Environ. Sci. Technol. 1999, 33, 1637-1644
Organic Matter Facies and Equilibrium Sorption of Phenanthrene SYBILLE KLEINEIDAM, HERMANN RU ¨ GNER, BERTRAND LIGOUIS, AND PETER GRATHWOHL* University of Tu ¨ bingen, Geological Institute, Applied Geology Group, Sigwartstrasse 10, 72076 Tu ¨ bingen, Germany
Remediation of groundwater contamination in unconsolidated aquifers by dissolved hydrophobic compounds (HOC) requires detailed information on the sorption parameters present in the sediments. Equilibrium sorption isotherms were measured for phenanthrene for a wide variety of lithocomponents (constituents of sand and gravel sediments) and unweathered rock fragments (limestones and sandstones). The lithocomponents were separated based on macroscopic appearance of different lithologies (e.g. limestones, sandstones, shales, mudstones, and igneous rocks) and characterized in terms of organic carbon content and specific surface area. In addition the organic matter (OM) was characterized using coal petrography methods (white and UV light microscopy). As confirmed by heat-treated samples sorption was solely due to OM. Organic carbon normalized sorption coefficients (KOC) varied by almost 3 orders of magnitude among the samples investigated. The different origin and maturity of isolated organic matter (organic facies) is believed to be responsible. For example, extremely high KOC values were found for particulate organic matter such as charcoal and coal particles which were preserved within the sandstone and limestone grains. In a second paper we report data on sorption kinetics of the samples used in this study (1).
Introduction Many early studies (2-5) showed that sorption of hydrophobic organic compounds (HOC) in soils and sediments is mainly controlled by the organic matter (OM) contained in these materials. More recent work has revealed that in addition to the organic carbon content the nature or quality of the organic matter has a significant impact onto the sorption capacity (6-9) and sorption nonlinearity (10-13). Organic carbon normalized sorption coefficients (KOC) are higher in geologically old (mature) organic matter in sedimentary rocks compared to relatively recent humic organic matter in soils (8-14). Empirical correlations established for soil organic matter (e.g. 2, 4) underestimated the sorption of HOC in samples containing mature organic matter (8, 15). Analogous to the Van Krevelen diagram which describes the chemical evolution of kerogen disseminated in sedimentary rocks (16, 17), H/C and O/C ratios have been used to * Corresponding author phone: 49-7071-2975429; fax: 49-70715059; e-mail:
[email protected]. 10.1021/es9806635 CCC: $18.00 Published on Web 04/10/1999
1999 American Chemical Society
characterize OM in terms of sorption behavior (8, 11, 15, 18). Also, OM with higher aromatic fractions (as measured by C13-NMR) enhanced the sorption of polycyclic aromatic hydrocarbons (PAH) (14, 19). The influence of organic matter quality on the sorption isotherm shape was shown by Weber and co-workers (20) for shale-containing soils which exhibit a higher degree of nonlinearity compared to other soils investigated. A nonlinear sorption behavior at low solute concentrations was explained by Spurlock and Biggar (21) by a “general partition model” assuming a limited number of high energy sites within the OM. The observed OM heterogeneity led to the division of soil OM into rubbery (amorphous) and glassy (condensed) regions (12, 22), analogous to the classification of polymers. Weber and Huang (23) made a similar but extended division and postulated three sorption site domains: inorganic surfaces, amorphous (soft) organic matter, and condensed (hard) organic matter. Young and Weber (10) suggested diagenetic transformation from amorphous to condensed OM, to explain the changes in nonlinearity and KOC for mature organic matter. OM Origin. The variability of OM in terms of origin, alteration, and maturation is a well-known and wellinvestigated issue in coal and petroleum science (16, 17, 2427. Organic matter in sediments can either be of autochthonous and/or allochthonous origin. In aqueous environments autochthonous OM is derived from phytoplankton (e.g. algae), zooplankton and microorganisms (e.g. bacteria). Depending on the dissolved oxygen concentration (aerobic/ anaerobic) in the sediment or sediment-water interface bacterial degradation may lead to the production of amorphous OM (anaerobic conditions). In contrast to the amorphous OM, the fraction of allochthonous OM in the sediments is predominantly particulate organic matter (POM) and of 2-fold origin: (i) POM reflecting the vegetation (and climate) of the sedimentation time (wood, spores, pollen, cuticle, charcoal from wildfires) and (ii) POM originating from the source rocks of the sediment (e.g. OM from coal seams or shale formations). For example an immature sediment may contain higher proportions of reworked OM of high maturity level (e.g. coal) than of indigenous, immature OM (e.g. humins). The transport of this POM can either be fluvial (accumulation in sand and siltstones) or aeolian (small charcoal particles in limestones and sandstones). Peters (28) suggests that the charcoal accumulates preferentially in sandstones, but more recent investigators (29-31) have shown the OM in marine deposits to contain between 10% and 90% of charcoal. The scope of this study was to gain a better understanding concerning the OM variability in sedimentary rocks having different origin and maturity (e.g. marine and terrestrial plant debris, lignite, bituminous coal, and charcoal) and to quantify the influence of different OM facies on the HOC sorption (sorption capacity and isotherm linearity). Instead of heterogeneous bulk samples, we used uniform lithocomponents in terms of mineralogy and/or sediment processes and rock fragments in sorption studies with phenanthrene, a frequent organic contaminant in soils and groundwater. OM was isolated from the samples using hydrochloric acid and hydrofluoric acid treatment (established coal petrography techniques to dissolve the inorganic carbon and the silica minerals present in the sample). The isolates then were prepared on thin sections for microscopic analysis and characterized using UV light and reflected white light. VOL. 33, NO. 10, 1999 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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TABLE 1: Description of the Samples and Their Abbreviationsd description of the lithocomponents dark (D) colored micritic to sparitc marine limestone (L) (in the case of MsKr, MsK triassic mud-flats) light (L) colored micritic to sparitic marine limestone (L) (in the case of JKr, JK jurassic carbonate shelf)
sample abbreviations DLH,a DLS, MsK,b MsKrc LLH, LLS JK,b JKr
dark (D) colored fine grained sandstone (S) having a carbonate matrix (dominated by cretaceous turbiditic marine sandstones)
DSH, DSS
light (L) colored fluvial and marine sandstone (S) with a carbonate cementation
LSS, LSH, SSb
carbonate free quartzitic sandstone from triassic fluvial sand deposits
BS,b RHr
igneous and metamorphic rocks (granite, gneiss, chiste, amphibolite)
MetH, MetS
monominerals (quartz, feldspar, and calcite occurring in rock joints and cleavages)
QzS, QzH
coal particles from a quaternary sand (CoalS) and a tertiary lignite seam (Bcoalr)
CoalS, Bcoal
marine bituminous shale (restricted marine basin in the lower jurassic)
LEr
Hu¨ ntwangen, samples collected by Siegenthaler and Huggenberger (43). b Horkheim (44). c r ) fresh rock fragments. d Letters relate to color, lithology and weathering, subscript letters indicate sampling location of the aquifer materials; H (Hu¨ ntwangen), S (Singen). a
Materials and Methods Origin of Sediments and Sedimentary Rocks (Table 1). Aquifer sediments are heterogeneous mixtures of sand and gravel sized rock fragments (lithocomponents). The petrography of the lithocomponents depends on the geological formations in the source areas (source rocks) and the transport distance of the sediments. Our investigations focused on alpine derived quaternary (glacio-) fluvial sediments (Hu ¨ ntwangen, Singen) in Switzerland and Southwest Germany and a quaternary river deposit in the Neckar Valley (Horkheim). The source area of the Horkheim sediments is relatively small, and the different lithocomponents can be allocated to distinct geological units, which are partially still exposed in outcrops in the near vicinity. In addition to the sand and gravel lithocomponents, which might be affected by weathering processes during transport and sedimentation, fresh (nonweathered) rocks were sampled from the source areas of the sediments (in sandstone, shale and limestone quarries and outcrops). Separation of the Samples. The fluvial sediments were air-dried. The grain size fractions used in the experiments were obtained by dry sieving. The bulk gravel was divided, based on visual appearance, into macroscopic homogeneous lithocomponent fractions such as limestones, sandstones, and igneous rock fragments. The fresh sedimentary rock samples were broken to sand and gravel sized fragments (1). Macroscopically homogeneous classes of lithocomponents and rock fragments were distinguished as listed in Table 1. Sample Characterization and Preparation. For the determination of the physico-chemical properties and the equilibrium sorption isotherms the samples (lithocomponents and nonweathered rock fragments) were pulverized for 30-40 min in a planet ball mill (Fritsch, Laborette). All mill components in contact with the samples (balls and cups) were of zirconium oxide in order to prevent contamination of the samples with carbon (e.g. from steel). Eighty percent of the pulverized material was < 0.063 mm as determined by sedimentation analysis in an Atterberg Cylinder. To quantify the contribution of mineral surfaces to the overall sorption, organic matter was removed from selected samples by heat-treatment at 550 and 750 °C (Heraeus muffle oven). Carbonate content was determined by titration using hydrochloric acid (1 N) and back-titration of the residual HCl with caustic soda (1 N). Phenolphthalein was used as the indicator. The carbonate content (CaCO3) was high in the limestone components and lower in the sandstone components where either carbonate cement (SS, LSS) or/and carbonate minerals (DSS, DSH) are responsible for the CaCO3. 1638
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The CaCO3 levels are negligible for the mineral fraction and the igneous and metamorphic rock fragments as well as the coal samples. For the marine bituminous shale (LEr) a carbonate content of 15% was measured. The organic carbon content (oc) was measured (after pretreatment of the samples with HCl in order to remove inorganic carbon) by dry combustion at 850 °C (Model 183 Boat Sampling Module, Rosemount) and quantified by an infrared detector for CO2 (Horiba PIR-2000). The specific surface area (SA) was determined using nitrogen adsorption and the BET-method (ASAP 2010, Micromeritics). For the equilibrium sorption isotherms batch experiments were conducted using phenanthrene as a chemical probe. Phenanthrene is frequently encountered in soil contamination and representative for many other HOCs (log KOW ) 4.57 (32); water solubility ) 1.29 mg/L (33)). It was obtained as a pure product (98%) from Aldrich Chemical Corp. and used as received. Concentrated methanol stock solutions were prepared and spiked to deionized, degassed, and filtersterilized water as needed. Methanol concentrations in the aqueous solution were always less than 0.5% (a level at which methanol has no measurable effect on sorption (e.g. 34). All batch experiments were conducted in triplicate in 10100 mL crimp top reaction glass vials sealed with PTFE-lined butyl rubber septa (Alltech). Phenanthrene concentrations in control vials were stable within an error range of 5% up to 200 days (for details see ref 1). Vials containing 0.5-50 g of the pulverized sample were filled with the phenanthrene spiked aqueous solution (CW ) 50 - 600 µg/L). The different solid-to-water ratios were necessary due to major differences in sorption capacity of the samples. Headspace was kept at a minimum. Sodium azide at a concentration level of 200 mg/L was added in order to inhibit bacterial growth. The samples were stored at 20 °C in the dark and shaken by hand every day. After 7 days, a time period which was found to be sufficient to establish sorption equilibrium with pulverized samples (Figure 1), an aliquot of 0.5-6 mL of the supernatant water was sampled. Phenanthrene was extracted from the water sample using cyclohexane. Naphthalene was added as an internal standard. Separation, detection, and quantification was done using HPLC and fluorescence detection (column: Grom PAH, 250 mm*4 mm, 5 µm C18 silica; mobile phase: 32% water 68% acetonitrile; emission/extinction wavelengths for phenanthrene ) 249/345, and naphthalene ) 278/324). To check the mass balance five selected pulverized samples were extracted after equilibration for 7 days using hot methanol (35). The recovery rate of phenanthrene was 95-105%.
The KOC as used in this paper represents the different sorptivities of distinct organic matter facies for a HOC such as phenanthrene at the trace concentration level of 1 µg/L.
Results and Discussion
FIGURE 1. Sorption kinetics in pulverized materials (grain size < 63 µm); equilibrium is reached after 7 days. Isolation and Characterization of OM. Organic matter was isolated from selected lithocomponents (DSS, DLS, Jkr, Mskr, LSS, LLH, DLH, LEr) by a standard demineralization procedure using hydrochloric and hydrofluoric acid (24).Grain size fractions from 2-5 mm were used to avoid destruction of the POM through pulverization (36). These OM samples were prepared on a glass slide. Microscopic investigations were carried out on a Leica DMRX photometer microscope. To investigate a possible organic coating on the grain surfaces a thin section of the sand fraction from Singen was prepared and studied using the same microscopy method. Optical techniques such as fluorescence microscopy (under UV excitation) provide information on the level of OM maturity (37). UV or blue light induces fluorescence of various constituents (e.g. spores, algae, and cuticles). The intensity and color spectrum of the visible light emitted by OM in response to the excitation allows the identification of different OM facies. Immature oil prone OM fluoresces yellow-green and as the OM becomes more mature, the color changes through yellow, orange, and brown until it ceases at high maturity levels. Polymerization, aromatization, and condensation through maturation result in increased quenching of fluorescence (37). The isolated OM as well as the OM of the thin section were identified and characterized using white light and UV illumination in transmitted and reflected light mode. Data Analysis. Phenanthrene sorption in batch experiments was calculated based on mass balance considerations. Equilibrium sorption data were fit by the Freundlich model
CS ) KFR CW1/n
(1)
where CS [µg/kg] and CW [µg/L] denote the concentrations in the solid and the aqueous phase, respectively. KFR [µg/kg: µg/L] and 1/n [-] are the Freundlich sorption coefficient and the Freundlich exponent, respectively. The parameters were obtained by a linear regression of the linearized form of eq 1 (log-transformed). Note that the distribution coefficient Kd as used in this paper denotes a concentration ratio (Kd ) CS/CW) which for nonlinear sorption isotherms is a function of the solute concentration (Kd ) KFRCW1/n-1; Kd equals KFR at an aqueous concentration of 1µg/L; phenanthrene concentrations in this low range are frequently encountered in soil and groundwater contaminationssthe legal limit in Germany is 0.2 µg/L). Organic carbon normalized sorption coefficients (KOC) were calculated based on the fraction of organic carbon fOC and KFR at an aqueous concentration of 1 µg/L:
KOC )
KFR fOC
(2)
Organic Carbon Content (oc). Table 2 summarizes oc along with other properties for the different lithocomponents. Organic carbon is low in sedimentary rocks from the photooxide zone in shallow marine environments (e.g. carbonate platforms, reefs; samples: JK, JKr, LLH, LLS) and aerobic sedimentary environments (continental realm, e.g. fluvial or marine sands; samples: SS, LSH, LSS) due to enhanced degradation of OM. In general these types of sedimentary rocks show light colors (Tables 1 and 2). In contrast, the sediments deposited in oxygen restricted or anoxic conditions (mudflats, restricted marine basins; samples: MsK, MsKr, DLH, DLS, LEr) show dark colors and significantly higher oc values. In the case of the dark turbiditic sandstone (DSS, DSH) the organic matter is preserved because of the high sedimentation rates in these marine environments which reduces the degradation rates of organic substances (Table 1 and 2). Differences in oc within different grain size fractions of the same macroscopic lithocomponent (e.g. DLS, DSS in Table 2) can be explained by the natural variation of oc within layers of the same stratigraphic unit (e.g. of a sandstone or limestone formation). The coal samples (CoalS, Bcoalr) are characterized by very high organic carbon values. Low oc values close to the detection limit were found in the monominerals (QzS, QzH), the quartz dominated sandstones (BS, RHr), and for the igneous and metamorphic rocks (MetH, MetS). Specific Surface Area (SA, Table 2). The SA in sedimentary rocks is a function of grain size, cementation, and diagenesis. Higher surface areas of 1-4 m2/g were measured for the sandstone samples (BS, SS, RHr, LSH, LSS) compared to 1-2 m2/g of the limestone samples (JK, JKr, MsK, MsKr, LLH, LLS, DLH, DLS). The specific surface area is highest (16 m2/g) for the bituminous shale (LEr) due to its high clay mineral content. The relative high surface areas of CoalS (7.6 m2/g) compared to Bcoal (2.5 m2/g) is probably also due to clay minerals associated with the CoalS particles. Heat Treated Samples. The significance of sorption to mineral surfaces in materials which are low inorganic carbon has long been an issue of discussion (e.g. refs 3 and 38). Since the specific surface areas measured for pulverized samples (Table 3) were generally slightly higher compared to the unaltered fine gravel samples (Table 2) we tried to quantify the contribution of the mineral surface to overall sorption using oc free samples. In order to remove OM the lithocomponents were heated for 2 h at 550 and 750 °C. At a temperature of 750 °C the carbonate content decreased (CaCO3) by 50-98% which probably indicates the beginning of thermal dissociation of the carbonate minerals (in limekilns typically temperatures of ca. 1000 °C are used for dissociation of CaCO3 into CaO). Single point Kd values were determined for the heat-treated samples and compared to untreated samples (Figure 2). Since heating at 750 °C resulted in a significant weight loss (20-70%), the Kd values were calculated based on the unaltered solid mass. Sorption of phenanthrene showed a minimum on samples that were heat-treated at 550 °C. Measured Kds were less than 5% of the values reached in the untreated samples (Figure 2, Table 3). After heat-treatment at 750 °C an increase of Kd was observed in all samples except MetS. Most likely this increase in sorption is due to thermal changes in mineral composition and the generation of reactive surfaces. A steeper increase in sorption was found for the carbonate rich limestone samples compared to the sandstone sample. No measurable sorption was found in the carbonate free igneous rock VOL. 33, NO. 10, 1999 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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TABLE 2: Physical and Chemical Properties of the Lithocomponents and Rock Fragments grain size d [mm]
lithocomponents dark limestonea(DLH) dark sandstone (DSH) light limestonea(LLH) light sandstone (LSH) quartz, feldspar (QzH) igneous rocks (MetH)
2-4 2-4 2-4 2-4 2-4 2-8
dark limestone (DLS) dark sandstone (DSS) light limestone (LLS) light sandstone (LSS) quartz, feldspar (QzS) igneous rocks a(MetS) reworked coal particles (CoalS)
specific surface area SA [m2/g] Hu1 ntwangen 0.89 2.0 1.8 3.1 0.09 0.84
0.86 0.37 0.81 0.33 0.025
