GOT PCBS? (IN YOUR SAND) - TROUBLESHOOTING A SEPARATION

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1 GOT PCBS? (IN YOUR SAND) - TROUBLESHOOTING A SEPARATION T.J. Estes 1 ABSTRACT At New Bedford harbor where PCBs are the principal contaminant of concern, all sediment dredged is being disposed off-site. As part of the sediment dewatering operation, sand is separated from fines using hydrocyclones. Significant disposal cost savings could be realized if PCBs concentrations in the sand fraction could be reduced below the TSCA threshold. However, carryover of fine particulates and carbonaceous phases in the cyclone underflow appear to be responsible for high residual PCBs concentrations in the sand. The objective of this study was to evaluate the relative contribution of these phases to the PCBs loading in the sand fraction and potential for separation with process modifications. A fractionation study was designed to evaluate PCBs concentrations in size and density fractions of the cyclone underflow. Sediment samples were separated by size into coarse and fine fractions, and by density into operationally defined organic and mineral fractions. PCB congeners, oil and grease (OG), condensed carbon phases (soot), and amorphous carbon phases (OC) were measured in each fraction. Contaminant mass associated with each sorptive phase was estimated based on statistical correlation. Grain size distributions for the coarse and organic sediment fractions were found to be similar and it is not clear that further modification of cyclone configuration or operation will achieve processing objectives. However, based on particle size distribution and measured specific gravity, a portion of the coarse fraction should have a settling velocity higher than that of the coarsest organic particulates. It appears that an upflow separation process could be effective in recovering the coarsest sand free of organic phases. Quantifying contaminant reduction achievable is complicated, however, by the inefficiency of the separations and the residual co-mingling of organic and mineral phases. Also, it would be reasonable to expect a concentration gradient within the organic fraction, as a function of particle size and surface area. This would influence the amount of organics that could be allowed to carry over into the sand. The contribution of organics to contaminant loading in the fines fraction and the size related contaminant gradient within the organic fraction are needed to better refine contaminant reduction estimates. Alternatively, bench or pilot testing to evaluate the effectiveness of an upflow separator would be a reasonable next step. Keywords: Hydrocyclones, size and density fractions, mineral and organic phases, contaminant carryover, contaminant distribution. INTRODUCTION Remediation dredging is taking place at New Bedford harbor where PCBs are the primary focus of the remediation. Dredging is currently being done hydraulically, using an 8 foot Elicot Mudcat auger dredge, and the sediment dewatered in preparation for transport off-site. The first step in the dewatering operation is hydrocyclone separation of sand from the slurry. Sand is removed from the slurry because it is very abrasive and potentially damaging to the dewatering equipment, but also because it can be effectively dewatered passively, by gravity drainage. Also, sand is typically relatively uncontaminated as compared to the fine sediment fraction. In this case, off-site transportation and disposal costs are approximately $60/ton less for materials with PCBs concentrations below 50 mg/kg (the threshold defining hazardous materials under the Toxic Substances Control Act (TSCA). To date, however, residual concentrations in the sand fraction have been higher than the TSCA threshold. Subsamples taken during plant operation indicate some carryover of fine particulates into the underflow, as well as a coarse particulate that appears to be carbonaceous in character. Field and bench testing was proposed to devise a means for reducing underflow PCBs concentrations below 50 mg/kg. Field testing of different hydrocyclone configurations and operating parameters was conducted by others, 1 Research Civil Engineer, United States Army Corps of Engineers Engineer Research and Development Center (ERDC), 3909 Halls Ferry Road, Vicksburg, MS , USA, T: , Fax: , Trudy.J.Estes@usace.army.mil. 99

2 and provided sediment samples for the subsequent bench testing effort conducted at ERDC. Size and density separations were conducted on these sediment samples, and the following parameters measured in the fractions: PCBs oil and grease (OG) condensed carbon phases (soot) amorphous carbon phases (OC) Contaminant mass associated with each phase was estimated based on statistical correlation. Separability of the contaminant bearing phases was evaluated based on measured physical properties. This report summarizes the results of the bench testing. METHODS AND MATERIALS Sediment samples For each of six tests, five gallon samples were collected of hydrocyclone feed, overflow and underflow. Samples were packaged in HDPE buckets, iced and shipped to ERDC for testing. Chain of custody procedures were observed. Samples were received in good condition, with temperature ranging from 4 to 5 deg C at check-in. Clear supernatant was decanted from all samples received at the lab, and they were then homogenized for 10 minutes using a clean Lightnin mixer. Samples received in multiple buckets were combined for homogenization then returned to separate buckets. Samples of homogenized material were taken for moisture content analysis. The remainder of the sediment samples were stored at 4 deg C until use. Bench Testing Because the analytical budget was limited, not all samples shipped to ERDC were used in the bench testing. Three tests were selected for evaluation based influent and underflow concentrations, with the objective that the effect of feed concentration and sorptive phases would be reflected in the fractionation testing, and the differences between the most effective test and the remainder of the tests captured. Table 1 summarizes some of the properties of the selected samples. Table 1. Property Summary for Selected Tests Cyclone Test Sample Solids 1 Organic 2 Fines 1 TOC 1 PCBs 1 Oil & Grease 1 (%) (mg/kg) 1 Feed <100 5 Feed Feed <100 KHC 3 5 UF UF UF HHC 4 5 UF UF UF From analysis of field generated samples analyzed by others. Subsequent data tables and figures included in this report were generated from ERDC bench testing samples and analysis., 2 Based on units conversion of TOC concentration, 3 Krebs hydrocyclone, 4 Harrisburg hydrocyclone Samples were taken for fractionation testing from the homogenized material for each of the selected tests. Table 2 summarizes the selected testing matrix and properties measured on the samples generated in the bench testing. 100

3 PCBs, oil and grease (O&G), total organic carbon (TOC) and soot were measured to provide data for statistical correlation. Grain size distribution was measured using a Coulter Counter (Model LS 100Q), which provides an estimate of the continuous particle size distribution. One aliquot of feed was taken for each test. One aliquot of underflow was taken and split, ½ for density separations and ½ for size separations. Table 2. Hydrocyclone Pilot Test Sample Matrix Test Feed HHC KHC No. No. Total UF UFL UFH UFC UFF UF UFL UFH UFC UFF Samples Tests No. per Samples Test Coulter PCBs Soot & OC Oil & Grease Particle Specific Gravity UF=underflow, L=S.G.<2.1, H=S.G.>2.1, C=>75um, F=<75um Density Separations Underflow samples were separated at a specific gravity of 2.1, the value which defines the upper end of density for carbonaceous materials. Samples were separated using Sodium Polytungstate, a water soluble heavy media with a neat specific gravity of 2.8 g/cm. Specific gravity of the heavy media was adjusted with distilled, deionized water (DDI), taking into account dilution effects of the pore water contained in the sediment samples. A minimum of 40 g of organic material was needed for analytical procedures. The amount of sediment and media required to generate sufficient organic fraction for analysis was estimated for each sample based on TOC measured in the field generated underflow samples. Density separations were not done on Test 5 underflow samples due to the low organic content and the commensurate heavy media requirements. In order to minimize heavy media requirements, some underflow samples were centrifuged prior to density separation. Samples were taken again for moisture content determination after centrifuging. The requisite amount of sediment and sodium polytungstate was then weighed into centrifuge bottles to give a 3:1 solution to dry solids ratio. (A 3:1 ratio was estimated to provide a sample sufficiently fluid to achieve separation and sufficient fluid volume for vertical separation of the density fractions in the centrifuge bottles.) Bottles were placed on a shaker table for approximately 45 minutes to thoroughly slurry the material, then transferred to an ultrasonic bath (Branson 3210 Ultrasonic Cleaner) for 20 minutes to initiate the density separation. Samples were then centrifuged for 30 minutes at 3000 rpm to separate the density fractions. The light solids and heavy media supernatant were decanted and filtered on a Millipore HAWP 45 um filter. Solids captured on the filter were rinsed with DDI and filtered again (heavy media interferes with the analytical instruments and must be removed). DDI was added to the heavy solids remaining in the centrifuge bottle, which were re-slurried, then centrifuged again and overlying fluid decanted. Subsamples of light and heavy fractions were placed in separate centrifuge tubes of heavy media, shaken and allowed to settle (or rise), in order to verify that a reasonably complete separation had been achieved. Remaining solids were collected in glass sample jars with Teflon lined lids, and stored at 4 degrees C until all samples had been processed. To minimize analytical variability, size and density fraction samples were sent for analysis as one batch. Size Separations As for the density separations, a minimum of 40 g of each size fraction was needed for analysis. Minimum sample size required to produce at least 40 g of both coarse and fine fractions was estimated based on the grain size analysis of the underflow. Samples were well stirred, then placed on a #200 (75 um) stainless steel sieve, and agitated gently with RO (reverse osmosis) water to facilitate separation. The coarse fraction was collected directly off the sieve. 101

4 Fine slurry passing through the sieve was centrifuged at 3000 rpm for approximately 1 hour, and clear supernatant decanted. Solids from both fractions were collected in glass sample jars with Teflon lined lids and stored at 4 degrees C prior to analysis. Specific Gravity Measurement ASTM Method D (Standard Test Methods for Specific Gravity of Soil Solids by Water Pycnometer) applicable to moist solids <4.75 mm in size, was modified to accommodate the small sample quantities available from the size and density separations. The procedure calls for a pycnometer with a minimum capacity of 250 ml, and specifies that the pycnometer volume be at least 2 to 3 times greater than the volume of the soil-water mixture used during the de-airing portion of the test. Depending upon soil type, recommended soil mass for a 250 ml pycnometer ranges from 35 to 60 g (dry weight). The procedure was instead performed in 25 ml bell shaped glass pycnometers, using the dry weight equivalent of 10 g of each sample. Pycnometer volume was calibrated according to the procedures specified in the method (ASTM D854-10). A standard deviation of 0.05 ml was required for the pycnometer to pass volumetric calibration. Wet samples were taken for moisture analysis and the amount of wet sample required yielding 10 g dry equivalent calculated. Weighed samples were then dispersed in the weighing tins with distilled, de-aired water, and poured into the pycnometers to approximately 1/3-1/2 the capacity of the pycnometer. Partially filled pycnometers were packed into a glass vacuum cabinet, the cabinet was sealed and placed on an orbital shaker, and vacuum applied to the cabinet to de-air the samples. The samples were de-aired under continuous vacuum and gentle agitation for a minimum period of 2 hours. After de-airing, pycnometers were plugged and filled with de-aired distilled water from the beaker in the insulated container. The exterior was carefully dried to prevent wicking water up through the cap apertures, and pycnometer and slurry weighed to the nearest 0.01 g using the same balance as used for calibration of the pycnometers. The plugs were removed from the pycnometers and temperature of the slurry recorded to the nearest 0.1 deg C. The slurry was rinsed from the thermometer and from the pycnometer into a tared weighing pan, and dried at 110 deg C to a constant mass (approximately 5 hours). Samples were cooled in a desiccator, and weight of sample plus tare recorded to the nearest 0.01 g, using the same balance as for all other steps. The mass of the solids was calculated and recorded to the nearest 0.01 g, and compared to dry mass calculated based on wet weight and moisture content to check for losses. Volume of water was determined from filled pycnometer weight, less weight of dry pycnometer and dry solids added. Volume of solids was determined by difference, and specific gravity of the solids was then calculated (dry mass/volume solids). The procedure was performed in triplicate, and the percent difference, mean and standard deviation (SD) of the specific gravity calculated for all samples. The maximum SD of the specific gravity values was g/cm 3, while the mean SD was g/cm 3. (For all but two samples, the SD was less than 0.06.) The procedure was therefore considered to provide an acceptable estimate of solids specific gravity based on small volume samples. Analysis Influent, underflow and underflow fractionation samples were analyzed for PCBs (SW-846 SW ), Oil and Grease (EPA 418.1), TOC (SW ) and soot (Gustafsson et al. 1997). Both solid and liquid samples intended for chemical analysis were stirred before obtaining subsamples for processing to assure representative samples would be acquired. Approximately 15 g solids subsamples were processed for PCBs and oil and grease analyses. Approximately 0.5 g of oven-dried and ground subsample was taken for TOC analyses. TOC was measured by organic carbon combustion analysis. PCBs were analyzed using a Hewlett Packard 5890 Gas Chromatograph. Samples were analyzed in batches of up to 20 samples. Each batch included a blank, a lab control (spiked blank) sample, and, based upon sample availability, a matrix spike (spiked batch sample). Surrogates were added to PCB batches to monitor extraction efficiency. Data discrepancies and anomalous readings occurring during analyses were indicated on a quality assurance form, and, if necessary, corrective actions were taken by the analyst. The fractionation data collectively was used to evaluate correspondence of contaminant concentrations to the various sorptive phases overall. Correlation of contaminant concentrations to concentration of sorptive phases in all samples was evaluated by entropy analysis using SAS (a statistical software package). Entropy analysis is helpful in 102

5 cases where the parameter of interest cannot be measured directly, where there is collinearity between parameters, the data is non-normal or the distribution is unknown, or when the coefficients obtained using linear regression are illogical. The regression was conducted on the following equation: C + i = acsoot + bco& G ccoc (1) Where C i = concentration of congener i in the fraction (μg/kg) C Soot = concentration of soot in the fraction (mg/kg) C O&G = concentration of oil and grease in the fraction (mg/kg) C OC = concentration of organic carbon in the fraction (mg/kg) a, b and c = coefficients returned by the regression (μg/mg) The coefficients returned by the regression for each of the variables represent the concentration of constituent i associated with each fraction. A fourth sorptive phase, mineral clay, has been shown to be a statistically significant sorptive phase for some sediments (Estes 2005). The importance of the mineral clay in terms of overall contaminant mass, however, is relatively small and it was not expected to be an important phase in this case based on previous testing. Results Total PCBs concentrations were calculated by summing the measured congener concentrations, and were all above the TSCA definition of 50 mg/kg in the underflow (Table 3). Total PCBs concentration for each sample fraction is also reported in Table 3. The lowest UF concentration achieved was for Test 5 (5HHC). Because feed concentrations for this test were between that of Test 1 and Test 6 (Figure 1), this would not appear to be attributable entirely to lower feed concentrations, although organic content of feed in Test 5 was only about 30% of that in Tests 1 and 6. The data was further evaluated to determine what other properties distinguish this sample from the others, and whether that information could be exploited to further reduce PCBs concentrations in the UF. Mean underflow grain size distribution was compared to the grain size distribution for the underflow sample from 5HHC. The particle size distribution of 5HHC is somewhat coarser overall than the mean distribution for the UF samples, but the proportion of fine silts and clay size particles is similar. This suggests that lower fines content was not responsible for lower PCBs concentrations in 5HHC. Also, PCBs concentrations in the fine fraction of Test 5 were higher than that in Test 6 and comparable to that in Test 1 (Table 3). Soot concentration was lowest in the 5HHC underflow sample (Figure 2) however, which may be significant. Lower OG concentration in 5HHC UF may play a role as well, though OG concentrations were not markedly different from other UF samples with higher PCBs concentrations (Figure 2). Regressions were used to further evaluate the role of the respective sorptive phases with respect to contaminant loading in the sediment fractions. Figure 3 illustrates, for sample 1HHC, the correspondence of predicted contaminant concentrations in the sorptive phases (obtained from the Entropy regression) to the measured contaminant concentrations in the sample, with concentrations in OG>>Soot>OC. Figure 4 illustrates the correspondence of the sum of the contaminant mass predicted for all three sorptive phases versus the measured contaminant mass in UF sample 1HHC. The relationship is strongly correlated (R2>.99), supporting the predicted sorptive phase concentrations, but is somewhat skewed. In this case, the implication is that the regression has underestimated concentrations in the sorptive phases somewhat. A more direct demonstration of the relationship intimated by the Entropy regression is correlation of the measured sorptive phase concentrations themselves to the measured PCBs concentrations, as illustrated in Figure 5. Figure 5 confirms the strength of the correspondence to O&G and soot concentrations, and the weakness of the correspondence to OC concentration, which is consistent with the results of the Entropy regression. The importance of soot and O&G to contaminant loading of the fractions is further underscored when the results are examined in terms of contaminant mass (Figures 6 and 7). Although predicted contaminant mass in OC is somewhat higher than for soot and OG in Figure 7, the mass of OC was also nearly twice that of soot, and many times that of OG in this sample (Figure 2). 103

6 Figure 1. Total PCBs concentration (sum of all congener concentrations) in feed and underflow samples Figure 2. Sorptive phase concentration in underflow samples 104

7 Table 3. Total PCBs concentration in feed, underflow and size and density fractions Total PCBs ( All Congener Concentrations) Fraction Percent of Underflow (by weight) 1 Test Fraction Cyclone (μg/kg) (%) 1 Coarse HHC 492, Coarse KHC 1,422, Coarse HHC 93, Coarse KHC 238, Coarse HHC 282, Coarse KHC 1,180, All Coarse Mean 618, Fine HHC 1,108, Fine KHC 747, Fine HHC 1,098, Fine KHC 1,071, Fine HHC 442, Fine KHC 276, All Fine Mean 790, Organic HHC 3,358, Organic KHC 3,469, Organic HHC N/A 2 N/A 2 5 Organic KHC N/A 2 N/A 2 6 Organic HHC 1,937, Organic KHC 1,720,084 9 All Organic Mean 2,621, Mineral HHC 249, Mineral KHC 333, Mineral HHC N/A 2 N/A 2 5 Mineral KHC N/A 2 N/A 2 6 Mineral HHC 114, Mineral KHC 96, All Mineral Mean 198, Feed N/A 2,375,141 5 Feed N/A 1,304,634 6 Feed N/A 701,559 All Feed Mean 1,460,445 1 UF HHC 709,548 1 UF KHC 825,598 5 UF HHC 182,477 5 UF KHC 474,515 6 UF HHC 313,746 6 UF KHC 447,856 All UF Mean 492,290 1 Based on normalized weight of respective sample fractions, 2 Organic content too low for density testing 105

8 Figure 3. Predicted contaminant concentration in sorptive phases versus measured contaminant concentration in UF (sample 1HHC) Figure 4. Sum predicted contaminant mass in sorptive phases versus measured contaminant mass in UF (sample 1HHC) 106

9 Figure 5. Correlation between measured sorptive phase concentrations and measured PCBs concentration (congener 105, all tests all fractions) Figure 6. Predicted contaminant mass in sorptive phases versus measured contaminant mass in underflow (sample 1HHC) 107

10 Figure 7. Predicted contaminant mass in sorptive phases versus measured contaminant mass in underflow (sample 5HHC) The question remains, how can these phases or sediment fractions be selectively removed without sacrificing clean sand particles, and will this result in acceptable PCB concentrations (<50 mg/kg) in the UF? Examining the relative grain size of the different fractions making up the underflow samples, reveals that the organic fraction (SG<2.1) is a relatively coarse fraction. Using the particle size distribution (cumulative percent less than by volume as measured by Coulter Counter), piecewise curve fits and measured SG of each fraction, settling velocity distributions were calculated (based on Stokes law) for each size and density fraction. Figure 8 illustrates the average values for fine, organic and coarse fractions. Based on Figure 8, it is apparent that (on average) approximately 14 percent of the coarse sediment fraction should be relatively free of discrete organic and fine particles, though some may persist as a coating on the aggregate. Based on the settling velocities calculated from aggregate fraction properties, a hydroseparator designed to skim off particles with a settling velocity less than 41 cm/s could be utilized to recover the clean coarse material. Because there are several simplifications involved in the calculations (Stoke s law assumes spherical particles, for example), confirmatory bench or pilot testing will be needed to verify this result. Figure 9 represents the particle size versus calculated settling velocity for coarse, organic and mineral fractions. The corresponding cumulative plots reflect the cumulative percent of each fraction that would be removed using a specific separation velocity or screen aperture. For example, a separation velocity of 20 cm/s would remove 86 percent of the organic material (or all organic particles less than 691 um in diameter), along with 85 percent of the mineral material (or all mineral particles less than 464 um in diameter). In theory, the plot should help in the establishing the parameters of the separation and the composition of the underflow and overflow. To provide predictive capability with respect to residual concentrations, however, more needs to be known about the concentration gradients in each fraction. For example, if concentrations are lower in the coarse particles of the organic fraction, some of the coarse organic might be allowed to carry over into the sand without compromising processing objectives. 108

11 Figure 8. Cumulative percent of fraction with settling velocity less than value specified Figure 9. Particle size versus estimated settling velocity (closed symbols) for three fractions and cumulative percent less than particle size (open symbols) 109

12 In the absence of gradient information, one must assume that concentrations are uniform throughout the organic fraction and, on that basis, only the percentage of coarse mineral clearly separable from the organic fraction would be considered recoverable. However, PCBs concentrations in the mineral fraction samples were also found to be above 50 mg/kg in the bench testing (Table 4). It is possible this is attributable to inefficiencies in the bench density separation, resulting in organic matter carrying over into the mineral fraction, or to the presence of contaminant bearing phases as a particulate coating. Assuming the latter, attritioning and surfactant washing may also be required in order to recover a sufficiently decontaminated sand fraction. This should also be evaluated in follow-on testing. CONCLUSIONS None of the field tested hydrocyclone configurations resulted in underflow PCB concentrations <50 mg/kg. Given these results and the apparent similarity of grain size in coarse and organic sediment fractions, it seems unlikely that further modification of cyclone configuration or operation will achieve processing objectives. However, it appears that approximately 14 percent (on average) of the coarse fraction should have a higher settling velocity than the organic fraction. It may be that if the hydrocyclones are sized to target the corresponding cut size, the separation could be improved. Alternatively, an upflow separator may be effective in recovering this portion of the sand free of organic/carbonaceous materials. Based on our experience with bench testing, oil and grease may persist in the sand, either as a separate phase or as a coating on the particulates. In that case, an additional washing step may be required, depending upon the residual concentrations that result. Because we believe that oil and grease is the most contaminated sorptive phase, this is anticipated to be problematic with respect to residual PCBs concentrations in the sand. Further bench or pilot scale testing is needed to confirm that a portion of the coarse fraction is recoverable with PCBs concentrations <50 mg/kg. Predictive capability would be improved with further evaluation of the size related gradients in the organic fraction and contribution of organics to contaminant concentrations in the fine fraction. REFERENCES Estes, T.J. (2005). "PAH and PCB Distribution in Sediment Fractions and Sorptive Phases", Dissertation, Louisiana State University, Baton Rouge, LA. Gustafsson, Ő. and Gschwend, P. M. (1997). Soot as a strong partition medium for polycyclic aromatic hydrocarbons in aquatic systems, Molecular Markers in Environmental Geochemistry, American Chemical Society Symposium Series 671; Eganhouse, R.P., Ed.; American Chemical Society; Washington, DC. pp