Development of Engineered Natural Organic Sorbents for

A Chemagnetics CMX solids NMR spectrometer using a cross polarization (CP) technique with magic-angle spinning (MAS) and high-power decoupling was ...
0 downloads 0 Views 281KB Size
Download PDF
Environ. Sci. Technol. 2006, 40, 1650-1656

Development of Engineered Natural Organic Sorbents for Environmental Applications. 1. Materials, Approaches, and Characterizations

carbon materials originating in the form of simple biopolymers deposited in essentially anoxic solid/liquid environments. It then proceeds through an extended sequence of condensation reactions that convert these highly amorphous substances into increasingly condensed forms such as humus, kerogen, and eventually coal-like solids; i.e., a slow transformation of what are commonly referred to as “soft” forms of carbon to more condensed or “hard” forms (6).

WALTER J. WEBER, JR.,* JIXIN TANG, AND QINGGUO HUANG

The progression of carbon along this pathway is motivated in nature by inputs of both mechanical and thermal energy. When measured and controlled, such energy inputs slowly effect a series of deoxygenation, aromatization, and condensation reactions. When uncontrolled and in the presence of gases containing modest amounts of oxygen (e.g., such events as forest fires, volcanic eruptions, power generation, and incineration), partially complete combustion processes convert both soft and hard carbonaceous materials into charand soot-like byproducts; i.e., so-called “black” carbons. The latter forms of environmentally produced carbon materials have recently been “rediscovered” with respect to the multiple roles that they can and often do play in contaminant fate and transport in environmental systems, as evidenced, for example, by the several-day symposium on black carbon held at the 2005 Fall Meeting of the American Chemical Society in Washington, DC. Although of apparent renewed interest, the sorptive properties of “black carbons” have in reality been recognized for centuries, and appropriately controlled partial combustion processes have for more than a half-century been used commercially to manufacture “activated” forms of black carbon from numerous raw organic carbon materials tailored to address a wide range of sorptive separation applications (8).

Department of Chemical Engineering, Energy and Environment Program, 4103 Engineering Research Building, The University of Michigan, Ann Arbor, Michigan 48109-2099

A technology for effecting diagenesis-like transformations in young natural organic matter (NOM) derived from common plant materials is described. Thirteen such materials were processed in liquid-phase water at superheated temperature/pressure conditions. In all cases significant changes in the physical and chemical properties of the raw NOM source materials occurred. Their carbon and nitrogen contents increased, their hydrogen and oxygen contents decreased, their surface areas initially increased dramatically, then sharply decreased, and their bulk densities and equilibrated aqueous phase pH values increased as functions of increasing temperature and pressure. Spectroscopic analyses confirmed marked changes in the molecular structures of the NOMs resulting from superheated water processing; e.g., fractions of alkyl and aromatic carbons increased while oxygen-associated functional organic carbon decreased. These changes in elemental composition and molecular structure indicate that the organic fractions of the raw materials became less polar, increasingly condensed, and more aromatic as a result of the reactions induced by superheated water processing, the changes correlating directly with the temperature/ pressure conditions employed. The results presented confirm that the long-term biogeochemical processes that in nature effect geologically slow advances in the chemical states of organic carbon can be simulated and markedly accelerated by superheated water processing.

Introduction That various forms of natural organic matter (NOM) associated with soils and sediments serve as dominant environmental “compartments” for sorption and accumulation of hydrophobic organic chemicals (HOCs) has long been recognized. It was determined more than a decade ago that the sorption/desorption behaviors of different forms of NOM vary markedly with respect to typical organic environmental contaminants, and it became evident soon thereafter that those behaviors correlate generally to the degree of natural diagenesis that a particular form of NOM has undergone (1-7). Diagenesis of organic matter in nature results in geologically slow advances in the oxidation state of its associated organic carbon. The process begins with highly amorphous * Corresponding author phone: (734)763-2274; fax: (734)936-4391; e-mail: [email protected]. 1650

9

ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 40, NO. 5, 2006

Various thermal treatment techniques have been employed to simulate diagenesis and to determine associated structural transformation mechanisms and rate processes (9, 10). A number of diagenetic-type reactions (e.g., deoxygenation, aromatization, and condensation) have recently been observed to occur rapidly for a variety of specific organic compounds in superheated water (11, 12). We have earlier demonstrated in fact that superheated water treatment can simulate and markedly accelerate certain diagenesis-like reactions in geologically young peat materials (5, 13). The foregoing considerations led us to this more extensive exploration of the feasibility of designing specifically restructured natural organic matter for sorption and sequestration of organic contaminants. Potential environmental applications envisioned for such tailored sorbents include remediation of contaminated soils and sediments, water purification, and waste treatment. The advantages of such materials might in each case include lower costs than such adsorbents as activated carbon and synthetic polymers, greater effectiveness than randomly generated black carbon residues such as chars or fly ashes, and greater environmental compatibility than either of these other classes of sorbent materials. Greater environmental compatibility would accrue to the fact that the materials produced are expected to continue to participate in ongoing humification and diagenesis processes when introduced to the environment. They may in particular therefore have unique potential for the irreversible incorporation and identity/behavior transformation of persistent toxic chemicals via such ongoing activity (e.g., ref 14). We examined a broad range of different types of raw materials and conditions and conducted a large number of detailed product evaluations. Examples of types of transformations and characterizations of the changes 10.1021/es051664h CCC: $33.50

 2006 American Chemical Society Published on Web 01/31/2006

observed in converting several natural organic materials into “engineered” natural organic sorbent materials are presented in this paper. The second paper of the series (15) evaluates and describes the performance characteristics of the engineered natural organic sorbents produced.

Experimental Section Organic Raw Materials. Thirteen natural organic materials comprising various forms of cellulose, hemicellulose, lignin, and humics were employed as raw materials for this study. All are readily available, inexpensive, and eventually incorporated into environmental systems by natural processes. Two young soil precursors, Canadian (CA) peat and Michigan (MI) peat, were selected as representatives of humiccontaining natural organic materials. CA peat is a longfibered, light-blond-type sphagnum moss peat produced in Baie-Comeau, Quebec, Canada, while MI peat is a weathered sphagnum moss peat collected in northern Michigan. Both peat materials are in the very early stages of diagenesis. A second group of organic materials deriving from woody plants, including almond husks, green ash leaves, oak leaves, pine cones, pine needles, pine bark, and red maple leaves, was also studied. The almond husks were purchased from a local market, and the remaining materials in this group collected on the North Campus of The University of Michigan in Ann Arbor. A third group of raw agricultural byproduct materials studied consisted of corn and soybean stalks, peanut husks, and wheat straws. Soybean stalks were obtained from the Michigan State University Cooperative Extension County Office in Ann Arbor, and the remaining materials in the group were obtained from local farm markets. The 13 raw materials were air-dried, ground using a conventional grinding mill, and stored in glass bottles until used in various phases of the experimental work. Superheated Water Processing. The term superheated water refers here to water having temperatures ranging from 100 to 300 °C and maintained in a liquid state under pressures equal to or greater than corresponding vapor pressures. At the outset of each treatment, an organic raw material was placed (with minimum headspace) in a bench scale LC series reactor (Pressure Products Industries, Inc., Warminster, PA), and N2-purged Milli-Q water was added to the reactor in an amount required to provide a predetermined moisture content. The reactor was then purged with nitrogen gas for at least 30 min to eliminate residual oxygen, after which the gas inlet and outlet valves were closed and heating of the reactor to a preset desired temperature was initiated. The organic materials were processed at various temperature and pressure conditions. Upon completion of a production run the processed materials were removed from the reactor, transferred to a fume hood, dried, ground, and passed through a 1-mm sieve. The bulk densities of the solid materials and the pH values of aqueous phases equilibrated with each were determined, and the remaining sample materials were then stored in glass bottles at room temperature for use in subsequent experiments. Elemental Compositions. A calibrated Leco CHN-1000 Analyzer (Leco Corporation, St. Joseph, MI) employing a hightemperature combustion technique was used to perform elemental analyses of carbon, hydrogen, and nitrogen on the organic materials before and after superheated water treatment. Oxygen contents were determined by mass balance and ash content (10), the latter of which was determined by high-temperature combustion using an OX500 biological material oxidizer (R. J. Harvey Instrument Co., Hillsdale, NJ). Each sample was measured in duplicate. Surface Areas and Morphologies. Specific surface areas (SSAs) were measured prior to and following processing by a N2 Brunauer-Emmett-Teller (BET) gas adsorption/ desorption method at liquid-N2 temperature (i.e., 77 K) using

a Micromeritics accelerated surface area and Hg porosimeter (ASAP-2010). Surface images of the MI peat samples were obtained using a Hitachi S3200N scanning electron microscope (SEM). A conventional high-resolution thermionic SEM with pressure control was placed into the chamber in which a specimen can be inserted directly and observed in its natural state. Organic samples were first mounted on a metal SEM stub using adhesive carbon paper and then coated with gold particles. Images of the organic samples were scanned in secondary electron mode under high-vacuum conditions. Fourier Transform Infrared Analysis. Fourier transform infrared (FTIR) analyses were performed using a Nexus 870 FTIR spectrometer (Thermo Nicolet, Madison, WI) equipped with dual channel collection and a KBr beam splitter. Treated and untreated solid organic samples were finely ground with potassium bromide salt (KBr, FTIR grade) with a mortar and pestle and dried in a freeze-dryer for 24 h before analysis. Spectra were acquired in the 4000-400 cm-1 wavenumber range with varying numbers of scans and resolutions. The background spectrum of finely ground KBr was also recorded at the same instrument setting. Peak area integration and spectral subtractions were obtained using OMNIC software. All spectra were normalized with respect to the interval given by minimum and maximum absorption. Solid-State 13C NMR Analysis. Solid-state 13C NMR spectra of the treated and untreated organic materials were provided by Dr. Francis Miknis, Western Research Institute, Laramie, WY. A Chemagnetics CMX solids NMR spectrometer using a cross polarization (CP) technique with magic-angle spinning (MAS) and high-power decoupling was employed, and the spectra recorded at a 13C frequency of 25 MHz, using a 90E pulse width of 5 µm, a contact time of 1 ms, a pulse delay of 1 s, and a sweep width of 16 kHz. These measurements were made using a 7.5-mm zirconium PENCILTM rotor at a spinning rate of 4.5 kHz, and 10 800 transients were recorded. A 50-Hz exponential multiplier was applied to the free induction decay of each 13C spectrum before integration.

Results and Discussion Changes in the physical and chemical properties of the natural organic materials tested prior to and following processing under different conditions are summarized below in terms of elemental compositions, surface areas and morphologies, and functional groups and molecular structures. Elemental Compositions. The elemental carbon (C), hydrogen (H), nitrogen (N), and oxygen (O) initial masspercent compositions of the raw organic materials varied somewhat (Table S-1, Support Information). C and N contents increased by factors ranging, respectively, from 1.23 to 1.46 and from 1.41 to 2.01. Conversely, H and O contents decreased by as much as 21.1% and 57.2%, respectively. To gain better insight to specific relationships between treatment temperatures and changes in elemental composition, the effects of processing the MI peat, ash leaves, maple leaves, oak leaves, corn stalks, and soybean stalks were determined over greater ranges of temperature. The data shown by way of example for MI peat in Figure 1 reveal that C and N contents increased linearly and H and O contents decreased linearly with increasing treatment temperature, although H contents decreased only slightly. Similar results were obtained for the other raw NOM sources cited above (Figure S-1, Support Information). Correlation coefficient (R2) values for the linear traces shown in Figures 1 and S-1 range from 0.849 to 0.996. As processing temperatures increased, C and O contents changed much more sharply than did H and N contents. Increasing C and decreasing H and O mass-percent compositions can be attributed to deoxygenation and dehydrogenation reactions, with oxygen-associated functional groups VOL. 40, NO. 5, 2006 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

9

1651

FIGURE 1. Example for MI peat of the effects of processing temperature on elemental composition. (Data are the average of two measurements with standard deviation less than 10% for all data.)

FIGURE 2. Relationships between oxygen/carbon and hydrogen/ carbon atomic ratios at different temperatures. such as carbonyl, carboxyl, and methoxyl groups being reduced and oxygenated aliphatic group carbons becoming more aromatic and condensed (16, 17). Indices of atomic ratios are useful for the identification of humic substances from different sources, the monitoring and elucidation of structural changes that occur as a result of specific geochemical reactions, and the degrees of humification, maturation, and oxidation of NOMs in specific environmental systems and situations (18, 19). As illustrated in Figure 2, a strong linear relationship between the H/C and the O/C atomic ratios was observed for various degrees of processing of ash leaves, corn stalks, maple leaves, MI peat, oak leaves, and soybean stalks, with correlation coefficients ranging from 0.989 to 0.999. Not surprisingly, these atomic ratios changed similarly with increased temperature for most of the raw organic materials tested, with correlation coefficients ranging from 0.806 to 0.998. The single exception was the N/H atomic ratio for MI peat, an exception that can be attributed to the relatively high initial nitrogen content of this raw source material. With increasing burial depth, time, and/or increasing temperature in natural systems, the removal of chemically bound oxygen to increase hydrocarbon content is one of the primary steps during production of oil and coal (20). Ibarra and Juan (21) reported decreased H/C and O/C atomic ratios in humic acids isolated from brown coals of increasing ranks. Lu et al. (22) reported that relationships between H/C and 1652

9

ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 40, NO. 5, 2006

O/C ratios are closely associated with the maturity of humic substances isolated from different depositional environments. Systematic characterizations of NOMs isolated from a number of soils and sediments have shown that kerogen has relatively low O/C and H/C atomic ratios and low contents of oxygen-containing functional groups (23) and that the O/C atomic ratios of various NOMs decrease with increasing age and diagenetic alteration (24). In light of these observations, the elemental analyses in the work reported here that demonstrate organic carbons become more condensed and aromatic after superheated water treatment strongly support the concept that such treatment closely mimics the evolution of NOM through natural diagenesis. Surface Areas and Morphologies. The BET surface areas of MI peat can be observed in part A of Figure 3 to have remained essentially unchanged by treatment at 50 and 100 °C for 2 h but then to have increased sharply by an order of magnitude when treatment temperature was increased to 150 °C. Surface areas subsequently decreased with further increases in treatment temperatures from 150 to 300 °C, although still remaining 4-7 times greater than that of the untreated sample. Similar results are shown in Figure 3 for soybean stalks (part B), CA peat, and pine needles (part C). These findings suggest that changes in the surface areas of natural organic materials of the types tested can be initiated only under conditions of superheating (>100 °C) of liquid water, a condition in which the chemistry (e.g., dielectric constant, ionic dissociation constant, and density) of this medium changes dramatically (25-27). While the available data do not allow rigorous interpretation of this significant surface area behavior at this juncture, the observation marks an interesting and important related area for further study. Scanning electronic microscopy (SEM) images revealed that the surface morphology of MI peat also was changed significantly with increased temperature of processing (Figure S-2, Support Information). While unprocessed MI peat surfaces were observed to be highly furrowed, processing at increased temperatures appears to render surface morphology smoother and more “condensed” in appearance. FTIR Analyses. Typical infrared radiation frequency ranges in which specific functional groups and structures observed for organic matter are listed in Table S-2 of the Support Information. The FTIR spectra shown by way of example for Michigan peat in Figure 4 reveal the presence of aliphatic, aromatic, carboxylic, and carbohydrate forms of organic carbon. Distinct changes are observed after processing of this material at different superheated water temperatures. While aliphatic C did not change significantly as the temperature was increased from 50 to 100 °C, it increased markedly in the range from 150 to 300 °C and formed two distinct and sharp peaks at wavenumbers of 2927 and 2850 cm-1. Aromatic C with CdC stretching at 1630 and 1242 cm-1 greatly increased as processing temperature was increased from 50 to 300 °C. As noted in Figure 4, there was initially an increase in the bonded OH group for carboxylic C at a temperature of 50 °C, followed by a substantial decrease as temperatures increased from 100 to 300 °C. The carboxylic C, aliphatic C, aromatic C, and carbohydrate C structures observed in raw corn and soybean stalks were markedly changed by superheated water processing at 100, 150, and 200 °C (Figure S-3, Supporting Information). For corn stalks, hydroxyl groups associated with OsH stretching at 3440 cm-1 slightly increased at 100 °C, then decreased from 150 and 200 °C. Aromatic C at 1604 cm-1, carboxylic C at 1704 cm-1, and aliphatic C at wavenumbers of 2931, 1340, 1191, and 1120 cm-1 increased as the temperature increased from 100 to 200 °C. Similar changes in carbon structures and functional groups for soybean stalks occurred when temperatures were increased from 100 to 200 °C. Significant changes as a result of processing were

FIGURE 3. Effects of thermal processing on specific surface areas of MI peat, soybean stalks, CA peat, and pine needles. (Data points represent means of two measurements.)

FIGURE 4. FTIR spectra for MI peat processed at different superheated water temperatures. also observed in the FTIR spectra of other natural organic materials tested (i.e., almond husks, pine bark, pine cones, pine needles, ash leaves, oak leaves, peanut husks, and wheat straw; Figures S-4 and S-5 in the Supporting Information). Solid-State 13C NMR Analyses. Changes in the molecular structures and functional groups of MI peat samples were further characterized using solid-state 13C NMR analysis before and after superheated water processing. Integrations were performed over the 0-210 ppm range of the chemical shift in NMR spectral regions. The NMR results are consistent with the FTIR results discussed above, confirming changes

in alkyl, methoxyl, O-alkyl, aromatic, phenolic, and carboxyl carbons as functions of processing temperatures. As shown in Figure 5 and in Table S-3 of the Supporting Information, alkyl C and aromatic C were increased significantly. Conversely, oxygenated aliphatic C, carbohydrate C, and carboxyl C gradually decreased as processing temperatures were increased from 50 to 250 °C. At 250 °C the functional carbon forms are dominated by aliphatic and aromatic carbons, and methoxyl, O-alkyl, carbohydrate, and carboxyl carbons are significantly reduced. The relative percentages of aliphatic and aromatic carbons increased respectively from 23.1% and VOL. 40, NO. 5, 2006 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

9

1653

FIGURE 5. Progression of solid-state 13C NMR spectra for MI peat with increasing processing temperature. 37.2% for the unprocessed peat to 36.6% and 56.7% for the peat processed at 250 °C. Percentages of carbohydrate, carboxyl, and carbonyl carbons decreased respectively from 29.1%, 6.9%, and 2.9% to 2.72%, 2.8%, and 1.2%. When the five functional forms of organic carbon for untreated peat and peat treated at 250 °C are compared, the total percentage increase in aliphatic and aromatic carbon is approximately equal to the total percentage decrease of oxygenated aliphatic, carboxyl, and carbonyl carbon, indicating that increases in aromatic and aliphatic carbons came at the expense of oxygen-containing carbons. The aromaticity index (IAr) and hydrophobicity index (IH) are often used, respectively, to quantify degrees of aromaticity and hydrophobicity of organic carbon in natural organic matter (28-31). These indices were calculated for MI peat from the 13C NMR spectra. IAr is determined by the ratio of the sum of the spectral peak areas for aromatic (100-140 ppm) and phenolic (140-165 ppm) carbons to the sum of the spectral peak areas for the aliphatic (0-48 ppm) and carbohydrate (50-100 ppm) carbons, while IH is determined by the ratio of peak areas of aliphatic and aromatic carbons to the peak areas of the carbohydrate C and phenolic and 1654

9

ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 40, NO. 5, 2006

carboxylic (160-190 ppm) carbons (31, 32)

∑(100-165 ppm) ∑(0-100 ppm)

(1)

∑[(0-48 ppm) + (100-140 ppm)] ∑[(50-100 ppm) + (140-190 ppm)]

(2)

IAr )

IH )

The results show that IAr values gradually increase as processing temperatures increase. For the MI peat they increase by a factor of 1.8, from 0.65 for the unprocessed control to 1.18 for the peat processed at 250 °C. The corresponding IH values increase even more significantly, i.e., by a factor of 5.0, from 0.81 for the control to 4.15 for peat processed at 250 °C. The plots of IAr and IH values as functions of treatment temperatures given in parts A and B of Figure 6 show strong linear correlations between both indices and processing temperature, with correlation coefficients of 0.976 and 0.938, respectively. Not surprisingly, strong relationships are also observed in parts C and D between the aromaticity

FIGURE 6. Relationships of hydrophobicity (IAr) and aromaticity (IH) indices for MI peat to processing temperatures and elemental atomic ratios. and the hydrophobicity indices and the corresponding elemental atomic ratios of O/C and H/O. As O/C ratios increase, both IAr and IH values decrease, and as H/O values increase both IAr and IH values increase. The structural and compositional changes upon superheated water processing observed in this work accord with those that occur in the natural humification and diagenesis of organic matter, and confirm the feasibility of simulating and markedly accelerating diagenetic transformations of NOM via superheated water processing. This is readily evident, for example, by comparison of the progressive changes in the 13CNMR spectra shown in Figure 5 and Table S-3 with differences in the 13C NMR spectra reported by Huang and Weber (33) for several peat materials and kerogens extracted from three different Anthrim shales of the late Devonian period. Consistent with the comparative sorption behaviors of peats and kerogens also reported by Huang and Weber (33, 23) and by Johnson et al. (5, 34), the second paper in this series (15) demonstrates that the accelerated transformation reactions described herein produce materials of increasingly greater capabilities with respect to the sorption and sequestration of organic contaminants.

Acknowledgments The authors thank former graduate student Sung Ho Kim and a number of undergraduate students at the University of Michigan for the diligent laboratory assistance that they provided during the experimental phases of the work. We also appreciate the expert support of Tom Yavaraski with analytical instrumentation. The research was supported in major part by the Environmental Management Science Program of the United States Department of Energy through Grant DE-FG07-02ER63488 and in part during its exploratory stages by Research Grant P42ES04911-14 from the National Institutes for Environmental and Health Sciences, and Research Grant U009439 from the Office of the Vice President for Research at The University of Michigan through the Michigan Sea Program. The conclusions set forth in the paper do not necessarily reflect the views of any of these funding sources.

Supporting Information Available Elemental compositions of source organic materials before and after superheated water processing, relative percentages of NMR signals for specific organic carbon functional groups, calculated hydrophobicity and aromaticity indices for MI peat, SEM images for MI peat processed at different temperatures in superheated water, and FTIR spectra for other test organic materials before and after processing. This material is available free of charge via the Internet at http:// pubs.acs.org.

Literature Cited (1) Grathwohl, P. Influence of organic matter from soils and sediments from various origins on the sorption of some chlorinated aliphatic hydrocarbons: Implications on KOC correlations. Environ. Sci. Technol. 1990, 24, 1687-1693. (2) Huang, W. L.; Young, T. M.; Schlautman, M. A.; Yu, H.; Weber, W. J., Jr. A distributed reactivity model for sorption by soils and sediments. 9. General isotherm nonlinearity and applicability of the dual reactive domain model. Environ. Sci. Technol. 1997, 31, 1703-1710. (3) Young, T. M.; Weber, W. J., Jr. A distributed reactivity model for sorption by soils and sediments. 3. Effects of diagenetic processes on sorption energetics. Environ. Sci. Technol. 1995, 27, 92-97. (4) Xing, B. S.; Chen, Z. Q. Spectroscopic evidence for condensed domains in soil organic matter. Soil Sci. 1999, 164, 40-47. (5) Johnson, M. D.; Huang, W. H.; Weber, W. J., Jr. A distributed reactivity model for sorption by soils and sediments. 13. Simulated diagenesis of natural sediment organic matter and its impact on sorption/desorption equilibria. Environ. Sci. Technol. 2001, 35, 1680-1687. (6) Weber, W. J., Jr.; McGinley, P. M.; Katz, L. E. A distributed reactivity model for sorption by soils and sediments. 1. Conceptual basis and equilibrium assessments. Environ. Sci. Technol. 1992, 26, 1955-1962. (7) McGinley, P. M.; Katz, L. E.; Weber, W. J., Jr. A distributed reactivity model for sorption by soils and sediments. 2. Multicomponent systems and competitive effects. Environ. Sci. Technol. 1993, 27, 1524-1531. (8) Weber, W. J., Jr. Evolution of a technology. J. Environ. Eng. 1984, 110, 899-917. (9) Buchanan, A.; Britt, P.; Struss, J. Investigation of reaction pathways involved in lignin maturation. Energy Fuels 1997, 11, 247-248. VOL. 40, NO. 5, 2006 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

9

1655

(10) Lu, X.; Hanna, J.; Johnson, W. Evidence of chemical pathways of humification: A study of aquatic humic substances heated at various temperatures. Chem. Geol. 2001, 177, 249-264. (11) Akiya, N.; Savage, P. E. Roles of water for chemical reactions in high-temperature water. Chem Rev. 2002, 102, 2725-2750. (12) Siskin, M.; Katritzky, A. R. Reactivity of organic compounds in superheated water: General background. Chem. Rev. 2001, 101, 825-835. (13) Johnson, M. D.; Huang, W. L.; Dang, Z.; Weber, W. J., Jr. A distributed reactivity model for sorption by soils and sediments. 12. Effects of subcritical water extraction and alterations of soil organic matter on sorption equilibria. Environ. Sci. Technol. 1999, 33, 1657-1663. (14) Huang, Q.; Weber, W. J., Jr. Inclusion of persistent organic pollutants in humification processes: Direct chemcial incorporation of phenanthrene via oxidative coupling. Envion. Sci. Technol. 2003, 37, 4221-4227. (15) Tang, J.; Weber, W. J., Jr. Development of engineered natural organic sorbents for environmental applications: 2. Sorption characteristics and capacities with respect to phenanthrene. Environ. Sci. Technol., 2006, 5, 1657-1663. (16) Ioselis, P.; Rubinsztain, Y.; Ikan, R.; Aizenshtat, Z.; Frenkel, M. Thermal characterization of natural and synthetic humic substances. Org. Geochem. 1985, 8, 95-101. (17) Lu, X.; Vassallo, A.; Johnson, W. Thermal stability of humic substances and their metal forms: An investigation using FTIR emission spectroscopy. J. Anal. Appl. Pyrolysis 1997, 43, 103113. (18) Steelink, C. Implications of elemental characteristics of humic substances. In Humic Substances in Soil, Sediment, and Water; Aiken, G. R., McKnight, D. M., Wershow, R. L., et al., Eds.; Wiley: New York, 1985; pp 457-476. (19) Belzile, N.; Joly, H. A.; Li, H. B. Characterization of humic substances extracted from Canadian lake sediments. Can. J. Chem. 1997, 75, 14-27. (20) Bennett, B.; Abbott, G. D. A natural pyrolysis experiments Hopanes from hopanoic acids? Org. Geochem. 1999, 30, 15091516. (21) Ibarra, J. V.; Juan, R. Structural-changes in humic acids during the coalification process. Fuel 1985, 64, 650-656. (22) Lu, X. Q.; Hanna, J. V.; Johnson, W. D. Source indicators of humic substances: An elemental composition, solid-state C-13 CP/MAS NMR and Py-GC/MS study. Appl. Geochem. 2000, 15, 1019-1033. (23) Song, J. Z.; Peng, P. A.; Huang, W. H. Black carbon and kerogen in soils and sediments. 1. Quantification and charaterization. Environ. Sci. Technol. 2002, 36, 3960-3967.

1656

9

ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 40, NO. 5, 2006

(24) Huang, W. H.; Weber, W. J., Jr. A distributed reactivity model for sorption by soils and sediments. 10. Relationships between desorption, hysteresis, and chemical characteristics of organic domains. Environ. Sci. Technol. 1997, 31, 2562-2569. (25) CRC Handbook of Chemistry and Physics, student ed.; CRC Press: Boca Raton, FL, 1988; Vol. 1. (26) Uematsu, M.; Franck, E. U. Static dielectric-constant of water and steam. J. Phys. Chem. Ref. Data 1980, 9, 1291-1306. (27) Marshall, W. L.; Franck, E. U. Ion product of water substance, 0 °C-1000 °C, 1-10,000 barssNew international formulation and its background. J. Phys. Chem. Ref. Data 1981, 10, 295-304. (28) Capriel, P. Hydrophobicity of organic matter in arable soils: Influence of management. 1997, 48, 457-462. (29) Kurkova, M.; Klika, Z.; Klikova, C.; Havel, J. Humic acids from oxidized coals I. Elemental composition, titration curves, heavy metals in HA samples, nuclear magnetic resonance spectra of HAs and infrared spectroscopy. Chemosphere 2004, 54, 12371245. (30) Agnelli, A.; Celi, L.; Degl’Innocenti, A.; Corti, G.; Ugolini, F. C. Chemical and spectroscopic characterization of the humic substances from sandstone-derived rock fragments. Soil Sci. 2000, 165, 314-327. (31) Conte, P.; Piccolo, A.; van Lagen, B.; Buurman, P.; de Jager, P. A. Quantitative differences in evaluating soil humic substances by liquid- and solid-state 13C-NMR spectroscopy. Geoderma 1997, 80, 339-352. (32) Hammond, T. E.; Dory, D. E.; M., R. W.; Morita, H. Highresolution solid state 13C NMR of Canadian peats. Fuel 1985, 64, 1687-1695. (33) Huang, W. L.; Weber, W. J. A distributed reactivity model for sorption by soils and sediments. 10. Relationships between desorption, hysteresis, and the chemical characteristics of organic domains (1997, Volume 31, Pages 2562-2569). Environ. Sci. Technol. 1999, 33, 972-972. (34) Johnson, M. D.; Keinath, T. M.; Weber, W. J., Jr. A distributed reactivity model for sorption by sails and sediments. 14. Characterization and modeling of phenanthrene desorption rates. Environ. Sci. Technol. 2001, 35, 1688-1695.

Received for review August 22, 2005. Revised manuscript received November 14, 2005. Accepted November 19, 2005. ES051664H