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Xi, Yunfei, Mallavarapu, Megharaj, & Naidu, Ravendra(2010)Adsorption of the herbicide 2,4-D on organo-palygorskite.Applied Clay Science, 49(3), pp. 255-261.

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1

Adsorption of the herbicide 2,4-D on organo-palygorskite

Yunfei Xi1,2, Megharaj Mallavarapu1,2 and Ravendra Naidu1,2

1 Centre for Environmental Risk Assessment and Remediation (CERAR), University of

South Australia, Mawson Lakes, SA 5095, Australia;

2 Cooperative Research Centre for Contamination Assessment and Remediation of the

Environment (CRC CARE), University of South Australia, Mawson Lakes, SA 5095,

Australia.

Corresponding author: Tel: +61-8-8302-6232,

Email address: yunfei.xi@unisa.edu.au (Y. Xi)

2

Abstract

Among the clay minerals, montmorillonite is the most extensively studied

material using as adsorbents, but palygorskite and its organically modified products

have been least explored for their potential use in contaminated water remediation. In

this study, an Australian palygorskite was modified with cationic surfactants octadecyl

trimethylammonium bromide and dioctadecyl dimethylammonium bromide at different

doses. A full structural characterization of prepared organo-palygorskite by X-ray

diffraction, infrared spectroscopy, surface analysis and thermogravimetric analysis was

performed. The morphological changes of palygorskite before and after modification

were recorded using scanning electron microscopy, which showed the surfactant

molecules can attach on the surface of rod-like crystals and thus can weaken the

interactions between palygorskite single crystals. Real surfactants loadings on organo-

palygorskites were also calculated based on thermogravimetric analysis. 1 CEC, 2 CEC

octadecyl trimethylammonium bromide modified palygorskites, 1 CEC and 2 CEC

dioctadecyl dimethylammonium bromide modified palygorskites absorbed as much as

12 mg/g, 42 mg/g, 9 mg/g and 25 mg/g of 2,4- dichlorophenoxyacetic acid respectively.

This study has shown a potential on organo-palygorskites for organic herbicide

adsorption especially anionic ones from waste water. In addition, equilibration time

effects and the Langmuir and Freundlich models fitting were also investigated in details.

Keywords: palygorskite; octadecyl trimethylammonium bromide; dioctadecyl

dimethylammonium bromide; 2,4- dichlorophenoxyacetic acid; adsorption; organoclay

3

1. Introduction

Due to its low cost and good selectivity, 2,4-dichlorophenoxyacetic acid (2,4-D)

is used as one of the most commonly used herbicides in the world for controlling a wide

range of broadleaf weeds. Thus it can be frequently detected in the environment from its

application on weed control in lawn, along fences/highway/railroad and in aquacultures,

with adverse impacts observed on the ecosystems (Aksu and Kabasakal, 2004; Chao et

al., 2008). It has been proved to be moderately toxic and potentially carcinogenic. It is

water soluble and due to its low pKa value at 2.73 (Chao et al., 2008; Nelson and Faust,

1969), it exists predominantly in anionic form which can badly affect the aquatic life in

water bodies and hamper the ecosystem; and its poor biodegradable feature makes it

stable in the environment (Carter, 2000). In general, anionic contaminants in ground

water are a great concern because they are weakly retained by most soil and subsurface

materials (Fontaine et al., 1991; Hermosin et al., 2006). Therefore, the efficient removal

of this kind of herbicide is attracting a lot of concerns; several physical, chemical and

biological processes have been evolved so far for the removal of 2,4-D. Among them,

adsorption has been proven to be an effective and attractive mechanism (Aksu and

Kabasakal, 2004; Hameed et al., 2009). In this context, the use of clay materials has

obtained considerable attention (Churchman et al., 2006). Clay minerals are suitable

candidates to immobilise herbicide from waste water due to their inexpensive

availability, environmental stability and high adsorptive and ion exchange properties.

However, the intrinsic negative surface charge of most natural clay minerals imparts

them repulsing interaction with this compound. These clay minerals are highly

hydrophilic and consequently show very low adsorption for hydrophobic molecules.

However, when the surfaces of these materials are modified by introducing long chain

organic surfactant, high adsorption of herbicides can be achieved. Additionally, the

4

cationic surfactant may impart positive surface charge to the clay minerals that can

effectively hold 2,4-D molecules.

Bentonite is the most studied material using as adsorbents (Harvey and Lagaly,

2006; Hermosin et al., 2006; Xi et al., 2010; Zhou et al., 2007). Palygorskite and its

organically modified products have been least explored for their potential use in

contaminated water remediation (Chang et al., 2009; Chen and Zhao, 2009). The present

study was aimed to evaluate the adsorption potential of organically modified

palygorskite for 2,4-D in aqueous solution. The modified palygorskites were

characterised by surface area analysis, XRD, FTIR, scanning electron microscopy (SEM)

and thermogravimetric analysis (TG). Adsorption of 2,4-D at different surfactant

loading rates was examined.

2. Experimental

2.1. Materials and preparation

An Australian palygorskite (Grade 050F) was supplied by Hudson Resource

Limited, which originates from Western Australia and was used as received without

further purification. The cation exchange capacity (CEC) was 0.17 meq/g as determined

by the ammonia electrode method (Borden and Giese, 2001). It had a specific surface

area of 97.3 m2/g as measured by the BET method on a Gemini 2380 surface analyser.

The surfactants selected for this study were octadecyl trimethylammonium bromide

(C21H46NBr, FW: 392.52, denoted as ODTMA) and dioctadecyl dimethylammonium

bromide (C38H80BrN, MW: 630.95, denoted as DODMA) as supplied by Sigma–Aldrich.

2,4-dichlorophenoxyacetic acid (denoted as 2,4-D, Cl2C6H3OCH2CO2H, MW: 221.04)

5

was also obtained from Sigma-Aldrich and all these chemicals were used without any

purification.

Palygorskite was modified by a similar procedure described previously (Frost et

al., 2008; Xi et al., 2010). The clarifying surfactant solution was obtained by adding

certain amounts of surfactants into hot deionized water. A certain amount of

palygorskite was dispersed in this solution and the mixture was stirred slightly to avoid

the yield of excess spume in an 80 °C water bath for 5 hours. The water/palygorskite

mass ratio was 10. All organo-palygorskites were washed free of bromide anions as

determined by the AgNO3 test, dried at 60°C (S.E.M. Pty Ltd, South Australia), ground

in an agate mortar and stored in a vacuum desiccator before use. The ODTMA and

DODMA modified palygorskites prepared at the concentration of 1 CEC and 2 CEC

were denoted as OP1CEC, OP2CEC, DP1CEC and DP2CEC, respectively.

Because of the reliability and simplicity of batch model, isotherm adsorption

studies were conducted by batch technique at room temperatures (25 °C). In a typical

experiment, 0.2 g of adsorbent material was placed in a 50 mL centrifuge tube

containing 40 mL of 2,4-D solution of known concentration and pH (measured with an

Orion 3 star pH meter from Thermo Electron Corporation). The mixture was shaken for

5 h on a rotating shaker (RATELC Instrument Pty LTD, Vic, Australia), then

centrifuged at 4000 rpm for 15 min (Multifuge 3 S-R, Hevaeus, Kendo Laboratory

Products, Germany). In equilibration time studies, 1 g of adsorbent was placed in a 250

ml beaker containing 200 ml of 100 mg/L 2,4-D solution, the mixture was stirred using

a magnetic stirrer for 5 h, at certain time intervals, certain volume of dispersion was

removed and filtered using a syringe with a 0.45 µm mixed cellulose ester (MCE) filter.

2,4-D concentration in filtered liquid was determined spectrophotometrically on an

Agilent 8453 UV-VIS Spectroscopy system by measuring absorbance at kmax of 282 nm

6

(Aksu and Kabasakal, 2004). The amount of 2,4-D adsorbed by palygorskite/organo-

palygorskites was calculated from the difference between the initial and

final/equilibrium solution concentrations; solid-phase loading of 2,4-D, qe (mg/g) was

computed from the mass balance: qe = V(Ci – Ce)/M; where, Ci and Ce are total

dissolved and equilibrium liquid phase concentration (mg/L), respectively, and M is the

dose of adsorbent (g/L), V is the volume of the solution (mL). All working solutions

were prepared from 2,4-D stock solution diluted with deionized water and all

experiments were carried out in duplicate.

2.2. Characterisation methods

2.2.1. X-ray diffraction (XRD)

Unmodified palygorskite and organo-palygorskites powder were pressed in

stainless steel sample holders. X-ray diffraction (XRD) patterns were recorded using

CuKα radiation (n = 1.5418 Å) on a Panalytical X’Pert (PW3040) diffractometer

operating at 40 kV and 50 mA between 1 and 65° (2θ) at a step size of 0.0167°.

2.2.2. Infrared spectroscopy (IR)

Infrared (IR) spectra were obtained using a Magna-IRTM spectrometer 750

(Nicolet Instrument Corp. USA) equipped with liquid nitrogen cooled mercury-

cadmium-telluride (MCT) detector and DRIFT (Diffuse Reflectance Infra-red Fourier

Transform) accessories. Spectra over the 4000–400 cm−1 range were obtained by the co-

addition of 64 scans with a resolution of 4 cm−1 and a mirror velocity of 0.6329 cm/s.

Peakfit software package (SPSS Inc.) was used to undertake band component analysis

that enabled the type of fitting function to be selected and allowed specific parameters

7

to be fixed or varied accordingly. Gauss–Lorentz cross-product function with the

minimum number of component bands was used for band fitting. The fitting was

undertaken until reproducible results were obtained with squared correlations of r2

greater than 0.98.

2.2.3. Scanning electron microscopy (SEM)

A Philips XL30 FEG scanning electron microscope (SEM) was used for

morphological studies. Palygorskite and the surfactant modified products were dried at

room temperature and coated with carbon for the SEM studies.

2.2.4. Surface analysis

Adsorption and desorption experiments using N2 were carried out at 77K on a

Gemini 2380 surface analyser. Prior to each measurement the samples were degassed at

353K for 24 h. The N2 isotherms were used to calculate the specific surface area (SA)

with the multipoint BET method.

2.2.5 Thermogravimetric analysis

Thermogravimetric analysis were undertaken with a TA Instruments Inc. TA

2950 high-resolution TGA operating at a ramp of 10 °C/min from room temperature to

900 °C in a high-purity flowing nitrogen atmosphere (50 cm3/min). Approximately 30

mg of the finely ground sample was heated each time in an open platinum crucible.

3. Results and discussion

3.1 X-ray diffraction (XRD)

8

The XRD patterns of unmodified and surfactant modified palygorskite are

shown in Fig. 1. An intense reflection at 2θ = 8.55° (10.34Å) was observed for

unmodified palygorskite, which was attributed to the (110) plane of palygorskite. In

addition, relative strong reflections at 2θ = 20.96° (4.24Å), 2θ = 20.04° (4.43Å) and 2θ

= 24.12° (3.69Å) were also observed, which represented the ‘040’, ‘121’ and ‘221’

planes respectively. One reflection observed at 2θ = 12.46° (7.10Å) corresponded to a

(hydr)oxide containing sodium and magnesium ions (Araujo Melo et al., 2002). The

most intense reflection at 2θ = 26.7° (3.34Å) was attributed to quartz (Onal and

SarIkaya, 2009). Other impurities were dolomite and illite. For organo-palygorskites,

similar reflections were observed at 10.49-10.51Å, 4.46-4.47Å, 4.24-4.25Å and 3.70Å

attributed to ‘110’, ‘121’, ‘040’ and ‘221’ planes, respectively. In general, no obvious

differences on the XRD patterns were observed between organo-palygorskites and

unmodified palygorskite, the changes of the (110) reflection position were also minor.

For instance, OP1CEC, OP2CEC, DP1CEC and DP2CEC showed corresponding

reflections at 10.49Å, 10.53Å, 10.55Å and 10.51Å, respectively. Thus, modification

with organic surfactants did not noticeably change the structure of palygorskite. The

surfactants attached on the surface of the rod-like crystals can reduce the aggregation of

the particle (Yuan et al., 2007). In addition, three "diagnostic regions," 4.0-4.5Å, 3.05-

3.3Å and 2.5-2.6Å (Chisholm, 1990; Chisholm, 1992; Yalcin and Bozkaya, 1995) were

also used to distinguish between the orthorhombic and monoclinic form of palygorskite.

The reflections at 4.24Å and 3.69Å revealed that this palygorskite mainly had the

orthorhombic crystal symmetry (Yalcin and Bozkaya, 1995).

3.2 Infrared spectroscopy (IR)

9

One section of FTIR spectra between 3100 cm-1 to 2700 cm-1 is shown in Fig. 2,

where the major changes of palygorskites before and after surfactants modifications

were observed. Compared with the spectrum of unmodified palygorskite, the bands

observed at ~2930 and ~2850 cm-1 were attributed to the CH2 antisymmetric stretching

vibration νas (CH2) and symmetric stretching vibration νs (CH2), the frequency of the

CH2-stretching mode of alkyl chains is extremely sensitive to the conformational

ordering of the chains (Xi et al., 2005; Xi et al., 2007). In addition, these bands were

sensitive to the changes in the gauche/trans conformer ratio and the chain–chain

interactions. Thus, when the loading of surfactant with ODTMA increased from 1 to 2

CEC, νas (CH2) shifted from 2928 cm−1 for OP1CEC to 2926 cm−1 for OP2CEC (See

Fig. 2, bands were fitted with Gauss-Lorentz function using Peakfit software). In

general, the frequency of νas (CH2) was sensitive to the gauche/trans conformer ratio and

the packing density of alkyl chains (Li and Ishida, 2003; Venkataraman and Vasudevan,

2001). Band shifting to higher wavenumbers was characteristic of the disorder by

gauche conformations, whereas shifting to lower wavenumbers indicated highly ordered

all-trans conformations. With the increase of surfactant loading, the frequency of νas

(CH2) decreased. The frequency of νas (CH2) of both the ODTMA organo-palygorskites

was higher than that of the pure surfactant with a band at 2925 cm−1. It reflected that the

surfactant in organo-palygorskites took some disordered conformations comparing with

the pure surfactant. In the case of DODMA modified palygorskites, the frequency

decreased from 2926 cm−1 for DP1CEC to 2924 cm−1 for DP2CEC, while the

antisymmetric stretching band had the same frequency as that of pure surfactant. At this

concentration it was probable that a large amount of surfactant was adsorbed on the

surface so that the value of the antisymmetric stretching vibration corresponded with

that of the pure surfactant. The frequency shift of the CH2 stretching vibrations can be

10

used as a guide to determine the molecular environment of the surfactant molecules in

the organo-palygorskites. The higher frequencies (trans/gauche disorder) represented a

liquid-like environment of surfactant molecules, while the lower frequencies

represented a solid-like environment of the surfactant molecules on palygorskite.

3.3 Scanning electron microscopy (SEM)

The morphological images of unmodified palygorskite and surfactants modified

ones are shown in Fig 3a-e. Unmodified palygorskite (Fig. 3a) appeared as flat and

straight fibres that were oriented randomly and usually presented in aggregates of

entangled bundles and sheet-like layers of rods. This was due to the strong interaction

between the fibrous crystals, and it was difficult to find the single fibrous crystals.

Similar morphology of palygorskite was also discussed in other studies where fibrous

texture and a cardboard or paper-like aggregates were observed (Aiban, 2006; Grim,

1968). The morphological differences between organo-palygorskites and unmodified

palygorskite were not very distinct, but noticeable. As shown in Fig. 3b-e the surfactant

molecules attached on the surface of rod-like crystals can weaken the interactions

between palygorskite single crystals and can ‘isolate’ these particles to some extent. As

a consequence, more single fibrous particles were observed (Yuan et al., 2007). The

type of surfactants had a strong effect on the morphological differences of DODMA-

palygorskites (Fig. 3d, 3e) showed more single fibrous particles than ODTMA-

palygorskites (Fig. 3b, 3c). DODMA with two long chains can ‘isolate’ (by attaching on

the surface) palygorskite particles more effectively. In conclusion, compared with

unmodified palygorskite, fibres were prevalent and much easier to be observed after

modification and they also had more open pore space and were less compacted.

11

3.4 Surface analysis

The specific surface area decreased in the order: palygorskite (97.3 m2/g) >

OP1CEC (43.5 m2/g) > DP1CEC (33.2 m2/g) > OP2CEC (25.6 m2/g) > DP2CEC (23.8

m2/g). With increasing surfactant amount added, the specific surface area decreased and

at the same level of surfactant concentrations, ODTMA modified samples had a larger

specific surface area than that of DODMA modified products, i.e. larger surfactant

cation may produce a smaller BET specific surface area. It is believed that the surfactant

molecules may block the surface or pores/tunnels and thus decrease the N2 accessible

surface area (Wang et al., 2004).

3.5 Thermogravimetric analysis

Several mass-loss steps were observed in the DTG curves for unmodified

palygorskite: The mass loss before 200 °C was attributed to the zeolitic water; bound

water (coordinated water) was lost between 250 °C and 400 °C, and dehydroxylation

occured above 400 °C (VanScoyoc et al., 1979). By comparing the DTG results of

organo-palygorskites with that of unmodified palygorskite, some mass losses occurring

from around 200 to 400 °C were attributed to the decomposition of the surfactant

molecules. And similar to our pervious study (Xi et al., 2007), there were differences

between the amounts of surfactant added and adsorbed. The amount of adsorbed

surfactant was determined by Eq. (1) (Xi et al., 2007) and the results were summarised

in Table 1 - where X is the surfactant amount adsorbed; m the total mass of organo-

palygorskite; M the molar mass of the surfactant; S the mass loss in percent of surfactant

of the organo-palygorskite; y is 0 (if all the Br ions remain) or 80 (no Br ions, the molar

mass of Br is 80), y = 0 or 80 is not possible to reach but it can help to work out the

range of X by calculating theoretical maximum and minimum values. The amount of

12

surfactant adsorbed was always smaller than the added amount. For ODTMA modified

palygorskite, the data fitted reasonably well to the theoretical values for both 1 and

2CEC modified ones, where the amounts surfactants adsorbed were around 0.66-0.83

CEC and 1.82-2.28 CEC respectively. For DODMA modified palygorskite, there were

noticeable less surfactant adsorbed than added. Only about 0.52-0.59 CEC and 1.19-

1.36 CEC were really adsorbed at 1 and 2 CEC.

(1)

3.6 2,4-D adsorption

3.6.1 Adsorption isotherms

Only negligible amounts of 2,4-D were adsorbed by unmodified palygorskite,

even after 24 h. In the presence of the modified palygorskites, the equilibrium

concentration was almost reached within 10 min (Fig. 5). For OP1CEC and DP1CEC,

equilibrium concentrations of 2,4-D were 48 mg/L and 51 mg/L after treatment. For

OP2CEC and DP2CEC, equilibrium concentrations of 2,4-D were 2 mg/L and 10 mg/L.

3.6.2 Isotherm study on 2,4-D adsorption

The palygorskites were equilibrated with 2,4-D within 5h at 25 °C. All isotherm

data were better fitted by the Langmuir model (Langmuir, 1918) than by the Freundlich

(Chiou and Li, 2002) (table 2). The linear form of the Langmuir model is:

Ce/qe = 1/(qmb) + ce/qm …………………………(2)

where, Ce is equilibrium 2,4-D concentration, qe amount of 2,4-D adsorbed, qm the

monolayer capacity and b the Langmuir constants. The maximum adsorption of 2,4-D

(M-y) · m · (100-S) · 10-2 · 17 · 10-3

m · S · 10-2 · 100

17 · (M-y) · (100-S)

S · 105

X = =

13

on OP2CEC and DP2CEC at 25 °C reached 42 and 26 mg/g, while OP1CEC and

DP1CEC adsorbed 12 and 9 mg/g. The good fitness of the data to the Langmuir model

suggested that the adsorption was limited with the monolayer coverage and the surface

was energetically homogeneous.

3.6.3 Adsorption mechanisms

ODTMA modified palygorskite - OP2CEC showed better adsorption than

DP2CEC. In general, 2 CEC surfactant modified palygorskites can remove higher

amounts of 2,4-D than that the 1 CEC modified palygorskites. Modification of

palygorskite with ODTMA or DMDOA not only changed the hydrophobic property of

the clay mineral surface, but also neutralized the negative charges which, if existed, can

repel anionic groups of 2,4-D. Van der Waals interactions can also contribute to the

adsorption of 2, 4-D by the organo-palygorskites (Hermosin et al., 2006). Thus, the

amount of surfactant used to modify palygorskite particles is a key factor controlling the

adsorption as the adsorption of 2,4-D was governed by the electrostatic attraction

between the anionic adsorbate and the positively charged surfactant molecules, so 2

CEC modified palygorskite had a higher adsorption capacity toward 2,4-D than 1 CEC

modified palygorskite: ODTMA modified palygorskites showed better adsorption of

2,4-D than that DODMA palygorskites. Probably the larger surfactant DODMA

blocked some channel regions and reduced the surface area accessible to the adsorptive.

It may also be possible that the higher trans/gauche disorder of the surfactants as

revealed by the FTIR spectra in the series favored the van der Waals interaction with

2,4-D.

4. Conclusions

14

Palygorskites was modified with octadecyl trimethylammonium bromide and

dioctadecyl dimethylammonium bromide ions to enhance the adsorption of the anionic

herbicide. Unmodified palygorskite adsorbed negligible amounts of 2,4-D due to its

negative surface charge. The adsorption capacity was significantly improved by

modifying palygorskite with the surfactants. Thus, the modified palygorskites have a

potential to be used as highly efficient materials for the removal of anionic herbicides

from wastewater.

Acknowledgements

The authors would like to acknowledge the financial and infrastructural support

of the Centre for Environmental Risk Assessment and Remediation (CERAR,

University of South Australia) and the Cooperative Research Centre for Contamination

Assessment and Remediation of the Environment (CRC CARE).

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17

Table 1 Sufactant content of the organo-palygorskites 1

Sample OP1CEC OP2CEC DP1CEC DP2CEC

CEC added 1.0 2.0 1.0 2.0

Surfactant (%) 4.24 10.81 5.27 11.31

Content (in CEC, assuming

the presence of the Br-ions)

0.66 1.82 0.52 1.19

Content (in CEC, assuming

the absence of the Br-ions)

0.83 2.28 0.59 1.36

2

Table 2 Fitting the adsorption isotherm by the Langmuir model 3

Sample Langmuir

qm(mg/g) b(L/mg) R2

OP1CEC 12.24 0.06 0.97

OP2CEC 42.02 0.094 0.99

DP1CEC 9.14 -0.21 0.94

DP2CEC 25.77 0.23 0.99

4

5

6

7

8

9

10

11

12

18

List of Figures 13

Fig. 1 X-ray diffraction patterns of palygorskite and organo-palygorskites 14

Fig. 2 Asymmetric and symmetric stretching vibrations of CH2 in the surfactants and 15

organo-palygorskites 16

Fig. 3 SEM images of a) unmodified palygorskite; b) OP1CEC; c) OP2CEC; d) 17

DP1CEC and e) DP2CEC 18

Fig. 4 DTG curves of palygorskite and organo-palygorskites 19

Fig. 5 Influence of equilibration time on 2,4-D equilibrium concentration in solution 20

Fig. 6 Langmuir isotherms for 2,4-D adsorption at 298K (adsorbent 0.2 g; initial 2,4-21

D 50 to 500 mg/L; time 300 min) 22

23

19

24

Fig. 1 X-ray diffraction patterns of palygorskite and organo-palygorskites 25

20

26

Fig. 2 Asymmetric and symmetric stretching vibrations of CH2 in the surfactants and 27

organo-palygorskites 28

29

21

30

Fig. 3a SEM image of unmodified palygorskite 31

32

Fig. 3b SEM image of OP1CEC 33

22

34

Fig. 3c SEM image of OP2CEC 35

36

Fig. 3d SEM image of DP1CEC 37

23

38

Fig. 3e SEM image of DP2CEC 39

40

24

41

Fig. 4 DTG curves of palygorskite and organo-palygorskites 42

43

44

0 200 400 600 800 1000

Der

ivat

ive

Mas

s %

Temperature/°C

Palygorskite

OP1CEC

OP2CEC

DP1CEC

DP2CEC

25

45

Fig. 5 Influence of equilibration time on 2,4-D equilibrium concentration in solution 46

47

48

Fig. 6 Langmuir isotherms for 2,4-D adsorption at 298K (adsorbent 0.2 g; initial 2,4-49

D 50 to 500 mg/L; time 300 min) 50

51

52

53

0

10

20

30

40

50

60

70

80

90

100

0 50 100 150 200 250 300

Equ

ilib

ium

con

cent

rati

on (m

g/L

)

Equilibration time (min)

DP1CEC

OP1CEC

DP2CEC

OP2CEC