Environ. Sci. Technol. 2002, 36, 4107-4113
Extraction of Polycyclic Aromatic Hydrocarbons from Soot and Sediment: Solvent Evaluation and Implications for Sorption Mechanism MICHIEL T. O. JONKER* AND ALBERT A. KOELMANS Aquatic Ecology and Water Quality Management Group, Department of Environmental Sciences, Wageningen University, P.O. Box 8080, 6700 DD Wageningen, The Netherlands
Soot contains high levels of toxic compounds such as polycyclic aromatic hydrocarbons (PAHs). Extraction of PAHs from soot for quantitative analysis is difficult because the compounds are extremely tightly bound to the sorbent matrix. This study was designed to investigate the effect of solvent type on PAH extraction yield, to identify the most optimal solvent for PAH extraction from soot, and to gain insight into the mechanism of PAH sorption to soot in aquatic environments. To that end, different types of soot as well as coal, charcoal, and sediments containing soot-like material were extracted with seven organic solvents. Large differences in extraction recoveries were observed among solvents, with relative values as low as 16% as compared to the best extracting solvent. These differences were much larger for soot than for sediments. Dichloromethane, which to date is the most widely used solvent for soot and sediment extractions, appeared to be the overall worst extractant, whereas toluene/ methanol (1:6) gave the best results. Based on extraction yields and solvent properties, extraction of PAHs from soot was explained by a two-step mechanism involving swelling of the sorbent matrix and subsequent displacement of sorbates by solvent molecules. Due to the low displacement capacity of water, desorption of PAHs from soot in the aquatic environment will be strongly limited. Moreover, a certain fraction of the total PAH mass on soot is suggested to be physically entrapped, making it unavailable for partitioning to the aqueous phase.
Introduction Soot has been the subject of numerous studies during the past decades. This material which is a product of incomplete combustion of fossil fuels and biomass has raised the interest of both toxicologists and environmental scientists because (a) it contains high levels of highly toxic compounds such as polycyclic aromatic hydrocarbons (PAHs), (b) the estimated current worldwide production is enormous (approximately 50-200 × 1012 g/yr (1)) which makes soot probably the most important source of PAHs in the environment, (c) it strongly optically absorbs solar radiation, influencing our climate, * Corresponding author phone: +31 317 485485; fax: +31 317 484411; e-mail:
[email protected]. 10.1021/es0103290 CCC: $22.00 Published on Web 08/23/2002
2002 American Chemical Society
and acts as a scavenger of volatile pollutants and as catalyst for various reactions while transported through the atmosphere (2, 3), (d) soot particles are relatively inert (2) which makes the material a long-term sink of carbon in the global carbon cycle (1) but also an excellent sedimentary tracer of fires and fossil fuel use over geologic time (2, 4), and (e) soot was recently shown to be a very strong sorption phase for PAHs and possibly also for several other organic contaminants in the aquatic environment (5-8). To be able to quantify toxicological properties or bioavailable fractions of sorbed PAHs, the concentration as well as the partitioning behavior of PAHs on soot need to be known. With respect to the latter, soot-water distribution coefficient measurements suggest very strong sorption (5, 6). Because PAHs are mainly formed during the combustion process it has been hypothesized that they are partly entrapped within the soot matrix, included in small pores or strongly sorbed to flat surfaces (9). To date, the exact mechanism, however, is not well-known. As a result of the entrapment or strong sorption, natively soot-associated PAHs are difficult to extract (2). Therefore, for quantitative analysis of PAHs on soot robust extraction methods such as Soxhlet extraction, Soxtec extraction (10), Supercritical Fluid Extraction (SFE) (11, 12), or Accelerated Solvent Extraction (ASE) (13) need to be applied, and less rigorous techniques (e.g. shaking or ultrasonic extraction) should be avoided. Among the methods mentioned, the classical Soxhlet extraction is the technique most frequently used. Partially, this is due to the fact that this method is relatively simple and cheap which makes it applicable in each laboratory. Most important, however, Soxhlet extraction recoveries are usually superior or approximately equal to recoveries of other (robust) techniques (11-16). Soxhlet extraction can be performed by using a variety of extraction solvents. Traditionally, soot is extracted using dichloromethane (e.g., refs 10, 11, 13, and 17-19), but occasionally other solvents such as toluene (20), hexane (21), cyclohexane (22), benzene (23), methanol (22), hexane/acetone (24), dichloromethane/methanol (25), dichloromethane/hexane (26), benzene/methanol (19, 27), and benzene/2-propanol (21) are used. Obviously, no consensus exists on the best solvent choice for soot extractions. A few studies were dedicated to comparing efficiencies of different solvents extracting natively sorbed PAHs from either diesel soot or air particulate matter. In general, these studies show that different solvents have different extraction recoveries and that solvent choice is a critical factor in extracting PAHs from soot (28). However, all studies recommend different extraction solvents, i.e., dichloromethane (28), methanol (29), either cyclohexane, acetone, or benzene (30), either dichloromethane or toluene (16), aromatic solvents such as toluene (31), and no single solvent is optimal (32). Therefore, based on these studies no universal best extractant for soot can be defined. Also sediments are extracted using different organic solvents. However, the most frequently used solvents during Soxhlet extractions of sediments are dichloromethane and hexane/acetone. The solvent choice for sediment extractions is however less critical than for soot because of the nature of the sorbing material. The majority of sorption sites in sediments for chemicals such as PAHs is located in natural organic matter. This degradation product of dead biomass consists of biopolymers such as fulvic and humic acids and humins. Extraction of this material is less difficult than extraction of soot. However, it has been reported that sediments frequently contain soot or soot-like materials (33, 34), and for such sediments solvent choice may still be very VOL. 36, NO. 19, 2002 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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important. Indeed, this was recently demonstrated for sediments from the Baltic Sea (35) which showed reduced extraction recoveries for polychlorinated biphenyls upon increasing soot carbon fraction. Use of toluene as extraction solvent proved to result in significantly higher recoveries than the use of hexane/acetone. In the present study, different types of soot and sediments containing soot were extracted with a series of wellconsidered organic solvents. Aim was to investigate the effect of solvent type on the extraction yield of PAHs to (a) select the best extracting solvent and (b) gain more insight into the bound state and desorption mechanism of soot associated PAHs in the aquatic environment.
Experimental Design Extraction solvents were selected on the basis of the following starting points: (a) use of solvents suitable for Soxhlet extraction (boiling point < 115 °C), (b) aromatic solvents are possibly better extractants for PAHs than aliphatic solvents because of direct displacement of the sorbed compounds (competition for adsorption sites) and the rule of thumb “like dissolves like”, (c) the combination of a polar with a nonpolar solvent probably is more effective than the use of separate solvents (36), and (d) use of some of the most frequently applied solvents. Accordingly, the following seven extractants were defined: dichloromethane, toluene, toluene/ methanol (1:6), toluene/ethanol (1:4), benzene/ethanol (3: 2), benzene/1-propanol (3:1), and hexane/acetone (3:1). The ratios mentioned in brackets follow on the composition of the formed azeotropes and the requirement that after concentration of the extract to 1 mL only polar solvents should remain. These solvents subsequently can be exchanged azeotropically to hexane, the solvent used during cleanup. Exception to this is the mixture hexane/acetone (3:1) which is directly concentrated to 1 mL of hexane. Fulfillment of the mentioned requirement for all solvent mixtures was carefully tested and approved by determining refractory indices of concentrated mixtures and comparing them to indices of pure solvents. Different types of “soot” were used as testing material: “traffic soot”, soot originating from the combustion of wood, of oil and of coal, and the soot-like materials charcoal and coal. Extracts of these six materials as well as extracts of four sediments containing soot-like material were analyzed for 13 PAHs, i.e., phenanthrene (Phen), anthracene (Ant), fluoranthene (Flu), pyrene (Pyr), benz[a]anthracene (BaA), chrysene (Chr), benzo[e]pyrene (BeP), benzo[b]fluoranthene (BbF), benzo[k]fluoranthene (BkF), benzo[a]pyrene (BaP), benzo[g,h,i]perylene (BghiPe), dibenz[a,h]anthracene (dBahA), and indeno[1,2,3-c,d]pyrene (InP). The study was divided into two phases. First, all materials were extracted once using all solvents. From the results, the three (sometimes four) most efficient extractants per material were selected. Then, the samples were extracted two more times using these solvents, yielding triplicate data on the three best extractants.
Material and Methods Chemicals. Solvents used were as follows: hexane and acetone (picograde; Promochem, Wesel, Germany), methanol (HPLC gradient grade; Mallinckrodt Baker, Deventer, The Netherlands), ethanol (p.a.; Merck, Darmstadt, Germany), acetonitrile (HPLC grade; Lab-Scan, Dublin, Ireland), 1-propanol (HPLC grade; Riedel-de Hae¨n, Seelze, Germany), benzene, toluene, and dichloromethane (HPLC grade; SigmaAldrich, Steinheim, Germany), and water (Nanopure; Barnstead, Dubuque, Iowa, U.S.A.). Other chemicals used were as follows: aluminum oxide-Super I (ICN Biomedicals, Eschwege, Germany) and 2-methylchrysene (99.2%; Community Bureau of Reference, Geel, Belgium). 4108
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Soot and Sediments. Pulverized charcoal (from combustion of bark, particles < 150 µm) was obtained from Boom BV, Meppel, The Netherlands. Coal (originating from Poland) was supplied by the coal-fired power station EPZ, Borssele, The Netherlands. Traffic soot was sampled from used exhaust pipes (n ≈ 70) at a local garage using a large test tube brush. Wood soot, oil soot, and coal soot were supplied by local chimney sweep companies. These materials originated from private domestic fires and were sampled from the chimneys. To gain soot that more closely resembles the fraction found in sediments, all six materials were passed through a 50 µm porosity sieve. Then, the fractions
