- Email: [email protected]
Ultrasonic dehalogenation and toxicity reduction of trichlorophenol
Ultrasonics Sonochemistry 12 (2005) 121–125 www.elsevier.com/locate/ultsonch
Ultrasonic dehalogenation and toxicity reduction of trichlorophenol Andreas Tiehm
a,*
, Uwe Neis
b
a
b
Department of Environmental Biotechnology, Water Technology Center, Karlsruher Str. 84, 76139 Karlsruhe, Germany Department of Sanitary Engineering, Technical University of Hamburg-Harburg, Eißendorfer Str. 42, 21073 Hamburg, Germany Received 13 February 2004; accepted 24 May 2004 Available online 3 August 2004
Abstract The study focussed on the effect of ultrasonic frequency and co-pollutants on dechlorination and toxicity reduction of a toxic model pollutant, i.e. 2,3,5-trichlorophenol (TCP). The effect of ultrasonic frequency on TCP degradation and chloride formation was studied at 41, 206, 360, 618, 1068, and 3217 kHz. Most efficient ultrasonic dechlorination was achieved at 360 kHz. The degradation of TCP and adsorbable organic halogens followed pseudo-first-order rate kinetics. Toxicity in the bioluminescence test increased during the initial sonication period, indicating the temporary formation of more toxic reaction products. Subsequently, toxicity was significantly reduced. Dehalogenation efficiency decreased in the presence of the hydrophobic radical scavenger t-butanol, whereas hydrophilic co-pollutants such as acetate or glucose did not interfere with ultrasonic dechlorination and toxicity reduction. After ultrasonic pre-treatment, a fast biodegradation of the remaining organic pollutants was observed. In conclusion, the results demonstrate the potential of integrated ultrasonic/biological approaches for the treatment of wastewaters containing toxic pollutants. 2004 Elsevier B.V. All rights reserved. Keywords: AOX; Biodegradation; Dehalogenation; Toxicity; Trichlorophenol; Ultrasound frequency
1. Introduction Chlorinated phenols are applied as disinfectants and pesticides, and represent hazardous pollutants due to their persistence in the environment [1]. Due to their high toxicity, a high concentration of chlorophenols, e.g. in industrial wastewater or landfill leachate, can cause upsets in the operation of the biological activated sludge process, or make a microbial treatment impossible [2]. A number of studies have shown that sonication is suitable to degrade environmental pollutants [3], including 2- and 4-chlorophenol [4–6]. However, a complete mineralization of chlorophenols or real wastewaters by ultrasound would require large amounts of *
Corresponding author. Tel.: +49 721 9678 220; fax: +49 721 9678 101. E-mail address: [email protected] (A. Tiehm). 1350-4177/$ - see front matter 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.ultsonch.2004.05.013
energy [4,5,7]. Therefore, a combination of ultrasonic pre-treatment followed by biodegradation is promising in order to develop an economically more favourable integrated technique [7–10]. Environmental applications of ultrasound have been reported for a broad range of ultrasonic frequencies, starting from 20 kHz up to several MHz [11,12]. At low frequency, e.g. 20 kHz, the hydromechanical forces produced by cavitation are most pronounced [13]. Fast sonochemical reaction rates were reported at frequencies of about 200–800 kHz [14]. Recently, degradation of chlorophenol also was reported at 1.7 MHz [15]. For chlorophenols, sonochemical degradation due to OH radicals entering the bulk liquid phase and pyrolytic combustion inside the cavitation bubble have been demonstrated [6,15]. At alkaline pH the ionic species dominate and react only with radicals that enter the bulk liquid. The molecular chlorophenol species present at
122
A. Tiehm, U. Neis / Ultrasonics Sonochemistry 12 (2005) 121–125
acidic pH diffuse more easily into the interfacial region of the cavitation bubble where the concentration of radicals is high. Therefore, sonochemical degradation of chlorophenols is more efficient at low pH [4,6]. However, the yield of sonochemical reactions also is influenced by the presence of other compounds that might act as radical scavengers [16]. The aim of this study was to examine sonochemical chlorophenol degradation and toxicity reduction under varying boundary conditions. 2,3,5-Trichlorophenol (TCP) was used as toxic chlorinated model pollutant. Ultrasonic irradiation was done at pH 4 and optimized with respect to ultrasound frequency. The effect of hydrophilic and hydrophobic co-pollutants on dehalogenation and toxicity reduction was evaluated.
2. Materials and methods 2.1. Ultrasonic treatment All sonochemical experiments were done in air-saturated 10 mM phosphate buffer solution, adjusted to pH 4.0 by addition of H2SO4. The initial concentration of TCP ranged from 0.80 to 0.95 mM (156–188 mg/l). The ultrasound transducers were purchased from L-3 Communications ELAC Nautik GmbH, Kiel, Germany. The disk transducers (transducer area 25 cm2) were fixed at the bottom of a cylindrical tube equipped with a stirrer and a thermostatic jacket (Fig. 1). Transducers of similar shape were used to study the effect of different frequencies between 41 kHz and 3.2 MHz. The ultrasonic experiments were done at 20 ± 5 C with 500 ml samples. Power input was adjusted by calorimetric measurement [17]. The power input of the transducers operating at 41 kHz and 3.2 MHz was limited to 40 W. Therefore, in experiments studying the effect of ultrasonic frequency, sonication was done
for 60 min with 40 W power input. In the other experiments, treatment was done at 360 kHz for 240 min with 100 W power input. 2.2. Chemical analysis 2,3,5-Trichlorophenol was monitored using a high performance liquid chromatograph (HPLC) equipped ¨ berlingen, with a UV-detector (Perkin Elmer, U Germany). A Nucleosil 120-C18 (100 mm · 4 mm) column and an eluent consisting of methanol/water (60/ 40) was used for separation. Measurement of adsorbable organic halogens (AOX) was done with an AOX-System 2000 C (Haberkorn and Braun, Mu¨nchen, Germany) according to the German standard method DIN 384109 H14 [18]. A 1-ml sample was transferred into 100 ml deionized water and 50 mg granular activated carbon (GAC) was added. The suspension was shaken for 1 h. The GAC loaded with AOX was filtered through 0.4 lm polycarbonate membrane filters and the retained GAC washed with 25 ml of a 10 mM nitrate solution to remove inorganic chloride. The GAC was combusted at 950 C in a stream of pure oxygen. The chloride produced by combustion was absorbed in a titration cell and quantified by microcoulometry. The chemical oxygen demand (COD) was determined by oxidation of the organic compounds with K2Cr2O7. The produced Cr3+ was analysed colorimetrically. Chloride analysis was done after reaction with Hg(SCN)2 in the presence of Fe(III). The colored reaction products were determined colorimetrically. COD and chloride were determined with reagents and equipment purchased from Dr. Lange GmbH, Du¨sseldorf, Germany. The standardized analytical methods were performed according to the German Standard Methods for Water and Wastewater Analysis [18]. 2.3. Toxicity analysis
solution to be sonicated vibrating plate cooling fluid generator transducer
Fig. 1. Experimental set-up for the sonication experiments.
Toxicity was determined in (i) the bioluminescence test and (ii) by monitoring biological oxygen consumption of sonicated samples. (i) The bioluminescence test was performed with Vibrio fisheri according to the European standard procedure EN ISO 11348. The marine bacterium V. fisheri emits light during physiological activity. A decreasing light intensity is correlated with an increasing toxicity of the sample tested. (ii) In order to study the biodegradability of the sonicated samples, mineral salts suitable for microbial growth were added and the pH was adjusted to 7.0. Inoculation was done with activated sludge obtained from a municipal wastewater treatment plant, and biological degradation was monitored in a respirometer at 20 C.
A. Tiehm, U. Neis / Ultrasonics Sonochemistry 12 (2005) 121–125
3. Results and discussion
123
100 TCP
3.1. Effect of ultrasound frequency on sonochemical degradation of TCP
3.2. Kinetics of TCP and AOX degradation The kinetics of TCP decomposition, of the reduction of the total adsorbable organic halogens (AOX), and chloride formation are shown in Fig. 3. These experiments were performed at the most favourable ultrasonic frequency, i.e. 360 kHz. A complete disappearance of
Chloride
[%]
A pronounced sonochemical decomposition was observed in the frequency range between 206 and 1068 kHz with the highest efficiency at 360 kHz. Only small effects were obtained at 41 kHz, and no sonochemical degradation was observed at 3.2 MHz (Fig. 2). Increasing TCP decomposition corresponded to an increasing formation of chloride. At 360 kHz, 42% reduction of TCP and 25% of the calculated maximum possible chloride formation were achieved. Obviously, chloride formation was lower than had been calculated from TCP degradation. However, the chemical oxygen demand (COD) was reduced by only 15% demonstrating that mineralization of the phenolic structure was low. Obviously, dechlorination was the predominating sonochemical process.
AOX
80 60 40 20 0
0
60
120
180
240
time [min]
Fig. 3. Kinetics of TCP and AOX degradation and formation of chloride (360 kHz, pH 4). The 100% value of chloride corresponds to the initial chlorine amount of TCP.
2,3,5-trichlorophenol was achieved within a sonication time of 180 min. After 240 min, the AOX were reduced to 31%. Corresponding to AOX reduction, the chloride concentration increased from zero to about 70% of the calculated maximum concentration possible. The sonochemical degradation rates followed pseudo-first-order kinetics (Table 1). In previous studies [10], sonochemical degradation of 2,4-dichlorophenol (DCP) had been studied under same conditions with the same equipment. The results are shown for comparison purposes (Table 1). Obviously, ultrasonic degradation of TCP and of the related AOX proceeded slower than sonochemical decomposition of DCP. Since the pKa values of diand trichlorophenols are beyond pH 6 [19] and sonication was done at pH 4, the molecular species of the chlorophenols dominated in both cases. Most probably the different rates were due to the limited amount of reactive species such as OH radicals produced by ultrasonic cavitation. Since more reactive species are required for TCP dechlorination as compared to DCP, the observed reaction rate is slower. 3.3. Effect of co-pollutants on TCP dechlorination
Fig. 2. Effect of ultrasonic frequency on TCP decomposition and chloride formation. The 100% value of chloride corresponds to the initial chlorine amount of TCP.
Sonochemical degradation of TCP was also studied in the presence of hydrophilic and hydrophobic co-pollutants. Addition of co-pollutants was normalized with respect to COD. Addition of 900 mg/l acetate, 900 mg/l glucose, or 385 mg/l t-butanol corresponded
Table 1 Rate coefficients for sonochemical degradation of 2,3,5-trichlorophenol (TCP) and the corresponding AOX Compound
Pseudo-first-order rate coefficients Original substance k (min 1)
2,3,5-Trichlorophenol 2,4-Dichlorophenol [10]
11.1 · 10 18.9 · 10
AOX r2
3 3
0.99 0.99
k (min 1) 4.1 · 10 8.2 · 10
3 3
r2 0.99 0.99
Rate coefficients for 2,4-dichlorophenol (DCP) were determined under identical experimental conditions and are shown for comparison purposes.
124
A. Tiehm, U. Neis / Ultrasonics Sonochemistry 12 (2005) 121–125
toxicity [1/EC 50]
100 80 60 40 20 0 0
to a COD of 1000 mg/l, a value often observed in industrial wastewater. The presence of the hydrophilic co-pollutants, i.e. acetate and glucose, only moderately affected the dechlorination of TCP (Fig. 4). In contrast, the presence of t-butanol reduced the rate of dechlorination significantly. This effect is explained by the accumulation of t-butanol in the interfacial region of the cavitation bubbles where the concentration of OH radicals is high. It has been shown previously that the reaction rates of radical scavengers in sonochemical experiments are correlated with their hydrophobicity [16].
more toxic reaction products. At the end of the experiments, toxicity was significantly reduced. Also in the presence of acetate or glucose a significant decrease in toxicity was achieved. A significantly lower reduction in toxicity was obtained in the presence of t-butanol (Fig. 6). These results corresponded to the different degrees of dechlorination observed in the presence of the co-pollutants (Fig. 4). Noteworthy, also in biodegradation of environmental pollutants often first an increase in toxicity is observed, before a significant reduction of toxicity is achieved [20]. In ultrasonic and biological measures, an appropri-
O2-consumption [mg/L]
O2 -consumption [mg/L]
Toxicity of TCP and the sonicated samples was examined with the bioluminescence test. The dilution factor resulting in a 50% reduction (EC50) of bioluminescence was determined. A higher EC50 corresponds to a lower toxicity. In Fig. 5, the reciprocal values 1/EC50 are plotted in order to better illustrate the effects. In this presentation, decreasing values correspond to a decreasing toxicity. During the first period of sonication, the toxicity of the sonicated samples increased, indicating the temporary formation of
after sonication 400
A 0 0
5
time [d]
10
240
Fig. 6. Effect of co-pollutants on the toxicity remaining after 240 min sonication.
3.4. Reduction of toxicity and improved biodegradation after ultrasonic pre-treatment
without sonication
120 180 time [min]
Fig. 5. Effect of ultrasonic treatment time on toxicity. A higher 1/EC50 corresponds to a higher toxicity.
Fig. 4. Effect of co-pollutants on TCP dechlorination. Addition of copollutants was normalized to a COD of 1000 mg/l, corresponding to 900 mg/l acetate, 900 mg/l glucose, and 385 mg/l t-butanol.
800
60
800
400
B 0 0
5
10
time [d]
Fig. 7. Biological oxygen consumption after 240 min sonication of (A) TCP + acetate and (B) TCP + glucose. No microbial activity was observed without ultrasonic pre-treatment.
A. Tiehm, U. Neis / Ultrasonics Sonochemistry 12 (2005) 121–125
ate treatment time is a prerequisite for beneficial effects. After ultrasonic pre-treatment of TCP, biological degradation was studied with the samples containing acetate or glucose. The respirometer test was not performed with the sample amended with t-butanol since t-butanol is known to be a poorly degradable compound due to its branched structure. The effect of ultrasound treatment on subsequent biological oxygen consumption is demonstrated in Fig. 7. Whereas no microbial activity was observed in untreated samples, in the sonochemically pre-treated samples an oxygen consumption (BOD10) of about 500 and 700 mg/l was obtained for the samples amended with acetate (Fig. 7A) or glucose (Fig. 7B), respectively. Ratios of COD/BOD10 ranged from 1.6 to 2.0, demonstrating good biodegradability of the sonicated samples.
4. Conclusions The study demonstrates that sonochemical treatment can be a suitable method to reduce the toxicity of environmental pollutants. Sonochemical degradation of the model compound 2,3,5-trichlorophenol was observed between 41 kHz and 1 MHz. Highest efficiency was obtained at about 360 kHz. Decreasing concentrations of trichlorophenol and the AOX did not directly correspond to the toxicity of irradiated samples. In the initial phase, sonication led to an increased toxicity. Longer irradiation periods resulted in a significant decrease in toxicity, making subsequent biodegradation possible. In contrast to t-butanol, the presence of hydrophilic copollutants such as acetate or glucose did not significantly interfere with trichlorophenol degradation and toxicity reduction. Even in the presence of high concentrations of the hydrophilic model co-pollutants acetate or glucose, dechlorination was the predominating process. In conclusion, the combination of ultrasonic treatment and biodegradation represents a promising new technique in the field of environmental engineering. Toxic compounds inhibiting microbial degradation processes can be selectively removed by ultrasound. However, the applicability and efficiency is affected by the composition of the wastewaters to be treated. More studies on real wastewater are encouraged in order to help develop large scale applications.
Acknowledgment Financial support from the German Federal Ministry for Education and Research (BMBF, grant no. 02WA9677/0) is gratefully acknowledged.
125
References [1] H. Beitz, H. Schmidt, F. Herzel, Occurrence, toxicological and ecotoxicological significance of pesticides in groundwater and surface water, in: H. Bo¨rner (Ed.), Pesticides in Ground and Surface Water, Chemistry of Plant Protection, vol. 9, SpringerVerlag, Berlin, 1994, pp. 1–56. [2] S. Ren, P.D. Frymier, Estimating the toxicities of organic chemicals to bioluminescent bacteria and activated sludge, Water Res. 36 (2002) 4406–4414. [3] A. Tiehm, Combination of ultrasonic and biological pollutant elimination, in: T. Mason, A. Tiehm (Eds.), Ultrasound in Environmental Protection, Advances in Sonochemistry, vol. 6, Elsevier Science, Amsterdam, 2001, pp. 25–58. [4] Y. Ku, K.-Y. Chen, K.-C. Lee, Ultrasonic destruction of 2-chlorophenol in aqueous solution, Water Res. 31 (4) (1997) 929–935. [5] J.-G. Lin, C.-N. Chang, J.-R. Wu, Y.-S. Ma, Enhancement of decomposition of 2-chlorophenol with ultrasound/H2O2 process, Water Sci. Technol. 34 (9) (1996) 41–48. [6] C. Pe´trier, Y. Jiang, M.-F. Lamy, Ultrasound and environment: sonochemical destruction of chloroaromatic derivatives, Environ. Sci. Technol. 32 (1998) 1316–1318. [7] E. Gonze, N. Commenges, Y. Gonthier, A. Bernis, High frequency ultrasound as a pre- or post-oxidation for paper mill wastewaters and landfill leachate treatment, Chem. Eng. J. 92 (2003) 215–225. [8] H. Bru¨ggemann, H. Ko¨ser, E. Meyer, T.-H. Nguyen, Sonolytic debromination of ambroxol process wastewater, Water Res. 37 (2003) 674–680. [9] G. Tezcanli-Guyer, N.H. Ince, Degradation and toxicity reduction of textile dyestuff by ultrasound, Ultrason. Sonochem. 10 (4–5) (2003) 235–240. [10] A. Tiehm, I. Kohnagel, U. Neis, Removal of chlorinated pollutants by a combination of ultrasound and biodegradation, Water Sci. Technol. 43 (2) (2001) 297–303. [11] A. Tiehm, U. Neis, Ultrasound in Environmental Engineering. Technical University Hamburg-Harburg Reports on Sanitary Engineering 25 (ISSN 0724-0783, ISBN 3-930400-23-5), GFEUVerlag, Hamburg, 1999. [12] T. Mason, A. Tiehm (Eds.), Ultrasound in Environmental Protection, Advances in Sonochemistry, vol. 6, Elsevier Science B.V., Amsterdam, 2001. [13] A. Tiehm, N. Nickel, M. Zellhorn, U. Neis, Ultrasonic waste activated sludge disintegration for improving anaerobic stabilization, Water Res. 35 (8) (2001) 2003–2009. [14] C. Pe´trier, A. Francony, Incidence of wave-frequency on the reaction rates during ultrasonic wastewater treatment, Water Sci. Technol. 35 (1997) 175–180. [15] H.W. Hao, Y.F. Chen, M.S. Wu, H. Wang, Y.W. Yin, Z.L. Lu, Sonochemistry of degrading p-chlorophenol in water by high frequency ultrasound, Ultrason. Sonochem. 11 (1) (2004) 43–46. [16] A. Henglein, Sonochemistry: historical developments and modern aspects, Ultrasonics 25 (1987) 6–16. [17] T. Mason, Practical Sonochemistry: UserÕs Guide to Applications in Chemistry and Chemical Engineering, Ellis Horword Ltd., Chichester, UK, 1991. [18] Deutsche Einheitsverfahren zur Wasser-, Abwasser- und Schlammuntersuchung [DEV], VCH Publishers, Weinheim, Germany, 1997. [19] F. Ullmann, Enzyklopa¨die der technischen Chemie, Band 3, Urban & Schwarzenberg Verlag, Berlin, 1986. [20] S. Belkin, M. Stieber, A. Tiehm, F.H. Frimmel, A. Abeliovich, P. Werner, S. Ulitzur, Toxicity and genotoxicity enhancement during polycyclic aromatic hydrocarbons biodegradation, Environ. Toxicol. Water Qual. 9 (1994) 303–309.
Ultrasonic dehalogenation and toxicity reduction of trichlorophenol Andreas Tiehm
a,*
, Uwe Neis
b
a
b
Department of Environmental Biotechnology, Water Technology Center, Karlsruher Str. 84, 76139 Karlsruhe, Germany Department of Sanitary Engineering, Technical University of Hamburg-Harburg, Eißendorfer Str. 42, 21073 Hamburg, Germany Received 13 February 2004; accepted 24 May 2004 Available online 3 August 2004
Abstract The study focussed on the effect of ultrasonic frequency and co-pollutants on dechlorination and toxicity reduction of a toxic model pollutant, i.e. 2,3,5-trichlorophenol (TCP). The effect of ultrasonic frequency on TCP degradation and chloride formation was studied at 41, 206, 360, 618, 1068, and 3217 kHz. Most efficient ultrasonic dechlorination was achieved at 360 kHz. The degradation of TCP and adsorbable organic halogens followed pseudo-first-order rate kinetics. Toxicity in the bioluminescence test increased during the initial sonication period, indicating the temporary formation of more toxic reaction products. Subsequently, toxicity was significantly reduced. Dehalogenation efficiency decreased in the presence of the hydrophobic radical scavenger t-butanol, whereas hydrophilic co-pollutants such as acetate or glucose did not interfere with ultrasonic dechlorination and toxicity reduction. After ultrasonic pre-treatment, a fast biodegradation of the remaining organic pollutants was observed. In conclusion, the results demonstrate the potential of integrated ultrasonic/biological approaches for the treatment of wastewaters containing toxic pollutants. 2004 Elsevier B.V. All rights reserved. Keywords: AOX; Biodegradation; Dehalogenation; Toxicity; Trichlorophenol; Ultrasound frequency
1. Introduction Chlorinated phenols are applied as disinfectants and pesticides, and represent hazardous pollutants due to their persistence in the environment [1]. Due to their high toxicity, a high concentration of chlorophenols, e.g. in industrial wastewater or landfill leachate, can cause upsets in the operation of the biological activated sludge process, or make a microbial treatment impossible [2]. A number of studies have shown that sonication is suitable to degrade environmental pollutants [3], including 2- and 4-chlorophenol [4–6]. However, a complete mineralization of chlorophenols or real wastewaters by ultrasound would require large amounts of *
Corresponding author. Tel.: +49 721 9678 220; fax: +49 721 9678 101. E-mail address: [email protected] (A. Tiehm). 1350-4177/$ - see front matter 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.ultsonch.2004.05.013
energy [4,5,7]. Therefore, a combination of ultrasonic pre-treatment followed by biodegradation is promising in order to develop an economically more favourable integrated technique [7–10]. Environmental applications of ultrasound have been reported for a broad range of ultrasonic frequencies, starting from 20 kHz up to several MHz [11,12]. At low frequency, e.g. 20 kHz, the hydromechanical forces produced by cavitation are most pronounced [13]. Fast sonochemical reaction rates were reported at frequencies of about 200–800 kHz [14]. Recently, degradation of chlorophenol also was reported at 1.7 MHz [15]. For chlorophenols, sonochemical degradation due to OH radicals entering the bulk liquid phase and pyrolytic combustion inside the cavitation bubble have been demonstrated [6,15]. At alkaline pH the ionic species dominate and react only with radicals that enter the bulk liquid. The molecular chlorophenol species present at
122
A. Tiehm, U. Neis / Ultrasonics Sonochemistry 12 (2005) 121–125
acidic pH diffuse more easily into the interfacial region of the cavitation bubble where the concentration of radicals is high. Therefore, sonochemical degradation of chlorophenols is more efficient at low pH [4,6]. However, the yield of sonochemical reactions also is influenced by the presence of other compounds that might act as radical scavengers [16]. The aim of this study was to examine sonochemical chlorophenol degradation and toxicity reduction under varying boundary conditions. 2,3,5-Trichlorophenol (TCP) was used as toxic chlorinated model pollutant. Ultrasonic irradiation was done at pH 4 and optimized with respect to ultrasound frequency. The effect of hydrophilic and hydrophobic co-pollutants on dehalogenation and toxicity reduction was evaluated.
2. Materials and methods 2.1. Ultrasonic treatment All sonochemical experiments were done in air-saturated 10 mM phosphate buffer solution, adjusted to pH 4.0 by addition of H2SO4. The initial concentration of TCP ranged from 0.80 to 0.95 mM (156–188 mg/l). The ultrasound transducers were purchased from L-3 Communications ELAC Nautik GmbH, Kiel, Germany. The disk transducers (transducer area 25 cm2) were fixed at the bottom of a cylindrical tube equipped with a stirrer and a thermostatic jacket (Fig. 1). Transducers of similar shape were used to study the effect of different frequencies between 41 kHz and 3.2 MHz. The ultrasonic experiments were done at 20 ± 5 C with 500 ml samples. Power input was adjusted by calorimetric measurement [17]. The power input of the transducers operating at 41 kHz and 3.2 MHz was limited to 40 W. Therefore, in experiments studying the effect of ultrasonic frequency, sonication was done
for 60 min with 40 W power input. In the other experiments, treatment was done at 360 kHz for 240 min with 100 W power input. 2.2. Chemical analysis 2,3,5-Trichlorophenol was monitored using a high performance liquid chromatograph (HPLC) equipped ¨ berlingen, with a UV-detector (Perkin Elmer, U Germany). A Nucleosil 120-C18 (100 mm · 4 mm) column and an eluent consisting of methanol/water (60/ 40) was used for separation. Measurement of adsorbable organic halogens (AOX) was done with an AOX-System 2000 C (Haberkorn and Braun, Mu¨nchen, Germany) according to the German standard method DIN 384109 H14 [18]. A 1-ml sample was transferred into 100 ml deionized water and 50 mg granular activated carbon (GAC) was added. The suspension was shaken for 1 h. The GAC loaded with AOX was filtered through 0.4 lm polycarbonate membrane filters and the retained GAC washed with 25 ml of a 10 mM nitrate solution to remove inorganic chloride. The GAC was combusted at 950 C in a stream of pure oxygen. The chloride produced by combustion was absorbed in a titration cell and quantified by microcoulometry. The chemical oxygen demand (COD) was determined by oxidation of the organic compounds with K2Cr2O7. The produced Cr3+ was analysed colorimetrically. Chloride analysis was done after reaction with Hg(SCN)2 in the presence of Fe(III). The colored reaction products were determined colorimetrically. COD and chloride were determined with reagents and equipment purchased from Dr. Lange GmbH, Du¨sseldorf, Germany. The standardized analytical methods were performed according to the German Standard Methods for Water and Wastewater Analysis [18]. 2.3. Toxicity analysis
solution to be sonicated vibrating plate cooling fluid generator transducer
Fig. 1. Experimental set-up for the sonication experiments.
Toxicity was determined in (i) the bioluminescence test and (ii) by monitoring biological oxygen consumption of sonicated samples. (i) The bioluminescence test was performed with Vibrio fisheri according to the European standard procedure EN ISO 11348. The marine bacterium V. fisheri emits light during physiological activity. A decreasing light intensity is correlated with an increasing toxicity of the sample tested. (ii) In order to study the biodegradability of the sonicated samples, mineral salts suitable for microbial growth were added and the pH was adjusted to 7.0. Inoculation was done with activated sludge obtained from a municipal wastewater treatment plant, and biological degradation was monitored in a respirometer at 20 C.
A. Tiehm, U. Neis / Ultrasonics Sonochemistry 12 (2005) 121–125
3. Results and discussion
123
100 TCP
3.1. Effect of ultrasound frequency on sonochemical degradation of TCP
3.2. Kinetics of TCP and AOX degradation The kinetics of TCP decomposition, of the reduction of the total adsorbable organic halogens (AOX), and chloride formation are shown in Fig. 3. These experiments were performed at the most favourable ultrasonic frequency, i.e. 360 kHz. A complete disappearance of
Chloride
[%]
A pronounced sonochemical decomposition was observed in the frequency range between 206 and 1068 kHz with the highest efficiency at 360 kHz. Only small effects were obtained at 41 kHz, and no sonochemical degradation was observed at 3.2 MHz (Fig. 2). Increasing TCP decomposition corresponded to an increasing formation of chloride. At 360 kHz, 42% reduction of TCP and 25% of the calculated maximum possible chloride formation were achieved. Obviously, chloride formation was lower than had been calculated from TCP degradation. However, the chemical oxygen demand (COD) was reduced by only 15% demonstrating that mineralization of the phenolic structure was low. Obviously, dechlorination was the predominating sonochemical process.
AOX
80 60 40 20 0
0
60
120
180
240
time [min]
Fig. 3. Kinetics of TCP and AOX degradation and formation of chloride (360 kHz, pH 4). The 100% value of chloride corresponds to the initial chlorine amount of TCP.
2,3,5-trichlorophenol was achieved within a sonication time of 180 min. After 240 min, the AOX were reduced to 31%. Corresponding to AOX reduction, the chloride concentration increased from zero to about 70% of the calculated maximum concentration possible. The sonochemical degradation rates followed pseudo-first-order kinetics (Table 1). In previous studies [10], sonochemical degradation of 2,4-dichlorophenol (DCP) had been studied under same conditions with the same equipment. The results are shown for comparison purposes (Table 1). Obviously, ultrasonic degradation of TCP and of the related AOX proceeded slower than sonochemical decomposition of DCP. Since the pKa values of diand trichlorophenols are beyond pH 6 [19] and sonication was done at pH 4, the molecular species of the chlorophenols dominated in both cases. Most probably the different rates were due to the limited amount of reactive species such as OH radicals produced by ultrasonic cavitation. Since more reactive species are required for TCP dechlorination as compared to DCP, the observed reaction rate is slower. 3.3. Effect of co-pollutants on TCP dechlorination
Fig. 2. Effect of ultrasonic frequency on TCP decomposition and chloride formation. The 100% value of chloride corresponds to the initial chlorine amount of TCP.
Sonochemical degradation of TCP was also studied in the presence of hydrophilic and hydrophobic co-pollutants. Addition of co-pollutants was normalized with respect to COD. Addition of 900 mg/l acetate, 900 mg/l glucose, or 385 mg/l t-butanol corresponded
Table 1 Rate coefficients for sonochemical degradation of 2,3,5-trichlorophenol (TCP) and the corresponding AOX Compound
Pseudo-first-order rate coefficients Original substance k (min 1)
2,3,5-Trichlorophenol 2,4-Dichlorophenol [10]
11.1 · 10 18.9 · 10
AOX r2
3 3
0.99 0.99
k (min 1) 4.1 · 10 8.2 · 10
3 3
r2 0.99 0.99
Rate coefficients for 2,4-dichlorophenol (DCP) were determined under identical experimental conditions and are shown for comparison purposes.
124
A. Tiehm, U. Neis / Ultrasonics Sonochemistry 12 (2005) 121–125
toxicity [1/EC 50]
100 80 60 40 20 0 0
to a COD of 1000 mg/l, a value often observed in industrial wastewater. The presence of the hydrophilic co-pollutants, i.e. acetate and glucose, only moderately affected the dechlorination of TCP (Fig. 4). In contrast, the presence of t-butanol reduced the rate of dechlorination significantly. This effect is explained by the accumulation of t-butanol in the interfacial region of the cavitation bubbles where the concentration of OH radicals is high. It has been shown previously that the reaction rates of radical scavengers in sonochemical experiments are correlated with their hydrophobicity [16].
more toxic reaction products. At the end of the experiments, toxicity was significantly reduced. Also in the presence of acetate or glucose a significant decrease in toxicity was achieved. A significantly lower reduction in toxicity was obtained in the presence of t-butanol (Fig. 6). These results corresponded to the different degrees of dechlorination observed in the presence of the co-pollutants (Fig. 4). Noteworthy, also in biodegradation of environmental pollutants often first an increase in toxicity is observed, before a significant reduction of toxicity is achieved [20]. In ultrasonic and biological measures, an appropri-
O2-consumption [mg/L]
O2 -consumption [mg/L]
Toxicity of TCP and the sonicated samples was examined with the bioluminescence test. The dilution factor resulting in a 50% reduction (EC50) of bioluminescence was determined. A higher EC50 corresponds to a lower toxicity. In Fig. 5, the reciprocal values 1/EC50 are plotted in order to better illustrate the effects. In this presentation, decreasing values correspond to a decreasing toxicity. During the first period of sonication, the toxicity of the sonicated samples increased, indicating the temporary formation of
after sonication 400
A 0 0
5
time [d]
10
240
Fig. 6. Effect of co-pollutants on the toxicity remaining after 240 min sonication.
3.4. Reduction of toxicity and improved biodegradation after ultrasonic pre-treatment
without sonication
120 180 time [min]
Fig. 5. Effect of ultrasonic treatment time on toxicity. A higher 1/EC50 corresponds to a higher toxicity.
Fig. 4. Effect of co-pollutants on TCP dechlorination. Addition of copollutants was normalized to a COD of 1000 mg/l, corresponding to 900 mg/l acetate, 900 mg/l glucose, and 385 mg/l t-butanol.
800
60
800
400
B 0 0
5
10
time [d]
Fig. 7. Biological oxygen consumption after 240 min sonication of (A) TCP + acetate and (B) TCP + glucose. No microbial activity was observed without ultrasonic pre-treatment.
A. Tiehm, U. Neis / Ultrasonics Sonochemistry 12 (2005) 121–125
ate treatment time is a prerequisite for beneficial effects. After ultrasonic pre-treatment of TCP, biological degradation was studied with the samples containing acetate or glucose. The respirometer test was not performed with the sample amended with t-butanol since t-butanol is known to be a poorly degradable compound due to its branched structure. The effect of ultrasound treatment on subsequent biological oxygen consumption is demonstrated in Fig. 7. Whereas no microbial activity was observed in untreated samples, in the sonochemically pre-treated samples an oxygen consumption (BOD10) of about 500 and 700 mg/l was obtained for the samples amended with acetate (Fig. 7A) or glucose (Fig. 7B), respectively. Ratios of COD/BOD10 ranged from 1.6 to 2.0, demonstrating good biodegradability of the sonicated samples.
4. Conclusions The study demonstrates that sonochemical treatment can be a suitable method to reduce the toxicity of environmental pollutants. Sonochemical degradation of the model compound 2,3,5-trichlorophenol was observed between 41 kHz and 1 MHz. Highest efficiency was obtained at about 360 kHz. Decreasing concentrations of trichlorophenol and the AOX did not directly correspond to the toxicity of irradiated samples. In the initial phase, sonication led to an increased toxicity. Longer irradiation periods resulted in a significant decrease in toxicity, making subsequent biodegradation possible. In contrast to t-butanol, the presence of hydrophilic copollutants such as acetate or glucose did not significantly interfere with trichlorophenol degradation and toxicity reduction. Even in the presence of high concentrations of the hydrophilic model co-pollutants acetate or glucose, dechlorination was the predominating process. In conclusion, the combination of ultrasonic treatment and biodegradation represents a promising new technique in the field of environmental engineering. Toxic compounds inhibiting microbial degradation processes can be selectively removed by ultrasound. However, the applicability and efficiency is affected by the composition of the wastewaters to be treated. More studies on real wastewater are encouraged in order to help develop large scale applications.
Acknowledgment Financial support from the German Federal Ministry for Education and Research (BMBF, grant no. 02WA9677/0) is gratefully acknowledged.
125
References [1] H. Beitz, H. Schmidt, F. Herzel, Occurrence, toxicological and ecotoxicological significance of pesticides in groundwater and surface water, in: H. Bo¨rner (Ed.), Pesticides in Ground and Surface Water, Chemistry of Plant Protection, vol. 9, SpringerVerlag, Berlin, 1994, pp. 1–56. [2] S. Ren, P.D. Frymier, Estimating the toxicities of organic chemicals to bioluminescent bacteria and activated sludge, Water Res. 36 (2002) 4406–4414. [3] A. Tiehm, Combination of ultrasonic and biological pollutant elimination, in: T. Mason, A. Tiehm (Eds.), Ultrasound in Environmental Protection, Advances in Sonochemistry, vol. 6, Elsevier Science, Amsterdam, 2001, pp. 25–58. [4] Y. Ku, K.-Y. Chen, K.-C. Lee, Ultrasonic destruction of 2-chlorophenol in aqueous solution, Water Res. 31 (4) (1997) 929–935. [5] J.-G. Lin, C.-N. Chang, J.-R. Wu, Y.-S. Ma, Enhancement of decomposition of 2-chlorophenol with ultrasound/H2O2 process, Water Sci. Technol. 34 (9) (1996) 41–48. [6] C. Pe´trier, Y. Jiang, M.-F. Lamy, Ultrasound and environment: sonochemical destruction of chloroaromatic derivatives, Environ. Sci. Technol. 32 (1998) 1316–1318. [7] E. Gonze, N. Commenges, Y. Gonthier, A. Bernis, High frequency ultrasound as a pre- or post-oxidation for paper mill wastewaters and landfill leachate treatment, Chem. Eng. J. 92 (2003) 215–225. [8] H. Bru¨ggemann, H. Ko¨ser, E. Meyer, T.-H. Nguyen, Sonolytic debromination of ambroxol process wastewater, Water Res. 37 (2003) 674–680. [9] G. Tezcanli-Guyer, N.H. Ince, Degradation and toxicity reduction of textile dyestuff by ultrasound, Ultrason. Sonochem. 10 (4–5) (2003) 235–240. [10] A. Tiehm, I. Kohnagel, U. Neis, Removal of chlorinated pollutants by a combination of ultrasound and biodegradation, Water Sci. Technol. 43 (2) (2001) 297–303. [11] A. Tiehm, U. Neis, Ultrasound in Environmental Engineering. Technical University Hamburg-Harburg Reports on Sanitary Engineering 25 (ISSN 0724-0783, ISBN 3-930400-23-5), GFEUVerlag, Hamburg, 1999. [12] T. Mason, A. Tiehm (Eds.), Ultrasound in Environmental Protection, Advances in Sonochemistry, vol. 6, Elsevier Science B.V., Amsterdam, 2001. [13] A. Tiehm, N. Nickel, M. Zellhorn, U. Neis, Ultrasonic waste activated sludge disintegration for improving anaerobic stabilization, Water Res. 35 (8) (2001) 2003–2009. [14] C. Pe´trier, A. Francony, Incidence of wave-frequency on the reaction rates during ultrasonic wastewater treatment, Water Sci. Technol. 35 (1997) 175–180. [15] H.W. Hao, Y.F. Chen, M.S. Wu, H. Wang, Y.W. Yin, Z.L. Lu, Sonochemistry of degrading p-chlorophenol in water by high frequency ultrasound, Ultrason. Sonochem. 11 (1) (2004) 43–46. [16] A. Henglein, Sonochemistry: historical developments and modern aspects, Ultrasonics 25 (1987) 6–16. [17] T. Mason, Practical Sonochemistry: UserÕs Guide to Applications in Chemistry and Chemical Engineering, Ellis Horword Ltd., Chichester, UK, 1991. [18] Deutsche Einheitsverfahren zur Wasser-, Abwasser- und Schlammuntersuchung [DEV], VCH Publishers, Weinheim, Germany, 1997. [19] F. Ullmann, Enzyklopa¨die der technischen Chemie, Band 3, Urban & Schwarzenberg Verlag, Berlin, 1986. [20] S. Belkin, M. Stieber, A. Tiehm, F.H. Frimmel, A. Abeliovich, P. Werner, S. Ulitzur, Toxicity and genotoxicity enhancement during polycyclic aromatic hydrocarbons biodegradation, Environ. Toxicol. Water Qual. 9 (1994) 303–309.