Graduate Thesis Or Dissertation
 

Eco-corona formation and impact on environmentally relevant micro- and nanoplastics

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https://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/d791sq67n

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  • Microplastics (MPs) are plastic particles less than 5 mm in diameter. These contaminants are increasingly found in the environment due to increased plastic production, use, and improper disposal. Detecting and measuring these contaminants in the environment has proven difficult, requiring the use of fate and transport models to assess MP risk. However, current research that informs these models often uses pristine, monodisperse microspheres in simplified media not representative of environmental samples. Further, environmental transformations such as eco-corona formation and UV photooxidation of polymers have been shown to directly impact the fate and transport of MPs. The objective of this research was to 1) generate microplastics fragments representative of secondary microplastics found in the environment, 2) identify the extent that protein-like and humic substance-like compounds form eco-coronas on aged and unaged polymer surfaces, and 3) investigate the impact of eco-coronas on the adsorption of copper and zinc on polystyrene (PS) and polyvinyl chloride (PVC) microplastics. To accomplish these objectives, cryomilling was utilized to generate heterodisperse micro- and nanosized fragments characteristic of environmental MPs. This technique resulted in a reproducible and reliable method to generate MPs used for subsequent experiments. In a second phase, organic matter interactions with unaged and photooxidized PS and PVC surfaces were characterized using Quartz Crystal Microbalance with Dissipation (QCM-D). Finally, batch adsorption experiments were utilized to investigate the effect of polymer type, photooxidation, and organic matter character on the adsorption of heavy metals to PS and PVC MPs. Organic matter character and photooxidation of polymer surfaces were found to impact the extent of eco-corona formation on PS and PVC surfaces. Photochemical weathering decreased the hydrophobicity of the polymers studied. Photooxidation increased the zeta potential of PS but decreased the zeta potential of PVC. Electrostatic interactions and hydrogen bonding were found to dominate bovine serum albumin (BSA) adsorption while hydrophobic interactions dictated humic acid (HA) adsorption to polymer surfaces. It was found that the presence of both BSA and HA impeded adsorption of organic matter to all polymer surfaces investigated. Polymer chemistry, photooxidation, and organic matter character also impacted the adsorption of zinc and copper onto MPs. UV aging and polymer polarity were positively correlated with the adsorption of heavy metals. Zinc adsorption was enhanced by the presence of BSA and hindered by the presence of HA. Zinc is then expected to be more particulate bound in protein rich waters similar to wastewater. Additionally, copper was more likely to adsorb to BSA than bare and BSA coated particles. As with zinc, HA impeded copper adsorption onto PVC. However, HA enhanced copper adsorption onto PS compared to bare plastics. As a result, it is expected that PS bound copper is more likely in environments similar to river water. Organic matter mixtures similar to those found in wastewater are expected to inhibit the co-transport of copper by PS and PVC MPs. The conclusions of this research identify important factors involved in MP-organic matter interactions. First, environmental MPs have distinct chemical and physical characteristics that are different from the pristine, uniform microspheres commonly used. These characteristics, such as wettability, polarity, and surface charge have been shown to influence the formation of eco-coronas and the adsorption of heavy metals. To better investigate MP behaviors, it is recommended that photochemically weathered particles be used in future experiments. Additionally, corona formation was impacted by the presence of multiple model organic matter compounds. Interactions between the organic compounds decreased the extent of corona formation. While the character of eco-coronas formed also influenced the extent of zinc and copper adsorption, polymer chemistry played a larger role. Here, the polymer-organic matter affinity affected eco-corona formation and also influenced the complexation of metals to aqueous phase organic compounds. Metals are then less likely to be MP bound when organic matter-metal complexation is a more favorable interaction. Additionally, unfavorable organic matter-MP interactions often furthered complexation between metals and aqueous organic matter. Ultimately, this work shows the importance of moving MP research towards environmental relevance through the use of transformed particles and complex aquatic media.
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