04-05 I кор.
Доклади на Българската академия на науките
Comptes rendus de l’Acad´emie bulgare des Sciences
Tome 70, No 4, 2017
BIOSORPTION OF Cd2+ IONS USING MENTHA SPICATA
L. AND RUTA GRAVEOLENS L.
Paunka S. Vassileva, Albena K. Detcheva, Lidia P. Ivanova,
Snezhanka K. Evtimova
(Submitted by Academician D. Klissurski on December 14, 2016)
The feasibility of using Mentha spicata L. (MS) and Ruta graveolens L.
(RG). for the removal of Cd2+ ions from aqueous solution was investigated.
Batch experiments were performed to evaluate the eﬀect of contact time, acid-
ity and initial metal concentration on Cd2+ removal from aqueous solutions.
Equilibrium experimental data were ﬁtted to linear Langmuir and Freundlich
models. It was established that Freundlich isotherm most adequately described
the adsorption process. Pseudo-ﬁrst order, pseudo-second order and intraparti-
cle diﬀusion models were used to analyze kinetic data. Maximum biosorption
capacities for MS and RG were found to be 19.23 mg/g and 16.95 mg/g re-
spectively. The study revealed that both plants MS and RG could be used as
eﬀective biosorbents for the removal of Cd2+ ions from aqueous media.
Key words: Mentha spicata L., Ruta graveolens L., biosorption of cad-
mium ions, adsorption isotherms
Introduction. Cadmium is a very toxic element aﬀecting environment and
humans. Pollution by cadmium usually comes from several industrial processes
such as electro-plating, plastics manufacturing, nickel-cadmium batteries, fertiliz-
ers, pigments, mining and metallurgical processes [1, 2]. Cadmium is transferred
to humans via the food chain. Cadmium accumulates mainly in the kidneys and
liver, but is also found in skeletal and muscular systems, and in the endocrine
glands . On the other hand, the International Agency for Research on Cancer
has classiﬁed cadmium as the most probable carcinogen to man . Therefore,
cadmium being a threat to living organisms has to be eliminated from industrial
wastewaters before discharging it into the environment.
The technologies employed for treatment of wastewaters containing cadmium
as chemical precipitation, ion exchange, solvent extraction, membrane technolo-
gies [1, 4] are expensive and incapable of meeting strict water quality standards
currently being imposed by public health authorities. The search for new tech-
nologies involving removal of toxic metals from wastewaters has directed attention
to biosorption, based on metal binding capacities of various biological materials.
The major advantages of biosorption over conventional treatment methods in-
clude: low cost, high eﬃciency, minimization of chemical and biological sludge,
no additional nutrient requirement, regeneration of biosorbent and possibility of
metal recovery. The removal of heavy metal ions by biosorption using biological
materials has been widely studied in the last decade due to its potential, partic-
ularly in wastewater treatment. In this aspect, some plant products [1, 4–10] have
been studied for the removal of Cd2+ from polluted waters. No data concerning
the adsorption of Cd2+ ions on the plants Mentha spicata L. and Ruta graveolens
L. have been reported in the literature so far.
The present study explores the utilization of Mentha spicata L. and Ruta
graveolens L. available in abundance as potential biosorbents for Cd2+ ions re-
moval from aqueous solutions.
Experimental. Materials. The leaves of Mentha spicata L. (denoted as
MS) and Ruta graveolens L. (denoted as RG) were washed with distilled water
several times to remove the surface adhered particles and water soluble particles
and dried at 60 ◦C in an electric oven for 48 h. The materials were then grinded
manually in an agate mortar to a size below 0.1 mm. No other physical or chemical
treatment was performed on the materials thus obtained.
Adsorption studies. The adsorption properties of both biosorbents with
respect to Cd2+ ions were determined by means of the batch method. Experi-
ments were carried out using stoppered 50 mL Erlenmeyer ﬂasks containing about
0.2 g ash sample and 20 mL of aqueous solution of Cd2+ ions. The mixture was
shaken at room temperature (22 ◦C) by an automatic shaker. On reaching equi-
librium, the material was removed by ﬁltration through a Millipore ﬁlter (0.2 µm).
The initial and equilibrium concentrations of the cadmium ions were determined
by ﬂame AAS on a Pye Unicam SP 192 ﬂame atomic absorption spectrometer
The eﬀect of acidity on Cd2+ removal eﬃciency of the studied materials
was investigated over the pH range 1.7–5.0 (pH-meter model pH 211, Hanna
instruments, Germany) employing an initial concentration of 300 mg L−1for
both materials. To determine the eﬀect of the initial metal ion concentration on
the adsorption capacity Cd2+ concentrations in the range 50-300 mg L−1at pH
5.0 were chosen.
Deionized water and analytical grade reagents were used in all experiments.
Working standard solutions of Cd2+ ions were prepared by stepwise dilution of
stock solution with the concentration of 1 g L−1(Merck, Darmstadt, Germany).
498 P. S. Vassileva, A. K. Detcheva, L. P. Ivanova et al.
All adsorption experiments were replicated and the average results were used in
Results and discussion. Eﬀect of pH. The acidity of the aqueous solution
is an important controlling parameter in biosorption processes. The amounts of
adsorbed Cd2+ ions onto the studied materials as a function of pH of initial
solutions are presented in Fig. 1(a). As expected, the pH of the solution had a
signiﬁcant eﬀect on Cd2+ adsorption for both samples. Upon increasing the pH
values the amount of adsorbed cadmium ions strongly increases to pH about 3
and then remains constant. Therefore the optimum pH value was found to be
at pH 3–5 for both samples. The observed decrease in the uptake at lower pH
values may be attributed to the partial protonation of the active groups on the
surface of biomaterial particles and the competition of H3O+ions with Cd2+ ions
for adsorption sites on the biosorbents. With increasing pH, the competing eﬀect
of hydronium ions decreases and the positively charged Cd2+ ions adsorb on the
free binding sites of the biosorbents. This trend was observed in many cases of
adsorption of metal cations on solid surface. The dependence of metal uptake
on pH is related to both the surface functional groups present on the biomass
and the metal chemistry in solution [11, 12 ]. At low pH, the surface ligands are
closely associated with the hydronium ions (H3O+) and restricted the approach
of metal cations as a result of the repulsive force . Furthermore, the pH
dependency on the metal ions uptake by biomasses can also be justiﬁed by the
association-dissociation of certain functional groups, such as the carboxyl and
hydroxyl groups present on the biomass. In fact, it is known that at low pH,
most of the carboxylic groups are not dissociated and cannot bind the metal ions
in solution, although they take part in complexation reactions .
1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0 5.5
0.0 0.1 0.2 0.3 0.4 0.5
Fig. 1. Eﬀect of pH (a) and contact time (b) on the amount Cd2+ ions adsorbed on MS and
Compt. rend. Acad. bulg. Sci., 70, No 4, 2017 499
In hydrochloric acid medium (pH <1.75), the cadmium ions are not ad-
sorbed. Hence, hydrochloric acid can be used as eluting solution.
Kinetic studies. The eﬀect of contact time on the amount of adsorbed
Cd2+ ions with initial concentrations of 300 mg L−1was studied at pH 3.0. The
relevant data are presented in Fig. 1(b). The extent of Cd2+ removal by both
biomaterials was found to increase with the increase in agitation time and reached
a maximum value within 10 min. This short time period required to attain
equilibrium suggests an excellent aﬃnity of the investigated biosorbents towards
Cd2+ ions from aqueous solution.
In order to determine the rate-controlling mechanism of the adsorption pro-
cess, two kinetic models were applied to the experimental data. The pseudo-ﬁrst-
order  and pseudo-second-order  rate equations are as follows:
(1) log(Qe−Qt) = log(Qe)−(k1/2.303)t
(2) (t/Qt) = (1/k2Qe) + (1/Qe)t
where Qt= amount of Cd2+ ions adsorbed for a certain time t(mg g−1) and
k1= rate constant of pseudo-ﬁrst-order adsorption (min−1); Qe= equilibrium
adsorption capacity (mg g−1) and k2= rate constant of pseudo-second-order
adsorption (g mg−1min−1).
Table 1 shows the kinetic parameters for Cd2+ adsorption on all studied
plant samples. The theoretical Qevalues calculated from the pseudo-ﬁrst-order
kinetic model diﬀer from the experimental values and the corresponding correla-
tion coeﬃcients were found to be lower than those for the pseudo-second-order
model (Table 1). On the other hand, the theoretical values obtained from the
pseudo-second-order kinetic model were very close to the experimental Qevalues.
Thus, we concluded that Cd2+ adsorption on all investigated materials could be
T a b l e 1
Kinetic parameters of adsorption of Cd2+ ions onto investigated biosorbents
kid (mg g−1min−1/2)
MS 6.52 3.98 0.9490 19.08 0.006 0.9998 4.606 16.73 0.9277
RG 4.60 3.21 0.6992 13.35 0.008 0.9998 7.000 9.94 0.7080
500 P. S. Vassileva, A. K. Detcheva, L. P. Ivanova et al.
described better by the pseudo-second-order kinetic mechanism. This indicated
that chemisorption was the rate-limiting mechanism through sharing or exchange
of an electron between biosorbent and adsorbate. Along with adsorption on the
outer adsorbent surface there was also possibility of intraparticle diﬀusion from
the outer surface into the pores of the studied materials. The intraparticle diﬀu-
sion is described [16 ] by the equation:
where C= intercept and kid = intraparticle diﬀusion rate constant (mg g−1
min1/2). In case of using this model, the plots Qtversus t1/2should be linear, if
the intraparticle diﬀusion was involved in the adsorption process. If these lines
pass through the origin, then the intraparticle diﬀusion is the rate-controlling
If the plots do not pass through the origin, the intraparticle diﬀusion is not
the only one rate-limiting step and some other kinetic models can simultaneously
inﬂuence the adsorption rate. The correlation coeﬃcients for the intraparticle
diﬀusion model were lower than those of the pseudo-second-order kinetic model.
These results conﬁrmed that the pseudo-second-order mechanism was predomi-
nant in the cases of adsorption of Cd2+ onto all investigated plant materials.
Adsorption isotherms. The equilibrium isotherms are very important in
designing adsorption systems. The adsorption isotherm describes the distribu-
tion of the adsorbed molecules between the liquid and solid phases at equilibrium
state. Concentration variation method is used to calculate the adsorption char-
acteristic of biosorbent and the process. The elucidation of isotherm data by
ﬁtting them to diﬀerent isotherm models is a substantial step in the adsorption
study. Experimental adsorption isotherms on both biomaterials are presented in
Fig. 2. The adsorption data were analyzed with the following linearized forms of
--20 0 20 40 60 80 100 120 140 160 180
Fig. 2. Adsorption isotherms of Cd2+ ions onto
4Compt. rend. Acad. bulg. Sci., 70, No 4, 2017 501
Langmuir and Freundlich isotherms:
Langmuir isotherm : Ce/Qe= 1/KLQ0+Ce/Q0
Freundlich isotherm : ln Qe= ln kF+ (1/n) ln Ce
where Ce= equilibrium concentration of metal ions (mg L−1); Qe= amount of
Cd2+ ions adsorbed (mg) per unit of mass of the adsorbent (g), Q0= adsorption
capacity (mg g−1); KL= Langmuir constant; kF= Freundlich constant; n=
intensity of adsorption. The values of Langmuir and Freundlich parameters were
obtained based on the linear correlation between the values of (i) (Ce/Qe) and
Ce and (ii) ln Qeand ln Ce, respectively (Table 2). The equilibrium adsorption
capacities were found to be 19.23 mg g−1, and 16.95 mg g−1for MS and RG,
respectively. Biosorbent MS showed higher adsorption eﬃciency towards Cd2+
T a b l e 2
Isotherm constants for Cd2+ adsorption onto investigated biomaterials
Langmuir parameters Freundlich parameters
(mg g−1) (L mg−1) (mg1−nLng−1) (L mg−1)
MS 19.23 0.063 0.8874 2.00 2.213 0.9877
RG 16.95 0.012 0.7599 0.73 1.562 0.9618
The Langmuir type of model is based on the assumption that all adsorption
sites are “equally active”, the surface is energetically homogeneous and a mono-
layer surface coverage is formed without any interaction between the adsorbed
molecules. The Freundlich type of model is valid for heterogeneous surfaces and
predicts an increase in the concentration of the ionic species, adsorbed on the
surface of the solid when the concentration of certain species in the liquid phase
is increased .
The correlation coeﬃcients r2(greater than 0.96) proved that the Freundlich
model was more adequate in describing the adsorption processes. It can therefore
be concluded that there are interactions between the adsorbed Cd2+ ions and, on
the other hand, the adsorption sites are distributed exponentially with respect to
the heat of adsorption. It also conﬁrms the existence of diﬀerent types of possible
adsorption sites on adsorbent surface with considerable diﬀerence in energy, e.g.
if the site is on an edge or is located on a defect position. The n-values are 2.21
and 1.56 for MS and RG, respectively, which indicates favourable adsorption onto
both investigated biomaterials [19, 20].
Conclusions. The biosorption of Cd2+ using dried Mentha spicata L. and
Ruta graveolens L. biomasses was studied using the batch method. The sorption
of metal ions rapidly reached the equilibrium within 10 min. The inﬂuence of
acidity of initial solutions on their adsorption was investigated and the optimum
502 P. S. Vassileva, A. K. Detcheva, L. P. Ivanova et al.
pH range is found to be above 3.0. The results from the kinetic studies conﬁrmed
that the pseudo-second-order mechanism was predominant for the Cd2+ adsorp-
tion on both investigated biomaterials. The adsorption isotherm of the Cd2+
ions exhibits mainly Freundlich behaviour, which indicates heterogeneous surface
binding. The maximum adsorption capacities were calculated. The highest equi-
librium adsorption capacity was registered for the biosorbent MS. Nevertheless,
both biomaterials could be used as eﬀective biosorbents for Cd2+ removal from
contaminated aqueous solutions.
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Institute of General and Inorganic Chemistry
Bulgarian Academy of Sciences
Acad. G. Bonchev St, Bl. 11
1113 Soﬁa, Bulgaria
504 P. S. Vassileva, A. K. Detcheva, L. P. Ivanova et al.