phương pháp mới nhằm loại bỏ Cd trong nước bị ô nhiễm
Trang 1Short 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 l1 solution at the efficiency of 92.7% and 97.9% respectively Maximum cadmium sorption was observed to be 39.2 mg g1at equilibrium (Ceq) of 112.8 mg l1by immobilized microalgal biomass as compared to 33.5 mg g1at Ceqof 116.5 mg l1by free biomass from initial concentration of 150 mg l1 In continuous liquid flow column, the cadmium sorption capacity of immobilized C sorokiniana was 192 mg g1, 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 lE m2s1 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 l1) of 100 ml Cd2þ solution with 0:1 0:003 g free
or immobilized microalgal biomass, shaken on an orbital shaker at 100 rpm for 60 min Residual concen-tration of Cd2þ 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: pcsir@brain.net.pk (M Iqbal).
0960-8524/03/$ - see front matter Ó 2002 Elsevier Science Ltd All rights reserved.
PII: S 0 9 6 0 - 8 5 2 4 ( 0 2 ) 0 0 2 8 9 - 4
Trang 2after 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 0:017 g immobilized C sorokiniana
biomass, packing height 28 cm For biosorption, 5
mg l1 Cd2þ solution was pumped upwards through the
collected after every 500 ml of the total 52 l Cd2þ
solu-tion passed Biosorpsolu-tion saturasolu-tion of the immobilized
biomass was indicated by the attainment of inlet–outlet
Cd2þequilibrium Cd2þ desorption was done by passing
500 ml 0.1 MHCl through the column bed in an upward
direction at the flow rate of 5 ml min1 The effluent
metal solution was collected after every 20 ml desorbent
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 Scanndur-ing electron
microscopy showed these cells to be aggregated along
the surface of the fibrous threads (Fig 1)
Biosorption of Cd2þ by C sorokiniana cells was done
at concentrations of 10 and 25 mg l1 At both these
concentrations the uptake of Cd2þ by microalgal cells
was rapid (Fig 2) Biosorption of Cd2þ from 10 mg l1
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 Cd2þ
The first rapid phase of sorption involves bulk transport
of Cd2þ (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 Cd2þ 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 Cd2þ to the binding sites for adsorption (Plette et al., 1996) Higher sorption of Cd2þby 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 Cd2þ to sorption sites A similar sorp-tion trend was observed at higher concentrasorp-tion of 25
mg l1 Cd2þ, which respectively by free and immobi-lized biomass after 5 min was 20.7 mg g1 (82.9%) and 22.7 mg g1 (90.8%), and after 60 min was 21.8 mg g1
(87.0%) and 23.94 mg g1 (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 Cd2þ sorption g1
biomass (q) increased as equilibrium Cd2þconcentration (Ceq) increased Ceq also increased as initial Cd2þ con-centration (Ci) increased (Fig 3) Maximum sorption (qmax), respectively for free and immobilized microalgal cells, was 33.5 and 39.2 mg g1 at the Ceq of 116.5 and 112.8 mg l1 at Ci of 150 mg l1 It may be concluded that both free and immobilized cells were saturated with
Cd2þ at 150 mg l1 at the fixed sorbent biomass of 1
from Ci of 100 mg l1 by 1 g l1 free and polyurethane foam-immobilized biomass of Rhizopus oligosporous at the Ceqvalue of 78.8 mg l1 (Aloysius et al., 1999), both free and immobilized C sorokiniana cells were signifi-cantly more efficient at the corresponding C value of
Fig 1 Scanning electron micrograph of C sorokiniana cells
immobi-lized along sponge fibres.
0 20 40 60 80 100 120
Contact time for biosorption (min)
Immobilized microalgal cells Free microalgal cells
Fig 2 Percentage biosorption of cadmium from 10 mg l 1 solution,
pH 5.0, by 1 g l 1 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
Trang 3100 mg l1, respectively showing Ceq of 69.5 and 68.4
mg l1, (Cd2þ q¼ 32:2 and 37.6 mg g1) The data
ob-tained in the present studies were observed to fit the
Langmuir isotherm model
0:017 g immobilized C sorokiniana showed sorption
capacity at saturation to be 192.0 and 188.7 mg g1
biomass from 5 mg l1Cd2þsolution, 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 Cd2þ solution passed through
fixed-bed column, which further indicates a good
re-usability potential of immobilized C sorokiniana cells
99.4% Cd2þ desorption of the immobilized microalgal
biomass was achieved with 500 ml 0.1 MHCl 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 Cd2þin a continuous liquid flow operation It also overcomes the operational difficulties associated with immobilization on polymeric gels
References
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Fig 3 Equilibrium biosorption isotherms for cadmium sorption from
solutions 2.5–200 mg l 1 by 1 g l 1 C sorokiniana biomass, free or
immobilized on vegetable sponge of L cylindrica showing cadmium
sorption mg g 1 biomass, specific (q) and maximum (q max ) at mg l 1
initial concentration (C i ) and equilibrium (C eq ).
N Akhtar et al / Bioresource Technology 88 (2003) 163–165 165