Envlron. Sci. Technol. 1994, 28, 253-258
Application of a Permeant/Polymer Diffusional Model to the Desorption of Polychlorinated Biphenyls from Hudson River Sediments Kenneth M. Carroll,'lt Mark R. Harkness,t Angelo A. Bracco,? and Robert R. Balcarcel*
General Electric Corporate Research and Development, Schenectady, New York 12301, and Department of Chemical Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139 The desorption of polychlorinated biphenyls (PCBs) from Hudson River sediments was examined using XAD-4 resin as a PCB adsorbent. Both a rapidly desorbing labile component and a more slowly desorbing resistant component were observed. The fraction of PCBs in the resistant component was found to be a function of PCB concentration and total organic carbon in the sediment, independent of inorganic particle size. Milling the sediment had no effect on PCB desorption, while pretreatment of the sediment using heat (with and without caustic addition) enhanced the rate of desorption and significantly decreased the resistant fraction. We propose that the labile and resistant fractions arise from disparate diffusional rates of PCBs from the swollen and condensed phases of the sediment organic matter. Diffusion coefficients of 2.6 X 10-l8 and 7.3 X cm2/s for PCBs in the condensed and swollen humic polymer are calculated using a permeant/polymer diffusional model, with calculated diffusional distances on the order of 1 X 10" cm.
Introduction Equilibrium partitioning of organic compounds between water, soils, and sediments has been the subject of extensive study over the last 2 decades (1-4).Morerecently, interest has grown in considering kinetic effects which govern the movement of contaminants in these media under nonequilibrium conditions (5-15). Kinetics are important in many transport or biodegradation processes which operate at shorter time scales than are required for equilibrium to be established. Sorbed nonionic organic compounds often display biphasic desorption kinetics from soils and sediments where a labile component of the compound desorbs readily and reversibly, while a resistant component desorbs orders of magnitude more slowly (7,8).This phenomena has been observed with PCBs in both spiked and environmentally contaminated sediments (16-18). The slow desorption step has been attributed to the retardation of contaminant diffusion due to microscale partitioning in sediment particle pores (11-13) or to diffusion in the sediment organic matter (6, 14). Soil organic matter is composed primarily of complex, macromolecular humic substances. Reported molecular weights for humic polymers vary from 300 to 200 000 (19), placing them in the same molecular weight class as many synthetic polymers. Flory-Huggins theory, developed to model solute/polymer interactions in amorphous synthetic polymers, has been successfully applied to describe partitioning behavior of solute molecules into the soil organic phase (20). While little is known about the diffusion of small molecules in humic polymer (5,II),there is a large body of literature concerned with transport in + General Electric Corporate Research and Development. f
Massachusetts Institute of Technology.
0013-936X/94/0928-0253$04.50/0
0 1994 Amerlcen Chemical Society
synthetic polymers (21,22). For example, the diffusional rate of a molecule in a polymer can be predicted based upon its size,the forces holdingthe polymer chains together (Permachor value), and the swollen (rubber-like) or condensed (glass-like) state of the polymer (23). In addition, models are available which allow diffusional distances to be calculated when diffusion coefficients and sorption/desorption kinetic curves are available (24). In this work, desorption kinetics of PCBs from environmentally contaminated Hudson River sediment were examined using XAD-4 (high BET surface area polystyrene resin beads) as a PCB adsorbent. The method is similar to the procedure utilizing Tenax polymeric beads used by Pignatello (9) but was developed independently. The desorption of PCBs from the sediments is measured under a variety of physical, thermal, and chemical conditions in order to probe the diffusion-controlling structure of the matrix. The desorption data generated are coupled with independent estimates of PCB-humic polymer diffusion coefficients and a sorptionldesorption model to permit diffusional distances of the PCBs in the humic polymer to be calculated.
Experimental Procedure Sediments contaminated with high, medium, or low levels of PCBs were obtained from the Hudson River near Moreau, NY. The sediment particle size distribution was determined by sedimentation (25). Total organic carbon (TOC) analysis was performed by Hudson Environmental Services (Queensbury, NY) using EPA Method SW8469060 and the EPA Lloyd Kahn Method. All the sediments collected were sieved through 1/4-in. screens to remove stones and plant detritus, air-dried, homogenized, and stored in glass jars until needed. A subsample of the highly PCB-contaminated sediment was further separated in size fractions by sieving the dry material through aseries of screens (3327-,990-, 293-, 123-, and 69-pm openings). A subsample of the sediment contaminated with the lowest level of PCBs was bar-milled by placing 50 g of the sediment in a 250-mL jar containing 10 4411. stainless steel bars and rolling the jar on a mill (Norton U S . Stoneware Inc.) at 30 rpm for 6 h. The XAD-4 resin (Rhom and Haas) was prepared by washing the resin successively with water, methanol, acetonelhexane (50/50 mixture by weight), methanol, and water to ensure that preservative agents (salts) and residual monomer were removed prior to use (26). The wet resin (50% water) was stored in a polyethylene jar to inhibit dehydration. Subsamples of the moderately PCB-contaminated sediment were treated with a 0.5 N NaOH (caustic) solution both at ambient and elevated temperatures. Caustic solution was added to 50 g of the dry sediment in a 1:l liquid-to-solid ratio and either mixed for 8 h at room Envlron. Scl. Technol., Vol. 28, No. 2, 1994 253
temperature on a reciprocating shaker or refluxed for the same time at 100 "C. Sediment in distilled water was refluxed for 8 h in order to separate the effect of the heat treatment from that of the caustic addition. The reflux apparatus consisted of a boiling vessel, a condenser, and a cold trap used to prevent substantial losses of volatilized PCB. At the end of the reflux pretreatments, the glass walls of the reflux apparatus were washed with acetone, which was concentrated to 1-2 mL and added to the sediment slurry. The caustic treated sediments were neutralized to pH 7 using 38% HC1. The sediment was air dried in preparation for the desorption experiments. PCB losses ranged from 8.7 % to 16.3% as a result of these pretreatments. Prior to the desorption experiments, 1 g of sediment was contacted with 1 mL of deionized water in a 9-mL glass vial sealed with a Teflon-lined cap and allowed to stand for 24 h in order to rehydrate the sediment. Desorption experiments were initiated by adding 1 g of XAD-4 resin and 5 mL of water to the vials, placing them on a reciprocating shaker, and mixing them at 30 rpm at room temperature. Time points were obtained by extracting whole vials. Prior to extraction, 0.4 g of potassium carbonate was added to the slurry to increase aqueous phase density and to cause the resin to float to the surface. The sedimenthand and the XAD-4 phases were separated after centrifugation. PCBs were extracted by adding 5.0-mL aliquots of ethyl acetate to the solid phases (sediment or XAD-4) slurried with 1mL of water. The samples were extracted overnight at 30 rpm on a reciprocating shaker. Three successive extractions were needed for complete extraction of the PCBs. The extracts were combined and treated with mercury to remove sulfur, which interferes with the gas chromatographic (GC) analysis. PCB analysis was performed using a Varian 3700 or Hewlett-Packard 5890 GC equipped with electron capture detectors (ECDs) and DB1 bonded poly(dimethylsilicone), fused silica (30 m X 0.25 mm i.d.) capillary columns (J&W Scientific). PCBs were quantified on a peak-by-peak basis using Aroclor 1242 amended with 2- and 4-chlorobiphenyl as an external reference standard (27). Reproducibility of PCB analysis on duplicate samples was f 1 0 % , primarily due to sample variability. The combined mass of PCBs in the resin and sediment phases after separation was within 10% of the initial mass of PCBs in the sediment, except where noted in the text. The aqueous phase was periodically analyzed for PCB content and consistently contained less than 1% of the total PCBs in the sample.
Results and Discussion Sediment Analysis. The three Hudson River sediments used in this study contained 25,64, or 205 mg/kg (dry weight) PCBs, with total organic carbon contents of 0.96 % ,3.43 % ,and 4.59 % ,respectively. ThePCBspresent in the sediments consisted of primarily mono- and dichlorinated biphenyls (60-70% of total) due to substantial natural dechlorination of the Aroclor 1242 originally released into the river (28). The inorganic component of the sediment was comprised of 82 % sand (57 % fine sand), 15% silt, and 3% clay. Desorption of PCBs from Untreated Hudson River Sediments. When desorption experiments were run using the Hudson River sediment containing 25 mg/kg PCBs, 55 % of the PCBs (labile component) desorbed from the sediment in the first day, while little of the remaining 254
Envlron. Sci. Technol., VoI. 28, No. 2, 1994
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Flgure 1. Short-term PCB desorption from Hudson River sediment contaminated with 25 mg/kg PCBs. Distribution of the PCBs between the sediment (W) and XAD-4 resin (0) Is shown, as well as the overall mass balance (A).
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Figure 2. Long-term PCB desorption from Hudson River sediment contaminated with 25 mg/kg PCBs. Distribution of the PCBs between the sedlment (W) and XAD-4 resin (0)is shown. The line represents a nonlinear regression of the data by the two-box model.
45% (resistant component) desorbed in 170 h (Figure 1). Over a period of 1year, approximately half of the remaining resistant fraction desorbed with a first-order rate constant of 0.005 day1 (Figure 2). This result is comparable to a rate constant of 0.003 day1determined by Witkowski (18) for resistant phase desorption of Aroclor 1242from spiked sediments. The resistant fraction was not affected by increasing the residsediment ratio (by a factor of 41, suggesting that kinetics and not thermodynamics were limiting PCB desorption from the sediment. Thermodynamic calculations on this system using the fugacity model of Mackay (29)indicated that 99.95% of the PCBs should reside in the resin at equilibrium, due to a 2-order of magnitude disparity in both mass and KO,between the resin and sediment organic matter. This suggests that even after 1 year the system was not approaching equilibrium. The effect of PCB loading on desorption kinetics was examined by performing desorption experiments on sediments with an &fold higher (205 mg/kg) PCB concentration. A total of 25% of the PCBs in the sediment remained in the resistant fraction after 7 days, as compared to -45% in the more lightly contaminated material. However,the absolute PCB concentration in the resistant fraction was four times higher in the more highly contaminated sediment (51 versus 11 mg/kg). When the experiment was extended over 6 months, PCBs continued to move from the sediment to the resin with a first-order
Table 1. P C B Desorption D a t a for Composite a n d Size-Classified Sediments
sample
(I
wt%
composite low PCB composite mod PCB composite high PCB 11.3 900-3327 16.5 293-990 pm 30.0 123-293 pm 37.2 69-123 pm 7.0
