Polymer density dominates the sedimentation behavior of microplastics in weakly dynamic water bodies: Synergistic regulation by photoaging degree and concentration of calcium ion

ZHANG Hao-tian; LI Pei-zhao; SHU Xiao-hua; RU Xuan; LI Zong-chen; ZHANG Qian

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China Environmental Science ›› 2026, Vol. 46 ›› Issue (1) : 440-449.

Polymer density dominates the sedimentation behavior of microplastics in weakly dynamic water bodies: Synergistic regulation by photoaging degree and concentration of calcium ion

ZHANG Hao-tian¹, LI Pei-zhao¹, SHU Xiao-hua², RU Xuan¹, LI Zong-chen¹, ZHANG Qian¹

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Abstract

The sedimentation behavior of microplastics (MPs) in water bodies is synergistically regulated by the intrinsic properties of the particles, the photoaging degree, and the chemical conditions of the solution. Existing studies have overlooked the sedimentation behavior for different types of aged MPs in karst small water bodies. Through simulation experiments, the sedimentation behavior of three types of MPs (polystyrene (PS), polyamide 6 (PA6), and polyethylene terephthalate (PET)) with different photoaging degrees wasinvestigated in weakly dynamic water bodies and environments with different concentrations of CaCl₂. The results showed that the sedimentation ratios of the three types of MPs exhibited a decreasing trend with the deepening of photoaging under pure water conditions,while the opposite result was observed in the CaCl₂ environment.Compared with pristine MPs, the sedimentation ratios of MPs aged for 1000h decreased by 38.8% to 56.0%in pure water. In contrast, the sedimentation ratios increased by 23.8% to 93.3% and 66.1% to 357.1% in 10mmol/L and 100mmol/L CaCl₂ solution, respectively. Ca²⁺ can compress the electrical double layer of MPs, reducing the electrostatic repulsion between particles. Simultaneously, It can also bridge with the oxygen-containing functional groups on the surface of aged MPs. These effects facilitate the homogeneous aggregation of particles, therebypromoting their sedimentation. The results of Pearson correlation analysis and DLVO theory calculations indicated that bridging of Ca²⁺ was an important external factor regulating the aggregation and sedimentation of MPs before and after aging. Although the photoaging degree and salt ions influenced the sedimentation process of MPs, the fact that Peclet values of the three polymer MPs were below 1demonstratedthat material densitywas the dominantfactor governing their sedimentation behavior. the sedimentation of MPs is primarily governed by polymer density. Photoaging regulates interparticle interactions by changing the oxygen-containing functional groups and size distribution of MPs. While the compression of the electrical double layer and bridging effects induced by Ca²⁺ further influence their aggregation and sedimentation behavior of MPs.

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microplastics / sedimentation behavior / polymer type / photoaging / multivalent cations

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ZHANG Hao-tian, LI Pei-zhao, SHU Xiao-hua, RU Xuan, LI Zong-chen, ZHANG Qian. Polymer density dominates the sedimentation behavior of microplastics in weakly dynamic water bodies: Synergistic regulation by photoaging degree and concentration of calcium ion[J]. China Environmental Science. 2026, 46(1): 440-449

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