Effect of Hydrophobicity on the Transport of Carbon Nanoparticles in Saturated and Unsaturated Porous Media

Carbon-based nanoparticles (CNPs) are increasingly used for environmental and industrial applications such as in pharmaceuticals, energy production, and water and wastewater treatment. Thus, it is crucial to understand their interactions and transport in porous media. Here, we examine the impact of CNP hydrophobicity and porous medium surface area on their transport. We use CNPs that are synthesized from citric acid and ethanolamine and are fluorescent. They exhibit synthesis-temperature-dependent hydrophobicity and were synthesized at four temperatures: 190 °C, 210 °C, 230 °C, and 250 °C. The experiments were conducted by flowing these CNPs in sand-packed columns under saturated and unsaturated conditions. To examine the impact of the surface area of sand on CNP transport, the sands packed in the columns had three surface areas. In addition, a particle transport model in HYDRUS 1D was used to model the transport.

Together, our experimental and modeling noted four important observations. The first observation indicated the importance of hydrophobicity on CNP transport. There was a 55% difference between the recovery of CNPs synthesized at 190 °C compared to those synthesized at 250 °C. Second, a five-fold increase in surface area yielded a 17% decrease in the recovery of CNPs, suggesting the role of sand surface area on CNP recovery. Third, due to the small size of CNPs relative to the water film on the sand surface, there were no significant differences in the mass recovery of CNPs under unsaturated and saturated conditions. Fourth, the particle transport model with a Langmuirian site blocking term successfully simulated the transport of CNPs. There was approximately a 10-fold increase in the adsorption coefficients for hydrophobic CNPs compared to hydrophilic ones. In sum, our observations and modeling demonstrated that hydrophobicity was a major factor that impacted the transport of CNPs.