Environ. Sci. Technol. 1999, 33, 3152-3159
Determining PCB Sorption/ Desorption Behavior on Sediments Using Selective Supercritical Fluid Extraction. 3. Sorption from Water S T E V E N B . H A W T H O R N E , * ,† ERLAND BJO ¨ RKLUND,‡ S Ø R E N B Ø W A D T , †,§ A N D LENNART MATHIASSON‡ Energy and Environmental Research Center, University of North Dakota, Grand Forks, North Dakota 58202-9018, and Department of Analytical Chemistry, University of Lund, P.O. Box 124, S-221 00 Lund, Sweden
Supercritical fluid extraction (SFE) with pure CO2 was used to quantitatively remove PCBs from historically contaminated sediments without substantially disturbing their bulk organic or inorganic matrix as evidenced by only small or undetectable changes in thermal gravimetric behavior, elemental (C, H, N, S) composition, ionic conductivity, and pH determined before and after SFE. The extracted PCBs were then spiked into water with the parent sediment, and sorption was allowed to occur for up to 18 days. The selective SFE conditions developed in part 1 were used to determine the proportion of PCBs which could be extracted under four conditions of increasing stringency. Comparing the selective SFE behavior of the PCBs from the water/sediment sorption samples to the original historically contaminated sediments demonstrated that 18 days was not sufficient for PCBs to migrate to the “slower” sediment-binding sites (those sites requiring more rigorous SFE conditions), which the PCBs had occupied in the historically contaminated sediments and that the adsorbed PCBs were primarly associated with the binding sites most easily extracted (“rapidly desorbed”) by SFE. Sediment/ water distribution coefficients at 18 days were similar for sediments with low contamination levels (98%) all PCBs from historically contaminated sediments used in this study. The extracted PCBs from each sediment were collected in acetone (kept at ∼10 mL by small additions during the extraction and then concentrated to 1 mL under nitrogen). A total of ∼25-30 g was extracted for CRM 536, SRM 1944, and the Lake Ja¨rnsjo¨n sediment, and each of these was used for an 18-day sorption test described below. Because of its higher PCB concentrations, only 8 g of the SRM 1939 sediment was prepared, and ∼2 g subsets were used for 2 h, 24 h, and 18-day sorption tests. Quantities of sediment were chosen so that the total PCB extract from each sediment could be added to the water used for sorption experiments without exceeding the water solubility of any of the PCB congeners. The extracted sediments were each suspended in 1 L of HPLC-grade water in a 1 L “EPA-certified clean” brown glass 10.1021/es981072h CCC: $18.00
1999 American Chemical Society Published on Web 08/05/1999
FIGURE 1. First derivative weight loss curves for the four test sediments before (bottom curve for each sample) and after SFE (top curve for each sample). The temperature program was 25-110 °C at 25 °C/min, hold at 110 °C for 10 min (to determine water content), then heat at 25 °C/min to 650 °C and hold until the weight loss stabilized. Weight loss between 110 and 650 °C under inert gas (argon) was defined as volatile organic matter. At this point, the air was introduced (still at 650 °C) to determine the fixed or combustible organic mass. The two curves for each figure offset by 1 wt %/min unit so that the shape of the curves can be more easily compared.
TABLE 2. Effect of SFE (150 °C, 400 bar, 1 h) on Characteristics of Sediments as Determined by Thermal Gravimetric Analysisa water content (wt %)
volatile organic matter (wt %)
combustible residue (wt %)
sample
before SFE
after SFE
before SFE
after SFE
before SFE
after SFE
CRM 536 NIST 1939 NIST 1944 Lake Ja¨ rnsjo¨ n
2.6 2.5 1.4 0.6
1.9 1.7 1.4 0.5
11.8 8.3 7.2 2.4
11.9 8.1 6.9 2.3
3.2 3.6 1.9 0.9
3.3 3.6 1.9 0.8
a Water content was determined by weight loss at 110 °C, volatile organic matter by weight loss from 110 to 650 °C under argon, and combustible residue by weight loss at 650 °C with the addition of air.
bottle with a Teflon-lined cap. The PCB-containing acetone fraction was then added to the bottle containing the parent sediment. All bottles were completely filled so no headspace remained. For each sample, the final concentrations of the added PCBs in the water (Table 1) were below the saturation level for all of the PCB congeners based on published solubilities (2). It should be noted that, since all of the sediments were real (collected in the environment), other semivolatile pollutants [e.g., pesticides, polycyclic aromatic hydrocarbons (PAHs)] were likely present and would have been extracted and respiked into the water in the same manner as the PCBs.
Each sediment/water mixture was mixed for 15 days on a shaker table at room temperature. The bottles were inverted daily by hand to ensure suspension of the sediment. Additional samples were prepared in an identical manner for SRM 1939 and mixed for 2 and 24 h. After 15 days, the sediments were allowed to settle for 3 days or, for the 2 and 24 samples, centrifuged for 1 h to separate the sediment and water. In all cases, the water fraction was removed and saved, and the wet sediment was allowed to air-dry overnight before performing the SFE extractions. Thermal Gravimetric Analysis. Thermal gravimetric analysis (3-5) was performed on all samples before (original historically contaminated) and after SFE removal of the PCBs (residue). The temperature program was 25-110 °C at 25 °C/min, held at 110 °C for 10 min (to determine water content), then heated at 25 °C/min to 650 °C and held until the weight loss stabilized (typically for 40 min). Weight loss between 110 and 650 °C under inert gas (argon) was defined as volatile organic matter content. At this point, the air was introduced (still at 650 °C) to determine the “fixed” or combustible organic mass. Conductivity, pH, and Elemental Analysis. For conductivity and pH measurements, 0.5 g of each sediment was equilibrated overnight with HPLC-grade water which had been purged with nitrogen to remove dissolved carbonates. Measurements were performed with standard pH and conductivity electrodes. Carbon, hydrogen, and nitrogen were determined using a Leeman Labs model CE440 elemental analyzer. Sulfur was determined by combustion and iodometric titration with a LECO model HF10 sulfur analyzer. VOL. 33, NO. 18, 1999 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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TABLE 3. Elemental Analysis of Sediments before and after SFE with Pure CO2 1939
carbon (wt %) hydrogen (wt %) nitrogen (wt %) sulfur (wt %) conductivity (µmhos)b pHb
1944
Lake Ja1 rnso1 n
536
before
after
before
after
before
after
before
after
5.5 ( 0.78 ( 0.05a 0.45 ( 0.01a 0.49 ( 0.03a 700 4.1
5.2 ( 0.02 0.67 ( 0.04 0.42 ( 0.02 0.43 ( 0.03 730 4.1
5.2 ( 0.2 0.54 ( 0.05 0.26 ( 0.02 1.05 ( 0.09 2200 6.3
4.6 ( 0.3 0.45 ( 0.04 0.24 ( 0.02 0.88 ( 0.06 2200 6.5
8.3 ( 0.3 0.85 ( 0.03 0.42 ( 0.04 0.53 ( 0.04 300 6.2
7.9 ( 0.5 0.79 ( 0.11 0.42 ( 0.04 0.44 ( 0.04 320 6.2
1.5 ( 0.1 0.19 ( 0.01 0.10 ( 0.01 0.16 ( 0.02 85 6.2
1.5 ( 0.4 0.19 ( 0.03 0.10 ( 0.02 0.06 ( 0.01 94 6.0
0.1a
a Standard deviations based on quadruplicate determinations for each sample. after SFE.
b
Determined in water equilibrated with sediment before and
maining in the water fractions were extracted with isooctane and analyzed as for the SFE extracts, and their concentrations in the water were compared to those found in the SFE extracts of the sediment from the same 1 L bottle sorption experiments. In addition, the total mass of PCBs found on the test sediments from the SFE profiles and the water phases was compared to the known masses of PCBs (determined by SFE and by the certifying agency as described in part 1) in the original historically contaminated samples.
Results and Discussion
FIGURE 2. Selective SFE extraction behavior for representative PCB congeners, PCB 28 (A) and PCB 180 (B), from the historically contaminated CRM 536 sediment (labeled original sample) and after 18 days of sorption from water of the same PCBs onto a duplicate sediment which had been precleaned by SFE. Selective SFE Conditions and Extract Analysis. The same sequentially stronger SFE conditions were used as described in detail in part 1 of this series (1). In brief, each sample was extracted for 1 h with pure CO2 at each of the following conditions: 40 °C and 120 bar (rapidly desorbing or fast PCBs), 40 °C and 400 bar (moderate), 100 °C and 400 bar (slow), and finally 150 °C and 400 bar (very slow). Eight fractions were collected for each selective condition, and extracts were analyzed by gas chromatography (GC) with electron capture detection (ECD) in the same manner as used for part 1. Verification of the extraction curves was also performed as described in part 1. To determine mass balance and apparent distribution coefficients (Kds) for the sorption experiments, PCBs re3154
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Effect of SFE on Matrix Organics. As shown in part 1 of this series, extraction with pure CO2 at temperatures up to 150 °C quantitatively removes the PCBs from all of the samples tested, in agreement with earlier literature (6-8). However, to be useful as a method to prepare PCB-free samples for water/sediment sorption studies, the SFE procedure must not significantly affect the bulk matrix organics (e.g, humic and fulvic acids) or result in significant changes in soil pH or ion content. In an attempt to determine changes in the sediment matrixes, thermal gravimetric analysis (TGA) was performed on each sample before and after the 150 °C SFE procedure used to prepare the PCB-free sediments. As shown in Table 2, the SFE procedure did not significantly affect the “volatile” organic matter content (weight loss to 650 °C under argon) or the “combustible” organic matter content (weight loss after adding air at 650 °C). Small losses of water after SFE were demonstrated (weight loss at 110 °C) for some of the sediments, especially NIST 1939 (Table 2), but any loss of water is unlikely to affect the subsequent PCB sorption experiments since they are performed by suspending the sediments in water. Although the weight loss vs temperature curves show little structure, the first derivative curves (wt % loss per minute) allow some comparison as to the type of matrix organics to be compared. For example, the types of organic matter functional groups removed at different temperatures have been described based on infrared spectroscopy of soils after TGA (4). As shown in Figure 1, the derivative curves show quite different rates of weight loss between 110 and 650 °C for each of the four samples used in this study, clearly demonstrating different organic compositions for the four sediment samples (4, 5). However, the derivative curves for each sample before and after the 150 °C SFE step are essentially unchanged. These results indicate that extraction with pure CO2 may have not substantially changed the bulk matrix organic material. In contrast, extraction with organic solvents or with CO2 mixed with organic modifiers substantially reduces the bulk organic content of sediments as evidenced by the dark brown to black extracts that result from the extraction of most sediments by either procedure. For example, when the CRM 536 was extracted using methylene chloride/acetone in a Soxhlet apparatus, the concentration of the bulk organic matrix was reduced from ∼15 to ∼2 wt %, clearly demonstrating that the use of organic solvents radically changes the organic matter content of
FIGURE 3. Water/sediment sorption behavior of PCBs to preextracted CRM 536 sediment after 18 days compared to the original historically contaminated sediment. The rapidly desorbing sites include the PCBs extracted with the mildest two SFE conditions (from 0 to 120 min) and the slowly desorbing sites include the PCBs extracted with the strongest SFE conditions (from 120 to 240 min). A percent readsorbed value of 100 indicates that the same quantity of that PCB congener was found on a particular type of site (rapidly or slowly desorbing) in the 18 day sample as was found in the original sample. PCB congener numbers are shown above the individual bars.
TABLE 4. Sediment/Water Partition Coefficients (KdS) for CRM 536 and SRM 1939 sediment/water Kd [(ng/g sediment)/(ng/g water)] PCB congener
1939 (2 h)
1939 (24 h)
1939 (18 days)
536 (18 days)
1944 (18 days)
28 52 101 138 149 153
3000 2600 1500 2300 1400
4500 5000 3500 4500 3400
6300 3200 2900 3800 3600 2700
8300
1200 870 400 540 400
7700 6400
sediments. An earlier report compared the mass of matrix material extracted from soils using Soxhlet extraction versus SFE by evaporating the Soxhlet solvents and SFE collection solvents and weighing the resultant residues (7). Residues from Soxhlet extraction were typically 20-fold higher than those from SFE, again demonstrating that SFE affects the sample matrix much less than Soxhlet extraction (7). It should be noted that in many SFE studies an increase in extraction efficiencies is achieved by adding polar modifiers (organic solvents such as methanol) to the CO2 (9-11). Such procedures will definitely cause some of the bulk organic matrix to be extracted (similar to the Soxhlet results discussed above) as evidenced by the generally dark color of the extracts obtained when modifiers are used. Therefore, it is critical that pure (not modified) CO2 be used to remove the PCBs when the goal is to not cause significant changes in the bulk matrix organic material. Fortunately for this study, extraction with pure CO2 at 150 °C and 400 bar quantitatively removes the PCBs from the sediments without apparently affecting the bulk organic matrix. Elemental analyses were also performed in order to determine if any measurable changes in carbon, hydrogen, nitrogen, or sulfur concentrations resulted from SFE. As shown in Table 3, some small drops in carbon, hydrogen, and nitrogen may have occurred, but none of the changes were substantial. Note also that the concentration of carbon is ∼1/2 of the total organic matter content determined by
TGA (Table 2), as is consistent with previous reports (4). Sulfur concentrations also remained fairly unchanged, except that the concentration in the Lake Ja¨rnso¨n sample dropped by ∼1/2. Since supercritical CO2 is quite effective at extracting elemental sulfur, but not other inorganic forms such as sulfate (12), these results demonstrate that elemental sulfur is not present in significant concentrations on these sediments (except possibly, for the Lake Ja¨rnso¨n sediment). One other possible modification of the matrix that could occur from the SFE procedure is the sorption or reaction of CO2 to form carbonates. However, the results shown in Figure 1 demonstrate that such reactions are not significant, if they occur at all. In addition, the values determined both for pH and electrical conductivity (Table 3) of water equilibrated with the sediments before and after SFE also demonstrate that carbonate formation did not occur, and that the types and concentrations of inorganic ions was not significantly changed by SFE. Mass Balance for Water/Sediment Sorption Experiments. As described above, all of the PCBs extracted by the SFE procedure were spiked back into the 1 L water bottles with their parent sediment for the water/sediment sorption experiments. Thus, 100% sorption of the spiked PCBs from the water to the sediment would result in PCB concentrations on the sediment exactly the same as the concentrations on the original (unextracted) sediment samples. Because of the possible loss of PCBs during the sorption time (and to calculate distribution coefficients), each sediment and supernatant water was analyzed to determine the total mass of each PCB congener present in the equilibration bottles after the 2 h to 18-day sorption times. For SRM 1939, SRM 1944, and the Lake Ja¨rnsjo¨n samples, mass balance was quite good, with total recoveries (compared to the spiked quantities which are based on the concentrations of each congener determined as described in part 1) of all of the individual PCB congeners typically ranging 85-110%. However, for CRM 536, the overall recovery was substantially lower, and ranged 64-77% for the individual congeners. The reason for the lower mass balance obtained for CRM 536 is thought to be a result of losses during the SFE VOL. 33, NO. 18, 1999 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 4. Water/sediment sorption behavior of PCBs to preextracted NIST 1944 sediment after 18 days compared to the original historically contaminated sediment. The rapidly desorbing sites include the PCBs extracted with the mildest two SFE conditions (from 0 to 120 min) and the slowly desorbing sites include the PCBs extracted with the strongest SFE conditions (from 120 to 240 min). A percent readsorbed value of 100 indicates that the same quantity of that PCB congener was found on a particular type of site (rapidly or slowly desorbing) in the 18 day sample as was found in the original sample. PCB congener numbers are shown above the individual bars. step used to originally remove the PCBs from the 30 g (three 10 g extractions) of sediment prepared for the sorption experiment. During the extraction of this sample, plugging of the outlet restrictor frequently occurred, which made it necessary to increase the restrictor heater temperature to its maximum of 240 °C. With the heated coaxial style restrictor and with acetone as collection solvent, quantitative collection of PCBs is achieved with lower restrictor temperatures (e.g., up to 90 °C). However, loss of PCBs with hotter restrictor temperatures can occur. Such losses were verified by the extraction of additional samples with a hotter restrictor. Accordingly, we expect that the 70% mass balance seen for the sorption experiments for CRM 536 is a result of poor collection during the initial SFE preparation step, and not during the water/sediment sorption procedure. (Note also that such losses do not occur during the four-step sequential SFE used to determine the extraction profiles since restrictor plugging was never encountered with the four-step procedure.) Therefore, for CRM 536, all subsequent discussions will base the 100% values on the actual amount of each PCB congener found by the four-step SFE procedure on the CRM 536 samples and the PCBs found in the supernatant water used for the water/sediment sorption experiments. Water/Sediment Distribution. For SRM 1944, CRM 536, and the Lake Ja¨rnsjo¨n sediments, most of the PCBs partitioned to the sediments during the 18-day equilibration (i.e.,
