Open Journal of Metal, 2013, 3, 62-67
http://dx.doi.org/10.4236/ojmetal.2013.32A1008 Published Online July 2013 (http://www.scirp.org/journal/ojmetal)
Comparative Study on Heavy Metals Biosorption
by Different Types of Bacteria
Eteri Gelagutashvili
Andronikashvili Institute of Physics, Ivane Javakhishvili Tbilisi State University, Tbilisi, Georgia
Received April 29, 2013; revised June 2, 2013; accepted June 12, 2013
Copyright © 2013 Eteri Gelagutashvili. This is an open access article distributed under the Creative Commons Attribution License,
which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
ABSTRACT
Biosorption of Cd(II), Ag(I) and Au(III) by cyanobacteria Spirulina platensis, of Au(II)—by Streptomyces spp. 19H,
and of Cr(VI) and Cr(III)—by Arthrobacter species was studied by using the dialysis and atomic absorption analysis
under various conditions. In particular, the impact of the following parameters on biosorption was studied: pH (for Ag,
Cd, Au), living and non-living cells (for Cr), heavy metal valence (for Cr), homogenized and non-homogenized cells
(for Au), Zn(II) ions (on Cr(VI)—Arthrobacter species). It was shown that biosorption efficiency of Cr(III), Cr(VI),
Cd(II), Au(III) and Ag(I) ions is likely to depend on the type of bacteria used as well as on the conditions under which
the uptake processes proceeded. It was shown that metal removal by microorganisms was influenced by physical-
chemical parameters. The pH value of 7.0 was optimum for the removal of Ag(I) and Cd(II) by Spirulina platensis. At a
low pH value of 5.5, Au (III) was by test algae more efficiently than Cd(II) and Ag(I).
Keywords: Biosorption; Heavy Metals; Cyanobacteria; Actinomycetes
1. Introduction
Biosorption can be defined as the removal of metals from
the solution by biological material.
The biosorbents can be bacteria, microalgae etc. Heavy
metal pollution represents an important environmental
problem because of toxic effects of metals. Interest in the
development of metal removal by biosorption using mi-
croorganisms is shown in literature [1-11]. Cyanobacteria
(blue green algae) represent the largest and most diverse
group of photosynthetic prokaryotes. They are excellent
organisms to serve as a model for the investigation of a
wide variety of biological problems, including the indi-
cators of environmental pollution. Spirulina platensis is a
cyanobacterial species that can potentiate the immune
system, leading to suppression of cancer development
and viral infection [1]. The cyano-bacteria Spirulina pla-
tensis has recently been reported to accumulate multiple
metals and to be a hyperaccumulator of Cd and Pb [2].
The interest in Cr is governed by the fact that its toxicity
depends critically on its oxidation state. While Cr(III) is
considered essential for lipid and protein metabolism,
Cr(VI) is known to be toxic to humans [7]. Gram-posi-
tive Arthrobacter bacteria can reduce Cr(VI) to Cr(III)
under aerobic growth, and there is great interest in Cr-
reducing bacteria. Silver nitrate was tested for antimicro-
bial activity in vitro. The bactericidal action of silver
with its broad spectrum of activity including bacterial,
fungal and viral agents is well known [8]. Silver ions
block the respiratory chain of microorganisms. Some
bacteria have evolved the mechanisms of detoxication of
heavy metals, and some even use them for respiration. In
[9] the recovery of gold using algae cells was investi-
gated, and in [10,11] the microorganism-gold interaction
was studied. The investigation of the efficacy of the
metal uptake by the microbial biomass is essential for the
industrial application of biosorption.
In spite of the fact that recently the interest in studying
the biosorption of metals by microorganisms has in-
creased, the exact mechanism by which microorganisms
take up the metal is still relatively unclear.
The objective of this paper is to study the influence of
different conditions on the biosorption of heavy metals
Cd, Ag, Cr, Au by actinomycetes: (Streptomyces spp.
19H), Actinomycetes belonging to Arthrobacter genera
(Arthrobacter oxidas and Arthrobacter globiformis), and
microalgae (Spirulina platensis).
2. Materials and Methods
Analytical grade reagents were used in all experiments:
K2CrO4, CrCl3, ZnSO4, CdS04, AgNO3, HAuCl4. Actin-
C
opyright © 2013 SciRes. OJMetal

E. GELAGUTASHVILI 63
omycetes belonging to Arthrobacter genera—Arthrobac-
ter globiformis 151B and Arthrobacter oxydans 61B (iso-
lated from the basalt rocks collected in the Kazreti region
of Georgia), blue-green algae Spirulina platensis (strain
IPPAS B-256) and actinomycetes Streptomyces spp. 19H
(isolated from the rhizosphere of soybeans grown in
Georgia) were used [12-14].
Arthrobacter bacteria were cultivated in the nutrient
medium without co-cations and loaded with Zn (50 mg/l)
(in the case of Cr(VI)) [12]. The cells were centrifuged at
12,000 rpm for 10 min and washed three times with
phosphate buffer (pH 7.1). The centrifuged cells were
dried without a supernatant solution until constant weight.
After solidification (dehydration) of cells (dry weight),
the solutions for dialysis were prepared by dissolving in
phosphate buffer. This buffer was used in all experi-
ments.
Spirulina platensis IPPAS B-256 strain from Timiri-
azev Institute of Plant Physiology of the Russian Acad-
emy of Sciences was cultivated in a standard Zaroukh
alkaline water-salt medium at 34˚C, illumination ~5000
lux, initial рН 8.7 and at constant mixing [13].
To study the biosorption process on the bacterial cells,
the methods of dialysis and atomic absorption analysis
were used. A known quantity of dried bacteria suspen-
sion was contacted with the solution containing a known
concentration of metal ions. The experiments of dialysis
were carried out in 5-ml cylindrical vessels made of or-
ganic glass. A cellophane membrane 30 m wide (type—
Visking, manufacturer—Serva) was used as a partition.
The duration of dialysis was 72 hours. The metal concen-
tration after the dialysis was measured by using the
atomic absorption spectrophotometer Analyst-900 at the
wavelength of  = 328.1 nm (Ag),  = 242.8 (Au) nm, 
= 357.9 nm (Cr).
 = 228 nm (Cd). For biosorption iso-
therm studies, the dry cell weight was kept constant (1
mg/ml), while the initial metal concentration in each
sample was varied in the interval 10‒3 - 10‒6 M. All ex-
periments were carried out at ambient temperature.
Data Analysis. The isotherm data were characterized
by the Freundlich equation [15]. 1n
bt
, where Cb is
the metal concentration adsorbed on either live or dried
cells of bacteria in mgg−1 dry weight, Ct is the equilib-
rium concentration of metal (mgl‒1) in the solution, K is
the empirical constant that provides an indication logCb
as a function of logCt of the adsorption capacity of either
live or dry cells, 1/n is the empirical constant that pro-
vides an indication of the intensity of adsorption. The
adsorption isotherms were obtained by plotting logCb as
a function of logCt.
CKC
3. Results and Discussion
Biosorption of heavy metals Cd(II), Ag(I),Cr(III),Cr(VI)
Au(III) by the blue-green algae Spirulina platensis, ac-
tinomycetes belonging to Arthrobacter genera—Arthro-
bacter globiformis 151B and Arthrobacter oxydans 61B,
few new bacterial strains of actinomycetes Streptomyces
spp. 19 H were studied at different conditions. In par-
ticular, the impact of the following parameters on bio-
sorption was studied for the microorganism-metal inter-
action: pH (for Ag, Cd, Au), living and non-living cells
(for Cr), heavy metal valence (for Cr), homogenized and
non-homogenized cells (for Au), Zn(II) ions (on Cr(VI)-
Arthrobacter species). Figure 1 shows the biosorption
isotherms as an example of Cd(II)-Spirulina platensis
interaction at different pH values. For A, B and C cases,
cyanobacteria were dissolved in phosphate buffer (pH
7.0), in nutrient medium (pH 8.6) and in water (pH 5.5),
respectively. The results of equilibrium batch sorption
experiments resulted are shown by the dots in the bio-
sorption isotherms, which were approximated by the
Freundlich model. Each dot is the average of three inde-
pendent values, and the standard deviation is less than
13% of the average value. In all cases, the correlation
between the experimental and the theoretical data is ob-
vious (R is more than 0.9). By means of Freundlich iso-
therms the biosorption constant (K) and the capacity (n)
were determined for Cd(II) Spirulina platensis. Similar
data were obtained for Ag(I) and Au(III) Spirulina plat-
ensis. The results of equilibrium sorption experiments are
listed in Table 1.
As seen from Table 1, Spirulina platensis showed dif-
ferent binding patterns for Cd(II), Ag(I) and Au(III). In
particular, the biosorption parameters changed with
changing pH value. Namely, in the case of high pH value
(pH 8.6), the biosorption constant of Ag(I) Spirulina
platensis K = 9.4  10‒4 exceeded the biosorption con-
-3.0 -2.5 -2.0 -1.5 -1.0 -0.50.00.5
-5.0
-4.5
-4.0
-3.5
-3.0
-2.5 Cd-Spirulina platensis
-3.0 -2.5 -2.0 -1.5 -1.0 -0.50.00.5
-4.8
-4.4
-4.0
-3.6
-3.2
-2.8
logCb
logCt
B
-3.0 -2.5 -2.0 -1.5-1.0 -0.50.00.5
-4.4
-4.0
-3.6
-3.2
logCb
logCt
C
A
logCb
logCt
Figure 1. Biosorption isotherms for Cd(II) Spirulina platen-
sis at different pH obtained by using the fitted Freundlich
odel. m

E. GELAGUTASHVILI
64
Table 1. Biosorption constants for Cd, Au and Ag biosorption by Spirulina platensis.
na platensis dissolved Spirulina platensis dissolved in the Spirulina platensis dissolved in the Spiruli
medium pH 8.6 phosphate buffer pH 7.0 in water pH 5.5
Biosorption constant K Ag 9.4  10‒4 13.0  10‒4 2.9  10‒4
Cd 5.1  10‒4 8.3  10‒4 3.6  10‒4
Au 8.7  10‒4 2.07  10‒4 4.87  10‒4
Biosorption capacity n
Correlation coefficient R Ag
Ag 1.67 5.27 2.78
Cd 1.82 1.45 2.7
Au 1.58 1.39 0.93
0.92 0.97 0.97
Cd 0.98 0.96 0.97
Au 0.96 0.97 0.98
tant for pH 5.5 (K = 2.9  10‒4), and the capacity dif- metal biosorption and the biosorption capacity of P.
tie
s
fered by a factor of 1.5. The pH dependence was also
obtained for Cd(II) and Au(III) Spirulina platensis. The
absorption of Cd(II) and Ag(I) increased at pH 7.0, while
Au(III) had very low sorption at pH 7.0. Gold(III) was
more effectively adsorbed by test algae at low pH value
(pH 5.5) than Cd(II) and Ag(I). This can be explained by
the fact that, as the pH value decreased, the surface
charge on the algae cells became positive, and the inter-
action of Au(III) in anionic forms with the binding sites
was primarily electrostatic. This would lead to electro-
static attractions between positively charged cations (Ag,
Cd) and negatively charged binding sites and hence to
the rapid increase in binding efficiency in the range pH
5.5 to pH 7.0. The maximum uptake of cadmium and sil-
ver biosorption was observed at pH 7.0. On the other
hand, the optimum biosorption for gold(III) was ob-
served at pH 8.6. Hence we can assume that the effect of
pH is related to different protonation of the functional
groups present in the cell membrane. They are several
chemical groups that attract metals in biomass: mainly
carboxyls and sulphates in polysaccharides of marine
algae. In [16] it was hypothesized that Cd, Cu and Co
biosorption by dead biomass of algae takes place through
electrostatic interactions between the metal ions in solu-
tion and the cell walls of microbial cells.
In acidic conditions, the absorption capacity was found
to be very low for gold (n = 0.93), but high for Cd and
Ag (2.7 and 2.8, respectively). For other pH values, the
difference between the capacities is insignificant. The
difference in the magnitude of capacity of metal ion
binding may be related to the properties of the algae and
the properties of the metal sorbents. Carboxylic, sulphy-
dryl, phosphate and thiol groups differ in their affinity
and specificity for metal binding.
Thus, the pH value exerted the most important effect
on the biosorption of metal ions. Similar results were
reported in [17], where it was revealed that the solution
pH and ionic strength were very important factors in the
aeruginosa AT18 for Cr(III), Cu(II), Mn(II) and Zn(II).
In this case, the biosorption increased with the increasing
pH value in the range pH 5.46 - 7.72. The metal uptake
capacity of Egyptian marine algae was studied using the
species of green and brown algae, namely, Ulva lactuca
L. and Sargassum la tifolium (Turner) C. Agardh, respec-
tively. The biosorption efficiency of Cu(II), Co(II), Ni(II),
Cd(II), Hg(II), Ag(I) and Pb(II) ions seems to depend on
the type of algae used as well as on the conditions [18].
In work [19], the potential of green marine macroalgae
for removal of Cd, Hg and Pb from aqueous solutions
was assessed. The results obtained in that study indicated
the highest absorption ability of Chaetomorpha sp. for
Cd and Pb.
The metal-removing ability of living or dry cells of
bacteria (Arthrobacter globifo rmis and Arthrobacter oxi-
das) was studied as a function of metal concentration.
The linearized absorption isotherms of Cr ions in anion
and cation forms for two kinds of Arthrobacter were ob-
tained by fitting the experimental dots. The Freundlich
parameters evaluated from the isotherms with the corre-
lation coefficients are given in Table 2. The data in Ta-
ble 2 show significant difference between the binding
constants for Cr(VI)-Arthrobacter oxidas and Cr(VI)-
Arthrobacter glob iformis. The decrease in bioavailability
was observed experimentally for Cr(VI)-Arthrobacter
globiformis as compared with Arthrobacter oxidas. It
was shown that the carboxyl groups were the main bind-
ing sites in the cell wall of gram-positive bacteria [20].
Such bond formation could be accompanied by the dis-
placement of protons and is dependent in part on the ex-
tent of protonation, which is determined by the pH value
[21].
Our results indicated that Cr(VI) sorption depended on
the species of Arthrobacter bacteria. The difference be-
tween Arthrobacter species in metal ion binding may be
related to the properties of metal sorbates and the proper-
s of bacteria (functional groups, structure and surface

E. GELAGUTASHVILI 65
Table 2. Biosorption characteristics for Cr(VI) and Cr(III) robater oxidas and Cr(VI) and Cr(III) Arthrobacter globi-
formis. Arth
Cr(VI) Cr(III)
B‒4‒4 iosorption characteristics (K, n) K  10 n R K  10 n R
Arthrobater oxidas (dry cells) 1. 0 1 0
A1.
A
A
4.6 25.98 26.0 .37.98
rthrobacter globiformis (dry cells) 3.4 35 0.96 20.2 1.23 0.98
rthrobater oxidas (living cells)1.0 1.25 0.94 - - -
rthrobacter globiformis (living cells) 1.3 1.35 0.91 - - -
Arthrobater oxidas + Zn(II) 6.6 1.08 0.98 - - -
Arthrobacter globiformis + Zn(II) 8.11.19 0.96 - - -
area, vaFunctional 2]
such ydryl, phate and
iol groups, differ in their affinity and specifity for
e n values are the same in both
ca
ria.
Th
creased in the presence of Zn ions in both cases (for
Cr(VI)-Abacter giformis anfor Cr(VI)-thro-
bacter oxidas). But for Cr(VI)-Arthrobacter globiformis
et cations by bac-
te
re equal to: K = 8.2 × 10‒4, n = 1.64 (gold-
rying in the species). groups [2,
as amino, carboxylic, suphosph
th
metal binding. The n values which reflect the intensity of
sorption represent the same trend, but, as obvious from
Table 2, for both Arthrobacter species the n values differ
insignificantly, and their sorption intensity indicators are
small (1.08 - 1.47).
Comparative Freundlich biosorption characteristics of
the Cr(VI)-Arthrobacter species in living and dry cells
(Table 2) show that th
ses. The dry cells have higher biosorption constants for
both species (K) (4.6 × 10‒4, 3.4 × 10‒4) than the living
ones (1.0 × 10‒4, 1.36 × 10‒4). This may confirm the hy-
pothesis that the metal sorption by these bacteria is inde-
pendent of the metabolic state of the organism [23].
The comparison of Cr(VI)- and Cr(III)-Arthrobacter
species interactions (Table 2) showed that Cr(III) was
absorbed more effectively than Cr(VI) by both bacte
e absorption capacity is the same for both chromium-
Arthrobacter systems. The biosorption constants for Cr(III)
are higher than for Cr(VI) by a factor of 5.65 - 5.88 for
both species. Cr(VI) is one of the most stable oxidation
states, the others being chromium(II) and chromium(III).
Cr(VI) can be reduced to Cr(III) by the biomass through
two different mechanisms [24]. The “uptake-reduction”
model for Chromium(VI) carcinogenicity is that tetrahe-
dral chromate is actively transported across the cell mem-
brane. Chromium(III) is not actively transported across
the cell membrane to lack of transport mechanisms for
these octahedral complexes. Thus, Cr(VI) may be ab-
sorbed by bacteria at a much lower degree than Cr(III).
The above mentioned is in good agreement with litera-
ture data according to which there is a significant differ-
ence in the efficiency of absorption in each species of
microorganisms, since the sorption depends on the nature
and the composition of the cell wall [25]. On the other
hand, gram-positive bacteria have greater sorptive capac-
ity due to their thicker layer of peptidoglycan, which
contains numerous sorptive sites [26].
It is seen from Table 2 that the bioavailability in-
this increase was more significant. The presence of other
cations increased the uptake of the targ
rthro lobd Ar
ria. Such an effect of the other cation (Zn(II)) suggests
that at least ion exchange is one of the mechanisms re-
sponsible for metal uptake by such Arthrobacter species.
This has implications for the selection of Arthrobacter
species for indrustrial applications. Biosorption is often
accompanied by a slower metal binding process in which
the additional metal ion is bound, often irreversibly. This
slow phase of metal uptake can be due to a number of
mechanisms, including covalent bonding, crystallization
on the cell surface or, most often, diffusion into the cell
interior and binding to proteins and other intercellular
sites [27].
In Figure 2 are shown biosorption isotherms for gold-
Streptomyces spp. 19 H cells (1—homogenized cells and
2—partially homogenized cells). By means of Freundlich
isotherms the biosorption constants (K) and the capacity
(n) were determined for gold-Streptomyces spp. 19H
cells. They a
-3.0 -2.5 -2.0 -1.5 -1.0 -0.50.00.5
-5.0
-4.5
-4.0
-3.5
-3.0
-2.5 gold _ Streptomyces sp p. 19H
1 - homo genized cells
2- particulate homogenized cells
1
logCb
logCt
2
Figure 2. Linearized Freundlich absorption isotherms for
Au(III) Streptomyces spp. 19H cells.

E. GELAGUTASHVILI
66
Streptomyces spp. 19H-homogenized cells) and K = 4.8
× 10‒4, n = 1.75 (gold-Streptomyces spp. 19H-partially
homogenized cells). It is clear that the biosorption c
stant for homogenized cells is greater than for particulate
homogenized cells, and in both cases the sorptive capac-
ity is greater.
Thus, different species of bacteria displayed differen
sorptive relationships. The obtained biosorption data are
in good agreement with the literature data according to
which biological ligands are generally polyfunctional
polyelectrolytic, with an average pK value within
6.0 [28].
4. Conclusion
The biosorption of four different bacteria: cyanobac er
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