Biosorption of uranium and thorium

7 downloads 0 Views 851KB Size Report
The biomass of Rhizopus arrhizus at pH 4 exhibited the highest uranium and thorium ... tailings.'* All 12 known isotopes of thorium are radioactive and, together.
Biosorption of Uranium and Thorium MARIOS TSEZOS and BOHUMIL VOLESKY ,* Department of Chemical Engineering, McGill University, Montreal, Quebec, Canada H3A 2A7

Summary Selected samples of waste microbial biomass originating from various industrial fermentation processes and biological treatment plants have been screened for biosorbent properties in conjunction with uranium and thorium in aqueous solutions. Biosorption isotherms have been used for the evaluation of biosorptive uptake capacity of the biomass which was also compared to an activated carbon and the ion exchange resin currently used in uranium production processes. Determined uranium and thorium biosorption isotherms were independent of the initial U or Th solution concentration. Solution pH affected the exhibited uptake. In general, lower biosorptive uptake was exhibited at pH 2 than at pH 4. No discernible difference in uptake was observed between pH 4 and pH 5 where the optimum pH for biosorption lies. The biomass of Rhizopus arrhizus at pH 4 exhibited the highest uranium and thorium biosorptive uptake capacity (g) in excess of 180 mgig. At an equilibrium uranium concentration of 30 mgiliter, R . arrhizus removed approximately 2.5 and 3.3 times more uranium than the ion exchange resin and activated carbon, respectively. Under the same conditions, R . arrhizus removed 20 times more thorium than the ion exchange resin and 2.3 times more than the activated carbon. R . arrhizus also exhibited higher uptake and a generally more favorable isotherm for both uranium and thorium than all other biomass types examined.

INTRODUCTION The Phenomenon of Biosorption

Living cells have been known to concentrate cations from their aqueous environment. Microbial biomass has been documented to exhibit a selective retention of heavy metal ions. Polikarpov’ in 1966 pointed out that radionuclides present in an aquatic (sea) environment are accumulated by marine microorganisms through direct adsorption from the water. He pointed out that the above property is independent of the life functions of the microbial cells since dead cells exhibited this property as well as or better than live ones. Tezuka’ in 1968 suggested that the reversible flocculation of activated sludge bacteria, with the help of bivalent cations like Ca’’ or Mg2+, is * To whom correspondence should be addressed. Biotechnology and Bioengineering, Vol. XXIII, Pp. 583-604 (1981) 0 1981 John Wiley & Sons, Inc. 0006-3592/81/0023-0583$01.20

TSEZOS AND VOLESKY

584

the result of ionic bond bridges formed among negatively charged cell surfaces and cations in solution. Cell walls of procaryotes and eucaryotes contain polysaccharides as basic building blocks. The ion exchange properties of natural polysaccharides have been studied in detail3 and it is a well-established fact that bivalent metal ions exchange with counterions of the polysaccharides as shown in the following example involving alginic acid: 2NaAlg

+ MZ+ e M(Alg), + 2Na+

With the help of an enrichment culture, Chiu4 isolated a fungal culture from sewage that could uptake uranium from solution. Uranium was taken up by living as well as dead mycelia, thus supporting the hypothesis of physical-chemical uranium ions retention mechanism by the microbial cells. Jilek et al.’ moved toward the technical application of the above property and patented a product claimed to have a uranium uptake capacity of about 100 mg/g. However, samples of the supposedly marketed product were not supplied when requested. Beveridge6 in 1977, working with pure cell wall preparations of Bacillus subtilis, reported that the microbial cell wall removed and retained ions of high-atomic-number elements. A physical-chemical mechanism of retention was hypothesized. Shumate et al.’ in 1978 reported rapid uptake of uranium from solution by resting Saccharomyces cerevisiae and Pseudomonas aeruginosa cells. The above information indicates that microbial cells possess the ability to bind with certain cations and remove them from solution. This potential is expressed even when the cells are dead and it is probably associated with the cell wall. The phenomenon of retention of cations from solution by dead microbial cells has been termed “biosorption.” The mechanism of bisorption is little understood and the largely qualitative information available is fragmented and rather limited. A number of microbial processes are presently being used by food and pharmaceutical industries. Biological wastewater treatment is also widely practiced. Byproducts of the above processes are large quantities of waste microbial mass that is in most cases being disposed of by landfilling or incineration. Waste biomass has, however, a biosorption potential and could be used as an inexpensive material for the development of a wastewater treatment scheme for the decontamination of waste streams containing ions of high-atomic-number elements such as uranium, thorium, radium, etc. The study of biosorption of uranium and thorium is the subject of the present work.

Uranium and Thorium Recovery The increasing demand for uranium has prompted a search for cheaper and more efficient methods for its recovery. Thorium, being a potential

BIOSORPTION O F URANIUM AND THORIUM

585

fuel for nuclear breeder reactors, is receiving similar attention. In addition to the ore processing recovery aspects, environmental problems resulting from intensified mining and ore processing activities call for the removal of radioactive elements from wastewater originating from these operations. Only about 15% of the total radioactivity in the ore leaves in the final uranium product,8-” whereas the remaining 85% is discharged in the tailings.’* All 12 known isotopes of thorium are radioactive and, together with other unearthed not recovered radionuclides such as radium, polonium, etc., find their way to the natural surface water bodies and environment. Uranium is also found in concentrations as high as 50 mg/liter in certain copper leach dumpsioand can be commercially extracted from the acidic wastewaters originating in the production of phosphoric acid from uranium bearing phosphate rock. A number of radionuclides and other heavy metal elements potentially can be extracted from seawater provided that powerful and cheap sequestering agents are available. Malfunctions and leaks of nuclear reactor cooling fluids containing highly radioactive elements of different kinds further demonstrate the urgent need for efficient and element-selective materials of this sort. Biosorption potentially could be used for recovery as well as for cleanup purposes after we improve our understanding of the phenomenon and of the parameters that influence it.

MATERIALS AND METHODS Samples of waste microbial biomass originating from industrial fermentations and wastewater treatment processes have been collected and processed. The samples were requested from most of the known largescale fermentation industries in Canada and the United States. Eight different types of biomass were made available and are listed in Table I. An ion exchange resin IRA-400 (Rohm-Haas Co.), used by most uranium production companies for selective separation of U from other ions, and TABLE I Materials Tested for Uptake of Uranium and Thorium 1) Aspergillus niger

2) 3) 4) 5) 6)

7) 8) 9) 10)

Aspergillus terreus Penicillium chrysogenum Pseudomonas fluorescens Streptomyces niveus Municipal wastewater activated sludge Industrial wastewater (phenolic), activated sludge Activated carbon Filtrasorb 400 (Calgon Co.) Ion exchange resin (anionic) IRA-400 (Rohm-Haas Co.) Rhizopus arrhizus

586

TSEZOS AND VOLESKY

Filtrasorb 400, an extensively used activated carbon (Calgon Co.), were also tested for comparison. Solutions of uranium and thorium were prepared by dissolving exact quantities of uranyl nitrate and thorium nitrate in distilled water, following the procedure suggested by Marcenko.I3 The standard method used for the determination of activated carbon adsorption isothermsI4 was also applied for the evaluation of uranium and thorium “biosorption” isotherms. A spectrophotometric technique, developed by M a r ~ e n k o was ’ ~ used for the determination of U and Th concentrations in solution. Arsenazo I11 was used as the color-developing agent. The chelate complexes formed by U6+ and Th4+ with Arsenazo I11 have molecular absorptivities of E = 1.27 x lo5 liter mol-’ cm-’’ at A = 656 nm and E = 1.15 x lo5 liter mol-’ cm-’ at A = 655 nm, respectively. A Spectronic 70 (Bausch and Lomb) spectrophotometer was used for the quantitative determinations. Initial solution concentrations ranged from 50 to 1000 mg/liter for U and from 30 to 100 mg/ liter for Th, all below the solubility limits determined by the pH values used. For each isotherm, initial uranium concentrations were kept constant while the dry cell weight in each sample was varied. Separation of the sorbents from the solution was accomplished by vacuum filtration using 0.45 pm Sartorius membrane filters, which present the least washable TOC.” Each filter membrane, before being used, was washed with distilled deionized water (DDW) and the first 10 ml of the filtrate were discarded in order to minimize possible change of the U/Th filtrate equilibrium concentration due to retention of U/Th by the filter or complexation by washable total organic carbon (TOC). The procedure suggested by Rizza and KornfeldI6 was followed for the disruption of mycelia of Rhizopus arrhizus. Dry R . arrhizus (1.4 g) was mixed with 50 g of 0.25-0.30 mm B-Braum glass beads, previously cleaned with HCI. The mixture was wetted with 15 ml of DDW. Mycelia were disrupted in a Bronwill Mechanical Cell Homogenizer maintained at low (15OC) temperature. After 25 min in the Bronwill homogenizer, the mixture was centrifuged at 100 g for 5 min and the precipitate (glass beads and intact cells) discarded. The supernatant was diluted with 4% w/v sodium lauryl phosphate (SLP) solution and boiled for 15 min to remove the cell membranes. The crude wall fraction was collected by centrifuging at 1000 g for 20 min. The precipitate underwent successive cycles of resuspensions in DDW, followed by 20-min centrifugations at 10,000g until no more foaming could be detected, thus indicating an SLP-free solution. Cell walls were subsequently freeze dried and yielded a slightly grey fluffy material. Experiments were conducted at pH values of 2, 4, and 5. The selection of the pH values was based on the fact that pH 2.5 is critical for the onset of hydrolysis of the uranyl ion: Above pH 2.5, composite hydrolyzed

BIOSORPTION OF URANIUM AND THORIUM

587

uranium ions of positive or negative charge predominate while below pH 2.5 only the simple UOz2+ ion exists in solution. At pH values higher than 5 , solubility of U, and especially of Th, is very low. Thorium hydrolyzes much less readily, yet the same pH values were used so that uranium and thorium uptake capacities could be compared at identical pH values. It was necessary to buffer the U/Th biomass contact systems as biomass usually shifted the solution pH to more alkaline values. Potassium biphthalate solutions were used as a buffering agent for pH 4 and 5 , and HCI-NaCl buffer for pH 2. Biphthalate was used after being tested for possible interference: I ) with the U/Th uptake capacity of the biomass by determining the biosorptive uptake capacity of 30 mg of R . arrhizus at pH 4 (C, = 100 mg/liter) with and without biphthalate (unbuffered solution pH at equilibrium was pH 3.9.); and 2) with the employed analytical technique by analyzing standard U solutions of equal uranium concentrations with and without biphthalate and comparing their absorbance values over the examined U concentration range. Biosorption isotherms were determined at a temperature of 23°C. In addition, Rhizopus arrhizus biosorption isotherms were determined at 5 and 40°C. RESULTS The samples of sorbent materials were examined for their sorption of uranium and thorium. The resulting biosorption isotherms, plots of loading (uptake) values q (in mg/g) against residual equilibrium concentration of the element remaining in the solution after contact, C,, (in mg/liter), served for quantitative comparison of the different materials. pH Effect

Uranium The examined biomass types can be separated into two groups. The first group comprises the samples that did not present a significant difference in U uptake between pH 2 and pH 4. Pseudomonasfluorescens (Fig. l), Penicillium chrysogenum, municipal waste activated sludge, and Aspergillus terreus belong to this group. The second group presented significantly lower U uptake at pH 2 than at pH 4. Industrial activated sludge (Fig. 2 ) , the ion exchange resin IRA400 (Fig. 3) and the activated carbon F-400 (Fig. 3) and, to a certain extent, also Streptomyces niveus (Fig. 1) form the second group. The latter two materials exhibited zero U uptake at pH 2. No discernible difference was observed in uranium uptake between pH 4 and pH 5 for all materials tested, as seen in Figures 1 , 2, and 3. Table I1 compares U uptake capacities of all examined materials at equilibrium (residual) concentrations C,, of 5, 30, and 700, respectively. The con-

588

TSEZOS AND VOLESKY

23OC

Fig. 1. Linearized uranium biosorption isotherms for Pseudomonas fluorescens ( q = 0.095 C’/0.823) and Strepiomyces niveus ( q = 5.55 C’’3.8”,pH 2; q = 7.27 C”3.75,pH 4,5).

centration of 5 mg/liter is specified as a maximum permissible uranium concentration in surface waters by the Canadian EPS. Rhizopus urrhizus showed the highest maximum uranium loadings in excess of 180 mg/g. Penicillium chrysogenum exhibited a similar maximum; however, its isotherm was not as steep, indicating lower attainable loadings at lower residual (equilibrium) concentrations of U. Selected major uranium isotherms are presented in Figure 4 in their nonlinearized form for better comparison. Thorium The individual thorium uptake capacity that was exhibited by each

tested material at pH 4, did not change when the pH was raised to 5. Three materials of microbial origin-Pseudomonas fluorescens and Streptomyces niveus (Fig. 5 ) , AspergilEus niger (Fig. 6)-and the two synthetic materials IRA-400 and act. carbon F-400 (Fig. 7) are presented as examples of this behavior.

BIOSORPTION OF URANIUM AND THORIUM

589

Reduced loadings q were exhibited by all materials (with the exception of Aspergillus niger) at pH 2 as compared to q at pH 4. This can be clearly seen, for example, in Figures 5 , 6, and 7. Figure 6 presents data for Aspergillus niger, the only exception which showed similar uptake capacity for thorium at pH 2 and pH 4. Thorium uptake capacities of all tested materials are compared in Table I11 at the common equilibrium concentrations of 5 , 30, and 70 mg/liter Th4+. Maximum permissible thorium concentration in water depends strongly on the atomic number of the radioisotope. For “natural” thorium, Th232,this has been proposed to be 5.5 mg/liter (600 pci/liter).*Rhizupus arrhizus showed the highest Th4+ uptake, reaching loadings of 185 mg/g at C,, = 30 mg/liter, featuring a desirable steeply rising isotherm which indicates a very active biosorption even at relatively low residual (equilibrium) concentrations of Th in solution. Selected major thorium isotherms are presented in Figure 8 in their nonlinearized form for better comparison.

=

Fig. 2. Linearized uranium biosorption isotherms for “phenolic” activated sludge [9 5.929 C’/’-’Oz,pH 2; 1 ) 9 = 3.328 C”’.h90,2) 9 = 6.909 C”2~72s, pH 4, 51.

590

TSEZOS AND VOLESKY

P Fig. 3. Linearized uranium biosorption isotherms for IRA-400 [ I ) q = 6.107 C'''.'%, 2) q = 22.295 C"4.79',pH 4, 5) and F-400 (q = 7.303 C''2.2'6,pH 4, 51.

Temperature Effect on q

The effect of temperature on equilibrium uptake capacity of Rhizopus arrhizus, at p H 4, was similar for uranium and thorium. Temperature increase favored the equilibrium uptake of U and Th within the concentration range examined. Temperature increase resulted in a small q increase. Figures 9 and 10 illustrate the effect of temperature on uranium and thorium uptake 3f R . arrhizus at pH 4 while Table IV presents numerically the percentage differences for selected equilibrium concentrations. Cell Wall Preparation Isotherm

In Figures 9 and 10 the uranium and thorium uptake exhibited by the cell wall preparation of Rhizopus arrhizus is presented. The cell wall preparation uptake is plotted on the same diagrams with the uptake of whole mycelia, so that comparison can be made easily. It can be seen that under

A. terreus

6 8

Residual conc. (mghiter)

5 30 700

10 17 22

A. niger

6 13 31

I I

1

A. niger

A. terreus

5 30 700

Residual conc. (mg/liter) I1 18 40

5 12 45

10 26 78

8 13 15

P . fluorexens 27 34

18

29 46 47

33 51 51

Municipal Phenolic activated activated S . niveus sludge sludge

Material

120 I42

P . chrysogenum

40 70 I65

P . chrysogenum

TABLE 111 Thorium Biosorption Uptake Capacity q (mg/g)

6 6

P . fluorescens

Municipal Phenolic activated activated S . niveus sludge sludge

Material

TABLE I1 Uranium Biosorption Uptake Capacity q (mg/g)

185

140

R. arrhizrrs

80 140 >I80

R. arrhizrrs

3 9 14

Ionex IRA-400

31 45 79

20 61

Activated carbon F-400

34 145

15

Activated lonex carbon IRA-400 F-400

2

20

z 9 zU

C

z

P

C

%

8z

c

Is

z0

TSEZOS AND VOLESKY

592

URANIUM

23O C pH= 4 ; S 200 -

Rhizopus arrhizus

chrysogenum

ionex I R A - 4 0 0

h, mg/t u+'

in solution

Fig. 4. Comparison of uranium uptake capacities for selected sorbent materiais.

the conditions tested, the cell wall preparations exhibited somewhat higher (by approximately 10%) uptake than the whole mycelia. Biosorption Isotherm Model Fitting

All determined biosorption isotherms exhibited a shape resembling that of common activated carbon adsorption isotherms. Hence, it was decided to fit the available biosorption data with two of the most widely accepted adsorption isotherm models, namely those of Langmuir and Freudlich. Model parameters and the standard error of estimate (SEE) values were determined for each isotherm. The value of SEE measures the deviation

BIOSORPTION OF URANIUM AND THORIUM

593

TABLE IV Temperature Effect (%) on the Equilibrium Uptake ( q ) by Rhizopus arrhizus Temperature (“C) C,, (mg/liter)

5 and 23

23 and 40

5 and 40

- 4% - 2% - 1%

+ 30% +21% + 14%

+ 24% + 24% + 16%

+ 14% + 16% + 18%

+ 28% + 23% + 17%

+ 46% + 43% + 38%

Uranium 50 65 80 Thorium 15 20 30

23O C S. niveus 0 A

1001

10-1

I

I

I

I

I I I I I

100

I

&,.

I

1

mg/l

I

I 1 1 1 1

Th+4

P.fluorescens

pH

2

0

4

0

5

A

I

I

I

l

l

l

l

l

10’ in solution

Fig. 5 . Linearized thorium biosorption isotherms for Pseudomonas Jluorescens (4 = 2.996 C”3.h19, 2: q = 6.264 C”4.x59, pH 4, 5 ) and for Strepromyces niveus (4 = 11.709 c114.846 , pH 2: pH q = 10.312 C”3.259, pH 4, 5 ) .

TSEZOS AND VOLESKY

594

104

lo-'

I

I

I

I

1

~

1

1

1

1

I

I

I I

I I I I

100 Ceq, mg/l

Th+4

I

I

I

1

1

1

I01 in solution

Fig. 6. Linearized thorium biosorption isotherms for Aspergillus niger ( q , PH 2, 4, 5 ) .

1

102

=

5.823

~"3.169

of the data from a particular regression line and it has been used as a criterion that indicates how well a particular model fits a set of data. Table V summarizes the calculated SEE values for uranium and thorium biosorption data for all materials tested, at different pH values. The determined values for the parameters of the Freundlich model are also included. Comparison of the SEE values calculated for the two adsorption models shows that although both fit reasonably well the biosorption data for the examined concentration range, the fit of Freundlich model equation usually resulted in somewhat lower SEE values, thus indicating slightly better suitability of this model. Both models can be linearized. Because of the better fit of the Freundlich equation, biosorption isotherms have been linearized by using a log-log plot.

BIOSORPTION OF URANIUM A N D THORIUM

595

DISCUSSION p H Effect

Uranium

Most of the microbial biomass samples either did not present a discernible difference in their individual U uptake between pH 2 and pH 4 or presented a small one. Industrial activated sludge, the ion exchange resin and the activated carbon, however, presented significantly lower U uptake at pH 2 than at pH 4. More specifically, IRA-400 exhibited zero U uptake at pH 2 and a maximum loading of about 80 mg/g at pH 4. This behavior of the IRA-400 could be explained by considering that the anionic resin cannot retain the positively charged U02+ions that exist at pH 2. It can retain, however, negatively charged complex hydrolyzed uranium ions that have been suggested as existing" at pH 4. Hydrolysis products

23O C

I

F-400

A

pH

IRA-400

2 4 5

0 A

10

)*

C,,

mg/l

Th+4

in solution

Fig. 7. Linearized thorium biosorption isotherms for IRA-400 (q 4, 5) and for F-400 ( 9 = 0.048 C""."", pH 2; 9 = 5.728 C'/1.34').

=

1.274 C'/'.840,pH

596

TSEZOS AND VOLESKY ~ _ _ _ _ _ _ TnORlUr Rhlzopus a r r h i z u s

-

ac8.cmrbon F - 4 0 0

Pnoudomonm8 fluoroeeonn 10n.X

IRA-400

0

are in simultaneous dynamic equilibria, hence the resin can reduce the uranium concentration in the solution down to zero. Activated carbon F-400 also presented zero loading at pH 2 and a maximum uptake of about 105 mg/g at pH 4. Activated carbon showed a preference for the absorption of the bigger hydrolyzed composite uranium ions that also exist above pH 2.5 rather than for the simple uranyl ion. It is possible that this behavior is the result of the narrow overall pore volume distribution of this activated carbon that'5 favors the adsorption of the bigger size composite ions. None of the examined materials presented a significant difference in U biosorptive uptake between pH 4 and pH 5.

BIOSORPTION OF URANIUM AND THORIUM

597

The U 0 2 * hydrolysis products' distribution curves available in literature'* suggest that for low total U6+ concentrations and for pH below 5, UOz2+ continues to be one of the major uranium ionic species in solution. The following equations have been suggested as best describing U6+ hydrolysis in non complexing media:l7.I8 +

UOZ2+ + H2O

U 0 2 ( 0 H ) + + H + (log k l

=

-5.8)

+ 2H20 (U02)2(OH)22f + 2 H + (log kZ = -5.62) 3uo2 + 5H20 $ (U02)3(OH)5+ f 5 H + (log kj = 15.63)

2U022f

We can assume that the same equations approximate the equilibria in the dilute uranium aqueous solutions used in the present work, and a uranium mass balance along with the three equilibrium expressions yield a system of four equations, the numerical solution of which allows an estimation of the distribution of U6+ among the principle uranium-hy-

I

Fig. 9. Uranium biosorptive uptake of Rkizopus urrhizus whole cells and cell walls. Temperature effect: q = 25.81 C"2.'9(23 and 5°C).

598

TSEZOS AND VOLESKY

R. arrhisus , p H = 4 &5 --

A

5°C 23OC 40°C

0

23OC

0

0

cellwalls

? caq;mg/l

TI.+^

in solution

Fig. 10. Thorium biosorptive uptake of Rhizopus arrhizus, whole cells and cell walls. Temperature effect: I ) q = 64.5 C”3.34(5”C), 2) q = 64.78 C”3.24(23”C), 3) q = 117.53 C” 5 s 7 (40°C).

drolyzed ionic species. For a total U6+ concentration of 100 mglliter, at pH 4 about 80% of U6+ exists in the form of U02’+, while at pH 5 it drops to about 9%. At pH 2 it is the only species present. Biosorptive uptake of U6+ at pH 2 takes place through the biosorption of U02’+. Some waste biomass samples have exhibited similar uptakes at pH values of 2 , 4 , and 5. In view of the information on U6+ hydrolysis, and assuming that the waste biomass samples were not altered by the pH change, such behavior could be explained by the hypothesis that U022+ is the primarily biosorbed uranium species and due to the dynamic nature of the simultaneous equilibria, U6+ uptake appears the same at pH 2, 4, and 5. This hypothesis can also explain the observed increase in solution pH as biosorption proceeds in unbuffered solutions. Reverse hydrolysis reactions consume H + , thus reducing the pH of the solution. “Phenolic” activated sludge, however, presented a statistically significant lower uptake at pH 2 than at pH 4 or 5, thus indicating that the

-

1.311

2.587

3.300

2.443

13.831

13.521

5.525

1.841

A . terrus

P.jluorescens

S. niveus

Municipal sludge

“Phenolic” sludge

R . arrhizus

P. chrysogenirm

IRA-400

F-400

Overall SEE values.

6.388

A . niger

a

Langmuir

Material

2.523 (5.33; 2.36) 13.488 (14.25; 1.37) 14.067 (33.52; 5.36) 6.102 (9.36; 2.14) 2.088 (8.15; 2.56)

1.307 (2.26; 5.26) 2.233 (5.72; 2.88) 1.733 (3.17; 2.50)

6.330 (2.09; 1.75) -

Freundlich (Q; n )

Uranium

12.56

1.85

15.34

47.44

2.52

9.98

3.385

2.207

3.465

0.841

-

-

13.52

13.83

1.61

3.30

I .09

-

I .30 (3.00; 30.24) 0.9 I (5.62; 3.37) 1.73 (10.75; 37.17) I .79 (8.52; 53.28) 13.49 (14.25; 1.37) 14.06 (33.52; 5.36) -

I .31

1.12 (6.92; 73.7)

( Q ;n )

-

1.17

I .29 (5.63; 3.14) 4.06 (5.82; 76.04) 0.77 (6.68; 5.61) 2.39 (10.37; 3.50) 8.29 (16.34; 2.74) 5.79 (7.81; 2.74) 22.09 ) (63.84; 16.01 (107.27; 9.48) 1.08 (1.31; 1.92) 12.82 (10.68; 1.87)

1.12

-

8.93

18.18

4.06

9.98

3.38

0. I4

-

0.84

1.10 (0.30; 1.29)

3.37 (21.44; 5.05) 18.18 (52.42; 5.43) 10.65 (92.08; 28.68) -

8.30 (19.31; 3.27)

1.30 (3.79; 77.78) 2.39 (10.76; 3.76)

1.29 (5.63; 3.14) -

( Q ;n )

Freundlic h

Thorium Langmuir

PH 2

-

Langmuir

Freundlich

Uranium

CQ; n )

Freundlich

Thorium Langmuir

PH 4 , 5

TABLE V SEE” Values for Biosorption Isotherms

t n

2

2

U

z +z

5

C p 9

%

2 5 z

;a

8

z

600

TSEZOS AND VOLESKY

presence of the hydrolyzed U6+ ions, may enhance the uranium biosorptive uptake of certain biomass samples. Biosorptive uptake of uranium appears independent of U solubility as all examined waste biomass types exhibited pH independent q values for the pH range between pH 4 and pH 5 (in certain cases even at p H 2) despite a reduction of uranium solubility with increasing pH within the range tested. The concentrations of U in solution in this work, however, never exceeded the solubility limits under the given conditions. In general, we can say that biosorptive uptake of uranium at pH 4 and 5 is equal to or greater than the uptake at pH 2.

Thorium Thorium hydrolysis is more complicated than uranium hydrolysis. Indeed thorium solubility decreases dramatically with pH. Along with the main hydrolysis products thorium also exists in solution in the form of colloidal Th(OH)4(aq). In essence thorium solutions above pH 3 are supersaturated even at Th4+ concentrations of IO-’M(ca. 3 mg/liter),18 the colloidal particles being below 300 A in size. l9 At pH 2, however, thorium solubility is high with Th4+ as the soluble species. As the solubility decreases, with pH increase, Th(OH)3 , Th(OH)22+,Th2(OH)26+,Th(OH)3 , Th,(OH)?: appear as more significant ionic species,I8 Th4+ becoming increasingly less important. The existence of negatively charged hydrolyzed thorium ions at pH 4 and 5 is suggested by the behavior of the anionic resin IRA-400. At pH 2, IRA-400 did not uptake any thorium, thus indicating the existence, in solution, of only positively charged thorium ions. At pH 4, a loading of about 15 mg/g was determined, thus indicating the presence of exchangeable negatively charged hydrolysis products. The overall uptake capacity of the ion exchange resin for thorium is much lower than that for uranium. Filtrasorb 400 also exhibited a very low loading of about 4 mg/g at pH 2 and a much higher loading (over 60 mg/g at C,, = 30 mg/liter) at pH 4. In most examined cases, biosorptive uptake of Th4+ was higher at p H 4 or 5 than at pH 2. The information on Th4+ hydrolysis data indicates that a pH 2 biosorption of thorium occurs via the Th4+ species. At pH 4 or 5 Th4+ still exists. However, higher uptake values were experienced. The hydrolyzed thorium isonic species, namely Th(OH)22+that predominates under the experimental conditions used, probably favor the biosorption of thorium by being biosorbed more efficiently than the simple Th4+ ion. From pH 4 to pH 5 the reduction of thorium solubility is small, with Th(OH)22+continuing to be the dominating species. The relatively small change of the chemistry of the solution within this pH range might be +

+

BIOSORPTION OF URANIUM A N D THORIUM

60 1

accepted as the reason for the equal thorium biosorptive uptake exhibited at pH 4 and pH 5. Temperature Effect on q Adsorption and ion exchange are exothermic phenomena. In the case of the biosorption system studied here, equilibrium biosorptive uptake of thorium increased slightly with temperature increase. Biosorption of uranium exhibited higher uptake at 40°C as compared to that at 23 and 5°C. Table IV summarizes the differences observed in biosorptive uptake by R. arrhizus within the examined different temperature changes and for different C,, values. The table gives the percent change with respect to the q at the lower temperature value. Observed differences are statistically significant, for the 5-40°C change. Cell Wall Preparation Isotherm The cell wall preparations of Rhizopus arrhizus exhibited marginally higher uptake capacities for uranium and thorium than whole mycelia. Higher uptake capacities by the cell wall preparation could be interpreted as indicating that the cell wall may be a more biosorptively active constituent of the cell structure of Rhizopus arrhizus. Indeed, cell walls make up a significant fraction of the total dry weight of the cell and, assuming that the same average quantity of uranium or thorium is biosorbed per cell wall, the marginally higher loading observed, q , could be expected. However, based on the values observed here no conclusion can be drawn and further information is being collected on this aspect as the work continues. Biosorption isotherm Model Fitting Table V indicates that the Freundlich model describes the available experimental biosorption data reasonably well and, in most cases, better than the Langmuir model. Both equations, however, have been originally derived to describe adsorptive equilibrium uptake and the constants (parameters) of each model have been assigned specific physical meaning, corresponding to the mechanism assumed for adsorption. The fact that the available biosorption data can be described by the Freudlich equation does not necessarily imply that the two parameters can be assigned again the same physical meaning, since the equation has been used as a mathematical function simply in order to describe the collected experimental data points. It is not a product of theoretical considerations on the mechanism of biosorption, hence it should not be used without any prior testing outside the examined concentration range. It must also be pointed out that whenever high SEE values were determined, as for example in the case of Rhizopus arrhizus (Figs. 4 and

602

TSEZOS AND VOLESKY

8), the relative biosorption isotherm comprised a number of points of different loadings at zero residual concentration. Points of that nature [ ( q l , O ) , (q2,0), (q3,0),etc.] obviously cannot be described by either of the two models, thus yielding high SEE values. The Freundlich equation can easily be linearized by taking the logarithms of both sides of the equation: q

=

KC;?

In q

=

In K

(1)

+ ( l / n )In Ceq

(2)

where k,n are constants, q is the uptake capacity, and C,, is the equilibrium concentration. A plot of In q vs. In Ceqyields a straight line with In K as the intercept and l / n as the slope. For some of the examined materials, linearization of the biosorption data yielded two intercepting lines (Figs. 2, 5 , and 9), indicating a change in the response of the biosorption system at a certain equilibrium concentration. Such a change could be the result of the different response of the biosorbing material to the different solutes in solution. Differentiation of the Freundlich equation gives

The left-hand term of the equation has units of (mg/g)/(mg/liter) and represents the distribution of the ion under consideration, between the liquid and the solid phase. Thus, from biosorption isotherm data, after fitting the model and determining the model parameters, distribution coefficients can be calculated for any residual ion concentration. Precision and Accuracy of Experimental Results

The precision of the analytical technique used was 1.5% for both uranium and thorium determinations. This value represents the percent ratio of the standard deviation of concentrations obtained from replicate analysis of the same solution, over the mean of all the measurements. The precision of the overall method used for the evaluation of U/Th biosorptive uptake capacities q was estimated at 4% for uranium and 3% for thorium. These precision values represent the percent ratio of the standard deviation of q values obtained by analyzing different replicate samples, over the mean of all q values determined. The accuracy of the employed analytical technique, expressed by the standard deviation of the percentage difference between actual and experimentally determined solution concentrations, was estimated at the level of 1.8% for uranium and 1.4% for thorium. The 95% confidence limit of the evaluated q values stands at 6.5% for uranium and at 2.2% for thorium computed at q values of 132 mg/g for U and 143 mg/g for Th.

BIOSORPTION O F URANIUM AND THORIUM

603

Comparison of Experimental Data with Results Reported in Literature

Some experimental data on biosorption of uranium by Penicillium chrysogenum were reported by Jilek et al.’ Their results indicated that at pH 3 , dried P . chrysogenum mycelium is capable of a maximum uranium uptake of 140 mg/g, while “natural” mycelium uptook approximately 170 mg/g, both after 24 hr of “culture time.” These results correspond well with the biosorptive uptake of about 165 mg/g (at pH 4 and C,, of about 700 mg/liter) determined in the present work. Chiu4 also reported uranium uptakes by dry dead mycelia of unknown species in the range of about 162 mg/g. His results, however, cannot be compared directly with the results of the present work since the microorganism species had not been identified. However vague his observation was, it can serve as an indication of the order of magnitude of uptake capacities that might be expected from some mycelia and, again in that sense, it correlates well with data obtained in the present work.

References I . G. C. Polikarpov, in Radioecology ofAquatic Organisms (North Holland, New York, 1966). 2. Y. Tezuka, Appl. Microbiol., 222 (1969). 3. Y. Tanaka and S . Skoryna, Organic Macromolecular Binders of Metal Ions (Gastrointestinal Research Laboratory of McGill University, Montreal, 1970). 4. Y. S. Chiu, Ph.D. Thesis, University of Western Ontario, London, Ontario, 1972. 5. R. Jilek, J. Fuska, and P. Nemec, Biologia, 33(3), 201 (1978). 6. T. J . Beveridge, Can. J . Microbiol., 24, 89 (1978). 7. S. E . Shumate, 11, Biotechnol. Bioeng. Symp., 8 (1978). 8. D. Moffett, CANMET Rep. 76-19, Ottawa, Ontario, 1976. 9. EPS Report 3-WP-75-5, Ottawa, Ontario, 1975. 10. A. Reed, H. C. Meeks, S. E. Pomeroy, and V. Hale, “Assessment of Environmental Aspects of Uranium Mining and Milling,” NTIS PB 266 413, Washington, D.C., 1976. 11. Anon., “Stream Surveys in the Vicinity of Uranium Mills, 11, Area of Moab, UtahAugust 1960,” NTIS PB 260 277, Washington, D.C., 1961. 12. D. A. Clark, “State of the Art: Uranium Mining, Milling, and Refining Industry,” EPA 660/2-74-038, Ottawa, Ontario, 1974. 13. Z. Marcenko, Spectrophotometric Determination of the Elements (Wiley, New York, I 976). 14. A. Benedek, “Carbon Evaluation and Process Design,” Proceedings of the PCT Activated Carbon Adsorption in Pollution Control, Sponsored by Environment Canada, Ottawa, Ontario, 1974. 15. M. Tsezos, “Adsorption of Bioresidual Organics in a Fluidized Bed Biological Reactor,” Master’s Thesis, McMaster University, Hamilton, Ontario, 1979. 16. V. Rizza and J. M. Kornfeld, J. Gen. Microbiol., 58, 307 (1969). 17. P. N. Palei, Analytical Chemistry of Uranium (Hamphrey Science, London, 1970). 18. F. C. Baes and E. R. Mesmer, in The Hydrolysis of Cations (Wiley, New York, 1976). 19. A. S. Pershin, Radiokhimiya, 14(1), 88 (1972). 20. B. Volesky and M. Tsezos, “Recovery of Radioactive Elements from Waste Waters. Biosorption of Uranium and Thorium,” paper presented at the Joint ACS-CSJ Conference in Honolulu, Hawaii, April 1-6, 1979. 21. J . A. Hearne and A. G. White, J . Chem. SOC.,2168 (1957).

604

TSEZOS AND VOLESKY

22. D. Ryabchikov and E. Golbraikh, The Analytical Chemistry of Thorium (Pergamon, New York, 1962). 23. R. L. Scott and R. M. Hays, “Inactive and Abandoned Underground Mines, Water Pollution Prevention and Control,” EPA 600/7-75-007, Ottawa, Ontario, 1975. 24. J. F. Stara and D. Waldron, “Diagnosis and Treatment of Deposited Radionucleides,” Medical Symposium held at Richland, Washington, May 15-17 (Excerpta Medica Foundation, Princeton, NJ, 1967).

Accepted for Publication May 23, 1980.