Sorption of polycyclic aromatic hydrocarbons to oil contaminated sediment: unresolved complex?

Oil is ubiquitous in aquatic sediments and may affect partitioning and bioavailability of hydrophobic organic chemicals (HOCs). In contrast to other sedimentary hydrophobic carbon phases (natural organic matter, soot-like materials), oil residues have hardly received any attention as far as it concerns effects on HOC sorption. This paper describes experimental work dealing with such effects of oil on polycyclic aromatic hydrocarbon (PAH) sorption to sediments. Three different oils were spiked to a marine sediment in concentrations between 0 and 100 g/kg. Sediment-water distribution coefficients (Kd) for six deuterated PAHs were then determined either directly after spiking the oil or after a semi-natural weathering process in the lab (lasting for more than 2 yr). Resulting Kd values demonstrated sorption-reducing (competitive) effects at relatively low oil concentrations and sorption-enhancing effects at high oil concentrations. The latter effects only occurred above a certain threshold [i.e., ca. 15% (w/w) of oil on a sedimentary organic carbon basis] marking the oil concentration at which the hydrocarbon mixture presumably starts forming separate phases. Assuming a two-domain (organic carbon + oil) distribution model, oil-water distribution coefficients (K(oil)) for PAHs were estimated. For fresh oils, log K(oil) values appeared to be very similar for different types of oils, proportional to log K(OW) values and indistinguishable from log K(OC) values. For weathered oils, K(oil) values were also rather independent of the type of oil, but the affinity of low molecular weight PAHs for weathered oil residues appeared to be extremely high, even higher than values reported for most types of soot. Because affinities of high molecular weight PAHs for oils had not changed upon weathering, sorption of all PAHs studied (comprising a log K(OW) range of 4.6-6.9) to the weathered oil residues appeared to be more or less constant (averaged log K(oil) = 7.0 +/- 0.24). These results demonstrate that it is crucial to take the presence of oil and its weathering state into account when assessing the actual fate of PAHs in aquatic environments.     

Modeling maximum adsorption capacities of soot and soot-like materials for PAHs and PCBs

Recent studies have shown that not partitioning but adsorption is the main mechanism for sorption of hydrophobic organic compounds to soot and soot-like materials. For compounds that adsorb by van derWaals forces only, variation in soot-water distribution coefficients will result from differences in these forces for adsorption, as well as the maximum number of accessible sites. This maximum number of accessible sites may a priori be expected to vary due to differences in both sorbent characteristics and sorbate dimensions. In this modeling study, variation in maximum adsorption capacities is explained from sorbent and sorbate properties. Maximum adsorption capacities were calculated using (a) literature values for soot-water distribution coefficients for polycyclic aromatic hydrocarbons and polychlorobiphenyls on 10 different soot and soot-like materials and (b) Langmuir affinities for adsorption at a carbonaceous surface estimated using a recently reported method. The variation in maximum adsorption capacities could be explained by the variation in sorbent specific surface area, sorbent organic carbon content, and the sorbent-sorbate contact area. Furthermore, increasing sorbate thickness was related to a decrease in maximum adsorption capacities, which points to adsorption in micropores. Maximum adsorption capacities decreased by 1-2 orders of magnitude as the contact area increased by 50%. This points to adsorption sites being hardly larger than sorbates.     

Sorption of polycyclic aromatic hydrocarbons and polychlorinated biphenyls to soot and soot-like materials in the aqueous environment: mechanistic considerations

Recent studies have shown that sorption of polycyclic aromatic hydrocarbons (PAHs) in soot-water systems is exceptionally strong. As a consequence, soot may fully control the actual fate of PAHs in the aquatic environment. However, sorption has only been characterized for a limited number of PAHs to diesel soot, and the mechanism is poorly understood. In this paper, we present an extensive data set of sorbent-water distribution coefficients (K(S), n = 236) for a series of PAHs (both native and added) and polychlorinated biphenyls (PCBs) to five different types of soot and five soot-like materials. Both Ks values and physicochemical properties of the sorbents show large variation. In general, sorption is very strong, with K(S) values up to 10(10), showing the highest distribution coefficients on a mass basis ever reported. Sorption of in particular PAHs is often over 1000 times as strong as sorption to amorphous sedimentary organic carbon. The variation in K(S) values cannot be explained by "soot carbon fractions" or specific surface areas of the sorbents. Instead, values for native PAHs are mostly determined by the sorbates' molar volume, and values for added PAHs and PCBs are determined by the sorbents' average pore diameter. From differences in K(S) values between native and added PAH analogues, it can be deduced that generally more than 50% (with values up to 97%) of the native PAH concentration in soot is not available for distribution to the aqueous phase. We conclude that this is caused by physical entrapment of the chemicals within the solid matrix. Furthermore, most sorbents appear to preferentially sorb PCBs with planar configurations, a phenomenon most likely driven by sorption in molecular-sized pores. Pore sorption is also concluded to be the most important sorption mechanism for added PAHs together with pi-pi interaction processes with flat aromatic sorbent surfaces. Frequently observed, slowly desorbing, resistant contaminant fractions in sediments may very well be explained on the basis of these results.     

Extraction of polycyclic aromatic hydrocarbons from soot and sediment: solvent evaluation and implications for sorption mechanism

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.     

PCB and PAH speciation among particle types in contaminated harbor sediments and effects on PAH bioavailability

This research provides particle-scale understanding of PCB and PAH distribution in sediments obtained from three urban locations in the United States: Hunters Point, CA; Milwaukee Harbor, WI; and Harbor Point, NY. The sediments comprised mineral grains (primarily sand, silt, and clays) and carbonaceous particles (primarily coal, coke, charcoal, pitch, cenospheres, and wood). The carbonaceous sediment fractions were separated from the mineral fractions based on their lower density and were identified by petrographic analysis. In all three sediments, carbonaceous particles contributed 5-7% of the total mass and 60-90% of the PCBs and PAHs. The production of carbonaceous particles is not known to be associated with PCB contamination, and it is very unlikely that these particles can be the source of PCBs in the environment Thus, it appears that carbonaceous particles preferentially accumulate PCBs acting as sorbents in the aqueous environment if PCBs are released directly to the sediment or if deposited as airborne soot particles. Aerobic bioslurry treatment resulted in negligible PAH loss from the carbonaceous coal-derived material in Milwaukee Harbor sediment but resulted in 80% of the PAHs being removed from carbonaceous particles in Harbor Point sediment. Microscale PAH extraction and analysis revealed that PAHs in Harbor Point sediment were associated mainly with coal tar pitch residue. PAHs present in semisolid coal tar pitch are more bioavailable than PAHs sorbed on carbonaceous particles such as coal, coke, charcoal, and cenosphere. Results of this study illustrate the importance of understanding particle-scale association of hydrophobic organic contaminants for explaining bioavailability differences among sediments.     

Importance of black carbon to sorption of native PAHs, PCBs, and PCDDs in Boston and New York harbor sediments

The solid-water distribution ratios (Kd values) of "native" PAHs, PCBs, and PCDDs in Boston and New York Harbor sediments were determined using small passive polyethylene samplers incubated for extended times in sediment-water suspensions. Observed solid-water distribution coefficients exceeded the corresponding f(oc)Koc products by 1-2 orders of magnitude. It was hypothesized that black carbon (fBC), measured in the Boston harbor sediment at about 0.6% and in the New York harbor sediment at about 0.3%, was responsible for the additional sorption. The overall partitioning was then attributed to absorption into the organic carbon and to adsorption onto the black carbon via Kd = f(oc)Koc + f(BC)K(BC)C(w)n-1 with Cw in microg/L. Predictions based on published Koc, K(BC), and n values for phenanthrene and pyrene showed good agreement with observed Kd,obs values. Thus, assuming this dual sorption model applied to the other native PAHs, PCBs, and PCDDs, black carbon-normalized adsorption coefficients, K(BC)S, were deduced forthese contaminants. Log K(BC) values correlated with sorbate hydrophobicity for PAHs in Boston harbor (log K(BC) approximately 0.83 log gamma w(sat) - 1.6; R2 = 0.99, N= 8). The inferred sorption to the sedimentary BC phase dominated the solid-water partitioning of these compound classes, and its inclusion in these sediments is necessary to make accurate estimates of the mobility and bioavailability of PAHs, PCBs, and PCDDs.     

Immobilization of soot particles in a silica matrix: A sorbent-carrier system for studying organic chemical sorption

A new method for studying sorption with diesel and hexane sootwas developed, tested, and applied. A commercial silica-based chromatography medium was used as an inert inorganic carrier for immobilization (entrapment) of soot particles and their aggregates, thus creating a combined sorbent for sorption of hydrophobic organic chemicals (HOCs). After precombustion to remove potential organic carbon contaminants, the silica particles and soot samples were mixed under dry conditions that allowed the soot to be incorporated within the pore structure of the much larger (> 180 microm) carrier particles. Unincorporated soot was removed by multiple rinses with Milli-Q water. Sorption rate and equilibrium experiments were conducted, using phenanthrene as a probe HOC. Strong nonlinear sorption of phenanthrene was observed, in agreement with results previously obtained using air-bridge and flocculation-based methods. Batch kinetic studies suggested that 60 d of prewetting is required to obtain full water saturation, as perhaps needed for proper assessment of phenanthrene uptake rate by soot in aqueous systems. Forthe determination of equilibrium phenanthrene sorption, however, 1-d prewetting is sufficient so long as final equilibration is for at least 60 d. The new method is a practical approach to sorption measurement that may prove especially useful for study of strongly sorbing chemicals.     

Investigation of selected persistent organic pollutants in farmed Atlantic salmon (Salmo salar), salmon aquaculture feed,and fish oil components of the feed

There is extensive literature documenting the bioaccumulation of persistent organic pollutants in the marine environment, but relatively little data are available on contamination pathways in aquaculture systems such as that for farmed salmon. In recent years,the salmon industry has grown significantly in Europe. This study reports on the determination of a wide range of polychlorinated biphenyls (PCBs), organochlorine pesticides, and polybrominated diphenyl ethers (PBDEs) in farmed and wild European Atlantic salmon fish, aquaculture feeds, and fish oils used to supplement the feeds. The study confirms previous reports of relatively high concentrations of PCBs and indicates moderate concentrations of organochlorine pesticides and PBDEs in farmed Scottish and European salmon. Concentrations of the selected persistent organic pollutants varied among the samples: PCBs (salmon, 145-460 ng/g lipid; salmon feeds, 76-1153 ng/g lipid; fish oils, 9-253 ng/g lipid), S DDTs (salmon, 5-250 ng/g lipid; salmon feeds, 34-52 ng/g lipid; fish oils, 11-218 ng/g lipid), and PBDEs (salmon, 1-85 ng/g lipid: salmon feeds, 8-24 ng/g lipid; fish oils, ND-13 ng/g lipid). Comparison of the samples for all groups of contaminants, except for HCHs, showed an increase in concentration in the order fish oil < feed < salmon. Homologue profiles were similar, with an increase in contribution of hepta- and octa-PCBs in the fish, and profiles of DDTs were similar in all three types of samples. With a constant contribution to the total PCB content, the ICES 7 PCBs appear to be reliable predictors of the PCB contamination profile through all the samples. For PBDEs, BDE 47 dominated the profiles, with no significant difference in the PBDE profiles for the three matrixes. Samples with higher PCB contents generally showed higher levels of the pesticide residues, but this was not the case with the PBDEs, indicating the existence of different pollution sources.     

Field-scale reduction of PCB bioavailability with activated carbon amendment to river sediments

Remediation of contaminated sediments remains a technological challenge because traditional approaches do not always achieve risk reduction goals for human health and ecosystem protection and can even be destructive for natural resources. Recent work has shown that uptake of persistent organic pollutants such as polychlorinated biphenyls (PCBs) in the food web is strongly influenced by the nature of contaminant binding, especially to black carbon surfaces in sediments. We demonstrate for the first time in a contaminated river that application of activated carbon to sediments in the field reduces biouptake of PCBs in benthic organisms. After treatment with activated carbon applied at a dose similar to the native organic carbon of sediment, bioaccumulation in freshwater oligochaete worms was reduced compared to preamendment conditions by 69 to 99%, and concentrations of PCBs in water at equilibrium with the sediment were reduced by greater than 93% at all treatment sites for up to three years of monitoring. By comparing measured reductions in bioaccumulation of tetra- and penta-chlorinated PCB congeners resulting from field application of activated carbon to a laboratory study where PCBs were preloaded onto activated carbon, it is evident that equilibrium sorption had not been achieved in the field. Although other remedies may be appropriate for some highly contaminated sites, we show through this pilot study that PCB exposure from moderately contaminated river sediments may be managed effectively through activated carbon amendment in sediments.     

Abundances, depositional fluxes, and homologue patterns of polychlorinated biphenyls in dated sediment cores from the Pearl River Delta, China

Despite the recent efforts to investigate the distribution and fate of persistent organic pollutants in the tropical and subtropical regions of Asia, very little was known about the temporal change of polychlorinated biphenyls (PCBs) in the environmental ecosystem of China. In this study, three dated sediment cores collected from the Pearl River Delta of southern China were analyzed for a large suite of PCB congeners, from which the temporal profiles of PCB abundances, fluxes, and homologue patterns were constructed. The sedimentary inventories of total PCBs at the sampling sites ranged from 480 to 1310 ng/cm2, at the low end of the worldwide figures. Although production and use of PCBs have been banned or highly restricted in China since the early 1980s, the fluxes of total PCBs continued to increase in the Pearl River Delta sediments. There was a concurrent increase of PCB fluxes and gross domestic product per capita in the region from 1980 to 1997, and a decline of agricultural land use was evident at the same time. Apparently, large-scale land transform since the early 1980s as well as emissions from the PCB-containing electrical equipments were responsible for the sharp rise of PCB fluxes in the recent sediments. The difference in the PCB homologue patterns from 1940 to the mid-1970s was probably indicative of the different timelines of PCB usage in Macao/Hong Kong and mainland China and the differenttypes of technical PCBs commercially used. PCBs were detectable in sediments deposited well before the time frame when production of PCBs began (before 1930) and were relatively enriched in the less chlorinated homologue groups (3Cl and 4Cl PCBs), suggesting the downward mobility of lightly chlorinated PCB congeners in the sediment column.     

Particle-water partitioning of PCBs in the photic zone: a 25-month study in the open Baltic Sea

From previous laboratory and field studies, it remains unclear whether partitioning of hydrophobic organic contaminants (HOCs) to phytoplankton from water is kinetically limited or may be treated as an equilibrium process. Here, we report on the partitioning of polychlorinated biphenyls (PCBs) to particulate organic carbon (POC), dominated by planktonic primary production, in the open Baltic Sea during a 25-month period. The organic carbon-normalized partition coefficient (Koc) was corrected for temperature, salinity, and sorption to filter-passing organic carbon. At all 21 sampling occasions, the log Koc-log Kow regression was significantly linear, despite a large variation in biogeochemical parameters such as POC concentration and composition, primary production, and phytoplankton species composition. These data strongly suggest that partitioning of PCBs to POC in temperate surface waters is equilibrated and therefore not kinetically limited by factors such as rapid phytoplankton growth rate or large cell size. The partitioning of PCBs to the POC was described throughout seasonal cycles by log Koc = 0.88 +/- 0.07 log Kow + 0.90 +/- 0.47 (95% confidence interval). The slope of the log Koc-log Kow regression for the single sampling occasions varied between 0.56 and 1.25, and there was a seasonal variation in the POC sorbent quality (e.g., log Koc for PCB 28 varied between 5.5 and 6.9; median 5.9). These variations reflect the variability in structural composition of the POC pool in such pelagic waters. Being able to predict particle-water partitioning of HOCs significantly reduces the required complexity of both food web uptake models and predictions of POC-mediated export of HOCs to the deep ocean.     

Long-term recovery of PCB-contaminated sediments at the Lake Hartwell superfund site: PCB dechlorination. 1. End-member characterization

Under anaerobic conditions, such as those typically found in buried sediments, the primary metabolic pathway for polychlorinated biphenyls (PCBs) is reductive dechlorination in which chlorine removal and substitution with hydrogen by bacteria result in a reduced organic compound with fewer chlorines. Vertical sediment cores were collected from Lake Hartwell (Pickens County, SC) and analyzed in 5-cm intervals for 107 PCB congeners in a total of more than 280 samples from 18 sediment cores and surface samples. This paper reports on extensive PCB dechlorination measured in Lake Hartwell sediments and the characterization of dechlorination end-member (EM) patterns using chemical forensic methods. PCB congener fingerprinting and a multivariate receptor modeling method, polytopic vector analysis (PVA), were used for identification and characterization of weathered and dechlorinated PCB congener patterns. Dechlorination resulted in a substantial shift in buried sediments from tetra- through decachlorobiphenyl congeners to mono- through trichlorobiphenyl congeners. Mono- through trichlorobiphenyls comprised approximately 80% of the PCBs in buried sediments that underwent maximum dechlorination as compared to approximately 20% in surface sediments. The major concentration decreases were seen in the tetra- through hexachlorobiphenyl homologues, which accounted for over 90% of the dechlorination. Octa- through decachlorobiphenyl congeners also were dechlorinated, but their overall contribution to dechlorination was relatively small due to their low initial concentrations (< 5%). The net accumulation of 2-CB, 2,2'/2,6-DCBs, 2,4'-DCB, 2,2',4-TCB, and 2,2',6-TCB at Lake Hartwell matched characteristic PCB dechlorination products reported in the literature, such as those for Processes M, Q, and C; and the persistence of tetrachlorobiphenyls (TeCBs) that contained 24- and 25-congener groups resembled dechlorination Processes H or H'. Although dechlorination tended to be very extensive in most of the cores, it was not always consistent from core to core or at various depth intervals within a single core. The reason for this variability in dechlorination extent could not be determined from the existing data and did not appear to correlate with such factors as PCB concentration, total organic carbon, or age. The authors used fingerprinting analysis and a PVA multivariate receptor model as exploratory data analysis tools to characterize PCB sources and their alteration patterns. Dominant sources and alteration patterns were determined in this large data set by comparing PVA EM patterns with known source patterns (i.e., Aroclors or Aroclor mixtures) and literature-reported alteration patterns. PVA also afforded an opportunity to characterize the vertical and lateral distributions of the weathered and unweathered PCB source patterns and dechlorination patterns, a task that would have been much more difficult to accomplish through comparison of chromatograms alone.     

Absorption and adsorption of hydrophobic organic contaminants to diesel and hexane soot

Soot particles vary in pore structure, surface properties, and content of authigenic (native) extractable organic chemicals. To better understand the effects of these properties on sorption, aqueous sorption isotherms for 14C-labeled phenanthrene and 1,2,4-trichlorobenzene were obtained for four soots of varying properties: two diesel reference soots, a hexane soot, and an ozonated hexane soot. Substantial isotherm nonlinearity was observed. In comparison to diesel soot SRM 2975, diesel soot SRM 1650b had a much higher content of extractable authigenic organic chemicals, showed less sorption of 14C-labeled sorbate at low relative concentrations (Ce/Sw), and showed higher sorption at high Ce/Sw. In comparison to normal hexane soot, the ozonated hexane soot had a higher surface O/C ratio and showed substantially less sorption at all concentrations studied. The sorption differences were attributed to the noted differences in properties, and results were interpreted through a dual-mode sorption model that included the possibility of both surface adsorption (modeled using a Polanyi-based approach) and simple phase partitioning (linear absorption). Generally, such modeling indicated that overall uptake at low concentrations in all four soots was dominated by surface adsorption but that sorption at higher sorbate concentrations in SRM 1650b was heavily influenced by linear absorption within the natively bound organic phase.     

Sorption of a diverse set of organic vapors to diesel soot and road tunnel aerosols

In a traffic-dominated environment sorption of organic pollutants to exhaust aerosols can strongly determine their further fate. The sorption properties of two aerosol samples representing different exhaust sources have been determined for a large set of diverse organic vapors. For pure diesel soot we could identify adsorption to elemental carbon (EC) as the dominant sorption process. We used our experimental equilibrium adsorption coefficients to derive a predictive model for adsorption on soot in line with adsorption models for other surfaces published earlier. On road tunnel aerosols, both adsorption to EC and absorption in organic matter (OM) governed the observed sorption and the data could not be further evaluated in terms of a specific sorption mechanism.     

Sorption and desorption behavior of organotin compounds in sediment-pore water systems

Sediments contaminated with organotin compounds (OTs), in particular triorganotins (TOTs), are abundant in areas with high shipping activities. To assess the possible remobilization of these highly toxic compounds from such sediments, a profound understanding of their sorption/desorption behavior is necessary. In this work the extent and reversibility of sorption of OTs to sediments has been investigated using contaminated freshwater harbor sediments and two certified OT containing marine sediments. Experiments conducted with perdeuterated OTs showed that sorption of OTs to sediments is a fast and reversible process involving primarily particulate organic matter (POM) constituents as sorbents. The organic carbon-normalized sediment-water distribution ratios (DOC, expressed in L/kgOC) determined in the laboratory were consistent with in-situ DOCs obtained from OT concentrations measured in sediment and pore water samples from two dated sediment cores. For both butyl- and phenyltin compounds the log DOC values were in the range of 4.7-6.1, and the following sequence was observed: DOC (tri-OT) > or = DOC (di-OT) > or = DOC (mono-OT). However, the differences were much less pronounced than would have been expected for hydrophobic partitioning of the corresponding compounds into POM. These results support our hypothesis from earlier work with dissolved humic acids that OT sorption to sediments occurs primarily by reversible formation of (innerspere) complexes between the tin atom and carboxylate and phenolate ligands present in POM. Because of the high DOC values (i.e. log DOC > or = 4) the diffusion of OTs from deeper sediments to the surface will be rather slow, and thus a major release from undisturbed sediments is not expected. However, because OTs readily desorb, any resuspension of contaminated sediments (e.g., by the tide, storms or dredging activities) will lead to enhanced OT concentrations in the overlaying water column. Furthermore, in contrastto polycyclic aromatic hydrocarbons (PAH) where large fractions may be tightly bound (in)to soot or other carbonaceous materials, OTs will be more readily bioavailable due to the fast and reversible sorption/desorption behavior.     

Influence of sediment-amendment with single-walled carbon nanotubes and diesel soot on bioaccumulation of hydrophobic organic contaminants by benthic invertebrates

Single-walled carbon nanotubes (SWNT) have extremely high affinity for hydrophobic organic contaminants, considerably higher than natural or refractory (e.g., soot and detrital) carbon found in sediments. To evaluate the effect of sediment-associated SWNT on contaminant uptake from sediments by infaunal deposit feeders, we have conducted a comparative bioaccumulation study using two infaunal estuarine invertebrates. The deposit-feeding meiobenthic copepod Amphiascus tenuiremis and the deposit/suspension-feeding polychaete Streblospio benedicti were exposed to hydrophobic organic contaminants (HOCs) including polycyclic aromatic hydrocarbons (PAHs), polychlorinated biphenyls, and polybrominated diphenyl ethers for 14 days in the presence of sediment amended with (1) SWNTs, (2) NIST diesel soot, or (3) no carbon amendment. Coaddition of SWNT to sediments significantly reduced bioaccumulation of HOCs in S. benedicti; however, soot addition tended to increase the bioaccumulation of these same compounds in the polychaete worm. Bioaccumulation of HOCs from sediments by copepods (A. tenuiremis) was less dependent on black carbon addition to sediment; neither SWNT nor soot significantly impacted bioaccumulation of PAHs from sediment by this organism. Exposure of both copepods and polychaetes to radiolabeled (14C) SWNT in estuarine sediments revealed that these organisms did not assimilate these materials into their tissues, although S. benedicti did ingest 14C-SWNT, as fecal rods from this organism contained identical 14C activity as that of the sediment to which the worms were exposed.     

Extremely slowly desorbing polycyclic aromatic hydrocarbons from soot and soot-like materials: evidence by supercritical fluid extraction

Combustion-derived PAHs are strongly sorbed to their particulate carrier (i.e., soot, charcoal), and therefore, very slow desorption kinetics of the chemicals might be anticipated. Measurements are however lacking, because conventional methods (Tenax, XAD, gas-purging) fail to accurately determine desorption kinetics due to practical problems. In this study, we used a mild supercritical fluid extraction (SFE) method, which mimics desorption into water and circumvents these problems, to quantify desorption kinetics of 13 native PAHs from pure charcoal, coal, and four types of soot. The results show that generally only very small PAH fractions are released. Desorption behavior was, however, not related to common sorbent/sorbate characteristics. Two-site model-derived "fast desorbing fractions" were <0.01 in the majority of cases, and for the dominant "slow sites", the calculated rate constants for desorption into water measured from 10(-7) to 10(-5) h(-1). These data suggest that desorption of coal and combustion-derived PAHs can be even slower than the "very slow" desorption observed in sediments. Estimated time scales required for removal of pyrogenic PAHs from these extremely slow sites into water amount to several millennia. Our results imply reduced chemical risks for soot and soot-like materials, casting doubts on current risk assessment procedures and environmental quality standards of pyrogenic PAHs.     

Extensive sorption of organic compounds to black carbon, coal, and kerogen in sediments and soils: mechanisms and consequences for distribution, bioaccumulation, and biodegradation

Evidence is accumulating that sorption of organic chemicals to soils and sediments can be described by "dual-mode sorption": absorption in amorphous organic matter (AOM) and adsorption to carbonaceous materials such as black carbon (BC), coal, and kerogen, collectively termed "carbonaceous geosorbents" (CG). Median BC contents as a fraction of total organic carbon are 9% for sediments (number of sediments, n approximately 300) and 4% for soils (n = 90). Adsorption of organic compounds to CG is nonlinear and generally exceeds absorption in AOM by a factor of 10-100. Sorption to CG is particularly extensive for organic compounds that can attain a more planar molecular configuration. The CG adsorption domain probably consists of surface sites and nanopores. In this review it is shown that nonlinear sorption to CG can completely dominate total sorption at low aqueous concentrations (<10(-6) of maximum solid solubility). Therefore, the presence of CG can explain (i) sorption to soils and sediments being up to 2 orders of magnitude higher than expected on the basis of sorption to AOM only (i.e., "AOM equilibrium partitioning"), (ii) low and variable biota to sediment accumulation factors, and (iii) limited potential for microbial degradation. On the basis of these consequences of sorption to CG, it is advocated that the use of generic organic carbon-water distribution coefficients in the risk assessment of organic compounds is not warranted and that bioremediation endpoints could be evaluated on the basis of freely dissolved concentrations instead of total concentrations in sediment/soil.     

Mineral oil and PCB/dioxin analysis in some European food contamination episodes

Following a contamination of the animal food chain in Belgium with PCBs, we investigated if there was a link between the presence of PCBs and the mineral oil content. The concentrations of mineral oil hydrocarbons in samples highly contaminated with PCBs were identical with those observed in samples with very low concentrations of PCB collected from non-contaminated farms or supermarkets. We concluded that the introduction of PCBs in the animal food chain was done by the use of pure transformer oil and not by waste oils (used edible, hydraulic or motor oils) collected at the recycling centres. The concentrations of mineral oil were lower than samples collected and analysed in Switzerland during the summer of 1999 when relatively high levels were measured.     

Absorption or adsorption? Insights from molecular probes n-alkanes and cycloalkanes into modes of sorption by environmental solid matrices

Environmental solid matrices such as soils and aerosols contain a variety of absorbents (e.g., organic matter) and adsorbents (rigid carbonaceous geosorbents, minerals) but the contribution of both modes of sorption to the overall sorption behavior is often uncertain. Absorption of a cycloalkane from air to bulk phases is generally stronger than that of the n-alkane of the same number of carbon atoms, while adsorption onto surfaces does not differ between these two compounds, or rather favors the n-alkane. The presentstudy explores this characteristic sorption behavior of alkanes and eventually claims that determination of n-alkane-to-cycloalkane sorption coefficient ratios (Kn/Kc) helps elucidate the mode of sorption by complex mixtures in the environment. Differences in sorption coefficients from air (K) between n- and cycloalkanes were explained based on the linear free energy relationship (LFER) models in the form log K = - a V+ b MR + constant, where V and MR are the molar volume and the molar refraction, respectively. The LFER models predict Kn/Kc < 1 for absorption and Kn/Kc approximately 1 for adsorption. An extensive number of experimental K values of C5--C8 alkanes for known ab- and adsorbents were evaluated. The data matched the model expectations and indeed exhibited a distinct difference in Kn/Kc, between ab- and adsorption. Steric factors due to sorbate and sorbent geometries generally favored adsorption of n-alkanes over that of cycloalkanes (Kn/Kc > 1), clarifying the contrast between the two sorption modes even more. The application to environmental solid matrices is demonstrated using sorption data for diesel soot, aerosols and snow. The results are in excellent agreement with previous discussions on the modes of sorption in these materials.