Reactivity and environmental fate of emerging contaminants in wastewater treatment systems: A reactive continuum framework approach

Emerging contaminants (ECs), including pharmaceuticals (e.g., antibiotics) and personal care products (PPCPs), hormones, artificial sweeteners, per- and polyfluoroalkyl substances (PFAS), microplastics (MPs), and antibiotic resistance genes (ARGs), are increasingly recognized as critical pollutants in wastewater treatment systems due to their persistence, bioaccumulation potential, and toxicity. Despite advances in wastewater treatment plants (WWTPs), the removal of ECs remains limited due to compound-specific behaviour, whereby contaminants undergo adsorption onto sludge solids, partial transformation into intermediates, and phase transfer to biosolids or colloids. The coexistence of ECs in complex wastewater matrices induces competitive interactions, matrix effects, and non-linear transformation pathways, reducing predictability and treatment efficiency. In this context, this review aims to provide a mechanistic understanding of EC fate and transformation in WWTPs, linking molecular properties to variability in removal across treatment configurations. It examines transformation pathways across treatment stages, integrates advances in detection with physicochemical and biological mechanisms, and highlights limitations of conventional WWTPs in achieving complete mineralization. It further proposes a reactive continuum framework (RCF) to classify contaminants by reactivity and transformation potential, thereby improving the prediction of treatment outcomes. The implications for bioaccumulation, biotransformation, and biomagnification are assessed to link contaminant behaviour with environmental risk. The RCF provides a falsifiable, molecular descriptor-based basis for predicting EC fate that removal-efficiency metrics cannot, with direct implications for reactivity-informed regulatory design under the EU Urban Wastewater Treatment Directive and equivalent frameworks globally.