Leaching of Polycyclic Aromatic Hydrocarbons Using a Column Method
Sammanfattning: Polycyclic aromatic hydrocarbons (PAHs) are a group of organic pollutants of great environmental concern, due to their toxicity, carcinogenicity and long persistence in the environment. To determine the potential risk associated with PAHs released from contaminated soil or waste materials, knowledge is required on the leachable concentrations of PAHs and the amounts available for release. The objectives of the work reported in this thesis were to develop a method to determine the leaching of PAHs from contaminated soils and other solid materials, and to study how differences in leaching conditions, such as temperature and flow rates, affect the release of PAHs. In addition, modelling was used to integrate data, hypotheses and experience gained from the experimental work. The leaching method applied consists of a glass column, containing the sample, through which sterile water is pumped, followed by on-line filtration and solid-phase extraction. The leaching behaviour of PAHs was studied for different materials: aged contaminated soil, green liquor sludge and different combustion residues. The method developed succeeded in providing reproducible results and proved to be useful for the evaluation of PAH (bio)availability in contaminated materials in two respects: i) leachable aqueous concentrations of contaminants could be determined as well as ii) the available fractions of total sequestered amounts. From the column leaching experiments on contaminated soil it was concluded that the release of PAHs originated from at least two different fractions; a very small fraction (only a few percent) of the total content of PAHs was initially released (during the first 120 days), followed by extended tailing of very low leachable concentrations (up to 4 orders of magnitude lower than solubility concentrations). Experiments conducted at different hydraulic retention times showed that the leached concentrations were close to distribution equilibrium concentrations. The applied average contact time was short (approx. 0.5 h), which indicates that the compounds were released with fast desorption kinetics. Modelling suggested that the desorption of PAHs followed a linear isotherm up to a cut-off value, at which the distribution equilibrium concentration appeared to be independent of the sorbed amount. Leaching experiments conducted at different temperatures (7, 15, 18 and 23 ?C) indicated that the concentrations of PAHs in soil-water, at a contaminated site in Sweden, will probably be about one order of magnitude lower than leachable concentrations obtained from laboratory experiments performed at room temperature. Estimated values of the activation energy for the desorption of PAHs from the initially released fraction were in the range of 50 to 80 kJ mol-1, indicating that these PAHs were associated to amorphous soil organic matter rather than to a dense organic matrix. This finding is in agreement with that of fast desorption kinetics. However, empirically obtained mass transfer coefficients for the PAHs studied increased with the molar volume of the compounds, which seems counterintuitive to the above findings. Considerably lower concentrations of PAHs were obtained in the leachates from the residual materials studied (well below the guideline values for drinking water). The application of the column leaching method to these materials showed that leaching experiments, together with total amounts of PAHs, can provide valuable information about the materials? stability and their potential for PAH leaching; knowledge that is very important when a material is considered for reuse, either as a filling material for construction, or as a nutrient source for forest soils.
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