This work demonstrates a new model to determine which mechanism(s) - adsorption or hydrolysis - is (are) responsible for the removal of oxytetracycline (OTC) from water in the presence of biomass-based activated carbons (ACs). OTC removal using potassium hydroxide-activated biochars follows pseudo-second-order kinetics, initially dominated by chemisorption. Overtime, hydrolysis increasingly dominates. Conversely, most CO2-ACs display weaker chemisorption, with hydrolysis dominating OTC disappearance. Chars prepared using higher temperature CO2 activation show that both hydrolysis and weak chemisorption are important in OTC removal. A computational model describes these concurrent pseudo-first- and second-order processes, rather than oft-employed sequential models.