Spatial retention, absorption, transport, and enrichment of microplastics in mangrove sediment complex system

Mangrove areas are the major sink of pollutants such as microplastics (MPs, less than 5000 μm in diameter). The spatial retention, transport, and accumulation of microplastics (MPs) within the complex mangrove sediment system has become a hotspot in the field of emerging contaminants. In this study, the Xiaoguansha mangrove forest in Guangxi Province, China, was selected as a representative case to investigate the horizontal and vertical distribution of MPs in sediments. To elucidate the processes of MP retention, accumulation, and their downward transport into deeper soil layers, a combination of statistical methods was employed, including the Kruskal-Wallis one-way ANOVA, correlation analysis, regression fitting, and Structural Equation Model (SEM). The results showed that: (1) The average abundance of MPs in the mangrove area (2414.0 ± 1570.8 items/kg) was significantly higher-by a factor of 2.24-than that in the tidal flat areas, suggesting that mangroves play a vital role in seawater purification.(2) The MPs in the smaller size range (0-1000 μm) tend to accumulate more readily in mangrove areas compared to larger particles (1000-5000 μm), implying a heightened potential risk to environmental and ecosystem health.(3) With the increase in soil depth, exhibited an exponentially decreasing trend, primarily due to the well-developed root systems of mangroves and the physicochemical adsorption capacity of the surrounding sediments. (4) Spatial retention and sediment absorption contributed 67.2 % and 32.8 %, respectively, to the enrichment of MPs in mangrove areas. The SEM analysis confirmed that the distribution of MPs was primarily governed by extensive root system and dense physical structure of mangrove. In addition, adsorption effects driven by the fundamental physicochemical properties of the sediments also contributed to MP retention. The findings contribute to a deeper understanding of the behavior of MPs in the mangrove-covered water-sediment system.