Azithromycin sorption and biodegradation in a simulated California river system

Area of Science:

• Environmental Chemistry
• Microbiology
• Ecotoxicology

Background:

• Azithromycin (AZ) is a widely used macrolide antibiotic frequently detected in natural waterways via sewage effluent.
• AZ is an emerging contaminant of concern, yet its environmental fate and transport in aquatic systems remain poorly understood.

Purpose of the Study:

• To investigate the degradation and sorption of azithromycin (AZ) in American River soils and water under simulated California watershed conditions (aerobic and anaerobic).
• To quantify AZ's degradation rate, persistence, and sorption behavior to predict its fate and transport in aquatic environments.

Main Methods:

• Microcosm experiments simulating flooded (anaerobic) and non-flooded (aerobic) conditions.
• Calculation of degradation rate constants (k), DT50 values, and sorption coefficients (log Kd, log Kf, log Koc).
• Microbial growth assessment via culturing despite autoclaved soil and antibiotic presence.

Main Results:

• Under aerobic conditions, AZ showed a slow degradation rate (k=0.0084 day⁻¹) and long DT50 (82.52 days), with less than 1% parent compound degradation over 150 days.
• High sorption was indicated by calculated coefficients (log Kd=2.18, log Kf=1.90, log Koc=4.25 Lkg⁻¹), suggesting AZ recalcitrance.
• Microbial growth occurred even in autoclaved soil with antibiotic present, indicating potential for microbial adaptation or introduction.

Conclusions:

• Azithromycin (AZ) is expected to exhibit very limited degradation in aquatic systems due to strong sorption to soil.
• AZ's transport will be influenced by soil-water saturation levels and sediment movement.
• Further research into microbial interactions and adaptation is warranted given observed microbial growth.