ORIGINAL ARTICLE

Sorption characteristics of ethyl tert-butyl ether to Chinesereference soils

Xiaoxing Zhu • Erping Bi

Received: 22 May 2010 / Accepted: 5 February 2011 / Published online: 19 February 2011

� Springer-Verlag 2011

Abstract As a gasoline additive, ethyl tert-butyl ether

(ETBE) has great market potential and its utilization might

cause groundwater contamination problem. However, little

research has been done on its sorption in soil. In this study,

the sorption characteristics of ETBE to Chinese reference

soils were studied in batch experiments. The results

showed that the ETBE sorption to six soils can be descri-

bed by linear sorption isotherm. The temperature influences

the sorption process of ETBE to soils. The negative sorp-

tion enthalpy (DH \ 0) indicated that the sorption process

was exothermic. Furthermore, DH is in a range from -8 to

-32 kJ/mol. This showed that van der Waals forces and

other specific interactions happened simultaneously in the

sorption process. With the increasing ionic strength, con-

tent of ETBE sorption to all soils decreased, which is

probably also an indication of other sorption mechanisms

besides ETBE partitioning into soil organic carbon.

Keywords Ethyl tert-butyl ether (ETBE) � Chinese

reference soil � Sorption � Sorption thermodynamics

Introduction

Methyl tert-butyl ether (MTBE) is a widely used gasoline

oxygenate. However, MTBE has high water solubility, and

has caused water contamination problems (Bi et al. 2005;

Chisala et al. 2007). Due to its adverse effects (Squillace

et al. 1997), the use of MTBE has declined. As a promising

alternative of MTBE, ethyl tert-butyl ether (ETBE) is used

for increasing octane and enhancing fuel burning rate

(Goldaniga et al. 1998; Segovia et al. 2010). Compared to

MTBE, ETBE has superior qualities as an octane enhancer

and could be produced from bio-ethanol, which is a

renewable source (Ancillotti and Fattore 1998; Ozbay and

Oktar 2009).

With the growing use of ETBE, the public paid more

attention to its environmental problems. Some works have

been done on ETBE environmental issues in the past

decades, such as water quality and ETBE use (Zogorski

et al. 1996), ETBE biodegradation (Hernandez-Perez et al.

2001; Kharoune et al. 2002; Steffan et al. 1997), ETBE

toxicity to animals and humans (McGregor 2007; Stanard

et al. 2003), transfer in water–air systems of ETBE (Arp

and Schmidt 2004), and effect of ETBE solubility by

temperature (Gonzalez-Olmos and Iglesias 2008).

In the groundwater system, sorption dominates the

migration and transformation of organic pollutants. To

better understand the environmental fate of ETBE in the

subsurface, further study of ETBE sorption and transfer in

water and soil systems needs to be carried out. Besides soil

properties, e.g., organic matter content and characters

(Greenwood et al. 2007; Shih 2007), and mineral sorption

of organic matter (Dontsova and Bigham 2005; Gu and

Karthikeyan 2005), sorption process can also be affected

by environmental factors such as temperature, presence of

dissolved organic matter, pH, ionic strength, type of solu-

tion cations (Abu and Dike 2008; Tremblay et al. 2005),

and the physicochemical properties of organic pollutants.

Yu et al. (2005) and Inal et al. (2009) have done some

research on sorption of ETBE and MTBE to granular

X. Zhu � E. Bi (&)

School of Water Resources and Environment, Beijing Key

Laboratory of Water Resources and Environment Engineering,

China University of Geosciences (Beijing), 29 Xueyuan Road,

Beijing 100083, People’s Republic of China

e-mail: bi@cugb.edu.cn

X. Zhu

e-mail: zhuxiaoxinggo@163.com

123

Environ Earth Sci (2011) 64:1335–1341

DOI 10.1007/s12665-011-0958-3

activated carbon, respectively. The results showed that the

sorption was nonlinear, and the Freundlich and Langmuir

isotherms described the equilibrium sorption of ETBE

well. However, little has been done on the sorption of

ETBE to a wide range of soils (e.g., reference soils of

China) under different conditions.

The main purpose of this study was to investigate the

sorption characteristics of ETBE to Chinese reference soils

through batch experiments. The effects of temperature and

ionic strength on the sorption were investigated to provide

reliable data and information for the prediction of ETBE

transport in the soil–groundwater system.

Materials and methods

Materials

Chinese reference soils used in the experiment were

obtained from Langfang Institute of Geochemistry and

Geophysics, Chinese Academy of Geosciences. Some

properties of the reference soils are given in Table 1.

ETBE (C99.0%, Sigma-Aldrich) stock solution was pre-

pared in methanol (HPLC grade, Burdick and Jackson) and

stored at 4�C in dark. Anhydrous CaCl2 (analytical grade,

Beijing Chemical Works) was dissolved with deionized

water for different ionic strength solutions.

Sample preparation

In batch experiments, the ETBE solution (0.005 M CaCl2as background electrolyte) volume in 20-mL crimp-top

headspace vial was 15 mL and the mass of each reference

soil added was 4.00 g. The kinetic experiment was per-

formed to determine the sorption equilibrium time. The

initial concentration range of ETBE was from 1.0 to

143.1 mg/L in an isothermic experiment (25�C, 0.005 M

CaCl2). Thermodynamic experiments were carried out at

temperatures of 15, 25, and 40�C, kept ionic strength at

0.005 M CaCl2. Different ionic strength solutions [i.e., 0 M

(deionized water), 0.005 M, and 0.05 M CaCl2] were used

to investigate the effect of ionic strength on sorption. All

vials were shaken on a horizontal shaker (HZQ-C, HDL) at

180 rpm. After sorption reached equilibrium, the samples

were centrifuged (TD5A-WS, Xiangyi) for 15 min at

3,200 rpm; then supernatant solution was removed into a

10-mL crimp-top headspace vial for analyses by a 5-mL

glass syringe (Zhongge, Shanghai). Blank controls (con-

taining only deionized water and ETBE) were used for

controlling volatilization in batch experiments.

Analytical methods

ETBE was analyzed by gas chromatography (HP6820 GC,

Agilent), equipped with automatic headspace sampler

(HP7694E, Agilent), a 30 m capillary column (HP-624,

0.53 mm ID, 3.0 lm film thickness) and a flame ionization

detector (FID). Split ratio was 10:1, and injector tempera-

ture was kept constant at 150�C. The column temperature

was programmed as follows: 2 min at 45�C; the tempera-

ture was increased at the rate 10�C/min to 90�C, hold time

3 min. The FID temperature was 150�C. The method

detection limit for ETBE is 0.05 mg/L.

Sorption isotherms

When the headspace method is used, it is necessary to

consider the ETBE mass in the gas phase; the concentration

of ETBE in the solid phase was calculated by following

equation:

Cs ¼ ðC0Vw � CeVw � CeVgH25Þ=ms ð1Þ

where Cs is the sorbent concentration in the solid phase

(mg/kg), C0 the initial aqueous concentration (mg/L),

Ce the equilibrium aqueous concentration (mg/L), Vw the

solution volume (mL), Vg the volume of gas phase (mL),

ms sorbent mass (g), and H25 is the Henry’s law constant at

25�C (0.06616 for ETBE, data from SRC PhysProp

Database).

Inal et al. (2009) and Yu et al. (2005) have reported that

sorption of ETBE to activated carbon can be described by

the Freundlich isotherm:

Table 1 Main physicochemical properties of Chinese reference soils

Sample Sampling location pH SiO2 (%) Al2O3 (%) Fe2O3 (%) OC (%)

GSS10 Songnen Plain 8.13 65.50 ± 0.12 13.80 ± 0.11 4.17 ± 0.03 1.35 ± 0.07

GSS11 Liaohe Plain 7.60 69.42 ± 0.28 13.14 ± 0.06 4.21 ± 0.06 1.07 ± 0.06

GSS13 North China Plain 8.74 64.88 ± 0.29 11.76 ± 0.10 4.11 ± 0.04 0.62 ± 0.08

GSS14 Sichuan Basin 8.10 64.51 ± 0.36 14.43 ± 0.13 5.32 ± 0.06 0.79 ± 0.07

GSS15 Yangtze Plain 7.77 63.63 ± 0.20 15.27 ± 0.10 6.44 ± 0.07 0.78 ± 0.05

GSS16 Pearl River Delta 7.14 63.81 ± 0.16 17.85 ± 0.12 5.44 ± 0.05 0.97 ± 0.12

Based on the information provided by the manufacturer

1336 Environ Earth Sci (2011) 64:1335–1341

123

Cs ¼ KfCne ð2Þ

where Kf is the Freundlich constant [(mg/kg)/(mg/L)-n],

n is the sorption nonlinearity index. When n = 1, the

Freundlich isotherm was changed to be a linear form:

Cs ¼ KdCe ð3Þ

where Kd is the sorption distribution coefficient (L/kg).

The linear isotherm has been often used in literature

(Schwarzenbach et al. 1993), and it is normally applicable

for narrow solute concentration range. In sorption of

ETBE to granular activated carbon, the isotherms can

also be described by the Langmuir isotherm (Inal et al.

2009):

Cs ¼ CsmKCe=ð1þ KCeÞ ð4Þ

where Csm is the maximum sorption capacity at monolayer

coverage (mg/kg), K is the affinity coefficient (L/mg). The

fitted Csm for isotherms should not be smaller than the

maximum experimental Cs value; and it is calculated by

solver in excel. The coefficient of determination (R2 value)

and the sum of residual squared error (SRSE) are used to

evaluate the fitting quality.

Results and discussions

Sorption kinetics

Kinetic experiments were conducted to determine the time

to reach sorption equilibrium. The results showed that

sorption of ETBE to six reference soils reached equilibrium

in approximately 48 h (Fig. 1). It was used as the equili-

brating time in the experiments for isotherm determination.

Sorption isotherms

The equilibrium sorption data of ETBE to six Chinese

reference soils were fitted by the linear, Freundlich and

Langmuir isotherms, respectively (Fig. 2; Table 2). In the

experimental concentration range of ETBE, the correlation

coefficients showed that the sorption data could be

appropriately described by the linear sorption isotherm,

which normally means that the partition of ETBE to soil

organic carbon dominates the sorption process.

Both correlation coefficients and SRSE values indicated

that the Freundlich isotherm fits the experimental data quite

well except that for soil GSS15 (Table 2). The exponent

(n) values are quite close to 1. Compared with the high

nonlinear ETBE sorption (n from 0.33 to 0.62) to granular

activated carbon (Inal et al. 2009), the ETBE sorption to

Chinese soils is almost in the linear range (n from 0.884 to

1.251).

The Langmuir isotherm reasonably described the sorp-

tion of ETBE to soils GSS11, GSS13, GSS14, and GSS16,

but underestimates the sorption to soils GSS10 and GSS15

over the whole concentration range. The fitted Csm values

ranged from 1,988 to 3,099 mg/kg, which is much higher

than the maximum measured values (117.7 mg/kg). This

means that the sorption ETBE in the experimental con-

centration range is in the linear part of the Langmuir iso-

therm, i.e., the lower aqueous concentration range.

Comparing the fitting results of three sorption isotherms,

the linear sorption isotherm was used to discuss the sorp-

tion characteristics of ETBE to different soils. The Kd

values of six reference soils decreased in the order

GSS10 [ GSS15 [ GSS11 [ GSS13 [ GSS14 [ GSS16

(Table 2). This is not consistent with the order of organic

carbon content in soils (Table 1), especially for soil

GSS15, which is probably an indication of the effect of the

soil organic carbon characteristics.

The organic carbon normalized sorption distribution

coefficient Koc of ETBE to the soils was not constant and

was in a range from 68.4 to 136.5 L/kg. It is often found

that the Koc is not constant for a single organic compound

due to different sorbents (Zhang et al. 2009). The variation

in Koc normally reflected the different origin and structure

of soil organic matter (SOM) composition. In previous

studies, it was found that the composition of SOM affects

the sorption of organic chemicals to soils (Chiou et al.

2000; Franco et al. 2006; Pan et al. 2007), and SOM

properties can be influenced by several factors (Bekins

et al. 2001; Martin-Neto et al. 1998; Spaccini et al. 2006).

012

3456

78

0 20 40 60 80 100

t (h)

Ce (

mg/

L)

GSS10

GSS11

GSS13012

3456

78

0 20 40 60 80 100

t (h)

Ce

(mg/

L)

GSS14

GSS15

GSS16

Fig. 1 Sorption kinetics of

ETBE to different Chinese

reference soils

Environ Earth Sci (2011) 64:1335–1341 1337

123

Koc values from GSS13 and GSS15 were much higher than

those from the other four soils (Fig. 3; Table 2). This might

have resulted from the climate impact on the forming

process of SOM in soils, especially the humic acid (HA)

and fulvic acid (FA). GSS13 and GSS15 were collected

from the North China Plain and the Yangtze River Plain,

respectively, where the climate is temperate. According to

Xiong (1987), the ratio of HA/FA of soil in the temperate

climate is high. The higher HA/FA ratios of soil GSS13

and GSS15 led to the higher Koc values than other soils

Linear Freundlich Langmuir

0

30

60

90

120

150

0 30 60 90 120 150

Ce (mg/L)

Cs

(mg/

kg)

GSS10

0

30

60

90

120

150

0 30 60 90 120 150

Ce (mg/L)

Cs

(mg/

kg) GSS11

0

30

60

90

120

150

0 30 60 90 120 150

Ce (mg/L)

Cs (

mg/

kg) GSS13

0

30

60

90

120

150

0 30 60 90 120 150

Ce (mg/L)

Cs (

mg/

kg) GSS14

0

30

60

90

120

150

0 30 60 90 120 150

Ce (mg/L)

Cs

(mg/

kg)

GSS15

0

30

60

90

120

150

0 30 60 90 120 150

Ce (mg/L)C

s (m

g/kg

) GSS16

Fig. 2 ETBE sorption

isotherms for different Chinese

reference soils

Table 2 Values of isotherm parameters for sorption of ETBE to reference soils

Models Parameters GSS10 GSS11 GSS13 GSS14 GSS15 GSS16

Linear Kd (L/kg) 1.120 0.999 0.846 0.729 1.057 0.663

Koca (L/kg) 82.963 93.364 136.452 92.278 135.513 68.351

R2 0.982 0.946 0.992 0.950 0.914 0.976

SRSEb 1.165 0.387 1.077 0.733 1.001 0.264

Freundlich Kf [(mg/kg)/(mg/L)-n] 0.379 1.179 0.776 0.665 0.724 0.665

n 1.251 0.959 1.016 1.022 1.088 1.008

R2 0.987 0.958 0.998 0.987 0.884 0.989

SRSE 0.115 0.582 0.042 0.482 0.708 0.226

Langmuir Csm (mg/g) 3.099 2.067 2.050 2.012 2.077 1.998

K (L/kg) 3.10E-04 4.74E-04 4.04E-04 3.32E-04 4.39E-04 3.14E-04

R2 0.993 0.972 0.995 0.975 0.958 0.988

SRSE 0.645 0.361 0.058 0.476 0.771 0.226

a Koc = Kd/foc

b SRSE ¼Pn

i¼1 ððfitted�measuredÞ=measuredÞ2; n is the number of data points

1338 Environ Earth Sci (2011) 64:1335–1341

123

because FA is normally thought to have little contribution

to the overall sorption.

Effect of temperature

To further investigate the ETBE sorption to soils GSS13

and GSS15, the influence of temperature on the sorption of

ETBE to GSS13 and GSS15 was studied and the results

were shown in Fig. 4. Sorption coefficients decreased with

increasing temperature, which indicates that the sorption

processes are exothermic. The enthalpy change (DH) of

sorption was calculated from the van’t Hoff equation:

ln Kd ¼ DS=R� DH=R� 1=T ð5Þ

where DS is the entropy change [kJ/(mol K)], DH the

enthalpy change (kJ/mol), R the gas constant [8.3145

J/(mol K)], and T is the temperature (K).

There were two DH values at two different solution

concentrations for soils GSS13 and GSS15. The lower

-DH value was obtained from the higher solution con-

centration, this was in line with previous findings (Abu and

Dike 2008; Spurlock et al. 1995). The DH values of GSS13

were in a range from -8.83 to -31.07 kJ/mol, and the

DH value range of GSS15 was from -13.91 to -22.92 kJ/

mol. The enthalpy change (DH) of all sorption was higher

than -32 kJ/mol. This is consistent with the absence of

covalent bonding (i.e., chemisorption, which generally

gives DH in the range from -60 to -80 kJ/mol; Delle

2001). However, the van der Waals forces are normally

corresponding to a DH from -4 to -8 kJ/mol (Delle

2001); therefore, it was proposed that specific interactions

besides partition play a role in the ETBE sorption to ref-

erence soils GSS13 and GSS15.

Effect of ionic strength

To investigate the effect of solution chemistry on the

sorption of ETBE to reference soils, the solution ionic

strength was changed. As shown in Fig. 5, with increase of

CaCl2 concentration from 0 to 0.05 M, the sorption coef-

ficients Kd of ETBE to reference soils decreased. Similar

result was found for other apolar compounds (Abu and

Dike 2008; Yuan and Xing 2001). Although cations in a

solution do not contribute to ETBE sorption directly, their

interaction with SOM may play a pivotal role in the

sorptive behavior of HA in soils. When ionic strength is

low, all organic molecules on the particle surfaces are fully

accessible for sorption. When ionic strength increases,

cations might form bridges between the anionic functional

group of HA and dissolved organic carbon (DOC)

(Schlautman and Morgan 1993; Yuan and Xing 2001).

Saturation of HA and DOC with cations might cause the

macromolecules to coagulate and become condensed,

which reduce the interaction between sorbate and SOM,

thus Kd values decreased.

The change of ionic strength of solution has different

effects on the ETBE sorption to soils (Fig. 5). Taking the

sorption at 0.005 M as reference, when ionic strength

increased from 0.005 to 0.05 M, it has quite similar inhi-

bition effects on the sorption to all soils. However, when

ionic strength decreased from 0.005 to 0 M, it has least

effects on the ETBE sorption to soil GSS13 but highest

S16

S15

S14

S13

S11 S10

0

50

100

150

200

0.5 1 1.5

f oc

Koc

(L/k

g)average of K oc

Fig. 3 Correlation between Koc for ETBE and organic carbon (foc) in

soils

-2.0

-1.5

-1.0

-0.5

0.0

0.5

0.0031 0.0032 0.0033 0.0034 0.0035

1/T(K-1)

lnK

(L

/kg)

d

GSS13 GSS15 (C0=35mg/L)

GSS13 GSS15 (C0=71mg/L)

Fig. 4 Sorption thermodynamics of ETBE to GSS13 and GSS15

(0.005 M CaCl2, initial concentrations 35 and 71 mg/L)

0

100

200

Soils

Kd

Cha

nge

(%)

0M 0.005M 0.05M

GSS16GSS15GSS14GSS13GSS10 GSS11

Fig. 5 Effect of CaCl2 concentrations on the sorption of ETBE to

reference soils (initial ETBE concentration 13 mg/L, 25�C)

Environ Earth Sci (2011) 64:1335–1341 1339

123

effect on that of soil GSS10, which probably resulted from

the difference in SOM properties of soils. A possible

explanation is that the interaction between SOM and cat-

ions (e.g., Ca2?) caused the change of sorption character-

istics of soils. To understand the mechanisms in detail,

further work needs to be carried out.

Conclusions

The sorption characteristics of ETBE to six Chinese ref-

erence soils were determined in batch experiments. Over-

all, the sorption can be described by linear sorption

isotherm indicating that partition dominates the sorption

process in the experimental concentration range. However,

the effects of temperature and solution ionic strength on

sorption indicated that other processes also play a role in

the sorption of ETBE to soils.

Acknowledgments This work was supported by the National

Natural Science Foundation of China (No. 40972161).

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