Factors controlling transport of graphene oxide nanoparticles in saturated sand columns

The authors conducted column experiments and a modeling study to understand the effects of several environmental factors on the aggregation and transport of graphene oxide nanoparticles (GONPs) in saturated quartz sand. The GONPs were negatively charged and stable under the test conditions (0-50 mM NaCl; pH 4.8-9.0), and the Derjaguin-Landau-Verwey-Overbeek (DLVO) calculation indicated that deposition of GONPs was under unfavorable attachment conditions. The GONPs exhibited high mobility even at an ionic strength of 25 mM NaCl. The transport of GONPs was insensitive to the changes of pH (from 5.1 to 9.0), but the presence of 10 mg/L Suwannee River humic acid (SRHA) considerably enhanced transport at high ionic strength (35 mM NaCl), likely via enhanced steric repulsion and significantly inhibited stacking of GO flakes. Varying flow velocity also enhanced transport at high ionic strength. In general, GONPs exhibit greater mobility compared with other carbon nanoparticles because the aggregation and transport of GONPs are more resilient to changes in solution chemistry and hydrodynamic forces that favor aggregation and deposition of nanoparticles. A 2-site transport model incorporating both the blocking-affected attachment process and straining effects can effectively model the transport of GONPs. The high mobility of GONPs should be given full consideration in assessing their environmental risks.