Out of the many pollutants present in wastewaters, dyes affect the
environment badly. Dyes are synthetic aromatic water soluble and dispersible
organic compounds, which cause coloration of natural water bodies when
released into the environment. They affect the aquatic fauna and flora
by reducing light transmission through water surface and in some cases
may be toxic to the aquatic biota due to the presence of aromatics, metals,
chlorides etc. Dyes usually have complex aromatic molecular structures,
which make them more stable and more difficult to biodegrade (Aksu, 2005).
Major classes of synthetic dyes include azo, anthraquinone and triphenyl-methane
dyes. Dyes are difficult to degrade biologically, so that removal of dyes
from aquatic environment has received considerable attention. About 10-15%
of all dyes are directly lost to wastewater in the dyeing process (Vaidya
and Datye, 1982). Thus the wastewater must be treated before releasing
into the natural environment. The members of triphenylmethane family are
animal carcinogens (Parshetti et al., 2006). The triphenylmethane
dye, crystal violet, has been extensively used in human and veterinary
medicine as a biological stain and in various commercial textile processes
as a dye (Bumpus and Brock, 1988). Crystal violet has been classified
as a recalcitrant molecule, thereby indicating that it is poorly metabolized
by microbes and consequently is long lived in a variety of environments
(Chen et al., 2007). An additional worrying factor is that some
triphenylmethane dyes including crystal violet are potent clastogens,
possibly responsible for promoting tumor growth in some species of fish
(Cho et al., 2003).
Many alternative processes aimed at removing crystal violet from wastewater
have been investigated including chemical oxidation and reduction, physical
precipitation and flocculation, photolysis, adsorption, electrochemical
treatment, advanced oxidation, reverse osmosis and biodegradation (Azmi
et al., 1998). Of these, adsorption is known to be a promising
technique, which has great importance due to the ease of operation and
comparable low cost of application in the decolorization process. Various
adsorbents have been tested and used for the removal of dyes from polluted
water such as activated carbon, silica gel, natural clay, peat, wood chips,
rice husk ash, living or dead microbial biomass etc. (Safarik et al.,
In the present research, the biosorption of crystal violet from aqueous
solution on leaf biomass of Calotropis procera has been investigated.
The phytochemistry of this plant has revealed that its leaves and stalks
contain calotropin, calotropagenin and phenolics. The aqueous extract
of the leaves contains D-glucose, D-arabinose, D-glucosamine and α-rhamnose.
Its latex contains uscharine, calotoxin, calactin, amyrin esters, uscharidin,
voruscharine, uzarigenin, syriogenin, proceroside and choline. The seeds
contain coroglaucigenin, frugoside and corotoxigenin. The root bark contains
benzoyllineolone and benzoylisolineolane while the flowers contain calotropenyl
acetate and procesterol.
MATERIALS AND METHODS
This study was conducted at the Department of Biotechnology, University
of Malakand, Chakdara, Pakistan, in December 2007.
Preparation of Biosorbent
The leaves of Calotropis procera (giant milkweed or Sodom apple),
an evergreen perennial shrub of the family Asclepiadaceae, were collected
from its plants in Tazagram, a village near the University campus. The
leaf biomass of the plant was chosen as biosorbent material because of
its easy availability in greater amounts. The leaves were washed with
distilled water and sun dried for seven days. The dried leaves were crushed
and sieved to a final particle size of 120-160 μm. The powdered biomass
obtained in this manner was kept in an oven at 105°C for 80 min in
order to evaporate moisture.
A stock dye solution (100 mg L-1) was prepared by dissolving
100 mg of the pure dye in one liter of distilled water. Subsequent dye
solutions were prepared by dilution of the stock dye solution. The property
wise data of the dye is shown below.
||Basic violet 3
||593 (Gurr, 1971), 588 (Aldrich)
Effect of Contact Time
One Hundred milliliter dye solution (30 mg L-1) was shaken
at 120 rpm at 20°C with 1 g of the adsorbent in 250 mL caped Erlenmeyer
flasks for 80 min. The adsorption progress was monitored by measuring
the absorbance of the solution at various time intervals, 5, 10, 15, 20,
25, 30, 40, 50, 60 and 80 min. Each time a sample was taken out of the
being treated solution, it was filtered using a disposable syringe filter
(0.45 μm) and the absorbance of the filtrate was measured at the
λmax of the dye (588 nm) using a UV-Visible Spectrophotometer
(UV-1700 Shimadzu). The extent of adsorption or removal of the dye was
calculated from the decrease in absorbance of the solution by using the
||Initial absorbance of the solution
|| Absorbance of the solution at time (t)
Effect of Initial Dye Concentration
For investigating the effect of initial dye concentration on adsorption,
different initial dye concentrations (10, 20, 30, 40 and 50 mg L-1)
were used. The experiments were carried out as described earlier but the
treatment was done for 60 min, the equilibrium time.
Effect of Adsorbent Dose
Hundred milliliter of dye solution (30 mg L-1) was treated
with different amounts of the adsorbent (0.1, 0.2, 0.4, 0.6, 0.8, 1.0,
1.5, 2.0, 2.5 and 3.0 g) for 60 min and the above mentioned procedure
All the experiments were carried out in duplicate and mean values were
RESULTS AND DISCUSSION
Effect of Contact Time
The adsorption data for the removal of crystal violet from aqueous
solution at a dye concentration of 30 mg L-1 (about 75 μM)
is shown in Fig. 1. The equilibrium time for the adsorption
of crystal violet on the adsorbent was 60 min (1 h). In the initial 5
min, the rate of adsorption was very rapid after which adsorption took
place gradually. Thus 57.76% of the dye was removed in the first 5 min.
The higher adsorption rate at the initial period (first 5 min) may be
due to a large number of vacant sites on the adsorbent surface available
at the initial stage (Uddin et al., 2007). As time passes, the
adsorption rate is decreased due to the accumulation of the dye molecules
in the vacant sites.
||Effect of contact time on the adsorption of crystal violet
onto Calotropis procera leaf biomass
||Effect of initial dye concentration on the adsorption of crystal
violet onto leaf biomass of Calotropis procera
Effect of Initial Dye Concentration
The adsorption of crystal violet, qe (mg g-1), was
calculated by using Eq. 1 (Mahvi et al., 2007).
||Adsorption density (mg of adsorbate adsorbed per g of adsorbent)
||Initial concentration of adsorbate (mg L-1)
||Equilibrium concentration of adsorbate (mg L-1)
||Volume of solution used (L)
||Mass of adsorbent used (g)
From the data, it is seen that the removal of crystal violet, qe
(mg g-1), increases with increasing initial dye concentration.
The effect of initial dye concentration on adsorption of crystal violet
onto leaf biomass of Calotropis procera is shown in Fig.
Effect of Adsorbent Dose
The percentage removal of crystal violet increased with increase of
adsorbent dose up to 1 g and then decreased. Figure 3
shows the effect of adsorbent dose on the adsorption of crystal violet
onto leaf biomass of Calotropis procera. The increase in percentage
removal with increase in adsorbent dose up to 1 g can be attributed to
increased adsorbent surface area and availability of more adsorption sites
resulting from the increase in adsorbent dose. The decrease in percentage
removal at adsorbent doses of more than 1 g may be due to aggregation
of the adsorbent particles thereby reducing the adsorbent surface area
and hence number of adsorption sites.
The equilibrium data for the adsorption of crystal violet onto leaf
biomass of Calotropis procera was analyzed by using the Langmuir
adsorption isotherm, which is the most widely used isotherm equation for
modeling of the adsorption data and is valid for monolayer adsorption
onto a surface with a finite number of identical sites. The Langmuir adsorption
isotherm equation is given by Eq. 2.
||Effect of adsorbent dose on the adsorption of crystal violet
on leaf biomass of Calotopis procera
||Langmuir plot for crystal violet adsorption from aqueous solution
on leaf biomass of Calotopis procera at 20°C
where, qo and KL are Langmuir parameters related
to maximum adsorption capacity and free energy of adsorption, respectively.
Ce is the equilibrium concentration in the aqueous solution
and qe is the equilibrium adsorption capacity of adsorbent.
The linearized form of Langmuir equation can be written as:
Straight line was obtained by plotting Ce/qe vs.
Ce as shown in Fig. 4. The applicability
of the Langmuir isotherm indicates good monolayer coverage of the dye
molecules on the surface of the leaf biomass of Calotropis procera,
which consequently suggests the formation of monolayer coverage of adsorbate
on the adsorbent surface in the concentration range studied. Langmuir
constants, qo and KL
||Lagergren plot for crystal violet adsorption on leaf biomass
of Calotropis procera at 20°C
were calculated from the slope and intercept of plot of Ce
/ qe vs. Ce, respectively. The values of qo
and KL are 4.14 mg g-1 and 0.1139 l mg-1,
respectively. Thus the maximum adsorption capacity of the leaf biomass
of Calotropis procera was found to be 4.14 mg g-1.
In order to analyze the biosorption kinetics of crystal violet, the
first order kinetic model was applied to the experimental data. The first
order rate expression of Lagergren can be expressed as:
where, qe and qt are the amounts of adsorbate adsorbed
(mg g-1) at equilibrium and at time t, respectively and k is
the overall rate constant. Straight line was obtained by plotting log
(qe – qt) vs. t as shown in Fig.
5. This indicates that crystal violet adsorption onto leaf biomass
of Calotropis procera follows first order kinetics. The value of
rate constant, k, was calculated from the slope of the plot of log (qe–qt)
vs. t. The value of k was found to be 0.0322 min-1.
Adsorption Mechanism, Nature of Adsorbent Sites and Type of Interaction
Between Sites and Crystal Violet
The FT-IR studies of the biomass of Calotropis procera have
confirmed the presence of aromatic groups, hydroxyl groups and carboxylic
groups (Pandey et al., 2007). The biosorption of crystal violet
on the leaf biomass of the plant may likely be due to electrostatic attraction
between these groups and the cationic dye molecules (CV+).
At pH above 4, the carboxyl groups are deprotonated and as such are negatively
charged. These negatively charged carboxylate ligands (-COO–)
can attract the positively charged crystal violet molecules and binding
can occur. Thus the CV+ binding to the biomass may be an ion-exchange
mechanism, which may involve electrostatic interaction between the negatively
charged groups in the cell walls and the dye cationic molecules.
The performance of the leaf biomass of Calotropis procera
as biosorbent for textile dyes can be evaluated by comparing its dye uptake
capacity with other biosorbents. Removal of crystal violet from aqueous
solution at different concentrations, pH and temperatures by neem saw
dust has been carried out. The percentage of dye adsorbed was found to
be 91.56 for the dye at a dye concentration of 6 mg L-1 at
a temperature of 30±1°C and pH 7.2 (Khattri and Singh, 2000).
Eren and Afsin (2006) investigated the adsorption of crystal violet from
aqueous solution onto raw and pre-treated bentonite surfaces. According
to their results, the amounts of crystal violet (CV+) adsorbed
at equilibrium at 298 K were 0.27, 0.37, 0.49 and 0.54 mmol g-1
bentonite for the raw, Ni-, Zn- and Co-saturated bentonite samples, respectively.
The biological decolorization of a structurally related dye, malachite
green using microalgae Cosmarium sp. was investigated by Daneshvar
et al. (2007). Their results show that the algae (4.5x106
cells mL-1) removed 80% of the dye at a dye concentration of
10 ppm at pH 9 and 25°C. Recently Ncibi et al. (2007) have
studied the adsorptive removal of another dye, reactive red 228 using
Posidonia oceanica fibrous biomass. According to them, the qo
(mg g-1) at pH 5 and 30°C using the Langmuir model is 5.74.
From the present study, it can be concluded that the leaf biomass of
Calotropis procera is a potentially good adsorbent for the removal
of crystal violet (a cationic dye) from aqueous solution. The cation binding
capacity of the adsorbent biomass can be enhanced by its treatment with
alkalis like sodium hydroxide. Most plant tissues have cellulose, hemicellulose
and lignin as their major constituents. These constituents contain methyl
esters, which do not bind metal ions (cations) significantly. The methyl
esters can be converted to carboxylate ligands by treatment of a biomass
with alkalis like sodium hydroxide. The resulting carboxylate ligands
are believed to be responsible for binding metal ions (cations) (Rehman
et al., 2006).