Environ. Sci. Technol. 2007, 41, 2901-2907
Development of Engineered Natural Organic Sorbents for Environmental Applications: 3. Reducing PAH Mobility and Bioavailability in Contaminated Soil and Sediment Systems JIXIN TANG, ELIJAH J. PETERSEN, QINGGUO HUANG, AND WALTER J. WEBER, JR.* Department of Chemical Engineering, Energy and Environment Program, 4103 Engineering Research Building, The University of Michigan, Ann Arbor, Michigan 48109 - 2099
The effects of engineered natural organic amendments on two measures of the environmental “availability” of polycyclic aromatic hydrocarbons (PAHs) associated with soil and sediment samples are assessed. Two soils spiked with pyrene alone and a sediment spiked with a mixture of 4 PAH compounds were amended with raw or superheated-water processed peat or soybean stalks, then aged for periods of either 62 or 105 days. The aged soils were then examined with respect to bioaccumulation of spiked pyrene by earthworms (Eisenia foetida), and to its human bioaccessibility as measured by extraction with simulated gastrointestinal fluid. Additions of processed amendments reduced both measures of availability by factors ranging from 7.6 to 27.0 for earthworm bioaccumulation and from 2.3 to 8.8 for gastrointestinal extractability. All PAH compounds spiked to the sediment were reduced to varying extents in their availabilities to E. foetida and leachabilities by water by both processed and raw organic amendments.
Introduction Natural organic matter (NOM), long recognized as a major compartment of terrestrial and aquatic systems for sorption and sequestration of persistent hydrophobic organic chemicals (HOCs), profoundly impacts the environmental availabilities and ultimate fates of such contaminants. Direct exposures of contaminants to ecological species, and thus associated environmental risks, are generally reduced in proportion to the extent to which they are sequestered by natural or synthetic sorbents (1-2). The sorption and retention properties of various forms of NOM for HOCs have been observed to correlate with their respective geochemical structures and properties, particularly their degrees of condensation and aromatization (1, 3-7). Geologically mature and more condensed or ”hard carbon” forms have greater HOC sorption and retention capacities than geologically young and more amorphous forms (3). Soil/sediment associated HOC bioavailabilities to microorganisms and * Corresponding author phone: (734)-763-2274; fax: (734)-9364391; e-mail:
[email protected]. 10.1021/es061736k CCC: $37.00 Published on Web 03/14/2007
2007 American Chemical Society
benthic organisms have been inversely related to the presence of kerogens, shales, soot, wood chars, waste coke, and unburned coal particles in those materials (8-13). Hard carbon materials, such as activated carbon, have long been known to exhibit strong sorption of HOC contaminants from water, and they have been examined and proposed as contaminated sediment amendments for reduction of associated HOC bioavailability (14), although cost and technical constraints on their large scale use for in-situ open-water applications are important considerations (15). The work described in this paper and in two earlier papers (16, 17) has pursued potential alternative amendments. The first stage of this work involved modification of readily available and inexpensive sources of geologically young NOM using modest superheated water (SHW) processing to yield engineered natural organic sorbent (ENOS) materials having intermediate diagenetic character (16). SHW processing of different natural organic materials was shown to result in marked increases in their carbon and nitrogen contents, and corresponding decreases in their respective hydrogen and oxygen contents. The materials produced thus became less polar, increasingly condensed, and more aromatic as a result of SHW processing. The second paper (17) presented experimental data confirming the enhanced sorption properties and behaviors of the products anticipated in the first paper’s results. The sorption capacities, isotherm nonlinearity, and retention effectiveness of the ENOS products with respect to phenanthrene, a common polycyclic aromatic hydrocarbon (PAH), were demonstrated to increase markedly with increasing degrees of SHW processing (17). The results led to a hypothesis that SHW processing of inexpensive and universally available biopolymers of plant origin can yield products having specifically enhanced sorptive properties and potentials for targeted environmental applications. Here we address the effectiveness of ENOS materials as soil or dredged sediment amendments designed to reduce the environmental availability of associated contaminants to ecological and human receptors. The work is predicated on the findings and conclusions reported in the first two papers (16, 17) and was undertaken to confirm the implications thereby inferred. The effects of ENOS amendments on several common measures of the environmental “availability” of soil- and sediment-associated PAHs are assessed. Test conditions employed are based on anticipated applications of these materials as either soil or dredged sediment spoil amendments. We neither examined nor imply applications to submerged open-water sediments.
Experimental Section A diagram summarizing the overall experimental design for the studies described is shown in Figure 1, and details of the experimental procedures are described below. The somewhat different protocols used for the soil and sediment samples relate to different anticipated dominant contamination circumstances and remediation approaches. Testing of the soils is predicated on their having been exposed to land surface contamination events, and that of the sediment to open-water contamination events and subsequent dredging and placement in a confined disposal facility. Soils and Sediment. Two near-surface sandy-loam field soils were examined, one having an organic carbon (OC) content of 2.47% and the other 5.95% based on dry weight. The former, collected on the North Campus of the University of Michigan, is designated “NC soil” and the latter, collected in Chelsea, Michigan, is referred to as “Chelsea soil”. A sand and clay sediment mixture, having an OC content of 0.66%, VOL. 41, NO. 8, 2007 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 1. Experimental Design. was collected from the Huron River in Ann Arbor, MI. and is referred to as “HR sediment”. After analyses to ensure that none of the samples contained trace amounts of target contaminants, each was air-dried and passed through a 2-mm mesh sieve. Both soil samples were then sterilized by 2.5Mrad γ-irradiation using a 60Co source prior to spiking. This was done to isolate and quantify the specific sorption/ sequestration effects of the ENOS amendments on pyrene availability in the absence of microbial degradation and over and above the effects of aging alone. The abiotic condition emphasizes the impact of the ENOS amendments by allowing the otherwise biologically available pyrene to be fully sorbed by the soil and its amendments. The sediment, on the other hand, was meant to represent a material contaminated by several PAHs and was aged under nonsterile submerged conditions before treatment with amendments. It was therefore neither sterilized nor assessed with respect to isolation and quantification of aging effects. ENOS Amendments. A peat soil precursor in its early stages of diagenesis and essentially fresh soybean stalks were selected as representative natural organic materials from which to produce ENOS amendments (16). Michigan (MI) peat, a weathered sphagnum peat, was collected in northern Michigan, while the soybean stalks were obtained in Ann Arbor. Both raw materials were air-dried and ground in a mill, processed in superheated water (SHW) (200 °C) for 5 h, air-dried, and passed through a 1-mm mesh sieve. These amendments had OC fractions of 45.1, 46.1, 60.4, and 62.5% for the raw peat, raw soybean stalks, SHW-processed peat, and SHW-processed soybean stalks, respectively. Detailed SHW treatment and characterizations of the physiochemical and structural changes and sorption/desorption behaviors of the ENOS products were presented in previous papers (16, 17). All raw and processed amendments were also extracted and no traces of the target contaminants were found. Test Organisms. Earthworms (Eisenia foetida), obtained from the Carolina Biological Supply Co. (Burlington, NC) served to assess the availability of the sorbed target PAH compounds to biological uptake. E. foetida was maintained on a worm bedding (Carolina Biological Supply) at 21 ( 2 °C and kept moist with deionized water. The worms were fed twice a week with worm food comprising a mixture of crude proteins and carbohydrates (Magic Worm Products, Amherst Junction, WI). Soil and Sediment Spiking and Treatment. As shown in Figure 1, the radiation-sterilized soils were spiked aseptically in sterilized 500-mL wide-mouth glass jars with pyrene in unlabeled and 14C-labeled (position 9) forms dissolved in dichloromethane. The jars were agitated on an end-over2902
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end shaker for 24 h to ensure thorough mixing of their contents, then they were placed in a fume hood, their lids were replaced with sterilized aluminum foil, and the solvent was allowed to evaporate for 24-48 h. Sterilized raw and processed MI peat and soybean stalks were then aseptically mixed with these soil samples in ratios of 0-20% (w/w) to yield a final pyrene soil concentration of 100 µg/g. Autoclaved Milli-Q water was added to provide a moisture level of 2022% (v/w), and the samples were then mixed and transferred aseptically to sterilized glass jars to fill them nearly full to minimize pyrene loss by volatilization. The jars were then sealed with sterilized screw caps fitted with Teflon liners and stored in the dark at 10 ( 1 °C to initiate a 105-day aging/ sequestration process. At the end of this amended soil aging process, comparative freshly spiked samples were made by adding pyrene dissolved in dicholoromethane to clean samples of the NC and Chelsea soils, the dichloromethane was allowed to evaporate, and the soils were moistened and held for 1 day prior to use. To simulate real-time contamination and treatment scenarios, two-hundred gram samples of dry unsterilized HR sediment were spiked in 3.8-L glass jars with a mixture of phenanthrene, anthracene, pyrene, and chrysene dissolved in acetone, the jars mixed on a tumbler for an hour, the acetone evaporated in a fume hood, an additional 1000 g of dry clean sediment added to each, and the jars again mixed overnight on the tumbler. A 2000-mL volume of river water obtained from the sediment collection site was added to each jar to cover the sediment samples and the samples then stored in the dark at 10 ( 1 °C for a 56-day aging period, and the aged sediment samples then separated from the water and freeze-dried. To assess their comparative effects on the availability of sediment contaminants, processed and raw MI peat were added at 5% (w/w) to 300-g samples in 500-mL glass jars, 150-mL of Milli-Q water added, and the jars closed with Teflon-lined caps. Similarly handled unamended sediment samples served as controls. Because some PAH degradation was noted during this 56-day 10 °C aging period, the samples were further aged in the dark for an additional 62 days at 4-5 °C to minimize microbial activity. Duplicate or triplicate samples of these aged soils and sediments were measured for moisture content, hightemperature combustion, extraction by methanol, and uptake and accumulation of each PAH by earthworms, physiological extraction, and water leachability. Earthworm Bioaccumulation. The bioavailability of pyrene in all aged, fresh, and amended and unamended soil samples was determined first by earthworm uptake and accumulation. Three adult worms were transferred to moist (21% water) aged soil samples (75 g) in 250-mL glass jars, the jars loosely closed with a cap to prevent worm escape and allow air exchange, then held in the dark for 14 days at 21 ( 1 °C for measurements of pyrene uptake. At least two replicates were tested for each soil sample. The bioavailabilities of the four PAH compounds in the aged HR sediment samples amended with and without raw or processed peat were also determined by measuring bioaccumulation by E. foetida in the same manner. After 14 days of exposure the PAH concentrations in the sediment samples and biological tissues were determined. No earthworm fatalities were observed during these experiments. Physiological Extraction. In vitro gastrointestinal extraction procedures have been employed as means for estimating the potential bioaccessibilities of soil-associated organic contaminants to humans (18, 19). Bioaccessibility is defined here as the fraction of soil-/sediment-associated contaminants that becomes available for absorption by fluids in the stomach and small intestines. A modified extraction method based upon physiological similitude with human gastrointestinal fluid was used to measure pyrene bioaccessibility
in aged and fresh soil samples (18, 19). Simulated gastric juices were prepared by mixing 15 g of glycine with 1 L of deionized water, then adding HCl to adjust the pH of the mixture to 1.5, a typical pH of human stomach fluids under fasting conditions (18). The bulk stomach solution was completed by adding pepsin (1 g), bovine serum albumin (5 g), and type III porcine mucine (2.5 g) to the gastric solution. The bioaccessibility of the pyrene was determined by adding 0.4 g of NC soil sample into 100 mL of stomach solution in 250-mL glass flasks. The mixture was shaken in a gyratory shaker at 50 rpm for 1 h at 37 °C to conclude the gastric component of the assay. To begin the small intestinal component of assay, the pH of the slurry was increased to 7.2 by adding 1 M NaOH and 0.1 mL of stock intestinal solution (18). This stock intestinal solution was prepared by mixing ten amino acids in physiological concentrations commonly found in humans (20). The slurry solution was shaken again at 50 rpm at 37 °C to mimic mixing that occurs within the human small intestine. Finally, the solution was centrifuged and 10 mL of supernatant transferred to scintillation vials for measurement of 14C-radioactivity using a LS6500 liquid scintillation counter (Beckman, Fullerton, CA). Sediment Sorbed PAH Leachabilities. The availabilities of the four PAHs in the spiked and aged HR sediment were determined by measuring their leaching into flowing water. Duplicate dried aged sediment samples (6.0 g) with/without MI peat were loosely packed in clean glass columns (8 × 150 mm) with glass wool at both bottom and top. The sediment samples were then leached for 5 days with a 0.5-mL/min flow of 0.01 M CaCl2 aqueous solution (with 200 mg/L of NaN3 to inhibit biological activity) using syringe pumps. Sequential leachate samples of about 1000 mL were collected in 1-L glass bottles. Because of the low PAH concentrations in the leachates, C18 membranes were used to collect samples for chemical analysis. The membranes were conditioned with methanol and Milli-Q water, then added to and mixed with the leachates in the collection bottles on a shaker for 48 h, removed and dried on paper towels, and PAHs then were eluted with methanol for analysis by HPLC. Preliminary experiments using C18 membranes for the PAHs revealed recoveries in the range 91.2-98.4%. Chemical Analysis and Data Analysis. Residual concentrations of PAH compounds were determined in all soil, sediment, and biological tissue samples before and after aging and uptake experiments. Portions (1-2 g) of soil and sediment samples were freeze-dried for measurement of total residual pyrene or all four PAH compounds. Earthworms taken from the soil samples were washed with Milli-Q water, transferred to wet filter paper in Petri dishes for 24 h in the dark to allow purging of gut contents, again rinsed with clean Milli-Q water, transferred to preweighed glass centrifuge tubes, freeze-dried, and weighed for analyses of 14C-radioactivity. Earthworms from the sediment uptake experiments were cleaned in the same manner described for the soil samples, then freezedried. Dried worm tissues were ground in a mortar and transferred to 25-mL glass centrifuge tubes for chemical extraction. Soil and worm tissue samples containing 14C-labeled pyrene were combusted in an OX-500 Biological Oxidizer (R.J. Harvey Instrument Co., Hillsdale, NJ), the resulting 14CO2 was recovered in 15 mL of scintillation cocktail, and radioactivity was measured by liquid scintillation counting. The PAHs in the corresponding sediment and worm tissue samples were extracted twice with methanol by sonication for 3 h at 50 °C in 25-mL glass centrifuge tubes. Preliminary tests showed the recoveries of PAH compounds yielded by the sonication extraction method ranged from 90.4 to 106.2%. PAH concentrations in the methanol extracts were analyzed using an Agilent 1100 high-performance liquid chromato-
graph with a 125 × 3.20 mm Envirosep PP C18 column (Phenomenex, Torrance, CA) and an ultraviolet or fluorescence detector. Acetonitrile-water (80:20, v/v) was used as the mobile phase at 0.8-1.0 mL/min. Quantities of each PAH compound accumulated in worm tissues and bioaccumulation factors (BAF; i.e., ratios of concentrations in biological tissue to residual concentrations in a soil or sediment sample, including amendment masses) were determined (21, 22). Statistically significant differences among means of duplicate or three replicates for different measurements were determined using a one-way analysis of variance (ANOVA) procedure in conjunction with a Duncan multiple range test (SAS Institute 2002).
Results and Discussion Recoveries of initially applied amounts of pyrene ranged from 89.3 to 102.7% for NC and Chelsea soil samples aged aseptically with MI peat or soybean stalks, indicating little degradation of pyrene under the aging conditions employed. Conversely, for HR sediment samples aged for 56 days under nonsterile conditions, recoveries of spiked compounds were 14.6% for phenanthrene, 55.3% for anthracene, 82.3% for pyrene, and 83.4% for chrysene, indicating for each some degradation, loss of extractability, or loss to overlying water during aging. Aging for an additional 62 days in the absence or presence of raw or treated MI peat resulted in subsequent recoveries of these compounds ranging from 38.7 to 90.8% of the total concentrations remaining after the first 56 day period. Pyrene Availability to Earthworms in Soils. It is evident from the data presented in Figure 2 that the availabilities of pyrene to E. foetida earthworms in unamended NC and Chelsea soils were decreased by aging alone, decreased further by the presence of raw material amendments at levels in excess of 10%, and decreased markedly in the presence of processed ENOS amendments at levels of only 5%. More specifically, the addition of the processed amendments decreased pyrene uptake by factors ranging from 7.6 to 27.0 compared to aged but unammended soils. Bioaccumulation factor (BAF) values for NC and Chelsea soil samples (Figure 2) were significantly reduced by aging alone, and further decreased in samples aged in the presence of both raw and processed MI peat and soybean stalks. Adding 10% raw MI peat decreased the BAF values for the NC and Chelsea soil by factors of 1.4 and 1.9, respectively, compared to the unamended soils, while the OC fraction increased by factors of 2.7 and 1.7, respectively. This decrease can be explained in large part as a result of increasing the total OC concentration in the soil/amendment mix, even though the added organic carbon was geologically young and exhibited relatively weak sorption properties. Conversely, it has been shown that earthworm Eisenia andrei preferentially feeds on organic matter in soil systems (23), and such behavior could increase the organism exposure to the contaminants sorbed to the amendments. BAF values after addition of the ENOS amendments, however, are significantly lower than those of the unmodified soil and the soil with raw amendments. Thus decreases in bioaccumulation values can be readily explained by the strong sorption properties of the ENOS amendments themselves, and not due simply to increases in the total OC concentration. For example, adding 20% processed MI peat to the NC soil increased the OC fraction by a factor of 5.7, but decreased the BAF value by a factor of 37. Physiological Extraction of Pyrene. As shown in Figure 3, aging significantly decreased the fraction of pyrene extracted by the gastrointestinal solution. This decrease mirrors the impact of aging on pyrene bioaccumulation by earthworms. Bioaccessible pyrene was decreased by addition of processed peat amendments by factors ranging from 2.3 to 8.8, and would thus markedly reduce the risk that these VOL. 41, NO. 8, 2007 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 2. Effects of organic amendments and 105 days of aging on pyrene uptake (I and II) and bioaccumulation factor (BAF) values (III and IV) by E. foetida from soils aged for 105 days (“Fresh” designates a soil neither aged nor amended, “None” a soil aged without organic amendments, “RMP” a soil aged with raw MI peat, “PMP” a soil aged with processed MI peat, “RSS” a soil aged with raw soybean stalks, and “PSS” a soil aged with processed soybean stalks). Error bars represent one standard deviation (n ) 2). Values for bars followed by the same letter or letters are not significantly different (P < 0.05).
FIGURE 3. Effects of raw and processed MI peat amendments during sample aging on the physiological extractability of pyrene from NC soil. Term abbreviations are as defined for Figure 1. Error bars represent one standard deviation (n ) 3). Values for bars followed by the same letter or letters are not significantly different (P < 0.05). chemicals would pose to humans upon exposure through soil ingestion. Conversely amendments of 10% raw peat actually increased pyrene extractability (9.9%) in two of the three cases examined. This might be attributed to the 2904
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reversible nature of pyrene sorption by the raw peat samples and the relative ease of its release in physiological extraction. This facile desorption stems from the young diagenetic organic carbon of the raw peat amendments.
FIGURE 4. Effects of raw and processed MI peat amendments during aging on the bioaccumulation of PAH compounds by E. foetida from Huron River sediment. Term abbreviations are as defined for Figure 1. Error bars represent one standard deviation (n ) 3). Values for the bars followed by the same letter are not significantly different (P < 0.05).
FIGURE 5. Effects of the presence of raw (RMP) and processed (PMP) MI peat amendments during aging on the water leaching of PAH compounds from Huron River sediment. Error bars represent one standard deviation (n ) 2). Biological Availabilities of PAH Compounds in Sediments. As shown in Figure 4, the uptakes of each of the four aged PAH compounds except phenanthrene by E. foetida from aged HR sediment samples were substantially reduced by amendments of MI peat, particularly the processed material. Biodegradation of phenanthrene during the aging interval might account for this difference in its bioaccumulation behavior. The addition of 5% processed MI peat resulted in pyrene concentrations in dry earthworm tissues
decreasing from 188.5 to 43.7µg/g dry worm after exposure to contaminated HR sediment samples, and BAF values similarly decreased by a factor of 4.1 (Table S-1, Supporting Information). The BAF value of pyrene after the addition of 5% PMP to NC soil decreased by a factor of 9.2 compared to the aged soil without amendments, but this larger decrease in the BAF value contrasts with the fact that the addition of the PMP increased the fraction of organic carbon more significantly for the HR sediment than for the NC soil. This VOL. 41, NO. 8, 2007 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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result could stem from the organic carbon in the sediment possessing a higher sorptive capacity than that in the NC soil, or from biodegradation of the biologically available pyrene during the sediment aging. It has been established that both biodegradation and aging decrease bioaccumulation more than either process alone (24), but the NC soil was aged in the presence of the ENOS amendments for longer (105 d) than the HR sediment (62 d). This suggests that the addition of the ENOS amendments should have had a stronger impact on the NC soil. The interplay between the effects of biodegradation, aging, and sorption are clearly complex and their relative importance is likely to vary based on the soils, amendments, and experimental design. Leachabilities of PAH Compounds from Sediments. Aging of HR sediment in the presence of both raw and processed MI peat further decreased percentages of the four PAH compounds leached, as illustrated in Figure 5. Figure 5 also shows that leaching patterns of aged PAH compounds varied as functions of leaching water volume; i.e, the leaching plateau for each was essentially reached after application of only approximately 1000 mL of water in the case of the sediment amended with processed MI peat. The PAH compounds were apparently sorbed so strongly by this processed ENOS amendment that leaching to the water phase was greatly diminished. Conversely, leaching from both unamended and raw peat amended sediment samples steadily increased with the exception of phenanthrene for which a relative plateau was reached after 3000 mL of water. Not unexpectedly, given their respective water solubilities, hydrophobicities, and initial concentrations, leaching percentages varied among the four PAH compounds, that for phenanthrene (solubility)1.1 mg/L) being highest and that for chrysene (solubility ) 0.003 mg/L) being lowest. These results for the amendments cannot be directly extrapolated to open-water applications, under which conditions the amendments’ small particle sizes and light bulk densities, may well render them hazardous materials of high potential environmental mobility (15). As was anticipated from the results of the work described in the first two papers in this series, the effects of ENOS amendments on contaminant leachability and biological availability appear generally to correlate with their respective sorption properties. Michigan Peat, for example, was demonstrated to be altered dramatically in its material characteristics (16) and significantly enhanced in its sorption properties with respect to PAHs (17) by mild SHW processing, while the results given here show concomitant and significant decreases in associated contaminant availability (Figures 2-4) and leachability (Figure 5). ENOS amendments thus appear to have strong potential for applications designed to reduce the risks of contaminated soils and dredged sediment spoils. Soil and sediment amendments with sorbents of various types comprise a potentially important strategy for environmental remediation, restoration, and stabilization applications. Selection and/or design of appropriate amendments are keys to realization of this potential. ENOS products provide attractive alternatives to relatively inert and costly sorbent amendments (e.g., activated carbons and adsorptive synthetic resins) proposed by others (9, 10, 14). Fundamental differences between the products described here and other alternative materials suggest several potential advantages for ENOS that merit consideration, particularly when eventual remediation of HOCs by biodegradation is a principal objective of a remediation campaign. Leuking et al. (1) demonstrated that certain natural sorbent amendments (e.g., shales) offer the advantages of retarded contaminant release, while concurrently supporting contaminant biodegradation. ENOS materials can be produced under relatively mild and readily adjustable heating conditions, and thus likely are 2906
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tunable to pre-selected levels of contaminant “reversibility”. Such products would better serve more complete remediation-oriented objectives by controlling local releases at ecologically acceptably but microbially available levels to enable ultimate sorbate biodegradation. Moreover, ENOS materials still contain high levels of reactive oxygen and nitrogen and are rich in functional groups (16). Earlier studies (25, 26) have indicated that these specific properties are important with respect to inducing and sustaining natural humification reactions that can result in chemical incorporation of contaminants into soil organic matter matrices, another important mechanism leading to their transformation in state and detoxification. These significant potential advantages merit further investigation.
Acknowledgments We appreciate the outstanding support of Laboratory Director Tom Yavaraski. The research was supported in part by the United States Department of Energy (DOE) through grant DE-FG07-02ER63488, in part by grant P42ES04911-14 from the National Institutes for Environmental and Health Sciences, and in part by grant U009439 Michigan Sea Grant Program. The conclusions set forth in the paper do not necessarily reflect the views of the funding agencies.
Supporting Information Available Effects of aging and ENOS amendments on E. foetida bioaccumulation factors of PAH compounds in the HR sediment. This material is available free of charge via the Internet at http://pubs.acs.org.
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Received for review July 21, 2006. Revised manuscript received January 9, 2007. Accepted January 10, 2007. ES061736K
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