Treatment of Materials Contaminated with Polychlorinated Biphenyls (Pcbs): Comparison of Traditional Method and Supercritical Fluid Extraction ()
1. Introduction
Polychlorinated biphenyls (PCBs) are highly toxic, chlorinated organic substances that are considered persistent organic pollutants. They do not easily degrade due to their high thermal and chemical stability. Therefore, when these substances are released into the environment, they accumulate in ecosystems and are incorporated into the food chain, resulting in biomagnication [1-3]. PCBs are composed of two, six-carbon aromatic rings containing multiple chlorine atoms (Figure 1) with 209 possible congeners. However, only 130 PCB congeners have been detected in commercial products [3,4].
The toxicity of individual PCBs is determined by their structure and number of chlorine atoms. PCBs with more chlorine atoms are more toxic [5]. Accumulation of PCBs in the body can cause a number of health problems, including cancer, immune suppression, reproductive damage, birth defects, and fetal death [5,6]. In the 1970s, PCBs were produced for a number of uses including thermal exchange fluids in heat exchangers, lubricants, isolating fluids in electrical equipment, resins and pesticides. Due to their high toxicity, in 1970s, the manufacture and commercial use of PCBs was banned [7,8]. Although the production of PCBs has been banned, they are still found in the environment. According to Brazilian regulations, materials contaminated with greater than 50 ppm PCBs must be stored under specific conditions or destroyed, usually by incineration in plasma furnaces with dual steps [9]. However, transportation of PCB contaminated materials requires specific conditions that are expensive and incineration itself is a costly process [10- 12]. Extraction of PCBs from contaminated samples using new methodologies [5,13] can be used to significantly decrease the amount of contaminated material requiring storage or incineration [5,14]. Soxhlet has been used as standard technique during more than one century. This technique is a very simple, cheap and provides good reproducibility. However, Soxhlet is not a selective process. Microwave-assisted extraction, accelerated solvent extraction and Supercritical Fluid Extraction (SFE) are techniques that use smaller amounts of organic solvents than conventional extraction methods [15,16]. Extraction of PCBs from seaweed samples and sediments using supercritical fluid extraction has been demonstrated previously [17-19]. Nevertheless, up to now the researches did not compare the extraction of PCBs from wood and soil contaminated using these different methodologies. Besides, most work with contaminated materials is related to soil and sediments, and not with wood. The objective of this study was to develop an experimental methodology for the removal of PCBs from contaminated materials, soil and wood, using conventional Soxhlet extraction methods and Supercritical Fluid Extraction (SFE).
2. Experimental
2.1. Simulated Contamination of Wood and Soil Materials
Simulated contamination of wood and soil samples for use in this study was carried out as below. Samples were macerated and then separated through sieves (<2 mm, 10 mesh). 40 × 10−3 kg of sample was suspended in 100 mL of hexane with 2.4 × 10−3 kg of a mixture of PCBs (Askarel®). The samples were then homogenized in a shaker (Kline Spencer Scientific, Model 109-1TC) for 3 h. The material was then left to rest for four days at room temperature and pressure to allow evaporation of all solvent. The simulated contamination resulted in materials with 60.000 mg PCBs/kg material.
The main properties of the soil, characterized in previous work [20], are presented in Table 1. The wood used was a mixture of Pine (Araucaria angustifolia), Jatoba (Hymenaea sp.) and Timborana (Clathrotropis macrocarpa). The material was collected from a carpentry in Cubatão-SP, without treatment or characterization.
2.2. Soxhlet Extraction
Samples of contaminated materials (3 ×10−3 kg) were extracted with 100 mL of solvent using a conventional Soxhlet apparatus (Gerhardt, Model Soxtherm Multistat/SX PC), with reflux of 10 - 420 minutes at 180˚C and atmospheric pressure. The solvents used were hexane (Synth, P.A, Lot 109169) and ethanol (CAAL, 99.5%, P.A., Lot 11460). After the required extraction time, the extracts were made up to 100 mL with solvent. All extractions were carried out in triplicate.
Figure 1. PCB Chemical Structure. x and y indicate the number of chlorides (x + y ≤ 10).
2.2.1. Preliminary Study
Before the systematic extraction experiments, a preliminary study of Soxhlet extraction was carried out to determine optimum conditions for the soil extractions. The objective of the preliminary study was to determine the optimum extraction time (from 10 to 420 minutes) and solvent (hexane and ethanol). Four experimental extractions (Tests 1 - 4) were carried out as follows. Test 1 was carried out using soil contaminated with 6.000 mg of Askarel®/kg soil with hexane as the solvent. Test 2 was carried out using soil contaminated with 60.000 mg of Askarel®/kg soil with hexane as the solvent. Test 3 and Test 4 were carried out under the same condition as Tests 1 and 2 respectively with ethanol as the solvent. All experiments were carried out using the Soxhlet method at 180˚C.
2.2.2. Experimental Design
After the preliminary tests, an experimental design experiment was carried out using two plans (H and E) and a full factorial design (22) with three replicates at the design centre point. The two independent factors were investigated at three different levels (−1, 0, +1) at initial PCB concentrations of 6.000 and 60.000 mg of Askarel®/kg of soil and at extraction time of 20 and 300 minutes (−1 and +1, respectively). The range and centre point values of the independent variables were chosen based on the results of preliminary study [21]. The difference between plans H and E was the solvent used: hexane was the solvent for plan H and ethanol was the solvent for plan E.
2.3. Supercritical Fluid Extraction
Around 5 × 10−3 kg of materials was used for each supercritical fluid extraction. The experiments were carried out using a SFE unit (Thar Technologies, Model SFE- 100 System), as illustrated in Figure 2. The apparatus contained a stainless steel extraction cell (100 mL capacity), flash stainless steel extraction collector (250 mL capacity), cooler, CO2 pump, co-solvent pump and heater. The temperature, pressure and CO2 flow rate of the process were controlled by software connected to the SFE apparatus. Liquid CO2 (≥99.98%, Ultra Pure, Air Product, Lot 1796) from a cylinder was cooled to −5˚C and subsequently pumped at 3 × 10−3 kg/min.
Before extraction, the system was equilibrated (40 min) at the same conditions as the extraction process. Extractions were carried out at 70˚C and 200 bar for 1 - 3 h (dynamic extraction), based on experimental data from literature [22]. The PCB mixture (Askarel®) was collected and quantified by gas chromatography-mass spectrometry (GC/MS).
2.4. Extract Analysis
Each extract was mixed with hexane to yield 100 mL of “pollutant solution”. An aliquot (1 mL) of the “pollutant solution” was placed in vials and analyzed by GC/MS (Varian, Model CG-CP 380 MS-Saturn 2200, VF-5MS) using an automatic injector on a capillary column (5%: 95% diphenyl: dimethyl-polysiloxane, 30 m × 0.25 mm × 0.25 μm). The column temperature program was: 100˚C for 5 min, 4˚C/min to 230˚C, 230˚C for 5 min, 35˚C/min to 280˚C, 280˚C for 5 min and 30˚C/min to 300˚C. The carrier gas was ultra-pure helium (1.0 mL/min) and the injection was performed at 260˚C (split 20:1). Identification and quantification of compounds was based on retention time comparisons with standard solutions. Retention times for the four congeners of polychlorinated biphenyls are shown in Table 2.
2.5. Statistical Analysis
2.5.1. Response Surface Methodology
The response surface methodology was used for statisti-
Table 2. GC/MS PCB congener retention times.
cal analysis using Statistica Software® (Version 7.0) for regression analyses. The first-order polynomial equation y = a0 + a1x1 + a2x1x2 was used to describe the behavior of the Soxhlet extraction method for plan H and E. In this polynomial equation, the variables a0, a1 and a2 are model parameters where a0 is the global average of PCB removal. x1 (initial soil PCB concentration) and x2 (extraction time) represent normalized variables and y is the percentage extraction of reference PCB congeners and total PCBs. The equation was fit to the experimental results at a 95% confidence level.
2.5.2. Two-Way Analysis of Variance
Statistical analysis of the wood extractions was performed using two-way analysis of variance (two-way ANOVA) to evaluate the three extraction methods (supercritical fluid extraction using carbon dioxide and Soxhlet extraction using hexane or ethanol as solvent). The five independent variables were the percentage of total PCB recovery from the soil and/or wood samples and the percentage recoveries of the penta-, hexa-, heptaand octa-chlorinated congeners. Statistics were carried out using MINITAB Statistical Software (Version 15).
3. Results and Discussion
3.1. PCB Extraction of Contaminated Soils
3.1.1. Preliminary Study
The percentage of PCB removal as a function of extraction time for different initial concentrations of contaminated soil using different solvent extractions are presented in Figure 3. In the first 10 minutes of extraction, a high percentage of PCB removal was obtained for all methods. By the third hour of extraction, the PCB removal slowed for all methods.
The best PCB removal (73% - 95%) was observed for
Figure 3. Preliminary Soxhlet extraction tests. (□) 6.000 mg Askarel®/kg soil, hexane extraction solvent, (■) 60.000 mg Askarel®/kg soil, hexane extraction solvent, (∆) 6.000 mg Askarel®/kg soil, ethanol extraction solvent, (▲) 60.000 mg Askarel®/kg soil, ethanol extraction solvent.
material with a high level of PCB contamination (60.000 mg Askarel®/kg soil) at extraction time of 10 - 420 minutes using hexane as the extraction solvent.
The initial soil PCB concentration influenced the extraction yield, probably due to the amount of accessible PCB at the surface of the contaminated material. The best results (95%) were obtained when hexane was used as the solvent for an extraction time of 420 minutes (7 hours). Unfortunately, hexane is a non-GRAS (Generally Recognized as Safe) solvent, and therefore its toxicity must be considered.
3.1.2. Statistical Experimental Design and Data Analysis for Conventional Extractions
Askarel® oil is a mixture of PCBs with some congeners. Therefore, the following five responses were used for each experimental design: percentage removal of total PCBs and percentage removal of penta-, hexa-, heptaand octachlorinated congeners. In both H and E plans, a significant difference between the independent factors (initial soil PCB concentration and extraction time) was observed in the five responses.
3.1.2.1. Plan H The response surface methodology was used to determine optimized variables for the removal of Askarel® oil by Soxhlet extraction and three-dimensional surface plots were constructed according to Equations (1) to (5).
(1)
(2)
(3)
(4)
(5)
Figure 4 shows the effect of the initial PCB concentration and extraction time on the removal of penta-chlorinated congener from soil samples. The response surfaces for the other congeners (hexa, hepta, octa-chlorinated and total PCB removal) exhibited behavior similar to that shown in Figure 4.
Analysis of the extraction obtained using this plan showed that both initial concentration and extraction time had significant effects on the extraction of penta-, hexa-, heptaand octa-chlorinated congeners. In contrast, for total PCB extraction, only extraction time had a significant effect. The interaction of extraction time and initial concentration was not significant for either the congener or total PCB responses. The model showed a satisfactory correlation between the experimental and calculated data, with coefficients of determination (R2) of 0.88, 0.91, 0.91, 0.89 and 0.87 for removal of penta-, hexa-, heptaand octa-chlorinated congeners and total PCB, respectively. The response surfaces also showed that extraction time is an important variable. The results showed that the optimal removal conditions by response surface methodology were initial PCB concentration of 60.000 mg Askarel®/kg soil and an extraction time of 300 minutes (5 hours). At these conditions, 92.0, 91.0, 92.8, 94.1 and 94.3% removal were obtained for penta-, hexa-, heptaand octa-chlorinated congeners and total PCBs, respectively.
3.1.2.2. Plan E The response surface methodology was used to determine optimized variables for the removal of Askarel® oil by Soxhlet extraction and three-dimensional surface plots were constructed according to the regression Equations (6) to (10). Equations (6) to (10) represent empirical relationships between the response (y) and the tested variables. The response surfaces obtained show similar behavior to that shown in Figure 4.
(6)
(7)
(8)
(9)
(10)
In this plan, both initial concentration and extraction time significantly affected the extraction of the penta-, hexaand hepta-chlorinated congeners and total PCBs. For removal of the octa-chlorinated congener, only extraction time had a significant effect.
Statistical analysis indicated that the proposed model
Figure 4. Effect of initial PCB concentration and extraction time on extraction of penta-chlorinated congeners from soil samples.
was adequate with coefficients of determination (R2) of 0.98, 0.98, 0.92, 0.88 and 0.91 for penta-, hexa-, heptaand octa-chlorinated congeners and total PCBs, respectively. These coefficients indicate that the model for plan E yielded a better model for comparing experimental and calculated data than that obtained for plan H.
According to the response surface methodology results, the optimal conditions for PCB extraction were observed when the extraction time and initial PCB concentration were maximal. Under these conditions, 77.9%, 78.0%, 74.0%, 77.4% and 78.3% extraction were obtained for penta-, hexa-, heptaand octa-chlorinated congeners and total PCBs, respectively.
3.1.3. Supercritical Fluid Extractions
Figure 5 shows the percentage extraction as a function of extraction time for supercritical fluid extractions. The highest extraction by SC-CO2 was observed after 3 hours of extraction, where the removals varied from 70.5% ± 0.9% for penta-chlorinated congeners to 50% ± 2% for octa-chlorinated congeners. 57.2% ± 0.2% of the total PCB congeners were removed under these same conditions.