Do initial concentration and activated sludge seasonality affect pharmaceutical biotransformation rate constants?

Area of Science:

• Environmental chemistry
• Environmental microbiology
• Environmental science

Background:

• Pharmaceuticals enter aquatic environments through wastewater treatment plants (WWTPs).
• Understanding pharmaceutical biotransformation in WWTPs is crucial for mitigating environmental risks, but mechanistic insights are limited.
• First-order biotransformation rate constants (k_b) are key parameters for predicting pharmaceutical fate.

Purpose of the Study:

• To investigate the relationship between pharmaceutical biotransformation rate constants (k_b) and initial compound concentrations.
• To assess the influence of sampling season (summer vs. winter) on k_b values.
• To evaluate the applicability of Michaelis-Menten kinetics for pharmaceutical biotransformation at environmentally relevant concentrations.

Main Methods:

• Activated sludge from WWTP Groesbeek was used in four-day bottle experiments.
• Nine pharmaceuticals were tested at two environmentally relevant concentrations (3 and 30 nM).
• Pharmaceutical concentrations were measured using LC-MS/MS; microbial communities were analyzed via 16S rRNA gene sequencing.

Main Results:

• Acetaminophen, metformin, metoprolol, terbutaline, and phenazone were biodegradable, while carbamazepine, diatrizoic acid, diclofenac, and fluoxetine were not.
• Biodegradable pharmaceuticals exhibited higher k_b values at increased initial concentrations (30 nM vs. 3 nM).
• Seasonal differences in activated sludge inocula led to minor variations in k_b, suggesting microbial activity, not just community composition, influences rates.

Conclusions:

• Pharmaceutical biotransformation rate constants are concentration-dependent, challenging the direct application of Michaelis-Menten kinetics at low environmental concentrations.
• Initial pharmaceutical concentration is a critical factor for accurately predicting biotransformation rates and environmental fate in WWTPs.
• While microbial activity is key, seasonal variations in WWTP inocula can subtly affect pharmaceutical biotransformation rates.