Date of Award
Master of Science
Plastics, due to their extensive production and inert nature, accumulate in the environment after their improper disposal. While being in the environment, these plastic-based products undergo different degradation processes resulting in smaller sized microplastics (MPs) and nanoscale plastics (NPs). MPs and NPs can adsorb various organic and inorganic contaminants due to their huge surface area to volume ratio and the presence of diverse functional groups. Hence, the presence of various contaminants in the environment can impact both the plastics and the adsorbed contaminants fate in the environment. In light of this, the current work investigates the co-transport behavior of polystyrene (PS) MPs and copper metal (Cu), as both PS MPs and Cu are commonly encountered in the environment. The co-transport behavior was observed by understanding the mobility and deposition behavior of both contaminants under various ionic strength conditions (IS) and salt types (NaCl, CaCl2). Bench scale packed column studies and batch adsorption experiments were used to observe the transport behavior. The quartz crystal microbalance (QCM) was used to determine the mechanisms responsible for the deposition behavior of MPs and NPs at the nanoscale level. Results showed that in the absence of Cu, the secondary energy minimum was responsible for the reduced PS mobility (40-60%) with increasing IS of NaCl and CaCl2 respectively. However, when Cu was present, PS mobility reduced even more (10-30%), which might result from the adsorption of PS-Cu complexes on porous media following their formation. Similarly, in the absence of PS, Cu had less mobility (nearly 10%) in all IS and salt types due to electrostatic attraction between sand and Cu. On the other hand, Cu demonstrated increasing mobility during co-transport owing to a lack of adsorption sites caused by the competing adsorption of PS and Cu. The batch adsorption results also revealed that MPs had a greater adsorption capacity on quartz sand in the presence of Cu, resulting in enhanced heavy metal mobility. QCM experiments also showed that with increasing IS and in absence of Cu, both MPs and NPs deposition on the silica surface increased due to compression of the electric double layer, following DLVO theory. However, in presence of Cu, PSMPs and PSNPs had 6.6 and 4.0-fold higher deposition respectively for NaCl and 1.5 and 4.8-fold respectively for CaCl2 under the high IS condition, than in absence of Cu. Positive metal ions can compress the electrostatic double-layer even more, lowering the energy barrier and form complexes with the PS causing greater PS deposition on the silica surfaces. Furthermore, QCM showed that regardless of the presence of heavy metals, NPs mass deposition was higher than MPs on the silica surface. According to DLVO theory, NPs had a lower energy barrier than MPs due to their smaller size, resulting in a higher deposition. In summary, the findings of this study showed that the interaction between PS and Cu can influence both their transport and deposition behaviors in the environment under different aquatic chemistry conditions. This work could be used to anticipate the fate and movement of MPs and NPs in the presence of other pollutants in the aquatic environment and allow necessary steps to be taken to prevent additional contamination and design their subsequent removal.
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