Novel and Environmentally Friendly Oil Spill Dispersant Based on the

May 5, 2016 - †Key Laboratory of Marine Chemistry Theory and Technology, Ministry of ... of natural biopolymer, Xanthan Gum (XG), and silica nanopar...
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Research Article pubs.acs.org/journal/ascecg

Novel and Environmentally Friendly Oil Spill Dispersant Based on the Synergy of Biopolymer Xanthan Gum and Silica Nanoparticles Guilu Pi,† Yiming Li,*,† Mutai Bao,† Lili Mao,‡ Haiyue Gong,† and Zhining Wang† †

Key Laboratory of Marine Chemistry Theory and Technology, Ministry of Education, and ‡College of Chemistry and Chemical Engineering, Ocean University of China, 238 Songling Road, Qingdao, 266100, Shandong Province, China S Supporting Information *

ABSTRACT: The potential toxicity of existing chemical dispersants on the marine environment has motivated the search for environmentally friendly dispersants with excellent dispersion ability. Here, an effective Pickering emulsifier is developed based on the synergy of natural biopolymer, Xanthan Gum (XG), and silica nanoparticles. The oil−in−seawater emulsion stabilized by a combination of XG and silica demonstrates great stability and smaller droplet size, which is favorable for the following natural degradation of oil. The synergistic emulsification mechanism has been investigated systematically. The presence of XG favors the adsorption of silica nanoparticles at the oil−seawater interface and also is considerably effective in enhancing the viscosity of continuous phase. These contributions of XG slow down the droplet coalescence and creaming significantly. Confocal laser scanning microscope (CLSM) and scanning electron microscope (SEM) images of emulsions indicate a thick layer of aggregated XG/silica particles at the oil− water interface. This thick layer provides an effective steric barrier. In this study, the synergy between XG and silica not only enhances the dispersion effectiveness, but also reduces the amount of nanoparticles dramatically. This finding opens up a new path for the development of a novel, high efficiency, ecologically acceptable, and cheaper dispersant for emulsifying crude oil following a spill. KEYWORDS: Oil spill dispersion, Pickering emulsion, Biopolymer, Nanoparticles, Synergy, Sustainable

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INTRODUCTION Every year, inevitable oil spill accidents lead to a massive amount of oil being released into the sea and significant contamination of the ocean and shoreline environments. If adequate oil spill response measures were not adopted, the health of plants and animals in the ecosystem would be influenced seriously.1 Among oil spill remediation methods, the addition of dispersant is a feasible one.2 For the Gulf of Mexico oil spill, about 2.1 million gallons of chemical dispersant were used.3 Typical oil spill dispersants are mixtures of surfactants and organic solvents. When these dispersants are applied, the spilled oils are broken down into tiny oil droplets under sufficient energy produced by waves because of the significantly reduced crude oil−seawater interfacial tension.4 The dispersion of an oil slick in water is essentially a process of emulsification. Another advantage of using dispersants is that the formation of tiny oil droplets dramatically increases the interfacial area available for bacteria attack, which stimulates the biodegradation process.5 However, concerns about the potential environmental impact and toxicity of chemical dispersants are growing because most these surfactants are synthesized chemically.6 Therefore, developing environmentally friendly and biodegradable dispersants, while keeping their high dispersion efficiency, is getting more and more important. Recently, food grade © XXXX American Chemical Society

surfactants such as lecithin have been investigated as the active agents in dispersant formulation.7,8 Emulsions stabilized by the adsorption of solid particles at the interface are called Pickering emulsions, which were first proposed by Ramsden and Pickering.9 The adsorption free energy (ΔadsF) for spherical particles at an oil−water interface is illustrated by ΔadsF = −πR2γo − w(1 − |cos θ|)2

(1)

where R is the particle radius, γo−w is the oil−water interfacial tension, and θ is the contact angle through either phase.10 The adsorbed solid particles at the oil−water interface will produce a particle layer and thus enhance emulsion stability by providing repulsive interactions between droplets,11 steric barriers,12 and interfacial viscosity.13 The potential applications of Pickering emulsion include drug delivery, oil recovery, food, material fabrication, and personal products.14−16 Various solid particles have been employed for preparing Pickering emulsion, such as silica,17 iron oxide,18 clay,19 polystyrene,20 chitin nanocrystals,21 and carbon black.22 In addition, one of the Received: January 11, 2016 Revised: April 26, 2016

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positively charged silica nanoparticles would result in a synergistic emulsification. Recently, Venkataraman et al. found that the emulsion stability was enhanced using a combination of chitosan and chemical dispersant.42 In this study, a stable oil−in−seawater emulsion was produced with a mixture of XG and silica nanoparticles. A model oil system, tetradecane, was first used to examine emulsification and mechanism, and then, we used these results to guide our work on the emulsification of crude oil. On the basis of the synergy of XG and silica nanoparticles, effective crude oil dispersion in seawater was obtained and corresponding mechanism was illustrated. Stable emulsion is formed even with only a small amount of silica nanoparticles present, which is far below the concentration used in other studies.43,44 Such an environmentally benign and effective dispersant system could be a viable alternative to conventional chemical surfactant dispersants.

exciting features of Pickering emulsion is that they are highly stable to coalescence even when the oil droplets are large.23 During the preparation of Pickering emulsion, surfactants are often combined with solid particles. The surfactant has multiple functions in the preparation of emulsion: to decrease the interfacial tension; to affect the wettability of particles and ; in some cases, to allow the flocculation of solid particles.24 Due to the two emulsifiers act synergistically or antagonistically with respect to emulsion stabilization, the choice of surfactant and mixing protocol would also influence the final emulsion.25 The synergy between surfactants and solid particles has been investigated widely in designing optimally stable emulsions.26−28 Despite the peculiar stability of Pickering emulsion, little attention has been paid to solid particle asoil dispersant in seawater. Only in the last three years, a few research groups began studying in this direction. Their researches include the use of surface modified carbon black particles,22 montmorillonite clay microparticles modified with bis(2-hydroxyethyl)oleylamine,29 a mixture of silica nanoparticles and zwitterionic surfactant.30 Olasehinde and Nyankson et al. integrated the stabilization of halloysite clay nanotubes together with their release of chemical dispersant, to replace traditional liquid oil spill dispersant formulations.31,32 In most of those studies, chemical surfactant was still needed to modify the surface of solid particles in order to obtain stable Pickering emulsion. To reduce the dispersants’ impact on environment, natural and biodegradable surfactants or biopolymer are always better choices.33 Our recent studies have shown that chitosanmodified bacterial cells and rhamnolipid-modified silica nanoparticles are good emulsifiers for dispersing crude oil in seawater.34,35 However, the low yield of rhamnolipid in industry restrictsits large-scale application in oil spills remediation. Further study is needed to design more efficient, nontoxic, and affordable emulsifier system. To this end, we focus on developing highly stable oil−in− seawater emulsions based on the synergy of biopolymer, Xanthan Gum (XG), and hydrophilic silica nanoparticles. It has been confirmed in previous studies that polymer facilitates the adsorption of solid particles at the oil−water interface and contributes to the stabilization of emulsions even without lowering the interfacial tension, such as polystyrene/carbon nanotubes, polystyrene/hydroxyapatite, triblock copolymer/ iron nanoparticles, and cellulose derivatives/cellulose nanocrystals mixture.33,36−38 Comparing with the chemical modification of nanoparticles, physical adsorption of polymer onto particles is an easy way to obtain synergistic stabilization. XG is an anionic extracellular polysaccharide produced by Xanthomonas campestris bacteria. XG, natural and nontoxic, functions as a hydrophilic colloid to thicken, suspend, and stabilize waterbased systems, resulting in its extensive application in food, personal care, pharmaceutical industry, and enhanced oil recovery.39 The approval by the FDA (Fed. Reg. 345376) in 1969 as a nontoxic and safe polymer allows the use of XG as thickener and stabilizer in many food products. The molecular conformation of XG in aqueous solution has been reported in a literature.40 Above a certain concentration, XG alone can stabilize emulsions by increasing the viscosity of aqueous phase and impeding the coalescence of oil droplets.41 XG is widely applied in food industry. However, to the best of our knowledge, no studies have described the application of XG in dispersing marine oil spills. It is expected that the strong electrostatic interactions between polyanionic XG and

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EXPERIMENTAL SECTION

Materials. Silica nanoparticles (30 wt %) suspension in H2O (LUDOXCL) was purchased from Sigma-Aldrich. XG (viscosity ≥ 1200 cPs) was purchased from Beijing Solarbio Science &Technology Company. The molecular structure of XG is shown in Scheme S1 in the Supporting Information. Tetradecane (98%, Aladdin, China) was used as the oil phase without further purification. According to the formula of Lyman and Fleming,45 artificial seawater (ASW) with a pH of 7.9 was prepared by (g/L): NaCl 26.726, MgCl2 2.260, MgSO4 3.248, NaHCO3 0.198, KCl 0.721, CaCl2 1.153. Deionized (DI) water was produced from a triply distilled water purification system. Crude oil with a viscosity of 72.9 Pa s−1 (25 °C, 3 r min−1), freezing point of 23.0 °C, and density of 0.85 g cm−3 was obtained from Shengli Oil field, China. Preparation and Characterization of Emulsions. Aqueous solutions of silica nanoparticles and XG in DI water and ASW were prepared, respectively. XG aqueous solution was left at room temperature for 24 h before use. Then, the dissolved XG was added to silica aqueous solution at room temperature and mixed vigorously with a stir bar for 5 min. After left 30 min to equilibrate, tetradecane was added to the XG/silica aqueous solution at a 1:3 ratio of oil to water. The mixture was immediately homogenized at 11 000 rpm for 2 min (IKA Ultra-Turrax T-10). Emulsions were observed by optical microscope (Leica DM1000 LED, Leica, Germany), and the average emulsion droplet size was determined by Nano Measurer software. The emulsification was determined by the percentage of the height of emulsion layer divided by the total height of the mixture over a period of 24 h, which was expressed as the Emulsification Index (EI24). The EI value was assessed through the movement of the emulsion−water interface with a Nikon camera.30 For crude oil−in−ASW emulsions, the ratio of crude oil to ASW was maintained at 1:16 by weight. Emulsion stability was determined by the variance of oil droplet size and EI value over time. Emulsion type was determined by conductivity measurements and also confirmed using the “drop” dilution test. Interfacial Tension Measurements. The interfacial tensions (γ) of tetradecane−aqueous solution were determined at room temperature (22 ± 2 °C) by the pendant drop method through droplet shape profile analysis (OCA instrument, Dataphysics ES, Germany). The surface tension of triply distilled water was 72.2 ± 0.3 mN m−1, which was measured by using the same pendant drop method. At least three independent measurements were performed. Data points showed the mean of three replicates, and error bars showed the standard deviation. Dynamic Light Scattering and Zeta Potential. The size and zeta potential of silica nanoparticle dispersions were recorded at a fixed scattering angle of 90° on a Nano Zetasizer ZS instrument (Malvern Instruments, UK) at room temperature (22 ± 2 °C). The zeta potentials of oil droplets were also measured using the same instrument. At least three independent measurements were performed. Transmission Electron Microscope (TEM). The morphologies of silica nanoparticle dispersion with and without XG were visualized B

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ACS Sustainable Chemistry & Engineering by TEM (JEM-2100, JEOL, Japan) at room temperature (22 ± 2 °C). A drop of fresh aqueous dispersion was deposited on a carbon-coated copper grid, and then the grid was air-dried prior to imaging. Fluorescence Microscopy Analysis. Confocal laser scanning microscope (CLSM, Fluo ViewTM FV1000, Olympus Corporation, Japan) was used to visualize the microstructure of emulsions. Prior to preparing emulsion, Rhodamine B was added to 0.1 wt % silica aqueous solution to fluorescently label the silica nanoparticles. Then, the labeled silica particles dispersion was mixed with XG solution. The final concentration of Rhodamine B in the aqueous solution was 1 × 10−6 mol L−1. This mixed XG and silica solution was used to prepare emulsions. Scanning Electron Microscope (SEM). Based on our previous work,35 we polymerized styrene emulsion droplets as a way to visualize the emulsion oil droplets under an SEM instrument (S-4800, Hitachi, Japan). V65 was used as an initiator. Here, 3 mL of oil phase was mixed with 6 mL of silica/XG aqueous suspension, and then the mixture was homogenized for 2 min at 11 000 rpm. The Pickering emulsion droplets were diluted by 4 mL of water. Then it was polymerized at 65 °C for 24 h after degassed with nitrogen gas for 5 min. The dried polystyrene particles were sprayed with Aurum and visualized by SEM. Rheology. Rheological properties of emulsions were performed using a HAAKE MARS III rheometer (ThermoFisher, Germany) with a parallel plate (35 mm diameter). A temperature of 25 ± 0.1 °C was maintained by a Peltier temperature control. With shear rates ranging from 1 to 1000 s−1, flow curves were measured. Oscillatory measurements were performed to determine the storage modulus (G′) and loss modulus (G″) for frequencies ranging from 0.01 to 50 Hz at a fixed strain of 0.1 Pa, which was confirmed to be in the linear viscoelastic range.

Figure 1. (a) Effect of XG concentration on the size distribution of 0.1 wt % silica particles in DI water. The insert figure is the size distribution of silica nanoparticles in the presence of 0.40 g/L XG in ASW. (b) Effect of XG concentration on zeta potential of 0.1 wt % silica nanoparticles suspended in DI water or ASW. SEM images of dried sedimentation of aggregated XG/silica particles in DI water (c) and ASW (d). In SEM experiments, 0.1 wt % silica and 0.40 g/L XG were used in sample preparation.

alone is approximately +45 mV in DI water, which is much higher than that in ASW. With XG concentration increasing, the zeta potential decreases. The zeta potential variance confirms the adsorption of XG onto silica surface. It should be noted that the zeta potential of silica particle becomes almost zero at 0.1 g/L XG in ASW. The XG concentration needed to coat nanoparticles to produce zero charge is almost the same with that needed to produce the maximum sedimentation extent in ASW, as expected. Some studies indicated that when the particle emulsifier was weakly flocculated either by surfactant or salt, fine emulsions of longterm stability may be achieved in the case of oil−in−water emulsions.44 The sedimentation at the bottom of the sample vessel was taken and dried for SEM analysis. The images are shown in Figure 1c and d. In DI water, quite large flocs of microrods were observed. However, the mixture of silica and XG in ASW formed small stacks of platelets with a particle size between 3− 5 μm, as a result of substantial particle agglomeration. On the other hand, the XG chains become more rigid in ASW due to the collapse of side chains along the polymer backbone.46 These factors indicate the different morphologies of sedimentation in DI water and ASW. Additional results on TEM image (see the Supporting Information, Figure S2) show that silica particles in DI water are distributed independently. However, those silica nanoparticles in XG solution aggregate relatively. This confirms that the adsorption of XG onto silica surface contributes to the aggregation of silica nanoparticles. Interfacial tensions were investigated as a function of XG concentration with and without silica nanoparticles (see the Supporting Information, Figure S3). The value of interfacial tension between tetradecane and DI water is 46.8 mN m−1. Addition of XG has negligible impact on the equilibrium interfacial tension, which means both XG and silica is not surface active. Emulsification Based on the Synergy of XG and Silica Nanoparticles. Tetradecane−in−DI water and tetradecane− in−ASW emulsion stabilized by silica, XG, and a mixture of silica and XG are compared, as shown in Figure 2. In DI water,

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RESULTS AND DISCUSSION Interactions between Silica Nanoparticles and XG. Since the isoelectric point of silica nanoparticles is ∼8.5, these nanoparticles possess positive charges at neutral aqueous solution. The presence of oppositely charged XG results in electrostatic attractions between XG and silica nanoparticles. In DI water, sedimentation is observed when XG and silica are mixed, because of their strong electrostatic interactions (see the Supporting Information, Figure S1a). High ionic strength in ASW results in the sedimentation of silica nanoparticles alone. The extent of sedimentation in ASW (sediment height) was evaluated with the photograph of vessels after 24 h storage. With XG concentration increasing, the extent of sedimentation in ASW increases until 0.1 g/L XG, after which it decreases instead (see the Supporting Information, Figure S1b). It is likely because that further adsorption of XG onto particle surface renders the nanoparticle more hydrophilic. This was also reported for the interaction between hexadecyltrimethylammonium bromide (CTAB) and negatively charged silica nanoparticles.43 Samples of the supernatant for XG/silica mixture in DI water and ASW were taken for DLS and zeta potential measurements which are shown in Figure 1. DLS results show that the increase of XG leads to a significantly increased nanoparticle size. Two factors likely contribute to the increased particle size. First, the electrostatic attractions between XG and silica result in the adsorption of XG onto particle surface, which increases particle size. Second, the adsorption of XG onto particles is followed by stronger agglomeration between particles due to the screened electrostatic charges. At the same XG concentration, the average size of silica nanoparticles in ASW is much larger than that in DI water, as a result of substantial nanoparticle agglomeration caused by high ionic strength (the inset in Figure 1a). The zeta potential of silica nanoparticle C

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Figure 2. Photograph and optical microscopy images of tetradecane emulsion stabilized by 0.1 wt % silica nanoparticles, 0.4 g/L XG, and a mixture of silica and XG, respectively. Figure 3. Variation of initial oil droplet size and EI24 value of emulsions stabilized by different combinations of silica nanoparticles and XG in DI water and ASW, respectively.

emulsification was not observed in system containing silica particles alone, which is because of the hydrophilic nature of silica and low energy of attaching to the oil−water interface. Sadeghpour et al. also found that the nontreated aqueous dispersions of silica particle are too hydrophilic to stabilize Pickering emulsions.10 For silica nanoparticles adsorbed at the tetradecane−DI water interface, eq 1 gives ΔadsF ≈ 9.5 × 10−19 J (R = 40 nm, γo−w = 47 mN m−1, and θ = 20°), which is lower relatively. It is worthy to note that 0.4 g/L XG alone in ASW is an effective emulsifier. The bivalent cations and high salinity in ASW lead to intramolecular cross-linking and chains contraction of XG, as revealed by Dario et al.47 The increased viscosity of continuous phase improves the emulsification of XG. The mean droplet size of emulsion stabilized by XG alone is 155.8 μm. A synergism is found between XG and silica nanoparticles, especially in ASW. Optical microscopy image of tetradecane−in−ASW emulsion stabilized by a mixture of silica and XG indicates a smaller oil droplet size of 17.8 μm, an indication of their synergistic effect. All the emulsions are o/w type, which is determined by conductivity measurements and “drop” dilution test. In ASW, flocculated silica nanoparticles alone are ineffective emulsifiers. However, the disappearance of flocculated silica nanoparticles in the resolved aqueous phase for emulsion stabilized by silica/XG mixture implies that flocculated silica nanoparticles also participate in forming stable emulsion in the presence of XG, as shown in Figure 2b. As discussed above, a synergism is found between silica and XG in forming stable emulsions both in DI water and in ASW. The influence of XG and silica concentration on the synergistic emulsification is investigated systematically. The EI24 value and initial droplet size of emulsions prepared from different combinations of XG and silica are shown in Figure 3. The appearance of corresponding emulsions is shown in the Supporting Information (Figure S4). It is indicated that the ASW system produces higher EI24 value and smaller droplet size, compared to the DI water system. This is valuable for their potential applications in marine oil spill dispersion. With XG concentration increasing, smaller droplet size and higher EI24 value are observed. Viscosity experiments indicate that the apparent viscosity of tetradecane−in−ASW emulsion is increased with XG concentration (see the Supporting Information, Figure S5a). The increased emulsion viscosity promotes droplet fragmentation during homogenization, due to increased disruptive shear stresses and decreased droplet recoalescence.48 On the other hand, the presence of XG may give enough time for adsorption of silica particles at the

interface by providing high continuous phase viscosity, thereby facilitating generation of smaller and uniform droplets. These factors contribute to the formation of smaller oil droplets, up to a threshold XG concentration (0.4 g/L) beyond which the droplet size is apparently limited by the emulsification process. Other studies have also reported a major decrease in droplet size for emulsions stabilized by surfactant when small amount of XG was added.49 Figure 3c and d shows a small influence of silica concentration on emulsification when it is higher than 0.1 wt %. Considering these results and possible cost, a mixture of 0.4 g/L XG and 0.1 wt % silica is used in the following studies. Zeta potential measurements of emulsion droplets (see the Supporting Information, Figure S6) indicate that the zeta potential becomes more negative with XG concentration and then reaches a plateau at 0.4 g/L XG. This suggests that at a given silica concentration, excessive XG molecules would not be trapped together with silica particles at the oil droplet interface. Instead, these excessive XG molecules are mainly used to increase the viscosity of continuous phase. However, at a given XG concentration, addition of silica gradually decreases the magnitude of negative zeta potential of oil droplet due to attachment of more positively charged silica particles. The decreased zeta potential gives a weakened electrostatic repulsive barrier between droplets. This supports our results in Figure 3d that higher silica concentration does not lead to desirable ability to emulsification. Stable crude oil−in−ASW emulsion was also prepared based on the synergism of XG and silica. In the presence of 0.4 g/L XG and 0.1 wt % silica, the conductivity of emulsion is 7.21 ± 0.2 ms/cm, indicating the o/w type of emulsion. The results on crude oil emulsification lead us to the same conclusion that higher XG concentration results in smaller oil droplet and higher EI24 value (see the Supporting Information, Figure S7). In emulsions prepared by using 0.1 wt % silica and 0.4 g/L XG, the initial average size of crude oil droplet is about 35 μm and the EI24 value is 48%, suggesting an effective emulsification. It is worthy to mention that the crude oil droplet size keeps lower than 100 μm even after 1 month storage (Figure 4d), which is an appropriate droplet size value for optimum consumption by bacteria.50 The long-term stabilities of emulsions stabilized by XG alone or by a mixture of XG and silica were investigated by D

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Figure 5. CLSM images of emulsions stabilized by a mixture of 0.1 wt % silica and 0.4 g/L XG in DI water (a) and ASW (b), respectively.

contribute to the effective emulsification of silica particles in the presence of XG: (i) adsorption of XG onto silica particle surface by electrostatic attractions increases the size of particle and the corresponding free energy of their attachment to the oil−water interface; (ii) the presence of XG may give enough time for adsorption of silica particles at the interface by providing high continuous phase viscosity; (iii) adsorption of XG onto particles is followed by stronger particle agglomeration. Those agglomerated particles easily adsorb at the oil− water interface, yielding fine emulsions of excellent long-term stability. Binks et al. showed that oil−in−water emulsions are more stable to both creaming and coalescence when particle emulsifiers are flocculated in some degree.44 These factors may lead to silica nanoparticles partitioned to the oil−water interface, after which the adsorbed layer around droplets impedes coalescence by providing a great steric and electrostatic barrier. On the other hand, the presence of XG is very important in forming a stable emulsion, by modifying the flow properties of emulsions. XG has a secondary structure that consists of a 5fold helical structure, which is why it shows high viscosity.52 XG can improve the colloidal stability of emulsions through enhancing continuous phase viscosity and retarding coalescence and creaming of oil droplets. Viscosity experiments not showed here indicate that the apparent viscosity of emulsion increases with XG and silica concentration (see the Supporting Information, Figure S5). For coalescence and creaming to happen, the water phase needs easily flow around the oil droplets and therefore intimate contact between the droplets is possible. A more viscous continuous phase will flow less easily and sometimes cause water to be trapped between droplets that are trying to coalesce. Creaming is the principal process by which the disperse phase separates from an emulsion. The creaming rate can be estimated from the Stokes’ equation:

Figure 4. Time dependence of EI value and oil droplet size of emulsions: (a) Tetradecane−in−DI water emulsion stabilized by 0.1 wt % silica and 0.4 g/L XG; (b) tetradecane−in−ASW emulsion stabilized by 0.1 wt % silica and 0.4 g/L XG; (c) tetradecane−in−ASW emulsion stabilized by 0.4 g/L XG alone; and (d) crude oil−in−ASW emulsion stabilized by 0.1 wt % silica and 0.4 g/L XG.

monitoring the change of EI value and droplet size over time. Emulsions were stored at 25 ± 2 °C for a period of 30 days. Figure 4 shows that the droplet size of emulsion increases and the corresponding EI value decreases steadily over time. However, in comparison with tetradecane−in−DI water emulsion (Figure 4a), only slight creaming and droplet size increase are observed in tetradecane−in−ASW emulsion (Figure 4b). Though XG alone was an effective emulsifier in ASW, a dramatically increased droplet size was found after 1 month of storage (Figure 4c). Combining XG and silica nanoparticles produces an emulsion with smaller change of droplet size over 1 month, indicating their synergistic effect on emulsion stability. The crude oil−in−ASW emulsion was also observed for 1 month in our lab. During the period, it is found that the droplet size of crude oil increases from 33 to 64 μm (Figure 4d). With 0.6 g/L XG, much smaller changes of droplet size and EI value for crude oil−in−ASW emulsion are observed after 1 month storage (see the Supporting Information, Figure S8). Based on the synergy, the mixture of XG and silica considerably limits the coalescence and creaming of emulsion, in comparison with Corexit 9500A, which is the commonly used chemical dispersant in oil spill dispersion. It was shown that Corexit 9500A is an effective emulsifier in dispersing different types of crude oil. However, the droplets dispersed using Corexit 9500A coalesce rapidly due to an insufficient electrostatic repulsive barrier.51 Emulsification Mechanism. A small amount of silica nanoparticle plays an important role in preparing stable oil− in−water emulsions. Prior to emulsification, silica nanoparticles were fluorescently labeled with Rhodamine B. The bright red rings in CLSM images shown in Figure 5 indicate the adsorption of fluorescently stained silica nanoparticles around the oil droplets both in DI water and in ASW when XG is present. In the emulsion prepared in ASW, smaller emulsion droplet is obtained, which is consistent with the result observed using optical microscopy. It has already been demonstrated that flocculated silica nanoparticles alone are ineffective emulsifier. However, in the presence of XG, the emulsification of silica nanoparticles is observed clearly. Various factors could

υ = 2r 2(ρ − ρ0 )g /9η

(2)

where, υ is the creaming rate, r is the droplet radius, ρ is the density of the droplet, ρ0 is the density of the dispersion medium, η is the viscosity of continuous phase, and g is the local acceleration due to gravity. The Stokes’ equation shows that creaming is inhibited by a small droplet radius and a highly viscous continuous phase.53 To understand the synergistic emulsification of XG and silica better, the rheological properties of tetradecane−in−ASW emulsions stabilized by XG alone and a mixture of silica and XG are compared in Figure 6. The emulsions demonstrate non-Newtonian behavior (Figure 6a). From the curve of apparent viscosity versus shear rate, addition of 0.1 wt % silica results in an enhanced apparent viscosity when compared with XG alone at the same shear rate. A E

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Thickening droplet surface layer through the interactions of XG with silica nanoparticles can be another important contribution to their synergistic stabilization. The visualization of the shell morphology of oil droplets is shown in Figure 7. For the emulsions prepared both in DI water and in ASW, the droplet surface consists almost entirely of aggregated XG/silica particles. In DI water, the XG/silica particles are present as microrod-shape flocs at the droplet surface, forming a closely packed layer. This morphology of particle layer is similar to the visualization in Figure 1c. At high magnification (Figure 7c), the aggregated XG/silica particles are found to have a high cross-linking with relatively homogeneous porous structure. However, the shell morphology of oil droplets in ASW is different from that in DI water. A layer of more closely packed XG/silica aggregates is observed, as shown in Figure 7d−f. The degree of particle agglomeration is more pronounced in ASW. An evidence of this is also found in Figure 1d. Such XG/silica aggregated particles should be harder to be displaced from oil− ASW interface. The almost full coverage of the droplet surface by a thicker particle layer prevents the coalescence between droplets by providing steric barriers. The mechanical strength of this adsorbed layer as well as the steric barrier of thick stabilizing layer also greatly contribute to the long-term stability of emulsions. Based on the synergy of XG and silica, the emulsion stability is substantially enhanced. Their synergy in enhancing the viscoelasticity of continuous phase and forming a thick layer at droplet surface leads to the decreased amount of XG and silica needed. The concentration of XG used in this study is far below the values of 1−3 g/L, which is often used in other papers.39,49,55 In a previous study, highly stable oil−in− seawater emulsions was achieved with addition of 0.5% w/v silica nanoparticles and 0.1% w/v caprylamidopropyl betaine.30 Binks et al. prepared stable emulsions with no oil released within 6 months using 2 wt % silica nanoparticles and SDS as an emulsifier.44 The lower silica concentration used here means a possible reduced adverse effect of fine nanoparticles on marine organisms. Recently, some researchers began to focus on evaluating the environmental impact of novel nanoparticle dispersants.56 Zhang et al. found that the large surface area of silica nanoparticles results in a high adsorption capability and

Figure 6. (a) Steady shear viscosity of tetradecane−in−ASW emulsion stabilized by XG alone and a mixture of XG and silica versus shear rate. (b) Frequency dependence of G′ (filled symbols) and G″ (open symbols) for different emulsions.

possible reason for the increased apparent viscosity is that the flocculated silica nanoparticles around the XG chain restrain the long-range motion of XG.54 The enhanced apparent viscosity slows down droplets movement and the number of collisions, and it also is an indication of formation of a stronger gel structure. The domination of elastic over viscous moduli indicates the existence of gel structure. For emulsions stabilized by XG alone, elastic moduli is slightly higher than viscous moduli, indicating a “weak gel” behavior. The presence of 0.1 wt % silica does not change the trend with frequency but increases elastic and viscous moduli, especially elastic moduli. This means that silica nanoparticles enhance the strength of the existing three-dimensional network structure of continuous phase. The combination of XG and silica has led to a synergistic stabilization. Previous studies on silica nanoparticles in polysaccharide solutions have found gel formation caused by the nanoparticles.49,54 In DI water and in ASW, the rheological properties of emulsion stabilized by a mixture of XG and silica are compared, respectively (see the Supporting Information, Figure S9). The apparent viscosity and elastic moduli of emulsion in ASW are significantly enhanced, which implies a more effective emulsification in ASW. Generally, high salinity in ASW leads to intramolecular cross-linking and chains contraction of XG, which increases the apparent viscosity and elastic moduli of emulsions.47 This means that it is a viable dispersant system for the remediation of marine oil spills.

Figure 7. SEM images of styrene droplets stabilized by a mixture of 0.1 wt % silica and 0.4 g/L XG in DI water (a−c) and in ASW (d−f), respectively. F

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ACS Sustainable Chemistry & Engineering allows the particles to participate in possible toxic carrier activities toward marine organisms.57 Therefore, the use of a small amount of nanoparticles in this study would greatly reduce their potential adverse environmental impacts.

Notes

CONCLUSIONS A mixture of XG and silica nanoparticle is proposed as a novel dispersant for the remediation of marine oil spills. Synergistic emulsification occurs through the electrostatic attractions between XG and oppositely charged silica nanoparticles. The key concepts are based on the Pickering emulsification of silica nanoparticles and effective stabilization of XG for oil droplets. XG and silica nanoparticles act synergistically, leading to oil− in−ASW emulsion with smaller oil droplet size and greater stability. Results and observations show that the synergy is a result of the following factors: (1) the presence of XG favors the adsorption of flocculated silica nanoparticles at the oil− water interface. These flocculated silica nanoparticles function as an effective Pickering emulsifier; (2) the enhanced viscoelasticity of continuous phase caused by XG and silica particles is responsible for the long-term stability of emulsion. (3) The almost full coverage of the droplet surface by a thicker layer consisting of closely packed XG/silica aggregates prevents the coalescence and creaming of droplets, by providing steric barrier. These factors contribute to the smaller oil droplets and slower coalescence rate. On the basis of the synergy of XG and silica nanoparticles, the concentrations of XG and silica required for effective oil dispersion are substantially reduced. Then, the potential adverse environmental impact of large-scale fine particles on marine organism will be reduced significantly. Such synergistic effect of natural biopolymer XG and a small amount of silica nanoparticles points to a novel, effective, and environmentally friendly marine oil spill dispersant. This study will inspire the design of particle-based dispersants for emulsifying oil and keeping them stable in the water columns after oil spill accidents.

This research is supported by the Natural Science Foundation of Shandong Province (ZR2014DQ026), the Applied Basic Research Programs of Qingdao in China (14-2-4-119-jch), and National Natural Science Foundation of China (41376084). This is MCTL contribution No.111.

The authors declare no competing financial interest.

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REFERENCES

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.6b00063. Chemical structure of XG, photograph of vessels containing a mixture of 0.1 wt % silica and XG in DI water and ASW, TEM images of 0.1 wt % silica dispersion without and with XG, effect of XG concentration on interfacial tension, photograph of emulsions stabilized by different combinations of XG and silica in DI water and ASW, viscosity of tetradecane− in−ASW emulsion, zeta potentials of oil droplet for tetradecane−in−ASW emulsion, photographs and corresponding initial oil droplet size and EI24 value of crude oil−in−ASW emulsion, time stability of crude oil−in− ASW emulsion stabilized by 0.1 wt % silica and 0.60 g/L XG, rheological properties of emulsions prepared in DI water and in ASW (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (Y.L.). G

DOI: 10.1021/acssuschemeng.6b00063 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

Research Article

ACS Sustainable Chemistry & Engineering

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DOI: 10.1021/acssuschemeng.6b00063 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX