Cadmium and zinc biosorption, alone or in combination, was investigated with sodium alginate immobilized Chlor~lla homosphaera cells. Concentrations ...
Biotechnology Letters Vol 13 ;~o ~ Received as revised 2nd July
559-562
(1991)
METALS BIOSORPTION BY SODIUM ALGINATE IMMOBILIZED CHLORELLA HOMOSPHAERA CELLS Antonio Carlos Augusto da Costa* and Selma Gomes Ferreira Leite** *Centro de Tecnologia MineraI-CETEM/CNPq, Rua 4, Quadra D, Cidade Universit~ria, 21949, Rio de Janeiro, RJ, Brasil (Corresponding author). **Universidade Federal do Rio de Janeiro, Escola de Qufmica. Dept. de Eng. Bioqu(mica SUMMARY
Cadmium and zinc biosorption, alone or in combination, was investigated with sodium alginate immobilized Chlor~lla homosphaeracells. Concentrations ranging from 20.0 to 41.0mg/I cadmiurn, 75.0 and 720.0mg/I zinc were tested and, in all cases, the metal removal achieved values near 100%. When these metals were put in combination a decrease in the rate of absorption was detected. Gold was also tested in the immobilized system and 90% of the initial metal added was recovered in solution containing 213.0mg/I of the metal, the alginic matrix being responsible for 40% gold uptake. INTRODUCTION Interest in developing new technologies for the treatment of effluents polluted with heavy metals is sti~-nulating studies in this field. These technologies should be based on processes less aggressive to the environment, preventing changes in established ecosystems. The process of metals biosorptio~ seems to be suitable for the final treatment of heavy metal contaminated effluents, enabling their discharge. In the case of an interest on noble metals, their recovery from effluents or any other
gold containing solution is feasible by its strategic interest and value (Darnall, 1986; Hosea, 1986). Several studies have been conducted employing green microalgae of the genus Chlorella with this purpose (Sakaguchi, 1979; Butler, 1980). The main metals of interest for biosorption can be divided basically in two groups: one group comprising noble metals (gold, platinum, silver) and a second group, constituted by toxic metals that must be extracted from solution due to their toxic action o~ the environment. Amongst the noble metals gold seems to be the one with greatest knowledge for recovery from solution by biological action (Hosea, 1986; Darnall, 1986, Greene, 1986). Concerning toxic metals there is a wide range of elements to be studied, nevertheless most researches are focused on the treatment of heavy metals found in metallurgical effluents (Costa, 1990A; Costa, 1990B; Harris, 1990; Gesweid, 1983; Sakaguchi, 1979). The knowledge of cell immobilization techniques greatly contributes to the improvement of metals biosorption studies (Durand, 1978; Vos, 1990). In the specific case of algae immobilization several polymeric matrices have already been studied, sodium alginate being the one with greatest accumulated knowledge (Dainty, 1986; Robinson, 1985), although a series of other supports, with lower cost, is being tried (Darnall, 1986). MATERIALS AND METHODS M i c r o o r g a n i s m - A green microalgae of the species Chlorella homosphaera isolated from a lake at Quinta da Boa Vista, Rio de Janeiro City, was employed in the present study. A l g a e c u l t i v a t i o n a n d m a i n t e n a n c e - The cells were grown in suitable medium with the following composition, in g/h sodium nitrate (1.0), dipotassium phosphate (0.25), magnesium sulphate
(0.51), ammonium chloride (0.05), calcium chloride (0.06) and glucose (10.0). The salts were dissolved and the final pH of the solution adjusted to 7.0. A 10% inoculum was added to the nutrients medium and agitated for 72 hours in a rotary shaker under illumination, at room temperature. For the culture maintenance agar-agar (10.0g/I)was added to the described medium and 10ml of the obtained solution incubated in small tubes at room temperature and under intense illumination (Costa,
1990A). 559
Cells i m m o b i l i z a t i o n - After growing the cells were centrifuged and washed three times with distilled water. The supernatant was discharged and 30ml of a solution containing 16.0g/i sodium alginate and 8.6g/I sodium chloride added to the centrifuged cells; the mixture was then extensively agitated and introduced in a 50ml syringe. The mixture was dropped in a salts solution containing calcium and sodium chloride, and the beads thus formed were washed, and maintained in calcium chloride solution. E x p e r i m e n t s - Experiments were conducted using a percolation column 30.0cm high and with 3.5cm internal diameter. The immobilized cells were added to the column, one third if its total volume being occupied with the alginate entrapped ceils. 200ml of the solution containing the metal(s), added as sulphate salts, was poured into the column and the absorption verified for sixty minutes. In the experiments with gold containing solution the absorption time was thirty minutes. From the fixed bed thus formed, samples were collected for determination of residual metal(s), periodically. The metal(s) concentration(s) tested was (were): 20.0, 27.0 and 41.0mg/I cadmium; 75.0 and 720.0mg/I zinc. When cadmium and zinc were simultaneously added the used concentrations were: 34.5mg/I cadmium and 75.0mg/I zinc; and 9.8mg/I cadmium and 37.0mg/I zinc. For gold studies the used solutions were obtained from gold acid digestions from solid matters. The obtained solution showed a concentration of 213.0mg/I gold, probably associated with silver and other trace metals. A n a l y t i c a l d e t e r m i n a t i o n s - Metals concentrations were determined using an absorption spectrophotometer Varian Techtron, Model AA6. RESULTS AND DISCUSSION The following Figures show the employement of immobilized cells for cadmium, zinc and gold biosorption. The results indicate an increasing absorption rate for all tested metals. It can be deduced from the results that the metals absorption, although continuous with time, is very efficient in the initial steps of the process. This fact is probably associated to the availability of reactional sites around or inside the cells, able to capture metals. In a second stage, with the gradual occupancy of these sites, the uptake becomes less effective. After this, the metals must be eluted, with the employement of a proper eluting solution, to liberate cells for further metals uptake. Figure 1 shows cadmium biosorption at three differents metals concentrations. For the concentrations tested the uptake efficiency was great, near 100%, the rate of absorption being smaller for the less concentrated solution. In these cases the final cadmium concentrations were in the range of 0.1-0.2mg/I. This may be explained in terms of cadmium distribution in the solution, this being the smaller metal concentration. At 30 minutes of contact between the cells and this solution the uptake efficiency achieves its maximum value; nevertheless, for the two other solutions this maximum value is achieved in a shorter period of time. Figure 2 shows the results for zinc biosorption. In both cases (75.0 and 720.0mg/I zinc) the biosorption efficiency was close to 100%, the final zinc concentration being 0.5mg/I for the less concentrated solution and 2.Tmg/I for the other tested solution. These concentrated solutions tested, compared to the ones used for cadmium biosorption, were efficiently treated by the immobilized system. The fact that zinc is metabolically used by the algal cells probably contributes to these good results. From Figure 3 interesting information can be collected about simultaneous bioabsorption of cadmium and zinc. As previously stated from cadmium biosorption, the greater the metal concentration, most effective was the uptake. The greatest zinc/cadmium ratio generated a better rate of uptake for both metals, this fact probably related to metals transport mechanism by the cells. These results have already been noted in previous experimentations (Hart, 1977). The competing effect between the metals during uptake was observed for the lower zinc/cadmium ratio; zinc was not so efficiently extracted from solution in the presence of cadmium. Experiments
560
conducted 1979) and conducted containing
by other authors revealed similar observations when using Chlorella pyrenoidosa (Hart, CMorella vulgaris (Harris, 1990). Preliminary studies on gold biosorption have also been employing the described system. Uptake experiments were conducted with a solution 213.0mg/I gold, the total absorption of the metal being 90% in a period of 30 minutes. 100
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A period of thirty minutes was used for gold biosorption based on the previously stated absorption curves for cadmium and zinc; in thirty minutes of contact between the solutions and the cells the process had already been stabilized. If the rates of absorption for cadmium, zinc and gold (when not added in combination) are compared, the rate for gold is slightly lower (Figure 4). The comparison of these results with other works conducted with CMor~lla cells, immobilized in the same support for gold recovery indicates the potential applicability of the system in the treatment of gold containing solutions (Hosea, 1986; Darnall, 1986; Greene, 1986). In the present study a blank experiment was conducted for gold biosorption. The results indicated 40% gold recovery associated to the presence of the alginic matrix, being the remaining uptake due to the presence of the algal ceils. These blank experiments were not conducted for cadmium and zinc as the literature states negligible uptake of these metals by the alginic matrix. The observed absorption by the polymer does not make it inadequate for this purpose, because in a next step, metals can be eluted from the cells efficiently (data not shown). The good results obtained in the experiments described above stimulating further studies in metals biosorption with the aim of recovering noble metals or treating solutions polluted with heavy metals. The way these experiments were conducted helps the scaling-up of the process concerning engineering operational parameters.
561
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BIBLIOGRAPHY
1. Costa, A.C.A.; Leite, S.G.F. (1990A) Biotechnology Letters, 12, 941-944. 2. Costa, A.C.A.; Leite, S.G.F. (1990B) In: Congreso Latinoamericano de Biotecnologia, Havana, Cuba. 3. Dainty, A.L.; Goulding, K.H. (1986) Biotechnology aud Bioengineerzng, 28, 210-216. 4. Darnail, D.W.; Greene, B. (1986) Environmental Science and Technoiogy, 20, 206-208. 5. Darnall, D.W. (1986) World Intellectual Property Organization, International Bureau. Patent n~-WO 86/03480. 5. Durand, G.; Navarro, J.M. (1978) Process Biochemistry, September, 14-23 I. Geisweid, H.J.; Urbach, W. (1983) Zentratblatt fur Pflanzenphysiologie Bd., 109, 127-141. 8. Greene, B.; Hosea, M. (1985) Environmental Science and Technology, 20, 627-532. 9. Harris, P.O.; Ramelow, G.J. (1990) Environmental Science and Technology, 24, 220-228. 10. Hart, B.A.; Scaife, B.D. (1977) Environmental Research, 14, 401-413. 11. Hart, B.A.; Bertram, P.E. (1979) Environmental Research, 18, 327-335. 12. Hosea, M.; Greene, B. (1985) Inorganica Chimica Acta, 123, 161-155. 13. Robinson, P.K.; Dainty, A.L. (1985) Enzyme and Microbial Technology, 7, 212-215. 14. Sakaguchi, T.; Nakajima, A. (1981) European Journal of Applied Microbiology and Biotechnology, 12, 84-89. 15. Vos, H.J.; Groen, D.J. (1990) Biotechnology and Bioengineering, 36, 357-375.
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