Sorption of short- and long-chain perfluoroalkyl surfactants on sewage sludges

Perfluoroalkyl surfactants (PASs) have attracted increasing concerns in recent years due to their global distribution, persistence, bioaccumulation and potential toxicity. Since sludge was a significant source of PASs to environment, the sorption of short (C2-C6) and long-chain (C7-C15) PASs on different sewage sludge was investigated in this study. The equilibrium data were well represented by the Freundlich isotherm and were generally nonlinear. In order to elucidate the sorption mechanism of PASs to sludge, effect of sludge property, solution chemistry and molecular structure were also investigated in details. The dominant sludge parameter influencing sorption of PASs was protein in extracellular polymeric substances. The sorption of PASs onto sludge increased as solution pH decreased. For all the PASs homologues, enhanced adsorption occurred with increasing calcium concentration in solution. For PASs with C5-C15, sorption on sludge increases with increasing alkyl chain length, while for PASs with C2-C5, the association of sludge decreases when the alkyl chain length increases. The perfluorinated sulfonic acid (PFSA) demonstrated substantially stronger sorption than perfluorinated carboxylic acid (PFCA) analog. Evidence for both hydrophobic and electrostatic interactions was found.