Research Journal of Chemical Sciences 
______
______________________________
______
____
 
ISSN 2231
-
606X
 
 
 
Vol. 
2
(
4
), 
1
3
-
1
7, 
April 
(201
2
)
 
 
Res.J.Chem.Sci.
 
 
 
International Science Congress Association
 
 
 
 
    
13
 
Studies on the effect of pH on the sorption of Al
3+
 
and Cr
6+
 
Ions from 
aqueous solutions by Almond Tree (
Terminalia catappa L
.) Biomass
 
 
Enemose Edith A. and Osakwe S.A.
*
 
Department of Chemistry, Delta State University, P. M. B 1, Abraka, Delta State, NIGERIA
 
 
Available online at: 
www.isca.in
 
(Received
 
20
th
 
January 
201
2
, revised
 
30
th
 
January 
201
2
, accepted
 
21
st
 
March 
201
2
)
 
 
Abstract
 
Environmental protection requires the use of natural products instead of chemicals to minimize pollution. This investigation 
studies the use of a non
-
useful plant material as naturally occurring biosorbents for the removal of cationic pollutants in 
wastewa
ter. The effect of pH on the sorption of Al
3+
 
and Cr
6+
 
ion unto Terminalia catappa biomass was investigated. The 
experimental results have been analysed in terms of Langmuir and Freundlich isotherms. The data showed that the maximum 
pH (pH max) for efficie
nt sorption of Al
3+
 
was 4.0 (85.5 mg/g) and for Cr
6+
 
2.0 (82.6 mg/g). Evaluation using Langmuir 
equation gave the monolayer sorption capacity as 1.12mg/g and 2.67mg/g at the respective pHmax for Al
3+
 
and Cr
6+
. The 
thermodynamic assessment of the metal ion 
–
 
Terminalia catappa L. biomass system indicates the feasibility and spontaneous 
nature of the process.
 
 
Keywords:
 
adsorption, heavy metal removal, almond tree, phytoremediation, water treatment.
 
 
 
Introduction
 
The conventional 
technologies for effluent treatment are not 
economically feasible for small
-
scale industries prevalent in 
developing economies due to huge capital investment. It is 
therefore necessary to search for alternative adsorbents which 
are low cost, often naturall
y occurring products that have good 
sorbent properties and no value to the people.
 
 
The presence of Al
3+
 
and Cr
6+
 
and other heavy metals in the 
environment has become a major threat to plant, animal and 
human life due to their bio
-
accumulating tendency and toxicity 
and therefore must be removed from municipal and industrial 
effluents before discharge. Resultantly, it
 
is necessary that there 
are technologies for controlling the concentrations of these 
metals in aqueous emissions.
 
 
A range of products has been examined. Few of these include 
pillared clay
1
, sago waste
2
, cassava waste
3
, banana pith
4
, 
Medicago sativa
5
, wil
t cocoyam
6
, and spagnum moss peat
7
.
 
 
The adsorbent used in this study is almond tree biomass. The 
almond tree belongs to the family called combretaceae 
(Combretum family). The botanical name of the tropical almond 
tree is 
Terminalia catappa L
. The almond t
ree (
Terminalia 
catappa L.
) has been known for its usefulness in the medical 
world. The use of the biomass as a biosorbent for trace metals in 
water and waste effluents is expected to solve environmental 
problems.
 
 
The equilibrium distribution of metal ion
s between the sorbent 
and the solution is important in determining the maximum 
sorption capacity. Two sorption models, (the Langmuir and 
Freundlich) will be used to assess the different isotherms and 
their ability to correlate experimental data. The Langmu
ir 
equation was chosen for the estimation of maximum adsorption 
capacity corresponding to complete monolayer coverage on the 
biomass surface. The Freundlich model was chosen to estimate 
the adsorption intensity of the sorbent towards the biomass. The 
major
 
aim of the present work is to investigate the potential use 
of the biomass of almond tree (
Terminalia catappa
 
L.) as a 
novel biosorbent for the sorption of valuable and toxic metal 
ions from aqueous media.
 
 
This paper reports the effect of pH on the abili
ty of the biomass 
of the almond tree (
Terminalia catappa
 
L.) to remove Al
3+
 
and 
Cr
6+
 
ions from single metal ion solutions. The effect of varying 
initial pH and equilibrium sorption on the potential ability of the 
Terminalia catappa
 
L. in removing these two
 
metal ions from 
solution was investigated.
 
 
Material and Methods
 
Collection and preparation of sample (biosorbent) 
investigated
: The almond tree (
Terminalia Catappa L
.) leaves 
were collected from Akarai
-
Etiti, Akarai
-
Obodo and Aboh in 
Ndokwa East Local Go
vernment Area of Delta State. The 
almond tree leaves were collected with clean polythene bags 
and then air
-
dried for one week under normal sunlight 
conditions. The dried almond tree leaves were initially crushed 
to smaller size with mortar and pestle and f
urther ground using a 
food processor (Magimix Cuisine System 5000) to 90
-
µm size 
to obtain a fine biomass which was then stored in clean, air tight 
plastic container and ready for use.
 
 
Research Journal of Chemical Sciences 
______
_
_
_______________________________
______________
_
____
 
ISSN 2231
-
606X
 
 
Vol. 
2
(
4
), 
1
3
-
1
7, 
April 
(201
2
)
 
  
Res.J.Chem.Sci
 
International Science Congress Association
 
 
14
 
Biosorbent activation
: The purpose of activation is to increase 
the po
rosity and open more pores in the biomass. The finely 
divided biomass was activated by soaking 5g biomass in excess 
0.03M HNO
3
 
for 25 hours, followed by washing thoroughly 
with deionized
 
water. The washing process continued until the 
filtrate gave a negative EDTA (Ethylenediaminetetraacetic acid) 
test for heavy metal ions. The test was carried out by the 
addition of 5 drops of 0.001M EDTA solution and 2 ml of 
NH
3
/NH
4
Cl buffer to 5 ml of t
he washing water filtrate. The 
appearance of blue color of the EDTA solution indicates the 
absence of metal ions. The filtered biomass was then oven
-
dried 
at 65
o
C to constant weight. 
 
 
Characterization 
o
f 
t
he Biomass
: 
The finely divided biomass 
was characterized for apparent density and porosity were 
determined by mercury intrusion porosimeter (Micrometrics 
model
-
9310) and specific gravity bottle respectively. Pore 
volume was estimated as the inverse of reaction of part
icle 
density (Horsfall et al, 2005), while the ash content was 
determined using the ignition method by burning 1.0g of 
biomass sample (placed in a thoroughly washed crucible) in a 
furnace which was pre
-
heated to 500
o
C for 3 hours. The 
crucible was removed 
and cooled in a desiccator and reweighed 
until a constant weight was obtained. The percentage ash 
content was calculated using the formula
6
:
 
% Ash = 
 
 
 
 
        
(1)
 
Where 
 
= mass of ash (g) and 
 
= mass of sample used 
(g). 
 
 
Preparation 
o
f Metal Solutions
: 
The aqueous solutions of the 
metal ions used were prepared by using analytical grade 
reagents provided by Fluka
 
(Switzerland). Individual stock 
metal ion solution of 1000mg/l concentration of Al
III
 
from 
Al(NO
3
)
3
. 9H
2
O and Cr
VI
 
from K
2
Cr
2
O
7
 
were prepared. Serial 
dilutions were made with double distilled water from these 
stock solutions. In order to prevent the forma
tion of metal 
hydroxide and allow all metal ion to be solution, the stock 
solutions were acidified with HNO
3
 
to 4H6.
 
 
Measurement
: 
pH was measured using X74081 Toptronic pH 
meter (Vetrerie ed Apparecchiature Per Laboratori Chmici, 
Milano, Italy). The me
tal ion concentrations were measured 
using Buck Scientific Flame Atomic Absorption Spectrometer 
(FAAS) model 300A, Germany). The samples were read in 
duplicates. The analytical wavelengths used for the metals 
studied were: 309.3 nm and 358.2nm for Aluminiu
m and 
Chromium respectively. Analytical grade standards were used to 
calibrate the instrument response. The correlation coefficient for 
calibration curve was about 98.6.
 
 
The batch experiments were performed in duplicates and the 
mean (or the slope and int
ercept) were computed for each set of 
values.
 
Experimental Procedure
: 
Adsorption isotherm studies
: 
Effect of pH:
 
The pH solutions ranging from 2.0 to 8.0 was 
studied, the solution were adjusted to the required pH values by 
adding a solution of In HCl or 1m
 
NaOH. All the flasks were 
reacted on a shaker for one hour. The suspensions were filtered 
using No. 45 Whatman filter paper and then centrifuge at 
2800rpm for five minutes in order to remove all traces of 
cloudiness. The metal ions investigated were deter
mined using 
FAAS model 300A.
 
 
Data Evaluation
: 
The amount of metal ion taken by the 
biomass was calculated using a mass balance equation which 
has been previously used by other researches in evaluating the 
amount of metal ion adsorbed by the Almond tree le
aves 
biomass. The mass balanced equation is given as
 
q
e
 
=
 
 
 
 
 
            
(2)
 
where
 
qe
 
= amount of metal ion removed by the almond tree waste 
biomass (mg/g), m = Mass of almond tree waste biomass used 
(g), v = Volume of initial ion solution used (ml)
1
 
Co = initial 
metal ion concentration (mg/l), Ce = Equilibrium metal ion 
concentration (mg/
l).
 
 
Adsorption Isotherms
: 
Two of the most sorption models were 
used to fit the experimental data. The Langmuir model which 
assumes that equilibrium is attained when a monolayer of the 
adsorbate molecules saturates the adsorbent. This model can be 
presente
d as in equation 3
 
q
r
 
= 
 
 
 
 
 
            
(
3
)
 
Where X
m
 
and K are the Langmuir constants and specifically 
Xm is the monolayer and sorption capacity of the biomass q
e
 
is 
the concentration of metal ion on the biomass (mg/g) at 
equilibrium
 
and C
e
 
as the concentration (in mg/l) remaining in 
solution at equilibrium.
 
 
The linear form of the Langmuir model is given in equation 4
 
 
 
 
 
 
           
(
4
)
 
The capacity of the biomass can be obtained if a plot of C
e
/q
e
 
against C
e
 
is made.
 
The second model is Fruendlich model 
which can be written as in equation 5. The mathematical 
equation is given as:
 
 
 
 
 
 
           
(
5
)
 
Where: X is the mass of metal ion adsorbed (mg), m is the mass 
of biomass used (g), C
e
 
is
 
the concentration of metal ion at 
equilibrium, n is the adsorption intensity and K is the adsorption 
constant.
 
 
The linear form of equation 5 takes the form (eqn 6)
 
Research Journal of Chemical Sciences 
______
_
_
_______________________________
______________
_
____
 
ISSN 2231
-
606X
 
 
Vol. 
2
(
4
), 
1
3
-
1
7, 
April 
(201
2
)
 
  
Res.J.Chem.Sci
 
International Science Congress Association
 
 
15
 
In 
 
 
 
 
            
(
6
)
 
A plot of 1n 
 
against 1nC
e
, will give a straight line which 
will confirm the Freudlich Isotherm.
 
 
Results and Discussion
s
 
The data in table 
-
1 gives some physical characteristics of the 
almond tree leaves biomass. These characteristics play some 
important role in the a
dsorption process of metal ions onto the 
biomass. The low value of the apparent density (1.17±0.06, 
g/cm
3
) is an indication of the ease of suspension of the biomass 
in aqueous solution, which is an essential factor in the 
interaction between metal ion in solution and a coagulation 
ligand. Apparent density closer to unity indicates higher contact 
between sorba
te and sorbent. The porosity and pore volume are 
important factor in characterizing the macrostructure properties 
of the biomass.
 
Table 
–
 
1
 
Some physical characteristics of the biomass
 
 
 
Parameters
 
Values
 
Apparent density, g/cm
3
 
1.17±0.06
 
Porosity %
 
43.3±0.08
 
Pore volume, cm
3
/g
 
0.79
 
Ash content, %
 
12.3±0.11
 
 
Table
-
2
 
The pH dependent data for sorption of Al(III) and Cr(VI)
 
pH
 
Al
3+
 
Cr
6+
 
2
 
0.081
 
0.083
 
4
 
0.086
 
0.076
 
6
 
0.071
 
0.063
 
8
 
0.063
 
0.059
 
Effect 
o
f pH
: 
Most plant materials are made up of complex 
organic residues such as lignin and cellulose that contain several 
types of polar functional groups. These groups can be involved 
in chemical bonding and may be responsible for the typical 
cation
-
exchange charact
eristics of most biomaterials.
 
 
The pH of the aqueous medium affects the surface charge of the 
adsorbent, the degree of ionization, and the species of the 
adsorbate
5
.
 
 
The pH dependence data for the sorption of the two metals 
under investigation are presen
ted on 
f
igure 1. The data revealed 
that, at pH 2.0, there was significant removal of the metal ions 
by the almond tree waste biomass; 85.5% for Al
3+
 
and 82.6% 
for Cr
6+
. However, when the pH was raised to 4.0, there was a 
large increase in the amount of 
metal ion removed. At pH 4.0, 
the biomass removed 85.5% Al
3+
 
and 76.1 Cr
6+
. Above pH 4.0, 
a gradual decrease in the amount of metal ions removed by the 
almond tree waste biomass was observed.
 
 
A close look at the metal ions investigated a pH 2 favours Cr
6+
 
removal with a percentage of up to 82.6% while a pH 4 favours 
Al
3+
 
as shown in figure 1.
 
 
The overall trend observed in the pH
-
dependent sorption 
behaviour suggests that metal uptake occurs through ion
-
exchange. Our results, thus, indicate that binding fo
llows an ion
-
exchange mechanism that involves electrostatic interaction 
between the negatively charged groups in the walls of the 
biomass and the metallic cations. At low pH, the surface of the 
adsorbent would closely be associated with hydroxonium ions 
(H
3
O
+
) that hinder the access of the metal ions to the surface 
functional groups, consequently decreasing the percentage of 
ion removal at low pH
4
.
 
 
 
 
Figure 
-
1
 
Graphical representation of the pH dependent 
data for sorption of Al(III) and
 
Cr(VI)
 
Research Journal of Chemical Sciences 
______
_
_
_______________________________
______________
_
____
 
ISSN 2231
-
606X
 
 
Vol. 
2
(
4
), 
1
3
-
1
7, 
April 
(201
2
)
 
  
Res.J.Chem.Sci
 
International Science Congress Association
 
 
16
 
Table
-
3
 
The values of experimental results as analysed in terms of Langmuir and Fruedlich isotherms
 
Metal ions
 
Langmuir
 
Fruedlich
 
KL
 
Xm
 
R2
 
KF
 
1/n
 
R2
 
A1III
 
2.37 X 10
-
2
 
36.25
 
0.94
 
1.45 X 10
-
2
 
1.14
 
0.98
 
CrVI
 
1.51 X 10
-
1
 
17.47
 
0.91
 
7.5 X 10
-
3
 
0.97
 
0.95
 
 
Figure 
-
2
 
Graphical representation of the analytical results in terms of Langmuir and Fruedlich isotherms
 
 
Figure
-
3
 
Graphical representation of the equilibrium metal ion concentrations against intensity
 
 
 
 
 
Ce/qe
 
Residual Cone, mg/l
 
Research Journal of Chemical Sciences 
______
_
_
_______________________________
______________
_
____
 
ISSN 2231
-
606X
 
 
Vol. 
2
(
4
), 
1
3
-
1
7, 
April 
(201
2
)
 
  
Res.J.Chem.Sci
 
International Science Congress Association
 
 
17
 
Surface of the adsorbent, would be closely associated with 
hydroxonium ions (H
3
O
+
) which hinder the access of the metal 
ions to the surface functional groups. Consequently, the 
percentage of metal ion removal may decrease at low pH.
 
 
So
rption Equilibrium
: 
The applicability of sorption processes 
as a unit operation can be evaluated using isotherm models. The 
equilibrium sorption data obtained were analysed in terms of the 
Langmuir 
and 
Freundlich equations. The data linearised to fit 
the L
angmuir equation and presents the constants derived by 
regression analysis for the equation. A comparison of the 
Langmuir sorption capacity at various initial pH was made by 
plotting q
max
 
(mg/g) against pH
in
. The results confirmed that at 
pH 2 and 4, the s
orption of Al
3+
 
and Cr
6+
 
on Terminalia catappa 
L. biomass was much less than at pH 6 and above, indicating 
that the Terminalia catappa L. biomass was much less effective 
sorbent at lower pH values. Hence, lowering the pH may aid the 
regeneration and recycl
ing of spent biomass. The R
2
 
values 
suggested that the Langmuir isotherm provides a good model of 
the sorption system. The sorption coefficient, K
L
, which is 
related to the apparent energy of sorption, was greater for Al
3+
 
than for Cr
6+
 
at all pH
in
. The hi
gh values of K
L
 
for Al
3+
 
and Cr
6+
 
at pH 2 and 4 respectively, indicate that the energy sorption is 
more favourable at these pH maxima than other pH values.
 
 
The linear Freundlich isotherm plots for the sorption of the two 
metals (Al
3+
 
and Cr
6+
) onto the 
Terminalia catappa L. biomass 
are presented. Examination of the plot suggests that the 
Freundlich isotherm is also an appropriate model for the 
sorption study of Al
3+
 
and Cr
6+
. Table below shows the linear 
Freundlich sorption isotherm constants and the coe
fficients of 
determination (R
2
). Values of I/n for Al
3+
 
and Cr
6+
 
were found 
to be less than unity at all pH respectively, indicating that 
significant sorption could take place at these initial pH values. 
Confirmation of the pH
max
 
was made by plotting the F
reundlich 
equation parameter, I/n, which is a measure of the sorption 
intensity against pH
in
. The plot further confirmed that, the pH 
maxima for the sorption of Al
3+
 
and Cr
6+
 
onto Terminalia 
catappa L. biomass are 2.0 and 4.0 respectively. Based on the 
coe
fficient of determination (R
2
) values, the linear form of the 
Freundlich isotherm appears to produce a reasonable model for 
the sorption of the two metals, with Al
3+
 
fitting the data better 
than Cr
6+
. The K
-
values suggests that Al
3+
 
has greater sorption 
te
ndency towards the Terminalia catappa L. biomass in neutral 
solutions. However, for Cr
6+
 
the greater affinity towards active 
groups on the biomaterials was observed at pH 4.0.
 
 
Conclusion
 
In conclusion, the data has shown that, the sorption process of 
Al
3+
 
and Cr
6+
 
ions on to Terminalia catappa L. biomass is 
feasible and spontaneous in nature. The metal ions binding 
capacity of the biomass was shown as a function of initial pH of 
the aqueous solution. The equilibrium data fitted the Langmuir 
and Freundhich 
isotherms very well. The two models confirmed 
that maximum sorption capacity onto the biomass occurred at 
pH 2.0 for Al
3+
 
and pH 4.0 for Cr
6+
. Thus efficient removal of 
the two metal ions in an effluent using 
Terminalia catappa L.
 
biomass may require pH ad
justment between 4.0 and 2.0. The 
equilibrium sorption of the two metal ions at their PH
max
 
was 
determined from the Langmuir equation and found to be 85.50 
mg/g and 82.60 mg/g for Al
3+
 
and Cr
6+
 
respectively. The data 
showed that, Terminalia catappa L. is a
 
successful biosorbent 
for treating heavy metal contaminated wastewater and may 
serve as an alternative adsorbent to conventional means. Hence, 
not only is Terminalia catappa L. readily available, it also has 
the potential for metal removal and recovery fr
om contaminated 
waters. This process will be environment friendly and convert 
the non
-
useful plant into an economic crop for local farmers. It 
may also provide an affordable technology for small and 
medium
-
scale industries. The pH study is particularly imp
ortant 
for determining the ligands that may be involved in the reaction 
for optimization in the design of sorption process units.
 
 
References
 
1.
 
 
Vinod V.P. and Antrudhan T.S
.,
 
Sorption of Tanic acid on 
zirconium pillared clay
, 
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-
101 (
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)
 
 
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Quek S.Y.
,
 
Wase D.A.J. and Forster C.F.
,
 
The use of Sago 
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the sorption of lead and copper,
 
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Horsfall M. Jnr. and Didi O.
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Removal of metals 
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,
 
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Medicago sativa (Alfalfa)
,
 
J. Hazard Mat,
 
57
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-
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) 29
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39 
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6.
 
 
Horsfall M.
,
 
Jnr and Abia A
.A.
,
 
Sorption of Cd(II) and 
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,
 
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, 
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-
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)
 
 
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Ho Y.S.
,
 
John Wase D.A. and Forster C.F.
,
 
Batch nickel 
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, 
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)
 
<end>