Natural colloids such as clays and natural organic matter (NOM) are universally present in environments, which could interact with microplastics (MPs) and thus alter the fate and transport of MPs in porous media. The co-effects of clays and NOM on MPs transport in saturated porous media were systematically explored at both low and high ionic strength (IS) conditions. Specifically, bentonite and humic acid (HA) were employed as representative clays and NOM. 5 mM NaCl and 1 mM CaCl solutions were used as low IS conditions, while 25 mM NaCl and 5 mM CaCl solutions were employed as high IS conditions. We found that formation of MPs-bentonite heteroaggregates had great effects on MPs transport under different conditions. Without HA, the small MPs-bentonite heteroaggregates formed under low IS increased MPs transport via serving as mobile carriers, while larger MPs-bentonite heteroaggregates formed at high IS led to the decreased MPs mobility. When both HA and bentonite were copresent in MPs suspension, we found that HA could inhibit the formation of larger sized MPs-bentonite heteroaggregates. Particularly, when the two types of natural colloids copresent in MPs suspensions, MPs transport behaviors were similar to those with only bentonite present in MPs suspensions at low IS, while MPs transport was greatly increased at high IS comparing with those only with bentonite in suspensions. Clearly, without HA in suspensions, bentonite played the dominant role on MPs transport under all examined conditions concerned in this study. Instead, when both HA and bentonite copresent in MPs suspensions, MPs transport was mainly controlled by bentonite at low IS, while both bentonite and HA had major contributions at high IS. The results showed that under solution conditions concerned in present study, MPs mobility in porous media would be greatly affected (either enhanced or inhibited) by the two types of natural colloids.

中文翻译：

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饱和多孔介质中微塑料颗粒的传输和沉积：粘土颗粒和天然有机质的共同作用

粘土和天然有机物（NOM）等天然胶体普遍存在于环境中，它们可能与微塑料（MP）相互作用，从而改变MP在多孔介质中的命运和传输。在低离子强度 (IS) 和高离子强度 (IS) 条件下系统地探讨了粘土和 NOM 对饱和多孔介质中 MP 传输的共同影响。具体而言，膨润土和腐植酸（HA）被用作代表性粘土和NOM。 5 mM NaCl 和 1 mM CaCl 溶液用作低 IS 条件，而 25 mM NaCl 和 5 mM CaCl 溶液用作高 IS 条件。我们发现MPs-膨润土杂聚集体的形成对不同条件下MPs的运输有很大影响。在没有HA的情况下，在低IS下形成的小MPs-膨润土杂聚集体通过充当移动载体增加了MPs的运输，而在高IS下形成的较大MPs-膨润土杂聚集体导致MPs移动性降低。当HA和膨润土共存于MPs悬浮液中时，我们发现HA可以抑制较大尺寸的MPs-膨润土杂聚集体的形成。特别是，当两种类型的天然胶体共存于MPs悬浮液中时，在低IS下，MPs的传输行为与MPs悬浮液中仅存在膨润土的情况相似，而在高IS下，与悬浮液中仅存在膨润土的情况相比，MPs的传输大大增加。显然，在本研究涉及的所有检查条件下，悬浮液中没有 HA，膨润土在 MP 运输中发挥着主导作用。相反，当HA和膨润土共存于MPs悬浮液中时，在低IS下MPs传输主要受膨润土控制，而在高IS下膨润土和HA均起主要作用。结果表明，在本研究涉及的溶液条件下，两类天然胶体会极大地影响（增强或抑制）MPs在多孔介质中的迁移率。