Degradation of fluorescent dye-Solvent Green 7 (HPTS) in wastewater by advanced oxidation process

Solvent Green 7 (HPTS) is a commonly used fluorescent dye, it is mainly used in the coloring of paints, plastic products, alkaline cleaners, and is the main ingredient of fluorescent marker ink (França et al. 2018; Kumar et al. 2018). In recent years, it has been gradually used for the fluorescent tracing of circulating cooling water systems (Wang et al. 2014).

In industry, the continuous consumption and concentration of circulating cooling water, as well as the mixing of dust, soluble gas, bacteria and fungi, has led to the deterioration of circulating water quality. Therefore, various water treatment agents, such as scale inhibitors (Dietzsch et al. 2013; Zhang et al. 2016b), corrosion inhibitors (Wei et al. 2020), bactericides and algaecides, flocculants, were widely used to increase the life of circulating cooling water. Recently, carbon quantum dots and various water treatment agents have been used to synthesize fluorescent water treatment agents, which can be used to monitor the concentration of water treatment agents in circulating cooling water online (Du et al. 2009). Even so, the circulating cooling water still needed to be discharged regularly after long-term use, otherwise it would reduce the efficiency and damage the equipment.

With the use of fluorescent water treatment agents, paints, and plastic products, HPTS continued to penetrate the environment. The emission of a small amount of HPTS in our daily life had little impact on the environment, but a large number of industrial emissions could not be ignored. It was difficult to degrade polycyclic aromatic hydrocarbons (PAHs) derivative by physical, chemical and biological methods, conventional wastewater treatment plants (WWTP) mainly used the adsorption of activated sludge to treat it (Liu et al. 2011; Sun et al. 2018; Zhou et al. 2018). If HPTS was integrated into natural water in quantities, it would affect the water quality and the biomes' physiological status (Qiao et al. 2018). Besides, the fluorescence characteristics of HPTS could also cause light pollution to the water, and fluorescent labeling also affects the food chains of the ecosystem when organisms are marked by HPTS.

Therefore, this article adopted advanced oxidation processes (AOPs) to degrade HPTS, based on the generation of hydroxyl radicals (Oller et al. 2011; Miklos et al. 2018; Sun et al. 2019; Wang & Zhuan 2020). Oxidant was hydrogen peroxide (H₂O₂: E⁰ = 1.78 V), which could produce hydroxyl radicals (•OH: E⁰ = 2.80 V) under ultraviolet light (Baeza & Knappe 2011; Cheng et al. 2016; Babu et al. 2019). Due to the high oxidation-reduction potential, hydroxyl radicals generated could degrade organics with low biodegradability in water (Homlok et al. 2020; Lankone et al. 2020). Fluorescence characteristics meant that a substance could absorb ultraviolet light and emit fluorescence with longer wavelength, which could not be ignored during the HPTS degradation process.

In summary, the main content of the experiments in this article was to: (1) investigated the fluorescence characteristics on HPTS degradation process, (2) investigated the degradation efficiency of HPTS under different conditions, (3) investigated the reactions' kinetics models and kinetics rate constants, (4) analyzed the degradation pathways of HPTS.

HPTS (purity >99%) and PTSA (purity >98%) were provided by Shandong Taihe Water Treatment Technologies Co., Ltd (China). NaCl, Na₂SO₄, NaNO₃, and H₂O₂ (30 wt%) were purchased from Sinopharm Chemical Reagent Co., Ltd (China). Methanol (HPLC-grade) was obtained from Fisher Scientific (USA). D₂O reagent (99.9 atom % D) was obtained from Qingdao Tenglong Weibo Technology Co., Ltd (China).

The experimental setup is shown in Figure 1. The UV lamp (15 W) was purchased from Shenzhen Ruixing Water Treatment Equipment Co., Ltd (China) and the main emission peak of the lamp was at 253.7 nm. During the experiment, after the solution was added into the equipment, cork was used to block the inlet and sampling port to form a closed system. When sampling, the sampling port was opened and the pipette was used to take out a small amount of solution for measuring.

HPTS, 8-hydroxy 1, 3, 6-pyrene trisulfonic acid sodium salt: its chemical structure is shown in Figure 2. Pyrene is a fluorescent group, which has good fluorescence performance because of its four benzene rings. The sulfonyl group and hydroxyl group attached to the pyrene make HPTS easily soluble in water. The sulfonyl group and the pyrene form a strong conjugated system, which has strong characteristic absorption of ultraviolet light.

HPTS is weakly acidic and susceptible to pH. As can be seen from Figure 3, there are two forms of HPTS existing in aqueous solution. When the pH changes from acidic to alkaline, the hydroxyl group on the pyrene ring changes into -O⁻, which enhances the ability of -OH to donate electrons, promotes the delocalization of π electrons and increases the molecular planarity. The spectral band positions in the absorption and emission spectrum of HPTS move to a longer wavelength, resulting in a bathochromic shift, thus enhancing the fluorescence of the dye.

For further research, the degradation products of HPTS were characterized. The methods used were high performance liquid chromatography-mass spectrometry (HPLC-MS) and nuclear magnetic resonance (NMR). The experimental methods were as follows:

For HPLC-MS there were four samples: 20 mg/L HPTS initial solution, 20 mg/L HPTS solutions degraded for 40, 80, 120 min under UV irradiation. Samples were analyzed by Thermo Scientific Ultimate 3000 HPLC system (USA) and Thermo LCQ Fleet Ion Trap Mass Spectrometer (USA). HPLC system equipped with an autosampler, a dual pump, a column oven. The mobile phase consisted of methanol (solvent A, 60%) and water (solvent B, 40%), The flow rate was 0.2 mL/min. The autosampler was maintained at 25 °C and the injection volume was 10 μL. Ionization was carried out in a negative mode by the electrospray method. Mass parameters were optimized with source temperature 320 °C, voltage 4,000 V, nitrogen was used as an auxiliary (pressure: 10 arb) and sheath gas (pressure: 30 psi).

For NMR there were two samples: 0.4 mL HPTS initial solution (10 g/L), 0.4 mL concentrated solution of 200 mL HPTS (10 g/L) irradiated with UV light for 10 h. The amount added of the D₂O reagent was 0.2 mL. All ¹H NMR and ¹³C NMR spectra were acquired on a 600 MHz NMR spectrometer (Bruker Avance NEO 600, Switzerland). ¹H NMR parameters: temperature: 298 K, number of scans: 512, number of dummy scans: 2, spectral width: 20 ppm, acquisition time: 2.75 s, delay d1: 1 s. ¹³C NMR parameters: temperature: 298 K, number of scans: 3,096, number of dummy scans: 4, spectral width: 240 ppm, acquisition time: 0.92 s, delay d1: 2 s. The tetramethylsilane (TSP) was used (δ 0.00 ppm) for quantification and calibration of the spectrum.

For the degradation of fluorescent dyes, its fluorescence characteristics could not be ignored. Therefore, it was necessary to investigate the effect of the fluorescence characteristics of HPTS on its degradation. PTSA (1,3,6,8-pyrenetetrasulfonic acid, sodium salt) and HPTS have similar structures; its chemical structure is shown in Figure 4. Due to the high symmetry of the PTSA molecular structure, its fluorescence characteristics are much stabler and higher than that of HPTS. By comparing the emission spectra and degradation kinetics of HPTS and PTSA, the effect of fluorescence characteristics of fluorescent dyes on the degradation of the UV/H₂O₂ process could be investigated (Xiang et al. 2016). Figure 5 shows the fluorescence emission spectra of HPTS and PTSA with different concentrations under the same conditions measured by fluorescence spectrophotometer (F-4600, Hitachi, Japan). It could be seen that the fluorescence intensity of PTSA at 254 nm was much higher than that of HPTS.

Figure 6 shows the degradation kinetics of HPTS and PTSA under different initial concentrations. As can be seen in Figure 6(a) and 6(b), the degradation efficiency of HPTS decreased with the increase of HPTS concentration. When the initial concentrations of HPTS were 10, 20, 30 and 40 mg/L, the degradation efficiency were 86.9%, 82.2%, 80.1%, and 77.6%, The HPTS degradation data fitted a pseudo-second-order kinetics model and the second-order kinetics rate constants were 0.1974, 0.063, 0.039, and 0.026 L·mg⁻¹·min⁻¹. As can be seen in Figure 6(c) and 6(d), when the initial concentrations of PTSA were 10, 20, 30 and 40 mg/L, the degradation efficiency were 73.6%, 51.7%, 38.4%, and 31.2%, The HPTS degradation data fitted a pseudo-first-order kinetics model and the first-order kinetics rate constants were 0.0114, 0.0063, 0.0043, and 0.0032 min⁻¹.

Through comparison, HPTS and PTSA only had great difference in fluorescence intensity, but the degradation efficiency and degradation kinetics constants in UV/H₂O₂ process were quite different. Therefore, it could be concluded that the fluorescence characteristics of degradation substrate have great influence on the degradation of UV/H₂O₂ process.

In section 3.1, the effect of different initial concentration of HPTS was investigated. There were two main reasons for explaining this (Figure 7): with the initial concentration increasing, (i) more ultraviolet light converted into visible light due to the fluorescence characteristics, which reducing the efficiency of UV light in the degradation process. (ii) the number of hydroxyl radicals allocated to a single HPTS molecule decreased, then competition between molecules increased.

By adding hydrogen peroxide, the degradation efficiency of HPTS could be further improved. In the experiment, the concentration of 20 mg/L HPTS was chosen as the substrate because the suitable degradation efficiency made the results more presentable. The effect of hydrogen peroxide concentration on the degradation kinetics is shown in Figure 8. As can be seen from Figure 8(a), the suitable hydrogen peroxide concentration is between 1 mg/L and 5 mg/L, and the degradation time reduces from 2 hours to 1 hour. As can be seen from Figure 8(c), the degradation efficiency of HPTS increases with the increase of hydrogen peroxide concentration. When the concentrations of H₂O₂ were 1.0, 2.0, 3.0, 4.0, and 5.0 mg/L, the degradation efficiencies were 82.74%, 89.53%, 96.40%, 98.4%, and 98.8%. However, excessive use of hydrogen peroxide would increase actual costs and the dosages of hydrogen peroxide needed to be control according to the actual situation. To achieve a degradation efficiency of HPTS more than 95%, it could be seen that when the concentration of hydrogen peroxide was 3 mg/L, the degradation process was the most economical and fastest.

As can be seen in Figure 8(b) and 8(d), the HPTS degradation data fits a pseudo-first-order kinetic model and the first-order kinetics rate constants are 0.0305, 0.0443, 0.0724, and 0.0842 min⁻¹_.

There were some interference factors in wastewater, such as pH, and inorganic anions, which might affect the degradation (Yang et al. 2014; Wang et al. 2016; Zhang et al. 2016a; Lee et al. 2018). The common inorganic anions in wastewater are chloride ions, sulfate ions, and nitrate ions, so the impact of pH, Cl⁻, SO²⁻₄, and NO⁻₃ on the degradation of HPTS were mainly investigated. As can be seen in Figure 9, the results are as follows:

• (i)

  pH: When the pH value of the solution was 1, 4, 7, and 10, the first-order kinetics rate constants were 0.0577, 0.0582, 0.0583, and 0.053 min⁻¹. It could be concluded that the pH conditions did not affect HPTS degradation efficiency.

• (ii)
  Cl⁻: When the chloride ions concentration was 500 mg/L, and the pH values of the solution were 1, 4, 7, and 10, the first-order kinetics rate constants were 0.0307, 0.0497, 0.0489, and 0.0466 min⁻¹. Under strongly acidic conditions, when there were large amounts of chloride ions in solution, the degradation efficiency of HPTS decreases and the reaction stops at about 40 minutes. The explanation was that the chloride ions react with the hydroxyl radicals and produce the secondary free radicals of chlorine. The oxidation effect of the secondary radicals was weaker than that of the hydroxyl radicals, which led to a decrease in the degradation effect of HPTS:

  

  (2)

  

  (3)

  

  (4)
• (iii)

  SO²⁻₄: When the sulfate ions concentration was 500 mg/L, and the pH values of the solution were 1, 4, 7, and 10, the first-order kinetics rate constants were 0.0508, 0.0534, 0.0527, and 0.0476 min⁻¹. It could be concluded that sulfate ions did not affect HPTS degradation efficiency.

• (iv)

  NO⁻₃: When the nitrate ions concentration was 50 mg/L, and the pH values of the solution were 1, 4, 7, and 10, the first-order kinetics rate constants were 0.0499, 0.0454, 0.0427, and 0.038 min⁻¹. It could be concluded that nitrate ions did not affect HPTS degradation efficiency.

There have been many types of research into the degradation of polycyclic aromatic hydrocarbons (PAHs) and their derivatives (Launen et al. 2000; Choi et al. 2014; Soni et al. 2017; Al Farraj et al. 2020; Li et al. 2020a, 2020b; Zhou et al. 2020a, 2020b). The most common view of pyrene degradation was that pyrene was oxidized to pyrene-4,5-diol, phenanthrene derivatives, and then further oxidized to naphthalene and benzene derivatives, and finally degraded to water and carbon dioxide. In our experiment, HPLC-MS and NMR were used to investigate the degradation pathways of HPTS and identify the intermediate products. The structures of the intermediates were determined by ¹H NMR and ¹³C NMR then verified by HPLC-MS. Through these intermediates, the degradation pathways of HPTS were further speculated. The results of NMR are shown in Figures 10 and 11. From the carbon spectrum, the overall position of the peak shifted before and after degradation, which might be due to the change of pH value of the solution (solvent effect, Table 1.). However, no new peaks were observed, indicating that no new complex structure appeared after degradation. From the hydrogen spectrum, only six peaks appeared in the spectrum of HPTS solution before degradation because hydrogen on the hydroxyl groups exchanged with deuterium on deuterium oxide. After degradation, there were new peaks (δ: 7̃8 ppm; δ: 10.5̃10.8 ppm). The peaks (δ: 7̃8 ppm) were related to naphthalene derivatives. The appearance of the peak (δ: 10.5̃10.8 ppm) was related to hydrogen on the phenolic hydroxyl group (1-hydroxypyrene) or aldehyde group (intermediate products with the aldehyde group). For the former, when the sulfonic group was lost from the pyrene ring of HPTS, the chemical shift of hydrogen on the hydroxyl group would increase. At the same time, the decrease of pyrene ring solubility would slow down the exchange rate between hydroxyl hydrogen and deuterium on deuterium oxide, so the intensity of the peaks (δ: 10.5̃10.8 ppm) increased.

Based on those, the degradation pathways of HPTS were speculated as Figure 12. Under the action of hydroxyl radical, HPTS firstly removed three sulfonic groups around the pyrene ring and formed into 1-hydroxypyrene; then, the hydroxyl group and its symmetrical position ring were oxidized and formed into 1,4,5,8-naphthalenetetracarboxylic acid. Due to decarboxylation reaction, 1,4,5,8-naphthalenetetracarboxylic acid produced 1-naphthalenecarboxylic acid and then naphthalene. After that, the naphthalene was further oxidized to form o-hydroxybenzoic and then benzoic acid. Finally, these products degraded to water and carbon dioxide. By HPLC-MS, most of the intermediate products had been found (Table S1 and Figures S3–S6). The comparison of the UV spectrum before and after the reaction also confirmed our last conjecture (Figure S7).

By comparing the total organic carbon (TOC) of HPTS solution before and after degradation, the degradation efficiency could be better understood. According to the experimental procedure on the effect of hydrogen peroxide concentration, the TOC removal of 20 mg/L HPTS solution under different hydrogen peroxide concentrations and fluence time was investigated. The experimental data were measured by a TOC analyzer (TOC-L CPH, Shimadzu, Japan) and the results are shown in Figure 13. When the UV fluence time was 1 h and the concentration of hydrogen peroxide was 3 mg/L, the TOC removal rate of HPTS was 47%, this indicated that there were still some intermediate products in the solution.

In this study, the effects of some factors on the photodegradation of HPTS were investigated and the degradation pathways were inferred. The conclusions are as follows:

• (i)

  Comparing HPTS with PTSA, it was found that the fluorescence characteristics decreased the degradation rate of fluorescent dyes. The degradation efficiency of HPTS decreased with the increase of initial concentration, which might be related to the fluorescence characteristics and the relative content of hydroxyl radicals and HPTS in solution.

• (ii)

  With the increasing H₂O₂ concentration, the HPTS degradation efficiency correspondingly increased. When the initial concentration of HPTS was 20 mg/L and the concentration of H₂O₂ was 3 mg/L, the efficiency and cost of degradation were most coordinated. At high concentration and work alone, chloride, sulfate, and nitrate ions did not affect degradation, but in strong acid solution, chloride ions had an obvious inhibition effect on degradation.

• (iii)

  The degradation pathways of HPTS were obtained: under the action of the hydroxyl radical, through oxidation and decarboxylation reaction, HPTS was gradually decomposed into naphthalene and benzene derivatives, and finally completely converted into water and carbon dioxide.

• (iv)

  The degradation kinetics model was fitted with the degradation data and the degradation kinetics rate constants were calculated. The TOC removal of the degradation was also calculated, when the concentration of H₂O₂ was 3 mg/L and fluence time was 1 h, The TOC removal of the degraded solution was 47%.

PTSA (1,3,6,8-pyrenetetrasulfonic acid, sodium salt) and HPTS have similar structures, but its fluorescence intensity was higher in aqueous solution. The study on the degradation of PTSA will confirm more about the effect of fluorescence characteristics in the AOPs (UV/H₂O₂) and the degradation mechanism of PAHs derivatives.

We thank Haiyan Sui and Xiaoju Li from State Key Laboratory of Microbial Technology in Shandong University for NMR measurements.

All relevant data are included in the paper or its Supplementary Information.