Phenol Degradation in Water by Pulsed Streamer Corona ... - CiteSeerX

Dors et al.

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Phenol Degradation in Water by Pulsed Streamer Corona Discharge and Fenton Reaction M. Dors, E. Metel, J. Mizeraczyk Centre for Plasma and Laser Engineering, The Szewalski Institute of Fluid Flow Machinery, Gdańsk, Poland Abstract—In this paper, results of phenol degradation in water induced by a positive pulsed streamer corona discharge are presented. A novelty of this work is studying an influence of the pulsed streamer corona discharge enhanced with the Fenton reaction on phenol degradation in the tap water. To our knowledge, the tap water has never been used before in such studies. The positive pulsed streamer corona discharge was generated between a stressed needle and a cylinder, both immersed in the water contaminated with phenol (0.62 mM). Iron ions in the form of FeSO4 (0.08 mM) were added to the water. They were supposed to enhance the phenol degradation through the Fenton reaction. It was found that the Fenton reaction enhanced significantly phenol degradation rate only in the distilled water of low conductivity (1 μS/cm). In the tap water, which has relatively high conductivity (600 μS/cm), the Fenton reaction was weak and had to be initiated by lowering the initial pH from alkaline (7.6) to acidic (4.1). During phenol degradation the water acidity increased due to organic acids, which were the secondary products of the phenol degradation. The primary products of the phenol degradation were dihydroxyphenols. Keywords—streamer corona discharge, water purification, Fenton reaction, phenol

I. INTRODUCTION A number of studies have utilized pulsed discharges generated either in the water bulk or in the gas phase over the water surface or inside gas bubbles introduced into the water bulk, to produce strongly oxidative species, such as hydroxyl radicals (OH), O atoms, ozone (O3) and hydrogen peroxide (H2O2) [1-16] with a goal of the degradation of aromatic compounds present in the water. The aromatic organic compounds present in the water are oxidized by these active species to simpler compounds, which further can be easily removed from the water using conventional bio-filters. It was already reported in [5, 12-16] that addition of iron salts to the water processed by the pulsed streamer corona discharge enhanced degradation of several organic compounds. This was due to the known Fenton reaction which have been widely used to the degradation of a wide range of organic compounds including herbicides [17], pesticides [18], dyes [19], phenol [20] and chlorinated phenols [21]. They have been utilized in combination with not only pulsed streamer corona discharge but also UV/TiO2 photocatalysts [22] and ultrasounds [23]. In the presence of iron salts in the water, hydrogen peroxide, the most abundantly produced oxidant by the streamer corona discharge, is converted to hydroxyl radical, which is much stronger oxidant than H2O2, through the Fenton reaction [12]: Fe2+ + H2O2 → Fe3+ + OH- + OH. (1) In most investigations concerning the application of the pulsed streamer corona discharges in water for organic compound decomposition, the deionised water with additives regulating conductivity was used [5, 9, 1214]. Since the studies on the pulsed streamer corona discharges in water are aimed at their application to the drinking water and wastewater purification we used also the tap water in our experiment. In this paper, results of Corresponding author: Mirosław Dors e-mail address: [email protected] Originally presented at ISNTPT-5, June 2006 Revised; November 20, 2006 and January 11, 2007, Accepted; January 18, 2007

the investigation of iron ions influence on phenol oxidation in both the distilled and tap water by pulsed positive streamer corona discharge are presented. To our knowledge, the tap water has never been used before in such studies. Thus, a novelty of this work is presenting an influence of the pulsed streamer corona discharge in combination with the Fenton reaction on phenol degradation in the tap water. II. METHODOLOGY The experimental set-up is presented in Figure 1. The pulsed positive streamer corona discharge was generated between a stressed stainless steel needle electrode and a grounded brass cylinder electrode. The needle-cylinder spacing was 55 mm. The electrodes were placed in a glass parallelepiped reactor (12.0 cm × 3.6 cm × 14.4 cm), which was filled with phenol-polluted water.

Fig. 1.

Experimental set-up.

The water was either distilled or tap water. The conductivity of the water was 1 μS/cm (pure distilled water), 200 μS/cm (distilled water with addition of NaCl) and 600 μS/cm (tap water). Initial concentration of phenol was 0.62 μM. Composition of the tap water, delivered from a deep-underground water intake, was as follows: chlorides – 17 mg/m3, sulfides – 31 mg/m3, Fe – 0.01 mg/m3, Mn – 0.022 mg/m3, F – 0.3 mg/m3, Ca – 92 mg/m3, Na – 37.4 mg/m3, K – 6.2 mg/m3, Mg – 16.2 mg/m3.

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International Journal of Plasma Environmental Science & Technology Vol.1, No.1, MARCH 2007

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Fig. 2. Typical images of the pulsed positive streamer discharge in the phenol-polluted water of different conductivities: (a) 1 μS/cm, (b) 200 μS/cm, (c) 600 μS/cm Initial phenol concentration 0.62 mM. Pulse energy 0.7 J.

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Fig. 3. Typical voltage and current pulses of the pulsed positive streamer discharge in the phenol-polluted water of different conductivities: (a) 1 μS/cm, (b) 200 μS/cm, (c) 600 μS/cm. Initial phenol concentration 0.62 mM. Pulse energy 0.7 J.

Initial concentration of Fe2SO4 was 0.08 mM. The water flowed through the reactor with a flow rate of 2.0 l/min and the external heat exchanger was used to keep the water temperature in the range of 20-25°C. The positive polarity high voltage pulses of 30 kV were applied to the needle electrode from a discharge capacitor C1 (2 nF), which was charged from a DC power supply through a resistor Rp (10 kΩ) and a capacitor C2 (22 nF). The repetition rate of the streamer corona pulses, determined by the rotation velocity of a rotary spark, was 50 Hz. The images of the streamer corona discharge in the phenol-polluted water are shown in Figs. 2a-c. Typical voltage and current pulses are shown in Figs. 3a-c. The concentration of phenol in the water was measured by a gas chromatography method before and after the streamer corona discharge processing. For this purpose a Shimadzu 17A gas chromatograph with Flame Ionization Detector and DB-WAX column heated from 150ºC up to 240ºC was used. A 2 μl sample of water was injected directly to the column. The water was analyzed also for the presence of hydrogen peroxide using the pertitanic acid formation method developed by Eisenberg [24] and modified by Lukes [25]. This method uses the reaction of H2O2 with titanyl ions giving yellow colored complex of pertitanic acid: (2) Ti4+ + H2O2 + 2 H2O → TiO2·H2O2 + 4 H+. The titanyl reagent was prepared by dissolving 4 g of titanium oxysulfate-sulfuric acid complex octahydrate TiOSO4·H2SO4·8H2O in 250 ml of sulfuric acid (50% solution). After dissolving of all hydrate the solution was diluted to 1 liter by sulfuric acid (33% solution). For measurement of H2O2 2 ml of sample and 1 ml of titanyl reagent were mixed and then the optical

absorbance was measured at 407 nm using Spekol 11 and Marcel 330 spectrometers. III. RESULTS AND DISCUSSION A. Phenol oxidation efficiency Generally, phenol oxidation efficiency decreased when the water conductivity increased. This effect is known and is attributed to the lower production of OH radicals in the water of higher conductivity (OH radicals are the main oxidant of phenol) [1, 4, 5]. In our experiment, when the phenol-polluted water (without iron ions) was processed by the pulsed streamer corona discharge the concentration of phenol in the water decreased during 90-minutes processing time by 42% and 10% in the distilled water of a conductivity 1 μS/cm and 200 μS/cm, respectively. In the tap water, the conductivity of which was 600 μS/cm, no phenol oxidation was observed (Fig. 4). In the above experiment, the amount of oxidized phenol was relatively small and we were not able to detect any primary phenol oxidation products, which usually are dihydroxyphenols (our dihydroxyphenols detection limit was 1 μg/cm3). Since the needle electrode was made of stainless steel one may suspect that in the processed water there were iron ions emitted from the needle electrode, which could participate in the Fenton reaction. If so, the above results obtained in the phenol-polluted water without addition of iron salt could be overestimated. However, analysis of a water without phenol (using the Atomic Absorption Spectroscopy) after 90-minute processing showed that concentration of iron ions in it was at most 4 μg/l. The enhancement of phenol oxidation by the Fenton reaction due to such a small amount of iron ions is negligible.

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Phenol concentration (mM)

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the water of different conductivities without phenol, processed by the pulsed streamer corona discharge. The same effect was reported earlier by other researchers [10, 11]. As it is seen in Fig. 6, in spite of the same conductivity of tap water and distilled water with NaCl (600 μS/cm), production of H2O2 in the tap water is higher. However, there was no significant difference in phenol oxidation efficiency when the tap water and distilled water of the same conductivity was processed by the corona discharge. 1 μS/cm (dist. water) 200 μS/cm (dist. water + NaCl) 600 μS/cm (tap water) 600 μS/cm (dist. water + NaCl)

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The presence of iron ions, added to the water in the form of Fe2SO4 (0.08 mM), improved the phenol oxidation efficiency due to enhanced OH radical production via the Fenton reaction (1). However, again the water conductivity influenced the phenol oxidation process. In the presence of iron ions the phenol concentration decreased by 78% and 52% in the distilled water of conductivity 1 μS/cm and 200 μS/cm, respectively (Fig. 5). However, our first processing of the phenol-polluted tap water, with iron ions showed no phenol oxidation at all. This was because the initial pH of tap water was 7.6, i.e. much far from the pH range of 3-6, in which the Fenton reaction works [26]. In order to initiate the Fenton reaction, the initial pH was lowered to 4.1 by addition of H2SO4 (this operation has changed the initial conductivity from 600 μS/cm to 680 μS/cm). As can be seen in Fig. 5, in such a water, phenol oxidation efficiency was 20%. 1 μS/cm (dist. water) 200 μS/cm (dist. water + NaCl) 600 μS/cm (tap water + H2SO4)

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Fig. 5. Concentration of phenol in the phenol-polluted water (with iron ions) of different conductivities: 1 μS/cm, 200 μS/cm, 600 μS/cm. Initial phenol concentration 0.62 mM. Initial concentration of Fe2SO4 0.08 mM. Water amount 500 cm3. Pulse energy 0.7 J.

The phenol oxidation efficiency decrease with water conductivity, shown in Fig. 5, is due to lower production of H2O2, which is the substrate in the Fenton reaction (1). The decrease in H2O2 production is shown in Fig. 6, which presents H2O2 concentration in

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Fig. 6. Concentration of hydrogen peroxide in pure water of different conductivity. Water amount 500 cm3. Pulse energy 0.7 J.

When the phenol oxidation, enhanced by the Fenton reaction, was relatively strong, we detected dihydroxyphenols in the water (Fig. 7). However, the concentration of dihydroxyphenols was much lower than suggested by reactions of phenol with OH radicals. This may mean that the phenol was converted also to other products, not measured in this experiment. As it is seen in Fig. 7, after 30-40 minutes of the pulsed discharge processing the increase in concentration of dihydroxyphenols stopped. A similar effect was observed by Kusič et al. [9] when a pulsed corona discharge above the water surface was employed for phenol degradation. Dihydrohyphenols concentration (mM)

Fig. 4. Concentration of phenol in the phenol-polluted water (without iron ions) of different conductivities: 1 μS/cm, 200 μS/cm, 600 μS/cm. Initial phenol concentration 0.62 mM. Water amount 500 cm3. Pulse energy 0.7 J.

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Fig. 7. Concentration of dihydroxyphenols in the phenol-polluted water (with iron ions) of different conductivities: 1 μS/cm, 200 μS/cm, 600 μS/cm. Initial phenol concentration 0.62 mM. Initial concentration of Fe2SO4 0.08 mM. Water amount 500 cm3. Pulse energy 0.7 J.

The initial increase (before 30-40 min.) and the following plateau or decrease (after 30-40 min.) in dihydroxyphenols concentration can be explained by two processes which occur simultaneously. It is known that dihydroxyphenols are mainly formed in the oxidation


reactions of phenol caused by OH radicals [1]. On the other hand, OH radicals react with dihydroxyphenols, converting them into organic aliphatic compounds (mostly into acids, such as muconic acid, oxalic acid, fumaric acid etc.). In the initial stage of the processing, when the phenol concentration is high, the production of dihydroxyphenols dominates over their conversion into the organic aliphatic compounds. Therefore, the increase in dihydroxyphenols concentration is observed. However, when phenol concentration became low (after 30-40 min. of the processing) the dihydroxyphenols production became equal or lower to their conversion into the aliphatic compounds, and their concentration reached plateau or decreased. B. Changes in pH Unfortunately, in our experiment we were not able to detect directly organic acids in the water. However, if in our experiment organic acids were really formed, then we would have to observe an increase in acidity of the water. Indeed, the measurements of the water pH showed an increase in its acidity (Fig. 8). The decrease in pH, observed in Fig. 8, corresponded to the amount of oxidized phenol. When there was no iron ions in the distilled water of conductivity 1 μS/cm decrease in phenol concentration was relatively high, so pH decreased from initial 6.3 to 4.9. In the distilled water of conductivity 200 μS/cm, change in the phenol concentration was lower and consequently pH decreased only to 6.0. Since in the tap water without iron ions there was no decrease in the phenol concentration, there was no change in the pH too. a)

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As seen in Fig. 8b, in the water with iron ions more phenol molecules were decomposed, presumably to organic acids, and consequently the pH was lower than in the water without iron ions. In contradiction to the case without iron ions, the pH decreased not linearly but with a rapid drop at the beginning of the processing and then with the slower decrease (Fig. 8b). A very similar shape of the pH curve in the water with iron ions, with a rapid decrease at the beginning of the process and the following slow decrease was predicted by the numerical model developed by Grymonpré et al. [12]. In our experiment, such a behaviour concerns the distilled water of conductivity 1 μS/cm and 200 μS/cm. In the case of the tap water with phenol and iron ions, the initial pH was decreased artificially to 4.1 by addition of H2SO4, what allowed to initiate the Fenton reaction. The pH drop observed after initiation of the Fenton reaction was 0.2 only, i.e. to 3.9, and corresponded to the relatively small amount of oxidized phenol.

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Fig. 8. The pH of the phenol-polluted water of different conductivities without (a) and with iron ions (b). Initial pH of the tap water of 7.6 was lowered to 4.1 by addition of H2SO4. Initial phenol concentration 0.62 mM. Water amount 500 cm3. Pulse energy 0.7 J.

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Fig. 9. Concentration of H2O2 in the phenol-polluted water with and without iron ions. Water conductivity: (a) 1 μS/cm, (b) 200 μS/cm, (c) 600 μS/cm. Initial phenol concentration 0.62 mM. Pulse energy 0.7 J.

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C. Production of hydrogen peroxide

D. Energy yields As it seen in Fig. 10, energy yields resulted from the experiment with phenol and iron ions in the water corresponds to the concentrations of phenol presented in Fig. 5. The highest energy yields were obtained at low water conductivity. In the case of tap water they did not exceed 1×10-9 mol/J.

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According to the Fenton reaction (1) most of H2O2 molecules formed during the discharge are supposed to be converted into OH radicals which are dominant oxidants of phenol. Therefore, H2O2 molecules are indirectly responsible for the phenol degradation. This is consistent with the results presented in Fig. 9, which shows that the concentration of H2O2 in the water containing iron ions, i.e. with the Fenton reaction present, during the streamer corona discharge processing is much lower than in the water without iron ions. On the other hand, the concentration of H2O2 in the pure water is only slightly higher than in water with phenol. This means that in the phenol-polluted water without iron ions there was very small consumption of H2O2 in chemical reactions (the Fenton reaction strongly converting H2O2 to OH was absent due to the lack of iron ions). In the distilled water of conductivity 1 μS/cm, when iron ions converting H2O2 to OH were present in the water, the concentration of H2O2 first increased with processing time and then decreased, being all the time much lower than that in the distilled water without iron ions (Fig. 9a). The explanation of the initial increase (before 30 min.) and the following decrease (after 30 min.) in H2O2 concentration is the Fenton reaction. It is known that the Fenton reaction rate is sensitive to the pH of water. The pH optimum for the Fenton reaction is in the range of 3-6 with the maximum reaction rate at 4.2 [25]. Since phenol was decomposed through dihydroxyphenols to organic acids, the pH of phenolpolluted water decreased with streamer corona discharge processing time. As seen in Fig. 8b, in the presence of iron ions the pH of the water decreased from initial value of 6.2 to about 4, which is the optimum value for the Fenton reaction rate. Thus, with processing time more organic acids were produced from phenol, and the increase in the Fenton reaction rate caused higher decomposition of H2O2 (Fig. 9a) and enhanced production of OH radicals, which oxidized the phenol molecules to organic acids. The same processes occur in the distilled water of conductivity 200 μS/cm and in the tap water (Fig. 9b, c). However, due to the lower production of H2O2 no maximum in its concentration was observed. In the tap water, the production of H2O2 molecules was so small that all H2O2 molecules were consumed in the Fenton reaction after its initiation by addition of H2SO4 (Fig. 9c).

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Fig. 10. Energy yields of phenol oxidation in the phenol-polluted water with iron ions of different conductivities: 1 μS/cm, 200 μS/cm, 600 μS/cm. Initial phenol concentration 0.62 mM. Initial concentration of Fe2SO4 0.08 mM. Water amount 500 cm3. Pulse energy 0.7 J.

Tab. I summarizes the results of phenol degradation experiments carried out by us and presented in [4, 5, 12], basing on the formula given in [6]. In order to compare the phenol degradation efficiency of the mentioned experiments, a G yield value has been calculated, expressing the number of converted phenol molecules divided by the energy input required. The G yield value at 50% phenol conversion is given by [6]: 0 .5 ⋅ C 0 ⋅ V , (3) G 50 = E p ⋅ f ⋅ t 50 where C0 is the phenol concentration at t = 0, V is the aqueous phase volume, Ep is the discharge pulse energy, f is the pulse repetition rate and t50 is the time required for 50% phenol conversion. TABLE I COMPARISON OF PHENOL CONVERSION USING DIFFERENT CORONA DISCHARGE SYSTEMS

Reference C0 (mM) Fe2+ (mM) O2 addition σ (μS/cm) V (cm3) tp (μs) Ipeak (A) Upeak (kV) Ep (J) f (Hz) t50 (min) G50 (mol/J)

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[12]

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this work

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0.62 0.08 200 500 2 23 37 0.7 50 60 2.3x10-9

C0 – initial phenol concentration, Fe2+ - addition of iron ions to the water, O2 – addition of oxygen to the water, σ – initial conductivity of water, V - water volume, tp – pulse duration measured at half-maximum, Upeak – discharge voltage peak, ,Ipeak – discharge current peak, Ep - discharge pulse energy, f- pulse repetition rate, t50 time required for 50% phenol conversion. G yield value is expressed in mol/J.

As it is seen from Table. I, energy yields in different corona discharge systems are in the same order of magnitude. It seems that the G50 values may be correlated with pulse energies. Since in our experiment the pulse energy was the lowest in comparison to other systems presented in Tab. I, consequently the G50 yield was the

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lowest. Moreover, initial concentration of iron salt could also influence the efficiency of phenol oxidation and resulted G50 yield. As was calculated by Grymonpré et al. [12], optimal concentration of iron salt is 0.5 mM. Both at lower and higher iron concentrations phenol oxidation efficiency is decreased. IV. CONCLUSIONS In this paper, results of the investigation of iron ions influence on phenol oxidation in both the distilled and tap water by pulsed positive streamer corona discharge are presented. To our knowledge, the tap water has never been used before in such studies. Thus, a novelty of this work is presenting an influence of the pulsed streamer corona discharge in combination with the Fenton reaction on phenol degradation in the tap water. The results of the investigations showed that: • The Fenton reaction enhanced significantly phenol degradation rate (from 42% to 78%) only in the distilled water of low conductivity. • In the tap water, which has relatively high conductivity (600 μS/cm), the Fenton reaction was weak and had to be initiated by lowering the initial pH from alkaline (7.6) to acidic (4.1). • Enhanced phenol degradation to organic acids resulted in lower pH (in the water of a low conductivity pH reached value optimal for the Fenton reaction, i.e. pH = 4.1). • The presence of iron ions in the water did not influence the phenol degradation products, which are dihydroxyphenols (primary products) and organic acids (secondary products). A general conclusion arises from the above experiment that the Fenton reaction combined with the pulsed streamer corona discharge is not effective in the purification of drinking water. Thus, other chemical activators or other types of discharges are needed for this purpose. ACKNOWLEDGEMENT This work was supported by the Institute of Fluid Flow Machinery, Polish Academy of Sciences under programme IMP PAN O3Z3T2. The designing and manufacturing of the pulsed power supply used in this experiment by R. Michalski and Prof. A. Wolny is acknowledged. REFERENCES [1] B.R. Locke, M. Sato, P. Sunka, M.R. Hoffmann, J.S. Chang, “Electrohydraulic discharge and nonthermal plasma for water treatment“, Ind. Eng. Chem. Res., vol. 45, pp. 882-905, 2006. [2] A.A. Joshi, B.R. Locke, P. Arce, W.C. Finney, ”Formation of hydroxyl radicals, hydrogen peroxide and aqueous electrons by pulsed streamer corona discharge in aqueous solution”, Journal of Hazardous Materials, vol. 41, pp. 3-30, 1995.

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