Colloids and Surfaces B: Biointerfaces 94 (2012) 362–368
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Colloids and Surfaces B: Biointerfaces
journal homepage: www.elsevier.com/locate/colsurfb
Isotherm kinetics of Cr(III) removal by non-viable cells of Acinetobacter
haemolyticus
Siti Khairunnisa Yahya a , Zainul Akmar Zakaria b , Jefri Samin c , A.S. Santhana Raj d , Wan Azlina Ahmad a,∗
a
Department of Chemistry, Faculty of Science, Universiti Teknologi Malaysia, 81310 UTM Johor Bahru, Johor, Malaysia
Institute of Bioproduct Development, Universiti Teknologi Malaysia, 81310 UTM Johor Bahru, Johor, Malaysia
Materials Science Laboratory, Faculty of Mechanical Engineering, Universiti Teknologi Malaysia, 81310 UTM Johor Bahru, Johor, Malaysia
d
Electron Microscopy Unit, Institute for Medical Research, Jalan Pahang, 50588 Kuala Lumpur, Malaysia
b
c
a r t i c l e
i n f o
Article history:
Received 28 September 2011
Received in revised form 27 January 2012
Accepted 14 February 2012
Available online 22 February 2012
Keywords:
Biosorption
Cr(III)
Acinetobacter
Removal
Wastewater
FTIR
TEM
a b s t r a c t
The potential use of non-viable biomass of a Gram negative bacterium i.e. Acinetobacter haemolyticus to remove Cr(III) species from aqueous environment was investigated. Highest Cr(III) removal of
198.80 mg g−1 was obtained at pH 5, biomass dosage of 15 mg cell dry weight, initial Cr(III) of 100 mg L−1
and 30 min of contact time. The Langmuir and Freundlich models fit the experimental data (R2 > 0.95)
while the kinetic data was best described using the pseudo second-order kinetic model (R2 > 0.99). Cr(III)
was successfully recovered from the bacterial biomass using either 1 M of CH3 COOH, HNO3 or H2 SO4
with 90% recovery. TEM and FTIR suggested the involvement of amine, carboxyl, hydroxyl and phosphate
groups during the biosorption of Cr(III) onto the cell surface of A. haemolyticus. A. haemolyticus was also
capable to remove 79.87 mg g−1 Cr(III) (around 22.75%) from raw leather tanning wastewater. This study
demonstrates the potential of using A. haemolyticus as biosorbent to remove Cr(III) from both synthetic
and industrial wastewater.
© 2012 Elsevier B.V. All rights reserved.
1. Introduction
Chromium is used in various industrial applications such as tanneries, electroplating, textile dying, wood preservation, petroleum
industry as well as metal finishing. Chromium is present in the
environment in either one of its two most stable forms i.e. the
highly soluble and toxic Cr(VI) or the less mobile and less toxic
Cr(III) [1]. Conventional methods to remove heavy metals are generally effective at metal concentrations greater than 100 mg L−1 . As
a consequence, these techniques may not be suitable to treat the
increasing volumes of wastewater containing low metal concentrations [2]. Therefore, it is imperative to develop a cost effective and
environmental-friendly treatment process. In recent years, much
of the research has been focused on the development of biological methods for the treatment of industrial effluents, some of
which are in the process of commercialization. From these methods, biosorption has been demonstrated to possess good potential
for the removal of heavy metals [3]. Amongst its advantages include
the reusability of biomaterial, low operating cost and improved
selectivity for specific metals of interest [4]. Abundant and inexpensive materials, such as algae, fungi, bacteria, chitosans and
∗ Corresponding author.
E-mail address: [email protected] (W.A. Ahmad).
0927-7765/$ – see front matter © 2012 Elsevier B.V. All rights reserved.
doi:10.1016/j.colsurfb.2012.02.016
zeolites have proven to have a reasonably high metal sequestering capacity which can be used as biosorbents as reported by other
researchers [5]. Therefore, it is the objective of this study to evaluate
the potential of using non-viable cells of Acinetobacter haemolyticus
(A. haemolyticus) as biosorbent for Cr(III) removal.
2. Materials and methods
2.1. Preparation of biosorbent
A. haemolyticus EF369508 was isolated from the Cr(VI)containing wastewater from a batek (textile-related) manufacturing premise in Kota Bharu, Kelantan, Malaysia [6]. The non-viable
bacterial biomass was prepared by harvesting the NB-grown bacterial cells at early stationery phase via centrifugation at 7000 rpm,
4 ◦ C for 5 min (B.Braun, SIGMA 4K-15). The supernatant was discarded while the pellet was suspended in minimal volume of
distilled deionized water (DDW) and inactivated by autoclaving at
121 ◦ C for 15 min.
2.2. Removal of Cr(III) from synthetic solution
The effect of some parameters affecting the biosorption process such as pH, biomass dosage, initial Cr(III) concentration and
contact time were evaluated as follows: 10 mg dry wt. of the
S.K. Yahya et al. / Colloids and Surfaces B: Biointerfaces 94 (2012) 362–368
Table 1
Physico-chemical characteristics of raw and treated leather tanning wastewater;
four different batches of sampling.
Parameters
Unit
pH
Suspended solid
Colour
Turbidity
COD
Cr(III)
Mg
K
Na
Zn
Pb
Cu
Co
Mn
–
mg L−1
ADMI
FAU
mg L−1
mg L−1
mg L−1
mg L−1
mg L−1
mg L−1
mg L−1
mg L−1
mg L−1
mg L−1
Raw
7.8–8.7
185.5–1067.0
167–8075
53–1100
1050–2626
121.5–20888
16.74–39.40
5.46–18.26
439.91–1788.98
1.27–4.05
0.78–0.26
0.010–0.196
0.010–0.196
0.15–5.67
Treated
6.0–6.3
2.0–56.0
17–248
2–31
111–482
1.91–6.05
13.37–39.40
10.73–26.39
889.60–1339.29
1.05–4.84
0.19–0.78
0.251–0.912
0.005–0.200
0.15–2.70
bacterial biomass suspension was transferred into a series of
250 mL Erlenmeyer flasks containing 50 mL of 50 mg L−1 Cr(III)
solutions at pH values between 4 and 6. The pH was varied using
either 1.0 M NaOH or 1.0 M HCl. The mixtures were then equilibrated for 24 h at 100 rpm in 25 ◦ C. Similar experimental setup was
used for the effect of biomass dosage where 100 mg L−1 of Cr(III)
solution was contacted with either 10, 15, 30 or 50 mg cell dry wt.
for 30 min. For the isotherm study, 5–200 mg L−1 Cr(III) was mixed
with 15 mg cell dry wt. at pH 5 and shaken at 100 rpm for 30 min. In
the kinetic study, 50 and 100 mg L−1 of Cr(III) was contacted with
15 mg cell dry wt. followed by periodical sampling. Total working
volume used in this study was 50 mL (mixture of Cr(III) solution and
biomass suspension). Cr(III) concentration was determined using
AAS. The stock solution for Cr(III), 1000 mg L−1 was prepared by
dissolving 1.286 g of CrCl3 .6H2 O in 250 mL of DDW.
2.3. Removal of Cr(III) from leather tanning wastewater
Biosorption of Cr(III) from leather tanning wastewater (LTW) by
non-viable A. haemolyticus was carried out using optimized conditions determined from the synthetic solution study. Two types of
samples i.e. raw and treated LTW, was determined for Cr(VI) and
total Cr concentrations. Since Cr(VI) was not detected in either of
the samples, total Cr concentration measured would directly relate
to the Cr(III) concentration (as total Cr equals to the sum of Cr(VI)
and Cr(III) present), which was determined at 121.15 mg L−1 for the
raw LTW and 1.91 mg L−1 for the treated LTW. Based on this observation, raw LTW was chosen for Cr(III) removal study because of the
higher Cr(III) concentration. The physico-chemical characteristics
of LTW are listed in Table 1.
2.4. Desorption of Cr(III) from non-viable bacterial biomass
The desorption of Cr(III) from Cr(III)-laden bacterial biomass was
carried out using either 0.1 M or 1.0 M H2 SO4 , HNO3 or CH3 COOH as
the desorption agent. Following the biosorption process, the bacterial cells were first harvested via centrifugation where pellets
obtained were washed using 25 mL DDW followed by contact with
25 mL of desorption agents at 200 rpm, 25 ◦ C for 2 h. The desorped
bacterial biomass (termed as regenerated biomass) was then used
in repeated cycles of adsorption-desorption process.
2.5. TEM analysis
TEM analysis of bacterial cell pellets before and after contacting
with Cr(III) were carried out as follows; firstly, the cell pellets were
fixed using 4% (v/v) of glutaraldehyde in 0.1 M phosphate buffered
saline (PBS) for 1 h at room temperature followed by washing using
363
0.1 M PBS and post-fixation with 2% (v/v) osmium tetroxide for 1 h.
Then, the cells were dehydrated using increasing concentrations of
99.9% (v/v) ethanol (30, 50, 70, 90 and 100%) at 5 min intervals followed by 2 min washing in 100% (v/v) acetone. The cells were then
embedded in 50 and 100% (v/v) of epoxy resin in acetone for 15 min
each. The cell pellets were again infiltrated with 100% (v/v) epoxy
resin and cured at 60 ◦ C overnight. Specimens of 90 nm thickness
were sectioned from the embedded blocks using an ultramicrotome (Leica-UltraCut UCT) and mounted on 200-mesh copper grids.
The specimens were stained with uranyl acetate, post-stained with
lead-citrate (5 min each) and viewed using a transmission electron
microscope (Tecnai G2 , Philips).
2.6. Fourier transform infrared (FTIR) analysis
Possible involvement of functional groups from non-viable cells
of A. haemolyticus during the removal of Cr(III) from aqueous solution was elucidated using the Fourier transform infra-red (FTIR)
analysis. Overnight dried (60 ◦ C) bacterial cell pellets were ground
with KBr (spectroscopic grade) at a ratio of 1:200 in a mortar before
pressed into 10 mm diameter disks under 6 tonnes of pressure. FTIR
spectra were obtained on a PerkinElmer FTIR-600 spectrometer.
The analysis conditions used were 16 scans at a resolution of 4 cm−1
measured between 400 and 4000 cm−1 .
2.7. Data analysis
2.7.1. Adsorption isotherms
Four types of adsorption isotherms namely Langmuir, Freundlich, Dubinin–Radushkevich (D–R) and BET were used to
elucidate the Cr(III) removal process by non-viable cells of A.
haemolyticus. The Langmuir adsorption isotherm model suggests
the removal of Cr(III) to occur on homogenous surfaces via
monolayer sorption without any interactions between the sorbed
analytes (Eq. (1)).
qe =
Qm Ce
1/b + Ce
(1)
where qe represents the equilibrium Cr(III) uptake on the biosorbent, Qm is the maximum adsorption capacity (mg g−1 ), Ce is the
residual concentration of Cr(III) at equilibrium (mg L−1 ) and b is
the Langmuir constant related to the energy of adsorption (L mg−1 ).
The maximum adsorption capacity, Qm and Langmuir constant, b
can be determined from the slope and intercept of linear plot of the
logarithmic equation. The essential features of Langmuir isotherm
can be expressed in terms of a dimensionless constant separation
factor, RL (Eq. (2)) where b is the Langmuir constant, C0 is the initial concentration and RL indicates the shape of the isotherm (RL > 1
unfavorable, RL = 1 linear; 0 < R < 1 favorable and RL = 0 irreversible)
[7].
RL =
1
1 + bC0
(2)
The Freundlich isotherm assumes a heterogeneous surface with
a non-uniform distribution of heat over the surface [8] with general
relationship as depicted by Eq. (3) where Kc is a constant for relative
adsorptive capacity and n is an affinity constant.
1/n
qe = Kf Ce
(3)
The Dubinin and Radushkevich (D–R) isotherm proposed an
equation for the analysis of isotherm to determine whether the
adsorption process occurred via a physical or chemical process. The
D–R equation is more general than the Langmuir model because
it does not assume a homogeneous surface, a constant sorption
potential nor absence or steric hindrance between adsorbed and
incoming particles [8]. The D–R equation is as shown in Equation
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S.K. Yahya et al. / Colloids and Surfaces B: Biointerfaces 94 (2012) 362–368
Table 2
Cr(III) biosorption using various biosorbents at optimum pH.
Type of biosorbent
Optimum pH
Streptomyces VITSVK9 spp.
Sorghum straw (acid washed)
Oats straw (acid washed)
Agave bagasse (acid washed)
Aerobic Granules
Nymphae sp.
Spirogyra spp.
Eichornia crassipes
A. haemolyticus
4.0
4.0
4.0
4.0
5.0
4.5–5.5
5.0
4.5–5.5
5.0
Cr(III) removal, mg g−1 (unless stated otherwise)
76%
6.96
12.97
11.44
37.80
6.11
81%
6.61
62.35
4 [9] where qeq is the biosorption capacity (mol g−1 ), ˇ is activity
coefficient constant (mol2 j−2 ) related to biosorption energy, qmax is
the maximum biosorption capacity (mol g−1 ) to form monolayer, ε
is polanyi potential, T is absolute temperature (K), R is a gas constant
(8.314 J mol−1 K−1 ) and Ceq is the equilibrium concentration.
2
qeq = qmax exp −ˇ(RT ln(1 + 1/Ceq )
(4)
The BET model is an extension of the Langmuir model for multilayer biosorption. It is based on the assumption that each adsorbate
in the first biosorbed layer serves as a biosorption site for the second
layer and beyond. General BET isotherm is given in Eq. (5) where
CBET is the constant relating to the energy of interaction with the
surface (L mg−1 ), Cs – saturation concentration of solute (mg L−1 ),
Ceq – concentration of solute remaining in the solution at equilibrium (mg L−1 ), qeq – amount of solute adsorbed per unit weight of
adsorbent and Q◦ max – maximum specific uptake corresponding to
monolayer saturation (mg g−1 ). Linearization of Eq. (5) would yield
the CBET and Qmax values which can be calculated from the slope
and the intercept respectively.
qeq =
Q ◦ max CBET Ceq
(CS − Ceq )[1 + (CBET − 1)(Ceq /CS )]
(5)
2.7.2. Pseudo-first order kinetic analysis
The biosorption process may proceed by diffusion of metal ions
through the boundary layer at the biosorbent surface and this
may be the rate determining step of the overall process [10]. Eq.
(6) shows the first-order rate expression where qt (mg g−1 ) is the
amount of adsorbed metal on the biosorbent at time t, k1,ad (min−1 )
is the rate constant of first-order adsorption and qeq is the amount
of metal sorbed at equilibrium (mg g−1 ).
log (qeq − qt ) = log qeq −
K1,ad
t
2.303
(6)
Linear plots of log (qeq − qt ) versus t indicate the applicability
of this model where the qeq and k1,ad can be determined from
the intercept and slope of the plot, respectively. If the differences
between the determined qeq value and the experimental values are
high, the reaction cannot be classified as first-order even though
high correlation coefficient was obtained [11].
2.7.3. Pseudo-second order kinetic analysis
For adsorption that proceeds via a pseudo second-order mechanism, the rate-limiting step may be chemical sorption involving
valency forces through the sharing or exchange of electrons
between sorbent and sorbate as covalent forces. The following
assumptions were made; adsorption via monolayer adsorption, the
sorption energy for each ion is similar and does not depend on
surface coverage, the sorption occurs only on localized sites, no
interactions between the adsorbed ions and the rate of sorption
is almost negligible in comparison with the initial rate of sorption [12]. The pseudo-second order relationship can be expressed
according to Eq. (7) where k2,ad (g mg−1 min−1 ) is the rate constant
for second-order adsorption. A straight line of t/q versus t would
Reference
[15]
[16]
[16]
[16]
[17]
[18]
[19]
[15]
This study
indicate the applicability of second-order kinetics where the qeq
and k2,ad values were determined from the slope and intercept of
the plot, respectively.
t
t
1
+
=
2
q
q
k2,ad qeq
eq
(7)
3. Results and discussion
3.1. Effect of pH
The non-viable cells of A. haemolyticus showed the highest Cr(III)
uptake at pH 5 (62.35 mg g−1 ) followed by pH 4 (37.38 mg g−1 ) and
pH 6 (35.15 mg g−1 ). High Cr(III) uptake at pH 5 can be explained
as follows; as the pH increases from 4 to 6, surface of the bacteria will be deprotonated, hence exposing more negatively charged
ligands which have the ability to bind the cationic Cr(III) species
such as Cr(OH)2 + and Cr(OH)2+ from solution [13]. This study was
carried out between pH 4 and 6 range as it was anticipated that
at pH < 3, predominant species in solution would be the free Cr(III)
ions [14]. In this condition, the free Cr(III) ions would compete with
H+ ions for the adsorption sites as well as the electrostatic repulsion
condition between the protonated bacterial surface and the Cr(III)
ions [13]. At pH > 6, the formation of hydroxylated chromium complex (Cr(OH)3 ) may reduce the availability of Cr(III) species to form
complex with the negatively charged groups present on the bacterial cells surface. Table 2 shows the comparison between Cr(III)
removal using A. haemolyticus and other biosorbents at optimum
pH values.
3.2. Effect of adsorbent dosage
Cr(III) adsorption increased with increasing amounts of adsorbent dosage (Fig. 1). This condition can be attributed to the increase
in adsorption area and the availability of free adsorption sites.
However, further increment in the biosorbent dose would lead
to the formation of aggregates that would reduce the availability of effective adsorption area, hence decreasing the overall metal
removal capacity. The adsorption sites remain unsaturated during
the biosorption process due to a lower adsorptive capacity utilization of biosorbent, which decreases the biosorption efficiency.
Highest Cr(III) uptake of 188.58 mg g−1 was obtained at a dosage
of 10 mg cells dry wt. while the highest Cr(III) removal of 56.44 at%
15 mg cells dry wt.
3.3. Effect of contact time and initial Cr(III) concentration
The non-viable cells of A. haemolyticus showed rapid Cr(III)
uptake within the first 0.5–1 h with maximum uptake of
51.08 mg g−1 (initial Cr(III) of 50 mg L−1 ) and 198.80 mg g−1 (initial
Cr(III) of 100 mg L−1 ). This could be due to the availability of large
number of vacant adsorption sites on the bacterial cells surface. This
was followed by a gradual decrease in Cr(III) uptake until an adsorption equilibrium condition was achieved. It is also worthy to note
S.K. Yahya et al. / Colloids and Surfaces B: Biointerfaces 94 (2012) 362–368
365
capacity due to the saturation of binding sites. The increase of
Cr(III) uptake capacity with increasing Cr(III) concentration (up to
175 mg L−1 ) could be due to the stronger driving force to overcome
all mass transfer resistances of Cr(III) ions between the aqueous
solution and the biosorbent, which results in higher probability of
collision between Cr(III) ions and the biosorbent thus leading to a
greater biosorption capacity [22].
3.4. Biosorption isotherm study
Fig. 1. Effect of biomass dosage on the uptake and removal of Cr(III) by A. haemolyticus; SDuptake = 0.48–21.35, SDremoval = 16.53, n = 2; Cr uptake (mg g−1 ) was calculated
using the normal mass balance equation; Cr removal (%) was derived from dividing
the residual and initial concentration of Cr(III) after equilibrium.
that higher initial Cr(III) concentration resulted in a higher Cr(III)
uptake (difference of 2–3 times as demonstrated in this study). This
can be explained from the increased adsorption rate and the higher
driving force generated at high Cr(III) concentration [21]. Similar
observation was reported by other researcher where the uptake
of Cr(III) by aerobic granule increased with increasing initial concentration [17]. The effect of initial Cr(III) concentration on Cr(III)
biosorption capacity of A. haemolyticus was further evaluated using
10–200 mg L−1 of Cr(III). The Cr(III) uptake capacity (Quptake ) and
the concentration equilibrium (Ceq ) of A. haemolyticus are listed in
Table 3.
The results showed that the Cr(III) uptake capacity increased
with increasing concentration of Cr(III) with maximum uptake
of 330.63 mg g−1 at initial Cr(III) of 175 mg L−1 . However, further
increase to 200 mg L−1 of Cr(III) cause a slight decrease in the uptake
Table 3
Equilibrium concentration (Ceq ) and Cr(III) uptake capacity (Quptake ) of non-viable
A. haemolyticus; SDCeq = 0.36 − 12.51, SDQuptake = 1.20 − 41.7 and n = 2; Cr uptake
(mg g−1 ) was calculated using the normal mass balance equation.
Initial Cr(III) (mg L−1 )
Ceq (mg L−1 )
10
15
25
50
75
100
125
150
175
200
8.17
10.88
15.12
33.18
46.45
39.70
38.35
50.33
65.63
109.05
±
±
±
±
±
±
±
±
±
±
0.36
0.47
1.89
1.31
2.19
12.51
10.50
3.22
1.63
3.22
Quptake (mg g−1 )
6.58
9.07
24.56
46.58
96.49
177.82
260.72
282.39
330.63
231.06
±
±
±
±
±
±
±
±
±
±
1.20
1.55
6.32
4.36
7.30
41.71
35.00
10.72
5.42
10.72
The Langmuir and Freundlich adsorption isotherm models fitted well with the Cr(III) removal data by non-viable A. haemolyticus
with R2 values of 0.9515 and 0.9742 respectively. For the Langmuir isotherm, maximum uptake capacity, Qe , of 20.03 mg g−1 is
lower than the predicted maximum uptake value (Qm ) of 44 mg g−1 .
This indicate that the Cr(III) removal by non-viable A. haemolyticus would proceed at higher Cr(III) concentrations. The constant
b value of 0.0166 indicates significant binding strength affinity
of Cr(III) towards non-viable A. haemolyticus. The RL (dimensionless separation factor) values of between 0 and 1 also suggested
the favourable homogenous monolayer mode of adsorption using
non-viable A. haemolyticus as biosorbent. Moreover, the Freundlich
isotherm model also gave the best fit to the experimental data with
R2 values of 0.9742. The n value of 2.0792 (which is within the
n of 1–10 interval) indicates that the biosorption of Cr(III) using
non-viable A. haemolyticus is favourable at the experimental condition used. On the other hand, the D–R and BET isotherms did
not fit well with the experimental data due to the low R2 values of
0.7877 and 0.096 respectively suggesting the unlikely occurrence of
multilayer adsorption, hence limiting the overall biosorption process to monolayer adsorption only. The mean free energy (Ea ) of
<8 kJ mol−1 suggests that physical sorption may also take place
during the biosorption process along with the primary chemical
binding. The adsorption isotherm parameters are summarized in
Table 4.
3.5. Pseudo-first order and second-order kinetic analysis
Analysis on the kinetic parameters i.e. theoretical uptake (qcal ),
experimental uptake (qexp ), correlation coefficient (R2 ) and the
kinetic rate constants (K1 and K2 ) are as summarized in Table 5
while the linearized form of pseudo-first order and pseudo-second
order is illustrated in Fig. 2.
It can be clearly seen that the pseudo second-order model fitted very well (R2 of 0.9814 and 0.9966) with the experimental data
compared to pseudo first-order kinetic model (R2 of 0.0071 and
0.0583). The qe(cal) values increased with increasing initial Cr(III)
concentration while the opposite trend was observed for the K2
values. The suitability of using the pseudo second-order Lagergren
kinetic model to describe the biosorption of metal ions by various
Fig. 2. Linearized form of the Lagergren (a) pseudo first-order and (b) pseudo second-order kinetic model for Cr(III) removal by non-viable cells of A. haemolyticus.
366
S.K. Yahya et al. / Colloids and Surfaces B: Biointerfaces 94 (2012) 362–368
Table 4
Isotherm constants of Langmuir, Freundlich, D–R and BET models for biosorption of
Cr(III) using A. haemolyticus.
Isotherm
Parameter
Langmuir
Qe,max (mg g−1 )
Qmax (mg g−1 )
b (L mg−1 )
R2
RL
Kf
N
R2
ˇ (mol2 J−2 )
E (kJ mol−1 )
R2
CBET (L mg−1 )
R2
Freundlich
Dubinin–Radushkevich
(D–R)
BET
A. haemolyticus
20.03
44
0.0166
0.9515
0.2525–0.8559
0.0811
2.0792
0.9742
5 × 10−5
0.1
0.7877
0.562
0.096
Fig. 3. Desorption of Cr(III) from Cr(III)-loaded A. haemolyticus using various concentrations of CH3 COOH, HNO3 , H2 SO4 ; initial Cr(III) of 100 mg L−1 .
types of biosorbent has also been reported for the biosorption of
Cu(II) using the chemically modified Uncaria gambir [23], biosorption of Zn(II) using living and non-living strain of Streptomyces
ciscaucasicus CCNWHX 72-14 [24] and biosorption of Cd(II) using
Pleurotus platypus [25]. Therefore, it is suggested that the biosorption of Cr(III) by A. haemolyticus can best be described using the
pseudo second-order kinetic model based on the assumption that
the rate limiting step may be chemical sorption or chemisorption
involving valency forces through sharing or exchange of electrons
between biosorbent and Cr(III) ions.
3.6. Desorption of Cr(III)
All acid solutions used showed good potential as desorption
agent for Cr(III) with more than 90% of recovery for 0.1 and 1.0 M
H2 SO4 , 1.0 M CH3 COOH and 1.0 M HNO3 (Fig. 3). This situation is
likely to occur via the exchange of protons with bound Cr(III) on
the bacterial cells surface, which can be supported from the low
pH values of each acids evaluated; 1.0 M H2 SO4 (pH 1.32), 0.1 M
H2 SO4 (pH 1.47), 1.0 M HNO3 (pH 1.26), 0.1 M HNO3 (pH 1.66),
1.0 M CH3 COOH (pH 1.86) and 0.1 M CH3 COOH (pH 2.03). Mineral
acids can be used as a potent desorbing agent based on its ability to
replace the adsorped metal ions on the biosorbent’s surface with
protons. In view of the weak nature of the metal-binding forces,
Fig. 4. Biosorption–desorption cycles of Cr(III) by cells of A. haemolyticus; initial
Cr(III) of 100 mg L−1 ; SDsorp = 0.89–8.31, SDdesorp = 0.33–5.68 and n = 2.
it is also possible to desorp the bound metal from the biosorbent
using desorbing solution containing other cations such as H+ and
Ca2+ . The effectiveness of desorption process strongly depends on
the binding strength of the metal ion to the active site. Mineral
Fig. 5. FESEM micrographs of non-viable cells of A. haemolyticus (a) before and (b) after desorption using 1.0 M CH3 COOH; bar = 1 ␮m.
Table 5
Pseudo first-order and pseudo second-order kinetic rate constants for the biosorption of Cr(III) by non-viable cells of A. haemolyticus.
Initial Cr(III) conc. (mg L−1 )
qe(exp) (mg g−1 )
Pseudo first-order rate constant
qe(cal) (mg g−1 )
50
100
38.43
169.43
5.499
1.683
Pseudo second-order rate constant
K1 (min−1 )
−3
1.15 × 10
2.99 × 10−3
R2
0.0583
0.0071
qe(cal) (mg g−1 )
36.63
120.48
K2 (min−1 )
−3
2.4 × 10
8.05 × 10−4
R2
0.9814
0.9966
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367
acids have also been reported to desorp 90% of Pb(II) and Cd(II)
from macrofungus [26].
3.7. Biosorption–desorption cycles
During the first biosorption–desorption cycle, non-viable
biomass of A. haemolyticus showed high percentage of Cr(III)
removal from initial Cr(III) of 100 mg L−1 with values ranging from
45 to 72% of Cr(III) removal. Upon desorption using HNO3 , H2 SO4
and CH3 COOH, only 5–15% of Cr(III) remained on the bacterial
cells surface (indicating Cr(III) recovery percentage of 85–95%).
However, the percentage of Cr(III) removal and recovery was
greatly reduced in the second cycle where the percentage removal
dropped to 8–18% while percentage recovery was 5–20% only
(Fig. 4). The decrease in the loading capacities following desorption could be due to various factors such as loss of biosorbent
from the reactor, structural damage of biosorbent and blockage
of binding sites by metal complex. This can be supported from
the FESEM micrographs of biosorbent before and after desorption
(Fig. 5). It can be observed that the biosorbent cells were deformed
and clumped after exposure to CH3 COOH (1.0 M) thus, resulting
in a decrease in surface area/volume ratio hence, less availability for binding. Also, the blockage of active binding sites by Cr(III)
residue hindered the interaction with newly adsorped Cr(III). The
adsorption of Mn(II) by Pseudomonas sp., Staphylococcus xylosus
and Blakeslea trispora cells were reported to decrease after the
first cycle of sorption-desorption when 0.1 M HNO3 was used as
the desorption agent [28] as well as a 50% reduction in terms of
Cr(III), Cd(II) and Cu(II) uptake capacities for the blue–green algae
Sprulina sp. when 1.0 M HNO3 was used as the desorption agent
[29].
Fig. 6. Biosorption of Cr(III) from leather tanning wastewater using non-viable cells
of A. haemolyticus; SDremoval = 0.20–2.28, SDuptake = 5.85–17.99 and n = 2.
3.8. Biosorption of Cr(III) from LTW
Around 79.87 mg g−1 of Cr(III) was removed from LTW after
three consecutive adsorption–desorption cycles (Fig. 6), which was
carried out in similar optimized condition as the simulated solution. However, this value is significantly lower compared to the
simulated solution (198.80 mg g−1 ) which can be attributed to the
presence of cations such as Na+ , K+ and Mg2+ (13–1340 mg L−1 ) that
interferes with Cr(III) binding to the functional groups present on
the bacterial cells surface. This condition can be supported from
one report [30] where the Cr(III) uptake capacity of Spirogyra condensata and Rhizoclonium hieroglyphicum were lower compared to
Fig. 7. TEM micrographs of non-viable cells of A. haemolyticus (a) cells only (b) in 25 mg L−1 Cr(III) (c) in 100 mg L−1 of Cr(III), bar = 500 nm and (d) in 100 mg L−1 of Cr(III),
bar = 200 nm; red arrows indicates Cr deposition as electron-dense microparticles.
368
S.K. Yahya et al. / Colloids and Surfaces B: Biointerfaces 94 (2012) 362–368
simulated Cr(III) solution due to interference from cations such as
Mg2+ .
3.9. TEM analysis
The bacterial cell structure remained intact even after autoclaving at 121 ◦ C. However, the disappearance of the bacterial cell
wall outer layer was noticeable. Uneven distribution of electrondense microparticles on the outer region of the bacterial cells
clearly indicates the deposition of Cr(III) onto the bacterial cells
functional groups without further translocation into the cytoplasmic region (Fig. 7). The absence of electron-dense structure at the
bacterial cytoplasmic region strongly suggest that Cr(III) removal
by non-viable A. haemolyticus proceeds via the non-metabolically
dependent biosorption followed by microprecipitation on the bacterial cells outer region.
3.10. FTIR analysis
The band assignments for native non-viable cells of A. haemolyticus showed strong absorption at 3414.3 cm−1 that can be attributed
to the overlapping stretching vibrations of NH and OH groups
[31], 2924.38 cm−1 due to CH2 asymmetric stretching vibration
[32], 1651.81 cm−1 for the typical amide band i.e. C O stretching, 1544.45 cm−1 which corresponded to amide II (combination of
NH bending and CN stretching modes), 1401.81 cm−1 for CO bending from carboxylate group and 1083.61–1240.13 cm−1 which were
due to the vibrational motion of the carboxyl and phosphate groups
[33].
Upon contacting with Cr(III), several peaks were significantly
shifted notably 3414.34 cm−1 to 3431.91 cm−1 indicating the
involvement of hydroxyl and amine group during the Cr(III)
removal process, the amide and amide II peaks at 1651 cm−1
and 1544.45 cm−1 that were slightly shifted to 1649 cm−1 and
1540.56 cm−1 respectively which suggested the complexation of
Cr(III) ions with functional groups from protein [17], 1401.81 cm−1
to 1388.88 cm−1 , 1240.13 cm−1 to 1234.84 cm−1 and 1083.61 cm−1
to 1071.49 cm−1 which may be due to the binding of Cr(III) with
carboxyl, phosphate and carbonyl groups respectively [34].
4. Conclusion
The non-viable cells of A. haemolyticus showed great potential to
be used as an alternative biosorbent to remove Cr(III) from aqueous solution. Its superior metal uptake ability compared to other
existing biosorbents warrants further investigation on its uptake
capacity in view of accelerating its industrial application.
Acknowledgments
The authors acknowledge the support from Ministry of Science,
Technology and Innovation (MOSTI), Malaysia for the National Science Fellowship (NSF) scholarship awarded to Siti Khairunnisa
Yahya, the Ministry of Higher Education (MOHE), Malaysia for funding of the project (FRGS Vote 78465 and 78532) and Universiti
Teknologi Malaysia for the GUP grant Q.J13000.7125.00H52.
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