DSpace at VNU: Removal of As(V) from aqueous solutions by iron coated rice husk

Fuel Processing Technology 106 (2013) 511–517

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Fuel Processing Technology
journal homepage: www.elsevier.com/locate/fuproc

Removal of As(V) from aqueous solutions by iron coated rice husk
E. Pehlivan a,⁎, T.H. Tran b, W.K.I. Ouédraogo c, C. Schmidt d, D. Zachmann d, M. Bahadir d
a

Department of Chemical Engineering, Selcuk University, Campus, 42079 Konya, Turkey
Hanoi University of Science, Hanoi, Vietnam
c
Laboratoire de Chimie Organique, Structure et Réactivité, UFR-SEA, Université de Ouagadougou, 03 BP 7021, Ouagadougou 03, Burkina Faso
d
Institute of Environmental and Sustainable Chemistry, Technische Universitaet Braunschweig, Germany
b

a r t i c l e

i n f o

Article history:
Received 5 December 2011
Received in revised form 15 August 2012
Accepted 6 September 2012
Available online 30 September 2012
Keywords:
Adsorption
Rice husk

As(V)
Fe(III)
Isotherms

a b s t r a c t
A lignocellulosic material extracted from rice husk (Oryza sativa), Vietnam, was modified as a new adsorbent
for the removal of As(V) ions from aqueous solution. Iron was coated onto this adsorbent by hydrolization of
ferric nitrate while adding an alkaline solution drop wise into the batch type reactor. The adsorption of As(V)
ions from aqueous solution on coated rice husk was then studied at varying pH, As(V) concentrations, contact
times, ionic strength, and adsorbent amounts. The minimum contact time to reach equilibrium is about 6 h.
The adsorption of As(V) anions on the coated rice husk was found to be highly pH dependent due to Coulomb
interactions between As(V) species in solution and positively charged surface groups RH-FeOOH, as well as
formation of chelate complexes with naturally occurring carboxyl and carbonyl functional groups in the matrix. As(V) adsorption on Fe(III)-coated rice husk (RH-FeOOH) from aqueous solution was studied in the pH
range 2–10. The main effects of pH on adsorption are estimated by considering both the behavior of As(V)
ions (hydrolysis and hydroxide precipitation) and the effect of pH on coordination. A strong effect of pH
was demonstrated at pH 4.0 with a maximum percentage for removal of As(V) ions 94%. Although both
Langmuir and Freundlich isotherms have been used to characterize the adsorption of As(V), the Langmuir
model fitted the equilibrium data better than Freundlich model and confirmed the surface homogeneity of
adsorbent. The maximum adsorption capacity is determined as 2.5 mg/g of adsorbent at pH 4.0 for the
Fe(III)-coated rice husk. It is concluded that initial As(V) concentration has an effect on the removal efficiency
of RH-FeOOH. Higher adsorption of As(V) was observed at lower initial concentrations. RH-FeOOH as a low
cost material is effective for the removal of As(V) ions and may become a valuable adsorbent to improve
the ground water quality in Vietnam.
© 2012 Elsevier B.V. All rights reserved.

1. Introduction
Arsenic in water streams was reported from over 70 countries to
pose serious threat to an estimated 150 million people world-wide
[1]. Water supply in many countries, e.g., in Bangladesh, India,
Taiwan, Vietnam, Burkina Faso, Mongolia, Mexico, Pakistan, France,

Italy, Chile, New Zealand and even in the United States contains
dissolved arsenic in excess amounts (>10 μg/L), which is the maximum acceptable level recommended by the USEPA [2,3]. Arsenic occurs in water stream in several forms depending upon pH value and
redox potential. The oxidation state of arsenic in dissolved phase
plays an important role since it determines the properties of the related chemical species, i.e., toxicity, sorption behavior, and mobility in
the aquatic environment [4]. Since the pH of the aqueous medium determines the predominant species, it is one of the important parameters for the arsenic removal from drinking and wastewater. At typical
pH values of natural water (pH 5–8), the two predominant forms of
⁎ Corresponding author. Tel.: +90 332 2232127; fax: +90 332 2410635.
E-mail address: (E. Pehlivan).
0378-3820/$ – see front matter © 2012 Elsevier B.V. All rights reserved.
/>
inorganic arsenic species in aqueous environments are the trivalent
arsenite, As(III), and the pentavalent arsenate, As(V). Arsenite mainly
exists as fully protonated form and arsenate remains as an anion,
which could be found in various ionic forms in dissolved species [4,5].
A number of treatment methods have been applied for the removal of arsenic from water such as precipitation, co-precipitation [6],
coagulation-microfiltration [7], ion-exchange [8], reverse osmosis
and nano-filtration [9]. Adsorption has been paid more attention
due to its high treatment efficiency and lower process costs compared
with the above mentioned ones. Industrial and agricultural byproducts play an important role for improving arsenic removal after
their simple and cheap chemical modifications that could be of particular interest for developing countries. Iron compounds are among the
most popular adsorbents being used for the removal of arsenic from
aqueous solutions. Rice husk (RH) is a promising adsorbent that has
interesting properties such as hydrophilic, porous, and high surface
area as well as high resistivity. Studies on applying RH as adsorbent
for arsenic are still scarce. Coating RH with iron can increase the removal capacity for As(V) from aqueous solutions. Arsenic removal
through iron oxyhydroxide (FeOOH)-coated matrices primarily


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E. Pehlivan et al. / Fuel Processing Technology 106 (2013) 511–517

depends on the number of iron sites on the surface of adsorbent. Iron
used for the modification of adsorbents can occur in various types
such as amorphous iron oxide, ferrihydrite, akaganeite, goethite, hematite, etc., depending upon conditions such as pH and temperature
[1–3,10].
There are a number of different materials for the adsorption of arsenic ions such as rice polish [11], zeolite [12], red mud [13], modified
fungi Aspergillus niger [14], activated alumina [15], surface-modified
carbon black [16], iron hydroxides and oxides [17,18], open celled cellulose sponge [19] and further adsorbents [20].
The aim of this study was to use RH as an agro-waste product that
is produced in huge amounts particularly in developing countries in
the tropics and subtropics. The adsorption capacity of RH should be
tested before and after being coated with iron oxyhydroxide
(RH-FeOOH) as another cheap material in order to remove As(V)
from aqueous solutions. The effects of different parameters like pH
of the initial solution, As(V) concentration, contact time, amount of
RH-FeOOH, and ionic strength should be investigated in easy to run
batch experiments that could be easily applied in those countries in
order to diminish the arsenic pollution in drinking and process
water, e.g., in rural areas.
2. Materials and methods
2.1. Treatment of rice husk (RH)
Air-dried RH was purchased from the vicinity of Hanoi, Vietnam, and
ground in a vibratory mill (BLB Braunschweig). The size fraction of
125–200 μm was collected, washed with deionized water, and airdried in an oven at 60 °C for one day before use. All chemicals and reagents used were purchased from Merck (Darmstadt, Germany) and
were of analytical grade. Aqueous solutions of 0.2 M NaOH and HCl
were used to adjust pH. MgSO4.7H2O (Fluka, Seelze, Germany), Na3PO4.12H2O (Sigma-Aldrich, Seelze, Germany), and NaNO3 (Merck)
were used for ionic strength studies. Dissolved As(V) stock solution (X
mg/L) was purchased from Merck. Iron Nitrate Fe(NO3)3 was used for
the modification of adsorbent. All glassware was cleaned by soaking in

diluted nitric acid and washing with deionized water.
2.2. RH treatment with ferric nitrate Fe(NO3)3 solution
The milled and air-dried RH (30 g) was pretreated (1) with 2 mol/L
H2SO4 (1:1 w/w of dry matter at 80 °C for 0.5 h) for removing starch,
proteins, and carbohydrates, and (2) with 0.5 mol/L NaOH (RH/NaOH
5:1, stirring for 24 h at room temperature) for removing the low molecular weight lignin type compounds. After filtration, the adsorbent was
air-dried in an oven at 50 °C for 24 h.
The prepared adsorbent was then coated with a ferric nitrate
Fe(NO3)3 solution. A mixture of 800 mL of 0.05 M ferric nitrate
Fe(NO3)3 in ultrapure water and 25 g RH was given in a 2 L beaker.
A 1 mol/L NaOH solution was slowly added into the reaction vessel
at a velocity 10 mL/h under continuous stirring. The mixture was
left for 1 day during this procedure. Thereby, pH was kept between
2.8 and 3.5 and readjusted if necessary. The suspension was then filtered using a cellulose acetate membrane of 0.45 μm pore size and
washed with ultrapure water several times until pH neutral,
air-dried in an oven at 50 °C, and stored in closed bottles at room
temperature until use.
2.3. Determination of iron amount loaded onto RH
The content of iron loaded onto RH, an important factor influencing the As(V) adsorption capacity, was determined by soaking RH in
1 M HCl for recovering all the loaded iron, and the solution was analyzed for iron concentration using HG-AAS. The amount of iron loaded
in RH was calculated as 1.2 mg/g of adsorbent.

2.4. Spectroscopic studies for raw and coated RH
In order to identify the main functional groups of the raw and
Fe-coated adsorbent responsible for As(V) adsorption FT-IR spectra
were recorded (Buker Tensor 27, diamond ATR at 4000–520 cm −1,
32 scans) (Fig. 1).
The broad and strong bands at 3340 cm −1 are the hydroxyl
(\OH) and the absorbance at 2890 cm −1 are due to \CH groups of
the adsorbents. The absorption at 1741 cm −1 is attributed to

stretching vibration of the carboxyl groups. The bands observed at
1024 cm −1 were assigned to C\OH stretching of alcohols and carboxylic acids. The coating of RH with FeOOH leads to some important
changes of the IR spectrum especially in the decrease of the ratio of
COOH (1741 cm −1) and COO − (1677 cm −1) group's intensities,
which can be explained by the complexation of Fe 3+ ions with carboxylate groups in the matrix. Intensity for lignin structure was observed to be decreased at 1263 cm −1. This can be ascribed to a
partial oxidation of lignin by Fe 3+ ions [10].
2.5. Preparation of standards, reagents and analyses
Ultrapure water was used for all experiments. All chemicals used for
the coating process and batch equilibrium studies were of analytical
grade and purchased from Merck (Darmstadt, Germany). The AAS standard solution of 1000 mg/L As(V) was prepared by transferring the contents of a Titrisol ampoule with As2O5 in H2O (Merck, Germany) into a
1 L volumetric flask, which was filled up to the mark at 20 °C according
to the instructions by Merck. Arsenic solutions with different concentration used in the batch studies were prepared by diluting the main stock
solution. The As analyses were performed with a Hitachi Atomic
Absorption Spectrophotometer (Series Z-2000; Hitachi Corporation,
Japan) which was connected to a hydride formation system (model
HFS-3; Hitachi). For hydride generation the following solutions
were used: (i) 1.2 M HCl (p.a., Merck); (ii) NaBH4–NaOH solution:
solute 10 g NaBH4 (p.a., Fluka) in 1 L H2O (Seralpure) by adding
4 g NaOH (p.a., Merck); the solution was prepared immediately before use; (iii) KI-solution as a reduction agent; 20% (w/v; reduction
to As(III)). All standards, reference solutions, and sample solutions
were adjusted to 0.24 N HCl and 2% KI. The reduction agent was
added at least 30 min before analysis. In general a 5-point calibration
was run before starting the analyses (0–20 μg/L). Argon was used as
carrier gas with a flow rate of 0.3 L/min for constant transfer of
As-hydride from the reaction cell to the cuvette. The 193.7 nm emission line of the As-hollow-cathode lamp was used. For the reduction
of As(V) into As(III), 2.5 mL of 30% HCl and 2.5 mL of 20% (w/v) KI
was added to 25 mL of the standard or sample solution and left for
15 min.
2.6. Batch adsorption experiments
Batch adsorption experiments were carried out in order to evaluate the performance of the adsorbent for As(V) removal. Batch experiments were performed in triplicate in sealed glass beakers by adding

RH-FeOOH in 50 mL of aqueous As(V) solution of desired initial pH,
As(V) ion concentration, and temperature. The pH of working solutions was controlled and adjusted by adding 0.2 M HCl or 0.2 M
NaOH as required. The beakers were shaken on a horizontal shaker
at 200 rpm for certain periods (15 min–24 h). The adsorbents were
then separated through filtration and the remaining filtrates were analyzed for As(V) concentration by hydride generation atomic absorption spectrometry with a Zeeman correction (HGAAS-Hitachi Z-2000
AAS). The amount of As(V) adsorbed per unit mass of the RH-FeOOH
(mg/g) was calculated using following Eq. (1):
Q e ¼ ðC i −C e ÞV=W

ð1Þ


E. Pehlivan et al. / Fuel Processing Technology 106 (2013) 511–517

513

Fig. 1. FTIR spectra of raw RH and coated RH-FeOOH (black: raw RH, red: RH-FOOH).

where Ci and Ce are the As(V) concentrations in mg/L initially and at
equilibrium, respectively; V is volume of the arsenic solution in mL;
and W is the weight of RH-FeOOH in mg.
For studying the effect of initial pH (2–10) on arsenic uptake by
RH-FeOOH, adsorption experiments were performed using 50 mL of solution with initial As(V) concentration of 5 mg/L and adsorbent dose of
4 g/L at 23 °C. Effect of variation of initial As(V) concentration was studied with different initial arsenic concentration of 1, 3, 5, 7.5, 10, 15, 20,
30, 50, and 75 mg/L; adsorbent dose of 4 g/L; pH 4; temperature
23 °C. Effect of contact time (15 min–24 h) and adsorbent amount
(0.1–0.3 g) was studied with initial As(V) concentration of 2 mg/L; pH
4; temperature 23 °C.

3. Results and discussion

3.1. Effect of pH on As(V) removal
Iron oxides have been considered already as effective materials for
removal of As(V) in water streams and sediments [21]. The adsorption process of As(V) on these materials takes place at the hydrous
oxide/water interface. The adsorption capacity for As(V) strongly depends on the chemical species and characteristics of the solid
supports.
RH consists of cellulose, hemicellulose and lignin. The cellulose and
hemicellulose are bound to lignin both by hydrogen and covalent
bonds. Cellulose is a common material in plant cell walls. Hemicellulose
consists of different monosaccharide units such as glucose, xylose, mannose, galactose, and arabinose. Lignin comprises a variety of functional
groups including aliphatic and phenolic hydroxyl-, methoxyl-, and carbonyl groups, which are able to transfer electron pairs from oxygen
atoms and forming coordination complexes with toxic metals. Raw
RH also contains some polar functional groups such as alcoholic, carbonyl, carboxylic and phenolic ones, which are potentially able to complex As(V).
pH value is considered as an important parameter in arsenic removal [22]. The effect of pH on adsorption process was studied at
22 °C in the range from 2.0 to 10.0. It was observed that the initial
pH of all solutions increased slightly after shaking the samples for
6 h. Fig. 2 shows the effect of equilibrium pH on the As(V) ion

adsorption of RH-FeOOH. The percentage of adsorption increases
slightly at the pH range of 2.0–4.0 and maximizes at pH 4.0 which
shows high selectivity of the modified adsorbent for As(V). After pH
6.0, the adsorption of As(V) decreases significantly until pH 10.0.
The mechanism for the removal of As(V) from solution phase is attributed to Coulomb interactions and formation of chelate complexes
between the As(V) ions and the charged functional groups on the
adsorbent's surface. At lower pH, the positive charge density on surface sites is rising which results in higher electrostatic attraction between (FeOH2+) and As(V) ions. This provides a higher adsorption
capacity for As(V). In contrast, while increasing the pH, the electrostatic repulsion is increasing due to the decrease of positive charge
density of proton on the adsorption sites. Moreover, the electrostatic
attraction between the positively charged surface groups (FeOH2+)
and As(V) species H2AsO4− and HAsO42− decreases and hinders the
formation of surface complexes resulting in lower adsorption capacity
for As(V).

The pH also affects the presence of various As(V) species in aqueous solution. It was reported that As(V) occurs in solution at different
pH in form of H3AsO4, H2AsO4−, HAsO42−, and AsO43− oxo anions. The
predominance of various As(V) species as a function of pH is shown in
Fig. 3. The different species of As(V) are present in solution based on
the following three equilibriums and their respective stability constants [22] (Eqs. (2)–(4)):
−

þ

K

H3 AsO4 ↔H2 AsO4 þ H ; P1 ¼ 2:3

−

2−

þ

K

H2 AsO4 ↔HAsO4 þ H ; P2 ¼ 6:7

2−

3−

þ

K

HAsO4 ↔AsO4 þ H ; P3 ¼ 11:6

ð2Þ

ð3Þ

ð4Þ

The pH influences the protonation or deprotonation of the adsorbent surface. The interactions of As(V) ions with the coated RH surface are due to Coulomb interactions, ligand exchange phenomena,
and formation chelate complexes. Narasimhan reported that goethite,
ferrihydrite and FeOOH loaded bio-sorbent have the same adsorption
mechanisms for As(V) [23]. Iron oxides can form oxy-hydroxides


514

E. Pehlivan et al. / Fuel Processing Technology 106 (2013) 511–517

As(V)

100

Sorption %

80
60
40
20
0

0

2

4

6

8

10

12

pH
Fig. 2. pH effect on the adsorption of As(V) on RH-FeOOH (initial As(V) concentration:
5 ppm; solvent volume: 50 mL; adsorbent amount: 0.2 g; temperature: 22 ± 2 °C, contact time: 6 h).

which can be protonated or deprotonated depending on the pH value
of solutions and as a result, a positive or negative surface can be
formed as the following [24] (Eqs. (5) and (6)):
þ

þ

`Fe\OH þ H ↔`Fe\OH2

ð5Þ

þ

ð6Þ

−

`Fe\OH↔H þ `FeO

In the pH range 2–7, the major arsenate species in aqueous solution is H2AsO4−. An adsorption takes place by the reaction between
the active hydrolyzed form of `FeOH on the surface of RH-FeOOH
and As(V) ions that leads to the formation of surface complexes
according to the following equilibriums [24] (Eqs. (7) and (8)):
þ

−

À

þ

2`FeOH2 þ H2 AsO4 →½ð`FeOÞ2 \AsOðOHފ þ H3 O þ H2 O

þ

−

−

`FeOH2 þ H2 AsO4 →½`Fe\ðOÞ2 \AsOðOHފ þ H3 O

þ

ð7Þ

ð8Þ

In the first reaction, non-specific Coulomb interactions
(outer-sphere adsorption) or ligand exchange reaction on the surface
(inner-sphere adsorption) with As(V) can occur [25] as the following
(Eqs. (9)–(13)):
þ

−

þ

−

`Fe\OH2 þ H2 AsO4 ↔`Fe\OH2 … O4 AsH2

−

`Fe\OH þ H2 AsO4 ↔`Fe\OAsO3 H2 þ OH

−

Fig. 3. Distribution (%) of various As(V) species as a function of solution pH [21].

3.2. Effect of agitation time on the removal of As(V)
The results of time effects on As(V) adsorption process on
RH-FeOOH matrix are given in Fig. 4. This graph demonstrates that

the As(V) adsorption reached an equilibrium 96–99% after shaking
for 6 h. It seems that there is not much difference in adsorption percentage thereafter. The adsorption of As(V) takes place at a pH
below the pHpzc that was found to be about 5.8–6.3 (Fig. 5). For
that reason, the experiments were carried out at pH below pHpzc of
adsorbent.
The scanning electron microscopic pictures (SEM) reveal the surface textures and porosities of RH and RH-FeOOH (Fig. 6). It shows
very fine particle sizes having pores within the particle of varying
size.
3.3. Effect of initial As(V) concentration
The influence of the initial concentration on the adsorption on
RH-FeOOH was studied at pH 4.0 after 6 h shaking with As(V) with
the concentrations in the range of 1–75 ppm (Fig. 7). The results
demonstrated that at higher concentrations, more As(V) ions
remained in dissolved phase due to the saturation of binding sites
(`Fe\OH) towards As(V). As(V) uptake by RH-FeOOH was 99.6 %
at a concentration of 3 ppm.
The data obtained from equilibrium isotherm that provides information on the sorption capacity are the most important factor in any

ð9Þ

120

As (V)

100

ð10Þ

þ

−

`Fe\OH2 þ H2 AsO4 ↔`Fe\OAsO3 H2 þ H2 O

ð11Þ

Sorption %

80

60

40

`Fe\OH þ

2−
−
HAsO4 ↔`Fe\OAsO3 H

þ OH

−

ð12Þ
20

2−

−

2`Fe\OH þ HAsO4 ↔`Fe2 \AsO2 H þ 2OH

ð13Þ

The protonation of active surface sites at low pH plays a significant
role in reducing the free As(V) ions in the dissolved phase. Above pH
7.0, adsorption of As(V) decreases because of the competition between
H2AsO4− ions and OH− ions for the reactive sites of RH-FeOOH [26,27].

0
0

5

10

15

20

25

30

Contact time (hours)
Fig. 4. Sorption isotherm of As(V) on RH–FeOOH as a function of contact time (initial
As(V) concentration: 2 ppm; solvent volume: 50 mL; pH: 4; adsorbent amount:
0.2 g; temperature: 22 ± 2 °C).

E. Pehlivan et al. / Fuel Processing Technology 106 (2013) 511–517

12

Arsenic uptake q(mg/g)

2

10
8

pH final

515

6
4
2

1,6
1,2

As(V)

0,8
0,4
0
0

0
0

1

2

3

4

5

6

7

8

9

10

11

pH initial

adsorption system. The As(V) adsorption capacity of RH-FeOOH was
calculated using Langmuir and Freundlich models [28]. These isotherms are related to As(V) uptake per unit weight of RH-FeOOH,
qe, and the equilibrium As(V) ion concentration in the dissolved

phase, Ce.
The Langmuir isotherm has been widely applied for adsorption processes to separate an analyte from aqueous solutions [29]. Langmuir isotherm model assumes that adsorption process forms a monolayer and
occurs at specific homogeneous adsorption sites. Intermolecular forces
decrease rapidly with the distance from the surface. The Langmuir
model is more popular since it contains the two reasonable parameters
(Kb and As) that are easy to interpret the adsorption [30–32].
The general form of Langmuir model is:

ð14Þ

where As (mol/g) and Kb (L/mol) are the coefficients, qe is the weight
adsorbed per unit weight of adsorbent and Ce is the analyte concentration in solution at equilibrium.
Freundlich equation can be used as another model for determination of As(V) adsorption capacity as shown below:
Freunlich
equation :
x
¼ kC e 1=n
m

10

15

20

25

Fig. 7. Sorption isotherm of As(V) on Fe-loaded rice husk (RH) as a function of initial
As(V) concentration (initial As(V) concentration: 1–75 ppm; solvent volume: 50 mL;
pH: 4; adsorbent amount: 0.2 g; temperature: 22 ± 2 °C; contact time: 6 h).

Fig. 5. The pH point zero of charge (pHpzc) of RH-FeOOH.

Langmuir equation :
Ce Ce
1
¼
þ
qe As As K b

5

Arsenate concentration Ce (ppm)

ð15Þ

where 1/n is the intensity of adsorption; k is the adsorption capacity,
x/m is the weight adsorbed per unit weight of adsorbent and Ce is the

analyte concentration at equilibrium in solution. The modified formula of this equation, Eq. (16) was also obtained.
log

x
m

¼ logk þ

1
logC e
n

ð16Þ

Langmuir and Freundlich constants and correlation coefficients
(R 2) are shown in Table 1. For the determination of these coefficients,
R 2 value was calculated from the linear form of Langmuir isotherm as
0.995 for As(V) ion adsorption. This result indicates that the As(V) ion
adsorption onto RH-FeOOH fits well the Langmuir model. Thereby,
the adsorption of As(V) ions onto RH-FeOOH is considered forming
a monolayer that takes place at the functional groups or binding
sites on the sorbent surface. The maximum adsorption capacity
(mg/g) of RH-FeOOH for As(V) was found to be 2.5 mg/g (Table 1).
Comparison of As(V) adsorption (mmol As/g adsorbent) of different adsorbents reported in the literature is given in Table 2. It appears
that RH-FeOOH has a reasonable potential as adsorbent for the removal of As(V) from aqueous solutions.
3.4. Desorption studies
The desorption of As(V) ion from the adsorbent (RH-FeOOH) was
investigated as well. Desorption studies can help to regenerate the
adsorbents for further reuse. Desorption efficiency of As(V) ions
from RH-FeOOH was studied with 30% HCl and 1 M NaOH. It was concluded that the desorption percentage of As(V) from adsorbent is
higher when using NaOH than HCl. Table 3 shows that desorption of
As(V) enhanced by the increase of pH. Maximum desorption was

Fig. 6. SEM of RH (a) and RH-FeOOH (b).


516

E. Pehlivan et al. / Fuel Processing Technology 106 (2013) 511–517

Table 1

Langmuir and Freundlich isotherm constants.
Langmuir isotherm parameters

Freundlich isotherm parameters

As (mg/g)

Kb

R2

Kf (mg/g)

N

R2

2.5

2.0

0.995

1.15

3.26

0.982

Table 2

Maximum adsorption capacities of some adsorbents reported in the literature.
Adsorbent

Max. As(V) adsorption
capacity (mmol As/g)

pH

Reference

Gibbsite
Fly ash
Hematite
Feldspar
Sulfate-modified iron
oxide-coated sand
RH-FeOOH

0.073
0.40
0.003
0.003
0.0017

3.0–7.0
4.0
4.2
6.2
4

[33]
[34]
[35]
[35]
[36]

0.033

4

Present study

observed at a pH range of 12–14. 90% of As(V) is recovered under
these conditions. These results demonstrate that adsorbed As(V)
can be desorbed from the RH-FeOOH using 1 M NaOH and thus successfully applied for the regeneration of the RH-FeOOH while shaking
for 20 h.
3.5. Effect of adsorbent amount on the As(V) removal
The amount of RH-FeOOH used for adsorption experiments was
varied from 0.1 to 0.3 g in 50 mL volume at an initial As(V) concentration of 2 ppm; contact time of 6 h at 22 ± 2 °C and pH 6. The
As(V) equilibrium concentration in dissolved phase decreased when
increasing adsorbent quantity (Fig. 8). Thus the optimum adsorbent
amount (RH-FeOOH) was found as 0.25 g by the given As(V) content.
3.6. Effects of ionic strength on As(V) removal
Ionic strength is one of the important factors influencing aqueous
phase equilibrium. The effects of the interfering sulfate, phosphate
and nitrate anions were evaluated for the As(V) adsorption. Adsorption process decreases when increasing ionic strength of the
dissolved phase. The results showed that there was significant decrease in As(V) adsorption when 50 ppm phosphate ions were
contained together with As(V) in the solution. However, the adsorption of As(V) was slightly decreased by addition of 50 ppm nitrate
and sulfate ions. The competition of phosphate with arsenate for the
same sorption sites is very likely due to similar molecular structures

of the two anions in the periodic system of elements. This fact has
to be considered if one wants to remove As(V) from natural waters
that contains also phosphate residues from wastewater or fertilizers.
4. Conclusion
The RH-FeOOH adsorbent was prepared using rice husk from
Vietnam. The characteristics of RH and RH-FeOOH were identified by

Fig. 8. Sorption isotherm of As(V) on Fe-loaded rice husk (RH) as a function of adsorbent amount (initial As(V) concentration: 2 ppm; solvent volume: 50 ml; pH: 6; adsorbent amount: 0.1 g–0.3 g; temperature: 22 ± 2 °C).

using FTIR technique. The adsorption capacity of prepared RH-FeOOH
was investigated by the batch adsorption experiments which revealed
that RH-FeOOH adsorbent was effectively removing As(V). The As(V)
removal capacity of the prepared RH-FeOOH material is 99.6% at pH 4.
This proves that the coated adsorbent has a remarkable capacity for removing As(V) from aqueous solutions.
The kinetic studies indicated that equilibrium of As(V) adsorption
on RH-FeOOH was reached after 6 h. As(V) adsorption increased with
an increase of As(V) in the solution. The optimum pH corresponding
to the maximum adsorption rates was found to be about pH 4 for
RH-FeOOH. The As(V) adsorption on RH-FeOOH was best described
using Langmuir isotherm model only. It was found that the desorption percentage of As(V) from adsorbent is high at pH above 12. The
presence of high phosphate concentrations decreases the As(V) adsorption due to the competition for the same sorption sites. Meanwhile, the adsorption capacity for As(V) was not affected when
adding nitrate and sulfate ions to the solution at the same amounts.
This laboratory investigation was performed under conditions that
can easily be scaled up and applied for the removal of As(V) in developing countries of tropics and subtropics suffering from high As contents in ground and drinking water and producing rice as main
agricultural crop at the same time (e.g., Vietnam and Burkina Faso).

Acknowledgements
This investigation was performed at the Guest Chair within the
project “Exceed – Excellence Center for Development Cooperation –
Sustainable Water Management in Developing Countries” at the

Technische Universitaet Braunschweig, Prof. Pehlivan being the visiting professor, and Ms. Hien and Mr. Ouedraogo being the international exchange staff members. The Exceed Project is granted by the
German Federal Ministry for Economic Cooperation and Development
(BMZ) and German Academic Exchange Service (DAAD), and their financial support we gratefully acknowledge.

References

Table 3
The relationship of desorption and pH values.
Leaching agent

pH

Desorption (%)

HCl (30 %)
NaOH (1 M)

1.5
7
9
12
14

12.3
50.3
70.2
85.6
90.6

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