Key Laboratory of Integrated Regulation and Resources Development on Shallow Lakes of Ministry of Education, College of Environment, Hohai University, Nanjing, 210098, China.
Key Laboratory of Integrated Regulation and Resources Development on Shallow Lakes of Ministry of Education, College of Environment, Hohai University, Nanjing, 210098, China.
Water Res. 2022 Jul 1;219:118609. doi: 10.1016/j.watres.2022.118609. Epub 2022 May 16.
The ecological risk of microplastics (MPs) usually depends on their environmental behavior, however, few studies focused on the impact of hydrodynamic perturbations on the fate of MPs in hyporheic zone. This study chose quartz sand (250-425 μm) as simulated porous medium to investigate the transport of 100 nm polystyrene nanoplastics (PSNPs) under hydrodynamic factors, including flow rates (0.5, 1.0, and 2.0 mL/min), flow orientations (up-flow, down-flow, and horizontal-flow), and water saturations (50%, 80%, and 100%), as well as different salinities and temperatures. The breakthrough curves (BTCs) and retained profiles (RPs) of PSNPs were compared and analyzed by Derjaguin-Landau-Verwey-Overbeek (DLVO) theory. Due to the small size and moderate density of PSNPs, as well as high flow rates, the flow orientation exhibited little effect on the PSNP transport. However, high flow rate, low salinity, high water saturation, and low temperature would facilitate the mobility of PSNPs. The increase in salinity from zero to 35 PSU (practical salinity units) caused the compression of electrical double layer and weakened the electrostatic repulsion between PSNPs and sands, which dramatically decreased the penetration rate from 100% to zero. Especially, the lower energy barrier of PSNPs-PSNPs at 3.5 and 35 PSU (16.45 kT and zero, respectively) facilitated the adsorption of PSNPs on sand via ripening mechanism. Due to the strong adsorption of PSNPs by sand at high salinity, the effect of flow rate on PSNP transport was more pronounced at low salinity. The mobility of PSNPs at 0.035 PSU was enhanced by 41.4%-75.3% as the flow rate increased from 0.5 to 2.0 mL/min, which was contributed from the reversible deposition in lower secondary energy minimum depth at low salinity and the stronger hydrodynamic drag force generated by the high flow rate. However, the sufficient molecular diffusion at low flow rate promoted the occupation of PSNPs on adsorption sites. In addition, the penetration rate of PSNPs decreased by 25.0% as the water saturation decreased from 100% to 50%, indicating that the film straining at the air-water interface would hinder the transport of PSNPs. Finally, temperature increase impeded the penetration of PSNPs by 6.26%-23.1% via blocking mechanism. Our results suggest that low-salinity, high-flow river systems may be at greater risk of MPs contamination due to enhanced vertical transport capability.
微塑料（MPs）的生态风险通常取决于其环境行为，但很少有研究关注水动力扰动对渗流区 MPs 命运的影响。本研究选择石英砂（250-425μm）作为模拟多孔介质，研究了在水动力因素（包括流速（0.5、1.0 和 2.0mL/min）、流动方向（上向流、下向流和水平流）和水饱和度（50%、80%和 100%）以及不同盐度和温度下，100nm 聚苯乙烯纳米塑料（PSNPs）的传输。通过德加古因-兰德劳-韦尔韦-奥弗贝克（DLVO）理论比较和分析了 PSNPs 的穿透曲线（BTC）和保留剖面（RP）。由于 PSNPs 的尺寸小、密度适中、流速高，流动方向对 PSNP 的传输影响不大。然而，高流速、低盐度、高水饱和度和低温会促进 PSNPs 的迁移。盐度从 0 增加到 35 PSU（实用盐度单位）会导致双电层压缩，减弱 PSNPs 和砂之间的静电排斥，从而使穿透率从 100%急剧下降到 0。特别是，3.5 和 35 PSU（分别为 16.45 kT 和 0）下 PSNPs-PSNPs 的较低能垒促进了 PSNPs 通过成核机制在砂上的吸附。由于 PSNPs 在高盐度下被砂强烈吸附，因此在低盐度下，流速对 PSNP 传输的影响更为明显。当流速从 0.5 增加到 2.0mL/min 时，0.035 PSU 下 PSNPs 的迁移率增加了 41.4%-75.3%，这归因于低盐度下较低二次能量最小值深度的可逆沉积和高流速产生的更强水动力拖曳力。然而，低流速下充分的分子扩散促进了 PSNPs 在吸附位点上的占据。此外，当水饱和度从 100%降低到 50%时，PSNPs 的穿透率降低了 25.0%，表明空气-水界面处的膜拉伸会阻碍 PSNPs 的传输。最后，通过阻塞机制，温度升高会阻碍 PSNPs 的渗透，渗透率降低 6.26%-23.1%。研究结果表明，由于垂直输运能力增强，低盐、高流速的河流系统可能面临更大的 MPs 污染风险。
