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Journal of Engineering Science and Technology Special Issue on SOMCHE 2014 & RSCE 2014 Conference, January (2015) 11 - 22 © School of Engineering, Taylor’s University

11

REMOVAL OF FLUORIDE USING MODIFIED KENAF AS ADSORBENT

S. R. M. YUSOF1, N. A. M. ZAHRI

1,*, Y. S. KOAY

2,

M. M. NOUROUZI3,4, L. A. CHUAH

1,4, T. S. Y. CHOONG

1

1Department of Chemical and Environmental Engineering, Faculty of Engineering,

Universiti Putra Malaysia, 43400 UPM Serdang, Selangor DE, Malaysia 2 Department of Bioproduct Research and Innovation, Institute of Bioproduct Development

(IBD), Universiti Teknologi Malaysia, 81310 UTM Johor Bahru, Johor, Malaysia 3Department of Environment, Islamic Azad University, Khorasgan branch, Isfahan, Iran

4Institute of Tropical Forestry and Forest Products (INTROP), Universiti Putra Malaysia,

43400 UPM Serdang, Selangor D.E, Malaysia

*Corresponding Author: [email protected]

Abstract

Kenaf core fibers (KCF), one of the agricultural wastes were chemically

modified by using N-(3-chloro-2-hydroxypropyl) trimethylammonium

chloride (CHMAC) as quaternizing agent. The potential of quaternized kenaf core fibers (KCF) as an adsorbent for fluoride from aqueous solution was

then studied. The modified kenaf core fibers (MKCF) were characterized by

Fourier transform infrared spectroscopy (FTIR) and Scanning electron

microscope (SEM). The effect of initial fluoride concentration, pH, contact

time, and adsorbent dose on the fluoride sequestration was investigated. The results showed that with increasing of the adsorbent amount and contact time,

improve the efficiency of fluoride removal. The maximum fluoride uptake

was obtained at pH 6 and contact time of 72 hours. This study also showed

that the maximum adsorption capacity per unit mass of MKCF was 13.98

mg/g. The adsorption equilibrium data were analyzed using Langmuir,

Freundlich, Redlich-Peterson, Sips, Toth and Koble-Corrigan isotherm models. The obtained results in batch studies were best fitted with Freundlich

isotherm. Eventually, MKCF is recommended as a suitable and low cost

adsorbent for fluoride removal from aqueous solutions.

Keywords: Fluoride removal, Quaternized kenaf, Characterization, Adsorption,

Isotherms.

12 S. R. M. Yusof et al.

Journal of Engineering Science and Technology Special Issue 4 1/2015

Nomenclatures

Ce Concentration of solute in solution at equilibrium

Co Initial concentration of solute in solution

qe Amount of fluoride adsorbed per gram of adsorbent at equilibrium

qm Maximum adsorption capacity per gram of adsorbent

qt Amount of fluoride adsorbed per gram of adsorbent at time t

R2 Correlation coefficient

Abbreviations

CHMAC 3-chloro-2-hydroxypropyl trimethyl ammonium chloride

KCF Kenaf core fibers

MKCF Modified kenaf core fibers

1. Introduction

Fluoride is a salt of the element fluorine. High fluoride levels wastewater produce

every day from industries that uses hydrofluoric acid as a cleaning agent such as

the semiconductor, solar cell or metals manufacturing industries or as a reactant

or catalyst in the plastics, pharmaceutical, petroleum refining and refrigeration

industries. Untreated high fluoride level wastewater is one of the key contributors

to groundwater and surface water pollution [1, 2].

Fluoride removal technology has been the interest among the researchers for

many years. Table 1 shows the comparison between the fluoride wastewater

treatment methods. Most of the treatment methods imposed high cost either in

installation or maintenance of the system. Even though some methods like reverse

osmosis is simple and low in operation cost, but the process to dispose the

secondary pollution generated will be costly. In contrast, the adsorption method

has been reported to be one of the most effective, economic and environment

friendly among various defluoridation techniques [3-5]. The advantage of

adsorption is simplicity in design and operation. However, the selections on

adsorbents play an important role in the adsorption process.

Table 1. A Summary on Method for Fluoride Treatment [6].

Method Advantages Disadvantages

Ion exchange No loss of sorbent High operating cost

Reverse Osmosis Simple and low costs Generation of toxic sludge

Electrodialysis Effective Complicated procedure

Electrochemical Effective High cost factor

Precipitation and

coagulation

Effective Generation of toxic sludge

Membrane process No additives Expensive in installing

Nalgonda technique Simple High residual of aluminum

Kenaf, Hibiscus cannabinus, is an ancient crop and has a long history of being

planted and used by human beings. It is considered as an alternative crop that

provides a good source for cellulose. Due to its biodegradability and

environmental compatibility, the usage of kenaf has increased [7].

Quaternization is a surface chemical treatment which adds NH4+ groups onto

surface of lignocellulosic fibers via epoxy substitution to increase affinity towards

Removal of Fluoride Using Modified Kenaf as Adsorbent 13


anionic substances by promoting ion-exchange adsorption [8, 9]. There are three

reaction steps in quaternization reaction namely interaction between cellulose and

sodium hydroxide, epoxy formation from 3-chloro-2-hydroxypropyl trimethyl

ammonium chloride (CHMAC) and reaction between epoxide and hydroxyl group

of cellulose to produce ether as shown in Fig. 1.

Fig. 1. Lignocellulose Quaternization Reaction Scheme:

(1) Cellulose (2) N- (3-chloro-2-hydroxypropyl) (3) Trimethylammonium

Chloride Epoxide (4) Quaternized Cellulose [10].

The objectives of this paper are to prepare kenaf core fibers (KCF) adsorbent

by quaternization using CHMAC as quaternization agent and to study the effect

of pH, contact time, sorbent dose, initial fluoride concentration and adsorption

isotherm for removal of fluoride in aqueous solution by modified kenaf core

fibers (MKCF).

2. Materials and Methods

Kenaf core fibers used in this project was obtained from Institute of Tropical

Forestry and Forest Product (INTROP), Serdang, Selangor. Kenaf core fibers

coarse powder was produced by grinding kenaf fibers (core chips) in a table type

pulverizing machine to obtain the size in range of 1 mm to 2 mm. The fluoride

solution was prepared from sodium fluoride (NaF) which was purchased from

Systerm Chem Ar. Fluoride stock solutions of 1000 mg/L were prepared by

weighing 2.2100 g (±0.01) of NaF into 1 L volumetric flask and dissolved with

distilled water. The stock solution would then be diluted to the desired

concentration. N-(3-chloro-2-hydroxypropyl) trimethylammonium chloride

(CHMAC) 60% in water was purchased from Sigma-Aldrich. Sodium hydroxide

(NaOH) was purchased from Merck. Acetone, acetic acid, and hydrochloric acid

were all analytical grade purchased from Acros. All chemicals were used without

further purification.



2.1. Preparation of modified kenaf core fiber (MKCF)

The kenaf fibers (core chips) was mercerized by soaking in 60 wt.% NaOH for 24

hours at room temperature. After the alkali pretreatment, mercerized kenaf fibers

was rinsed with distilled water and dried at 60°C. Dried mercerized kenaf fiber

was weighed. Each gram of kenaf fiber was reacted with reaction mixture

consisting of 1.5: 4: 2.5 wt./wt. ratio of NaOH, CHMAC, and water, respectively.

Hence, the reaction mixture contains 37 mmol of NaOH and 35 mmol of

CHMAC per gram of KCF. The mixture was left at room temperature for 24

hours. Then, the modified kenaf fibers (MKCF) was rinsed with 0.2% acetic acid

solution to halt the reaction, followed by distilled water until pH 7 was achieved.

Granular MKCF was dried at 60°C and was kept in desiccator [11].

2.2. Characterization of MKCF

Surface morphology was acquired using Scanning Electron Microscopy (SEM)

(Hitachi S-3400N). Samples were coated with a thin layer of Au/Pd. SEM was

operated at 5 kV and scanning electron photographs were recorded at a

magnification of 1000X to 5000X depending on the nature of the sample. Infrared

(IR) spectra were recorded on a Fourier Transform-Infrared (FT-IR) Spectrometer

(Perkin Elmer 1750X). The sampling technique used was attenuated total

reflectance (ATR). All the spectra were plotted as the percentage of transmittance

versus wavenumber (cm-1) for the range of 400 to 4000 cm-1 at room temperature.

Functional groups of samples were determined from the FTIR spectrum.

2.3. Adsorption experiments

All the experiments were completed in batch system using 250 ml Erlenmeyer

flasks. The volume of fluoride solution in each flask was 100 ml. All flasks were

sealed properly to prevent leakage and evaporation. All samples were shaken

mechanically by digital orbital shaker (Wise Shake SHO-2D, Korea) at 150 rpm

(±2 rpm) at 25°C (±2°C). Fluoride ion was measured by fluoride meter as per

standard methods by using Portable Fluoride Meter (HACH, USA) working at

fixed wavelength of 570 nm. The adsorption performance of modified kenaf core

fibers was characterized by fluoride uptake. The adsorption parameters studied

are initial pH, contact time, sorbent dose and initial fluoride concentration.

The effect of initial concentration on MKCF adsorption was studied using

100 mL of fluoride solution with initial fluoride concentration of 5 mg/L, 20

mg/L, 40 mg/L, 60 mg/L and 80 mg/L were prepared respectively and poured

into 250 mL Erlenmeyer flasks (Pyrex, England). 0.1 g of MKCF was added to

each flask and sealed properly. The samples were shaken for 72 hours to ensure

equilibrium was reached.

The effect of MKCF dosage was investigated by running batch experiments

with 0.2 g/L, 0.4 g/L, 0.6 g/L, 0.8 g/L, 1.0 g/L, 1.2 g/L, 1.4 g/L and 1.6 g/L of

MKCF was added to 100 mL of 20 mg/L fluoride solution respectively.

The effect of initial pH was investigated by adjusting the pH of fluoride

solution (20 mg/L) to pH of 2, 3, 4, 5, 6, 7, 8 and 9 by using 0.1 N HCl and 0.1 N

NaOH solution.



3. Results and Discussion

3.1. Characterization

3.1.1. Scanning electron microscopy (SEM) analysis

The textural structure of kenaf core fibers (KCF) and modified kenaf core fibers

(MKCF) were observed by SEM images (Fig. 2. (a)-(c)). From the figures, it is

clearly illustrated that surface of raw kenaf particles were fibrous and hairy. More

micropores can be observed in MKCF after modification and it resemble broken

pieces of porous wall. Hence, the high surface area of MKCF could provide active

site for the adsorption to occur.

(a) (b)

(c)

Fig. 2. SEM micrograph with 300x magnification of (a) raw KCF (b)

mercerization KCF and 1000x magnification of (c) quartenized KCF.

3.1.2. Chemical analysis

The FTIR spectrum is an essential tool to identify the surface functional groups in

this case the NR4+ group which is the key functional group that contribute

extensively to enhance adsorption efficiency. The major and minor peaks

recorded for kenaf core fibers (KCF) and modified kenaf core fibers (MKCF)

were shown in Fig. 3 and listed in Table 2.

In MKCF spectrum, the C-N stretching vibration is present at peak of 1455

cm-1

but do not present in KCF spectrum, proving that the NR4+ group was

successfully quaternized in MKCF [12, 13]. The broad band at 3431 cm-1

in

MKCF spectrum attributed to O-H stretching vibration due to inter and

intramolecular hydrogen bonding of polymeric compounds. The weak peak at

1715 cm-1

in MKCF spectrum corresponded to stretching vibration of C=O in

carboxylic acid group attributed to the trace amount of acetic acid used for



washing [12,13]. Furthermore, in MKCF spectrum, the bands 1424 cm-1 and 1326

cm-1 with nearly equal intensity are assigned to -CH2 scissoring and -OH vibration

respectively. The peaks of 898 cm-1

and 839 cm-1

are assigned to C-H out plane

deformation.

Table 2. Analysis of FTIR spectra of KCF and MKCF.

IR

peak

KCF

(cm-1)

MKCF

(cm-1)

Assignment

1

3215

681 C-H out of plane bonding

2 3431 O-H stretching vibration

3 1455 C-N stretching vibration

4 898,839 C-H out of plane deformation

Fig. 3. FTIR Spectra of KCF and MKCF.

3.2. Adsorption experiment

3.2.1. Effect of dosage

The effect of adsorbent dose was studied at temperature of 25°C (±2°C) with

contact time of 72 hours. The initial concentration of fluoride in this solution was

constant at 20 mg/L and the varied dosage were 0.2 g/L, 0.4 g/L, 0.6 g/L, 0.8 g/L,

1.0 g/L, 1.2 g/L, 1.4 g/L and 1.6 g/L respectively at pH 5.2.

Based on Fig. 4, it was observed that the percentage of fluoride removal

increase with respect to MKCF dosage until 1.2 g/L, further increase in dosage

resulting in constant percentage of fluoride removal due saturation point. MKCF

could remove 77.5% of fluoride with dose of 1.2 g/L. Therefore, the adsorbent

dosage was optimized at 1.2 g/L of MKCF.



Fig. 4. Effect of MKCF dosage on fluoride removal

(pH:5.2; speed:150 rpm (±2 rpm); T:25°C (±2°C)).

3.2.2. Effect of pH

The initial solution pH is one of the most important parameter that directly

influences the adsorption of cation and anion from solution [14]. The effect of pH

on the adsorption of fluoride by MKCF was observed at range of pH 2 to pH 12,

while keeping other parameters constant like initial concentration, time,

temperature and agitation rate. The effect of solution pH on adsorption of fluoride

was examined and the result was illustrated in Fig. 5. The adsorption of fluoride

at pH 2 to pH 4 is low and remains plateau, then the removal of fluoride increases

sharply with increasing pH until reach the optimum removal at pH 6, the

adsorption decreases in alkaline condition until pH 9 and reach a plateau when

further increase in pH. The optimum fluoride removal was 65% at pH 6 and

dosage of 0.1 g/L.

In addition, the surface chemical treatment has influence on the accessibility

and the degree of protonation of functional groups on the adsorbent [15, 16].

These functional groups such as amines, were positively charged when

protonated and electro-statistically bind with negatively charged species, which

are OH- and F

- [17].

A sharp decrease in fluoride removal in basic condition may be due to the due

to the competitiveness of the OH- and F

- ions in the bulk, at high pH value [17,

18]. Theoretically, the following reactions describe the fluoride adsorption

behavior under different pH [17, 19].

HF ↔ H+ + F−

≡ SOH + H+ ↔ ≡ SOH2

+

≡ SOH + OH− ↔ ≡ SO

− + H2O

≡ SOH2+ + F

− ↔ ≡ SF + H2O

≡ SOH + F− ↔ ≡ SF + OH−

where

≡ SOH, ≡ SOH2+ and ≡ SO

− are the

neutral, protonated and

deprotonated sites on MKCF and

≡ SF is the active site of fluoride

complex (S = MKCF).



Fig. 5. Effect of pH on fluoride removal by MKCF (Dosage:0.1 g/L; initial

concentration:20 mg/L; speed:150 rpm (±2 rpm); T:25°C (±2°C))

3.2.3. Effect of contact time

The effect of contact time on the fluoride adsorption onto modified kenaf core

fibers (MKCF) was investigated by measuring the removal of fluoride in different

time interval along the adsorption process. Initial fluoride concentration of 20

mg/L, 40 mg/L, 60 mg/L and 80 mg/L were tested with 0.1 g/L of adsorbent

dosage, 150 rpm (±2 rpm) of agitation speed and temperature at 25°C (± 2°C).

As shown in Fig. 6, the adsorption of fluoride with increase respect to time

period until reached a steady state in 30 hours.

Fig. 6. Effect of contact time on equilibrium uptake of fluoride by MKCF

(Dosage:0.1 g/L; pH:5.2; speed:150 rpm (±2 rpm); T:25°C (± 2°C)).

3.2.4. Isotherm study

As shown in Fig. 7, the removal percentage of fluoride decreases with the

increases of initial fluoride concentration. The percentage of fluoride removal for

5 mg/L, 20 mg/L, 40 mg/L, 60 mg/L and 80 mg/L are recorded as 60%, 35%,

23%, 18% and 15% respectively. The decrease is due to constant dosage applied.

0

18

35

53

70

88

0 3 6 9 12

qe(%

)

pH



From the Fig. 8, the amount of fluoride adsorbed (mg/g) on MKCF was 3

mg/g, 7 mg/g, 9 mg/g, 11 mg/g and 12 mg/g respectively. This illustrated that the

adsorption capacities (mg/g) of fluoride onto MKCF increases with increasing

initial concentration of fluoride. This is due to a raise in the driving force of the

concentration gradient.

Fig. 7. Effect of initial fluoride

concentration, Co on removal of

fluoride ions by MKCF (Dosage: 0.1

g/L; pH:5.2; speed:150 rpm (±2

rpm); T:25°C (±2°C)).

Fig. 8. Effect of initial fluoride

concentration, Co on adsorption

capacity of MKCF (Dosage: 0.1 g/L;

pH:5.2; speed:150 rpm (±2 rpm);

T:25°C (± 2°C)).

The adsorption isotherm is important to study the relationship between the

amount of adsorbate adsorbed per unit gram of adsorbent and the concentration of

adsorbate in the bulk liquid phase during adsorption equilibrium. Six isotherm

models: Langmuir, Freundlich, Redlich-Peterson, Sips, Toth and Koble-Corrigan

isotherms were used to analyze this adsorption system. The isotherm parameters

listed in Table 3 were identified using a non-linear method. The sum of the square

of errors function (ERRSQ) (Eq. 1) was used together with solver add-in in

Microsoft’s Excel spreadsheet to minimize the error function across the

concentration range studied in order to determine the constants [20]. A non-linear

method was chosen over a linear method to eliminate the inherent bias resulting

from linearization.

( )∑ −=2

exp__ ecalce qqERRSQ

(1)

where, qe_exp is adsorption capacity from the experiment and qe_calc is the

adsorption capacity calculated by using isotherm models.

According to the Langmuir isotherm, maximum adsorption capacity (qm) for

adsorption of fluoride was 13.984 mg/g. Calculation from Sips isotherm also

shows qm value of 17.883 mg/g. From Fig. 9, the Freundlich isotherm model is

the best model in describing the adsorption of fluoride onto MKCF because it

gives the highest correlation coefficient (R2) which is 0.9879 and also the lowest

SSE value which is 0.6187 among others isotherm. From Freundlich isotherm

model, the value of the slope (1/n) was near to zero indicated that the surface of

MKCF is more heterogeneous. The maximum adsorption capacity (qm) for

adsorption of fluoride calculated from Langmuir isotherm model is 13.984 mg/g,

which is comparable with other adsorbents for fluoride removal (Table 4).

0

18

35

53

70

0 20 40 60 80

qe(%

)

Co (mg/L)

0

3

6

9

12

15

0 20 40 60 80

qm(m

g/g)

Co (mg/L)



Fig. 9. Isotherm curve for adsorption of fluoride onto MKCF

(Dosage=0.1g/L; pH=5.2; speed=150 rpm (±2 rpm); T=25°C (±2°C)).

Table 3. Isotherm models constants of fluoride adsorption onto MKCF.

Langmuir Redlich-Peterson Sips

KL (L/g)

αL (L/mg)

R2

SSE

1.0250

0.0733

0.9583

2.1336

kR (L/g)

αR (L/mg)

β

R2

SSE

1.7009

0.2951

0.8011

0.9839

0.8251

kLF (L/g)

αLF (L/mg)

nLF

R2

SSE

1.5880

0.0888

0.7375

0.9838

0.8275

Freundlich Toth Koble-Corrigan

kF (L/g)

1/n

R2

SSE

2.2386

0.4045

0.9879

0.6187

kT (L/g)

αT (L/mg)

1/t

R2

SSE

4.3870

4.2350

0.7512

0.9835

0.8427

A

B

n

R2

SSE

1.5880

0.0888

0.7375

0.9838

0.8275

Table 4. Maximum adsorption capacity, qm (mg/g)

from various adsorbents for removal of fluoride [5].

Adsorbents qm (mg/g) References

Aluminum impregnated carbon

Manganese oxide coated alumina (MOCA)

Manganese dioxide-coated activated alumina

Alumina

La (III) incorporated carboxylated chitosan beads

Fe-Al-Ce nano-adsorbent

1.07

0.16

2.85

1.08

11.91

2.22

[21]

[22]

[23]

[23]

[24]

[25]

4. Conclusions

As a conclusion, modified kenaf core fiber (MKCF) was successfully synthesized

and can be used as alternative adsorbent with lower cost and sustainable. The

adsorption of fluoride was best represented by Freundlich isotherm model with R2



value of 0.9879 and lowest SSE value which is 0.6187. pH 6 was the optimum pH

for removal of fluoride on MKCF. The maximum adsorption capacity per unit

mass of MKCF was 13.98 mg/g.

Acknowledgement

The authors are grateful to Universiti Putra Malaysia (UPM) and Research

University Grant Scheme (RUGS) fund for their supports on this project.

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