Modelling sorption and dissipation kinetics of ciprofloxacin and enrofloxacin antibiotics in New Zealand pastoral soils

Abstract

Fluoroquinolones are a class of widely used antibiotics for veterinary purposes and are known to have moderate to high persistence in soil and aquatic bodies, leading to their bioaccumulation in the environment. Studies on their environmental fate are absent for New Zealand soils, which often receive animal waste effluent including poultry litter. Laboratory studies were performed to evaluate the sorption and dissipation behavior of two fluoroquinolones (ciprofloxacin and enrofloxacin) in three New Zealand pastoral soils with and without poultry litter amendment at low and high levels (1 and 5%, w/w). Results of the batch sorption studies suggest that isotherms were best described by Freundlich model, Freundlich coefficients for the compounds varied from 312 to 62,163 ​g^1−N L^N kg⁻¹ in the soils, with cation exchange being the dominant sorption mechanism. Addition of poultry litter (5%, w/w) decreased the sorption affinity of fluoroquinolones to soils, which was mainly related to pH-induced changes in compound speciation. Results of incubation studies performed under aerobic conditions show that dissipation half-life was highly variable (15–378 days). A dehydrogenase assay performed as an indicative of microbial activity during incubation studies indicates that despite lower microbial activity, dissipation was faster in subsoils (30–40 ​cm), attributing to higher compound bioavailability due to lower sorption ability of the subsoils. Augmenting poultry litter (5%, w/w) increased dissipation, due to a combination of lower sorption and higher microbial activity. High sorption in New Zealand soils may reduce fluoroquinolone's toxicity to exposed terrestrial organisms. However, high persistence may represent a relevant antibiotic reservoir in a long term, posing risks to terrestrial and aquatic ecosystem health and thus warranting further elucidation.