GEOCHEMICAL FATE AND TRANSPORT OF DIPHENHYDRAMINE IN NATURAL SOILS

Common pharmaceuticals like diphenhydramine have been detected in natural streams and groundwater as a result of sewage overflows, runoff, or treated wastewater discharge. In this study, we investigated geochemical behavior of diphenhydramine in natural soils and determined the reactivity of main soil components, including clay minerals and organic matter. Diphenhydramine is an antihistamine used to treat allergies and common cold symptoms, reduce cough, induce sleep, and treat motion sickness. It 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 determine sorption and transport behavior of diphenhydramine in natural soils. Two types of soils - an organic-rich A-horizon (organic matter 6-8%), and a clay-mineral-rich B-horizon (clay mineral content ~20%) - were used to perform batch sorption isotherm and column transport experiments. Diphenhydramine was analyzed using a high performance liquid chromatograph (HPLC) and an liquid chromatography mass spectrometer (LC-MS).

Batch sorption studies were conducted to determine the adsorption isotherm of diphenhydramine onto natural soils. These experiments showed nonlinear sorption relationship in both organic-rich and clay-rich soils and also that diphenhydramine sorbs stronger to clay-rich soils than to organic-rich soils. This could be due to the ionic bonding between the positively-charged amine functional groups (at pH<9.0) in diphenhydramine and the negative charges of clay mineral surfaces in soil. Breakthrough curves obtained from glass chromatography experiments further confirmed this behavior by strongly retaining diphenhydramine in both soils. The results imply that soils can be natural filter agents to remove complex chemical compounds like diphenhydramine.