Desorption kinetics of ciprofloxacin in municipal biosolids determined by diffusion gradient in thin films

Ciprofloxacin (CIP) is a commonly-prescribed antibiotic that is largely excreted by the body, and is often found at elevated concentrations in treated sewage sludge (biosolids) at municipal wastewater treatment plants. When biosolids are applied to soils, they could release CIP to surface runoff, which could adversely affect growth of aquatic organisms that inhabit receiving water bodies. The hazard risk largely depends on the amount of antibiotic in the solid phase that can be released to solution (labile CIP), its diffusion coefficient, and sorption/desorption exchange rates in biosolids particles. In this study, these processes were evaluated in a Class A Exceptional Quality Biosolids using a diffusion gradient in thin films (DGT) sampler that continuously removed CIP from solution, which induced desorption and diffusion in biosolids. Mass accumulation of antibiotic in the sampler over time was fit by a diffusion transport and exchange model available in the software tool 2D-DIFS to derive the distribution coefficient of labile CIP (K_dl) and sorption/desorption rate constants in the biosolids. The K_dl was 13 mL g⁻¹, which equated to 16% of total CIP in the labile pool. Although the proportion of labile CIP was considerable, release rates to solution were constrained by slow desorption kinetics (desorption rate constant = 4 × 10⁻⁶ s⁻¹) and diffusion rate (effective diffusion coefficient = 6 × 10⁻⁹ cm² s⁻¹. Studies are needed to investigate how changes in temperature, water content, pH and other physical and chemical characteristics can influence antibiotic release kinetics and availability and mobility in biosolid-amended soils.