Carbonate mineral precipitation enhances microplastic deposition in karst rivers globally

Rivers represent major pathways for microplastic (MP) transport from land to the oceans. MP transport and deposition processes in rivers are influenced by complex hydraulic flow behavior of the river and its interactions with streambed, vegetation and infrastructure as well as the physicochemical properties of MPs themselves. While there is increasing understanding of the hydrodynamic controls on the transport behavior of MPs of different density, size and shape, the impact of geochemical interaction of MPs with other compounds, including possible co-precipitation with minerals remains poorly understood. We here reveal the potential for enhanced MP deposition from the rivers in karst regions through co-precipitation with carbonate minerals through a large-scale field observation covering the entire Pearl River Basin. This is supported by the substantially greater difference in MP mean density between the river water and sediments in carbonate-dominated basins compared to non-carbonate-dominated basins. Laboratory experiments demonstrate that co-precipitation of CaCO₃ and MPs can occur in rivers of carbonate-dominated basins, which enhances MP deposition on the streambed. The negative surface charge of MPs adsorbs Ca²⁺ and increases the possibility of CaCO₃ precipitation on the surface of MPs. MPs of smaller size and higher electronegativity, such as polyamide (PA), show greater affinity for Ca²⁺ and higher deposition ratio from the water column via co-precipitation with CaCO₃. At global scale, regions with high proportion of carbonate bedrocks are characterized by reduced riverine MP exports to the ocean as more of them deposited at the streambed. Our findings underscore the critical role of carbonate mineral precipitation depending on the geological background in regulating fluvial MP deposition and retardation in rivers. This newly described mechanism provides a theoretical basis for future MP mitigation strategies in rivers with different bedrock geology.