Electrochemical, Photochemical, and Photoelectrochemical Treatment of Sodium p-Cumenesulfonate
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The degradation of sodium p-cumenesulfonate (SCS) by electrochemical, photochemical, and photoelectrochemical methods in aqueous solution of NaClO4, NaCl, and NaClO has been studied. It was found that as a result of NaClO4 electroreduction and photodecomposition, the ions Cl− and ClO3− are formed. These ions undergo transformations into radicals, mainly Cl•, Cl2•−, ClO•−, ClO2•−, and ClO3•−, due to electrochemical and photochemical reactions. It was shown that the interpretation of results of the studies over mineralization processes carried out in the presence of ClO4− cannot be adequate without taking into consideration the reduction of ClO4− to Cl− and ClO3−. Therefore, previous works presented in the literature should be rediscussed on the basis of the new data. Photoelectrochemical mineralization of substrate in NaCl solution at the concentration of 16 mmol L−1 is comparable with the efficiency of the reaction in NaClO4 solution containing more than 8 mmol L−1 of NaClO. Total SCS mineralization was obtained in the photoelectrochemical reactor with a UV immersion lamp with a power 15 W in the period of 135 min and current intensity of 350 mA. In such conditions, the power consumption was about 1.2 kWh per g of TOC removed.
KeywordsCyclic voltammetry NaClO4 decomposition Photoelectrochemical process Sodium p-cumenesulfonate
Hydrotropes are mild surface active amphiphilic organic salts with hydrophobic part comparatively smaller than in the case of conventional surfactant. Hydrotropes exhibit a higher and often more selective ability to solubilization of organic compounds in water. The self-aggregation of the hydrotropes has been considered to be a pre-requisite for a number of applications in various fields such as drugs solubilization (Yalkowsky 1981), chemical reactions (Khadilkar et al. 1995), and separation of organic compounds (Gaikar and Phatak 1999). Hydrotropes are among over 5,000 high production volume chemical substances listed by the Organization for Economic Cooperation and Development (OECD). The consumption of these substances in 2005 reaches ca. 29,000 metric tons in the USA, 17,000 in Europe, and 1,100 in Australia. These compounds are used as additives for household cleaners such as laundry powders and liquids, laundry bleach, dishwashing liquid, toilet cleaners and liquid, powder, gel, and spray surface cleaners. The use of hydrotropes in household laundry and cleaning products raises no safety concerns for the consumers. They are also used as corrosion inhibitors, components of electroplating baths, and as a support for an extraction process (Stanton et al. 2010). The studies carried out on individual hydrotropes or surfactant-hydrotrope mixtures support the fact that hydrotropes are able to form self-aggregates in aqueous solutions known as minimum hydrotrope concentration (MHC), analogous to critical micelle concentration (CMC) for surfactants (Bhat and Gaikar 1999; Bhat and Gaikar 2000).
The electrocatalytic activity of RuO2 coupled with oxygen evolution is very favorable for the photochemical process of pollutants degradation, because the oxygen superoxide anion radicals (O2•−) formed during the photochemical process can produce additional amounts of hydroxyl radicals (Pelegrini et al. 1999).
So far, the literature data concerning the degradations of SCS are not numerous. Kimura and Ogata (1983) studied the decomposition of SCS by the photochemical oxidation (UV light λ > 290 nm) in alkaline aqueous solution with an addition of sodium hypochlorite (NaClO). The SCS mineralization was about 38 % after 45 min reaction at room temperature and the following products were detected: cumene, 2-isopropylphenol, 4-isopropylphenol, and 2-phenyl-2-propanol.
SCS very often coexists in wastewater with chloride ions, for example in zinc electroplating baths (Kozłowski et al. 1990). In many studies, the effect of chloride ions on the photoelectrochemical degradation of pollutants was investigated. Xiao et al. (2009) found that the presence of chlorides accelerates degradation of organic compounds in wastewater.
This work presents the results of sodium p-cumenesulfonate degradation in the solutions of NaClO4, NaCl, and the mixtures of NaClO4 and NaClO or NaCl using electrochemical, photochemical, and photoelectrochemical methods.
2 Materials and Methods
Purified water (double purification using Millipore Milli-Q Plus system)
Sodium p-cumenesulfonate, purity 93 % (Huntsman Holland BV, The Netherlands)
Sodium perchlorate, purity 99 % (Sigma-Aldrich)
Sodium chloride (NaCl), sodium hypochlorite (NaClO), sodium chlorate (NaClO3), and sodium perchlorate (NaClO4), purity 99 % (Chempur, Poland)
Standards for the analysis: sodium hydrogen carbonate (NaHCO3), sodium carbonate (Na2CO3), and sodium acetate (CH3COONa)—(Metrohm Poland).
2.2 Analytical Methods
In order to assess the mechanism of electrochemical oxidation and reduction of SCS, the cyclic (CV) and differential pulse (DPV) voltammetry measurements were carried out using an Autolab Potentiostat PGstat30 (EcoChemie, Holland). A three-electrode cell was applied in all experiments; platinum and titanium covered with TiO2 (70 %) and RuO2 (30 %) electrodes were used as a working electrodes for the oxidation process. The mercury electrode was used as a working electrode for the reduction process. The potential of the working electrode was measured vs. the saturated calomel electrode (SCE), whose standard potential is defined as 0.244 V vs. the standard hydrogen electrode (SHE). Before the measurements, solutions were purged with argon in order to remove dissolved oxygen. During measurements, an argon blanket was kept over solutions.
Differential capacity was measured using the method of electrochemical impedance spectroscopy (module FRA in Autolab, PGstat30). Electrode potential was changed every 10 mV in the applied potential range. Each measurement was taken on a new drop.
In order to assess the degradation of SCS, the following analytical methods were performed:
Where αTOC is TOC conversion (%), C0 is organic carbon content in initial solution (mg L−1), and C is organic carbon content in solution after reaction (mg L−1).
UV/Vis spectra were recorded in the wavelength range from 190 to 800 nm using UV/Vis Spectrophotometer Shimadzu UV-24001 PC.
Sulfate ion concentration was analyzed by means of high performance ion chromatography (HPIC), using Methorm apparatus equipped with a column (Metrosep A Supp 3—250/4.6). The mobile phase contained acetone (50 %) and water solution (50 %) of sodium hydrogen carbonate (4 mmol L−1) and sodium carbonate (1 mmol L−1).
The total organic carbon (TOC), UV/Vis spectra, and sulfate ion concentration in the solutions were recorded before and after the process.
3 Results and Discussion
3.1 Electrooxidation and Electroreduction
The electrooxidation and electroreduction reactions of SCS at the platinum and mercury electrode were studied by cyclic and differential pulse voltammetry in NaClO4 solution. Differential pulse voltammetry is the method with higher resolution, which enables better separation of peaks characterizing subsequent steps of the electrode reaction. Half-wave potential (E1/2) corresponds to the potential of the peak occurring in a differential pulse curve and is characteristic for each of the subsequent steps of the investigated electrode reaction. Basic information about the course of electrochemical reaction is provided by the dependence of the current on the potential.
The above results indicate that the reduction of SCS starts at the potential of −0.8 V vs. SCE and an oxidation at the potential of 1.7 V; however, the oxidation potential of SCS is higher than the potential of oxygen evolution. The current density used in the electrochemical treatment was tenfold higher than it followed from the cyclic voltammograms (from 5 × 10−3 A cm−2 to 1.5 × 10−2 A cm−2). The electrochemical process in water can generate hydroxyl radicals (•OH). The direct evidence for •OH formation was obtained by the electron spin resonance method (YanQing et al. 2007). The powerful •OH generated electrochemically could effectively degrade organic pollutants, but during the electrochemical degradation of SCS only slight change of αTOC about 5 % was observed. It may be caused by the strong adsorption of substrate on the electrode surface, which is associated with the generation of hydroxyl radicals.
3.2 Photochemical Mineralization
In order to improve the degradation of SCS, electrochemical and photochemical methods were combined.
3.3 Photoelectrochemical Mineralization
UV light and chlorination are two of the most common disinfection methods of water and wastewater. Hypochlorite acid can form hydroxyl and hypochlorite radicals by irradiation with UV light in the range from 200 to 400 nm (Xiao et al. 2009).
Electrochemical and photoelectrochemical studies of oxidation and reduction of organic compounds are often performed in NaClO4 aqueous solution as a basic electrolyte. Kim and Anderson (1994) observed that the presence of ClO4− ions in solution significantly slows down the reaction of photocatalytic oxidation of formic acid. The degradation degree was much higher in the solution without an addition of ClO4− ions. In the work of Zhang et al. (2005), it was observed that during the photochemical and photoelectrocatalytic oxidation of the reactive Brillant Orange K-R, the presence of ClO4− ions lowers the rate of degradation of the compound, whereas the presence of Cl− ions considerably improves the efficiency of the reaction, particularly at higher concentrations of chloride ions.
3.4 Photoelectrochemical Treatment of SCS in Rayonet Photoreactor
Due to a different assessment of the influence of NaClO4 on the effect of photochemical and photoelectrochemical reactions, the studies of SCS degradation in four different solutions containing: NaClO4 (100 mmol L−1), NaClO4 (100 mmol L−1) with the addition of NaClO or NaCl in the concentration range from 0 to 16 mmol L−1, and NaCl in the concentration range from 2 to 16 mmol L−1 were performed. The studies were carried out in an electrolyzer placed in the photoreactor containing 16 lamps emitting radiation at the wavelength of 254 nm, the solution volume was 80 mL and current intensity of 0.3 A.
In (NaClO4 + NaCl) solution (curve 3) along with the increase in concentration of chloride ions, an almost linear increase in αTOC up to about 90 % at Cl− of 16 mmol L−1 is observed. In the case of (NaClO4 + NaClO) solution (curve 1) two ranges of increase in αTOC can be distinguished depending on the concentration of ClO− ions: a relatively sharp within the concentration range from 0 to 6 mmol L−1 and much lower in the range from 6 to 16 mmol L−1. The changes of αTOC during the SCS reaction in the NaCl solution (curve 2) are the same as in (NaClO4 + NaClO) mixture.
The electrons occupy the vacancies in the valence band of TiO2 left by the electrons which have been excited to the conduction band of TiO2. Such processes will be repeated upon the illumination of the cell (Rahman et al. 2007).
Practically the linear dependence of the αTOC on the reaction time in NaClO4 solution and the proportional increase in αTOC with the increase in the concentration of chloride ions in NaClO4 solution suggests that oxidation of the substrate in monocomponent NaClO4 solution can be basically caused by radicals formed from the products of NaClO4 electrochemical reduction and photodecomposition.
To summarize, the discussed results indicate that the efficiency of SCS degradation in each of the considered cases is mainly connected with the Cl• and Cl2•− radicals formed from the products of electrochemical, chemical and photochemical reactions of the basic electrolyte and Cl− and ClO− ions added. According to Brown (1986) the perchlorate ion is electrochemically reduced to the chloride ion at an active titanium electrode in aqueous solutions. Negative current values can be observed on the positive sweep in the potential range from 200 to 700 mV.
Therefore, further studies of SCS degradation in a photoelectrochemical reactor with an immersion lamp in the solutions: NaClO4 (100 mmol L−1), NaClO4 (100 mmol L−1) with the addition of NaClO (2 mmol L−1), and NaCl (16 mmol L−1) were performed.
Additionally, the products of electroreduction, photodestruction, and photoelectrochemical changes of the electrolyte were analyzed.
3.5 Photoelectrochemical Treatment of SCS in Heraeus Photoreactor
Due to the increase in the rate of the substrate destruction in the presence of Cl− and ClO− ions, the studies of photoelectrochemical degradation of SCS were carried out for a larger volume of solution (160 mL) in the reactor with an immersion mercury lamp. The measurements were taken in the following solutions: NaClO4 (100 mmol L−1), NaClO4 containing NaClO (2 mmol L−1, NaClO to SCS ratio 1:1) and NaCl (16 mmol L−1). In the reaction products, TOC and the concentration of SCS, SO42−, and ClO3− ions were analyzed.
Perchlorate can be generated as a final product of the photochemical reaction of aqueous solutions containing NaClO, NaClO2, and NaClO3 salts exposed to UV radiation. However, it can only be formed if the concentration of precursors (NaClO, NaClO2, and NaClO3) is higher than 1,000 mg L−1. Perchlorate is a thermodynamically stable molecule in comparison with NaClO, NaClO2, and NaClO3 (Kang et al. 2006). Therefore, one can conclude that under experimental conditions, ClO3− and Cl− or/and ClO− are the final products of photoelectrochemical changes of NaClO4.
3.6 Optimization of Photoelectrochemical Process with Brandon Method
SCS is reduced on mercury electrode in three steps (E1/2 = −1.25, −1.35, and −1.42 V vs. SCE) in the potential range lower than the potential at which hydrogen evolution started. The substrate in acetonitrile is oxidized in two stages (E1/2 = 2 and 2.25 V vs. SCE) on the platinum electrode. SCS is adsorbed on applied electrodes surface during the electrooxidation and electreduction process. Adsorption and a high potential of oxidation and reduction cause low mineralization of substrate during the electrochemical reaction (about 5 %).
It was proved that during electrochemical, photochemical and photoelectrochemical reactions, NaClO4 decomposes forming mainly Cl− and ClO3− ions. It was concluded that the main cause of synergy in the photoelectrochemical destruction of SCS in NaClO4 solution is its photochemical decomposition leading to the generation of Cl•, ClO•−, ClO2•−, and ClO3•− radicals. The increase in the αTOC is proportional to the increase in the concentration of added Cl− ions into NaClO4 solution which is caused by the increase in the number of radicals resulting from the electrochemical, photochemical and photoelectrochemical reactions of Cl− ions and their inhibitory effect on ClO4− reaction on the electrode surface. The photoelectrochemical mineralization of the substrate in NaCl solution at the concentration of 16 mmol L−1 is comparable with the efficiency of the reaction in NaClO4 solution containing more than 8 mmol L−1 of NaClO. Total mineralization of SCS was achieved in the photoelectrochemical reactor with an immersion UV lamp 15 W in the period of 135 minutes and current intensity of 350 mA. Under these conditions the energy consumption was about 1.2 kWh per g of TOC removed.
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