Adsorption, Bioavailability and Microbial Toxicity of Diclofenac in Agricultural Soil

Persistence and environmental implication of pharmaceuticals in agricultural soil is determined depending on adsorption, bioavailability and toxicity. This study aims to assess adsorption/partitioning behaviour of diclofenac (DCF) and its impact on microbial activity in four agricultural soils, differing in pH, organic carbon content, and cation exchange capacity. Results from batch studies suggests that soil/water partition coefficients of DCF are essentially nonlinear, i.e. depends on drug amount (p = 0.001), and positively correlated with soil organic carbon (p = 0.008). The adsorption data can effectively be modelled using Freundlich isotherm (regression coefficients between 0.84 and 0.90). In soil incubation studies, DCF could not be detected after 6 days of spiking (20 µg/g) in all soil types, including abiotic control. This suggests an interplay of combined biotic/abiotic process in DCF removal. Though microbial activity (based on tetrazolium reduction) declined with incubation time, but was not correlated with DCF exposure, particularly in soils rich in organic carbon.