Decolorization of Reactive Red 2 in Fenton and Fenton-like systems: effects of ultrasound and ultraviolet irradiation

Abstract

This study utilized Fenton, Fenton-like, photo-Fenton, photo-Fenton-like, sono-Fenton, and sono-Fenton-like systems for the decolorization of Reactive Red 2 (RR2) dye. Fe²⁺ and Fe³⁺ were used to catalyze the actions of oxidants H₂O₂ and Na₂S₂O₈. Pseudo-first order rate constants (k) were obtained by fitting the decolorization kinetics of RR2. With added oxidant at 0.3 mM and iron ion at 0.03 mM, the k values of H₂O₂/Fe²⁺, H₂O₂/Fe³⁺, Na₂S₂O₈/Fe²⁺, and Na₂S₂O₈/Fe³⁺ were 13, 0.67, 6.2, and 0.080 r⁻¹, in order. Higher oxidant and iron concentrations resulted in faster decolorization in Fenton and Fenton-like systems. Additionally, decolorization was accelerated by ultraviolet (UV) and ultrasound (US) irradiation. When ethanol was used as a scavenger for the generated hydroxyl and sulfate radicals, it significantly inhibited the decolorization process in photo-Fenton and photo-Fenton-like systems. The k values obtained for the UV/H₂O₂/Fe²⁺, UV/Na₂S₂O₈/Fe²⁺, UV/H₂O₂/Fe²⁺/C₂H₅OH, and UV/Na₂S₂O₈/Fe²⁺/C₂H₅OH systems were 23, 14, 0.36, and 0.69 h⁻¹, in order.

Introduction

Wastewaters from the textile dyeing industry typically contain a high concentration of color compounds. Azo dyes containing one or more azo bonds are most common among the synthetic dyes and are a major pollutant in wastewaters. Dyes are readily discernible at small quantities and adversely affect the water environment. Therefore, the removal of color from wastewater often takes priority over the treatment of other colorless organics. Reactive Red 2 (RR2), a common azo dye with the dichlorotriazine group, was selected as the contaminant compound in this study.

Conventional physical (adsorption, filtration, and flotation), chemical (coagulation, oxidation, reduction, and electrolysis), and biological processes have been applied to treat wastewaters containing organic dyes and pigments. Advanced oxidation processes (AOPs) are treatment processes based on the generation of radicals that are highly reactive and nonselective oxidants towards a wide range of organic compounds in water. These processes were significantly advanced in recent years for the treatment of textile effluents [1–14]. Among the AOPs, Fenton-type processes have shown pollutant removal efficiencies >90% [2–4, 15–22]. The Fenton-type process combines iron (Fe²⁺ or Fe³⁺) with hydrogen peroxide to produce hydroxyl radicals. The general mechanism involves Fenton reagents that utilize Fe²⁺ (Fenton) or Fe³⁺ (Fenton-like) ions as a catalyst to decompose hydrogen peroxide. Several studies have investigated the effectiveness of Fenton-type processes according to H₂O₂ dosage [2, 3, 15, 16, 23, 24], catalyst dosage [2, 3, 15–17, 23, 24], iron speciation [4, 15, 18], the presence or absence of ultraviolet illumination (UV) [2, 3, 15–17, 23–25], and UV intensity [3, 26].

Various AOP combinations often result in synergistic effects that significantly reduce the required reaction time [5, 6, 8–10]. In recent years, attention has been focused on the application of ultrasonic energy to meet wastewater treatment challenges. Ultrasound (US) causes acoustic cavitation, resulting in bubble collapses that cause intense local heating, pressurization, and very short lifetimes of the bubbles; these transient, localized hot spots are useful in driving high-energy chemical reactions [27]. The solute or solvent molecules in contact with or inside these cavities in the vapor phase undergo fragmentation, yielding free radicals that degrade organic compounds. Degradation of organic compounds is typically achieved via a combination of pyrolytic reactions that occur inside or near the bubble and radical reactions in the bulk liquid [28, 29]. The combinations of US with other techniques such as UV [5], Fe⁰ [30–32], Fe²⁺ [29, 30, 32, 33], Fe³⁺ [33], TiO₂ [6, 7, 31, 34, 35], H₂O₂ [8], UV/ZnO [5], UV/TiO₂ [6, 7], UV/H₂O₂ [8], H₂O₂/Fe²⁺ [36–38], ${\text{S}}_{ 2} {\text{O}}_{ 8}^{2 - }$/Fe²⁺ [37], and ${\text{S}}_{ 2} {\text{O}}_{ 8}^{2 - }$/H₂O₂/Fe²⁺ [39] have been shown to enhance contaminant degradation.

Several studies have explored the performance of US/Fenton [36–39], US/Fenton-like [37, 39], UV/Fenton [15–19, 23–26], and UV/Fenton-like [15, 18, 19] systems in the degradation of different compounds. Additionally, UV/US/TiO₂ [6, 7]; O₃ [40, 41]; UV/O₃ [40]; UV/TiO₂ [40, 42–44]; H₂O₂/Fe²⁺ [14, 45] and H₂O₂/Fe³⁺ [46] systems have been used to decolorize RR2. However, none have been undertaken to compare all of these systems based on the degradation of a single contaminant compound. This study compares US/Fenton, US/Fenton-like, UV/Fenton, and UV/Fenton-like systems with similar oxidant and catalyst dosages for their degradative performance on RR2. Both Fe²⁺ and Fe³⁺ were used to catalyze H₂O₂ and Na₂S₂O₈. Fenton (H₂O₂/Fe²⁺), Fenton-like (H₂O₂/Fe³⁺, ${\text{S}}_{ 2} {\text{O}}_{ 8}^{2 - }$/Fe²⁺ and ${\text{S}}_{ 2} {\text{O}}_{ 8}^{2 - }$/Fe³⁺), photo-Fenton (UV/H₂O₂/Fe²⁺), photo-Fenton-like (UV/H₂O₂/Fe³⁺, UV/${\text{S}}_{ 2} {\text{O}}_{ 8}^{2 - }$/Fe²⁺ and UV/${\text{S}}_{ 2} {\text{O}}_{ 8}^{2 - }$/Fe³⁺), sono-Fenton (US/H₂O₂/Fe²⁺), and sono-Fenton-like (US/H₂O₂/Fe³⁺, US/${\text{S}}_{ 2} {\text{O}}_{ 8}^{2 - }$/Fe²⁺, and US/${\text{S}}_{ 2} {\text{O}}_{ 8}^{2 - }$/Fe³⁺) systems were tested. The objectives of this study are to: (i) determine the effects of oxidant and iron dosages in Fenton and Fenton-like systems, and (ii) evaluate the effects of UV and US irradiation in Fenton and Fenton-like systems.

Experimental

Materials

The parent compound RR2 was purchased from Aldrich. The formula, molecular weight, and maximum light absorption wavelength of RR2 were C₁₉H₁₀Cl₂N₆Na₂O₇S₂, 615 g/mol, and 538 nm, respectively. Ferrous sulfate (FeSO₄) and ferric sulfate (Fe₂(SO₄)₃) (Merck) were used to supply Fe²⁺ and Fe³⁺, respectively. Oxidants sodium persulfate (Na₂S₂O₈) and hydrogen peroxide (H₂O₂, 30% w/w) were also purchased from Merck. Ethanol (C₂H₅OH) was used as a hydroxyl and sulfate radical scavenger. The solution pH was controlled by adding HNO₃ or NaOH through an automatic titrator. All reagents were of analytical grade and used as purchased. Deionized, double-distilled water from a Milli-Q system was used.

Decolorization experiments

The decolorization of RR2 in the UV, US, H₂O₂, Na₂S₂O₈, Fe²⁺, and Fe³⁺ systems was individually tested in control experiments. RR2 at 20 mg/L was used in all experiments. Several studies had indicated pH 3 being optimal for Fenton and photo-Fenton systems [18, 23, 26] and thus pH 3 was used in all decolorization experiments. These experiments were performed with a 2.5-L hollow cylindrical glass reactor. An 8-W UV lamp (254 or 365 nm, Philips) was placed inside the quartz tube as the irradiation source. The intensity of the light from the lamps at wavelengths of 245 and 365 nm was 3.92 and 0.57 mW/cm², respectively. The ultrasonic bath was operated at 40 kHz and 400 W (Delta, DC 400H). The distance between the reactor bottom and the ultrasonic bath was maintained at 2 cm. A calorimetric method was used to determine the acoustic power of sonication over 40 min. The ultrasonic power received by the system was calculated by [47, 48]:

$$ {\text{Power }} = \, \left( {{\text{dT}}/{\text{dt}}} \right) \times {\text{C}}_{\text{p}} \times {\text{M}} $$

(1)

where (dT/dt) is the temperature rise rate of the water during sonication (K s⁻¹), C_p the heat capacity of water (4.2 J g⁻¹ K⁻¹), and M the water mass (g). The acoustic power received by the reactor was determined to be 13.0 W. Oxidant doses of 0.3, 0.5, 1, and 2 mM along with iron doses of 0.01, 0.03, and 0.05 mM were used to evaluate the dosage effects in the Fenton and Fenton-like systems. In experiments to determine the effects of US and UV irradiation, the doses of oxidant and iron were 0.3 and 0.03 mM, respectively. Ethanol was added at 1,200 mg/L as a radical scavenger to determine the involvement of free radicals in the Fenton, Fenton-like, photo-Fenton, and photo-Fenton-like systems. All experiments were stirred at 100 rpm and aerated continuously and the temperature was controlled at 25 °C using a water circulation system. Aliquots (15 mL) were withdrawn from the photoreactor at predetermined time intervals. The solution was filtered through a 0.22-μm filter (Millipore). RR2 concentration was measured using a UV–vis spectrophotometer (Hitachi U-2001) at 538 nm. Decolorization of RR2 was quantified by the difference in absorbance measured before and after treatment. Most experiments were performed in duplicate and the averages shown.

Results and discussion

Effects of oxidant and iron doses in Fenton and Fenton-like systems

After 120 min, the extents of decolorization of RR2 by UV (254 nm), UV (365 nm), and US irradiation were 18, 4, and 19%, respectively. The extents of decolorization of RR2 when individually subjected to Fe²⁺, H₂O₂, and Na₂S₂O₈ at the highest test concentration (i.e. 2 mM of oxidant or 0.05 mM of iron) were 2, 2, and 5%, respectively. The decolorization extents were small (i.e., <5% after 120 min) compared to subsequent results with UV and US irradiation that would demonstrate catalytic, photocatalytic, or sonocatalytic effect. The synergistic effects of added oxidants in Fenton and Fenton-like systems are shown in Table 1. RR2 decolorization in the Fenton and Fenton-like systems was found to be well described by pseudo-first order kinetics, as previously reported for dye decolorization by various investigators [5–8, 18, 31, 49].

Table 1 Effects of oxidant concentration in Fenton and Fenton-like systems ([RR2] = 20 mg/L, [iron ion] = 0.03 mM, pH = 3, and 25 °C)

The H₂O₂/Fe²⁺ system involves several cyclical reactions with catalyst regeneration, as shown in Eqs. 2–9. In H₂O₂/Fe³⁺, the reaction sequence begins with Eq. 3 [2, 15, 25, 37, 39]. The reaction mechanisms of Na₂S₂O₈/Fe²⁺ are shown in Eqs. 10–14, while those of Na₂S₂O₈/Fe³⁺ systems are shown in Eqs. 11–14 [37, 39, 50, 51].

$$ {\text{Fe}}^{ 2+ } + {\text{H}}_{ 2} {\text{O}}_{ 2} \to {\text{Fe}}^{ 3+ } + {\text{OH}}^{ - } + {\text{HO}}^{ \bullet } $$

(2)

$$ {\text{Fe}}^{ 3+ } + {\text{H}}_{ 2} {\text{O}}_{ 2} \to {\text{Fe}}^{ 2+ } + {\text{H}}^{ + } + {\text{HO}}_{ 2}^{ \bullet } $$

(3)

$$ {\text{Fe}}^{ 3+ } + {\text{H}}_{ 2} {\text{O}}_{ 2} \to {\text{Fe}{-}\text{O}}_{ 2} {\text{H}}^{ 2+ } + {\text{H}}^{ + } $$

(4)

$$ {\text{Fe}}^{ 3+ } + {\text{H}}_{ 2} {\text{O}} \leftrightarrow {\text{FeOH}}^{ 2+ } + {\text{H}}^{ + } $$

(5)

$$ {\text{Fe}}^{ 2+ } + {\text{ HO}}_{ 2}^{ \bullet } \to {\text{Fe}}^{ 3+ } + {\text{ HO}}_{ 2}^{ - } $$

(6)

$$ {\text{Fe}}^{ 3+ } + {\text{HO}}_{ 2}^{ \bullet } \to {\text{Fe}}^{ 2+ } + {\text{H}}^{ + } + {\text{O}}_{ 2} $$

(7)

$$ {\text{HO}}^{ \bullet } + {\text{H}}_{ 2} {\text{O}}_{ 2} \to {\text{HO}}_{ 2}^{ \bullet } + {\text{H}}_{ 2} {\text{O}} $$

(8)

$$ 2 {\text{ HO}}^{ \bullet } \to {\text{H}}_{ 2} {\text{O}}_{ 2} $$

(9)

$$ {\text{Fe}}^{ 2+ } + {\text{S}}_{ 2} {\text{O}}_{ 8}^{ 2- } \to {\text{Fe}}^{ 3+ } + {\text{ SO}}_{ 4}^{ 2- } + {\text{ SO}}_{ 4}^{ -\bullet } $$

(10)

$$ {\text{Fe}}^{ 3+ } + {\text{S}}_{ 2} {\text{O}}_{ 8}^{ 2- } \to {\text{Fe}}^{ 2+ } + {\text{ 2 SO}}_{ 4}^{ -\bullet } $$

(11)

$$ {\text{Fe}}^{ 2+ } + {\text{ SO}}_{ 4}^{ - \bullet } \to {\text{Fe}}^{ 3+ } + {\text{ SO}}_{ 4}^{2 - } $$

(12)

$$ {\text{SO}}_{ 4}^{ - \bullet } + {\text{H}}_{ 2} {\text{O}} \to {\text{HO}}^{ \bullet } + {\text{H}}^{ + } + {\text{SO}}_{ 4}^{ 2- } $$

(13)

$$ {\text{SO}}_{ 4}^{ 2- } + {\text{HO}}^{ \bullet } \to {\text{SO}}_{ 4}^{ - \bullet } + {\text{ OH}}^{ - } $$

(14)

Increasing the oxidant concentration (from 0.3 to 2 mM) enhanced generation of radicals and therefore increased RR2 decolorization in AOP systems of H₂O₂/Fe²⁺ (Eqs. 2 and 3), H₂O₂/Fe³⁺ (Eq. 3), and Na₂S₂O₈/Fe²⁺ (Eqs. 10, 11, 13, and 14). In the Na₂S₂O₈/Fe³⁺ system, the k value increased with increasing oxidant concentration up to 1 mM, beyond which it decreased possibly due to recombination of the free radicals (Table 1).

The effects of catalyst concentration in Fenton and Fenton-like systems are presented in Table 2. The results revealed that the k values increased with Fe concentration in all tested Fenton and Fenton-like systems. Fe served as a catalyst and initiated the decomposition of hydrogen peroxide and persulfate for the generation of HO^• and HO ^•₂ in the Fenton system and SO ^−•₄ and HO^• in the Fenton-like system. More free radicals were produced with an increase of available Fe.

Table 2 Effects of catalyst concentration (Fe²⁺ or Fe³⁺) in Fenton and Fenton-like systems ([RR2] = 20 mg/L, [oxidant] = 0.3 mM, pH = 3 and 25 °C)

When compared at the same oxidant and iron doses, the rate of RR2 decolorization (k) followed the order of H₂O₂/Fe²⁺ > Na₂S₂O₈/Fe²⁺ > H₂O₂/Fe³⁺ > Na₂S₂O₈/Fe³⁺ (Tables 1, 2). Sulfate radical is a very strong oxidant with redox potential of 2.6 V, next only to hydroxyl radical with redox potential of 2.8 V [51]. Generally, the oxidizing properties of peroxide oxidants are measured by the tendency of electron transfer to a lower unoccupied molecular orbital (LUMO) [52]. Miroshnichenko and Lunenok-Burmakina [52] have indicated that the energy of LUMO of ${\text{S}}_{ 2} {\text{O}}_{ 8}^{ 2- }$ is higher than that of H₂O₂, implying that H₂O₂ accepts an electron more readily than does ${\text{S}}_{ 2} {\text{O}}_{ 8}^{ 2- }$. It has also been suggested that the Fe catalyst activates H₂O₂ more readily than ${\text{S}}_{ 2} {\text{O}}_{ 8}^{ 2- }$ [22, 52]. Hence, at the same Fe dose, H₂O₂-related systems would be more active than ${\text{S}}_{ 2} {\text{O}}_{ 8}^{ 2- }$-related systems. Based on Eqs. 3–7, 11, and 12, Fe³⁺-related mechanisms are side reactions of Fe²⁺-related mechanisms; therefore, the Fe²⁺-related systems had higher k values than the Fe³⁺-related systems. This was corroborated by results of Orozco et al. [4] that showed higher decolorization rate of azo-dye Reactive Blue 69 by H₂O₂/Fe²⁺ than by H₂O₂/Fe³⁺.

Effects of UV and US irradiation in Fenton and Fenton-like systems

The decolorization of RR2 in the H₂O₂/Fe²⁺, H₂O₂/Fe³⁺, Na₂S₂O₈/Fe²⁺, and Na₂S₂O₈/Fe³⁺ systems when coupled with US and UV (254 and 365 nm) irradiation is shown in Figs. 1, 2, 3, and 4. In 5 min, the extent of RR2 decolorization was 70, 84, 80, 73, 42, 67, 58, and 46% for the H₂O₂/Fe²⁺, UV (254 nm)/H₂O₂/Fe²⁺, UV (365 nm)/H₂O₂/Fe²⁺, US/H₂O₂/Fe²⁺, Na₂S₂O₈/Fe²⁺, UV (254 nm)/Na₂S₂O₈/Fe²⁺, UV (365 nm)/Na₂S₂O₈/Fe²⁺, and US/Na₂S₂O₈/Fe²⁺ systems, respectively (Figs. 1, 3). After 20 min, the extent of decolorization was 10, 83, 20, 54, 2, 70, 31, and 45% for the H₂O₂/Fe³⁺, UV (254 nm)/H₂O₂/Fe³⁺, UV (365 nm)/H₂O₂/Fe³⁺, US/H₂O₂/Fe³⁺, Na₂S₂O₈/Fe³⁺, UV (254 nm)/Na₂S₂O₈/Fe³⁺, UV (365 nm)/Na₂S₂O₈/Fe³⁺, and US/Na₂S₂O₈/Fe³⁺ systems, respectively (Figs. 2, 4). The Fe²⁺-related systems were more effective in decolorizing RR2 than were Fe³⁺-related systems, as shown by higher decolorization extents within a shorter contact time in the former. Kusic et al. [15], Neamtu et al. [18], and Orozco et al. [4] also reported that the degradation rate with UV/H₂O₂/Fe²⁺ was higher than with the UV/H₂O₂/Fe³⁺ system. In Fe²⁺-related systems, the k values followed the order of UV (254 nm)/oxidant/Fe²⁺ > UV (365 nm)/oxidant/Fe²⁺ > US/oxidant/Fe²⁺ > oxidant/Fe²⁺. Moreover, the trend of UV (254 nm)/oxidant/Fe³⁺ > US/oxidant/Fe³⁺ > UV (365 nm)/oxidant/Fe³⁺ > oxidant/Fe³⁺ was also found in Fe³⁺-related systems (Table 3). Therefore, these experimental results indicate that UV (254 nm) activates these systems more than US. In some cases (e.g. UV (365 nm)/oxidant/Fe²⁺), UV (365 nm) irradiation was better as well.

Fig. 1

RR2 decolorization in H₂O₂/Fe²⁺ systems with US and UV (254 and 365 nm) ([RR2] = 20 mg/L, [H₂O₂] = 0.3 mM, [Fe²⁺] = 0.03 mM, pH = 3, and 25 °C)

Fig. 2

RR2 decolorization in H₂O₂/Fe³⁺ systems with US and UV (254 and 365 nm) ([RR2] = 20 mg/L, [H₂O₂] = 0.3 mM, [Fe³⁺] = 0.03 mM, pH = 3, and 25 °C)

Fig. 3

RR2 decolorization in Na₂S₂O₈/Fe²⁺ systems with US and UV (254 and 365 nm) ([RR2] = 20 mg/L, [Na₂S₂O₈] = 0.3 mM, [Fe²⁺] = 0.03 mM, pH = 3, and 25 °C)

Fig. 4

RR2 decolorization in Na₂S₂O₈/Fe³⁺ systems with US and UV (254 and 365 nm) ([RR2] = 20 mg/L, [Na₂S₂O₈] = 0.3 mM, [Fe³⁺] = 0.03 mM, pH = 3, and 25 °C)

Table 3 Effects of US and UV in Fenton and Fenton-like systems ([RR2] = 20 mg/L, [oxidant] = 0.3 mM, pH = 3 and 25 °C)

Wang et al. [34, 35] proposed that US irradiation caused the formation of US-induced luminescence at wavelength below 375 nm, decompose H₂O₂ and ${\text{S}}_{ 2} {\text{O}}_{ 8}^{ 2- }$, and produce HO^•and ${\text{S}}_{ 2} {\text{O}}_{ 8}^{ - \bullet }$. In Fenton and Fenton-like systems with US or UV, the decomposition of H₂O₂, S₂O₈ ²⁻, Fe-O₂H²⁺, and FeOH²⁺ to generate the reactive free radicals is shown by Eqs. 15–18:

$$ {\text{H}}_{ 2} {\text{O}}_{ 2} + {\text{ UV }}\left( {\text{or US}} \right) \to 2 {\text{ HO}}^{\bullet} $$

(15)

$$ {\text{S}}_{ 2} {\text{O}}_{ 8}^{ 2- } + {\text{ UV }}\left( {\text{or US}} \right) \to 2 {\text{ SO}}_{ 4}^{ -\bullet } $$

(16)

$$ {\text{Fe}}{-}{\text{O}}_{ 2} {\text{H}}^{ 2+ } + {\text{ UV }}\left( {\text{or US}} \right) \to {\text{Fe}}^{ 2+ } + {\text{ HO}}_{ 2}^{\bullet} $$

(17)

$$ {\text{FeOH}}^{ 2+ } + {\text{ UV }}\left( {\text{or US}} \right) \to {\text{Fe}}^{ 2+ } + {\text{ HO}}^{\bullet} $$

(18)

US or UV apparently favored the formation of HO ^•₂ and HO^• in the Fenton or Fenton-like cycle (Eqs. (17) and (18)), accelerating decolorization. Sun et al. [36] and Pradhan and Gogate [38] also reported more effective oxidation with US/H₂O₂/Fe²⁺ than with H₂O₂/Fe²⁺.

In photo-Fenton, photo-Fenton-like, sono-Fenton, and sono-Fenton-like systems, decolorization was accelerated with a higher Fe dose (Table 3). Additionally, the decolorization rate at 254 nm exceeded that at 365 nm in the photo-Fenton and photo-Fenton-like systems (Table 3). This finding was attributed to the fact that the energy and intensity of UV (254 nm) light exceeded those of UV (365 nm). Lo et al. [53] and Lin et al. [54] found increasing photodegradation of 4-chlorophenol with decreasing illumination wavelength. Similar results were reported for the photodegradative decolorization of C.I. Reactive Red 198 by Wu and Wu [10].

Fig. 5a, b display the UV–Vis spectral changes of RR2 in UV (254 nm)/H₂O₂/Fe²⁺ and UV (254 nm)/Na₂S₂O₈/Fe²⁺ systems, respectively. RR2 had a major absorption peak at 538 nm, whose intensity declined after 20 min of photocatalytic reaction, suggesting that some intermediates may have been generated following the photocatalytic reaction, but this work did not further elucidate this possibility. Wu et al. [55] found that the degradation products of RR2 under photooxidation treatment were substituted benzene and naphthalene. Hu et al. [42] concluded that the intermediates of RR2 photodegradation were mainly organic aromatic and aliphatic carboxylic acids. Moreover, acetyl benzoic acid, diethyl phthalate and phthalic anhydride have also been identified as intermediates in RR2 photodegradation [41]. Both Wu et al. [55] and Hu et al. [42] found that the byproducts can be further oxidized and finally mineralized to form carbon dioxide.

Fig. 5

UV-Vis spectral changes of RR2 in various systems a UV/H₂O₂/Fe²⁺ and b UV/Na₂S₂O₈/Fe²⁺ ([RR2] = 20 mg/L, [oxidant] = 0.3 mM, [Fe²⁺] = 0.03 mM, UV = 254 nm, pH 3, and 25 °C)

Effects of ethanol addition

Fig. 6 presents the effects of added ethanol in the H₂O₂/Fe²⁺, UV/H₂O₂/Fe²⁺, Na₂S₂O₈/Fe²⁺, and UV/Na₂S₂O₈/Fe²⁺ systems. After 60 min, RR2 was decolorized by 3.7, 15, 32, and 45% in the H₂O₂/Fe²⁺/C₂H₅OH, Na₂S₂O₈/Fe²⁺/C₂H₅OH, UV/H₂O₂/Fe²⁺/C₂H₅OH, and UV/Na₂S₂O₈/Fe²⁺/C₂H₅OH systems, respectively. Ethanol is known to scavenge hydroxyl [56–58] and sulfate radicals [59]. Excess ethanol (1,200 mg/L) suppressed decolorization of RR2 in all tested systems. The results suggest that hydroxyl and sulfate radicals have been significantly involved in RR2 decolorization. Furthermore, the fact that RR2 decolorization was almost completely inhibited with ethanol in the H₂O₂/Fe²⁺ system indicated a major role of HO^• in decolorization. The fitted k values of the UV/H₂O₂/Fe²⁺, UV/Na₂S₂O₈/Fe²⁺, UV/H₂O₂/Fe²⁺/C₂H₅OH, and UV/Na₂S₂O₈/Fe²⁺/C₂H₅OH systems were 23, 14, 0.36, and 0.69 h⁻¹, in order. For the UV/H₂O₂/Fe²⁺ system, the k value with ethanol was approximately 60 times lower, which was similarly observed for the H₂O₂/Fe²⁺ system, suggesting again a major decolorization pathway via the attack by HO^•. However, decolorization was only partially inhibited by ethanol even at 1,200 mg/L in the UV/Na₂S₂O₈/Fe²⁺ system, as evidenced by k values reduced by only 20 times. The second-order rate constants of hydroxyl radicals and sulfate radicals with ethanol were reported to be 1.9 × 10⁹ M⁻¹ s⁻¹ [58] and 7.8 × 10⁷ M⁻¹ s⁻¹ [59], respectively. This indicates that ethanol is more inhibitive toward hydroxyl radicals than toward sulfate radicals. Therefore, inhibition by added ethanol was more significant in the UV/H₂O₂/Fe²⁺ system than in the UV/Na₂S₂O₈/Fe²⁺ system. Sulfate radicals are produced in Na₂S₂O₈/Fe²⁺ (Eqs. 10, 11, and 14) and UV/Na₂S₂O₈/Fe²⁺ (Eqs. 10, 11, 14, and 16) systems. The results suggest that decolorization proceeds primarily via HO^• in the H₂O₂/Fe²⁺, UV/H₂O₂/Fe²⁺, Na₂S₂O₈/Fe²⁺, and UV/Na₂S₂O₈/Fe²⁺ systems; however, ${\text{S}}_{ 2} {\text{O}}_{ 4}^{ - \bullet }$ is also be involved in the Na₂S₂O₈/Fe²⁺ and UV/Na₂S₂O₈/Fe²⁺ systems.

Fig. 6

Effects of added C₂H₅OH on decolorization a H₂O₂/Fe²⁺ and UV/H₂O₂/Fe²⁺ systems and b Na₂S₂O₈/Fe²⁺ and UV/Na₂S₂O₈/Fe²⁺ systems ([RR2] = 20 mg/L, [oxidant] = 0.3 mM, [Fe²⁺] = 0.03 mM, UV = 254 nm, C₂H₅OH = 1,200 mg/L, pH 3, and 25 °C)

Conclusion

Increasing the concentration of oxidant or catalyst increased the generation of reactive free radicals and thus enhanced RR2 decolorization in Fenton and Fenton-like systems. At the same oxidant and Fe doses, the fitted k values reveal the trend of H₂O₂/Fe²⁺ > Na₂S₂O₈/Fe²⁺ > H₂O₂/Fe³⁺ > Na₂S₂O₈/Fe³⁺. Fe²⁺-related systems are more effective than Fe³⁺-related systems in decolorizing RR2. At the same kind and dose of Fe, H₂O₂-related systems are more effective than ${\text{S}}_{ 2} {\text{O}}_{ 8}^{2 - }$-related systems. Both US and UV promote the effectiveness of Fenton and Fenton-like systems.