TRANSPORT OF SINGLE-LAYERED GRAPHENE OXIDE THROUGH POROUS MEDIA

Graphene oxide (GO) nanosheets are comprised of six-membered carbon ring lattices interrupted and/or decorated with covalently-bonded epoxide, ketone, and hydroxyl functional groups on the basal planes, and carboxylic and phenolic functional groups at the edges. When completely exfoliated, single-layered GO nanosheets exhibit a very high degree of anisotropy, being generally two to three micrometers in width and approximately one nanometer in thickness. As a result of this extraordinarily large surface area-to-mass ratio and the presence of numerous proton-active functional groups, GO nanosheets demonstrate a tremendous capacity to adsorb metals and other contaminants from aqueous solutions and are thus often suggested for use in in situ remediation efforts. The potential importance of GO nanosheets as an adsorbent in soil and groundwater necessitates a detailed understanding of their mobility in realistic environmental systems, and this topic remains largely unexplored. Hence, the objective of this study was to investigate the transport behavior of single-layered GO nanosheets through well-characterized saturated porous media. We used replicate glass columns packed with either uncoated or Fe-coated quartz sands and within each experimental treatment we varied pH (5.6 to 8.3) and ionic strength (IS; 10 mM and 50 mM NaClO₄) to establish the relative influences of each electrochemical property on GO nanosheet transport. Break-through of GO nanosheets from each treatment column was continuously monitored using a flow-through quartz cuvette and UV-Vis absorbance at 230 nm. Transport of GO nanosheets through quartz sands occurred relatively rapidly at 10 mM IS with greater retention observed at 50 mM IS. In addition, retention of GO nanosheets in the columns increased with decreasing pH and in those columns packed with Fe-coated sands. These observed trends are considered in relation to our detailed understanding of the surface properties (e.g., ζ-potential) demonstrated by the sand grains and GO nanosheets upon exposure to each treatment condition.