Sorption-desorption equilibrium and diffusion of tetracycline in poultry litter and municipal biosolids soil amendments

Tetracycline (TET) is commonly used to treat bacterial diseases in humans and chickens (Gallus gallus domesticus), is largely excreted, and is found at elevated concentrations in treated sewage sludge (biosolids) and poultry litter (excrement plus bedding materials). Routine application of these nutrient-and carbon-enriched materials to soils improves fertility and other characteristics, but the presence of antibiotics (and other pharmaceuticals) in amendments raises questions about potential adverse effects on biota and development of antibiotic resistance in the environment. Hazard risks are largely dictated by sorption-desorption and diffusion behavior in amendments, so these processes were evaluated from sorption-desorption equilibrium isotherm and diffusion cell experiments with four types amendments (biosolids, poultry manure, wood chip litter, and rice hull litter) at three temperatures (8 °C, 20 °C and 32 °C). Linear sorption-desorption equilibrium distribution constants (Kd) in native amendments ranged between 124–2418 L kg⁻¹. TET sorption was significantly increased after treatment with alum, and there was a strong exponential relationship between Kd and the concentration of bound Al³⁺ in amendments (R² = 0.94), which indicated that amendments contained functional groups capable of chelating Al³⁺ and forming metal bridges with TET. Effective diffusion coefficients of TET in amendments ranged between 0.1 and 5.2 × 10⁻⁶ cm² s⁻¹, which were positively related to temperature and inversely related to Kd by a multiple regression model (R² = 0.86). Treatment of organic amendments with alum greatly increased Kd, would decrease D_s, and so would greatly reduce hazard risks of applying these organic amendments with this antibiotic to soils.