Polycyclic aromatic hydrocarbons sorption on soils: some anomalous isotherms

The sorption of four polycyclic aromatic hydrocarbons (PAHs), namely acenaphthene (Ac), phenanthrene (Ph), anthracene (An), and fluoranthene (Fl), on soil has been investigated. The kinetics of the sorption is characterised by the presence of two distinct periods. A fast initial stage followed by a second slower sorption process. Various kinetic models (i.e., Elvoich, Lagergren, second order and double exponential models) have been used to fit experimental data. The sorption equilibrium of individual PAHs has been assessed in the 298-333 K temperature range. Unlike Ac, Ph at 333 K and An and Fl at any temperature showed anomalous isotherms. The reason seems to rely on the "trapping" of dissolved PAHs by soil organic matter (SOM) released to water. This abnormal trend was not experienced when the isotherms were obtained for four PAHs mixture. Apparently, the most soluble Ac was capable of binding all the released material so no effect was thereafter observed.