Sorption of phenanthrene by sewage sludge during composting in relation to potentially bioavailable contaminant content

The aim of the present study was to determine to what degree the sewage sludge sorption capacity to phenanthrene influences on bioavailability of this compound during composting. Sewage sludges were composted for 76 days. The content of the potentially bioavailable phenanthrene fraction was determined by: mild-solvent extraction with n-butanol (BTL) and non-exhaustive extraction technique with hydroxypropyl[β] cyclodextrin (HPCD). Batch experiments were used to construct phenanthrene sorption isotherms. The contribution of the potentially bioavailable phenanthrene fraction in individual sewage sludges ranged from 32 to 48% (BTL) and from 5.1 to 80.3% (HPCD). The direction of changes in the potentially bioavailable fraction resulting from composting also depended on the sewage sludge and the extraction method applied. Isotherms demonstrated a good fit to the Freundlich isotherm model. Sorption coefficients (log KF) and organic carbon-normalized distribution coefficients (log Koc) of phenanthrene by sewage sludges ranged from 3.42 to 3.98 and from 4.14 to 4.70, respectively. Sewage sludges exhibited relatively strong affinity for sorption large amounts of phenanthrene. In the case of two sludges, a strong relationship between phenanthrene sorption capacity (log KF and log Koc) and the content of the bioavailable fraction of this compound was observed.