Characterizing the Interaction between Antibiotics and Humic Acid by Fluorescence Quenching Method

Area of Science:

• Environmental Chemistry
• Analytical Chemistry
• Biochemistry

Background:

• Humic acid (HA) is a complex organic matter found in soil and aquatic environments.
• Antibiotics, such as oxytetracycline (OTC) and sulfadiazine (SD), are frequently detected in environmental matrices.
• Understanding the interaction between HA and antibiotics is crucial for assessing their environmental fate and bioavailability.

Purpose of the Study:

• To elucidate the interaction mechanism between humic acid (HA) and antibiotics oxytetracycline (OTC) and sulfadiazine (SD).
• To investigate the influence of antibiotic concentration, temperature, and pH on the binding strength between HA and antibiotics.
• To compare the binding affinity and stability of the complexes formed between HA and OTC versus HA and SD.

Main Methods:

• Utilized Three-Dimensional Excitation Emission Matrix Fluorescence Spectrophotometry to analyze HA-antibiotic interactions.
• Systematically varied antibiotic concentrations, temperature (298 K to 313 K), and pH conditions.
• Quantified quenching constants (K) and binding constants (Kb) to characterize the interaction.

Main Results:

• Observed a static quenching mechanism for both OTC and SD binding to HA, indicating complex formation.
• Identified hydrogen bonds and van der Waals forces as the primary interaction forces.
• Found that HA exhibited a stronger quenching effect and higher binding affinity for OTC compared to SD (K_OTC > K_SD, Kb_OTC > Kb_SD).
• Determined that lower temperatures and neutral to alkaline pH conditions favored stronger HA-antibiotic interactions and complex stability.
• Quantified binding constants: K for HA-OTC ranged from 3.223 × 10³ to 12.784 × 10³ L·mol⁻¹, and K for HA-SD ranged from 2.051 × 10³ to 5.533 L·mol⁻¹.

Conclusions:

• The interaction between HA and OTC is more pronounced and results in a more stable complex than with SD.
• Environmental factors such as temperature and pH significantly influence the binding dynamics and stability of HA-antibiotic complexes.
• These findings provide critical insights into the environmental behavior and potential ecological risks of antibiotic contaminants in the presence of natural organic matter.