Transport of graphene quantum dots (GQDs) in saturated porous media

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Abstract

Graphene quantum dots (GQDs), a new type of carbon-based nanoparticles are widely used in scientific applications in the recent years. With the size less than 10 nm, and the strong hydrophilic characteristics GQDs exhibit high mobility in the environment, and can easily transmit to the groundwater environment. Hence, a good understanding of the transport behaviors of GQDs in porous media can provide guidance for assessment and prediction of its environmental risks. Saturated packed column experiments were conducted to investigate the effect of ionic strength (IS,1 and 100 mM), pH (4,7 and 9) and initial concentration (9 and 18 mg L−1) on the fate and transport of GQDs in saturated porous media. Advection-dispersion-reaction model with the second-order kinetics simulated the process well. The retention of GQDs was promoted with increased ionic strength in sand column, which could be explained with classic filtration and XDLVO theories. The increase of pH slightly reduced the GQDs deposition in saturated porous media, due to that there were no secondary minima in XDLVO calculation, and GQDs still showed high mobility. Higher input concentration slightly increased the GQDs retention due to the ripening process. The recovery rate of GQDs were more than 90 % in the environment with low salt content, exhibiting extremely high migration capacity.

Introduction

Carbon-based nanomaterials have developed rapidly in scientific applications in the recent years due to their excellent conductivity, magnetism, optical properties, etc. [1]. Graphene is a type of hexagonal planar two-dimensional carbon material with honeycomb lattice composed of carbon atoms and sp2 hybrid orbits, and it exhibits excellent high mechanical strength and good thermal properties [2]. When the transverse size of graphene is reduced to 10 nm, the movement of electrons in the material is limited, which has obvious quantum confinement effect [3]. This nano-sized graphene material is known as graphene quantum dots (GQDs). GQDs are generally about 10 nm in diameter and 0.5–1.0 nm in thickness. The surface of GQDs contains carboxyl and hydroxyl groups, which makes it water-soluble. Since GQDs and its hybrids have high sensitivity and good selectivity, so they play an important role in bioimaging, fluorescence sensing, photocatalysis, fuel cells, electronic devices, agricultural food and other fields [[4], [5], [6], [7], [8], [9], [10], [11], [12], [13]]. The increased use of GQDs, as they are released into the environment, will pose significant threat to the ecosystem, public health and future sustainability of water resources [14].

Underground aquifer is one of the precious resources on the earth. As a new carbon nanomaterial, still very little is understood about GQDs’ deposition and transport processes in porous media. However, numerous investigations have been conducted on the transport behavior of other carbon-based nanomaterials in porous media. The research found factors, such as ionic strength(IS) [15,16], pH [17], initial concentration [18], grain size [19], surface heterogeneities [20], flow rate [21], etc. affect the migration of carbon-based nanoparticles. Based on XDLVO theory, the chemical solution condition---IS and pH, has significant effect on the transport of carbon-based nanomaterials. For fullerenes [22], graphene oxide (GO) [23] and carbon dots (CDs) [24], their deposition process in saturated porous media enhanced with increasing IS. Sharma et al. found at the critical deposition IS of 4 mM NaCl, exceeded 99 % carbon nanotubes (CNTs) was filtrated from a 15 cm length column [25].Wang et al. found that sensitivity of GO to temperature was depending on IS. At low IS, temperature has almost no effect on the retention and transport of GO, but when the IS increases to 10 mM, the temperature showed significant effect on the retention and migration of GO in saturated porous media [26]. In term of pH, both Tian et al. and Sharma et al. proved that the increase of pH value resulted in the increase of carbon nanotubes (CNTs) mobility [25,27];however, graphene (GR) is not sensitive to the pH condition. Liu et al. found that graphene (GR) has a high mobility in acid-cleaned sand, regardless of whether the pH is acidic or alkaline, although the recovery rate under alkaline condition is 1.1 % higher than under acidic condition [17].

There are no consistent conclusions on the effect of input concentration on the mobility of carbon-based nanomaterials. Sun et al. showed that higher input concentration of GO produced 'blocking' mechanism, and as a result reduced the particle retention rate [28]. On the contrary, Zhang et al. found when IS greater than 0.1 mM, higher GO input concentrations resulted in increased colloid retention [29].

There has been no studies investigated the fate and transport of GQDs, only carbon dots were illustrated by Kamrani et al. that the mobility of carbon dots(CDs) in saturated porous media, increased with the increase of pH, the decrease of IS, the increase of sand particle size and the increase of initial concentration [24]. Both GQDs and CDs are nanoparticles with size smaller than 10 nm, but the structure of GQD is similar to layered structure, while CDs is spherical structure. In contrast to the hydrophobicity of CDs, GQDs have good hydrophilicity and high mobility [30,31]. Thus, their transport behaviors would be different. Since IS, pH and initial concentration are the key factors to understand the retention and transport of nanoparticles in porous media, the objectives of this study are, 1)understanding the effect of IS on GQDs transport in saturated porous media;2)exploring the retention of GQDs in saturated porous media under neutral, acidic and alkaline conditions of pH; 3)investigating the effects of initial input concentration on GQDs migration in saturated porous media, and at the end environmental guidance is provided.

Section snippets

Materials

GQDs used were purchased from manufacturer (ALADDIN Biochemical Technology, Inc.), with layered diameter range of 3−6 nm. Its density is 1 mg/cm3 and stock concentration 1000 mg/L. GQDs size were measured by transmission electron microscopy (TEM, JEOL JEM-2100). The GQDs input concentration levels of 18 mg/L or 9 mg/L in the experiment, were adjusted by diluting the stock GQDs suspension. Before injection to the column, GQDs were sonicated (KH-500DB) for 2 h for thorough dispersion. Sodium

Characterization and stability of GQDs

GQDs were stable under all experimental conditions (Table 1) across the experimental period. The TEM images (Fig. 1a, b) showed GQDs were monodispersed in DI water, and the average particle size of GQDs was about 5 nm.The GODs concentration kept constant (Fig. 1c) over a 3 h period. Meanwhile, the XDLVO energy barrier between GQDs particles (Fig. 1d) under all experimental conditions were higher than 10 mJ/m2, which is difficult to overcome to aggregate for GQDs. Although second minimum energy

Conclusions

In this study, a number of experiments were conducted to study the transport and removal of GQDs in saturated porous media under different ionic strength, pH and initial concentration conditions. A mathematical model was used to simulate the experimental data and help explore the transport and removal mechanism. It was found that ionic strength is the key factor in determining the migration of GQDs in saturated porous media. The retention rate of GQDs increased significantly with the increase

Authors’ contributions

CRY and XG wrote the manuscript, XG conducted the experiments, JJ provided the Zeta potential data, XTG and ZBY provided important advices for the investigation. All authors reviewed the final manuscript.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (Grant No. 51509069); the Fundamental Research Funds for the Central Universities in China (Grant No. 2019B10814); the Open Research Fund of State Key Laboratory of Hydrology-Water Resources and Hydraulic Engineering, Hohai University (Grant No. 2017490811), the Special Fund of State Key Laboratory of Hydrology-Water Resources and Hydraulic Engineering, Hohai University (Grant No. 20195025612).

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