Environ. Sci. Technol. 2009, 43, 5060–5066
Oil-in-Water Emulsions for Encapsulated Delivery of Reactive Iron Particles NICOLE D. BERGE AND C. ANDREW RAMSBURG* Department of Civil and Environmental Engineering, Tufts University, 200 College Avenue, Room 113 Anderson Hall, Medford, Massachusetts 02155
Received February 4, 2009. Revised manuscript received April 28, 2009. Accepted May 4, 2009.
Treatment of dense nonaqueous phase liquid (DNAPL) source zones using suspensions of reactive iron particles relies upon effective transport of the nano- to submicrometer scale iron particles within the subsurface. Recognition that poor subsurface transport of iron particles results from particle-particle and particle-soil interactions permits development of strategies which increase transport. In this work, experiments were conducted to assess a novel approach for encapsulated delivery of iron particles within porous media using oil-in-water emulsions. Objectives of this study included feasibility demonstration of producing kinetically stable, iron-containing, oil-in-water emulsions and evaluating the transport of these iron-containing, oil-in-water emulsions within water-saturated porous media. Emulsions developed in this study have mean droplet diameters between 1 and 2 µm, remain kinetically stable for >1.5 h, and possess densities (0.996-1.00 g/mL at 22 °C) and dynamic viscosities (2.4-9.3 mPa · s at 22 °C and 20 s-1) that are favorable to transport within DNAPL source zones. Breakthrough curves and post-experiment extractions from column experiments conducted with medium and fine sands suggest little emulsion retention (300c
τ (min)a
interval for τ b
not measured 29.3 ( 0.4 25.1 ( 0.5 35.4 ( 0.3 21.6 ( 0.1 not measuredd
not applicable 30 to 55 min 52 to 67 min 91 to 136 min 96 to 122 min not applicable
a Value ( standard error. b Not determined as a result of immediate destabilization. c Visual assessment suggests time of destabilization is on the order of days. d Not measured due to stability >300 min.
containing, oil-in-water emulsions within medium- and finegrained sandy porous media under conditions that theoretically limit the potential for DNAPL mobilization.
Experimental Methods Iron-in-Oil Dispersion Stability. Stability of iron-in-oil dispersions was assessed by monitoring light transmission at a wavelength of 508 nm for a period of 300 min using a Lambda 25 spectrophotomer (PerkinElmer, Inc.) (7). Additional details on materials used to create the dispersions, including iron-coating procedures, are included in the SI. Emulsion Development. Assessment of iron-containing, oil-in-water emulsions for subsurface transport within a DNAPL source zone was based upon kinetic stability, density, viscosity, interfacial tension with TCE-DNAPL, and droplet size distribution. Detailed methods for each analysis may be found in the SI. Kinetic stability is important for emulsion transport as flocculation and coalescence during delivery have deleterious effects on droplet sizes and, therefore, mobility. Droplet sizes smaller than the pore diameters of the porous medium are necessary to promote mobility (22). Density, viscosity, and interfacial tension of the emulsion are important physicochemical properties when considering flow in DNAPL source zones. Emulsion components were selected based upon their suitability to generate kinetically stable emulsions and their nontoxic nature (food-grade components are preferred). The Tween and Span series of surfactants are nontoxic (foodgrade status) nonionic surfactants that have been used previously in subsurface remediation efforts (e.g., refs 17 and 21) and have been shown to produce a synergistic emulsifying relationship (21). Pluronic F108 and oleic acid (OA) are not designated as food-grade but have been employed together to stabilize suspensions of magnetic particles (9). Soybean oil is a food-grade component which has been used as slowrelease substrate in bioremediation efforts (e.g., refs 19 and 23). Surfactant concentrations in the oil-in-water emulsions were selected to maintain emulsion hydrophilic-lipophilic balance (HLB) between 12 and 15, a range documented to be suitable to promote stability of oil-in-water emulsions (24). RNIP (Toda America, Inc.), a commercially available product comprising iron particles that are 40 to 60 nm in size at the time of manufacturing (3), was used as the source of iron in all experiments. Additional details on materials used to create the emulsions as well as the methods employed for emulsification are included in the SI. Emulsion Transport. Column experiments were conducted to evaluate emulsion transport in Ottawa Federal Fine and F-95 sands. Details on the experimental methods and properties of these sands are available in the SI. Subsequent to a nonreactive tracer test, emulsion was introduced upward through the sandy medium (in the absence of DNAPL) at a constant Darcy velocity. Influent and effluent samples were collected over the course of each experiment and analyzed for density, iron concentration, and droplet size distribution using the methods detailed in the SI. Upon conclusion of the experiment, columns were
immediately disassembled and sectioned at 1-cm intervals for subsequent iron and emulsion analysis. Emulsion content describes nonvolatile components (oil, surfactant(s), and iron particles) present within the samples.
Results and Discussion Iron-in-Oil Dispersion Stability. Evaluation of the kinetic stability of iron-in-oil suspensions is a necessary prerequisite for successful particle encapsulation. Suspension stability (iron particles within soybean oil) was analyzed by interpreting settling curves (Figure SI-S2) to determine (i) the time to destabilization and (ii) the time-constant for sedimentation (6, 25, 26). These metrics provide a quantitative basis for selecting an iron-in-oil suspension to use in experiments related to encapsulation and transport. The time to destabilization (Table 1) characterizes the lag time evident in the destabilization curves shown in Figure SI-S2. Suspensions of unmodified iron particles in soybean oil (nonpolar) immediately destabilized (Figure SI-S2). The small increase in stability (to 29 min) upon sonication of the iron-in-oil dispersions was attributed to the breakup of any pre-existing aggregates of the magnetic particles. Specific comparison of the stability of uncoated RNIP suspended in soybean oil and that reported in water is made difficult by the fact that aggregation kinetics depend upon the number of particles in a suspension (6). Destabilization of concentrated suspensions of uncoated RNIP in water (e.g., 1130 mg/L, (6)), however, typically occurs over a period of seconds. The apparent difference in stability between observations for sedimentation in water and soybean oil results from two important differences between the fluids - dielectric constant and viscosity. Nonpolar fluids are characterized by low dielectric constants. Although weak electrostatic interactions may occur within a nonpolar fluid, the contribution of electrostatic repulsion to suspension stability is far less than is typically experienced in polar fluids (27). In the absence of strong electrostatic forces, solvation forces (i.e., the molecular-level interaction between the fluid and solid) must be considered (28). Interactions between metal oxide surfaces and nonpolar fluids tend to be thermodynamically unfavorable, due to the propensity of oxide surfaces to adsorb water (29), and thus should not contribute stability to the suspension. Therefore, particle motion resulting from magnetic, solvation, and gravitational forces is thought to be moderated by the greater viscous drag experienced within the soybean oil (28, 30). This results in a viscous-type stabilization of the suspension. Surface coatings have been employed to introduce steric, solvation, and electrosteric barriers to particle aggregation, thereby increasing the stability of suspensions of particles in aqueous and nonaqueous media (e.g., refs 7 and 31-33). Here, Aerosol MA (MA), Aerosol OT (AOT), oleic acid (OA), and Span 80 coatings (see the SI for details of the coating procedure) were evaluated for enhancement of iron-in-oil dispersion stability. Anionic surfactants MA and AOT were selected to reduce the particle-particle interactions by introducing necessary steric barriers for stabilization. OA was VOL. 43, NO. 13, 2009 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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TABLE 2. Selected Iron-Containing Oil-in-Water Emulsionsa emulsion compositionb water Emulsion A: 10% soybean oil + 6.2% Tween 20 + 2% Span 80 Emulsion B: 10% soybean oil + 6.2% Tween 20 + 2% Span 80 Emulsion C: 10% soybean oil + 3.2% F108 + 2.6% oleic acid Emulsion D: 10% soybean oil + 6.4% F108 + 5.3% oleic acid
RNIPb (% wt)
stabilityc (min)
density (g/mL)
viscosityd (mPa · s)
IFTe (mN/m)
NA
NA
0.998
0.95
35.2 ( 0.3
MA coated (0.25%)
100
1.00 ( 0.01
2.7 ( 0.5
7.54 ( 0.24
AOT coated (0.25%)
90
1.00 ( 0.03
2.4 ( 0.5
7.07 ( 0.14
OA coated (0.25%)
7
NM
NM
NM
OA coated (0.25%)
90
0.996 ( 0.01
9.3 ( 0.5
8.46 ( 0.13
a All properties were measured at 22 ( 3° C and reported as mean ( standard error. b Compositions in percent weight of total emulsion and comprise weight of particles plus surfactant coating (∼2.5 g/L). c Kinetic stability as determined using the time to first visible coalescence in undisturbed samples. d Viscosity measured at 20 s-1. e Interfacial tension with TCE-NAPL; NA ) not applicable; NM ) not measured because of low kinetic stability.
selected because this fatty acid has been shown to be an effective stabilizer of magnetic-fluids (34). Span 80 was selected because hydrophobicity of this surfactant results from an OA moiety. Results indicate that coating the iron particles with Span 80, MA, AOT, and OA increases dispersion stability (Table 1). Note that the addition of MA, AOT, and OA did not influence the viscosity of the iron-in-oil dispersion (60 mPa · s). The use of Span 80 as a coating was found to increase the viscosity of the soybean oil to 80 mPa · s; however, the 33.3% increase in viscosity cannot alone explain the 80% increase in kinetic stability observed for the Span 80 system. Thus, stability enhancements observed in these particle-coated dispersions are hypothesized to result from the combination of solvation, osmotic, and steric barriers to aggregation (6, 27). The time constant for sedimentation is based upon the concept that sedimentation can be modeled as a series of first order processes each having a characteristic time (τ) (e.g., refs 6 and 26). Values of τ calculated for each suspension are shown in Table 1. Phenrat et al. (6, 11) assessed aggregation kinetics for aqueous suspensions of RNIP by characterizing the temporal dependence of aggregate shape and permeability - parameters required to infer particle size from the sedimentation curve (6, 26) and obtained through microscopic visualization. While future studies aimed at optimizing any successful encapsulation approach may find utility in mechanistic, microscale description of RNIP aggregation within soybean oil, focus here is placed on a phenomenological assessment. Interestingly, all but the OA coated particles exhibited similar sedimentation dynamics (i.e., values for τ), suggesting that the aggregates may grow to sizes that are of a similar order of magnitude. Different times to destabilization indicate that the growth process occurs over much different time scales. The longer time to destabilization observed for the coated particles suggests that the Aerosol MA, Aerosol OT, and oleic acid are effective in decreasing the probability of particle-particle attachment to a level that permits time for emulsification of the particlecontaining soybean oil via phase inversion (11). Emulsion Development. Emulsions contained a coated particle loading of 0.25% wt (particle and coating mass, ∼2.5 g/L), which is similar to particle concentrations reported in previous investigations of RNIP transport in porous media (see Table SI-S1). Emulsions A and B exhibited undisturbed kinetic stabilities of 100 and 90 min, respectively (Table 2 and Figure SI-S3). Emulsion C, created using soybean oil containing a 10:1 mass ratio of OA to iron, exhibited rapid creaming and iron sedimentation indicating that this amount of OA is insufficient to simultaneously stabilize the oil droplets and iron particles within the oil droplets. Kinetic stability of 90 min, however, was achieved by increasing the OA content 5062
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to 5.3% wt (an OA to iron mass ratio of 20:1) in Emulsion D. The F108 content in Emulsion D was also increased to 6.4% to maintain the design HLB of 13.6. Continuous agitation (at 140 rpm) was observed to increase the kinetic stability of all emulsions, suggesting that the shear expected during transport through a porous medium may act to maintain emulsion stability (21). The potential for DNAPL mobilization is a critical, though not always appreciated, consideration when developing or assessing remedial amendments for injection within DNAPL source zones. Conditions that suppress DNAPL mobilization can be estimated using the total trapping number (NT) concept (17). NT calculations have aided the design or evaluation of many remedial amendments including surfactant solutions (e.g., refs 17 and 35), macroemulsions (21), solutions of chemical oxidants (36), and pure soybean oil (37). Values of NT were calculated over a range of Darcy velocities (0.01 to 10 m/day) for Emulsions A, B, and D in fine sand (Figure 1), as well as medium and coarse sands (Figure SI-S4). Data used in these calculations are available in Tables 1, SI-S2, and SI-S3. Results suggest that Emulsions A and B have low potential for mobilizing TCE-DNAPL located within medium and fine sands when Darcy velocities are less than 1 m/d (Figures 1 and SI-S4). In addition, the NT analysis suggests emulsion-based delivery of amendments may be less appropriate in coarse sands containing TCE-DNAPL. For comparison, values of NT were also calculated based upon reported properties for a macroemulsion (21), EZVI (5), and surfactant solutions designed for mobilization and solubilization (35). The importance of hydraulic design in the delivery of remedial amendments to DNAPL source zones can be seen in Figure 1. At slow velocities, the magnitude of NT is controlled by amendment density and interfacial tension. As Darcy velocity increases (Figure 1), the steepness in the rise of NT is controlled by viscosity. Thus, the use of higher viscosity amendments such as EZVI (1942 cP, (5)) requires slower delivery or careful containment/recovery of mobilized DNAPL. Emulsions A and B were found to have similar kinetic stability, density, viscosity, and mobilization potential; however, Emulsion B was selected for further analysis based upon the greater hydrophobicity of the AOT coating. The droplet size distribution of Emulsion B was assessed using micrographs from cryogenic SEM and light microscopy (see the SI for experimental details). Two methods were used here because measurement techniques that require dilution or plating of the emulsion may alter droplet sizes. The methods employed here provide lines of evidence that the range of droplet sizes is 0.3-3.0 µm (Figure 2). Differences in the range of the two histograms result from analytical detection limits and the effects of freezing the droplets. The
FIGURE 1. Potential for mobilization of entrapped TCE-DNAPL during delivery of fluids employed for source zone remediation in a fine sand having intrinsic permeability of 2.8 × 10-8 cm2.
FIGURE 2. Oil-droplet distributions for emulsion B: (a) cryogenic SEM micrograph (scale bar is 1 µm) with diameters adjusted for contraction due to freezing, (b) light microscopy micrograph (320 × magnification), and (c) histograms of oil-droplet distributions associated with SEM and light microscopy micrographs. detection limit for the light microscopy analysis is 0.96 µm, limited by camera pixilation. Diameters obtained with cryogenic SEM were adjusted using coefficient of thermal expansion for soybean oil (7.24 × 10-4 °C-1) to account for oil contraction (38); however alterations to droplet size due
to changes in shape and proximity during freezing are also probable. The polydispersivity index, defined as the standard deviation of droplet size divided by the average droplet size (39), of this emulsion is 0.55, which is consistent with those reported by Coulibaly and Borden for oil-in-water emulsions VOL. 43, NO. 13, 2009 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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TABLE 3. Conditions Employed for Emulsion Transport Experiments parameter
experiment I
experiment II
experiment III
experiment IV
Ottawa sand permeability (cm2) porosity (-) packed length (cm) pore volume (mL) Darcy velocity (cm/min) dispersivity (cm) flow direction emulsion introduced (number of pore volumes) effluent emulsion recovery (% wt) effluent iron recovery (% wt) emulsion retained (% wt) iron retained (% wt) emulsion mass balance (%) iron mass balance (%)a
Federal Fine 4.7 × 10-7 0.36 11.6 75.3 0.028 0.091 upward 3.19 101 92 0.18 0.23 101 95
F-95 2.8 × 10-8 0.37 11.0 73.1 0.028 0.054 upward 3.34 101 96 0.12 0.30 101 99
Federal Fine 4.7 × 10-7 0.37 9.8 65.6 0.028 0.150 upward 2.88 100 89 0.09 0.30 100 91
F-95 2.8 × 10-8 0.37 11.8 79.6 0.028 0.067 upward 3.01 101 86 0.19 0.17 101 89
a
Includes iron mass retained in the inlet manifold of the column apparatus.
employed to enhance subsurface microbial activity (19). Light microscopy images indicate the absence of iron aggregates in the continuous phase suggesting the iron particles are encapsulated within the oil. Emulsion Transport. Emulsion transport was evaluated at a Darcy velocity (0.4 m/day) that, based upon results from the NT analysis, poses little risk for DNAPL mobilization. Importantly, the 0.4 m/day Darcy velocity is significantly slower than those employed for previous investigations of iron-particle transport within porous media (Table SI-S1). During experiments I and II (see Table 3), influent and effluent emulsions were sampled to determine whether or not emulsion droplet size distributions are significantly altered during transport through these porous media. Influent droplet size data indicate negligible droplet coalescence in the inlet reservoir during the emulsion flood (∼8 h), suggesting the emulsion remains kinetically stable during the period of introduction (Figure SIS5). The emulsion used in experiment II was more graded than that used in experiment I (Figures 3 and SI-S5). This variation in droplet size distribution was likely the result of less intense mixing during emulsion generation and presented an opportunity to explore how graded droplet size distributions influence transport in sandy media. Effluent breakthrough curves for both the emulsion and iron in experiments I and II are shown in Figure 3 (data from experiments III and IV are shown in Figures SI-S6 and -S7, respectively). Breakthrough of the emulsion in all experiments mimics that of a conservative, nonreactive tracer, suggesting that the oil-in-water emulsion is transported through a sandy porous media without appreciable retention. Results from post-experiment column extractions confirm that the sandy media contained negligible amounts of emulsion (
