Microplastics are being used in the cosmetics and clothing industries at increasingly great rates, and as microplastics use increases, the overall accumulation of microplastics in aqueous systems also increases. The effects of microplastics alone on aqueous systems are well documented; however, aqueous systems are too complex to justifiably ignore interactions between microplastics and other types of pollutants. Two of these pollutants of interest are amoxicillin (AMX) and ciprofloxacin (CPX). Both AMX and CPX are listed as two of the top 150 most prescribed antibiotics in the United States and are consistently observed in surface water pollution studies, with concentrations reaching up to 120 μg/L. To understand the impacts that these antibiotics may have on the physicochemical properties of microplastics, which ultimately could impact their behavior in aqueous environments, we used dynamic light scattering, laser doppler microelectrophoresis, and ultraviolet-visible spectroscopy to study the effects of AMX and CPX in simplified salt based aqueous solutions at a pH of 7.4. From DLS and zeta potential measurements we investigated the impact of AMX and CPX on the size, charge, and aggregation rates of microplastics in various salt solutions. From UV-vis spectroscopy, we analyzed drops in absorbance values over time due to settling out of the microplastics in various antibiotic and salt combinations. At pH 7.4, AMX and CPX both exist in a zwitterionic state. This electrostatic similarity in our choice of antibiotics, along with studying both positively- and negatively-charged microplastics allows us to closely observe the role electrostatics play in microplastic-antibiotic interactions. Preliminary results suggest both antibiotics have the potential to stabilize the microplastic suspension, but at different rates, suggesting electrostatic interactions are not the sole driving force in these interactions. These results, along with the impacts of the two antibiotics on size and charge of the particles, will be discussed.

Role of electrostatics in interactions between positively and negatively charged microplastics and zwitterionic antibiotics

ACS Spring 2022

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