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A Facile and Cost-Effective Method for Separation of OilWater Mixtures Using Polymer-Coated Iron Oxide Nanoparticles Soubantika Palchoudhury, and Jamie R Lead Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/es5037755 • Publication Date (Web): 19 Nov 2014 Downloaded from http://pubs.acs.org on November 24, 2014

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Environmental Science & Technology

1

A Facile and Cost-Effective Method for Separation

2

of Oil-Water Mixtures Using Polymer-Coated Iron

3

Oxide Nanoparticles

4

5

Soubantika Palchoudhury and Jamie R. Lead*

6

Center for Environmental Nanoscience and Risk (CENR), Department of Environmental Health

7

Sciences, Arnold School of Public Health, University of South Carolina, Columbia, 29208, USA

8 9

TABLE OF CONTENTS

10

11

ACS Paragon Plus Environment


12

ABSTRACT

13

Catastrophic oil spills and oil from waste waters such as bilge and fracking waters pose major

14

environmental concerns. The limitations of existing cleanup techniques for benign oil

15

remediation has inspired a recent scientific impetus to develop oil-absorbing, smart

16

nanomaterials. Magnetic nanocomposites were here designed to allow easy recovery from

17

various systems. In this study, sorption of reference MC252 oil with easy-to-synthesize and low-

18

cost hydrophilic polyvinylpyrrolidone-coated iron oxide nanoparticles is reported for the first

19

time. The one-step modified polyol synthesis in air directly generates water-soluble

20

nanoparticles. Stable polyvinylpyrrolidone-coatings are known to minimize environmental

21

alterations of nanoparticles from aggregation and other processes. Iron oxide provides effective

22

magnetic actuation, while both PVP and iron oxide have low toxicity. These nanoparticles gave

23

quantitative (near 100%) oil removal under optimized conditions. The facile synthesis and ease

24

of use represents a significant improvement over existing techniques.

25


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INTRODUCTION

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Recent catastrophic oil spills like the Deepwater Horizon, first Gulf War, Exxon Valdez, and

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IXTOC 1 induce significant damage to the marine ecosystem in the form of dead sea-birds,

29

otters, sea-turtles, marine mammals, contaminated planktons, and affected corals.1,2 Oil from

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wastewaters like bilge water and industrial oil spills from facility repairs add to this

31

environmental threat. Additionally, hydrocarbon mixed water from modern extraction techniques

32

like directional drilling and hydraulic fracturing significantly affects contaminant level in nearby

33

wells and aquifers.3,4 The long-term detrimental impact of different oil contaminations to the

34

food-chain is a huge concern. To mitigate the harmful environmental effect of fast spreading oil

35

spills and wastes, rapid removal of oil from the discharge and receiving waters is important.

36

However, there are potential challenges to the existing cleanup methods. For instance, current

37

methods for oil spill remediation result in a low efficiency and labor intensive clean-up, with

38

possible increased toxicity.5,6 Excessive use of dispersants adds to the water contamination and

39

limits the efficacy of other oil removal routes. Mechanical skimmers are limited to oil removal

40

near the deployment area. The booms cannot prevent sinking of the recovered oil, causing

41

damage to the marine biota. The sorbent materials are widely used, but require improved oil pick

42

up rate and retention. For bioremediation routes, the microbial biosurfactants must be field tested

43

and should show higher oil removal capacity for practical applicability. Biosurfactants

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essentially function like dispersants and cannot remove the oil. These limitations highlight the

45

necessity to develop new nanomaterials for cost-effective and benign oil removal.7,8

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Magnetic nanocomposites have been used to address the limitation in collecting conventional

47

oil adsorbents.9,10 For example, polysiloxane coated Fe2O3@C core-shell nanoparticles (NPs)

48

were used for enhanced selectivity in oil-water separation.11 In a recently reported iron oxide-



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collagen nanobiocomposite, inexpensive collagen from industrial waste served as an improved

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oil absorbing agent and the iron oxide core provided magnetic actuation.12 Calcagnile et. al.

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found that the weakly bound iron oxide NPs in surface modified polyurethane foam facilitated

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oil absorption, easy recovery, and reusability.13 Additionally, magnetite nanofillers infused in

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low epoxidized natural rubber showed excellent absorption capacity for petrol oil.14

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concentration of iron oxide NPs within alkyd resin biopolymer substantially increased the oil

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absorption capacity of the composite.15 These studies highlight the potential of iron oxide NPs

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for oil removal.16 These applications are particularly attractive because iron oxide is of inherently

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low toxicity.17-20 However, the practical applicability of the materials for submerged oil, for

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instance, is questionable. In addition, many methods require complex syntheses, using large

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energy and material inputs for production. A facile and cheap method of production is required

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to directly generate stable, water soluble iron oxide NPs to minimize environmental impact21 and

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facilitate an easy scale-up for oil remediation applications.

High

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In this study, oil sorption efficiency of polyvinylpyrrolidone (PVP)-coated iron oxide NPs is

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investigated. Compared to the multi-step complex synthesis methods, these water-soluble iron

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oxide NPs were formed via a cost-effective one-step protocol under ambient air with low energy

65

consumption. In addition, PVP-stabilized NPs are known to show low toxicity and excellent

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temporal stability in terms of aggregation and dissolution changes in changing environmental

67

milieux.22,23 The NPs were systematically evaluated for separation of oil-water mixtures. In

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particular, oil samples from the Deepwater Horizon spill were chosen for this study to facilitate

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practical applicability. This is the first report of using easy-to-synthesize, cost-effective, and

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directly water-soluble PVP-coated iron oxide NPs for oil removal. These PVP-iron oxide NPs


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could potentially open the route for an easily scalable and benign oil remediation technique,

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given that practically relevant oil samples were used.

73 74 75

EXPERIMENTAL SECTION

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Nanoparticle Synthesis. Iron oxide NPs were synthesized using a modified polyol method.24

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Specifically, the capping molecule, PVP (MW 10 kDa, 0.03 mmol, GBiosciences) was heated to

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dissolution in tri-ethylene glycol (TREG, 99%, 1 mL, Alfa Aesar) at 90 °C for 10 min. The iron

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precursor, iron (III) acetylacetonate (Fe(acac)3, 99%, 2 mmol, 0.7 g, Acros Organics) was added

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to this solution and mixed for 10 min, prior to thermal decomposition at 260 °C for 1 h. The

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entire synthesis was conducted in air without inert gas protection. To study the effect of PVP

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coating on oil sorption, the iron oxide NPs were synthesized with different precursor-to-capping

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agent molar ratios (e.g., 66.7:1, 22.2:1, and 12.5:1) and with a different PVP polymer (MW 40

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kDa, 0.03 mmol, Sigma-Aldrich), keeping all other parameters the same. Additionally, to

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investigate the influence of NP size on oil removal, different reaction times (e.g., 1 and 2 h) were

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used, keeping all other conditions the same (Table S1). The as-synthesized NPs were

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magnetically collected and redispersed in ultra-pure water (Milipore) to remove excess solvent.

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The target concentration of NPs was set to 20 g/L. The aqueous NP products were subsequently

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used for oil removal studies.

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Oil Removal. In a typical oil removal experiment, crude oil from the Deepwater Horizon spill

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(Aecom Environment) was mixed well with ultra-pure water (Milipore) in a vial via sonication

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(Branson 2800, 40 kHz, ambient conditions) for 30 min to prepare the oil samples (0.6-0.9 g/L,

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pH 6-8). PVP-iron oxide NPs were added to the oil samples at target oil:NP weight ratios of



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1.4:1, 2.6:1, 4.3:1, 6:1, and 7.3:1, respectively. To investigate the oil extraction capacity of NPs,

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the oil-NP solutions were magnetically separated for different time periods (3 h, 5 h, 7 h, 12 h,

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and 72 h) using a 1” cubic neodymium magnetic collector. The remaining clear solutions in each

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vial were collected for subsequent analysis to quantify the amount of oil removed. These clear

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solutions were ultracentrifuged (45000 rpm, 1 h, Sorvall MTX Micro-ultracentrifuge) and

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measured again in some cases to remove interferences from remnant NPs. The oil-coated NPs

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magnetically collected to the side of the vial were sonicated in methyl tert-butyl ether (MTBE, ≥

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99%) for oil recovery. The oil sample, oil-coated NPs, and the final clear solution batches were

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further characterized via UV-visible spectroscopy (UV-vis), gas chromatography-mass

103

spectrometry (GC/MS), dynamic light scattering (DLS), and atomic force microscopy (AFM)

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imaging to measure oil removal.

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Characterization and Quantification. The size and morphology of the PVP-iron oxide NPs

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before and after oil absorption were examined on an Asylum Cypher AFM operating in non-

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contact mode in ambient air. Noncontact mode silicon AFM cantilevers (ACM240TM,

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Olympus) were used for the characterization. Samples were carefully prepared via an adsorption

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technique.25 Specifically, the sample solutions (~ 50 μL) were dropped on freshly sliced mica

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slides via micro-pipettes and washed with ultra-pure water, prior to drying in an enclosed vial to

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prevent airborne contamination. Data collection and analysis were conducted with Asylum

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software package. To obtain size histograms, the AFM images were analyzed quantitatively with

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section analysis in the AFM software.26 The measured sizes of 300 particles were classified into

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intervals of 5 nm width for a representative size plot. In addition, a 200 kV Hitachi H8000

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transmission electron microscope (TEM) was used to confirm the size and morphology of PVP-

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iron oxide NPs. The hydrodynamic size and zeta potential of PVP-iron oxide NPs and oil coated


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PVP-iron oxide NPs were measured on a Malvern Zetasizer Nano DLS. Three consecutive

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measurements were recorded at 25 °C to calculate a Z-average size for each sample.

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A Shimadzu UV-2600 UV-vis was used to measure the concentration of oil in the different

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samples, based on the absorbance spectra. A calibration plot was obtained with known oil

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concentrations. The UV spectra were carefully measured for the initial oil-water samples, NPs

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samples used for oil studies, oil-water-NP solution samples, oil recovery samples after sonication

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of oil-coated NPs, and the final separated solution samples. Based on the height of absorbance

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peaks for the different samples and the calibration plot, the oil removal efficiency was quantified.

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Pictures were taken for each sample prior to UV measurement.

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The concentrations of oil samples before and after the NP-based separation were further

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measured on a double focusing magnetic sector VG 70S GC/MS. The samples were prepared via

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extraction in MTBE (≥ 99%, 1 mL, Sigma-Aldrich). The chromatograph oven was operated from

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60 °C to 300 °C rising at 10 °C/min. Compounds were separated on a Restek RTx-5 (length 30

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m, 0.25 mm ID, and 0.2-mm film). The electron ionization was set at 70 eV with a helium carrier

131

gas at 10 psi operating pressure. An injection volume of 1 μL was used. Ion chromatograms were

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recorded for mass-to-charge ratios (m/z) of 40 to 450 for qualitative identification of the

133

spectrum and 208 to 370 for quantitative analysis of the oil samples, particularly the high

134

percentage of aliphatics. A full scan mode was used for both cases. To compare the oil samples

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before and after NP-based separation, the C25H52, C22H46, C18H38, and C15H32 were monitored in

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the closer scan (208 to 370) for quantitative measurement.

137 138



139

RESULTS AND DISCUSSION

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PVP-Iron Oxide Nanoparticles. Iron oxide NPs are attractive for their inherent low toxicity

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and excellent magnetic properties.27 The magnetic property of iron oxide NPs is easily tunable

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via control over size, morphology, or ligand coating.28 Iron oxide NPs are currently used in

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several bio-applications e.g., FDA approved magnetic resonance imaging (MRI) contrast

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agents.29 Typically, sterically stabilized polymer-coated NPs, such as PVP coating, are known to

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remain largely unaffected in complex environmental milieux and also pose little hazard.23,22 The

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PVP-iron oxide NPs are potentially attractive candidates for benign oil remediation material

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where PVP serves to absorb both aliphatic and aromatic components of oil while the iron oxide

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acts as the structural support and subsequently allows easy magnetic separation from the aqueous

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phase. Both materials exhibit low toxicity. Here, a modified polyol method was used to directly

150

synthesize water dispersible and crystalline PVP-iron oxide NPs. This cheap and facile method

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has advantages in that it is a relatively low energy method requiring small amounts of low

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toxicity reactants. Other methods suffer from some issues in practical use e.g., the thermal

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decomposition in non-polar solvents can form highly crystalline and monodisperse iron oxide

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NPs, but requires additional aqueous phase transfer.30 Iron oxide NP products from the co-

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precipitation route are hydrophilic but can lack crystallinity. The outer PVP-coating in this study

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rendered the NPs hydrophilic.31 To reduce synthesis cost, the quantity of TREG was minimized

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(Table S1, Supporting Information). Figure 1 shows the size and morphology of the PVP-iron

158

oxide NPs via AFM and DLS. Based on the AFM image, the NPs are discrete NPs of size 13 ±

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0.2 nm (Figure 1a and b). The hydrodynamic diameter was 69 nm with a pdi of 0.1 as measured

160

by DLS (Figure 1c). The single peak below 100 nm suggested good colloidal stability of the

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aqueous NP dispersion (Figure 1d). The NPs were sterically stabilized via PVP, as expected,


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with a zeta-potential (4.1 mV) insufficient to provide colloidal stabilization.23 Figure 1d shows

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the stable aqueous dispersion of the PVP-iron oxide NPs. The size and spherical morphology of

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these highly stable and water-soluble PVP-iron oxide NPs could also be confirmed via TEM

165

(Figure S2).

166 167

Figure 1. PVP-iron oxide NPs (a) topographical AFM image, (b) size distribution histogram

168

from AFM (n=300), (c) DLS plot, and (d) NP solution.

169 170

Measurement of Oil Removal with UV-vis spectroscopy. The potential of these PVP-iron

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oxide NPs to remove oil was systematically investigated using Surrogate Deepwater Horizon oil

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samples prepared at concentrations relevant to, but slightly higher than, typical oil spill

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concentrations and waste waters (0.6 – 0.9 g/L).32,33 Typically, the surrogate oil was mixed in

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ultrapure water via sonication to make the target oil samples. Different amounts of PVP-iron



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oxide NPs were added to the oil-water samples to give a range of oil: NP ratios, well-mixed via

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sonication and left to magnetically separate overnight using a standard 1”x1” neodymium

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laboratory magnet. The treated clear sample was retrieved for measurement. The oil-coated PVP-

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iron oxide NPs attracted to the magnet were sonicated in MTBE to measure oil recovery. UV-vis

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was used as a simple, rapid, and inexpensive method to measure mass concentrations, although

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we acknowledge the limitations of this method, primarily related to background NP signal

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overlapping with the oil signal.34 Both the aliphatic and aromatic hydrocarbons of oil-water

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sample were likely detected by the UV-vis, based on the control hexane-water and xylene-water

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samples. Figure 2a shows sample UV-vis plots. The concentration is measured from the

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difference in peak heights based on a calibration plot. The oil-water samples for the calibration

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curve were separately prepared via careful dilution of the same sample to minimize error. As

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seen from the UV-vis data, the typical absorbance curve for oil is reduced to near zero for the

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final treated sample (oil remaining), suggesting near complete oil removal. The absorbance for

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water and PVP-iron oxide NPs before addition to oil are plotted for reference. However, it was

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challenging to quantify the oil recovered from the oil-coated NPs because of strong interference

190

from the NPs. Figure 2b shows the oil sample at each stages of the treatment with PVP-iron

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oxide NPs. Oil removal was near 100%, as suggested by the transparent final sample compared

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to the cloudy oil-water initial mixture. Similar results were observed for oil-water mixtures at

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different pH (Figure S3).


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Figure 2. Measurement of oil removal with UV-vis (pH 7) (a) absorbance plots of samples and

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(b) images of samples used in a.

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AFM and DLS Characterization for Oil Removal. The oil-coated NPs used for oil recovery

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measurements were further characterized to confirm oil absorption on the NPs. The morphology

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of the NPs after oil sorption is completely different from those observed for pristine PVP-iron

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oxide NPs (Figure 1a). Figures 3a and b show the topographical and amplitude AFM images of

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the oil-coated iron oxide NPs. Compared to the initial discrete small NPs (Figure 1a), an

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aggregated morphology is observed, indicative of oil sorption. Additionally, the size distribution

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obtained from the AFM for the oil-coated NPs is broader with a peak centered at 30 nm with a

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pdi of 2 (Figure 3c). The increased size and polydispersity of the PVP-iron oxide NPs suggest oil

206

sorption on the NPs. The hydrodynamic diameter measured on the DLS increased from 69 ± 0.1



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nm to 389 ± 0.194 nm for the oil-coated NPs (Figure 3d). Oil bound to the NP surfaces was

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quantified based on the DLS measurements and geometric considerations. For a given volume,

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each NP sorbed approximately 178 times its own volume of oil, indicating a promising oil

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separation capacity of the PVP-iron oxide NPs. The aromatic and aliphatic portions of the outer

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PVP ligand layer likely serves as a suitable oil sorption interface such that the oil forms a second

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surface layer to encapsulate an ensemble of NPs, driven by hydrophobic considerations. It should

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be noted that the NPs were sonicated for 30 min prior to DLS measurements to eliminate any

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aggregation effects due to prolonged magnetic separation of the oil-coated NPs. Figure 3d, insert

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shows a well-sonicated aqueous dispersion of oil-coated PVP-iron oxide NPs.

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Figure 3. AFM images and DLS plot of PVP-iron oxide NPs after oil absorption (a)

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topographical AFM, (b) amplitude image, (c) AFM based size histogram, and (d) DLS, insert,

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photo of oil-NP sample.


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Measurement of Oil Removal with GC-MS. To accurately measure the oil removal with

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PVP-iron oxide NPs, GC-MS was used, providing a more reliable quantification. The same

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samples analyzed on the UV-vis were used for GC-MS measurements to maintain comparability.

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Specifically, the initial surrogate oil-water sample (0.8 g/L) and the clear final sample after NP

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treatment were extracted into volatile methyl tert-butyl ether (MTBE, 1 mL) solvent to prepare

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samples suitable for GC-MS (Figure 4). The chromatograms for oil sample showed prominent

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aliphatic peaks, characteristic of crude oil (Figure 4a).35,36 The peaks are indexed as Cn in the

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chromatogram. The peaks are significantly reduced in the NP-treated sample (Figure 4b). Oil

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removal was calculated based on the reduction in chromatogram peak area. Minimum remnant

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oil was observed in the treated sample e.g., 0.66% of the original docosane (C22H46) and 1% of

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the initial pentacosane (C25H52) remained. Most oil constituents were below the limit of detection

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in the cleaned water. The GC-MS data, therefore confirmed the near 100% oil sorption by the

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PVP-iron oxide NPs.

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Figure 4. GC-MS plots (a) oil-water sample and (b) oil-water sample after NP based separation.



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The aim was to maximize the oil removal using minimum NPs and time for cost-effective

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practical application. To find the best conditions for oil absorption with PVP-iron oxide NPs, the

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magnetic separation time and the oil: NP weight ratios were varied. Table 1 summarizes selected

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oil sorption efficiency of the NPs at each reaction condition, as obtained from the UV-vis data.

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Based on the results, in batch studies a minimum of 307.6 mg NPs (oil:NP wt. ratio, 2.6:1) is

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required to fully remove approximately 0.9 g L-1 in 12 hours. Interference from remnant PVP or

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PVP coated iron oxide NPs in the final cleaned solution was a potential issue to measure oil

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removal with UV-vis in some cases. Ultracentrifugation was used to remove these remnant NPs

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before measurement. However, under optimal conditions, there was near 100% oil removal by

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the NPs and no difference observed before and after ultracentrifugation. Maximum and stable oil

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removal was observed ~ 12 h, based on the oil sorption of the NPs measured after different

247

periods of magnetic interaction (e.g., 3 h, 5 h, 7 h, 12 h, and 72 h; Figure S4). At earlier times (3-

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7 h), the measured oil removal capacities were affected from remaining NPs in the solution.

249 250

Table 1: Oil removal with PVP-iron oxide NPs under different conditions, based on UV-vis Time (h)

Wt. of NPs (mg)

Oil conc. (g/L)

% Removal

12

128.6

0.9388

97.83

12

145.6

0.8658

96.58

12

206.6

0.899

93.13

12

307.6

0.8

100

12

623.9

0.8851

100

72

151.3

0.9

98.73

72

151.9

0.6733

95.86


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Finally, the PVP-coated iron oxide NPs provide a simple, low hazard but powerful tool to

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potentially remove oil from oil-water mixtures caused by, for instance, oil discharges such as

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those that occurred at the Deepwater Horizon spill. Approximately 100% oil removal under

255

optimum conditions was observed. A major implication of this study is the facile, easy-to-scale-

256

up, inexpensive synthesis, and oil removal analysis methods developed for the highly effective

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PVP-iron oxide NPs. The reported method holds great potential for designing smart materials to

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remediate oil with minimal environmental impact.

259 260

ASSOCIATED CONTENT

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Supporting Information

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Additional information on synthesis parameters and colloidal stability of different PVP-iron

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oxide NP formulations tested for oil sorption is provided in SI. DLS calculations for oil sorption

264

are also provided. This material is available free of charge via the Internet at http://pubs.acs.org.

265 266

AUTHOR INFORMATION

267

Corresponding Author

268

*Phone: (803) 777-0091. E-mail: [email protected].

269 270

ACKNOWLEDGMENT



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The authors thank Dr. Mike Walla and the Mass Spectrometry Centre at the University of

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South Carolina for use of GC-MS. We thank Aecom for providing reference MC252 surrogate

273

oil. We also thank Seyyedali Mirshahghassemi for TEM imaging.

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