In Situ Antibiotic Accumulation on Microplastics in Wastewater Treatment Plants: Roles of Biofilm Colonization, Water Chemistry, and Substrate Aging

Abstract

Microplastics (MPs) and antibiotics co-occur in wastewater treatment plants (WWTPs), yet pristine-polymer adsorption does not adequately capture accumulation on field-aged MPs. We performed 3- and 12-month in situ deployment of polyamide (PA), polyethylene (PE), polyhydroxyalkanoate (PHA), and polycaprolactone (PCL) in WWTP oxidation and disinfection ponds along with sulfamethoxazole (SMX) and tetracycline (TC) adsorption experiments. Surface antibiotic loadings increased from 6.91–34.18 ng kg–1 at 3 months to 1.27 × 104–2.87 × 105 ng kg–1 at 12 months, while surface-associated antibiotic profiles showed limited overlap with contemporaneous water-phase profiles. PHA had the highest 12-month burdens in both ponds, whereas the variable behavior of PCL showed that biodegradability alone did not explain accumulation. On 3-month biofilm-colonized MPs, adsorption capacities were 0.06–0.27 mg g–1 for SMX and 0.01–2.07 mg g–1 for TC, and across polymers, higher biofilm biomass did not translate into higher adsorption. Isotherm and two-dimensional correlation spectroscopy analyses resolved binding modes: SMX retention involved partitioning-dominated weak interactions, whereas TC bound interface, specifically via ester, carbonyl, and amide groups, on polymer and biofilm domains. TC adsorption was strongly associated with O/C ratio (ρ = 0.940, p < 0.01) and SMX with polysaccharide content (ρ = 0.727, p < 0.01), whereas pH and representative ions modulated these compound-specific adsorption pathways through distinct interfacial mechanisms. After biofilm removal, TC adsorption on aged MPs was associated more closely with roughness and wettability than with BET surface area or O/C ratio. These findings support a dynamic-interface framework in which biofilm colonization, water chemistry, and substrate aging collectively influence adsorption on field-aged MPs.