Transient transport of surfactant in a calcaeous and clayey sand

Abstract

The efficiency of in-situ aquifer washing processes using surfactants and the fate and transport of these active agents in the environment is largely determined by their retention in soil and groundwater. The main heterogeneous reactions leading to the retention of anionic and nonionic surfactants are (1) adsorption at the mineral-aqueous solution interface, (2) adsorption at the soil organic matter-aqueous solution interface, and (3) partition of surfactants between the non-aqueous phase liquids and the aqueous phases. Moreover, anionic surfactants may precipitate with cations present in solution. These interactions are affected by different factors: in particular solution acidity and nature of the predominant cations affect both precipitation and cation exchange equilibria on clays. These factors can be controlled in some cases. Surfactant hydrophobicity determines the main mechanism that controls the retention in porous media: either precipitation (hydrophilic anionic surfactant) or adsorption (hydrophobic anionic and nonionic surfactant). A modelling approach to describe surfactant behaviour is proposed. Retention model may easily be integrated in a general multiphase transport computer code and be used to optimize a soil washing process in terms of surfactant loss.