H.A.J. Govers

Elucidation of the metabolic pathway of fluorene and cometabolic pathways of phenanthrene, fluoranthene, anthracene and dibenzothiophene by Sphingomonas sp. LB126.

The metabolic pathway of the PAH fluorene and the cometabolic pathway of the PAHs phenanthrene, fluoranthene, anthracene and dibenzothiophene in Sphingomonas sp. LB126 were examined. To our knowledge this is the first study on the cometabolic degradation of the three-ring PAHs phenanthrene, anthracene and the four-ring PAH fluoranthene by a fluorene-utilizing species. Metabolism of fluorene was shown to proceed via the 9-fluorenone pathway to form o-phthalic acid and protocatechuic acid. The cometabolic mono-hydroxylation found for phenanthrene, fluoranthene and anthracene shows similarity with the hydroxylation of fluorene. Several mono- and dihydroxy products and ring-cleavage products were identified for phenanthrene, fluoranthene and anthracene. It appeared that the cometabolism of those three compounds is a non-specific process, in contrast to the metabolism of fluorene. For dibenzothiophene the metabolites dibenzothiophene-5-oxide and dibenzothiophene-5,5-dioxide were identified; these compounds appeared to be the products of a dead-end pathway. Since apart from dibenzothiophene no metabolites were found in very high concentrations for any of the other substrates, complete degradation is suggested, even for the cometabolic degradation of phenanthrene, fluoranthene and anthracene.

Bioaccumulation of cadmium by the freshwater isopod Asellus aquaticus (L.) from aqueous and dietary sources

Experiments were conducted to determine the kinetics and relative importance of aqueous and dietary uptake of cadmium by the freshwater isopod Asellus aquaticus (L.). Test animals were exposed during 30 days to aqueous Cd in a continuous flow system (exposure levels: 0.2 - 10 microg litre(-1)) and kept on a diet of previously contaminated Elodea sp. (range of Cd concentrations: 2-350 microg g(-1), dry weight). Preceding semi-static experiments on dosage-control of the dietary factor revealed a rapid uptake of Cd by Elodea, with relatively high concentration factors (CF), which ranged from 4.8 to 5.5 (dry weight log (CF) after 16 days). For Asellus uptake from water appeared to be the predominant route. Highly significant bioconcentration of cadmium from water was observed in the animals, even at exposure levels below 1.0 microg litre(-1). In the various treatments, direct uptake from water accounted for 50-98% of the body burdens after 30 days exposure. The experimental results were described with a first order one-compartment bioaccumulation model. Model parameter estimates (mean +/- standard error) were obtained for rate constant of uptake (560 +/- 110 day(-1)), rate constant of elimination (0.032 +/- 0.017 day(-1)) and assimilation efficiency of Cd uptake from food (1.1 +/- 0.7%). The (dry weight) bioconcentration factor (BCF) and bioaccumulation factor (BAF) for extrapolated steady state conditions were estimated at 18 000 (BCF) and 0.08 (BAF). Experiments conducted at two different pH levels (5.9 versus 7.6) revealed no significant effects of pH on the uptake of aqueous Cd by the isopods. The results are discussed in relation to their potential significance to the field situation.

Degradation of anthracene by Mycobacterium sp. strain LB501T proceeds via a novel pathway, through o-phthalic acid.

ABSTRACT Mycobacterium sp. strain LB501T utilizes anthracene as a sole carbon and energy source. We analyzed cultures of the wild-type strain and of UV-generated mutants impaired in anthracene utilization for metabolites to determine the anthracene degradation pathway. Identification of metabolites by comparison with authentic standards and transient accumulation of o-phthalic acid by the wild-type strain during growth on anthracene suggest a pathway through o-phthalic acid and protocatechuic acid. As the only productive degradation pathway known so far for anthracene proceeds through 2,3-dihydroxynaphthalene and the naphthalene degradation pathway to form salicylate, this indicates the existence of a novel anthracene catabolic pathway in Mycobacterium sp. LB501T.

Thermal Degradation Characteristics of High Impact Polystyrene/Decabromodiphenylether/Antimony Oxide Studied by Derivative Thermogravimetry and Temperature Resolved Pyrolysis-Mass Spectrometry. Formation of Polybrominated Dibenzofurans, Antimony (oxy) Bromides and Brominated Styrene Oligomers

Abstract The thermal stability and the thermal degradation products of high impact polystyrene / decabromodiphenylether / antimony oxide (HIPS FR) have been studied in situ with derivative thermogravimetry (DTG), temperature resolved pyrolysis—mass spectrometry (Py-MS) and pyrolysis—gas chromatography / mass spectrometry (Py-GC /MS). With in-source temperature resolved Py-MS (negative ions) the thermal degradation processes of HIPS FR have been studied dynamically and antimony (oxy)bromides and brominated higher styrene oligomers upto n = 15 have been detected. During degradation of the HIPS FR polymer matrix several processes take place, such as debromination of the flame retardant decabromodiphenylether to form less brominated diphenylethers, bromination of polystyrene and formation of antimony bromides and antimony oxybromides. The formation of toxic polybrominated dibenzofurans (PBDFs) has been shown to occur in the temperature range in which the HIPS FR polymer matrix degrades (350–400°C). This is explained by debromination of decabromodiphenylether to form less brominated diphenylethers which are much more reactive towards formation of PBDFs.

Determination and Theoretical Aspects of the Equilibrium Between Dissolved Organic Matter and Hydrophobic Organic Micropollutants in Water (Kdoc)

Literature on the equilibrium constant for distribution between dissolved organic carbon (DOC) (Kdoc) data of strongly hydrophobic organic contaminants were collected and critically analyzed. About 900 Kdoc entries for experimental values were retrieved and tabulated, including those factors that can influence them. In addition, quantitative structure-activity relationship (QSAR) prediction equations were retrieved and tabulated. Whether a partition or association process between the contaminant and DOC takes place could not be fully established, but indications toward an association process are strong in several cases. Equilibrium between a contaminant and DOC in solution was shown to be achieved within a minute. When the equilibrium shifts in time, this was caused by either a physical or chemical change of the DOC, affecting the lighter fractions most. Adsorption isotherms turned out to be linear in the contaminant concentration for the relevant DOC concentration up to 100 mg of C/L. Eighteen experimental methods have been developed for the determination of the pertinent distribution constant. Experimental Kdoc values revealed the expected high correlation with partition coefficients over n-octanol and water (Kow) for all experimental methods, except for the HPLC and apparent solubility (AS) method. Only fluorescence quenching (FQ) and solid-phase microextraction (SPME) methods could quantify fast equilibration. Only 21% of the experimental values had a 95% confidence interval, which was statistically significantly different from zero. Variation in Kdoc values was shown to be high, caused mainly by the large variation of DOC in water samples. Even DOC from one sample gave different equilibrium constants for different DOC fractions. Measured Kdoc values should, therefore, be regarded as average values. Kdoc was shown to increase on increasing molecular mass, indicating that the molecular mass distribution is a proper normalization function for the average Kdoc at the current state of knowledge. The weakly bound fraction could easily be desorbed when other adsorbing media, such as a SepPak column or living organism, are present. The amount that moves from the DOC to the other medium will depend, among other reasons, on the size of the labile DOC fraction and the equilibrium constant of the other medium. Variation of Kdoc with temperature turned out to be small, probably caused by a small enthalpy of transfer from water to DOC. Ionic strength turned out to be more important, leading to changes of a factor of 2-5. The direction of this effect depends on the type of ion. With respect to QSAR relationships between Kdoc and macroscopic or molecular descriptors, it was concluded that only a small number of equations are available in the literature, for apolar compounds only, and with poor statistics and predictive power. Therefore, a first requirement is the improvement of the availability and quality of experimental data. Along with this, theoretical (mechanistic) models for the relationship between DOC plus contaminant descriptors on the one side and Kdoc on the other should be further developed. Correlations between Kdoc and Kow and those between the soil-water partition constant (Koc) and Kow were significantly different only in the case of natural aquatic DOC, pointing at substantial differences between these two types of organic material and at a high correspondence for other types of commercial and natural DOC.

Slow desorption of PCBs and chlorobenzenes from soils and sediments: relations with sorbent and sorbate characteristics.

The kinetics of slow desorption were studied for four soils and four sediments with widely varying characteristics [organic carbon (OC) content 0.5-50%, organic matter (OM) aromatic content (7-37%)] for three chlorobenzenes and five polychlorinated biphenyls (PCBs). Slowly and very slowly desorbing fractions ranged from 1 to 50% (slow) and 3 to 40% (very slow) of the total amount sorbed, and were observed for all compounds and all soils and sediments. In spite of the wide variations in sorbate K(OW) (factor 1000) and sorbent characteristics, the rate constants of slow (k(slow), around 10(-3) h(-1)) and very slow (k(very slow), 10(-5)-10(-4) h(-1)) desorption appeared to be rather constant among the sorbates and sorbents (both within a factor of 5). There was a good correlation (r(2) above 0.9) between the distribution over the slow, very slow and rapid sediment fractions and log K(OC), indicating that sorbate hydrophobicity may be important for this distribution. No correlation could be found between sorbent characteristics [OC, N, and O in the organic matter, polarity index C/(N+O), OC aromaticity as determined by CP-MAS (13)C-NMR] and slow desorption parameters (slowly/very slowly desorbing fractions+corresponding rate constants). The absence of (1) a correlation between k(slow) and k(very slow), respectively, and OC content, and (2) the narrow range of k(slow) and k(very slow) values, indicates that intra-OM diffusion is not the mechanism of slow or very slow desorption, because on the basis of this mechanism it would be expected that increasing OC content would lead to longer diffusion pathlengths and, consequently, to smaller rate constants. In addition, it was tested whether differential scanning calorimetry would reveal a glass transition in the soils/sediments. In spite of the sensitivity of the equipment used (changes in heat flow in the micro-Watt range were measurable), a glass transition was not observed. This means that activation enthalpies of slow desorption can be calculated from desorption measurements at various temperatures. In the present study these values ranged from 60 to 100 kJ/mol among the various soils and sediments studied.

The formation of polybrominated dibenzo-p-dioxins (PBDDs) and dibenzofurans (PBDFs) during pyrolysis of polymer blends containing brominated flame retardants

Abstract The formation of polybrominated dibenzo-p-dioxins and -dibenzofurans (PBDDs and PBDFs) during pyrolysis of two polymer blends containing decabromobiphenyl and tetrabromobisphenol-A, was investigated at different temperatures and carrier gas compositions. During pyrolysis of a polybutylene terephthalate/decabromobiphenyl blend PBDFs are formed at a ppm level. Even when there is oxygen available in the carrier gas, PBDDs are formed at a much lower level than PBDFs. In the presence of 10% oxygen the maximum yield of tetra- to octabromodibenzofurans is 70 ppm at 600°C. The contribution of 2,3,7,8-tetrabromodibenzofuran and 1,2,3,7,8-pentabromodibenzofuran is less than 4.5 ppm. During pyrolysis of acrylonitrile/butadiene/styrene with tetrabromobisphenol-A mono- to pentabrominated dibenzofurans were formed at a ppb level. The optimum temperature of formation of PBDDs and PBDFs was 600°C. The thermal degradation processes of these two polymer blends were investigated in a thermogravimetric analysis. Both flame retardants do not exert an influence on the elementary chemical degradation processes of the polymer blends. The flame retardant activity of decabromobiphenyl and tetrabromobisphenol-A consists of the emission of brominated species into the gas phase which scavenge the propagation radicals and reduce inflammability. The mechanism of formation of PBDDs and PBDFs from decabromobiphenyl in polybutyleneterephthalate consists of a combination of a condensed phase and a gas phase mechanism. In the case of tetrabromobisphenol-A only a gas phase mechanism of formation of PBDDs and PBDFs was observed.

Bioconcentration of polycyclic heteroaromatic hydrocarbons in the guppy (Poecilia reticula)

Abstract Bioconcentration factors (BCFs) of 19 parent and hetero polycyclic aromatic hydrocarbons (PAH) were obtained from static bioconcentration experiments with Poecilia reticulata by monitoring the decrease in the initial aqueous concentration. The BCFs were obtained by fitting the observed concentrations to a one-compartment first-order kinetic model. The BCFs thus obtained showed a satisfactory linear relationship with the experimental n-octanol/water partition coefficient, log Kow (r = 0.91, s = 0.30). By comparison with a limited number of semi-static tests, it was concluded that the static bioconcentration experiment is a satisfactory alternative for more laborious bioconcentration tests to obtain BCFs of (hetero) PAH. However, the static tests are less valuable for the acquisition of kinetic bioconcentration parameters such as, e.g., uptake or elimination rate constants. For several of the compounds in this study, the estimates resulting from the static tests did not match with those from a limited number of depuration experiments carried out in this study or with values expected from the relationship between the depuration rate constant and the log Kow, reported in the literature. The distribution of three parent PAH within the fish after a 7-day semi-static exposure was also investigated. Relatively high concentrations were found in liver and kidneys. The major part of the body burden was found in muscle and the gastro-intestinal tract.

Membrane-water partitioning of polychlorinated biphenyls in small unilamellar vesicles of four saturated phosphatidylcholines.

Experimental data are presented on the partition coefficients of 14 polychlorinated biphenyls (PCBs) between four species of saturated 1,2-diacyl-sn-glycero3-phosphocholine membrane vesicles and aqueous buffer. Small unilamellar vesicles of dimyristoylphosphatidylcholine (DMPC), dipalmitoylphosphatidylcholine (DPPC), distearoylphosphatidylcholine (DSPC), and diarachidoylphosphatidylcholine (DAPC) show a high affinity for extreme hydrophobic xenobiotics such as the PCBs in this study. Observed membrane-water partition coefficients K mw of the PCBs, with log K ow values ranging from 5.3 to 8.26, in membrane lipid vesicles were in the following order : DPPC > DMPC > DSPC > DAPC. Beside the hydrophobicity of the PCBs, the order of the K mw values is primarily determined by the fluidity of the membrane bilayer. Atthe experimental temperature of 37°C for every PCB, a correlation exists between log K mw and the membrane lipid main phase transition temperature T m . In comparison to the bulk solvent 1 -octanol, the membrane lipid phase is highly structured and the correlation between log K mw and both the molar liquid volume MLV of the PCBs and their log K ow is nonlinear.

Enhanced kinetics of solid-phase microextraction and biodegradation of polycyclic aromatic hydrocarbons in the presence of dissolved organic matter†

The uptake kinetics of fluorene, phenanthrene, fluoranthene, pyrene, and benzo[e]pyrene by solid-phase microextraction fibers was studied in the presence of dissolved organic matter (DOM) obtained from sediment pore water and resulted in increased fiber absorption and desorption rate coefficients. Compared to the control without DOM, these rate coefficients were increased at a DOM concentration of 36.62 mg/L by a factor of 1.27 to 2.21 and 1.31 to 2.10 for fluorene and benzo[e]pyrene, respectively. The calculated values for the fiber absorption and desorption rate coefficients show that diffusion through an unstirred boundary layer (UBL) surrounding the fiber probably forms the rate-limiting step of the process. The mineralization of aqueous-phase phenanthrene and pyrene by a representative polycyclic aromatic hydrocarbon (PAH)-degrading bacterium (Mycobacterium gilvum VM552) also was found to be enhanced by DOM. The initial degradation rates of phenanthrene (9.03 μg/L) and pyrene (1.96 μg/L) were significantly higher compared to the control values and were enhanced by a factor of 1.32 and 1.26 at a DOM concentration of 43.14 and 42.15 mg/L, respectively. We suggest that such an enhancement results from the combination of faster uptake kinetics of the water-dissolved compounds in the UBL surrounding microbial cells and direct access of the bacteria to DOM-associated PAHs. These enhanced kinetic effects of DOM may have strong implications in sediment processes like desorption, nonequilibrium exposure, and biodegradation.