Transport Of Graphene Oxide Research Articles

Transport of graphene oxide in the capillary fringe: Insights from sandbox experiments and numerical simulation

Studying contaminant transport in the capillary fringe (CF), a crucial part of the vadose zone, offers insights into the mechanisms controlling pollution in soils and groundwater aquifers. This paper investigated contaminant transport in the CF by continuously injecting a conservative tracer (NaCl) and graphene oxide nanoparticle (GONP), an adsorptive contaminant, into a sandbox. After entering the CF from the unsaturated zone, both NaCl and GONP underwent lateral transport. The breakthrough curves (BTCs) for NaCl and GONP were derived from water samples collected at predetermined sampling holes. Subsequently, contaminant transport in the CF was modeled using a one-dimensional–two-dimensional (1D-2D) coupled hydrodynamic model. This model incorporated lateral dispersivity (αL = 1.198 cm) and longitudinal dispersivity (αT = 0.286 cm), calculated using a point-by-point method. The hydrodynamic dispersion coefficients obtained were then applied to the Brooks and Corey (BC) and the van Genuchten (VG) parametric models. The BC model more accurately simulated the NaCl migration compared to the VG model, leading to its application in simulating GONP transport in the CF. However, the simulated BTCs for GONP showed a lag behind the measured data, especially at high ionic strengths. This discrepancy was attributed to the variable adsorption partition coefficient of GONP under different ionic conditions. During the experiment, GONP adsorption onto the porous media's surface altered the capillary dynamics, notably increasing capillary rise height, decreasing seepage velocity, and reducing GONP dispersion. Therefore, it is necessary to consider the adsorption capacity of the contaminants in order to accurately assess their transport within the vadose zone.

Effects of polyamide microplastic on the transport of graphene oxide in porous media

With the rapid development of the nano-material and chemical industry, more and more microplastic (MP) and nano-material were discharged into the environment. In this study, a two-dimensional (2D) surface of Extended Darjaguin-Landau-Verwe-Overbeek (XDLVO) is proposed to quantitatively investigate the effect of polyamide (PA) on the transport of graphene oxide (GO) in porous media. The influences of mass fraction of PA, flow rate, GO concentration, ionic type and strength on the migration of GO in saturated porous media are investigated by column experiments and numerical models. The two-dimensional (2D) surfaces of XDLVO interaction energy between GO and GO, GO and QS, GO and PA, are firstly calculated to analyze the transport of GO in saturated porous media. Experimental results suggest the mobility of GO is enhanced when flow velocity and initial concentration of GO are increased. However, the mobility of GO is inhibited when the mass fraction of PA and ionic strength are increased. More important, the inhibitory effect of divalent cations on GO migration is stronger than that of monovalent cations. Simultaneously, XDLVO results suggest that ionic types and strengths are important factors affecting the mobility of GO in porous media, and the critical ionic strength is observed from the continuous variation of the secondary minimum trap of XDLVO interaction energy. Model results show that there is a linear relationship between the logarithm of the secondary minimum trap of XDLVO interaction energy and the parameters related to GO mobility, which suggests XDLVO energy surface has an important application significance in the accurate quantification of GO mobility in porous media. These findings contribute to GO transport affected by microplastic in porous media, thus laying a significant foundation for the environmental risk and contamination remediation.

Effect of root exudates on the stability and transport of graphene oxide in saturated porous media

Root exudates are a major source of dissolved organic matters that strongly affect the stability and transport behaviors of nanomaterials in porous media. This study investigated the effect of citric acid (CA) and oxalic acid (OA), two common low molecular weight root exudates, on the stability and transport of graphene oxide (GO) in saturated sand columns under different combinations of pH (4.5, 7.0), ionic strength (IS: 10, 50 mM), and organic acid concentrations (10, 25 mM). Both OA and CA accelerated GO aggregation, especially under high IS and acid concentration conditions. With the presence of OA/CA (≥ 10 mM), the transport of GO was higher at pH of 7.0 than 4.5, and the GO mobility decreased with increasing IS and OA/CA concentrations, whereas, enhanced GO transport was observed at a low concentration of OA/CA (0.1 mM), indicating that the influence of organic acid was concentration-dependent. All the results suggest that perturbations of surface potential of GO and sand, as well as the chemical structure of organic acids under different solution chemistry conditions are crucial in controlling GO stability and transport behaviors. Mathematical models based on the advection-dispersion equation with one-site kinetics simulated the experimental breakthrough curves of GO very well.

Graphene oxide transport and retention in biochar media

This study explores the use of biochar (BC), an inexpensive filtration media, for removing graphene oxide (GO) contaminants from the aquatic subsurface environments. Mass balance approaches and column dissection tests were used to analyze the retention behavior of GO in a series of model fixed-bed columns as a function of ionic strength (IS) and flowrate. The column based on the biochar media (BC) displayed 3.6-fold higher retention compared to the quartz sand (control). To overcome the challenges of unfavorable electrostatic interactions between GO and BC, we used a facile functionalization strategy to modify the BC surfaces with nanoscale zero-valent iron (BC-nZVI). The BC-nZVI (5:1, w/w) retained 2.6-fold higher amounts of GO compared with bare biochar. Furthermore, the performance of BC-nZVI increased with decreasing values of IS, attributed to the attachment of GO to nZVI where nZVI was partially dissolved by the presence of higher chloride ion at high IS. A better GO retention (86%) at higher IS was observed in BC where the GO was primarily retained due to the higher aggregation via straining.

Transport of graphene oxide in saturated quartz sand containing iron oxides

The environmental implications of graphene oxide (GO) have received much attention. Transport of GO in subsurface environment is a critical process affecting the migration and potential risks of this important class of carbonaceous nanomaterials. To date, the effects of heterogeneity in porous media, in particular, iron oxides, on GO transport are not well studied. In this study, we investigated the transport properties of GO in saturate quartz sand as affected by the presence of iron oxides, using goethite, hematite and ferrihydrite as the model iron oxide species, and applied a two-site transport model (which accounts for both attachment and straining) to fit the transport data. We found that iron oxide coating on sand surfaces markedly inhibited GO transport, mainly due to increased electrostatic attraction between particles and collectors, as the positively charged iron oxides provided favorable deposition sites for the negatively charged GO nanosheets. Additionally, increased surface roughness was likely an additional mechanism leading to the enhanced GO deposition. The extent of transport inhibition by iron oxides also depended on the morphology iron oxides, in that at the same Fe loading a larger effect was observed when iron oxides existed as the coating on sand surface than as discreet particles. The presence of iron oxide coatings (tested using goethite) could magnify the effects of cations on GO transport. Specifically, the presence of goethite facilitated the accumulation of cations on the surface of sand, and in the case of Ca2+, the binding of GO via the cation-bridging mechanism was enhanced, as goethite contained abundant surface hydroxyl groups that are strong metal-complexing moieties.

Insights into aggregation and transport of graphene oxide in aqueous and saturated porous media: Complex effects of cations with different molecular weight fractionated natural organic matter

The stability of nanomaterials in aquatic environment is a critical factor that governs their fate and ecotoxicity. Meanwhile, the interaction between nanomaterials and ubiquitous natural organic matter (NOM) is a vital process that influences the transport and biological effects of nanomaterials in the environment. However, impacts of NOM on the aggregation and transport of two-dimensional nanomaterials, especially for the increasingly used graphene oxide (GO), are not well understood. Particularly, there is lack of exploration on potential impacts of the heterogeneous properties of NOM on GO behaviour, especially that induced by the wide molecular weight (MW) span of NOM. In this study, effects of several kinds of well-characterized MW fractionated Suwannee River NOM (Mf-SRNOMs) on the aggregation and transport of GO in aqueous media and saturated porous media were investigated. Our results suggest that the stability and migration capacity of GO under most investigated electrolyte conditions are promoted by all Mf-SRNOMs, and efficiencies of different Mf-SRNOMs are generally positively correlated with their MW. Primarily, mechanisms including MW-dependent steric hindrance and sorption of Mf-SRNOMs onto GO are critical in stabilizing GO, and thus facilitating its transport. However, the stronger sorption of higher Mf-SRNOMs onto the GO basal plane through π-π interaction further facilitated the cation bridging between both ends of Mf-SRNOM and GO, and resulted in heteroaggregation of NOM-GO. Moreover, the weight analysis indicated that despite the fact that high Mf-SRNOMs only occupied a small percentage of pristine-SRNOM, they showed a stronger contribution towards pristine-SRNOM's capacity in stabilizing GO, when compared with that of lower MW counterpart. These findings pointed out that complex effects of the heterogeneities of NOM and cations should be highly relevant when the aggregation and transport behaviour of two-dimensional nanomaterials is investigated, and NOM fractions that are highly aromatic and of a higher MW should receive greater attention.

Effect of physicochemical factors on transport and retention of graphene oxide in saturated media.

Fate and transport of graphene oxide (GO) have received much attention recently with the increase of GO applications. This study investigated the effect of salt concentration on the transport and retention behavior of GO particles in heterogeneous saturated porous media. Transport experiments were conducted in NaCl solutions with three concentrations (1, 20, and 50 mM) using six structurally packed columns (two homogeneous and four heterogeneous) which were made of fine and coarse grains. The results showed that GO particles had high mobility in all the homogeneous and heterogeneous columns when solution ionic strength (IS) was low. When IS was high, GO particles showed distinct transport ability in six structurally heterogeneous porous media. In homogeneous columns, decreasing ionic strength and increasing grain size increased the mobility of GO. For the column containing coarse-grained channel, the preferential flow path resulted in an early breakthrough of GO, and further larger contact area between coarse and fine grains caused a lower breakthrough peak and a stronger tailing at different IS. In the layered column, there was significant GO retention at coarse-fine grain interface where water flowed from coarse grain to fine grain. Our results indicated that the fate and transport of GO particles in the natural heterogeneous porous media was highly related to the coupled effect of medium structure and salt solution concentration.

Concurrent aggregation and transport of graphene oxide in saturated porous media: Roles of temperature, cation type, and electrolyte concentration.

Simultaneous aggregation and retention of nanoparticles can occur during their transport in porous media. In this work, the concurrent aggregation and transport of GO in saturated porous media were investigated under the conditions of different combinations of temperature, cation type (valence), and electrolyte concentration. Increasing temperature (6-24 °C) at a relatively high electrolyte concentration (i.e., 50 mM for Na+, 1 mM for Ca2+, 1.75 mM for Mg2+, and 0.03 and 0.05 mM for Al3+) resulted in enhanced GO retention in the porous media. For instance, when the temperature increased from 6 to 24 °C, GO recovery rate decreased from 31.08% to 6.53% for 0.03 mM Al3+ and from 27.11% to 0 for 0.05 mM Al3+. At the same temperature, increasing cation valence and electrolyte concentration also promoted GO retention. Although GO aggregation occurred in the electrolytes during the transport, the deposition mechanisms of GO retention in the media depended on cation type (valence). For 50 mM Na+, surface deposition via secondary minima was the dominant GO retention mechanism. For multivalent cation electrolytes, GO aggregation was rapid and thus other mechanisms such as physical straining and sedimentation also played important roles in controlling GO retention in the media. After passing through the columns, the GO particles in the effluents showed better stability with lower initial aggregation rates. This was probably because less stable GO particles with lower surface charge densities in the porewater were filtered by the porous media, resulting in more stable GO particle with higher surface charge densities in the effluents. An advection-dispersion-reaction model was applied to simulate GO breakthrough curves and the simulations matched all the experimental data well.

Macroscopic and molecular study of the sorption and co-sorption of graphene oxide and Eu(III) onto layered double hydroxides

In recent years, the fate and transport of graphene oxide (GO) has received considerable attention owing to its inevitable release into the natural ecosystem during production and applications, wherein mineral particles and radionuclides are generally co-presented. This paper aimed to clarify the sorption and co-sorption of Eu(III) and GO onto Mg/Al layered double hydroxides (LDH) using batch and extended X-ray absorption fine structure (EXAFS) studies. The results showed that Eu(III) and GO displayed interactively enhancement roles in their co-sorption on LDH. Eu(III) enhanced GO co-sorption on LDH which was mainly due to the formation of “LDH-Eu(III)-GO” ternary surface complexes (i.e., Eu(III)-bridging). While, GO increased Eu(III) co-sorption on LDH which was primarily due to the formation of “LDH-GO-Eu(III)” ternary surface complexes (i.e., GO-bridging). EXAFS fitting results exhibited that the presence of GO could suppress the formation of outer-sphere complexes of Eu(III) on LDH at low pH values, while inhibit the formation of surface precipitates of Eu(III) on LDH at high pH values. The enhanced co-sorption of Eu(III) and GO on LDH as they co-presented might decrease their mobility in the natural environment. These new findings are important to understand the environmental behavior and impact of GO in real ecosystem, and the natural purification of radionuclides in ecological environments.

Retention and transport of graphene oxide in water-saturated limestone media

In this work, column experiments were conducted to investigate the transport characteristics of graphene oxide (GO) nanoparticles in limestone media under various electrolytes, solution pH, and humic acid (HA) concentration conditions. In the limestone media, GO exhibited relatively low mobility with the mass recovery rate lower than 65.2%, even when solution ionic strength was low. The presence of HA enhanced its mobility. In addition, the presence of S2-, a divalent anion, also promoted GO transport in limestone media compared to Cl− under similar ionic strength conditions through neutralizing more positive charge and thus diminishing the cation bridging. Solution pH showed slight effect on the transport of GO in limestone with the mass recovery range from 40.3% to 51.7%. Over all, decreases in solution pH, HA concentration and increases in solution ionic strength reduced the mobility of GO in the limestone media under the tested conditions. These results indicated both environmental conditions and media characteristics played important roles in controlling GO fate and transport in porous media. The one-site kinetic deposition model was applied to describe the interactions between the GO and limestone media and model simulations fitted the observed experimental data very well. As limestone is an important component of aquiferous media in subsurface, findings from this study elucidated the key factors and processes controlling the fate of GO particles in limestone media, which can inform the prediction and assessment of the risks of GO in groundwater environment.

Transport, retention, and size perturbation of graphene oxide in saturated porous media: Effects of input concentration and grain size

Accurately predicting the fate and transport of graphene oxide (GO) in porous media is critical to assess its environmental impact. In this work, sand column experiments were conducted to determine the effect of input concentration and grain size on transport, retention, and size perturbation of GO in saturated porous media. The mobility of GO in the sand columns reduced with decreasing grain size and almost all GO were retained in fine sand columns for all of the tested conditions. This result can be explained with colloid filtration and XDLVO theories. Input concentration also influenced the retention and transport of GO in the sand columns because of the ‘blocking’ mechanism that reduces the particle retention rate. After passing through the column, average GO sizes increased dramatically. In addition, the sizes of GO retained in the sand also increased with travel distance. These results suggested that transport through the porous media induced GO aggregation. A mathematical model based on the advection–dispersion equation coupled with the second-order kinetics to reflect the blocking effect simulated the experimental data well.

Deposition and transport of graphene oxide in saturated and unsaturated porous media

In this work, sand and bubble column experiments were conducted to explore the deposition mechanisms of graphene oxide (GO) particles in porous media with various combinations of moisture content and ionic strength. Sand column experimental results indicated that retention and transport of GO in porous media were strongly dependent on solution ionic strength. Particularly, GO showed high mobility under low ionic strength conditions in both saturated and unsaturated porous media. Increasing ionic strength dramatically increased the retention of GO particles in porous media, mainly through secondary-minimum deposition as indicated in the XDLVO interaction energy profiles. Recovery rates of GO in unsaturated sand columns were lower than that in saturated columns under the same ionic strength conditions, suggesting moisture content also played an important role in the retention of GO in porous media. Findings from the bubble column experiments showed that the GO did not attach to the air–water interface, which is consistent with the XDLVO predictions. Additional retention mechanisms, such as film straining, thus could be responsible to the reduced mobility of GO in unsaturated porous media. The experimental data of GO transport through saturated and unsaturated porous media could be accurately simulated by an advection–dispersion-reaction model.

Effects of Solution Chemistry on the Transport of Graphene Oxide in Saturated Porous Media

A transport study was performed in saturated porous media through a packed bed column to simulate fate of graphene oxide nanoparticles (GONPs) in the subsurface environment. Transport experiments, along with mass balances and column dissections, were conducted as a function of ionic strength (IS, 10(-3)-10(-1) M). Additionally, an extensive evaluation of the electrokinetic properties and hydrodynamic diameters of GONPs were determined as a function of IS and pH. The measured hydrodynamic diameter and the electrophoretic mobility (EPM) of GONPs indicated an insensitivity to pH, although IS did play a role. Results from a stability study indicated that the hydrodynamic diameter of GONPs was stable and unchanging at the lower range of IS (10(-3) and 10(-2) M) then became unstable when IS ≥ 10(-1.5) M KCl was achieved. Specifically, for IS ≥ 10(-1.5) M KCl, the hydrodynamic diameter became greater and showed a larger size range of particles than at the lower IS range (10(-3) and 10(-2) M). In addition, the EPM of GONPs became less negative over the IS range of 10(-3) and 10(-2) M KCl. Furthermore, GONPs were found to be increasingly mobile for IS ≤ 10(-2) M KCl. When GONPs were passed through the packed bed column at 10(-2) and 10(-1) M KCl, 5% and 100% of the GONPs were retained in the column, respectively. Finally, mass balances and column dissections revealed that in the first cm of the column 7% and 95% of the GONPs were deposited at 10(-2) and 10(-1) M KCl, respectively, confirming that the transport of GONPs is a function of IS. The fraction of GONPs eluted during the transport experiments provides insight into the contribution of aggregation and reversibly bound fraction of GONPs in saturated porous media.