Biosorption of Uranium and Thorium
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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 fermen-
tation 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 inde-
pendent 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 dis-
cernible 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 gen-
erally 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 se-
lective 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$01.20584 TSEZOS AND VOLESKY
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 polysac-
charides 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 prop-
erty 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 re-
tention 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 waste-
water treatment scheme for the decontamination of waste streams con-
taining ions of high-atomic-number elements such as uranium, thorium,
radium, etc. The study of biosorption of uranium and thorium is the sub-
ject 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 potentialBIOSORPTION 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 opera-
tions. 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, polon-
ium, etc., find their way to the natural surface water bodies and envi-
ronment.
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 ura-
nium 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 ele-
ments 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 clean-
up 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 fer-
mentations and wastewater treatment processes have been collected and
processed. The samples were requested from most of the known large-
scale fermentation industries in Canada and the United States. Eight dif-
ferent 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) Aspergillus terreus
3) Penicillium chrysogenum
4) Pseudomonas fluorescens
5) Streptomyces niveus
6) Municipal wastewater activated sludge
7) Industrial wastewater (phenolic), activated sludge
8) Activated carbon Filtrasorb 400 (Calgon Co.)
9) Ion exchange resin (anionic) IRA-400 (Rohm-Haas Co.)
10) Rhizopus arrhizus586 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 deter- mined by the pH values used. For each isotherm, initial uranium con- centrations were kept constant while the dry cell weight in each sample was varied. Separation of the sorbents from the solution was accomplished by vac- uum 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 sub- sequently 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 hy-
drolyzes 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. Po-
tassium 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 ana-
lytical technique by analyzing standard U solutions of equal uranium con-
centrations 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 load-
ing (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 dif-
ference 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 IRA-
400 (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 =
and Strepiomyces niveus ( q = 5.55 C’’3.8”,pH 2; q = 7.27 C”3.75,pH 4,5).
0.095 C’/0.823)
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 is-
otherm was not as steep, indicating lower attainable loadings at lower
residual (equilibrium) concentrations of U. Selected major uranium is-
otherms 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 Strep-
tomyces niveus (Fig. 5 ) , AspergilEus niger (Fig. 6)-and the two synthetic
materials IRA-400 and act. carbon F-400 (Fig. 7) are presented as ex-
amples 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 As-
pergillus 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” thor-
ium, Th232,this has been proposed to be 5.5 mg/liter (600 pci/liter).*Rhi-
zupus 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 concen-
tration range examined. Temperature increase resulted in a small q in-
crease. Figures 9 and 10 illustrate the effect of temperature on uranium
and thorium uptake 3f R . arrhizus at pH 4 while Table IV presents nu-
merically the percentage differences for selected equilibrium concentra-
tions.
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 prep-
aration 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 underTABLE I1
Uranium Biosorption Uptake Capacity q (mg/g)
Material
Municipal Phenolic Activated
z0
Residual activated activated lonex carbon Is
conc. (mg/liter) A. terreus A. niger P . fluorescens S . niveus sludge sludge P . chrysogenum R. arrhizrrs IRA-400 F-400 c
5 I 6 I1 5 10 40 80 31 15 8z
30 I 13 6 18 12 26 70 140 45 34
700 1 31 6 40 45 78 I65 >I80 79 145 %
C
P
z
C
TABLE 111 z
Thorium Biosorption Uptake Capacity q (mg/g) 9
zU
Material
Municipal Phenolic Activated
20
Residual activated activated Ionex carbon
conc. (mghiter) A. terreus A. niger P . fluorexens S . niveus sludge sludge P . chrysogenum R. arrhizrrs IRA-400 F-400 2
5 10 8 18 29 33 120 140 3 20
30 6 17 13 27 46 51 I42 185 9 61
700 8 22 15 34 47 51 14592 TSEZOS AND VOLESKY
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 deviationBIOSORPTION 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
Uranium
50 - 4% + 30% + 24%
65 - 2% +21% + 24%
80 - 1% + 14% + 16%
Thorium
15 + 14% + 28% + 46%
20 + 16% + 23% + 43%
30 + 18% + 17% + 38%
23O C
S. niveus pH P.fluorescens
0 2 0
4 0
A 5 A
1001 I I I I I I I I I I I 1 I I 1 1 1 1 I I I l l l l l
10-1 100 10’
&,. mg/l Th+4 in solution
Fig. 5 . Linearized thorium biosorption isotherms for Pseudomonas Jluorescens (4 =
2.996 C”3.h19,
c114.846 , pH 2: pH pH 4, 5 ) and for Strepromyces niveus (4 = 11.709
2: q = 6.264 C”4.x59,
pH 4, 5 ) .
q = 10.312 C”3.259,594 TSEZOS AND VOLESKY
104 I I I I 1 ~ 1 1 1 1 I I I I I I I I I I I 1 1 1 1
lo-' 100 I01 102
Ceq, mg/l Th+4 in solution
Fig. 6. Linearized thorium biosorption isotherms for Aspergillus niger ( q = 5.823
~"3.169
, PH 2, 4, 5 ) .
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 de-
termined values for the parameters of the Freundlich model are also in-
cluded.
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 usu-
ally 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 Freund-
lich 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 dis-
cernible 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 pH IRA-400
2
4 0
A 5 A
10
)*
C,, mg/l Th+4 in solution
Fig. 7. Linearized thorium biosorption isotherms for IRA-400 (q = 1.274 C'/'.840,pH
4, 5) and for F-400 ( 9 = 0.048 C""."", pH 2; 9 = 5.728 C'/1.34').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 max-
imum 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 pre-
sented 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 liter-
+
ature'* suggest that for low total U6+ concentrations and for pH below
5, UOz2+ continues to be one of the major uranium ionic species in so-
lution. 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)
2U022f + 2H20 (U02)2(OH)22f + 2 H + (log kZ = -5.62)
3uo2 + 5H20 $ (U02)3(OH)5+ f 5 H + (log kj = 15.63)
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
0 5°C
0 23OC
A 40°C
0 23OC 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 signif-
icant lower uptake at pH 2 than at pH 4 or 5, thus indicating that theTABLE V
SEE” Values for Biosorption Isotherms
PH 4 , 5 PH 2
Uranium Thorium Uranium Thorium
Freundlich Freundlich Freundlich Freundlic h z
Material Langmuir (Q; n ) Langmuir CQ; n ) Langmuir Langmuir
( Q ;n ) ( Q ;n ) 8
;a
A . niger 6.388 6.330 0.841 I .29 1.17 1.12 0.84 1.29
(2.09; 1.75) (5.63; 3.14) (6.92; 73.7) (5.63; 3.14)
2
5
A . terrus - - 3.465 4.06 - - - - z
(5.82; 76.04)
P.jluorescens 1.311 1.307 2.207 0.77 I .31 I .30 0. I4 1.30
%
C
(2.26; 5.26) (6.68; 5.61) (3.00; 30.24) (3.79; 77.78) p
S. niveus 2.587 2.233 3.385 2.39 I .09 0.9 I 3.38 2.39 9
(5.72; 2.88) (10.37; 3.50) (5.62; 3.37) (10.76; 3.76) 5
Municipal sludge 3.300 1.733 9.98 8.29 3.30 1.73 9.98 8.30 z
(3.17; 2.50) (16.34; 2.74) (10.75; 37.17) (19.31; 3.27) +z
“Phenolic” sludge 2.443 2.523 2.52 5.79 1.61 I .79 4.06 3.37 U
(5.33; 2.36) (7.81; 2.74) (8.52; 53.28) (21.44; 5.05)
R . arrhizus 13.831 13.488 47.44 22.09 13.83 13.49 18.18 18.18 2
(14.25; 1.37) (63.84; ) (14.25; 1.37) (52.42; 5.43)
P. chrysogenirm 13.521 14.067 15.34 16.01 13.52 14.06 8.93 10.65
(33.52; 5.36) (107.27; 9.48) (33.52; 5.36) (92.08; 28.68) 2
IRA-400 5.525 6.102 1.85 1.08 - - - -
(9.36; 2.14) (1.31; 1.92)
F-400 1.841 2.088 12.56 12.82 - - 1.12 1.10
(8.15; 2.56) (10.68; 1.87) (0.30; 1.29)
a Overall SEE values.
t n600 TSEZOS AND VOLESKY
presence of the hydrolyzed U6+ ions, may enhance the uranium bio-
sorptive 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. In-
deed 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 su-
persaturated 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 de-
creases, 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 hydro-
lyzed 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 thor-
ium 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 thor-
ium 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 predom-
inates under the experimental conditions used, probably favor the bio-
sorption 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 beBIOSORPTION 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 con-
stituent 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 con-
tinues.
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 (pa-
rameters) 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 math-
ematical function simply in order to describe the collected experimental
data points. It is not a product of theoretical considerations on the mech-
anism 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 de-
termined, as for example in the case of Rhizopus arrhizus (Figs. 4 and602 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 loga-
rithms of both sides of the equation:
q = KC;? (1)
In q = In K + ( l / n )In Ceq (2)
where k,n are constants, q is the uptake capacity, and C,, is the equilib-
rium 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 ma-
terials, 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 dif-
ferent solutes in solution.
Differentiation of the Freundlich equation gives
The left-hand term of the equation has units of (mg/g)/(mg/liter) and rep-
resents 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 ura-
nium and thorium determinations. This value represents the percent ratio
of the standard deviation of concentrations obtained from replicate anal-
ysis 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 ex-
perimentally 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 chry-
sogenum 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 microor-
ganism 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.
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