Carbon dots (CDs) are likely to encounter heavy metal ions (i.e., Cd²⁺) because of their widespread application and inevitable release into the environment, wherein the ubiquitous low-molecular-weight organic acids (LMWOAs) may affect their co-transport behaviors in aquifer systems. Herein, citric acid was used as a typical LMWOA. The effect of citric acid on the CD-mediated transport of Cd²⁺ through sand columns was investigated. The results demonstrated that CDs could serve as carriers to markedly enhance the transport of Cd²⁺, which was attributed to the abundant adsorption sites of nanoparticles for Cd²⁺ and the excellent mobility of CDs. Citric acid promoted the CD-facilitated transport of Cd²⁺ due to the formation of water-soluble metal–citric acid complexes, the facilitation of the adsorption of Cd²⁺ on the CDs by citric acid, and the high mobility of the CDs. Interestingly, the enhanced role of citric acid in CD-facilitated transport declined as the pH was raised from 6.0 to 8.0, since the predominant Cd²⁺ species (e.g., free Cd²⁺) in the influents preferred to deposit onto sand surfaces at a higher pH. The findings provide meaningful insights into understanding the co-transport of carbon-based nanoparticles and heavy metals affected by the presence of LMWOAs in the natural subsurface environment.

中文翻译：

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柠檬酸对碳点介导的 Cd2+ 通过饱和多孔介质传输的影响

碳点（CDs）由于其广泛的应用和不可避免的释放到环境中，很可能遇到重金属离子（即Cd ²⁺），其中无处不在的低分子量有机酸（LMWOA）可能会影响它们的协同运输含水层系统中的行为。在此，柠檬酸被用作典型的LMWOA。研究了柠檬酸对CD ²⁺通过沙柱的CD 介导转运的影响。结果表明，CDs可以作为载体显着增强Cd ²⁺的传输，这归因于纳米粒子对Cd ²⁺的大量吸附位点和CDs优异的迁移率。柠檬酸促进 CD 促进的 Cd 转运²⁺由于水溶性金属-柠檬酸络合物的形成，柠檬酸促进 Cd ²⁺在 CD 上的吸附，以及 CD 的高迁移率。有趣的是，柠檬酸在 CD 促进转运中的增强作用随着 pH 从 6.0 升高到 8.0 而下降，因为流入物中的主要 Cd ²⁺物质（例如，游离 Cd ²⁺）倾向于以一定的速度沉积在沙子表面上。较高的 pH 值。这些发现为理解受自然地下环境中存在 LMWOA 影响的碳基纳米颗粒和重金属的共同运输提供了有意义的见解。