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Research Papers

# Chemical Methods for the Separation of Copper Oxide Nanoparticles From Colloidal Suspension in DodecaneOPEN ACCESS

[+] Author and Article Information
Mohammed H. Sheikh

Aerospace Engineering and
Mechanics Department,
The University of Alabama,
Tuscaloosa, AL 35487-0280
e-mail: msheikh@crimson.ua.edu

Mem. ASME
Aerospace Engineering and
Mechanics Department,
The University of Alabama,
Tuscaloosa, AL 35487-0280
e-mail: msharif@eng.ua.edu

Paul A. Rupar

Chemistry Department,
The University of Alabama,
Tuscaloosa, AL 35487-0336
e-mail: parupar@bama.ua.edu

1Corresponding author.

Manuscript received April 25, 2014; final manuscript received August 11, 2014; published online September 4, 2014. Assoc. Editor: Debjyoti Banerjee.

J. Nanotechnol. Eng. Med 5(2), 021007 (Sep 04, 2014) (8 pages) Paper No: NANO-14-1037; doi: 10.1115/1.4028284 History: Received April 25, 2014; Revised August 11, 2014

## Abstract

Several chemical methods for the separation of nanoparticles from a colloidal mixture in a phase change material (PCM) have been developed and systematically investigated. The phase changing property of the colloidal mixture is used in energy storage applications and the mixture is labeled as the nanostructure enhanced phase change materials (NEPCM). The objective is to investigate viable methods for the separation and reclamation of the nanoparticles from the NEPCM before its disposal after its useful life. The goal is to find, design, test, and evaluate separation methods which are simple, safe, effective, and economical. The specific NEPCM considered in this study is a colloidal mixture of dodecane (C12H26) and CuO nanoparticles of 1–5% mass fraction and 5–15 nm size distribution. The nanoparticles are coated with a surfactant to maintain colloidal stability. Various methods for separating the nanoparticles from the NEPCM are explored. The identified methods are: (i) chemical destabilization of nanoparticle surfactants to facilitate gravitational precipitation, (ii) silica column chromatography, and (iii) adsorption on silica particle surface. These different methods have been pursued, tested, and analyzed; and the results are presented in this article. These methods are found to be highly efficient, simple, safe, and economical.

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## Introduction

Phase change materials (PCMs) are used in thermal energy storage applications where energy is stored (as latent heat of fusion) by melting the PCM and is released during solidification [1-3]. Dispersing highly conductive nanoparticles into the PCM enhances the effective thermal conductivity of the PCM, which in turn significantly improves the energy storage capability [4-6]. The resulting colloidal mixture with the nanoparticles in suspension is referred to as nanostructure enhanced phase change materials (NEPCM). A commonly used PCM for energy storage application is the family of paraffin (CnH2n+2). Mixing copper oxide (CuO) nanoparticles (treated with surfactants for stability) in paraffin produces a stable and highly conductive NEPCM for energy storage. However, after numerous application cycles, the functionality of the NEPCM deteriorates and it is required to replace it with fresh supply. Disposal of the used NEPCM containing the nanoparticles is a matter of great concern as it cannot be discarded directly into the environment because of the short and long-term environmental and health hazards [7-10]. Due to the widespread application potential of NEPCM, it is very important to develop proper technology to separate the nanoparticles before the disposal of the NEPCM. This is the motivation behind the present study.

The separation method for a specific class of solid–liquid mixture depends on various factors such as the size of the particles, type/class of the liquid, mass/volume fraction of the particles in the mixture, physical properties of the base liquid and the particles. Filtration, distillation, centrifugation, electrophoresis, magnetic separation, chromatography, and chemical methods are among the most widely used methods to separate particulate matter from liquid–solid mixture. Every separation method has various operating requirements and the most critical ones being physical, chemical, and electrical properties of entities to be separated and the mixture to be processed [11-16]. By studying and analyzing such properties of NEPCM, various methods for the nanoparticle separation from the NEPCM have been identified. The identified methods include: (i) distillation under atmospheric and reduced pressure, (ii) mixing with alcohol mixture solvent, (iii) high speed centrifugation, (iv) chemical destabilization of nanoparticle surfactants to facilitate gravitational precipitation, (v) silica column chromatography, and (vi) adsorption on silica particle surfaces. These different nanoparticle separation methods have been pursued, tested, and evaluated. The results for the first three methods, which can be classified as physical separation methods, are presented in an earlier article [17]. The next three methods, viz., chemical destabilization of nanoparticle surfactants, silica column chromatography, and adsorption on silica particle surface are chemical separation methods. The results for these three methods are analyzed and presented in this paper.

## Methodology

###### NEPCM Configuration.

In this research, dodecane (C12H26) was used as the PCM base fluid and CuO nanoparticles were employed to enhance thermal conductivity of dodecane. Ninety-nine percent pure technical grade dodecane was used to obtain the NEPCM. Dodecane is a colorless liquid with a slight gasoline-like odor, has density of 0.753 g/cm3, and boiling point of 216 °C [18]. The CuO nanoparticles are of approximately spherical shape whose size varies from 5 to 15 nm with majority of them being about 9 nm. The density of CuO is 6.31 g/cm3 and they appear black in color. The nanoparticles used in this research was manufactured and provided by the Chemistry Department of the Auburn University [19].

Mixing the bare nanoparticles in the base fluid does not create a stable colloidal suspension. The particles coagulate together and quickly precipitate. In order to disperse the nanoparticles evenly in the base fluid and to achieve a stable nanocolloid, it is required to prevent agglomeration of particles using suitable treatment of the particles. As the CuO particles do not carry any charge leading to electrically and magnetically neutral behavior, a viable method is to restrict the agglomeration by placing a barrier between two approaching particles. This is achieved by coating the nanoparticle surface with stabilizing surfactant also known as ligands which act like a cushion among the particles. Sodium oleate (C18H33NaO2) is used as the stabilizer [19]. This forms numerous ligands on the particle surface which has a thread/string like structure, schematically shown in Fig. 1. The resulting coated nanoparticles have average mass composition of 69 ± 1.4% copper oxide and 31 ± 1.4% sodium oleate. The polar head of these ligands associates with particle surface whereas the nonpolar tail interacts with the base fluid. The ligands on particle surface provide a physical barrier which prevents particle to particle contact and subsequent agglomeration. Moreover, the string-like ligands exert additional opposing viscous drag on the particles against gravity preventing gravitational precipitation. Thus, a highly stable colloidal mixture of the nanoparticles and base fluid is obtained where the particles remain in the desirable homogeneous suspension for a long time (more than 8 months) with no or negligible precipitation [19]. This form of stabilization is known as steric stabilization.

###### Preparation of the NEPCM.

Nearly homogeneous NEPCM samples were prepared by dispersing desired amount of CuO nanoparticles into the base PCM, i.e., dodecane. The samples were prepared by following a mass fraction approach. In this approach, density of the nanoparticles and dodecane was used to find a required nanoparticle concentration (by mass) for a particular volume of NEPCM. The volume fraction, ϕvol, of CuO nanoparticles required for the preparation of NEPCM is calculated using law of mixtures given by Display Formula

(1)$φvol=wpρp/(wpρp+wbfρbf)$

where w and ρ are the weight and density and the subscripts p and bf are referred to nanoparticles and base fluid, respectively. In case the corresponding weight or mass fraction, ϕwt, is of interest, the following relation is used for conversion for a two component system: Display Formula

(2)$φwt=φvolρp/[φvolρp+(1-φvol)ρbf]$

The total mass of the particles, mp, required to be mixed in a certain volume of the base fluid, Vbf, to get the mass fraction, ϕwt, is calculated as Display Formula

(3)$mp=Vbfρbfφwt/(1-φwt)$

After determining the mass of the nanoparticles required, next step followed is dispersing this nanoparticle mass in required volume of dodecane, then heating and stirring vigorously at 60 °C on a hot-plate magnetic stirrer (SP131325Q, Thermo Fisher, Dubuque, IA), shown in Fig. 2, for 30 min to attain a stable colloidal suspension. This mixing in combination with heating produced a stabilized NEPCM, which appeared like a black ink and was used for further experiments. Using the oleate stabilized CuO nanoparticles, long-term stability of colloidal suspensions in various hydrocarbons (hexane, octane, dodecane, and eicosane) was observed both qualitatively and quantitatively for mass fraction up to 20% corresponding to volume fraction of about 3% [19]. In this work, only dodecane was considered as the base fluid to prepare the NEPCM and to conduct different trials.

###### Ultra Violet-Visualization (UV-Vis).

For each method the processed NEPCM samples after particle separation are analyzed to detect the presence of any trace of the nanoparticles for assessing the separation efficiency of the specific method. The UV-Vis spectroscopy was employed for this analysis. This technique was chosen over the other methods like electron microscopy due to its simplicity, versatility, speed, accuracy, and cost-effectiveness. A “Varian Cary 100” UV-Vis spectrophotometer was employed to determine absorption wavelengths. It is a double beam, recording spectrophotometer controlled by a computer operating under Windows 2000 and WinUV software. It hosts tungsten halogen as a visible light source and deuterium arc for ultra violet light and has wavelength range from 190 to 900 nm. A 1 cm cell was used to feed the sample in the sample holder. A wavelength range of 300 nm to 800 nm was used to capture the data. UV-Vis scan rate was set to 600 nm/min and data interval was 1 nm. The UV-Vis spectra were obtained for the pure dodecane and the processed sample to compare the results. Each specific trial was repeated at least 2–3 times and the UV-Vis spectrum was collected for each trial using two different subsamples to acquire highly accurate and undistinguishable spectrographs. Comparing the spectrographs for the pure dodecane and the processed sample after separation (clear liquid resembling dodecane), against a benchmark NEPCM sample containing the nanoparticles, the extent of nanoparticles remaining after the separation process can be determined. The reference NEPCM sample comprised of a mixture of 0.0165 wt.% concentration of the CuO nanoparticles in dodecane. The CuO nanoparticles absorb strongly at shorter wavelengths of the order of 300 nm, allowing for quick determination of their concentration. For the NEPCM with higher concentrations (∼0.5 wt.%), the absorbance values are very high at lower wavelengths, which makes the ordinate of the spectrograph disproportionately large. For this reason the spectrograph for a low concentration (0.0165 wt.%) is included as reference in the comparison of the spectrographs. To achieve 100% nanoparticle removal, the spectrographs for the pure dodecane and the processed sample should follow the same overlapping trend.

## Results and Discussion

In this section, the methods for the separation of the CuO nanoparticles from the NEPCM by chemical destabilization, silica column chromatography, and adsorption on silica particle surfaces, are described. The experimental results are discussed, analyzed, and presented.

###### Chemical Destabilization of Nanoparticle Ligands/Surfactants.

If the stabilizing ligand ends, attached to nanoparticle surfaces, can be detached by some process, then steric stabilization will break down leading to nanoparticle destabilization and consequential precipitation. After conducting several experiments using HCl and H2SO4, and getting unsatisfactory results, interaction of potassium hydroxide (KOH) with the NEPCM in the presence of ethanol produced the desired result. It was observed that KOH interaction with NEPCM, results in steric stabilization break down which makes nanoparticles fall down primarily due to gravity.

An initial trial was conducted using 5 ml of 0.5 CuO wt.% NEPCM, 3 ml of saturated aqueous solution of KOH, and 0.5 ml of ethanol. The 5 ml of NEPCM was taken in a test-tube and 3 ml of KOH solution was added followed by manual shaking for about 1 min. At this point little mixing was observed with no sedimentation of nanoparticles. A few drops of ethanol were added followed by continuous shaking. Nanoparticles were observed to form a dense black layer at test-tube bottom leaving behind a colorless top layer. Figure 3(a) shows the end result of trial 1. The clear layer on top of the precipitated nanoparticles is thought of as a mixture of base fluid dodecane and ethanol. Hexane was added to wash the test-tube walls and to make layers look distinct.

The removal of the oleate ligands from the CuO nanoparticles occurs via the reaction mechanism illustrated in Fig. 3(b). On left-hand side, a CuO nanoparticle is shown with the oleate group attached to its surface. It is hypothesized that KOH saponifies the oleate ester linkage, resulting in the formation of potassium oleate. The detachment of the oleate from the nanoparticle surfaces makes the particles unstable and results in gravitational precipitation as shown in Fig. 3(a). After drying out the hexane, the top clear layer in the test tube in Fig. 3(a) was analyzed by UV-Vis spectroscopy to detect the presence of nanoparticles. To record the data, a freshly prepared sample was irradiated at 20 °C by a dual source of light. The solution was removed at fixed times and its UV spectrum recorded until no further spectral changes occurred. Pure dodecane (99% technical grade) was used as reference sample (baseline). For the same sample, UV–Vis spectrum was recorded and presented in Fig. 4.

Absorbance spectrum of the sample from run 1 very closely resembles to the spectrum of baseline (pure dodecane), whereas the NEPCM spectrum possess very different behavior with high absorbance values. Hence it can be concluded that the treated sample is free from the CuO nanoparticles. This demonstrates that the aqueous KOH in the presence of ethanol is capable of removing the oleate ligands from the particle surfaces resulting in the gravitational precipitation of all of the nanoparticles from the sample NEPCM yielding 100% separation efficiency.

###### Silica Column Chromatography.

Chromatography is a separation process in which sample mixture to be processed is distributed between two phases in the chromatography bed (column or plane). One phase is stationary and known as the stationary phase while the other phase passes through the chromatography column and is known as mobile phase or eluent. The substances in the mixture to be processed or separated must have different affinities for these two phases. The substance with a relatively higher affinity for the stationary phase moves with a lower velocity through the chromatography column than the substance with lower affinity. This difference in migration velocities of the components of the sample ultimately leads to physical separation of the components [20,21]. The component of the sample mixture that leaves the stationary phase and moves with mobile phase is said to be eluted in a process known as elution.

A manual column chromatography using powder silicate was used to carry out NEPCM chromatography trials. A glass chromatography column of 10.5 mm internal diameter and 200 mm length was purchased from “The Lab Depot, Inc.,” which came with a straight stopcock and a poly-tetrafluoroethylene plug. A cotton plug, sand (coarse), and silica grain size (230–400 mesh) were used to establish the column where cotton plug and coarse sand served as column base. In this formation, silica gel was used as stationary phase and hexane was used as mobile phase. Figure 5 shows the NEPCM chromatography setup. A silica column, approximately 101.6 mm. long, was used for all the trials. After the column was established, NEPCM sample to be processed was added followed by hexane as the mobile phase to aid in the elusion process. Hexane was added to fill to the top end of the glass column (5 ml approx.) on the top of the NEPCM sample to help in faster elusion. Due to the pressure exerted by the hexane column, movement of the NEPCM was observed through silica column, which was easy to observe due to its black color. After reaching a certain length of the column the nanoparticle elusion stopped. At this point, nanoparticles are considered to be trapped by silica matrix or column allowing only transport of base fluid dodecane under action of hexane as an eluting agent.

Table 1 shows the chromatography data for different volume and concentrations of the NEPCM used. It was observed that elusion length of the column varies in relation with NEPCM volume and concentration, whereas the time required for the complete elusion is a function of elutant volume used. The elusion length here is the distance traveled by the nanoparticles in the silica column which was visually observed and measured with a scale.

UV-Vis spectrometry was employed to confirm complete removal of nanoparticles. Figure 6 shows UV-Vis spectrum of the elutant obtained for trial 3 in Table 1. To record the data, a freshly prepared sample of elutant was irradiated at 20 °C by a dual source of light. The solution was removed at fixed times and its UV spectrum recorded until no further spectral changes occurred. To capture the data, pure dodecane was used as a base or reference solution. UV-Vis spectral range captured (Fig. 6), incorporates the UV and visible portions of the electromagnetic spectrum from 300 to 800 nm. This spectrum was compared with pure dodecane (baseline) and NEPCM spectra as shown in Fig. 6. Comparing the three spectra reveals that, after the chromatography, sample spectrum resembles very close to the baseline spectrum where no light absorbance is detected for the entire wavelength spectrum indicating absence of any particulate matter. Difference in NEPCM spectrum and sample spectrum provides additional evidence that processed sample is free from nanoparticles. Hence silica gel chromatography is proved to be an efficient, simple, and cost-effective alternative for nanoparticle separation from the NEPCM.

###### Adsorption on Silica Particle Surfaces.

Silica, with 40–63 μm grain size (230–400 mesh) and pore size 60 Å in dry form, was purchased from SILICYCLE, UltraPure SILICA GELS, Canada and was used as is. The black color of the nanoparticles present in the NEPCM sample served as an indicator during these trials. For these trials, a known volume and concentration of the NEPCM, as given in Table 2, was taken as a sample in a test-tube and a measured amount of silica (Table 2) was added in steps of 0.1 gm followed by vigorous manual shaking for 1 min. A digital scale (Mettler Toledo MS204, USA) was employed to measure the weight of silica. Solution was allowed to settle for 15 s followed by addition of more silica in the mentioned measured amount until no settling of nanoparticles with silica was observed. Once enough silica was added to adsorb all of the nanoparticles, two distinct layers were observed in the test-tube; a clear top layer of base fluid (dodecane) and black layer at bottom containing silica and adsorbed nanoparticles. A Whatman grade no. 1 filter paper of diameter 9 cm (Whatman 1001-090) was used in a funnel to separate the two phases. Hexane was used as a washing agent and as an aid in filtration. The filtrate, which is basically a mixture of dodecane and hexane, was air dried to get rid of excess hexane. The final solution containing dodecane and traces of hexane was analyzed using UV-Vis photo-spectrometry to check and confirm nanoparticles are completely removed.

Figure 7 shows steps followed during the experiment while Table 2 presents summary of the trials done for different concentrations and volumes of the NEPCM. The same UV-Vis procedure, as explained in the Silica Column Chromatography section, was followed to analyze the processed sample. The total mass of silica particle added for each case tried was noted and is plotted against the NEPCM wt.% in Fig. 8. The amount of silica required to adsorb all nanoparticles monotonically increases with nanoparticle mass concentration as evident from Fig. 8. This is in accordance with the adsorption mechanism explained. As the concentration of the nanoparticles increases, the silica surface area (the mass of the silica particles of given size) required for adsorption increases. As adsorption is a surface phenomenon, grain size of silica is a critical factor to determine the amount (weight) of silica required. For the same amount of mass, silica with bigger grain offers less surface area for adsorption as compared to one with smaller grains. This work pertains to silica with grain size of 40–63 μm. Silica grains of further smaller size will provide more surface area for CuO nanoparticle adsorption. Hence, it is anticipated that using silica of smaller grain size, the amount of silica required to capture the nanoparticles in a given NEPCM volume with a given nanoparticle wt.% will be lesser.

The UV-Vis spectrum of trial 1 sample, along with reference dodecane and NEPCM sample, is shown in Fig. 9. Pure dodecane was used as a reference to capture UV spectrum of sample 1. Resulting spectrum for sample 1 was recorded three times and same behavior is observed. Comparing the three spectra, it is clear that sample 1 shows very similar behavior and absorbance values as that of base fluid dodecane and unlike NEPCM which exhibits higher absorbance values. Hence it can be said that sample 1 does not contain any impurities or particles as such to absorb light. This result confirms that sample so obtained after running trial 1 is free from nanoparticles and all the particles were adsorbed by the silica.

For a closer look, the silica particles were examined before and after adsorption by transmission electron microscope (TEM). Figure 10(a) shows the TEM image of silica grains with no nanoparticles which clearly indicates size of silica grains which ranges from 40–63 μm as per manufacturer specifications. Figure 10(b) shows the surface structure of individual silica grain without nanoparticles. It can be observed that the selected silica grain has a clean surface with negligible irregularities. Figures 10(c) and 10(d) show the silica grains after trial 1 indicating adsorption of the nanoparticles on the surface of the silica grains. Figure 10(c) shows zoomed in view at a magnification of 35,000 where the presence of nanoparticles on silica grain surface is clearly visible. Any further close-up view (higher magnification of 140,000) like Fig. 10(d) gives a better picture clearly depicting agglomeration of nanoparticles on the silica surface.

In order to make maximum number of nanoparticles come in contact with silica particle surface and saturate it, shaking of the sample mixture in a test tube was employed. It is anticipated that higher intensity and duration of shaking would result in more nanoparticles coming in contact with a given silica particle surface resulting into higher adsorption of nanoparticles on a silica particle surface. However, it should be noted that once a given silica particle surface gets saturated with adsorbed nanoparticles, no further shaking would result in higher adsorption. It is also to be noted that, for a given amount silica particle mass, smaller the particle size larger the total particle surface area available. Thus, the efficiency of adsorption depends on several factors such as mass fraction of the nanoparticles, mechanism, intensity and duration of the shaking, silica particle size, etc. It is very difficult to determine optimum shaking condition of these parameters for adsorption to establish a norm to quantify shaking intensity required to adsorb maximum number of nanoparticles on a silica particle surface. In order to successfully demonstrate the principle of the method, manual shaking was adopted in this work. For industrial upgrading of the method, the optimum shaking process and intensity needs to be determined by systematic trial and error process by varying the other pertinent parameters mentioned above.

###### Adsorption on Dirt Soil Particle Surface.

The previous trials for the nanoparticle separation methods performed using highly purified silica (SiO2) grains in the chromatography column and as a bare adsorbing agent, proved successful by yielding complete nanoparticle separation. As highly purified silica adds to the cost of resources required for the trials, an economical approach was sought. Similar to adsorption on silica particle surface, dirt soil was used to do the trials instead of the pure silica particles. The dirt sample, shown in Fig. 11, was obtained from a construction site. According to American Association of State Highway and Transportation officials method the used dirt soil sample is classified as Clayey Gravel and Sand, Symbol A-2-6. This chunk of dirt sample was dried out in the oven for 48 hours. About 20 g of the powdered dirt was added in two samples of 5 ml NEPCM (0.5 wt.% and 1 wt.%) in test tubes. These test tubes were shaken for 15 s and were kept undisturbed for several days.

Figure 12(a) shows the initial stage of the experiment before shaking where the dirt is not visible due to the black color of the 1 wt.% NEPCM. Figure 12(b) shows the settlement of the dirt particles with the nanoparticles absorbed on their surfaces after 6 days. Most of the nanoparticles are absorbed which is evident from the clear appearance of the fluid surrounding the dirt particles and black color of the dirt particles. Figure 12(c) shows settlement after 10 days which shows highly clear fluid surrounding dirt particles.

The mixture in Fig. 12(c) was filtered using Whatman grade no. 1 (Whatman 1001-090) filter paper to separate dirt pieces and base fluid for further processing. The filtered fluid was analyzed using UV-Vis photo-spectrometry to detect any remaining trace of the nanoparticles. The UV-Vis spectrums of the collected clear liquid along with reference dodecane and NEPCM sample are shown in Fig. 13. Resulting spectrum for the sample was recorded three times and same behavior was observed. Comparing the three spectra presented in Fig. 13, it can be concluded the clear liquid does not contain any nanoparticles implicating almost 100% particle removal efficiency.

###### Comparison of the Nanoparticle Separation Methods.

Based on the experiments conducted and the analysis performed and presented here, separation methods have been compared on the basis of economical and functional parameters. Table 3 presents the comparison summary. The comparison has been done strictly referring to the equipment used and practices followed to conduct the trials.

## Conclusions

Separation of nanoparticles from used nanofluids, before their disposal, is a big environmental concern. Three different chemical methods for the separation of copper oxide nanoparticles from a colloidal mixture in dodecane have been tested, analyzed, and evaluated and the results are presented in this article. These simple, economic, safe, and efficient methods have been proven to be very successful in a laboratory scale with 100% particle removal efficiency. Their implementation on the industrial scale seems very conceivable.

Referring to the results presented in the Results and Discussion section and the comparison of different methods presented in Table 3, the following conclusions can be drawn:

1. (1)Chemical destabilization of the stabilizing ligands/surfactants by saturated aqueous KOH solution in the presence of ethanol is a very effective separation method. The surfactant reacts with the KOH making the CuO nanoparticles unstable for suspension in the base fluid dodecane. This results in rapid precipitation of the nanoparticles which settle at the bottom of the container from where the particles can be collected and reclaimed. UV-Vis spectrometry of the collected clear liquid after particle separation confirms 100% particle removal efficiency of this method. The method is safe and simple with minimal equipment and material cost.
2. (2)Silica column chromatography is also a simple and safe method yielding 100% particle removal efficiency as evidenced from the UV-Vis spectrometry. The equipment and material cost is moderate. However, the nanoparticles cannot be reclaimed since they get trapped in the silica gel.
3. (3)Adsorption of the copper oxide nanoparticles on silica particle surfaces is also a simple, safe, and economic method. The equipment and material cost is low and the particle removal efficiency is 100%. Since pure silica can be somewhat expensive, trials were also conducted with dirt soil particles which produced similar results. However, in these methods utilizing oxide nanoparticle adsorption on silica particle surface, reclamation of the nanoparticles is difficult since they are deposited on the silica particle surfaces. These adsorption methods are suitable for oxide nanoparticles due to the affinity between the oxide particles and silica surface. It may not work for nonoxide particles.

## Acknowledgements

Dr. German Mills, Chemistry Department, Auburn University; Dr. Jay Khodadadi, Mechanical Engineering Department, Auburn University; Dr. Hank Heath, Biology Department, The University of Alabama; Dr. Patrick Frantom, Chemistry Department, The University of Alabama; and Dr. Andrew J. Graettinger, Civil Engineering Department, The University of Alabama have been consulted on many occasions during the progress of this study. This material is based upon the work supported by the U.S. Department of Energy (DOE) under Award No. DE-SC0002470 (http://www.eng.auburn.edu/nepcm). This report was prepared as an account of work sponsored by an agency of the United States Government. Neither the United States Government nor any agency thereof, nor any of their employees makes any warranty, express or implied, or assumes any legal liability or responsibility for the accuracy, completeness, or usefulness of any information, apparatus, product, or process disclosed, or represents that its use would not infringe privately owned rights. References herein to any specific commercial product, process, or service by trade name, trademark, manufacturer, or otherwise do not necessarily constitute or imply its endorsement, recommendation, or favoring by the United States Government or any agency thereof. The views and opinions of authors expressed herein do not necessarily state or reflect those of the United States Government or any agency thereof.

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## Figures

Fig. 1

(a) Schematic diagram of a nanoparticle with long ligands coated on its surface serving as the stabilizing cushion layer and (b) ligands on particle surface to provide a physical barrier (cushion) which prevents particle contact and subsequent agglomeration

Fig. 2

NEPCM preparation

Fig. 3

(a) Nanoparticle ligands destabilization using saturated KOH solution and (b) schematic of the chemical reaction between oleate and KOH

Fig. 4

Chemical destabilization of the nanoparticle ligands: UV-Vis spectrums of the NEPCM before processing, pure baseline dodecane, and the processed sample (clear liquid collected after processing)

Fig. 5

Silica column chromatography setup

Fig. 6

Silica column chromatography: UV-Vis spectrums of the NEPCM before processing, pure baseline dodecane, and the processed sample (clear liquid collected after processing)

Fig. 7

Adsorption on silica particle surfaces; (a) precipitation of the silica particles with adsorbed nanoparticles after mixing the silica particles in the NEPCM and (b) filtering out the clear liquid after nanoparticles are adsorbed

Fig. 8

Adsorption on silica particle surfaces; mass of silica required for different nanoparticle concentration (wt.%) in the NEPCM

Fig. 9

Adsorption on silica particle surfaces: UV-Vis spectrums of the NEPCM before processing, pure baseline dodecane, and the processed sample (clear liquid collected after processing)

Fig. 10

TEM images of the silica grains (image scales are shown at the lower right corner of each figure); (a) pure silica grains, (b) magnified view of the pure silica grain surface, and (c) and (d) silica grain surface showing adsorbed nanoparticles at a magnification of 35,000 and 140,000, respectively

Fig. 11

Dirt soil sample

Fig. 12

(a) Mixing of dirt soil in 5 ml of 1 wt.% NEPCM, (b) adsorption of the nanoparticles on the dirt particle surfaces after 6 days, and (c) adsorption of the nanoparticles on the dirt particle surfaces after 10 days

Fig. 13

Adsorption on dirt soil particle surfaces: UV-Vis spectrums of the NEPCM before processing, pure baseline dodecane, and the processed sample (clear liquid collected after processing)

## Tables

Table 1 Silica column chromatography results
Table 2 Summary of the trials for adsorption on silica particle surfaces
Table 3 Separation methods comparison

## Discussions

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