Insight into the distribution of pharmaceuticals in soil-water-plant systems

Pharmaceuticals in agricultural soils originating from irrigation with treated wastewater and land-applied biosolids can enter field crops. However, little is known about the role of pore water in plant uptake of pharmaceuticals from soils. In this study, the fate, uptake and distribution of fifteen commonly used pharmaceuticals in soil-water-radish systems were investigated to examine the relationship between the accumulation and their physicochemical processes in soils. The results indicate that the distribution of pharmaceuticals between soil and pore water, as well as their biodegradation, combined to govern the bioavailability of pharmaceuticals to plant uptake. Fourteen out of 15 pharmaceuticals could enter radish tissues in which the accumulation ranged from 2.1 to 14080 ng/g. Comparison of bioconcentration factors (BCFs) on the basis of pharmaceutical concentration in bulk soil vs. in pore water implies that pharmaceuticals present in soil pore water are the major bioavailable fractions to plant uptake. The pore water-based BCFs exhibited a positive linear relationship with log D_ow for the pharmaceuticals with >90% as neutral species in soil pore water, while such relationship was not observed between bulk soil-based BCFs and log D_ow mainly due to sorption by soil. Other than hydrophobicity, the dissociation of ionizable pharmaceuticals in the soil pore water and (or) root cells may lead to the "ion-trap" effects and thus influence the uptake and translocation process. The large molecular-size pharmaceuticals (e.g., tylosin) manifested a minimum uptake due plausibly to the limited permeability of cell membranes.