Speciation and Persistence of Tetracycline Antibiotics in the Aquatic Environment: Characterization in terms of a Linear Rate Model

Abstract

The present work aimed at studying the degradation of tetracycline antibiotics in the aquatic environment with a view of arriving at a linear rate model taking into account microbial, photolytic and hydrolytic degradation, as well as adsorption/desorption equilibria. The degradation of the antibiotics was monitored both in river water and sediment of the aquatic microcosm experiments, as well as in control experiments consisting of distilled water over a period of 90 days. Ultrasonic assisted dispersive solid phase extraction was used to extract the antibiotics from water and sediment samples. High performance liquid chromatography coupled to a variable ultra violet detector was used to determine the changes in concentration of antibiotics over the period of 90 days. An initial loss of up to 35% at most, due to adsorption by the sediment was observed in the microcosm experiments soon after charging. Triphasic linear rates attributed to microbial degradation of free and sediment or colloidal particle 1 and 2 adsorbed antibiotic for both water phase and sediment phase of the aquatic microcosm experiments were observed for oxytetracycline, chlortetracycline and tetracycline, while biphasic kinetics attributed to degradation of free, colloidal or sediment particle bound antibiotic were observed for doxycycline. The initial rates of degradation ranged from 1.35 - 3.07 x 10-2 μg/g/day (water phase), and 7.90 x 10-3 to 4.79 x 10-2 μg/g/day (sediment phase). Oxytetracycline exhibited the highest rate of degradation while that for tetracycline was the least. The covered distilled water control experiments for all the antibiotics showed a biphasic degradation pattern attributed to hydrolysis ranging from 2 x 10-6 to 5 x 10-4μg/g/day and microbial degradation ranging from 1.8 - 2.7 x 10-3 μg/g/day. In the distilled water exposed to natural light experiments, monophasic degradation (6.9 x 10-3μg/g/day) was observed for oxytetracycline, while biphasic degradation was observed for the other antibiotics. Addition of nitrates increased slightly the initial degradation rates for the light exposed distilled water experiments while the rates increased significantly in the microcosm experiments. The slight increase observed in the control experiments consisting of distilled water is attributed to photosensitization by the nitrate ions, which form hydroxyl radicals that further degrade the antibiotics while the huge increase in the microcosm experiments is attributed to increased population of microorganisms, due to availability of nutrients (added nitrates). The addition of nitrates did not affect the subsequent slow degradation rates. This is because the degradation rates depend on the rate of desorption of the antibiotic from colloidal and sediment particle surfaces. A kinetic model taking into account hydrolysis, photolysis, microbial degradation, as well as adsorption/ desorption equilibrium is presented to explain the observed zero order kinetics in the present study.