Overlooked Role of Bulk Nanobubbles in the Alteration and Motion of Microplastics in the Ocean Environment

Abstract

The increasing enrichment of microplastics (MPs) in the shoreline environment poses both ecological and social-economic risks. The alteration and motion of MPs in the ocean under the effect of bulk nanobubbles (NBs) have been less extensively studied. In this study, we explored the behavior and movement of various MPs in the presence of bulk NBs. The role of salinity and external energy in the interactions between NBs and MPs was evaluated, and the mechanism underlying these interactions was analyzed. In the presence of NBs, the binding of MPs and NBs resulted in an increase in the measured average particle size and concentration. Meanwhile, NBs reduced the aggregation between MPs, while the NBs present combined with MPs to make them more stable in suspensions. The velocity of motion of MPs driven by NBs varies under different salinity conditions. The increase in ionic strength reduced the energy barrier between particles and promoted their aggregation. Thus, the binding of NBs and MPs became more stable, which in turn affected the movement of MPs in suspensions. Polyethylene (PE1) with small particle size was mainly affected by Brownian motion, and its rising was limited; therefore, polyethylene (PE2) with large particle size rose faster than PE1 in suspension, especially in the presence of NBs. The rising velocity of poly­(tetrafluoroethylene) (PTFE) was higher than that of PE1 and PE2. However, when NBs were present, the trend of the change in velocity was the opposite compared to the absence of NBs for PTFE. Moreover, various types of MPs were found to be affected distinctly by external energy. The presence of NBs had a clear effect on PE under shaking conditions, whereas the effect on PTFE was less obvious.

Revealing the Freezing-Induced Alteration in Microplastic Behavior and Its Implication for the Microplastics Released from Seasonal Ice

“…Salinity plays a crucial role in governing the behavior of suspended particles in water by affecting the compression of the electrostatic double layer . The reduction in electrostatic repulsion due to increased salinity can accelerate particle aggregation and disrupt particle stability.…”

“…However, the difference between the freezing and control groups in freshwater was more significant for volume concentrations compared to mass concentrations. This may be attributable to the augmentation in particle size resulting from aggregation induced by freezing, which can lead to an overestimation of volume concentrations when measured using the LD method . As discussed in Section , the increase in PE concentration after a freezing–thawing cycle in freshwater can be attributed to wetting alterations enhanced by ice crystal compression.…”

“…This may be attributable to the augmentation in particle size resulting from aggregation induced by freezing, which can lead to an overestimation of volume concentrations when measured using the LD method. 16 As discussed in Section 3.1, the increase in PE concentration after a freezing−thawing cycle in freshwater can be attributed to wetting alterations enhanced by ice crystal compression. However, the introduction of salinity creates brine channels that mitigate the pressure generated by ice crystal growth.…”

“…Salinity plays a crucial role in governing the behavior of suspended particles in water by affecting the compression of the electrostatic double layer. 16 The reduction in electrostatic repulsion due to increased salinity can accelerate particle aggregation and disrupt particle stability. Moreover, salinity influences the entrainment of MPs in ice, as the formation of brine channels between ice crystals creates narrow spaces in the ice in which for MP entrapment to occur.…”

Unveiling the Vertical Migration of Microplastics with Suspended Particulate Matter in the Estuarine Environment: Roles of Salinity, Particle Properties, and Hydrodynamics

“…Saltwater intrusion is a prevalent phenomenon in estuaries, mainly caused by strong tidal flows. Part of the intruding seawater will mix with river water forming salinity structures (i.e., well mixed, partially mixed, or salt wedge). , The ionic strength in the water column can vary considerably under different salinity structures, where ionic strength has been demonstrated to influence particle aggregation and stability. − Salinity is therefore a critical factor in MP settling in estuaries, which warrants further investigation. Moreover, estuaries are energetic areas subject to tidal flow, estuarine circulation, and river outflow.…”