Real-time visualization of carbon quantum dot transport in homogeneous and heterogeneous porous media†

The widespread applications of carbon quantum dots (CQDs) have attracted much attention. This study presents a novel research system to study the transport and retention of CQDs in homogeneous and heterogeneous porous media. The light transmission visualization technique was used to visualize the real-time distribution and transport of CQDs. Results showed that the increase in quartz sand particle size and increased pH significantly enhanced the transport of CQDs. Due to the negative surface charge of CQDs shielded by high IS, the agglomeration of CQDs enhanced the clogging of CQDs. Particularly, significant aggregated fluorescence quenching of CQDs occurred at IS = 100 mM and IS = 200 mM. In heterogeneous media, the layer structure alteration and preferential flow contribute significantly in the transport of CQDs. Compared to the fine sand layer, most of the CQDs outflow from the coarse sand layer. The breakthrough curves for CQD transport in porous media can be matched by a simplified double-Monod model with high accuracy (R² > 0.92). Moreover, the DLVO theory and clogging mechanism well explain the environmental behavior of CQDs in 2D porous media. This study visualized the fate of CQDs in 2D porous media, enabling us to further assess and predict their environmental risks.