Đang tải... (xem toàn văn)
phương pháp mới nhằm loại bỏ Cd trong nước bị ô nhiễm
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: pcsir@brain.net.pk (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. References Aloysius, R., Karim, M.I.A., Ariff, A.B., 1999. The mechanism of cadmium removal from aqueous solution by nonmetabolizing free and immobilized live biomass of Rhizopus oligosporus. World J. Microbiol. Biotechnol. 15, 571–578. de Rome, L., Gadd, G.M., 1987. Copper adsorption by Rhizopus arrhizus, Cladosporium resinae and Penicillium italicum. Appl. Microbiol. Biotechnol. 26, 84–90. Gadd, G.M., 1988. Accumulation of metals by microorganisms and algae. In: Rehm, H J. (Ed.), Biotechnology. VCH, Weinheim, Germany, pp. 401–433. Garnham, G.W., 1997. The use of algae as metal biosorbents. In: Wase, D.A.J., Forster, C.F. (Eds.), Biosorbents of Metal Ions. Taylor and Francis Ltd., London, pp. 11–37. Greene, B., Bedell, G.W., 1990. Algal gels or immobilized algae for metal recovery. In: Akatsuka, I. (Ed.), Introduction to Applied Phycology. SPB Academic Publishing Co., The Hague, The Netherlands, pp. 137–149. Iqbal, M., Zafar, S.I., 1993a. The use of fibrous network of matured dried fruit of Luffa aegyptiaca as immobilizing agent. Biotechnol. Tech. 7, 15–18. Iqbal, M., Zafar, S.I., 1993b. Bioactivity of immobilized microalgal cells: application potential of vegetable sponge in microbial biotechnology. Lett. Appl. Microbiol. 17, 289–291. Rai, L.C., Mallick, N., 1992. Removal and assessment of toxicity of Cu and Fe to Anabaena doliolum and Chlorella vulgaris using free and immobilized cells. World J. Microbiol. Biotechnol. 8, 110–114. Plette, A.C.C., Benedetti, M.F., Riemsdjik, W.H., 1996. Competitive binding of protons, calcium, cadmium and zinc to isolated cell walls of a gram-positive soil bacterium. Environ. Sci. Technol. 30, 1902–1910. Robinson, P.K., 1998. Immobilized algal technology for wastewater treatment purposes. In: Wong, Y.S., Tam, F.Y. (Eds.), Wastewater Treatment with Algae. Springer-Verlag, Berlin, Heidelberg and Landes Biosciences, Georgetown, USA, pp. 1–16. Tsezos, M., 1986. Adsorption by microbial biomass as a process for removal of ions from process or waste solutions. In: Eccles, H., Hunt, S. (Eds.), Immobilization of Ions by Biosorption. Ellis Horwood, Chichester, UK, pp. 200–209. Fig. 3. Equilibrium biosorption isotherms for cadmium sorption from solutions 2.5–200 mg l À1 by 1 g l À1 C. sorokiniana biomass, free or immobilized on vegetable sponge of L. cylindrica showing cadmium sorption mg g À1 biomass, specific (q) and maximum (q max )atmgl À1 initial concentration (C i ) and equilibrium (C eq ). N. Akhtar et al. / Bioresource Technology 88 (2003) 163–165 165 . Communication Chlorella sorokiniana immobilized on the biomatrix of vegetable sponge of Lu a cylindrica: a new system to remove cadmium from contaminated aqueous. aqueous medium Nasreen Akhtar a , Asma Saeed b , Muhammed Iqbal b, * a Department of Biology, Government Islamia College for Women, Cooper Road, Lahore 54550,