Aggregation and Stability of Reduced Graphene Oxide: Complex Roles of Divalent Cations, pH, and Natural Organic Matter

Abstract

The aggregation and stability of graphene oxide (GO) and three successively reduced GO (rGO) nanomaterials were investigated. Reduced GO species were partially reduced GO (rGO-1h), intermediately reduced GO (rGO-2h), and fully reduced GO (rGO-5h). Specifically, influence of pH, ionic strength, ion valence, and presence of natural organic matter (NOM) were studied. Results show that stability of GO in water decreases with successive reduction of functional groups, with pH having the greatest influence on rGO stability. Stability is also dependent on ion valence and the concentration of surface functional groups. While pH did not noticeably affect stability of GO in the presence of 10 mM NaCl, adding 0.1 mM CaCl2 reduced stability of GO with increased pH. This is due to adsorption of Ca(2+) ions on the surface functional groups of GO which reduces the surface charge of GO. As the concentration of rGO functional groups decreased, so did the influence of Ca(2+) ions on rGO stability. Critical coagulation concentrations (CCC) of GO, rGO-1h, and rGO-2h were determined to be ∼ 200 mM, 35 mM, and 30 mM NaCl, respectively. In the presence of CaCl2, CCC values of GO and rGO are quite similar, however. Long-term studies show that a significant amount of rGO-1h and rGO-2h remain stable in Call's Creek surface water, while effluent wastewater readily destabilizes rGO. In the presence NOM and divalent cations (Ca(2+), Mg(2+)), GO aggregates settle from suspension due to GO functional group bridging with NOM and divalent ions. However, rGO-1h and rGO-2h remain suspended due to their lower functional group concentration and resultant reduced NOM-divalent cation bridging. Overall, pH, divalent cations, and NOM can play complex roles in the fate of rGO and GO.

Heteroaggregation of multiwalled carbon nanotubes with sediments

“…This mechanism has been posited in earlier aggregation and deposition studies where functionalized MWCNT (Yi and Chen, 2011) exhibited greater deposition than the unfunctionalized species in CaCl 2 but not NaCl systems. A recent study with variably oxidized forms of graphene also observed that in the presence NOM and divalent cations (Ca 2+ , Mg 2+ ), highly oxidized graphene rapidly aggregated due to functional group bridging with NOM and divalent ions, but less highly oxidized graphene remained suspended due lower functional group concentration and resultant reduced NOM-divalent cation bridging (Chowdhury et al, 2015). As observed in these earlier studies and the work presented here, ion valence clearly plays an important role in both nanoparticle deposition and release beyond classic charge shielding.…”

Graphene-family nanomaterials in wastewater treatment plants

“…In opposition to pretreatment, where no literature dealing with wastewater exists, three papers have been found that contain data on the sedimentation behaviour of GFNs suspended in sewage: those by Chowdhury et al [40], and Ren et al [31] for GO and that by Chowdhury et al [41] for RGO.…”

“…The anion associated to the cation also interacts with the nanoflake, making the GFN more negatively charged, but it seems that, when the electrolyte concentration increases, cations have a more pronounced effect on the suspension behaviour than anions do [31]. Regarding NOM, GFNs readily adsorb it on those areas of the basal plane without oxygenated groups via hydrophobic interactions or π -π bonding [12], and two opposing processes could happen: better aggregation (because organic matter is negatively charged, and provided more binding sites for cations) or steric hindrance (due to the size of the adsorbed organic molecules, which hampers the aggregation of the nanosheets), but the second one is the predominant in simple solutions [24,35,37,40,41]. Furthermore, adsorption of organic matter might cover the oxygen-containing groups of GO, obstructing the reduction process [35].…”

“…The work upon which this paper is based on was co-financed by Spanish MINECO Hydrodynamic diameter of Oconee effluent was not depicted since it was around 7000 nm in the first minutes and the nanomaterial was not detectable after day 1. and synthetic wastewater without organic matter (b) according to Chowdhury et al [41]. • Incubation of GO and activated sludge for 20 h before determining the amounts of ammonia and phosphate by colourimetry.…”

Heteroaggregation of CeO2 and TiO2 engineered nanoparticles in the aqueous phase: Application of turbiscan stability index and fluorescence excitation-emission matrix (EEM) spectra

“…The basic principle of Turbiscan is to measure both transmittance and backscatter of near-infrared pulsed light (λ = 880 nm) from the bottom to the neck of a container containing the suspension [59,60]. The physical principle of Turbiscan is based on the Lambert-Beer theory, as shown in formula (1)- (2).…”