Retention and Release of Graphene Oxide in Structured Heterogeneous Porous Media under Saturated and Unsaturated Conditions

In this work, saturated and unsaturated structured heterogeneous sand columns were used to examine the fate of graphene oxide (GO) nanoparticles in heterogeneous porous media under various conditions. A two-domain model considering mass exchange between zones was applied to describe GO retention and transport in structured, heterogeneous porous media, which matched the transport experimental breakthroughs well. Experimental and model results showed that GO retention and transport in all the heterogeneous columns were dominated by the preferential flow phenomena. Under saturated conditions, the coarse sand with higher hydraulic conductivity was the fast-flow domain (FFD), and the fine sand was the slow-flow domain (SFD), and both FFD and SFD affected GO particles fate in structured heterogeneous media. When the heterogeneous columns were drained, the fine sand with higher moisture content became the FFD, and the coarse sand was the SFD, however, preferential flows in the FFD dominated GO retention and transport processes. For all the columns, the mobility of GO decreased with the increasing ionic strength (IS), and the previous retained particles were released by reducing solution IS, indicating part of the retained particles were trapped in the secondary minimum energy well.