A Promising Surfactant for Enhanced Sorption and Desorption of

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A Promising Surfactant for Enhanced Sorption and Desorption of Polycyclic Aromatic Hydrocarbons Jia Wei,*,† Guohe Huang,‡ Jun Li,† and Xiujie Wang† †

Key Laboratory of Beijing for Water Quality Science and Water Environment Recovery Engineering, Beijing University of Technology, Beijing 100124, China ‡ Faculty of Engineering and Applied Science, University of Regina, Regina, Saskatchewan S4S 0A2, Canada ABSTRACT: Gemini surfactants, an innovative class of amphiphilic molecules, are of increasingly scientific interest due in part to their effectiveness in soil/water remediation. This study was carried out to investigate the overall partitioning of three representative polycyclic aromatic hydrocarbons (PAHs) in the soil-water− surfactant system which takes into consideration the soil-sorbed cationic Gemini surfactant, presence of Gemini micelles, and related developed coefficients. The results indicated that the adsorption of Gemini surfactants onto soil particles through both cation exchange and hydrophobic interaction contribute to the bi or multilayer formation. The sorbed C12−3−12 studied herein, is a highly effective partitioning media for PAHs to adsorb onto the soil phase from the aqueous phase and thus, can be considered as a good adjuvant for an enhanced sorption zone. The partitioning behavior of PAHs in the soil−water−Gemini surfactant has a strong relationship with their Kow. The experimental results from this research will be used to gain an understanding of the effect of cationic Gemini surfactant on the distribution of HOCs in a soil-water system and provide some fundamental and valuable information in remediation of contaminated soils and waters.

1. INTRODUCTION PAHs enter the environment via uncontrolled discharges from petroleum fields, chemical factories, etc. On one hand, PAHs, as with other organic pollutants, can absorb to some soils or sediments,1 allowing them to persist in the soil for extremely long periods of time. On the other hand, because of their limited capacity for sorption of organic contaminants by some soils or clays, they are ineffective in retarding them. The contaminants can transport to other environmental compartments such as a deeper soil layer or groundwater, resulting in human health risks though bioaccumulation and bioconcentration in the food-chain system.2 Therefore, surfactant enhanced remediation (SER) has been suggested as an economically and technically feasible technology for the remediation of contaminated soils and groundwater,3 using two different methods designed for various contaminants and in situ conditions, that is, mobilization and immobilization. Strong interest has been shown in these two opposite effects on contaminant behavior. In terms of mobilization, the ability of anionic surfactants to improve the water solubility of organic compounds provides a potential means of enhancing soil washing treatment efficiency for HOCs contaminated soils or ex-situ washing.4 Unlike anionic surfactants, cationic surfactants can help form the pseudo-organic phase on the solid substrate because of the positive charge of cationics which can be easily combined with soil particles possessing unusually negative charges.5 The tightly sorbed cationic surfactants facilitate the transport of solubilized HOCs and increase retardation of HOCs partitioning to immobile sorbed surfactants favoring the © XXXX American Chemical Society

environmental remediation of HOCs at the localized scale and enhanced accessibility of HOCs to microorganisms.6 Therefore, the distribution of pollutants in each phase, under the impact of surfactants, must be considered to maximize the performance efficiency and minimize the remediation costs. Presently, the majority of surfactants studied with regard to partitioning of HOCs, are conventional surfactants7 and their mixtures,8 which have a single hydrophobic tail and a single hydrophilic headgroup. A more recent and innovative class of amphiphilic molecules to aid the sorption or desorption of HOCs from solid is Gemini surfactants.9 They have become a topic of scientific interest due in part to their effectiveness in the modification of interfacial properties but also because their molecular geometries lead to interesting aggregate structures, both in solution and at the solid-aqueous interface.10 Gemini surfactant molecules consist of two hydrophobic chains and two hydrophilic headgroups connected through a relatively short (rigid or flexible) spacer group.11 The spacer group controls the separation between the headgroups that may be greater or less than the average separation of the corresponding monomer in an aggregate, which changes the mobility and the packing geometry of the surfactant within an aggregate, whether in solution or at a surface.12 It is also well-known that partitioning of organic compounds between a solid and an Received: October 21, 2015 Revised: April 10, 2016 Accepted: April 11, 2016

A

DOI: 10.1021/acs.iecr.5b03964 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Industrial & Engineering Chemistry Research Table 1. Selected Physicochemical Properties of Surfactant and PAHs in This Study

a

Surfactant data from ref 32. bPAHs data from ref 27.

purity >98% (Aldrich products). Gemini cationic surfactant C12−3−12 was obtained from Chengdu Organic Chemicals Co., Ltd., Chinese Academy of Science, with a purity of 95%. Selected physicochemical properties of surfactant and PAHs are included in Table 1. Concentrated PAH stock solutions were prepared in HPLC-grade methanol and stored in a dark place at 4 °C in an amber borosilicate bottle to minimize photodegradation and/or volatilization. Fresh Gemini surfactant stock solutions were prepared by dissolving the relevant surfactants in deionized water. Then, desired mole fractions were obtained by mixing precalculated volumes of the stock solutions, and the following experimental procedures were performed. The conductivity and the surface tension of deionized water are 2−3 μs/cm and 71.87 mN at 25 °C, respectively. The soil sample of this study was a natural soil and was collected from the top (0−25 cm) layer of an area in the Province of Saskatchewan, Canada, not having contamination. The soil pH value was measured in slurries made up at a 1:2.5 soil/water ratio. The texture of the soil is loamy clay containing 1.33% organic carbon (0.0133 g g−1), 39.96% sand, 27.26% silt, and 32.79% clay, respectively. The organic carbon of soil was determined by employing the LECO TruPec CN determinator at the condition of 50% relative humidity and 25 °C. 2.2. Methods. 2.2.1. Solubilization Measurement. The solubilization of PAHs by Gemini surfactant was carried out, subsequently, in batch mode. In each test, excess amounts of PAHs were separately spiked into each vial containing a series of 10 mL of surfactant solutions, having a range of concentrations below and above the CMC to ensure maximum solubility. The sample vials were sealed with a screw cap fitted with a Teflon lined septum to prevent any volatilization loss of PAHs from the surfactant. Triplicate samples were prepared for each surfactant concentration. These samples were then equilibrated for a period of 24 h on a reciprocating shaker, maintained at a temperature of 25 ± 0.5 °C. Following this step, the samples were subsequently centrifuged at 5000 rpm for 30 min to completely separate the undissolved PAHs, at the same temperature. An appropriate aliquot of the supernatant was then carefully withdrawn with a volumetric pipet and diluted to 10 mL in flasks containing 1 mL of methanol and the

aqueous phase depend upon the organic carbon content of the solid phase.13 Since each monomer of a Gemini surfactant possesses two hydrocarbon chains, a Gemini surfactant has twice as much organic carbons per molecule than the conventional surfactant molecule of the same species for a given number of carbon atoms per chain. Therefore, for the same moles of surfactant sorbed onto a solid substrate, the Gemini surfactant will form the better sorbent phase for HOCs in the aqueous phase than the conventional surfactant. Yang et al.14 found that organoclays modified with Gemini surfactants exhibited better efficacy in removing organic contaminants from wastewater in comparison to the monomer-modified clays. Prarat et al.15 synthesized and modifid the surface of mesoporous silica with adsorption of a cationic Gemini surfactant, which had been proven to be a promising adsorbent for removing hydrophobic organic compounds in environmental water. In addition, Gemini surfactants are n−s−m type quaternary ammonium surfactants. It is reported16 that quaternary ammonium surfactants have been widely used in industrial applications and pharmaceutical, cosmetic, and household products preparations, owing to their excellent cell membrane penetration properties, low toxicity, good environmental stability, nonirritation, low corrosivity, and extended residence time and biological activity. However, only sporadic attempts have so far been made to explain the properties of Gemini surfactants in the domain of remediation processes, especially in the soil environment. This study was undertaken to investigate the overall partitioning of three representative polycyclic aromatic hydrocarbons (PAHs) in the soil-water−surfactant system, which takes into consideration the soil-sorbed cationic Gemini surfactant, presence of Gemini micelles, and related developed coefficients. The experimental results from this research will be used to gain an understanding of the effect of cationic Gemini surfactant on the distribution of HOCs in a soil−water system, predict the activity and mobility of the sorbed solutes, and provide some fundamental and valuable information in designing SER applications for contaminated soils.

2. MATERIALS AND METHODS 2.1. Materials. Three representative PAHs were considered: naphthalene, acenaphthene, and phenanthrene, all with a B


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Industrial & Engineering Chemistry Research remainder with the corresponding surfactant−water solution for PAHs analysis. 2.2.2. Surfactant and PAHs Sorption. Batch experiments for sorption of Gemini surfactant, as well as PAHs sorption onto soil from water, in the presence or absence of surfactant, were conducted in duplicate. Soil samples of 0.1 g were weighed into 20 mL standard sample vials with Teflon-lined septa and screw caps, to which 10 mL of distilled water or a given concentration of C12−3−12 solution was added with 0.01 M CaCl2 and 0.01 M NaN3. The CaCl2 was used as an electrolyte to poise the ionic strength, and the NaN3 was used as an inhibitor for bacterial growth.17 The initial surfactant concentration spanned a large range of values below and above the nominal CMC of the Gemini surfactant. Then, the samples were spiked with a known mass of PAHs, prepared in methanol and ensured as being lower than their solubilities. The content in methanol was about 2% in volume so that it could not have an effect upon adsorption.18 The initial surfactant concentration spanned a large range below and above the surfactant CMCs. The sample vials were then equilibrated in a reciprocating chamber at 25 ± 0.5 °C for 24 h (our preliminary test showed apparent sorption equilibrium occurred in less than 20 h) and then centrifuged at 5000 rpm for 30 min. Following centrifugation, an appropriate aliquot of the supernatant was sampled for PAHs and Gemini surfactant analysis. Duplicate blank samples were analyzed as controls for each surfactant concentration. The loss of compounds by photochemical decomposition, volatilization, and sorption to the tube was found to be negligible. All experiments were performed at room temperature, 22−26 °C. 2.2.3. Analytical Method. The concentration of three PAHs in solutions was detected with a Varian spectrophotometer (Cary 300) by measuring the absorbance at 220, 226, and 334 nm wavelength. The typical error in the determination was less than 5%. Cationic Gemini surfactant analysis was carried out at room temperature using a G20 automatic titrator, furnished with a 20 mL autoburet, a stirrer, a surfactant sensitive electrode and a reference electrode. A 1 mL surfactant sample was added to 20 mL of distilled water and then was placed in a 100 mL titration vessel. The solution was titrated with 4 mM SDS dropwise added from the buret at a rate of 10 mL min−1. The intersection point in a titration was distinguished by the titrator, automatically. Thus, the sorbed concentrations of PAHs and surfactants could be determined from the difference of the initially added mass of the surfactant and PAHs and the final mass in the aqueous phase at equilibrium. The data in the figures are all presented as an average of the two replicates. The typical error was less than 5% for solubilization determination and 10% for adsorption determination.

Figure 1. Water solubility enhancement of PAHs as a function of the concentration of C12−3−12 at 25 °C.

activity of the surfactant is changed due to the introduction of the solubilizates. The molar solubilization (MSR) is characterized as the number of moles of compound solubilized by one mole of micellized surfactant and denotes the effectiveness of a particular surfactant in solubilizing a given solute. It can be expressed as follows:20 MSR =

Ss − Scmc Cs − CMC

(1)

where Ss is the total apparent solubility of PAH in a given surfactant solution, at a specified surfactant concentration Cs (the surfactant concentration above CMC at which Sm is evaluated), and Scmc is the apparent solubility of PAH at CMC. In the presence of excess PAH, MSR values could be obtained from the slope of the linear fitted line in which the concentration of PAH is plotted against a surfactant concentration above the CMC (surfactant concentration in mM vs PAH concentration in mM) given in Figure 1. The solubilization capacities of the selected C12−3−12 surfactant for PAHs, quantified by MSR, follow the order of naphthalene > phenanthrene > acenaphthene. The higher solubilization power of the C12−3−12 surfactant, in the present case, may be attributed to the two-head-group with a positive charge, which has a widespread interaction with π-electrons of PAHs and consists of a more hydrophobic content, assisting in excessive micellar core solubilization. To further characterize the effectiveness of solubilization, the micelle−water partition coefficient (Kmc L M−1),21 which is defined as the distribution of organic solutes between the surfactant micelles and the aqueous phase, may be calculated from experimental measurement, using the following formula:

3. RESULTS AND DISCUSSION 3.1. Solubilization of PAHs in the Presence of Gemini Surfactant. The solubilization capability is evaluated by measuring the apparent solubility of naphthalene, acenaphthene, and phenanthrene in various concentrations of Gemini surfactant, as shown in Figure 1. The equilibrium solubility of PAHs starts to increase significantly and linearly to a different extent when the surfactant concentrations exceeded the CMC, which indicates that solubilization is related to micellization. Moreover, the starting point of micellization for Gemini surfactant changed slightly in the presence of different PAHs, as evidenced from Figure 1. It coincides with the Rosen’s research19 that the CMC of surfactant in the aqueous phase in equilibrium with solubilizates should change because the

K mc =

Sm MSR = CmSw Sw

(2)

where Sm and Sw are the concentration of solubilizate in the micelles and water phase (mM), respectively. Cm is the concentration of surfactant in micellar form (mM). The calculated Kmc of three PAHs by C12−3−12 surfactant are presented in Table 2. The values of Kmc and Kow follow a straight line relationship with the slope being 0.5038, as shown C


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Industrial & Engineering Chemistry Research Table 2. Parameters of the Distribution of PAHs in a Soil−Water−Gemini Surfactant Systema

a

Kmc

Kd

Kd*

Koc

Kss

compound

Kow

L M−1

mL g−1

mL g−1

mL g−1

mL g−1

naphthalene acenaphthene phenanthrene

2291 8318 37154

852 3149 16722

28 50 129

551 1283 2225

2169 3817 9847

5634 35776 215864

The average errors in the Kmc, Kd, Kd*, Koc, and Kss are ±2.7%, ± 7.9%, ± 8.3%, ± 3.5%, and ±6.6%, respectively.

in Figure 2, thus indicating that solubilizate with larger Kow has a greater tendency to participate in the micellar phase.

the average. Surfactant sorption exceeds 90% of the added Gemini surfactant at concentrations below the effective CMC and is observed to be relatively weak up to equilibrium, when the added Gemini surfactant concentration goes beyond the CMC. In this study, the sorption isotherm follows a typical Langmuir-type behavior.22 According to the isotherm fitting, the maximum sorption amount of Gemini surfactant, on soil, is approximately 133.31 mg g−1, which in terms of organic carbon, is 77.31 mg g−1 (soil, 13.1 mg g−1), at an equilibrium surfactant concentration of 623.01 mg L−1, slightly higher than the CMC of C12−3−12 (596.87 mg L−1). It is obvious that the surface coverage is superior to the monolayer, indicating that the sorption of C12−3−12 to soils could not only be affected appreciably by the adsorption onto soil particles but also by the partition into SOM. As more organic carbon can be bound to soil with cationic Gemini surfactant, more hydrophobic organic sites are created, leading to more HOCs partitioning into the Gemini surfactant-treated soil particles. It is believed that the cationic surfactant molecules are retained by a cation exchange mechanism, that is, displacing the soil exchangeable inorganic cations. The isotherm plateau studied herein, corresponds to 106 mequiv kg−1, below the CEC of the soil (158 mequiv kg−1), demonstrating that about 67% of the total CEC sites were occupied by Gemini monomers, which is in tune with the result of Pen Wang et al. in which only a fraction of the CEC is generally available for cationic surfactant sorption. Evidently, the C12−3−12 surfactant saturation sorption occurs near the CMC as the micelles begin to form in the solution, indicating that the surfactant monomers are the main sorbing species because only surfactant monomers exist below the CMC. However, the formation of Gemini surfactant monolayer, bilayers, or vesicle-like structures on solid surfaces is not well understood and is still being researched. In this study, since the saturation sorption occurred above the CMC, it is reasonable to presume that bilayers may be formed by Gemini monomers. The highly nonlinear nature of the sorption isotherm signifies that an in situ immobile zone within an aquifer can be formed by Gemini cationic surfactant. Through the powerful interaction of Gemini surfactant with aquifer material, the movement of the surfactant-modified zone becomes progressively slower as the plateau of the isotherm is approached, until finally an enhanced sorption zone is emplaced in situ. Thus, the desorption of contaminants can be restricted, undoubtedly. 3.3. Distribution of PAHs in a Soil−Water−Surfactant System. The solubilization of HOCs into aqueous phase surfactant micelles, the partition of HOCs into the soil organic matter (SOM) phase, and the sorption of HOCs by the soilsorbed surfactant are the three main mechanisms governing the sorption of HOCs occurring in a soil−water system containing surfactant.13 Thus, the intrinsic distribution coefficient, without surfactant (Kd) and the apparent HOCs soil−water distribution

Figure 2. Correlation of the micelle−water partition coefficient (Kmc) with the octanol−water distribution coefficient (Kow) for PAHs.

3.2. Gemini Surfactant Sorption onto soil. Before gaining insight into the partition of PAHs in the water− surfactant−soil system, the sorption of cationic Gemini surfactant is investigated first. The sorption isotherm of Gemini surfactant onto natural soil is presented in Figure 3. It shows a steep initial slope at low concentrations and eventually the slope reaches a plateau at an elevated equilibrium concentration, indicating the saturation of the binding surface. All the standard errors are less than 10% of

Figure 3. Adsorption isotherm of C12−3−12 onto natural soil. D


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Industrial & Engineering Chemistry Research coefficient with surfactant (K*d ), can be well employed to describe the balance of these effects. The soil−water distribution coefficient, without surfactant (Kd), is expressed as24 Kd =

Cs Cw

mobilization or immobilization abilities of soil, with the help of surfactant, depend upon the final competition of these two effects. Obviously, in order to improve the retention capability of soil, effort should be directed into decreasing the undesirable surfactant micelles in the aqueous phase and increasing the amount of soil-sorbed surfactant and their interaction with solid phase in a surfactant-enhanced sorption zone by employing new surfactant systems or exploring suitable conditions for effective and safe soil remediation. Figure 5 illustrates the dependence of Kd* values of three PAHs on the Gemini surfactant equilibrium concentration,

(3)

where Cs (mg g−1) and Cw (mg L−1) are the solid-phase and aqueous-phase equilibrium solute concentrations, respectively. Kd is a fundamental parameter to characterize the mobility and fate of HOCs in soil−water systems. In this study, the observed Kd values for three PAHs in the order of naphthalene < acenaphthene < phenanthrene correlate directly with Kow and inversely with Sw of the solutes. The Kd also can be expressed as Kd = Koc foc, where Koc is the organic-carbon normalized distribution coefficient, and foc is the fractional organic carbon content of the soil. The Koc and Kow are also found to have a good linear relationship (0.2596) as shown in Figure 4, demonstrating that the larger is the Kow of PAHs, the greater is the tendency of PAHs to participate into the solid phase.

Figure 5. Measured apparent sorption coefficients (Kd*) of PAHs versus the equilibrium concentration of C12−3−12.

describing the immobilization of the PAHs within the water− soil−Gemini surfactant systems. As shown, the sorption capacities of naphthalene, phenanthrene, and acenaphthene on C12−3−12-sorbed soils are enhanced, with the maximal Kd* values of 19, 20, and 22 times that of the corresponding Kd, respectively, which indicates that the levels of Gemini surfactant applied decreases the contaminant mobility dramatically. The Kd* values increase sharply with increasing surfactant concentration until they reach a maximum. It is attributed to the strong affinity of PAHs to the head-on sorption of surfactant in the solid phase, which creates many hydrophobic organic sites, making the sorbed C12−3−12 a more effective and sorptive phase for the solutes than natural organic matter. As a matter of fact, the surfactants are present as monomers within this surfactant concentration range (i.e., before CMC), which prefer absorbing on the surface of the hydrophobic solid phase to being present in the water. Thus, the increased added surfactant concentration brings about higher sorbed surfactant concentration until the sorption equilibrium, which consequently resulted in more sorbed PAHs, suggesting that the chosen Gemini surfactant is more suitable for in situ immobilization of PAHs. As a consequence to further increases of surfactant concentration, surfactant sorption becomes negligible. Micelles begin forming in the aqueous solution, competing with the sorbed surfactant for PAH molecules, thereby diminishing the partitioning of PAHs from the soilsorbed surfactant. This result is consistent with previous research. However, it is worth noting that the same tendencies are presented for three PAHs wherein the sorbed PAHs reach

Figure 4. Correlation of the organic−carbon normalized distribution coefficient (Koc) with the octanol−water distribution coefficient (Kow) for PAHs.

Although Kd, for a given HOC, may vary by orders of magnitude in different soils, Koc usually differs only by a factor of 2 to 3.25 Thus, the degree of HOC sorption is principally controlled by soil properties such as foc. Introducing a surfactant and its subsequent sorption onto the surface of a natural solid phase should increase foc and enhance HOC sorption on the treated soil particles. The parameter Kd* is introduced to describe the solute mass balance between solid and solution phases in the presence of surfactant and can be expressed as24 Kd* =

Cs* Cw*

(4)

where Cs* and Cw* are the compound concentration in the solid phase (mg g−1) and the aqueous phase (mg L−1), respectively. This equation is an embodiment of two competitive effects inhibiting the soil−surfactant−contaminant systems, that is, the promoted sorption of organic pollutants by the soil organic matter and surfactant-derived organic matter and the weakened sorption by surfactant micelles in the aqueous phase. The E


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partitioning fraction of PAHs into adsorbed C12−3−12 and an adsorption fraction of PAHs on unoccupied soil surfaces. The C12−3−12+ cations in the hemimicelle occupy the CEC and generate a hydrophobic surface, which not only adsorb organic contaminants but also subsequently uptakes additional monomers from solution by a tail-to-tail interaction mechanism leading to a more dense coverage than the monolayer forming admicelles (Figure 6C). Worth noting is that surfactant monomers, carried in hemimicelle by hydrophobic bonding, are powerfully bound in comparison with those in the admicelles.25 Therefore, it is speculated that the formation of admicelles before CMC leads to the reduced sorption of PAHs. With the increased Gemini surfactant concentration, the nonretained surfactant monomers pass to the solution leading to the formation of Gemini micelles in the aqueous phase, which are capable of desorbing the retained PAHs from the soil phase because they offer a good hydrophobic environment to which the organic solutes can partition and therefore bring about the decreased sorbed amount of PAHs (Figure 6D). Additionally, both the horizontal and vertical differences among three PAHs can be explained by their different water− octanol partitioning coefficient; that is, (a) both the increase and decrease percent in Kd* and the maximum Kd* values (Figure 5) appeared to follow the Kow patterns (Figure 7):

maximum concentrations with increasing sorbed C12−3−12 concentrations before CMC, which is expected with regard to an in situ sorption zone forming with less usage of surfactant. As illustrated schematically (Figure 6A), cation exchange sites on the soil surface are ordinarily occupied by inorganic

Figure 7. Correlation of the apparent HOCs soil−water distribution coefficient with surfactant (Kd*) with the octanol−water distribution coefficient (Kow) for PAHs.

phenanthrene > acenaphthene > naphthalene, in accordance with PAHs hydrophobicity; (b) the occurred intersection under various sorbed surfactant concentrations appeared to be positively related to the Kow of PAHs. It is indicated that the original and obtained soil organic matter content affects more the hydrophobic than the hydrophilic substances while the aqueous surfactant has the opposite effect; that is, the PAHs, which are highly soluble in water, are poorly retained by Gemini surfactant-modified soils. The result is out of accord with the research27 on the distribution of PAHs in the soil-water in the presence of nonionic surfactant and differs with the study of Michael J. Brown,25 but is supported by the previous report28 on the enhanced sorption of pesticide with different Kow in the presence of the conventional cationic surfactant. HernándezSoriano et al.29 also confirmed that the cationic surfactant

Figure 6. Schematic representations of PAHs participation in a soil− water−Gemini surfactant system.

cations, dominantly Ca2+. C12−3−12+, organic cations studied herein, are more preferred than the available cation exchange sites, and thereby, easily displace the inorganic cations (Figure 6B). With the increase of surfactant concentrations, the Gemini monomers adsorbed on solid surfaces begin to aggregate and form micelle-like structures called “hemimicelles”, as defined by Yeskie and Harwell26 (Figure 6B). Thus, the sorbed hemimicelles may create many hydrophobic organic sites, which are more homogeneous and more effective for the sorption of PAHs than NOM, bringing about the highest concentration of sorbed PAHs, while the total sorption of PAHs by C12−3−12 modified soils may be considered as a combination of a F


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reaching a maximum Kss at a certain amount of sorbed surfactant before the surfactant saturation sorption is reached. Furthermore, investigating Figure 8 immediately reveals two distinct features. First, the Kss of the sorbed C12−3−12 for a PAH passed a maximum well before the C12−3−12 saturation sorption is observed, which presents the same tendency with Kd* values. Second, the greater are the Kow values of PAHs, the earlier the maximal points are reached and thus, higher affinity to the sorbed C12−3−12 phase. Another way to look at Figure 8 is that, the greatest Kss values of PAHs also follow the same order of Kow, as indicated by Figure 9. Moreover, the discrepancy of Kss

HDTMA had the highest effect for the more hydrophobic compounds in terms of immobilization. Hence, general conclusions ought to be avoided because each system including soil, water, surfactant, and organic compound may behave in a nonpredicted way, requiring a deeper evaluation, respectively. To precisely study the role of sorbed surfactant on the partitioning of PAHs, an important parameter, the carbonnormalized solute distribution coefficient with the soil-sorbed surfactant, Kss (mL g−1) needs to be studied which is derived from the following equation:30 Kd* =

Kd + fsoc K ss 1 + CsmK mc

(5)

where Csm, the equilibrium concentration of surfactant micelles, equals the difference of surfactant concentrators in solution and CMC (mM), fsoc is the surfactant organic-carbon fraction in the soil calculated by the following formula:

fsoc = Q ss × C%

(6) −1

where Qss is the amount of surfactant sorbed (mg g ), and C% is the percentage carbon within each surfactant molecule. Then, eq 5 can be rearranged to obtain an expression for Kss: K ss =

Kd*(1 + CsmK mc) − Kd Q ss × C %

(7)

From our previously determined parameters, Kd, K*d , and Kmc, the only remaining term Kss can be calculated for subsequent analyses. It is found that the Kss values studied herein are much larger than the corresponding micellar Kmc and solid Koc values which may be due to the geometric difference between sorbed and dissolved surfactants. 31 The variation of K ss for naphthalene, acenaphthene, and phenanthrene with the sorbed C12−3−12 concentration is presented in Figure 8. All studied

Figure 9. Correlation of the carbon-normalized solute distribution coefficient (Kss) with the octanol−water distribution coefficient (Kow) for PAHs.

is expected to vary with fsoc, due to presumably a structure change of the sorbed surfactant with the loaded mass. Because the adsorbed Gemini surfactant molecules may undergo a structural alteration from submonolayer to bilayer or multilayer on the solid phase, the state of which having a substantial consequence on the ability of the adsorbed Gemini surfactant molecules to uptake organic compounds. The Kss values are greater for these PAHs than the corresponding Koc, indicating that the soil-sorbed Gemini is more effective per unit mass as a partitioning medium than native soil organic matter. As the sorption of C12−3−12 increases, maximal Kss takes place when the monolayer formation is completed by sorbed C12−3−12. Then a bilayer starts forming. Within this step, the cationic Gemini surfactant continues to sorb onto the monolayered surfactant molecules and present as an “end-up” configuration leading to a gradually increasing amount of sorbed surfactant, and ultimately resulting in a decreasing Kss23 (Figure 8 region beyond maximal Kss and before surfactant sorption saturation). Thus, for an in situ practical application, a sorption zone can be formed with relatively small amounts of Gemini surfactant, which is lower than the fully sorbed surfactant, since maximum retardation efficiency occurs before surfactant saturation, as indicated by Kss.

Figure 8. Variation of Kss for PAHs with the sorbed C12−3−12 concentration.

PAHs behave in a similar manner, although they partition to a different extent. Compared with Figure 3 (C12−3−12 sorption), it can be observed that for each PAH, the respective Kss was a nonlinear function of the amount of surfactant sorbed, with Kss increasing initially as the amount of sorbed surfactant increased,

4. CONCLUSIONS In this study, a new Gemini surfactant was used to evaluate its effect upon the partition of PAHs in a soil-water−surfactant system. The results indicated that the selected Gemini G


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surfactant can be used in SER technology, as an amendment to the formation of a surfactant enhanced sorption zone. Even a relatively small mass of adsorbed Gemini surfactant can substantially enhance sorption of organic contaminants. Results of the laboratory experiments, presented here, show the following: (1) Solubilization and sorption ability of Gemini surfactant determines its effect upon the mobilization of an immobilization application. (2) The partitioning behavior of PAHs in the soil-water-Gemini surfactant has a strong relationship with their Kow. (3) The sorbed C12−3−12, studied herein, is a highly effective partitioning media for PAHs to adsorb onto the soil phase from the aqueous phase and thus can be considered a good adjuvant for an enhanced sorption zone. (4) Kss is determined by several factors such as the availability of CEC sites for sorption of the Gemini cationic surfactant, the amount of surfactant sorbed, and the characteristic of organic compounds.

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AUTHOR INFORMATION

Corresponding Author

*Tel.: (010) 67391726. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was supported by the National Natural Science Foundation (51209088) and the Postdoctoral Science Foundation. We are also grateful for anonymous reviewers for their helpful suggestions and advices.

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REFERENCES

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A Promising Surfactant for Enhanced Sorption and Desorption of

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