Environ. Sci. Technol. 2008, 42, 1458–1464
The Role of Condensed Carbonaceous Materials on the Sorption of Hydrophobic Organic Contaminants in Subsurface Sediments
of values determined for materials high in kerogen and humin. This work demonstrates the advantage of using both sequential chemical treatment and petrographic analysis to analyze the sorption contributions of different CMs in natural soils and sediments, and the importance of sorption to natural geopolymers in groundwater sediments not impacted by anthropogenic sources of black carbon.
BY SANGJO JEONG,† MICHELLE M. WANDER,‡ SYBILLE KLEINEIDAM,§ PETER GRATHWOHL,§ BERTRAND LIGOUIS,| AND C H A R L E S J . W E R T H * ,† Department of Civil and Environmental Engineering, University of Illinois at Urbana–Champaign, Department of Natural Resources and Environmental Sciences, University of Illinois at Urbana–Champaign, Center for Applied Geosciences, Tübingen University, Tübingen, Germany, and LAOP (Laboratories for Applied Organic Petrology), Tübingen University, Tübingen, Germany
The sorption of hydrophobic organic contaminants (HOCs) in soils and sediments is greatly affected by the type and amount of different carbonaceous materials (CMs), which in turn can affect HOC transport and fate (1–8). Significant efforts have been made to identify the CMs that control sorption in soils and sediments. Recently, condensed forms of CMs such as kerogen and black carbon (BC) have been shown to dominate sorption in soils and surface water sediments (ref (1, 8) and references therein). The role of condensed forms of CMs on HOC sorption to geosorbents in subsurface sediments, where anthropogenic inputs of BC (e.g., vehicular soot, coal char) may be negligible, has been studied in only a few groundwater sediments (9). In these studies, enriched fractions of more condensed carbonaceous materials (e.g., kerogen, coaly particulate organic matter) were found to have greater sorption capacity than the parent materials. While valuable, these studies did not quantify the contributions of kerogen, black carbon, and other CM forms (e.g., humic and fulvic acid) to sorption in groundwater environments. CMs include recent solid phase natural organic matter (SNOM), geopolymers formed through alteration of recent SNOM, and black carbon. Recent SNOM is derived from plant and animal remains and is characterized by physical appearance (e.g., cuticles, spores, algae, wood), and biochemical structures (e.g., polysaccharides, lignins, proteins, lipids). Recent SNOM initially undergoes diagenesis with burial, where it becomes more condensed and increasingly aromatic over geologic time through microbial action, heat, and pressure. Poorly defined polymeric compounds formed through diagenetic alteration of recent SNOM are called humic substances. Humic substances are operationally split into humin, fulvic acids, and humic acids (10). In marine and lacustrine sediments, CMs as humic substances and diagenetically altered but identifiable source materials are dispersed throughout the mineral matrix and are the precursors of kerogens (11). Kerogens form during diagenesis and are operationally defined as SNOM that cannot be extracted with an organic solvent, acid, or base. In ancient estuarine swamps or lacustrine bogs, these same source materials initially form peat during diagenesis (12). As diagenesis continues, peat undergoes condensation reactions to form lignite (i.e., brown coal), and then bituminous coal. In contrast, black carbon is a byproduct of incomplete combustion or pyrolysis of biomass and fossil fuels. BC includes solid phase residuals (e.g., charcoal) and condensed volatiles (e.g., soot) (13). In the presence of oxygen, CMs are subject to weathering. Weathering acts in opposition to geological evolution, resulting in less condensed and less aromatic materials with more hydrophilic functional groups. Black carbon is subject to weathering, but it is less reactive than SNOM (14). Sorption of HOCs is generally greater to SNOM that has undergone more extensive geological evolution (2, 15). Garbarini and Lion (16) measured ∼2, 30, and 70 times greater organic carbon normalized distribution coefficients, KOC, for
Received August 9, 2007. Revised manuscript received November 8, 2007. Accepted November 16, 2007.
The identification and characterization of carbonaceous materials (CMs) that control hydrophobic organic chemical (HOC) sorption is essential to predict the fate and transport of HOCs in soils and sediments. The objectives of this paper are to determine the types of CMs that control HOC sorption in the oxidized and reduced zones of a glacially deposited groundwater sediment in central Illinois, with a special emphasis on the roles of kerogen and black carbon. After collection, the sediments were treated to obtain fractions of the sediment samples enriched in different types of CMs (e.g., humic acid, kerogen, black carbon), and selected fractions were subject to quantitative petrographic analysis. The original sediments and their enrichment fractions were evaluated for their ability to sorb trichloroethene (TCE), a common groundwater pollutant. Isotherm results and mass fractions of CM enrichments were used to calculate sorption contributions of different CMs. The results indicate that CMs in the heavy fractions dominate sorption because of their greater mass. Black carbon mass fractions of total CMs in the reduced sediments were calculated and used to estimate the sorption contribution of these materials. Results indicate that in the reduced sediments, black carbon may sequester as much as 32% of the sorbed TCE mass, but that kerogen and humin are the dominant sorption environments. Organic carbon normalized sorption coefficients (KOC) were compared to literature values. Values for the central Illinois sediments are relatively large and in the range * Corresponding author phone: 217-333-3822; e-mail: werth@ uiuc.edu. † Department of Civil and Environmental Engineering, University of Illinois at Urbana–Champaign. ‡ Department of Natural Resources and Environmental Sciences, University of Illinois at Urbana–Champaign. § Center for Applied Geosciences, Tübingen University. | LAOP (Laboratories for Applied Organic Petrology), Tübingen University. 1458
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ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 42, NO. 5, 2008
Introduction
10.1021/es0719879 CCC: $40.75
2008 American Chemical Society
Published on Web 02/05/2008
fulvic acid, humic acid, and humin extracted from soil, respectively, than for cellulose. Weber and co-workers measured KOC values for kerogen that are 1-2 orders of magnitude greater than those for humic substances (17, 18). Sorption is generally less to SNOM that has undergone extensive weathering. Grathwohl (2) determined that the KOC value for a weathered shale was approximately 1 order of magnitude smaller than that for an unweathered shale. Binger et al. (5) determined that the KOC value for an oxidized sediment was smaller than that for a reduced sediment from the same geological formation. Values of KOC for HOCs have also been measured for materials rich in black carbon, and in many soils and lake sediments these sorbents are thought to control sorption. Bucheli and Gustafsson (19) determined that KOC values for a soot standard (SRM-1650) were 35–250 times greater than predicted by the organic matter-partitioning model. Ghosh et al. (20) found that sorption to black carbon in a harbor sediment is more favorable than to kerogen. They observed that more than 60% of PAHs are sorbed on the coal/wood derived particles of the lake sediment, even though they constitute only 5% of the sediment mass. According to a recent study (1), median BC contents as fractions of total organic carbon (TOC) are 9% (quartile range 5–18%) and 4% (quartile range 2–13%) for surface water sediments (number of sediments, n ≈ 300) and surface soils (number of soils, n ≈ 90), respectively. Reliable estimates of BC contents in subsurface sediments are not available, and their effect on HOC sorption is not clear. The primary objective of this study is to determine whether black carbon, kerogen, or less condensed CMs control sorption in the oxidized and reduced zones of a glacially deposited groundwater sediment in central Illinois. The subsurface sediments were collected and subjected to density separation and a sequence of chemical treatments to obtain fractions enriched in one or more types of CM. The amounts of different CMs were quantified by mass differences between enrichment fraction, and independently via quantitative petrographic analysis. Sorption isotherms for trichloroethene on the bulk sediments and the enrichment fractions were determined and used to interpret the role of different CMs to sorption. Other attempts to determine the dominant CM sorbents in natural soils and surface water sediments have been made in coarse grain materials (9, 21), or in materials with large organic carbon contents (22, 23).
Experimental Section Sample Description. Uncontaminated glacially deposited subsurface sediment samples were obtained adjacent to a hazardous waste site contaminated by chlorinated solvents at Chanute Air Force Base (CAFB) in Rantoul, Illinois. The sediments were collected in cores from the Wisconsin till formation, a 12-15 m thick predominantly clay-rich matrix with interstratified lenses of silt, sands, and gravels (24). All cores were divided into oxidized, reduced, and sandy layers. Oxidized (approximately 1∼5 m depth) and reduced (approximately 5∼10 m depth) sediments, mainly composed of silt and clay, were used for this study. Details of sample collection are provided in the Supporting Information. Sample Treatment. The reduced and oxidized CAFB sediments were subject to a series of physical and chemical treatment steps to obtain fractions enriched in one or more carbonaceous materials. The sediments were placed in a solution of 1.6 g/cm3 sodium polytungstate and, after gentle shaking, the light fraction (LFr) was poured off (25, 26). The remaining heavy material was again placed in a solution of 1.6 g/cm3 and, after vigorous shaking to break apart aggregates, the LFr was again poured off. The first LFr is referred to as loose particulate (LP) material, the second as occluded
particulate (OP) material. The sample remaining after removal of LP and OP is the humified, mineral associated fraction (HFr). Light and heavy fractions were pulverized and passed through a 100-mesh sieve to enhance reaction kinetics of subsequent treatments. They were first treated with HCl to remove carbonates and HF and HCl to remove silicate minerals (HCl/HF) (27). These steps also removed the less recalcitrant acid soluble CMs. Next, the sediments were treated with trifluoroacetic acid (TFA) and HCl to remove easily hydrolyzable organic matter (28), NaOH to remove fulvic and humic acids (29), and acid dichromate to remove the more recalcitrant humin and kerogen (30, 31). Additional treatment details are in the Supporting Information. Samples enriched in CMs after treatment through HF/ HCl are referred to as acid insoluble (AI) carbonaceous materials (e.g., HFr-AI, LP-AI, OP-AI). Samples enriched in humic and fulvic acid, kerogen, humin and black carbon after subsequent treatment with TFA/HCl are referred to as HAKBC. Samples enriched in kerogen, humin, and black carbon after treatment with NaOH are referred to as KBC. Lastly, samples treated with acid dichromate are referred to as BC. Mass fractions of different CM enrichments were calculated from changes in mass after each treatment step. Jeong and Werth (32) showed that sequential chemical treatment through TFA/HCl did not significantly change the sorption properties of a purified humic acid, and sequential chemical treatment through extraction with NaOH did not significantly change the sorption properties of laboratory made char and two soots. However, treatment through acid dichromate did change the sorption properties of the char and two soots due to oxidation of sorbent surfaces. Hence, sorption properties of acid dichromate enrichment fractions are not reported or evaluated. Jeong and Werth (32) also showed that approximately 19% of the purified humic acid was lost during treatment through TFA/HCl, 0% of the char was lost during treatment through acid dichromate, and 17% of the soot was lost during this same treatment. The materials were pulverized before treatment (as in this work) and mass losses are presumably due to fines being lost during filtration (0.2 µm pore size filter) and some nontarget CM reactivity. These results indicate that mass losses of up to 20% are possible in this study during sequential chemical treatment. Sample Characterization. The carbon contents, specific surface areas, and microporosities of original (i.e., bulk) sediments and all enrichment fractions were determined as described in the Supporting Information. The carbon-tooxygen ratios of all sample surfaces were determined using both energy-dispersive spectroscopy (EDS) and X-ray photoelectron spectroscopy (XPS). Functional groups of all samples were analyzed using diffuse reflectance Fourier transform infrared spectroscopy. Details of all methods and results are in the Supporting Information. Petrographic Analysis. Quantitative petrographic analysis was performed on density separated light and heavy fractions of the reduced sediments, and on only the heavy fraction of the oxidized sediments, all after treatment to remove carbonate and silicate minerals (i.e., after treatment with HCl/HF). It was also performed on the density separated heavy fraction of the reduced sediment after treatment through NaOH. CMs were classified according to maceral groups and subgroups as described in the Supporting Information. The accuracy of two maceral analyses made by any one operator on the same polished surface is approximately (1.5% by volume (33, 34) (at the 95% confidence level). For analysis on two different specimens (i.e., two different polished sections) of the same sample, the accuracy is somewhat less, due to errors that arise from differences between the two specimens. The overall accuracy has been determined to be approximately (2%. VOL. 42, NO. 5, 2008 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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Sorption Isotherms. Sorption isotherms of bulk sediments, the heavy sediments before and after each treatment step, and the light sediments before and after treatment with HCl/HF were measured using trichloroethene (TCE) following Li and Werth (35). Sorption isotherms of light fractions after treatment through TFA were not measured because of insufficient mass. TCE was used because it is one of the most common contaminants in groundwater,and it is frequently used as a probe chemical. Details of the method, in addition to data that supports the equilibration times used in this study (Figures S-4 and S-5) are in the Supporting Information. Sorption Contributions. The sorption contributions of different CM enrichment fractions to sorption in the bulk material were calculated as follows: sci(%) )
smi × 100 smBulk
(1)
where, smi is the mass sorbed to fraction i, and smBulk is the total mass sorbed to bulk sediment. Details are provided in the Supporting Information. This approach assumes that at a fixed aqueous concentration, the mass sorbed to CM enrichment fraction i is the same when this fraction is alone, or as part of the bulk sediment.
Results CM Composition. The compositions of CMs in CAFB sediments were analyzed by measuring the changes of total and organic carbon mass after each treatment step, and from direct calculation of CM volume percentages using petrographic analysis. The changes of total and organic carbon mass of sediments after each chemical treatment step are shown in Figure S-1, and listed in Tables S-1 and S-2, all in the Supporting Information. The volume fractions of all CMs identified from petrographic anlaysis are listed in Table S-3 (Supporting Information). Major trends are discussed here, and mass fractions as a percent of acid insoluble CMs are presented in Figure 1. The masses of reduced and oxidized bulk sediments are dominated by the heavy fractions (>99%); less than 1% are light fractions. The total CM fractions of bulk sediments (i.e., equivalent to the fraction of organic carbon, fOC) are 0.39% (reduced) and 0.51% (oxidized). Heavy fractions contain the majority of total CMs for the reduced (90%) and oxidized (51%) sediments; light fractions contain only small percentages of total CMs (
