Effects of surfactant type and concentration on graphene retention and transport in saturated porous media

Knowledge of the fate and transport of graphene (GR) nanosheets in porous media is essential to understand their environmental impacts. In this work, sand column experiments were conducted to investigate the retention and transport of surfactant-dispersed GR nanoparticles under various conditions. An anionic surfactant, sodium dodecylbenzene sulfonate (SDBS) and a cationic surfactant, cetyltrimethylammonium bromide (CTAB) were used to disperse and stabilize GR in aqueous solutions. Both surfactants were effective in stabilizing the GR particles, even at low concentration (0.004% w:v) because the surfactant coating introduced negative (SDBS) or positive (CTAB) charges on the GR surfaces. As a result, the SDBS- and CTAB-GR showed different retention and transport behaviors in the saturated porous media. At low surfactant concentration, the transport of SDBS-GR was much higher than that of the CTAB-GR, which was almost immobile in the sand columns with mass recovery rate only about 4%. When the surfactant concentration increased from 0.004% to 0.4%, it reduced the transport of SDBS-GR and dramatically enhanced the mobility of the CTAB-GR (with mass recovery rate of 91%). It is suggested that the presence of 'free' SDBS ions may reduce the electrosteric repulsions between SDBS-GR and sand surfaces by compressing the electrical double layer. The 'free' CTAB ions, however, may compete with the CTAB-GR for adsorption sites on the sand surfaces. Findings from this study indicated that the dispersion method plays an important role in affecting the environmental fate and transport of GR particles. (C) 2014 Elsevier B.V. All rights reserved.