Transport of two naphthoic acids and salicylic acid in soil: Experimental study and empirical modeling

In contrast to the parent compounds, the mechanisms responsible for the transport of natural metabolites of polycyclic aromatic hydrocarbons (PAH) in contaminated soils have been scarcely investigated. In this study, the sorption of three aromatic acids (1-naphthoic acid (NA), 1-hydroxy-2-naphthoic acid (HNA) and salicylic acid (SA)) was examined on soil, in a batch equilibrium single-system, with varying pH and acid concentrations. Continuous flow experiments were also carried out under steady-state water flow. The adsorption behavior of naphthoic and benzoic acids was affected by ligand functionality and molecular structure. All modeling options (equilibrium, chemical nonequilibrium, i.e. chemical kinetics, physical nonequilibrium, i.e. surface sites in the immobile water fraction, and both chemical and physical nonequilibrium) were tested in order to describe the breakthrough behavior of organic compounds in homogeneously packed soil columns. Tracer experiments showed a small fractionation of flow into mobile and immobile compartments, and the related hydrodynamic parameters were used for the modeling of reactive transport. In all cases, the isotherm parameters obtained from column tests differed from those derived from the batch experiments. The best accurate modeling was obtained considering nonequilibrium for the three organic compounds. Both chemical and physical nonequilibrium led to appropriate modeling for HNA and NA, while chemical nonequilibrium was the sole option for SA. SA sorption occurs mainly in mobile water and results from the concomitancy of instantaneous and kinetically limited sites. For all organic compounds, retention is contact condition dependent and differs between batch and column experiments. Such results show that preponderant mechanisms are solute dependent and kinetically limited, which has important implications for the fate and transport of carboxylated aromatic compounds in contaminated soils.

Experimental and calculated breakthrough curves of tracer Br− (a–b), HNA and NA (c–d–e–f) in soil-packed columns. Data: points and models: lines. Inflow concentration of NA or HNA = 0.25 mM; pH 6.6 ± 0.1.Figure optionsDownload high-quality image (176 K)Download as PowerPoint slideHighlights
► The transport is solute dependent and kinetically limited.
► The isolation of a fraction of sites in immobile water may affect solute transport.
► Kinetic limitations seemed to impact more significantly HNA mobility.
► The impact of flow fractionation seemed more pronounced for SA.
► Accurate modeling obtained by optimizing both physical and chemical kinetics.