Degradation of indigo carmine by coupling Fe(II)-activated sodium persulfate and ozone in a rotor-stator reactor

Degradation of indigo carmine by coupling Fe(II)-activated sodium persulfate and ozone in a rotor-stator reactor

Chemical Engineering & Processing: Process Intensification 148 (2020) 107791 Contents lists available at ScienceDirect Chemical Engineering & Proces...

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Chemical Engineering & Processing: Process Intensification 148 (2020) 107791

Contents lists available at ScienceDirect

Chemical Engineering & Processing: Process Intensification journal homepage: www.elsevier.com/locate/cep

Degradation of indigo carmine by coupling Fe(II)-activated sodium persulfate and ozone in a rotor-stator reactor

T

Ziye Zhaoa,b, Lei Wanga,b, Jinmeng Fana,b, Yunhua Songa,b, Guangwen Chua,b, Lei Shaoa,b,* a b

State Key Laboratory of Organic-Inorganic Composites, Beijing University of Chemical Technology, Beijing 100029, China Research Center of the Ministry of Education for High Gravity Engineering and Technology, Beijing University of Chemical Technology, Beijing 100029, China

A R T I C LE I N FO

A B S T R A C T

Keywords: Rotor-stator reactor Indigo carmine Ozone Persulfate Ferrous ion Degradation

Rotor-stator reactor (RSR), a novel high-gravity device, was employed in the O3/persulfate (PS)/Fe2+ process to enhance the degradation of indigo carmine (IC) in simulated wastewater. It was found that IC degradation efficiency increased with the increase of Fe2+ concentration, RSR rotation speed, O3 concentration and gasliquid ratio, but decreased with the increase of IC concentration and temperature. When the rotation speed of RSR increased from zero to 1200 rpm, IC degradation efficiency increased from 25.6% to 64.2%. Because of the synergetic effects of the oxidative species (O3, %OH, SO4−% etc.), IC degradation efficiency in the O3/PS/Fe2+ process was significantly higher than that in the O3 or PS/Fe2+ process. The high IC degradation efficiency can also be attributed to the good micromixing effect of the RSR. When the O3/PS/Fe2+ process in the RSR was adopted before the sequencing batch reactor activated sludge process, both IC and COD degradation efficiencies increased significantly as a result of the improved biodegradability of the IC wastewater. Furthermore, a possible degradation mechanism of IC in the O3/PS/Fe2+ process was proposed.

1. Introduction Synthetic dyes are one of the most important chemical products and widely used in many fields. However, they are also one of the main pollutants in water. Textile, dyeing, paper and pulp, tannery and paint, as well as dye manufacture industries discharge a large amount of wastewater with dyes to the environment [1]. Most synthetic dyes have the azo, anthraquinone, indigoid, triphenylmethyl or phthalocyanine structure, and thus the dye wastewater is toxic and carcinogenic. Besides, dyes in wastewater can be resistant to sunlight and hinder the photosynthesis of aquatic plants and algae [2,3]. Therefore, it is necessary to eliminate dyes in wastewater in order to prevent their harmfulness to human health and maintain the balance of natural ecology [4]. Dye wastewater can be treated by physical, chemical and biological methods [5–7]. Advanced oxidation processes (AOPs) are efficient chemical methods and have been widely used for dye wastewater treatment. AOPs are defined as the oxidation processes based on radicals such as hydroxyl radicals (%OH) and sulfate radicals (SO4−%) [8]. SO4−% (E0 = 2.5–3.1 V, half-life ˜ 4 s) can be produced through activation of persulfate anion (S2O82−) by heat [9], ultraviolet irradiation [10], or transitional metals [11]. Compared to %OH (E0 = 2.7 V, halflife ˜ 10−4 s), SO4−% have a much longer half-life, which makes SO4−%⁎

based AOPs a promising approach to either partially or completely mineralize organic compounds [12]. In addition, S2O82− itself is also a strong oxidant with the oxidation potential of 2.0 V and can react with organic pollutants directly [13]. Persulfate (PS) oxidation is considered to be relatively harmless and environment-friendly because of the high stability, solubility and safety of PS. In order to enhance the removal of pollutants, PS oxidation is often combined with other techniques to take advantage of their synergetic effects [14]. It is found that the combination of PS and O3 had better performances on leachate treatment than PS only or O3 only, where 72% of COD, 93% of color and 55% of NH3-N were removed under optimal conditions. Beltrán et al. [15] reported that the mass transfer of O3 is the limiting step in the O3-based processes in wastewater treatment. Therefore, a reactor with good gas-liquid mass transfer performance is desirable in the PS and O3 coupled processes. Rotor-stator reactor (RSR) is a novel high-gravity device and has excellent mass transfer and micromixing effects [16,17]. RSR comprises a series of concentric rotor-rings and stator-rings alternately configured in the radial direction. When the rotor-rings rotate, a simulated high gravity environment is generated by centrifugal force, where fluids experience violent turbulence and liquid is split into tiny droplets or thin films. Thus, the gas-liquid mass transfer process is greatly enhanced [18].

Corresponding author at: State Key Laboratory of Organic-Inorganic Composites, Beijing University of Chemical Technology, Beijing 100029, China. E-mail address: [email protected] (L. Shao).

https://doi.org/10.1016/j.cep.2019.107791 Received 3 June 2019; Received in revised form 18 November 2019; Accepted 13 December 2019 Available online 23 December 2019 0255-2701/ © 2019 Elsevier B.V. All rights reserved.

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countercurrently in the RSR to achieve the absorption of ozone and degradation of IC by O3/PS/Fe2+. Liquid samples were taken for IC concentration, COD or the liquid chromatography-mass spectrometry (LC–MS) analyses after all the IC solution passed the RSR. The experiments with the PS/Fe2+ process or O3 process were performed in the same way as those with the O3/PS/Fe2+ process but without the addition of O3 or PS/Fe2+.

In this work, indigo carmine (IC), one of the most important and oldest dyes, was used as the target pollutant to simulate dye wastewater. The effects of Fe2+ concentration, RSR rotation speed, O3 concentration, gas-liquid ratio, IC concentration, and temperature on IC degradation with the O3/PS/Fe2+ process in an RSR were investigated. Meanwhile, IC degradation efficiency in the O3/PS/Fe2+ process was compared with that in the O3 or PS/Fe2+ process. In addition, the sequencing batch reactor activated sludge process (SBR) experiments following the O3/PS/Fe2+ pre-oxidation process were carried out to simulate the process of real wastewater treatment plants where biological methods are generally employed. Furthermore, the possible degradation mechanism of IC in the O3/PS/Fe2+ process was proposed.

2.2.2. SBR experiments Firstly, the activated sludge had stood in a beaker for 24 h before the supernatant was removed. The effluent from the O3/PS/Fe2+ process was added into the activated sludge beaker to reach a volume of 1 L (SV30 = 25–30%). Subsequently, the suspension was aerated by an air pump. The IC and COD degradation efficiencies were tested every 24 h. After each test was completed, about one quarter of the suspension was removed and equal amount of fresh effluent from the O3/PS/Fe2+ process was supplemented. Varying amount of glucose was added into the suspension every 24 h according to the activity of the sludge. Finally, the average values of IC and COD degradation efficiencies were calculated based on the data of about 20 days.

2. Materials and methods 2.1. Materials and reagents Indigo carmine (C16H8N2Na2O8S2, CAS:860-22-0, 96%) was purchased from Aladdin Ind. Co., Shanghai, China. Sodium persulfate (Na2S2O8, CAS:7775-27-1, AR) and iron(II) sulfate heptahydrate (FeSO4·7H2O, CAS:7782-63-0, AR) were purchased from Sinopharm Chemical Reagent Co., Ltd., China. Glucose (C6H12O6·H2O, CAS: 50-997, AR) was purchased from Beijing Chemical Works, China. Activated sludge was taken from the secondary clarifier of a wastewater treatment plant in China. All aqueous solutions were prepared with deionized water.

2.3. Analytical methods The concentration of IC was measured with a UV–vis spectrophotometer (DR6000, Hach, America) at 610 nm, and the IC degradation efficiency (λIC) is defined as

λIC =

2.2. Experimental

(CIC 0 − CIC ) × 100% CIC 0

(1) −1

2.2.1. RSR experiments The schematic diagram of the RSR experiments with the O3/PS/ Fe2+ process is shown in Fig. 1. The IC solution, with an initial concentration, pH and COD of 200 mg L−1, 7.3 and 200 mg L−1 respectively, was divided into two equal parts: one was added with Na2S2O8 while the other with FeSO4·7H2O. The solutions were stirred and heated by a magnetic stirrer in a thermostat. Ozone was produced from air using an ozone generator (Tonglin Tech. Co. Ltd., Beijing, China) and gaseous ozone concentration was monitored by an ozone concentration analyzer (Tonglin Tech. Co. Ltd., Beijing, China). The ozone-containing gas was introduced into the RSR via the gas inlet when O3 concentration in the gas stream reached stable. Then the IC/Na2S2O8 solution and IC/FeSO4·7H2O solution were pumped into the RSR via two liquid inlets respectively with the same liquid flow rate and flowed outwards in the packing under the action of centrifugal force. The gas and the liquid streams contacted

where CIC0 is the initial IC concentration (mg L ), CIC is the IC concentration after treatment (mg L−1). Chemical Oxygen Demand (COD) was determined spectrophotometrically by a COD analyzer (5B-3A, Lian-hua Tech. Co. Ltd., Lanzhou, China) according to a Chinese national standard method (HJ/ T399-2007). The COD degradation efficiency (ηCOD) is defined as

ηCOD =

(ηCOD0 − ηCOD ) ηCOD0

× 100%

(2) −1

where ηCOD0 is the initial COD of the IC solution (mg L ), ηCOD is the COD of the IC solution after treatment (mg L−1). The intermediates in IC degradation were determined by LC–MS (Waters Xevo G2-XS QTof, America). One microliter of the sample was injected into the device and separated by a C18 column at 40 °C. The mobile phase consisted of water and acetonitrile (gradient elution: 95% H2O for 0−2 min, 60% H2O for 2−6 min, 5% H2O for 6–9.1 min, 95% H2O for 9.1−11 min) with a flow rate of 0.3 mL min−1. Mass spectra were acquired over the m/z range of 50–1200 in the negative mode. 3. Results and discussion 3.1. Effects of operating parameters on IC degradation with the O3/PS/ Fe2+ process in RSR The effects of Fe2+ concentration, RSR rotation speed, O3 concentration, gas-liquid ratio, IC concentration, and temperature on IC degradation with the O3/PS/Fe2+ process in the RSR were investigated, and the results are shown in Fig. 2. Fig. 2a shows the effect of Fe2+ concentration on IC degradation efficiency, which increased from 40.0% to 67.6% when Fe2+ concentration increased from 0 to 0.2 mmol L−1. PS can be activated by Fe2+ rapidly via the following reaction [19]:

S2 O82- + Fe2+→Fe3+ + SO24− + SO−4 • 2+

An increasing Fe concentration promoted the formation of which is a strong oxidant, thus more IC was degraded by

Fig. 1. Diagram of RSR experiments. 2

(3) SO4−%, SO4−%,

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Fig. 2. Effects of operating parameters on IC degradation with the O3/PS/Fe2+ process in RSR (all parameters except that in investigation are as follows: T = 25 °C, R = 600 rpm, pH = 7.3, CIC = 200 mg L−1, CPS = 0.2 mmol L−1, CFe2+ = 0.05 mmol L−1, LIC = 0.5 L min−1, CO3 = 2.5 mg L−1, GO3 = 0.5 L min−1): (a) Fe2+ concentration. (b) RSR rotation speed. (c) O3 concentration. (d) Gas-liquid ratio. (e) IC concentration. (f) Temperature.

phases because of the low solubility of O3. The driving force for O3 mass transfer was elevated with an increasing O3 concentration, resulting in more O3 transferring into liquid and reacting with IC. Besides, more % OH were generated with an increasing O3 concentration and caused a higher IC degradation efficiency [15,16]. Compared with our group’s study on methyl orange degradation with the O3/PS/Fe2+ process in a rotating packed bed (RPB) [20], a higher degradation efficiency of IC was achieved with a low O3 concentration in the RSR. Methyl orange degradation percentage was 84% with an O3 concentration of 40 mg L−1 in the RPB, while IC degradation percentage reached 93.8% with an O3 concentration of 12.5 mg L−1 in the RSR. The higher degradation efficiency of IC in the RSR may be ascribed to the better micromixing effect of RSR than RPB [17], allowing a good contact of the oxidative species (O3, %OH, SO4−% etc.) with IC. Because the chromophores are C]C bond in IC and N]N bond in methyl orange, these results also suggest that the cleavage of the C]C chromophore in IC by the oxidative species may be easier than the cleavage of the N]N chromophore in methyl orange. Fig. 2d illustrates the effect of gas-liquid ratio on IC degradation efficiency. The gas-liquid ratio was adjusted by changing the gas flow rate at a liquid flow rate of 0.5 L min−1. IC degradation efficiency

exhibiting a positive correlation between IC degradation efficiency and Fe2+ concentration. Fig. 2b presents the effect of RSR rotation speed on IC degradation efficiency. About 25.6% of IC was degraded when the RSR did not rotate. When the RSR rotated at 200 rpm, IC degradation efficiency went up significantly to 52.2%, and further increased to 64.2% when the rotation speed increased to 1200 rpm. The limiting step of ozonation process is the mass transfer between gas and liquid, and good mass transfer favors the degradation of pollutants [15]. With an increasing rotation speed, liquid was dispersed better and split into smaller droplets as well as thinner liquid films, resulting in a larger contact area between gas and liquid. Also, a higher renewal rate of the gas-liquid interface was achieved, which was beneficial to the mass transfer between O3 and IC solution. In addition, two streams of IC solution containing Fe2+ and PS respectively were mixed more efficiently, facilitating the generation of SO4−%. All these factors contributed to a higher IC degradation efficiency when the RSR rotation speed increased. Fig. 2c indicates that the effect of O3 concentration on IC degradation efficiency, which increased from 55.2% to 93.8% with an increasing O3 concentration from 2.5 to 12.5 mg L−1. The degradation process is limited by the mass transfer between the gas and liquid 3

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increased from 55.2% to 75.3% with an increasing gas-liquid ratio from 1 to 3. A higher gas-liquid ratio is beneficial to the mass transfer between gas and liquid phases [20], leading to an increased absorption of ozone and a higher IC degradation efficiency. Fig. 2e demonstrates the effect of IC concentration on IC degradation efficiency. The degradation efficiency of IC decreased significantly from 86.5% to 11.4% with an increasing IC concentration from 100 to 500 mg L−1. The IC amount to be treated in unit time and volume increased with a rising IC concentration while the oxidant supply was fixed. Therefore, a higher IC concentration caused a lower IC degradation efficiency. Fig. 2f depicts the effect of temperature on IC degradation efficiency. The degradation efficiency decreased from 55.2% to 17.2% when temperature increased from 25 to 65 °C. PS can be activated by heat, even at room temperature [9], because the energy input can cause the fission of OeO bond to form SO4−% [21]. An increasing temperature favors the activation of PS and boosts reaction rate, which can increase the degradation efficiency of pollutants. However, the increasing temperature also leads to the quenching reaction of SO4−%, decreasing the removal efficiency of targeted pollutants [22]. In addition, high temperature also causes a low O3 solubility, which is unfavorable for pollutants removal. In this work, the adverse effect of elevated temperature predominated during the degradation process of IC, resulting in a decreasing IC degradation efficiency.

Table 2 Comparison of IC and COD degradation efficiencies in the O3/PS/Fe2+, SBR and O3/PS/Fe2++SBR processesa.

(FeO)2+ + H2 O→Fe3+ + •OH + OH−

(5)

Fe2+ + O3→Fe3+ + O−3

(6)

O−3 + H+→HO3→O2 + •OH

(7)

O3/PS/Fe2+

λIC

40.0%

41.9%

96.9%

55.2% 6.3%

32.7% 23.9%

99.5% 36.3%

The degradation mechanism of IC in the O3/PS/Fe2+ process was speculated with the help of LC–MS analyses. The main oxidants are SO4−% and %OH, which are formed through the activation of PS and O3 by Fe2+. Ozone molecules are also present as the oxidant. In negative ion mode LC–MS analyses, IC and its degradation products exist mainly in the form of anions in water. Analyses on the initial IC solution revealed that m/z 209.9861 represents the double-charged anion of IC. After the IC solution was treated in the O3/PS/Fe2+ process, the peak at m/z 209.9861 disappeared, indicating that IC was consumed (Fig. 3). Besides, peaks at m/z 96.9589, 197.9805, 225.9802 and 243.9905 can be ascribed to HSO4−, C7H4NO4S, C8H4NO5S and C8H6NO6S respectively. Based on the LCeMS analyses, the degradation mechanism of IC can be inferred as follows. The C]C chromophore in IC was attacked firstly by oxidants during the degradation of IC (Fig. 4). After the bond was cleaved, isatin sulfonic acid (C8H4NO5S−) was detected by LCeMS. As the oxidation process continued, it was deduced that C8H4NO5S produced two kinds of single-charge anions: 2-amino-α-oxo-5-sulfo-benzeneacetic acid (C8H6NO6S−) and 2-amino-5-sulfo-yl-benzoate (C7H4NO4S−). Carbonyl group and carboxyl group were subsequently broken. Dissociation of sulfo group on aromatic ring resulted in the generation of hydrogen sulfate ions and anilines, and oxidation of anilines led to the generation of phenols and benzoquinones. Further oxidation broke the aromatic rings into chains, then they were oxidized to form many small inorganic compounds such as CO2 and H2O [25–32]. 4. Conclusion

Table 1 Comparison of IC degradation efficiency in the O3, PS/Fe2+, and O3/PS/Fe2+ processesa. PS/Fe2+

λIC ηCOD

3.4. Degradation mechanism of IC in the O3/PS/Fe2+ process

The AOPs, as pre-treatment or post-treatment processes, are generally combined with the biological methods to balance efficiency and cost. To simulate the real processes in wastewater treatment plant, this work combined the O3/PS/Fe2+ and SBR processes, and compared the performances of the O3/PS/Fe2+, SBR, and O3/PS/Fe2++SBR processes (Table 2). Table 2 shows that the degradation efficiency of the IC solution by SBR increased significantly after a pre-treatment by the O3/PS/Fe2+

O3

O3/PS/Fe2+ + SBR

process. Although O3 and PS concentrations in this study were much lower than those in our previous work [20], IC and COD degradation efficiencies rose to 99.5% and 36.3% respectively in the O3/PS/Fe2+ + SBR process compared with only 32.7% and 23.9% respectively in the SBR process. It can also be seen from Table 2 that IC and COD degradation efficiencies in the O3/PS/Fe2+ process were 55.2% and 6.3% respectively, indicating that the O3/PS/Fe2+ process enhanced the biodegradability of the IC solution and caused a significant boost of the IC and COD removal when a subsequent SBR treatment was employed. The application of microorganisms on wastewater treatment depends on the properties of pollutants because some toxic or recalcitrant organic compounds are adverse to the biological process [24]. In this work, the oxidative species in the O3/PS/Fe2+ process reacted with IC and produced biodegradable intermediates, which were easily degraded in the SBR treatment. Therefore, the O3/PS/Fe2+ + SBR process showed an improved performance on IC and COD degradation.

3.3. Effects of the O3/PS/Fe2+ process on the SBR treatment

Processes

SBR

T = 25 °C, pH = 7.3, CIC = 200 mg L−1, CPS = 0.2 mmol L−1, CFe2+ = 0.05 mmol L−1, CO3 = 2.5 mg L−1, R = 600 rpm, G/L = 1.

IC degradation efficiencies with the O3, PS/Fe2+ and O3/PS/Fe2+ processes in the RSR are shown in Table 1. The degradation efficiency reached 96.9% in the O3/PS/Fe2+ process compared with 40.0% and 41.9% in the O3 and PS/Fe2+ processes, respectively. The O3/PS/Fe2+ process exhibited a much better effect than the O3 and PS/Fe2+ processes probably because more %OH and SO4−% were generated, benefiting the removal of IC. It is deduced that the synergetic effects of O3/ PS/Fe2+ promoted the formation of these oxidative species. PS can be activated by Fe2+ to produce SO4− %as shown in Eq.3, and O3 can be activated by Fe2+ to produce %OH as follows [23] (4)

O3/PS/Fe2+

a

3.2. Comparison of the O3, PS/Fe2+ and O3/PS/Fe2+ processes for IC degradation in RSR

Fe2+ + O3→O2 + (FeO)2+

Processes

In this work, the RSR experiments were conducted with the O3/PS/ Fe2+ process to investigate the effects of different factors on IC degradation. IC degradation efficiency increased with the increase of Fe2+ concentration, RSR rotation speed, O3 concentration and gas-liquid ratio, but decreased with the increase of IC concentration and temperature. The O3/PS/Fe2+ process had a higher IC degradation efficiency than the O3 or PS/Fe2+ process because of the synergetic effects

a T = 25 °C, pH = 7.3, CIC = 200 mg L−1, CO3 = 2.5 mg L−1, CFe2+ = CPS = 0.8 mmol L−1, R = 600 rpm, G/L = 1.

4

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Fig. 3. Liquid chromatogram and mass spectrum of the IC solution after treatment in the O3/PS/Fe2+ process. (Samples were prepared at T = 25 °C, pH = 7.3, CIC = 200 mg L−1, CPS = 0.2 mmol L−1, CFe2+ = 0.05 mmol L−1, CO3 = 2.5 mg L−1, R = 600 rpm, G/L = 1).

collected and verified most of the data, analyzed the LC–MS data and proposed the degradation pathway, contributed to the writing of the manuscript. Lei Wang gave advices on experiment design and assisted Ziye Zhao in measurements, interpreted the results, drafted the original manuscript and completed the final version. Ziye Zhao and Lei Wang had an equal contribution to this study. They worked together on experiments procedure, results analyses and manuscript writing. Jinmeng Fan assisted in conducting the experiments and data analyses. Yunhua Song designed and developed the rotor-stator reactor (RSR) used in this work. Guangwen Chu helped design and develop the RSR used in this work. Lei shao led this study as the corresponding author. He conceived and supervised the study, managed the execution during all the process and gave critical help on manuscript writing. Declaration of Competing Interest The authors declare that they have no conflict of interest. Acknowledgement This work was supported by the National Natural Science Foundation of China (No. 21676008). Appendix A. Supplementary data Supplementary material related to this article can be found, in the online version, at doi:https://doi.org/10.1016/j.cep.2019.107791. Fig. 4. Degradation mechanism of IC in the O3/PS/Fe2+ process.

References

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