Settling velocity of submillimeter microplastic fibers in still water

Microplastic fibers (MPFs) are one of the most important MP contaminants of aquatic environments. However, little research has been conducted on the movement of submillimeter MPFs in water. Herein, the settling of 519 submillimeter MPFs in still water was measured and the settling velocity was analyzed. Observations of the settling velocity of MPFs with lengths of 300, 500, and 600 mu m showed that most MPFs settled individually or in pairs. The sedimentation of a single fiber could be divided into three patterns, that is, horizontal, inclined, and vertical. The average settling velocity increased with an increase in the MPFs length and orientation angle. As the MPFs length increased, the probability of inclined settlement decreased but that of horizontal settlement increased. The horizontal velocity of single fibers also was investigated, and the horizontal and vertical settling of MPFs exhibited minimal horizontal velocity. Because of the considerable difference between the calculated drag coefficients from existing drag coefficient models and experimental values, a drag coefficient model was developed with a deviation of <3 %. Four settling patterns were identified for two fibers, that is, X shaped, inverted-T shaped, cross shaped, and overlapping. The average velocity of the overlapping settlement of two fibers was considerably higher than that of the other three settling patterns. The average settling velocity of 600-mu m two fibers was 1.47 times that of single fibers, indicating that their corresponding drag coefficient was similar to 46 % that of a single fiber.

Automated summary

Microplastic fibers are important contaminants in aquatic environments, but there is little research on the movement of submillimeter fibers in water. This study measured the settling of submillimeter fibers in still water and analyzed their settling velocities. It was found that the settling velocity increased with fiber length and orientation angle. Different settling patterns were identified, and a new drag coefficient model was developed. The study also found significant differences between calculated and experimental drag coefficients.