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nonionic mixed surfactant
Environmental Pollution 147 (2007) 350e357 www.elsevier.com/locate/envpol
Enhanced desorption of phenanthrene from contaminated soil using anionic/nonionic mixed surfactant Wenjun Zhou, Lizhong Zhu* Department of Environmental Science, Zhejiang University, Hangzhou, Zhejiang 310028, PR China Received 31 March 2006; accepted 24 May 2006
Anionic/nonionic mixed surfactants appear to be a better choice than single surfactants for the application of surfactant remediation technology. Abstract A new approach using an anionic/nonionic mixed surfactant, sodium dodecyl sulphate (SDS) with Triton X-100 (TX100), was utilized for the desorption of phenanthrene from an artificial contaminated natural soil in an aim to improve the efficiency of surfactant remediation technology. The experimental results showed that the presence of SDS not only reduced the sorption of TX100 onto the natural soil, but also enhanced the solubilization of TX100 for phenanthrene, both of which resulted in the distribution of phenanthrene in soilewater systems decreasing with increasing mole fraction of SDS in surfactant solutions. These results can be attributed to the formation of mixed micelles in surfactant solution and the corresponding decrease in the critical micelle concentration of TX100 in mixed solution. The batch desorption experiments showed that the desorption percentage of phenanthrene from the contaminated soil with mixed solution was greater than that with single TX100 solution and appeared to be positively related to the mole fraction of SDS in surfactant solution. Thus, the anionic/nonionic mixed surfactants are more effective for the desorption of phenanthrene from the contaminated soil than a single nonionic surfactant. Ó 2006 Elsevier Ltd. All rights reserved. Keywords: Surfactant remediation; Phenanthrene; Contaminated soil; Mixed surfactant; Desorption percentage
1. Introduction The contamination of soils and groundwater by toxic and/or hazardous hydrophobic organic compounds (HOCs) is a widespread environmental problem. Various physical, chemical, biological technologies and their combinations have been attempted to remediate HOC contaminated soils and groundwater. Surfactant remediation technology has been suggested as the promising technology for the remediation of contaminated soils and groundwater (West and Harwell, 1992; Harwell et al., 1999). Surfactants enhance the remediation of HOCs contaminated soils by increasing the HOC’s aqueous-phase concentration via micelle solubilization (Kile and Chiou,
* Corresponding author: Tel./fax: þ86 571 88273733. E-mail address: [email protected] (L. Zhu). 0269-7491/$ - see front matter Ó 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.envpol.2006.05.025
1989; Edwards et al., 1991; Diallo et al., 1994; Jafvert et al., 1994; Pennell et al., 1997) and the mobilization of HOCs from solid into aqueous phases (Deitsch and Smith, 1995; Yeom et al., 1996; Johnson et al., 1999), thus improving the bioavailability of HOCs for microbial remediation and phytoremediation (Bury and Miller, 1993; Tsomides et al., 1995; Guha and Jaffe, 1996). Anionic and nonionic surfactants have usually been chosen in surfactant remediation technology. The solubility enhancement properties of HOCs in single anionic and nonionic surfactant solution are well defined (Kile and Chiou, 1989; Edwards et al., 1991; Diallo et al., 1994; Jafvert et al., 1994; Pennell et al., 1997). Compared with anionic surfactant, nonionic surfactants have greater solubilization enhancement for HOCs and show higher remediation efficiency for contaminated soils. However, recent soil experiments have suggested that precipitation for anionic surfactants (Jafvert and Heath,
W. Zhou, L. Zhu / Environmental Pollution 147 (2007) 350e357
1991) and the sorption of nonionic surfactant into soil (Liu et al., 1992; Edwards et al., 1994; Sun and Inskeep, 1995; Ko et al., 1998) may occur in soilewater systems. In particular, nonionic surfactants can be sorbed onto the solid matrix and thereby lead to HOCs partitioning into immobile sorbed surfactants and, thus, enhanced HOC retardation (Edwards et al., 1994; Sun and Inskeep, 1995; Ko et al., 1998). Edwards et al. (1994) found that the sorbed Triton X-100 acts to enhance phenanthrene sorption and, on a carbon-normalized basis, the sorbed surfactant is much more effective as a sorbent for phenanthrene than is humic matter. Sun and Inskeep (1995) observed that the sorbed surfactant enhanced HOC’s partitioning relative to the untreated soils and the increase or decrease in HOC’s distribution coefficients depend on the net effects of both soil-sorbed and aqueous phase surfactant fractions. Thus, the sorption of nonionic surfactant onto soil would decrease the remediation efficiency and result in an increase in remediation times and costs. The soils and groundwater may be contaminated by surfactants via sorption. Meanwhile, the environment factors of temperature, salinity and pH have obvious effects on the solubilization of individual anionic or nonionic surfactant solution for HOCs. Thus, an improved strategy for surfactant remediation technology for contaminated soils is to reduce the sorption of surfactant onto soils and to enhance the desorption of HOCs from contaminated soils in order to obtain an optimal remediation efficiency with the minimum surfactant dose. Currently, mixed surfactants are of great interest in scientific and industrial applications. Surfactants used in practical applications almost always consist of mixtures of surfactants. Commercial products are inexpensive, easily available and exhibit superior performance compared with the individual detergents. Mixed surfactants could be employed over a wider range of temperature, salinity, and hardness conditions than the individual surfactant (Holland and Rubingh, 1992). In particular, some researchers (Thibaut et al., 2000; Penfold et al., 2002) found that the sorption of nonionic surfactant at the hydrophilic silicon surface was strongly restricted with the presence of anionic surfactant as the formation of mixed micelle and the sorption amount of nonionic surfactant decreased with the increasing mole fraction of anionic surfactant in mixed solution. The synergistic solubilization of anionic/nonionic mixed surfactant for HOCs was observed in some studies (Zhu and Chiou, 2001; Zhu and Feng, 2003). Thus, in the practical applications of surfactant remediation technology for contaminated soils, the anionic-nonionic mixed surfactant may have a better performance than that of single surfactant. However, little information is known about the sorption of anionic-nonionic mixed surfactants onto natural soils and their performance in the remediation of contaminated soils by HOCs. Therefore, a study of the desorption of HOCs from contaminated soils by anionic-nonionic mixed surfactant is necessary for evaluating the performance of anionic/nonionic mixed surfactants in soil remediation with the aim of improving the efficiency of surfactant remediation technology. The objectives of this study are (1) to investigate the effect of the presence of anionic surfactant on the sorption
351
of nonionic surfactant onto a natural soil; (2) to evaluate the efficiency of anionic-nonionic mixed surfactant for the desorption of HOC contaminated soil. The experimental results can be used to understand the performance of anionic/nonionic mixed surfactants in the remediation of contaminated soil and to provide valuable information in designing surfactant remediation technology for contaminated soils. 2. Materials and methods 2.1. Materials Phenanthrene was selected as a representative polycyclic aromatic hydrocarbon (PAH) to model the hydrophobic organic contaminants; it was obtained from Acros Organics, with a purity of 99%. Sodium dodecyl sulphate (SDS) was obtained from Acros Organics, with purity 98%. Triton X-100 (TX100), a nonionic surfactant, was obtained from Sigma Chemical Company and used without further treatment. Selected physicochemical properties of the compounds are presented in Table 1. Mixed surfactant solutions were prepared by dissolving SDS and TX100 in deionized water with different mole ratios and the composition of mixed surfactants is expressed with the mole ratios of SDS to TX100. An uncontaminated natural soil was collected from Hangzhou City, China. The soil was air-dried and sieved to obtain particles less than 1 mm in all experiments. The soil contained 3.9% sand, 71.5% silt and 24.6% clay. The organic carbon content was 0.52%. The contaminated soil was prepared by dissolving an appropriate quantity of phenanthrene in petroleum ether and a known weight of soil was added slowly, with continuous mixing. This slurry was mixed thoroughly and the solvent was allowed to evaporate slowly. The dry contaminated soil was transferred into a bottle and tumbled for about a week before the experiments. The resulting contaminated soil had a final concentration of 308 mg/kg of phenanthrene, which was used directly in the desorption experiments.
2.2. Enhanced solubilization by surfactant solutions The solubility enhancement of phenanthrene by SDSeTX100 surfactant mixture was conducted in 25 ml Corex centrifuge tubes with Teflon-lined screw caps, in which a volume of 20 ml of solutions with a series of surfactants concentrations was placed and phenanthrene was subsequently added to each tube in an amount slightly more than required to saturate the solution. The tubes were equilibrated on a reciprocating shaker for 24 h at 20 1 C and subsequently centrifuged at 5000 rpm (7295 g) for 30 min to separate completely the undissolved solute. An appropriate aliquot of the supernatant was removed and analysed for phenanthrene by HPLC.
2.3. Sorption of TX100 onto soil from surfactant solutions Batch experiments were conducted in duplicate to determine surfactant equilibrium sorption isotherms using centrifuge tubes. Two grams of soil sample were weighed into each centrifuge tube, to which 20 ml of surfactant solution was added. All aqueous solutions for soil tests contained 0.01 M NaCl to keep a constant ionic strength and 0.01% w/w NaN3 to inhibit microbial growth. The initial surfactant concentration spanned over a large range of values below and above the nominal CMC of surfactants. These samples were then equilibrated on a reciprocating shaker for 24 h at 20 1 C and subsequently centrifuged at 5000 rpm for 30 min to completely separate the solution and solid phase. An appropriate aliquot of the supernatant was removed and analysed for TX100 by HPLC. The sorption amount of TX100 on soil was computed simply from the difference of the initial and final surfactant concentrations.
2.4. Distribution of phenanthrene in soilewateresurfactant systems The equilibrium distribution coefficient of PAHs in the soilewater system with or without surfactant was determined by a batch equilibrium experiment.
W. Zhou, L. Zhu / Environmental Pollution 147 (2007) 350e357
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Table 1 Selected physicochemical properties of compounds in this studya Compounds
Molecular formula
MW
logKow Sw CMC (mM) (mM)
Phenanthrene C14H10 178.23 4.57 Triton X-100 C8H17C6H4O(OCH2CH2)9.5H 628 288.38 SDS C12H25SO4Na a
6.62 0.29 5.4
Phenanthrene data from Yaws (1999).
Varied quantities of phenanthrene were added to the centrifuge tubes, which contained 2.0 g of clean soil sample and 20 ml of a given concentration of surfactant solution. The centrifuge tubes were shaken on a reciprocating shaker for 24 h at 20 1 C and subsequently centrifuged at 5000 rpm for 30 min to completely separate the solution and solid phase. An appropriate aliquot of the supernatant was removed and analysed for phenanthrene by HPLC. The sorbed amounts of phenanthrene were computed simply from the difference of the initial and final phenanthrene concentrations. The equilibrium distribution coefficients for phenanthrene in all soilewater systems were obtained via linear regression using the various sorption amounts and the equilibrium concentrations in aqueous phase.
2.5. Batch desorption experiments The percentage and extent of phenanthrene desorption from contaminated soil with different surfactant solutions were measured in batch experiments. A weight of 2.0 g of contaminated soil sample was weighed into each centrifuge tube, to which 20 ml of surfactant solution with different concentration and composition was added. The centrifuge tubes were then shaken on a reciprocating shaker for 24 h at 20 1 C. The solution and solid phase were separated by centrifugation at 5000 rpm for 30 min. An appropriate aliquot of the supernatant was removed and analysed for phenanthrene by HPLC.
2.6. Analytical methods Aqueous phenanthrene and TX100 concentrations were quantified by an Agilent HPLC fitted with UV detector and an Agilent Eclipse XDB-C8 column (4.5 150 mm, 5 mm) using methanolewater (75:25) as the mobile phase at a flow rate of 1 ml/min. Chromatography was performed at 30 C. The UV wavelengths were set at 250 and 224 nm for phenanthrene and TX100, respectively.
3. Results and discussion
dramatically as the surface micelle (admicelle) or bilayers form on the adsorbent through association or hydrophobic interactions between the hydrocarbon chains of the surfactants, and a plateau is reached corresponding to a maximum sorption amount with the surfactant concentration around the CMC. Similar sorption isotherms were obtained for TX100 from mixed surfactant solutions, in which the mole ratio of SDS to TX100 is 1:4, 1:2, 1:1 and 2:1, respectively. However, the presence of anionic surfactant, SDS, strongly affected the sorption amount of pure TX100. The addition of 20 mol% SDS to TX100 resulted in a 30% decrease in the maximum sorption amount for TX100, from 11.6 mmol/kg to 8.25 mmol/kg. When the mole ratio of SDS to TX100 in mixed surfactant solution was 2:1, the maximum sorption amount of TX100 onto soil was only about one-fourth of the value for TX100 from the single surfactant solution. Thus, the higher the mole fraction of SDS in mixed surfactant solution, the greater the decrease in the maximum sorption amount for TX100. These results show that when SDS was mixed together with TX100, the sorption of nonionic surfactant onto soil was severely restricted and a higher mole fraction of SDS in surfactant solution means that a lower plateau sorption can be reached with a smaller TX100 concentration in surfactant solution. As mentioned earlier, the sorption of nonionic surfactant proceeds through the sorption of surfactant monomer, the micelles are not directly sorbed. In mixed surfactant solution, the formation of mixed micelles would affect the CMC and the monomer concentration of component surfactant in mixed solution and then their sorption into the soil. The regular solution theory (Holland and Rubingh, 1983) has been proven to be remarkably successful in expressing the characteristics of mixed surfactant solutions. With the regular solution theory, the CMC values of a binary surfactant solution, CMC*, can be calculated by the following equation: 1 X1 X2 ¼ þ CMC f1 CMC1 f2 CMC2
ð1Þ
3.1. Effect of SDS on the sorption of TX100 onto soil 14 TX100 S:T = 1:4 S:T = 1:2 S:T = 1:1 S:T = 2:1
12
Sorbed TX100 (mmol/kg)
The sorption isotherms of TX100 onto the natural soil from different surfactant systems are shown in Fig. 1, which illustrates the effect of SDS on the sorption of TX100 onto soil. The sorption isotherm of TX100 from the single surfactant solution was nonlinear with typical S-shape curves, reaching a plateau in sorption amount at surfactant equilibrium concentration around the CMC, which is in accord with the results of other experimental studies on the sorption of nonionic surfactant (Liu et al., 1992; Edwards et al., 1994; Sun and Inskeep, 1995; Paria and Khilar, 2004). A conceptual model based on numerous experimental observations can account for the sorption of nonionic surfactant onto a hydrophilic surface. At low surfactant concentration, the nonionic surfactants are sorbed as monomers and lie parallel to the solid surface through surface interactions with both types of surfactant moieties. With the increase in the surfactant concentration, the sorption increases
10 8 6 4 2 0
0
1
2
3
4
Equilibrium Concentration of TX100 (mmol/L) Fig. 1. The sorption isotherms of TX100 onto soil from different surfactant systems with various mole ratios of SDS (S) to TX100 (T).
W. Zhou, L. Zhu / Environmental Pollution 147 (2007) 350e357
2 f1 ¼ exp b 1 X1m
ð2Þ
2 f2 ¼ exp b 1 X2m
ð3Þ
At mixed surfactant concentration above the CMC*, the micellar mole fraction of component 1 for every solution composition of mixed surfactant can be obtained by the following equation (Zhou and Zhu, 2005): ln
ð1 X1 ÞCMC1 X1m ¼ b 2X1m 1 m X1 CMC2 1 X1
ð4Þ
Thus, the critical micelle concentration of component 1 (CMCm 1 ) in the mixed solution is given by CMC1 ¼ X1 CMC
ð5Þ
For the SDSeTX100 mixed surfactant system, a value of 3.0 was used for the interaction parameter b, representing an average of values quoted for SDSealkylethoxylate mixed surfactant systems (Rosen, 2004). The critical micelle concentration of TX100 in every mixed surfactant systems was calculated using Eqs. (1)e(5) and is shown in Table 2. The observed decrease in the sorption amount of TX100 should result from the reduction of the TX100 critical micelle concentration as a consequence of mixed micelle formation. This explanation is confirmed by Table 2, which compares the maximum sorption amount with the critical micelle concentration of TX100 in mixed solution as a function of the mole fraction of SDS. Table 2 shows a parallel decrease in the critical micelle concentration and the maximum sorption amount of TX100 when the mixed surfactant solution contained relatively more SDS, which indicated that the formation of mixed micelle decreased the critical micelle concentration of TX100 in mixed surfactant solution and then restricted the sorption of TX100 onto the natural soil. In addition, Table 2 Parameters for the desorption of phenanthrene from the contaminated soil by different surfactant systems with various mole ratios of SDS to TX100 SDSe TX100
CMCT (mmol/L)a
Csorb (mmol/kg)b
logKmc (L/mol)
CDC (mM)c
0 1:4 1:2 1:1 2:1
0.29 0.25 0.22 0.19 0.16
11.6 8.25 6.41 4.90 3.33
4.19 4.22 4.24 4.28 4.36
1.6 1.2 1.0 0.8 0.6
a
The critical micelle concentration of TX100 in mixed solution. The maximum sorption amount of TX100 onto soil from different surfactant solutions. c The critical desorption concentration of TX100 for phenanthrene. b
Thibaut et al. (2000) observed that the addition of an anionic/nonionic surfactant mixture to silica precoated with the nonionic surfactant resulted in the release of the preadsorbed nonionic surfactant and the free energy of mixed micellization was more negative than that of nonionic surfactant sorption, which indicated that the mixed micellization in solution is more favourable than the sorption of nonionic surfactant onto soil. 3.2. Solubilization of phenanthrene in mixed surfactant solutions The solubilization of phenanthrene by SDSeTX100 mixed surfactant with different composition was determined. The apparent solubilities of phenanthrene in mixed surfactants are shown in Fig. 2 as a function of TX100 concentrations and compared with that in single TX100 solution. Obviously, the apparent solubilities of phenanthrene increased linearly over the range of TX100 concentrations in solution. The same as single surfactant, anionic-nonionic mixed surfactants also have the potential capacity to enhance the solubilization of phenanthrene in water. The behaviour is generally attributed to the incorporation or partitioning of organic solutes within mixed surfactant micelles. It was also observed that the apparent solubilities of phenanthrene in mixed solutions were higher than those in single TX100 solution at comparable TX100 concentration and increased with the increasing mole fraction of SDS in mixed solution. The apparent solubilities Sw of HOCs in surfactant solutions can be expressed as follows (Jafvert et al., 1994; Almgren et al., 1979): Sw ¼ Sw þ Cmic Kmc Sw
ð6Þ
where Sw is the solubility of phenanthrene in pure water; Cmic is the concentration of surfactant in micellar form; Kmc is the micelleewater partition coefficient. 0.6
Sw* of Phenanthrene (mmol/L)
where subscripts 1 and 2 indicate the components 1 and 2, respectively. CMC is the critical micelle concentration of individual pure component. X is the mole fraction of each component in mixed surfactant solutions. f is the activity coefficient of each component and can be expressed in terms of the micelle composition Xm and a single interaction parameter b.
353
0.5
TX100 S:T = 1:4 S:T = 1:2 S:T = 1:1
0.4
S:T = 2:1
0.3
0.2
0.1
0.0
1
2
3
4
TX100 Concentartion (mmol/L) Fig. 2. The apparent solubilities Sw of phenanthrene by different surfactant systems with various mole ratios of SDS (S) to TX100 (T).
W. Zhou, L. Zhu / Environmental Pollution 147 (2007) 350e357
The micelleewater partition coefficient Kmc is a parameter that indicates the partitioning of organic solutes between the surfactant micelles and the water phase (Jafvert et al., 1994; Almgren et al., 1979): Smic Kmc ¼ Sw Cmic
ð7Þ
where Smic is the concentration of solute in the micelle phase (M). The Kmc values for phenanthrene in all surfactant systems studied were calculated according to the corresponding solubilization data and are shown in Table 2. The Kmc values for phenanthrene in mixed solutions were higher than those in single TX100 solution and increased with the increasing mole fraction of SDS in mixed surfactant. In this study, the concentrations of SDS in mixed surfactant are less than the CMC values of SDS and the solubilization enhancement of SDS in mixed solution for phenanthrene is negligible. However, when SDS was mixed together with TX100, the critical micelle concentration of TX100 decreased and then the micelle concentration increased. At the same time, the formation of mixed micelle enhances the partition of phenanthrene into surfactant micelle. Both the increased Kmc values and the micelle concentration resulted in increased apparent solubilities of phenanthrene in mixed surfactant solutions. 3.3. Distribution of phenanthrene in the soilewater system with mixed surfactants The distribution of HOCs in a surfactant-free soilewater system is governed by a mechanism where the HOC molecules partition into the soil organic matter (SOM) (Chiou et al., 1979; Karickhoff et al., 1979) and can be evaluated with the soilewater distribution coefficient Kd. However, with the addition of nonionic surfactants, the sorption of nonionic surfactant onto soils and the partition of HOCs into the soil-sorbed surfactant would affect the distribution of solute in soilewater systems. The apparent soilewater distribution coefficients Kd for phenanthrene were obtained from the slope of the sorption isotherms (not shown) of phenanthrene in all soilewateresurfactant systems in this study. The changes of Kd for phenanthrene in soilewater systems containing single TX100 or SDSeTX100 mixed solution are shown in Fig. 3 as a function of TX100 concentration in solution. In all cases, phenanthrene showed a similar distribution behaviour pattern with increasing TX100 concentration in solution. That is, relative to the corresponding intrinsic Kd in a surfactant-free system, Kd increased gradually to reach a maximum value and then followed by a gradual decrease. The increase in Kd is apparently caused by the strong sorption of TX100 by the solid phase and the functionality of the sorbed TX100 as a sorptive phase for the solutes. The corresponding equilibrium concentrations of TX100 with the maximum Kd in all systems were in accord with that corresponding to the onset of the sorption plateau of TX100 onto soils.
TX100 S:T = 1:4
120
Kd* for Phenanthrene (L/kg)
354
S:T = 1:2 S:T = 1:1 S:T = 2:1
100 80 60 40 20 0
0
1
2
3
4
TX100 Concentration (mmol/L) Fig. 3. The apparent distribution coefficients Kd of phenanthrene in the soilewateresurfactant system as a function of the TX100 concentration in different surfactant systems with various mole ratios of SDS (S) to TX100 (T).
Although having similar change trends, the Kd values for phenanthrene with mixed solution appeared to be inversely related to the mole fraction of SDS in surfactant solutions. For example, the maximum Kd values for phenanthrene with SDSeTX100 mixed surfactant, in which the mole ratios of SDS to TX100 were 1:4, 1:2, 1:1 and 2:1, were about 82%, 68%, 60% and 48% of that with the single TX100 solution, respectively. With the same TX100 concentration of 4 mmol/L in surfactant solution, the corresponding Kd values for phenanthrene with SDSeTX100 mixed solution were about 72%, 55%, 42% and 27% respectively, of that with the single TX100 solution and decreased with an increasing mole fraction of SDS in mixed surfactant solution. Thus, the presence of SDS in surfactant solution reduced the distribution of phenanthrene into soil and the higher the amount of SDS in surfactant solution, the greater the phenanthrene is in aqueous phase. In the presence of surfactant, the distribution of solute in each phase (i.e., water, surfactant micelle, sorbed surfactant, soil solid) shows complex behaviour depending on the distribution of surfactants in solideaqueous systems. Thus, the Kd in the soilewateresurfactant system can be expressed as (Edwards et al., 1994; Sun and Inskeep, 1995): Kd ¼
Kd þ Csorb Ksf 1 þ Cmic Kmc
ð8Þ
where Csorb is the sorption amount of surfactant onto soil, Ksf is the solute distribution coefficient with the soil-sorbed surfactant. Eq. (8) can be used to describe accurately the functional dependence of Kd on the TX100 equilibrium concentration for all cases. At low TX100 equilibrium concentration, Kd values increased with increasing TX100 equilibrium concentration because the amount of soil-sorbed TX100 increases rapidly (Fig. 1) prior to the formation of TX100 micelles in solution and the sorbed TX100 is very effective for HOC partitioning. When the TX100 equilibrium concentration in aqueous phase reaches its CMC, the sorption
W. Zhou, L. Zhu / Environmental Pollution 147 (2007) 350e357
amount of TX100 reaches a plateau, and TX100 micelles in the aqueous phase become an important partition phase with increasing equilibrium concentration. As a result, micelles begin competing for HOC molecules, thereby causing a decrease in Kd . According to Eq. (8), the important features affecting the distribution of HOCs in soilewateresurfactant systems include the sorption of surfactant onto soil with the corresponding partition of HOCs into the soil-sorbed surfactant (CsorbKsf) and the enhanced solubilization of HOCs by aqueous surfactant solution (CmicKmc). The presence of SDS not only reduced the sorption of TX100 onto the natural soil (Fig. 1), but also enhanced the solubilization of TX100 for phenanthrene (Fig. 2), both of which resulted in the Kd for phenanthrene decreasing with an increase in the mole fraction of SDS in surfactant solutions. The smaller Kd means that the soil has weak sorption for phenanthrene and more phenanthrene would be in the aqueous phase. 3.4. Phenanthrene desorption studies Desorption studies were used to evaluate the efficiency of surfactant solution to desorb phenanthrene from contaminated soil. The aqueous phase phenanthrene concentrations, which were desorbed from soil, with the addition of mixed surfactant are shown in Fig. 4 as a function of TX100 concentrations in solution and compared with those of single TX100 solution. Fig. 5 shows the plot of the desorption percentage of phenanthrene from the contaminated natural soil by different surfactant systems with the TX100 concentration in surfactant solutions. From Figs. 4 and 5, it can be observed that both the aqueous phase phenanthrene concentrations and the phenanthrene desorption percentage shared a similar trend and had a sharp increase with the increasing TX100 concentration in solution. The corresponding TX100 concentrations in mixed solution
355
with sharp increases can be defined as the critical desorption concentration (CDC) and phenanthrene desorption from contaminated soil would be favourable when the TX100 concentration is greater than the CDC. The CDC values of TX100 for phenanthrene in different surfactant systems were estimated from Figs. 4 and 5 and are listed in Table 2. The CDC values of TX100 for phenanthrene with mixed surfactants were less than that with single TX100 solution and decreased with the increasing mole fraction of SDS in mixed solution. The sharp increase and the CDC can be rationally explained as the sorption of TX100 onto soil from surfactant solution and were attributed to the formation of surfactant micelles in aqueous phase. Before the sorption plateau of TX100 onto soil, TX100 existed in aqueous phase as a monomer and there were no surfactant micelles in aqueous phase. Thus, the aqueous phenanthrene concentrations desorbed from soil contributed to the phenanthrene water solubility. With the sorption of TX100 in plateau, the surfactant micelles began to form in aqueous phase, which enhanced the solubilities of phenanthrene in aqueous phase and thus increased the desorption percentage. For SDSeTX100 mixed surfactant systems, the sorption of nonionic surfactant onto soil was severely restricted and a higher mole fraction of SDS in surfactant solution means that a lower sorption plateau can be reached with a smaller TX100 concentration in mixed surfactant solution. Thus, the higher the mole fraction of SDS in surfactant solution, the lower the CDC values of TX100 for phenanthrene. From Fig. 4, the aqueous phenanthrene concentrations with the single TX100 solution were significantly less than the solubilities of phenanthrene in corresponding TX100 solution. It seems that the sorption of TX100 onto soil and the partition of phenanthrene into the sorbed TX100 limited the desorption of phenanthrene from the contaminated soil and resulted in the low aqueous concentration and the desorption percentage. With the addition of SDS into TX100 solution, the sorption
30 100
80 20
15
10
TX100 S:T = 1:4 S:T = 1:2
5
0
S:T = 1:1 S:T = 2:1 0
1
2
3
4
TX100 Concentration (mmol/L) Fig. 4. The aqueous phase concentration (Caq) of phenanthrene in the desorption experiments by different surfactant systems with various mole ratios of SDS (S) to TX100 (T).
Rd of Phenanthrene ( )
Caq of Phenanthrene (mg/L)
25
60
40 TX100 S:T = 1:4 S:T = 1:2 S:T = 1:1 S:T = 2:1
20
0
0
1
2
3
4
TX100 Concentration (mmol/L) Fig. 5. The desorption percentage (Rd) of phenanthrene by different surfactant systems with various mole ratios of SDS (S) to TX100 (T).
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W. Zhou, L. Zhu / Environmental Pollution 147 (2007) 350e357
of TX100 onto soil was reduced and the solubilization of phenanthrene in TX100 solution was enhanced. Thus, the distribution of phenanthrene in soilewater was decreased. Thus, the aqueous phenanthrene concentrations with SDSe TX100 mixed solution were greater than that with single TX100 solution at comparable TX100 concentration in solution and increased with the mole fraction of SDS in solution. The higher aqueous phase of phenanthrene meant a greater desorption percentage of phenanthrene from the contaminated soil. From Fig. 5, it can be seen that the phenanthrene desorption percentage by mixed surfactants was obviously greater than that by single TX100 solution and increased as the mole fraction of SDS in mixed surfactant solution increased. For example, using the same TX100 concentration in surfactant solution of 4 mmol/L, the desorption percentage of phenanthrene were 49.5% with single TX100 solution and 62.5%, 72.5%, 78.5% and 87.5% with mixed surfactant, in which the mole ratios of SDS to TX100 were 1:4, 1:2, 1:1 and 2:1, respectively. To reach the same phenanthrene desorption percentage of 50% from the contaminated soil, the corresponding concentration of TX100 is about 4 mmol/L for single TX100 solution and 2.6, 2.1, 1.6, 1.1 mmol/L for mixed solution with the composition of 1:4, 1:2, 1:1, 2:1, respectively. These results indicated that the SDSeTX100 mixed solution was more effective for the desorption of phenanthrene from the contaminated soil than the single TX100 solution. 4. Conclusions This study focused on a new approach to the desorption of phenanthrene from an artificial contaminated natural soil by using an anionic/nonionic mixed surfactant, SDSeTX100, with the aim of improving the efficiency of surfactant remediation technology. The sorption amount of TX100 onto soil from the mixed solution was less than that of single TX100 solution and decreased as the mole fraction of SDS in solution increased. In addition, the apparent solubilities of phenanthrene in mixed surfactant appeared to be greater than that in single TX100 solution and were positively related to the mole fraction of SDS in mixed surfactant solution. Both resulted in the distribution of phenanthrene in soilewater system with TX100 being reduced with the presence of SDS and the aqueous phase concentration of phenanthrene appeared to be positively related to the mole fraction of SDS in solution. Thus, the desorption percentage of phenanthrene from the contaminated soil with SDSeTX100 mixed solution was greater than that with single TX100 solution and appeared to be positively related to the mole fraction of SDS in solution. Then, SDSeTX100 mixed solution was more effective for the desorption of phenanthrene from the contaminated soil than the single TX100 solution. All the experimental results from this study indicated that the anionic/nonionic mixed surfactants may improve the performance of surfactant remediation technology by reducing the sorption of surfactants onto soils and increasing the solubilization of HOCs in single surfactants. Thus, anionic/nonionic mixed surfactants may be a better
choice for the applications of surfactant remediation technology. Acknowledgements This study was financially supported by the National Natural Science Foundation of China (40571143, 20337010, 20125719) and the National Basic Research Priorities Program of China (2003CB415004). References Almgren, M.A., Grteser, F., Thomas, J.K., 1979. Dynamic and static aspects of solubilization of neutral arenas in ionic micellar solutions. J. Am. Chem. Soc. 101, 279e291. Bury, S.J., Miller, C.A., 1993. Effect of micellar solubilization on biodegradation rates of hydrocarbons. Environ. Sci. Technol. 27, 104e110. Chiou, C.T., Peters, L.J., Freed, V.H., 1979. A physical concept of soil-water equilibria for nonionic organic compounds. Science 206, 831. Deitsch, J.J., Smith, J.A., 1995. Effect of Triton X-100 on the rate of trichloroethene desorption from soil to water. Environ. Sci. Technol. 29, 1069e1080. Diallo, M.S., Abriola, L.M., Weber, W.J., 1994. Solubilization of nonaqueous phase liquid hydrocarbons in micellar solutions of dodecyl alcohol ethoxylates. Environ. Sci. Technol. 28, 1829e1837. Edwards, D.A., Luithy, R.G., Liu, Z., 1991. Solubilization of polycyclic aromatic hydrocarbons in micellar nonionic surfactant solutions. Environ. Sci. Technol. 25, 127e133. Edwards, D.A., Adeel, Z., Luthy, R.G., 1994. Distribution of nonionic surfactant and phenanthrene in a sediment/aqueous system. Environ. Sci. Technol. 28, 1550e1560. Guha, S., Jaffe, P.R., 1996. Bioavailability of hydrophobic compounds partitioned into the micellar phase of nonionic surfactants. Environ. Sci. Technol. 30, 1382e1391. Harwell, J.H., Sabatini, D.A., Knox, R.C., 1999. Surfactants for ground water remediation. Colloids Surf. A: Phys. Eng. Asp. 151, 255e268. Holland, P.M., Rubingh, D.N., 1983. Nonideal multicomponent mixed micelle model. J. Phys. Chem. 87, 1984e1990. Holland, P.M., Rubingh, D.N., 1992. Mixed Surfactant Systems. American Chemical Society, Washington, DC. Jafvert, C.T., Heath, J.K., 1991. Sediment- and saturated-soil-associated reactions involving an anionic surfactant (dodecylsulfate). 1. Precipitation and micelle formation. Environ. Sci. Technol. 25, 1031e1038. Jafvert, C.T., Van Hoof, P.L., Heath, J., 1994. Solubilization of non-polar compounds by non-ionic surfactant micelles. Water Res. 28, 1009e1017. Johnson, J.C., Sun, S., Jaffe, P., 1999. Surfactant enhanced perchloroethylene dissolution in porous media: the effect on mass transfer rate coefficients. Environ. Sci. Technol. 35, 1286e1292. Karickhoff, S.W., Brown, D.S., Scott, T.A., 1979. Sorption of hydrophobic pollutants on natural sediments. Water Res. 13, 241e248. Kile, D.E., Chiou, C.T., 1989. Water solubility enhancements of DDT and trichlorobenzene by some surfactants below and above the critical micelle concentration. Environ. Sci. Technol. 23, 832e838. Ko, S.-O., Schlautman, M.A., Carraway, E.R., 1998. Partitioning of hydrophobic organic compounds to sorbed surfactants. 1. Experimental studies. Environ. Sci. Technol. 32, 2769e2775. Liu, Z., Edwards, D.A., Luthy, R.G., 1992. Sorption of non-ionic surfactants onto soil. Water Res. 26, 1337e1345. Paria, S., Khilar, K.C., 2004. A review on experimental studies of surfactant sorption at the hydrophilic solid-water interface. Adv. Colloid Interface Sci. 110, 75e95. Penfold, J., Staples, E., Tucker, I., Thomas, R.K., 2002. Sorption of mixed anionic and nonionic surfactants at the hydrophilic silicon surface. Langmuir 18, 5755e5760.
W. Zhou, L. Zhu / Environmental Pollution 147 (2007) 350e357 Pennell, K.D., Adinolfi, A.M., Abriola, L.M., Diallo, M.S., 1997. Solubilization of dodecane, tetrachloroethylzene in micellar solutions of ethoxylated nonionic surfactants. Environ. Sci. Technol. 31, 1382e1389. Rosen, M.J., 2004. Surfactants and Interfacial Phenomena, third ed. John Wiley & Sons, Hoboken. Sun, S., Inskeep, W.P., 1995. Sorption of nonionic organic compounds in soile water systems containing a micelle-forming surfactant. Environ. Sci. Technol. 29, 903e913. Thibaut, A., Misselyn-Bauduin, A.M., Grandjean, J., Broze, G., Je´roˆme, R., 2000. Sorption of an aqueous mixture of surfactants on silica. Langmuir 16, 9192e9198. Tsomides, H.J., Hughes, J.B., Thomas, J.M., Ward, C.H., 1995. Effect of surfactant addition on phenanthrene biodegradation in sediments. Environ. Toxicol. Chem. 14, 953e959.
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West, C.C., Harwell, J.H., 1992. Surfactants and subsurface remediation. Environ. Sci. Technol. 26, 2324e2340. Yaws, C.L., 1999. Chemical Properties Handbook. McGraw-Hill, Beijing. Yeom, I.T., Ghosh, M.M., Cox, C.D., 1996. Kinetic aspects of surfactant solubilization of soil-bound polycyclic aromatic hydrocarbons. Environ. Sci. Technol. 30, 1589e1595. Zhou, W., Zhu, L., 2005. Solubilization of polycyclic aromatic hydrocarbons by anionic-nonionic mixed surfactant. Colloids Surf. A 255, 145e152. Zhu, L., Chiou, C.T., 2001. Water solubility enhancements of pyrene by single and mixed surfactant solutions. J. Environ. Sci. 13, 491e496. Zhu, L., Feng, S., 2003. Synergistic solubilization of polycyclic aromatic hydrocarbons by mixed anionic-nonionic surfactants. Chemosphere 53, 459e467.
Enhanced desorption of phenanthrene from contaminated soil using anionic/nonionic mixed surfactant Wenjun Zhou, Lizhong Zhu* Department of Environmental Science, Zhejiang University, Hangzhou, Zhejiang 310028, PR China Received 31 March 2006; accepted 24 May 2006
Anionic/nonionic mixed surfactants appear to be a better choice than single surfactants for the application of surfactant remediation technology. Abstract A new approach using an anionic/nonionic mixed surfactant, sodium dodecyl sulphate (SDS) with Triton X-100 (TX100), was utilized for the desorption of phenanthrene from an artificial contaminated natural soil in an aim to improve the efficiency of surfactant remediation technology. The experimental results showed that the presence of SDS not only reduced the sorption of TX100 onto the natural soil, but also enhanced the solubilization of TX100 for phenanthrene, both of which resulted in the distribution of phenanthrene in soilewater systems decreasing with increasing mole fraction of SDS in surfactant solutions. These results can be attributed to the formation of mixed micelles in surfactant solution and the corresponding decrease in the critical micelle concentration of TX100 in mixed solution. The batch desorption experiments showed that the desorption percentage of phenanthrene from the contaminated soil with mixed solution was greater than that with single TX100 solution and appeared to be positively related to the mole fraction of SDS in surfactant solution. Thus, the anionic/nonionic mixed surfactants are more effective for the desorption of phenanthrene from the contaminated soil than a single nonionic surfactant. Ó 2006 Elsevier Ltd. All rights reserved. Keywords: Surfactant remediation; Phenanthrene; Contaminated soil; Mixed surfactant; Desorption percentage
1. Introduction The contamination of soils and groundwater by toxic and/or hazardous hydrophobic organic compounds (HOCs) is a widespread environmental problem. Various physical, chemical, biological technologies and their combinations have been attempted to remediate HOC contaminated soils and groundwater. Surfactant remediation technology has been suggested as the promising technology for the remediation of contaminated soils and groundwater (West and Harwell, 1992; Harwell et al., 1999). Surfactants enhance the remediation of HOCs contaminated soils by increasing the HOC’s aqueous-phase concentration via micelle solubilization (Kile and Chiou,
* Corresponding author: Tel./fax: þ86 571 88273733. E-mail address: [email protected] (L. Zhu). 0269-7491/$ - see front matter Ó 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.envpol.2006.05.025
1989; Edwards et al., 1991; Diallo et al., 1994; Jafvert et al., 1994; Pennell et al., 1997) and the mobilization of HOCs from solid into aqueous phases (Deitsch and Smith, 1995; Yeom et al., 1996; Johnson et al., 1999), thus improving the bioavailability of HOCs for microbial remediation and phytoremediation (Bury and Miller, 1993; Tsomides et al., 1995; Guha and Jaffe, 1996). Anionic and nonionic surfactants have usually been chosen in surfactant remediation technology. The solubility enhancement properties of HOCs in single anionic and nonionic surfactant solution are well defined (Kile and Chiou, 1989; Edwards et al., 1991; Diallo et al., 1994; Jafvert et al., 1994; Pennell et al., 1997). Compared with anionic surfactant, nonionic surfactants have greater solubilization enhancement for HOCs and show higher remediation efficiency for contaminated soils. However, recent soil experiments have suggested that precipitation for anionic surfactants (Jafvert and Heath,
W. Zhou, L. Zhu / Environmental Pollution 147 (2007) 350e357
1991) and the sorption of nonionic surfactant into soil (Liu et al., 1992; Edwards et al., 1994; Sun and Inskeep, 1995; Ko et al., 1998) may occur in soilewater systems. In particular, nonionic surfactants can be sorbed onto the solid matrix and thereby lead to HOCs partitioning into immobile sorbed surfactants and, thus, enhanced HOC retardation (Edwards et al., 1994; Sun and Inskeep, 1995; Ko et al., 1998). Edwards et al. (1994) found that the sorbed Triton X-100 acts to enhance phenanthrene sorption and, on a carbon-normalized basis, the sorbed surfactant is much more effective as a sorbent for phenanthrene than is humic matter. Sun and Inskeep (1995) observed that the sorbed surfactant enhanced HOC’s partitioning relative to the untreated soils and the increase or decrease in HOC’s distribution coefficients depend on the net effects of both soil-sorbed and aqueous phase surfactant fractions. Thus, the sorption of nonionic surfactant onto soil would decrease the remediation efficiency and result in an increase in remediation times and costs. The soils and groundwater may be contaminated by surfactants via sorption. Meanwhile, the environment factors of temperature, salinity and pH have obvious effects on the solubilization of individual anionic or nonionic surfactant solution for HOCs. Thus, an improved strategy for surfactant remediation technology for contaminated soils is to reduce the sorption of surfactant onto soils and to enhance the desorption of HOCs from contaminated soils in order to obtain an optimal remediation efficiency with the minimum surfactant dose. Currently, mixed surfactants are of great interest in scientific and industrial applications. Surfactants used in practical applications almost always consist of mixtures of surfactants. Commercial products are inexpensive, easily available and exhibit superior performance compared with the individual detergents. Mixed surfactants could be employed over a wider range of temperature, salinity, and hardness conditions than the individual surfactant (Holland and Rubingh, 1992). In particular, some researchers (Thibaut et al., 2000; Penfold et al., 2002) found that the sorption of nonionic surfactant at the hydrophilic silicon surface was strongly restricted with the presence of anionic surfactant as the formation of mixed micelle and the sorption amount of nonionic surfactant decreased with the increasing mole fraction of anionic surfactant in mixed solution. The synergistic solubilization of anionic/nonionic mixed surfactant for HOCs was observed in some studies (Zhu and Chiou, 2001; Zhu and Feng, 2003). Thus, in the practical applications of surfactant remediation technology for contaminated soils, the anionic-nonionic mixed surfactant may have a better performance than that of single surfactant. However, little information is known about the sorption of anionic-nonionic mixed surfactants onto natural soils and their performance in the remediation of contaminated soils by HOCs. Therefore, a study of the desorption of HOCs from contaminated soils by anionic-nonionic mixed surfactant is necessary for evaluating the performance of anionic/nonionic mixed surfactants in soil remediation with the aim of improving the efficiency of surfactant remediation technology. The objectives of this study are (1) to investigate the effect of the presence of anionic surfactant on the sorption
351
of nonionic surfactant onto a natural soil; (2) to evaluate the efficiency of anionic-nonionic mixed surfactant for the desorption of HOC contaminated soil. The experimental results can be used to understand the performance of anionic/nonionic mixed surfactants in the remediation of contaminated soil and to provide valuable information in designing surfactant remediation technology for contaminated soils. 2. Materials and methods 2.1. Materials Phenanthrene was selected as a representative polycyclic aromatic hydrocarbon (PAH) to model the hydrophobic organic contaminants; it was obtained from Acros Organics, with a purity of 99%. Sodium dodecyl sulphate (SDS) was obtained from Acros Organics, with purity 98%. Triton X-100 (TX100), a nonionic surfactant, was obtained from Sigma Chemical Company and used without further treatment. Selected physicochemical properties of the compounds are presented in Table 1. Mixed surfactant solutions were prepared by dissolving SDS and TX100 in deionized water with different mole ratios and the composition of mixed surfactants is expressed with the mole ratios of SDS to TX100. An uncontaminated natural soil was collected from Hangzhou City, China. The soil was air-dried and sieved to obtain particles less than 1 mm in all experiments. The soil contained 3.9% sand, 71.5% silt and 24.6% clay. The organic carbon content was 0.52%. The contaminated soil was prepared by dissolving an appropriate quantity of phenanthrene in petroleum ether and a known weight of soil was added slowly, with continuous mixing. This slurry was mixed thoroughly and the solvent was allowed to evaporate slowly. The dry contaminated soil was transferred into a bottle and tumbled for about a week before the experiments. The resulting contaminated soil had a final concentration of 308 mg/kg of phenanthrene, which was used directly in the desorption experiments.
2.2. Enhanced solubilization by surfactant solutions The solubility enhancement of phenanthrene by SDSeTX100 surfactant mixture was conducted in 25 ml Corex centrifuge tubes with Teflon-lined screw caps, in which a volume of 20 ml of solutions with a series of surfactants concentrations was placed and phenanthrene was subsequently added to each tube in an amount slightly more than required to saturate the solution. The tubes were equilibrated on a reciprocating shaker for 24 h at 20 1 C and subsequently centrifuged at 5000 rpm (7295 g) for 30 min to separate completely the undissolved solute. An appropriate aliquot of the supernatant was removed and analysed for phenanthrene by HPLC.
2.3. Sorption of TX100 onto soil from surfactant solutions Batch experiments were conducted in duplicate to determine surfactant equilibrium sorption isotherms using centrifuge tubes. Two grams of soil sample were weighed into each centrifuge tube, to which 20 ml of surfactant solution was added. All aqueous solutions for soil tests contained 0.01 M NaCl to keep a constant ionic strength and 0.01% w/w NaN3 to inhibit microbial growth. The initial surfactant concentration spanned over a large range of values below and above the nominal CMC of surfactants. These samples were then equilibrated on a reciprocating shaker for 24 h at 20 1 C and subsequently centrifuged at 5000 rpm for 30 min to completely separate the solution and solid phase. An appropriate aliquot of the supernatant was removed and analysed for TX100 by HPLC. The sorption amount of TX100 on soil was computed simply from the difference of the initial and final surfactant concentrations.
2.4. Distribution of phenanthrene in soilewateresurfactant systems The equilibrium distribution coefficient of PAHs in the soilewater system with or without surfactant was determined by a batch equilibrium experiment.
W. Zhou, L. Zhu / Environmental Pollution 147 (2007) 350e357
352
Table 1 Selected physicochemical properties of compounds in this studya Compounds
Molecular formula
MW
logKow Sw CMC (mM) (mM)
Phenanthrene C14H10 178.23 4.57 Triton X-100 C8H17C6H4O(OCH2CH2)9.5H 628 288.38 SDS C12H25SO4Na a
6.62 0.29 5.4
Phenanthrene data from Yaws (1999).
Varied quantities of phenanthrene were added to the centrifuge tubes, which contained 2.0 g of clean soil sample and 20 ml of a given concentration of surfactant solution. The centrifuge tubes were shaken on a reciprocating shaker for 24 h at 20 1 C and subsequently centrifuged at 5000 rpm for 30 min to completely separate the solution and solid phase. An appropriate aliquot of the supernatant was removed and analysed for phenanthrene by HPLC. The sorbed amounts of phenanthrene were computed simply from the difference of the initial and final phenanthrene concentrations. The equilibrium distribution coefficients for phenanthrene in all soilewater systems were obtained via linear regression using the various sorption amounts and the equilibrium concentrations in aqueous phase.
2.5. Batch desorption experiments The percentage and extent of phenanthrene desorption from contaminated soil with different surfactant solutions were measured in batch experiments. A weight of 2.0 g of contaminated soil sample was weighed into each centrifuge tube, to which 20 ml of surfactant solution with different concentration and composition was added. The centrifuge tubes were then shaken on a reciprocating shaker for 24 h at 20 1 C. The solution and solid phase were separated by centrifugation at 5000 rpm for 30 min. An appropriate aliquot of the supernatant was removed and analysed for phenanthrene by HPLC.
2.6. Analytical methods Aqueous phenanthrene and TX100 concentrations were quantified by an Agilent HPLC fitted with UV detector and an Agilent Eclipse XDB-C8 column (4.5 150 mm, 5 mm) using methanolewater (75:25) as the mobile phase at a flow rate of 1 ml/min. Chromatography was performed at 30 C. The UV wavelengths were set at 250 and 224 nm for phenanthrene and TX100, respectively.
3. Results and discussion
dramatically as the surface micelle (admicelle) or bilayers form on the adsorbent through association or hydrophobic interactions between the hydrocarbon chains of the surfactants, and a plateau is reached corresponding to a maximum sorption amount with the surfactant concentration around the CMC. Similar sorption isotherms were obtained for TX100 from mixed surfactant solutions, in which the mole ratio of SDS to TX100 is 1:4, 1:2, 1:1 and 2:1, respectively. However, the presence of anionic surfactant, SDS, strongly affected the sorption amount of pure TX100. The addition of 20 mol% SDS to TX100 resulted in a 30% decrease in the maximum sorption amount for TX100, from 11.6 mmol/kg to 8.25 mmol/kg. When the mole ratio of SDS to TX100 in mixed surfactant solution was 2:1, the maximum sorption amount of TX100 onto soil was only about one-fourth of the value for TX100 from the single surfactant solution. Thus, the higher the mole fraction of SDS in mixed surfactant solution, the greater the decrease in the maximum sorption amount for TX100. These results show that when SDS was mixed together with TX100, the sorption of nonionic surfactant onto soil was severely restricted and a higher mole fraction of SDS in surfactant solution means that a lower plateau sorption can be reached with a smaller TX100 concentration in surfactant solution. As mentioned earlier, the sorption of nonionic surfactant proceeds through the sorption of surfactant monomer, the micelles are not directly sorbed. In mixed surfactant solution, the formation of mixed micelles would affect the CMC and the monomer concentration of component surfactant in mixed solution and then their sorption into the soil. The regular solution theory (Holland and Rubingh, 1983) has been proven to be remarkably successful in expressing the characteristics of mixed surfactant solutions. With the regular solution theory, the CMC values of a binary surfactant solution, CMC*, can be calculated by the following equation: 1 X1 X2 ¼ þ CMC f1 CMC1 f2 CMC2
ð1Þ
3.1. Effect of SDS on the sorption of TX100 onto soil 14 TX100 S:T = 1:4 S:T = 1:2 S:T = 1:1 S:T = 2:1
12
Sorbed TX100 (mmol/kg)
The sorption isotherms of TX100 onto the natural soil from different surfactant systems are shown in Fig. 1, which illustrates the effect of SDS on the sorption of TX100 onto soil. The sorption isotherm of TX100 from the single surfactant solution was nonlinear with typical S-shape curves, reaching a plateau in sorption amount at surfactant equilibrium concentration around the CMC, which is in accord with the results of other experimental studies on the sorption of nonionic surfactant (Liu et al., 1992; Edwards et al., 1994; Sun and Inskeep, 1995; Paria and Khilar, 2004). A conceptual model based on numerous experimental observations can account for the sorption of nonionic surfactant onto a hydrophilic surface. At low surfactant concentration, the nonionic surfactants are sorbed as monomers and lie parallel to the solid surface through surface interactions with both types of surfactant moieties. With the increase in the surfactant concentration, the sorption increases
10 8 6 4 2 0
0
1
2
3
4
Equilibrium Concentration of TX100 (mmol/L) Fig. 1. The sorption isotherms of TX100 onto soil from different surfactant systems with various mole ratios of SDS (S) to TX100 (T).
W. Zhou, L. Zhu / Environmental Pollution 147 (2007) 350e357
2 f1 ¼ exp b 1 X1m
ð2Þ
2 f2 ¼ exp b 1 X2m
ð3Þ
At mixed surfactant concentration above the CMC*, the micellar mole fraction of component 1 for every solution composition of mixed surfactant can be obtained by the following equation (Zhou and Zhu, 2005): ln
ð1 X1 ÞCMC1 X1m ¼ b 2X1m 1 m X1 CMC2 1 X1
ð4Þ
Thus, the critical micelle concentration of component 1 (CMCm 1 ) in the mixed solution is given by CMC1 ¼ X1 CMC
ð5Þ
For the SDSeTX100 mixed surfactant system, a value of 3.0 was used for the interaction parameter b, representing an average of values quoted for SDSealkylethoxylate mixed surfactant systems (Rosen, 2004). The critical micelle concentration of TX100 in every mixed surfactant systems was calculated using Eqs. (1)e(5) and is shown in Table 2. The observed decrease in the sorption amount of TX100 should result from the reduction of the TX100 critical micelle concentration as a consequence of mixed micelle formation. This explanation is confirmed by Table 2, which compares the maximum sorption amount with the critical micelle concentration of TX100 in mixed solution as a function of the mole fraction of SDS. Table 2 shows a parallel decrease in the critical micelle concentration and the maximum sorption amount of TX100 when the mixed surfactant solution contained relatively more SDS, which indicated that the formation of mixed micelle decreased the critical micelle concentration of TX100 in mixed surfactant solution and then restricted the sorption of TX100 onto the natural soil. In addition, Table 2 Parameters for the desorption of phenanthrene from the contaminated soil by different surfactant systems with various mole ratios of SDS to TX100 SDSe TX100
CMCT (mmol/L)a
Csorb (mmol/kg)b
logKmc (L/mol)
CDC (mM)c
0 1:4 1:2 1:1 2:1
0.29 0.25 0.22 0.19 0.16
11.6 8.25 6.41 4.90 3.33
4.19 4.22 4.24 4.28 4.36
1.6 1.2 1.0 0.8 0.6
a
The critical micelle concentration of TX100 in mixed solution. The maximum sorption amount of TX100 onto soil from different surfactant solutions. c The critical desorption concentration of TX100 for phenanthrene. b
Thibaut et al. (2000) observed that the addition of an anionic/nonionic surfactant mixture to silica precoated with the nonionic surfactant resulted in the release of the preadsorbed nonionic surfactant and the free energy of mixed micellization was more negative than that of nonionic surfactant sorption, which indicated that the mixed micellization in solution is more favourable than the sorption of nonionic surfactant onto soil. 3.2. Solubilization of phenanthrene in mixed surfactant solutions The solubilization of phenanthrene by SDSeTX100 mixed surfactant with different composition was determined. The apparent solubilities of phenanthrene in mixed surfactants are shown in Fig. 2 as a function of TX100 concentrations and compared with that in single TX100 solution. Obviously, the apparent solubilities of phenanthrene increased linearly over the range of TX100 concentrations in solution. The same as single surfactant, anionic-nonionic mixed surfactants also have the potential capacity to enhance the solubilization of phenanthrene in water. The behaviour is generally attributed to the incorporation or partitioning of organic solutes within mixed surfactant micelles. It was also observed that the apparent solubilities of phenanthrene in mixed solutions were higher than those in single TX100 solution at comparable TX100 concentration and increased with the increasing mole fraction of SDS in mixed solution. The apparent solubilities Sw of HOCs in surfactant solutions can be expressed as follows (Jafvert et al., 1994; Almgren et al., 1979): Sw ¼ Sw þ Cmic Kmc Sw
ð6Þ
where Sw is the solubility of phenanthrene in pure water; Cmic is the concentration of surfactant in micellar form; Kmc is the micelleewater partition coefficient. 0.6
Sw* of Phenanthrene (mmol/L)
where subscripts 1 and 2 indicate the components 1 and 2, respectively. CMC is the critical micelle concentration of individual pure component. X is the mole fraction of each component in mixed surfactant solutions. f is the activity coefficient of each component and can be expressed in terms of the micelle composition Xm and a single interaction parameter b.
353
0.5
TX100 S:T = 1:4 S:T = 1:2 S:T = 1:1
0.4
S:T = 2:1
0.3
0.2
0.1
0.0
1
2
3
4
TX100 Concentartion (mmol/L) Fig. 2. The apparent solubilities Sw of phenanthrene by different surfactant systems with various mole ratios of SDS (S) to TX100 (T).
W. Zhou, L. Zhu / Environmental Pollution 147 (2007) 350e357
The micelleewater partition coefficient Kmc is a parameter that indicates the partitioning of organic solutes between the surfactant micelles and the water phase (Jafvert et al., 1994; Almgren et al., 1979): Smic Kmc ¼ Sw Cmic
ð7Þ
where Smic is the concentration of solute in the micelle phase (M). The Kmc values for phenanthrene in all surfactant systems studied were calculated according to the corresponding solubilization data and are shown in Table 2. The Kmc values for phenanthrene in mixed solutions were higher than those in single TX100 solution and increased with the increasing mole fraction of SDS in mixed surfactant. In this study, the concentrations of SDS in mixed surfactant are less than the CMC values of SDS and the solubilization enhancement of SDS in mixed solution for phenanthrene is negligible. However, when SDS was mixed together with TX100, the critical micelle concentration of TX100 decreased and then the micelle concentration increased. At the same time, the formation of mixed micelle enhances the partition of phenanthrene into surfactant micelle. Both the increased Kmc values and the micelle concentration resulted in increased apparent solubilities of phenanthrene in mixed surfactant solutions. 3.3. Distribution of phenanthrene in the soilewater system with mixed surfactants The distribution of HOCs in a surfactant-free soilewater system is governed by a mechanism where the HOC molecules partition into the soil organic matter (SOM) (Chiou et al., 1979; Karickhoff et al., 1979) and can be evaluated with the soilewater distribution coefficient Kd. However, with the addition of nonionic surfactants, the sorption of nonionic surfactant onto soils and the partition of HOCs into the soil-sorbed surfactant would affect the distribution of solute in soilewater systems. The apparent soilewater distribution coefficients Kd for phenanthrene were obtained from the slope of the sorption isotherms (not shown) of phenanthrene in all soilewateresurfactant systems in this study. The changes of Kd for phenanthrene in soilewater systems containing single TX100 or SDSeTX100 mixed solution are shown in Fig. 3 as a function of TX100 concentration in solution. In all cases, phenanthrene showed a similar distribution behaviour pattern with increasing TX100 concentration in solution. That is, relative to the corresponding intrinsic Kd in a surfactant-free system, Kd increased gradually to reach a maximum value and then followed by a gradual decrease. The increase in Kd is apparently caused by the strong sorption of TX100 by the solid phase and the functionality of the sorbed TX100 as a sorptive phase for the solutes. The corresponding equilibrium concentrations of TX100 with the maximum Kd in all systems were in accord with that corresponding to the onset of the sorption plateau of TX100 onto soils.
TX100 S:T = 1:4
120
Kd* for Phenanthrene (L/kg)
354
S:T = 1:2 S:T = 1:1 S:T = 2:1
100 80 60 40 20 0
0
1
2
3
4
TX100 Concentration (mmol/L) Fig. 3. The apparent distribution coefficients Kd of phenanthrene in the soilewateresurfactant system as a function of the TX100 concentration in different surfactant systems with various mole ratios of SDS (S) to TX100 (T).
Although having similar change trends, the Kd values for phenanthrene with mixed solution appeared to be inversely related to the mole fraction of SDS in surfactant solutions. For example, the maximum Kd values for phenanthrene with SDSeTX100 mixed surfactant, in which the mole ratios of SDS to TX100 were 1:4, 1:2, 1:1 and 2:1, were about 82%, 68%, 60% and 48% of that with the single TX100 solution, respectively. With the same TX100 concentration of 4 mmol/L in surfactant solution, the corresponding Kd values for phenanthrene with SDSeTX100 mixed solution were about 72%, 55%, 42% and 27% respectively, of that with the single TX100 solution and decreased with an increasing mole fraction of SDS in mixed surfactant solution. Thus, the presence of SDS in surfactant solution reduced the distribution of phenanthrene into soil and the higher the amount of SDS in surfactant solution, the greater the phenanthrene is in aqueous phase. In the presence of surfactant, the distribution of solute in each phase (i.e., water, surfactant micelle, sorbed surfactant, soil solid) shows complex behaviour depending on the distribution of surfactants in solideaqueous systems. Thus, the Kd in the soilewateresurfactant system can be expressed as (Edwards et al., 1994; Sun and Inskeep, 1995): Kd ¼
Kd þ Csorb Ksf 1 þ Cmic Kmc
ð8Þ
where Csorb is the sorption amount of surfactant onto soil, Ksf is the solute distribution coefficient with the soil-sorbed surfactant. Eq. (8) can be used to describe accurately the functional dependence of Kd on the TX100 equilibrium concentration for all cases. At low TX100 equilibrium concentration, Kd values increased with increasing TX100 equilibrium concentration because the amount of soil-sorbed TX100 increases rapidly (Fig. 1) prior to the formation of TX100 micelles in solution and the sorbed TX100 is very effective for HOC partitioning. When the TX100 equilibrium concentration in aqueous phase reaches its CMC, the sorption
W. Zhou, L. Zhu / Environmental Pollution 147 (2007) 350e357
amount of TX100 reaches a plateau, and TX100 micelles in the aqueous phase become an important partition phase with increasing equilibrium concentration. As a result, micelles begin competing for HOC molecules, thereby causing a decrease in Kd . According to Eq. (8), the important features affecting the distribution of HOCs in soilewateresurfactant systems include the sorption of surfactant onto soil with the corresponding partition of HOCs into the soil-sorbed surfactant (CsorbKsf) and the enhanced solubilization of HOCs by aqueous surfactant solution (CmicKmc). The presence of SDS not only reduced the sorption of TX100 onto the natural soil (Fig. 1), but also enhanced the solubilization of TX100 for phenanthrene (Fig. 2), both of which resulted in the Kd for phenanthrene decreasing with an increase in the mole fraction of SDS in surfactant solutions. The smaller Kd means that the soil has weak sorption for phenanthrene and more phenanthrene would be in the aqueous phase. 3.4. Phenanthrene desorption studies Desorption studies were used to evaluate the efficiency of surfactant solution to desorb phenanthrene from contaminated soil. The aqueous phase phenanthrene concentrations, which were desorbed from soil, with the addition of mixed surfactant are shown in Fig. 4 as a function of TX100 concentrations in solution and compared with those of single TX100 solution. Fig. 5 shows the plot of the desorption percentage of phenanthrene from the contaminated natural soil by different surfactant systems with the TX100 concentration in surfactant solutions. From Figs. 4 and 5, it can be observed that both the aqueous phase phenanthrene concentrations and the phenanthrene desorption percentage shared a similar trend and had a sharp increase with the increasing TX100 concentration in solution. The corresponding TX100 concentrations in mixed solution
355
with sharp increases can be defined as the critical desorption concentration (CDC) and phenanthrene desorption from contaminated soil would be favourable when the TX100 concentration is greater than the CDC. The CDC values of TX100 for phenanthrene in different surfactant systems were estimated from Figs. 4 and 5 and are listed in Table 2. The CDC values of TX100 for phenanthrene with mixed surfactants were less than that with single TX100 solution and decreased with the increasing mole fraction of SDS in mixed solution. The sharp increase and the CDC can be rationally explained as the sorption of TX100 onto soil from surfactant solution and were attributed to the formation of surfactant micelles in aqueous phase. Before the sorption plateau of TX100 onto soil, TX100 existed in aqueous phase as a monomer and there were no surfactant micelles in aqueous phase. Thus, the aqueous phenanthrene concentrations desorbed from soil contributed to the phenanthrene water solubility. With the sorption of TX100 in plateau, the surfactant micelles began to form in aqueous phase, which enhanced the solubilities of phenanthrene in aqueous phase and thus increased the desorption percentage. For SDSeTX100 mixed surfactant systems, the sorption of nonionic surfactant onto soil was severely restricted and a higher mole fraction of SDS in surfactant solution means that a lower sorption plateau can be reached with a smaller TX100 concentration in mixed surfactant solution. Thus, the higher the mole fraction of SDS in surfactant solution, the lower the CDC values of TX100 for phenanthrene. From Fig. 4, the aqueous phenanthrene concentrations with the single TX100 solution were significantly less than the solubilities of phenanthrene in corresponding TX100 solution. It seems that the sorption of TX100 onto soil and the partition of phenanthrene into the sorbed TX100 limited the desorption of phenanthrene from the contaminated soil and resulted in the low aqueous concentration and the desorption percentage. With the addition of SDS into TX100 solution, the sorption
30 100
80 20
15
10
TX100 S:T = 1:4 S:T = 1:2
5
0
S:T = 1:1 S:T = 2:1 0
1
2
3
4
TX100 Concentration (mmol/L) Fig. 4. The aqueous phase concentration (Caq) of phenanthrene in the desorption experiments by different surfactant systems with various mole ratios of SDS (S) to TX100 (T).
Rd of Phenanthrene ( )
Caq of Phenanthrene (mg/L)
25
60
40 TX100 S:T = 1:4 S:T = 1:2 S:T = 1:1 S:T = 2:1
20
0
0
1
2
3
4
TX100 Concentration (mmol/L) Fig. 5. The desorption percentage (Rd) of phenanthrene by different surfactant systems with various mole ratios of SDS (S) to TX100 (T).
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W. Zhou, L. Zhu / Environmental Pollution 147 (2007) 350e357
of TX100 onto soil was reduced and the solubilization of phenanthrene in TX100 solution was enhanced. Thus, the distribution of phenanthrene in soilewater was decreased. Thus, the aqueous phenanthrene concentrations with SDSe TX100 mixed solution were greater than that with single TX100 solution at comparable TX100 concentration in solution and increased with the mole fraction of SDS in solution. The higher aqueous phase of phenanthrene meant a greater desorption percentage of phenanthrene from the contaminated soil. From Fig. 5, it can be seen that the phenanthrene desorption percentage by mixed surfactants was obviously greater than that by single TX100 solution and increased as the mole fraction of SDS in mixed surfactant solution increased. For example, using the same TX100 concentration in surfactant solution of 4 mmol/L, the desorption percentage of phenanthrene were 49.5% with single TX100 solution and 62.5%, 72.5%, 78.5% and 87.5% with mixed surfactant, in which the mole ratios of SDS to TX100 were 1:4, 1:2, 1:1 and 2:1, respectively. To reach the same phenanthrene desorption percentage of 50% from the contaminated soil, the corresponding concentration of TX100 is about 4 mmol/L for single TX100 solution and 2.6, 2.1, 1.6, 1.1 mmol/L for mixed solution with the composition of 1:4, 1:2, 1:1, 2:1, respectively. These results indicated that the SDSeTX100 mixed solution was more effective for the desorption of phenanthrene from the contaminated soil than the single TX100 solution. 4. Conclusions This study focused on a new approach to the desorption of phenanthrene from an artificial contaminated natural soil by using an anionic/nonionic mixed surfactant, SDSeTX100, with the aim of improving the efficiency of surfactant remediation technology. The sorption amount of TX100 onto soil from the mixed solution was less than that of single TX100 solution and decreased as the mole fraction of SDS in solution increased. In addition, the apparent solubilities of phenanthrene in mixed surfactant appeared to be greater than that in single TX100 solution and were positively related to the mole fraction of SDS in mixed surfactant solution. Both resulted in the distribution of phenanthrene in soilewater system with TX100 being reduced with the presence of SDS and the aqueous phase concentration of phenanthrene appeared to be positively related to the mole fraction of SDS in solution. Thus, the desorption percentage of phenanthrene from the contaminated soil with SDSeTX100 mixed solution was greater than that with single TX100 solution and appeared to be positively related to the mole fraction of SDS in solution. Then, SDSeTX100 mixed solution was more effective for the desorption of phenanthrene from the contaminated soil than the single TX100 solution. All the experimental results from this study indicated that the anionic/nonionic mixed surfactants may improve the performance of surfactant remediation technology by reducing the sorption of surfactants onto soils and increasing the solubilization of HOCs in single surfactants. Thus, anionic/nonionic mixed surfactants may be a better
choice for the applications of surfactant remediation technology. Acknowledgements This study was financially supported by the National Natural Science Foundation of China (40571143, 20337010, 20125719) and the National Basic Research Priorities Program of China (2003CB415004). References Almgren, M.A., Grteser, F., Thomas, J.K., 1979. Dynamic and static aspects of solubilization of neutral arenas in ionic micellar solutions. J. Am. Chem. Soc. 101, 279e291. Bury, S.J., Miller, C.A., 1993. Effect of micellar solubilization on biodegradation rates of hydrocarbons. Environ. Sci. Technol. 27, 104e110. Chiou, C.T., Peters, L.J., Freed, V.H., 1979. A physical concept of soil-water equilibria for nonionic organic compounds. Science 206, 831. Deitsch, J.J., Smith, J.A., 1995. Effect of Triton X-100 on the rate of trichloroethene desorption from soil to water. Environ. Sci. Technol. 29, 1069e1080. Diallo, M.S., Abriola, L.M., Weber, W.J., 1994. Solubilization of nonaqueous phase liquid hydrocarbons in micellar solutions of dodecyl alcohol ethoxylates. Environ. Sci. Technol. 28, 1829e1837. Edwards, D.A., Luithy, R.G., Liu, Z., 1991. Solubilization of polycyclic aromatic hydrocarbons in micellar nonionic surfactant solutions. Environ. Sci. Technol. 25, 127e133. Edwards, D.A., Adeel, Z., Luthy, R.G., 1994. Distribution of nonionic surfactant and phenanthrene in a sediment/aqueous system. Environ. Sci. Technol. 28, 1550e1560. Guha, S., Jaffe, P.R., 1996. Bioavailability of hydrophobic compounds partitioned into the micellar phase of nonionic surfactants. Environ. Sci. Technol. 30, 1382e1391. Harwell, J.H., Sabatini, D.A., Knox, R.C., 1999. Surfactants for ground water remediation. Colloids Surf. A: Phys. Eng. Asp. 151, 255e268. Holland, P.M., Rubingh, D.N., 1983. Nonideal multicomponent mixed micelle model. J. Phys. Chem. 87, 1984e1990. Holland, P.M., Rubingh, D.N., 1992. Mixed Surfactant Systems. American Chemical Society, Washington, DC. Jafvert, C.T., Heath, J.K., 1991. Sediment- and saturated-soil-associated reactions involving an anionic surfactant (dodecylsulfate). 1. Precipitation and micelle formation. Environ. Sci. Technol. 25, 1031e1038. Jafvert, C.T., Van Hoof, P.L., Heath, J., 1994. Solubilization of non-polar compounds by non-ionic surfactant micelles. Water Res. 28, 1009e1017. Johnson, J.C., Sun, S., Jaffe, P., 1999. Surfactant enhanced perchloroethylene dissolution in porous media: the effect on mass transfer rate coefficients. Environ. Sci. Technol. 35, 1286e1292. Karickhoff, S.W., Brown, D.S., Scott, T.A., 1979. Sorption of hydrophobic pollutants on natural sediments. Water Res. 13, 241e248. Kile, D.E., Chiou, C.T., 1989. Water solubility enhancements of DDT and trichlorobenzene by some surfactants below and above the critical micelle concentration. Environ. Sci. Technol. 23, 832e838. Ko, S.-O., Schlautman, M.A., Carraway, E.R., 1998. Partitioning of hydrophobic organic compounds to sorbed surfactants. 1. Experimental studies. Environ. Sci. Technol. 32, 2769e2775. Liu, Z., Edwards, D.A., Luthy, R.G., 1992. Sorption of non-ionic surfactants onto soil. Water Res. 26, 1337e1345. Paria, S., Khilar, K.C., 2004. A review on experimental studies of surfactant sorption at the hydrophilic solid-water interface. Adv. Colloid Interface Sci. 110, 75e95. Penfold, J., Staples, E., Tucker, I., Thomas, R.K., 2002. Sorption of mixed anionic and nonionic surfactants at the hydrophilic silicon surface. Langmuir 18, 5755e5760.
W. Zhou, L. Zhu / Environmental Pollution 147 (2007) 350e357 Pennell, K.D., Adinolfi, A.M., Abriola, L.M., Diallo, M.S., 1997. Solubilization of dodecane, tetrachloroethylzene in micellar solutions of ethoxylated nonionic surfactants. Environ. Sci. Technol. 31, 1382e1389. Rosen, M.J., 2004. Surfactants and Interfacial Phenomena, third ed. John Wiley & Sons, Hoboken. Sun, S., Inskeep, W.P., 1995. Sorption of nonionic organic compounds in soile water systems containing a micelle-forming surfactant. Environ. Sci. Technol. 29, 903e913. Thibaut, A., Misselyn-Bauduin, A.M., Grandjean, J., Broze, G., Je´roˆme, R., 2000. Sorption of an aqueous mixture of surfactants on silica. Langmuir 16, 9192e9198. Tsomides, H.J., Hughes, J.B., Thomas, J.M., Ward, C.H., 1995. Effect of surfactant addition on phenanthrene biodegradation in sediments. Environ. Toxicol. Chem. 14, 953e959.
357
West, C.C., Harwell, J.H., 1992. Surfactants and subsurface remediation. Environ. Sci. Technol. 26, 2324e2340. Yaws, C.L., 1999. Chemical Properties Handbook. McGraw-Hill, Beijing. Yeom, I.T., Ghosh, M.M., Cox, C.D., 1996. Kinetic aspects of surfactant solubilization of soil-bound polycyclic aromatic hydrocarbons. Environ. Sci. Technol. 30, 1589e1595. Zhou, W., Zhu, L., 2005. Solubilization of polycyclic aromatic hydrocarbons by anionic-nonionic mixed surfactant. Colloids Surf. A 255, 145e152. Zhu, L., Chiou, C.T., 2001. Water solubility enhancements of pyrene by single and mixed surfactant solutions. J. Environ. Sci. 13, 491e496. Zhu, L., Feng, S., 2003. Synergistic solubilization of polycyclic aromatic hydrocarbons by mixed anionic-nonionic surfactants. Chemosphere 53, 459e467.