Aggregation Kinetics of Graphene Oxides in Aqueous Solutions: Experiments, Mechanisms, and Modeling

Abstract

Although graphene oxide (GO) has been used in many applications to improve human life quality, its environmental fate and behavior are still largely unknown. In this work, a novel approach that combines experimental measurements and theoretical calculations was used to determine the aggregation kinetics of GO sheets in aqueous solutions under different chemistry conditions (e.g., cation valence and pH). Experimental data showed that both cation valence and pH showed significant effect on the aggregation of GO sheets. The measured critical coagulation concentrations were in good agreement with the predictions of the extended Schulze-Hardy rule. Ca(2+) and Mg(2+) were more effective than Na(+) in aggregating the GO sheets, which could be attributed to the cross-linking between GO sheets by the divalent cations through "bridging" the functional groups at the edges of the GO sheets. When solution pH increases, deprotonation of carboxylic groups was found to play a key role in increasing GO sheet stability and surface charge development. These results suggested that edge-to-edge and face-to-face interactions were the dominant modes of GO aggregation in the presence of divalent metal ions and H(+), respectively. A modified attachment efficiency (α) model was developed on the basis of the Maxwell approach with considerations of both primary and secondary minima. The model predictions matched the experimental measurements of the aggregation kinetics of GO sheets in aqueous solutions under all of the tested experimental conditions well.

Blocking effect of colloids on arsenate adsorption during co-transport through saturated sand columns

“…However, promoted As(V) transport increased with increasing soil colloid concentration at pH 9.8, although a high ionic strength was associated with a high concentration of soil colloid. According to the DLVO theory (Feriancikova and Xu, 2012;Wu et al, 2013), when the colloids approached the surface of porous media, the electric double layer repulsion would prevent colloids from landing on the surface of porous media at a certain pH (pH 9.8 in this study), and thus colloidal transport was active in porous media. Based on this investigation, the hypothesis was developed and shown in Fig.…”

“…DLVO theory was used to calculate the interaction forces between the soil colloid and the sand, assuming plate-plate interactions (Feriancikova and Xu, 2012;Wu et al, 2013), in order to explain the mechanisms of soil colloid transport and retention in porous media. The DLVO energy interactions, the sum of van der Waals attraction and electric double layer repulsion between the soil colloid and the sand surface, were calculated.…”

Sensitivity to Heavy-Metal Ions of Unfolded Fullerene Quantum Dots

“…We can speculate that As(III) induces a different type of aggregation, in comparison with Cu 2+ and Pb 2+ , due to the different chemical reactivity and the presence in water of arsenic as oxy-compounds. In fact, as suggested by different studies [32,47], aggregation can occur either through functional groups located at the edges [46], such as the carboxyl groups (as depicted in Scheme 1), or by multiple interaction with the π states and -OH groups on the surface of nanosheets [45,49], thus giving rise to face-to-face aggregates with a different geometrical configuration (stacking). While further studies are needed to confirm this hypothesis, it should be pointed out that the different behaviour of the three ions, as regards the PL quenching and the transmittance variations, makes it possible to discriminate between the three species by using simple optical measurements.…”

“…In this way, possible artefacts due to inner-filter effects and transmittance variations affecting the PL quenching experiment [33] can be ruled out. [45], for instance with an edge-to-edge geometry as depicted in Scheme 1 [46]. Once an aggregate is formed, the fluorescent emission can be quenched both by energy transfer between the UFQDs and the chelated metal ion and by different radiationless mechanisms arising inside the aggregate itself.…”

Photochlorination-induced transformation of graphene oxide: Mechanism and environmental fate

“…Concurrent or subsequent sunlight irradiation of chlorinated GO may result in further transformation, affecting further the physicochemical properties (and thus, environmental implications) of GO. This potential transformation of GO by photochlorination has not received much attention as most existing studies on the environmental fate of GO focused on its colloidal stability and sunlightinduced transformations (Chowdhury et al, 2013Wu et al, 2013;Hou et al, 2015).…”