Concentration-Dependent Regimes in Sorption and Transport of a

Chapter 4

Concentration-Dependent Regimes in Sorption and Transport of a Nonionic Surfactant in Sand—Aqueous Systems

Downloaded by UNIV OF PITTSBURGH on June 13, 2013 | http://pubs.acs.org Publication Date: May 5, 1995 | doi: 10.1021/bk-1995-0594.ch004

Zafar Adeel and Richard G. Luthy Department of Civil and Environmental Engineering, Carnegie Mellon University, Pittsburgh, PA 15213

Sorption and transport of a nonionic surfactant in sand/aqueous systems appear to be controlled by concentration-dependent phenomena, resulting in two different sorption and transport regimes. Experiments were conducted in batch and column systems to evaluate the sorption isotherm and kinetics of sorption of Triton X-100 (C8PE9.5) onto Lincoln fine sand. The transition from one equilibrium sorption regime to the other occurred at an approximate surface coverage of 150 Å /molecule. A n unusual two-step breakthrough curve was observed in column transport tests. A n early surfactant breakthrough occurred at a fraction of the influent surfactant concentration; this was followed by a prolonged plateau in the effluent surfactant concentration. A transition from this plateau concentration to a second breakthrough segment was observed as surfactant surface coverage approached 180 Å /molecule. A two-stage empirical kinetic model for surfactant transport provided a reasonable fit to the experimental data. 2

2

The use of surfactants to assist removal of organic contaminants in in-situ aquifer remediation and ex-situ soil treatment systems has been discussed in recent papers (1-3). In this context, the principal points discussed in this chapter outline results from recent research, including the findings that: (i) nonionic surfactant sorption exhibits different characteristics for soil and aquifer sediment, (ii) the sorption of a hydrophobic organic compound (HOC) onto a solid is influenced by the sorption of nonionic surfactant, and appears to depend on the surface conformation of the sorbed surfactant, and (iii) the kinetics of nonionic surfactant transport through an aquifer sediment apparently are dependent also on sorbed surfactant conformation.

0097-6156/95/0594-0038$12.00/0 © 1995 American Chemical Society

In Surfactant-Enhanced Subsurface Remediation; Sabatini, D., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1995.


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Regimes in Sand-Aqueous Systems

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The sorption of surfactant onto solid media is an important consideration in surfactant-assisted remediation for sediment-aqueous systems (4, 5). It is important to be able to understand and predict these sorption processes, as surfactant sorption onto sediment directly affects the partitioning of organic contaminants between the aqueous, micellar, and solid phases. Equilibrium sorption of nonionic surfactants onto silica, silica gels, soils and clean sands has been studied by various researchers, and the equilibrium sorption isotherms have been described by a variety of mathematical formulations (4 - 10). Recent research has indicated that the sorption phenomena controlling partitioning of a nonionic surfactant between natural media and aqueous phases are different for soils and sands. It is proposed that surfactant sorption may be governed by the amount of naturally-occurring organic matter associated with the solid phase, the mineral composition of the solid medium, and the surfactant concentration (5, 11). Experimental evidence suggests that the conformation of nonionic surfactant molecules sorbed onto a surface may be important in affecting the degree of hydrophobicity of the surface, as well as affecting the affinity of the surface for sorbing HOC molecules (5, 9, 10, 12). In results summarized in this chapter, experiments comprising equilibrium sorption of Triton X-100 onto sand have shown that two distinct regimes of sorption may exist, and that such regimes may be dependent on the sorbed surfactant concentration. The two sorption regimes may correspond to difference in molecular conformations of the sorbed surfactant. In this regard, the extent of H O C sorption onto natural media containing sorbed nonionic surfactant at various concentrations has been used as an indicator of the degree of hydrophobicity of the surface. The data from H O C sorption experiments performed with various concentrations of a nonionic surfactant and an aquifer sand support the concept of different molecular conformations of sorbed surfactant and of two distinct sorption regimes. Laboratory-scale column transport studies suggest that the conformation of sorbed surfactant molecules may play an important role in surfactant sorption kinetics. Abdul and Gibson (13) have reported that the degree of retardation for nonionic surfactant transport in laboratory-scale columns containing a natural sediment is dependent on the influent surfactant concentration. In the results reported in this chapter, experimental observations for transport of a nonionic surfactant in sand columns suggest that two different sorption regimes may control the transport process, in which surfactant sorption in each stage is governed by separate kinetic parameters. The two governing kinetic parameters appear to depend on the sorbed surfactant concentration and surfactant molecular conformation. This observation of two regimes is qualitatively supported by the corresponding equilibrium sorption data (5) and reported data for transport of rhodamine W T in alluvial sediments (14)·

Experimental Procedures Clean, Lincoln fine sand passing U.S. standard sieve no. 10 (2 mm) was used as a solid medium for batch sorption and column transport experiments. The properties of Lincoln fine sand are shown in Table I.


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SURFACTANT-ENHANCED SUBSURFACE REMEDIATION

Table I. Properties of Lincoln fine sand. Value Units Property 5 xl0" Organic carbon (g/g) 2 xl0Clay (g/g) 6.4 pH (1:1 0.01M C a C l ) 3.5 CEC (g/cm ) 1.7 Bulk density 3.0 Surface area (m /g) Determined by Walkley-Black Method (15) From Wilson et al., (16) Determined for a packed column Measured according to B E T method (17) 4

a

2

6

6

2

6

3

c

d

2

α

6

c


d

Triton X-100 (C8Hi7-C6H -0(CH CH 0)9.5H) was selected as a representa tive nonionic surfactant due to its ability to enhance solubilization of organic com pounds (4, 5), and because of this surfactant being studied by other researchers (6j 8-10, 18), and its availability in radio-labeled form. A n aqueous solution con taining H-labeled and non-labeled Triton X-100 was used in column and batch experiments. Phenanthrene was obtained from Aldrich Chemical Company at 98% purity; C-labeled phenanthrene was acquired from Amersham Corp. A n aqueous solution containing C-labeled and non-labeled phenanthrene was used in some batch sorption experiments. 0.01 M C a C l was added to facilitate solidliquid separation. The properties of Triton X-100 and phenanthrene are shown in Table II. 4

2

2

3

14

14

2

Table II. Properties of Triton X-100 and phenanthrene. Property Triton X-100 Phenanthrene MW 625 178 Solubility 7xl0~ mol/L log K 4.57 CMC 1.8 x l O " mol/L Aggregation No. 140 Cloud Point* 65°C From Edwards et al., (5) From surface tension measurements From Robson and Dennis (18) From Partykaet al., (6) Q

0

6

a

ow

6

4

c

α

6

c

d


4. ADEEL & LUTHY

3


41

Regimes in Sand-Aqueous Systems 1 4

Aqueous solutions were counted for H or C activity with 10 mL of Packard Optifluor scintillation cocktail and a Beckman LS 5000 T D liquid scintillation counter (LSC). The radioactivity was measured as disintigerations per minute per mL of liquid sample. Batch surfactant sorption tests were conducted in 50 mL centrifuge tubes with P T F E septa, each containing 5 g of Lincoln fine sand and 30 mL of aqueous surfactant solution at various concentrations. For experiments with phenanthrene, an aqueous solution containing phenanthrene was equilibrated with the sand for 24 hours and the surfactant solution was then added to the tubes. The tubes were rotated end over end for at least 24 hours, followed by centrifuging at 1600 g for 30 min. In order to remove any suspended particles, the supernatant was expressed through Acrodisc P T F E filters (1 μτη pore size), which were conditioned by wasting an initial 5 mL, and then directly dispensing the filtrate into LSC vials containing scintillation cocktail. These vials were counted for H or C activity and the sorbed concentrations of Triton X-100 or phenanthrene, respectively, were determined by mass balance. One-dimensional column transport experiments were performed with a 7.53 cm long, 2.20 cm I.D., stainless steel column, packed with Lincoln fine sand in 18-20 layers. A number of pore volumes of de-ionized water containing 0.01 M CaCl2 were flushed through column to condition the soil prior to pumping the surfac tant solution. The aqueous surfactant solution was pumped from a stainless steel reservoir by an H P L C pump. The effluent surfactant solution from the column was accumulated in 10 mL tubes by an Eldex fraction collector; the volume of each sample collection depended on the flow rate and the duration of sampling. The H activity in these samples was determined by L S C counting. Pumping of surfactant solution was discontinued after the effluent concentration became equal to the influent concentration. Clean, de-ionized water containing 0.01 M CaCl2 was then pumped and surfactant desorption was measured in the manner described above for column effluent analysis. 3

1 4

3

Sorption of Surfactant and Phenanthrene This section describes equilibrium sorption of Triton X-100 onto Lincoln fine sand as well as sorption of phenanthrene onto Lincoln fine sand in the presence of Triton X-100. A mathematical formulation describing equilibrium sorption of Triton X-100 is also presented. Sorption of Nonionic Surfactant onto Sand. In results described below, it is shown that Triton X-100 molecules may sorb directly onto the solid surface, or may interact with sorbed surfactant molecules, the sorption mechanism appar ently being dependent on the nature of the sorbent and the surfactant dose (5). In the case of a mineral surface, or low-organic content aquifer sediment with very few sorbed surfactant molecules, the sorption of Triton X-100 surfactant molecules may occur mainly due to van der Waals interactions between the hydrophobic and the hydrophilic moieties of the surfactant and the surface (19). In comparison, at higher surfactant doses such sorption may occur through more structured sur-


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SURFACTANT-ENHANCED SUBSURFACE REMEDIATION


factant arrangements including the formation of monomer surfactant clusters on the surface or the formation of admicelles or bilayers. These arrangements may be governed mainly by interactions between hydrophobic moieties of the surfactant molecules (5, 19). The surface arrangements of surfactant molecules may be patchy rather than uniform. Figure 1 shows the sorption isotherm obtained from batch experiments for Triton X-100 and Lincoln fine sand. The data are expressed as the logarithm of sorbed phase surfactant concentration (mol/g solid) versus the logarithm of the bulk aqueous-phase surfactant concentration (mol/L). It is observed that sorption continues well beyond the point at which C M C occurs in the aqueous phase.

Triton X-100 Sorption ^

30 mL bulk solution 5 g Lincoln fine sand

-5

-6

i Log (Cint)

I -7 H

Log ( C M C ) Int. Reg.

-8 H .Region 1

-5

Region 2

H -4

-3

-2

-1

Log (Bulk Aqueous Cone, mol/L) Figure 1. Batch experimental data for sorption of Triton X-100 onto Lincoln fine sand, (o) show experimental data.

The sorption data reveal some complex relationships. A t low surfactant concentrations in region 1 the sorption is Freundlich-type, up to the point at which the critical micelle concentration (CMC) is reached in the aqueous phase. There is an intermediate region, beyond which the sorption appears to be Freundlichtype in region 2. The transition from the intermediate region to region 2 occurs at a bulk aqueous-phase surfactant concentration identified as Cint. The sorbed-phase surface concentration corresponding to C < is computed to be approximately 150 Â /molecule. It is envisioned that the orientation of the sorbed surfactant molecules undergoes a transition from a more-or-less patchy flat-lying tn

2


4. ADEEL & LUTHY

Regimes in Sand—Aqueous Systems

43

conformation to a patchy bilayer conformation as surfactant concentration is increased from region 1 to region 2 (5, 11). At the highest end of the sorption isotherm, i.e., that corresponding to an aqueous concentration of 150 times the C M C , the surface coverage is about 77 Â /molecule. This conceptualization of the orientation of sorbed surfactant molecules is illustrated schematically in Figure 2, where stages 1 and 2 correspond to regions 1 and 2 of Figure 1. 2


Aqueous phase

STAGE 1

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Monomers

kf1 Saod'

/ -

Aqueous phase

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STAGE 2

Micelles

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Concentration-Dependent Regimes in Sorption and Transport of a

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