Environ. Sci. Technol. 1999, 33, 489-495
Selective Biosorption of Lanthanide (La, Eu, Yb) Ions by Pseudomonas aeruginosa ANNE-CLAIRE TEXIER,† Y V E S A N D R EÅ S , ‡ A N D P I E R R E L E C L O I R E C * ,† De´partement Syste`mes Energe´tiques et Environnement and Subatech UMR 6457, Ecole des Mines de Nantes, 4 rue Alfred Kastler, BP 20722, 44307 Nantes, Cedex 03, France
The ability of Pseudomonas aeruginosa to adsorb selectively La3+, Eu3+, and Yb3+ from aqueous solution was investigated. The lanthanide biosorption equilibrium obeyed the Brunauer-Emmett-Teller isotherm model, indicating multilayer adsorption. Determined levels of maximum adsorption capacities were 397 µmol/g for lanthanum, 290 µmol/g for europium and 326 µmol/g for ytterbium ((10%). The results indicated that there were about 100 preferential sites for lanthanum per g of dry biomass. Experiments with mixed-cation solutions showed that the sequence of preferential biosorption was Eu3+ ) Yb3+ > La3+. Biomasses dried at 37 and 70 °C showed the same selective behavior as wet biomass. Inert microbial biomass dried at 37 °C appeared to be the most efficient form for experimental use. The uptake of lanthanide by P. aeruginosa cells was not affected by the presence of sodium, potassium, calcium, chloride, sulfate and nitrate ions. Aluminum was a strong inhibitor of lanthanide ions biosorption. 87% of the total Al3+ was removed from the 3 mM solution, whereas only 8%, 20% and 3% of the total La3+, Eu3+, and Yb3+, respectively, were sorbed from 3 mM solutions. The results suggested that cells of Pseudomonas aeruginosa may find promising applications for removal and separation of lanthanide ions from aqueous effluents.
Introduction Processing steps in the nuclear fuel cycle generate wastewater streams containing a variety of dissolved heavy metals, including lanthanide and actinide elements. The problem lies in the continuous production of high-volume, often lowactivity, waste requiring continuous treatment. Conventional methods for removing and/or recycling heavy metals from process streams have been shown to be either ineffective or expensive for large volume dilute wastewater applications. The search for new technologies has focused attention on the metal binding capacities of various microorganisms (110) and biopolymers (11-15). Major advantages in the use of biosorbent materials are relatively low cost and good metal uptake capacities, which may in some cases be even highly specific for a certain metal of particular interest (16-18). Several investigations have shown that Pseudomonas aerugi* Corresponding author phone: 33 (0) 2 51 85 82 50; fax: 33 (0) 2 51 85 82 99; e-mail:
[email protected]. † De ´ partement Syste`mes Energe´tiques et Environnement. ‡ Subatech UMR. 10.1021/es9807744 CCC: $18.00 Published on Web 12/24/1998
1999 American Chemical Society
nosa has high efficiency for metal uptake (19-23). Chang and Hong (24) have found that the amount of mercury adsorbed on Pseudomonas aeruginosa biomass was higher than that bound to a cation-exchange resin (AG 50W-X8 resin) with 180 mg Hg/g dry cell and 100 mg Hg/g dry resin, respectively. These authors concluded that the biosorption of mercury was not a simple ion exchange process but an affinity-based selective adsorption. Hu et al. (25) have identified Pseudomonas aeruginosa strain CSU as the best candidate among different biosorbents tested for uranium biosorption. P. aeruginosa CSU showed the highest affinity and maximal capacity (100 mg U/g, dry weight) and was also competitive with commercial cation-exchange resins. The use of Pseudomonas is interesting owing to its economically features and its versatility in nutritional requirements (26). The aim of this work was to investigate the selective sorption of three trivalent lanthanide (La, Eu, and Yb) ions by biomass from P. aeruginosa. The stable lanthanides ressemble closely the trivalent actinides, with similar ionic radii and separative chemistry. Moreover, they have the advantage of being easier to handle than actinides. Continued investigation of the selective biosorption of trivalent lanthanide ions in batch systems is necessary for enabling predictions of the likely applicability of biosystems to trivalent actinide removal.
Materials and Methods Bacterial Cultures. Pseudomonas aeruginosa (strain A 22, Institut Pasteur, Paris, France) was grown aerobically with agitation in a nutrient broth (tryptone 10 g/L, meat extract 5 g/L, and NaCl 5 g/L) at 30 °C. Bacteria were precultured for 12 h, and 6 mL of the growing cell preculture was aseptically transferred to 300 mL of growth medium in a 1 L flask. All bacterial cells used in this study were harvested at the stationary phase to maintain consistent culture conditions. After approximately 24 h of culture, cells in this growth phase (with an optical density at 660 nm of between 2.5 and 4.0) were harvested by centrifugation at 10 000g for 15 min. The yield of biomass was approximately 6 g of wet biomass per liter of growth medium. For each harvest, it was verified that the cultured biomass was not contaminated with extraneous microorganisms. The paste was washed three times and resuspended in its weight of NaCl 9‰ solution. Cell batches dried at 37 and 70 °C to constant weight and ground to a powder within a granulometric range of 5001000 µm were also used. Cation Solutions. Lanthanides as the nitrate derivatives La(NO3)3, 6H2O, Eu(NO3)3, 5H2O, and Yb(NO3)3, 5H2O were purchased from Aldrich Chemical Co. Cation solutions were prepared in distilled water adjusted to pH 5.0 by the addition of nitric acid. Adsorption Studies. For dynamic studies with wet biomass, 4 g of material was introduced into 100 g (4% w/w) of a 4 mΜ metal solution and stirred vigorously by magnetic means. For kinetic studies on dried biomass, approximately 600 mg of dry biomass from P. aeruginosa (37 or 70 °C) was resuspended in 50 mL of distilled water adjusted to pH 5.0. After homogenization at 500 rpm for 2 h, the suspension was mixed with 50 mL of an 8 mM metal solution. This gave an initial metal concentration of 4 mM for experiments. Samples of the homogeneous suspension were withdrawn with a 5 mL syringe at regular time intervals and forced through a 0.2 µm membrane microfilter. Dissolved metal in the filtrates was determined by colorimetry using arsenazo III (27). Tests were conducted to ensure that the filter apparatus did not adsorb dissolved lanthanide cation (data not shown). VOL. 33, NO. 3, 1999 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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For lanthanide sorption isotherms, initial concentrations were chosen in the range 1-7 mM. The biomass slurry was placed in contact with the metal cation solution to the extent of 0.8 g of biomass to 20 g of solution (4% w/w). Biomasses dried at 37 and 70 °C (0.1 g dry weight) were resuspended in 10 mL of distilled water adjusted to pH 5.0 and stirred at 500 rpm for 2 h prior to introduction into 10 mL of metal cation solution. The suspension was shaken at 500 rpm at room temperature (20 ( 5 °C) for 3 h. After centrifugation at 15 000g for 10 min, lanthanide ion remaining in solution was determined by an energy-dispersive X-ray fluorescence apparatus (Oxford ED 2000). It was verified that separation by centrifugation was sufficiently efficient in comparison with membrane filtration (0.2 µm). Only negligible differences were observed. The walls of Nalgene plastic containers used in our experiments did not adsorb metal ions from the solution. The reported experimental values of the metal uptake capacities (qe) by biosorption are the means of duplicate experiments; the reproducibility was 10% of the stated value. TOC Measurements. After separation by centrifugation of the bacterial/media phases, TOC measurements of the final metal solution were performed using a TOC-meter (Shimadzu TOC-5000A). Ion Effects on the Uptake of Lanthanide. The effect of various cations on the uptake of lanthanide by P. aeruginosa cells was investigated. Cells (4% w/w) were suspended in 20 mL of a solution containing 3 mM lanthanide element and 3 mM of one other metal ion. Potassium, sodium, calcium, and aluminum ions were added as KNO3, NaNO3, Ca(NO3)2, 4H2O, and Al(NO3)3, 9H2O, respectively. The effect of various anions such as Cl-, SO42-, and NO3- (applied as lanthanide salts) on the uptake of lanthanide was also studied. Cells (4% w/w) were suspended in 20 mL of a solution containing 18 mM of one anion and 6 mM lanthanide element at pH 5.0. The suspensions were stirred continuously for 3 h at 20 ( 5 °C. The cells were collected by centrifugation. The amount of lanthanide taken up by the cells was estimated from the metal ion content in the residual solution by X-ray fluorescence. Final concentrations of Al3+ were determined by atomic absorption with a Perkin-Elmer Model 2280 spectrometer. Final solutions were adjusted to pH 2.0 (HNO3) for measurement of all aluminum in the cationic state. Traces of biomass that may have remained in suspension after centrifugation were removed by membrane filtration (0.2 µm). Preparations were frozen until analysis. Possible loss of metals due to adsorption on laboratory glassware was continuously monitored using blanks.
Results and Discussion Adsorption Kinetics on Wet Biomass from P. aeruginosa. Figure 1 shows the adsorption kinetics for lanthanum. Cells of P. aeruginosa enabled rapid removal of lanthanum ions under these experimental conditions. After 3 h, equilibrium conditions were attained, as evidenced by a plateau. The kinetics were measured for europium and ytterbium under the same experimental conditions. They were similar to the sorption kinetics for lanthanum. Consequently, a contact time of 3 h in batch reactors was chosen for establishing the adsorption isotherms. It was observed that the amount of metallic cations taken up by cells increased slightly after 20 h. Metabolic uptake mechanisms may also contribute to the removal of metal and increase the adsorption capacities by bioaccumulation. The Langmuir-Hinshelwood (LH) kinetics model (28, 29) was used to describe the adsorption kinetics of metal ions on biomass. The LH kinetics expression is 490
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FIGURE 1. Adsorption kinetics on wet biomass from P. aeruginosa at pH 5.0 for 4 mM La3+.
TABLE 1. Results of Single Ion Langmuir-Hinshelwood Kinetic Plots of Three Lanthanide Metal Ions (4 mM) Adsorbed on Pseudomonas aeruginosa Biomass metal ion
k1 (min-1)
ko (L/mol)
k1/ko (mol‚L-1‚min-1)
La3+ Eu3+ Yb3+
5.52 × 10-4 4.24 × 10-4 1.13 × 10-4
0.32 0.31 0.28
1.72 × 10-3 1.37 × 10-3 4.04 × 10-4
-rM ) -
k1‚CM dCM ) dt 1 + k0‚CM
(1)
with the following variables: k0, reaction rate constant of LH model (L/mol); k1, reaction rate constant of LH model (min-1); rM, reaction rate, (mol‚L-1 min-1); CM, metal concentration in solution (mol/L); C0, initial metal concentration in solution (mol/L); and t, time (min). Upon rearranging the equation, the relation 1 becomes
( )
C0 k1‚t CM + k0 ) C0 - CM C 0 - CM ln
(2)
If we plot [ln(C0/CM)]/(C0 - CM) versus t/(C0 - CM), a straight line can be obtained, where the slope is k1 and the intercept is k0. Quantitative parameters, such as k0, k1, and the maximum reaction rate, -rM,max, can be calculated from the straight lines obtained. k0 represents the initial sorption rate constant of the LH kinetics model, which is similar to a zero-order rate constant. k1 represents the rate constant that occurs after the sorption maximum, i.e., the inflection point of the sorption isotherm, and is similar to a first-order rate constant. Negative k1 values can be interpreted as metal ions that cannot be sorbed on biomass. The LH plots of lanthanide ions on P. aeruginosa biomass are shown in Figure 2. A comparison of the results is shown in Table 1. Positive k1 values show that the three lanthanide ions can be adsorbed on the biomass. Among the three cations, Yb3+ had the smallest k1, which implies that less Yb3+ than other metal ions could be adsorbed onto the biomass. Lanthanum and europium showed larger amounts adsorbed on biomass, with higher k1/k0 values. Ytterbium cations thus appear to present slower adsorption kinetics than those of lanthanum and europium ions.
FIGURE 2. Langmuir-Hinshelwood kinetics plots of single ion sorption for three lanthanide ions (4 mM).
FIGURE 3. Effect of initial pH on lanthanum adsorption. Effect of Initial pH on Lanthanum Removal. As shown in Figure 3, the pH of the solution (adjusted with HNO3 and NaOH 0.1 M) significantly affected the lanthanum sorption dynamics only for pH 2.0 (approximately 100 µmol/g, dry weight). At pH values between 3.0 and 6.0, the uptake of lanthanum was slightly affected. Maximum adsorption of lanthanum occurred at pH 5.0. At lower pH levels (e.g., pH 2.0), the cell walls may be protonated, resulting in a weak complexation affinity between the cell wall and lanthanum cations. The reduction in cation loading capacities at pH 2.0 may also be due to damage in cell-wall structure (25). At pH 2.0, the total organic carbon in solution present in the filtrate was 6.2% (w/w), while at pH G 3.0, this value decreased by half (2.8% w/w). To optimize biosorption of lanthanide cations, the initial pH was adjusted to pH 5.0 for all adsorption experiments. Total organic carbon present in the final solution was measured at the state corresponding to the end of each adsorption isotherm and was carried out on wet biomass. An average percentage of 1.73% (w/w) was obtained with a standard deviation of 0.04%. It was verified that the medium at pH 5.0 (HNO3) had a negligible influence on the biomass. Evolution of Initial pH. No effort was made to control the pH of the solution during the experiments. The initial pH gradually changed in the course of 3 h to a value between
6.0 and 7.0 as metal ion was adsorbed by the cells, whereas the initial pH remained constant in a test medium without adsorbant. Moreover, the initial pH of 5.0 increased to pH 6.0 for adsorption kinetics on wet biomass, although the initial pH did not change when dried biomasses were used. It was found that only 0.1% of the bacterial population treated at 37 °C can survive compared with the initial population of live cells. It appears that the pH increase may be associated with cellular viability. Under our experimental conditions, P. aeruginosa cells used in wet form expectedly maintain some degree of metabolic activity, which would induce an increase in pH in aqueous solution. Marque`s et al. (30) in their study on uranium accumulation by Pseudomonas sp considered that the pH increase was related to cellular viability. They observed that the release of free hydroxyl ions was not dependent on the presence of uranyl ions in the medium. pH values for the precipitation of the lanthanide (La3+, 3+ Eu , Yb3+) ions by hydroxyl ions are 7.82, 6.82, and 6.30, respectively (31). To prevent the formation of lanthanide precipitates during adsorption experiments, it would appear preferable to use dried biomass in order to maintain constancy of the initial pH. Adsorption of Lanthanide (La, Eu, Yb) Ions in Individual Solutions. Adsorption isotherms of the type qe vs Ce were used first to verify that adsorption was favorable for each element. For modeling of metal uptake from aqueous solutions of a single system, the classical isotherm equation of Brunauer-Emmett-Teller (BET) (32) was employed. The BET equation has the following form
[
qe )
qm‚Ce
(Cs - Ce)‚ 1 + (b - 1)‚
( )]
(3)
Ce Cs
where qe is the adsorption capacity at equilibrium (µmol/g), Ce is the solution concentration at equilibrium (µmol/L), qm is the maximum adsorption capacity (µmol/g), Cs is the saturation concentration of the solute (µmol/L), and b is a constant relating to the energy of interaction with the surface. On rearrangement to a linear form
Ce (Cs - Ce)‚qe
)
( )( )
b - 1 Ce 1 + ‚ b‚qm b‚qm Cs
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FIGURE 4. Brunauer-Emmett-Teller adsorption isotherms for lanthanum, europium, and ytterbium ions.
TABLE 2. BET Parameters of Lanthanide-Ion Adsorption by Pseudomonas aeruginosa Biomass lanthanum
Correlation Coefficients 0.99 0.99
r qm b a
europium
397 0.10
Constants 290 0.72
0.99 326 0.04
qm: µmol/g; b: L/µmol.
9
cations
qe ( 10% (µmol/g dry weight)
% decrease in qe
La3+ Eu3+ Yb3+ La3+ Eu3+ La3+ Yb3+ Eu3+ Yb3+ La3+ a Eu3+ Yb3+ La3+ b Eu3+ Yb3+ La3+ c Eu3+ Yb3+ La3+ d Eu3+ Yb3+
417 313 298 122 255 94 174 142 129 87 191 173 109 106 104 150 288 239 139 267 230
71 18 77 42 55 57 79 39 42 74 66 65 64 8 20 67 15 23
ytterbium
A plot of Ce/(Cs - Ce)‚qe against Ce/Cs gives a straight line of slope (b - 1)/b‚qm and intercept 1/b‚qm. The BET adsorption isotherms for lanthanide cations are shown in Figure 4. The values of correlation coefficients and of the parameters qm and b are presented in Table 2. These results show that adsorption of lanthanide (La, Eu, Yb) ions by biomass from P. aeruginosa obeyed the BET model, which assumes a multilayer adsorption process in which one layer need not necessarily be completely filled before another is commenced. Moreover, each adsorption layer of the BET model can be reduced to Langmuir behavior with homogeneous surface energy. P. aeruginosa showed high affinities at low concentrations of lanthanide cations under our experimental conditions. The cell wall seems to present more accessible sites for lanthanum than for europium and ytterbium. Mullen et al. (5) found that at pH 4.0 139 µmol La/g were removed by P. aeruginosa, whereas our work indicated 216 µmol/g ((10%) for the same initial concentration (1 mM) at pH 5.0. According to Andre`s et al. (33), bacterial adsorption of La, Eu, and Yb cations by Mycobacterium smegmatis (5% w/w, biomass/ solution) in 2 mM solution at equilibrium (180 min) and at pH 1.0 was 55, 49, and 69 µmol/g, respectively. Previous studies (34) on Mycobacterium smegmatis dried at 37 °C enabled determination of Langmuir parameters and showed maximum adsorption capacities (qm) of 24 µmol/g for lanthanum and 126 µmol/g for europium. Plots were established for the Ruzic model (35), which permits evaluation of the stability constant (Ks) at metalbacterial complexing sites. It was found that biosorption of La, Eu, and Yb ions obeyed the Ruzic model, as evidenced by a straight line for each metallic ion and correlation coefficients of 0.99. The pKd values for La, Eu, and Yb ions 492
TABLE 3. Bacterial Adsorption of Lanthanide Cations at pH 5.0 in Individual and Mixed Solutions (6 mM) at Equilibrium (3 h)
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a On wet biomass (4% w/w), at equimolar ionic concentrations. b On wet biomass (4% w/w), at initial concentrations of lanthanum twice those of europium or ytterbium. c On biomass dried at 37 °C, at equimolar ionic concentrations. d On biomass dried at 70 °C, at equimolar ionic concentrations.
were 4.7, 5.2, and 4.0, respectively. The stability of the lanthanide-bacterial site complex was assessed and was found to decrease in the order Eu3+ > La3+ > Yb3+. High adsorption capacities of P. aeruginosa biomass for lanthanide cations were confirmed and a study was conducted to evaluate the selectivity of the cell wall. Adsorption of Lanthanide (La, Eu, and Yb) Ions in Mixed Solutions. Adsorption isotherms were determined for each element of the pairs La + Eu, La + Yb, and Eu + Yb and of the mixed solutions La + Eu + Yb. Figure 5 shows the adsorption isotherms of La3+ and Yb3+ in mixed solutions, and Figure 6 shows the adsorption isotherms of the three elements in mixed solutions. Adsorption on biomass from P. aeruginosa was found to be favorable in multicomponent systems. Results are presented in Table 3. Lanthanum
FIGURE 5. Adsorption isotherms of La3+ and Yb3+ in mixed solutions at equimolar ionic concentrations on wet biomass (4% w/w).
FIGURE 6. Adsorption isotherms of La3+, Eu3+, and Yb3+ in mixed solutions at equimolar ionic concentrations on wet biomass (4% w/w). biosorption was strongly affected by the presence of europium (-71%) and ytterbium (-77%), whereas the decrease in uptake of Eu3+ ions is negligible in the presence of lanthanum. The extent of removal of ytterbium decreased with the addition of La ions (-42%). Biosorption of europium decreased by half when ytterbium was present and vice versa. From experiments conducted on wet biomass with solutions containing two or three types of cations at equimolar concentrations, the following sequence of preferential biosorption was obtained: Eu3+ ) Yb3+ > La3+. Adsorption isotherms for individual solutions showed that the cell wall presents more accessible sites for lanthanum (about 112 ( 7 µmol/g more) than for europium or ytterbium. For each experiment in which lanthanum was in competition with europium or ytterbium, it appeared that some sites (about 100 per g of dry biomass) remained accessible for La3+ ions. Apparently, this biomass presents preferential sites for lanthanum which are not accessible for europium and ytterbium.
Biomasses dried at 37 and 70 °C showed the same selective behavior as wet biomass. The highest biosorption capacities were found for biomass dried at 37 °C. Processes with nonliving microorganisms are attractive because of the absence of nutritional requirements. According to Pearson (36), lanthanum, europium, and ytterbium can be classified as hard metals and are assigned to class A. Pearson’s reasoning suggests that the most significant degree of ionic competition occurs for metals belonging to the same class. Our results are in agreement with this interpretation and have shown that lanthanide elements such as La, Eu, and Yb probably compete for the same uptake sites on the biomass. In our previous studies with Mycobacterium smegmatis (34), it was found that bacteria exhibited preferential and selective properties of biosorption for europium with respect to lanthanum. The preferential affinity of Eu for the sites was attributed to the ionic radius of this element, which is smaller than that of La. The radii (Å) of the trivalent lanthanide ions La, Eu, and Yb are 1.032, VOL. 33, NO. 3, 1999 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 7. Effect of various cations on lanthanide biosorption. Control, single-cation solution (3 mM); Na+, NaNO3 (3 mM); K+, KNO3 (3 mM); Ca2+, Ca(NO3)2, 4H2O (3 mM); Al3+, Al(NO3)3, 9H2O (3 mM). 0.947, and 0.868, respectively (37). In the present work, the results cannot be interpreted on the basis of the ionic radius parameter alone. Although ytterbium ions are smaller than lanthanum ions, in the presence of equimolar concentrations of lanthanum, the biosorptive uptake capacity for Yb was depressed significantly. It was found previously (Table 1) that lanthanum ions were adsorbed faster than ytterbium cations, suggesting that adsorption kinetics can contribute to the selective uptake of lanthanide ions. Moreover, the effective ionic radius of a metallic ion can vary according to its coordination number. Formerly, coordination numbers of the tripositive lanthanide ions in aqueous solutions were not known with certainty. It was generally considered that the coordination number was nine for the lighter lanthanide ions, such as that of lanthanum, but eight for the heavier members, such as ytterbium (38). The lanthanum ion stands apart from those of europium and ytterbium with regard to its coordination number, implying the existence of a larger complex. Metal adsorption by bacteria can be influenced by metal speciation in the aqueous phase and also by surface properties, such as charge and orientation of the functional groups on the cell surface. Premuzic et al. (39) showed that, in addition to metal selectivity, there was also a speciesdependent differentiation in the uptake capacity. This can be explained by the fact that chemical and structural characteristics of cell membranes vary with species. These authors also found significant differences in the uptake of uranium and thorium by two strains of Pseudomonas aeruginosa, suggesting that metal uptake was also dependent on the bacterial strain employed. The same investigators suggested that these variations reflect the overall molecular organization and associated stereochemical characteristics of cell-wall constituents and their monomeric and polymeric forms. Spatial and sequential arrangements as well as the relative concentrations of these molecules should expectedly determine the overall chemistry of cell-wall interactions. A given bacterial strain, cultivated and used under constant experimental conditions, possesses a specific cell-wall composition with specific adsorption characteristics (40). Molecular, stereochemical, and spatial organization of the cellwall constituents very likely contribute to the adsorption selectivity. To shed further light on the notion of selective sites on bacterial membranes, our attention has also been directed toward the formation of complexes of crown ethers with lanthanide ions since these systems illustrate the dependence of coordination number on cationic radius and ligand-cavity diameter. Bu ¨ nzli, Wessner, and Klein (41) have provided evidence for a significantly increased stability of 494
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TABLE 4. Percentage Inhibition of Uptake in Cation Competition Studiesa primary cation
competing cation
% decrease in uptake qe
La3+
Al3+ Ca2+ Na+ K+
90 0 23 18
Eu3+
Al3+ Ca2+ Na+ K+
58 0 0 0
Yb3+
Al3+ Ca2+ Na+ K+
94 3 12 6
a Ion competition experiments were conducted at equimolar ionic concentrations (3 mM).
these complexes in the case of the larger lanthanides. It was found that the stability of the complex was determined by the relation between the size of the lanthanide ions in solution and the size of the cavity of the ligand (42). Preferential sites for lanthanum might accordingly have a cavity diameter that would be too large to bind europium and ytterbium with a corresponding degree of stability. These observations appear compatible with the selective behavior of the P. aeruginosa cell wall with respect to the trivalent lanthanide (La, Eu, Yb) ions. Ionic Competition. Before considering a practical application of P. aeruginosa biomass, possible interference by other ions must be taken into account. The presence of some competing ions could inhibit the sorption of contaminants such as heavy metals to biomass and reduce the efficiency of contaminant removal. Experiments were designed to study the possible interfering effect of the elements Na+, K+, Ca2+, Al3+, NO3-, SO42- and Cl-, which can be also present in authentic wastewater streams, on lanthanum, europium, and ytterbium biosorption. Pertinent results are shown in Figure 7 and summarized in terms of percentage inhibition of uptake in Table 4. Aluminum caused a marked abatement of lanthanide removal. Cells showed a high biosorption capacity for Al3+ ions, with qe ) 2250 µmol/g. The species AlOH2+ is most likely the only stable hydrolysis product which can be formed in dilute solution (3 mM) at pH 5.0. Most of the active binding sites on the biomass would probably be shielded by
TABLE 5. qe Values in Anion Competitiona nitrate
sulfate
chloride
405 294 323
397 298 377
387 441 362
La3+ Eu3+ Yb3+ a
The initial lanthanide concentration was 6 mM.
the aluminum. Al3+ ions inhibit especially La3+ and Yb3+ uptake. It is noteworthy that, as shown previously by the Ruzic model, the complexes of Eu with bacterial ligands were found to be more stable than the corresponding complexes with La and Yb. Therefore, europium biosorption was less affected by the presence of Al3+ ions. Under our conditions, the presence of calcium, sodium, or potassium did not significantly affect lanthanide biosorption by P. aeruginosa. The resulting lanthanide adsorption capacities at equilibrium in the presence of different anions are presented in Table 5. Our results did not show a significant effect of anions on the biosorption of lanthanide ions by P. aeruginosa. The presence of aluminum ion, however, definitely hindered the removal of lanthanide by the biomass. Consequently, in the design of a process for wastewater treatment, it would appear necessary to introduce a preliminary step for removal of most of the aluminum that may be present. It is also apparent that the biomass of P. aeruginosa is a strong biosorbent for aluminum and it could find promising applications for the removal of aluminum from wastewater streams.
Acknowledgments The authors wish to thank Dr. H. J. MacCordick for critically reviewing the English manuscript.
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Received for review July 28, 1998. Revised manuscript received November 4, 1998. Accepted November 9, 1998. ES9807744
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