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Method Article
Surface properties of a nanoparticle are important for their interaction with the surrounding medium. Therefore the surface modification of carbon nanotubes can be critical for their transport and retention through porous media. Here, lab scale column experiments are used to understand the possible transport and retention of these nanoparticles.
Carbon nanotubes (CNTs) are widely manufactured nanoparticles, which are being utilized in a number of consumer products, such as sporting goods, electronics and biomedical applications. Due to their accelerating production and use, CNTs constitute a potential environmental risk if they are released to soil and groundwater systems. It is therefore essential to improve the current understanding of environmental fate and transport of CNTs. The transport and retention of CNTs in both natural and artificial media have been reported in literature, but the findings widely vary and are thus not conclusive. There are a number of physical and chemical parameters responsible for variation in retention and transport. In this study, a complete procedure of selected multiwalled carbon nanotubes (MWCNTs) is presented starting from their surface modification to a complete set of laboratory column experiments at critical physical and chemical scenarios. Results indicate that the stability of the commercially available MWCNTs are critical with their attached surface functional group which can also influence the transport and retention of MWCNT through the surrounding medium.
With the recent development in nanotechnology that uses various types of nanoparticles to improve a number of technologies in industries such as information technology, energy, environmental science, medicine, homeland security, food safety, and transportation; a thorough understanding of the transport and retention of nanoparticles in soil and groundwater is critical for risk assessment as well as environmental applications of engineered nanoparticles1-3. Carbon nanotubes (CNTs) are one of the most produced carbon-based nanoparticles2,4. CNTs are the long and cylindrical form of graphene with a diameter typically below 100 nm and a length in the range of 100 nm to 50 µm. They have unique properties, which have accelerated their use in many applications, such as electronics, optics, cosmetics, and biomedical technology (e.g., composite materials)5. With increased use, there is also an increased risk to human exposure and effect on health as well as adverse ecological consequences following CNT and other carbon based nanomaterials disposal to the environment5-8.
With no surface modifications (unfunctionalized), CNTs are extremely hydrophobic and tend to aggregate in an aqueous solution. Functionalized CNTs can, however, remain dispersed and stable in aqueous solutions and are used for biomedical purposes such as drug delivery9. Here it is essential that the CNTs remain dispersed and mobilized, so the drug can be delivered within the human body10. On the other hand, to reduce environmental risks, there is a need for studies focusing on how to immobilize the CNTs in order to avoid their entrance into aquifers and drinking water resources11. Recent studies have reported the toxic effect of CNTs on living organisms and also risks to ecosystems in terms of CNTs entering and accumulating in the food chains, since CNTs are hard to biodegrade5,8. Even with barrier systems in landfills containing CNTs, it may be possible for CNTs to pass through the barriers. In such cases CNTs could enter into groundwater reservoirs and surface water bodies. As CNT disposal regulations are not well defined and transport mechanisms are poorly understood, an improved understanding of mobility of CNTs is necessary to formulate and design appropriate disposal systems12. Therefore, it is important to study and understand the fate and transport of CNTs in porous media and the effect of physical and chemical factors commonly present in the subsurface environment on surface modified CNT retention.
A number of research has been carried out about the effect of collector grain size13-15, flow rate16, and surface properties of the grains17 on transport of nanoparticles in porous media. However, systematic investigations on the effect of solution chemistry (such as pH and ionic strength) on possible deposition onto the collector surfaces are still limited18-20. Additionally, the combined impact of physical factors, solution chemistry of the medium, and surface properties of carbon nanotubes is not well understood and vary in different literature. In this study, a preparation method for surface modification of MWCNTs will be demonstrated along with a systematic laboratory-scale column packed with acid-cleaned quartz sand will be used to investigate the transport, retention and remobilization of surface-modified CNTs in saturated porous media.
1. Functionalization of Multiwalled Carbon Nanotubes
2. Porous Media for Transport Experiments
3. Column Experiments
Effect of MWCNT Functionalization
The functionalized and dispersed MWCNT solution was sealed in the beaker to allow the solution to reach equilibrium. There was neither sedimentation nor aggregation observed in the stock solution after sonication, as the hydrodynamic diameter of MWCNT (1,619 ± 262 nm) in the solution remained the same for six months of sonication (Figure 2). To investigate the effect of functionalization of MWCNTs on their mobility, two s...
Effect of MWCNT Functionalization
As Figure 2 confirms the stability of functionalized MWCNTs, the observed difference in eluted volume of MWCNT was due to functionalization and particularly due to the addition of carboxyl (-COOH) groups to the surface of the MWCNTs (Figures 3 and 4). In the similar functionalization process, the presence of oxygen was confirmed by X-ray photoelectron spectroscopy14. It has been fou...
The authors declare that they have no competing financial interests.
The authors would like to acknowledge the support from the Department of Earth Sciences, Uppsala University for supporting part of this research.
Name | Company | Catalog Number | Comments |
MWCNT | Cheap Tubes Inc., USA | sku-03040304 | Purchased as semi-functionlized powder |
Quartz sand | Sibelco Nordic, Baskarp, Sweden | B44 | Purchased with more than 91% silica sand |
H2SO4 | VWR | 1.01833.2500 | 95%-97% purity |
HNO3 | VWR | 1.00441.1000 | 70% purity |
HCl | VWR | 1.00317.2500 | 37%-38% purity |
H2O2 | VWR | 23615.248 | 30% purity |
NaCl | VWR | 1.06404.0500 | 99.5% purity |
NaOH | Sigma-Aldrich | S8045-500G | 99.99% pur pellets |
Ultrasonic Homogenizer | Biologics Inc. Manassas, Virginia | Model 3000, 0-127-0002 | Operated for fix time interval |
Sonicator (bath) | Kerry Ultrasonic Ltd | 1808 | Common bath sonicator |
Peristaltic pump | Ismantec, Glattbrugg, Switzerland | ISM931 | Work with tygon tubing in the pump |
Spectrophotometer | Hach Lange | DR500, LPV408.99.0001 | Operate with manual cuvette as well as automated sampling |
pH meter | Metrohm | 781 | pH analysis |
Glass column | Chromaflex | 420830-1510 | Column with adjustable cap |
Fraction collector | Spectrum Labs Europe | CF-2, 124846 | Fixed at regular interval of time |
Fraction collector tubes | VWR | 212-9599 | 6 ml volume glass tube |
Hot plate stir | Thermo Scientific | SP131320-33 | Adjustable tempurature |
Oven | Elektro Helios | 259 | For oven dry of sand |
Balance | Mettler Toledo | AE 160 | For accurate weight |
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