Modeling and predicting competitive sorption of organic compounds in soil

Abstract

Binary systems consisting of 1,2-dichlorobenzene (12DCB) + competitor were investigated over a range of concentrations of competitor in three natural sorbents with distinct characteristics. Two models, the ideal adsorbed solution theory (IAST) and the potential theory (Polanyi-based multisolute model), widely used in the prediction of multisolute sorption equilibrium from single-solute data, were used to simulate competitive sorption in our systems. The goal was to determine which multisolute model best represented the experimentally obtained multisolute data in natural sorbents of varied properties. Results suggested that for the sorbents and sorbates studied, the IAST model provided much better results. On average, the IAST model provided lower errors (23%) than the potential model (45%). The effect of competitor structure on the degree of competition was also investigated to identify any relationships between competition and structure using molecular descriptors. The competitors chlorobenzene, naphthalene, 1,4-dichlorobenzene, 1,2,4-trichlorobenzene all showed very similar degrees of competition, while benzene, phenanthrene, and pyrene were the least effective competitors toward 12DCB across all sorbents. Different sorption sites or sorption mechanisms might be involved in the sorption of these molecules leading to a lack of competitive behavior. A significant relationship between competitor structure and the degree of competition was observed at environmentally relevant sorbed competitor concentrations for the soil containing the highest fraction of hard carbon (Forbes soil).

Figure 1

Multi-solute experimental data (1,2-dichlorobenzene + competitor), Ideal adsorbed solution theory (IAST) model and Potential model results for sorbates benzene (BZ), chlorobenzene (CB), naphthalene (NP), 1,4-dichlorobenzene (14DCB), 1,2,4-trichlorobenzene (TCB), 1,2,3,4-tetrachlorobenzene (TeCB), phenanthrene (PHN) and pyrene (PY) as competitors in Forbes soil.

Figure 2

Competitive effect shown for all sorbates in Forbes soil. Refer to Figure 1 legend for other definitions.