Equilibrium partitioning of a non-ionic surfactant and pentachlorophenol between water and a non-aqueous phase liquid

ARTICLE IN PRESS

Water Research 37 (2003) 3412–3420

Equilibrium partitioning of a non-ionic surfactant and pentachlorophenol between water and a non-aqueous phase liquid Sung-Kil Parka, Angela R. Bielefeldtb,* a

POSCO, Department of Environment and Energy, Goedong-dong, Nam-gu, Pohang, Gyeongbuk 790-785, Pohang P.O. Box 36, South Korea b Department of Civil, Environmental and Architectural Engineering, University of Colorado, Boulder, Campus Box 428, Boulder, CO 80309-0428, USA Received 18 April 2002; accepted 10 April 2003

Abstract The partitioning of the non-ionic surfactant Tergitol NP-10 (TNP10) and pentachlorophenol (PCP) into a mineral oil light non-aqueous phase liquid (NAPL) were quantified in batch tests. Due to the ionizable nature of PCP, the effects of pH and ionic strength (m) on the equilibrium partitioning were evaluated. NAPL:water partition coefficients (Kn:w ) of TNP10 ranged from 3 to 7 Lwater/LNAPL. Enhanced PCP dissolution into water from the NAPL was achieved at aqueous TNP10 concentrations X200 mg/L. Surfactant addition of 1200 mg/L TNP10 increased the aqueous PCP concentrations by 14-fold at pH 5 versus 2 to 3-fold at pH 7 as compared to PCP aqueous solubility. The more significant response at the lower pH is likely due to the greater hydrophobicity of PCP at the lower pH, which is approaching PCP’s pKa of 4.7. Higher ionic strength (m 0.11 versus 0.001 M) increased Kn:w of PCP by 10–33% without surfactant, compared to a more than 150% increase with a dose of 4000 mg/L TNP10. This work contributes information relevant to the application of surfactants to remediate sites contaminated with NAPLs. r 2003 Elsevier Science Ltd. All rights reserved. Keywords: Non-ionic surfactant; Pentachlorophenol; NAPL; Partitioning; pH

1. Introduction A number of locations are contaminated with toxic organic compounds present as non-aqueous phase liquids (NAPLs), which are particularly challenging to remediate. These NAPLs may be composed of a single compound, such as trichloroethene (TCE), or a mixture of compounds, such as gasoline and coal tar. In some cases, the bulk of the mixed NAPL may be relatively non-toxic and have low water solubility. This is true in gasoline where BTEX (o5% initial NAPL by mass) is *Corresponding author. Tel.: +1-303-492-8433; fax: +1303-492-7317. E-mail address: [email protected] (A.R. Bielefeldt).

toxic while the remainder of the alkanes, etc. are fairly non-toxic. Therefore, cases may occur where selectively removing higher solubility and/or toxic compounds from mixed NAPLs rather than the entire NAPL can achieve acceptable site clean-up. The remediation of organic contaminants (OCs) associated with NAPLs can be enhanced by moving them into the aqueous phase. Surfactants have been shown to enhance the water solubility of pesticides, alkanes, chlorinated solvents, polychlorinated biphenyls, and polycyclic aromatic hydrocarbons [1,2]. Surfactants are amphiphilic chemicals with polar and non-polar regions, and can be classified based on the net charge of the hydrophilic head. Generally, cationic and anionic surfactants are more toxic to bacteria than non-ionic surfactants [3]. Alkylphenol ethoxylate non-ionic

0043-1354/03/$ - see front matter r 2003 Elsevier Science Ltd. All rights reserved. doi:10.1016/S0043-1354(03)00237-9

ARTICLE IN PRESS S.-K. Park, A.R. Bielefeldt / Water Research 37 (2003) 3412–3420

surfactants have been used for remediation due to their relatively high solubilization capacity [4]. Since emulsification of NAPLs is not generally desired due to the risk of spreading contamination, surfactants with a hydrophilic/lipophilic balance (HLB) of 12–18 are generally used in order to enhance solubilization. At bulk aqueous solution surfactant concentrations greater than a specific threshold value, known as the critical micellar concentration (CMC), the hydrophobic moieties of surfactant molecules are attracted to each other and create clusters with hydrophobic interiors called micelles. The CMC is a function of surfactant structure, solution pH, temperature, ionic strength (m), and the presence of co-solvents [4]. The primary mechanism for enhanced aqueous solubility is the partitioning of OCs into surfactant micelles, a process referred to as solubilization [4]. This solubility-enhancing effect can increase the rate and extent of dissolution of subsurface NAPLs. Zimmerman et al. [5] showed that a non-ionic surfactant could enhance the solubility of TCE by approximately 5 times. The groundwater with micelle-solubilized OCs can be pumped out and treated in an ex situ reactor. Alternatively, the OCs may be more bioavailable for bacterial degradation. Relatively high surfactant concentrations (2.5–8% by mass) have generally been applied for subsurface remediation [6,7]. This is important because a significant fraction of the total non-ionic surfactant mass (as much as 10% by weight [8]) may partition into NAPLs, reducing the effectiveness of the applied dose for achieving enhanced contaminant solubilization [9,5]. However, higher surfactant concentrations are not always beneficial due to potential colloid mobilization [10], clay swelling that reduces aquifer permeability [11], and macroemulsion formation [8]. Therefore, it is important to understand surfactant interactions with NAPLs in order to optimize surfactant concentrations used for remediation. Pentachlorophenol (PCP), a representative OC, is a widespread environmental contaminant due to its use for wood preservation and as a pesticide [12]. For wood preserving, PCP was generally formulated as a 5% solution in petroleum solvents [13]. Therefore, many sites contaminated with PCP also contain a significant amount of NAPL [14,15] with as much as 9000– 13,200 mg
PCP/kg
soil [16,17]. Because PCP is a weak acid (approximate pKa 4.7% [18]), it is generally present in the environment as both phenol and the phenolate anion. Other properties of PCP are summarized in Table 1. Because the water solubility and octanol water distribution coefficient (Kow) of PCP vary with pH and ionic strength [19,20], these parameters will likely effect PCP partitioning in NAPL:surfactant systems. Boyd and Sun [14] found that the NAPL:water partition coefficient of PCP (Kn:w;PCP ) to the oil and grease in contaminated soils was about 10 times more than the

3413

distribution coefficient to the natural soil organic matter (Kom ), but water chemistry effects on these values were not investigated. The objective of this study was to demonstrate the effect of non-ionic surfactant concentration, pH, and ionic strength on PCP partitioning between water and NAPL. Of various non-ionic surfactants previously tested, Tergitol NP-10 (TNP10) was the least inhibitory to PCP biodegradation by a pure culture of aerobic bacteria [21]. It was hypothesized that TNP10 would enhance PCP dissolution from NAPLs by micellar solubilization and that this effect would be more significant at low pH where PCP is predominantly protonated and thus more hydrophobic. In addition, higher solution ionic strength should decrease PCP dissolution from NAPLs due to competitive ion effects with pentachlorophenolate when the pH is above the pKa. Because dosed surfactant is only ‘‘active’’ to enhance solubilization in the aqueous phase, surfactant partitioning into the NAPL was also quantified. However, it was hypothesized that the non-ionic surfactant partitioning to the NAPL would be insensitive to varying pH and ionic strength of the aqueous phase due to the neutral characteristics of the surfactant and mineral oil.

2. Materials and methods 2.1. Reagents and chemicals TNP10 (99% pure, Aldrich Chemical Company) is an alkylphenol ethoxylate non-ionic surfactant with an average molecular formula of C9H19(C6H4)O(CH2CH2O)10.5H and an average molecular weight of 683 [22]. PCP (95% pure, Fisher Scientific) was used as received. The NAPL used in all of the experiments was heavy mineral oil (Fisher Scientific) with a density of 0.90 g/cm3 and an average molecular weight of 350 [23]. Aqueous solution pH was varied using potassium phosphate dibasic (K2HPO4) and monobasic (KH2PO4) in nanofiltered deionized water (DI). 2.2. Determination of surfactant CMC The possible NAPL influence on the CMC of TNP10 was studied. NAPL (3 mL) and DI water (30 mL) containing 10–160 mg/L TNP10 was combined in 40mL glass vials with Teflon-lined septa. The samples were allowed to equilibrate at lab temperature (2572 C) on a rotary shaker table (120 rpm) in the dark for a minimum of 5–7 days. Vials were centrifuged on a GS-6 Centrifuge for 30 min at 14,000 rpm to separate the NAPL from the aqueous solution. Aqueous phase TNP10 concentrations were measured. Solution surface tension was measured using a Fisher Scientific Model 20 Surface

ARTICLE IN PRESS S.-K. Park, A.R. Bielefeldt / Water Research 37 (2003) 3412–3420

3414

Table 1 Literature reported properties of TNP10 and PCP Compound

Molecular weight

Chemical formula

pKa

Water solubility @ 25 Ca, (mg/L) @ pH, mðM)

Log Kow

PCP

266.34

C6Cl5OH

4.6–5.3b 4.570.3a

44 @ 5.8, 0.0003 260 @ 6.6, 0.002 1275 @ 7.2, 0.01

57 @ 5.6, 0.2 273 @ 6.3, 0.2 2789@ 7.2, 0.2

5.1 n; 2.6–2.7 ic B4c; 3.6 @ pH 6d 2.3 @ pH 6.9d

CMC (mg/L)

HLB

30f, 34–37e, 82g

13.5 (calculated)

TNP10

683e

C9H19(C6H4)O (CH2CH2O)10.5He

a

[20]. [18]. c [43]. d [19] (m 0.1 M; n=phenol, i=phenolate). e [26]. f [21]. g [27]. b

Tensiometer. Surface tension was also measured with 3– 300 mg/L TNP10 in DI water. The CMC is the surfactant concentration at which the surface tension no longer decreases with increasing surfactant concentration. 2.3. Batch tests Batch experiments were conducted in glass bottles (40-mL or 100-mL) with Teflon-lined septa. To determine if surfactant addition would decrease the amount of PCP partitioning into mineral oil as a representative NAPL, 30 mL DI water containing 8.6 mg/L PCP and 0–1000 mg/L TNP10 was added to 2 mL of clean mineral oil. The effect of surfactant (0–1600 mg/L TNP10) and pH (5–7.5) on PCP partitioning was studied in systems with a NAPL:water volume ratio of 1:10 (3 or 9 mL oil: 30 or 90 mL water, respectively). The NAPL initially contained 0–7200 mg/L PCP. The pH was measured at the beginning and end of each experiment, and varied less than 0.1 unit. Batch systems were incubated at lab temperature (2572 C) on a rotary shaker table (120 rpm) in the dark to avoid PCP losses via photolysis. After selected times ranging from 5 h to 100 days, vials were centrifuged (GS-6 Centrifuge; 30 min at 14,000 rpm) and 2-mL of the aqueous solution were sampled for analysis of TNP10 and PCP concentrations. Controls without NAPL were run to account for glass sorption and other potential losses of TNP10 and PCP. By correcting for sampling effects and control losses, the TNP10 and PCP concentrations in the NAPL over time were calculated. Equilibrium was determined when the concentrations in samples from multiple dates did not vary. Equilibrium was generally reached within 1–3 days.

2.4. Analytic methods The quantification of TNP10 concentrations in the centrifuged supernatants was performed with a SpectraPhysics SP8880 high performance liquid chromatography (HPLC) system with UV detection (Spectra 100 UV-Vis absorbance detector) at 254 nm. A HewlettPackard Zorbax SB-C18 column (250 mm L  4.6 mm ID) in reverse phase was used with a gradient method of 30% phosphoric acid buffer and 70% acetonitrile at a flow rate of 1.0 mL/min (typical operating pressure B2000 psi). Aqueous phase TNP10 concentrations were determined by comparison to external standard curves. The minimum quantification limit for TNP10 was 15 mg/L based on the lowest standard concentration detected. Aqueous phase PCP concentrations (including both free PCP in solution and micellized PCP) were determined using a Hewlett Packard model 5890A gas chromatograph equipped with an electron-capture detector (GC/ECD), autosampler, and a J&W Scientific DB-5 column (30 m L  0.53 mm ID; 3.0 mM film thickness). Operating conditions include: nitrogen carrier gas at 3 mL/min, 1 mL injection volume, injector temperature 250 C, and detector temperature 380 C. The column was isothermal at 175 C for 5 min, ramped at 5 C/min to 225 C, and isothermal at 225 C for 5 min. Aqueous samples (1 mL) were acetylated and extracted using the method of Perkins et al. [24]. PCP concentrations were determined using an internal standard (2bromophenol) and a six-point external standard curve. The minimum quantification limit for PCP was 0.01 mg/ L. Alternatively, PCP was measured using the same HPLC method that was used for TNP10 quantification. The PCP detection limit on the HPLC was approximately 0.2 mg/L. Solution pH was measured with an

ARTICLE IN PRESS S.-K. Park, A.R. Bielefeldt / Water Research 37 (2003) 3412–3420

Orion Model 520A pH meter equipped with a Ross Sure-Flow electrode.

3. Results and discussion 3.1. Effect of NAPL on the CMC of surfactant The measured surface tension as a function of aqueous TNP10 concentration ([TNP10]aq) is shown in Fig. 1. In the system containing only DI water (pH 5.75; mB0 M), the CMC is 40 mg/L. In batch systems containing dissolved mineral oil, pH 6.75, and m 0.112 M, the CMC was 50 mg/L. Literature CMC values for TNP10 range from 30 to 82 mg/L [25,21,26,27]. The CMC of surfactants can vary depending on temperature, pH, ionic strength and concentration of co-solvents [4]. However, experiments with TNP10 showed that pH of 5–7 and ionic strength of 0–0.1 did not change the CMC (data not shown). Since TNP10 is a molecularly heterogeneous surfactant, it may exhibit preferential partitioning of more hydrophobic components to the NAPL, leaving more hydrophilic molecules in the aqueous phase [9] and contributing to the increase in the TNP10 CMC in the oil-water system. Ysambertt et al. [28] observed that as the polarity of the organic phase increased, the selective partitioning of hydrophobic surfactant components decreased. In these experiments micelles likely formed at [TNP10]aq exceeding 30– 50 mg/L. The presence of dissolved mineral oil at equilibrium with the water (total organic carbon of 0.4 mg/L) did not change the measured surface tension of the water (64– 65 dynes/cm). However, the combination of TNP10 and the mineral oil resulted in a higher minimum surface tension that was measured at the same aqueous TNP10 concentrations. This may be due to the fact that dissolved mineral oil interacted with the surfactant monomers to change the apparent surface tension.

Surface tension, dynes/cm

55 Water only system Clean oil plus water system

50

literature CMC range

45 40 35 30 25 1

10

100

1000

Aqueous TNP10, mg/L Fig. 1. Measured aqueous phase surface tension as a function of [TNP10]aq for systems with and without NAPL.

3415

3.2. Surfactant partitioning into NAPL The equilibrium partitioning of TNP10 into the mineral oil NAPL containing 0, 1800, or 4500 mg/L PCP was measured in batch tests at pH ranging from 6 to 7.5. As shown in Fig. 2, below 700 mg/L [TNP10]aq all of the data follows a similar linear trend with: TNP10 concentration in oil (C0)=5.37 [TNP10]aq [r2 ¼ 0:93]. Including all the data at [TNP10]aq up to 3353 mg/L the 2 Freundlich isotherm (C0 ¼ 9:18 [TNP]0.87 aq ; r ¼ 0:89) provides a better data fit than either a linear (r2 ¼ 0:77) or Langmuir (r2 ¼ 0:74) isotherm. Because TNP10 is molecularly heterogeneous, it is also likely that there was selective partitioning of the more hydrophobic surfactant components into the NAPL. Although all the data followed a similar trend, differences due to aqueous pH and PCP concentration in the oil are evident. Kn:w was calculated using linear fits to the data forced through zero that produced rsq >0.90; values are summarized in Table 2. Statistically significant differences in the Kn:w values at a 95% confidence were determined using a student’s t-test [29]. With 1800 and 4500 mg/L PCP in the oil, the Kn:w was significantly higher at a lower pH for [TNP10]aq below 1025 and 311 mg/L, respectively. At a lower pH, a greater amount of the PCP would be in the more hydrophobic unionized form, such that higher Kn:w;TNP10 is due to interaction with PCP. In general, pH would be expected to have minimal effects on non-ionic surfactant partitioning into most NAPLs. The surfactant interaction with PCP is also indicated by the higher Kn:w;TNP10 for the 0.2% PCP versus clean oil at pH 6.75. However, at a single dose of 4000 mg/L TNP10 over a range of 1800, 2700, 4500, and 7200 mg/L PCP in oil, the [TNP10]aq was not significantly different at the different PCP levels (data not shown). The reason that a PCP effect was not observed at high TNP10 concentrations is likely due to the saturated of the NAPL with surfactant, as is evident in Langmuir-type distribution. Ionic strength had a minimal effect on TNP10 partitioning into mineral oil in the presence of PCP. Kn:w;TNP10 for high m (0.112 M) at pH 6.75 (1800 mg PCP/Loil) of 6.94 was 14% higher than Kn:w;TNP10 of 3.03–5.99 at low m (0.003–0.006 M) over pH 6–7 (4500 mg PCP/Loil). ANOVA analysis of the data using SYSTAT confirmed that ionic strength effects were significant at a 90% confidence. In tests at a 4000 mg/L TNP10 dose the partitioning into mineral oil was similar at m 0.001 versus 0.112 M, with equilibrated [TNP10]aq not significantly different. Over the ionic strength range of typical groundwater of 0.001–0.05 M [30], significant changes in non-ionic surfactant partitioning into NAPLs is not expected. The surfactant Kn:w values measured in this work of 3–7 into mineral oil are well below the reported Kn:w values of 10–90 for ethoxylated alcohol surfactants into chlorinated NAPLs [9,31].

ARTICLE IN PRESS S.-K. Park, A.R. Bielefeldt / Water Research 37 (2003) 3412–3420

3416

7000 % PCP in oil; pH; ionic strength,M 0; 6.75; 0.112 0.2; 6.75; 0.112 0.2; 7.5; 0.116 0.5; 6; 0.006 0.5; 7; 0.003

Equilibrium NAPL phase TNP10, mg TNP10/L oil

6000 5000

0; 6.75; 0.112 0.2; 6.75; 0.112

2000

4000

1500 3000

1000 2000

500

1000

Co=5.56[T NP10]aq-69.80 R2 = 0.93

0 0

100

200

300

0 0

500

1000

1500

2000

2500

3000

3500

4000

Equilibrium aqueous TNP10, mg/L

Fig. 2. TNP10 partitioning into clean or PCP-containing NAPL at different pH and PCP fractions. Inset shows the same parameters for clean and 0.2% PCP fraction.

Table 2 NAPL:water distribution coefficients for TNP10 at varying PCP concentration, pH, and ionic strength (distribution coefficients represent linear fits to data over the aqueous TNP10 concentrations shown) Initial mg PCP/Loil

pH

m (M)

[TNP10]aq (mg/L)

Kn:w;TNP10

rsq

0 1800 1800

6.75 6.75 7.5

0.11 0.11 0.11

4500 4500

6.0 7.0

0.006 0.003

34–281 28–682 46–655 46–1025 113–260 142–1208

6.94 5.75 5.45 4.69 5.99 3.03

0.96 0.95 0.96 0.92 0.97 0.97

3.3. Effect of surfactant addition on PCP partitioning into oil

Table 3 Oil–water partitioning coefficient (Kn:w ) of PCP at pH 5.75 (mB0)

Batch tests were conducted to determine if surfactant addition would decrease the amount of PCP partitioning into the mineral oil NAPL. Calculated Kn:w;PCP values (with appropriate corrections for sampling effects) are shown in Table 3. With higher TNP10 doses, the PCP partition coefficients decreased from 15.0 to 2.5. The partitioning of PCP into the oil will be influenced by pH due to the relative concentrations of phenol and phenolate. Fall et al. [32] presented a non-linear model to predicting partitioning values based on pH and nonionic surfactant (Triton X-100) dose: 1 f¼ ; ð1Þ ð1 þ 10ðpHpKa Þ Þ

Initial TNP10 conc (mg/L)

Kn:w; PCP ¼ fKn:wn ea surf þ ð1  fÞ Kn:wi eb surf ;

ð2Þ

where Kn:w;PCP is the NAPL:water equilibrium constant for PCP (Lwater =Loil ); f is the fraction of non-ionized PCP; Kn:wn and Kn:wi are the partition constants for

0 0 100 500 1000

Kn:w of PCP (Lwater =Loil ) Measureda

Pred using pKa 4.75b

Pred using pKa 4.5b

15.0 11.7 9.9 2.8 2.5

59.4 59.4 9.9 2.6 2.6

36.0 36.0 9.9 2.6 2.6

a

Measured average values on day 4. Predicted values from Eq. (2) with surf=aqueous surfactant concentration [32]. b

neutral and ionized PCP, respectively; surf is the dose of surfactant, % (v/v); and a and b are empirical constants. The model was tested by Fall et al. [32] at surfactant concentrations of 1500 and 5000 mg/L, which are well above the CMC (43 mg/L [33] ) and the 100–1000 mg/L

ARTICLE IN PRESS S.-K. Park, A.R. Bielefeldt / Water Research 37 (2003) 3412–3420

used in this work. In addition, NAPL-surfactant interaction was not considered. Since a significant fraction of the TNP10 dose (7–48%) dissolved into the oil, the aqueous surfactant concentration might be more appropriate in Eq. (2) than surfactant dose. Kn:w values of PCP at pH 2.5 and 11 (mB0:002 M) were measured as 625 and 2.85 (log Kn:w 2.80 and 0.45), respectively, to represent neutral and ionized PCP (data not shown). For comparison, partitioning values for PCP in soils were approximately 3.90 for log Kocn and 2.85 for log Kocn where Koc represents the organic carbon partition coefficient [32]. Most reported values of log Kow for PCP at unspecified pH lie between 3.69 and 5.86 [18]. Kn:w is significantly different due to the characteristics (i.e., polarity) of mineral oil compared to the organic matter in soils or octanol [34]. Using the data set from DI water at pH 5.75, the exponents a and b (calculated using [TNP10]aq instead of dose) were estimated to be 571 and 0.0351, respectively. Table 3 shows that the Kn:wp values predicted by Eq. (2) using these coefficients (pKa of PCP=4.75) is in reasonable agreement with the measured data in the systems containing surfactant. Poor agreement at 0 surfactant dose may be due to using the Kn:wi and Kn:wn measured in ionic solutions versus DI water. As the literature values for the pKa of PCP vary significantly, the data was also modeled using the Wightman and Fein [20] pKa of 4.5. This resulted in improved prediction at 0 surfactant dose and gave a and b of 426 and 0.615, respectively (Table 3). 3.4. Effect of surfactant addition on PCP partitioning out of NAPL The partitioning of PCP out of the mineral oil NAPL into aqueous solution containing 0–4000 mg/L surfactant was measured. Initial conditions included PCP concentrations in the oil ranging from 1800 to 7200 mg PCP/L oil, pH of 5.75–7.5, and m from B0 to 0.12 M. In the absence of surfactant, Kn:w;PCP values ranged from 5 to 66 (Table 4). As aqueous phase pH increased from 6.75 to 7.5 at similar ionic strength (mB0:11 M), the

3417

Kn:w, PCP decreased by a factor of 8, as expected due to the higher solubility of the phenolate anion. Higher ionic strength (m 0.112 versus 0.001 M) decreased PCP partitioning out of the oil resulting in 5–30% higher Kn:w;PCP values. A higher equilibrium aqueous concentration of PCP ([PCP]aq) with a higher initial PCP fraction in the oil is expected based on Raoult’s law. The liquid PCP solubility at different pH levels was derived from literature data [18,35,36]; however, the effects of temperature (literature 20–27 C) and ionic strength were not considered. Using the calculated mole fraction and an assumed activity coefficient of 1 the predicted [PCP]aq values were 25–27% lower than measured concentrations, indicating that PCP and mineral oil may be a non-ideal mixture. Alternatively, co-solvent effects of the mineral oil components on PCP solubility may be significant. Fig. 3 shows that increasing [TNP]aq increased the [PCP]aq that dissolved out of NAPL initially containing 1800 mg
PCP/Loil (closed symbols). Enhanced solubilization of PCP was measured at 190–700 mg/L [TNP10]aq. Due the TNP10 partitioning into the NAPL, this was an initial dose of X300 mg/L TNP10 at both pH levels (open symbols). The result shows that the [TNP10]aq required for significant PCP micellar solubilization in the presence of NAPL was about 4 times larger than the CMC. At pH 6.75 and 7.5 with surfactant doses X300 mg/L, X50 and 84% of the PCP initially in the oil had partitioned into the bulk aqueous phase compared to B32 and 69% in water alone system, respectively (open symbols). At pH 7.5 and 1000 mg/L TNP10 dose (650 mg/L [TNP10]aq) almost 100% of the PCP mass originally present in the mineral oil was measured in the aqueous phase. 3.5. Effect of pH and ionic strength on PCP partitioning The effect of pH on PCP partitioning out of the NAPL in the presence of different quantities of TNP10 was studied in batch tests. As shown in Fig. 3, [PCP]aq was higher for all surfactant doses at pH 7.5 versus 6.75

Table 4 Partitioning coefficients for PCP in mineral oil as a function of solution pH and ionic strength (no surfactant present) Initial PCP in oil (% in w/w) a

0

If all into water (mg PCP/Lwater )

pH

Ionic strength, m (M)

Kn:w;PCP

8.5

5.75 6.75 7.5 6.75 6.7 6.75 6.75

B0 0.001, 0.116 0.001, 0.006, 0.001, 0.001,

12–15 30, 39 5 41, 49 46, 22 55, 58 58, 66

0.2b

180

0.3b 0.5b 0.8b

270 450 720

a b

Measured average values on day 4. Predicated values from Eq. (2) with surf=aqueous surfactant concentration [32].

0.112 0.112 0.003 0.112 0.112

ARTICLE IN PRESS

Equilibrium aqueous PCP, mg/L

80 150 60 100 40 50 [PCP] aq vs . [PCP] aq ; % vs. Dose ;

0 0

200

400

600

p H 6 .75 p H 7.5 p H 6 .75 p H 7.5

800

20

% partitioning out of PCP from NAPL

100

200

0 1000

Equilibrium aqueous PCP, mg/L

S.-K. Park, A.R. Bielefeldt / Water Research 37 (2003) 3412–3420

3418

400

pH; ionic strength,M 5; 0.04 6; 0.006 7; 0.003

350 300 250 200 150 100 50 0 0

500 1000 1500 Equilibrium aqueous TNP10, mg/L

Fig. 4. Effect of pH on [PCP]aq from 0.5% PCP initially present in oil with varying amounts of [TNP10]aq.

Equilibrium aqueous TNP10 or dose, mg/L

(high mB0:11 M). However, the increase in [PCP]aq due to surfactant addition was greater at pH 6.75. Due to the nearly complete PCP solubilization achieved, another experiment with 4500 mg PCP/Loil initially in the NAPL and pH 5, 6, and 7 was conducted. If 100% of the PCP partitioned out of the NAPL [PCP]aq would be B450 mg/L. Fig. 4 shows that at low ionic strength (m 0.003–0.04 M), pH had a significant effect on PCP partitioning. The slope of [PCP]aq versus [TNP10]aq is similar for all pH levels, indicating that micellar enhanced solubilization itself (mass
PCP/ mass
surfactant-micelle) was independent of pH. At pH 5 1,200 mg/L [TNP]aq was needed to cause the same [PCP]aq as increasing the pH to 7. This indicates that pH changes may be as effective as surfactant addition for PCP remediation. However, at pH X6 where X94.7% of PCP is already present as phenolate, pH changes should have only minimal benefits toward increasing PCP solubility. This also indicates that surfactant addition may be most beneficial for PCP remediation at low pH sites. Another important factor that affected PCP partitioning was ionic strength. Tests were conducted at low (m 0.0012–0.04 M) and high (m 0.112–0.116 M) ionic strengths using a K2HPO4–KH2PO4 matrix at pH 6.75. As shown in Fig. 5, without surfactant as m increased from 0.001 to 0.112 M partitioning increased 1.1–1.3 times at all levels of PCP in oil (1800–7200 mg/L). For the m increase of interest, Karickhoff [37] predicted that

100

T NP10 dose, mg/L; ionic strength, M 0; 0.112 0; 0.001 4,000; 0.112 4,000; 0.001

80 Kn:w,PCP, Lwater/Loil

Fig. 3. [PCP]aq from 0.2% PCP in oil with varying [TNP10]aq at pH 6.75 (closed circles; left ordinate axis) and 7.5 (closed squares; left ordinate axis). Values shown are means of three independently processed samples with error bars corresponding to the standard deviation. Also, equilibrium % PCP partitioning out of NAPL with varying surfactant dose at pH 6.75 (open circles; right ordinate axis) and 7.5 (open squares; right ordinate axis).

60 40 20 0 0

0.2 0.4 0.6 0.8 Initial % PCP in oil by mass

1

Fig. 5. PCP partitioning coefficient Kn:w with varying amounts of initial PCP in oil at low and high ionic strength (pH 6.75) and at 0 and 4000 mg/L TNP10 dose.

the soil sorption coefficient should increase by no more than a factor of 1.2. Lee et al. [38] reported that at pH >7 PCP soil sorption increased by a factor of B6 over the m range of 0.01–1.4 M. Pavlostathis and Jaglal [39] found no effect of solution ionic strength on the soil desorption of TCE below m 0.001 M. For more hydrophobic compounds, such as PAHs (i.e., benzo[a] pyrene), the ionic strength of the aqueous phase had a strong effect on desorption from soil [40]. Partitioning of the neutral species can be altered by the effects of m on aqueous activity [41], termed the ‘‘salting out’’ effect [42]. As shown in Table 5, less [TNP10]aq was required to achieve enhanced PCP solubilization at lower ionic strength (low m 2.3–2.9 times the versus high m 3.7–4.1 times the CMC). In the presence of B3400 mg/L [TNP10]aq (Fig. 5), a greater decrease in Kn:w;PCP compared to the surfactant-free value was achieved at low m: Ionic strength has been shown to be of

ARTICLE IN PRESS S.-K. Park, A.R. Bielefeldt / Water Research 37 (2003) 3412–3420 Table 5 Effect of ionic strength on the amount of TNP10 required to achieve enhanced PCP solubilization out of the NAPL Ionic strength, m (M)

pH

[TNP10]aq for enhanced PCP solubilization/CMCa

0.003 0.006 0.11 0.12

6 7 6.75 7.5

2.3 2.9 3.7 4.1

a

CMC=50 mg/L [TNP10]aq.

importance for the partitioning of PCP from NAPL into surfactant micelles, with TNP10 being more effective at low ionic strength.

4. Conclusions The work presented here demonstrates that the partitioning of non-ionic surfactants into NAPLs can be significant, representing a ‘‘loss’’ of aqueous surfactant injected for remediation. Aqueous surfactant concentrations at X4 times the CMC significantly increased the partitioning of PCP into the aqueous phase from NAPL, with as much as >99% mass extracted. Increased pH over 0.75–1 pH unit decreased Kn:w;TNP10 by 30–50%. Increased pH also increased PCP partitioning into the aqueous phase. Ionic strength significantly affected the partitioning behavior of PCP between water and NAPL, with lower desorption of PCP from the NAPL at higher ionic strength regardless of surfactant. Given the complexity of environmental applications of surfactants, a detailed understanding of the behavior of surfactants in the subsurface as well as operational factors such as pH and ionic strength is essential for the proper implementation of surfactant remediation technologies. The work presented indicates that in some cases, pH may be more easily or cost efficiently manipulated than surfactant addition for PCP remediation. The extent to which surfactants influence PCP distribution in environmental systems will depend on the combined effects of PCP and surfactant sorption reactions with the aquifer solids. Such investigations are underway and will be addressed in a future paper.

References [1] Diallo MS, Abriola LM, Weber Jr. WJ. Solubilization of nonaqueous phase liquid hydrocarbons in micellar solutions of dodecyl alcohol ethoxylates. Environ Sci Technol 1994;28(11):1829–37.

3419

[2] Deitch JJ, Smith JA. Effect of Triton X-100 on the rate of trichloroethene desorption from soil to water. Environ Sci Technol 1995;29(4):1069–80. [3] Rouse JD, Sabatini DA, Suflita JM, Harwell JH. Influence of surfactants on microbial degradation of organic compounds. Crit Rev Environ Sci Technol 1994;24(4): 325–70. [4] Rosen MJ. Surfactants and interfacial phenomena, 2nd ed.. New York, NY: Wiley; 1989. [5] Zimmerman JB, Kibbey TCG, Cowell MA, Hayes KF. Partitioning of ethoxylated nonionic surfactants into nonaqueous-phase organic liquids: influence on solubilization behavior. Environ Sci Technol 1999;33(1):169–76. [6] Brown CL, Delshad M, Dwarakanath V, Mckinney DC, Pope GA. Design of a field-scale surfactant enhanced remediation of a DNAPL contaminated aquifer. I&EC Special Symposium American Chemical Society, Birmingham, AL, September 9–12, 1996. [7] Clarke AN, Mutch Jr. RD, Wilson DJ, Oma KH. Design and implementation of pilot scale surfactant washing/ flushing technologies including surfactant reuse. Water Sci Technol 1992;26(1–2):127–35. [8] Okuda I, McBride JF, Gleyzer SN, Miller CT. Physicochemical processes affecting the removal of residual DNAPL by nonionic surfactant solutions. Environ Sci Technol 1996;30(6):1852–60. [9] Butler EC, Hayes KF. Micellar solubilization of nonaqueous phase liquid contaminants by nonionic surfactant mixtures: effects of sorption, partitioning and mixing. Water Res 1998;32(5):1345–54. [10] Abdul AS, Gibson TL, Rai DN. Laboratory studies of the flow of some organic solvents and their aqueous solutions through bentonite and kaolin clays. Ground Water 1990;25:524–33. [11] Gardner KH, Arias MS. Clay swelling and formation permeability reductions induced by a nonionic surfactant. Environ Sci Technol 2000;34(1):160–6. [12] US EPA, 1992. Contaminants and remedial options at wood-preserving sites. EPA/600/R-92/182 US EPA. Washington, DC. [13] Crosby DG. Environmental chemistry of pentachlorophenol: a special report on pentachlorophenol in the environment. In: Commission on pesticide chemistry, Department of Environmental Toxicology, University of California, Davis, CA, 1980. p. 1052–80. [14] Boyd SA, Sun S. Residual petroleum and polychlorobiphenyl oils as sorptive phase for organic contaminants in soils. Environ Sci Technol 1990;24(1):142–4. [15] Tobia RJ, Camacho JM, Augustin P, Griffiths RA, Frederick RM. Washing studies for PCP and creosote contaminated soil. J Hazardous Mater 1994;38:145–61. [16] Compeau GC, Mahaffey WD, Patras L. Full-scale bioremediation of contaminated soil and water. In: Sayler G, Fox R, Blackburn JW, Editors. Environmental biotechnology for waste treatment: environmental science research, vol. 41. New York: Plenum Press, 1991. p. 91–109. [17] Litchfield CD, Chieruzzi GO, Foster DR, Middleton DL. A biotreatment train approach to a PCP-contaminated site: in-situ bioremediation coupled with an aboveground BIFAR system using nitrate as the electron acceptor. In: Hinchee RE, Leeson A, Semprini L, Ong SK, editors.

ARTICLE IN PRESS 3420

[18]

[19]

[20]

[21]

[22]

[23]

[24]

[25]

[26]

[27]

[28]

[29]

S.-K. Park, A.R. Bielefeldt / Water Research 37 (2003) 3412–3420 Bioremediation of chlorinated and polycyclic aromatic hydrocarbon compounds, 1994. p. 239–47. Shiu WY, Ma KC, Varhanickova D, Mackay D. Chlorophenols and alkylphenols: a review and correlation of environmentally relevant properties and fate in an evaluative environment. Chemosphere 1994;29(6): 1155–224. Nowosielski BE, Fein JB. Experimental study of octanolwater partition coefficients for 2,4,6-trichlorophenol and pentachlorophenol: derivation of an empirical model of chlorophenol partitioning behavior. Appl Geochem 1998;13(7):893–904. Wightman PG, Fein JB. Experimental study of 2,4,6trichlorophenol and pentachlorophenol solubilities in aqueous solutions: derivation of a speciation-based chlorophenol solubility model. Appl Geochem 1999;14:319–31. Cort T, Bielefeldt AR. Effect of surfactants and temperature on PCP biodegradation. J Environ Eng 2000; 126(7):635–43. Weinheimer RM, Varineau PE. Polyoxyethylene alkylphenols. In: van Os NM, editor. Nonionic surfactant: Organic chemistry; surfactant science series, vol. 72. New York: Marcel Dekker, 1998. p. 39–85. Meyer E. White mineral oil, petrolatum and related products. New York: Chemical Publishing Company Inc.; 1968. Perkins PS, Komisar SJ, Puhakka JA, Ferguson JF. Effects of electron donors and inhibitors on reductive dechlorination of 2,2,6-TCP. Water Res 1994;28(10): 2101–7. Cort T. Effects of surfactant on pentachlorophenol biodegradation by Sphingomonas Chlorophenolicum sp. Strain RA2, Ph.D. Thesis, University of Colorado, Boulder, CO, 2000. Edwards DA, Liu Z, Luthy RG, Liu Z. Solubilization of polycyclic aromatic hydrocarbons in micellar nonionic surfactant solutions. Environ Sci Technol 1991;25(1): 127–33. Grimberg SJ, Nagel J, Aitken MD. Kinetics of phenanthrene dissolution into water in the presence of nonionic surfactants. Environ Sci Technol 1995;29(6):1480–7. Ysambertt F, Anon R, Salager JL. Retrograde transition in the phase behavior of surfactant-oil-water systems produced by an oil equivalent alkane carbon number scan. Colloids Surfaces A 1997;125:131–6. Zar JH. Biostatistical analysis, 4th ed.. Upper Saddle River, NJ: Prentice-Hall; 1999.

[30] Mays LW. Water resources handbook. New York, NY: McGraw-Hill Inc.; 1996. [31] Cowell MA, Kibbey TCG, Zimmerman JB, Hayes KF. Partitioning of ethoxylated nonionic surfactants in water/ NAPL systems: effects of surfactant and NAPL properties. Environ Sci Technol 2000;34(8):1583–8. [32] Fall C, Chavarie C, Chaouki J. Generalized model of pentachlorophenol distribution in amended soil–water systems. Water Environ Res 2001;73(1):110–7. [33] Guha S, Jaffe P. Bioavailability of hydrophobic compounds partitioned into the micellar phase of nonionic surfactants. Environ Sci Technol 1996;30(4): 1382–91. [34] Kishi H, Hasimoto Y. Contribution of soil constituents in adsorption of chemicals. In: Arendt F, Hinsenveld M, van den Brink WJ, editors. Contaminated soils. Kluwer Academic Publishers: Dordrecht, The Netherland; 1990. p. 331. [35] Arcand Y, Hawari J, Guiot SR. Solubility of pentachlorophenol in aqueous solutions: the pH effect. Water Res 1995;29(1):131–6. [36] Schwarzenbach RP, Gschwend PM, Imboden DM. Environmental organic chemistry. New York, NY: Wiley, 1993. p. 291–341. [37] Karickhoff SW. Organic pollutant sorption in aquatic systems. J Hydraul Eng 1984;110(6):707–35. [38] Lee SL, Roa PSC, Nkedi-Kizza P, Delfino JJ. Influence of solvent and sorbent characteristics on the distribution of pentachlorophenol in octanol, water and soil-water systems. Environ Sci Technol 1990;24(5):654–61. [39] Pavlostathis SP, Jaglal K. Desorptive behavior of trichloroethylene in contaminated soil. Environ Sci Technol 1991;25(2):274–9. . [40] Kogel-Knabner I, Totsche KU, Raber B. Desorption of polycyclic aromatic hydrocarbons from soil in the presence of dissolved organic matter: effect of solution composition and aging. J Environ Qual 2000;29(3):906–16. [41] Chiou TC, Schmedding DW. Partitioning of organic compounds in octanol-water systems. Environ Sci Technol 1982;16(1):4–9. [42] Whitehouse BG. The effects of temperature and salinity on the aqueous solubility of polynuclear aromatic hydrocarbons. Mar Chem 1984;14:319–32. [43] Jafvert CT, Westall JC, Grieder E, Schwarzenbach RP. Distribution of hydrophobic ionogenic organic compounds between octanol and water: organic acids. Environ Sci Technol 1990;24(12):1795–803.