1 © IWA Publishing 2015 Journal of Water Reuse and Desalination | in press | 2015

Uncorrected Proof

Kinetics and equilibrium studies for removal of fluoride

from underground water using cryptocrystalline

magnesite

Vhahangwele Masindi, Wilson Mugera Gitari and Tholiso Ngulube

ABSTRACT

In the present study, the defluoridation capabilities and adsorption mechanisms of cryptocrystalline

magnesite were evaluated. All experiments were done by batch procedure. Conditions assessed

include time, dosage, concentration, pH and the effects of competing ions. Optimum defluoridation

conditions were observed to be 20 g/L magnesite, 2:100 solid:liquid ratio, 20 min of agitation and

60 mg/L fluoride concentration. Adsorption of fluoride by magnesite was observed to be

independent of pH. Cryptocrystalline magnesite showed >99% efficiency for fluoride removal.

Adsorption kinetics fitted better to a pseudo-second order than a pseudo-first-order thus confirming

chemisorption. Adsorption data fitted better to a Langmuir than a Freundlich adsorption isotherm

thus confirming monolayer adsorption. Cryptocrystalline magnesite successfully removed excess

fluoride from aqueous solution to below Department of Water Affairs and Forestry water quality

guidelines. As such, this material can be used for a point source defluoridation technique in rural

areas and households in South Africa and other developing countries. Based on comparison studies,

cryptocrystalline magnesite proved to have high adsorption capacity for fluoride removal and can be

used as a substitute for conventional treatment methods.

doi: 10.2166/wrd.2015.080

Vhahangwele Masindi (corresponding author)Wilson Mugera GitariTholiso NgulubeEnvironmental Remediation and Water Pollution

Chemistry Research Group,Department of Ecology and Resources

Management,University of Venda,Private bag X5050,Thohoyandou,0950 Limpopo,South AfricaE-mail: vmasindi@csir.co.za,

masindivhahangwele@gmail.com

Vhahangwele MasindiCouncil of Scientific and Industrial Research (CSIR),Building Science and Technology (BST), Built

Environment,P.O. BOX 395,Pretoria 0001,South Africa

Key words | adsorption, defluoridation, fluoride, isotherms, kinetics, magnesite

INTRODUCTION

Excessive uptake of water rich in fluoride has led to serious

health problems (Shen & Schäfer ). Depending on the

concentration, fluoride can be beneficial or harmful to

human health. Permitted levels of fluoride assist in bone

and tooth formation. Fluoride also prevents the development

of dental cavities. However, if it is consumed in excess, it can

lead to the development of dental and skeletal fluorosis (Tor

; Tor et al. ; Zhu et al. ; Trikha & Sharma ;

Vhahangwele et al. ; Zhang et al. ; Zhou et al. ;

Gitari et al. ; Zhao et al. ). According to the Depart-

ment of Water Affairs and Forestry (DWAF), recommended

fluoride concentrations in drinking water are around 1 mg/L

(Vhahangwele et al. ; Gitari et al. ).

Contamination of groundwater by fluoride may be due

to anthropogenic activities or the geochemical environment.

It enters the aquatic ecosystems naturally, through

weathering of fluoride-bearing rock, such as fluorapatite,

sellaite and cryolite. It enters the environment artificially,

through use of fluoride-containing materials in manufactur-

ing industries, fertilizers, leaching and cleaning processes.

All these activities lead to accelerated fluoride pollution.

To remedy this, fluoride need to be removed from drinking

water prior to human exposure (Xiuru et al. ; Meenak-

shi & Maheshwari ; Wang et al. , ; Kamble

et al. ; Zhu et al. ; Karthikeyan et al. ; Miretzky

& Cirelli ; Sujana & Anand ; Wu et al. ; Srivastav

et al. ; Tomar et al. ; Vhahangwele et al. ; Zhou

et al. ; Gitari et al. ; Yu et al. ; Zhao et al. ).

Recently, scientific communities have been searching

for practical ways of removing fluoride from underground

water. Several low-cost and point-based technologies have

been developed but the cost factor, unsustainable treatment

2 V. Masindi et al. | Defluoridation of groundwater using cryptocrystalline magnesite Journal of Water Reuse and Desalination | in press | 2015

Uncorrected Proof

processes, inefficient treatment capabilities and generation

of huge secondary sludge limit their application in the

defluoridation process. An array of materials and technol-

ogies have been tested, and they entail ion exchange,

adsorption, precipitation, reverse osmosis, distillation, bio-

sorption, filtration, flocculation and coagulation (Choi &

Chen ; Chaturvedi et al. ; Gaciri & Davies ;

Czarnowski et al. ; Dorenbos ; Çengeloglu et al.

; Fan et al. ; Fletcher et al. ; Daifullah et al.

; Bansiwal et al. ; Biswas et al. ; Zhu et al.

; Camacho et al. ; Chen et al. a, 2010b, ;

Bhatnagar et al. ; Chang et al. ; Brunson & Sabatini

; Dayananda et al. ; Gwala et al. ).

Calcium-based materials and modified clay have been

used for defluoridation (Tor ; Viswanathan & Meenak-

shi ; Sujana & Anand ; Vhahangwele et al. ). To

date, no study has reported on use of cryptocrystalline min-

erals for defluoridation of contaminated water. The primary

aim of this study was to explore the feasibility of using cryp-

tocrystalline magnesite as an adsorbent for defluoridation of

groundwater. A study by Masindi et al. () documented

that Folovhodwe region, in Limpopo province, South

Africa has large deposits of magnesite (18 MT) which have

been prospected to be mined for the coming 20 years

unless the demand increase.

EXPERIMENTAL METHODS

Sampling and preparations of magnesite adsorbent

Raw magnesite rocks were collected prior to any processing

at the mine from the Folovhodwe Magnesite Mine, Limpopo

Province, South Africa (22W35047.000S; 30W2503300E). Magne-

site samples were milled to a fine powder using a Retsch

RS 200 miller and passed through a 32-μm particle size

sieve.

Physiochemical characterization of the adsorbent

Elemental characteristics of the magnesite samples were

measured by X-ray fluorescence spectroscopy (XRF) using

a Philips PW 1480 X-ray spectrometer. Mineralogical

characteristics of the magnesite samples were ascertained

using a Philips X-ray diffractometer with Cu-Kα radiation.

Phase identification was performed by searching and match-

ing obtained spectra with the powder diffraction file data

base with the help of JCPDS (Joint Committee of Powder

Diffraction Standards) files for inorganic compounds. Mor-

phology and elemental composition of magnesite were

investigated by energy dispersive X-ray spectrometry (EDS:

focused ion beam, Auriga from Carl Zeiss) attached to a

scanning electron microscope (SEM: JEOL JSM7500 micro-

scope) (SEM-EDS).

Preparation of stock solution and working solution

The accuracy of the analysis was monitored by analysis of

National Institute of Standards and Technology water stan-

dards. Simulated fluoride-rich water was prepared using

sodium fluoride salt. Fluoride stock solution (1,000 mg/L)

was prepared by dissolving 2.2 g NaF in 1,000 mL ultra-

pure water. A working solution (10 mg/L) was prepared by

diluting the stock solution 1:100 in ultra-pure water.

Optimization of defluoridation conditions

Effects of agitation time

Removal of F� with contact time was evaluated by using

10 mg/L F� solution and 1:100 solid:liquid (S:L) ratio.

Nine 100 mL aliquots of the 10 mg/L F� solution were

pipetted into 250 mL bottles and magnesite was added at a

dose of 10 g/L. The mixtures were agitated for 1, 5, 10, 15,

20, 30, 60, 180 and 360 min at 250 rpm using a Stuart reci-

procating shaker. After equilibration, the mixture was

filtered through a 0.45 μm pore size nitrate cellulose filter

membrane. The samples were analyzed for F� by a

CRISON ion selective electrode.

Effects of magnesite dosage

Removal of F� according to the magnesite dose was evalu-

ated using 10 mg L F� solution. One hundred millilitre

aliquots of the 10 mg/L F� solution were pipetted into

eight 250 mL bottles and 0.1, 0.3, 0.5, 1, 2, 4, 8 and 10 g of

magnesite, respectively, were added to the flasks. The mix-

tures were then agitated for 20 min at 250 rpm using a

3 V. Masindi et al. | Defluoridation of groundwater using cryptocrystalline magnesite Journal of Water Reuse and Desalination | in press | 2015

Uncorrected Proof

Stuart reciprocating shaker. After equilibration, the mixtures

were filtered through a 0.45 μm pore size nitrate cellulose

filter membrane. The samples were analyzed for F� by a

CRISON ion selective electrode.

Effects of ion concentration

Removal of F� with concentration was evaluated at 20 min

of contact time, 20 g/L of magnesite dosage, 2:100 S:L and

250 rpm mixing. Ten 100 mL samples at initial concen-

tration of 2, 4, 6, 8, 15, 20, 30, 40, 50 and 60 mg/L F�

solution were pipetted into 250 mL bottles and 20 g/L of

magnesite was added to each. The mixtures were then agi-

tated for 60 min at 250 rpm using a Stuart reciprocating

shaker. After equilibration, the mixtures were filtered

through a 0.45 μm pore size nitrate cellulose filter mem-

brane. The samples were analyzed for F� by CRISON ion

selective electrode.

Effects of supernatants pH

Removal of fluoride as a function of pH was evaluated at

20 min of contact time, 20 g/L of magnesite, 2/100 S:L

and 60 mg/L F�. One hundred millilitre aliquots of 60 mg/

L F� solution were pipetted into six 250 mL bottles. The

pH of the solution was adjusted to 2, 4, 6, 8, 10 and 12

using 0.1 M of HCl and 0.1 M NaOH. Four grams of magne-

site was added to each bottle. The mixture was then agitated

for 60 min using a reciprocating shaker at 250 rpm. After

equilibration, the mixtures were filtered through a 0.45 μm

pore size nitrate cellulose filter membrane. The samples

were analyzed for F� by CRISON ion selective electrode.

Effects of competing ions

Removal of F� with competing ions was evaluated at 20 min

of contact time, 20 g/L of magnesite dosage, 2/100 S:L and

250 rpm. One hundred millilitres of solution containing of

fluoride, sulphate, nitrate, chloride and bromide, all at

60 mg/L, was mixed with 20 g/L of cryptocrystalline magne-

site. The mixture was equilibrated for 20 min at 250 rpm

using a Stuart reciprocating shaker. After equilibration, the

mixtures were filtered through a 0.45 μm pore size nitrate

cellulose filter membrane. The samples were analyzed for

anions by CRISON ion selective electrode and ion

chromatography.

Adsorption of F� from borehole water at optimized

conditions

The optimized conditions of 20 min of contact time, 20 g/L

of magnesite dosage, 2/100 S:L, 25 WC and 250 rpm shaking

were used for defluoridating groundwater. The borehole

water came from Siloam, Nzhelele, Limpopo province,

South Africa.

Modelling of analytical results

Adsorption kinetics

The pseudo-first order is a kinetic model described by the

following equation

log qe � qtð Þ ¼ log qe � K2:303

� �t (1)

where qe (mg g�1) is adsorption capacity at equilibrium, qt(mg g�1) is the adsorption capacity at time t, and K

(min�1) is the rate constant of pseudo-first order. The

value of K1 can be obtained from the slope by plotting t

vs. log (qe–qt).

The pseudo-second order model is used when the appli-

cability of the first-order kinetics becomes untenable. The

equation of the pseudo-second-order model is

1qt

¼ 1K2qe

þ 1qe

� �t (2)

This equation is used to obtain K2, the second order rate

from the plots t vs. t/qe.

Adsorption isotherms

The relationship between the amount of fluoride adsorbed

and the fluoride concentration remaining in solution is

described by an isotherm. The two most common isotherm

types for describing this type of system are Langmuir and

Freundlich adsorption isotherms. These models describe

4 V. Masindi et al. | Defluoridation of groundwater using cryptocrystalline magnesite Journal of Water Reuse and Desalination | in press | 2015

Uncorrected Proof

adsorption processes on a homogenous (monolayer) or het-

erogeneous (multilayer) surface respectively. The most

important model of monolayer adsorption came from Lang-

muir. This isotherm is given as follows:

qe ¼ Q0bCe

1þ bCe(3)

The constants Q0 and b are characteristics of the Lang-

muir equation and can be determined from a linearized form

of Equation (4). The Langmuir isotherm is valid for mono-

layer sorption due to a surface with finite number of

identical sites and can be expressed in the following linear

form:

Ce

qe¼ 1

Q0bþ Ce

Q0(4)

where, Ce¼ equilibrium concentration (mg L�1), Qe¼amount adsorbed at equilibrium (mg g�1), Qm¼Langmuir

constant related to adsorption capacity (mg g�1) and b¼Langmuir constant related to energy of adsorption (L mg�1).

A plot of Ce vs. Ce/Qe should be linear if the data is described

by the Langmuir isotherm. The value of Qm is determined

from the slope and the intercept of the plot. It is used to

derive the maximum adsorption capacity and b is deter-

mined from the original equation and it represents the

intensity of adsorption.

The Freundlich adsorption isotherm describes the het-

erogeneous surface energy by multilayer adsorption. The

Freundlich isotherm can be formulated as follows:

qe ¼ kCe1=n (5)

The equation may be linearized by taking the logarithm

of both sides of the equation and can be expressed in linear

form as follow:

log qe ¼ 1nlogC þ logK (6)

where Ce¼ equilibrium concentration (mg L�1), qe¼amount adsorbed at equilibrium (mg g�1), K¼ partition

coefficient (mg g�1) and n¼ intensity of adsorption. The

linear plot of log Ce vs. log qe indicates if the data is

described by Freundlich isotherm. The value of K implies

that the energy of adsorption on a homogeneous surface is

independent of surface coverage and n is an adsorption con-

stant which reveals the rate at which adsorption is taking

place. These two constants are determined from the slope

and intercept of the plot of each isotherm. An error analysis

is required in order to evaluate the fit of the adsorption iso-

therms to experimental data. In the present study, the linear

coefficient of determination of (R2) was employed for the

error analysis. The linear coefficient of determination is cal-

culated by using the equation

r ¼ nΣxy� (Σx)(Σy)ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffin(Σx2)� (Σx)2

q�

ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi(Σy2)� (Σy)2

q (7)

Theoretically, the values of R2 range from 0 to 1. A R2

value of 1 shows that 100% of the variation of experimental

data is explained by the regression equation. The coefficient

of determination R2 was applied to determine the relation-

ship between the experimental data and the isotherms in

most studies.

RESULTS AND DISCUSSION

Elemental composition of magnesite by XRF

XRF analysis showed that cryptocrystalline magnesite is

composed of MgO (92%), SiO2 (5%), CaO (2%), FeO3

(0.5%) and MnO (0.5%).

Thus magnesite from Folovhodwe is dominated by MgO

as the main element. It also contains impurities of silicon,

calcium, iron and manganese which might have been incor-

porated during deposition processes. The results

corroborated previous findings (Masindi et al. ).

Mineralogical composition by X-ray diffraction

The mineralogical composition of cryptocrystalline magne-

site is presented in Figure 1 and the quantitative results

are shown in Table 1.

Figure 1 | Mineralogical composition of cryptocrystalline magnesite.

Table 1 | Percentage composition (weight) of magnesite

Mineral phases Weight% 3 σ error

Brucite 15.44 0.48

Forsterite 3.11 0.22

Periclase 81.45 0.48

5 V. Masindi et al. | Defluoridation of groundwater using cryptocrystalline magnesite Journal of Water Reuse and Desalination | in press | 2015

Uncorrected Proof

As shown in Figure 1, cryptocrystalline magnesite con-

tains periclase and brucite as the main crystalline

Figure 2 | EDS (left) and SEM (right) analysis of cryptocrystalline magnesite.

minerals. It also contains impurities of forsterite and monti-

cellite. The amorphous phases were not detected on the

analysis. Amorphous phases were quantified by XRF, Four-

ier transform infrared spectroscopy and EDS.

Elemental composition by EDS and morphology by SEM

The elemental composition and morphology of cryptocrys-

talline magnesite is shown in Figure 2.

Q1

6 V. Masindi et al. | Defluoridation of groundwater using cryptocrystalline magnesite Journal of Water Reuse and Desalination | in press | 2015

Uncorrected Proof

SEM-EDS results show that cryptocrystalline magnesite

contains Mg, C, O, Si and Ca, respectively. This corroborates

results reported by Sparks () who state that theoreti-

cally, pure magnesite contain 47.8% MgO and 52.2% CO2.

The sum of Mg, C and O in the material under study indicate

that the material is a carbonate of amorphous nature since it

was detected by the XRD. SEM reveals the morphology of

cryptocrystalline magnesite to be characterized by spherical

shapes. Granular and rough surfaces promote the inter-

action of the surface and aqueous solution (Sparks ;

Selim & Sparks ; Artiola et al. ; Harrison ;

Vicente et al. ).

Batch experiments

Effects of contact time

Figure 3 shows the variation of fluoride concentration with

agitation time. Fluoride removal was evaluated at 10 g/L of

cryptocrystalline magnesite, 10 mg/L of fluoride concen-

tration, 25 WC ambient temperature and 1 g/100 mL S:L

ratio. The shaking time was varied from 1 to 360 min.

Figure 3 shows that there was a gradual increase in the

percentage removal of fluoride from aqueous solution with

an increase in contact time. From the first minute of contact,

the reaction kinetics were very rapid and then stabilized

after 20 min (approximately 90% fluoride removal). After

20 min, no further significant removal of fluoride was

observed, indicating that the reaction has reached an equili-

brium. Therefore, 20 min was taken as the optimum contact

time and was used for subsequent experiments. Chen et al.

(b) reported that ceramic adsorbent takes 48 h to

remove fluoride from wastewater. Biswas et al. ()

Figure 3 | Effect of contact time on defluoridation of borehole water.

noted that 1.5 h is required to remove fluoride from aqueous

solution using synthetic iron, aluminium and chromium (III)

ternary mixed oxide. This study managed to remove fluoride

in the shortest time of 20 min as compared to selected con-

ventional methods.

Effects of magnesite dosage

Figure 4 shows the variation of fluoride concentration with

dose of magnesite added to borehole water. Fluoride

removal was evaluated at 20 min of agitation, 10 mg/L fluor-

ide concentration, and 25 WC ambient temperature. The

doses of magnesite were varied from 0.1 to 10 g.

The percentage removal of fluoride from aqueous solution

was observed to increasewith an increase in dosage of magne-

site. As dosage increases, more surfaces which are suitable for

the uptake of fluoride become more available. From 1 to 2 g

per sample, the fluoride uptake was observed to be gradually

increasing until it reached a steady state at 2 g (% removal

�100%). This is an indication that more surfaces were present

to remove all the fluoride from the aqueous solution. After 2 g,

no further reaction was observed hence indicating that 4 g of

magnesite provide enough surfaces to remove approximately

10 mg/L of fluoride. At low adsorbent dosage, the fluoride

adsorption rate is rapid since the active sites are easily avail-

able and at high adsorbent dosage, the adsorbate species

find it increasingly difficult to access the adsorption sites

and equilibrium is established. As such, it was noted that the

optimum dosage suitable for defluoridation is 2 g/100 mL of

magnesite. Therefore, 20 min of agitation and 20 g/L of mag-

nesite are the optimum defluoridation conditions which will

be used in subsequent experiments.

Figure 4 | Effect of dosage on defluoridation of borehole water.

Figure 6 | Variation of initial and final pH on defluoridation of borehole water.

Figure 7 | Effect of pH on defluoridation of borehole water.

7 V. Masindi et al. | Defluoridation of groundwater using cryptocrystalline magnesite Journal of Water Reuse and Desalination | in press | 2015

Uncorrected Proof

Effects of ion concentration

Figure 5 shows the variation of percentage removal of fluoride

from aqueous media with varying fluoride concentration.

Fluoride removal was evaluated for 20 min contact time,

20 g/L of dosage, 2/100 S:L ratio and 25 WC ambient tempera-

ture. Concentrations were varied from 2 to 60 mg/L.

Figure 5 shows a decrease in percentage removal of fluor-

ide with an increase in fluoride concentration. The general

observation is that when the concentration was increased

the percentage removal decreased. However, from 2 to

10 mg/L, there was a slight increase in the adsorption of flu-

oride because initially the adsorption rate may be higher

due to an increase in the number of vacant sites available

resulting in an increased concentration gradient between

the sorbate in the solution and that of the sorbent surface.

With time, the concentration gradient is reduced owing to

the fluoride adsorption onto the vacant sites leading to the

decreased adsorption during the later stages. Over the con-

centration range 2–15 mg/L magnesite managed to remove

approximately 99% of fluoride from an aqueous medium.

Over the concentration range 15–60 mg/L, it was capable

of removing >95% of fluoride from the solution. This shows

that magnesite has strong affinity for fluoride. From the

obtained results, it can be concluded that 20 min of contact

time, 20 g/L of dosage, and 60 mg/L of fluoride solution are

the optimum conditions for defluoridation of borehole water.

Effect of supernatant pH

Figures 6 and 7 shows the variation of pH and percentage

removal of fluoride from aqueous media with varying pH

Figure 5 | Effect of concentration on defluoridation of borehole water.

ranges. Fluoride removal was evaluated at 20 min contact

time, 20 g/L of dosage, 60 mg/L F�, 2/100 S:L ratio and

25 WC ambient temperature. The pH varied from 2 to 12.

Figure 6 shows an increase in pH after the interaction of

magnesite and fluoride-rich water. Final pH for all solutions

ranged from 10 to 11. Removal of fluoride was >95.0% at all

pH ranges except pH 2 which was slightly lower than 94.5%.

The percentage removal did not vary significantly at various

initial pH suggesting the removal of fluoride using crypto-

crystalline magnesite is independent of pH of media.

Furthermore, it was observed that adsorption of fluoride

using magnesite is independent of initial pH (Figure 7).

Effects of competing ions

The effect of competing ions on fluoride removal is shown in

Figure 8 (20 min contact time, 20 g/L dosage, 60 mg/L,

2:100 S:L ratio and 25 WC ambient temperature).

Figure 8 | Effect of competing ions on defluoridation of borehole water (60 min contact

time, 20 g/L dosage, 60 mg/L, 2/100 S:L ratios and 25W

C ambient

temperature).

8 V. Masindi et al. | Defluoridation of groundwater using cryptocrystalline magnesite Journal of Water Reuse and Desalination | in press | 2015

Uncorrected Proof

The effects of co-existing nitrate, sulphates, chlorite, and

bromide anions on fluoride removal was examined

(Figure 8). In the presence of other anions, removal of fluor-

ide by cryptocrystalline magnesite was greater than 99%.

Sulphate was also observed to be adsorbed onto magnesite

matrices hence showing that the anionic species has affinity

to magnesite. As shown in Figure 8, the competing anions

have no effects on the removal of fluoride from aqueous

solution.

Possible fluoride removal mechanism

The mechanism of fluoride sorption on magnesite surfaces

can be given by the following expression:

¼ Mg�OHþ F� ↔¼ Mg� FþOH� (8)

MgCO3 þ 2F� þH2O ↔¼ MgF2 þHCO�3 þOH� (9)

It is expected that similar process occur on the presence

study using cryptocrystalline magnesite.

Adsorption kinetics

The pseudo-first and second-order adsorption kinetics is

shown in Figure 9.

To evaluate the mechanisms of adsorption by magne-

site, pseudo-first order and second-order kinetic models

were applied (Figures 9(a) and 9(b)). This was done in

an attempt to gain insight on the mechanism and steps

controlling removal of fluoride. The kinetics data

showed better correlation with the pseudo-second-order

model because of the high values of correlation coeffi-

cients based on the linear regression (R2¼ 1). This

confirms chemisorption, since the rate limiting step is a

chemical sorption.

Adsorption isotherms

The Langmuir and Freundlich adsorption isotherm is shown

by Figure 10.

The R2 values for the linear form of the Langmuir and

Freundlich isotherms are 0.99 and 0.90, respectively (Figures 8

(a) and 8(b)). According to R2 values, the Langmuir isotherm

best represents the equilibrium adsorption of fluoride onto

cryptocrystallinemagnesite; it thereforemeans that the adsorp-

tion process occurred on a heterogeneous surface energy by

multilayer adsorption. The RL between 0 and 1 means the

adsorption is favourable (Kamble et al. ). As such, adsorp-

tion of fluoride onto magnesite matrices is favorable.

Removal of fluoride under optimized conditions

Table 3 shows the removal efficiency of fluoride from aqu-

eous media using magnesite under optimized and natural

conditions. The fluoride-rich groundwater had a slightly

alkaline pH of 9.2. Magnesite was observed to remove fluor-

ide from groundwater to below DWAF water quality

guidelines. This shows that magnesite is an effective material

that can be used for removal of fluoride in wastewaters or

groundwater.

Table 4 shows the different adsorption capacities of var-

ious adsorbents of fluoride reported in the literature. Even

though the adsorption capacities were obtained at different

pHs and temperatures, they offer a useful criterion to com-

pare the different adsorption capacities. From the table, it

is clear that magnesite has a higher adsorption capacity

than the other adsorbents. Therefore magnesite can be effec-

tively applied for defluoridation of groundwater because of

its ability to adsorb fluoride.

CONCLUSIONS AND RECOMMENDATIONS

This study arrived at the following conclusions:

Figure 10 | Langmuir (a) and Freundlich (b) adsorption isotherm (20 g/L of the magnesite, 20 min of equilibration and 250 rpm). Concentration was varied from 2 to 60 mg L�1. The

Langmuir and Freundlich adsorption isotherms are shown in Table 2.

Figure 9 | Pseudo-first order (a) and second-order kinetics (b) (20 g/L of the magnesite, 10 mg L�1 of fluoride, 250 rpm and at ambient temperature). Time: 1–360 min.

Table 2 | Langmuir and Freundlich adsorption isotherm

Langmuir adsorption isothermFreundlich adsorptionisotherm

Species R2 Qm b RL R2 k n

F� 0.99 9.2 5.9 0.003 0.90 5.0 2.4

9 V. Masindi et al. | Defluoridation of groundwater using cryptocrystalline magnesite Journal of Water Reuse and Desalination | in press | 2015

Uncorrected Proof

1. Adsorption of fluoride by magnesite is independent of

pH.

2. Maximum fluoride removal was found at 20 min of con-

tact time, 20 g/L of dosage, 60 mg/L of fluoride

concentration, 2/100 S:L and 25 WC ambient temperature.

Table 3 | Physiochemical conditions of borehole water before and after defluoridation

Parameter Untreated water Treated water

pH 9.2 10.26

EC (μS cm�1) 100 45.3

TDS (mg L�1) 66.4 185.3

F� (mg L�1) 10 0.01

Bromide 11 9

Sulphate 47 0.01

Nitrate 65 64

Chloride 140 141

EC, electrical conductivity; TDS, total dissolved solids.

Table 4 | Comparison of different adsorption capacities (mg/g) of different adsorbents for

fluoride

AdsorbentAdsorptioncapacity Source

Cryptocrystallinemagnesite

9.2 Present study

Alum-bent 5.7 Vhahangwele et al.()

Al-oxide origin alumina 2 Kamble et al. ()

Mg2þ bentonite 2.3 Thakre et al. ()

polymer/bio-polymercomposites

15 Karthikeyan et al.()

Activated alumina 1.1 Maliyekkal et al.()

Lanthanide impregnatedsilica gel

3.8 Zhou et al. ()

Fe3þ modified bentoniteclay

2.9 Gitari et al. ()

10 V. Masindi et al. | Defluoridation of groundwater using cryptocrystalline magnesite Journal of Water Reuse and Desalination | in press | 2015

Uncorrected Proof

3. Kinetic studies revealed that the fluoride removal fol-

lowed a pseudo-second order than pseudo-first order

hence confirming chemisorption as the rate-limiting step.

4. The adsorption data fitted well to Langmuir than Freun-

dlich adsorption isotherms hence confirming monolayer

adsorption

5. To this end, it can be concluded that, effectiveness, avail-

ability and low cost of cryptocrystalline magnesite makes

this material a potential candidate for defluoridation of

groundwater.

6. Cryptocrystalline magnesite managed to remove fluoride

to below DWAF drinking water quality guidelines. As

such, this technology can be used as a point source

defluoridation technique in rural areas and households

in South Africa and other developing countries.

ACKNOWLEDGEMENTS

The authors wish to convey their sincere acknowledgement

to the council of scientific and industrial research (CSIR),

SASOL-INZALO, ESKOM-TESP, National Research

Foundation (NRF), and Department of Science and

Technology (DST) and University of Venda research and

publication committee for funding this project.

REFERENCES

Artiola, J. F., Pepper, I. L. & Brusseau, M. L. EnvironmentalMonitoring and Characterization. Elsevier Academic Press,Burlington, MA, USA.

Bansiwal, A., Thakre, D., Labhshetwar, N., Meshram, S. & Rayalu, S. Fluoride removal using lanthanum incorporatedchitosan beads. Colloid. Surf. B. 74, 216–224.

Bhatnagar, A., Kumar, E. & Sillanpää, M. Fluoride removalfrom water by adsorption-A review. Chem. Eng. J. 171,811–840.

Biswas, K., Gupta, K. & Ghosh, U. C. Adsorption of fluorideby hydrous iron(III)–tin(IV) bimetal mixed oxide from theaqueous solutions. Chem. Eng. J. 149, 196–206.

Biswas, K., Gupta, K., Goswami, A. & Ghosh, U. C. Fluorideremoval efficiency from aqueous solution by synthetic iron(III)–aluminum(III)–chromium(III) ternary mixed oxide.Desalination 255, 44–51.

Brunson, L. R. & Sabatini, D. A. Practical considerations,column studies and natural organic material competition forfluoride removal with bone char and aluminum amendedmaterials in the Main Ethiopian Rift Valley. Sci. TotalEnviron. 488–489, 580–587.

Camacho, L. M., Torres, A., Saha, D. & Deng, S. Adsorptionequilibrium and kinetics of fluoride on sol–gel-derivedactivated alumina adsorbents. J. Colloid Interf. Sci. 349,307–313.

Çengeloglu, Y., Kır, E. & Ersöz, M. Removal of fluoride fromaqueous solution by using red mud. Sep. Purif. Technol. 28,81–86.

Chang, C. F., Chang, C. Y. & Hsu, T. L. Removal of fluoridefrom aqueous solution with the superparamagnetic zirconiamaterial. Desalination 279, 375–382.

Chaturvedi, A. K., Pathak, K. C. & Singh, V. N. Fluorideremoval from water by adsorption on china clay. Appl. ClaySci. 3, 337–346.

Q2

Q3

Q4

11 V. Masindi et al. | Defluoridation of groundwater using cryptocrystalline magnesite Journal of Water Reuse and Desalination | in press | 2015

Uncorrected Proof

Chen, N., Zhang, Z., Feng, C., Sugiura, N., Li, M. & Chen, R. aFluoride removal from water by granular ceramic adsorption.Colloid Interf. Sci. 348, 579–584.

Chen, N., Zhang, Z., Feng, C., Li, M., Zhu, D., Chen, R. & Sugiura,N. b An excellent fluoride sorption behavior of ceramicadsorbent. J. Hazard. Mater. 183, 460–465.

Chen, L., Wang, T. -J., Wu, H. -X., Jin, Y., Zhang, Y. & Dou, X. -M. Optimization of a Fe–Al–Ce nano-adsorbent granulationprocess that used spray coating in a fluidized bed for fluorideremoval from drinking water. Powder Technol. 206, 291–296.

Choi, W. W. & Chen, K. Y. Removal of fluoride from watersby adsorption. J. Am. Water Works Ass. 71, 562–570.

Czarnowski, W., Wrzesniowska, K. & Krechniak, J. Fluoridein drinking water and human urine in Northern and CentralPoland. Sci. Total Environ. 191, 177–184.

Daifullah, A. A. M., Yakout, S. M. & Elreefy, S. A. Adsorptionof fluoride in aqueous solutions using KMnO4-modifiedactivated carbon derived from steam pyrolysis of rice straw.J. Hazard. Mater. 147, 633–643.

Dayananda, D., Sarva, V. R., Prasad, S. V., Arunachalam, J. &Ghosh, N. N. Preparation of CaO loaded mesoporousAl2O3: Efficient adsorbent for fluoride removal from water.Chem. Eng. J. 248, 430–439.

Dorenbos, P. 5d-level energies of Ce3þ and the crystallineenvironment. I. Fluoride compounds. Phys. Rev. B 62,15640–15649.

Fan, X., Parker, D. J. & Smith, M. D. Adsorption kinetics offluoride on low cost materials. Water Res. 37, 4929–4937.

Fletcher, H. R., Smith, D. W. & Pivonka, P. Modeling thesorption of fluoride onto alumina. J. Environ. Eng. 132,229–246.

Gaciri, S. J. & Davies, T. C. The occurrence and geochemistryof fluoride in some natural waters of Kenya. J. Hydrol. 143,395–412.

Gitari, W. M., Ngulube, T., Masindi, V. & Gumbo, J. R. Defluoridation of groundwater using Fe3þ-modified bentoniteclay: optimization of adsorption conditions. DesalinationWater Treat. 53, 1578–1590.

Gwala, P., Andey, S., Nagarnaik, P., Ghosh, S. P., Pal, P.,Deshmukh, P. & Labhasetwar, P. Design anddevelopment of sustainable remediation process formitigation of fluoride contamination in ground water andfield application for domestic use. Sci. Total Environ.488–489, 588–594.

Harrison, R. M. An Introduction to Pollution Science. RSCPublishing, Cambridge, UK.

Kamble, S. P., Dixit, P., Rayalu, S. S. & Labhsetwar, N. K. Defluoridation of drinking water using chemically modifiedbentonite clay. Desalination 249, 687–693.

Kamble, S. P., Deshpande, G., Barve, P. P., Rayalu, S., Labhsetwar,N. K., Malyshew, A. & Kulkarni, B. D. Adsorption offluoride from aqueous solution by alumina of alkoxide nature:batch and continuous operation. Desalination 264, 15–23.

Karthikeyan, M., Kumar, K. K. S. & Elango, K. P. Batchsorption studies on the removal of fluoride ions from water

using eco-friendly conducting polymer/bio-polymercomposites. Desalination 267, 49–56.

Maliyekkal, S. M., Sharma, A. K. & Philip, L. Manganese-oxide-coated alumina: a promising sorbent for defluoridationof water. Water Res. 40, 3497–3506.

Masindi, V., Gitari, M. W., Tutu, H. & De Beer, M. Application of magnesite–bentonite clay composite as analternative technology for removal of arsenic from industrialeffluents. Toxicol. Environ. Chem. 1–17. Doi: 10.1080/02772248.2014.966714.

Meenakshi, R. C. Fluoride in drinking water and its removal.J. Hazard. Mater. 137, 456–463.

Miretzky, P. & Cirelli, A. F. Fluoride removal from water bychitosan derivatives and composites: A review. J. FluorineChem. 132, 231–240.

Selim,H.M. E.&Sparks,D. L. Physical andChemical ProcessesofWater and Solute Transport/Retention in Soils: Proceedings ofa Symposium Sponsored by Divisions S-1 and S-2 of the SoilScience Society of America in Baltimore, MD, 18 to 22 October1998. Soil Science Society of America, Madison, WI, USA.

Shen, J. & Schäfer, A. Removal of fluoride and uranium bynanofiltration and reverse osmosis: a review. Chemosphere117, 679–691.

Sparks, D. L. Environmental Soil Chemistry. Academic Press,Amsterdam.

Sparks, D. L. Environmental Soil Chemistry. Academic Press,Amsterdam.

Srivastav, A. L., Singh, P. K., Srivastava, V. & Sharma, Y. C. Application of a new adsorbent for fluoride removal fromaqueous solutions. J. Hazard. Mater. 263, 342–352.

Sujana, M. G. & Anand, S. Fluoride removal studies fromcontaminated ground water by using bauxite. Desalination267, 222–227.

Thakre, D., Rayalu, S., Kawade, R., Meshram, S., Subrt, J. &Labhsetwar,N. Magnesium incorporated bentonite clay fordefluoridation of drinkingwater. J.Hazard.Mater. 180, 122–130.

Tomar, V., Prasad, S. & Kumar, D. Adsorptive removal offluoride from aqueous media using Citrus limonum (lemon)leaf. Microchem. J. 112, 97–103.

Tor, A. Removal of fluoride from an aqueous solution byusing montmorillonite. Desalination 201, 267–276.

Tor, A., Danaoglu, N., Arslan, G. & Cengeloglu, Y. Removalof fluoride from water by using granular red mud: Batch andcolumn studies. J. Hazard. Mater. 164, 271–278.

Trikha, R. & Sharma, B. K. Studies on factors affectingfluoride removal from water using passive system. J. Environ.Chem. Eng. 2, 172–176.

Vhahangwele, M., Mugera, G. W. & Tholiso, N. Defluoridation of drinking water using Al3þ-modifiedbentonite clay: optimization of fluoride adsorptionconditions. Toxicol. Environ. Chem. 1–16. Doi: 10.1080/02772248.2014.977289.

Vicente, M. A., Gil, A. & Bergaya, F. Developments in clayscience. (B. Faïza & L. Gerhard, eds.). Elsevier, Amsterdam,pp. 523–557.

12 V. Masindi et al. | Defluoridation of groundwater using cryptocrystalline magnesite Journal of Water Reuse and Desalination | in press | 2015

Uncorrected Proof

Viswanathan, N. & Meenakshi, S. Enriched fluoride sorptionusing alumina/chitosan composite. J. Hazard. Mater. 178,226–232.

Wang, H., Chen, J., Cai, Y., Ji, J., Liu, L. & Teng, H. H. Defluoridation of drinking water by Mg/Al hydrotalcite-likecompounds and their calcined products. Appl. Clay Sci. 35,59–66.

Wang, J., Xu, W., Chen, L., Jia, Y., Wang, L., Huang, X. -J. & Liu, J. Excellent fluoride removal performance by CeO2–ZrO2

nanocages in water environment. Chem. Eng. J. 231, 198–205.

Wu, H. -X., Wang, T. -J., Chen, L., Jin, Y., Zhang, Y. & Dou, X. -M. Granulation of Fe–Al–Ce hydroxide nano-adsorbent byimmobilization in porous polyvinyl alcohol for fluorideremoval in drinking water. Powder Technol. 209, 92–97.

Xiuru, Y., Kuanxiu, S., Jianping, W., Liuchang, H. & Zhaohui, Y. Preparation of CeO2–TiO2/SiO2 and its removalproperties for fluoride ion. J. Rare Earths 16, 279–280.

Yu, Y., Yu, L. & Paul Chen, J. Adsorption of fluoride by Fe–Mg–La triple-metal composite: adsorbent preparation,

illustration of performance and study of mechanisms. Chem.Eng. J. 262, 839–846.

Zhang, S., Lu, Y., Lin, X., Su, X. & Zhang, Y. Removal offluoride from groundwater by adsorption onto La(III)- Al(III)loaded scoria adsorbent. Appl. Surf. Sci. 303, 1–5.

Zhao, X., Zhang, L., Xiong, P., Ma, W., Qian, N. & Lu, W. Anovel method for synthesis of Co–Al layered doublehydroxides and their conversions to mesoporous CoAl2O4

nanostructures for applications in adsorption removal offluoride ions. Micropor. Mesopor. Mat. 201, 91–98.

Zhou, Y., Yu, C. & Shan, Y. Adsorption of fluoride fromaqueous solution on La3þ-impregnated cross-linked gelatin.Sep. Purif. Technol. 36, 89–94.

Zhou, Q., Lin, X., Li, B. & Luo, X. Fluoride adsorption fromaqueous solution by aluminum alginate particles prepared viaelectrostatic spinning device. Chem. Eng. J. 256, 306–315.

Zhu, P., Wang, H., Sun, B., Deng, P., Hou, S. & Yu, Y. Adsorption of fluoride from aqueous solution by magnesia-amended silicon dioxide granules. J Chem. Technol.Biotechnol. 84, 1449–1455.

First received 21 September 2014; accepted in revised form 28 December 2014. Available online 9 April 2015

Author QueriesJournal: Journal of Water Reuse and Desalination

Manuscript: JWRD-D-14-00080

Q1 Please confirm the change of citation from Selim et al. (2001) to Selim and Sparks (2001) as per the referencelist.

Q2 Please provide volume number for Masindi et al. (2014).

Q3 Please provide volume number for Vhahangwele et al. (2014).

Q4 Please provide book title for Vicente et al. (2013).