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Frontier of Environmental Science September 2014, Volume 3, Issue 3, PP.97-108

Preparation of Nanometer Magnesia and Its Properties for Fluoride Removal Huifang Zhou

1,2, Wen Chen

1,2#, Xianghui Zhang

1,2, Dan Shi

1

1. College of Materials and Chemistry & Chemical Engineering, Chengdu University of Technology, Chengdu, Sichuan 610059,

China

2. Mineral Chemistry Key Laboratory of Sichuan Higher Education Institution, Chengdu, Sichuan 610059, China

# Email: chenwen2010@foxmail.com

Abstract

Nano-MgO powders were prepared by direct chemical precipitation using polyethylene glycol (PEG 400) as dispersing agent. The

technological conditions for nano-MgO preparation were ammonia, Mg2+, PEG, reaction at 60 ℃, reaction time in 1.5 h and aging

time in 2 h, calcination at 500 ℃ after filtrating and vacuum drying. The optimum conditions for removal of fluoride are as

follows: pH range of 6.0~7.0, temperature is 25 ℃, adsorbent dosage is 20 mg/10 ml, initial concentration is 40 µg/mL, and ionic

strength is 0.1 mol/L KNO3. It was found that the nano-MgO had the largest adsorption capacity of 74 mg/g. The adsorption data

were well described by the Langmuir isotherm model and the two-constant rate equation kinetic model. According to these studies,

the nano-MgO has potential application in fluoride ions removal from water.

Keywords: Nano Magnesium; Defluorinate; Adsorption; Kinetics; Thermodynami

1 INTRODUCTION

The quality of drinking water is very important for public safety and the living quality. Excess fluoride in drinking

water causes dental, skeletal fluorosis, decreases growth and intelligence [1]. For example, skeletal fluorosis is

caused by continuous, excessive exposure to fluoride and is characterized by axial osteosclerosis, joint pain,

ligamentous ossification and fractures [2]. Fluoride enters the human body mainly through food and water intake.

Studies indicate that soluble fluoride in drinking water is the highest contributor to daily fluoride intake [3], and that

drinking water is thus the most significant source of human fluoride ingestion [4]. High fluoride level in groundwater

is a worldwide problem; India and China have the highest fluorosis prevalence in humans in Asia [5]. Fluorosis is a

serious public health problem throughout most of China. In 2010 there were 41.76 million fluorosis cases in 1325

different counties, out of which 58.2 % were caused by chronic exposure to high levels of fluoride in drinking

water[6].

Yuanmou County in Yunnan Province, China, has been identified by the Center for Disease Control and Prevention

(CDC) as an area where endemic fluorosis caused by using groundwater high in fluoride content for drinking

purposes, prevails [7]. Therefore, it is necessary to treat fluoride-contaminated water and control the fluoride

concentration to a permissible limit, which is 1.5 mg/L in the WHO guideline [8]. And The Chinese national

guideline value for fluoride in drinking water is 1.0 mg/L.

Compared with the common defluoridation methods, adsorption which is simple, high-efficiency, versatile and

economical is regarded as the most appropriate method [9]. It is reported that many adsorbents have been used in

defluoridation. Synthetic and biomass materials such as different types of aluminas [10], aluminum fluoride

complexation [11], Zr(IV)-loaded GP gel [12], stilbite zeolite modified with Fe(III) [13], Mg–Al–Fe [14], hydrous

zirconium oxide [15] , granular ceramic adsorption [16], Hydrous bismuth oxides (HBOs) [17], bauxite [18], Zr(IV)–

ethylenediamine [19], Zr–Mn composite material [20], and hydroxyapatite [21] .

These adsorbents have shown a certain degree of fluoride adsorption capacity but some of them can only be used in a

narrow pH range (5~6) and some of them are too expensive to be considered for full-scale water treatments.

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Recently, considering their large specific area and great mass transfer efficiency, nanosized materials, such as

zirconium-loaded garlic peel [22], nano-goethite [23], CeO2-ZrO2 nanocages [24], polypyrrole/Fe3O4 magnetic

nanocomposite [25], Fe-Ti oxide nano-adsorbent [26], cellulose/hydroxyapatite nanocomposites [27], Fe-Al-Ce

nano-adsorbent [28], Fe3O4/Al2O3 nanoparticles [29] have been developed for effective removal of fluoride. The use

of nanoparticles in water treatment, however, is limited by the costly synthetic process , separation process.

Furthermore, some of them need a high temperature; interfering with coexisting ion and resulting in a supplementary

difficulty in eliminating residual chemicals in excess bring about secondary pollution.

MgO is avirulence and magnesium resources rich in China, not only exists in sea water, bittern, Well Brine etc liquid

resources, but also is very rich with magnesite, dolomite, brucite, serpentine etc natural mineral resources, these

provide a solid resource advantage for magnesium oxide adsorbent products. And China’s energy is abundant and

low-cost, providing a guarantee for production dedicated magnesia material. Thus, nano-MgO has a high potential to

be applied in adsorption systems.

Up to now, A few scholars had done some researches of active magnesium oxide for removal of fluoride. These

include conditions of active magnesium oxide for defluorination [30], Preparation of activated magnesium oxide

defluorination adsorbents by microwave radiation method [31], fluorine removal of fluorine-containing mineral

spring water [32]. However, active magnesium oxide is no match for nano-Mgo which has a larger specific surface

area. The study indicated that nano-MgO be able to achieve an adsorption capacity of 20.66 mg /g [33]. Now nano-

MgO’s application in thick films of Bi2Sr2CaCu2O8 [34], removal of phosphate and nitrate from aqueous solutions

were investigated [35]. There were no literature reported the kinetics and thermodynamics of the F- adsorption on

nano-MgO in drinking water. It is an important part of the explanation nano-MgO adsorption mechanism, which is

crucial to further research and its application.

In this study, nano-MgO(100 nm) was obtained by high temperature calcination. The optimum removal of fluoride

ions on nano-MgO was evaluated. For this, batch and continuous mode sorption experiments were conducted by

using nano-MgO as the sorbent. Attempts have also been made to learn the adsorption kinetics and thermodynamics

of adsorption.

2 EXPERIMENTAL

2.1 Materials

PEG 400, NaOH, HCl, MgCl2 • 6H2O and absolute ethanol used were analytical grade. A stock solution (500.0

µg/mL) was prepared by dissolving 0.5525 g NaF (analytical grade, obtained after forging at 2 h in the temperature

range of 120 ℃) in 500 ml of distilled water. All the solutions for fluoride removal experiments and analysis were

prepared by an appropriate dilution from the stock solution.

2.2 Synthesis of Nano-Mgo Procedure

10.17 g MgCl2 • 6H2O and 13.7 mL PEG 400 (30 % mass of magnesium chloride) was dissolved in 50 mL of

absolute ethanol. 15 ml ammonia solution-ethanol (1:1) solution (with MgCl2 ratio of 2:1) was slowly added into the

mixed solution until the solution was heating to 60 ℃. Adding at a rate of one drop per second with continuous

magnetic stirring, then white precipitate formed. The solution keeps thermal insulation at 60 ℃ for 1.5 h. Then the

solution becomes ageing at room temperature for 2 h. The precipitate formed was in vacuum drying at 60 ℃ at the

end of the precipitate was separated by filter. This was followed by repeated washing with distilled water and ethanol.

The nano-MgO was calcined at 500 ℃ for 1.5 h, and then the nano-MgOs were obtained. The synthesis reaction

equations may be expressed by:

Mg2+

+2OH-=Mg(OH)2↓ (1)

Mg(OH)2=MgO+H2O (2)

2.3 Characterization of Nano-Mgo Precursor

A NETZSCH STA409PC simultaneous thermal analyzer was used for thermogravimetry (TG) and differential

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thermal analysis (DTA) with different scanning rates (5, 10, 15, 20 K/min) from 30 to 1100 ℃. An alumina crucible

container was used in air. The gas flow rate was 30 ml/min. Sample weight was around 12 mg. The decomposition

product was collected at the temperature that was shown on the DTA-TG curves. Prior to experiments, temperature

calibration was performed. All experiments were performed twice to show the reproducibility. The thermokinetic

calculation was conducted with the help of NETZSCH thermokinetic software, using the detected TG data.

The X-ray diffraction (XRD) patterns of each sample were obtained from random mounts using a Rigaku D/MAX-

ШC powder diffractometer. The X-ray generator is equipped with a Ni filter and generates a beam of CuKa radiation

(λ=0.15418 nm). The operational settings for all the XRD scans are voltage: 40 kV, current: 30 mA, range: 5< 2θ

<70°, scanning speed: 0.6°/s.

Morphology of the samples was determined by scanning electron microscope (SEM) with S-3000N (Hitachi, Japan).

2.4 Determination of Fluoride Adsorption Capacity

10 ml 40 µg/mL fluoride solutions were added separately into 50 ml plastic pipe, with KNO3 as the background

electrolyte in a concentration of 0.1 mol/L, The solid-liquid ratio is 1:500 g/mL. The pH of the test solution was kept

at 6.0~7.0 by titrating with 2.0 mol/L HCl or 1.0 mol/L NaOH solution. The test solution was shaken at 25 ℃ for 0.5

h and standing for 24 h during the adsorption test. The blank control should be measured using 10 ml distilled water.

After adsorption, the solutions were filtered and the filtrate was analyzed. The fluoride ion concentration remaining

in the filtrate was measured with a JP-303 polarograph. The second order derivative linear sweep voltammetric peak

currents (ip〞) were recorded in the potential range from −0.30 V to −1.30 V (vs. SCE) at room temperature,

scanning rate and standing time was 500 mvs and 8 s respectively.

The adsorption capacity Q (µg/g); and fluoride removal efficiency P (%) was calculated from the mass balance

equation respective presented by:

Q=[(C0-C)/m]V (3)

P=[(C0-C)/C0]100% (4)

where C0 (µg/mL) is the initial fluoride concentrat ion and C (g/mL) is the final fluoride concentration after

adsorption, V is the volume of the solution containing fluoride ions, and m is the adsorbent granules dose of 20

mg/10 mL.

3 RESULTS AND DISCUSSION

3.1 Thermal Decomposition Process of As-Prepared Products

1250

250

500

1000

0

750

10 20 30 40 50 60 70 80

(a)

Inte

nsi

ty (

Cou

nts

)

2-Theta (°)

(b)

FIG. 1 XRD PATTERN AND SEM PHOTO OF THE AS-PREPARED PRODUCTS:

(a) XRD PATTERN; (b) SEM IMAGE

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The XRD patterns and SEM image of the as-prepared products were given in Fig.1. The XRD pattern shows that all

the diffraction peaks in the pattern are in agreement with those of the hexagonal type of Mg(OH)2 with space group

Pmp21(18) and lattice constants a= 3.144Å, b= 3.144 Å,c=4.777Å (standard card JCPD No. 44-1482). No

diffraction peaks of impurities are observed. Hence the single phase Mg(OH)2 is prepared successfully.

The SEM image (Fig.1b) shows that the Mg(OH)2 particles are uniform, consisting of plane-like grains.

Fig.2 shows the DTA-TG-DTG curves of the as-prepared products at a heating rate of 10 K/min in air below

1000 ℃. The thermal decomposition of the as-prepared products occurs in three steps in concordance with the DTG

peaks, and all steps are endothermic reaction.

The first step is from 80 to 220 ℃( Tmax=116.9 ℃ , WL=2.44%) and The second step is from 202 to 400 ℃

(Tmax=373.6 ℃, WL=31.58%), respectively, attributed for the evaporation of adsorbed water and hydroxide of the

magnesium and the formation of magnesium oxide. But the whole weight loss is 34.03%, higher than the theoretical

value (31.03%). we attempted to explain this due to the reason that the loss of the water of crystallization in the

magnesium hydroxide. The corresponding XRD patterns support the reaction process, a pure MgO (standard card

JCPD No.JCPD No.65-0476) is observed in Fig.3a. The SEM image (Fig. 3b) shows that the MgO particles are

uniform; consisting of sphericity-like grains about 20nm in size. Hence the nano-MgO is prepared successfully.

The third step is from 400 to 1000 ℃ (WL=0%, Tmax=779.4 ℃) without mass loss shows the phase transition process

of magnesium oxide.

FIG. 2 DTA-TG-DTG CURVES OF THE Mg(OH)2 AT A HEATING RATE of 10K/min in AIR

FIG. 3 XRD PATTERN AND SEM IMAGE OF THE AS-PREPARED NANO-MgO

(a) XRD PATTERN; (b) SEM IMAGE

30 1

0 2

0

40 5

0

6

0

70 8

0

0

(a)

2-Theta (°)

250

500

750

1000

Inte

nsi

ty (

Cou

nts

)

(b)

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3.2 Fluoride Removal Studies.

1) Effect of pH and Ionic Strength

The pH of the medium is one of the important parameters that significantly affect the extent of fluoride adsorption.

Other parameters were equal to the fluoride adsorption capacity determination test. The effect of pH on fluoride

removal from aqueous solution by the nano-MgO was studied at various pH values (ranging from 4 to 9). Fig. 4

shows the adsorption capacity of fluorinion onto the nano-MgO in different pH. The nano-MgO exhibited a strong

adsorption of fluoride when the pH was 4.0. However, nano-MgO is alkaline in the water.

When the pH was 4.0, MgO has been partially dissolved, it will combine with fluorine ion in the solution of MgF2,

reduce fluorine ion concentration in the solution, showing the illusion of a large amount of adsorption. When the pH

was between 6.0 and 7.0, the nano-magnesium on fluorine adsorption capacity is high and stable. After pH 9, the

fluoride the adsorption adsorption capacity is decreased because the change in surface negative charge of the

adsorbent. removal is mainly governed by ion-exchange mechanism. So in the study, we selected the acidity between

6.0 and 7.0. The distilled water we use whose pH is between 6.0 and 7.0, so the experiments without pH adjustment.

To observe the effect of ionic strength on adsorption capacity, 0-0.5 mol/L KNO3 concentration was studied. Ionic

strength is very important variable in adsorption. Take the test solution 0.1 mL measured experimental results shown

in Fig. 5. When the ion concentration was 0.2 mol/L, the adsorption capacity of fluoride on nano-magnesium was

nearly balanced. However, in real life, the ion concentration of the solution reached a highest value of 0.1 mol/L, so

we chosen the ion concentration of 0.1 mol/L.

2) Effect of Temperature and Initial Fluoride Concentration

When other parameters were unchanged. The samples obtained at temperatures of 25℃, 35℃, and 45℃,

respectively. The adsorption rates keep balanced with the temperature changed. The adsorption rates is high when

the reaction temperature was 25℃. So the best temperature of this experiment is setting for room temperature, which

is 25 ℃.

Equal masses of nano-MgO were put into contact with solutions of different concentrations of fluoride-ion solutions

(from 10 to 60 µg/mL). Fig. 6 shows that as fluoride concentration increases, adsorbed amounts of fluoride increase

too at first. At 20 µg/mL, the adsorption reached equilibrium and the adsorption rates decreased more than 40 µg/mL.

This plateau occurs due to the saturation of active sites on the adsorbent surfaces. At low fluoride concentrations, the

ratio of surface active sites to fluoride ions is high. After the The adsorption quantity saturated, the adsorption rate is

reduced with the increase of initial fluoride concentration increase(c>40μg/mL). So the optimal initial concentration

is 40 µg/mL.

4 5 6 7 8 90

5

10

15

20

adso

rpti

on c

apac

ity

/(m

g/g)

pH

FIG. 4 EFFECT OF pH ON THE ADSORPTION

CAPACITY (THE INITIAL F− CONCENTRATION

WAS 40 µg/ml; ADSORBENT DOSAGE 20 mg/10

mL; 298 K)

0.0 0.1 0.2 0.3 0.4 0.512

13

14

15

16

17

18

19

adso

rpti

on c

apac

ity/

(mg/

g)

KNO3 concentration/(mol/L)

FIG. 5 EFFECT OF IONIC STRENGTH ON THE

ADSORPTION CAPACITY (THE pH=6.0~7.0; THE

INITIAL F− CONCENTRATION WAS 40 µg/ml;

ADSORBENT DOSAGE 20 mg/10 mL; 298 K).

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3) Effect of Adsorbent Dosage

Changing adsorbent dosage from 5 mg/10 ml to 40 mg/10 ml in the solution, take the test solution 0.5 mL, measured

experimental. Fig. 7 shows the adsorption effect to flouride was increased with the increase of the amounts of nano-

MgO.

It is evident that the percent of fluoride removal increased with the increase of the adsorbent concentration which is

due to the fact that a greater amount of adsorbent provides greater number of available binding sites. When the

adsorbent dosage is 20 mg/10 ml, there were reaching a value of steady. Based on the above discussion, the

adsorbent dosage of 20 mg/10 ml was used as the optimum value for the fluoride ion adsorption experiments.

By the study of relation between single experimental factor such as solid-liquid ratio, initial concentration, initial pH

of solution, adsorption temperature and adsorption effect, the optimum adsorptive removal rate can be obtained:

adsorbent dosage = 20 mg/10 ml, pH=6.0 ~ 7.0, C0=40 mg/L, T=25 ℃, ionic strength was 0.1 mol/L. Adsorption

capacity of fluoride removal reaches 74 mg/g.

3.3 Effects of Coexisting Ions in Ground Water

Contaminated groundwater usually contains several other co-existing ions along with fluoride, which may compete

with fluoride for the active adsorption sites. Hence, it is imperative to investigate the possible interference of these

ions (HCO3−, Cl

−, NO3

−, SO4

2−, PO4

3−, Ca

2+, Mg

2+, Fe

3+ and Mn

2+) on fluoride removal by nano-MgO.

The adsorption studies were carried out in the presence of distilled water compare with ground water. This may

contain chloride, sulfate, nitrate, bicarbonate, phosphate and calcium. At an initial fluoride concentration of 20

µg/mL, the effects of these co-existing ions on fluoride removal in the ground water did not significantly affect the

removal of fluoride. Under the optimum conditions, in the distilled water, the adsorption rates is 96.53 %, and the

concentration of fluoride solution is 0.70 µg/mL by defluorinated. Regard to groundwater, the adsorption rates is

96.67 %; the concentration of fluoride solution is 0.67 µg/mL by defluorinated. All these were lower than drinking

water standard of China (1.0 µg/mL).

3.4 Adsorption behavior and possible mechanism of fluoride

Nano-MgO has smaller particle size than ordinary magnesium oxide, thus shows different surface effect which

ordinary magnesium oxide doesn’t have. Nano-MgO’s surface extremely increases when its particle size decreases,

and then making the surface adsorption sites increased greatly. The fluoride adsorption may be due to the combined

effect of both chemical and electrostatic interaction between the oxide surface and fluoride ion and also availability

of active sites on the oxide surfaces. As reported earlier, anion adsorption on metal oxide surface is through

0 5 10 15 20 25 30 35 40 45

20

30

40

50

60

70

80

90

100

adso

rpti

on

rat

e(%

)

adsorbent dosage /(mg/10 ml)

FIG..7 EFFECT of SOLID-LIQUID RATIO on THE

ADSORPTION RATE (THE INITIAL F−

CONCENTRATION WAS 40 µg/ml;

pH=6.0~7.0; 298 K).

10 20 30 40 50 6040

50

60

70

80

90

100

adso

rpti

on r

ate（

%）

initial concentration/（g/ml）

FIG.6 EFFECT of INITIAL FLUORIDE

CONCENTRATION on THE ADSORPTION RATE

(ADSORBENT DOSAGE 20 mg/10 mL;

pH=6.0~7.0; 298 K).

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columbic forces and/or ligand exchange reactions, where the anions displace OH− or H2O from the surface [37]. In

our studies it was observed that the pH of the equilibrated solution slightly increased in acidic pH range, while it

slightly decreased when the initial solution was alkaline. Hence, the fluoride adsorption on nano-MgO may be due to

anion exchange at acidic pH and by van der Waals forces at alkaline pH ranges (Fig. 4). The most probable

mechanism for fluoride adsorption in acidic conditionis mainly as [38]:

MOH + H2O + F− ↔ MOH2

+ + F

− + OH

− ↔ MOH2–F + OH

−↔ MF + H2O+ OH

− (5)

3.5 Adsorption Isotherm Studies

Adsorption capacity is one of the basic parameters required for the design of any batch or fixed-bed adsorption

system. The equilibrium adsorption study of fluoride by nano-MgO was carried out at optimum adsorptive condition

which obtained above. The initial fluoride concentration was varied over a wide range from 10 µg/mL to 400 µg/mL,

in order to attain complete exhaustion of the adsorbent. The saturated adsorption capacity of fluoride on nano-MgO

powder was showed (Fig. 8).

The data obtained from this study are analyzed with the help of two isotherm models, including the well-known

Langmuir and Freundlich. The linearized form of the Langmuir and Freundlich isotherm are given below.

Langmuir model:

Ce/qe = Ce/qm + 1/(KL×qm) (6)

Freundlich model:

lgqe = lgKF + 1/nlgCe (7)

where qe is the amount of solute adsorbed per unit weight of adsorbent at equilibrium (mg/g); Ce is equilibrium

concentration of the adsorbate in the solution (mg/L); qm represent maximum specific up take capacities at

equilibrium of Langmuir (mg/g); KL is the Langmuir isotherm constants (L/mg); KF is the Freundlich isotherm

constant [(mg/g) (m/g)−1/n

].

Fig. 9-10 show the Langmuir isotherm model and Freundlich isotherm model fit for fluoride sorption on the

adsorbents.

The Langmuir isotherm in its linear form is represented by:

qe =7.913Ce/ (1+0.102Ce). (8)

The Freundlich isotherm in its linear form is represented by:

qe=9.471Ce1/2.107

. (9)

FIG. 8 THE INFLUENCE OF DIFFERENT INITIAL CONCENTRATION ON ADSORPTION

CAPACITY (ADSORBENT DOSAGE 20 mg/10 mL; pH=6.0~7.0; 298 K)

0 50 100 150 200 250 300 350 400 450

0

10

20

30

40

50

60

70

80

90

q t / ( m

g / g

)

C/(μg/mL)

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The Langmuir isotherm constants calculated are follows: qm=77.58, KL=0.102. The saturated adsorption capacity

depends on the quantity of adsorption activity sites, which directly related to the nano-MgO specific surface area of

the adsorbent. It is clear that the qmax of Langmuir model fitted reach to 77.58 mg/g. The Freundlich isotherm

constants calculated are follows: KF=9.471; n=2.107. It is believed to when n＜0.5, the adsorption is difficult to

reaction. now n=2.107, the adsorption is an easy reaction. From Figure 9-10, Langmuir model fitted the isotherm

data well, yielding a correlation coefficient of 0.998. The adsorption of defluorination on nano-MgO obeys the

Langmuir monolayer adsorption.

The thermodynamic feasibility of the process in the present fluoride-iron oxide system was evaluated from the free

energy change following Eq. (10)

ΔG= -RT lnKd (10)

where ΔG is the change in free energy (kJ/mol); T is the absolute temperature (K); R is the universal gas constant

(8.314J/K·mol); Kd is the Langmuir isotherm constants (L/mg).

When the sorption equilibrium at studied systems is established, the fluoride adsorbed onto nano-MgO is in

equilibrium with the residual fluoride concentration remaining in the liquid phase. The value of the observed

equilibrium constants (Kd) of the adsorption process, the constants of F- distribution between the solid and liquid

phases at the equilibrium, were calculated with respect to temperature using the method of Khan and Singh,[36] by

plotting ln(qe/Ce) versus qe and extrapolating to zero qe. Kd =2.094, ΔG values was estimated at -1.83 kJ/mol. This

indicates that the adsorption process is spontaneous. From above indicated that nano-MgO is a wonderful absorbent

material for defluorination.

3.6 Adsorption Kinetic Studie

In this study, kinetics of adsorption of fluoride at various initial concentrations was carried out by using nano-MgO

as adsorbents. We chose three initial concentrations: 30 µg/mL, 100 µg/mL, and 200 µg/mL. Fig. 11 shows the time

dependence of fluoride sorption onto nano-MgO at these initial concentrations.

From Fig. 11, In the case of nano-MgO, most of sorption has taken place within 3 h of contact time. After 3 h, the

rate of adsorption was negligible and residual fluoride concentration reached almost a constant value. We can also

know that the higher initial concentration of absorption liquid, the higher equilibrium adsorption quantity is.

After the equilibrium period, the amount of adsorbed fluoride did not significantly change with time. Theoretically,

adsorption kinetics of fluoride onto solid particles is controlled by different mechanisms. These mechanisms involve

diffusion or transport of fluoride from bulk solution to exterior surface of the adsorbent particle. The rate of sorption

onto a solid surface depends upon a number of parameters such as structural properties of the sorbent, initial

concentrations of the solute, and the interaction between the solute and the active sites of the sorbent. Therefore,

FIG. 9 LANGMUIR ISOTHERM FITS FOR ADSORPTION

OF FLUORIDE ONTO NANO-MgO (INITIAL FLUORIDE

CONCENTRATIONS 40 µg/ml; ADSORBENT DOSAGE 20

mg/10 mL; pH=6.0~7.0).

0 50 100 150 200 250

0

1

2

3

4

y=0.01289x+0.1261

R2=0.9984

Ce/q

e (mg/

ml)

Ce/ (μg/ml)

FIG.10 FREUNDLICH ISOTHERM FITS FOR

ADSORPTION OF FLUORIDE ONTO NANO-MgO

(INITIAL FLUORIDE CONCENTRATIONS 40 µg/ml;

ADSORBENT DOSAGE 20 mg/10 mL; pH=6.0~7.0).

-0.5 0.0 0.5 1.0 1.5 2.0 2.5

0.8

1.2

1.6

2.0

2.4

y=0.4746x+0.9764

R2=0.8493

logq

e

logCe

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kinetics dates analyses for fluoride sorption onto nano-MgO were studied with different models. The pseudo first

order dynamic model, pseudo secondary dynamic model, banerm equation model and two-constant rate equation

models were tested to describe the kinetic process. The mathematical representations of models are given in Eqs (11),

(12), (13) and (14).

Pseudo-first-order adsorption kinetic model:

ln (qe−qt) = ln qe−k1t (11)

Pseudo-second-order adsorption kinetic model:

t/qt =1/(k2 qe2)+ t / qe (12)

banerm equation kinetic model:

ln qe/(qe-qt ) = k3 tn (13)

two-constant rate equation kinetic model:

ln qt = a+ k4 ln t (14)

where qe is the amount of solute adsorbed per unit weight of adsorbent at equilibrium (mg /g); qt is the amount of

fluoride sorbed by adsorption at time t (mg/g); k1, k2, k3, k4, a, n are the constants.

The adsorption kinetic model parameters obtained from the above equations are given (Fig. 12). From these results,

it is evident that kinetic datas of the fluoride adsorption onto nano-MgO fitted well with two-constant rate equation.

Yielding a correlation coefficient of 0.9 (Fig. 12d), this suggests that the adsorption kinetic of magnesium oxide

process is not a simple first order kinetics of reversible reaction, it is a complex reaction process, may be include

adsorption diffusion and chemical reaction process.

FIG. 11 THE TIME DEPENDENCE OF FLUORIDE SORPTION ONTO

NANO-MGO at 30 µg/ml, 100 µg/ml, 200 µg/ml INITIAL

CONCENTRATIONS

-50 0 50 100 150 200 250 300 350 400

0

5

10

15

20

25

30

35

40

45

50

55

60

C0=100mg/ml

t/(min)

C0=30mg/ml

q t / (m

g/g)

C0=200mg/ml

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3.7 The Desorption of Fluoride on Nano-MgO Adsorbent

Under the optimum conditions, the adsorptions test was done at an initial fluoride concentration of 30 µg/mL.

Separate MgO from solution after oscillating, assaying the fluoride concentration in the solution. The nano-MgO in

plastic tubes was washed with distilled water, adjusts the pH of 6, 7, 8, 9, 10, 11, 12 and distilled water respectely,

the blank control be measured using distilled water, a total volume of 10 mL. The elution conditions: 3 hour of

shaking time and 25 ℃ temperature. The fluoride ions remaining in the test solution were measured with JP-303

Polarograph. The results show that pH has a nonsignificant influence on desorption of fluoride on nano-MgO

adsorbent. The hightest desorption rate was 0.73% in the pH of 8.0. It is also do slightly influence in distilled water.

So it can be learn that the fixation of F- by nano-MgO was the very strong. It is suitable for the fluoride adsorption

on nano-MgO, Which is hard to desorb of fluoride on nano-MgO adsorbent.

4 CONCLUSIONS

1) The calcination product of nano-MgO can be an efficient adsorbent for fluoride sequestering from water. The

optimal parameters for producing high performance adsorbent nano-MgO were at the adsorption temperature of 25

°C, a adsorbent dosage of 20 mg/10 ml, and an ionic strength of 0.1 mol/L KNO3. This nano-MgO had an adsorption

capacity for fluoride of 74 mg/g for water with an initial fluoride concentration of 40 µg/ml that was treated at pH

6.0~7.0. That is higher than usual adsorbents.

2) The adsorption process is well described by the Langmuir isotherm model and the two-constant rate equation

model, as assessed by the correlation coefficient values. This suggests that the adsorption of defluorination on nano-

MgO obeys the Langmuir monolayer adsorption; the adsorption kinetic of magnesium oxide process is not a simple

first order kinetics of reversible reaction, it is a complex reaction process, may be include adsorption diffusion and

chemical reaction process.

3) The interference of common ions in ground water is also investigated. The experiment showed that it been the

difficultly desorbed F- after adsorption. So the nano-MgO has a significant amount of potential by promising

environmental materials for fluoride removal from water.

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FIG. 12 KINETICS OF DIFFERENT CONCENTRATIONS F- ADSORPTION ON THE NANO-MgO (A), (B), (C)

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