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Ind. Eng. Chem. Res. 1999, 38, 380-384
Dechlorination of Polychlorinated Biphenyls: A Kinetic Study of Removal of PCBs from Mineral Oils P. De Filippis,* M. Scarsella, and F. Pochetti Chemical Engineering Department, University of Rome “La Sapienza”, Via Eudossiana 18, 00184 Rome, Italy
A kinetic study was done of the dechlorination of polychlorinated biphenyls (PCBs) eliminated from contaminated dielectric oils by using the potassium poly(ethylene glycolate) (KPEG) process. Experimental runs at laboratory scale showed that the kinetics of the removal reaction was first-order for each PCB present and first-order with respect to the KPEG concentration. The PCB elimination grade was also affected by the KOH/PEG ratio. An exponential correlation was found between the kinetic constant for each congener and its respective gas chromatographic relative retention time. Introduction It is widely recognized that polychlorinated biphenyls (PCBs) are one of mankind’s most dangerous pollutants.1,2 The extremely high toxicity of this class of compounds results from the compounds’ high stability to chemical agents (resistance to degradation) and their propensity to bioaccumulation in living organisms.3,4 In the past 20 years, many investigations focused on the removal and destruction of PCBs from contaminated products (mainly dielectric oils). Numerous methods were developed for processing these compounds.5,6 High-efficiency incineration is the recommended technology for destruction of PCB-contaminated products.7 This technology converts PCBs to HCl, CO2, and H2O and minimizes the formation of toxic compounds such as dioxins. Unfortunately, high-temperature incineration is expensive and is limited to pure PCBs mixtures and hydrocarbons containing very high concentrations of PCBs. Other problems arise with the handling and transporting of contaminated material and the wasteful destruction of reusable materials. Interest in recovery of reusable materials, and the necessity to treat contaminated products containing low concentrations of PCBs, have spurred renewed interest in the selective destruction or removal of PCBs to yield reusable products.8-12 Several selective destruction technologies are now available, based on chemical dechlorinations, photochemical degradations, biological treatments, and electrochemical processes. Among nondestructive, industrial-scale decontamination processes, the reaction of PCBs with potassium poly(ethylene glycolate) (KPEG) appears promising. This technology was initially applied by using molten sodium metal dispersed in poly(ethylene glycol)s.13,14 The reaction is rapid and completely decomposes PCBs. However, this reaction is somewhat dangerous. Metallic sodium requires special handling precautions since trace amounts of water could activate dangerous side reactions. This problem was resolved by substituting metallic sodium with either sodium hydroxide or potassium * To whom correspondence should be addressed. E-mail:
[email protected].
hydroxide. This procedure is safe, effective, and tolerant of water and other contaminants.15 Currently, this technology has been successfully applied to PCBs decontamination of dielectric oils. The application of KPEG processes for the decontamination of lube-used oils is more difficult than that for dielectric oils, because of the various additives of lube oils (detergents, antioxidant and emulsifying agents) and the presence of water (up to 10 wt %). These additives can influence the kinetics of the PCB decontamination reactions by reducing the yields of dechlorination. Previous work showed the effectiveness of KPEG technology in the treatment of both dielectric and lube contaminated oils and the importance of the PEG/oil and KOH/PEG ratios, and of the PCBs chlorination degree, on the efficiency of the KPEG treatment.16 This paper describes a kinetic study of the removal of PCBs from mineral oils by the KPEG dechlorination process at different temperatures. Experimental Procedure Experiments were performed on a dielectric oil containing a mixture of the PCBs Aroclor 1420, 1450, and 1460.The dechlorinating agent was poly(ethylene glycol) alkoxide. These reactions were conducted in a magnetically stirred, thermostated 300-mL glass vessel. The experimental procedure was as follows: contaminated dielectric oil containing 1015 ppm of PCBs was preheated in the vessel to a selected temperature while being stirred. After the addition of KOH-PEG, previously prepared by dissolving KOH in PEG at 70 °C, the run was started. Experimental conditions used in this work are listed in Table 1. All runs were batch modes ranging between 2.5 and 5 h. Approximately 5-mL aliquots of liquid samples were withdrawn from the reactor at fixed time intervals to determine the residual PCBs content in the oil. Prior to analysis, all the samples remained static for several hours to allow separation of the glycol and oil phases, i.e., the polar heavy phase and the nonpolar light phase, respectively. PCBs contents in the oil and glycol phases were determined using gas-liquid chromatography (GLC).
10.1021/ie9803422 CCC: $18.00 © 1999 American Chemical Society Published on Web 01/14/1999
Ind. Eng. Chem. Res., Vol. 38, No. 2, 1999 381 Table 1. Experimental Conditions of the Designed Runs run
KOH/PEG molar ratio
PEG/oil weight ratio
temp (°C)
1 2 3 4 5 6 7 8 9 10
0.85 0.85 0.85 0.85 0.85 0.85 0.85 0.85 0.42 1.70
0.2 0.2 0.2 0.2 0.2 0.25 0.15 0.1 0.2 0.2
80 100 110 120 125 120 120 120 100 100
Table 2. PCBs Removal Results residual PCBs (%) after run 15 min 30 min 45 min 60 min 150 min 240 min 300 min 1 2 3 4 5 6 7 8 9 10
71 52 29 19 12 7 27 34 82 22
62 40 22 13 9
54 33 16 10 6
75 15
50 25 14 7 4 5 10 13
36 14 8 5 3 3 7 10 58 6
28 12 5 3 1 1 5 8 52 3
20 8 3 1 >1
For the oil phase, sample cleanup and preparation and GLC analysis were carried out according to the ASTM D 4059/91 analytical procedures. The glycol phase was extracted with isooctane (Baker, Resi-Analized reagent, for pesticides analysis) and analyzed with the same GLC procedure as used for the oil phase. A Fisons 800 gas chromatograph, fitted with a split/ splitless injector, an ECD detector, and an SE-30 30 (0.25-µm film)-m × 0.32-mm-i.d. capillary column, was used for the analysis. The main parameters used were the following: nitrogen as carrier gas and as makeup gas with flow rates of 1.5 and 30 mL/min, respectively; injection temperature, 280 °C; detector temperature, 310 °C. The temperature program was 110 °C for 1 min; 110-160 °C at 15 °C/min; 160-270 °C at 5 °C/min; 270 °C for 4 min; 270-300 °C at 20 °C/min; 300 °C for 10 min. The PCB congeners, peaks were identified by comparison with a calibration standard solution and by their respective retention times; congener 209 was chosen as the reference peak for determining the relative retention time. Poly(ethylene glycol) 400 (PM 400) technical grade (Enichem) and KOH 97% (Baker reagent grade) were used as reagents. Each run was repeated three times: the results reported in Tables 2 and 5 are the mean values of the experimental data. Results and Discussion Table 2 summarizes the results obtained for runs 1-10. Runs 1-5 were carried out at five different temperatures (80-125 °C), runs 4 and 6-8 at four different KPEG/oil ratios, and runs 2, 9, and 10 at three different KOH/PEG ratios, keeping constant during each run the other two variables, as reported in Table 1. PCBs removal in a nonpolar medium, using KPEG as the dechlorinating agent, involves the nucleophilic aromatic substitution of the chlorides of the PCBs by
poly(ethylene glycol) alkoxide to produce an aryl poly(ethylene glycolate) and KCl:15
In the studied two-phase system, the displacement of a chloride ion from the chlorinated substrate should be rapid only if the glycol alkoxide reagent and the PCB are in the same phase. It is uncertain whether nucleophilic substitution takes place in the oil or in the glycol phase. The reaction was hypothesized to be a typical phase-transfer reaction, with transfer of potassium poly(ethylene glycol) alkoxide in the nonpolar phase, followed by the substitution reaction. The phase transfer would be enabled by complexing the potassium ion and by surrounding the potassium alkoxide group by its own long polyether chain. Alternatively, being that PCBs are miscible with PEGs, the substitution reaction could occur in the glycol phase, after extraction of PCBs in the polar phase. However, this can be excluded because the CLG analysis of the glycolic phases did not show a detectable presence of PCBs. This could be attributed to the high KOH/PEG molar ratios used in the experimental runs, which can affect the miscibility of PCBs in PEG. Although the global reaction mechanism is still not known, the kinetics of PCBs removal from a mineral oil can be studied by considering the two-phase system as homogeneous, under the condition of very efficient stirring required by the treatment. To investigate the kinetics of removal of PCBs, it is also necessary to consider the composition of the contaminant: the mixture of Aroclor 1420, 1450, and 1460 used in this work contained chlorinated biphenyls ranging from dichlorinated isomers to octachlorinated ones. The gas chromatographic spectrum of the original PCBs distribution in the contaminated oil is reported in Figure 1. It has been reported in the literature15 that the displacement of a single chloride ion with a poly(ethylene glycol) alkoxide ion in a PCB congener is sufficient to remove it from the nonpolar phase. The recovered aryl poly(ethylene glycol)ates, more or less chlorinated, are slightly toxic.17 Therefore, to study the kinetics of removal of PCBs from a mineral oil, only the monodechlorination reactions need to be considered, as all the reactions of monodechlorination on the different congeners were considered to take place independently. In a previous work,16 it was shown that the highly chlorinated biphenyls were more quickly eliminated than the slightly chlorinated ones. A number of identified PCB congeners were considered in this study, to investigate the kinetics of PCBs disappearance by the reduction of the gas chromatographic response area of their peaks. Thus, a number of gas chromatographic peaks corresponding to homologous congeners were selected in this report, to study the influence on the removal rate of the present chlorine atoms. The main characteristics of each designated peak are reported in Table 3, where the response factor was assumed as the mean value of the response factors reported in the literature for the congeners in the same peak.
382 Ind. Eng. Chem. Res., Vol. 38, No. 2, 1999
Figure 1. Typical gas chromatographic spectrum of the PCBs mixture used in this study. Table 3. Characteristics of the Considered GLC Peaks congener number
IUPAC name
chlorine atoms
relative retention time
relative response factor
28 52 90; 101 132; 153 138; 160 187
2,4,4′-trichlorobiphenyl 2,2′,5,5′-tetrachlorobiphenyl 2,2′,3,4′,5- and 2,2′,4,5,5′-pentachlorobiphenyl 2,2′,3,3′,4,6′- and 2,2′,4,4′,5,5′-hexachlorobiphenyl 2,2′,3,4,4′,5′- and 2,3,3′,4,5,6-hexachlorobiphenyl 2,2′,3,4′,5,5′,6-heptachlorobiphenyl
3 4 5 6 6 7
0.384 0.434 0.554 0.670 0.705 0.7292
0.854 0.418 0.639 0.710 1.009 1.122
Table 4. GLC Areas Measured for Selected Congeners during Run 2
a
congener number
0 min
15 min
45 min
28 52 90; 101 132; 153 138; 160 187
18731 14473 49910 121537 124488 78645
17818 14299 41688 78274 43445 8167
17590 13200 22202 26055 8332 1150
peak area responsea 60 min 150 min 17161 12938 19110 14714 4269 nd
16780 11393 5698 nd nd nd
240 min
300 min
15379 11274 2638 nd nd nd
12454 9544 nd nd nd nd
nd, not detectable by GLC.
Table 5. Calculated Kinetic Constants and Activation Energies congener number
run 1
run 2
28 52 90; 101 132; 153 138; 160 187
1.40 × 10-4 1.98 × 10-4 1.72 × 10-3 6.01 × 10-3 1.46 × 10-2 2.53 × 10-2
1.00 × 10-3 1.20 × 10-3 1.24 × 10-2 3.60 × 10-2 5.63 × 10-2 9.06 × 10-2
kinetic constant (min-1) run 3 run 4 1.70 × 10-3 2.00 × 10-3 2.90 × 10-2 9.50 × 10-2 1.39 × 10-1 1.86 × 10-1
For runs 1-5, the kinetics of removal of the selected PCB congeners were determined, evaluating the decrease with time of the corresponding peak areas. Table 4 reports as an example the gas chromatographic area values measured for run 2 at different times. When the kinetics of the heterogeneous reaction is expressed in terms of pseudohomogeneous rate, for each congener, the form of the kinetic expression that best fits the experimental data is
r)-
d[PCBcong] ) -kcong[PCBcong] dt
(1)
The kinetics of PCB removal is first-order regarding each congener. Table 5 lists the mean values of the kinetic constants calculated from the below-reported kinetic equation for the considered reactions.
3.80 × 10-3 4.00 × 10-3 5.90 × 10-2 1.86 × 10-1 2.02 × 10-1 2.95 × 10-1
run 5
activation energy (kJ/mol)
4.62 × 10-3 5.80 × 10-3 8.46 × 10-2 2.85 × 10-1 3.18 × 10-1 4.32 × 10-1
91 ( 5 86 ( 4 101 ( 3 100 ( 2 79 ( 4 78 ( 2
Table 5 also reports the apparent activation energy for the removal of each PCB congener, calculated from the Arrhenius plot for the five considered temperatures. It should be noted that all activation energies are in the same range, suggesting that for all the PCB congeners the same removal reaction mechanism applies. The dependence of the kinetic constant on the chlorine content was analyzed. From the kcong calculated for the hexachlorinated homologues (132-153 and 138-160), the initial hypothesis about a correlation between the number of chlorine atoms and kcong was not verified. To the contrary, it was found that the reaction kinetic constants increase exponentially with the relative retention time (RRT) for each reaction temperature, as is shown in Figure 2. This correlation between kcong and the RRT suggests that the reactivity of each PCB is dependent not only
Ind. Eng. Chem. Res., Vol. 38, No. 2, 1999 383
(i.e., over the range of RRTs), the whole range of RRTs was divided into 14 intervals, corresponding to an RRT interval of 0.05: the total chromatographic peak area in each RRT interval was then considered as the response of a hypothetical PCB (pseudocongener) present with a concentration proportional to such area and having a RRTpseudocong equal to the mean value for the considered interval. For all the RRTpseudocong values, the kinetic constant kpseudocong was calculated by applying the equations reported in Figure 2. Table 6 reports the considered RRTpseudocong, the calculated GLC areas for each of the pseudocongeners in the utilized oil-PCBs mixture, and the calculated kpseudocong for run 2. The evolution of PCBs concentration with time considering the hypothesized pseudocongeners can be written as follows:
Figure 2. Kinetic constant vs relative retention time. Table 6. Pseudocongeners Definition and Characteristics pseudocongener relative retention pseudocongener pseudocongener kinetic time intervals RRT zero time area constant (min-1) 0.250-0.300 0.300-0.350 0.350-0.400 0.400-0.450 0.450-0.500 0.500-0.550 0.550-0.600 0.600-0.650 0.650-0.700 0.700-0.750 0.750-0.800 0.800-0.850 0.850-0.900 0.900-0.950
0.275 0.325 0.375 0.425 0.475 0.525 0.575 0.625 0.675 0.725 0.775 0.825 0.875 0.925
8.77 × 10-5 1.88 × 10-4 4.02 × 10-4 8.61 × 10-4 1.84 × 10-3 3.95 × 10-3 8.45 × 10-3 1.81 × 10-2 3.87 × 10-2 8.29 × 10-2 1.78 × 10-1 3.80 × 10-1 8.14 × 10-1 1.74
4 576 15 894 25 101 24 010 21 223 132 806 87 139 118 620 263 691 185 360 78 611 108 799 91 129 26 834
on the number of activated chlorine atoms but also on the PCB structure and conformation via the steric and electronic factors related within them. It is reported in the literature that the general trend is an increase of the RRT with chlorine content, but for homologues of PCBs the RRT is dependent on structure and increases when chlorines are placed farther from the biphenyl bridge; the same is true for those homologues that can more readily assume a planar conformation.18,19 The influence of the KPEG/oil ratio was also considered. When the KPEG/oil ratio increases between 0.1 and 0.25 (runs 4, 6-8), an almost linear increase of the PCBs elimination rate occurs (Table 2), suggesting that the studied reaction is first-order with KPEG concentration. Therefore, the overall reaction kinetics for the removal of PCBs mixtures from a mineral oil is
r)-
d[PCB] ) -k′( dt
∑kcong[PCBcong])[KPEG]
(2)
To validate the exponential correlation between the kcong and RRTs found for the selected six considered chromatographic peaks, over all the PCBs distribution
[PCB]t )
∑([PCBpseudocong]0e(-k
pseudocongt)
)
(3)
This assumption is confirmed from the experimental data for the overall PCBs removed in runs 2, 3, and 4. In Figure 3 is reported the evolution of the hypothesized pseudocongeners with time, calculated from eq 3 for run 2. Table 7 shows a comparison between the experimental and calculated data. The square root of the mean-squared error was used to evaluate the proposed model. For all runs considered, the value was
