Sorption of PAHs and PAH derivatives in peat soil is affected by prehydration status: the role of SOM and sorbate properties

Abstract

Abstract Purpose Sorption of organic compounds to soil largely determines their mobility and bioavailability in ecosystems. It is well known that molecular properties of the organic sorbates affect the sorbed amounts and sorption kinetics. But also changes in the hydration status of soils alter the physicochemical and sorptive properties of soil organic matter (SOM), which is the main sorbent for organic compounds in soils. This study elucidates the effects of varying SOM prehydration status on sorption of PAHs and PAH derivatives in peat soil. Materials and methods For sorption experiments, topsoil samples of a peat soil with 51% SOM were adjusted to water contents of 15, 30, and 50% (w/w based on dry soil mass) and conditioned for different water contact times of up to 2 years at one constant water content. Sorption kinetics and isotherms of naphthalene, three naphthalene derivatives, phenanthrene, and pyrene to these samples were investigated in batch experiments. Effects of the sorbates’ properties and SOM hydration on sorption were analyzed. Results and discussion Sorption to the peat soil was nonlinear and varied among sorbates and differently prehydrated soil samples. Sorbate polarizability, molecular volume, and weight increased the sorbed amount. Sorption kinetics were two-phased with a fast and a slow sorbing fraction. Hydroxyl groups in sorbates acting as H-donor or acceptor led to an increase of the slow sorbing fraction. Increasing total water contents, amounts of non-freezable water in SOM, increasing pore sizes, and decreasing hydrophobicity of SOM decreased the total sorbed amount and the fast sorbing fraction while increasing the slow sorbing fraction. The latter effects increased with increasing polarity and dipole moment of the sorbate. The SOM matrix rigidity varied with prehydration status; higher SOM matrix rigidity led to non-ideal sorption processes, namely, higher sorption nonlinearity and slower sorption. Conclusions The study revealed the effects and interplay of SOM prehydration status and molecular properties of the sorbates on sorption of PAHs and PAH derivatives. As sorbed amounts may decrease and become non-ideal upon aging at the presence of water, estimations of the mobility of organic compounds in the environment need to consider SOM prehydration status in high organic soils.