Colloidal Properties and Stability of Graphene Oxide Nanomaterials in the Aquatic Environment

Abstract

While graphene oxide (GO) has been found to be the most toxic graphene-based nanomaterial, its environmental fate is still unexplored. In this study, the aggregation kinetics and stability of GO were investigated using time-resolved dynamic light scattering over a wide range of aquatic chemistries (pH, salt types (NaCl, MgCl2, CaCl2), ionic strength) relevant to natural and engineered systems. Although pH did not have a notable influence on GO stability from pH 4 to 10, salt type and ionic strength had significant effects on GO stability due to electrical double layer compression, similar to other colloidal particles. The critical coagulation concentration (CCC) values of GO were determined to be 44 mM NaCl, 0.9 mM CaCl2, and 1.3 mM MgCl2. Aggregation and stability of GO in the aquatic environment followed colloidal theory (DLVO and Schulze-Hardy rule), even though GO's shape is not spherical. CCC values of GO were lower than reported fullerene CCC values and higher than reported carbon nanotube CCC values. CaCl2 destabilized GO more aggressively than MgCl2 and NaCl due to the binding capacity of Ca(2+) ions with hydroxyl and carbonyl functional groups of GO. Natural organic matter significantly improved the stability of GO in water primarily due to steric repulsion. Long-term stability studies demonstrated that GO was highly stable in both natural and synthetic surface waters, although it settled quickly in synthetic groundwater. While GO remained stable in synthetic influent wastewater, effluent wastewater collected from a treatment plant rapidly destabilized GO, indicating GO will settle out during the wastewater treatment process and likely accumulate in biosolids and sludge. Overall, our findings indicate that GO nanomaterials will be stable in the natural aquatic environment and that significant aqueous transport of GO is possible.