关键词: 氧化石墨烯, 天然有机物, 钙镁离子, 凝聚动力学, 沉降动力学, 再分散

Abstract:

Graphene oxide (GO) as a kind of carbon nanomaterials is wildly used in many industrial fields benefiting from its remarkable physicochemical properties. However, GO easily leaks and is stable in the aquatic environment because of its hydrophilicity and biological toxicity, which lead to potential ecological and environmental risks. When GO enters water, its morphology and stability are changed and influenced by complex environmental factors, which in turn alter its bioavailability and toxicity. Among these, natural organic matter (NOMs) and electrolytes are the most critical factors affecting the stability of GO in natural aquatic environments. However, the effects of NOMs properties and complex interactions with electrolytes on the stability of GO in water have not been thoroughly investigated in previous studies. The relationship between GO aggregation and sedimentation remains unclear, and the effect of aggregate redispersion after sedimentation on the stability of GO has not yet been reported. The microstructure of GO was characterized using scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Fourier transform infrared spectroscopy (FTIR) was employed to analyze the surface functional groups of GO, and the full spectrum of the GO stable suspensions was scanned using a UV-Vis spectrophotometer. In addition, the particle size and zeta potential of the GO were measured using a nanoparticle size and zeta potential analyzer. To examine the effects of NOMs characteristics and the interaction between NOMs and electrolytes on the stability of GO in water, a series of experiments were conducted using Mg²⁺ and Ca²⁺, which are common in natural aquatic environments. Additionally, humic acid (HA), sodium alginate (alginate), and polyacrylic acid (PAA) were selected as NOMs to construct GO aggregation and sedimentation systems. The complex mechanisms of the effects of NOMs and electrolytes on the stability of GO in water were investigated by analyzing the kinetics of GO aggregation and sedimentation in water, as well as the effect of redispersion on GO aggregation and sedimentation. The results showed that GO exists as two-dimensional sheets with a variety of oxygen-containing functional groups on the surface and exhibits characteristic ultraviolet absorption peaks. The zeta potential of GO was less than −30 mV, indicating its strong electronegativity, which allowed it to be stably dispersed in water. Mg²⁺ and Ca²⁺ induce GO aggregation in water by compressing the electric double layer. The aggregation speed increased with increasing electrolyte concentration and the particle size increased after aggregation. The attachment efficiency (α) first increased and then stabilized. The aggregation process was divided into two stages: slow aggregation and fast aggregation, which is consistent with DLVO theory. The critical coagulation concentrations of Mg²⁺ and Ca²⁺ were 4.5 mmol·L⁻¹ and 1.8 mmol·L⁻¹, respectively. After aggregation, the GO settled and separated from the water. The sedimentation rate increased with increasing electrolyte concentration, and was positively correlated with the aggregation rate. Compared with Mg²⁺, Ca²⁺ exhibited a stronger affinity for GO, resulting in faster aggregation, larger aggregate sizes, tighter structures, and higher sedimentation rates. The NOMs did not significantly alter the surface charge characteristics of GO and did not directly trigger aggregation. In Mg²⁺ aggregation systems, HA and Alginate inhibited GO aggregation through steric hindrance after adsorption by GO, which reduced the attachment efficiency and slowed down aggregations; however, aggregation processes could still be divided into two stages. Compared with Alginate, HA is more easily adsorbed by GO because its ring structure is similar to that of GO, thereby exerting a stronger inhibitory effect on aggregation. Ca²⁺ can form a complex with HA, resulting in its aggregation. In the coexistence system of GO, Ca²⁺, and HA, two primary aggregation processes occurred: aggregation of GO caused by the compressed electric double layer and aggregation of HA caused by the complexation reaction. These primary processes led to the formation of complex secondary aggregates. The synergistic effects of the multiple aggregation processes accelerated the overall aggregation of the system. PAA of different molecular weights inhibited the aggregation and sedimentation of GO in water. The inhibitory effect of PAA with a high molecular weight (450 kDa) is significantly stronger than that of PAA with a low molecular weight (2 kDa). This suggests that organic matter with a high molecular weight is more effective in promoting stable dispersion of GO in water. In Ca²⁺ aggregation systems, PAA did not induce enhanced aggregation similar to HA. This indicated that the various oxygen-containing functional groups attached to the aromatic ring structure of HA might be the primary active sites for complexation reactions with Ca²⁺. The settled GO aggregates were decomposed and redispersed via mechanical stirring. After redispersion, the particle size of GO increased significantly compared to its initial size, and GO spontaneously aggregated and sedimented. Affected by enhanced hydration and reduced Brownian motion, the aggregation and sedimentation of the redispersed GO were slower than the initial aggregation and sedimentation. The aggregation and sedimentation of GO after redispersion in the presence of organic matter are consistent with those observed in the absence of organic matter. However, the particle size after dispersion was larger, and the aggregation and sedimentation were slower. The complex mechanisms of the effects of NOMs and electrolytes on the aggregation and sedimentation of GO in water were systematically investigated in this study, which could provide a theoretical basis for comprehensively evaluating the stability and ecological risk of GO in aquatic environments.

Key words: graphene oxide, natural organic matters, calcium and magnesium ions, aggregation kinetics, sedimentation kinetics, redispersion