Transport of Surface-modified Carbon Nanotubes through a Soil Column

Summary

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.

Abstract

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.

Introduction

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 nanoparticles^1-3. Carbon nanotubes (CNTs) are one of the most produced carbon-based nanoparticles^2,4. 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 environment^5-8.

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 biodegrade^5,8. 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 size^13-15, 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 limited^18-20. 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.

Protocol

1. Functionalization of Multiwalled Carbon Nanotubes

1. Perform the entire functionalization step inside a fume hood, using safety eye glasses, gloves and lab coat. Measure 24 ml of sulfuric acid and 8 ml of nitrate acid using a graduated cylinder, and then transfer them into a beaker. Add 32 mg of untreated MWCNTs into a beaker using tin foil container at an analytical balance (final concentration should be 1 mg/ml of acid mixture).
2. First, keep the beaker with MWCNT and acid mixture in the ultrasonic cleaner (bath) for 2 hr at RT. Then, heat and stir the MWCNT-acid solution for 5 hr at 90 °C using a hot plate.
3. Filter the CNT suspension with a 0.2 µm pore diameter PTFE filter membrane placed on a filter holder, and use vacuum to help filtration. Perform the filtration portion by portion and use several filter membranes (approximately 1/4^th of above mixture per portion for one filter). Add boiling water during filtration process to filter out the acid solution until the pH of the mixture becomes greater than 5.
4. Always break the vacuum before it is turned off and not to introduce anything into the vacuum system. Use a conical beaker to collect the waste liquid.
5. Pour filtrated acid into a waste container (send the waste container to a waste handling facility or dilute the liquid before dumping it into the sink by adding at least ten times of tap water).
6. Transfer the filter membranes with retained MWCNTs into evaporating dishes and put the dishes into the desiccator (contains approximately 100 g of silica gel) and create a vacuum environment (leave the vacuum on for about 1 hr) for CNT to complete drying (about 24 hr).
   1. Scrape the CNTs out of the membranes carefully using spatula and transfer the particles into a clean container. Weigh the MWCNTs powder and label the container for future use.

2. Porous Media for Transport Experiments

1. Prepare 0.1 M HCl solution for acid washing of silica sand.
   1. Perform all these steps inside a fume hood with safety eye glasses, gloves, and lab coat. Add 1 L de-ionized water to a 2 L flask. Measure 8 ml of 37% HCl using a graduated cylinder.
   2. Add the HCl into the de-ionized water carefully. Shake the flask carefully to help mixing.
2. Wash the sand with the prepared HCl solution.
   1. Weigh about 1,000 g sand. Add 1/3 of sand into the flask with the HCl solution and shake the flask twice to help mixing then add rest of the sand (1/3 of the sand each time).
   2. Shake the flask three times and leave the acid with sand for 30 min.
   3. Pour the liquid out of the flask to acid waste container and rinse the sand with de-ionized water at least 8 times.
3. Wash the sand with a H₂O₂ solution.
   1. Add 700 ml of de-ionized water into the flask with sand then measure 40 ml of 30% H₂O₂ solution using a graduated cylinder.
   2. Add the H₂O₂ solution into flask with sand and shake twice to help mixing. Then add another 40 ml of 30% H₂O₂ solution 3 times until there is 160 ml H₂O₂ total in the flask.
   3. Shake and mix the solution and sand each time and leave the H₂O₂ solution with sand for 40 min to allow the reaction to be completed. Shake the flask and stir the sand with a plastic rod every 10 min.
   4. Decant the liquid down to the sink and run tap water for 30 sec.
4. Rinse and dry the sand.
   1. Rinse sand with de-ionized water at least 8 times to get rid of any solution or left over reaction products. Shake and stir thoroughly when rinsing.
   2. Put flask with rinsed sand into an oven (105 °C) for 24 hr to dry, then take sand out of the oven using oven-mitten and leave at the counter for 2 hr for the sand to cool down.
   3. Transfer the clean sand into a plastic container. Mark the container and place it in an appropriate shelf to be ready for use.

3. Column Experiments

1. Preparation of background solution.
   1. Prepare appropriate background solution chemistry for the column experiment.
   2. Use 0.1 M HCl and 0.1 M NaOH solutions to adjust the pH and NaCl salt to achieve appropriate ionic strength for the following experiment.
2. Column selection.
   1. Choose a glass column of 2.5 cm diameter and 15 cm length for this experiment (pH: 5 and ionic strength: 2 mM in the current study). Use a steel mesh filter (0.2 mm) on both sides of the glass column.
   2. Flush the tubes connected to the column and fill with background solution (or MWCNTs solution until the 3-way valve to control the type of liquid flow (MWCNTs solution or background solution) as shown in Figure 1.
3. Wet-packing of the column.
   1. Weigh the clean sand on a scale and take 124 g of clean sand for the selected column size.
   2. Use a high precision peristaltic pump. Calibrate the pump to achieve 2 ml/min of liquid flow.
   3. Start the pump to fill the column from the bottom until the water level is a couple of centimeters above the bottom of the column. Put approximately 1/10^th of the measured sand at a time into the column but make sure that the sand level doesn’t come above the water level in the column. Continue the water flow to the column continuously to stay above the sand level.
   4. Close the column cap with appropriate filter mesh after complete filling.
   5. Allow the packed column to flow for at least 1 hr. The individual parameters of the column are indicated in Table 1.
4. Tracer test.
   1. Start the column experiment with a tracer test prior to the MWCNT solution experiments.
   2. Switch the 3-way valve to the tracer solution (using food color tracer at 20 mg/L) to start the experiment.
   3. Collect the outflow samples from the column at every 2 min (i.e., 4 ml/samples in each sampling tube) using the connected fraction collector as shown in Figure 1.
   4. Continue to inject the tracer solution for a 4.32 pore volume (i.e., solution passes 4.32 times of the total empty pore space in the sand packed column), which is also called phase I of the experiment.
   5. Switch the 3-way valve to flow background solution (DI water in the case of tracer experiment) for another 4.32 pore volume.
5. Preparation of MWCNT solution.
   1. Make a dispersed, functionalized MWCNTs solution by placing 15 mg of functionalized MWCNTs in a 300 ml beaker containing 200 ml of aqueous solution (with desired solution chemistry i.e., pH 5 and 2 mM ionic strength in the current experimental condition) and using an ultrasonic homogenizer probe placed in the beaker (with 40% power output for 15 min). Mix the dispersed MWCNTs solution with another 800 ml of the same aqueous solution to achieve the MWCNT concentration of 15 mg/L.
   2. Perform scanning electron microscopy (SEM) image analysis of stock solution for their size and shape of the nanoparticle after functionalization.
6. MWCNT transport experiment.
   1. Switch the 3-way valve to MWCNT solution to start the column experiment.
   2. Collect the outflow samples from the column at every 2 min using the connected fraction collector.
   3. Inject the MWCNT solution for a 4.32 pore volume (phase I of the experiment).
   4. Switch the 3-way valve to flow background solution for another 4.32 pore volume, which is called phase II of the experiment.
   5. Change the injection tube of background solution into DI water bottle (after stopping the pump for a moment to avoid the entrance of air from the tube) and continue the flow for another 4.32 pore volume, which is called phase III of the experiment.
7. Sample analysis.
   1. Transfer all the tube samples from fraction collector into a tube rack.
   2. Prepare a UV/VIS spectrophotometer for sample analysis, i.e., find out the appropriate scanning wavelength for the quantification of the collected samples. Use 400 nm for a MWCNT solution and a 333 nm wavelength for tracer solution.
   3. Scan all the samples collected from the column during phases I, II, and III using a cuvette at 400 nm wavelength (or a different wavelength if deemed more appropriate in the previous step) and store the data.
   4. Collect the data from the spectrophotometer and plot them vs time or pore volume to obtain breakthrough curves as shown in the representative results (for example, Figure 3).
   5. Perform size analysis (hydrodynamic diameter) of inflow and outflow samples using zeta sizer and conduct the visualization studied for both inflow and outflow samples using scanning electron microscopy.

Results

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...

Discussion

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...

Materials

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