Analysis of Volatile and Semivolatile Organic Compounds in Municipal Wastewater Using Headspace Solid-Phase Microextraction and Gas Chromatography
By Antoniou, Chrysoula V Koukouraki, Elisavet E; Diamadopoulos, Evan
ABSTRACT: The aim of this work was to develop a simple and fast analytical method for the determination of a wide range of organic compounds (volatile and semivolatile compounds) in municipal wastewater. The headspace-solid-phase microextraction (HS-SPME) and gas chromatography (with mass spectroscopy) was used for determination of the organic compounds. In this study, 39 organic compounds were determined, including 3 sulfur compounds, 28 substituted benzenes, and 8 substituted phenols. The extraction parameters, such as types of SPME fiber, extraction temperature, extraction time, desorption time, salt effect, and magnetic stirring, were investigated. The method had very good repeatability, because the relative standard deviations ranged from 0.5 to 12%. The detection limit of each compound was at or below the microgram-per- liter level. This method was applied for determination of the organic compounds in raw wastewater, primary effluent, secondary effluent, and chlorinated secondary effluent samples from the Chania Municipal Wastewater Treatment Plant (Crete, Greece). Water Environ. Res., 79, 921 (2007).
KEYWORDS: wastewater, volatile, semivolatile, organic compounds, solid-phase microextraction, gas chromatography.
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A large number of synthetic organic compounds present in wastewater may be hazardous to human health and aquatic life (Almeida and Boas, 2004). Chlorinated benzenes are used as solvents and chemical intermediates in the production of agricultural chemicals. They can be dechlorinated by photodegadation and biodegradation to form less chlorinated isomers (He et al., 2000). Also, benzene; toluene; xylene; 1,3,5-trimethylbenzene; 1,4- dichlorobenzene; and tetrachloethylene are the most common organic compounds found at wastewater treatment plants (Suschka et al., 1996). As reported in the literature, solid-phase microextraction (SPME) with gas chromatography (GC) can be used for the determination of aromatic volatile organic compounds (Cho et al., 2003; Ji et al., 2006; Menendez et al., 2000).
Sulfur compounds are typically responsible for the unpleasant odor in municipal wastewater treatment plants (Cheng et al., 2005; Glindemann et al., 2006). Dimethyl sulfide and dimethyl disulfide are produced by many living organisms (microorganisms, algae, plants, and animals) through various mechanisms (Bentley and Chasteen, 2004; Bouillon and Miller, 2005). The determination of sulfur compounds is performed by gas chromatography.
Phenolic compounds are often found in the environment, as a result of various processes, such as the production of pesticides, petrochemical products, and dyes, and they are considered some of the most important contaminants. They are typically analyzed by liquidliquid extraction (LLE) or solid-phase extraction (SPE) followed by gas chromatography (GC) or high-performance liquid chromatography (HPLC) (Zhou et al., 2005). Solid-phase microextraction has also been used for the determination of phenolic compounds in wastewater (Moder et al., 1997; Ozkaya, 2005; Penalver et al., 2002; Pincheiro and Esteves da Silva, 2005; Portillo et al., 2005).
Solid-phase microextraction is a fast, solventless procedure not requiring complex instrumentation. In this technique, a phase- coated fused silica fiber contained within a syringe is exposed to the sample. The analytes are adsorbed on the fiber coating. After adsorption equilibrium has been reached, the fiber is withdrawn into the needle, and the needle is removed from the sample vial and introduced to the gas chromatography injector, where the analytes are thermally desorbed and analyzed (Pawliszyn, 1997).
Sulfur compounds, substituted phenols, and substituted benzenes are present in municipal wastewater and can be hazardous; therefore, it would be useful to have a simple method for their simultaneous determination. In this study, a fast and simple (with minimum sample pretreatment) analytical method using HS-SPME-GC was developed for the determination of 39 organic compounds. Volatile organic compounds (dimethyl sulfide, dimethyl disulfide, carbon disulfide and substituted benzenes, such as BTEX, alkyl-, and chloro- benzenes); semivolatile; and polar compounds (substituted phenols, such as alkyl- and chloro- phenols) can be analyzed with this method. The developed method was applied in raw wastewater, primary effluent, secondary effluent, and chlorinated secondary effluent samples from the municipal wastewater treatment plant of Chania, Crete, Greece.
Reagents and Standards. The 200 [mu]g/mL volatile aromatic compounds mixture-503/502/524 (benzene; bromobenzene; nbutylbenzene; sec-butyl benzene; tert-butyl benzene; chlorobenzene; 2- chlorotoluene; 4-chlorotoluene; 1 ,2-dichlorobenzene; 1,3- dichlorobenzene; 1 ,4-dichlorobenzene; ethylbenzene; hexachloro1,3- butadiene; isopropyl benzene; p-isopropyltoluene; naphthalene; n- propylbenzene; styrene; tetrachloroethene; toluene; 1,2,3- trichlorobenzene; 1,2,4-trichlorobenzene; trichloroethene; 1,2,4- trimethylbenzene; 1,3,5-trimethylbenzene; o-xylene; p-xylene; and m- xylene) in methanol and the 2000 pg/mL phenols mixture # 2-8040 (2- chlorophenol; 2,6-dichlorophenol; 2,4-dimethylphenol; 2- methylphenol; 4-methylphenol; 2,4-dinitrophenol; 2,3,4,6- tetra- chlorophenol; 2,4,5-trichlorophenol; and dinoseb) were purchased from Chem Service (West Chester, Pennsylvania). Dimethyl sulfide (>99%); dimethyl disulfide (>98%); and 1,4-dibromobenzene were obtained from Fluka (Buchs, Switzerland). The 5000 [mu]g/mL carbon disulfide and the 2000 [mu]g/mL EPA 8040 surrogate standard mix (2- fluorophenol and 2,4,6-tribromophenol) were purchased from Supelco (Taufkirchen, Germany). Sodium chloride was purchased from Merck (Darmstadt, Germany). The working standard solutions were prepared by diluting stocks in methanol.
Instrumentation. Gas chromatographic analysis was carried out on a Shimadzu QP5050 GC-mass spectroscopy (MS) system (Shimadzu, Kyoto, Japan). The analytical column was DB-5MS+DG, 30 m + 10 m Duragard, 0.25-mm internal diameter, and 0.25-[mu]m film thickness (J&W Scientific Agilent, Wilmington, Delaware). Injections were performed in the splitless mode; the mass spectrometer was used in the electron impact mode (7OeV); and the carrier gas (helium) flow was 1.0 mL/min. The interface temperature was 300[degrees]C. The temperature program of the gas Chromatograph was from 35[degrees]C (1 minute) to 60[degrees]C (2 minutes) at 5[degrees]C/min, from 60 to 80[degrees]C (2 minutes) at 3[degrees]C/min, from 80 to 140[degrees]C at 5[degrees]C/min, and from 140 to 300[degrees]C (1 minutes) at 10[degrees]C/min. The injector temperature was 250[degrees]C.
Total suspended solids (TSS) of the samples was measured according to Standard Methods (APHA et al., 1992). Chemical oxygen demand (COD) of the samples was measured using the Merck COD kit (25 to 1500 mg/L [25 to 1500 ppm] and 10 to 150 mg/L [10 to 150 ppm]).
Sample Collection. All samples were collected in 60-mL amber glass vials with a screw-cap and polytetrafluoroethylene (PTFE)- silicon septum. Each vial was completely filled with the sample. Also, 4 mg sodium thiosulfate (Na^sub 2^S^sub 2^O^sub 3^), as a dechlorination agent, was added in the vials before the introduction of the chlorinated secondary effluent samples. The samples were analyzed immediately after sampling or stored, for a maximum of 5 days, at 4[degrees]C.
HS-SPME Procedure. The SPME holder and fiber assemblies used for the SPME extraction were obtained from Supelco. Before use, all fibers were conditioned according to the manufacturer’s user guide. Twenty-five mL of water sample was placed in a vial (40 mL, Supelco) sealed with a screw-cap and PTFE-silicon septa. The water sample was spiked with an appropriate amount of the standard solution and with 6.25 g sodium chloride. The pH was adjusted to 4 by adding an appropriate amount of 2 N sulfuric acid. The overall methanolic concentration during the experiments was always less than 0.1% (v/ v). The sample vials were placed in a water bath to maintain a constant temperature. The needle of the SPME device pierced the septum of the vial, and the fiber was exposed for 30 minutes to the headspace of the vial at 35[degrees]C, while the water sample was stirred with a PTFE-coated magnetic bar. Then, the fiber was withdrawn inside the needle, and the needle was removed from the sample vial. Subsequently, the needle was manually inserted to the gas chromatography injector, where the analytes were thermally desorbed and analyzed. The desorption time was 10 minutes at 250[degrees]C.
The concentration of compounds during SPME development was 2 [mu]g/L volatile aromatic compounds mixture, 400 [mu]g/L phenols mixture, 2 [mu]g/L dimethyl sulfide, 0.02 [mu]g/L dimethyl disulfide, and 120 [mu]g/L carbon disulfide. Also, 400 [mu]g/L surrogate standard mix and 8 [mu]g/L 1 ,4-dibromobenzene were used as internal standards. Distilled water was used for the preparation of the water samples during the SPME method development and the qualitative analysis.
The same SPME procedure was used for the analysis of the wastewater samples.
Results and Discussion
HS-SPME Development In this study, the following six types of fibers were tested: 85 [mu]m poly aery late (85 [mu]m PA), 100 [mu]m polydimethyl siloxane (100 [mu]m PDMS), 70 [mu]m carbowax/ divinylbenzene (70 [mu]m CW/DVB), 65 pm polydimethyl siloxane/ divinylbenzene (65 [mu]m PDMS/DVB), 85 pm carboxen/polydimethyl siloxane (85 [mu]m CAR/PDMS), and 50/30 pm divinylbenzene/ carboxen/ polydimethyl siloxane (50/30[mu]m DVB/CAR/PDMS). The extraction efficiency of the six fibers was checked for all the analytes, as shown in Figure 1. The best extraction efficiency for most of the volatile aromatic compounds mixture and the sulfur compounds was achieved using the fiber 85 pm CAR/PDMS, whereas the extraction efficiency for the phenols mixture was very low using this fiber. Also, the best extraction for the phenols mixture efficiency was obtained using the fiber 65 pm PDMS/DVB, whereas the recoveries for the volatile aromatic compounds mixture and the sulfur compounds were good using this fiber. Therefore, the fiber 65 pm PDMS/DVB was chosen for the SPME procedure. The volume of the gaseous phase in the HS-SPME should be minimized for higher recoveries, according to the SPME theory (Pawliszyn, 1997). To optimize the ratio of sample to headspace volume, a 40-mL vial was used, while the water volume ranged from 15 to 25 mL (headspace volume from 25 to 15 mL, accordingly). The best results were obtained at a 25/15 mL volume ratio of sample to headspace.
The addition of salt can improve the extraction of the more polar compounds (Pawliszyn, 1997). Thus, the addition of 12.5 and 25 w/v % sodium chloride was studied. The results of these experiments showed that the optimum recoveries of the compounds were obtained with the addition of 25 w/v % sodium chloride.
The pH of the water sample was set at 4, because, in acidic pH, the recoveries of the phenolic compounds increased. There was no significant effect on the recoveries of the other compounds by changing the pH.
Stirring the water sample can increase extraction efficiency, because stirring can speed up the transfer of the compounds from water to headspace. The water samples were stirred at 750 and 1000 r/ min, and the best recoveries for all compounds were observed at 1000 r/min.
The influence of temperature on the extraction yield of the compounds was studied varying the temperature between 20 and 70[degrees]C, using 65 pm PDMS/DVB and 30-minute extraction time. The peak areas at different extraction temperatures are shown in Figure 2. Each analyte has different diffusion rate into the fiber coating, resulting in different responses in the extraction yield. At temperatures higher than 50[degrees]C, the ability of the fiber to adsorb begins to decrease for most of the analytes. This is because extraction by SPME is an exothermic process. Thus, with increasing temperature, the distribution constant decreases (Pawliszyn, 1997). No single temperature gave the best recoveries; therefore, the decision was made to maintain the temperature for all experiments at 35[degrees]C, because this temperature was a reasonable compromise with high recoveries for all compounds.
The HS-SPME is an equilibrium process of the analytes between the vapor phase and the fiber coating, so it is important to determine the time it takes for analytes to reach equilibrium. Analytes with high molecular weight or low Henry’s constant values need longer times to reach equilibrium (Pawliszyn, 1997). To evaluate the extraction efficiency for all the analytes, the extraction time ranged from 5 to 120 minutes at 35[degrees]C, using 65 [mu]m PDMS/ DVB, as shown in Figure 3. Acceptable equilibrium states were achieved for most of the volatile aromatic compounds mixture and the sulfur compounds at 30 minutes, while, for some of the phenols, equilibrium was reached after 50 minutes. Therefore, the extraction time was kept at 30 minutes. For those substances in which the extraction time had not reached equilibrium, the measurements were also valid, as long as the extraction time remained constant for all analyses (Pawliszyn, 1997).
The desorption time was studied using the 65 [mu]m PDMS/DVB fiber, with constant extraction time of 30 minutes at 35[degrees]C. The desorption time ranged from 0.5 to 15 minutes at 250[degrees]C. The results showed that all compounds were completely desorbed at 10 minutes at this temperature.
The gas chromatogram of HS-SPME GC-MS analysis of a standard solution of the 39 compounds is presented in Figure 4. Before the chromatographic analysis, SPME pretreatment was applied using the 65 pm PDMS/DVB SPME fiber, by addition of 25 w/v % sodium chloride and agitation. The extraction time was 30 minutes; the extraction temperature was 35[degrees]C; and the desorption time was 15 minutes at 250[degrees]C. The only analyte that could not be determined with this method was the 2,4-dinitrophenol.
The method conditions for HS-SPME and GC-MS analysis that were used for the analysis are summarized in Table 1.
Qualitative Analysis. The linear range of the HS-SPME was established by plotting the area of the analyte peak versus the concentration of each analyte. The correlation coefficients (r^sup 2^) for all compounds were good, because they ranged from 0.988 to 0.999. The relative standard deviation (RSD%) ranged from 0.5 to 12.1%, indicating that the method had good repeatability.
The limit of detection (LOD) and the limit of quantitation (LOQ) are defined as statistical values (APHA et al., 1992; Keith et al., 1983). In particular, LOQ and LOD are given by 10s and 3s, respectively, where s is the standard deviation of seven repeat measurements of the standard solution performed at the lower point of the linear range. The LODs for the sulfur compounds ranged from 0.00006 to 0.27 [mu]g/L, for the volatile aromatic compounds mixture ranged from 0.0006 to 0.031 [mu]g/L, and for the phenols mixture ranged from 0.033 to 17 [mu]g/L.
The retention times, LODs, LOQs, linear range, and correlation coefficients of the compounds are presented in Table 2.
To investigate the matrix effects on extraction, the relative recovery of each analyte was calculated in raw wastewater, belt press filtrate, primary effluent, secondary effluent, and chlorinated secondary effluent samples. Three different concentration levels (low, medium, and high concentrations according to the calibration curve) of all the analytes were used for the calculation of relative recovery in each type of matrix. The samples were analyzed in triplicate at each concentration level. The relative recovery was estimated as follows:
The spiked concentrations were 0.5 [mu]g/L volatile aromatic compounds mixture; 50 [mu]g/L phenols mixture; 0.1 [mu]g/L dimethyl sulfide; 0.01 [mu]g/L dimethyl disulfide; 50 [mu]g/L carbon disulfide; 400 [mu]g/L surrogate standard mix; and 8 [mu]g/L 1 ,4- dibromobenzene; and the relative recoveries of the compounds in all samples are presented in Table 3. AU the relative recoveries were compared witii those in distilled water. The relative recoveries of the analytes in the secondary effluent and in the chlorinated secondary effluent were high, as they ranged from 70 to 118%. The relative recoveries of the compounds in raw wastewater, belt press filtrate, and primary effluent samples ranged from 50 to 111%. The low relative recoveries were the result of the complex matrix of these samples. Therefore, quantitative analysis of these samples should be done using the standard addition method.
The LOD for all compounds were at the microgram-per-liter level, and the method had very good repeatability, as the RSD% ranged from 0.5 to 12.1%.
Analysis of Wastewater Samples. The previously described HS-SPME method was used for the determination of volatile organic compounds in raw wastewater, belt press filtrate, primary effluent, secondary effluent, and chlorinated secondary effluent samples from the Chania Municipal Wastewater Treatment Plant. The measurements of COD, TSS, and pH of the wastewater samples are presented in Table 4. The values of these parameters in the raw wastewater are typical of untreated domestic wastewater (Metcalf & Eddy, 1991), while the equivalent values in the treated effluent are below minimum legislation standards.
Typical chromatograms of HS-SPME-GC-MS analysis of raw wastewater and secondary effluent samples are shown in Figure 5. A reduction in the concentrations of all analytes from the raw to secondary effluent samples can be observed. For the analytes with concentrations outside the calibration curve (toluene, tetrachloroethene, and p-isopropyltoluene), an appropriate dilution of the sample with distilled water took place. The dilution was 1:5 to 1:30, depending on the concentration of the analyte.
The analysis of the samples (sampling date: July 5, 2005) showed the presence of volatile organic compounds (Table 5). The sulfur compounds were present in raw wastewater and primary effluent at low concentrations. Also, alkyl-substituted benzenes, naphthalene, tetrachloroethylene, trichloroethylene, and toluene were present in all samples. The 2-chlorophenol and 4-methylphenol were determined in the raw wastewater sample, but only 4-methylphenol was at a high concentration. The concentrations of all analytes decreased from raw wastewater to the chlorinated secondary effluent. The reduction of the concentrations of five analytes (dimethyl disulfide, toluene, tetrachoroethylene, p-isopropyl-toluene, and 4-methyl phenol) during the treatment process is presented in Figure 6. Also, the concentrations of mese five analytes, from July 5, 2005, to March 9, 2006, in the raw wastewater, are presented in Figure 7. As seen in Figure 7, 4-methyl phenol is the compound with the highest concentration variations in the raw wastewater.
The concentrations of toluene, tetrachloroethylene, trichloroethylene, and ethylbenzene determined in the raw wastewater sample were at the same levels as those reported in the literature (Atasoy et al., 2004; Escalas et al., 2003; Tansel and Eyma, 1999). Also, the results obtained for these compounds in the primary effluent and secondary effluent samples are also comparable with those reported in the literature (Escalas et al., 2003; Tansel and Eyma, 1999). However, the concentration of dimethyl disulfide in the samples (raw wastewater 0.4 [mu]g/L and primary effluent 0.25 ng/L) was lower than these reported by Escalas et al. (2003) (raw wastewater 1.6 [mu]g/L and primary effluent 2.8 [mu]g/L). The lower concentration of dimethyl disulfide found in the present study possibly occurred because of the lower concentration of sulfur compounds in the original organic matter and because most of this highly volatile substance may have been stripped in the aerated grit chamber. The GC-MS analysis of the raw wastewater (sampling date: March 9, 2006) showed the presence of unknown peaks with high peak intensity. The characteristic fragments ions of these compounds were m/z 57, m/z 71, and m/z 85. These compounds were found to be tetradecane (C^sub 14^H^sub 30^), nonadecane (C^sub 19^H^sub 40^), pentatriacontane (C^sub 35^H^sub 72^), and docosane (C^sub 22^H^sub 46^), according to the NIST21 MS Library (Database of National Institute for Standard Technology [NIST] for the MS library provided with QP5000 GC-MS software by Shimadzu), with a 91% similarity level. The peak intensity of these compounds decreased in the primary effluent, secondary effluent, and chlorinated secondary effluent. These compounds may have been found in the raw wastewater because of the disposal of oily wastes into the sewers.
In conclusion, a simple and fast analytical method was developed that could be used for monitoring volatile and semivolatile organic compounds in municipal wastewater treatment plants. Sample pretreatment was done by means of SPME, which is a solventless, fast, and low-cost technique. Each fiber was used for at least 60 analyses, without having observed any carryover problems. However, some reproducibility problems occurred after the completion of 60 analyses during the standard solution check. Total time of analysis, including the SPME procedure and GC-MS analysis, takes approximately 75 minutes.
This study was supported by the Greek Ministry of Education (Athens) and the European Union in the Framework of “Iraklitos- Fellowships for Research-ENVIRONMENT”.
Submitted for publication May 10, 2006; revised manuscript submitted November 15, 2006; accepted for publication December 19, 2006.
The deadline to submit Discussions of this paper is November 15, 2007.
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Chrysoula V. Antoniou, Elisavet E. Koukouraki, Evan Diamadopoulos*
Department of Environmental Engineering, Technical University of Crete, Greece.
* Department of Environmental Engineering, Technical University of Crete, University Campus, Kounoupidiana, 73100 Chania, Crete, Greece; e-mail: email@example.com.
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