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Resolving the Thermoinduced Electrochemistry for an In-Depth Understanding of the STEP Degradation of SDBS Di Gu, Yiyang Zhang, Lingyue Zhu, and Baohui Wang* Provincial Key Laboratory of Oil & Gas Chemical Technology, Institute of New Energy Chemistry and Environmental Science, College of Chemistry and Chemical Engineering, Northeast Petroleum University, Daqing 163318, P. R. China S Supporting Information *

ABSTRACT: The solar thermal electrochemical process (STEP) has sustainably accounted for the solar thermo- and electrochemical oxidation of sodium dodecyl benzene sulfonate (SDBS) fully driven by solar energy, gaining a high efficiency with a fast rate by the combination of thermochemistry and electrochemistry. In this article, thermoinduced electrochemistry was resolved for an in-depth understanding of the STEP degradation of SDBS. We employed thermodependent cyclic voltammetry, temperature-dependent fluorescence-electrochemical spectroscopy, and time-dependent electrochemical current spectroscopy for studying the electrochemistry, including the reaction, pathway, and mechanism. First, thermodependent cyclic voltammetric spectra indicated that the SDBS in sodium chloride solution is oxidized via an indirect process initialized by active chlorine, substantially accelerating and completing the oxidation process. Second, temperature-dependent fluorescence-electrochemical spectra displayed the pathway and kinetics by finding the initial desulfonation and the subsequent breaking of the alkyl side chain and benzene ring. Finally, time-dependent electrochemical current spectra demonstrated that the initial desulfonation is the fast step by generating the high current and the subsequent breaking is the slow one by a low current response, which is in agreement with the temperature-dependent fluorescenceelectrochemical spectra. A panoramic view is proposed and schemed for fully understanding the process and mechanism of the STEP degradation of SDBS. Moreover, the efficiency and effectiveness of SDBS degradation were proven to be significantly enhanced by using the STEP in outdoor and indoor tests. It is a novel and energy-free route for wastewater treatment, accomplished by the synergistic use of solar energy without any other input of energy.

1. INTRODUCTION At the present time, solar energy is considered a green, sustainable, safe, and abundant energy source, leading to efforts to focus on efficient solar-to-electrical and solar-to-heat conversions.1,2 The solar thermal electrochemical process (STEP) is an established and comprehensive solar thermoelectrochemical strategy to efficiently use solar energy in chemistry.3−5 By utilizing the energy obtained from solar-toheat conversion to increase the temperature of the degradation system, significant revolution has been achieved in the degradation rate of wastewater and COD. Here, we employ the STEP for a study of the efficient treatment of sodium dodecyl benzene sulfonate (SDBS) wastewater, which makes SDBS mineralize into CO2 and H2O completely, specifically by resolving the thermoinduced electrochemistry. SDBS as a kind of anionic surfactant with good performance, with the molecular structure presented in Figure 1, is commonly used as a raw material in a variety of detergents.6 The SDBS molecule consists of benzene and an alkyl side chain with sulfonic acid composition. Among them, the alkyl side chain showed hydrophobic properties; sulfonic acid showed hydrophilic properties. With a passivation agent on the benzene © 2017 American Chemical Society

ring, the molecular structure of SDBS becomes very stable. SDBS does not easily oxidize under normal conditions, thus resulting in its incomplete degradation, causing serious environmental pollution.7,8 The conventional wastewater treatment methods are relatively inefficient in SDBS removal,9 leading to SDBS detection in the natural water environment.10,11 The response studies of organisms to the toxicity of SDBS showed that the enzymatic activity and brain acetyl cholinesterase were influenced by SDBS.12 The activity of aquatic reduced immediately after exposure in SDBS with the concentration of 12.5 mg/L, and someone performed abnormally behavior and high speed swimming when the concentration increased, some aquatic even died after exposure in SDBS with the concentration of 20 mg/L for 71−80 min.13 Therefore, handling SDBS wastewaters is a serious environmental challenge, and much effort is required to develop an efficient and fast process serving as an energy-saving and sustainable treatment. Received: December 6, 2016 Revised: February 3, 2017 Published: February 7, 2017 1900

DOI: 10.1021/acs.jpcb.6b12272 J. Phys. Chem. B 2017, 121, 1900−1907

Article

The Journal of Physical Chemistry B

Figure 1. Structure of the SDBS molecule.

Figure 2. Thermodependent cyclic voltammetric spectra of SDBS in sodium sulfate solution (a) and sodium chloride solution (b).

2. EXPERIMENTAL SECTION 2.1. Chemicals and Materials. The SDBS wastewater was prepared with deionized water. SDBS (C18H29NaO3S, 99.0%), sodium sulfate (Na2SO4, AR), and sodium chloride (NaCl, AR) were purchased from DM Tianjing Reagent Co., Ltd. and were used as received. 2.2. Measurement of the Thermodependent Cyclic Voltammetric Spectra. Thermodependent cyclic voltammetry experiments were performed on a BAS Epsilon-EC electrochemical workstation at a sweep rate of 50 mV s−1. A conventional three-electrode system was used, with Pt sheets (2 cm × 2 cm) as both working electrode and counter electrode, and Ag/AgCl as the reference electrode (CH111, ChenHua). Cyclic voltammetry was performed with a high concentration of SDBS (1000 mg/L, SI) to distinguish the peaks with a change in temperature. 2.3. Measurement of Thermodependent Electrochemical Spectra. 2.3.1. Indoor Studies of STEP SDBS Oxidation. A Pt sheet (2 cm × 2 cm) working electrode and a stainless steel sheet counter electrode were used in the SDBS wastewater degradation experiment. The concentration of SDBS was 50 mg/L in sodium chloride (NaCl, 5 g L−1) as the electrolyte. 2.3.2. Outdoor Tests of STEP SDBS Oxidation. The experimental outdoor apparatus, described in our previous paper15 and detailed in the Supporting Information (SI), consisted of three components, photoelectric, photothermal,

Organic pollutants were oxidized and decomposed rapidly after electrolysis at high temperatures with the adoption of the solar STEP concept.14 The thermodependent cyclic voltammetric spectra showed that different electrolytes have different effects on the degradation efficiency, indicating that SDBS in sodium chloride solution is oxidized via an indirect process initialized by active chlorine, significantly accelerating and completing the oxidation. Meanwhile, an indoor experiment was carried out to find the best concentration of sodium chloride solution in the electrolyte and the pH of the electrolyte and to study their effect on the degradation rate of SDBS and the COD. Outdoor pilot tests demonstrated that the STEP SDBS oxidation system can work successfully and efficiently without any other input of energy other than sunlight and without human intervention. Moreover, the pathway and kinetics were investigated and analyzed by the thermodependent cyclic voltammetric spectra, temperature-dependent fluorescence-electrochemical spectra, and time-dependent electrochemical current spectra. The mechanism of the STEP degradation of SDBS was proposed and discussed. The rate of SDBS degradation is significantly enhanced. Thus, a novel and energy-free route for wastewater treatment is accomplished by the synergistic use of solar energy without any other input of energy. 1901


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has been proven17 to occur more easily with increasing temperature in the presence of H+; therefore, part of SDBS is converted into alkylbenzene when the temperature goes up to 90 °C before the application of the electric field, in agreement with the fact that peak 1 at 90 °C is lower in intensity than those at 50 and 70 °C. This favors further electrolysis of alkylbenzene, corresponding to peak 2, to achieve energy saving and high-efficiency solar energy utilization. In addition, it can be seen from the right corner of Figure 2a,b that the oxidation peak of desulfonation began to decrease at a temperature of 70 °C and continued to decrease until 90 °C. This is due to the existence of active chlorine in the reaction system to strengthen the H+ attack on the benzene ring. Peaks 2 at 30 and 50 °C in Figure 2a,b showed that the peak intensity of anodic oxidation, which cannot be clearly distinguished from the peak of oxygen evolution, increased with increasing temperature, and simultaneously, the anodic oxidation peak potential decreased. The decrease in oxidation peak potential is consistent with the synergy of the thermodynamic potential drop with the temperature increase, combined with a kinetic overpotential drop expected with facilitated electron transfer at higher temperatures. The results are conducive to the inclusion of solar thermal energy to improve the degradation efficiency and decrease the energy for the STEP degradation of SDBS wastewater. For a particular case, both main reaction and side reaction occur in the oxidation of SDBS at the same time

and electrochemical units (Thermo-Electro-Reactor), to treat the wastewater containing SDBS, as shown in Figure 10. Timedependent electrochemical current spectroscopy (VICTOR multimeter, VC86E) was used to demonstrate the change in the current in the reaction system. The maximum output heat per unit time of the concentrator in the outdoor experiment was 55 kJ/min, and the amount of heat required for the reactor was only 27.3 kJ for heating from room temperature to 90 °C and thus more energy could be supplied to the reaction. An appropriate amount of energy is obtained by changing the axis of the symmetric face of the concentrator toward the Sun. 2.4. Measurement of Temperature-Dependent Fluorescence-Electrochemical Spectra. The pathway and kinetics were investigated and analyzed by thermodependent cyclic voltammetry and temperature-dependent fluorescenceelectrochemical spectroscopy (LS-55, PerkinElmer). The degradation rates of SDBS at different temperatures were determined using X-ray fluorescence spectrometry at a sweep rate of 50 nm min−1.

3. RESULTS AND DISCUSSION 3.1. Thermodependent Cyclic Voltammetric Spectra of SDBS. Cyclic voltammetry is a type of potentiodynamic

SDBS → CO2 (oxidation of SDBS, main reaction) water → O2 + H 2(water splitting, side reaction)

ESDBS‑oxidation can be equal to or greater than Ewater‑splitting in the majority of reactions. Hence, on the basis of thermodynamics, the remarkable shift will be attributed to the adjustment of the potential for a suitable separation of ESDBS‑oxidation and Ewater‑splitting, which is clearly shown in Figure 2a,b at temperatures of 70 and 90 °C. The reduction peaks in Figure 2 illustrate that the intermediate products of SDBS are reduced; however, the reduction peaks in Figure 2b disappear, corresponding to peak 3 in Figure 2a, indicating that the degradation intermediates of SDBS in sodium chloride solution are oxidized by active chlorine, accelerating the oxidation process of SDBS and making the oxidation more intensive. Compared with the cyclic voltammetric spectra of SDBS in sodium sulfate solution in Figure 2a, the position of the oxidation peak in Figure 2b is basically the same. However, there is an oxidation peak at 30 °C corresponding to peak 1 in Figure 3 (red line), illustrating that the oxidation of SDBS takes place in sodium chloride solution at the temperature of 30 °C. It can be concluded that SDBS can be oxidized in sodium chloride electrolyte at a low temperature. Therefore, SDBS is more easily oxidized with sodium chloride as the electrolyte. The thermodependent cyclic voltammetric spectra in Figures 2 and 3 (i) imply the feasibility of STEP theory used in the degradation of SDBS in wastewater; (ii) describe in detail the processes of thermo- and electrochemisty in different electrolytes, indicating that SDBS in sodium chloride solution is oxidized via an indirect process initialized by active chlorine, significantly accelerating and completing the oxidation process; (iii) guide the actual wastewater treatment process to choose the appropriate electrolytic potential and electrolytes. The STEP results are conducive to the inclusion of solar thermal

Figure 3. Cyclic voltammetric spectra of SDBS in different electrolytes at 30 °C.

electrochemical measurement used for detecting redox (reduction and oxidation) processes.16 Thermodependent cyclic voltammetric spectra were employed to study the thermoassisted electrochemical process in our investigations. The peak corresponds to one redox reaction; furthermore, the intensity of the peak corresponds to the intensity of the current, which in turn depends on the extent of the redox reaction. It can be seen from Figure 2a (and also from Figure 3) that there is no redox reaction taking place at the temperature of 30 °C, and corresponding to that there is no degradation of SDBS at this temperature in the system of sodium sulfate solution as the electrolyte. However, there are two peaks of anodic oxidation (peaks 1 and 2) at temperatures around 50 °C in a potential range toward positive scanning in Figure 2a,b, less than the oxygen evolution potential. When the electrolysis reaction is carried out under the condition of high temperatures, H+ as electrophilic reagents can directly attack the benzene ring to produce alkylbenzene via an electrophilic substitution reaction corresponding to peak 1. Desulfonation 1902


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Figure 4. Model of STEP degradation of SDBS in wastewater.

Figure 6. Degradation rates of SDBS and COD at different pH values. Figure 5. Degradation rate of SDBS and COD with different sodium chloride concentrations.

2Cl− + 2H 2O → Cl 2 ↑ +2OH− + 2e− −

−

Cl 2 + 2H 2O → ClO + Cl + 2OH

energy to improve the degradation efficiency and decrease the energy for the STEP electrolytic degradation of SDBS. This method demonstrates a substantial improvement in SDBS oxidation with the increase in solar heating and a decrease in the SDBS electrolysis potential with the increase in temperature. 3.2. Resolving the STEP Mechanism Originated from Electrolytes by the Thermodependent Cyclic Voltammetric Spectra. The direct oxidation reaction of organic compounds occurs in the system with sodium sulfate as the electrolyte (middle figure in Figure 4). Only a small amount of SDBS oxidation occurs at the anode this time, and it results in excessive production of intermediates. The situation is more complicated with sodium chloride as the electrolyte, where the following reactions will occur in the system18,19

−

(1) (2)

6ClO− + 3H 2O → 2HClO3 + 4HCl + 3[O] + 6e−

(3)

ClO− + H 2O + 2e− → Cl− + 2OH−

(4)

In the presence of sodium chloride, the degradation process in wastewater becomes fast. Part of Cl− is oxidized to Cl2 first, and then reacts with water to form an oxidizing agent, ClO− (eq 2), and part of ClO− reacts with water to generate HClO3 and [O] finally (eq 3). The formation of active chlorine is a transient process, which makes the mineralization become an indirect process, significantly accelerating and finishing the oxidation process. The presence of hydroquinone as the main intermediate shows that the mineralization mechanism is radically changed by the mediator, with respect to direct 1903


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Figure 7. Temperature-dependent fluorescence-electrochemical spectra of SDBS (scan rate 50 nm min−1). (a) Excitation and emission spectra of SDBS; fluorescence-electrochemical spectra of SDBS with the degradation temperature of 30 °C (b), 60 °C (c), and 90 °C (d).

Figure 8. Concentration−time curve (a) and the first-order kinetics of SDBS degradation (b).

ization themselves.19 Accordingly, the combined effect of strong oxidization of these products accelerated the degradation rate of SDBS in wastewater, as shown in the right figure in Figure 4.

oxidation. In contrast, in the active chlorine mediated oxidation, phenols, quinone, and other carboxylic acids are very unstable intermediate products, undergoing fast and complete mineral1904


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Electrochemical Spectra. Every carbon atom in the benzene ring has a 2p orbital, and the overlap of 2p orbital electrons generated a conjugated π bond, which can produce fluorescence in SDBS molecules. The emission spectrum of SDBS was investigated from 240 to 400 nm during the temperature-dependent electrochemical degradation process, and the results are shown in Figure 7. It can be seen from the figure that the maximum emission peak at 290 nm, which represents the benzene ring in SDBS, disappears gradually during the electrochemical oxidation process. Moreover, the peaks at 340 nm, which represent the sulfonic group in SDBS, disappear rapidly within 30 min even at a low temperature of 30 °C. It can be concluded that desulfonation is a fast reaction and the ring-opening reaction is a slow reaction. This discovery accords with the study based on the time-dependent electrochemical current spectra, reflecting that the reaction pathway of SDBS degradation begins with desulfonation followed by the ring-opening reaction. The concentration of SDBS decreases when the peak height at 290 nm is lower, and there is no definite increase in the degradation rate from (peak height at λ = 290 nm) 30 to 60 °C, compared with the obvious increase in the degradation degree from 60 to 90 °C, indicating that the STEP can effectively remove SDBS from wastewater. Treatment of SDBS wastewater using the STEP technology can effectively destroy the molecular structure of SDBS, and finally achieve complete mineralization. The 90 °C STEP has a significant effect on the degradation of SDBS in wastewater, even in a very short time (30 min), with the peak of the benzene ring decreasing rapidly. The reaction order is the basic parameter of chemical kinetics. Obviously, increased temperature leads to a huge enhancement in the rate of chemical reactions. The experimental results show that under different temperature conditions, ln(c0/ct) showed a good linear relationship with the reaction time, illustrating that the STEP degradation of SDBS is a first-order reaction, and the slope of the curve gives the reaction rate at the corresponding temperature. It is clearly observed from Figure 8 and Table 1 that the reaction maintains first-order kinetics from 30 to 90 °C. The same mechanism of the reaction must be followed. The rate is increased to k90 °C/ k30 °C = 2 times by an increment of 60 °C. The event obeys the Arrhenius equation of temperature dependence. In the present

Table 1. Kinetic Equation of SDBS Degradation under Different Temperature temperature (°C) 30 50 70 90

kinetic equation y y y y

= = = =

0.00572x 0.00724x 0.00996x 0.01199x

+ + + +

0.04208 0.07502 0.05455 0.15327

correlation degree 0.95482 0.96545 0.98248 0.96654

3.3. Indoor Studies of STEP SDBS Oxidation. The SDBS degradation rate and the COD removal rate were analyzed, with the initial concentration of SDBS being 50 mg L−1 and the current density being 20 mA cm−2, to investigate the effect of different concentrations of sodium chloride and the pH of the electrolyte on the degradation efficiency. 3.3.1. Effect of Sodium Chloride Concentration on the Degradation Rate. As can be seen from Figure 5, with the increase in the concentration of sodium chloride, the SDBS degradation rate and the COD removal rate were increased. When the concentration of sodium chloride is higher than 6 g L−1, the increased electrolyte concentration prevents the collisions between electrons and ions, reducing the generation of active substances in solution, thus decreasing the SDBS treatment efficiency. Therefore, although the increase in electrolyte concentration can improve the efficiency of SDBS treatment and reduce energy consumption, excessive concentration of electrolyte is not suitable. Moreover, the high concentration of sodium chloride will increase the cost of the subsequent sewage treatment. Thus, in the range of the experimental conditions, the best electrolyte concentration should be 4−6 g L−1. 3.3.2. Effect of pH on the Degradation Rate. As can be seen from Figure 6, with the increase in the pH of the electrolyte, the degradation rates of SDBS and COD were decreased. This is because with the decrease in the pH value, SDBS is more easily heated with water to undergo desulfonation,17 accelerating the oxidation reaction. Furthermore, hypochloric acid and hypochlorite ions have stronger oxidation ability under acidic conditions, which can remove the SDBS more effectively from the wastewater. 3.4. Resolving the Pathway and Kinetics of SDBS Degradation by Temperature-Dependent Fluorescence-

Figure 9. Time-dependent reaction temperature spectrum (a) and time-dependent electrochemical current spectrum (b). 1905


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Figure 10. Experimental apparatus used for the outdoor STEP wastewater experiments (a) and the PS10 solar power plant (b) in Andalucia,́ Spain (from Wikipedia).

3.5.2. Experimental Apparatus and Expanding Application. The variations of time-dependent reaction temperature spectrum and time-dependent electrochemical current spectrum in Figure 9a,b are basically consistent with the experimental results in laboratory simulations. The experimental apparatus used for the outdoor STEP wastewater experiments is shown in Figure 10a, corresponding to the apparatus of the PS10 solar power plant (Figure 10b) in Andalucia,́ Spain, which concentrates sunlight from a field of heliostats onto a central solar power tower, where lenses and mirrors concentrate sunlight to produce heat energy. The outdoor experimental results and solar power plant showed that the application of STEP treatment of wastewater containing SDBS is feasible and effective. There has been a renewed interest in STEP treatment of wastewater in recent years which is evident from the fact of experimental results that involved in designing experimental method and pathway. This is an important step toward the ultimate goal of developing commercial wastewater treatment methods with high efficiency using solar energy.

work, the STEP degradation of SDBS is investigated with emphasis on the effect of several operating parameters as well as various matrix components on SDBS degradation kinetics. Solar radiation provides energy to increase the degradation rate of SDBS in wastewater; thus, a novel and energy-free route for wastewater treatment is accomplished by the synergistic use of solar energy. 3.5. Outdoor Tests of STEP SDBS Oxidation. 3.5.1. Resolving the Mechanism of SDBS Degradation by Current Spectra. For testing and verifying the role of the thermoelectrochemical combination characteristic of the STEP, fixedvolume experiments were performed in the outdoor facility. On the basis of the STEP theory, it is well known that decreasing the electronic energy (the electrolysis potential) of an endothermic reaction by increasing the temperature leads to energy saving in the STEP in which the high temperature is provided by solar radiation. The time-dependent reaction temperature spectrum and time-dependent electrochemical current spectrum are shown in Figure 9a,b. As can be seen in Figure 9a, the temperature increased rapidly during the application of the STEP to SDBS degradation, which reached 80 °C within 10 min. The increased temperature increases the energy of reactant molecules, allowing more of them to overcome the activation barrier, causing the electrochemical reaction to occur because the reaction pathway is changed at the high temperature. The outdoor test equipment can heat up quickly reaching the required reaction temperature, and it is conducive to the actual application of industrial water pollution treatment. Correspondingly, the electrochemical current increases with the temperature at the beginning of the experiment, as shown in Figure 9b, reaching a peak of 23 mA in about 15 min and then decreasing and stabilizing within 30 min. Desulfonation has been proven to occur more easily with increasing temperature in the presence of H+; therefore, part of SDBS is converted into alkylbenzene rapidly, corresponding to the increase in the initial current, which is also reflected in peak 1 of the thermodependent CV in Figure 2. The desulfonation reaction proceeds easily and rapidly; however, the ring-opening reaction is difficult and slow, corresponding to the decrease in the current and its stabilization within 30 min. This discovery reflects that the reaction pathway of SDBS degradation begins with desulfonation followed by ring opening.

4. CONCLUSIONS To face the problem of surfactant in wastewater, there is a drive to replace the conventional energy-driven technique by a renewable energy driven one with a particular focus. The degradation of SDBS was fully driven by solar energy without the input of any other forms of energy and chemical additives. The thermodependent cyclic voltammetric spectra showed that different electrolytes have different effects on the degradation efficiency, and applying sodium chloride as an electrolyte is more conducive to high-efficiency degradation. The optimal degradation conditions were obtained by studying the effect of concentration of the electrolyte and pH on the degradation rate of SDBS and COD in indoor experiments, from which the optimal concentration of sodium chloride was found to be 5−6 g L−1 and the optimal pH value was found to be 3. The rate of SDBS degradation was significantly enhanced by solar heating, and the formation of reduction products was inhibited, as evidenced by the significant increase in yield at 90 °C compared with that at 30 °C. Solar heating provides energy to increase the degradation rate of SDBS in wastewater; thus, a novel and energy-free route for SDBS wastewater treatment is accomplished by the synergistic use of solar-to-heat and solar-toelectrical conversions. The pathway and kinetics were investigated by resolving the three spectra, illustrating that by 1906


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applying the STEP technology, the degradation of SDBS follows a first-order reaction. Meanwhile, the outdoor pilot experiment demonstrated that the STEP SDBS oxidation system can work smartly and efficiently without the input of energy other than sunlight and without human intervention. In the present work, the STEP is investigated with emphasis on the effect of several operating parameters as well as various matrix components on SDBS degradation. The mechanism of the STEP degradation of SDBS is proposed and discussed based on resolving the thermoinduced electrochemistry from both indoor and outdoor pilot experiments, thus providing a reasonable and highly efficient method of using solar energy by analyzing and designing the wastewater treatment mechanism.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcb.6b12272. Details of the experimental outdoor apparatus (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel: +86 0459 6503498. ORCID

Baohui Wang: 0000-0003-2547-9954 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported jointly by the Innovative Team of Science and Technology in Heilongjiang Higher Education Institutes (No. 2013TD004), National Nature Science Foundation of P. R. China (project Nos. 21376049, 21201128, and 21306022), and Northeast Petroleum University Foundation (project No. NEPUQN2015-1-06).

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