Rhodotorula glutinis에 의한 중금속의 흡착

조 대행, 김 의용

서울시립대학교 화학공학과

Biosorption of Heavy Metals from Aqueous Solutions

by Rhodotorula glutinis

Dae Haeng Cho, Eui Yong Kim

Dept. of Chemical Engineering, University of Seoul

Introduction

The accumulation of heavy metal contaminants in environment has become a concern due to growing health risks to the public. The metals of most immediate concern are Pb, Cd, Cr, Mn, Fe, Cu, Zn and Hg. For humans, poisoning by most of these metals causes severe dysfunction of kidney, reproductive system, liver, brain and central nervous system.

Conventional methods for heavy metal removal are chemical precipitation, ion-exchange and reverse osmosis processes. These methods have several disadvantages such as less effective metal ions removal, high regent requirements, high costs, the generation of toxic sludges and the problem of the safe disposal of the materials. The potential for using microorganism in the treatment of metal-bearing wastewater has been studied intensively and many microorganisms including bacteria, fungi and algae have been found to remove metals from solutions.[1] The uptake of heavy metal ions by microorganisms can take place by entrapment in the cellular structure and subsequent sorption on to the binding sites present in the cellular structure and also involve its passage into the cell across the cell membrane through the cell metabolic cycle.

Among the yeast, Saccharomyces cerevisiae has been the most studied in mechanism of adsorption, cellular physiology, and environmental conditions on metal uptake.[2] However S. cerevisiae biomass has been referred to as a mediocre biosorbent. It was reported that Rhodotorula sp. including R. rubra, R. mucilaginosa and R. minuta was able to uptake cadmium, lead and copper. However the available information about the removal of heavy metals by R. glutinis is limited. Therefore, the aim of the present work was to determine the ability of R. glutinis a newly isolated from soil on the removal of heavy metals and investigate the adsorption isotherms of this strain.

Materials and method

Microorganism : Rhodotorula glutinis was isolated from soil and identified according to carbon and nitrogen assimilation tests.

Medium and culture conditions : The growth medium contained glucose 10g/l, peptone 5g/l, yeast extract 3g/l, malt extract 3g/l. Cultures were incubated at 25℃ on a reciprocating shaker at 120 rpm for 36hr.

Biomass preparation : The broth was centrifuged at 10,000×g for 5 min at 4℃. The supernatant was removed and the cell pellet was washed twice with distilled water. It was dried at 70℃ for 24hr then grounded in mortar and pestle.

Metal biosorption experiments : Stock metal ion solutions (1000mg/l) were prepared using nitrate salts of the metals. Aliquots (50ml) of each metal solution were pipetted into 250 ml Erlenmeyer flasks. Before adding biomass (0.1g dry weight), the pH of the metal solutions were adjusted using 0.1N NaOH and HNO3. In all cases, the flasks were incubated at a constant temperature (25℃) on a reciprocating shaker at 150 rpm. After incubation, cells were separated by centrifugation at 10,000×g for 5 min, and supernatant was used for metal analysis. The heavy metal concentrations were determined with Varian AA-220FS spectrophotometer.

Adsorption isotherm : The metal uptake value (q) was calculated using the following equation: q = V(Ci - Cf)/1000m, where q is the metal uptake(mg metal / g of biomass), V is the volume of metal solution(ml), Ci and Cf are the initial and final concentration of metal respectively, m is the mass of the biomass. The biosorption equilibrium-isotherm was obtained using the Langmuir sorption model:

q = qmaxbCf/(1+bCf), where b is the Langmuir constant and qmax the maximum metal uptake.

Results and discussion

Figure 1 demonstrates that the heavy metal uptake by R. gutinis is very rapid for Pb and Cd. Binding occurred within five minutes and relatively stable thereafter. The rapid uptake of Pb, Cd from dilute solution suggests that the binding sites are cell wall components and the metal ions are not diffusing through the cell wall. The effect of pH on heavy metal adsorption is shown in Fig. 2. The uptake of Pb and Cd by R. glutinis was significantly affected by the initial pH of the solution. At low pH, removal of Pb and Cd decreased, while removal of both metals increased as pH increased. The optimum initial pH values were 4.5-6.0 and 4.0 for uptake of Pb and Cd, respectively. The metal biosorption depends on the protonation or deprotonation of the cell wall functional groups. At low pH, the protonation of the functional groups gives an overall positive charge to the polymer molecules unable to adsorb positively charged heavy metal ions. The adsorption is reduced. Increasing the pH reduced the electrostatic repulsion, exposing more ligands carrying negative charges. The adsorption is hence increased at high pH.

The adsorption isotherms for R. glutinis for Pb and Cd are shown in Fig. 3 and Fig. 4. The isotherms are rather steep at lower equilibrium concentrations implying the suitability to treat dilute heavy metal solutions. The biosorption capacity of the biomass increased first with increasing of the initial concentrations of metal ions and reached a saturation value. Langmuir sorption model was used to evaluate the sorption behavior of biomass and Langmuir parameters are summarized in Table 1. Metal uptakes at equilibrium residual concentrations of 10 mg/l (q10) and 200 mg/l (q200) were selected for comparison with other biosorbents, representing the choice of low and high metal concentrations. The table 1 also show the Langmuir conctant b related to the affinity of the sorbent material for the metal sorbate. A q10 value for Pb uptake by R. glutinis were 12.9 mg/g. This value is higher than the q10 value for R. rubra (8.28 mg/g), Rhizopus arrhizus (8 mg/g) and Penicillium chrysogenum (10 mg/g).[3][4] The Cd uptake capacity by R. glutinis was less significant than the Pb uptake. The q10 value for Cd uptake was 1.02 mg/g. This value is lower than that of R. rubra (9.90 mg/g), R. arrhizus (7 mg/g) and Penicillium chrysogenum (2 mg/g).

Fig. 1. Time course of Pb and Cd uptake by R. glutinis

Fig. 2. Effect of pH on Pb and Cd uptake by R. glutinis

It was known that R. glutinis produces an extracellular polysaccharide (EPS). The EPS produced from R. glutinis newly isolated from soil was identified as a uronic acid/mannose rich exopolymer in our early study. Uronic acids confer a net negative charge to the polymer and also play an important role in the binding capacities of the polymer. The acidic polysaccharides containing uronic acid have been expected to have some heavy metal binding capability.

This polymer could contribute to the higher adsorption capacity of R. glutinis. The play a role of this polysaccharide in metal uptake is not fully understood and needs to be studied in greater datail.

Table.1. Langmuir parameters for Pb and Cd uptake by R. glutinis

	qmax (mg/g)	b (l/mg)	q10 (mg/g)	q200 (mg/g)
Pb	71.9	2.18×10-2	12.96	58.51
Cd	18.4	0.59×10-2	1.02	9.53

Fig. 4. Cd sorption isotherm for R. glutinis

Fig. 3. Pb sorption isotherm for R. glutinis

References

1. Gadd, G. M. "Fungi and yeast for metal accumulation" McGarw-Hill. (1990)

2. Avery, S. V., Tobin, J. M. Appl. Environ. Microbiol. 59. 2851-2856 (1985)

3. Salinas, E., Elorza, M., Rezza, I., Martinez, L., Machesvky, E., Sanz, M.            Bioresource. Technol. 72. 107-112 (2000)

4. Horan, Z. R., Volesky, B.  Appl. Biochem. Biotech. 53. 133-146 (1995)