Journal of Analytical Sciences, Methods and Instrumentation, 2011, 1, 25-30
doi:10.4236/jasmi.2011.12004 Published Online December 2011 (http://www.SciRP.org/journal/jasmi)
25
Removal of As (III) from Aqueous Solutions Using
Montmorillonite
Ansar Anjum, Punnuswamy Lokeswari, Manpreet Kaur, Monika Datta
Department of Chemistry, University of Delhi, Delhi, India.
Received September 6th, 2011; revised October 26th, 2011; accepted November 11th, 2011.
ABSTRACT
Arsenic (III) adsorption has been studied as a function of concentration of arsenic (III) in the solution, pH of the solu-
tion, contact time during the batch extraction process using montmorillonite (MMT) and surfactant modified MMT
(CPC-MMT and CTAB-MMT) from aqueous solution It has been observed that up to 90% of arsenic (III) can be ex-
tracted from a solution containing 100 ppm of As (III) at pH 8.0, within a contact time of 10 minutes. The lowest level of
As (III) that could be extracted was found to be 0.4 ppm.
Keywords: Arsenic (III), MMT, Surfactant Modified MMT, Adsorption Isotherm
1. Introduction
Arsenic, cadmium, chromium, cobalt, copper, lead, man-
ganese, mercury, nickel and zinc, etc. are the metals of
major environmental concern in the present day world. Ar-
senic has been classified as one of the most toxic and car-
cinogenic metal and has therefore been recorded by the
World Health Organization as a first priority issue [1].
Arsenic is ubiquitous and ranks 20th in natural abun-
dance, comprising about 0.00005% of the earth’s crust.
Most environmental problems are due to the mobilization
of arsenic under natural conditions (natural weathering
reactions, biological activity, geochemical reactions, vol-
canic emissions and other anthropogenic activities).
Arsenic is known to exist in various oxidation states as
arsenious acids, arsenic acids, arsenites, arsenates, methy-
larsenic acid, dimethylarsinic acid, etc., in water Two forms
that are commonly found in natural water are arsenite
(3
As ), As (III) and arsenate (4), As (V). Arsenic
(V) dominates in oxic water, while arsenic (III) is more
likely to occur in anoxic water [2]. Of both the forms As
(III) is known to be more toxic than As (V) and is pre-
dominant in ground waters [3].
3
O3 AsO 
The occurrence of arsenic in natural water is a world-
wide problem. Arsenic pollution has been reported in the
Argentina, Bangladesh, Canada, China, India, Japan,
Mexico, New Zealand, Taiwan and USA. Fifty districts
of Bangladesh and nine in West Bengal (India) have been
reported to have the concentration of arsenic above the
World Health Organization’s arsenic guideline value of
10 mg·L−1 [4,5] in the drinking water.
Prolonged consumption of water containing arsenic is
known to produce long term health effects, including der-
mal changes and respiratory, cardiovascular, gastrointesti-
nal, genotoxic, mutagenic, and carcinogenic effects [6].
Long term exposure to arsenic via drinking water causes
skin, lung, bladder, and kidney cancer, skin thickening (hy-
perkeratosis) neurological disorders, muscular weakness,
loss of appetite, and nausea.
Various methods that have been reported for the re-
moval of arsenic from aqueous solutions include coagu-
lation and flocculation, precipitation, adsorption and ion
exchange, membrane filtration, ozone oxidation, biore-
mediation and electrochemical treatments. Most of these
methods involve production of high arsenic contaminated
sludge [7], high maintenance cost and require relatively
expensive mineral adsorbents which offset performance
and efficiency advantages [8].
Of all the methods adsorption has been most extensi-
vely used as reported by Abhijit and Sunando [9]. As cited
by Deliyanni, Peleka and Matis [10] selective adsorption
utilizing biological materials, mineral oxides, clay min-
erals, zeolites, fly ash, activated carbons, or polymer res-
ins, has generated increasing interest. In the present work,
montmorillonite (a member of the smectite group of the
clay mineral) has been selected as the host material for
the removal of arsenic from aqueous solutions. MMT is
known for its low cost, high surface area, high chemical
stability, high sorption properties and rich intercalation
chemistry. To the best of our knowledge, the best result

Removal of As (III) from Aqueous Solutions Using Montmorillonite
26
achieved so far accounts for the detection of 10 ppm of
arsenic using montmorillonite [1].
2. Experimental
2.1. Materials and Methods
Montmorillonite (MMT-KSF) was obtained from Sigma
Aldrich (USA). Cetylpyridinium chloride (CPC) and cetyl
trimethylammonium bromide (CTAB) were obtained from
Merck (Germany) and British Drug House respectively.
Arsenious oxide and potassium antimonyl tartarate were
obtained from British Drug House (England), Sodium
hydroxide; hydrochloric acid and potassium iodate were
obtained from Qualingens (India). Leuco crystal violet
(LCV) was obtained from Sigma Aldrich Pvt. Ltd. (Ger-
many).The AR grade chemicals/reagents and double dis-
tilled water was used throughout the experiment.
2.2. Synthesis of Organoclay
Surfactant modified clay (CPC-MMT and CTAB-MMT)
were synthesized by the modification of the reported pro-
cedures by Khalaf, Bouras and Perrichon [11].
In order to facilitate intercalation, 5.0 g of dried (80˚C)
MMT was dispersed in 250 ml of double distilled water and
was kept under constant stirring for a period of 24 hours.
To this 1% aqueous surfactant (CPC and CTAB) solution
was gradually added with constant stirring over a period
of 3 hours at NTP. The clay suspension was separated by
centrifugation and washed with double distilled water for
the complete removal of unreacted surfactant. The resi-
es thus obtained were dried at 80◦C and was labelled as
CPC-MMT and CTAB-MMT.
2.3. Characterization
X-ray diffractograms were recorded on X-ray Diffracto-
ter (Philips PW3710) using CuKα radiations. The specific
surface areas were measured by adsorption of nitrogen
according to the BET method. UV-VIS studies were per-
rmed on Systronics UV/VIS spectrophotometer. pH was
monitored using a pH meter (Eutech instruments, pH 510).
2.4. Experimental
2.4.1. Pre paration of As (I II) Solution
As (III) stock solution (1000 mg/L) was repared by dis-
lving 1.320 g of arsenious oxide (As2O3) in 25 mL of 1
M NaOH in double distilled water. The solution was then
diluted to about 100 mL with double distilled water and two
drops of 0.2% phenolphthalein was added to this solution
followed by neutralization of the solution using 1 M HCl.
The volume of the solution was made up to 1 L in a vo-
metric standard flask.
2.4.2. Spectrophotometric Determination of As (III)
The concentration of arsenic (III) was determined spec-
trophotometrically procedure using LCV as reported [12].
The reagent blank gave negligible absorbance at the
above wavelength. A rectilinear calibration graph was ob-
ined by measuring the absorbance of the solution at 592
mover a known concentration range and concentration of
As (III) in the experimental solution was calculated from
the calibration curve.
2.4.3. Behavior of As (III) Adsorption on Clay and
Modified C l ay
The adsorption experiments were carried out using MMT
and CPC-MMT and CTAB-MMT as an adsorbent. The
batch experiments were carried out with 0.1 g of the ad-
sorbent and 50 mL of (100 ppm) As (III) solution, as a func-
tion of pH, contact time and concentration. The concen-
tration of arsenic in the supernatant was estimated spec-
trophotometrically. The percentage of the metal adsorbed
onto the adsorbent was calculated using the relation:
 

iei
%As III adsorbedCCC*100



Amount of metal adsorbed (qe) was calculated from
the relationship
   
eie
qCC*V m
where Ci was the initial As (III) concentration (mg/L), Ce
was the final concentration of As (III) in the solution after
equilibrium was attained (mg/L), V was the volume of the
As (III) solution (L) and m was the mass of the adsorbent
(g) used.
3. Results and Discussions
3.1. XRD Analysis
The X-ray diffraction pattern of the clay and the organo
clay is shown in Figure 1. The decrease in the 2 theta va-
lue with respect to MMT was observed in case of CPC-
MMT and CTAB-MMT. The interlayer (d001) spacing of
MMT, CPC-MMT and CTAB-MMT is 12 Å, 19.5 Å and
2theta = 7.2
2theta=3.5
2theta = 4.2
Figure 1. XRD pattern of (a) MMT; (b) CTAB-MMT; (c)
CPC-MMT.

Removal of As (III) from Aqueous Solutions Using Montmorillonite
27
ctively. The increase in d spacing of CPC-
nt mechanisms
of
ethod indicates that the
17.6 Å respe
MMT and CTAB-MMT with respect to MMT is 7.5 Å
and 5.6 Å respectively. This increase in the basal spacing
is attributed to intercalation of the surfactants into the
interlayer region of the host clay, MMT.
Generally, there are two most importa
the cationic surfactant adsorption on solids: 1) ion ex-
change and 2) hydrophobic interactions [13]. Thus, there
are two well-distinguished phases of CPC and CTAB
adsorption on MMT, first can be ascribed to incomplete
monolayer formation. After the monolayer onset of the
surfactant is completed bilayers are formed. The CPC ad-
sorption arrangements are illustrated in Figure (2). Fig-
ures 2(a)-(c) illustrates the CPC adsorption on external
MMT surface and Figures 2(d) and (e) simply manifest
the intercalation, as also observed by the X-ray diffracttion
analysis. Similar adsorption mechanism is also followed
by the CTAB on MMT surface. The alkyl ammonium bi-
layer structure (Figure 2(c)) is positively charged, which
makes possible the adsorption of arsenite ions [14].
3.2. Surface Area Analysis
The Nitrogen BET adsorption m
surface area of MMT, CPC-MMT and CTAB-MMT are
28,700 cm2/g, 82,000 cm2/g and 98,200 cm2/g, respec-
tively. This increase in surface area of CPC-MMT and
CTAB-MMT clays with respect to MMT is 53,000 cm2/g
and 69,500 cm2/g respectively. The increase in surface
area is due to an increase in the interlayer spacing.
3.3. Effect of pH on As (III) Adsorption
The effect of pH on the amount of arsenic uptake by clay
and organo clay (CPC-MMT and CTAB-MMT) was stud-
ied in the pH range of 1 - 12, using 0.1 gram of the adsor-
bent and 100 ppm at As (III) solution with contact time
of 60 minutes (Figure 3).
The percentage of As (III) adsorbed increased from
47.5% (23.5 mg/g) to 70.25% (35.15 mg/g) in the pH range
of 1 - 8, and then decreased to 56.88% (28.45 mg/g) at pH
12 in MMT. A similar behaviour was observed for CPC-
MMT and CTAB-MMT where the percentage of As (III)
adsorbed in the pH range of 1 - 8, increased from 89%
(44.5 mg/g) to 94.4% (47.20 mg/g) in case of CPC-MMT
and 88% (44 mg/g) to 92.6% (46.32 mg/g) in case of
CTAB-MMT, followed by a decrease in As (III) adsorp-
tion up to pH 12 in both the cases.
Figure 2. Arrangement of CPC adsorbed onto MMT (a) incomplete monolayer (b) monolayer (c) bilayer (d) monolayer in-
tercalated (e) bilayer intercalated.
Figure 3. Adsorption as a function of pH. Conc. of As (III) solution = 100 ppm, mass of adsorbent = 0.1 gm, pH 1 - 12, contact
time = 60 minutes.

Removal of As (III) from Aqueous Solutions Using Montmorillonite
28
These results show that in a highly acidic medium, where
h
TAB-MMT
w
which maxi-
m
t of Contact Time on As (III) Adsorption
by clay (MMT) and organoclay (CPC-MMT and CTAB-
m
67
y
oc
n on As (III)
The concentration on the amount of arse-
II) adsorbed was
ob
te adsorbent surfaces are highly protonated and As (III)
mainly exists in the form of neutral H3AsO3 species, ad-
sorption of As (III) is not favorable. This is due to the weak
interaction that occurs between the adsorbent and H3AsO3.
Therefore, the only driving force between H3AsO3 and
the adsorbent is the physical adsorption, thereby resulting
in less adsorption. At near neutral pH values (7 - 9), slow
dissociation of H3AsO3 producing arsenite ions begins
and it reaches a maximum at pH 8.0. These partially neu-
tral and partially negatively charged arsenite ions are at-
tracted to the positively charged surface of the adsorbent
thereby resulting in high As (III) uptake by the adsorbent
in this pH range. At pH 10 and above, the adsorbent sur-
faces become negatively charged. Hence, the interaction
between the adsorbent and arsenite ions decreases lead-
ing to low As (III) uptake by the adsorbent.
The As (III) uptake by CPC-MMT and C
as observed to be higher than MMT. This is because,
the surface of the organo clay composites is highly posi-
tively charged than MMT, as indicated by the zeta poten-
tial values (MMT = 12.9 mV, CPC-MMT = 17.2 mV,
CTAB-MMT = 22.6 mV). This in turn facilitates stronger
interaction between the positively charged clay surface
and the existing species of arsenite ions in the solution,
thereby leading to a higher As (III) uptake.
Therefore pH (8.0) was fixed as the pH at
um As (III) adsorption on the clay and organoclay had
occurred.
3.4. Effec
The effect of contact time on the amount of arsenic uptake
MMT) was studied as a function of time in the range of 10 -
120 minutes, using 0.1 gram of the adsorbent, at pH 8.0 with
initial concentration of As (III) at100 ppm (Figure 4).
The percentage of As (III) adsorbed increased fro
% (33.5 mg/g) at 10 minutes to 70% (35 mg/g) at 60
minutes on MMT and decreases thereafter. A similar be-
haviour was observed for CPC-MMT and CTAB-MMT
where the percentage of As (III) adsorbed increased from
88% (44 mg/g) at 10 minutes to 94% (47.44 mg/g) at 60
minutes in the former and 89% (44.75 mg/g) at 10 min-
utes to 92.5% (46.25 mg/g) at 60 minutes in the latter.
The adsorption of As (III) on the clay and organocla
curred very quickly in the initial phase of the experi-
ment. This could be due to the availability of a large number
of adsorption sites on the clay surface. Maximum adsor-
ption of the As (III) was observed in the time duration of
60 minutes and beyond this no further increase in adsor-
ption was observed. Therefore, 60 minutes was fixed as
the time at which maximum As (III) adsorption on the
clay and modified clay had occurred.
3.5. Effect of Initial Concentratio
Adsorption
effect of initial
nic uptake by clay (MMT) and organoclay (CPC-MMT
and CTAB-MMT) was studied using 0.1 gram of the ad-
sorbent and by varying the concentration of As (III) solu-
tion from 0.2 - 100 ppm and maintaining pH 8.0 and a
contact time of 60 minutes. (Figur e 5)
An increase in the percentage of As (I
served from 24% to 71% in MMT on varying the metal
Figure 4. Adsorption as a function of co ntact time. Conc. of As (III) solution = 100 ppm, mass of adsorbent = 0.1 gm, pH 8.0,
contact time = 10 – 120 minutes.
Figure 5. Adsorption as a function of As (III) concentration. Conc. of As (III) solution = 10 ppm – 100 ppm, mass of adsorb-
ent = 0.1 gm, pH 8.0, contact time = 60 minutes.

Removal of As (III) from Aqueous Solutions Using Montmorillonite29
-MMT and
a het-
er
of the widely used ma-
sually fits the experimen-
concentration from 10 - 100 ppm. The percentage of As (III)
dsorbed increased from 21% to 94.4% in CPCa
21% to 92.3% in CTAB-MMT on varying the metal
concentration from 0.2 - 100 ppm. The amount of As (III)
adsorbed increased from 0.066 mg/g to 47.43 mg/g in case
of CPC-MMT and 0.065mg/g to 46.25 mg/g in CTAB-
MMT composites. Whereas, in case of MMT the amount
of As (III) adsorbed increases from 1.85 mg/g to 35.75
mg/g on varying concentration from 10 - 100 ppm.
The increase in As (III) adsorption on all the three ad-
sorbents in the given concentration range indicates
ogeneous system where adsorption is not restricted to
monolayer formation. The percentage of As (III) adsorbed
in case of surfactant organoclay was observed to be higher
than that of the host clay. This can be explained on the
basis of zeta potential studies. The zeta potential values
of CPC-MMT and CTAB-MMT were found to be +22.6
mV and +17.2 mV respectively. The zeta potential values
suggest that the adsorption is therefore due to the elec-
trostatic attraction between the positively charged adsor-
bent (organoclay) and the negatively charged adsorbate
(arsenite). In case of MMT, the zeta potential value is +12.9
mV and is therefore less positively charged as compared
to CPC-MMT and CTAB-MMT. Therefore, the interact-
tion between the adsorbent and arsenite ions is less, thereby
leading to a low uptake.
3.6. Adsorption Isotherms
The Freundlich isotherm is one
thematical descriptions and it u
tal data over a wide range of concentrations. This isotherm
gives an expression encompassing the surface heteroge-
neity and the exponential distribution of active sites and
their energies. It describes a heterogeneous system and
reversible adsorption process that is not restricted to the
monolayer formation. This model predicts that adsorption
of arsenic on the organoclay increases with increase in
arsenic concentration in the solution.
The Freundlich equation is as follows: 1n
efe
qKC
The linear form of the above equation is given as:
ef e
here Kf is the Freundlich constant (mg/g) and 1/n is
logq= log K+1nlog C
wthe
heterogeneity factor. A plot of log qe and log Ce giv
were y = 1.25689x + 1.67829 (R = 0.999
an
values indicate a
favorable adsorption process.
es a
straight line and the values of Kf and n can be calculated
from the intercept and slope of the plots respectively
(Figure 6).
The linear equation obtained for CPC-MMT and
CTAB-MMT 22)
d y = 1.48656x + 1.66462 (R2 = 0.9951) respectively.
The Kf values obtained from the equation were 47.74 mg/g
and 46.19 mg/g respectively.
The value of n obtained from the above equation was
0.7956 and 0.6727 respectively. These
Figure 6. Freundlich plot for the adsorption of As (III) on
MMT, CPC-MMT and CTAB-MMT.
4. Conclusions
and 46.15 mg/g
. The adsorption capacity of CPC-
MT was observed to be higher than
eparation and
Purification Technology. 1, 2007, pp. 90-
100. doi:10.10
The results obtained indicate that the maximum As (III)
uptake on MMT, CPC-MMT and CTAB-MMT was found
be 35.75 mg/g (71%), 47.43 mg/g (95%)to
(92%) respectively
MMT and CTAB-M
that of MMT. This is because of the higher positively
charged organoclay surface which facilitates stronger in-
teraction between the adsorbent and the adsorbate, thereby
leading to a higher As (III) uptake. The lowest level of
detection of As (III) from aqueous solutions was found to
be 0.4 ppm using both the clay composites (CPC-MMT
and CTAB-MMT). This process was found to be highly
effective over a wide range of concentration and pH. Thus,
these composites can be successfully employed for the
removal of As (III) from aqueous solutions.
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