Environmental Adsorption/Desorption Behavior of Antimicrobial Contaminants and PFAS on Virgin and Weathered Microplastics

Microplastics (MPs), derived from the degradation of larger plastic debris, dominate aquatic plastic pollution, yet the knowledge of naturally weathered MPs is limited. MPs in the natural environment are exposed to several weathering processes, including ultraviolet (UV) radiation, mechanical abrasion, thermal oxidation, and biological degradation. These processes can change the surface morphology, chemical structure and surface charge properties of MPs. Due to their widespread occurrence in aquatic environments, MPs can co-occur with other pollutants and are recognized as carriers for a diverse range of contaminants through various interactions. Triclosan (TCS), sulfamethoxazole (SMX), and per- and polyfluoroalkyl substances (PFAS) are among the most frequently detected co-occurring contaminants. The main objective of this dissertation was to conduct comprehensive research on i) the characterization of a diverse set of MPs representing multiple polymer types, source and weathering state, ii) the adsorption/ desorption interactions between MPs and TCS, SMX and PFAS under environmentally relevant conditions. The second objective was examined under three different study plans. First, physicochemical properties of MPs were investigated through a set of experiments, including zeta potential, point of zero charge (pHpzc), water contact angle (hydrophobicity), glass transition temperature, specific surface area, elemental composition, and Fourier-transform infrared spectroscopy (FTIR) spectra, across a diverse set of MPs. The selected set of MPs includes 35 MPs representing several polymer types and multiple virgin or weathered variants within the same polymer group. The pHpzc of the MPs ranged from 3 to 5, and the surfaces were negatively charged at the experimental pH (5-9). Measured pHpzc values showed variability within the same polymer type, indicating that aging processes, surface oxidation, and manufacturing conditions, such as additives and stabilizers used, significantly influence MP surface characteristics. Most MPs exhibited hydrophobic characteristics, with contact angles around 90°, consistent with the non-polar nature of polymers such as polyethylene (PE), polypropylene (PP), and polystyrene (PS). Polyamide 66 (PA66) showed relatively low contact angles around 60°, indicating hydrophilic surfaces. The elemental composition of weathered MPs was consistent with the relevant polymer structure. The BET surface areas of the MPs were relatively low, ranging from 1 to 3 m2/g. FTIR analysis revealed surface functional groups, providing insight into possible interaction mechanisms with the contaminants. Second, adsorption/desorption of TCS and SMX by virgin and weathered MPs was investigated under different environmental conditions. Polyamide 6 (PA6) (virgin) MPs showed the highest uptake of TCS and SMX, attributed to their amide groups and low water contact angle, followed by PE MPs with high rubbery domains. The desorption behavior of antimicrobials showed an inverse relationship with adsorption: PA6 showed the highest uptake and desorbed only 1% of the TCS adsorbed, whereas the polyethylene terephthalate (PET) type of MPs showed varying adsorption uptake of around 20-100 µg/g and desorbed up to 62% of the adsorbed TCS. Natural organic matter (NOM) competed with the antimicrobials for available surface sites and decreased the adsorption of both antimicrobials. However, the presence of NOM in the desorption solution did not significantly affect the desorption of TCS and SMX from the surface. Increasing the MP dose or solution pH decreased both adsorption and desorption. Desorption of TCS from MPs was relatively low compared to adsorption, ranging from 1 to 32 µg/g across all MPs. Additionally, there was an inverse relation between adsorption and desorption. These results suggest that virgin and weathered MPs exhibit distinct adsorption and desorption behaviors, driven by variations in surface characteristics and the type and degree of weathering. Third, PFAS structure and MPs polymer characteristics determined the adsorption and desorption mechanisms. For seven of the PFAS tested, PA6 MPs showed the highest adsorption (77-104 µg/g), followed by PA66 MPs, and both of the polymer types showed the lowest desorption percentages, demonstrating the contribution of hydrophobic interactions and possible weak hydrogen-bonding mechanisms. PET, PE, and poly(methyl methacrylate) (PMMA) exhibited moderate adsorption (< 60 µg/g), while acrylonitrile butadiene styrene (ABS), PS, PP, and polylactic acid (PLA) consistently showed low uptake (< 15 µg/g) and higher desorption percentages. A difference was observed between the virgin and the weathered form with the same polymer type, possibly due to variation in plastic production processes and weathering processes, which alter the surface properties of MPs and consequently affect PFAS interactions. Long-chain and sulfonate PFAS exhibited stronger adsorption compared to short-chain and carboxylate PFAS, indicating the important contribution of hydrophobic interaction and van der Waals forces to MPs -PFAS interactions. Lastly, a pH increase negatively affected adsorption uptake levels while promoting desorption percentages, and similarly, NOM suppressed the PFAS adsorption while increasing the desorption percentages. Overall, the findings illustrated that complex interactions between PFAS and MPs, and their dependence on environmental factors, PFAS structure, and polymer surface characteristics, are crucial for understanding their environmental fate. Fourth, the effects of ion composition and strength on PFAS adsorption onto MPs were investigated. Specifically, monovalent (Na+, K+ ) and divalent (Ca2+, Mg2+) cations, as well as common anions (Cl-, NO3-, SO42-, HCO3-), were evaluated over a wide concentration range (0.5, 2, 6, 10 and 100 meq/L) and NOM presence (5 mg C/L). Perfluorooctanoic acid (PFOA) adsorption onto virgin PA6 was around 101 ug/g, decreasing to 94-97 ug/g with increasing concentrations of calcium chloride (CaCl2), magnesium chloride (MgCl2), sodium chloride (NaCl), potassium chloride (KCl) or calcium sulfate (CaSO4). The most adverse effects of ion composition were obtained when HCO3- and NO3- anions were present in the background. Similarly, HCO3- and NO3- anions caused a significant decrease (60–70%) in PFOA adsorption onto PA66-1. The effect of calcium nitrate (Ca(NO3)2), potassium nitrate (KNO3), and sodium bicarbonate (NaHCO3) on PFOA adsorption on virgin PET and weathered PET-2 showed different trends, indicating that weathering can alter the dominant adsorption mechanism and the ion effect. Adsorption affinity of MPs followed the order of long-chain> short-chain and sulfonic head > carboxylic head. There was no effect of CaCl2, NaCl, Ca(NO3)2, and NaHCO3 at ion concentrations of 0, 0.5, and 10 meq/L for perfluorooctanesulfonic acid (PFOS) adsorption onto PA6. Similar to PFOA, PFOS adsorption onto PA66-1 was negatively affected by ion compositions and concentrations. On the other hand, increasing the CaCl2 and NaCl concentrations in the solution increased PFOS adsorption onto PET- and PE-type microplastics regardless of weathering state. Adsorption of perfluorobutanesulfonic acid (PFBS) and perfluorobutanoic acid (PFBA) on PA6 and PA66-1 exhibited similar behavior to that observed for PFOA and PFOS. PFBS adsorption decreased from 90 µg/g to around 60 µg/g with CaCl2and NaCl, while CaNO3 and NaHCO3 decreased to adsorption uptakes of 19 µg/g and 30 µg/g, respectively. PFBA showed the most severe ion sensitivity even at the lowest concentrations (0.5 meq/L), adsorption uptake declined from ~ 48 µg/g to ~10-20 ug/g, and with further increase of ion concentrations to 10 meq/L, uptake was reduced to ~ 3 to 5 ug/g, reaching a 90% decrease in adsorption. Results showed that NOM in the background further suppressed PFAS adsorption, with the impact being polymer-dependent. Overall, the observed variability in physicochemical properties demonstrated that MPs cannot be treated as uniform materials based solely on polymer type. Instead, surface heterogeneity induced by environmental weathering played a critical role in determining interaction mechanisms. Environmental parameters, such as pH, NOM, and ionic strength, as well as the contaminants' chemical characteristics, strongly influenced the adsorption and desorption by MPs.