋅[Fe2þ] – 0.315⋅[H2O2]⋅[Fe3þ] – 0.076⋅ [Fe2þ]2–0.383⋅[Fe2þ]⋅[Fe3þ] þ 0.441⋅[Fe3þ]2 (5)
With regard to the oxidation state of the remaining carbon in the solution shown in Table 2, a higher and more positive oxidation state corresponds to ternary UV/Fenþ/H2O2 and binary Fe2þ/H2O2 processes. An oxidation state between þ1 and þ 1.5 would make us think about aldehyde and ketone intermediates.
The positive or negative sign indicates the favourable or unfav ourable influence of the effect. The greater the absolute value of the regression coefficient, the greater the influence of the factor on the target variable. Clearly the initial concentrations of H2O2 and Fe2þ have a positive effect on tartrazine removal. Increasing the concentration of H2O2 increases the formation of �OH radicals, which are the true oxi dants for organic compounds. An increase in the concentration of Fe2þ ion accelerates the oxidation reactions, as it acts as a catalyst (Eq. (4)). Table 4 shows the optimum values of the operating variables, as well as the study range. As shown, there is an optimum value within the study region for all operating variables. In terms of the interaction between variables, the graphical analysis of the results obtained showed very little interactions between the operating variables. Fig. 3 shows, as an example, the response surface and level curve for the influence of H2O2 and Fe2þ on the target variable. As shown, the response surface is convex within the region. Similarly, the initial con centration of Fe3þ has a positive effect, while the pH has a negative effect. Clearly, tartrazine removal increases as the hydrogen peroxide concentration increases until the maximum point is reached, where an increase in this parameter would be detrimental to the process (Eqs. (6) and (7)).
3.3. Cost evaluation Taking into account the results obtained in the previous section, the Fenton system (Fe2þ/H2O2) could be one of the most suitable oxidation processes. After conducting an extensive bibliographical review of the economic aspects of different oxidation processes, Beltr� an de Heredia et al. (2002) showed, there are two very different operating costs scales, processes that do not use UV-radiation and processes that use it. While in the first group, operating costs for the Fenton system ranged from 0.2 to 2.1 €⋅m 3, the second UV group ranged from 14 €⋅m 3 (O3/UV/H2O2/Fe2þ) to 265 €⋅m 3 (when the single UV-radiation was used). Although LED technology could improve the costs of this second group of technologies, the emitted radiation is at a greater wavelength (>350 nm) and therefore the efficiency of these systems decreases considerably. Taking into account these estimations, and according to the optimal results obtained in the previous section, we decided to carry out a deeper study on the Fenton system (using design of experiments). 3.4. Fenton system multivariable optimization The main objectives of this study are, to establish the influence of the operating variables, as well as the possible interactions between them, and to determine the optimum values for these operating conditions. Dye removal (%), TOC elimination (%) and COD elimination (%) were chosen as target variables. Likewise, the selected operating variables were: initial hydrogen peroxide concentration, initial ferric concentra tion, initial concentration of ferrous ion, and pH. The experimental design was designed to analyze the relationship between the selected variables and to find the optimal value of these operating conditions that would achieve the maximum value for each target variable, and also to determine the possible interactions between the operating variables. A factorial central composite orthogonal and rotatable design of experiments (FCCORD), coupled with response sur face analysis was employed. The study region (variables range), the central value and the steps are shown in Table 3.
H2 O2 þ ⋅ OH→HO�2 þ H2 O2
(6)
Fe2þ þ ⋅ OH→Fe3þ þ OH
(7)
3.4.1.1. The physical meaning of the experimental results. There are several interpretations of the influence of variables. For example, in a study focusing on the degradation of methyl dye by Fenton’s reagent ~ a et al., 2014), the efficiency of the process increased with an (Saldan Table 4 Optimal coded and real values of variables for the elimination of tartrazine by Fenton’s reagent. [Tartrazine]o ¼ 1⋅10 4 M; T ¼ 20 � C. Parameter pH [H2O2]0 (M) [Fe2þ]0 (M) [Fe3þ]0 (M)
3.4.1. Removal of dye as the target variable This section shows the results obtained for the different experiments 4
Low 2.0 2.0 2.0 2.0
High
Coded optimum
Coded optimum
2.0 2.0 2.0 2.0
0.674 1.095 0.200 0.036
3 1.095⋅10 6.876⋅10 6.365⋅10
4
M M 5 M 5
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Journal of Environmental Management 256 (2020) 109972
Fig. 4. Response surface for COD removal. [Tartrazine]o ¼ 1⋅10 ¼ 6.25 � 10 5 M; T ¼ 20 � C; pH ¼ 3.5.
Fig. 3. Estimated response surface and level curve for tartrazine removal using Fenton’s reagent. [Tartrazine]o ¼ 1⋅10 4 M; [Fe3þ]o ¼ 6.25⋅10 5 M; T ¼ 20 � C; pH ¼ 3.5.
4
M; [Fe2þ]o
If we consider TOC removal as a target variable. Fig. 5 shows, as an example, the influence of the initial concentrations of H2O2 and Fe2þ. As in the case of COD removal, the highest TOC removal was reached with higher initial H2O2 and Fe2þ concentrations. It should be empha sized that in this case, the influence of the concentration of ferrous ion is very small. Regarding the influence of pH, at a higher H2O2 concentration and lower pH, a higher TOC removal is achieved. Also, a higher TOC removal is achieved by increasing the hydrogen peroxide and reducing the Fe3þ concentration (the influence of ferric ion is very small). The influence of the pH is also very small (in the range 2–5), and it acts contrary to COD and TOC. This can be explained by taking into account the fact that at a lower pH, the solubility of CO2 (final product of mineralization) de creases, thereby favouring TOC removal.
increase in the Fe2þ concentration and with a decrease in the concen tration of H2O2. However, Mitsika et al. (2013) concluded that the variable with the most significant effect on the degradation of Acet amiprid was the concentration of iron followed by the concentration of H2O2. Guo et al. (2018) studied the oxidation of benzene dye in termediates and concluded that the most influential variable was pH. Therefore, by increasing the concentration of Fe2þ and H2O2 the elim ination of the dye was improved. In a study conducted by Domínguez et al. (2014) on oxidation by Fenton’s reagent using the response surface method, the removal levels increase as the initial concentration of H2O2 and Fe2þ increases in all cases. Similar results were obtained in the present work, although in this case we only obtained significance for the initial H2O2 concentration.
3.4.3. Experimental confirmation of the theoretical maxima predicted An experiment was conducted on the optimal conditions predicted by this statistical study in order to determine the level of elimination of tartrazine, COD and TOC. Table 5 shows the optimal, coded and real values, and the elimination level obtained under these conditions.
3.4.2. Chemical oxygen demand (COD) and total organic carbon (TOC) removals as target variables By applying ANOVA test to the obtained results, it can be concluded that the statistically significant factors with a probability of 95% are as follows:
4. Conclusions
- For COD removal: the initial concentration of H2O2, Fe2þ, Fe3þ, the square of all variables and interactions of pH- Fe3þ, H2O2- Fe2þ and Fe2þ- Fe3þ. - For TOC removal: the initial concentration of H2O2, pH, the squares of these variables, and the interactions of pH- H2O2 and Fe2þ- Fe3þ.
The following conclusions can be deduced from the study. The fastest processes, with the highest percentage of dye removal and mineraliza tion (higher percentages of TOC and COD removal), are those of binary systems (or higher) in which UV-radiation and/or Fenton reagent acts. The Fenton-like reactive mediated processes (Fe3þ/H2O2) are usually the slowest and least efficient. The addiction of oxalate to the photo catalyzed process with VIS-light and Fe3þ considerably improves the system. The Fenton reagent (Fe2þ/H2O2) achieves the highest mineral ization values and also achieves the highest value of oxidation state of the residual organic carbon (þ1,25). A statistical design of experiments was implemented in order to optimize the Fenton’s process. This optimization was carried out using the response surface methodology. When we considered dye removal as the target variable, the most influential factor was the initial
Clearly, in this case, a wide variety of experimental factors affects TOC and COD removals, which makes the response surface methodology application more useful and interesting. The coefficients of the polynomial that adjusts the experimental values for each of the factors appear in Eqs. (8) and (9). COD removal (%) ¼ 31.91 þ 1.749⋅pH þ 14.32⋅[H2O2] þ 3.555⋅ [Fe2þ] – 2.865⋅[Fe3þ] – 3.109 pH2 þ 1.922⋅pH⋅[H2O2] – 2.188⋅pH⋅[Fe2þ] þ 6.811⋅pH⋅[Fe3þ] – 0.477⋅[H2O2]2 þ 5.844⋅[H2O2]⋅[Fe2þ] – 0.512⋅[H2O2]⋅ [Fe3þ] þ 4.183⋅[Fe2þ]2–4.073⋅[Fe2þ]⋅[Fe3þ] þ 3.081⋅[Fe3þ]2 (8) TOC removal (%) ¼ 5.435–3.424⋅pH þ 17.79⋅[H2O2] þ 1.921⋅[Fe2þ] þ0.224⋅[Fe3þ] þ5.372 pH2 – 14.184⋅pH⋅[H2O2] – 0.396⋅pH⋅[Fe2þ] 1.887⋅pH⋅[Fe3þ] þ7.928⋅[H2O2]2 þ 1.581⋅[H2O2]⋅[Fe2þ] – 1.271⋅[H2O2]⋅ [Fe3þ] þ1.581 3362⋅[Fe2þ]2–3.362⋅[Fe2þ]⋅[Fe3þ] þ 1.027⋅[Fe3þ]2 (9) Fig. 4 reflects the response surface for COD removal. As shown in Fig. 4, Fe3þ concentration mildly affects COD removal in the negative direction. A higher concentration of H2O2 and a lower Fe3þ concentra tion achieve a higher COD removal. Further, there is an optimum pH value. The highest COD removal was achieved with higher H2O2 and Fe2þ concentrations. This can be explained considering that hydrogen peroxide and Fe2þ concentrations contribute to the generation of hydroxyl radicals (Eq. (4)).
Fig. 5. Influence of H2O2 and Fe2þ initial concentrations on TOC removal. Coded variables. [Tartrazine]o ¼ 1⋅10 4 M; [Fe3þ]o ¼ 6.25⋅10 5 M; T ¼ 20 � C; pH ¼ 3.5. 5
T. Gonz� alez et al.
Journal of Environmental Management 256 (2020) 109972 Beltr� an de Heredia, J., Domínguez, J.R., Peres, J., 2002. Eliminaci� on de compuestos fen� olicos por oxidaci� on avanzada. Estim Costes. Ing. Quím. 142–146. Chekir, N., Tassalit, D., Benhabiles, O., Kasbadji Merzouk, N., Ghenna, M., Abdessemed, A., Issaadi, R., 2017. A comparative study of tartrazine degradation using UV and solar fixed bed reactors. Int. J. Hydrogen Energy 42 (13), 8948–8954. https://doi.org/10.1016/j.ijhydene.2016.11.057. Collivignarelli, M.C., Abb� a, A., Carnevale Miino, M., Damiani, S., 2019. Treatments for color removal from wastewater: state of the art. J. Environ. Manag. 236, 727–745. https://doi.org/10.1016/j.jenvman.2018.11.094. Dasgupta, J., Sikder, J., Chakraborty, S., Curcio, S., Drioli, E., 2015. Remediation of textile effluents by membrane based treatment techniques: a state of the art review. J. Environ. Manag. 147, 55–72. https://doi.org/10.1016/j.jenvman.2014.08.008. De Oliveira, D.M., Cavalcante, R.P., da Silva, L. de M., Sans, C., Esplugas, S., de Oliveira, S.C., Junior, A.M., 2019. Identification of intermediates, acute toxicity removal, and kinetics investigation to the Ametryn treatment by direct photolysis (UV254), UV254/H2O2, Fenton, and photo-Fenton processes. Environ. Sci. Pollut. Res. 26 (5), 4348–4366. https://doi.org/10.1007/s11356-018-1342-6. Domínguez, J.R., Beltr� an, J., Rodríguez, O., 2005. Vis and UV photocatalytic detoxification methods (using TiO2, TiO2/H2O2, TiO2/O3, TiO2/S2O82 , O3, H2O2, S2O82 , Fe3þ/H2O2 and Fe3þ/H2O2/C2O42 ) for dyes treatment. Catal. Today 101 (3–4), 389–395. https://doi.org/10.1016/j.cattod.2005.03.010. Domínguez, J.R., Mu~ noz, M.J., Palo, P., Gonzalez, T., Peres, J., Cuerda-Correa, E.M., 2014. Fenton advanced oxidation of emerging pollutants: parabens. Int. J. Energy Environ. Eng. 5 (2–3), 1–10. https://doi.org/10.1007/s40095-014-0089-1. EFSA, 2009. Panel on food additives and nutrient sources added to food, scientific opinion on the re-evaluation of tartrazine (E 102) as a food additive. EFSA J. 7 https://doi.org/10.2903/j.efsa.2009.1331. EFSA, 2008. Scientific opinion of the panel on food additives, flavourings, processing aids and food contact materials (AFC): assessment of the results of the study by McCann et al. (2007) on the effect of some colours and sodium benzoate on children’s behavior. EFSA 660, 1–54. https://doi.org/10.2903/j.efsa.2008.660. El Hassani, K., Kalnina, D., Turks, M., Beakou, B.H., Anouar, A., 2019. Enhanced degradation of an azo dye by catalytic ozonation over Ni-containing layered double hydroxide nanocatalyst. Separ. Purif. Technol. 210, 764–774. https://doi.org/ 10.1016/j.seppur.2018.08.074. Guimaraes, J.R., Guedes Maniero, M., Nogueira de Araújo, R., 2012. A comparative study on the degradation of RB-19 dye in an aqueous medium by advanced oxidation processes. J. Environ. Manag. 110, 33–39. https://doi.org/10.1016/j. jenvman.2012.05.020. Guinea, E., Arias, C., Cabot, P., Garrido, J., Rodríguez, R., Centellas, F., Brillas, E., 2008. Mineralization of salicylic acid in acidic aqueous medium by electrochemical advanced oxidation processes using platinum and boron-doped diamond as anode and cathodically generated hydrogen peroxide. Water Res. 42 (1–2), 499–511. https://doi.org/10.1016/j.watres.2007.07.046. Guo, Y., Xue, Q., Zhang, H., Wang, N., Chang, S., Wang, H., Pang, H., Chen, H., 2018. Treatment of real benzene dye intermediates wastewater by the Fenton method: characteristics and multi-response optimization. RSC Adv. 8 (1), 80–90. https://doi. org/10.1039/c7ra09404c. Huang, H., Zeng, S., Dong, X., Li, D., Zhang, Y., He, M., Du, P., 2019. Diverse and abundant antibiotics and antibiotic resistance genes in an urban water system. J. Environ. Manag. 231, 494–503. https://doi.org/10.1016/j.jenvman.2018.10.051. 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Table 5 Optimal values (coded and real). Oxidation by Fenton’s reagent. [Tartrazine]o ¼ 1⋅10 4 M; T ¼ 20 � C. Factor
Codified Optimum
Real Optimum
pH [H2O2]o [Fe2þ]o [Fe3þ]o COD removal (%) TOC removal (%) Tartrazine removal (%)
1.2132 1.1614 0.0575 0.1219
2.6 9.879⋅10 6.070⋅10 5.869⋅10 52.4 33.5 98.13
4
M M 5 M 5
concentration of hydrogen peroxide; the rest of the variables were not significant. When we considered TOC removal as the target variable, the most influential variables were the initial concentration of H2O2, pH, the squares of these variables, and interactions of pH- H2O2 and Fe2þ- Fe3þ; the most important factor (positive) was the initial concentration of hydrogen peroxide. No optimal operating conditions were found within the study region and only trends can be talked about. An increase in H2O2 and Fe2þ concentrations had a positive effect on mineralization, while an increase in pH or concentration of Fe3þ had a negative effect. When we considered COD removal as the target variable, the most influential variables were: initial concentration of H2O2, Fe2þ, Fe3þ, the squares of pH, Fe2þ and Fe3þ variables, and interactions of pH- Fe3þ, H2O2 - Fe2þ and Fe2þ- Fe3þ; the most important factor (positive) was the initial concentration of hydrogen peroxide. We have found optimal conditions only for pH. Author contribution statement T. Gonzalez: Conceptualization, Methodology, Writing- Original draft preparation, Writing- Reviewing and Editing, J.R. Dominguez: Supervision, Writing- Original draft preparation, Conceptualization, Methodology, E.M. Cuerda Correa: Supervision, Software, Validation, S. E. Correia: Writing- Original draft preparation, Writing- Reviewing and Editing, G. Donoso: Investigation, Validation. Acknowledgements The authors gratefully acknowledge financial support of this �n Interministerial de Ciencia y Tec research work through the Comisio nología (CICYT)-CTM 2016-75873-R project-as well as through Junta de Extremadura under GR15067 and IB16016 projects and Fondo Europeo de Desarrollo Regional. References Abe, F.R., Machado, A.L., Soares, A.M.V.M., Oliveira, D. P. de, Pestana, J.L.T., 2019. Life history and behavior effects of synthetic and natural dyes on Daphnia magna. Chemosphere 236, 124390. https://doi.org/10.1016/j.chemosphere.2019.124390. Alcocer, S., Picos, A., Uribe, A.R., P�erez, T., Peralta-Hern� andez, J.M., 2018. Comparative study for degradation of industrial dyes by electrochemical advanced oxidation processes with BDD anode in a laboratory stirred tank reactor. Chemosphere 205, 682–689. https://doi.org/10.1016/j.chemosphere.2018.04.155. Ali, S.R., Kumar, R., Kadabinakatti, S.K., Arya, M.C., 2018. Enhanced UV and visible light-driven photocatalytic degradation of tartrazine by nickel-doped cerium oxide nanoparticles. Mater. Res. Express 6 (2), 025513. https://doi.org/10.1088/20531591/aaee44. Ba^eta, B.E.L., Luna, H.J., Sanson, A.L., Silva, S.Q., Aquino, S.F., 2013. Degradation of a model azo dye in submerged anaerobic membrane bioreactor (SAMBR) operated with powdered activated carbon (PAC). J. Environ. Manag. 128, 462–470. https:// doi.org/10.1016/j.jenvman.2013.05.038. Baiju, A., Gandhimathi, R., Ramesh, S.T., Nidheesh, P.V., 2018. Combined heterogeneous Electro-Fenton and biological process for the treatment of stabilized landfill leachate. J. Environ. Manag. 210, 328–337. https://doi.org/10.1016/j. jenvman.2018.01.019. Barcel� o, L., L� opez de Alda, M., 2011. Contaminaci� on y calidad química del agua: el problema de los contaminantes emergentes. Panel científico- T� ecnico de seguimiento de la política de aguas. Barcelona: Fundaci� on nueva cultura del agua, pp. 1–26.
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Research article
Selecting and improving activated homogeneous catalytic processes for pollutant removal. Kinetics, mineralization and optimization T. Gonz�alez a, *, J.R. Dominguez a, E.M. Cuerda-Correa b, S.E. Correia a, G. Donoso a a b
Dept. Chemical Engineering and Physical Chemistry, University of Extremadura, Avda. Elvas, 06006, Badajoz, Spain Dept. Organic and Inorganic Chemistry, University of Extremadura, Avda. Elvas, 06006, Badajoz, Spain
A R T I C L E I N F O
A B S T R A C T
Keywords: Activated homogeneous catalytic processes Fenton Photolysis Kinetic rate constants Mineralization Oxidation path-ways
The degradation of a model pollutant, tartrazine, very used in food industry and usually present in WWTPs effluents and surface waters, was investigated by nine activated homogeneous catalytic processes, namely, Fe3þ/ H2O2, Fe2þ/H2O2, UV/H2O2, UV/S2O28 , UV/Fe2þ/H2O2, UV/Fe3þ/H2O2, UV, VIS/Fe3þ/H2O2, and VIS/Fe3þ/ H2O2/C2O24 . In order to compare the mineralization and oxidation ability of each process, the removal of dye, chemical oxygen demand (COD) and total organic carbon (TOC) were analyzed, as well as the overall kinetic rate constant. Also, the different oxidation path-ways (direct photolysis and/or oxidation by free radicals) were estimated for each system. After the comparison, the Fenton process, which had the highest mineralization values, was tested in luminous and dark phases using designed experiments, and the influences of all operating variables were studied by RSM.
1. Introduction For decades, the scientific community had focused its efforts on the study of chemical pollutants whose presence in the environment is regulated by different legislations, mostly non-polar, toxic, persistent, and bioaccumulative substances, such as polycyclic aromatic hydro carbons (HPAs), biphenyls (PCBs) and dioxins. However, in recent years, the development of new and more effective detection methods has made it possible to detect the presence of other potentially dangerous con � and taminants, known globally as “emerging pollutants (EPs)” (Barcelo �pez de Alda, 2011; Huang et al., 2019; Moreau et al., 2019). Lo Among them, synthetic dyes represent a group of recalcitrant com pounds that are discharged in the aquatic environment in large quan tities primarily by manufacturing and industrial activities (Ngulube et al., 2017). Its accumulation in the environment constitutes a risk to flora and aquatic fauna reducing the penetration of light radiation, altering photosynthetic activity (Collivignarelli et al., 2019), and also, reducing oxygen transfer into waters (Tee et al., 2015). These com pounds are very difficult to degrade, mainly because they usually possess a very complex structure that makes them quite stable and, consequently, they are not readily biodegradable (Li et al., 2016). Therefore, the treatment of synthetic dyes in aqueous medium consti tutes one of the greatest challenges in the water treatment field due to
their visual impact and ability to noticeably increase organic and toxicity parameters. Currently, there are different methods to degrade these compounds, such as ozonation (El Hassani et al., 2019), electro chemical oxidation (Singh et al., 2016), sonochemical-oxidation (Tunc Dede et al., 2019), ultraviolet irradiation in presence or absence of catalysts (Pascariu et al., 2019; Zuorro et al., 2013), membrane-based treatment techniques (Dasgupta et al., 2015), and adsorption pro cesses (Islam et al., 2017). The azo dyes group is the most common group among the synthetic dyes (Abe et al., 2019; Ba^eta et al., 2013). Their production is relatively economical, and they can be used in a wide variety of sectors, including textile, food, paper and pharmaceutical sectors, among others. They are characterized by the presence of one or several azo groups chromophore – ¼N-). All azo dyes were obtained by chemical synthesis, and none (-N– of them exists in nature. The synthesis takes place through the diazoti zation of a primary aryl-amine in the presence of sodium nitrite in hy drochloric acid, obtaining a diazonium salt (Umape et al., 2013). Subsequently, it is reacted with an aromatic amine or an alcoholic compound. Tartrazine, Yellow 5, Acid Yellow 23, Food yellow 4 (FDA-USA), E102 (UE), CI 19140 is an azo sulfonate dye used in the food industry to give colour between yellow and bright orange to food products. It is also used in cosmetic and medicinal products (Tekin et al., 2018). It is
* Corresponding author. E-mail address: [email protected] (T. Gonz� alez). https://doi.org/10.1016/j.jenvman.2019.109972 Received 9 October 2019; Received in revised form 25 November 2019; Accepted 6 December 2019 Available online 13 December 2019 0301-4797/© 2019 Published by Elsevier Ltd.
T. Gonz� alez et al.
Journal of Environmental Management 256 (2020) 109972
marketed in the form of powder and is soluble in water. Its commercial potential has increased considerably in recent years because in addition to the yellow-orange tones when mixed with other dyes such as bright blue (E133) or green S (E142) various greenish shades are obtained. It has been widely used since 1916 in soft drinks, cereals, cake combina tions, soups, jam, sauces, ice cream, some rice, candy, chomping gum, marzipan, jelly, gelatins, mustard, marmalade, yogurt, noodles, chips, glycerin, lemon and honey products. Tartrazine was evaluated by the Joint FAO/WHO Expert Committee on Food Additives (JECFA) in 1966 and by the EU-SCF in 1975 and 1984. Both committees established an ADI of 0–7.5 mg/kg of body weight per day (EFSA, 2009). Many studies have been conducted to reevaluate tartrazine, including that carried out by Sasaki et al. (2002), which reported its effects on the migration of nuclear DNA in mice, and the study by McCann et al. (2007), which revealed that mixtures of colourants and/or preservatives including AY 23, induce an increase in hyperactivity in children (EFSA, 2008). Advanced oxidation processes (AOPs) are water purification tech nologies that have been widely used in the last 15 years due to their versatility and a broad spectrum of applicability. They are very efficient methods for water and wastewater treatments (Baiju et al., 2018; Sathya € et al., 2018). These are methods that use the et al., 2019; Sillanp€ aa extraordinary oxidizing ability of the radicals �OH. These AOPs are technologies that are compatible with the environment, and their viability depends on the efficacy of the generation of �OH, which is considered to have the highest oxidizing power after fluorine (Guinea et al., 2008). This radical acts as a non-selective oxidant on the organic contaminants. It is very effective in transforming them and, in many cases, it leads to their complete mineralization (Alcocer et al., 2018; Tarkwa et al., 2019; Wohlmuth da Silva et al., 2018). In this sense, AOPs can be classified into two large blocks, homogeneous and heterogeneous catalytic processes. This research is focused on the first group, activated homogeneous catalytic processes (AHCPs), namely, Fe3þ/H2O2, Fe2þ/H2O2, UV/H2O2, UV/S2O28 , UV/Fe2þ/H2O2, UV/Fe3þ/H2O2, UV, VIS/Fe3þ/H2O2, and VIS/Fe3þ/H2O2/C2O24
model FOC 225E, that allows it to operate in a temperature range of 3 to 50 � C. This procedure was carried out in the luminous phase (allowing the dye dissolution to be in contact with visible light) and in dark phase (the flask was covered with aluminium foil). Once each experiment was initiated, several samples were taken out periodically, and the remain ing tartrazine concentration was determined by absorbance measure ments at 428 nm. For chemical oxygen demand (COD) measurement, the Hanna In struments HI 83099 photometer and cuvettes required for this purpose (for a concentration range between 0 and 140 mg O2⋅L 1) were used. Total organic carbon (TOC) measurements were carried out by a Lange photometer, model DR-2800, and the corresponding commercial cu vettes, for a concentration range between 3 and 30 mg O2⋅L 1.
2. Materials and methods
εC ¼ 4
2.1. Chemicals
This parameter will give an idea of the mineralization degree reached for this dye. The maximum state of oxidation is equal to þ4, which corresponds to total mineralization to CO2. The initial values of the oxidation state ԑc of the organic carbon as well as the initial values of TOC and COD (for an initial dye concentration of 10 4 M) are equal to 0.4 (and corresponds to values of 15.15 mg L 1 and 45.4 mg L 1 respectively).
3. Results and discussion Removal percentages of compound, TOC, and COD, were used to compare the nine homogeneous catalytic processes applied to the aqueous solution of the dye. Also, the pseudo-first-order kinetic rate constant was determined for each oxidation system (Eq. (1)): ln
CA0 ¼ k⋅t CA
(1)
where k is the global pseudo-first-order kinetic rate constant of each process, which can be considered as the sum of two contributions that are part of the global oxidation process, and can be written as follows (Eq. (2)): (2)
k ¼ kUV þ kR
where kUV refers to the possible direct photolysis of the compound and kR refers to the oxidation contribution by free radicals (mainly �OH radicals) in the global process of oxidation. Also, it is of great importance to know that the state of oxidation of the remaining carbon in the solution ԑc is defined as shown in Eq. (3). COD0 32 TOC0 12 = =
Tartrazine, (4E)-5-oxo-1-(4-Sulfonatophenyl)-4-[(4-Sulfonato phenyl)-Hydrazino]-3-Pyrazole Carboxylate trisodium was obtained from Sigma-Aldrich. Hydrogen peroxide (30% w/v), sodium oxalate, iron(II) sulfate heptahydrate, iron(III) chloride hexahydrate, sodium perchlorate hydrate, perchloric acid and potassium persulfate were obtained from Merck.
4
(3)
3.1. Tartrazine removal and kinetic
2.2. Experimental procedure
Fig. 1 shows the removal of normalized dye concentration versus time for each homogeneous chemical oxidation process. As shown in Fig. 1, the highest dye removal percentages were ob tained for the UV-photocatalytic processes. The oxidation rates for UV, VIS/Fe3þ/H2O2 and Fe3þ/H2O2 processes were very low, not surpassing for any case, the removal of 10% after an hour of treatment. These results corroborate those achieved by Ali et al. (2018) and Chekir et al. (2017) demonstrating high stability of this azo dye for these processes. On the other hand, Guimaraes et al. (2012) and Kalikeri et al. (2018) studied the degradation of the azo dye RB-19 by UV obtaining a very slow removal rate. Also, it is demonstrated that the photo-Fenton process with VIS-light is much less effective than when UV-light is used. This disadvantage could be solved in part with the addition of oxalate. This is because ferrioxalate anion has a higher absorption co efficient at higher wavelengths due to the generation of hydroxyl radi cals with a high quantum yield (Domínguez et al., 2005). Despite this, the removal rate obtained by the UV-light system (UV/Fe3þ/H2O2) was not reached with the activated VIS-light process
Experiments were performed in a reactor in which the radiation source was located axially. The reactor was always charged with 350 cm3 aqueous tartrazine solution (10 4 M). pH was adjusted by adding sodium perchlorate hydrate and perchloric acid. The temperature in all cases remained constant at 20 � 0.5 � C. To achieve this, a Frigiterm ultra-thermostat was used. The UV radiation source was a Heraeus TNN-15W low pressure mercury vapour lamp, which emits 2.0⋅10 6 E s 1. The Vis-radiation source was a halogen lamp Philips Capsuleline Pro-50W (936 cd/sr). In the experiments with hydrogen peroxide, the calculated amount of this oxidant was added to the solution. In all the oxidation systems involving hydrogen peroxide, 20 μL of 0.1 M sodium thiosulfate was added to each sample to stop the reaction. When oxalate and/or persulfate compounds were used, they were dis solved at concentrations of 2⋅10 4 M and 5⋅10 4 M respectively. The oxidation experiments, using Fenton or Fenton-like reagents were car ried out in a 350 cm3 flask introduced into a Velp Scientifica chamber, 2
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Journal of Environmental Management 256 (2020) 109972
2003), which are reduced again by the dye to ferrous ions. Meanwhile, ferric ions can also react with H2O2, producing hydroperoxide radicals, causing a second oxidation process. (4)
Fe2þ þ H2 O2 → Fe3þ þ ⋅ OH þ OH
In Fenton-like reaction, ferric ions react with H2O2 to produce ferrous ions at a very slow rate (k ¼ 0.001–0.01 M 1 s 1) (Neyens and Baeyens, 2003); so only a few ferrous ions are generated and, conse quently, the Fenton reaction is very limited. Other studies have demonstrated that organic oxidation is faster with Fenton reagent than with Fenton-like due to the immediate formation of hydroxyl radicals in the first case (Wang, 2008). This fact is evident in the values of the ki netic rate constant, which is two hundred times higher in Fenton reac tion. Similar results were observed (see Table 1) by comparing the Photo Fenton (UV/Fe2þ/H2O2) and photo Fenton-like (UV/Fe3þ/H2O2) processes. Previous studies carried out with Tartrazine for the UV/H2O2 process (Oancea and Meltzer, 2014) show the dependence of the ratio [dye]: [H2O2] on the kinetic rate constant and the percentage of dye removal the ratio is 1:40 for the best values for both parameters after 180 min of treatment (k ¼ 4.746 ⋅ 10 2 min 1 and 97% of removal). In this work (1:5 ratio and 80 min of reaction time) we obtained results very close to those obtained by these authors using a 1:10 ratio (k ¼ 1.482 ⋅ 10 2 min 1 and 180 min). This result is relevant because it would reduce the operation costs by decreasing the H2O2 concentration. Therefore, the process that shows a higher reaction rate is the photoFenton (UV/Fe2þ/H2O2). The processes, ranked from lowest to highest reaction rate are as follows: Fe3þ/H2O2 � UV � VIS/Fe3þ/H2O2 < Vis/ Fe3þ/H2O2/C2O24 < UV/S2O28 < UV/H2O2 < UV/Fe3þ/H2O2 < Fe2þ/ H2O2 < UV/Fe2þ/H2O2.
Fig. 1. Removal of normalized dye concentration versus time for each homo geneous chemical oxidation process. [H2O2]o ¼ 5⋅10 4 M; [Tartrazine]o ¼ 1⋅10 4 M; [S2O48 ]o ¼ 5⋅10 4 M; [C2O24 ]o ¼ 2⋅10 4 M; [Fe2þ]o ¼ [Fe3þ]o ¼ 5⋅10 5 M. T ¼ 20 � C.
(UV/Fe3þ/H2O2/C2O24 ). Among the UV-light activated photocatalytic processes, the UV/S2O28 achieved the lowest efficiency, less than 60%. Table 1 shows the global pseudo-first-order kinetic rate constant (k) and its radical pathway constant (kR) for each process, calculated ac cording to Eqs. (1) and (2), as well as the removal achieved after 30 and 60 min of treatment. As shown, UV-photocatalytic treatments produced a much faster degradation than VIS-photocatalytic processes. A comparison of the Fenton type processes shows that the slower and less efficient process was the Fenton-like reagent (Fe3þ/H2O2). The use of VIS-light did not considerably improve the kinetics and removal percentage. Better re sults the dye elimination were obtained only when oxalate was added. This is due, as pointed out previously, to the generated oxalate-Fe3þ complex, which absorbs radiation in the VIS region (λ > 300 nm). This fact explains the formation of �OH radicals in this spectral region, and consequently, the possible applications of this system with solar radia tion. Therefore, the highest reaction rates were obtained by combining UV-light with Fenton-like processes. These advantages were previously demonstrated with different pollutants, showing that UV radiation ac celerates the classic Fenton reaction (Fe2þ/H2O2), obtaining higher pollutant degradation rates (Verma and Haritash, 2019) and even higher mineralization rates (De Oliveira et al., 2019). Comparison between Fenton (Fe2þ/H2O2) and Fenton-like (Fe3þ/H2O2) processes shows that there is hardly any compound degradation with Fenton-like. It is well known, that Fenton type processes generate very oxidizing active spe cies (hydroxyl and hydroperoxide radicals mainly). Hydroperoxide radicals have a lower oxidation potential than hy droxyl radicals (Malika and Saha, 2003). Thus, during the Fenton re action, ferrous ions can react quickly with H2O2 to produce hydroxyl radicals and ferric ions (Eq. (4)), k ¼ 70 M 1 s 1 (Neyens and Baeyens,
3.2. Mineralization efficiency The results obtained for the removal of TOC and COD expressed in percentage as well as the oxidation state of the remaining organic carbon in the solution are shown in Table 2 and Fig. 2. As shown in Fig. 2, the most efficient process (from a mineralization point of view) was the Fenton process, which obtains higher values than UV/Fe2þ/H2O2 and UV/H2O2 processes, which have notable differences with the Fe3þ systems. This is because, as demonstrated above, the processes involving Fe2þ are faster, which would imply greater miner alization of organic matter. Also, for these two processes that use Fe2þ, the COD removals are higher than 40%, and the change in the remaining organic carbon oxidation state (ranging from 0.7 to 1.25) is also noticeable. The Fe3þ/H2O2 and UV/S2O28 processes obtained mineral ization values (COT removal was less than 6% and COD removal was about 10%). Table 2 Mineralization obtained at 60 min [H2O2]o ¼ 5⋅10 4 M; [Tartrazine]o ¼ 1⋅10 4 M; [S2O48]o ¼ 5⋅10 4 M; [C2O24 ]o ¼ 2⋅10 4 M; [Fe2þ]o ¼ [Fe3þ]o ¼ 5⋅10 5 M. T ¼ 20 � C.
Table 1 Pseudo-first order kinetic rate constants and tartrazine removals at 30 and 60 min of reaction time. [H2O2]o ¼ 5⋅10 4 M; [Tartrazine]o ¼ 1⋅10 4 M; [S2O48 ]o ¼ 5⋅10 4 M; [C2O24 ]o ¼ 2⋅10 4 M; [Fe2þ]o ¼ [Fe3þ]o ¼ 5⋅10 5 M. T ¼ 20 � C. Process 2þ
UV/Fe /H2O2 Fe2þ/H2O2 UV/Fe3þ/H2O2 UV/H2O2 UV/S2O28 VIS/Fe3þ/H2O2/C2O4VIS/Fe3þ/H2O2 UV Fe3þ/H2O2
k⋅100 (min
1
)
7.1 � 0.1 3.7 � 0.2 2.12 � 0.09 1.27 � 0.07 0.84 � 0.02 0.33 � 0.02 0.116 � 0.004 0.081 � 0.002 0.017 � 0.002
kR⋅100 (min
1
)
7.0 � 0.1 3.7 � 0.2 2.04 � 0.09 1.19 � 0.07 0.76 � 0.02 0.33 � 0.02 0.116 � 0.004 0.017 � 0.002
X30 (%)
X60 (%)
78.1 51.5 43.2 22.1 19.5 4.2 1.9 1.6 0.5
87.2 68.4 92.3 50.4 34.1 11.9 4.0 3.6 0.9
3
Process
TOC removal (%)
COD removal (%)
ԑc
Fe2þ/H2O2 UV/H2O2 UV/Fe2þ/H2O2 UV/Fe3þ/H2O2 UV/S2O28 VIS/Fe3þ/H2O2 UV Fe3þ/H2O2 VIS/Fe3þ/H2O2/C2O24
24.5 22.3 22.3 7.8 5.8 5.8 4.5 3.9 3.2
52.7 34.8 42.3 41.2 10.4 20.3 2.4 12.3 19.8
1.25 0.3 0.7 0.8 0.2 0.3 0.5 0.01 0.4
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Table 3 Design of Experiments. Operating levels region. [Tartrazine]o ¼ 1⋅10 20 � C.
4
M; T ¼
Parameter
Low value
High value
Center point
Step value
pH [H2O2]0 (M) [Fe2þ]0 (M) [Fe3þ]0 (M)
2.0 0.0 0.0 0.0
5 1.25⋅10 1.25⋅10 1.25⋅10
3.5 6.25⋅10 6.25⋅10 6.25⋅10
0.75 3.125⋅10 3.125⋅10 3.125⋅10
3 4 4
4 5 5
4 5 5
(32 runs) after 24 h of reaction time. It can be concluded the statistically significant factors with a probability of 95% that would modify the target variable with their variations are the initial concentration of H2O2 and its square. Likewise, the values of R2 and R2 adjusted were 98.58% and 97.64%, respectively, standard error ¼ 3.029 and medium error ¼ 1.633, which indicate the goodness of the adjusted equations. Once it has been determined by the ANOVA test that the model is adequate to represent effectively the influence of the variables, ac cording to the obtained p-value, it is possible to calculate the coefficients of the polynomial that adjusts the experimental values Eq. (5).
Fig. 2. TOC and COD removals and oxidation state of remaining organic car bon (εc) at 120 min of reaction. [H2O2]o ¼ 5⋅10 4 M; [Tartrazine]o ¼ 1⋅10 4 M; [S2O48 ]o ¼ 5⋅10 4 M; [C2O24 ]o ¼ 2⋅10 4 M; [Fe2þ]o ¼ [Fe3þ]o ¼ 5⋅10 5 M; T ¼ 20 � C; pH ¼ 3.5.
Removal (%) ¼ 87.89–0.430⋅pH þ 20.89⋅[H2O2] þ 0.454⋅[Fe2þ] – 0.035⋅ [Fe3þ] þ 0.990 pH2 þ 0.086⋅pH⋅[H2O2] – 0.317⋅pH⋅[Fe2þ] þ 0.247⋅pH⋅[Fe3þ] – 9.553⋅[H2O2]2–0.472⋅[H2O2]⋅[Fe2þ] – 0.315⋅[H2O2]⋅[Fe3þ] – 0.076⋅ [Fe2þ]2–0.383⋅[Fe2þ]⋅[Fe3þ] þ 0.441⋅[Fe3þ]2 (5)
With regard to the oxidation state of the remaining carbon in the solution shown in Table 2, a higher and more positive oxidation state corresponds to ternary UV/Fenþ/H2O2 and binary Fe2þ/H2O2 processes. An oxidation state between þ1 and þ 1.5 would make us think about aldehyde and ketone intermediates.
The positive or negative sign indicates the favourable or unfav ourable influence of the effect. The greater the absolute value of the regression coefficient, the greater the influence of the factor on the target variable. Clearly the initial concentrations of H2O2 and Fe2þ have a positive effect on tartrazine removal. Increasing the concentration of H2O2 increases the formation of �OH radicals, which are the true oxi dants for organic compounds. An increase in the concentration of Fe2þ ion accelerates the oxidation reactions, as it acts as a catalyst (Eq. (4)). Table 4 shows the optimum values of the operating variables, as well as the study range. As shown, there is an optimum value within the study region for all operating variables. In terms of the interaction between variables, the graphical analysis of the results obtained showed very little interactions between the operating variables. Fig. 3 shows, as an example, the response surface and level curve for the influence of H2O2 and Fe2þ on the target variable. As shown, the response surface is convex within the region. Similarly, the initial con centration of Fe3þ has a positive effect, while the pH has a negative effect. Clearly, tartrazine removal increases as the hydrogen peroxide concentration increases until the maximum point is reached, where an increase in this parameter would be detrimental to the process (Eqs. (6) and (7)).
3.3. Cost evaluation Taking into account the results obtained in the previous section, the Fenton system (Fe2þ/H2O2) could be one of the most suitable oxidation processes. After conducting an extensive bibliographical review of the economic aspects of different oxidation processes, Beltr� an de Heredia et al. (2002) showed, there are two very different operating costs scales, processes that do not use UV-radiation and processes that use it. While in the first group, operating costs for the Fenton system ranged from 0.2 to 2.1 €⋅m 3, the second UV group ranged from 14 €⋅m 3 (O3/UV/H2O2/Fe2þ) to 265 €⋅m 3 (when the single UV-radiation was used). Although LED technology could improve the costs of this second group of technologies, the emitted radiation is at a greater wavelength (>350 nm) and therefore the efficiency of these systems decreases considerably. Taking into account these estimations, and according to the optimal results obtained in the previous section, we decided to carry out a deeper study on the Fenton system (using design of experiments). 3.4. Fenton system multivariable optimization The main objectives of this study are, to establish the influence of the operating variables, as well as the possible interactions between them, and to determine the optimum values for these operating conditions. Dye removal (%), TOC elimination (%) and COD elimination (%) were chosen as target variables. Likewise, the selected operating variables were: initial hydrogen peroxide concentration, initial ferric concentra tion, initial concentration of ferrous ion, and pH. The experimental design was designed to analyze the relationship between the selected variables and to find the optimal value of these operating conditions that would achieve the maximum value for each target variable, and also to determine the possible interactions between the operating variables. A factorial central composite orthogonal and rotatable design of experiments (FCCORD), coupled with response sur face analysis was employed. The study region (variables range), the central value and the steps are shown in Table 3.
H2 O2 þ ⋅ OH→HO�2 þ H2 O2
(6)
Fe2þ þ ⋅ OH→Fe3þ þ OH
(7)
3.4.1.1. The physical meaning of the experimental results. There are several interpretations of the influence of variables. For example, in a study focusing on the degradation of methyl dye by Fenton’s reagent ~ a et al., 2014), the efficiency of the process increased with an (Saldan Table 4 Optimal coded and real values of variables for the elimination of tartrazine by Fenton’s reagent. [Tartrazine]o ¼ 1⋅10 4 M; T ¼ 20 � C. Parameter pH [H2O2]0 (M) [Fe2þ]0 (M) [Fe3þ]0 (M)
3.4.1. Removal of dye as the target variable This section shows the results obtained for the different experiments 4
Low 2.0 2.0 2.0 2.0
High
Coded optimum
Coded optimum
2.0 2.0 2.0 2.0
0.674 1.095 0.200 0.036
3 1.095⋅10 6.876⋅10 6.365⋅10
4
M M 5 M 5
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Journal of Environmental Management 256 (2020) 109972
Fig. 4. Response surface for COD removal. [Tartrazine]o ¼ 1⋅10 ¼ 6.25 � 10 5 M; T ¼ 20 � C; pH ¼ 3.5.
Fig. 3. Estimated response surface and level curve for tartrazine removal using Fenton’s reagent. [Tartrazine]o ¼ 1⋅10 4 M; [Fe3þ]o ¼ 6.25⋅10 5 M; T ¼ 20 � C; pH ¼ 3.5.
4
M; [Fe2þ]o
If we consider TOC removal as a target variable. Fig. 5 shows, as an example, the influence of the initial concentrations of H2O2 and Fe2þ. As in the case of COD removal, the highest TOC removal was reached with higher initial H2O2 and Fe2þ concentrations. It should be empha sized that in this case, the influence of the concentration of ferrous ion is very small. Regarding the influence of pH, at a higher H2O2 concentration and lower pH, a higher TOC removal is achieved. Also, a higher TOC removal is achieved by increasing the hydrogen peroxide and reducing the Fe3þ concentration (the influence of ferric ion is very small). The influence of the pH is also very small (in the range 2–5), and it acts contrary to COD and TOC. This can be explained by taking into account the fact that at a lower pH, the solubility of CO2 (final product of mineralization) de creases, thereby favouring TOC removal.
increase in the Fe2þ concentration and with a decrease in the concen tration of H2O2. However, Mitsika et al. (2013) concluded that the variable with the most significant effect on the degradation of Acet amiprid was the concentration of iron followed by the concentration of H2O2. Guo et al. (2018) studied the oxidation of benzene dye in termediates and concluded that the most influential variable was pH. Therefore, by increasing the concentration of Fe2þ and H2O2 the elim ination of the dye was improved. In a study conducted by Domínguez et al. (2014) on oxidation by Fenton’s reagent using the response surface method, the removal levels increase as the initial concentration of H2O2 and Fe2þ increases in all cases. Similar results were obtained in the present work, although in this case we only obtained significance for the initial H2O2 concentration.
3.4.3. Experimental confirmation of the theoretical maxima predicted An experiment was conducted on the optimal conditions predicted by this statistical study in order to determine the level of elimination of tartrazine, COD and TOC. Table 5 shows the optimal, coded and real values, and the elimination level obtained under these conditions.
3.4.2. Chemical oxygen demand (COD) and total organic carbon (TOC) removals as target variables By applying ANOVA test to the obtained results, it can be concluded that the statistically significant factors with a probability of 95% are as follows:
4. Conclusions
- For COD removal: the initial concentration of H2O2, Fe2þ, Fe3þ, the square of all variables and interactions of pH- Fe3þ, H2O2- Fe2þ and Fe2þ- Fe3þ. - For TOC removal: the initial concentration of H2O2, pH, the squares of these variables, and the interactions of pH- H2O2 and Fe2þ- Fe3þ.
The following conclusions can be deduced from the study. The fastest processes, with the highest percentage of dye removal and mineraliza tion (higher percentages of TOC and COD removal), are those of binary systems (or higher) in which UV-radiation and/or Fenton reagent acts. The Fenton-like reactive mediated processes (Fe3þ/H2O2) are usually the slowest and least efficient. The addiction of oxalate to the photo catalyzed process with VIS-light and Fe3þ considerably improves the system. The Fenton reagent (Fe2þ/H2O2) achieves the highest mineral ization values and also achieves the highest value of oxidation state of the residual organic carbon (þ1,25). A statistical design of experiments was implemented in order to optimize the Fenton’s process. This optimization was carried out using the response surface methodology. When we considered dye removal as the target variable, the most influential factor was the initial
Clearly, in this case, a wide variety of experimental factors affects TOC and COD removals, which makes the response surface methodology application more useful and interesting. The coefficients of the polynomial that adjusts the experimental values for each of the factors appear in Eqs. (8) and (9). COD removal (%) ¼ 31.91 þ 1.749⋅pH þ 14.32⋅[H2O2] þ 3.555⋅ [Fe2þ] – 2.865⋅[Fe3þ] – 3.109 pH2 þ 1.922⋅pH⋅[H2O2] – 2.188⋅pH⋅[Fe2þ] þ 6.811⋅pH⋅[Fe3þ] – 0.477⋅[H2O2]2 þ 5.844⋅[H2O2]⋅[Fe2þ] – 0.512⋅[H2O2]⋅ [Fe3þ] þ 4.183⋅[Fe2þ]2–4.073⋅[Fe2þ]⋅[Fe3þ] þ 3.081⋅[Fe3þ]2 (8) TOC removal (%) ¼ 5.435–3.424⋅pH þ 17.79⋅[H2O2] þ 1.921⋅[Fe2þ] þ0.224⋅[Fe3þ] þ5.372 pH2 – 14.184⋅pH⋅[H2O2] – 0.396⋅pH⋅[Fe2þ] 1.887⋅pH⋅[Fe3þ] þ7.928⋅[H2O2]2 þ 1.581⋅[H2O2]⋅[Fe2þ] – 1.271⋅[H2O2]⋅ [Fe3þ] þ1.581 3362⋅[Fe2þ]2–3.362⋅[Fe2þ]⋅[Fe3þ] þ 1.027⋅[Fe3þ]2 (9) Fig. 4 reflects the response surface for COD removal. As shown in Fig. 4, Fe3þ concentration mildly affects COD removal in the negative direction. A higher concentration of H2O2 and a lower Fe3þ concentra tion achieve a higher COD removal. Further, there is an optimum pH value. The highest COD removal was achieved with higher H2O2 and Fe2þ concentrations. This can be explained considering that hydrogen peroxide and Fe2þ concentrations contribute to the generation of hydroxyl radicals (Eq. (4)).
Fig. 5. Influence of H2O2 and Fe2þ initial concentrations on TOC removal. Coded variables. [Tartrazine]o ¼ 1⋅10 4 M; [Fe3þ]o ¼ 6.25⋅10 5 M; T ¼ 20 � C; pH ¼ 3.5. 5
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Table 5 Optimal values (coded and real). Oxidation by Fenton’s reagent. [Tartrazine]o ¼ 1⋅10 4 M; T ¼ 20 � C. Factor
Codified Optimum
Real Optimum
pH [H2O2]o [Fe2þ]o [Fe3þ]o COD removal (%) TOC removal (%) Tartrazine removal (%)
1.2132 1.1614 0.0575 0.1219
2.6 9.879⋅10 6.070⋅10 5.869⋅10 52.4 33.5 98.13
4
M M 5 M 5
concentration of hydrogen peroxide; the rest of the variables were not significant. When we considered TOC removal as the target variable, the most influential variables were the initial concentration of H2O2, pH, the squares of these variables, and interactions of pH- H2O2 and Fe2þ- Fe3þ; the most important factor (positive) was the initial concentration of hydrogen peroxide. No optimal operating conditions were found within the study region and only trends can be talked about. An increase in H2O2 and Fe2þ concentrations had a positive effect on mineralization, while an increase in pH or concentration of Fe3þ had a negative effect. When we considered COD removal as the target variable, the most influential variables were: initial concentration of H2O2, Fe2þ, Fe3þ, the squares of pH, Fe2þ and Fe3þ variables, and interactions of pH- Fe3þ, H2O2 - Fe2þ and Fe2þ- Fe3þ; the most important factor (positive) was the initial concentration of hydrogen peroxide. We have found optimal conditions only for pH. Author contribution statement T. Gonzalez: Conceptualization, Methodology, Writing- Original draft preparation, Writing- Reviewing and Editing, J.R. Dominguez: Supervision, Writing- Original draft preparation, Conceptualization, Methodology, E.M. Cuerda Correa: Supervision, Software, Validation, S. E. Correia: Writing- Original draft preparation, Writing- Reviewing and Editing, G. Donoso: Investigation, Validation. Acknowledgements The authors gratefully acknowledge financial support of this �n Interministerial de Ciencia y Tec research work through the Comisio nología (CICYT)-CTM 2016-75873-R project-as well as through Junta de Extremadura under GR15067 and IB16016 projects and Fondo Europeo de Desarrollo Regional. References Abe, F.R., Machado, A.L., Soares, A.M.V.M., Oliveira, D. P. de, Pestana, J.L.T., 2019. Life history and behavior effects of synthetic and natural dyes on Daphnia magna. Chemosphere 236, 124390. https://doi.org/10.1016/j.chemosphere.2019.124390. Alcocer, S., Picos, A., Uribe, A.R., P�erez, T., Peralta-Hern� andez, J.M., 2018. Comparative study for degradation of industrial dyes by electrochemical advanced oxidation processes with BDD anode in a laboratory stirred tank reactor. Chemosphere 205, 682–689. https://doi.org/10.1016/j.chemosphere.2018.04.155. Ali, S.R., Kumar, R., Kadabinakatti, S.K., Arya, M.C., 2018. Enhanced UV and visible light-driven photocatalytic degradation of tartrazine by nickel-doped cerium oxide nanoparticles. Mater. Res. Express 6 (2), 025513. https://doi.org/10.1088/20531591/aaee44. Ba^eta, B.E.L., Luna, H.J., Sanson, A.L., Silva, S.Q., Aquino, S.F., 2013. Degradation of a model azo dye in submerged anaerobic membrane bioreactor (SAMBR) operated with powdered activated carbon (PAC). J. Environ. Manag. 128, 462–470. https:// doi.org/10.1016/j.jenvman.2013.05.038. Baiju, A., Gandhimathi, R., Ramesh, S.T., Nidheesh, P.V., 2018. Combined heterogeneous Electro-Fenton and biological process for the treatment of stabilized landfill leachate. J. Environ. Manag. 210, 328–337. https://doi.org/10.1016/j. jenvman.2018.01.019. Barcel� o, L., L� opez de Alda, M., 2011. Contaminaci� on y calidad química del agua: el problema de los contaminantes emergentes. Panel científico- T� ecnico de seguimiento de la política de aguas. Barcelona: Fundaci� on nueva cultura del agua, pp. 1–26.
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