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.

Effects of humic acid on copper adsorption onto few-layer reduced graphene oxide and few-layer graphene oxide

“…S6. It is shown that the increase of HA concentrations had a negligible effect on the EMPs of FGO, consistent with the result of Chowdhury et al [36]. Addition of HA, however, enhanced the stabilization of FGO in water (Fig.…”

“…S8). Because HA did not affect the EMPs of FGO, steric repulsion imparted by the adsorbed HA macromolecules may be the dominant mechanism in FGO stabilization [36,37].…”

Stability and Transport of Graphene Oxide Nanoparticles in Groundwater and Surface Water

“…The resulting GONPs had a planar structure with an average perimeter of 781 -502 nm, an average height of 0.9 -0.2 nm, and an average square root of the surface area of 179 -112 nm (Chowdhury et al, 2013;Duch et al, 2011). The modified Hummer's method was used to synthesize the GONPs used in this study and were reported to contain oxygen-bearing functional groups including carboxylic, carbonyl, and hydroxyl (Duch et al, 2011).…”

“…The GONP suspensions were created at an unadjusted pH since previous studies (Chowdhury et al, 2013;Lanphere et al, 2013) have shown that there is no significant effect on stability from pH 5 to 9. The artificial groundwater (Bolster et al, 1999) and surface water (Yip et al, 2011) suspensions were developed using recipes found in the literature and had an ionic strength (IS) of 3.78 and 1.74 mM, respectively.…”

“…For carbon-based materials, humic acid has been reported to greatly increase the stability of fullerene (nC 60 ) nanoparticles as a result of steric hindrance (Qu et al, 2010). In another study, the stability of GONPs was increased in the presence of humic acid at concentrations of 1-10 mg/L Suwannee River humic acid (SRHA) (Chowdhury et al, 2013). A plausible explanation for the increased transport and reduced aggregation is steric stabilization Morales et al, 2011), which involves polymers present in humic acid that increase colloidal stability at higher NOM concentrations (e.g., >1 mg/L), which are typically found in surface water environments (Volk, 2001).…”