Bioavailability of organic contaminants in sediments.
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The bioavailability of polycyclic aromatic hydrocarbons (PAHs) and chlorobiphenyls (CBs) in sediments is largely dependent on the freely dissolved concentration of these pollutants. However, measuring these is challenging, due to the low concentrations of lipophilic contaminants in the environment and their strong affinity for particles and for traditional sampling (filtration and centrifugation) equipment. An equilibrium passive sampling device made of silicone rubber was developed in this research to measure the freely dissolved concentrations of lipophilic contaminants and other parameters (water extractable proportions and sediment-water partition coefficients) that describe the availability of these contaminants in the environment. Equilibration between sampler and sediment for PAHs and CBs was found to be adequately achieved after 20 days shaking of a silicone rubber sampler in sediment slurry on an orbital shaker at 200 rpm. The reproducibility of uptake was better than 5 %. Silicone rubber-water partition coefficients for 34 PAHs and 32 CBs were measured in the laboratory using a co-solvent method using methanol as co solvent. Strong linear correlations of log sr w K , with octanol-water partition coefficients (log ow K ) ( log 0.97 log 0.01; 2 0.94 , K = K − r = sr w ow & log 1.17 log 1.82; 2 0.90 , K = K − r = sr w ow ) were found for PAHs and CBs, with a systematic difference in correlations observed for the different classes of compounds which was attributed to structural differences of the compounds. The silicone rubber samplers were then used to measure concentrations of PAHs in the pore water of sediments from the Fladen Ground of the North Sea, Loch Shell, Firth of Forth, Firth of Clyde, Loch Etive and Aberdeen Harbour in Scotland and the Vefsn fjord, Norway. A proportion of the PAHs were found to be unavailable for exchange into the aqueous phase, and this was reflected in the high log oc K measured in all the sediments studied. The sediment-water partition coefficients also correlated positively with the octanol-water partition coefficients. Accumulation of PAHs in Nereis virens from sediments was better predicted from literature bio concentration factors and pore water concentrations obtained using the silicone rubber samplers than from sediment concentrations traditionally used in risk assessments. Participation in an International Council for the Exploration of the Seas (ICES) passive sampling trial survey using silicone rubber in sediments and water is described, and demonstrated the potential of passive sampling in monitoring environmental pollution. The log BCF (bio concentration factor) for PAHs in mussels increased with increasing log ow K at both Loch Etive and Aberdeen Harbour locations, and could be used to estimate concentrations in mussels directly. The survey data also showed the use of silicone rubber in assessing the diffusive exchange of PAHs across sediment-water interfaces.