Tetracycline desorption kinetics in municipal biosolids and poultry litter amendments determined by diffusive gradients in thin films (DGT)

Tetracycline (TET) is commonly used to treat bacterial diseases in humans and chickens (Gallus gallus domesticus), is largely excreted, and is often found at elevated concentrations in treated sewage sludge (biosolids) and poultry litter (excrement plus bedding materials). Land spreading of these materials is practiced worldwide to improve soil fertility, but the practice raises questions about whether TET could be released to the environment and cause adverse effects. Hazard risks largely depend on the concentration in the solid phase that can be released to the solution phase (labile TET), it's desorption rate constant, and diffusion rate of dissolved TET in amendments. In this study, these quantities were evaluated in biosolids and three types of litter amendments by combinations of equilibrium sorption-desorption isotherm and desorption kinetic studies using diffusive gradient in thin films (DGT) samplers. Results from isotherm experiments showed that TET partitioning was inhibited at the high dissolved organic carbon (DOC) concentrations in amendments (6-15% of dry mass). Despite low partition coefficients determined at high particle/DOC concentrations of amendments (K_d = 9-46 mL g^-1), results from DGT experiments revealed that TET release by desorption and diffusion would be slow and short-lived (<3 d) due to small effective diffusion coefficients (<8 × 10^-8 cm² s^-1) and low concentrations of labile TET in amendments (<5% of total TET). Despite this, evaluations of antibiotic uptake during microbial colonization and plant root interception of amendment surfaces are highly warranted.