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Journal of the Chinese Institute of Engineers, Vol. 28, No. 2, pp. 375-379 (2005) 375

Short Paper

EFFECTS OF SOIL PROPERTIES ON SURFACTANT

ADSORPTION

Jiunn-Fwu Lee*, Ming-Hung Hsu, Chung-Kung Lee, Huan-Ping Chao, and Bai-Her Chen

ABSTRACT

The effects of surface area, soil organic matter (SOM) content, and cation ex-

change capacity (CEC) of natural soils and clays on the adsorption capacity of cationic,anionic, and nonionic surfactants in water-solid systems were investigated based on

the adsorption isotherm analysis. The sorption capacity for a cationic surfactant was

proportional to the CEC of the solids. For both anionic and nonionic surfactants, the

sorption capacity was related to the soil mineral fraction. However, other soil proper-

ties probably affect the practical sorption. The investigated soil properties were treated

case by case.

Key Words: soil properties, surfactants, adsorption capacity, adsorption model.

*Corresponding author. (Tel: 886-3-4227151-34658; Fax: 886-

3-4226742; Email: [email protected])

J. F. Lee, M. H. Hsu, H. P. Chao, and B. H. Chen are with the

Graduate Institute of Environmental Engineering, National Cen-

tral University, Chungli, Taiwan 320, R.O.C.

C. K. Lee is with the Department of Environmental Engineering,Van-Nung Institute of Technology, Chungli, Taiwan 320, R.O.C.

I. INTRODUCTION

A better understanding of surfactant adsorption

by clays and soils is of great importance due to the

wide spread use of these compounds in household and

industrial activities. For subsaturated organic con-

taminants in a soil-water-surfactant system, the sur-

factants enable the micelle to greatly promote the ap-

parent aqueous solubility of relatively insoluble

solutes. As such, it has potential as a means of re-

moving pollutants from soil during a remediation

treatment process. However, surfactants in a soil-

water system are also considered to have potential to

be adsorbed by soils to increase contaminant parti-

tioning in the soils (Lee et al., 2000). Thus, prior tousing surfactants for subsaturated contaminant

remediation in soil, it is prudent to establish the rela-

tion between the soil properties and the surfactant

adsorption characteristics, as a guide to proper

surfactant selection and the best concentration for

maximum performance. For this, in addition to the

quantitative adsorption measurement, and endeavor-

ing to theoretically describe the adsorption isotherms,numerous instrumental methods have been used to

characterize the adsorption process, the formation of

bonds, the structure of interfacial layers, overall

dispersion, and organocomplex properties (Goloub et

al ., 1996). The experimental data basis for surfac-

tant sorption by soils and clays is quite extensive. It

was found that there are some internal factors, such

as the length and nature of the surfactant polar chain

and the nature of the solid surface that must be taken

into account. In this study, various natural clays and

soils were used. The effects of their properties on

the adsorption of cationic, anionic, and nonionic sur-

factants were examined to give a more comprehen-sive description of the adsorption profile of surfac-

tants in soils and clays. These adsorbents offer a wide

range of physicochemical properties. We think that

this is an ideal platform from which to critically ex-

amine the relationships between the clay and soil

characteristics and their adsorption capacity for the

surfactants.

II. EXPERIMENTS

1. Natural Solids

Six agricultural soils and two natural clays



376 Journal of the Chinese Institute of Engineers, Vol. 28, No. 2 (2005)

(Ca-montmorillonite and Na-kaolinite) were selected

for the sorption experiments. All of the soil samples

were collected from Taiwan. The Ca-montmorillo-

nite and Na-kaolinite samples were purchased from

the University of Missouri-Columbia, Source Clay

Minerals Repository. The properties of the above

adsorbents are given in Table 1.

2. Surfactants

The cationic, anionic, and nonionic surfactants

used were domiphen bromide (DB or D+ Br–), sodium

dodecylbenzene sulfonate (DBS or DB –Na +), and

octylphenol polyethoxylate with an average ethylene

oxide chain length of n=9.5 (TX-100). All of these

compounds were of analytical grade or better andwere used as received. Some of the properties of the

surfactants used are listed in Table 2.

3. Adsorption Measurements

A few grams of solid were placed in a 25 ml

tube containing the surfactant water solutions of

known concentrations. Equilibrium was reached by

shaking for 24 h with a reciprocating shaker at 25°C.

The solution and solid phases were separated using a

high-speed centrifuge (Sorvall Co., Model RC-5C) at

8000 rpm for 25 min. A 15-ml aliquot of the super-natant was removed and analyzed using the UV wave

lengths (see Table 2). The specific excess amount

was calculated, using the relation Q=V ∆C / m, where

V is the volume of the liquid phase, m is the mass of the solid, and ∆C is computed as the difference be-

tween the initial and final UV readings.

III. RESULTS AND DISCUSSION

The most frequently exhibited adsorption iso-

therms of surfactants in soils and clays are three types:

the Langmuir (L-type); the S-shape (S-type) and the

“two plateaus” shape (LS-type). According to the

Freundlich equation, the adsorptive amount of the

surfactants relative to equilbrium concentration in the

liquid phase is expressed as,

Q =xm = KC

1/ n (1)

Here n>1, n=1, and n<1 represent the L-type, the lin-

ear-type and the S-type, respectively. In addition, a

two-step adsorption model (LS types) has been pro-

posed (Zhu and Gu, 1991). In the first step the sur-

factants are adsorbed as individual ions or molecules

(depending on the type of surfactant involved) in the

first layer of the solid surface, through electrostatic

attraction (this is only present in the case of cationic

surfactants) and/or specific attraction (i.e., van der

Waals or hydrogen bonding between hydrophilicgroups in the surfactants and the mineral surface). In

the second step, the adsorbed surfactant molecules,

on the adsorbent, through association, or hydropho-

bic interactions between the hydrocarbon chains of

the surfactants, increase dramatically.

Although the two-step adsorption model may

provide a theoretical basis for the mechanism of sur-

factants adsorbed into soils and clays, the adsorption

capacity of surfactants for solids correlated with the

properties of the adsorbent, the surfactant character-

istics, and the solution’s properties and should be dis-

cussed further. In this study, three types of thesurfactants, and eight solids were used to evaluate the

Table 2 Properties of the studied surfactants:

MW=molecular weight, CMC=critical

micelle concentration, and WL=UV ab-sorption wave length

Surfactant MW CMC (mol/m3) WL (nm)

DB 414 1.77 268.4

DBS 349 1.50 260.8

TX-100 624 0.21 274.8

Table 1 Properties of the studied solid samples

Sand Silt Clay BET Pore volume Pore size SOM CECSolid (Abbreviation)

(%) (%) (%) (m2 /g) (cm3 /g) (nm) (%) (meq/100g)

Kuaikuan bottom soil (KKB) 10 56 34 4.73 0.018 13.61 20.0 41.9

Kuaikuan top soil (KKT) 14 54 32 13.28 0.034 9.88 5.6 19.5Shamou Mountain soil (SM) 46 46 8 57.23 0.079 5.18 27.3 53.9

Chinsing Mountain soil (CS) 68 22 10 10.14 0.019 8.00 14.8 19.8

Li Mountain soil (LM) 28 44 28 15.35 0.041 10.42 0.5 6.8

Hsiushui soil (HS) 6 68 26 2.10 0.008 14.09 3.9 9.9

Ca-montmorillonite (Ca-Mon) — — 99 80.79 0.146 6.97 0.03 120.2

Na-kaolinite (Na-Kao) — — 99 11.52 0.056 17.05 0.02 2.3



J. F. Lee et al.: Effect of Soil Properties on Surfactant Adsorption 377

adsorption characteristics of the surfactants on the

basis of their respective isotherm types. The solids

with striking differences in the physicochemical

properties, relative to the surfactant adsorption, were

selected to describe the adsorption mechanisms.

1. Cationic Surfactants

As shown in Fig. 1, all of the exhibited adsorp-

tion isotherms are L-type and the maximum adsorp-

tion capacity is ordered as follows: Ca-Mon>KKB>

SM>KKT≈CS>HS>LM>Na-Kao, which is generally

consistent with the order of the CEC values, except

for the KKB and SM sequences (see Table 1). Elec-

trostatic interaction and hydrophobic bonding are

important driving forces for cationic surfactant ad-

sorption (Xu and Boyd, 1995). In our results, the ad-

sorption isotherms of the DB only reach a plateau in

the given concentration range, indicating the absence

of two-step adsorption when the aqueous DB concen-tration reaches the CMC.

It is well known that the CEC plays a key role

in the adsorption of cationic surfactants by solids.

Since both the mineral surfaces and the SOM have a

number of exchangeable cation sites, the overall DB

adsorption is weakly related to the surface area or the

SOM. However, in some cases, another factor must

also be taken into consideration. For example, al-

though SM has a larger CEC than KKB, the satura-

tion capacity of KKB is larger than that of SM. The

reason may be ascribed to the small SM pore size,

which may inhibit some of the DB from entering thepores and thus prevent adsorption by the pores. The

DB, therefore, remains primarily on the external sur-

face of the substrate.

2. Anionic Surfactants

The DBS adsorption isotherms for the five sol-

ids are presented in Fig. 2. The shapes of the surfac-tant adsorption isotherms for the selected solids can

be basically divided into two types: the S-shape (S-

type) and the “two plateaus” shape (LS-type). The

adsorption capacity of the examined soils and clays

is in the following order: Ca-Mon>SM>KKB>Na-

Kao>CS. In general, anionic surfactants are com-

posed of highly polar and non-polar functional groups.

Hence, hydrogen bonding, adsorption by the polar-

ization of π electrons, adsorption by dispersion forces,

and hydrophobic bonding (partitioning), may all be

operative mechanisms. The isotherms were S- types

representing low affinity of DBS to the soils under

low surfactant concentration. This is because soilsin aqueous solutions often contain negative charges

on the surface to produce repulsion with anionic

surfactants. The LS-type isotherms may be explained

with the above-mentioned two-step adsorption model.

For Ca-Mon and SM, this higher surface area is

a dominant factor, which leads to higher adsorptive

amounts. Another probably influential factor is the

organic matter content. The KKB with the high SOM,

onto which DBS can partition, indicates the relatively

higher adsorptive amounts with respect to its low sur-

face area. Thus, the anionic surfactants adsorbing on

natural soils theoretically increase as the SOMincreases, regardless of the surface area. However,

20

15

10

5

0

0 2

KKT (n=9.2, K =0.15)

SM (n=10.7, K =0.16)

CS (n=14.7, K =0.16)

LM (n=7.1, K =0.06)

HS (n=8.3, K =0.10)

Na-Kao (n=2.4, K =0.04)

4

CMC

6 8

Equilibrium DB concntration, X (mol/m3)

D B u p t a k e ,

Q ( m

o l × 1 0 2 / k g )

10 12 14 16

20

15

10

5

00 2

KKB (n=11.0, K =0.28)Ca-Mon (n=9.2, K =2.38)

4

CMC

6 8

Equilibrium DB concntration, X (mol/m3)

D B u p t a k e ,

Q ( m

o l × 1 0 1 / k g )

10 12 14

Fig. 1 The DB sorption isotherms of the selected natural solids, n and K are noted in Eq. (1)



378 Journal of the Chinese Institute of Engineers, Vol. 28, No. 2 (2005)

an unknown SOM ingredient may complicate the par-titioning behavior of the surfactants to produce a dis-

parate result. We thus suggest that the influential fac-

tors on anionic surfactant adsorption have the following

order: surface area >SOM>other factors.

3. Non-Ionic Surfactants

The TX-100 adsorption isotherms for seven

natural solids are shown in Fig. 3. Ca-Mon exhibited

the highest uptake, and the others, following order,

are KKB, KKT, CS/LM, SM, and Na-Kao. The TX-

100 uptake reached a plateau when the concentrationX was 1- 2 times the nominal CMC in pure water.

Fig. 2 The DBS sorption isotherms of the selected natural solids n and K are noted in Eq. (1)

40

30

20

10

00 1

KKB (left Y axis, n=0.3, K =0.006)

SM (left Y axis, n=1.8, K =0.013)

CS (left Y axis, n=0.7, K =0.001)

Ca-Mon (right Y axis,n=0.7, K =0.014)Na-Kao (left Y axis,n=1.4, K =0.004)

CMC 2 3

Equilibrium DBS concntration, X (mol/m3)

D B S u p t a k e ,

Q ( m

o l ×

1 0 3 / k g )

120

100

80

60

40

20

0

D B S u p t a k e ,

Q ( m

o l ×

1 0 3 / k g )

4 5 6

Fig. 3 The TX-100 sorption isotherms of the selected natural solids n and K are noted in Eq. (1)

12

9

6

3

00.0 0.6

SM (n=4.3, K =0.007)

CS (n=2.3, K =0.012)

LM (n=6.8, K =0.010)

Na-Kao (n=1.6, K =0.005)

1.2

CMC

1.8 2.4

Equilibrium TX-100 concntration, X (mol/m3)

T X - 1 0 0

u p t a k e ,

Q ( m

o l × 1 0 3 / k g )

3.0

20

16

12

8

4

0

0.0 0.6

KKB (n=4.1, K =0.046)

KKT (n=4.4, K =0.023)

Ca-Mon (n=1.7, K =0.22)

1.2

CMC

1.8 2.4 3.0

Equilibrium TX-100 concntration, X (mol/m3)

T X - 1 0 0

u p t a k e ,

Q ( m

o l × 1 0 2 / k g )

3.6

All of the adsorption isotherms are “L type”, appear-ing to take place more by adsorption than by

partitioning.

For Ca-Mon, LM, and Na-Kao, the adsorption

capacity is closely related to the solid surface area.

However, the effects of the SOM content are beyond

the surface area for TX-100 uptake on KKB and KKT.

One of the possible reasons is that the surface area

difference between KKB and KKT is small and the

van der Waals force between TX-100 and SOM is more

than that between TX-100 and soil mineral matter.

Since both SM and CS have comparable SOMs

and surface areas, the TX-100 uptake involves ad-sorption of the mineral matter, and partitioning or



J. F. Lee et al.: Effect of Soil Properties on Surfactant Adsorption 379

adsorption onto the SOM. This indicates that the

overall TX-100 adsorption is not only related to the

surface area but also weakly related to the SOM. Both

the SOM and the surface area for SM are the highest,

but its saturation capacity is lower than most above

soils, indicating that another factor must be taken into

consideration. Although the TX-100 adsorption ca-

pacity was not closely related to the SOM, which is

in contrast to the findings of Haigh (1996), this re-

sult should be limited at the specific SOM range. It

can be concluded that the nonionic surfactant sorp-

tion is proportional to the soil mineral matter content

unless the soil has a fairly high organic matter content.

IV. CONCLUSIONS

Form this study, it can be concluded that cat-

ionic surfactants tended to be strongly adsorbed tosoils and clays via the electrostatic interactions and

thus the DB adsorption capacities were directly pro-

portional to the CEC values of the examined solids.

The possible sorption mechanism for the anionic sur-

factant is the adsorption by van der Waals force or

hydrogen bonding and partitioning into the SOM. The

adsorption of nonionic surfactants usually showed a

correlation with the soil mineral properties, but the

relatively higher SOM soils may produce divergent

results.

NOMENCLATURE

C equilbrium concentration of solutes in the so-

lution mol/m3

∆C the difference between the initial and final sur-

factant concentration mol/m3

K equilibrium constant

m mass of the solid adsorbent kg

n experimental constant

Q adsorption amount mol/kg adsorbent

V volume of the liquid phase m3

X equilibrium surfactant concentration mol/m3

x solute amount on the soils or clays mol

REFERENCES

Goloub, T. P., Koopal, L. K., Bijsterbosch, B. H., and

Sidorova, M. P., 1996, “Adsorption of Cationic

Surfactants on Silica Surface Charge Effects,”

Langmuir , Vol. 12, No. 13, pp. 3188-3194.

Haigh, S. D., 1996, “A Review of the Interaction of

Surfactants with Organic Contaminants in Soil,”

Science of The Total Environment , Vol. 185, No.

1-3, pp. 161-170.

Lee, J. F., Liao, P. M., Kuo, C. C., Yang, H. T., andChiou, C. T., 2000, “Influence of a Nonionic Sur-

factant (Triton X-100) on Contaminant Distribu-

tion between Water and Several Soil Solids,”

Journal of Colloid and Interface Science, Vol.

229, No. 2, pp. 445-452.

Xu, S., and Boyd, S. A., 1995, “Alternative Model

for Cationic Surfactant Adsorption by Layer

Silicates,” Environmental Science & Technology,

Vol. 29, No. 12, pp. 3022-3028.

Zhu, B. Y., and Gu, T., 1991, “Surfactant Adsorption

at Solid-Liquid Interfaces,” Advances in Col loid

and Interface Science, Vol. 37, Nos. 1-2, pp. 1-

32.

Manuscript Received: Feb. 09, 2004

Revision Received: Jun. 18, 2004

and Accepted: Jul . 09, 2004

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