Vol. 7, No. 2, p. 163167, June 2003
Comparison of pH and counter-ion effect in surfactant-assisted remediation
ABSTRACT: In order to determine whether pH or counter-ion is more effective in modifying surfactant effectiveness, column exper- iments were performed. Ottawa sand was selected for model soil and toluene was used as a model contaminant. DOSL (anionic sur- factant) was used for this study. The greatest recovery of toluene in column tests was 92%, which was obtained with a surfactant + NaOH. The effect of NaCl in changing effectiveness was less than that of NaOH. Much greater effectiveness was observed using sur- factant solutions containing NaOH. The effect of NaOH in chang- ing effectiveness was not due to Na+ effects, but to the OH−−−− as shown by these experimental results. The effect of counter-ion (Na+) was small, and was much less than that of pH in surfactant- assisted remediation.
Key words: pH, counter-ion, surfactant, recovery, toluene, remediation 1. INTRODUCTION
Contamination of soils and groundwater with hydropho- bic chemicals is a widespread problem. The removal of hydrophobic organic substances from contaminated soils and groundwater is difficult because of low solubilities and high interfacial tensions (Sabatini et al., 1997, Lee et al., 2001a). A common method for remediation of aquifers con- taminated with hydrophobic organic substances is pump- and-treat. This method is neither effective nor economical.
An extensive study on soil and groundwater remediation has demonstrated that surfactant (surface active agents) flushing is a viable alternative for improving the efficiency of pump-and-treat remediation (Pennell et al., 1997, Lee et al., 2001a, 2002a). These studies show that aqueous sur- factant solutions significantly enhance the removal of hydrophobic contaminants from soil and groundwater.
Surfactants have the ability to both solubilize hydropho- bic substances and to lower interfacial tensions and increase mobility of the contaminants. However, surfactant-based technologies are still ineffective on silts and clays due to their small particle size and large specific surfaces. Further, aqueous surfactant solutions are affected by environmental conditions such as pH, counter-ion conditions, temperature, and the clay type and percentage etc. Rosen (1989) and Harwell (1992) observed that micelle formation of surfac- tant is also enhanced by high pH conditions. Chang and Rosano (1984) reported that generally the higher pH, the
lower surface tension of a surfactant solution. Lee et al.
(2002b) suggested that the effect of electrolyte of target area in surface-based remediation should be considered for remediation design. The addition of an electrolyte such as NaCl to an anionic surfactant solution can both increase the micelle aggregation number (size of micelle) and decrease the CMC (Hiemenz, 1986), thereby modifying surfactant effectiveness for remediation. The addition of NaOH for pH adjustment adds two ions, Na+ and OH−, to the surfactant solution. Changes in effectiveness for remediation could be caused either by pH changes or by changes on the counter- ion Na+. More laboratory work and/or pilot scale tests are needed to understand the modifying surfactant effective- ness. In order to determine whether OH− or Na+ more effec- tive in modifying surfactant effectiveness, experiments were performed with electrolyte (NaCl, NaOH) added to the surfactant solutions.
2. MATERIALS AND METHODS 2.1. Materials
Materials used as model soil require high permeability, low cation exchange capacity, and low total organic carbon content. Ottawa sand was selected because it met these cri- teria, and because of its uniformity, simple mineralogy, and availability. It was obtained from the U.S. Silica Company (Ottawa, IL, USA). The mean grain diameter of Ottawa sand is 0.45 mm, and the specific surface area is 0.007 m2/g (Lee, 1999). Ottawa sand was washed with deionized water to remove fine materials and air dried prior to use.
Toluene (C6H5CH3) was used for the model contamina- tion. Toluene (reagent grade), a non-chlorinated aromatic hydrocarbon, is a major industrial hydrophobic organic compound and is commonly found at waste disposal sites.
It is similar to benzene but is less toxic. Toluene is com- pletely soluble in organic solvents such as hexane so that it can be easily extracted. It was obtained from Fisher Scien- tific (Chicago, IL, USA). The characteristics of toluene are shown in Table 1.
4% (v/v) DOSL (Dowfax 8390 anionic surfactant) was selected for this study. Surfactant concentration of 4% (v/v) provides the best removal efficiency in a previous study (Lee, 1999). DOSL has good solubilizing abilities for Dal-Heui Lee*
Jong-Sik Ryu
Eun-Sik Kim } School of Earth and Environmental Sciences, Seoul National University, Seoul 151-747, Korea
*Corresponding author: [email protected]
hydrophobic organic molecules (Rouse et al., 1993). Anionic surfactants are usually chosen for surfactant-based remedi- ation procedures because of their lower degree of adsorp- tion on soil than that by cationic and nonionic surfactant (Lee et al., 2002b). DOSL (sodium diphenyl oxide disul- fonate) was obtained Dowfax Chemical (Midland, MI, USA) and meets criteria as an indirect food additive under U.S.
Food and Drug Administration (FDA) (Regulation 21CFR- 178.3400) and is rapidly biodegradable by soil and/or aquatic microorganisms. The characteristics of DOSL are shown in Table 2.
2.2. Methods
The glass column used in this study was 5 cm (O.D.) in diameter and 30 in height. This column was obtained from Supelco Company. Experimental methods and procedures were similar to those performed in prior research (Lee et al., 2001b). Glass wool and glass beads were placed at the col- umns base, and 350 g of Ottawa sand was placed over the beads. Compaction of the dry soil in 0.5 cm layers was standardized by tapping the side of the column 25 times.
After a column was packed, deionized water was pimped at
a rate of 4 ml/min into the column for 3 hours to saturate the soil (Fig. 1). The contaminant (5 ml of toluene) was then injected by syringe into the upper middle of the col- umn, which is a method of contamination analogous to a point source of contamination at the field scale. Then 4%
(v/v) surfactant solutions with/without 2% NaCl or deion- ized water was pumped into the columns top at a rate of 4 /min. The same number of Na+ moles were added as those from NaOH in this experiments (Lee et al., 2001a, b). In order to evaluate the effects of pH on surfactant-based lab- scale remediation, the pH of surfactant solutions was varied by adjusting pH with a 10% NaOH (2.5 M) solution. pH values of the 4% (v/v) DOSL solutions were adjusted to 10 based on prior study (Lee et al., 2002a, b). Effluent was col- lected in 5 individual 250 ml fractions to give a total vol- ume of 1250 ml. Each treatment was evaluated in duplicate columns.
2.3. Analytical Methods
Toluene in aqueous samples was extracted by solvent extraction with hexane using standard separatory funnel method 3510 (Lee, 1999; Lee et al., 2001b) and analysed by gas chromatography with split/splitless injection system (Hewlett Packard Model 5890 series II) (Lee, 1999). The solid samples were extracted using a soxhlet extraction Table 1. Characteristics of model organic contaminant used.
Name Toluenea
Formula C6H5CH3
Molecular weight 92.1
Liquid density (g/cm3) 0.87
Aqueous solubilityb (mg/l) 510
Log octanol-water partition coefficient 2.69 Liquid-air interfacial tension (dyne/cm) 29 Liquid-water interfacial tension (dyne/cm) 36
Total gas density (kg/m3) 1.27
Viscosity (cP) 0.9
aData from Fisher Scientific, Chicago, IL.
b20−25oC, atmospheric pressure.
Table 2. The Characteristics of used surfactant.
Trade Name DOSL (Dowfax 8390)
Chemical name Diphenyl oxide disulfonates
Molecular weight 642
HLBa 13
CMCb (mM) 0.5
Molecular formula C16H33C12H7O(SO3Na)2
Type Anionic
Company Dow Chemical, Midland, MI
Water solubility Completely miscible Specific gravity 1.03−1.13
Data come from manufacturers.
aHLB=hydrophile-lipophile balance.
bCMC=critical micelle concentration.
Fig. 1. Schematic diagram of continuous leaching experimental apparatus.
method. Prior to the analysis of the extracted samples, the response factor and linearity of detection for the internal standard and contaminant were determined. After calculat- ing the response factor, a calibration graph was prepared.
The quantitative determination of contaminant concentra- tion was based on these internal standard reference com- pounds, so that sample peak areas were compared with those of their respective internal standards (ethylbenzene for toluene) (Lee, 1999).
3. RESULTS AND DISCUSSION
Figure 2 and Table 3 show the effect of NaOH in removal effectiveness of toluene. The increase was 16% for anionic DOSL, 12% for anionic Sandopan JA36 (Lee et al., 2001a), and 9% for the nonionic Triton X100 (Lee et al., 2001a).
This increase was much less than that observed for another sand column experiments (Lee, 1997, 1999). In this study, much greater effectiveness was observed using surfactant solutions containing NaOH. NaOH increased the effective- ness of a nonionic surfactant (Pluronic L44) by 50%, an anionic surfactant (Sandopan JA36) by 57%, and water by 10% in a pure sand column. The model contaminant was toluene in this study (Lee, 1997). Also, monitored pH val- ues of effluent solutions during experiments were almost same as the initial solution pH at the start of each test.
Chang and Rosano (1984) and Rosen (1989) found that micelle formation in surfactant solutions was enhanced by high pH conditions and the surface tension of surfactant solutions was decreased by high pH. Both factors may have affected our results. However, the cause of lesser affects compared to Lee’s (1997) results is not known although
probably resulted from the different model surfactants.
Hydrogen ion concentration in solution was an important factor in surfactant effectiveness (Rosen, 1989). Figure 2 shows that DOSL surfactant solution with NaOH affects clearly toluene removal from contaminated soil columns.
The addition of small amounts of pH neutral electrolyte such as NaCl to an anionic surfactant solution can both increase the micelle aggregation number (size of micelle) and decrease the CMC (Critical Micelle Concentration), thereby modifying surfactant effectiveness for remediation (Harwell, 1992). Also Rosen (1989) showed that the addi- tion of small amounts of pH neutral electrolyte to solution of ionic surfactants appears to increase the extent of solu- bilization of hydrocarbons that are solubilized in the inner core of the micelle. Increased binding of cations (Na+) should cause the CMC of the surfactant to decrease and the aggregation number to increase with the stabilization of micelles. Therefore, experiments were conducted to exam- ine the effect of cations (Na+) in the removal toluene from the Ottawa sand. Figure 3 and Table 4 show the effect of NaCl in removal effectiveness of toluene. In contrast, Na+ as 2% NaCl (wt NaCl per volume solution) had almost no effect on DOSL effectiveness compared to surfactant solu- tions without the added NaCl. We should also point out that
Fig. 2. Effect of NaOH on leaching of toluene from sand.
Table 3. The effect of 10% NaOH (2.5 M) in removal (%) of toluene.
Surfactant Without NaOH With NaOH Effectiveness
DOSL 79 92 16% increase
Sandopan JA36* 74 83 12% increase
Triton X100* 74 81 9% increase
*from Lee et al. (2001a)
DOSL is considered as a low Na+ surfactant containing about 0.1%−0.3% NaCl (Rouse et al., 1993) so that counter-ion effects of Na+ from the DOSL itself should be very minor.
The effect of Na+ was small, and was much less than that of pH (Table 5). The effect of NaOH in changing effec- tiveness is not due to Na+ effects, but rather to the OH− as shown by these experimental results.
Based on results of these column tests, DOSL with elec- trolyte (especially NaOH) can be a good candidate for sur- factant-assisted soil or groundwater remediation. In addition to the high percentage removal of toluene observed in our
column experiments, DOSL has good solubilizing abilities for 1,2,4-trichlorobenzene, phenanthrene, naphthalene and related substances (Deshpande et al., 2000). Also, DOSL should show minimal loss during surfactant flushing because of low sorption to soil particle and resistance to precipita- tion with cations (Na+).
4. CONCLUSIONS
1) Much greater effectiveness was observed using sur- factant solutions containing NaOH. 2) The effect of NaCl in changing effectiveness was less than that of NaOH. 3) The effect of NaOH in changing effectiveness was not due to Na+ counter-ion effects, but rather to the OH−. Subsur- face aquifers will rarely contain such high pH waters, and buffering of pH in surfactant solutions to pH 10 may prove difficult and expensive for many aquifers. If possible, how- ever, maintaining a high pH surfactant solution in field remediation is desirable because it will enhance contami- nant removal.
Fig. 3. Effect of NaCl on leaching of toluene from sand.
Table 4. The effect of 2% NaCl in removal (%) of toluene.
Surfactant Without NaCl With NaCl Effectiveness
DOSL 80 80 No increase
Sandopan JA36* 75 79 5% increase
Triton X100* 75 76 1% increase
*from Lee et al. (2001a)
Table 5. Comparison of NaOH and NaCl removal (%) effect in surfactant-based remediation.
NaOH or NaCl DOSL (anionic) Sandopan JA36 (anionic) Triton X100 (nonionic)
Unadjusted pH and without NaCl 79 74 74
pH 10 with NaOH and without NaCl 92 83 81
pH 11 with NaOH and without NaClc 88 81 80
pH 10 with NaOH and with NaCla,c 89 79 76
pH 11 with NaOH and with NaClb,c 85 79 76
aNa+ from NaCl equivalent to pH 10 Na+ in NaOH
bNa+ from NaCl equivalent to pH 11 Na+ in NaOH
cfrom Lee (1997, 1999)
ACKNOWLEDGMENTS: We would like to thank Dr. R.D. Cody of Iowa State University, U.S.A. for the discussion. Authors also express appreciation to the members of the Environmental Geochemistry Labo- ratory, Seoul National University, Korea. This study was supported by the BK21 project, Ministry of Education.
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Manuscript received August 27, 2002 Manuscript accepted March 30, 2003
