Oxidation Degradation of Rhodamine B in Aqueous by UV/S₂O₈²⁻ Treatment System

Abstract

The UV photolysis of persulfate (S₂O₈²⁻) is a novel advanced oxidation technologies (AOTs), which leads to the formation of strong oxidizing radicals, sulfate radicals (SO₄^•−). The effect of oxidant S₂O₈²⁻ concentration, initial dye concentration, initial pH of solution, and various inorganic anions (Cl⁻, H₂PO₄⁻, and HCO₃⁻) were investigated using Rhodamine B (RhB), a kind of xanthene dye, as a model pollutant. With the increase of oxidant S₂O₈²⁻, more SO₄^•− produced to attack RhB molecules and result in the increase of RhB degradation. While the improvement was not sustained above a critical value, beyond which degradation rate does not increase. Initial pH of solution had great effect on the RhB degradation rate during UV/S₂O₈²⁻ system. SO₄^•− is rather stable in acidic solutions, while increasing system pH results in the transformation of SO₄^•− to ^•OH. The effects of three inorganic anions (Cl⁻, H₂PO₄⁻, and HCO₃⁻) all had some negative effect on the degradation of RhB. Based on the RhB solution changes of the UV-vis absorption intensity during the UV/S₂O₈²⁻ treatment, decolorization of RhB accompanied the destruction of aromatic ring structures of RhB molecules.

1. Introduction

Dyes are widely used in textile, leather, pharmaceutical, plastic, paint, and food industries [1].

Nearly more than 800,000 tons dye are produced per year, and over 15% of the synthetic textile dyes used are lost during manufacturing or processing operations and released as effluents [2, 3]. The effluents will produce adverse effects on the ecoenvironments due to their nonbiodegradability, toxicity, potential carcinogenic and mutagenic nature [3, 4]. For the treatment of dye-containing wastewater, traditional physicochemical techniques such as adsorption on activated carbon, membrane separation and coagulation have difficulties in the complete destruction of dye pollutants [5].

Advanced oxidation technologies (AOTs) are attractive alternatives to nondestructive physical water treatment processes because they are able to mineralize organic contaminants. AOTs, such as Fenton reagent, photo-Fenton, UV/O₃, UV/H₂O₂, and TiO₂-mediated photocatalysis processes based on the generation reactive hydroxyl radicals (^•OH), have emerged to be promising alternatives for dye-off wastewaters [6–9].

In recently, sulfate radicals-based AOTs have come forth for the degradation of nonbiodegradable compounds [10–14]. The rapid destruction of organic contaminants by sulfate radicals (SO₄^•−) at a near diffusion-controlled rate (10⁹ M⁻¹·s⁻¹) has gained great interest among practitioners as a potential method for on-site hazardous waste remediation [15]. Compared to ^•OH, SO₄^•− demonstrate higher reduction potential at neutral pH and are more selective for oxidizing organic pollutants [16]. In general, SO₄^•− can be generated by scission of peroxide bond by radiolytic, photolytic, and thermal activation of persulfate (S₂O₈²⁻), or electron transfer by transition-metal activation of S₂O₈²⁻ or peroxymonosulfate [17–20]. Among them, UV-mediated decomposition of persulfate (UV/S₂O₈²⁻) is an efficient oxidative system that can form SO₄^•− as major oxidizing species. Similar to H₂O₂, S₂O₈²⁻ shows significant photochemical activity under the UV irradiation. S₂O₈²⁻ is the most important oxidant used as SO₄^•−-based oxidants and thus has drawn increasing attention as an alternative oxidant in the chemical oxidation of contaminants [11–15, 19–21]. The use of S₂O₈²⁻ has several advantages. S₂O₈²⁻ itself is a strong oxidant with a standard oxidation potential of 2.01 V comparable to O₃ (2.07 V), nonselectively reactive, and relatively stable at room temperature [10, 19–21]. Moreover, a low cost of S₂O₈²⁻ facilitates SO₄^•−-based AOTs for potential application in wastewater treatment.

In this work, a model refractory organic dye pollutant Rhodamine B (RhB), which contains four N-ethyl groups at either side of the xanthene ring, was chosen as the target pollutant to examine the degradation behavior of the UV/S₂O₈²⁻ system. RhB is an important representative of xanthene dye, widely used as a colorant in textiles and food stuffs, and is also a well-known water tracer fluorescent, which has the property of carcinogenicity, reproductive and developmental toxicity, neurotoxicity, and chronic toxicity towards humans and animals [6, 22]. The objective of this work was to investigate the factors that influence the degradation rate of RhB in UV/S₂O₈²⁻ treatment system such as oxidant doses, initial dye concentration, and the effects of anions (Cl⁻, H₂PO₄⁻, and HCO₃⁻) that are ubiquitous coexistent in waters on the degradation of RhB with the UV/S₂O₈²⁻ system.

2. Experimental

2.1. Chemicals

RhB (Figure 1) was purchased from Alfa Aesar, A Johnson Matthey Company. K₂S₂O₈ was purchased from Tianjing Guangfu Company, China. All other chemicals were analytical grade and used as received without further purification. For pH adjustment, 0.1 M sulfuric acid and 0.1 M sodium hydroxide were used, and all the experiments were conducted in an air-conditioned room at 25±2°C. Milli-Q water was used throughout this study. Triplicate (𝑛=3) experiments were conducted under the identical condition.

Figure 1 

Structure of Rhodamine B.

2.2. Photoreactor and Light Source

The photodegradation experiments were performed in an XPA-7 merry-go-round photochemical reactor (Xujiang Electromechanical Plant, Nanjing, China) with quartz tubes containing the reaction solutions. A 300 W high-pressure mercury lamp used for the photodegradation experiment under UV-vis irradiation immersed inside water-cooled quartz well. The light source irradiance spectra (Figure 2) were measured with a monochromator (Acton, SP-300). The light intensity (200–420 nm) in the center of the reactive solutions was 4.10 mW/cm² for the mercury lamp.

Figure 2 

Irradiance spectra of the high-pressure mercury lamp light sources.

2.3. Experimental Procedure and Analysis

At specific intervals, samples were removed from exposure for UV-vis spectra measurement. The RhB degradation was characterized by measuring absorbency at 554 nm, the peak wavelength of RhB in the visible region. UV-vis spectra variations were recorded between 190 and 800 nm employing a Shimaduz 1700 UV-vis spectrometer.

3. Results and Discussion

3.1. Effect of Oxidants K₂S₂O₈ on the Photodegradation of RhB

The effect of oxidant K₂S₂O₈ on the degradation of RhB was investigated at natural pH, and the results were shown in Figure 3. Initially, control experiments were carried out in the absence and presence of either UV light or K₂S₂O₈ alone. Results showed that mere K₂S₂O₈ was not sufficient for the degradation of RhB. There was no observable color loss in the absence of UV irradiation. Only UV light irradiation, there was about 35% degradation of RhB with 60 min reaction time, while there was 85% with UV/K₂S₂O₈ ([K₂S₂O₈] = 0.2 mM) treatment system at the same reaction time, which is indicating that UV/K₂S₂O₈ was superior to UV alone in terms of the dye removal efficiency. The RhB degradation rate increased with the increasing of concentration of oxidant K₂S₂O₈ from 0.2 mM to 0.8 mM. While the improvement was not sustained above a critical value ([K₂S₂O₈] = 1.0 mM), beyond which the reaction rate decreased. This is mainly because that oxidant K₂S₂O₈ has two opposing effects [23, 24]. As the concentration of oxidant K₂S₂O₈ is increased, more SO₄^•− will be available to attack RhB molecules. Therefore, the rate of reaction increases. On the other hand, when K₂S₂O₈ is used in excess, SO₄^•− efficiently reacts with K₂S₂O₈ produce to S₂O₈^•−, and SO₄^•− with SO₄^•− can dimerize to generate S₂O₈²⁻.

Figure 3 

RhB degradation rate during UV/K2S2O8 treatment system at different oxidant concentration. Conditions: [RhB] = 0.02 mM, no pH adjustment.

3.2. Effect of the Initial RhB Concentration

The effect of dye initial concentration on the degradation efficiency was monitored, and the results were presented in Figure 4. It can be seen that photooxidation efficiency decreased as initial dye concentration is increased when the concentration of K₂S₂O₈ is constant. At high concentration of RhB, the penetration of photons entering into the solution decreases, so that an inner filter effect is induced, and hence the solution becomes more and more impermeable to UV radiation, consequently SO₄^•− concentration decreases. On the other hand, since the oxidant K₂S₂O₈ is constant and high concentration of RhB would consume more SO₄^•−, the degradation rate would be decrease with the increase of the initial RhB concentration.

Figure 4 

RhB degradation rate during the UV/K2S2O8 treatment system at different dye initial concentration. Conditions: [K2S2O8]=0.4 mM, no pH adjustment.

3.3. Effect of the Initial pH

To illustrate the effect of pH in the UV/K₂S₂O₈ treatment, the pH was adjusted to 4.20, 7.16, and 9.01, respectively. The results of pH dependency on the RhB degradation rate during UV/K₂S₂O₈ system at various initial pH were presented in Figure 5. When the initial pH was 4.21 and 9.01, the degradation efficiency was ~67.7% and 55.5% at 30 min, respectively. From the results, we can observe that acid initial pH has slightly positive effect on the RhB degradation during the UV/K₂S₂O₈ system. That is perhaps mainly because that SO₄^•− start to decompose and transform into ^•OH rapidly with the pH increase [20, 25]. On the contrary, SO₄^•− are rather stable in acidic solutions. At low pH, the dominant active species is SO₄^•−. In general, different pH values have not significant effect on the degradation of UV/K₂S₂O₈ system. Hence, the application of the combined UV/K₂S₂O₈ system will be of great significance because dye wastewater is usually in a broad range of pH.

Figure 5 

Effect of initial pH on the RhB degradation rate during the UV/K2S2O8 treatment system. Conditions: [RhB] = 0.02 mM, [K2S2O8] = 0.4 mM, no pH adjustment.

3.4. Effect of Inorganic Ions

This study also examined the effect of anions on the decolorization of RhB such as Cl⁻, H₂PO₄⁻, and HCO₃⁻, which are usually found in textile waste streams. The effect of three typical inorganic anion on the degradation of RhB with UV/K₂S₂O₈ was illustrated in Figure 6. It can be seen that all the anions used in this work inhibited the RhB degradation. It is well known that SO₄^•− can directly oxidize Cl⁻, H₂PO₄⁻, and HCO₃⁻ vis electron transfer reactions to produce less-active species [13, 26–28]. It is well known that SO₄^•− can directly oxidize Cl⁻, H₂PO₄⁻ and HCO₃⁻ vis electron transfer reactions to produce less-active species [13, 26–28]. The reaction rate constants for the reactions of SO₄^•− with HCO₃⁻, H₂PO₄⁻, and Cl⁻ are ~10⁶ L/mol·s, ~10⁵ L/mol·s and ~10⁸ L/mol·s, respectively [28, 29]. The reaction rate constants of SO₄^•− with HCO₃⁻ and H₂PO₄⁻ are less than that of Cl⁻ with SO₄^•−. Hence, the inhibiting effect on RhB degradation of Cl⁻ is more obvious than HCO₃⁻ and H₂PO₄⁻⋅SO₄^•− were the dominant oxidative species in the UV/K₂S₂O₈ process, these anions were able to compete with RhB for SO₄^•− at various rate and thus led to the decrease of degradation of RhB.

Figure 6 

Influence of Cl−, H2PO4− and HCO3− anions on the degradation of with UV/K2S2O8 system. Conditions: [RhB] = 0.02 mM, [K2S2O8] = 0.4 mM.

3.5. The UV-Visible Absorption Spectra Analysis

In order to investigate the change in the molecule of RhB with the reaction time, UV-visible absorption spectra changes in the dye solution were observed, and the corresponding spectra are shown in Figure 7. It can be seen that there are two main characteristic absorption bands of RhB solution. One is UV region (259 nm), and another is in the visible region (554 nm). The UV band absorbance of dye solution at 554 nm (𝑛→𝜋 transition of C=N, C=O groups) is due to the color of the dye solution, and it is used to monitor the decolorization of dye. Absorbance at 259 nm represents the aromatic content of RB, and the decrease of absorbance at this band indicates the degradation of aromatic part of dye [30]. During the UV/K₂S₂O₈ treatment system, absorption intensity of RhB in solution became weaker along with the reaction time. It can be explained that the aromatic ring structures of RhB molecules were destructed with reaction process.

Figure 7 

UV/Vis spectral changes of RhB with UV/K2S2O8 treatment system. Conditions: [RhB] = 0.02 mmol/L, [K2S2O8] = 0.4 mmol/L, no pH adjustment.

4. Conclusion

In this study, application of the UV/K₂S₂O₈ treatment processes to the aqueous degradation of RhB was investigated. The efficiency of the UV/K₂S₂O₈ system was investigated in terms of RhB decay. The effects of oxidants doses, initial RhB concentration anions, initial pH, and three kinds of anions (Cl⁻, H₂PO₄⁻ and HCO₃⁻) on the degradation rate in the UV/K₂S₂O₈ system were investigated. The RhB degradation rate increased with the increasing of concentration of oxidant K₂S₂O₈ from 0.2 mM to 0.8 mM. While the improvement was not sustained above a critical value ([K₂S₂O₈]=1.0 mM), photooxidation efficiency decreased as initial dye concentration increased at the same concentration of K₂S₂O₈. Acid initial pH facilitated the RhB degradation during the UV/K₂S₂O₈ system because that SO₄^•− are the dominant active species in this pH value area. In the UV/K₂S₂O₈ system, the observed adverse effects of inorganic anions could be the result of competitive with dye and the inhibiting effect followed the order of Cl⁻< H₂PO₄⁻<HCO₃⁻. During the UV/K₂S₂O₈ treatment system, absorption intensity of RhB in solution became weaker along with the reaction time, which can be explained that the aromatic ring structures of RhB molecules were destructed with reaction process.

Acknowledgment

The study was supported by Zhejiang Provincial Natural Science Foundation (no. Y5110338) and the National Natural Science Foundation (no. 20907045) of China.