A new system to remove cd from contaminated aqueous medium

Short Communication
Chlorella sorokiniana immobilized on the biomatrix
of vegetable sponge of Luffa cylindrica: a new system
to remove cadmium from contaminated aqueous medium
Nasreen Akhtar
a
, Asma Saeed
b
, Muhammed Iqbal
b,
*
a
Department of Biology, Government Islamia College for Women, Cooper Road, Lahore 54550, Pakistan
b
Environment Biotechnology Group, Biotechnology and Food Research Center, PCSIR Laboratories Complex, Lahore 54600, Pakistan
Received 21 August 2002; received in revised form 6 October 2002; accepted 12 October 2002
Abstract
A new sorption system of microalgal cells immobilized on the biostructural matrix of Luffa cylindrica for sequestering cadmium is
reported. Free and immobilized Chlorella sorokiniana removed cadmium from 10 mg l
À1
solution at the efficiency of 92.7% and
97.9% respectively. Maximum cadmium sorption was observed to be 39.2 mg g
À1
at equilibrium (C
eq
) of 112.8 mg l
À1
by immobilized
microalgal biomass as compared to 33.5 mg g
À1
at C

eq
of 116.5 mg l
À1
by free biomass from initial concentration of 150 mg l
À1
.In
continuous liquid flow column, the cadmium sorption capacity of immobilized C. sorokiniana was 192 mg g
À1
, which was 73.2% of
the total metal passed in 51.5 l. Metal desorption with 0.1 M HCl was 100% and the desorbed immobilized system was reusable with
a similar efficiency in the subsequent cycle.
Ó 2002 Elsevier Science Ltd. All rights reserved.
Keywords: Biosorption; Chlorella sorokiniana; Luffa sponge; Immobilization; Cadmium; Wastewater treatment
1. Introduction
Many microalgal species have been investigated for
metal sorption from industrial wastewaters (Garnham,
1997). Freely dispersed microalgal cells, nevertheless,
present several disadvantages for large-scale applica-
tions, which include blockage of flow lines and clogged
filters (Tsezos, 1986). This has led to interest in the use
of immobilized microalgal cells for metal biosorption.
Several immobilization media, such as alginates, car-
rageenans and polyacrylamide gel have been used for
this purpose (Robinson, 1998). Immobilization matrices
based on these polymeric metabolites, however, result in
restricted diffusion due to closed embedding structures
with low mechanical strength. These difficulties were
overcome by immobilizing the red alga Porphyridium
cruentum within the sponge of Luffa cylindrica (Iqbal
and Zafar, 1993a,b). The sponge is made up of an open

network of fibrous support, providing it strength and
instant contact of immobilized cells to the surrounding
aqueous medium. Luffa sponge is thus ideally suited for
the immobilization of microalgal cells to biosorb toxic
metals. The potential of Chlorella sorokiniana is re-
ported as an active metal sequester, which is the first
study on the biosorption of cadmium by microalgal cells
immobilized in a structured matrix.
2. Methods
Axenic culture of C. sorokiniana was isolated from a
local wastewater body. Biomass was grown to stationary
phase in an orbital shaker under continuous illumination
of 50 lEm
À2
s
À1
. Microalgal immobilization in the luffa
sponge was done as reported earlier (Iqbal and Zafar,
1993a,b). The immobilized and free cell biomass was
freeze dried for later studies on cadmium biosorption.
Biosorption capacity of C. sorokiniana was deter-
mined by contacting various concentrations (2.5–200
mg l
À1
) of 100 ml Cd
2þ
solution with 0: 1 Æ 0:003 g free
or immobilized microalgal biomass, shaken on an
orbital shaker at 100 rpm for 60 min. Residual concen-
tration of Cd

2þ
in the metal supernatant solutions was
determined using atomic absorption spectrophotometer
Bioresource Technology 88 (2003) 163–165
*
Corresponding author. Tel.: +92-42-9230704; fax: +92-42-
9230705.
E-mail address: (M. Iqbal).
0960-8524/03/$ - see front matter Ó 2002 Elsevier Science Ltd. All rights reserved.
PII: S 0 960-8524 ( 0 2 ) 0 0289-4
after contact periods between 5 and 60 min. Biosorption
in a continuous flow system was done in a fixed-bed
column bioreactor (2.7 cm inner diameter, 30 cm length)
packed with 1:0 Æ 0:017 g immobilized C. sorokiniana
biomass, packing height 28 cm. For biosorption, 5
mg l
À1
Cd
2þ
solution was pumped upwards through the
column at the flow rate of 5 ml min
À1
. Effluent was
collected after every 500 ml of the total 52 l Cd
2þ
solu-
tion passed. Biosorption saturation of the immobilized
biomass was indicated by the attainment of inlet–outlet
Cd
2þ

equilibrium. Cd
2þ
desorption was done by passing
500 ml 0.1 M HCl through the column bed in an upward
direction at the flow rate of 5 ml min
À1
. The effluent
metal solution was collected after every 20 ml desorbent
HCl passed and was analysed for Cd
2þ
content. The
desorbed immobilized algal biomass was reused in the
next biosorption cycle.
3. Results and discussion
The fibrous network of the luffa sponge was com-
pletely covered by immobilized C. sorokiniana cells dur-
ing an incubation period of 24 days. Scanning electron
microscopy showed these cells to be aggregated along
the surface of the fibrous threads (Fig. 1).
Biosorption of Cd
2þ
by C. sorokiniana cells was done
at concentrations of 10 and 25 mg l
À1
. At both these
concentrations the uptake of Cd
2þ
by microalgal cells
was rapid (Fig. 2). Biosorption of Cd
2þ

from 10 mg l
À1
solution, respectively by free and immobilized cells was
89.7% and 93.5% within 5 min, and in 60 min was 92.7%
and 97.9%. These observations indicate that C. soroki-
niana has active and efficient sorption affinity for Cd
2þ
.
The first rapid phase of sorption involves bulk transport
of Cd
2þ
(Gadd, 1988), which is followed by intracellular
uptake in the passive phase of sorption (Rai and Mal-
lick, 1992). The statistically significant smaller uptake
of Cd
2þ
by free cells may be attributed to their aggre-
gation, thus reducing their three dimensional surface
area for sorption. Raw, non-living algal cells, as were C.
sorokiniana cells used in present studies, tend to clump
together (Greene and Bedell, 1990). The structural mi-
crobarrier so created limits accessibility of Cd
2þ
to the
binding sites for adsorption (Plette et al., 1996). Higher
sorption of Cd
2þ
by immobilized microalgal biomass, on
the other hand, is due to cell immobilization along the
surface of the fibrous threads, little or no interaction

with other immobilized cells in the biomass, no clump-
ing, and the reticulated open network of immobilization
matrix, together contributed to enhanced surface area
and free access of Cd
2þ
to sorption sites. A similar sorp-
tion trend was observed at higher concentration of 25
mg l
À1
Cd
2þ
, which respectively by free and immobi-
lized biomass after 5 min was 20.7 mg g
À1
(82.9%) and
22.7 mg g
À1
(90.8%), and after 60 min was 21.8 mg g
À1
(87.0%) and 23.94 mg g
À1
(95.8%). The slight reduction
in sorption may be due to increase in metal ion con-
centration at constant biomass resulting in an intensive
competition for sorption sites, the availability of which
reduces, becoming a limiting factor at saturation (de
Rome and Gadd, 1987; Rai and Mallick, 1992).
Equilibrium sorption isotherms for free and immo-
bilized algal biomass showed that Cd
2þ

sorption g
À1
biomass (q) increased as equilibrium Cd
2þ
concentration
(C
eq
) increased. C
eq
also increased as initial Cd
2þ
con-
centration (C
i
) increased (Fig. 3). Maximum sorption
(q
max
), respectively for free and immobilized microalgal
cells, was 33.5 and 39.2 mg g
À1
at the C
eq
of 116.5 and
112.8 mg l
À1
at C
i
of 150 mg l
À1
. It may be concluded

that both free and immobilized cells were saturated with
Cd
2þ
at 150 mg l
À1
at the fixed sorbent biomass of 1
gl
À1
. When compared with maximum Cd
2þ
sorption
from C
i
of 100 mg l
À1
by 1 g l
À1
free and polyurethane
foam-immobilized biomass of Rhizopus oligosporous at
the C
eq
value of 78.8 mg l
À1
(Aloysius et al., 1999), both
free and immobilized C. sorokiniana cells were signifi-
cantly more efficient at the corresponding C
i
value of
Fig. 1. Scanning electron micrograph of C. sorokiniana cells immobi-
lized along sponge fibres.

0
20
40
60
80
100
120
0 102030405060
Contact time for biosorption (min)
% Cd adsorbed
Immobilized microalgal cells
Free microalgal cells
Fig. 2. Percentage biosorption of cadmium from 10 mg l
À1
solution,
pH 5.0, by 1 g l
À1
microalgal cell biomass of C. sorokiniana free or
immobilized in vegetable sponge of L. cylindrica as related to the time
of contact during orbital shaking at 100 rpm at 25 °C.
164 N. Akhtar et al. / Bioresource Technology 88 (2003) 163–165
100 mg l
À1
, respectively showing C
eq
of 69.5 and 68.4
mg l
À1
, (Cd
2þ

q ¼ 32:2 and 37.6 mg g
À1
). The data ob-
tained in the present studies were observed to fit the
Langmuir isotherm model.
A fixed-bed column bioreactor packed with 1:017 Æ
0:017 g immobilized C. sorokiniana showed sorption
capacity at saturation to be 192.0 and 188.7 mg g
À1
biomass from 5 mg l
À1
Cd
2þ
solution, respectively in the
first and second cycle. This amount of metal was sorbed
out of 262.1 and 260.6 mg, amounting to 73.2% and
71.9% removal, respectively during the first and second
cycle of 51.5 and 48.5 l Cd
2þ
solution passed through
fixed-bed column, which further indicates a good re-
usability potential of immobilized C. sorokiniana cells.
99.4% Cd
2þ
desorption of the immobilized microalgal
biomass was achieved with 500 ml 0.1 M HCl. The re-
generated biomass of C. sorokiniana was reusable having
sorption efficiency of 98.3%. C. sorokiniana immobilized
on luffa sponge, as a compact immobilized biomatrix
system, has thus shown the potential to efficiently re-

move Cd
2þ
in a continuous liquid flow operation. It also
overcomes the operational difficulties associated with
immobilization on polymeric gels.
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N. Akhtar et al. / Bioresource Technology 88 (2003) 163–165 165