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Reversible solubilization of typical polycyclic aromatic hydrocarbons by a photoresponsive surfactant
Colloids and Surfaces A: Physicochem. Eng. Aspects 454 (2014) 172–179
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Colloids and Surfaces A: Physicochemical and Engineering Aspects journal homepage: www.elsevier.com/locate/colsurfa
Reversible solubilization of typical polycyclic aromatic hydrocarbons by a photoresponsive surfactant Jian Long, Senlin Tian ∗ , Yanhua Niu, Guang Li, Ping Ning Faculty of Environmental Science and Engineering, Kunming University of Science and Technology, Kunming, Yunnan, China
h i g h l i g h t s
g r a p h i c a l
a b s t r a c t
• A novel principle for reversible surfactant-enhanced remediation was explicated. • The feasibility of this principle for organics-polluted soil was preliminarily verified. • Photoresponsive surfactant AZTMA was suitable for reversible solubilizing PAHs. • Solubilization-release cycle of PAHs by AZTMA manipulated promptly under UV–vis irradiation.
a r t i c l e
i n f o
Article history: Received 1 February 2014 Received in revised form 3 April 2014 Accepted 10 April 2014 Available online 21 April 2014 Keywords: Reversible solubilization Polycyclic aromatic hydrocarbons Photoresponsive surfactant Surfactant-enhanced remediation
a b s t r a c t For the purpose of surfactant-enhanced remediation (SER) as a means of removing polycyclic aromatic hydrocarbons (PAHs) from polluted soils, a new method of reversibly solubilizing PAHs has been developed based on modulating the surface activity of the photoresponsive surfactant 4butylazobenzene-4 -(oxyethyl)trimethylammonium bromide (AZTMA). AZTMA undergoes reversible isomerization between trans and cis isomers upon irradiation with ultraviolet (UV) and visible lights. The considerable disparity in critical micelle concentrations (CMCs) between trans- and cis-AZTMA implies that the surface activity of this surfactant may be easily manipulated. Kinetic studies have indicated that the solubilization mechanism of PAHs by AZTMA is similar to that of the conventional surfactants and can be described by an adsorption–desorption model. The molar solubilization ratios (MSRs) of pyrene, phenanthrene and acenaphthene in 10 mmol L−1 trans-AZTMA solution were 0.66 × 10−2 , 3.92 × 10−2 and 5.02 × 10−2 , respectively, much higher than those with cis-AZTMA. More than 70% of the solubilizate in trans-AZTMA solution could be released through irradiation with 365 nm UV light at an appropriate surfactant concentration. The presented results provide a theoretical foundation for reversible surfactantenhanced remediation (RSER) based on a photoresponsive switchable surfactant. © 2014 Elsevier B.V. All rights reserved.
1. Introduction The contamination of soil by toxic and hazardous organic pollutants is a worldwide environmental problem. Polycyclic aromatic
∗ Corresponding author. Tel.: +86 871 65920528/65107698; fax: +86 871 65920528. E-mail address: [email protected] (S. Tian). http://dx.doi.org/10.1016/j.colsurfa.2014.04.033 0927-7757/© 2014 Elsevier B.V. All rights reserved.
hydrocarbons (PAHs) are of particular concern owing to their longterm persistence in soil and adverse effects on human health [1]. Various physical, chemical, and biological technologies, as well as combinations thereof, have been evaluated for their removal. However, PAHs are very difficult to degrade because they adsorb strongly to soils or sediments. As a consequence, remediation of PAH-contaminated soil is often geared towards desorption of the PAHs [2,3]. It is well known that surfactants can improve the masstransfer of PAHs from the solid phase or a non-aqueous phase
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Fig. 1. Photoisomerization of AZTMA.
liquid (NAPL) to the aqueous phase by decreasing the interfacial tension and accumulating the hydrophobic compounds in micelles [4,5]. A variety of researches have also indicated the potential of using micellar solubilization to clean up residual and adsorbed contaminants from soils [5–8]. Surfactant-enhanced remediation (SER) has been proposed as a promising technology for the treatment of organic-contaminated soil [5,9,10]. Numerous studies have demonstrated that SER is very effective for the removal of PAHs from soil [11–16]. Demonstration projects around the world have been carried out to show that SER technology has great potential for the remediation of soil and groundwater [16–20]. Generally, during the SER process, after completing its specific function, the surfactant has to be separated from the solubilization system for reuse. Otherwise, there may be negative effects on the environment caused by its direct emission and costs would be high. Hence, the recycling efficiency of the surfactant must be taken into account in contaminated soil remediation. If the formation and disintegration of molecular assemblies could be reversibly controlled by external stimuli, it might be possible to regulate the solubilization and release of contaminants. In this way, the reuse of a surfactant could be achieved. It has been reported that switchable surfactants can be reversibly controlled through the formation and disruption of vesicles by external stimuli, including photochemical means [21,22], electrical potential [23–26], the presence of gases [27,28], and so on. In our previous studies, a nonionic switchable surfactant FPEG was synthesized and the hydrodynamic diameter and aggregation number of its micelles could be reversibly controlled by an electrochemical approach [29]. Li et al. [30] verified that a ferrocene-modified amphiphilic molecule Fc14 could efficiently solubilize VOCs, and that contaminants in the micelles could be easily separated after solubilization. In the present study, we have sought a surfactant for which the micelles could be reversibly and rapidly controlled without the addition of a third substance. For this purpose, a photoswitchable surfactant seemed to be a good choice. In order to obtain an appropriate switchable surfactant that could be conveniently regulated, azobenzene-modified cationic 4-butylazobenzene-4 (oxyethyl)trimethylammonium bromide (AZTMA) was synthesized. This molecule has good photoresponsive characteristics, with trans–cis photoisomerization occurring under irradiation with UV or visible light, and its surface chemical properties change accordingly [31,32]. As reported previously [33], the release of an oily
substance, ethylbenzene, could be reversibly controlled by the change in the molecular structure of AZTMA. Therefore, we envisaged reversible surfactant-enhanced remediation (RSER) based on this switchable surfactant. In the process of soil remediation, the ion-exchange capacity of AZTMA may limit the potential significance of this technology. However, Zhang et al. [34] found that the use of cationic–nonionic mixed surfactant could decrease the adsorption of cationic surfactant, and cationic surfactants can be used for soil remediation when mixed with other surfactants [35]. Furthermore, Matsumura et al. [31] reported that the formation and disintegration of mixed micelles of AZTMA and other surfactants could be regulated by light. Hence, AZTMA appeared to have good prospects for the proposed technique. However, owing to their lower solubility in water, the mechanism of solubilization of PAHs using photoresponsive amphiphilic molecules containing azobenzene groups has rarely been expounded. Understanding of the water solubility enhancement of organic pollutants by switchable surfactants and the release efficiency of organic compounds following contaminant solubilization in surfactant micelles might allow extending of the scope of contaminant remediation. In this study, due to their ubiquity and persistence in the environment, three typical PAHs (pyrene, phenanthrene, and acenaphthene) with various water solubilities have been employed as target pollutants. Experiments have been carried out to expound the mechanism of solubilization of PAHs using AZTMA and to ascertain whether this azobenzene-modified surfactant might be used in SER. The applicability of light-driven release of pollutants contained by AZTMA micelles has also been investigated. The photoisomerization of AZTMA is illustrated in Fig. 1.
2. Materials and methods 2.1. Materials The photosensitive surfactant AZTMA was synthesized and purified according to the procedures similar to those described by Hayashita et al. [36]. The identity of the cationic surfactant with an azobenzene moiety in its hydrophobic group was verified by NMR analysis in CDCl3 solution. The physicochemical properties of the target solubilizates pyrene, phenanthrene, and acenaphthene (98% pure, Aldrich Chemicals) are shown in Table SI-1.
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2.2. Surface tension measurements The surface tensions of trans-AZTMA and cis-AZTMA were measured by the Wilhelmy plate method (Fangrui Co. Ltd., Shanghai, China) at 25 ◦ C. The output from the surface tensiometer was continuously monitored until the surface tension was constant. The whole process was performed in the dark, so as to avoid changes in the structure of AZTMA caused by light irradiation. 2.3. Photoisomerization of AZTMA An XPA-7 photochemical reactor (Yuhua Co., Ltd., Gongyi, China) equipped with a 500 W mercury–xenon lamp was employed as the light source to induce the photoisomerization of AZTMA. UV (365 nm) and visible light (≥420 nm) irradiation were selected by the use of appropriate filters. The trans/cis isomerization of AZTMA in aqueous solution could be monitored spectrophotometrically (Shimadzu UV-2450) in the range 200–650 nm. Measurements were performed using a quartz cuvette with an optical pathlength of 10 mm. The micelle sizes of AZTMA in aqueous solution were measured by the dynamic light-scattering method using a light-scattering apparatus (BI-200SM-B) at 25 ◦ C. A BI-LRM laser (532 nm) was employed as the light source and the measuring angle was set at 90◦ . The distribution of micelle sizes was evaluated by the analysis of a non-negatively constrained least-squares (NNLS) algorithm. 2.4. Solubilization of PAHs Solubilization of the typical PAHs in AZTMA solutions was conducted in 40 mL Corex centrifuge tubes with Teflon-lined screw caps. Briefly, 10 mL aliquots of solutions with a series of surfactant concentrations (0.1–10 mmol L−1 ) were added to centrifuge tubes, and then pyrene, phenanthrene, or acenaphthene was added to each tube in an amount slightly greater than that required to saturate the solution. Duplicate samples were prepared for each concentration of surfactant and dose of PAHs. The samples were subsequently vibrated on a reciprocating shaker for 24 h at 25 ◦ C, which proved to be sufficient to achieve solubilization equilibrium. The solution and solid phase were then separated by centrifugation at 3000 rpm for 30 min. An appropriate aliquot (1 mL) of the supernatant was removed and extracted with hexane (20 mL). Pyrene, phenanthrene, and acenaphthene were then quantified spectrophotometrically at 333, 250, and 227 nm, respectively.
Fig. 2. Effect of UV–vis irradiation on the absorbance of AZTMA.
three different conditions were recorded and are shown in Fig. 2. The characteristic absorption peak for trans-AZTMA was seen at 348 nm; after UV irradiation for 2 h, this peak disappeared and a new absorption peak appeared at 435 nm, which indicated that the trans isomer had been transformed to the cis isomer. The peak at 348 nm assigned to the trans isomer reappeared after irradiation with visible light for 30 min. This verified that AZTMA undergoes reversible isomerization between the trans and cis isomers through irradiation with UV (365 nm) and visible (≥420 nm) lights, respectively. Fig. 3 shows the evolution of the photoisomerization processes versus irradiation time. On irradiation with UV light, the concentration of trans-AZTMA in an initially 10 mmol L−1 solution decreased rapidly to approach zero, which suggested that the isomerization was ongoing, and the process reached a photostationary state in 50 min. Conversely, cis-AZTMA was transformed into the trans isomer under irradiation with visible light, and the equilibration time was measured as 90 min, somewhat longer than that for the UV-induced process. The results showed that the structure of AZTMA could be conveniently controlled, the reversible reactions proceeding rapidly without the need for a third substance. However, the equilibrium concentration of re-trans-AZTMA was lower than original trans-AZTMA. This was mainly because the existence of energy barriers in inversion pathway could hinder the proceeding of photoisomerization [38]. In this state, both trans- and cis-AZTMA and the intermediate of photoisomerization reaction
2.5. The controllable release of PAHs Photochemically controlled release of the PAHs from the micelles of trans-AZTMA was performed in a photochemical reactor using a UV filter (365 nm). During these experiments, the photochemical reactor was kept at 25 ◦ C as in the solubilization experiments. Solutions of trans-AZTMA were irradiated with UV light for 2 h to achieve a photostationary state. The solubilization of PAHs was then performed as described above. The concentrations of PAHs in the surfactant solutions were analyzed spectrophotometrically [37]. The cumulative release efficiencies of the PAHs solubilized in the micelles were calculated from the difference in the solubilized concentrations in the two isomers. 3. Results and discussion 3.1. Verification of trans/cis photoisomerization of AZTMA in solution In order to verify the photoisomerization of AZTMA, the UV–vis absorption spectra of a 0.05 mmol L−1 AZTMA solution under
Fig. 3. Variation of the absorbance of AZTMA upon irradiation with UV light and visible light.
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Fig. 5. Equilibration curves for the solubilization of PAHs in trans-AZTMA solution. Fig. 4. Relationship between the surface tension and the surfactant concentration of trans-AZTMA/cis-AZTMA solution.
3.3. Solubilization kinetics of PAHs by trans-AZTMA Generally, solubilization involves a series of processes: the diffusion of PAH molecules and surfactant micelles in the solution, and the adsorption and desorption of micelles at the PAH/water interface. The micelle adsorption–desorption model proposed by Chan and Cussler [39] in 1976 and the solute diffusion model proposed by Carroll [40] in 1981 have been widely used to describe the dynamic process of solubilization. The two models may be represented by a pseudo-first-order kinetic equation [40–42] and a pseudo-second-order kinetic equation [42], as shown in Eqs. (1) and (3), respectively. Pseudo-first-order kinetic equation
existed in the surfactant solution. Additionally, we continued to find this cycle manipulated promptly with continued treatment of light, and there was still a small proportion of cis-AZTMA that could not convert into its trans-isomer, but this proportion of “stable” cis-AZTMA would not increase as the reversible reaction progress, which revealed that AZTMA possesses well reversible characteristics. According to these, if the surface chemical properties of an AZTMA solution change with variation of the structure, the solubilization and release of organics by micelles might be achieved.
dCt = k1 (Ce − Ct ) dt 3.2. Surface chemical properties of AZTMA
(1)
The linear expression derived from this equation is Ct = Ce (1 − e−k1 t )
The surface activity and photochemically reversible characteristics of the two isomers of AZTMA were compared. The relationship between surface tension and concentration for aqueous solutions of AZTMA at 25 ◦ C is shown in Fig. 4. Within a certain range, the surface tension decreased with increasing concentration of AZTMA, but the trend of the curve changed abruptly at a certain concentration for each isomer. As determined from the turning points on the surface tension curves, the critical micelle concentration (CMC) for the cis isomer was 5.1 mmol L−1 , whereas that for the trans isomer was 2.0 mmol L−1 . According to the structure of AZTMA, an increase in CMC was favored by the bulkier structure and the shorter apparent hydrophobic tail length of cis-AZTMA. The considerable disparity in the CMC values between transAZTMA and cis-AZTMA implies that the surface chemical properties of AZTMA can be easily manipulated by changes in the incident light. Hence, if the concentration of AZTMA was appropriately selected to be somewhere between the CMCs of the two isomers, UV light irradiation would disrupt the micelles in solution into surfactant monomers. If contaminants such as PAHs had been solubilized in micelles of trans-AZTMA, irradiation would also bring about their release.
(2)
Pseudo-second-order kinetic equation dCt = k2 (Ce − Ct )2 dt
(3)
The linear expression derived from this equation is 1 t 1 = + Ct Ce k2 Ce2
(4)
where Ce and Ct are the concentrations of the PAH at equilibrium and at time t (h), respectively; k1 and k2 are the reaction rate constants of pseudo-first-order kinetics and pseudo-second-order kinetics, respectively; and t is the time (h). The solubilization kinetics of the three typical PAHs fitted with the different kinetic equations are shown in Fig. 5, and the fitted parameters are shown in Table 1. The equilibration time of the PAHs in the surfactant solution was about 12 h. The correlation coefficients of the three kinetic equations follow the order: pseudo-second-order equation > pseudo-first-order equation. During the solubilization process, the diffusion of PAH molecules can be ignored because of their low solubility in water.
Table 1 Fitted parameters of different kinetic models. PAHs
Pyrene Pherantherene Acenaphthene
Pseudo first-order equation
Pseudo second-order equation
Ce
k1
R2
Ce
k2
R2
11.63 55.32 65.41
0.11 0.21 0.25
0.878 0.9764 0.9818
15.62 67.2 79.97
6.2 × 10−3 3.35 × 10−3 3.28 × 10−3
0.9331 0.9863 0.9818
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Fig. 6. The solubilization of PAHs by trans-AZTMA solution.
A surfactant solution with high concentration contains more surfactant micelles, and hence the likelihood of micelles coming into contact with the PAH is increased. Since AZTMA micelles have a strong capacity for encapsulation, adsorption–desorption at the PAH/water interface is the dominant factor, rather than the diffusion of PAH molecules into the micelles via the aqueous phase. As is evident from the above results, the solubilization mechanism of AZTMA is similar to that of conventional surfactants [40]. This suggests that the novel switchable surfactant AZTMA may indeed be applied for SER. 3.4. Solubilization of PAHs controlled by light irradiation The apparent enhancement in water solubility of a PAH in a surfactant solution can be expressed as follows [43]: f =
∗ Sw = 1 + Xmn Kmn + Xmc Kmc Sw
(5)
∗ is the apparent solute solubility at a surfactant concenwhere Sw tration of X; Sw is the solubility of the PAH in water at 25 ◦ C; Xmn is the concentration of the surfactant as a monomer in water; Xmc is the concentration of the surfactant as a micelle in water; Kmn is the partition coefficient of the solute between the surfactant monomer and water; and Kmc is the solute partition coefficient between the aqueous micellar phase and water. The changes in the solubility of the PAHs with the addition of trans-AZTMA solution are shown in Fig. 6. As is clear from Fig. 6, the solubilities of the PAHs in water were significantly enhanced by trans-AZTMA and increased linearly over the range of surfactant concentrations above the CMC. The apparent solubilities of pyrene, phenanthrene, and acenaphthene in 10 mmol L−1 AZTMA solution were about 69, 24, and 14 times higher than those in water, respectively. The water solubility enhancements of these three typical PAHs by cis-AZTMA were evaluated and compared with those in transAZTMA. As shown in Fig. 7, the water solubilities of the PAHs in cis-AZTMA solutions were lower than those in trans-AZTMA solu∗ determined tions at comparable concentrations. The values of Sw for pyrene, phenanthrene, and acenaphthene in 10 mmol L−1 cisAZTMA solution were 4.6, 29.6, and 33.8 mg L−1 , respectively. These results indicated that the uptake by and release from the micelles of these PAHs could be controlled through light irradiation, making use of the difference in CMC between the trans and cis isomers. After UV irradiation, the apparent hydrophobic tail length was shorter than before, and the micelles were disintegrated into surfactant monomers.
Fig. 7. The solubilization of PAHs by AZTMA solution: (a) pyrene, (b) phenanthrene, and (c) acenaphthene.
In order to determine the effectiveness of a given surfactant in solubilizing PAHs, the molar solubilization ratio (MSR) can be evaluated according to the following equation [12]: MSR =
S − SCMC Cs − CMC
(6)
where S is the apparent solubility of a PAH at a particular surfactant concentration, Cs , which is greater than the CMC; and SCMC is the apparent solubility of the solute at the CMC. The partitioning of a solute between aqueous solution and surfactant micelles can be characterized by a mole fraction micellephase/aqueous-phase partition coefficient, Kmc = Xm /Xa , where Xm is the mole fraction of the solute in the micellar phase and Xa is the mole fraction of the solute in the aqueous phase. Here, Kmc depends on the surface properties of AZTMA and the temperature. In terms of MSR, Kmc can be calculated from experimental measurements using the following formula: Kmc =
55.4MSR SCMC (1 + MSR)
(7)
The MSR and log Kmc values of the studied PAHs in cis- and trans-AZTMA solutions are shown in Table 2, and the solubilization
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Table 2 MSR and Kmc of PAHs in AZTMA solutions. Pyrene
trans AZTMA cis AZTMA CTAB
Pherantherene
Acenaphthene
MSR
log Kmc
MSR
log Kmc
MSR
log Kmc
0.66 × 10−2 0.048 × 10−2 1.6 × 10−2
2.23 1.83 2.65
3.92 × 10−2 2.59 × 10−2 2.7 × 10−2
2.02 1.50 2.09
5.02 × 10−2 3.17 × 10−2 4.4 × 10−2
1.76 1.25 1.81
potentials of the selected PAHs in the typical cationic surfactant hexadecyltrimethylammonium bromide (CTAB) are also summarized. According to the MSR and log Kmc values for AZTMA and CTAB, the solubilization capacities of the azobenzene surfactant and CTAB were almost the same. Moreover, AZTMA showed higher solubilization capacities for phenanthrene and acenaphthene, which confirmed that it could meet the requirements of the SER process. The results also demonstrated that the solubilizing power of trans-AZTMA is much higher than that of cis-AZTMA. This suggested that AZTMA micelles have the potential to release the solubilized organics. The degrees of water solubility enhancement of the PAHs by AZTMA follow the order pyrene > phenanthrene > acenaphthene, mirroring the octanol/water distribution coefficients (Kow ) of the PAHs. As shown in Fig. 8, excellent linearity was obtained. 3.5. Photochemical control of PAH release
with visible light, the micelles reformed and entrapped the substances (Fig. 10c). Finally, the entrapped PAH molecules could be released by irradiation of the AZTMA solution with UV light once more (Fig. 10d). By cycling the photoisomerization of AZTMA, the surfactant could be reused for the loading and release process, and this reversible process could be broadly applicable for various substances. The cumulative release efficiency of a PAH solubilized in the micelles by photoisomerizing trans-AZTMA to cis-AZTMA was evaluated according to the following equation: R=
∗ − S∗ Sw1 w2 ∗ Sw1
× 100%
(8)
∗ and S ∗ are the apparent water solubilities of the PAHs where Sw1 w2 in trans-AZTMA and cis-AZTMA, respectively (mg L−1 ). As shown in Fig. 11, the values of R increased with increasing surfactant concentration in the range 2–4 mmol L−1 . On further
Particle size distribution is a key criterion in judging the formation of micelles in surfactant solutions [31]. Fig. 9 shows the effects of UV–vis irradiation on the size distribution of molecular assemblies in 3.0 mmol L−1 AZTMA solutions. The mean micellar size in trans-AZTMA solution was found to be 2.4 nm. After UV irradiation, it changed from 2.4 to 1.7 nm. Thereafter, the mean micellar size reverted to 2.3 nm upon irradiation with visible light. As mentioned above, the structure of the azobenzene group is suggested to strongly affect the surface properties of AZTMA solution. Photochemical control of the solubilization and release of contaminants by AZTMA micelles may be realized if the changes in the surface properties caused by the photoisomerization of the azobenzene group of the surfactant can be exploited. The loading and release of the micelles, using PAHs as a typical model substrate, are demonstrated in Fig. 10. In the original trans-AZTMA solution, the micelles were in a shrunken state and the PAHs were stably solubilized within them (Fig. 10a). After UV irradiation, because of the bulkier structure and shorter apparent hydrophobic tail length, the micelles in cis-AZTMA disassembled and the micellar size was decreased, which permitted the outward diffusion of the PAH molecules (Fig. 10b). Thereafter, upon irradiation
Fig. 8. Correlation between log Kow and log Kmc for PAHs in AZTMA solutions.
Fig. 9. Particle size distributions of 3 mmol L−1 AZTMA micellar solutions: (a) trans isomer, (b) cis isomer, and (c) restored trans isomer.
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Fig. 10. Redox-mediated loading and release of PAHs.
increasing the surfactant concentration beyond 4 mmol L−1 the release efficiency began to decrease. About 68% of the acenaphthene could be released after UV irradiation for 2 h at a concentration of 4 mmol L−1 . Fig. 10 also shows that the cumulative release efficiencies of selected PAHs followed the order pyrene > phenanthrene > acenaphthene, which was consistent with the results of water solubility enhancements of PAHs by AZTMA. Note here that the consistency was most probably due to the intrinsic solubility and partitioning interaction of the selected PAHs. A lower intrinsic solubility and weaker partitioning interaction would lead to greater release efficiency. In the case of pyrene, about 13% of the solubilizate was not released within 2 h, even at a concentration of 4 mmol L−1 . It is believed that the unreleased substance can stably interact with a thick shell of AZTMA through hydrogen bonding and other interactions.
4. Conclusions In summary, this research reveals that the photosensitive surfactant AZTMA can reversibly undergo trans/cis photoisomerization upon irradiation with UV and visible lights. Because of its good surface activity, AZTMA has a strong solubilization capacity for PAHs to meet the requirements of the SER process. PAHs solubilized with AZTMA can be released from the micelles through UV irradiation, provided that the initial surfactant concentration is between the two CMCs for changing its surface properties. Thus, the azobenzene surfactant AZTMA may be reversibly used in surfactant-enhanced remediation technology through photochemical control. That is to say, aiming to reduce the cost and prevent repeated pollution, the switchable azobenzene-modified surfactant provides a convenient means of realizing reversible surfactant-enhanced remediation. Acknowledgments The authors would like to thank the National Natural Science Foundation of China (Nos. 21077048 and 20607008) and the Specialized Research Fund for the Doctoral Program of Higher Education (No. 20115314110011) for financial support. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.colsurfa. 2014.04.033. References
Fig. 11. Cumulative release of PAHs as a function of surfactant concentration.
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Colloids and Surfaces A: Physicochemical and Engineering Aspects journal homepage: www.elsevier.com/locate/colsurfa
Reversible solubilization of typical polycyclic aromatic hydrocarbons by a photoresponsive surfactant Jian Long, Senlin Tian ∗ , Yanhua Niu, Guang Li, Ping Ning Faculty of Environmental Science and Engineering, Kunming University of Science and Technology, Kunming, Yunnan, China
h i g h l i g h t s
g r a p h i c a l
a b s t r a c t
• A novel principle for reversible surfactant-enhanced remediation was explicated. • The feasibility of this principle for organics-polluted soil was preliminarily verified. • Photoresponsive surfactant AZTMA was suitable for reversible solubilizing PAHs. • Solubilization-release cycle of PAHs by AZTMA manipulated promptly under UV–vis irradiation.
a r t i c l e
i n f o
Article history: Received 1 February 2014 Received in revised form 3 April 2014 Accepted 10 April 2014 Available online 21 April 2014 Keywords: Reversible solubilization Polycyclic aromatic hydrocarbons Photoresponsive surfactant Surfactant-enhanced remediation
a b s t r a c t For the purpose of surfactant-enhanced remediation (SER) as a means of removing polycyclic aromatic hydrocarbons (PAHs) from polluted soils, a new method of reversibly solubilizing PAHs has been developed based on modulating the surface activity of the photoresponsive surfactant 4butylazobenzene-4 -(oxyethyl)trimethylammonium bromide (AZTMA). AZTMA undergoes reversible isomerization between trans and cis isomers upon irradiation with ultraviolet (UV) and visible lights. The considerable disparity in critical micelle concentrations (CMCs) between trans- and cis-AZTMA implies that the surface activity of this surfactant may be easily manipulated. Kinetic studies have indicated that the solubilization mechanism of PAHs by AZTMA is similar to that of the conventional surfactants and can be described by an adsorption–desorption model. The molar solubilization ratios (MSRs) of pyrene, phenanthrene and acenaphthene in 10 mmol L−1 trans-AZTMA solution were 0.66 × 10−2 , 3.92 × 10−2 and 5.02 × 10−2 , respectively, much higher than those with cis-AZTMA. More than 70% of the solubilizate in trans-AZTMA solution could be released through irradiation with 365 nm UV light at an appropriate surfactant concentration. The presented results provide a theoretical foundation for reversible surfactantenhanced remediation (RSER) based on a photoresponsive switchable surfactant. © 2014 Elsevier B.V. All rights reserved.
1. Introduction The contamination of soil by toxic and hazardous organic pollutants is a worldwide environmental problem. Polycyclic aromatic
∗ Corresponding author. Tel.: +86 871 65920528/65107698; fax: +86 871 65920528. E-mail address: [email protected] (S. Tian). http://dx.doi.org/10.1016/j.colsurfa.2014.04.033 0927-7757/© 2014 Elsevier B.V. All rights reserved.
hydrocarbons (PAHs) are of particular concern owing to their longterm persistence in soil and adverse effects on human health [1]. Various physical, chemical, and biological technologies, as well as combinations thereof, have been evaluated for their removal. However, PAHs are very difficult to degrade because they adsorb strongly to soils or sediments. As a consequence, remediation of PAH-contaminated soil is often geared towards desorption of the PAHs [2,3]. It is well known that surfactants can improve the masstransfer of PAHs from the solid phase or a non-aqueous phase
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Fig. 1. Photoisomerization of AZTMA.
liquid (NAPL) to the aqueous phase by decreasing the interfacial tension and accumulating the hydrophobic compounds in micelles [4,5]. A variety of researches have also indicated the potential of using micellar solubilization to clean up residual and adsorbed contaminants from soils [5–8]. Surfactant-enhanced remediation (SER) has been proposed as a promising technology for the treatment of organic-contaminated soil [5,9,10]. Numerous studies have demonstrated that SER is very effective for the removal of PAHs from soil [11–16]. Demonstration projects around the world have been carried out to show that SER technology has great potential for the remediation of soil and groundwater [16–20]. Generally, during the SER process, after completing its specific function, the surfactant has to be separated from the solubilization system for reuse. Otherwise, there may be negative effects on the environment caused by its direct emission and costs would be high. Hence, the recycling efficiency of the surfactant must be taken into account in contaminated soil remediation. If the formation and disintegration of molecular assemblies could be reversibly controlled by external stimuli, it might be possible to regulate the solubilization and release of contaminants. In this way, the reuse of a surfactant could be achieved. It has been reported that switchable surfactants can be reversibly controlled through the formation and disruption of vesicles by external stimuli, including photochemical means [21,22], electrical potential [23–26], the presence of gases [27,28], and so on. In our previous studies, a nonionic switchable surfactant FPEG was synthesized and the hydrodynamic diameter and aggregation number of its micelles could be reversibly controlled by an electrochemical approach [29]. Li et al. [30] verified that a ferrocene-modified amphiphilic molecule Fc14 could efficiently solubilize VOCs, and that contaminants in the micelles could be easily separated after solubilization. In the present study, we have sought a surfactant for which the micelles could be reversibly and rapidly controlled without the addition of a third substance. For this purpose, a photoswitchable surfactant seemed to be a good choice. In order to obtain an appropriate switchable surfactant that could be conveniently regulated, azobenzene-modified cationic 4-butylazobenzene-4 (oxyethyl)trimethylammonium bromide (AZTMA) was synthesized. This molecule has good photoresponsive characteristics, with trans–cis photoisomerization occurring under irradiation with UV or visible light, and its surface chemical properties change accordingly [31,32]. As reported previously [33], the release of an oily
substance, ethylbenzene, could be reversibly controlled by the change in the molecular structure of AZTMA. Therefore, we envisaged reversible surfactant-enhanced remediation (RSER) based on this switchable surfactant. In the process of soil remediation, the ion-exchange capacity of AZTMA may limit the potential significance of this technology. However, Zhang et al. [34] found that the use of cationic–nonionic mixed surfactant could decrease the adsorption of cationic surfactant, and cationic surfactants can be used for soil remediation when mixed with other surfactants [35]. Furthermore, Matsumura et al. [31] reported that the formation and disintegration of mixed micelles of AZTMA and other surfactants could be regulated by light. Hence, AZTMA appeared to have good prospects for the proposed technique. However, owing to their lower solubility in water, the mechanism of solubilization of PAHs using photoresponsive amphiphilic molecules containing azobenzene groups has rarely been expounded. Understanding of the water solubility enhancement of organic pollutants by switchable surfactants and the release efficiency of organic compounds following contaminant solubilization in surfactant micelles might allow extending of the scope of contaminant remediation. In this study, due to their ubiquity and persistence in the environment, three typical PAHs (pyrene, phenanthrene, and acenaphthene) with various water solubilities have been employed as target pollutants. Experiments have been carried out to expound the mechanism of solubilization of PAHs using AZTMA and to ascertain whether this azobenzene-modified surfactant might be used in SER. The applicability of light-driven release of pollutants contained by AZTMA micelles has also been investigated. The photoisomerization of AZTMA is illustrated in Fig. 1.
2. Materials and methods 2.1. Materials The photosensitive surfactant AZTMA was synthesized and purified according to the procedures similar to those described by Hayashita et al. [36]. The identity of the cationic surfactant with an azobenzene moiety in its hydrophobic group was verified by NMR analysis in CDCl3 solution. The physicochemical properties of the target solubilizates pyrene, phenanthrene, and acenaphthene (98% pure, Aldrich Chemicals) are shown in Table SI-1.
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2.2. Surface tension measurements The surface tensions of trans-AZTMA and cis-AZTMA were measured by the Wilhelmy plate method (Fangrui Co. Ltd., Shanghai, China) at 25 ◦ C. The output from the surface tensiometer was continuously monitored until the surface tension was constant. The whole process was performed in the dark, so as to avoid changes in the structure of AZTMA caused by light irradiation. 2.3. Photoisomerization of AZTMA An XPA-7 photochemical reactor (Yuhua Co., Ltd., Gongyi, China) equipped with a 500 W mercury–xenon lamp was employed as the light source to induce the photoisomerization of AZTMA. UV (365 nm) and visible light (≥420 nm) irradiation were selected by the use of appropriate filters. The trans/cis isomerization of AZTMA in aqueous solution could be monitored spectrophotometrically (Shimadzu UV-2450) in the range 200–650 nm. Measurements were performed using a quartz cuvette with an optical pathlength of 10 mm. The micelle sizes of AZTMA in aqueous solution were measured by the dynamic light-scattering method using a light-scattering apparatus (BI-200SM-B) at 25 ◦ C. A BI-LRM laser (532 nm) was employed as the light source and the measuring angle was set at 90◦ . The distribution of micelle sizes was evaluated by the analysis of a non-negatively constrained least-squares (NNLS) algorithm. 2.4. Solubilization of PAHs Solubilization of the typical PAHs in AZTMA solutions was conducted in 40 mL Corex centrifuge tubes with Teflon-lined screw caps. Briefly, 10 mL aliquots of solutions with a series of surfactant concentrations (0.1–10 mmol L−1 ) were added to centrifuge tubes, and then pyrene, phenanthrene, or acenaphthene was added to each tube in an amount slightly greater than that required to saturate the solution. Duplicate samples were prepared for each concentration of surfactant and dose of PAHs. The samples were subsequently vibrated on a reciprocating shaker for 24 h at 25 ◦ C, which proved to be sufficient to achieve solubilization equilibrium. The solution and solid phase were then separated by centrifugation at 3000 rpm for 30 min. An appropriate aliquot (1 mL) of the supernatant was removed and extracted with hexane (20 mL). Pyrene, phenanthrene, and acenaphthene were then quantified spectrophotometrically at 333, 250, and 227 nm, respectively.
Fig. 2. Effect of UV–vis irradiation on the absorbance of AZTMA.
three different conditions were recorded and are shown in Fig. 2. The characteristic absorption peak for trans-AZTMA was seen at 348 nm; after UV irradiation for 2 h, this peak disappeared and a new absorption peak appeared at 435 nm, which indicated that the trans isomer had been transformed to the cis isomer. The peak at 348 nm assigned to the trans isomer reappeared after irradiation with visible light for 30 min. This verified that AZTMA undergoes reversible isomerization between the trans and cis isomers through irradiation with UV (365 nm) and visible (≥420 nm) lights, respectively. Fig. 3 shows the evolution of the photoisomerization processes versus irradiation time. On irradiation with UV light, the concentration of trans-AZTMA in an initially 10 mmol L−1 solution decreased rapidly to approach zero, which suggested that the isomerization was ongoing, and the process reached a photostationary state in 50 min. Conversely, cis-AZTMA was transformed into the trans isomer under irradiation with visible light, and the equilibration time was measured as 90 min, somewhat longer than that for the UV-induced process. The results showed that the structure of AZTMA could be conveniently controlled, the reversible reactions proceeding rapidly without the need for a third substance. However, the equilibrium concentration of re-trans-AZTMA was lower than original trans-AZTMA. This was mainly because the existence of energy barriers in inversion pathway could hinder the proceeding of photoisomerization [38]. In this state, both trans- and cis-AZTMA and the intermediate of photoisomerization reaction
2.5. The controllable release of PAHs Photochemically controlled release of the PAHs from the micelles of trans-AZTMA was performed in a photochemical reactor using a UV filter (365 nm). During these experiments, the photochemical reactor was kept at 25 ◦ C as in the solubilization experiments. Solutions of trans-AZTMA were irradiated with UV light for 2 h to achieve a photostationary state. The solubilization of PAHs was then performed as described above. The concentrations of PAHs in the surfactant solutions were analyzed spectrophotometrically [37]. The cumulative release efficiencies of the PAHs solubilized in the micelles were calculated from the difference in the solubilized concentrations in the two isomers. 3. Results and discussion 3.1. Verification of trans/cis photoisomerization of AZTMA in solution In order to verify the photoisomerization of AZTMA, the UV–vis absorption spectra of a 0.05 mmol L−1 AZTMA solution under
Fig. 3. Variation of the absorbance of AZTMA upon irradiation with UV light and visible light.
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Fig. 5. Equilibration curves for the solubilization of PAHs in trans-AZTMA solution. Fig. 4. Relationship between the surface tension and the surfactant concentration of trans-AZTMA/cis-AZTMA solution.
3.3. Solubilization kinetics of PAHs by trans-AZTMA Generally, solubilization involves a series of processes: the diffusion of PAH molecules and surfactant micelles in the solution, and the adsorption and desorption of micelles at the PAH/water interface. The micelle adsorption–desorption model proposed by Chan and Cussler [39] in 1976 and the solute diffusion model proposed by Carroll [40] in 1981 have been widely used to describe the dynamic process of solubilization. The two models may be represented by a pseudo-first-order kinetic equation [40–42] and a pseudo-second-order kinetic equation [42], as shown in Eqs. (1) and (3), respectively. Pseudo-first-order kinetic equation
existed in the surfactant solution. Additionally, we continued to find this cycle manipulated promptly with continued treatment of light, and there was still a small proportion of cis-AZTMA that could not convert into its trans-isomer, but this proportion of “stable” cis-AZTMA would not increase as the reversible reaction progress, which revealed that AZTMA possesses well reversible characteristics. According to these, if the surface chemical properties of an AZTMA solution change with variation of the structure, the solubilization and release of organics by micelles might be achieved.
dCt = k1 (Ce − Ct ) dt 3.2. Surface chemical properties of AZTMA
(1)
The linear expression derived from this equation is Ct = Ce (1 − e−k1 t )
The surface activity and photochemically reversible characteristics of the two isomers of AZTMA were compared. The relationship between surface tension and concentration for aqueous solutions of AZTMA at 25 ◦ C is shown in Fig. 4. Within a certain range, the surface tension decreased with increasing concentration of AZTMA, but the trend of the curve changed abruptly at a certain concentration for each isomer. As determined from the turning points on the surface tension curves, the critical micelle concentration (CMC) for the cis isomer was 5.1 mmol L−1 , whereas that for the trans isomer was 2.0 mmol L−1 . According to the structure of AZTMA, an increase in CMC was favored by the bulkier structure and the shorter apparent hydrophobic tail length of cis-AZTMA. The considerable disparity in the CMC values between transAZTMA and cis-AZTMA implies that the surface chemical properties of AZTMA can be easily manipulated by changes in the incident light. Hence, if the concentration of AZTMA was appropriately selected to be somewhere between the CMCs of the two isomers, UV light irradiation would disrupt the micelles in solution into surfactant monomers. If contaminants such as PAHs had been solubilized in micelles of trans-AZTMA, irradiation would also bring about their release.
(2)
Pseudo-second-order kinetic equation dCt = k2 (Ce − Ct )2 dt
(3)
The linear expression derived from this equation is 1 t 1 = + Ct Ce k2 Ce2
(4)
where Ce and Ct are the concentrations of the PAH at equilibrium and at time t (h), respectively; k1 and k2 are the reaction rate constants of pseudo-first-order kinetics and pseudo-second-order kinetics, respectively; and t is the time (h). The solubilization kinetics of the three typical PAHs fitted with the different kinetic equations are shown in Fig. 5, and the fitted parameters are shown in Table 1. The equilibration time of the PAHs in the surfactant solution was about 12 h. The correlation coefficients of the three kinetic equations follow the order: pseudo-second-order equation > pseudo-first-order equation. During the solubilization process, the diffusion of PAH molecules can be ignored because of their low solubility in water.
Table 1 Fitted parameters of different kinetic models. PAHs
Pyrene Pherantherene Acenaphthene
Pseudo first-order equation
Pseudo second-order equation
Ce
k1
R2
Ce
k2
R2
11.63 55.32 65.41
0.11 0.21 0.25
0.878 0.9764 0.9818
15.62 67.2 79.97
6.2 × 10−3 3.35 × 10−3 3.28 × 10−3
0.9331 0.9863 0.9818
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Fig. 6. The solubilization of PAHs by trans-AZTMA solution.
A surfactant solution with high concentration contains more surfactant micelles, and hence the likelihood of micelles coming into contact with the PAH is increased. Since AZTMA micelles have a strong capacity for encapsulation, adsorption–desorption at the PAH/water interface is the dominant factor, rather than the diffusion of PAH molecules into the micelles via the aqueous phase. As is evident from the above results, the solubilization mechanism of AZTMA is similar to that of conventional surfactants [40]. This suggests that the novel switchable surfactant AZTMA may indeed be applied for SER. 3.4. Solubilization of PAHs controlled by light irradiation The apparent enhancement in water solubility of a PAH in a surfactant solution can be expressed as follows [43]: f =
∗ Sw = 1 + Xmn Kmn + Xmc Kmc Sw
(5)
∗ is the apparent solute solubility at a surfactant concenwhere Sw tration of X; Sw is the solubility of the PAH in water at 25 ◦ C; Xmn is the concentration of the surfactant as a monomer in water; Xmc is the concentration of the surfactant as a micelle in water; Kmn is the partition coefficient of the solute between the surfactant monomer and water; and Kmc is the solute partition coefficient between the aqueous micellar phase and water. The changes in the solubility of the PAHs with the addition of trans-AZTMA solution are shown in Fig. 6. As is clear from Fig. 6, the solubilities of the PAHs in water were significantly enhanced by trans-AZTMA and increased linearly over the range of surfactant concentrations above the CMC. The apparent solubilities of pyrene, phenanthrene, and acenaphthene in 10 mmol L−1 AZTMA solution were about 69, 24, and 14 times higher than those in water, respectively. The water solubility enhancements of these three typical PAHs by cis-AZTMA were evaluated and compared with those in transAZTMA. As shown in Fig. 7, the water solubilities of the PAHs in cis-AZTMA solutions were lower than those in trans-AZTMA solu∗ determined tions at comparable concentrations. The values of Sw for pyrene, phenanthrene, and acenaphthene in 10 mmol L−1 cisAZTMA solution were 4.6, 29.6, and 33.8 mg L−1 , respectively. These results indicated that the uptake by and release from the micelles of these PAHs could be controlled through light irradiation, making use of the difference in CMC between the trans and cis isomers. After UV irradiation, the apparent hydrophobic tail length was shorter than before, and the micelles were disintegrated into surfactant monomers.
Fig. 7. The solubilization of PAHs by AZTMA solution: (a) pyrene, (b) phenanthrene, and (c) acenaphthene.
In order to determine the effectiveness of a given surfactant in solubilizing PAHs, the molar solubilization ratio (MSR) can be evaluated according to the following equation [12]: MSR =
S − SCMC Cs − CMC
(6)
where S is the apparent solubility of a PAH at a particular surfactant concentration, Cs , which is greater than the CMC; and SCMC is the apparent solubility of the solute at the CMC. The partitioning of a solute between aqueous solution and surfactant micelles can be characterized by a mole fraction micellephase/aqueous-phase partition coefficient, Kmc = Xm /Xa , where Xm is the mole fraction of the solute in the micellar phase and Xa is the mole fraction of the solute in the aqueous phase. Here, Kmc depends on the surface properties of AZTMA and the temperature. In terms of MSR, Kmc can be calculated from experimental measurements using the following formula: Kmc =
55.4MSR SCMC (1 + MSR)
(7)
The MSR and log Kmc values of the studied PAHs in cis- and trans-AZTMA solutions are shown in Table 2, and the solubilization
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Table 2 MSR and Kmc of PAHs in AZTMA solutions. Pyrene
trans AZTMA cis AZTMA CTAB
Pherantherene
Acenaphthene
MSR
log Kmc
MSR
log Kmc
MSR
log Kmc
0.66 × 10−2 0.048 × 10−2 1.6 × 10−2
2.23 1.83 2.65
3.92 × 10−2 2.59 × 10−2 2.7 × 10−2
2.02 1.50 2.09
5.02 × 10−2 3.17 × 10−2 4.4 × 10−2
1.76 1.25 1.81
potentials of the selected PAHs in the typical cationic surfactant hexadecyltrimethylammonium bromide (CTAB) are also summarized. According to the MSR and log Kmc values for AZTMA and CTAB, the solubilization capacities of the azobenzene surfactant and CTAB were almost the same. Moreover, AZTMA showed higher solubilization capacities for phenanthrene and acenaphthene, which confirmed that it could meet the requirements of the SER process. The results also demonstrated that the solubilizing power of trans-AZTMA is much higher than that of cis-AZTMA. This suggested that AZTMA micelles have the potential to release the solubilized organics. The degrees of water solubility enhancement of the PAHs by AZTMA follow the order pyrene > phenanthrene > acenaphthene, mirroring the octanol/water distribution coefficients (Kow ) of the PAHs. As shown in Fig. 8, excellent linearity was obtained. 3.5. Photochemical control of PAH release
with visible light, the micelles reformed and entrapped the substances (Fig. 10c). Finally, the entrapped PAH molecules could be released by irradiation of the AZTMA solution with UV light once more (Fig. 10d). By cycling the photoisomerization of AZTMA, the surfactant could be reused for the loading and release process, and this reversible process could be broadly applicable for various substances. The cumulative release efficiency of a PAH solubilized in the micelles by photoisomerizing trans-AZTMA to cis-AZTMA was evaluated according to the following equation: R=
∗ − S∗ Sw1 w2 ∗ Sw1
× 100%
(8)
∗ and S ∗ are the apparent water solubilities of the PAHs where Sw1 w2 in trans-AZTMA and cis-AZTMA, respectively (mg L−1 ). As shown in Fig. 11, the values of R increased with increasing surfactant concentration in the range 2–4 mmol L−1 . On further
Particle size distribution is a key criterion in judging the formation of micelles in surfactant solutions [31]. Fig. 9 shows the effects of UV–vis irradiation on the size distribution of molecular assemblies in 3.0 mmol L−1 AZTMA solutions. The mean micellar size in trans-AZTMA solution was found to be 2.4 nm. After UV irradiation, it changed from 2.4 to 1.7 nm. Thereafter, the mean micellar size reverted to 2.3 nm upon irradiation with visible light. As mentioned above, the structure of the azobenzene group is suggested to strongly affect the surface properties of AZTMA solution. Photochemical control of the solubilization and release of contaminants by AZTMA micelles may be realized if the changes in the surface properties caused by the photoisomerization of the azobenzene group of the surfactant can be exploited. The loading and release of the micelles, using PAHs as a typical model substrate, are demonstrated in Fig. 10. In the original trans-AZTMA solution, the micelles were in a shrunken state and the PAHs were stably solubilized within them (Fig. 10a). After UV irradiation, because of the bulkier structure and shorter apparent hydrophobic tail length, the micelles in cis-AZTMA disassembled and the micellar size was decreased, which permitted the outward diffusion of the PAH molecules (Fig. 10b). Thereafter, upon irradiation
Fig. 8. Correlation between log Kow and log Kmc for PAHs in AZTMA solutions.
Fig. 9. Particle size distributions of 3 mmol L−1 AZTMA micellar solutions: (a) trans isomer, (b) cis isomer, and (c) restored trans isomer.
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Fig. 10. Redox-mediated loading and release of PAHs.
increasing the surfactant concentration beyond 4 mmol L−1 the release efficiency began to decrease. About 68% of the acenaphthene could be released after UV irradiation for 2 h at a concentration of 4 mmol L−1 . Fig. 10 also shows that the cumulative release efficiencies of selected PAHs followed the order pyrene > phenanthrene > acenaphthene, which was consistent with the results of water solubility enhancements of PAHs by AZTMA. Note here that the consistency was most probably due to the intrinsic solubility and partitioning interaction of the selected PAHs. A lower intrinsic solubility and weaker partitioning interaction would lead to greater release efficiency. In the case of pyrene, about 13% of the solubilizate was not released within 2 h, even at a concentration of 4 mmol L−1 . It is believed that the unreleased substance can stably interact with a thick shell of AZTMA through hydrogen bonding and other interactions.
4. Conclusions In summary, this research reveals that the photosensitive surfactant AZTMA can reversibly undergo trans/cis photoisomerization upon irradiation with UV and visible lights. Because of its good surface activity, AZTMA has a strong solubilization capacity for PAHs to meet the requirements of the SER process. PAHs solubilized with AZTMA can be released from the micelles through UV irradiation, provided that the initial surfactant concentration is between the two CMCs for changing its surface properties. Thus, the azobenzene surfactant AZTMA may be reversibly used in surfactant-enhanced remediation technology through photochemical control. That is to say, aiming to reduce the cost and prevent repeated pollution, the switchable azobenzene-modified surfactant provides a convenient means of realizing reversible surfactant-enhanced remediation. Acknowledgments The authors would like to thank the National Natural Science Foundation of China (Nos. 21077048 and 20607008) and the Specialized Research Fund for the Doctoral Program of Higher Education (No. 20115314110011) for financial support. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.colsurfa. 2014.04.033. References
Fig. 11. Cumulative release of PAHs as a function of surfactant concentration.
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