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Rivers have been recognized as the primary conveyors of microplastics to the oceans, and seaward transport flux of riverine microplastics is an issue of global attention. However, there is a significant discrepancy in how microplastic concentration is expressed in field occurrence investigations (number concentration) and in mass flux (mass concentration). Of urgent need is to establish efficient conversion models to correlate these two important paradigms. Here, we first established an abundant environmental microplastic dataset and then employed a deep neural residual network (ResNet50) to successfully separate microplastics into fiber, fragment, and pellet shapes with 92.67% accuracy. We also used the circularity ( C ) parameter to represent the surface shape alteration of pellet-shaped microplastics, which always have a more uneven surface than other shapes. Furthermore, we added thickness information to two-dimensional images, which has been ignored by most prior research because labor-intensive processes were required. Eventually, a set of accurate models for microplastic mass conversion was developed, with absolute estimation errors of 7.1, 3.1, 0.2, and 0.9% for pellet (0.50 ≤ C < 0.75), pellet (0.75 ≤ C ≤ 1.00), fiber, and fragment microplastics, respectively; environmental samples have validated that this set is significantly faster (saves ∼2 h/100 MPs) and less biased (7-fold lower estimation errors) compared to previous empirical models.

Biofilm-coated microplastics are omnipresent in aquatic environments, carrying different organic matter (OM) that in turn influences the flocculation and settling of microplastic aggregates. In this study, the effects of chitosan, guar gum, humic acid, and xanthan gum on the flocculation of anthropogenic microplastics are examined under controlled shear through the mixing chamber experiments. The results show that all of the selected OMs have positive effects on biofilm culturing and thus enhance the growth of microplastic flocs, with more evident promoting effects for cationic and neutral OMs (i.e., chitosan and guar gum) than anionic OMs (i.e., humic acid and xanthan). No critical shear rate is observed in the size vs. shear relationship based on our measurements. In addition, the quadrature-based two-class population balance model is employed to track the development of bimodal floc size distributions (FSDs) composed of small and large microplastic flocs. The model predictions show reasonable agreement with the observed FSDs. The largest error of settling flux from the two-class model is 7.8% in contrast with the reference value measured by the camera-based FSDs with 30 bins. This study highlights the role of different OMs on microplastic flocculation and indicates that a two-class model may be sufficient to describe microplastic transport processes in estuaries., Competing Interests: Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2023. Published by Elsevier Ltd.)

The aggregation and deposition processes of marine microplastics are extremely important in marine ecosystems. The main effect of these two physical processes is the transfer of surface microplastics to the deep sea, and the underlying kinetics can be significantly affected by the organic matter in the ocean. The morphology of and interaction force on 20-μm polystyrene microplastics in the presence of organic matter were studied by using environmental scanning electron microscopy (ESEM) and atomic force microscopy (AFM), respectively. Experiments were performed using organic matter of various concentrations, and the results showed that humic acid formed a translucent organic film around polystyrene microplastics. With increasing total organic content (TOC), the average overall size of the microplastic coated with biofilm increased up to 11 % (at a TOC of 50 mg/L) and then decreased slightly. The biofilm formed by humic acid decreases the repulsion force between two particles and thus could promote the aggregation process significantly. A modified formulation of eXtended Derjaguin-Landau-Verwey-Overbeek (XDLVO) theory, describing the interaction force of microplastics with the influences of biofilms was proposed based on the measured results., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2022. Published by Elsevier B.V.)

Mussels are suggested as bioindicators of marine microplastic pollution. However, they are selective in regards to accumulation of microplastics. To make studies more targeted and comparable, ultimately helping to determine the suitability of the mussel as a bioindicator species for microplastic exposure, we review the published literature that has directly or indirectly demonstrated particle selection in mussels. The reported difference between microplastic levels in mussel tissues and environmental matrices provides evidence for their selective uptake characteristics. Both the organ-specific fate characteristics of microplastics, and the different movement patterns of microplastics in the same organ, show that selective translocation processes take place. The selective elimination is reflected in multiple aspects which include (1) the different characteristics of microplastics in excretion and mussel body; (2) the different retention time of various microplastics in mussels; and (3) the tissue-specific change in the numbers of microplastics during the depuration process. This selectivity is affected by the characteristics of the microplastics, the environmental, or laboratory exposure concentrations, feeding status, and other factors. There are still many research gaps and contradictory viewpoints in this field due to this complexity. The current methodology needs improvement and a breakthrough in standardization. [Display omitted] • Selective accumulation of microplastics in mussels is reflected in multiple aspects. • Size of microplastics is the most implicated factor affecting selective accumulation. • Selective mechanisms of mussels for microplastics remain knowledge gaps. • More targeted and comparable experiment is needed for mussel's selectivity research. Mussels have particle selection for microplastics. [ABSTRACT FROM AUTHOR]