Environ. Sci. Technol. 1993, 27, 2559-2565
Partitioning of Three Nonionic Organic Compounds between Adsorbed Surfactants, Micelles, and Water Jae-Woo Park and Peter R. Jaff6’
Water Resources Program, Department of Civil Engineering and Operations Research, Princeton University, Princeton, New Jersey 08544
The use of the surfactants to alter the transport properties of nonionic organic compounds in porous media is of significant interest for several environmental applications. In this study, we have investigated the interactions of a system consisting of oxide particles, an anionic surfactant, and nonionic organic compounds. The combination of oxides as the solid matrix and anionic surfactants may also have practical applications for watertreatment processes in the removal of nonionic organic compounds from water. Oxides exhibit surface charges in the aqueous environment. The zero point of charge (ZPC) of an oxide is the pH at which the solid surface charges from all sources are zero. If the pH is lower than the ZPC of an oxide, the oxide will take on positive surface charges and will act as an anion exchanger. If the pH is higher than the oxide’s ZPC, the oxide will take on negative surface charges and will act as a cation exchanger (2). A positively-charged oxide with an adsorbed anionic surfactant layer on its surface, called organo oxide, acts as a sorbent for nonionic organic compounds since these compounds will be partitioned into the organicphase of the sorbent. This sorbent can be easily regenerated in situ by increasing the pH above its ZPC, which changes the surface charge of the oxide, thereby desorbing the anionic surfactant phase, including the nonionic organic compounds, that has partitioned into the adsorbed surfactant phase. Even though this organo oxide might not be as efficient as activated carbon in terms of the bulk treatment of water (3), it has several advantages that might be of interest in specific applications: (1)selective removal of a certain contaminant may be achieved if a specific surfactant that sorbs the contaminant selectively is used; (2) the solute
that is removed from water by the organo oxide can be recovered if this is desired. Anionic surfactants can be adsorbed onto oxides in an acid environment where the oxides have positive charges (pH below ZPC), while they can be desorbed from oxides where the oxides have negative charges (pH above ZPC) (2,4). While the surfactant monomers dissolved in water form micelles at the concentrations above the critical micelle concentration (CMC), the surfactant monomers adsorbed onto the oxidesform amonolayer (a hemimicelle) composed of surfactant monomers adsorbed to the surface with their hydrophobic ends sticking out into solution and a bilayer of surfactant (an admicelle), which forms when additional surfactant monomers sorb to the hemimicelle extending their hydrophilic ends into solution (5-7). Incorporation of nonionic organic compounds into micelles, resulting in an increased “apparent” solubility, has been observed by several researchers (I, 3,8-11). Kile and Chiou (8)showed the solubility enhancement of two hydrophobic compounds by some surfactants below and above the CMC. Valsaraj and Thibodeaux (9)related the partition coefficients between micelles and water for some hydrophobic nonpolar chemicals to their octanol-water partition coefficients. Several researchers have investigated the combination of oxides and anionic surfactants to study the sorption capacity of the aggregates of the adsorbed surfactants. Valsaraj (12)showed that the partition coefficients of the anionic surfactants adsorbed on the aluminum oxide (AlzOs) correlated well with the octanol-water partition coefficients and solubilities in water of the hydrophobic compounds. Holsen et al. (7) reported that slightly soluble organic chemicals can be removed from the aqueous phase using anionic surfactant-coated ferrihydrite in batch experiments. Scamehorn and Harwell(13) proposed the possible use of fixed-bed admicellar chromatography as a water-treatment system. Park and Jaff6 (3,4) developed experimental units and demonstrated the technical feasibility of such a process, using organo oxides in a continuous-flow water-treatment process for the removal of nonionic organic compounds. The objective of this research is (1) to describe the distribution of nonionic organiccompounds between water, micelles in the aqueous phase, and aggregatesof surfactants adsorbed onto oxides; (2) to investigate the performances of continuous-flow,laboratory-scale organo oxide columns for removal of nonionic organic compounds from water; and (3) to determine if the coefficientsobtained from batch experiments that describe the sorption of the anionic surfactants onto the oxide as a function of the pH, and the partitioning of nonionic organic substances between water and micelles and between water and sorbed surfactant, can be used to describe a dynamic column experiment.
0 1993 American Chemical Society
Environ. Scl. Technol., Vol. 27, No. 12, 1993 2559
The partitioning of three nonionic organic compounds between aggregates of an anionic surfactant adsorbed onto positively-charged aluminum oxide, the micellar phase of the anionicsurfactant,and water was investigated. Uptake of the nonionic organic compounds from water was observed by both the surfactant layer adsorbed onto the oxide and the micelles of the surfactant in solution. As the molecular sizes of the solutes increased and their solubility in water decreased, their uptake from water per unit mass of surfactant became more efficient for the micellar phase than for the surfactant layer sorbed onto the oxide. Continuous-flow column experiments were performed first, to demonstrate that the partitioning measured in batch experiments between the nonionic organics, the different surfactant phases, and water could be used to describe a dynamic system; and second, to illustrate a potential water-treatment process that is based on these dynamics.
Introduction
0013-936X/93/0927-2559$04.00/0
Table I. Chemical Structure of Emcol CNP-604
Table 11. Selected Properties of the Nonionic Organic Compounds Used in This Study (20)
Emcol CNP-60 (MW = 542.7)
Emcol NP-60 (MW = 496.7)
compound
formula
MW
carbon tetrachloride naphthalene phenanthrene
CC4 CioHs Ci6Hi0
153.8 128.2 178.2
solubility in water (mg/L) log KO, 7.85 X 102 3.44 X 10' 1.29 X 10'
2.64 3.37 4.46
mass of the adsorbed surfactant by defining another partition coefficient as TheproductEmcolCNP-60consistsofapproximately70%Emcol CNPBO, -18% Emcol NP-60,10%HzO, and -1 % inorganic salts.
Sorption of Surfactant and Nonionic Organic Compound in the System
A general expression for the mass balance of surfactant in the system considered in this research is
T" = Sa+ M,"
+ M,"
(1) where TB is the total mass of surfactant added, Ss is the mass of surfactant adsorbed onto the oxide particles, Mls is the mass of surfactant dissolved in water in micellar form, and MzS is the mass of surfactant dissolved in water in monomeric form. If the adsorption of the solute onto the bare oxide surface is negligible, the mass balance of a nonionic organic compound is written as
+
I" = sc M,C
+ M,c + CC
(2)
where TC is the total mass of the solute added, Sc is the mass of the solute sorbed into the organic phase of the organo oxide, M1C is the mass of the solute sorbed into the micelles in the aqueous phase, MzCis the mass of the solute associated with the monomers in the aqueous phase, and Cc is the mass of the solute dissolved in water. Only extremely water-insoluble organic solutes such as DDT (KO, = 6.36) are known to be associated to a significant degree with monomers in the aqueous phase (8,14-17). Since the most hydrophobic compound in this research is phenanthrene (KO,= 4.461,MzCcan be neglected in this study, and the originalmass balance (eq 2) is reduced to
Tc = sc+ M,C + cc
(3)
Two partition coefficients can be defined that describe the partitioning equilibrium of the solute between the different organic phases and the aqueous phase as
K,, = C,/C,
(4)
where K,, is the partition coefficient of the solute between the organo oxide and the aqueous phase (L/kg), C, is the concentration of the solute sorbed onto the organo oxide (mg/kg of organo oxide), C , is the concentration of the solute dissolved in water, K,, is the partition coefficient of the solute between the micellar phase and the aqueous phase (L/kg), and C, is the concentration of the solute in micellar phase (mg/kg of micelle). If the adsorption of the solute onto the bare oxide surface is negligible, K,, can be expressed with respect to the 2560
Envlron. Scl. Technol., Vol. 27, No. 12, 1993
K,, = c,/c, (6) where Kaw is the partition coefficient of the solute between the adsorbed surfactant layer and the aqueous phase (L/ kg), and C, is the concentration of the solute in the adsorbed surfactant phase (mg/kg of the adsorbed surfactant layer). Therefore, K,, and K a w would be K,, = K,, (mass of the adsorbed surfactant layer)/ (mass of the organo oxide) (7) Using eq 4 and 5 to express eq 3 in terms of aqueous concentrations, we obtain I" = C, (mass of organo oxide) + C , (mass of micelles) + C, (volume of water) = K,,C, (mass of organo oxide) + K,,C, (mass of micelles) + C, (volume of water) (8) As shown in eq 8, if K,, and Kmw are known, the distribution of the solute between each phase in equilibrium can be predicted.
Experimental Section Materials and Analytical Methods. The anionic surfactant, Emcol CNP-60, was obtained from Witco Corp., and its chemical structure is shown in Table I. The oxide used in this study was activated and weakly acidic aluminum oxide (A1203) from Aldrich Chemical Co. The particle size distribution of the aluminum oxide was specified to be less than 150mesh with a BET surface area of 155m2/gdetermined by nitrogen (N2) adsorption. The anionic surfactant and aluminum oxide were used as received from the suppliers. Three nonionic organic compounds used are carbon tetrachloride, naphthalene, and phenanthrene. Selected properties of the nonionic organic compounds are listed in Table 11. A total of 250 pCi of [l4C1carbontetrachloride (specific activity equal to 4.3 mCi/mmol) was obtained from Dupont NEN. [l4C1Carbon tetrachloride was mixed with nonradioactive carbon tetrachloride to yield a 1.0 mL radiolabeled stock solution. Some 250 pCi of [14C]naphthalene (specificactivity equal to 8.0 mCi/mmol) and 100 pCi of [Wlphenanthrene (specific activity = 8.3 mCi/mmol) were acquired from Sigma Co. Both [l4C1naphthalene and [Wlphenanthrene were supplied as solids and were subsequently dissolved in spectrophotometric grade hexanes (from Mallinckrodt). An appropriate volume of each radiolabeled solution was added to a vial containing the same nonradioactive compound, after which the hexanes were allowed to evaporate. After the hexanes were completely evaporated, 1 mL of methanol (from Baker) was added to each vial in order to prepare the final radiolabeled stock solutions of naphthalene and phenanthrene.
Adsorption isotherms of the surfactant onto the oxide as a function of the pH were quantified from hatch experiments. For this purpose, 4 g of aluminum oxide and 56 mL of the surfactant solution with varying concentrations were placed in a series of 60-mL glass centrifuge tubes with Teflon-lined caps. Similarly, sorption isotherms of the nonionic organic compounds onto the organo oxide were also quantified from hatch experiments. For this purpose, 1g of the oxide, 14 mL of the surfactant solution, and a varying mass of the respective "C-labeled nonionic organic compound were placed into a series of 15-mL glass centrifuge tubes with Teflon-lined caps. This was repeated for different concentrations of the surfactant solution. In all cases, the samples were prepared in duplicate, equilibrated in the dark at 20 "C, and rotated continuously in a mechanical tumbler to facilitate mixing. Even though surfactant equilibrium between the aqueous and solid phases and solute equilibrium between the aqueous and organo oxide phases is a 48-h incubation time was used in all reached fast (7,18), samples. After incubation, the samples were centrifuged for 60 min at 650g (g = 9.81 m2/s), and the concentration of the supernatant was analyzed. In all cases, the sorbed mass was determined by subtracting the dissolved mass from the mass added. Blank samples containing only a solution of the sorbate were prepared and handled in parallel with each set of the hatch experiments to quantify losses. Aqueous surfactant concentrations were determined by measuring the surface tension of the aqueous supernatant withaFisher ScientificModel21surface tensiometer that employs the du Nouyring method. Surface tensionvalues were taken when stable readings were obtained for a given sample, as indicated by at least two consecutive measurements having the same value. After each measurement, the ring was washed by dipping it into benzene and then squirting it with acetone. Because Emcol CNPBO is a mixture of two surface-active agents, of which each may affect the surface tension differently and may sorb onto the organo oxide to a different extent, quantification of the surfactant mass in sorption experiments through surface-tension measurements may lead to erroneous results. For this reason the value of the highest sorbed surfactant m m estimated from surface tension measurements and expressed as total sorbed organic carbon was compared to that obtained from direct organic carbon measurements. Since the difference between both methods was 3 % ,it was concluded that surface-tension measurements were an appropriate analytical technique for this research. A Packard Tri-Carh 1900CAliquid-scintillationanalyzer was used to measure the radioactivity in the "C-labeled samples. The concentrations of the nonionic organic compounds in the supernatant were then determined with previously prepared standard curves relating disintegration per minute to aqueous concentrations. After the sorption experiments of the surfactant onto the oxide were completed, the organic carbon content of the organo oxide was quantified in duplicate by Huffman Laboratories, Golden, CO; whereas, the oxide's organic matter content was computed based on the mass of surfactant sorbed and quantified hy surface-tension measurements. Continuous-Flow Column Exgeriments. To study the oxide, surfactant, nonionic organic system under
r----
- - - -I
SWWPmP I 1"on-ionic organic compound) I I I I I I
r
ourno
I I I I
I I
I
I
I I
_I
\
io g of alumurn m d c
I
CalvmnDirnenrion 1 5 sm(lD)*lQcm[L)
d
Penstdm pYmp
Flpure 1. Column experlmenl setup.
dynamic conditions, duplicate column experiments were conducted for each nonionic organic compound. The experimental setup is shown in Figure 1. Glass columns with an inner diameter of 1.5 cm and a length of 10 cm werefilled withlOgofaluminumoxide. After thecolumns were packed, several pore volumes of de-aired and deionized water were flushed through them to obtain fully saturated conditions. A peristaltic pump (Manostat Cassette pump) was used to pump the solutions into the columns. Togenerate theadsorbedsurfactantlayeron the surface of the aluminum oxide, a solution of Emcol CNP-60 with aconcentrationoflZ.&g/L(CMC =59mg/L) waspumped into the columns from the top at a flow rate of 27 mL/h. After the effluent surfactant concentration was approximately 40% of the influent concentration, clean water waspumpedintothecolumnforanadditional 12-hperiod to remove the aqueous-phase surfactant in the column. The effluent surfactant concentration was monitored during this process in order to perform a proper surfactant mass balance. Subsequently, a solution containing one of the nonionic organic compounds was pumped into the columns from the top at a rate of 27 mL/h. The nonionic organic compound was added to the water stream with a syringe pump at a rate of 8.05 pL/h. The setup in the dotted box, shown in Figure 1, was used to keep the nonionic organic compound concentration in the influent constant. F'umpingwasdiscontinuedafterthe water contaminated with the nonionic organic compoundwas pumped through a column for 6 h. One pore volume (14 mL) of a 2.5 N sodium hydroxide (NaOH) solution (pH 13.8) was then pumped into the column in an up-flow mode to change the surface charges of the oxide from positive to negative in order to desorb the anionic surfactant. After this, the column was placed into a tumbler for 10 min, in order to achieve maximum contact between the oxide and the sodium hydroxide solution. This procedure was repeated twice. Each pore volume obtained in this manner was analyzed to determine the concentration of the surfactant and the nonionic organic compound in the aqueous phase. Em*wr. Sd. Tschnol.. Vol. 27. No. 12. 1983 ZM1
200 ~~
~
~
I
I
No pH adjustment pH 6.4 0 pH 9.5 A pH 13.8
0
Y
Eg .-5:
V
loo00 8000 6000
4000 2000 0 0
20000 aqueous Emcol CNP-60 (mgU 10000
30000
0
100
200 300 aqueous CC14
400
(mg/L)
Flgure 2. Adsorption of Emcol CNP-BO onto aluminumoxide at several pHs.
Flgure 3. Sorption isotherm of carbon tetrachloride onto the organo oxide with a 7.1 % organic carbon content at nonadjusted pH.
It was shown by Park and Jaffi! ( 4 ) that, without mechanical mixing during this desorption step, the mass of surfactant that desorbs per unit volume of NaOH solution and, hence, that of the nonionic organiccompound is significantly lower.
compounds, the organic matter normalized partition coefficients between the organo oxide and the dissolved phase, shown in Figure 4,is about the same as the octanolwater partition coefficients (KO,),shown in Table 11.The magnitude of the partition coefficients and the linearity of the sorption isotherm indicate that the uptake of the nonionic organic compounds by the organo oxide is a partitioning process rather than an adsorption process. This is in agreement with the results reported by Smith et al. (18),Boyd et al. (19),and Smith and Jaffi! (23),who showed that organo clays formed with long carbon-chain surfactants, similar in size to Emcol CNP-60, provide a partitioning media. An adsorption isotherm of the surfactant onto the aluminum oxide and a series of sorption isotherms of each nonionic organic compound onto the organo oxide were conducted at the pH at which the regeneration of the organo oxides in the column experiments was performed (pH = 13.8). Five to six sorption isotherms onto the organo oxide were quantified for each of the nonionic organic compounds using different quantities of excess surfactant. Sufficient surfactant was added to form micelles in the aqueous phase after the oxide was fully coated with the surfactant. Two of these isotherms for carbon tetrachloride and for the lowest and the highest of the five micellar concentrations are shown in Figure 5. The micellar concentration is defined here as the dissolved surfactant concentration in excess of CMC, or MIs per volume of solution. The results show that as the micellar concentration of the surfactant becomes larger, the mass of the solute sorbed onto the sorbent (Sc)gets smaller. This is because more mass of the solute is sorbed into micelles (MlC) rather than onto the organo oxide. Since only the total mass of the solute in the aqueous phase, which is the sum of mass of the solute sorbed into the micelles in the aqueous phase ( A l l C )and mass of the solute dissolved in water (Cc),could be determined with the analytical technique described earlier, the concentration of the solute dissolved in water (C,) had to be estimated indirectly. The sorbed concentration (C,) was still determined from a direct mass balance, as described earlier. The dissolved concentration (C,) was estimated from the sorbed concentration (C,) using eq 4. The corresponding K,, was estimated as follows: The mass of surfactant sorbed onto the oxide at a pH of 13.8, and therefore the oxide’s organic matter content, was determined from the results shown in Figure 2. K , could be estimated for that specific organic matter content from
Results and Discussions Batch Experiments. Adsorption isotherms of the anionic surfactant onto aluminum oxide at different pH’s are shown in Figure 2. The pH of the solution was controlled by adding the required amount of NaOH to each vial after the surfactant solution was added to the oxide. Because one of the surface-active agents of Emcol CNP-60 is a weak acid, the pH of the solutions without any adjustment varied from 5.2 for the lowest surfactant concentration to 3.6 for the highest surfactant concentration. The maximum organic carbon content obtained in these sorption experiments was 9.1% for the nonbuffered experiment. The isotherms shown in Figure 2 are similar to those reported by Valsaraj (12). These results show that when surfactants are added to oxides in water the adsorbed surfactant layer will form first, and the micellar phase in solution will form after the sorption capacity of the oxide has been satisfied and the concentration of the surfactant in solution exceeds CMC. The internal structure of the hemimicelles is similar to that of bulk micelles but has a higher microviscosity than the micellar interior (5, 21). Also, the adsorbed surfactant monomers are more constrained in motion than monomers in micelles (22). A sorption isotherm of carbon tetrachloride onto the organo oxide with a 7.1 % organic carbon content is shown in Figure 3, where the slope of the sorption isotherm is K,, for carbon tetrachloride and that specific organic carbon content. To obtain K,, for different values of the organic carbon content of the sorbent, a set of batch sorption experiments was conducted for each of the three nonionic organic compounds without adjusting the pH and for varying amounts of surfactant mass sorbed to the oxide. The results are shown in Figure 4 where the logarithms of K,, and the logarithms of K,, normalized to the organic matter content are plotted as a function of the organo oxide’s organic matter content. Note that for the pH at which these experiments were conducted, essentially all of the surfactant is adsorbed onto the oxide, hence the surfactant concentration in solution is much smaller than CMC. For each of the nonionic organic 2562
Envlron. Sci. Technol., Vol. 27, No. 12, 1993
log(Kxw)-CCl4 logWxw/fomWCl4 log(Kxw)-naphthalene log(Kxw/fom)-naphthalene log(Kxw)-phenanthrene log(Kxw/fom)-p henanthrene s-1 ! ' 0.00 ij
s
. .
'
- .
'
=
I
0.05
1
.
'
-
0.10
'
I 0.15
fom (organic matter content) Flgure 4. Partltion coefficient of nonionic organic compounds between the organo oxide and water. Expressed as a function of the organic matter content and normailzed wRh respect to the organlc matter content.
loo00 I
1
a
I
v
01
8
0
lo00
3000
2000
aqueous CC14
(mgiL) Figure 5. Partltioningof carbon tetrachloride into the organo oxlde at two micellar concentrations (3.831 and 18.516 g/L) at pH 13.8. The crlticel mlcelie concentrationof Emcoi CNP-60 is 59 mg/L. For these conditions, the relative mass of surfactant sorbed onto the oxide to that In the micellar phase was 0.30 and 0.08, respectlvely.
A
-r:
__
.-
r-i
2 ' 0 0
- -
. .... . . -e log(Kaw)-CCl4 log(Kaw)-naphthalene . log(Kaw)-phenanthrcne
A
A
a
-
m*-
m -d-*.
100
200
300
micelles or adsorbed surfactants (mg)
Flgure 8. Partition coefflcient between the micellar phase and water (KmJ and between the adsorbed surfactant phase and water (Kaw)for three nonionicorganic compoundsvs massof micellesor hemimiceiles/ admlcelles.
the results shown in Figure 4. Once the concentration of the solute dissolved in water (C,) was obtained, the concentration of the solute in micellar phase (C,) was determined by subtractingthe mass of the solute dissolved in water (C), from the measured total mass of the solute in the aqueous phase. Knowing C, and C m , Kmw could be estimated independently, using eq 4, for each one of the sorption isotherms quantified in the presence of an excess surfactant mass. The results are presented in Figure 6, where the logarithm of K,, is plotted as a function of the mass of the micelles in the system. Also shown in Figure 6 are the values of Ka,, or the partition coefficient of the nonionic pollutants between the adsorbed surfactant phase and water. These Kawvalues were estimated directly from the K,, values of Figure 4.
Ka, values are normalized here per unit mass of adsorbed surfactant and can, therefore, be compared directly with the values of K,, shown in Figure 6, which are also normalized per unit mass of surfactant in micellar phase. It is interesting to note that, for the three nonionic organic compounds examined, the difference between K,, and Kmwbecomes larger as the molecular size of the nonionic organic compound (or its KO,) increases. A possible explanation for this effect is that larger molecules may be incorporated easier into a less rigid phase or a phase in which the rearrangement of solvent molecules requires less energy. The movement of monomers in hemimicelles is physically more restricted compared to that in micelles, given that monomers in hemimicelles are bound to the oxide surface, resulting in a highly structured and almost rigid phase (5,22). It is also possible that the equivalent micellar number of the hemimicelles is smaller than that of the micelles and that this difference does not affect the partitioning of smaller nonionic organics, but has an effect on the larger molecules (24). Continuous-Flow Column Experiments. Columns were operated as described above. By monitoring the surfactant and each solute concentration in the inflow and effluent, the mass of the surfactant and of each nonionic organic compound sorbed in the columns was determined. The total initial mass of the surfactant sorbed in each column, in terms of the overall sorption capacity, averaged for each of the two replicate column experiments was 18.8%,14.8%,and 12.8%for the carbon tetrachloride, naphthalene, and phenanthrene experiments, respectively. The difference in surfactant mass sorbed between experiments is because the mass of surfactant applied to the columns between experiments was not identical. The mass of surfactant and of each nonionic organic compound desorbed per unit pore volume of hydroxide solution was determined by analyzing the dissolved-phase concentration in the columns after the column was placed into the tumbler for 10 min. The measured and estimated fractions of surfactant and carbon tetrachloride that desorbed with each pore volume of flushing solution are shown in Figures 7 and 8 for two replicate column experiments. The predicted results were obtained using the mass balance models described earlier and the partition coefficients obtained from the batch experiments. Since the total mass of the surfactant in the column (T")was known at the beginning of each desorption step, the mass of the surfactant adsorbed onto the oxide particles (Sa)and the mass of the surfactant in the aqueous phase (MI* M28)
+
Environ. Scl. Technol., Vol. 27, No. 12, 1993 2563
0.8
7 I
v Y
f
-*E
0.6
E l 3rd p.v.
m
s . F
E
0.4
III 2nd p.v.
rn
1st p.v.
actual predicted Column 2
0.8 1
1
F
actual predicted Solute
Flgure g. Total actualand predicteddesorbedfractions of the surfactant and three nonionic organic compounds. The bars labeledCCi, represent the set of experiments where carbon tetrachloride was the nonionic organic compound; naphthalene representsthe set where naphthalene was the solute; and phenanthrene represents the set where phenanthrene was the solute.
0.6
I
0.4
F
E 3rd p.v. [D 2nd p.v. rn 1st p.v.
0.2
U
0.0
actual predicted actual predicted Column 1 Column 2 Flgure 8. Recovery of the carbon tetrachloride from the columns during the desorption procedure.
could be determined from the isotherm at pH 13.8 shown in Figure 2, assuming equilibrium was achieved and because the pH of the flushing solution did not deviate significantly from 13.8during the flushing process. Since S8was known, K, and Kmwcould be determined from the results shown in Figures 4 and 6. Since the total mass of each nonionic organic compound in the column (T")was known at the start of each desorption step, the mass of the solute sorbed into the organic phase of the organo oxide (Sc) and the mass of the solute in the aqueous phase ( M l c + Cc)could be calculated from eq 7 using the two partition coefficients (Kxwand Kmw)from Figures 4 and 6. The predicted fractions of surfactant or nonionic organic compound desorbed in the columns were calculated separately for each desorption step in which 1 pore vol was replaced with a fresh sodium hydroxide solution. As indicated in Figure 9, the results from the actual experiments and the predicted results from the mass balance models matched well for all three nonionic organic compounds. Note that during regeneration, the total mass of the nonionic organic compounds per unit volume of hydroxide solution can be much larger than their solubility in water. This is possible because the corresponding surfactant concentration in solution is much larger than the critical micelle concentration, allowing for a facilitated transport out of the column. To achieve a satisfactory regeneration of the column, it is therefore important that the mass of surfactant and nonionic organic compound initially sorbed onto the column is such that upon desorption of both, the nonionic organic compound can be in the aqueous phase either dissolved or dissolved and partitioned into micelles. The formation of a separate phase during regeneration of the column will be avoided as long as (1)the regeneration solution does not dilute the surfactant below its critical 2564
phenanthrene
0.2
actual predicted Surfactant
Flgure 7. Recovery of the surfactant from the columns during the desorption procedure. Carbon tetrachloride was the nonionic organic compound.
8
v
f
CC14
03 naphthalene
0.0
actual predicted Column 1
f
rn 0.4
U
0.0
8
0.6
0
9 g
0.2
U
5-
--
Envlron, Scl. Technol., Vol. 27, No. 12, 1993
micelle concentration, (2) the overall partitioning of the nonionic organic compound between micelles in solution and water is equal or larger than that between hemimicelles and water, and (3) sufficient mixing is provided for all phases to reach equilibrium. The results of this work indicate that the partition coefficient of the nonionic organic compounds between micelles and water is equal or larger than that between hemimicelles and water.
Conclusions Partitioning of three nonionic organic compounds from water into micelles in solution as well as hemimicelles sorbed to aluminum oxide were studied as a function of the solution pH and the surfactant mass. The results have shown that the partitioning of the three nonionic organic compounds from water into micelles in solution as well as hemimicelles was directly proportional to the mass of monomers in these structures. The partition coefficient for all three nonionic organic compounds between micelles and water appears to be larger than that between hemimicelles and water. This difference appears to increase as the molecular size of the nonionic organic compound increases and its octanol water partitioning coefficient increases. Using these partition coefficients, one can estimate the performance of organo oxides in continuous-flow columns during the removal of nonionic organic pollutants from water and during the regeneration procedure of the organo oxide. Mechanical mixing of the organo oxides in the columns during the regeneration procedure is required to reach equilibrium conditions.
Acknowledgments This research was funded by a grant from CarterWallace, Inc. The anionic surfactants and their chemical structures were provided by Witco Corp.
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