an investigation into the adsorption removal of ammonium (2014 1-s2.0-s1876107013001168-main)

8/10/2019 An Investigation Into the Adsorption Removal of Ammonium (2014 1-s2.0-S1876107013001168-Main)

http:///reader/full/an-investigation-into-the-adsorption-removal-of-ammonium-2014-1-s20-s1876107013001168-m 1/11

An investigation into the adsorption removal of ammonium by salt activated

Chinese

(Hulaodu)

natural

zeolite:

Kinetics,

isotherms,

and

thermodynamics

Aref Alshameri a, Chunjie Yan a,*, Yasir Al-Anib, Ammar Salman Dawoodb, Abdullateef Ibrahim c,Chunyu Zhoua, Hongquan Wanga

aEngineering Research Center of Nano-geomaterial of Education Ministry, China University of Geosciences, Wuhan 430074, Chinab School of Environmental Studies, China University of Geosciences, Wuhan 430074, Chinac State Key Laboratory of Biogeology and Environmental Geology, China University of Geosciences, Wuhan 430074, China

1. Introduction

Clinoptilolite (Na,K,Ca)6(Si,Al)36O7220H2O is one of the natu-

rally existing zeolites. It is rich in silica and has a lower ion-

exchange capacity than other zeolites as well as less than that of

many of the available synthetic ion-exchange resins. It generally

exhibits a high selectivity for ammonium and metallic ions [1,2].

Clinoptilolite from different deposits has been widely reported as

adsorbent for ammonium removal from wastewaters [3,4]. Natural

zeolite, on the other hand, needs to be purified and modified in

order to improve its ion-exchange [5] and adsorption properties

before it can be used to remove ammonium effectively. Nitrogen

compounds in aqueous environments are usually found in the form

of ammonium ions (NH4+). Important sources of NH4

+ include

effluent from municipal sewage treatment plants, the application

of fertilizer in agricultural practices and industrial processes all of

which contributes to the accelerated incidence of eutrophication

resulting in algal bloom in lakes and rivers [610]. Complete

removal of NH4+ is required due to its toxicity to the majority of

aquatic life. For example, the ammonium nitrogen concentration

for most fish species must not exceed 1.5 mg NH4+ ion [11,12].

NH4+ concentration, in certain surface waters serving as a source of

potable water, is much higher than the permissible level, due to

large quantities of industrial and municipal wastewater being

discharged into existing water resources [5,13]. This threatens the

availability of safe drinking water and, thus, human health. For this

reason, the prevention of nitrogen pollution with NH4+ removal

from wastewater is of great importance [7,9,14,15].

Various methods including air stripping, biological methods

and activated carbon have been used for NH4+ removal [1619].

However, since biological methods do not respond well to shock

loads of ammonium, unacceptable peaks of NH4+ over the

discharging levels may frequently appear in the effluent. Also,

high costs, poor regeneration and uncertainty of outcome are some

of the frequently encountered limitations in the application of the

biological method. Moreover, there is a high risk to safety during

Journal of the Taiwan Institute of Chemical Engineers 45 (2014) 554564

A R T I C L E I N F O

Article history:

Received 2 February 2013

Received in revised form 2 May 2013

Accepted 6 May 2013

Available online 12 June 2013

Keywords:

Zeolite (clinoptilolite)

Adsorption

Activated zeolite

Ammonium removal

Kinetics and isotherm

A B S T R A C T

The development of the process of sodium activation of zeolite has been an effective technique for

enhancing the efficiency of ammonium removal. In this research, the optimum conditions for the

activation of Chinese (Hulaodu) zeolite of themost effective parameters such as sodium concentration,

stirring time, and temperature were determined. Themost efficient conditions were selected according

to the highest ammonium removal capacity. The characteristics of activated zeolite (ActZ) and its

mechanism of ammonium removal were investigated and compared with that of natural zeolite (NZ).

Additionally, both zeolites were analyzed by scanning electronmicroscopy (SEM), Zeta potential, X-ray

diffraction (XRD), thermogravimetry (TG) and BET surface analysis. The activated zeolite revealed the

highest ammonium removal efficiency reaching up to 98%based on stirring time, zeolite loading, initial

ammonium concentration, temperature and pH. The adsorption kinetic was explored and fitted best

with the pseudo-second-order model, whereas adsorption isotherm results illustrated that Langmuir

model (LM) provided thebestfit forthe equilibriumdata.Moreover,thermodynamicparameters such as

change in free energy (DG8), enthalpy (DH8) and entropy (DS8) were also calculated. The parametersrevealed that the exchange of ammonium ion by activated zeolite occurred spontaneously at ambient

conditions (25 8C). It was concluded that when Chinese (Hulaodu) zeolite is activated under thecondition of 1 M NaCl, 70 8C and stirring time of 30min, an excellent removal of NH4

+ was obtained.

2013 Taiwan Institute of Chemical Engineers. Published by Elsevier B.V. All rights reserved.

* Corresponding author. Tel.: +86 18971579917; fax: +86 027 67885098.

E-mail address: [email protected] (C. Yan).

Contents

lists

available

at

SciVerse

ScienceDirect

Journal of the Taiwan Institute of Chemical Engineers

journal homepage : www.elsev ier . com/loc ate / j t ice

1876-1070/$ see front matter 2013 Taiwan Institute of Chemical Engineers. Published by Elsevier B.V. All rights reserved.

http://dx.doi.org/10.1016/j.jtice.2013.05.008
http://dx.doi.org/10.1016/j.jtice.2013.05.008http://dx.doi.org/10.1016/j.jtice.2013.05.008http://dx.doi.org/10.1016/j.jtice.2013.05.008http://dx.doi.org/10.1016/j.jtice.2013.05.008http://dx.doi.org/10.1016/j.jtice.2013.05.008http://dx.doi.org/10.1016/j.jtice.2013.05.008http://dx.doi.org/10.1016/j.jtice.2013.05.008http://dx.doi.org/10.1016/j.jtice.2013.05.008http://dx.doi.org/10.1016/j.jtice.2013.05.008http://dx.doi.org/10.1016/j.jtice.2013.05.008http://dx.doi.org/10.1016/j.jtice.2013.05.008http://dx.doi.org/10.1016/j.jtice.2013.05.008http://dx.doi.org/10.1016/j.jtice.2013.05.008http://dx.doi.org/10.1016/j.jtice.2013.05.008http://dx.doi.org/10.1016/j.jtice.2013.05.008http://dx.doi.org/10.1016/j.jtice.2013.05.008http://dx.doi.org/10.1016/j.jtice.2013.05.008http://dx.doi.org/10.1016/j.jtice.2013.05.008http://dx.doi.org/10.1016/j.jtice.2013.05.008http://dx.doi.org/10.1016/j.jtice.2013.05.008http://dx.doi.org/10.1016/j.jtice.2013.05.008mailto:[email protected]:[email protected]://www.sciencedirect.com/science/journal/18761070http://www.sciencedirect.com/science/journal/18761070http://www.sciencedirect.com/science/journal/18761070http://dx.doi.org/10.1016/j.jtice.2013.05.008http://dx.doi.org/10.1016/j.jtice.2013.05.008http://www.sciencedirect.com/science/journal/18761070mailto:[email protected]://dx.doi.org/10.1016/j.jtice.2013.05.008



the subsequent processing such as the aeration process which

causes stripping effects for volatile compounds resulting in

accidental releases, often causing odor and aerosol with health

implications [20,21].

Contingency on temperature and climate conditions constitutes

another disadvantage in this process [22]. Compared with the

above mentioned methods, high safety, low cost [13,2326] and

relative simplicity of application and operation are some of the

attributes that are attracting an increasing focus on the use of

zeolite for environment applications [2,8,11].

The factors that influence ammonium removal performances

are mainly pH, temperature, reaction time, initial concentration

of NH4+, and adsorbent dosage. Although many previous studieshave focused on these factors collectively [3,27,28], however,

comparison of the results from the available literature indicates

significant variability in the reported behavior.

It seems that natural clinoptilolites from different places have

different characteristics [9,10,29,30]. These differences in the

characteristics of clinoptilolites are probably attributed to the

differences in the geological formation of zeolite sources

[21,31,32]. Therefore, each special zeolite material has its own

special characteristics and still requires to be researched individ-

ually [11,27,31].

The main aim of this study is to determine the optimal

conditions for the activation of Chinese (Hulaodu) zeolite to get the

best results for the adsorption of NH4+ by salt treatment. The main

focus

of

the

study

is

the

transformation

of

low

grade

Hulaoduszeolite to a high cation exchanger under appropriate activation

conditions. An investigation and comparison of characteristics and

the equilibrium removal of NH4+ ion onto both zeolites was also

carried out.

The specific objectives of this research is to study the sodium

activation of zeolite samples and the effect of various parameters

on zeolite activation such as sodium concentration, stirring time,

and temperature. And to identify the key processes controlling the

rate of ammonium adsorption by zeolite. In addition, the effects of

pH, stirring times, initial concentration, adsorbent dosages and

temperature on NH4+ removal for both natural and activated

samples were investigated and compared. Adsorption isotherms,

thermodynamics and reusability of zeolite for the removal of NH4+

ions

were

examined.

2. Materials and methods

2.1. Raw materials

A natural zeolite (NZ) was collected from Huludao city in China.

Analytical grade inorganic chemicals, such as ammonium chloride

(NH4Cl), sodium chloride (NaCl), sodium hydroxide (NaOH) and

hydrochloric acid (HCl), were used.

2.2.

Preparation

and

activation

of

zeolite

A natural zeolite (NZ) was ground and passed through 200

230 mesh sieves. The material was washed with distilled water to

remove any non-adhesive impurities, and then dried in an oven at

100 8C for 24 h and finely crushed. The activation process was

carried out by mixing NZ powder material with an aqueous

solution of sodium chloride under the different conditions detailed

below:

Effect of sodium concentration: To study the effect of sodium

concentration and batch activation, the sodium chloride

concentration was varied from 0.5 to 2 mol/L with zeolite/

solution ratio maintained as 1 g/10 ml. The suspension was

stirred in conical flasks (500 ml) using a magnetic stirrerwater

bath at a rate of 120 rpm and 90 8C for 2 h. Subsequently,

the suspension was filtered and washed with distilled water.

The wet activated material was dried at 70 8C in an oven for 24 h

and used in batch adsorption experiments at an initial

ammonium

concentration

of

100

mg/L

with

a

pH

7

at

25

8C

and stirring time of 24 h. The best sodium ion concentration for

the activation of zeolite was selected as that corresponding to

the highest NH4+ removal capacity (mg/g).

Effect

of

time: For

determining

the

optimum

time

required

for

activation zeolite from aqueous solution, a weighed quantity of

zeolite (1 g) was added into the solution of 1 mol/L sodium

chloride

concentration

and

stirring

time

ranging

from

0.5

to

3

h.

The

conditions

applied

above

on

the

effect

of

sodium

concentration experiment were repeated. The optimum stirringtime for the activation of zeolite was selected as that

corresponding

to

the

highest

NH4+ removal

capacity

(mg/g).

Effect

of

temperature:

The

effect

of

temperature

on

activation

of

zeolite was investigated at different temperature values

ranging from 10 to 90 8C. The sodium chloride concentration

and

stirring

time

were

kept

constant

at

1

mol/L

and

0.5

h,

respectively.

The

optimum

temperature

for

the

activation

of

zeolite was selected as that corresponding to the highest NH4+

removal capacity (mg/g).

2.3.

Analysis

and

characterization

The activated and natural zeolites were characterized by XRD,

SEM,

EDX,

TG,

Zeta

potential,

chemical

analysis

and

specific

surfacearea

(BET).

Identification

of

mineral

species

in

the

zeolite

samples

was

carried

out

by

XRD

pattern

using

a

Germany

D8-FOCOS

X-ray

diffractometer with Cu Ka (l = 0.154 nm) radiation operating at40 kV and 40 mA and a step width of 0.058. Semi-quantitative

weight percentages of samples were calculated by using mineral

intensity

factors.

Textural

characteristics

of

the

activated

and

natural zeolites were performed using a Japanese Netherlands

FESEM Quanta SU8010 electron microscope, operating at an

accelerating voltage of 15 kV for photomicrographs as well as to

analyze the Chinese-zeolite composition by Energy Dispersion X-

ray Spectrometry (EDS), USA, Apolloxp. The sample was initially

placed in a vacuum chamber for coating with a thin layer (a few

nanometers) of gold (Au). The specific surface area, pore size and

volume of the material were evaluated by the nitrogen gas

Nomenclature

C0 the starting equilibrium concentrations (mg/L)

Ce the final equilibrium concentrations (mg/L)

V

the

volume

of

the

working

solution

(L)

M the mass of added zeolite (g)

k1 and k2 (1/min) and (g/mg min) are constants of

adsorptionh

the

rate

of

adsorption

(mg/g

min)

K

Langmuir

constant

(L/mg)

KF Freundlich adsorbent capacity (mg/g (L/mg)1/n)

n the reciprocal of reaction order

qt adsorption capacity at time t (mg/g)

qe adsorption capacity at equilibrium conditions

(mg/g)

qmax maximum adsorption capacity (mg/g)

Kid constant of intraparticle diffusion (mg/g min1/2)

a and k0 constants of Bangham equation

V the volume of solution (ml) of Bangham equation

m

the

weight

of

adsorbent

(g/L)

of

Bangham

equation

A. Alshameri et al./Journal of the Taiwan Institute of Chemical Engineers 45 (2014) 554564 555



adsorption method, with a heating rate of 10 8C/min. N2adsorptiondesorption experiments were performed at 77 K with

an Automatic Volumetric Sorption Analyzer (ASAP2020, TSI,

America), employing multipoint BET isotherm adsorption data

fitting. Zeta potential measurements for the natural and activated

zeolites, as a function of medium pH, were determined using Zeta

Plusk equipment (zetasizer Nano ZS 90, Malvern, UK). A 103mol/L

solution of KNO3 and 60 mg/L of ammonium concentration were

used. The medium pH was controlled by adding HCl (pH 7) solutions, separately. The water content and zeolite

decomposed were determined by a Thermogravimetric (TG)

analyzer. TG/DSC analysis was performed on a NEZSCH, STA409-

PC-Germany, and thermal analysis system was in the range 30

1200 8C. In addition, the composition of zeolite was analyzed by

chemical method. Absorbance values of ammonium ion concen-

tration in solution were read using a Chinese-Shimadzu UV-723Vis

spectrophotometer. Quality control testing includes experiments

with blanks and duplicates.

2.4. Adsorption rate, batch sorption studies

All

adsorptions

in

batch

experiments

were

carried

out

using

stopper

conical

flasks

(500

ml),

zeolite/liquid

ratio

of

1

g/100

ml,

magnetic stirring water bath, a stirring rate of 120 rpm and a

temperature at 25 8C. A stock solution (1000 mg/L) was prepared

by

dissolving

NH4Cl in distilled water.

For

determining

the

optimum

time

required

for

ammonium

removal by natural zeolite (NZ) and activated zeolite (ActZ) from

aqueous solution, a weighed quantity of adsorbent (1 g) was added

into

solution

of

80

mg/L

ammonium

concentration

and

was

stirred

for

10420

min

and

560

min

of

NZ

and

ActZ,

respectively,

at

a

fixed

pH of 7. The effect of initial ammonium concentration in batch

adsorption experiments was carried out by using initial ammonium

concentration

in

the

range

of

10240

mg/L

and

10400

mg/L

for

300

min

and

40

min

for

NZ

and

ActZ,

respectively.

The

effect

of

pH

on

adsorption was investigated at initial ammonium concentration of

80 mg/L and performed in different pH values (210) at 25 8C. Batch

adsorption was conducted at 300 and 40 min for NZ and ActZ,respectively. The pH of the solution was adjusted by 1 M HCl or

NaOH solution. For determining zeolites loading effect, zeolite

loading

was

varied

from

0.2

to

2.2

g/100

ml

at

initial

ammonium

concentration

of

80

mg/L.

The

zeoliteliquid

was

then

stirred

for

300

and 40 min for NZ and ActZ, respectively, at temperature 25 8C.

Temperature of adsorption isotherms was studied at 25, 35 and

45

8C.

A

100

ml

of

ammonium

chloride

solution

of

80

mg/L

concentration

was

equilibrated

with

1

g

zeolite

for

40

min

for

ActZ. Samples were filtered through a 0.45 mm filter membraneafter adsorption.

The

residual

concentration

of

ammonium

was

determined

by

Nesslers

reagent

spectrophotometry

method

at

420

nm.

The removal efficiency (%) of zeolite and the amount of

adsorbed ammonium ions (qe) were calculated, respectively, usingthe

following

equations:

Removal efficiency % C0 CeC0

100 (1)

qe C0 Ce

M (2)

3. Results and discussion

3.1.

Activated

zeolite

Due to different results in activation of different zeolite

samples,

parameters,

such

as

sodium

concentration,

stirring

time

and temperature, had to be investigated to obtain optimum

activation conditions, and the results are shown in Fig. 1ac. As can

be seen from batch adsorption experiments (Fig. 1ac), the

temperature of 70 8C, stirring time of 30 min and 1 mol/L

concentration of NaCl were the most effective in adsorption of

NH4+ ions on Hulaodu natural zeolite (NZ). The optimum

conditions obtained above for the activation of zeolite using NaCl

were applied on NaOH for the adsorption of NH4+ ions at 100 mg/L

of ammonium concentration, pH 7 and 25 8C. Its NH4+ adsorption

capacity (mg/g) was compared with NaCl activation as shown in

Table 1. The results showed that NaCl-activated zeolite (ActZ) had

a higher NH4+ adsorption capacity value than NaOH-activated

zeolite. This result was confirmed by EDS analysis (Fig. 2a and b)

which showed that the Na+ content of the NaOH-activated zeolite

(1.33%) was lower than that of NaCl-activated zeolite (2.10%).

Higher NH4+ adsorption capacity means higher Na+ content

[3234]. From the above discussion, it can be concluded that ion

exchange promoted by NaCl-activated zeolite (ActZ) is a more

attractive zeolite preparation method than that of NaOH-activated

zeolite. Therefore, NaCl-activated zeolite (ActZ) was selected for

further study.

3.2.

Natural

and

activated

zeolite

characteristics

SEM and the qualitative composition analysis of EDS obtained

from the grain of natural and activated zeolites are shown in Fig. 2.

Fig. 2c illustrates that the main chemical elements (Al, Si, O, Na, Mg,

K, Ca and Fe) are present in the structure of this natural zeolite, in

agreement with the chemical composition (Table 2). Quantitative

tests were also performed on both zeolites. The results from

elemental analysis by EDS of NZ and ActZ as shown in Fig. 2a and c

indicate that the Na+ content in NZ increased from 0.21% to 2.10%

after activation. Meanwhile, the content of Ca2+, Mg2+ and K+

decreased from 3.44%, 0.81% and 1.53% to 1.08%, 0.58% and 0.76%,

respectively. For this reason, the ammonium capacity of combined

activated zeolite increased sharply in microscale [33,34].

The

XRD

patterns

of

NZ

and

ActZ

are

shown

in

Fig.

3a.

X-ray

diffraction did not show any changes in zeolite structure afteractivation. Mineralogical analysis of the zeolite samples was

carried out using X-Ray Diffraction (XRD). The results showed that

the

natural

zeolite

contained

clinoptilolite

in

the

majority

93%,

and

small

quantity

of

quartz

7%.

Chemical components of natural zeolite are shown in Table 2.

The Si/Al ratio calculated from these data was found to be 4.8. The

ratio

of

Si/Al

is

an

important

factor

in

understanding

the

zeolite

structure.

When

the

ratio

is

over

than

4.0,

then

the

zeolite

is

a

clinoptilolite-type and as such, the structure would not be broken

easily at high temperature.

This

result

was

confirmed

by

thermogravimetric

(TG)

analysis

as

shown

in

Fig.

3b.

It

shows

that

the

zeolite

could

stand

temperatures of up to 869.2 8C without being decomposed and that

it

only

undergoes

a

weight

reduction

of

0.07%

at

this

temperature.It

also,

shows

that

the

water

content

of

zeolite

was

lost

at

71.2

8C.

The

surface

analysis

of

activated

zeolite

was

investigated

by

Zeta potential and BET standard method and compared with that of

natural zeolite. Fig. 3c shows that zeolite surface groups are mainly

negative

in

the

studied

pH

range

and

that

the

ammonium

removal

does

not

interfere

much

with

the

Zeta

potential

measurements,

confirming the theory that the mechanism is not electrostatic

(charge neutralization) but a result of an ion-exchange reaction.

Moreover,

the

figure

shows

that

activated

zeolite

is

more

negative

than

natural

zeolite.

The BET specific surface area, total pore volume, and average

width pore size of the natural zeolite were measured to be

25.88

m2/g, 0.0032

cm3/g

and 8.72264mm, respectively. How-

ever,

these

values did not

change significantly after

activation

of

A. Alshameri et al./Journal of the Taiwan Institute of Chemical Engineers 45 (2014) 554564556



zeolite, 26.7074 m2/g of specific surface area, 0.004167 cm3/g of

total pore volume and 9.64864 mm of average width pore size.These

characteristics

of

activated

zeolite

have disadvantages for

physical adsorption,

but the results from equilibrium experi-

ments do not match with this fact because the removal

mechanism of ammonium by zeolites follows the ion-exchange

reaction. Therefore, ion-exchange capacities of

ammonium

depend

on

dielectric strength between

ammonium

as

well as

their affinities to zeolite. The size of micropores of the natural

zeolite is in the range of 310 A and micropores of the NaCl-

modified

zeolite

were

more

developed

at

the range

of

5

A than

other

cases. This implies

that the

NaClzeolite could

selectively

remove NH4+ ions which have a specific size. Also, owing to its

small surface area, this zeolite is very stable against heat and

acidity

as

reported by Tehrani

and Salari [29]

and Gottardi

and

Galli [35] who

found that

heat andacid

activation doesnot result

Fig. 1. Removal capacity as a function of sodium concentrations (a), stirring time (b)

and temperature (c). Fig. 2. SEM and EDS spectra analyses zeolite grain of NaCl-activated zeolite (ActZ)

(a), NaOH-activated zeolite (b) and natural zeolite (NZ) (c).




in drastic changes in zeolite structure but increases Si/Al ratio, as

a consequence of heat or acid activation.

3.3. Effect of stirring time

As shown in Fig. 4a and b, the removal efficiency of NH4+ ions

increased with increasing stirring time. 40% and 92% of NH4+ ions

removal were completed within 10 min for NZ and ActZ,

respectively, which also confirmed larger adsorption capacity of

ActZ compared to NZ.

The experimental data show that NZ could exceed 78%

percentage removal at 300 min, but then, the removal efficiency

plateaus. As to ActZ, the ammonium removal rate exceeds 98%

percentage removal at 40 min and became increasingly slow with

increasing stirring time. This may be attributed to the utilization ofthe most readily available adsorption sites of the zeolite that leads

to a fast diffusion and rapid equilibrium attainment .On the basis of

these results, 300 min and 40 min stirring period was selected for

all further studies of NZ and ActZ, respectively. Beyond this level

there is no noticeable increase in the adsorption and it is thus fixed

as the equilibrium time. It can be said that the NH4+ ions were

adsorbed by the exterior surface of the adsorbent. When the

adsorption of exterior surface of the adsorbent reached the

saturation point, the NH4+ ions enter the adsorbent pores and

are adsorbed by the interior surface of the particles [34,36,37].

3.4. Effect of initial ammonium concentration

As shown in Fig. 5, the increment of removal efficiency wasachieved in the ranges of 1050 mg/L of NH4

+ concentrations for

both NZ and ActZ. This result is similar with that reported by

Sarioglu

[11]

who

concluded

that

the

increase

in

removal

efficiency

was

achieved

between

8.8

and

40

mg/L

of

ammonium

concentrations, indicating that the initial NH4+ concentration plays

an important role in the adsorption of ammonium onto zeolites.

Increasing

the

initial

NH4+ concentration

would

increase

the

mass

transfer

driving

force

and

therefore

the

rate

at

which

ammonium ions pass from the bulk solution to the particle surface

[37]. The result can be generally expected for clinoptilolite having

micropores

and

macropores

[7].

After

50

mg/L

NH4+ concentration

of

both

zeolites,

the

removal

efficiency

of

ammonium

decreased

with increased initial NH4+ concentration. This is because the high

initial

ammonium

concentration

provided

a

greater

driving

force[38].

As

a

result,

the

NH4+ ion

could

migrate

from

the

external

surface

to

the

internal

micropores

of

the

zeolite

within

a

given

stirring time [32]. The equilibrium was achieved when all the

exchangeable ammonium and cation on the external and internal

surfaces

of

the

zeolite

were

reached

[31].

Fig.

5

shows

that

the

adsorption

capacity

of

the

activated

zeolite

was

higher

than

that

of

natural zeolite at each initial ammonium concentration.

3.5.

Effect

of

solution

pH

and

the

mechanism

of

adsorption

The pH of the aqueous solution is an important factor

controlling

ammonium

adsorption

[21].

As

shown

in

Fig.

6, pH

played

an

important

role

for

NH4+ adsorption

of

NZ

and

ActZ.

The

removal efficiency of both zeolites increased with increasing pH

Table 2

Chemical composition of the natural zeolite (wt.%).

NZ SiO2 Al2O3 TFe2O3 TiO2 MgO Na2O CaO K2O P2O5 MnO H2O L.O.I.

66.34 12.23 0.99 0.16 0.98 0.73 3.17 1.37 0.027 0.026 5.06 13.88

Fig. 3. The XRD of activated and natural zeolites (a), TG of natural zeolite (b) and

Zeta potential of activated and natural zeolites (c).

Table 1

Comparison of NaCl and NaOH in zeolite activation at 100mg/L of ammonium

concentration, pH 7, 25 8C and stirring time of 24h.

Sodium ion used for activation Sodium concentration

(mol/L)

Ammonium removal

capacity (mg/g)

NaCl-activated zeolite 1 5.921

NaOH-activated zeolite 1 4.552




from 2 to 7 and then it decreased gradually from pH 8 to 10 with

the maximum value being achieved at pH 7. An almost similar

trend has been reported for ammonium adsorption onto zeolite by

different researchers [21,30]. The behavior of ammonium as a

function of waters pH can be explained by considering the change

in density of hydrogen ions, the dominant ionic species of

ammonium and the surface charge of zeolite as a function of

waters pH. The pHzpcof natural zeolite and activated zeolite were7.7 and 7.8, respectively, implying that the zeolite particles surface

is uncharged at water pH of 7.7 and 7.8 of natural and activated

zeolites; the zeolites particles surface has a positive charge at

water pH below 7.7 and 7.8, and it is negatively charged at water

pH above 7.7 and 7.8. This indicates that the surface of zeolite is

negatively charged at pH of above 7 [39]. For ammonium

adsorption, according to the relation of ammonium dissociation

in water as a function of pH, NH4+ ions is the dominant species of

ammonia nitrogen in water at pH of below 7 while the molecular

form, ammonia (NH3) is dominant at alkaline pH. By considering

the above facts, the increase in adsorption of ammonium with the

increase in waters pH up to the maximum point (pH 7) can be

attributed

to

a

decrease

of

hydrogen

ions

in

solution

corresponding

to an increase in pH, and thus a reduction of the competitionbetween hydrogen ions and NH4

+ ions for adsorption/exchanging

sites onto zeolite particles [6,40]. The decrease of ammonium

adsorption

with

the

increase

of

waters

pH

above

7

is

related

to

the

increase

in

percentage

of

molecular

ammonium,

which

resulted

in

the reduction of ionexchange potential [10].

3.6. Effect of adsorbent dosage

As illustrated in Fig. 7, the removal efficiency of NH4+ ions by

both zeolites increases with increasing amount of both zeolites.

This effect can be attributed to an increased surface area and

number of adsorption sites [41]. As can be seen in Fig. 7, the

ammonium removal rate of ActZ increases more rapidly than that

of NZ and attains a plateau at 98.73% when the adsorbent dosagewas 1 g indicating that the NH4

+ ions removal was negligible at

higher adsorbent dosage. The natural zeolite attains a plateau at

94.5% when the dosage was 1.8 g as shown in Fig. 7. Thus, both of

them reached a balance of approximately 98% when the adsorbent

dosage for ActZ and NZ was 1 g and 1.8 g, respectively. This may be

attributed to a large adsorbent amount which effectively reduces

the unsaturation of the adsorption sites and correspondingly, the

number of such sites per unit mass comes down resulting in

comparatively less adsorption at higher adsorbent amount [37,41].

Hence, when the ammonium is exchanged completely with cations

on the zeolite surface at a certain amount of zeolite loading, the

NH4+ removal reached equilibrium.

3.7. Kinetics of ammonium exchange

To identify the key process controlling the adsorption rate,

several

models

must

be

checked

for suitability

and

consistency

over

a

broad

range

of

the

system

parameters.

Pseudo-first-

and

-second-

order and Bangham equations as well as the diffusion-based

Fig. 4. Effect of stirring time on NH4+ removal capacity of natural zeolite (a) and activated zeolite (b) (initial NH4

+ concentration: 80 mg/L, 25 8C and pH 7).

Fig. 5. Effect of initial ammonium concentration on NH4+ removal capacity of the

ActZ and NZ (stirring time: 40 min and 300 min for ActZ and NZ, respectively, at

25 8C and pH 7).

Fig. 6. Effect of pH on the removal of NH4+ ions (adsorbent dosage: 1 g/100 ml;

stirring time: 40 min and 300 min for ActZ and NZ, respectively; initial NH4+

concentration: 80 mg/L at 25 8C).




WeberMorris model are used to fit the experimental data and canbe

summarized

as

follows.

The

linear

form

of

pseudo-first,

pseudo-second

order

models

for

boundary conditions of q = 0 at t = 0 and qt= qe at t = te are as

follows:

The

pseudo-first-order

Eq: : lnqe qt lnqe k1t (3)

The

pseudo-second-order

Eq: : t

qt

1

k2q2e1

qet

h

k2q2e (4)

Bangham Eq: : log log C0C0 qtm

log

k0m

230V

a log t (5)

The adsorption kinetics of ammonium by ActZ and NZ are

presented

in

Fig.

8ae.

The

kinetic

data

were

better

fitted

by

thepseudo

second

order

model

than

other

models

as

indicated

by

higher R2 values (Table 3). Also it shows a higher sorption rate for

ActZ than NZ.

The pseudo second order model indicates that chemisorption

dominated

in

the

adsorption

process

[42]. The

difference

in

the

adsorbed concentration of adsorbate at equilibrium (qe) and at

time t (qt) is the key driving force for the adsorption, and the

adsorption

capacity

is

proportional

to

the

number

of

active

adsorption

sites

on

the

adsorbent

[43].

There

are

three

steps

involved in pseudo second order kinetic model: (i) the ammonium

ions diffuse from liquid phase to liquidsolid interface; (ii) the

ammonium

ions

move

from

liquidsolid

interface

to

solid

surfaces;

and

(iii)

the

ammonium

ions

diffuse

into

the

particle

pores

[23].

Herein,

the

diffusion

of

ammonium

ions

from

aqueousphase was much faster than the surface and intraparticle diffusion

processes

because

the

adsorption

was

performed

under

stirring

conditions

[22].

To reveal the relative contribution of surface and intraparticle

diffusion to the kinetic process, the kinetic adsorption data were

further

fitted

with

the

WeberMorris

model

using

Eq.

(6).

WeberMorriss Eq: : qt kidt1=2 C (6)

Intraparticle diffusion is assumed to be the sole rate-controlling

step if the regression of qtversus t1/2 is linear and the plot passes

through

the

origin

[44].

Our

fitting

results

show

that

the

regression

was linearly, but the plot did not pass through the origin (C 6 0).

Therefore, the adsorption kinetics of NH4+ ions on zeolite was

regulated

by

both

surface

and

intraparticle

diffusion

processes.

As

can be seen from Fig. 8d and e, ammonium exchange by both

zeolites involves two stages. These two stages suggest that the

ammonium exchange process proceeds by surface sorption and

intraparticle diffusion. It has been suggested that the first one can

be attributed to the instantaneous occupation of most available

surface sites by exchanging NH4+ ions onto zeolites particles. The

surface of zeolites was negatively charged at water pH below 7

(Fig. 6)and its rate is very fast. The second region is due to a gradual

adsorption stage, where the ammonium ions enter into zeolites

particle by intraparticle diffusion through the pores. The values of

intercept C provide information about the thickness of the

boundary layer and the resistance to the external mass transfer

increases as the intercept increases. The constant C was found to

increase from NZ (1.3642 mg/g) to ActZ (3.7406 mg/g) as shown in

Table 3, which indicates the increase of the thickness of the

boundary layer and decrease of the chance of the external mass

transfer and hence increase of the chance of internal mass transfer

[37,41].

Table 3 presents the results of fitting experimental data to the

pseudo-first, pseudo-second-order, Bangham and intraparticle

diffusion models. It can be seen from Table 3 that the correlation

coefficient R2 varies in the order: pseudo-second order >

Bangham > intraparticle diffusion > pseudo-first order model

under all experimental conditions, which indicates that thepseudo-second-order model is most suitable in describing the

adsorption kinetics of ammonium on zeolite.

3.8. Ammonium exchange isotherms

Isotherm fitting with model equations is a key issue to explore

adsorption mechanisms. Langmuir model (LM) and Freundlich

model (FM) were evaluated as follows:

LM is based on the assumption that each active site can only

hold one adsorbate molecule. The linear form of LM is expressed

as:

Ce

q

1

kqmax

1

qmax

Ce (7)

FM

endorses

the

heterogeneity

of

the

surface

and

assumes

that

the

adsorption

occurs

at

sites

with

different

energy

of

adsorption.

It is described as:

logq logkF1

nlogCe (8)

The linear plot of Langmuir isotherm of ActZ and NZ is shown in

Fig. 9a. It is noted that the values of qmax and k were calculated from

the slope and the intercept of the plot using Eq. (7) and are given in

Table 4. It will be seen that applicability of the simple Langmuir

equation for the present isotherm data indicates that the Langmuir

equation was able to properly describe the isotherm of ammonium

on the two zeolites (correlations coefficient >0.97). As shown in

Table

4,

the

ActZ

had

much

higher

ion

exchange

capacity

than

NZ.

Acomparison with other zeolites from various literature reviews,

Nguyen and Tanner [14] reported a qmax value of 5.76 mg/g

ammonium adsorption on Australian natural zeolite. It has also

been reported that the qmaxof ammonium removal using a Turkish

natural clinoptilolite was 8.1 mg/g [8]. Meanwhile a qmaxvalue of

0.085 mg/g was reported by Demir et al. [36].

Fig. 9b shows the linearized Freundlich adsorption isotherm of

ammonium curve and the Freundlich parameters is presented in

Table 4. The FM of ActZ provides a slightly more consistent fit to the

data compared with the FM of NZ. Similar values of 1/n which are

less than 1 have been reported for NH4+ removal using natural

zeolites from different countries [11,36].

To sum up, for the two zeolites, the experimental data are better

fitted by the LM than FM as can be seen from the higher R2 in

Fig. 7. Effect of adsorbent dosage on the removal of NH4+ ion (stirring time: 40 and

300 min for ActZ and NZ, respectively; initial NH4+ concentration: 80 mg/L at 25 8C

and pH 7).




Table 4. In Langmuir and Freundlich equations, coefficients K andKFrepresent the maximum amount that can be sorbed. Both values

for K and KFindicated that the activated zeolite has the higher

sorption

capacity

than

the

natural

zeolite

as

shown

in

Table

4.

3.9. Thermodynamic study

The effect of temperature on ammonium exchange was studied

at

25,

35

and

45

8C.

Fig.

10

indicates

that

the

amount

of

ammonium

exchanged

onto

zeolite

increases

with

a

decrease

in

temperature.

A

similar trend was also observed of some adsorbents, including

Turkish clinoptilolite [8], Turkish zeolite [3], flyash and sepiolite

[40],

and

NaA

zeolite

[45].

This

may

be

due

to

a

tendency

for

the

ammonium

molecules

to

escape

from

the

solid

phase

to

the

bulk

phase with the solution temperature increase [3,45]. In contrast,

increases in the NH4+ adsorption capacity with increasing

temperature

have

been

reported

for

some

other

adsorbents

[2,9].

Therefore,

by

comparing

the

results

of

the

present

work

with those of the literature, it can be concluded that the effect of

temperature

on

the

adsorption

of

ammonium

depends

on

both

thenature

of

the

adsorbent

and

the

selected

experimental

conditions

[21]. Furthermore,

ammonium

exchange

capacity

decreases

with

increasing temperature due to a weakening of the attractive forces

between NH4+ and adsorbent sites [41] and when the temperature

increases,

solubility

of

ammonium

increases

and

its

adsorption

decreases

[37].

The

thermodynamic

parameters,

such

as

Gibbs

energy (DG8), enthalpy (DH8), and entropy (DS8), for the adsorptionof ammonium on zeolites were calculated using the following

equations:

K0 qeCe

(9)

DG RT lnK0 (10)

Fig. 8. Pseudo-first order (a), pseudo-second-order (b), Bangham kinetic plots (c) and intra-particle diffusion of both zeolites ActZ (d) and NZ (e) for NH4+ removal.

Table 3

Kinetic parameters for NH4+ removal using various kinetic models.

Kinetic model Parameters

Pseudo-first order K1(min1) qe (mg/g) R

2

Absorbent

ActZ 0.0888 0.119123 0.954

NZ 0.0165 2.5126 0.918


Pseudo-second order h (mg/gmin) qe (mg/g) R2

Absorbent

ActZ 2.7156 3.821 0.999

NZ 0.1646 3.28 0.997


Bangham model K0 a R2

Absorbent

ActZ 201.609 0.0379 0.979

NZ 25.616 0.2981 0.958


Intraparticle diffusion model Kid C R2

Absorbent

ActZ 0.0102 3.7406 0.957

NZ 0.0977 1.3642 0.966




DG DH T DS (11)

lnK0 DH

RT

DS

R (12)

Eq. (12) represents a mathematical relationship betweenK0 and

1/T.

The

values

of

K0, DG8, DH8 andDS8 parameters are summarized

in

Table

5. Change

in

the

standard

free

energy

DG8

has

negativevalues (2.8662 and 1.3052 kJ/mol) at 25 and 35 8C, respectively,

but positive value (0.224 kJ/mol) at 45 8C. The negative values of

free

energy

change

(DG8) indicate

that

this

adsorption

process

is

spontaneous;

therefore,

no

energy

input

to

the

system

is

required.

The higher negative value reflects a more energetically

favorable adsorption [46]. For that reason, more energetically

favorable adsorption occurs at 25 8C. Change in the standard

enthalpy

DH8 indicates

a

negative

value

of

49.384

kJ/mol;

therefore, ammonium exchange is an exothermic process. Also,

the negative value of the standard entropy changeDS8 (0.1561 kJ/mol) suggests that the randomness decreases the removal of NH4

+

ions

on

the

clinoptilolite

[47].

The

linear

plot

of

LM

and

FM

isotherm

of

ActZ

at

25,

35

and

45 8C is shown in Fig. 9c and d. The LM and FM parameters are

presented in Table 5. The maximum value of K and KF at 25 8C

indicates

that

the

NH4+ adsorption

process

is

most

effective

at

25

8C.

Comparing

the

correlation

coefficients

in

Table

5

reflects

Fig. 9. The linearized Langmuir (a), Freundlich (b) of ActZ and NZ and Langmuir (c), Freundlich (d) adsorption isotherm of NH4+ curve at different temperature of ActZ.

Table 4

Constants for equilibrium isotherm models of NZ and ActZ.

Isotherm model Parameters

Langmuir Qmax(mg/g) K (L/mg) R2

ActZ

25 8C 9.515 0.444 0.9982

35 8C 9.533 0.313 0.9986

45 8C 9.794 0.123 0.967

NZ

25 8C 3.445 0.1998 0.9772

Isotherm model Parameters

Freundlich KF((mg/g)/(mg/L)1/n) 1/n R2

ActZ

25 8C 3.5743 0.2992 0.9716

35 8C 3.0634 0.3043 0.9442

45 8C 2.0305 0.3938 0.8867

NZ

25 8C 1.252 0.5192 0.7363Fig. 10. Effect temperature on the exchange isotherm of NH4

+ on activated zeolite

(ActZ).

Table 5

Change of thermodynamic parameters with temperature.

Temperature (8C) K0 DG8 (kJ/mol) DH8 (kJ/mol) DS8 (kJ/mol)

25 2.89 2.8662 49.384 0.1561

35 2.01 1.3052

45 0.82 0.224



http:///reader/full/an-investigation-into-the-adsorption-removal-of-ammonium-2014-1-s20-s1876107013001168- 10/11

that LM yields a much better (R2 = 0.9670.998) fit than that of the

FM (R2 = 0.8870.971).

3.10. Desorption and reusability

Accordingly, the ion-exchange and electrostatic adsorptions are

probably the main mechanisms for NH4+ ions removal by zeolite

under the selected conditions, through the following reactions

(Eq. (13)):

Ammoniumremoval : zeoliteNa NH4

! zeoliteNH4 Na (13)

In

this

study,

adsorption

experiments

were

performed

using

1

g

zeolite

and

100

ml

of

60

mg/L

NH4+

at

25

8C

for

40

min,

anddesorption of adsorbed NH4

+ onto zeolite was studied using 1 mol/

L

NaCl,

with

zeolite/liquid

ratio

of

1

g/10

ml

for

30

min

at

70

8C,

and

consecutive

adsorptiondesorption

cycles

were

repeated

four

times. The results are shown in Table 6. There was a slight decrease

with the increase of cycle times in adsorption efficiency from

5.633

mg/g

for

the

first

cycle

to

3.11

mg/g

for

the

fifth

cycle.

The

zeolite

that

was

regenerated

by

four

cycles

in

sodium

solution

had

high ammonium-removal efficiency with its adsorption efficiency

of 3.11 mg/g which is close to that of natural zeolite (3.344 mg/g).

4.

Conclusion

The characteristics of Chinese (Hulaodu) natural zeolite (NZ),

activated

zeolite

(ActZ)

and

their

efficacy

in

removing

ammonium

were

investigated.

The

following

conclusions

were

made

from

the

experimental results:

1

mol/L

of

NaCl,

stirring

time

of

30

min

and

70

8C

were

found

to

be the optimum activation conditions for the zeolite with

excellent removal of NH4+.

The

highest

adsorption

capacity

was

obtained

at

pH

7

for

both

zeolites

and

the

maximum

ammonium

adsorption

was

rapidly

attained within 40 min and 300 min for activated zeolite and

natural zeolite, respectively.

Langmuir

adsorption

isotherm

of

both

zeolites

fit

well

with

the

equilibrium

adsorption

data,

and

this

adsorption

process

agrees

very well with pseudo-second-order kinetics rate model.

DG8, DH8 and DS8 values reveal the exothermic and spontaneousnature

of

the

process

and

low

temperature

(25

8C)

favors

the

NH4+ removal

on

the

zeolite.

The adsorption capacity of activated zeolite is decreased to

3.11 mg/g which is close to that of natural zeolite (3.344 mg/g)

after

being

regenerated

four

times.

Based

on

these

results,

the

study

shows

that

Chinese

(Hulaodu)

natural

zeolite

can

be

used

as cheap, efficient and ecofriendly adsorbent for removing

ammonium

from

water

and

wastewaters.

Acknowledgment

This study was supported by Engineering Research Center of

Nano-Geomaterial of Education Ministry, China University of

Geosciences, Wuhan.

References

[1] Shavandi MA,Haddadian Z, Ismail MHS, Abdullah N, Abidin ZZ. Removal of Fe(III), Mn (II) and Zn (II) from palm oil mill effluent (POME) by natural zeolite. JTaiwan Inst Chem Eng 2012;43:7509.

[2] Du Q, Liu S, Cao Z, Wang Y. Ammonia removal from aqueous solution usingnatural Chinese clinoptilolite. Sep Purif Technol 2005;44:22934.

[3] Saltali K, Sari A, Aydin M.Removal of ammonium ion from aqueous solution bynatural Turkish (Yildizeli) zeolite for environmental quality. J Hazard Mater2007;141:25863.

[4] Arslan A, Veli S. Zeolite 13X for adsorption of ammonium ions from aqueous

solutions

and

hen

slaughterhouse

wastewaters.

J

Taiwan

Inst

Chem

Eng2012;43:3938.

[5] Li M, Zhu X, Zhu F, Ren G, Cao G, Song L. Application of modified zeolite forammonium removal from drinking water. Desalination 2011;271:295300.

[6] Huang H, Xiao X, Yan B, Yang L. Ammonium removal from aqueous solutionsby using natural Chinese (Chende) zeolite as adsorbent. J Taiwan Inst ChemEng 2010;175:24752.

[7] Jorgensen TC, Weatherley LR. Ammonia removal from wastewater by ionexchange in the presence of organic contaminants. Water Res 2003;37:17238.

[8] Karadag D, Koc Y, Turan M, Armagan B. Removal of ammonium ion fromaqueous solution usingnatural Turkish clinoptilolite.J Hazard Mater 2006;136:6049.

[9] Leyva-Ramos R, Monsivais-Rocha JE, Aragon-Pina A, Berber-Mendoza MS,Guerrero-Coronado RM, Alonso-Davila P, et al. Removal of ammonium fromaqueous solution by ion exchange on natural and modified chabazite. J EnvironManage 2010;91:26628.

[10] Wang S, Zhu ZH. Characterization and environmental application of an Aus-tralian natural zeolite for basic dye removal from aqueous solution. J HazardMater 2006;136:94652.

[11] Sarioglu M. Removal of ammonium from municipal wastewater using naturalTurkish (Dogantepe) zeolite. Sep Purif Technol 2005;41:111.

[12] Barry FJ. The evolution of the enforcement provisions of the federal waterpollution control act: a study of the difficulty in developing effective legisla-tion. Mich L Rev 1970;68:110330.

[13] Dimirkou A, Doula MK. Use of clinoptilolite and an Fe-overexchanged clin-optilolite in Zn2+ and Mn2+ removal from drinking water. Desalination2008;224:28092.

[14] Nguyen ML, Tanner CC. Ammonium removal from wastewaters using naturalNew Zealand zeolites. New Zeal J Agric Res 1998;41:42746.

[15] Gupta VK, Rastogi A, Nayak A. Biosorption of nickel onto treated alga (Oedo-gonium hatei): application of isotherm and kinetic models. J Colloid InterfaceSci 2010;342:5339.

[16] Gupta VK, Gupta B, Rastogi A, Agarwal S, Nayak A. A comparative investigationon adsorption performances of mesoporous activated carbon prepared fromwaste rubber tire and activated carbon for a hazardous azo dyeAcid Blue 113.J Hazard Mater 2011;186:891901.

[17] Mittal A, Mittal J, Malviya A, Kaur D, Gupta VK. Adsorption of hazardous dye

crystal

violet

from

wastewater

by

waste

materials.

J

Colloid

Interface

Sci2010;343:46373.[18] Moradi O, Yari M, Zare K, Mirza B, Najafi F. Carbon nanotubes: a review of

chemistry principles and reactions. Fuller Nanotub Carbon Nanostruct2012;20:13851.

[19] Moradi O, Zare K. Adsorption of ammonium ion by multi-walled carbonnanotube: kinetics and thermodynamic studies. Fuller Nanotub CarbonNanostruct 2013;21:44959.

[20] European Commission. IPPC reference document on best available techniquesin common waste water and waste gas treatment/management systems in thechemical sector. Sevilla: European IPPC Bureau; 2003.

[21] Moussavi G, Talebi S, Farrokhi M, Sabouti RM.The investigation of mechanism,kinetic and isotherm of ammonia and humic acid co-adsorption onto naturalzeolite. Chem Eng J 2011;171:115969.

[22] Liao P, Yuan S, Xie W, Zhang W, Tong M, Wang K. Adsorption of nitrogen-heterocyclic compounds on bamboo charcoal: kinetics, thermodynamics andmicrowave regeneration. J Colloid Interface Sci 2013;390:18995.

[23] Liao P, Ismael ZM, Zhang W,Yuan S, Tong M, Wang K, et al. Adsorption of dyesfrom aqueous solutions by microwave modified bamboo charcoal. Chem EngJ

2012;195:33946.[24] Alshameri A, Abood AR, Yan C, Muhammad AM. Characteristics, modification

and environmental application of Yemens natural bentonite. Arab J Geosci2013;113.

[25] Gupta VK,Jain R, Nayak A, Agarwal S, Shrivastava M. Removal of the hazardousdye-tartrazine by photodegradation on titanium dioxide surface. Mater SciEng C 2011;31:10627.

[26] Jain AK, Gupta VK, Bhatnagar A, Suhas. A comparative study of adsorbentsprepared from industrial wastes for removal of dyes. Sep Sci Technol 2003;38:46381.

[27] Wang YF, Lin F, Pang WQ. Ammonium exchange in aqueous solution usingChinese natural clinoptilolite and modified zeolite.J Hazard Mater 2007;142:1604.

[28] Gupta VK,Jain R, Mittal A, Saleh TA, Nayak A, Agarwal S, et al. Photo-catalyticdegradation of toxic dye amaranth on TiO2/UV in aqueous suspensions. MaterSci Eng C 2012;32:127.

[29] Tehrani RMA, Salari AA. The study of dehumidifying of carbon monoxide andammonia adsorption by Iranian natural clinoptilolite zeolite. Appl Surf Sci2005;252:86670.

Table 6

Data about adsorption and regeneration of zeolite.

Cycles

1 2 3 4 5

Ammonium adsorption

capacity (mg/g)

5.633 5.03 4.51 3.78 3.11

http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0005http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0005http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0005http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0005http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0005http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0005http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0005http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0005http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0005http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0005http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0005http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0005http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0005http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0005http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0005http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0005http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0005http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0005http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0005http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0005http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0005http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0005http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0005http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0005http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0005http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0005http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0005http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0005http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0005http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0005http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0005http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0005http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0005http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0005http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0005http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0005http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0005http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0005http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0005http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0005http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0005http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0005http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0005http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0005http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0005http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0005http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0005http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0005http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0005http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0005http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0005http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0005http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0005http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0005http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0005http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0005http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0005http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0005http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0005http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0005http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0005http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0010http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0010http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0010http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0010http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0010http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0010http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0010http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0010http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0010http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0010http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0010http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0010http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0010http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0010http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0010http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0010http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0010http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0010http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0010http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0010http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0010http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0010http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0010http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0010http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0010http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0010http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0010http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0010http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0010http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0010http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0010http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0010http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0010http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0010http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0010http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0010http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0010http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0010http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0010http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0010http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0015http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0015http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0015http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0015http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0015http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0015http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0015http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0015http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0015http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0015http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0015http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0015http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0015http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0015http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0015http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0015http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0015http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0015http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0015http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0015http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0015http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0015http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0015http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0015http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0015http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0015http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0015http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0015http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0015http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0015http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0015http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0015http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0015http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0015http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0015http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0015http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0015http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0015http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0015http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0015http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0015http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0015http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0015http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0015http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0015http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0020http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0020http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0020http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0020http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0020http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0020http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0020http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0020http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0020http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0020http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0020http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0020http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0020http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0020http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0020http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0020http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0020http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0020http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0020http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0020http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0020http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0020http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0020http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0020http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0020http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0020http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0020http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0020http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0020http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0020http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0020http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0020http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0020http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0020http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0020http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0020http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0020http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0020http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0020http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0020http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0020http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0020http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0020http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0020http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0020http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0025http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0025http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0025http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0025http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0025http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0025http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0025http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0025http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0025http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0025http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0025http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0025http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0025http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0025http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0025http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0025http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0025http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0025http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0025http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0025http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0025http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0025http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0025http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0025http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0025http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0025http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0025http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0025http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0025http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0025http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0025http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0025http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0025http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0025http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0025http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0025http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0025http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0025http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0025http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0025http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0025http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0025http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0025http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0025http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0030http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0030http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0030http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0030http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0030http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0030http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0030http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0030http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0030http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0030http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0030http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0030http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0030http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0030http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0030http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0030http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0030http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0030http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0030http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0030http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0030http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0030http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0030http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0030http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0030http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0030http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0030http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0030http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0030http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0030http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0030http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0030http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0030http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0030http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0030http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0030http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0030http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0030http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0030http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0030http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0030http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0030http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0030http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0030http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0030http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0030http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0030http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0030http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0030http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0035http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0035http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0035http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0035http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0035http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0035http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0035http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0035http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0035http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0035http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0035http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0035http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0035http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0035http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0035http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0035http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0035http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0035http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0035http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0035http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0035http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0035http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0035http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0035http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0035http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0035http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0035http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0035http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0035http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0035http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0035http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0035http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0035http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0035http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0035http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0035http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0040http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0040http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0040http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0040http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0040http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0040http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0040http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0040http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0040http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0040http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0040http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0040http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0040http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0040http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0040http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0040http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0040http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0040http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0040http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0040http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0040http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0040http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0040http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0040http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0040http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0040http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0040http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0040http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0040http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0040http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0040http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0040http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0040http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0040http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0040http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0040http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0040http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0040http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0040http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0040http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0040http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0045http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0045http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0045http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0045http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0045http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0045http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0045http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0045http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0045http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0045http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0045http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0045http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0045http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0045http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0045http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0045http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0045http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0045http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0045http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0045http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0045http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0045http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0045http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0045http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0045http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0045http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0045http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0045http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0045http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0045http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0045http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0045http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0045http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0045http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0045http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0045http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0045http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0045http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0045http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0045http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0045http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0045http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0045http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0045http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0045http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0045http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0045http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0045http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0045http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0045http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0045http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0045http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0045http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0045http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0045http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0045http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0045http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0045http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0045http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0045http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0050http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0050http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0050http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0050http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0050http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0050http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0050http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0050http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0050http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0050http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0050http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0050http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0050http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0050http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0050http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0050http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0050http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0050http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0050http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0050http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0050http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0050http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0050http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0050http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0050http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0050http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0050http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0050http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0050http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0050http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0050http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0050http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0050http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0050http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0050http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0050http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0050http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0050http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0050http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0050http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0050http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0050http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0050http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0050http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0050http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0050http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0050http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0055http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0055http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0055http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0055http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0055http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0055http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0055http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0055http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0055http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0055http://refhub.elsevier.com/S1876-1070(13)00116-8/sbref0055http://refhub.elsevier.com/S1876-1070(13)00116-8/sbr

an investigation into the adsorption removal of ammonium (2014 1-s2.0-s1876107013001168-main)

Documents