Anodic oxidation of isothiazolin-3-ones in aqueous medium by using boron-doped diamond electrode

Diamond & Related Materials 69 (2016) 152–159

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Anodic oxidation of isothiazolin-3-ones in aqueous medium by using boron-doped diamond electrode V. Kandavelu a,b, S. Yoshihara b,⁎, M. Kumaravel c, M. Murugananthan c,⁎ a b c

Department of Chemistry, Sri Shakthi Institute of Engineering and Technology, Coimbatore 641 062, India Department of Material and Environmental Chemistry, Graduate School of Engineering, Utsunomiya University, 7-1-2 Yoto, Utsunomiya, Tochigi 321-8585, Japan Department of Chemistry, PSG College of Technology, Peelamedu, Coimbatore 641 004, India

a r t i c l e

i n f o

Article history: Received 3 July 2016 Received in revised form 12 August 2016 Accepted 23 August 2016 Available online 29 August 2016 Keywords: BDD Biocide Isothiazolin-3-ones Hydroxyl radicals Mineralization

a b s t r a c t Electrochemical degradation of biocide compound, isothiazolin-3-ones, was studied in aqueous medium, with Na2SO4 supporting electrolyte using boron-doped diamond (BDD) anode. The redox response of isothiazolin3-ones at BDD was examined by cyclic voltammetric study. The degradation of isothiazolin-3-ones and its mineralization trend were monitored by UV–vis spectrophotometric method and total organic carbon (TOC) analyzer, respectively. The effect of operating parameters such as applied current density, biocide concentration, electrolyte pH and nature of supporting electrolytes (Na2SO4, NaNO3 and NaCl) on degradation rate was studied in detail. It was established that the hydroxyl radicals (•OH) generated at BDD surface were responsible for the degradation and the mineralization of the biocide contaminant. The rate of degradation was almost independent of electrolyte pH but became faster as the applied current density increased and the biocide concentration decreased. The kinetic studies revealed that the biocide decay follows a pseudo-first-order rate. The apparent rate constant for the oxidation of isothiazolin-3-ones was determined to be 2.65 × 10−4 s−1 at an applied current density of 25 mA cm−2 in the presence of 0.1 mol dm−3 Na2SO4 at pH 6.0. A poor mineralization efficiency was observed in the case of NaCl as supporting electrolyte which cause in-situ generation of chlorine based mediated oxidants such as Cl2 and OCl− which have negligible influence in mineralizing the isothiazolin-3-ones compared to peroxodisulfate (S2O2− 8 ) oxidants that formed in the case of Na2SO4. The oxidizing ability of the BDD anode was compared with those of Pt and glassy carbon anodes under similar experimental conditions. © 2016 Elsevier B.V. All rights reserved.

1. Introduction In recent years, research on negative consequences of chemicals used as pesticides, biocides, fertilizers etc., which caused increased health problems to humankind and the decline in the population of aquatic species, has become more significant among environmental scientists. Isothiazolin-3-ones, known biocides, have been found to exhibit biological activity towards several kinds of microorganisms. The isothiazolin-3-one compounds such as 5-chloro-2-methyl-4isothiazolin-3-one (CMI) and 2-methyl-4-isothiazolin-3-one (MI) are highly bioactive especially their chloro derivative CMI. These are used as preservatives for increasing the shelf life of latex paints [1] in pharmaceutical/self hygiene products (sunscreens, shampoos, baby wipes and creams/lotions/gels), cosmetics (moisturizers, eye shadows, and make-up removers), toiletries and, household and industrial products (detergents, printing inks, fabric softeners, and adhesives). Additionally, they are being used in metal working fluids, slimicides of cooling ⁎ Corresponding authors. E-mail addresses: [email protected] (S. Yoshihara), [email protected] (M. Murugananthan).

http://dx.doi.org/10.1016/j.diamond.2016.08.008 0925-9635/© 2016 Elsevier B.V. All rights reserved.

towers, paper mills, recirculating chillers, cutting oils and jet heating fuels [2]. Despite their versatile applications, epidemiological studies proved that these isothiazolin-3-ones, even at a very low concentration (0.2%), can create allergic contact dermatitis to humankind [3–5]. The source of contamination of these compounds in the natural water body system is due not only to its application, but also to the contaminated wastes that are released from the manufacturing industries. Mitigating the problem of isothiazolin-3-ones is really a tough task and a nightmare to environmental scientists as its natural decay in the biotic system is a very slow process and the average time taken is predicted to be several days to weeks [6,7]. The continued increase in the production of these compounds for mandate applications increases their load in the water environment [1]. A couple of attempts have been tried for the natural degradation of isothiazolin-3-ones, in aqueous media, as a function of temperature and pH [8–9]. At pH N 8.5, the half-life period was found to be 4.6 days at 40 °C and it increased to 46 days when the temperature falls down to 24 °C. In a pH controlled (neutral pH) photolytic degradation under natural sunlight, the half-life was determined as 158 h and 266 h for MI and CMI, respectively. Hence, the problem caused by the contamination of isothiozoline-3-ones cannot be mitigated by natural degradation and the biological approach. Thus, an

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effective and viable treatment technique is needed to mineralize the biocide material in to completely harmless products. In an attempt of a biodegradation of CMI with two different initial concentrations viz. 0.3 and 0.03 mg l−1, the respective CO2 evolution was measured as 38 and 62% after 28 days [10]. The successful removal of isothiazolin-3ones from aqueous solution was achieved [11] by heterogeneous photocatalysis technique. However, the main drawback for using photocatalysts is the difficulty in separating the semiconducting fine particles from the treated solution. Emerging technologies such as advanced oxidation processes (AOPs) using novel electrodes are proved to be a promising one owing to the in-situ production of •OH that results in total mineralization of persistent organic contaminants in aqueous system. The •OH species is a powerful oxidizing agent and its fast non-selective reaction with organic pollutants gives dehydrogenated or hydroxylated intermediates that can undergo complete mineralization into CO2, H2O and fragmented inorganic ions [12–14]. Among the AOPs, electrochemical oxidation has been proved to be an effective technique which offers many advantages such as low operational cost and high mineralization efficiency compared to other known chemical and photochemical processes [12,15–17]. Electrochemical oxidation allows the destruction of organic pollutants of an aqueous system by •OH formed at the surface of a high oxygen overvoltage anode (M) from water decomposition. MðH2 OÞMð OHÞ þ Hþ þ e−

ð1Þ

The application of electrochemical AOPs using a state-of- the art electrode material viz. boron doped diamond (BDD) has received great attention [18] as it exhibits the following important features; (i) an inert surface with a less adsorptive nature towards •OH; (ii) remarkable dimensional stability; (iii) corrosion resistive properties with greater durability; (iv) low and stable background current [19] and (v) an extremely wide potential window in aqueous and non-aqueous media [20]. The much higher oxygen overpotential of BDD compared to conventional electrodes (Pt, graphitic carbon electrodes and metal oxide coated electrodes such as SnO2, PbO2, RuO2, etc.) favors the large production of •OH on its surface thus making the BDD anode more effective in mineralizing the organic pollutants [21–23]. This fact has been proved by many recent studies on the degradation of several organic dyes and pollutants in aqueous media using a BDD anode that showed a complete mineralization into CO2 and H2O [23–28]. Thus, the anodic oxidation technique using BDD could be an appropriate choice for the treatment of water containing isothiazolin-3-one compounds. To the best of our knowledge, there has not been any work addressing the degradation of isothiazolin-3-ones by an electrochemical approach. Hence, in the present study, the degradation of iosthiazolin-3-ones from aqueous media was tried using BDD electrode. The influence of experimental parameters such as the initial concentration of the biocide, applied current density, electrolyte pH and different supporting electrolytes on the rate of degradation of isothiazolin-3-ones was systematically examined. Also, a comparative study with Pt and glassy carbon (GC) anodes was made to confirm the superior oxidation power of the BDD electrode. A reaction sequence for the mineralization of iosthiazolin-3ones at BDD was proposed.

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source, B2O3 (99.9%) was from KOJUNDO Chemical Laboratory. The high purity water, obtained from Millipore Milli-Q system, was used for the preparation of all the solutions. Diluted H2SO4 or NaOH solutions were used for adjusting the pH of the electrolyte solution. BDD anode was prepared by a microwave assisted plasma chemical vapour deposition (MPCVD) technique (Model AX2115, Astex Corp.) on p-Si(111) substrates. The fabrication process detail is described elsewhere [29]. The conditions for the fabrication of BDD thin film on a Si substrate are; temperature — 540 °C; pressure — 70 Torr; carrier gas — ultra pure H2; carbon and boron source — mixture of acetone and methanol (9:1 v/v) having B2O3 with a B/C ratio of 104 ppm; and deposition time — 24 h. The thickness of the film fabricated was 20 μm. The electrode surface was cleaned by sonication in ethanol followed by deionized water. The other working electrodes viz., Pt and GC were procured from Nilaco Corporation, Japan. 2.2. Electrolytic studies The molecular weight of CMI and MI is 149.59 and 115.1 g/mole, respectively. The gift sample obtained for this study (referred as isothiazolin-3-ones) was an aqueous mixture of CMI and MI with a ratio of 3:1. The sample was pale yellow in color with a density of 1.02 g/cm3. The working concentration used in this study was a mixture of 5.5 × 10−4 mol dm−3 of CMI and 1.7 × 10−4 mol dm−3 of MI. Since both the compounds have an almost identical structure (as seen in Fig. 2) and showed similar trends of photocatalytic degradation [11], the total concentration of isothiazolin-3-ones in the mixture was conveniently taken as 7.2 × 10−4 mol dm−3, for easy interpretations, predictions and calculations of reaction kinetic studies. The working volume of isothiazolin-3-one (7.2 × 10−4 mol dm−3) solution was 220 mL with a supporting electrolyte concentration of 0.1 mol dm−3. A computer controlled electrochemical work station (potentiostat/ galvanostat — Model HZ-5000, Hokuto Denko Ltd., Japan) with an undivided three electrode reaction cell was used for all the electrochemical studies. The working, counter and reference electrodes were BDD, Pt and Hg/Hg2Cl2·KCl (sat.) (SCE), respectively. Deposition of BDD thin film was made over a square shape Si wafer (2 cm × 2 cm). The ohmic contact was made on the pristine side of the Si wafer using gold and silver paste. Upon making ohmic contact, the pristine side was completely masked using adhesive tape. The surface area of the BDD thin film, i.e., the area exposed to the electrolyte was then measured to be exactly 4 cm2. The Pt strip used as a counter electrode was a rectangle type (1 cm × 2 cm) with an effective surface area of 4 cm2. The inter electrode gap between the working and counter electrode was maintained as 1 cm. The degradation experiments were conducted at galvanostatic mode under uniform stirring. For anodic oxidation experiments using Pt and GC electrodes, the respective cathode material was BDD and Pt with a constant surface area. The temperature of the electrolyte solution was maintained at 25 °C by a thermo-regulated water bath. Prior to every experimental run, an anodic polarization was carried out for the BDD electrode in 0.1 mol dm−3 H2SO4 solution at 100 mA for a period of 10 min while Pt was soaked in HNO3 for 10 min to remove any kind of deposition and/or impurities from the surface. The constant cell potential during the galvanostatic electrolysis indicated that the activity of the electrode was unaltered and retained throughout the experiment.

2. Experimental 2.3. Analyses 2.1. Materials ZONEN-C, biocide solutions, containing CMI (10.65%) and MI (3.31%) with magnesium nitrate as stabilizer, were obtained from Chemicrea (Japan) as gift samples. Magnesium stabilizes isothiazolin-3-ones by forming adducts through oxygen of carbonyl group. The electrolyte salts, organic solvents and other chemicals used in this study purchased from Kanto chemicals were of high purity and used as such. The boron

The degradation trend of the isothiazolin-3-ones with respect to electrolysis period was monitored in terms of the intensity of the absorbance peak (λmax observed at 273 nm) using a UV–vis spectrophotometer (Hitachi U-2000). The reproducibility of the obtained results was confirmed by a duplicate run with an error % of ± 2. The electrolyte pH was measured with a TOA HM-30S pH meter. The mineralization of the isothiazolin-3-ones solution was followed from the decay of

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total organic carbon (TOC) content using Analytik Jena Multi N/C 2100S TOC analyzer. From these values, the mineralization current efficiency (MCE) for an electrolyzed solution at a given period (t) was calculated following Eq. (2): MCE ð%Þ ¼

ΔðTOCÞ exp ΔðTOCÞtheor

 100

ð2Þ

where Δ(TOC)exp. is the experimental TOC removal at time t and Δ(TOC)theor is the theoretically calculated TOC removal considering that applied electric charge (=current × time) was consumed only to yield the reaction products shown in reactions (3) and (4). Assuming that the overall mineralization of isothiazolin-3-ones involves its conversion into CO2, Cl−, SO24 − and mainly NO− 3 from N degradation as the nitrogen has linked with extractable hydrogen [30], the reaction can be represented as reactions (3) and (4) −

2− þ C4 H4 NSOCl þ 14H2 O → 4CO2 þ NO− 3 þ SO4 þ Cl þ 32H þ 28e−

ð3Þ

and 2− þ − C4 H5 NSO þ 14H2 O → 4CO2 þ NO− 3 þ SO4 þ 33H þ 30e

ð4Þ

Taking into account the finding that the rate of degradation of both CMI and MI are equal [11] and the relatively higher concentration of CMI rather than the latter, the number of electrons involved in the complete mineralization of isothiazolin-3-ones can be conveniently taken as 28 to calculate the MCE of isothiazolin-3-one degradation. 3. Results and discussion The plan of the present work was as follows: (1) voltammetric study of isothiazolin-3-ones; (2) degradation kinetics of isothiazolin-3-ones; (3) mineralization of isothiazolin-3-ones as a function of applied current density and its current efficiency, and (4) impact of electrolyte pH and nature of electrolytes.

the first cycle of anodic sweep, a well defined anodic peak at 1.72 V and a shoulder at 1.58 V vs. SCE observed were attributed to the oxidation of isothiazolin-3-ones. The appearance of peak and shoulder, rather than a prominent single peak, was due to the overlapping of the two characteristic peaks against CMI and MI compounds in the solution. The peak current density for CMI and MI was observed to be 0.0088 mA/cm2 and 0.0032 mA/cm2, respectively. The ratio of the current densities was calculated as 2.8:1 against the concentration ratio of isothiazolin-3-ones at 3.22:1. The increase in the current density was almost proportional with respect to its concentration. The absence of any reduction peak at negative potential window (not shown) evidences the irreversible oxidation of isothiazolin-3-ones. Thus, this study confirms that the oxidation of isothiazolin-3-ones takes place at around 1.55 to 1.75 V by direct electron transfer reaction. The current density observed for the oxygen evolution peak in the presence of isothiazolin-3-ones, was slightly reduced by ~ 0.015 mA compared to the blank run i.e., using only Na2SO4 (0.1 M dm− 3) electrolyte. It is due to the direct electron transfer oxidation of isothiazolin-3-ones on its surface which leads to a deposit of oxidized product that in turn affects the activity of the surface of the BDD electrode [31]. It was further confirmed by the decrease in anodic peak current observed with respect to the subsequent cycles. As the deposition occurs on the surface, the characteristic anodic peak against the oxidation of isothiazolin-3-ones was reduced due to a fouling phenomenon. Quantitatively, if there is no fouling, the ratio between the current densities of the 3rd and 1st cycles would be close to unity, but, in the present case, it was 0.830 for CMI, and 0.804 for MI, indicating that passivation occurs to some extent. However, the activity could be restored by decomposing the fouling layer by an anodic polarization, in the potential region of water decomposition (N2.3 V) [32,33,34]. Since the applied current density employed in the present study falls in the region of water decomposition potential, no such passivation behavior was observed during the galvanostatic oxidation of isothiazolin-3-ones, which could be confirmed by a constant cell potential observed during the process [24]. 3.2. Electrochemical degradation study of isothiazolin-3-ones

As seen in Fig. 1, the cyclic voltammetric experiments were carried out with a solution containing isothiazolin-3-ones (7.2 × 10− 4 mol dm− 3) and Na2SO4 (0.1 M dm−3) at a scan rate of 50 mV s−1. For a comparative purpose, a blank run was carried out i.e., without isothiazolin-3-ones in which the oxygen evolution potential of the electrode was found to be at a maximum of 2.49 V. During

The electrochemical degradation of isothiazolin-3-ones was carried out using BDD anode concentration with 0.1 mol dm−3 Na2SO4 solution at their natural pH (6.0). The chemical structure of isothiazolin-3-ones and its UV–vis absorbance peak (0 min peak) are shown in Fig. 2. Both the MI and CMI absorb the light in the UV region and exhibit a single peak showing λmax at 273 nm. The electrochemical degradation of isothiazolin-3-ones using the BDD anode was efficient with 0.1 mol dm−3 Na2SO4, and a complete degradation could be achieved within 160 min of the electrolysis period at an applied current density

Fig. 1. Cyclic voltammograms for the oxidation of isothiazolinones at Si/BDD electrode (7.2 × 10−4 mol dm−3 isothiazolinones in 0.1 mol dm−3 Na2SO4, scan rate: 50 mV s−1, pH 6; T, 25 °C). Inset: magnified view of the oxidation peaks.

Fig. 2. UV–vis spectra of isothiazolinones at different time intervals of electrochemical oxidation on BDD (7.2 × 10−4 mol dm−3 isothiazolinones in 0.1 mol dm−3 Na2SO4, pH 6; T, 25 °C, 25.0 mA cm−2). Inset: molecular structure of isothiazolinones: 2-methyl (A) and 5-chloro-2-methyl (B) isothiazolin-3-ones.

3.1. Voltammetric study of isothiazolin-3-ones

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of 25 mA cm−2 as shown in Fig. 2 and Fig. 3. It clearly indicates that the •OH produced on the surface of the BDD anode destructs the compound and the technique can very well be adapted as a rapid degradation technique to decompose these compounds into aliphatic fragments [32]. It should be noted that the present experiments were carried out at 25 °C and therefore the thermal contributions could be considered as negligible. The oxidative degradation of organic pollutants at BDD electrode only takes place within the range of water decomposition potential [35], thus a lower value of the applied current was chosen so as to attain the anodic potential higher than 2.3 V vs. SCE. In this manner, the degradation experiment was performed with an aqueous solution containing isothiazolin-3-ones (7.2 × 10−4 mol dm− 3) and Na2SO4 (0.1 mol dm−3) as a supporting electrolyte salt by varying the applied current densities from 12.5 to 37.5 mA cm−2 and the corresponding results are shown in Fig. 3. The gradual disappearance of characteristic peak of isothiazolin-3-ones was observed over a period of 260 min electrolysis time at an applied current density of 12.5 mA cm−2. It clearly indicates that the isothiazolin-3-ones were degraded completely at 260 min, however, the degradation product molecules could remain in the solutions. The complete degradation of isothiazolin-3-ones could always be achievable due to the in-situ and concomitant generation of •OH according to the reaction (1). The absorbance peak vanished within 100 min of electrolysis period with an increase of the applied current density from 12.5 to 37.5 mA cm−2, and confirmed the accelerated oxidation reaction at higher current density owing to a progressive generation of •OH. It was quite clear that in the applied current density range employed, the rate of degradation mainly depends on the electro-generation of •OH rather than the mass transfer of the pollutant molecules. As seen in the inset of Fig. 3, the kinetic analysis of the degradation of isothiazolin3-ones follows the pseudo-first-order kinetics by fitting a linear relation with electrolysis time and the R2 values are found to be N0.99. The pseudo-first-order rate constants at different current densities were calculated to be 1.65, 2.65 and 5.18 × 10−4 s−1 for 12.5, 25.0 and 37.5 mA cm−2, respectively. Obviously, the rate constant increased linearly with an increase in the applied current density. It confirms that the degradation process is a bimolecular reaction between isothiazolin3-ones and •OH. Also, it can be understood that the energy applied is completely utilized for the effective degradation and, the formation of a secondary oxidant, peroxydisulfate (S2O2− 8 ), which is normally produced at the cost of •OH as a parasite reaction, which is also strong enough to oxidize the isothiazolin-3-ones unlike Cl2, OCl−, H2O2 and molecular O2. The nature of the supporting electrolyte decides the efficiency of the reaction to a greater extent and this will be discussed in the forthcoming Section 3.5.

The disappearance of the characteristic UV absorbance peak of isothiazolin-3-ones is not actually ensuring the complete destruction of the entire compound into CO2, H2O and inorganic species. Thus, to further confirm the mineralization of the metabolite compounds, the TOC analyses were carried out for the resultant solutions corresponding to Fig. 3, i.e., at three different current densities viz., 12.5, 25.0 and 37.5 mA cm−2. The TOC removal results are shown in Fig. 5a. The continuous decrease of TOC confirmed the progressive mineralization of isothiazolin-3-ones and its metabolites. It proceeds via opening of the ring in the isothiazolin-3-ones molecule and subsequently, it splits up into smaller fragments [11]. Under similar experimental conditions, almost complete mineralization (92%) was achieved at a current density of 37.5 mA cm− 2 after 350 min electrolysis period and, on the other hand, the TOC removal was 58 and 73% for 12.5 and 25.0 mA cm−2, respectively at the same electrolysis period. This study clearly demonstrates the greater oxidizing power of BDD(•OH) on the complete mineralization of isothiazolin-3-ones and it showed better mineralization under a higher current density at a fixed electrolysis time, which is due to the accelerated generation of •OH by the increased current density [36]. At the same time, it could be noted that the specific electrical charge (Q) required for an equal amount of TOC removal is increased with the applied current. This feature can be accounted for the fact that highly electro-generated •OH could be wasted to some extent at higher applied current densities by parallel parasite reactions such as O2, S2O2− 8

Fig. 3. Effect of Iappl on anodic oxidation of isothiazolinones on BDD (7.2 × 10−4 mol dm−3 isothiazolinones in 0.1 mol dm−3 Na2SO4, pH 6; T, 25 °C, current density: (♦) 12.5 mA cm−2, (▲) 25.0 mA cm−2 and (●) 37.5 mA cm−2). Inset: kinetic plots for isothiazolinone initial degradation assuming pseudo-first order reaction.

Fig. 4. Effect of initial concentration of isothiazolinones by anodic oxidation with BDD at 25.0 mA cm−2 (isothiazolinones in 0.1 mol dm−3 Na2SO4, pH 6; T, 25 °C, [isothiazolinones]: (♦) 8.6 × 10−4 mol dm−3, (▲) 7.2 × 10−4 mol dm−3 mM and (●) 5.9 × 10−4 mol dm−3). Inset: kinetic plots for isothiazolinone initial degradation assuming pseudo-first order reaction.

To have a better understanding of the degradation kinetics of isothiazolin-3-ones, the experiment was carried out with three different concentrations of isothiazolin-3-ones i.e., 5.9, 7.2 and 8.6 × 10−4 mol dm−3 in a 0.1 mol dm−3 Na2SO4 at a fixed current density of 25.0 mA cm−2 and the results are shown in Fig. 4. It is seen that the rate of degradation decreased with the increase in the concentration of isothiazolin-3-ones. The initial degradation rate constant for the process is found to be 3.02, 2.65 and 1.90 × 10−4 s−1 for the 5.9, 7.2 and 8.6 × 10−4 mol dm−3 concentrations, respectively. The decline in the rate of degradation with respect to increasing concentration could be due to the fact that the number of •OH available for the oxidation of isothiazolin-3-ones molecule is quantitatively more at a lower concentrated isothiazolin-3-one solution. In addition, at higher initial concentration, the possibility of reaction between •OH and isothiazolin-3ones is comparatively less due to more metabolite/intermediate products from isothiazolin-3-ones decomposition. It is now very clear that the oxidation reaction undergoes a current controlled reaction process, thus, the higher applied current density favors a faster degradation rate. 3.3. Mineralization study of isothiazolin-3-ones and its current efficiency

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the fact that the acceleration of secondary oxidant generations from •OH at a higher applied current results in relatively less efficiency. These secondary reactions involve, the oxidation of BDD(•OH) to O2 by reaction (5) and the dimerization of •OH to H2O2 by reaction (7) or its decay with H2O2 by reaction (8) [38]. H2 O2 þ OH →

HO2  þ H2 O

ð8Þ

Conclusively, working at a lower current density would be an advantageous one in terms of energy consumption and the complete mineralization of isothiazolin-3-ones could be achieved by anodic oxidation with BDD, even at a lower applied current density. 3.4. Effect of pH and supporting electrolytes on isothiazolin-3-ones degradation

Fig. 5. (a) Effect of current densities on TOC decay at different time intervals for the treatment of isothiazolinones on a BDD anode (7.2 × 10−4 mol dm−3 isothiazolinones in 0.1 mol dm−3 Na2SO4, pH 6; T, 25 °C, current density: (♦) 12.5 mA cm−2, (▲) 25.0 mA cm−2 and (●) 37.5 mA cm−2). (b) Change of mineralization current efficiency (calculated using Eq. 2) with specific electrical charge for the TOC decay at various current densities.

To find the role of pH, the degradation and mineralization of isothiazolin-3-ones at the BDD surface was studied at three different conditions of pH viz., 4.0, 6.0 and 9.5 by applying a constant current density of 25 mA cm−2. As seen in Fig. 6, the initial degradation rate constants at pH 9.5, 6.0 and 4.0 were 2.90, 2.65 and 1.95 × 10− 4 s−1, respectively. The degradation kinetics of isothiazolin-3-ones was slightly affected in acidic medium i.e. a notable decrease in the degradation rate was observed for pH 4.0. However, in these pH conditions, a complete degradation of isothiazolin-3-ones takes place within the same electrolysis period. Thus, it can be proposed that the difference observed could be due to the neutralization of •OH by H+ ion presented in the acidic medium. The initial experimental pH was found to move towards acidic pH with the progressive degradation. This can be attributed to the generation of HCl a decomposition product as it was evidenced by the

and H2O2 production as in reactions (5, 6, and 7). BDDð OHÞ 2SO2− 4

→ BDD½O2 þ Hþ þ e−

− → S2 O2− 8 þ 2e

2 OH → H2 O2

ð5Þ ð6Þ ð7Þ

However, many of these secondary mediated oxidants (O2, and H2O2), fail to degrade refractory organic molecules [30]. Thus, an increase in the applied current will always lead to the secondary anodic reactions to a certain extent at the expense of degradation of the pollutant molecules, which leads to comparatively lesser efficiencies. Based on the TOC removal values, the mineralization current efficiency (MCE) was calculated against the corresponding specific electric charge applied and the trend is shown in Fig. 5b. In all the cases, an increasing trend of MCE was observed in the initial phase of the electrolysis, and after a maximum value, the current efficiency started declining slowly, but continuously with respect to a specific applied charge (Q). This indicates that the oxidizing ability of the process becomes less effective as the reaction proceeds due to a gradual decline in the concentration of the pollutant molecules, and secondly to the formation of stable oxidation products such as short chain carboxylic acids that are hard to get oxidized with •OH than the metabolites formed at the initial phase [37]. With an increase in the applied current, the current efficiency was found to be decreased. For example, the MCE values at Q = 3.67 × 10−2 Ah dm−3, are 5.37, 3.27 and 2.33% for the current densities 12.5, 25.0 and 37.5 mA cm−2, respectively. The reason for the difference in MCE values is due to the fact that the mineralization was observed to be a current controlled process i.e. the electrical energy is effectively utilized for the mineralization of the pollutant with priority at a lower current density. The loss in MCE by raising the current can be explained by

Fig. 6. (a) Effect of initial pH on the oxidation of isothiazolinones at BDD anode (7.2 × 10−4 mol dm−3 isothiazolinones in 0.1 mol dm−3 Na2SO4, current density: 25.0 mA cm−2, T, 25 °C, pH: (♦) 4, (▲) 6 and (●) 9.5). (b) Effect of initial pH on TOC decay at different time intervals for the treatment of isothiazolinones on BDD anode (7.2 × 10−4 mol dm−3 isothiazolinones in 0.1 mol dm−3 Na2SO4, current density: 25.0 mA cm−2, T, 25 °C, pH: (♦) 4, (▲) 6 and (●) 9.5).

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photodegradation of isothiazolin-3-ones and biodegradation of CMI [11]. The absence of any significant difference in the TOC removal rate of isothiazolin-3-ones confirmed that the influence of pH in the mineralization process was only marginal. These findings indicate that the decomposition of isothiazolin-3-ones can be performed at any pH value between 4.0 and 9.5 without any significant loss in oxidation efficiency of the system. Considering the contradictory results reported in the literatures, the influence of electrolyte pH generally depends on the nature of the compounds undergoing degradation [32,39,40]. To examine the influence of the supporting electrolyte on the degradation kinetics and mineralization of isothiazolin-3-ones, the degradation experiments were performed with a BDD anode at a pH of 6.0 (natural) and a fixed current density of 25 mA cm− 2 employing 0.1 mol dm− 3 of different electrolyte such as Na2SO4, NaNO3 and NaCl. The results are shown in Figs. 7 and 8. The degradation kinetics of isothiazolin-3-ones was followed by measuring the intensity of its characteristic absorbance peak exhibited at 273 nm (Fig. 2 and Fig. 7b). In the presence of Cl−, the appearance of an additional new peak at 292 nm, characteristics of a hypochlorite (ClO−) ion, was observed and its intensity was found to increase with an increase in electrolysis time (Fig. 7b) [24]. The initial degradation rate constants in the presence of Na2SO4, NaNO3 and NaCl were calculated to be 2.65, 1.12 and 36.53 × 10−4 s−1, respectively. It is clear from the data (Fig. 7a) that the complete degradation of isothiazolin-3-ones in the presence of nitrate requires two fold electrolysis time when compared to that in the presence of SO2− whereas the rate is 15 times faster in the presence 4 of Cl− than that in the SO2− medium. The electrolyte containing Cl− 4 does not guarantee the actual decomposition of the isothiazolin-3-

157

Fig. 8. Effect of supporting electrolyte on TOC decay at different time intervals for the treatment of isothiazolinones (7.2 × 10−4 mol dm−3 isothiazolinones in 0.1 mol dm−3 electrolyte, current density: 25.0 mA cm−2, pH: 6.0, T, 25 °C, (♦) BDD-Na2SO4, (▲) PtNa2SO4, (●) BDD-NaNO3 and (□) BDD-NaCl).

ones and it could be a rapid transformation of the parent compound into its unopened ring based metabolite molecules [11]. It can further be explained well with the support of its organic carbon removal. As seen in Fig. 8, the TOC removal in the presence of Cl− was very poor and in particular, almost no organic carbon was eliminated till a reaction period (≈10 min) i.e., till the complete disappearance of the absorbance peak corresponding to isothiazolin-3-ones. Among the electrolytes employed, the mineralization process was found to be strongly influenced by the SO2− 4 ion and the degree of mineralization in the presence of SO2− 4 ion was 73% at 350 min while it was only 30% for NaNO3 and 8% for NaCl as electrolyte. The difference in the trend of mineralization compared to the degradation with respect to supporting electrolyte could be accounted for the various oxidant species generated during the oxidation process. The higher oxidizing ability in the SO2− 4 medium is due to the in-situ generation of S2O2− either by reaction (6) or reac8 tion (9), during the electrolysis involving the SO2− ion [38]. The S2O2− 4 8 can also be produced at the expense of •OH by reaction (10). 2SO4 −

ð9Þ

→ S2 O2− 8

 2SO2− 4 þ OH

→ SO4 − þ OH−

ð10Þ

The oxidants generated from SO2− were more reactive and strong 4 enough to degrade the parent compound as well as ring opened/aliphatic metabolites which leads to an effective TOC removal. The inert nature of NO− 3 does not produce any such oxidants and thus moderate mineralization was observed in the case of NaNO3 as supporting electrolyte. The presence of chloride leads to the formation of chlorine based oxidants as per reactions (11–15). BDDð OHÞ þ Cl 2Cl

−

−

→ BDD þ ½Cl2ðgÞ þ OH−

→ Cl2ðgÞ þ 2e−

ð12Þ

−

−

Cl2ðgÞ þ H2 O ↔ ClO þ Cl þ 2Hþ 

ClOH

Fig. 7. (a) Effect of supporting electrolytes on the degradation of isothiazolinones on BDD anode (7.2 × 10−4 mol dm−3 isothiazolinones in 0.1 mol dm−3 electrolyte, current density: 25.0 mA cm−2, pH: 6; T, 25 °C, electrolytes: Na2SO4 (♦), NaNO3 (▲) and NaCl (●)). (b) UV–vis spectra of isothiazolinone degradation and the formation of OCl− during electrolysis (7.2 × 10−4 mol dm−3 isothiazolinones in 0.1 mol dm−3 NaCl, pH 6; T, 25 °C, current density: 25.0 mA cm−2).

−

OH þ Cl −

→ ClOH

ð11Þ

ð13Þ

−

ð14Þ



ð15Þ

→ OH− þ Cl

As seen in Fig. 7b, the hypochlorite (ClO−) ion formed as per reactions (12) and (13), was confirmed by the appearance of a peak at 292 nm in UV–vis spectra [24], and believed to be so reactive towards the parent compound to cause a rapid degradation of isothiazolin-3ones within no time. Further, the very poor mineralization observed could be inferred with the continuous oxidation of Cl− at a relatively

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lower potential which deteriorates the •OH formation and secondly, the generated chlorine based oxidant such as OCl− was not good enough at degrading the metabolites formed, probably short chain chlorinated aliphatic compounds, during the progressive electrolysis. Importantly, the nature of the oxidants generated from Cl− oxidation, which ultimately either favors TOC removal or accumulates as an unreactive molecule, was found to depend on the concentration of Cl− [30]. In addition, the formation of stable and chlorinated refractory intermediates could also cause poor mineralization to some extent which are more stable towards chlorine based oxidants. It is worthy to mention that no characteristic peaks for ClO− at 292 nm was observed in the case of Na2SO4 and NaNO3 medium even after a prolonged electrolysis period up to a complete mineralization. This result indicates that the anodic oxidation of isothiazolin-3-ones, particularly CMI, produces a negligible amount of inorganic Cl− and thus, these do not expect to have an impact on chlorine based oxidant generations and the process. 3.5. Influence of anode materials on isothiazolin-3-one degradation To confirm the superior nature of the BDD anode, a comparative study was carried out with three different anode materials viz. BDD, Pt and GC at a constant current density of 25 mA cm − 2 in 0.1 mol dm− 3 Na2SO4. The degradation and mineralization trend observed with respect to electrolysis time are shown in Figs. 9 and 8. The initial degradation rate constants of BDD, Pt and GC anodes were calculated to be 2.65, 1.57 and 0.28 × 10− 4 s− 1, respectively. The observed trend is easily understandable and could be inferred to the difference in the •OH generation efficiency of the respective anodic materials. The TOC analysis for the GC anode was not carried out as the degradation was very marginal. The mineralization of isothiazolin-3-ones at the BDD and Pt electrodes (Fig. 8) followed almost the same trend as for the degradation. On the BDD electrode, about 73% of TOC removal was found at 350 min and it was only 48% for Pt under similar experimental conditions. The poor degradation performance of GC was due to the severe deterioration of the electrode surface as it was dimensionally unstable. The moderate efficiency of the Pt anode could be attributed to the comparatively less generation of •OH. This is due to the fact that the Pt electrode is known to be an active material and its surface has a strong interaction with electrogenerated •OH, which leads to the quick evolution of molecular oxygen and concomitant generation of weaker oxidants instead of •OH. The BDD exhibits a non-active behavior towards electrogenerated •OH that results in a larger quantity of •OH generation rather than derivative oxidants from •OH. These results confirmed the superiority of the BDD anode in mineralizing the

Fig. 9. Comparison of anode materials on the degradation of isothiazolinones (7.2 × 10−4 mol dm−3 isothiazolinones in 0.1 mol dm−3 Na2SO4, current density: 25.0 mA cm−2, pH: 6; T, 25 °C, electrodes: BDD (♦), Pt (▲) and GC (●) having equal surface area of 4 cm2).

isothiazolin-3-ones within a shorter electrolysis period to the anodes employed. 3.6. Mineralization reaction pathway mechanism Though the generation of secondary oxidants (H2O2, S2O2− 8 ) is possible in the case of the SO2− 4 medium, that contributes towards the degradation to some extent, the highly reactive •OH, greatly produced as a result of galvanostatic electrolysis are conveniently expected to facilitate the decomposition of isothiazolin-3-ones and eventually a complete mineralization. The plausible reaction sequence for the isothiazolin-3-one oxidation was proposed as shown in Fig. 10 on the basis of the results obtained in the present study as well as the previous study of our group on the degradation of isothiazolin-3-ones by the photocatalytic method [11]. The degradation of isothiazolin-3-one molecules proceeds via the removal of a chloride ion from CMI and the ring opening takes place through the nitrogen (N)–sulfur (S) bond breaking. Since the S\\N bond is an ambiphilic reaction center [41], an electrophilic attack at nitrogen and a nucleophilic attack at a sulfur atom are quite possible. A subsequent hydrolysis and oxidation leads to the formation of SO2− and N-containing derivatives having a lesser number 4 of C atoms. The complete mineralization rates of the fragmented compounds were much slower than the ring opening (disappearance) of the original molecular compound. It is known that the organic compounds containing a nitrogen heteroatom are more difficult to get degraded than the compounds containing sulfur or oxygen [42]. Finally, N-containing derivatives having a smaller number of C atoms are converted to CO2 and NO− 3 with BDD(•OH) simultaneously but more slowly than the generated carboxylic acids, that accounts for the smaller rate of TOC removal at the final phase of the isothiazolin-3-one mineralization. 4. Conclusions For the first time, the feasibility of the electrochemical degradation of isothiazolin-3-ones at the BDD anode was extensively carried out in an aqueous solution containing Na2SO4 as a supporting electrolyte. The cyclic voltammetric studies have revealed that the oxidation of isothiazolin-3-ones occurs at potentials of 1.58 (MI) and 1.72 V (CMI) by a direct electron transfer reaction. The kinetic studies showed that the isothiazolin-3-one degradation mainly depends on the current density applied and the supporting electrolyte employed. The degradation rate at initial stages follows pseudo first order kinetics. The pseudo first order rate constants calculated were found to be increased linearly by applying current density. The TOC analysis confirmed the progressive mineralization of isothiazolin-3-one compounds. The mineralization current efficiency was found to be increased with decreasing applied current density, thus a low applied current would be beneficial to remove isothiazolin-3-ones with less consumption of energy. The decline trend of mineralization current efficiency with respect to a specific applied charge was due to the formation of persistent metabolites such as short chain carboxylic acids. The role of pH was very marginal and the mineralization was found to be almost equal in the pH conditions studied. The nature of the supporting electrolytes was found to influence the degradation of isothiazolin-3-ones to a greater extent and among which, the electrolyte containing SO2− ion showed better effi4 ciency in mineralizing the isothiazolin-3-one compounds. In the case of NaCl as a supporting electrolyte, the mineralization rate was found to be very poor, due to the generation of weaker chlorine based oxidants at a lower potential region at the expense of highly reactive •OH formation. The BDD anode showed better efficiency towards isothiazolin-3one degradation compared to the Pt and GC anodes. The five membered − ring was destroyed and finally converted into CO2, SO2− 4 , NO3 and HCl. Conclusively, anodic oxidation of an aqueous solution of isothiazolin-3ones, which are not naturally degradable and hardly biodegradable, using the BDD electrode Na2SO4, is feasible and an effective method.

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159

Fig. 10. A possible reaction pathway of isothiazolinone degradation.

Acknowledgements One of the authors, V. Kandavelu, is thankful to the Creative Department for Innovation (CDI) of Utsunomiya University, Utsunomiya, Japan, for the financial support of his work and to I. Nakazawa, H. Yamato and Y. Watanabe for their support in the work.

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