MODELING OF COMPETITIVE SORPTION AND TRANSPORT OF CETIRIZINE AND DIPHENHYDRAMINE IN NATURAL SOILS

We investigated the geochemical behavior of cetirizine (CET) and diphenhydramine (DPH) in natural soils and determined their reactivity with major soil components, including clay minerals and organic matter. These compounds are common antihistamines used to treat allergies and common cold symptoms and are increasingly found in natural environments. CET is a racemic compound with a water solubility of 1.0 g/L, and pKa of 7.6. DPH is a polar organic molecule with a reactive amine functional group and two benzene rings, aqueous solubility of 3.1 g/L, and pKa of 9.0.

The main objective of this study was to model the data collected for sorption and transport behavior of CET and DPH in natural soils. To model the transport data, equilibrium convection-dispersion-reactive transport model was used. Sorption data was modeled using nonlinear model (Freundlich isotherm). Two types of soils - an organic-rich A-horizon (clay-mineral content ~6%, organic matter 6 - 8%), and a clay-mineral-rich B-horizon (clay-mineral content ~20%, organic matter 1.7%) - were used to perform batch and column transport experiments. Soils utilized for this research were collected from undisturbed areas in Francis Marion National Forest, Charleston, SC. CET and DPH in effluent solutions were analyzed using a high-performance liquid chromatography (HPLC) equipped with an UV detector.

Batch sorption studies showed nonlinear sorption relationship in both organic and clay-rich soils, but also that both compounds sorbed stronger in organic and clay-rich soils. This behavior is attributed to the ionic bonding between the positively-charged amine functional groups (at pH<9.0) in CET and DPH and the negative charges of clay mineral surfaces in soil. Breakthrough curves obtained from glass chromatography experiments further confirmed this behavior by strongly retaining CET and DPH in both soils. In addition, these two chemicals displayed competitive sorption behavior when present in the same soil-water solution. Batch sorption studies indicated lower sorption in both A- and B-horizons compared with sorption without competition. This behavior was verified by glass chromatography experiments. The results from this study have strong implications for future environmental management of pharmaceutical chemical effluents and their disposal.