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Development of Engineered Natural Organic Sorbents for Environmental Applications. 2. Sorption Characteristics and Capacities with Respect to Phenanthrene JIXIN TANG AND WALTER J. WEBER, JR.* Energy and Environment Program, Department of Chemical Engineering, 4103 Engineering Research Building, The University of Michigan, Ann Arbor, Michigan 48109-2099
The effects of superheated water processing on enhancement of phenanthrene sorption by various source materials of natural organic matter (NOM) were systematically examined. Sorption capacities and subsequent phenanthrene retention characteristics of all organic materials tested were markedly increased by superheated water processing. Temperature effects on enhancement of the sorption behaviors of the test materials were greater than those of processing time, moisture content, and the presence of mineral catalysts. Greatest enhancement was observed for processing at 250 °C for 5 h with a moisture content of 50%. Strong correlations of sorption capacities and isotherm nonlinearities with processing temperature, and with the atomic ratios of oxygen to carbon (O/C), aromaticity, and hydrophobicity of the processed materials were observed. The sorption/desorption hysteresis indices of the processed materials also increased linearly with processing temperatures and O/C ratios. This is consistent with our observations of increased condensation and aromatization of NOM during superheated water processing presented in the first paper of this series. The relationships described provide direct experimental evidence that the sorption-desorption properties of NOM geosorbents are closely related to their degree of aromatization and condensation, and the work suggests strong potential for production of efficient and cost-effective engineered natural organic sorbents for environmental applications.
Introduction As noted in the first paper of this series (1), various forms of natural organic matter (NOM) associated with soils and sediments have long been known to serve as dominant environmental “compartments” for sorption and accumulation of hydrophobic organic chemicals. It has, over the past fifteen years, further become common knowledge that variations in the physicochemical forms of NOM relating to its geochemical origin and history markedly influence their respective contaminant sorption/desorption behaviors (28). At approximately the same time that the diversity of sorption behaviors manifested by different forms of NOM became evident, engineering application opportunities began * Corresponding author phone: (734)-763-2274; fax: (734)-9364391; e-mail:
[email protected]. 10.1021/es051665+ CCC: $33.50 Published on Web 01/31/2006
2006 American Chemical Society
to develop with respect to the use of inexpensive sorbent materials for hazardous waste treatment. Specifically, it was recognized that the uptake and retention of organic contaminants in various types of soil/sediment environments could be increased by admixing the associated soils and/or sediments with various forms of readily available and inexpensive materials containing more strongly sorptive forms of organic carbon. One of the earlier application instances of this concept employed fly ashes (i.e., waste ”black” carbon residues of incomplete combustion processes) to prevent seepage and diffusion of organic contaminants through slurry-wall soil/sediment containment barriers (9). This was soon followed by the use of more environmentally friendly and abundant naturally occurring intermediate products of diagenesis; e.g., the “kerogen” carbons commonly found in cold-water shales (10). The general background, foundation, and current setting for the work described here and in part one (1) were established by and are predicated on the studies and reports discussed above. In more relevant recent predecessor work involving studies of thermochemical treatments of geologically immature constituents of natural soils and sediments we observed the sorptive properties of ordinary peat materials for organic contaminants to be markedly enhanced by relatively mild superheated water treatment (8, 11). That work led directly to the investigations described in this series; i.e., a comprehensive evaluation of the feasibility of designing and implementing specifically processed and restructured natural organic matter for potential environmental applications. In this significantly expanded effort we have examined a broader range of raw materials and treatment conditions and conducted a larger number of detailed product evaluations. The results of the various treatments of different materials were described in the first paper of the series, and this second paper evaluates and describes the performance characteristics of the engineered natural organic sorbent materials generated by such processing.
Methods and Procedures Sorption and Desorption Measurements. The effects of superheated water treatment on the sorption/desorption properties of the test organic materials for phenanthrene (98% purity, Sigma-Aldrich Chemical Co, Inc., St. Louis, MO) were determined in two principal ways. The first was by measuring single-point equilibrium uptake of the compound by the test solids from essentially saturated aqueous background solutions, followed by equilibrated desorption from the solid phase into an initially phenanthrene-free background solution having otherwise the same composition as the background solution from which uptake occurred. The uptake and release experiments were conducted using preweighed 125-mL or 250-mL glass bottles with Teflonlined caps operated as completely mixed batch reactors (CMBRs). Aqueous background solutions contained 0.01 M CaCl2 as a mineral constituent, 200 mg/L of NaN3 to control biological activity, and 100 mg/L of NaHCO3 to buffer at approximately neutral pH. Phenanthrene dissolved in methanol was added to the background solution in amounts required to obtain an essentially saturated phenanthrene concentration of 800 µg/L. The amounts of methanol added were in all cases less than 0.1 % (v/v) of the aqueous solutions to which they were added. The test organic materials were added to the CMBRs at a constant sorbent/water ratio for each test material up to 1:5000 (g/mL). Because of different sorption capacities of test organic materials, these ratios were determined in preliminary tests. VOL. 40, NO. 5, 2006 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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The second principal manner in which sorption and desorption measurements were conducted was through multi-point isotherm experiments determined using a single decant-refill technique with 50-mL glass tubes operated as CMBRs. Initial concentrations of phenanthrene in the aqueous phase were varied to span approximately 2 orders of magnitude in order to measure phenanthrene sorption over as large a residual concentration range as possible. Mass additions of organic sorbent materials were designed to yield 20-80% reduction in the initial aqueous-phase phenanthrene concentrations in each sorbent-solution system. The bottle and/or tube CMBRs were horizontally shaken on a gyratory shaker at 100 rpm for 14-21 days to ensure complete mixing and equilibrium between the aqueous and solid phases. Preliminary tests indicated that a 10-14 day period was adequate to obtain apparent sorption equilibrium of phenanthrene for all test organic materials. All bottles or tubes were taken from the shaker and weighed, then allowed to stand for 24 h to settle solid particles from aqueous solution. A 1.5-mL aliquot of supernatant was then drawn by glass pipette from each bottle or tube and placed in an HPLC vial for chemical analysis. Aqueous phenanthrene solution was added to bottles or tubes without sorbent materials as controls to determine phenanthrene losses to glass walls and caps. The supernatant remaining in the bottles or tubes at the end of the sorption steps was gently decanted using a glass pipet, with 1-2 mL of supernatant left in the bottles or tubes, and the bottles or tubes were weighed again to obtain the volumes of the remaining aqueous solutions by mass balance, then refilled with a fresh clean background solution for subsequent desorption experiments of sorbed phenanthrene. The bottles or tubes were recapped, shaken again for a period of another 14 days to ensure equilibration, and sampled. The sampling and measurement of the aqueous phase was identical to the sorption steps described above. Chemical Analyses. Phenanthrene concentrations were analyzed using an Agilent model 1100 high-performance liquid chromatograph with a 125 × 3.20 mm Envirosep PP C18 column (Phenomenex, Torrance, CA). An ultraviolet detector (254 nm) was employed for concentrations between 50 and 1000 µg/L and a fluorescence detector was used for concentrations of 1.0 to 50 µg/L. Acetonitrile/water (80:20, v/v) was used as the mobile phase at a rate of 0.8 mL/min. Data Analyses. The data reported are averages for either duplicate or triplicate measurements for each processing condition. The amounts of phenanthrene associated with the sorbent materials at the conclusion of each sorption/ desorption experiment were determined by mass balance. For the single point uptake/release experiments the phenanthrene associated with the solid phase was calculated from the relationship
qe )
V (C - Ce) WfOC 0
(1)
where qe (µg/g OC) is the organic-carbon normalized phenanthrene equilibrium “concentration” associated with the sorbent solid phase, C0 and Ce (µg/L) are, respectively, the initial and equilibrium concentrations of phenanthrene remaining in the aqueous phase, W (g) is the amount of organic sorbent, fOC is the organic carbon fraction of the sorbent, and V is the solution volume. Single-point distribution coefficients (KOC) were calculated by dividing the organiccarbon normalized equilibrium concentration of phenanthrene in the solid phase (qe) by its concentration in the aqueous phase (Ce) using the relationship
KOC ) 1658
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qe V ) (C - Ce) Ce WfOCCe 0
(2)
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Note that KOC has the units of L/g OC and represents the number of liters of water that would contain the same amount of phenanthrene as would a gram of organic carbon at equilibrium. Interpretation of the sorption/desorption isotherms was facilitated by fitting all sets of multi-point sorption/desorption equilibrium data to the Freundlich model
qe ) KF Cne
(3)
where KF is the unit-capacity coefficient and n is a parameter related to both the relative magnitude and diversity of energies associated with a particular sorption process (12). The higher the KF, the higher the sorption capacity of a sorbent, while the lower the n value, the more nonlinear is an isotherm. Both KF and n were determined by linear regression of log-transformed data.
Results and Discussion Effects of Temperature. Michigan (MI) peat and soybean stalk samples treated for 5 hrs at 50% (v/w) moisture content at temperatures ranging from 100 to 300 °C under a nitrogen atmosphere were exposed to the essentially saturated phenanthrene aqueous solution for 21 days. As illustrated in part A of Figure 1, the percentages of the initial phenanthrene in the solution that were sorbed by MI peat samples increased markedly as a function of increased processing temperature, ranging from 58% for unprocessed (22 °C, i.e. room temperature) peat to 97% (values rounded to 1%) for that processed at 300 °C. Moreover, the percentages of sorbed phenanthrene that were released in the subsequent desorption step of the experiment decreased by factors of up to nearly 30, ranging from 42.7% to 1.45%. Part B of Figure 1 shows the same results expressed in terms of solid-phase equilibrium phenanthrene concentrations. It can be noted also that the sorption capacity of MI peat for phenanthrene reached its maximum at 250 °C. Parts C and D of Figure 1 show comparable data regarding the effects of processing temperature on soybean stalk sorption/desorption behavior, and similar results were observed for corn stalks, oak leaves, almond shells, ash leaves, maple leaves, peanut husk, pine bark, pine cones, pine needles, and wheat straw (Figures S-1 and S-2, Supporting Information). Effects of Processing Time. The effects of the duration of processing on the capacities of MI peat for sorption of phenanthrene were determined on samples processed at a temperature of 250 °C and 50% moisture content for 1-24 h. The results (Figure S-3, Supporting Information) revealed that the percentages of phenanthrene sorbed almost doubled from 45% for raw MI peat to 86% for peat samples processed for only 1 h. As processing time was increased from 3 to 5 h, these percentages increased marginally to 92% and 96%, after which longer processing times of 10 and 24 h resulted in essentially no further increases in sorption capacity. The subsequent desorption of the phenanthrene was also reduced significantly by superheated water processing. More than half (58%) of the phenanthrene sorbed by the raw peat samples was released during the desorption step of the experiment, whereas only 11%, 4%, and 3% of the phenanthrene sorbed on the processed peat samples treated for 1, 5, and 24 h, respectively, desorbed. Effects of Moisture Content. MI peat samples were treated at 250 °C for 5 h with moisture contents of 0, 25, 50, 75 and 100% water (v/w). Sorption of phenanthrene from the near saturated aqueous solution was observed to increase as water content during processing was increased from 0 to 50%, and then to decrease slightly as water content was further increased to 75% and 100%. More specifically, 72% of the initial solute was sorbed by peat processed without water,
FIGURE 1. Effect of treatment temperatures on the sorption and desorption of phenanthrene by MI peat and soybean stalks treated in superheated water (water/solids, 1:2) for 5 h. (The sorption percentages were calculated based on initial phenanthrene amount in the aqueous solution and the desorption percentages were calculated based on the sorbed phenanthrene amounts on the sorbents after sorption.) the sorption percentages increased to 85% and 92% for peat processed with 25% and 50% moisture, respectively, and then decreased to 88% and 85% for peat processed with 75% and 100% moisture, respectively (Figure S-3, Supporting Information). Desorption of the sorbed phenanthrene initially decreased as a function of increased moisture content during processing from 21% for the peat treated without water to 7% for that processed at 50% moisture level, and then increased modestly to 11% for the peat processed at 100% moisture.
Effects of Potential Mineral Catalysts. The effects of the presence of common minerals during superheated processing on the sorption capacities of processed MI peat were also examined. MI peat samples were treated for 5 h at 250 °C and 50% moisture level in the singular presence of each of seven different minerals, i.e., ZnCl2, KOH, Na4P2O7, Al2O3, Fe2O3, birnessitte, and bentonite, at 5% of the MI peat mass (w/w). Each mineral catalyst was thoroughly mixed with the raw organic material before superheated water treatment. As shown in Figure S-4 of the Supporting Information, all of the MI peat samples processed in the presence of the minerals exhibited much higher sorption capacities for phenanthrene than did raw MI peat, again based on both sorption percentages of the initially applied phenanthrene and phenanthrene concentrations in the solid phase, and no significant differences in phenanthrene sorption were observed among peat samples treated in the presence of different minerals compared with peat treated in the absence of minerals. The above data reveal that the degree of enhancement of sorption capacities of MI peat samples for phenanthrene were dependent on a number of treatment conditions. It is clear that treatment temperature is the most important factor in improving their physicochemical properties and sorption behaviors. The greatest enhancement in sorption was observed for the MI peat treated at 200-250 °C with 50% moisture content for 5 h in batch reactors under inert atmosphere. These treatment conditions were used for treating peat and other organic materials in further experiments. Sorption and Desorption Isotherms. The effects of temperatures ranging from 50 to 300 °C during processing at 50% moisture levels on the sorption capacities of MI peat were evaluated in greater detail by development of isotherms for sorption and desorption of phenanthrene. Equilibrium sorption and desorption data and best-fit Freundlich model traces are shown in Figure 2, and related Freundlich model parameters are summarized in Table S-1. As processing temperatures were increased the Freundlich unit-capacity coefficients (KF) increased dramatically, while the best-fit model n values decreased for both sorption and desorption. Increased values of KF indicate increased sorption capacities, while decreasing n values indicate increasing isotherm nonlinearity. The n values for sorption and desorption decreased from 0.97 for raw peat to 0.67 for peat treated at 300 °C. This is not unexpected because the peat samples became more condensed and aromatized as processing temperatures increased (1), a factor previously demonstrated as impacting the sorption behaviors of organic sorbents (8, 11). While the above results clearly show that unit sorption capacities as measured by the Freundlich KF parameter increased with increased processing temperature, it is not entirely appropriate to directly compare KF values derived from sorption/desorption isotherms having different n values (12). Thus, to facilitate direct comparisons, single-point organic carbon (OC)-normalized distribution coefficients, expressed as KOC,Ce to distinguish the fact that this parameter is determined by single-point sorption/desorption tests, were calculated from KF and n values derived from the Freundlich model using the relationship
KOC,Ce )
qe ) KF Cn-1 e Ce
(4)
KOC,Ce values were calculated for phenanthrene sorption and desorption by raw and processed MI peat samples at two specific aqueous phase equilibrium concentrations; i.e., Ce ) 10 and 100 µg/L. At Ce ) 10 µg/L, sorption KOC,Ce values increased by more than 8-fold, ranging from 41.5 L/g OC for raw peat to 341.9 L/g OC (values rounded to 0.1 L/g OC) for VOL. 40, NO. 5, 2006 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 2. Effect of treatment temperatures on phenanthrene sorption (A) and desorption (B) isotherms (µg/g OC) for MI peat treated in superheated water for 5 h. peat treated at 300 °C and 50% moisture level for 5 h, while desorption KOC,Ce values increased from 43.8 to 386.6 L/g OC (Table S-1, Supporting Information). Sorption and desorption KOC,Ce values at a Ce value of 100 µg/L increased similarly. Sorption and desorption isotherms were also determined for almond shells, peanut husks, pine bark, pine cones, and pine needles processed for 5 h at 250 °C and a 50% moisture level, and all data fitted with the Freudlich model with coefficient (R2) values greater than 0.978 (Figure S-5 and Table S-2, Supporting Information). Sorption and desorption isotherm KF values greatly increased for all processed materials. Sorption isotherm n values decreased, ranging from 0.83-0.95 for the untreated materials to 0.66-0.72 for the treated materials, and desorption isotherm n values decreased, ranging from 0.96-0.98 for the raw materials to 0.710.78 for the treated materials, indicating that the sorption and desorption isotherms of the test organic materials both became more nonlinear after processing with superheated water. The KOC,Ce values for all test materials also markedly increased after processing (Table S-2, Supporting Information). At Ce ) 10 µg/L sorption KOC,Ce values increased by factors of from 3.4 to 8.2, ranging from 18.7 to 40.3 L/g OC for the raw materials and from 88.7 to 278.9 L/g OC for the processed materials, and desorption KOC,Ce values at this Ce level increased similarly by factors of 3.3-8.2. Correlations between Sorption Capacities, Processing Temperature, and Product Properties. To better delineate relationships between the observed sorption-desorption behaviors of the raw and processed materials, processing 1660
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conditions, and changes in sorbent properties, sorption capacities (as represented by logKOC,Ce values), and isotherm linearity (as represented by Freundlich n values) are plotted versus processing temperature, O/C atomic ratios, and the aromaticity and hydrophobicity indices in Figure 3. Strong correlations between logKOC,Ce and n values and treatment temperatures are observed (part A), with correlation coefficients (R2) ranging from 0.977 to 0.992 for both sorption and desorption data. This is clear evidence that the sorption capacities of MI peat for phenanthrene log-linearly increase and that isotherm nonlinearity increases directly with superheated water processing temperature. As reported in the first paper in this series (1), increased temperatures of superheated water processing leads to increased carbon contents of natural plant materials and corresponding decreases in their oxygen contents during superheated water treatment. It is thus not surprising that strong correlations between LogKOC,Ce and n values and O/C atomic ratio values for MI peat are observed in part B of Figure 3, indicating that sorption capacities and isotherm nonlinearity both increase markedly as O/C atomic ratios decrease. Strong correlations were also observed between log KOC values, treatment temperatures, and O/C atomic ratios of soybean stalks and oak leaves (Figure S-6, Supporting Information). Relationships between the sorption affinities of different NOMs for hydrophobic organic contaminants and their respective chemically condensed and aromatized compositions and structures have been examined extensively (6-8, 13-16). For example, strong positive correlations have been found between pyrene binding coefficients and the aromatic content of aquatic organic matter (17). Xing (18) also found a positive correlation between the aromaticity of various humic acid samples, their isotherm nonlinearity for sorption of naphthalene and phenanthrene, and the condensed domains comprising their aromatic moieties. Two recent studies have provided direct NMR spectroscopic evidence for the existence of condensed sorption domains in NOM, and that aromatic fractions likely comprise these domains (7, 19). In the first part of this two-part series we reported that the aromaticity and hydrophobicity of organic carbon associated with MI peat was found to increase as superheated water processing temperatures increased (1). As illustrated in parts C and D of Figure 3, the sorption capacities (e.g., logKOC,Ce values at Ce ) 10 µg/L) for MI peat increased andthe nonlinearity of its sorption and desorption isotherms increased linearly, as its aromaticity and hydrophobicity were increased by superheated water processing. These relationships provide direct experimental evidence that the sorptiondesorption properties of NOM geosorbents are related to their degrees of aromatization and condensation. That the Freundlich model KF values for the desorption isotherms for the processed MI peat samples were somewhat greater than those for its sorption isotherms (Table S-1, Supporting Information) suggests that there may be hysteresis-like effects associated with the sorption and desorption of phenanthrene by these materials. To pursue this issue and quantify its effect, the apparent sorption-desorption hysteresis index (HI) defined by Huang et al. (16) was invoked. This index has the parametric form
HI )
qde - qse |T,Ce qse
(5)
where qse and qde are solid-phase phenanthrene concentrations for the sorption and desorption phases of the process, respectively, and the subscripts T and Ce are sorption/ desorption temperatures (22 °C in this study) and aqueous concentrations of phenanthrene at equilibrium, respectively. Hysteresis indices at two apparent equilibrium concentra-
FIGURE 3. Relationships between isotherm nonlinearity and single-point distribution coefficients (Ce ) 10 µg/L) of phenanthrene sorption and desorption and treatment temperature (A), O/C atomic ratios (B), and the aromaticity (C) and hydrophobicity (D) indices of MI peat treated anaerobically in superheated water for 5 h. tions (i.e., Ce ) 10 and 100 µg/L) were calculated for MI peat treated in superheated water at different temperatures. As shown in Figure 4, the hysteresis index of MI peat increased linearly with increasing superheated water processing temperature and decreasing O/C atomic ratios. These data indicate that the more condensed NOMs in the MI peat samples processed at higher temperatures exhibited greater apparent hysteresis in the sorption-desorption of phenanthrene. We previously postulated that entrapment or hindered diffusion within the layers of condensed aromatic carbon matrixes of geologically mature kerogens is one possible mechanism responsible for desorption hysteresis in natural geosorbents (16). In another study we pointed out (8) that the aromatic fractions of organic matter in superheated water processed NOMs may not have been as densely packed as those of typical natural kerogens because they were formed by rapid dehydration rather than by condensation over geological time (8). The 13C NMR and FTIR data presented in the first paper of this two-part series showed that the raw organic materials examined in this study were composed principally of alkyl carbons and oxygenated aliphatic carbons. These forms of carbon are highly oxidized, of low bulk density, and likely exist principally in a highly amorphous or “rubbery” state (13, 20). Such “soft” (3) carbons have relatively large diameter mesopores and micropores spaces, allowing these highly
oxidized local regions to become extensively hydrated in aqueous solutions. These highly amorphous and swollen sorption domains thus act as relatively readily accessible liquid-like partitioning domains for large hydrophobic substances such as phenanthrene. When these aliphatic carbons are reduced and converted to more aromatic forms by slow diagenesis or rapid superheated water processing these domains are transformed into “glassy” states. Local regions in such “hard” (3) carbon domains are less likely hydrated or swollen in aqueous solutions. Their sorption reactions thus favor solute-sorbent interactions dominated by van der Waals forces rather than interactions with more polar water molecules. Relatively large mesopore and micropore channels of access became smaller and extremely small micropores having relatively well-defined geometries are generated by diagenesis or superheated water processing. Sorption of large hydrophobic solute molecules in such rigid pores likely follows for the most part a capacity-limited Langmuir-type adsorption or “hole filling” process” (21, 22). This special process, coupled with the heterogeneity of nonspecific sorption-site energies resulted from different pore sizes and pore surfaces, would be expected to generate nonlinear sorption and desorption isotherms, as we observe in Figures 2 and 3. The results obtained in this study provide direct evidence that the sorption capacities and isotherm nonlinearities of VOL. 40, NO. 5, 2006 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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Supporting Information Available Phenanthrene sorption and desorption behaviors by the test organic materials before and after treatment at different conditions and their isotherm Freundlich parameters. This material is available free of charge via the Internet at http:// pubs.acs.org.
Literature Cited
FIGURE 4. Relationships between hysteresis index (HI) (calculated at Ce ) 10 µg/L and Ce ) 100 µg/L) and treatment temperatures (A with correlation coefficient (R2) are 874 and 0.903) and O/C atomic ratios (B with correlation coefficient (R2) are 0.921 and 0.957) of MI peat treated in superheated water for 5 h at different temperatures.
different types of NOMs are functions of the degree of condensation and aromatization that they undergo, either during natural diagenesis over geological time or via rapid and relatively mild superheated water processing. The study suggests that it is possible to produce smart natural organic sorbents by modifying and restructuring readily available and low-cost natural plant materials. When such specially designed and engineered sorbents are intermixed with natural soils and sediments, they are expected not only to be environmentally benign, but also in fact to improve the structure, quality, and environmental behaviors of those soils and sediments via gradual incorporation therein through natural humification processes.
Acknowledgments We thank former graduate student Sung Ho Kim and a number of part-time undergraduate students for their diligent laboratory assistance during the experimental phases of the work. We also appreciate the excellent support of Tom Yavaraski with analytical instrumentation. The research was supported in part by the Environmental Management Science Program of the United States Department of Energy (DOE) through Grant DE-FG07-02ER63488, in part by Research Grant P42ES04911-14 from the National Institutes for Environmental and Health Sciences, and in part by Research Grant U009439 from the Office of the Vice President for Research at The University of Michigan through the Michigan Sea Grant Program. The conclusions set forth in the paper do not necessarily reflect the views of any of these funding agencies. 1662
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(20) Cuypers, C.; Grotenhuis, T.; Nierop, K. G. J.; Franco, E. M.; de Jager, A.; Rulkens, W. Aromphous and condensed organic matter domains: the effect of persulfate oxidattion on the composition of soil/sediment organic matter. Chemosphere 2002, 48, 919931.
(22) Xing, B.; Pingatello, J. J. Time-dependent isotherm shape of organic compounds in soil organic matter. Implications for sorption mechanism. Environ. Toxicol. Chem. 1996, 15, 12821288.
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Received for review August 22, 2005. Revised manuscript received December 22, 2005. Accepted December 23, 2005. ES051665+
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