Buletinul Ştiinţific al Universităţii “Politehnica” din Timisoara, ROMÂNIA Seria CHIMIE ŞI INGINERIA MEDIULUI Chem. Bull. "POLITEHNICA" Univ. (Timişoara)
Volume 53(67), 1-2, 2008
Removal of Nonylphenol Polyethoxylate by Electrochemical Oxidation at Modified SnO2 Electrodes M. Ihoş*, F. Manea**, A. Iovi** *
National Research and Development Institute for Industrial Ecology–ECOIND, P-ta Victoriei nr.2, 300006 Timisoara, Romania, Phone/Fax: ++40256220369, E-Mail:
[email protected]
** “
Politehnica” University of Timisoara, Faculty of Industrial Chemistry and Environmental Engineering, P-ta Victoriei nr.2, 300006 Timisoara, Romania, Phone/Fax: ++40256403070, E-Mail:
[email protected]
Abstract: Release of nonionic surfactants like nonylphenol polyethoxylates in the environment generates metabolites that cause a number of estrogenic responses on aquatic organisms and thus they have been classified as endocrine disruptors. This paper deals with the electrochemical oxidation of nonylphenol ethoxylate with 9 ethoxy units (NP9EO) at Ti/RuO2/SnO2-Sb2O5-RuO2 electrodes. The doped SnO2 electrocatalytic film and the intermediate layer of RuO2, respectively, were obtained by thermal decomposition of appropriate chlorides at 550 °C and 450 °C, respectively. The electrocatalytic film was characterized by Fourier transform infrared spectroscopy (FT-IR) and scanning electron microscopy (SEM). The electrodes were used for the removal of pollutant by electrochemical oxidation. The electrolysis were carried out by using 0.1 g/L NP9EO in 0.1 M Na2SO4 as supporting electrolyte, applied current densities of 25, 50, 75 and 100 A/m2, electrolysis time of 15, 30, 60, 120 minutes. The residual concentration of the pollutant was assessed by direct spectrophotometric determination in solution. For an applied current density of 100 A/m2 and 120 minutes of electrolysis the residual concentration of NP9EO was 33.34 mg/L. Keywords: electrochemical oxidation, modified SnO2 electrodes, nonylphenol ethoxylates
1. Introduction Nonylphenol ethoxylates (NPEs) are nonionic surfactants and they are used as wetting, dispersing, washing and emulsifier agents. NPEs are produced for high-volume use in many industrial sectors, including industrial laundering; textile processing; mining, flotation and petroleum productions; metallurgical industry; building and building materials industry; rubber, plastics and synthetic resins industry; paints and lacquers formulation; pulp and paper industry; food industry; cosmetics and pharmaceutical production and agrochemicals products. The largest quantity of NPEs is used in cleaning products, especially detergents and cleaners. NPEs biodegrades very slowly [1] and additionally this type of substances released in the environment generate metabolites with variable estrogenic activity such as nonylphenol (NP), nonylphenol ethoxylate (NP1EO), nonylphenol diethoxylate (NP2EO) [2-5]. These metabolites disrupt normal hormonal functioning in the body and thus are considered endocrine-disrupting chemicals. A large amount of NPEs metabolites enters the aquatic system and because of their toxicity is strongly necessary to apply appropriates processes for removal or improvement the biodegradability of NPEs metabolites. Electrochemical processes such as electrocoagulation [2,6], electrochemical Fenton [2] and electrochemical oxidation with Co2+ -promoted PbO2 anode [3] or carbon fibre anode [7] have been applied with promising results.
The electrochemical oxidation of NPEs at DSA type electrode has not been sufficient studied. The aim of this study was the degradation of NP9EO at modified SnO2 anodes prepared by thermal decomposition of appropriate precursors.
2. Experimental 2.1. Ti/RuO2/SnO2-Sb2O5-RuO2 anodes preparation The anodes were prepared by using titanium plates as a support. The plates were treated by sand-blasting followed by a chemical treatment with technical hydrochloric acid 15% for 20 minutes under boiling. Then, the plates were washed with water, distilled water and absolute ethylic alcohol reagent degree (Chimopar Bucureşti) followed by drying in open air. The precursors solution was brushed onto plates surface. The intermediate layer of RuO2 was obtained by thermal decomposition of a precursor solution obtained by dissolving RuCl3.nH2O (Fluka) in isopropanol reagent degree (Chimopar Bucureşti) at 450 ºC. The electrocatalytic layer of SnO2-Sb2O5-RuO2 was prepared as follows: the precursors solution was obtained by dissolving the appropriate amount of SnCl4.5H2O (Aldrich), SbCl3 (Aldrich) and RuCl3.nH2O (Fluka) in the solution containing hydrochloric acid (37%) and isopropanol reagent degree (Chimopar Bucureşti). The molar ratio Sn:Sb: Ru in precursors solution was 94:3:3.
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The anodic material was further treated by heating to 110 ºC in a drying cabinet. After that, the electrode was treated at 550 ºC for 10 minutes to decompose the salts from precursors solution. This cycle brushing – drying – thermal decomposition was repeated 15 times. Finally, a thermal treatment was applied for 1 hour to stabilize the oxide film. 2.2. Measurement characterization
techniques
for
anodes
FT-IR spectrum of thin film was recorded on a Shimadzu model IR Prestige-21 FT-IR spectrophotometer, IR Solution version 3.1 software. The spectrum was recorded from 4000 to 400 cm-1 for diffuse reflexion mode. SEM images of the thin film were obtained at 2,200 magnification by using a Philips CM30T scanning electron microscope. The microscope was operated at an accelerating voltage of 20 kV. 2.3. Electrolysis Electrochemical degradation of NP9EO was carried out in a plexiglass cell at Ti/RuO2/SnO2-Sb2O5-RuO2 anodes. Two anodes and three stainless steel cathodes were inserted at 1 cm gap. The active surface area was 38 cm2. Experiments were carried out by applying current densities of 25, 50, 75 and 100 A/m2 at various electrolysis times: 15, 30, 60 and 120 minutes. Experiments were carried out in solutions of 100 mg/L NP9EO in 0.1 M Na2SO4 as supporting electrolyte. NP9EO was a technical product and it was used as the supplier provided it. Solutions were prepared with distilled water and Na2SO4 (Chimopar Bucureşti) reagent grade. Chemical structure of NP9EO is shown in Fig. 1.
O(CH2CH2O)9H
Figure 2. UV spectrum of NP9EO in 0.1 M Na2SO4
3. Results and discussion 3.1. Anodes characterization Fig. 3 shows the FT-IR spectrum of the thin film. The bands at 594 and 771 cm-1 in the FT-IR spectrum showed the presence of metallic oxides in the electrocatalytic film, which is in line with the literature data that assign the metal-oxide vibration in the spectral range 800-600 cm-1 [8]. Also, previous works assigned the bands below 800 cm-1 to SnO2 [9] and the bands at 660, 620 şi 540 cm-1 to Sn-O stretching mode region [10]. The absence of the characteristics bands for the antimony and bismuth oxides is probably due to the low content in the electrocatalytic film, as it was mentioned the molar ratio Sn: dopping metal in the precursors solution was 94:3. The SEM image (Fig.4) exhibited a relative compact surface morphology, the coating having few mud-like cracks and pores. The presence of the cracks will facilitate the formation of TiO2 with poor conductivity. As a result, the resistance of the anode surface during the electrolysis will increase and the activity of the electrode would be also reduced [11]. 3.2. Removal experiments
C9H19 Figure1. Chemical structure of NP9EO
2.4. UV spectra of the electrolysed solutions The UV spectra of the electrolysed solutions were recorded by a Jasco V-530 spectrophotometer controlled by computer. The residual concentration of NP9EO within electrolysed solutions was assessed by direct spectrophotometric determination in solution by using a calibration curve at absorbance of 225 nm (A225). The UV spectrum of NP9EO in 0.1 M Na2SO4 is shown in Fig.2.
The spectra of the electrolysed solutions showed the same shape like the initial solution of NP9OE for all applied current densities and any electrolysis times. As the electrolysis time increased, the value of the peak absorbance decreased and no additional new peaks developed. For exemplification, Fig. 5 shows the recorded spectra at an applied current density of 100 A/m2 and various electrolysis times. The experiments proved that results are in accordance with the literature data. Comninellis [12] has demonstrated that the mechanism of the electrochemical oxidation of phenol at SnO2 electrodes involves a first stage of phenol hydroxylation and its transformation in aromatic intermediates, a second stage of aromatic ring opening with the aliphatic acids formation and a third stage of acids oxidation to CO2.
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100 Transmittance / %
30
Transmittance / %
80
60
-1
771 cm
25
-1
594 cm
20 15 10 5 0 800
750
700
650
600
Wavenumber / cm
550
-1
Table 1 shows the residual concentration of NP9EO and the working conditions during the electrochemical degradation at Ti/RuO2/SnO2-Sb2O5-RuO2 anodes. It can be easily observed that the best result for NP9EO residual concentration was 33.34 mg/L and it was obtained at an applied current density of 100 A/m2 and 120 minutes of electrolysis.
40
TABLE 1. Working conditions, values of absorbance at 225 nm and NP9EO residual concentration; 20
Current density / (A/m2)
0
4000
3500
3000
2500
2000
Wavenumber / cm
1500
1000
Time / (min)
Cell voltage / (V)
A225
*NP9EO residual concentration / mg/L
500
-1
Figure 3. FT-IR spectrum of Ti/RuO2/SnO2-Sb2O5-RuO2
15 2,9 60 3,0 120 3,0 15 3,3 50 30 3,3 120 3,3 30 3,4 75 60 3,4 120 3,4 30 3,6 100 60 3,6 120 3,6 *NP9EO initial concentration: 100 mg/L 25
1,20 1,10 0,97 1,20 1,16 0,73 1,15 0,96 0,59 1,11 0,85 0,48
85,89 78,59 69,10 85,89 82,97 51,58 82,24 68,37 41,36 79,32 60,34 33,34
4. Conclusions
Figure 4. SEM image of Ti/RuO2/SnO2-Sb2O5-RuO2
Figure 5. UV spectra of NP9OE in 0.1 M Na2SO4: applied curent density 100 A/m2; i - initial solution; 1 – 30 min; 2 – 60 min; 3 – 120 min
The aim of this paper was the removal of NP9EO by electrochemical oxidation at SnO2 modified electrodes, Ti/RuO2/SnO2-Sb2O5-RuO2. Thus, the doped SnO2 electrocatalytic film and the intermediate layer of RuO2, respectively, were obtained by thermal decomposition of appropriate chlorides at 550 °C and 450 °C, respectively. The molar ratio Sn:Sb: Ru in precursors solution was 94:3:3. The electrocatalytic film was characterized by Fourier transform infrared spectroscopy (FT-IR) and scanning electron microscopy (SEM). The FT-IR spectrum showed the presence of SnO2 in the electrocatalytic film in accordance with literature data that assigned the bands below 800 cm-1 to SnO2. The absence of the characteristics bands for the antimony and bismuth oxides is probably due to their low content in the electrocatalytic film. The SEM image exhibited a relative compact surface morphology, the coating having few mud-like cracks and pores. The electrolysis were conducted as following: 0.1 g/L NP9EO in 0.1 M Na2SO4 as supporting electrolyte, applied current densities of 25, 50, 75 and 100 A/m2, electrolysis time of 15, 30, 60, 120 minutes. The residual concentration of the pollutant was assessed by direct spectrophotometric determination in solution. For an applied current density of 100 A/m2 and 120 minutes of electrolysis the residual concentration of NP9EO was 33.34 mg/L.
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6. Ciorba, G.A., Radovan, C., Vlaicu, I., Mâşu, S., J. Appl. Electrochem, 2002, 32, 651. 7. Kuramitz, H., Saitoh, J., Hattori, T., Tanaka, S., Water Res., 2002, 36, 3323. 8. Vazquez-Gomez, L., Horvath, E., Kristof, J., Appl. Surf. Sci., 2006, 253, 1178. 9. Huang, L., Wei, H.B., F.S. K, Cai, J.S., Fan, X.Y., Sun, S.G., Colloids Surf., A, 2007, 308, 87. 10. Ha, H.W., Kim, K., Borniol, M., Toupance, T., J. Solid State Chem., 2006, 179, 702. 11. Ding, H.Y., Feng, Y.J., Liu, J.F., Mater. Lett. 2007, 61, 4920. 12. Ch. Comninellis, Environmental Electrochemistry, Elsevier, Amsterdam, 1994.
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