Environ. Sci. Technol. 2001, 35, 1680-1687
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 MARTIN D. JOHNSON,† WEILIN HUANG,‡ AND W A L T E R J . W E B E R , J R . * ,† Environmental and Water Resources Engineering, Department of Civil and Environmental Engineering, The University of Michigan, Ann Arbor, Michigan 48109-2125, and School of Environmental Science, Engineering, and Policy, Drexel University, Philadelphia, Pennsylvania 19104
Subcritical water treatment was used to effect rapid compositional and functional changes to peat organic matter that mimic those of the natural diagenesis process. Elemental, solid state 13C NMR, FTIR, and calorimetry analyses all indicated that the organic matter of the artificially aged peat was chemically similar to that of geologically mature coal kerogens. This paper extends the work of the previous paper in this series, which investigated the effects of subcritical water treatment of humic topsoil on subsequent phenanthrene sorption and desorption equilibria. As opposed to the previous study, however, changes in sorptive reactivity herein were unequivocally related to changes in organic matter rather than other soil constituents, and organic matter functional changes due to the simulated diagenesis were more accurately characterized. Phenanthrene sorption capacity and isotherm nonlinearity both increased with increasing degrees of artificial aging, supporting the viewpoint that hydrophobic organic contaminant sorption equilibrium properties can be directly related to the degree of diagenesis of geosorbent organic matter. In addition, this work investigated effects of subcritical water treatment of a geologically mature, kerogen-containing shale sample. In contrast to the peat, the functional characteristics of the shale were unchanged by this treatment, and subsequent phenanthrene sorption equilibria were altered far less.
Introduction Soil and sediment organic matter (SOM) is generally the dominant sorption domain for hydrophobic organic contaminants in subsurface systems, and the molecular characteristics of SOM affect sorption properties (1-15). To design appropriate remediation strategies and predict fate and transport of organic chemicals in a contaminated area, a mechanistic understanding of associated sorption and de* Corresponding author phone: (734)763-1464; fax: (734)763-2275; e-mail:
[email protected]. † The University of Michigan. ‡ Drexel University. 1680
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ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 35, NO. 8, 2001
sorption processes is essential. Conclusions have been made relating sorption behavior to the degree of natural organic matter maturation via such diagenesis reactions as condensation and aromatization (3, 5, 10, 12, 14, 15). Diagenesis is the natural process by which kerogen is formed from biopolymer source materials. Carbohydrates, proteins, and other biopolymers from decaying plants, animals, and microorganisms are broken down by enzymatic microbial degradation into individual amino acid and sugar monomers, which can either be used by the microorganisms or preserved by random polymerization and condensation, along with lipids and hydrocarbons, to form humic acids, fulvic acids, and humin. These heterogeneous geopolymers are ultimately condensed to kerogen and bitumen over geologic time (1618). Sorption capacity, isotherm nonlinearity, and apparent desorption hysteresis have been correlated with the oxygen/ carbon and hydrogen/carbon atomic ratios of sediment organic matter, both of which decrease with increasing degree of diagenesis (3, 5, 10). The foregoing conclusions have been drawn from studies employing a variety of soils, shales, kerogens, and peat samples, likely originating from an equally wide range of source materials. While this previous work is convincing, it would be yet more conclusive to conduct sorption studies on samples originating from the same source but spanning a range of degrees of diagenesis. Xing and Chen (14) extracted humic acids from different depths of a soil profile and subsequently used them in naphthalene sorption studies. It was found that condensed aromatic domains in the humic acids increased with natural burial and that degree of condensation and aromaticity correlated with isotherm nonlinearity. These findings support our previous work relating SOM sorption properties to degree of diagenesis. To overcome the geological time barrier, we have in this study mimicked the natural diagenesis of a geologically young peat sample rapidly through subcritical water treatment; i.e., extraction and reaction. Laboratory aquathermolysis reactions of model organic compounds in hot water have been used to rapidly study petroleum formation from source materials (19, 20). For example, pyridine and 3-methylpyridine are stable in 250 °C liquid water, but pyridine-3-carboxylic acid, pyridine-3carboxaldehyde, and 3-pyridylmethanol all undergo cleavage reactions to form reduced pyridine and methyl pyridines (21). Carboxylic, carboxaldehyde, and alcohol functional groups are common in geologically young natural sediment organic matter, while pyridine and alkyl functional groups are common among natural kerogens. Geraniol, a biologically synthesized olefinic alcohol, is rapidly condensed in hot water to form limonene and other condensed products (22). Acidcatalyzed aromatization reactions of limonene occur in 175 °C water with clay catalysts in less than 2 h without addition of acids (23). While limonene is representative of terpenes abundant in essential oils of plants, aromatic reaction products are representative of organic carbon groups in reduced kerogen. In addition, hydrous pyrolysis utilizes liquid subcritical water to rapidly simulate artificial aging of kerogens and release of fuels (24-29). This paper builds upon the idea that subcritical water conditions can induce rapid diagenesis-like reactions that alter the physicochemical properties of geologically young soil organic matter and, coupled with subsequent measurements of the sorption characteristics of altered samples, provide valuable mechanistic sorption information. Changes in the properties of water at elevated temperatures and pressures help explain why reactions of organic molecules can take place rapidly in this medium. The most 10.1021/es001390s CCC: $20.00
2001 American Chemical Society Published on Web 03/09/2001
TABLE 1. Subcritical Water Treatment of Peat and Shalea
a
sample name
sorbent type
time (h)
phase
pressure (atm)
extracted TOC (%)
N2-BET surface area (m2/g)
SWPeat-1 SWPeat-2 SWPeat-3 SWPeat-4 SWLach-1 SWLach-2 original peat original shale
C. peat C. peat C. peat C. peat L. shale L. shale
0.5 3 10 3 3 3
liquid liquid liquid steam liquid steam
70-100 90-110 90-110 3-5 90-110 3-5
51 56 60 30 3 4
nd nd 36.6 nd 12.3 nd 1.7 11.1
Temperature ) 250 °C and water flow ) 1.0 g/min in all cases.
significant changes that occur as the temperature of water is increased with enough pressure to maintain liquid phase are decreasing dielectric constant (30-32), increasing solubility of hydrophobic organics (33-38), and increasing ion dissociation product (31, 39). This paper, although similar in many ways to its predecessor in the series (7), presents significant new information. It was shown in our earlier work that subcritical water affected the sorption properties of Chelsea soil, and these changes were attributed to compositional and functional alterations of soil organic matter. However, the soil contained only 5.6% organic carbon, so changes to other soil constituents could not be ruled out as possible alternative explanations. This study, on the other hand, examines Canadian peat, which is composed almost entirely of geologically immature natural organic matter. Changes in the sorption properties of this material can thus be directly related to changes in sorbent organic matter. Furthermore, the high organic content of peat facilitates more descriptive characteristic analyses of the original and altered organic matter. In addition, this study investigates artificial aging of Lachine shale, a kerogencontaining ancient sediment that has already undergone diagenesis. If subcritical water does in fact mimic natural diagenetic changes, then its effects on a geologically mature kerogen sample should be minimal.
Experimental Section Sorbents and Their Characterization. Long fibered, light blond type sphagnum peat moss (particle size
