# Biosorption of Uranium and Rare Earth Elements Using Biomass of Algae

Authors: Nobuo Sakamoto; Naoki Kano; Hiroshi Imaizumi
Journal: Bioinorganic Chemistry and Applications
 (2008)

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## Abstract

In order to investigate the behavior of rare earth elements (REEs) and uranium (U) in marine organism, the concentrations of REEs and U in some brown algae samples taken on the coast of Niigata Prefecture were determined. In addition, laboratory model experiment to uptake these elements using living and dried algae (Undaria pinnatifida and Sargassum hemiphyllum) was also carried out to survey the uptake and bioaccumulation mechanism of REEs and U in algae. Consequently, the following matters have been mainly clarified. (1) The order of the concentration of REEs for each organ in Sargassum hemiphyllum is “main branch” > “leaf” > “vesicle,” however for U, the order is “leaf” > “vesicle” > “main branch.” (2) The concentration of REEs in Sargassum hemiphyllum may be strongly affected by suspended solid in seawater. (3) The uptake and/or accumulate mechanism of REEs in brown algae may be different from that of U.


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## Body

## 1. Introduction

Thorium (Th) and uranium (U) are
natural radionuclides and are widely distributed in nature due to the nuclear power
production as well as to a number of human activities (e.g., mining, production,
and use of phosphate fertilizers, copper metallurgy, and military activities) [1, 2]. These elements are possible harmful pollutants in the
environment. Hence, investigating the concentrations of Th and
U in the
environment is significant from a radiation and pollution protection viewpoints
[3].On the other hand, determination
of rare earth elements (REEs) in marine organism such as seaweed is important
in studying the extent of these REEs to marine environment. REEs are considered
important tracers for studying the circulation of materials in the biosphere [4, 5]. Our knowledge about the environmental behavior of the REE has increased because
of the development of new analytical techniques.Marine organism such as seaweed
is well known to concentrate metals and has been used as a monitor of seawater
pollution [6–9]. Moreover, biosorption
studies using living biomass including seaweed have been widely performed in
large parts of the world recently [10–13].It is known that alginate is an
exopolymer extracted mainly from brown algae and various bacteria that have been used both as an
immobilization material and as a biosorbent of several heavy metals. Moreover,
alginate is one of the constituents of the cell walls of brown algae, and it consists
of mannuronic and guluronic acid monomers. Oligopolymeric blocks of guluronic
acid show a high selectivity for heavy metal ions [14, 15].The coasts in the vicinity of Niigata Prefecture (including Sado Island) are located in the geographic
position where they are affected by both the cold ocean and the warm ocean
current systems, so many kinds of marine organisms occur. However, in studies
of seaweed in Japan, most of
previous works have been conducted on the Pacific coast, and the chemical data for
seaweed on the coast of the Japan Sea
are extremely limited
(particularly regarding the determination of REEs, Th, and U).Therefore, we have determined the
concentrations of REEs [16], Th, and U [17] in various kinds of seaweed taken
on the coast of Niigata
Prefecture, and consequently the following matters have been mainly clarified. The concentrations of REEs in seaweed species from Niigata Prefecture
were generally about 102-103 times higher than those in seawater, and the
enrichment factors of REEs were larger in heavy REE (HREE) than light REE
(LREE). A significant concentration difference of REEs, Th, and U was found
among species even in the same phylum. The concentration of U was generally
higher in brown algae and was the highest in Undaria pinnatifida.Considering the above mentioned,
the concentrations of REEs and U
in some brown algae samples taken on the coast of Niigata Prefecture were
determined in present work to investigate the behavior and/or the biological
concentration of REEs and U in marine organism in more detail. Furthermore, to survey the uptake and
bioaccumulation mechanism of REEs and U in algae, laboratory model experiment to uptake REEs and U using living
and dried algae was also done. Among REEs, lanthanides (i.e., La-Lu)
were selected in analysis of environmental
samples (seaweed and seawater). The lanthanides (La-Lu), particularly, are of
great interest because of their similar chemical behavior that allows them to
be used as a tracer of a wide variety of geochemical processes. The REE pattern,
where the abundance of each lanthanide relative to that of a chondrite or shale
is plotted on a logarithmic scale against the atomic number, is therefore regarded
as a “finger-print” of a geological sample [18, 19]. In case of laboratory model experiment, La, Eu,
and Yb were selected as the representatives of light REE (LREE), medium REE
(MREE), and heavy REE (HREE) based on Diniz and Volesky’s work [11]. Two
species of brown algae: Sargassum hemiphyllum and Undaria pinnatifida, were chosen in this model experiment, because they are general
species in Japan (particularly, U. p. is used as a part
of daily diet in Japan), and
easily found at almost all sampling locations on the coast in Niigata Prefecture.


## 2. Experimental

### 2.1. Reagents

Lanthanide elements and U standard solutions used for making the calibration
curve were prepared by diluting the standard solutions (XSTC-1 for REEs and
XSTC-13 for 31 elements including U; both 10 mg dm-3 5% HNO3 solutions) purchased from SPEX CertiPrep, Inc. (NJ, USA). Alginate acid was
purchased from Acros Organics (NJ, USA).
All other  chemical reagents were purchased from Kanto
Chemical Co., Inc. (Tokyo, Japan). All reagents used were of
analytical grade, and deionized and distilled water was used.


### 2.2. Samples

Five species of brown algae:Dictyota dichotoma, Ecklonia stolonifera, Sargassum
hemiphyllum (abbreviated as S. h. below), Sargassum honeri, and Undaria pinnatifida (abbreviated as U. p. below), were
collected along several coasts in Niigata Prefecture in Japan
(Figure 1) from 2004 to 2007.
All samples were sampled at rock reefs about 100 m from the coast. For example, S. h. grew on rocks in the littoral zone. On the other hand, U. p. grew on rocks between the littoral zone and the infralittoral zone. Each
seaweed sample was washed in the surrounding seawater to remove attachment at
sampling place and was well washed by filtered seawater and deionized water in our
laboratory as outlined in the work of Kato et al. [20].Figure 1
Location of sampling points. (1) Sampoku town (2) Niigata city(3) Izumozaki town (4) Kashiwazaki city (5) Itoigawa city (6) Sado city.Seawater samples were also
collected from each sampling point. Each seawater sample was filtered through a
0.45μm membrane filter immediately after sampling. The basic physical parameters such as water temperature,
pH, electric conductivity (EC), oxidation-reduction potential (ORP),
and dissolved oxygen (DO) of the samples were measured by a handy pH/COND and pH/DO
meter (HORIBA, D-24, and D-25) at each sampling
point.


## 2.1. Reagents

Lanthanide elements and U standard solutions used for making the calibration
curve were prepared by diluting the standard solutions (XSTC-1 for REEs and
XSTC-13 for 31 elements including U; both 10 mg dm-3 5% HNO3 solutions) purchased from SPEX CertiPrep, Inc. (NJ, USA). Alginate acid was
purchased from Acros Organics (NJ, USA).
All other  chemical reagents were purchased from Kanto
Chemical Co., Inc. (Tokyo, Japan). All reagents used were of
analytical grade, and deionized and distilled water was used.


## 2.2. Samples

Five species of brown algae:Dictyota dichotoma, Ecklonia stolonifera, Sargassum
hemiphyllum (abbreviated as S. h. below), Sargassum honeri, and Undaria pinnatifida (abbreviated as U. p. below), were
collected along several coasts in Niigata Prefecture in Japan
(Figure 1) from 2004 to 2007.
All samples were sampled at rock reefs about 100 m from the coast. For example, S. h. grew on rocks in the littoral zone. On the other hand, U. p. grew on rocks between the littoral zone and the infralittoral zone. Each
seaweed sample was washed in the surrounding seawater to remove attachment at
sampling place and was well washed by filtered seawater and deionized water in our
laboratory as outlined in the work of Kato et al. [20].Figure 1
Location of sampling points. (1) Sampoku town (2) Niigata city(3) Izumozaki town (4) Kashiwazaki city (5) Itoigawa city (6) Sado city.Seawater samples were also
collected from each sampling point. Each seawater sample was filtered through a
0.45μm membrane filter immediately after sampling. The basic physical parameters such as water temperature,
pH, electric conductivity (EC), oxidation-reduction potential (ORP),
and dissolved oxygen (DO) of the samples were measured by a handy pH/COND and pH/DO
meter (HORIBA, D-24, and D-25) at each sampling
point.


## 3. Analytical Method

### 3.1. Determination of Rees and U in Seawater Samples

The preconcentration of lanthanides
and U was carried out according to Takaku et al. [21], and the procedure is
briefly described as follows. These elements in seawater samples were separated
from matrix and concentrated by a chelate disk (47 φmm) (Empore Sumitomo
3M Co., Tokyo, Japan).
In the separation process, the disk was placed in an ordinary disk holder. Each
sample was run through the disk after adjusting the pH to 3 by using ammonium
acetate (CH3COONH4) and nitric acid (HNO3).
Then, lanthanides and U on the disk were eluted by 1.5 mol dm-3 HNO3 (20 cm3). Quantitative recovery of REEs and U from seawater was
determined by
“addition and recovery testing”: adding
the subject element of known concentration to a sample, and measuring the concentrations
of the element in both this additional sample and a nonadditional sample
(comparing the analytical result of this additional sample with that of nonadditional
sample). It was confirmed that quantitative recovery of lanthanides and U in
seawater samples was obtained.  The suspended
solid on membrane filter was dissolved  with 10 cm3 HNO3, 5 cm3 HF  and, a proper amount of  H2O2  in a PTFE beaker. The
sampling solution was evaporated to dryness. After that, the residue was
dissolved with 50 cm3 of 1 mol dm-3 HNO3 again.
After the preconcentration procedure, the concentrations of lanthanides and U
in samples were measured with an ICP-MS (HP4500; Yokogawa Analytical Systems, Tokyo, Japan).
The operating condition of ICP-MS (including the measured isotope) is the same
as that in our previous work [17].


### 3.2. Determination of Rees and U in Seaweed Samples

Each seaweed sample was dried at 110°C
for 24 hours, and was ashed at 550°C for 48 hours. In addition, dry S. h. sample was separated into some
different organs such as leaf, main branch, and vesicle.  The ash (ca. 0.5 g) and each
organ of dry S. h. (ca. 1.0 g) were
dissolved  with 10 cm3 HNO3, 5 cm3 HF, and a proper amount of H2O2 in a PTFE beaker. The sample solution was evaporated to dryness. After that, the residue was
dissolved again with 50 cm3 of 1 mol dm-3 HNO3.
The decomposition of seaweed was based on the procedure described by Fu et al.
[22] and Sakao et al. [6]. After the above-mentioned procedure, the
concentrations of lanthanides and U in these samples were measured with ICP-MS.


### 3.3. Determination of Alginate Content in Seaweed Samples

The extraction of alginate was based on the work of Tamiya and
Watanabe [23], and the procedure is briefly described as follows. S. h. and U. p. samples were washed with tap water
and then dried at 50°C for 48 hours. Each
sample (ca. 10 g) was
shaken with 1 dm3 of 0.2 N H2SO4 for 24 hours.
The solution was filtered through membrane (Filter Paper Qualitative 2,
Advantec), and the residue was shaken with 1 dm3 of 1% Na2CO3 for 24 hours. The solution was filtered again and was stirred with 1 dm3 of ethanol to precipitate alginic acid as sodium alginate. Suspension was
washed with 0.2-0.3 dm3 of
ethanol and diethyl ether and was dried at 30°C for 24 hours. The weight
of dried sodium alginate was measured.


### 3.4. Model Experiment Using Fresh and Dry Seaweeds

Samples used in this model experiment were freshS. h. and U. p. In addition to alginic acid, dried U. p. was also used as reference. Dried sample was washed by
deionized water and dried for 48 hours at 50°C. Alginic acid was
shaken in filtered seawater for 24 hours. Seawater used in this study was filtered
by Advantec TCW-3N-PPSE filter. The initial concentration of lanthanides and U in
seawater was adjusted approximately to 6 ppb by adding lanthanide and U standard solution. Biomass
samples (ca. 0.8 g
dried weight) and alginic acid (ca. 0.4 g) were shaken in 200 cm3 prepared seawater
on prescribed time (10, 20, 30, 40, 50, 60, 120, 360, 720, 1080, 1440, and 1800
minutes) at 15°C. Afterwards, matrix ions such as Na and K were
removed, and the subject elements such as lanthanide and U were concentrated by
using chelating cation-exchange resin (BioRad Chelex 100 Resin). Initial and final
concentrations of lanthanides and U in solution were also measured with ICP-MS.
The uptake amount (A) of lanthanides and U due to  each sample was estimated by subtracting
the final concentrations (Cf) from initial concentrations (Ci)
in the liquid phase expressed as the following
equation: (1)A=(Ci−Cf)V/Mm[moleg−1], where V is the volume of
the solution (0.2 dm3), M is the atomic weight of each element, and m
is the dry weight of each sample.


## 3.1. Determination of Rees and U in Seawater Samples

The preconcentration of lanthanides
and U was carried out according to Takaku et al. [21], and the procedure is
briefly described as follows. These elements in seawater samples were separated
from matrix and concentrated by a chelate disk (47 φmm) (Empore Sumitomo
3M Co., Tokyo, Japan).
In the separation process, the disk was placed in an ordinary disk holder. Each
sample was run through the disk after adjusting the pH to 3 by using ammonium
acetate (CH3COONH4) and nitric acid (HNO3).
Then, lanthanides and U on the disk were eluted by 1.5 mol dm-3 HNO3 (20 cm3). Quantitative recovery of REEs and U from seawater was
determined by
“addition and recovery testing”: adding
the subject element of known concentration to a sample, and measuring the concentrations
of the element in both this additional sample and a nonadditional sample
(comparing the analytical result of this additional sample with that of nonadditional
sample). It was confirmed that quantitative recovery of lanthanides and U in
seawater samples was obtained.  The suspended
solid on membrane filter was dissolved  with 10 cm3 HNO3, 5 cm3 HF  and, a proper amount of  H2O2  in a PTFE beaker. The
sampling solution was evaporated to dryness. After that, the residue was
dissolved with 50 cm3 of 1 mol dm-3 HNO3 again.
After the preconcentration procedure, the concentrations of lanthanides and U
in samples were measured with an ICP-MS (HP4500; Yokogawa Analytical Systems, Tokyo, Japan).
The operating condition of ICP-MS (including the measured isotope) is the same
as that in our previous work [17].


## 3.2. Determination of Rees and U in Seaweed Samples

Each seaweed sample was dried at 110°C
for 24 hours, and was ashed at 550°C for 48 hours. In addition, dry S. h. sample was separated into some
different organs such as leaf, main branch, and vesicle.  The ash (ca. 0.5 g) and each
organ of dry S. h. (ca. 1.0 g) were
dissolved  with 10 cm3 HNO3, 5 cm3 HF, and a proper amount of H2O2 in a PTFE beaker. The sample solution was evaporated to dryness. After that, the residue was
dissolved again with 50 cm3 of 1 mol dm-3 HNO3.
The decomposition of seaweed was based on the procedure described by Fu et al.
[22] and Sakao et al. [6]. After the above-mentioned procedure, the
concentrations of lanthanides and U in these samples were measured with ICP-MS.


## 3.3. Determination of Alginate Content in Seaweed Samples

The extraction of alginate was based on the work of Tamiya and
Watanabe [23], and the procedure is briefly described as follows. S. h. and U. p. samples were washed with tap water
and then dried at 50°C for 48 hours. Each
sample (ca. 10 g) was
shaken with 1 dm3 of 0.2 N H2SO4 for 24 hours.
The solution was filtered through membrane (Filter Paper Qualitative 2,
Advantec), and the residue was shaken with 1 dm3 of 1% Na2CO3 for 24 hours. The solution was filtered again and was stirred with 1 dm3 of ethanol to precipitate alginic acid as sodium alginate. Suspension was
washed with 0.2-0.3 dm3 of
ethanol and diethyl ether and was dried at 30°C for 24 hours. The weight
of dried sodium alginate was measured.


## 3.4. Model Experiment Using Fresh and Dry Seaweeds

Samples used in this model experiment were freshS. h. and U. p. In addition to alginic acid, dried U. p. was also used as reference. Dried sample was washed by
deionized water and dried for 48 hours at 50°C. Alginic acid was
shaken in filtered seawater for 24 hours. Seawater used in this study was filtered
by Advantec TCW-3N-PPSE filter. The initial concentration of lanthanides and U in
seawater was adjusted approximately to 6 ppb by adding lanthanide and U standard solution. Biomass
samples (ca. 0.8 g
dried weight) and alginic acid (ca. 0.4 g) were shaken in 200 cm3 prepared seawater
on prescribed time (10, 20, 30, 40, 50, 60, 120, 360, 720, 1080, 1440, and 1800
minutes) at 15°C. Afterwards, matrix ions such as Na and K were
removed, and the subject elements such as lanthanide and U were concentrated by
using chelating cation-exchange resin (BioRad Chelex 100 Resin). Initial and final
concentrations of lanthanides and U in solution were also measured with ICP-MS.
The uptake amount (A) of lanthanides and U due to  each sample was estimated by subtracting
the final concentrations (Cf) from initial concentrations (Ci)
in the liquid phase expressed as the following
equation: (1)A=(Ci−Cf)V/Mm[moleg−1], where V is the volume of
the solution (0.2 dm3), M is the atomic weight of each element, and m
is the dry weight of each sample.


## 4. Results and Discussion

### 4.1. Behavior of Rees and U in Marine Environment

The
concentrations of lanthanides and U in five
kinds of brown algae taken on the coast of Niigata
Prefecture are shown in Table1. The
relative standard deviation (RSD) of three replicated analyses of each sample
was less than 10%. In Table 1, the concentrations
of elements in seaweed are expressed on the basis of dry weight (in addition to
ash weight). From this table, it is noted that U. p. showed highest concentration of U in five samples. In regard to lanthanide
elements, the concentrations of elements of even atomic numbers (i.e., Ce, Nd,
Sm, Gd, Dy, Er, and Yb) were generally larger than those of neighboring  elements of odd atomic numbers (i.e., La, Pr,
Eu, Tb, Ho, Tm, and Lu), obeying Oddo-Harkins’ law.Table 1
The concentrations (ng g-1) of lanthanide and U in brown algae taken on the coast of Niigata  prefecture.
Concentration/ng g-1*Dictyota dichotoma(n=2)Ecklonia stolonifera(n=1)Sargassum hemiphyllum(n=5)Sargassum honeri(n=5)Undaria pinnatifida(n=5)La(1.1–1.3) × 1021.0 × 102(1.9–2.5) × 102(1.4–3.0) × 102(0.93–1.3) × 10235–382038–5333–6630–48Ce(2.4–2.6) × 1024.4 × 102(3.3–4.0) × 102(3.1–4.4) × 102(1.9–2.8) × 10274–799271–1.0 × 10277–9567–1.0 × 102Pr38–4026(1.9–4.4) × 10237–60(2.1–3.3) × 10212-135.44.9–9.79.3–135.3–11Nd(2.1–2.4) × 1021.3 × 102(1.9–2.2) × 102(15–23) × 10292–1.5 × 10-268–702740–5134–4923–44Sm85–998040–5336–5214–3428-291610–138.2–123.6–12Eu41–493.1 × 10212–1512–225.0–1113-14642.7–3.92.6–4.81.9–3.9Gd(1.8-1.9) × 1021.5 × 10249–5837–7016–3955–623110–158.0–153.8–12Tb30–361.1 × 1027.1–106.1–102.4–8.79.7–10231.4–2.41.3–2.26.0 × 10-1–3.1Dy(2.3–2.8) × 1021.4 × 10250–6634–699.6–2977–802811–169.4–153.4–9.9Ho53-541.2 × 1028.5–128.9–163.5–8.416-17241.9–3.22.0–3.59.0 × 10-1–2.8Er(1.5–1.8) × 1021.3 × 10231–4013–2213–2450–53287.8–9.22.9–4.83.8–7.4Tm22–271.2 × 1022.8–3.63.0–7.89.5 × 10-1–2.87.2–7.825(5.9–9.1) × 10-16.5 × 10-1–1.6(2.4–9.8) × 10-1Yb(1.4–1.8) × 1021.4 × 10230–3224–2912–2448–53306.2–7.96.1–7.33.8–8.5Lu21–281.4 × 1024.4–6.12.8–7.22.7–6.37.0–9.0289.0 × 10-1–1.67.1 × 10-1–1.57.6 × 10-1–2.3U(1.4–1.7) × 1031.3 × 103(1.4–1.8) × 103(1.0–1.5) × 103(1.8–2.5) × 103(4.6–5.0) × 1022.7 × 102(2.9–4.8) × 102(2.4–4.2) × 102(4.9–8.2) × 102*The concentrations of elements are
expressed on the basis of both ash weight (the upper line) and dry weight
(the lower line) of each brown algae species.The concentrations of lanthanides in each sample
were normalized to Leedey chondrite, and the relative concentrations are
plotted in logarithmic scale against atomic numbers (i.e., REEs patterns). REEs
patterns for five species of brown algae in this work were shown
in Figure2. In this figure, the average concentrations of each sample are indicated.Figure 2
Normalized REE concentrations for brown algae samples (∘: Dictyota dichotoma,   Δ: Ecklonia stolonifera, □ Sargassum hemiphyllum, ⋄: Sargassum honeri, •: Undaria pinnatifida).From this figure, the tendency of light REE
(LREE) enrichment was generally found for many samples exceptDictyota
dichotoma. It is
suggested that these samples are affected by crustal source (i.e., soil origins)
to some extent. Moreover, large deviation of Eu in Ecklonia stolonifera is found. Most of the lanthanide
elements have the valency of +3 on the earth, while only Ce and Eu are known to
take different valencies; Eu may have valency of +2 besides +3 [24]. Then, the
deviation of Eu in Ecklonia stolonifera may be closely related to the anomalous behavior of Eu
(e.g., difference in solubility or stability of compounds) due to different
valencies, although the cause of Eu deviation in this sample has yet to be
sufficiently clarified in our work.The concentrations of La, Eu, Yb, and U inS. h. and U.
p. bulk samples were
shown in Figure 3(a). Compared to U. p. sample, the concentrations
of REEs (i.e., La, Eu, and Yb) in S. h. were comparatively
higher, although the concentration of U is slightly smaller.The concentrations of REEs and U in brown algae sample. (a) Bulk sample; (): S. h., (□): U. p.; (b) each organ in S. h.; (): leaf, (□): main branch, (): vesicle.
(a)(b)The concentrations of La, Eu, Yb,
and U for each organ inS. h. are shown in
Figure 3(b). From this figure, it is found that the order of the concentration
of U is “leaf” > “vesicle” > “main branch.” On the other hand, the order
of the concentration of REEs (i.e., La, Eu, and Yb) is “main branch” > “leaf”
> “vesicle.” In case of REEs, the difference of accumulation among organs is
remarkably observed. It is suggested that the degree of uptake and/or
accumulation for each organ may strongly depend on the character of elements.It is considered that alginate is
significant for the uptake and/or accumulation of metals in brown algae. Hence,
the content of alginate inS. h. and U. p. was determined (Table 2).Table 2
Alginate content (%) in dryS. h. and U. p. samples.
SpeciesAlginate content  (%)S. h.(n=2)43.9±4.1U. p.(n=2)48.3±2.6As shown in Table2, no definite
difference of alginate content between these two kinds of algae was observed, although
the content of alginate in U. p. was slightly larger
than that in S. h. Furthermore, Table 2 shows that the content
of alginate comprises close to half of the representative brown algae.The
concentrations of La, Eu, Yb, and U in seawater samples are
shown in Figure4(a) (dissolved fraction) and Figure 4(b) (suspended solid
fraction). From Figure 4(a), it is found that the concentration of U in
seawater (dissolved fraction) is about 102-103 times higher than that of lanthanides. On the other hand, the concentration of La
in suspended solid fraction in seawater is larger than that of Eu, Yb, and U.
It is suggested that the concentration of REEs, particularly LREE (light REE)
such as La in S. h. may be strongly affected by suspended solid in seawater.The concentrations of REEs and U in seawater samples. (a) Dissolved fraction; (b) suspended solid fraction. (): Itoigawa city, (□): Izumozaki town, (): Kashiwazaki city.
(a)(b)In
this work, the concentration factor is defined as the ratio of “the
concentrations of elements in sample such as seaweed (ngg-1)” to
“the concentrations of elements in seawater (μg dm-3)” based on the work of Koyama et al.
[7]. The average concentration factor of La, Eu, Yb,
or U in S. h. and U. p. is shown
in Table 3.Table 3
The average concentration factor of La, Eu, Yb, or U
inS. h. and U. p.
LaEuYbUS.
h.2.4 × 1039.2 × 1022.8 × 1031.4 × 102U.
p.1.8 × 1037.9 × 1022.4 × 1032.5 × 102For both algae, the concentration factors of lanthanides (La, Eu, Yb) are much larger than those of U. In addition, it is noteworthy that the concentration
factors of lanthanides
are larger inS. h. than in U. p., whereas those of U are smaller in S. h.


### 4.2. Behavior of Rees and U in Model Experiment

Time dependency of the amount of REEs
(i.e., La, Eu, and Yb) adsorption to dryU. p. is shown in Figure 5. From this figure, it is found that 12 hours were required to achieve
equilibrium. The amount of adsorption varies depending on each element, and the
order of adsorption capacity was Eu ≥ La > Yb. This order was in accordance with
that of Diniz’s work [11]. They suggested that it may be attributable to the distribution
coefficient and affinity to biomass. However, the adsorption behavior is similar among three elements.
Based on this result, La was used as the representative of REEs in model
experiment hereinafter.Figure 5
The amount of REEs adsorption to driedS. h. sample. (∘): La, (Δ): Eu, (□): Yb.The adsorption amount of La or U to
each sample is shown in Figure6
(a) for La and Figure 6
(b) for U. Figure 6(a) is replotted in logarithmic
scale until the equilibrium is attained (Figure 6(c)). The adsorption amount of
La on living biomass was similar
to that on dry biomass. However, the difference of reaction rate between
living biomass and dry biomass was remarkably observed. On the other hand, 24
hours were required to achieve equilibrium in case of the adsorption of U (Figure
6(b)).  Figure 6(b) is also replotted in
logarithmic scale until the equilibrium is attained (Figure 6(d)). From this
figure, the difference of reaction rate of U among these samples was slightly
found, however the difference was not as large as that of REEs.The amount of (a) La or (b) U adsorption to each sample. (∘): S. h., (Δ): U. p., (□): dry U. p., (⋄): alginic acid. The amount of La or U adsorption to each sample. (∘): S. h., (Δ): U. p., (□): dry U. p., (⋄): alginic acid   ((c): La in logarithmical scale, (d): U in logarithmical scale).
(a)(b)(c)(d)The adsorption capacity of La
and U on each sample is shown in Table4. No large difference of adsorption capacity
between S. h. and U. p. was observed for both
elements, although S. h. showed slightly
higher adsorption capacity than U.
p. As shown in Figure 3(a), the concentration of La in S. h. is larger than that of U.
p. However, large difference of adsorption capacity of La between two species was
not observed in this model experiment.Table 4
Adsorption capacity
(nmol g-1/dry wt) of La and U on each sample.
S. h.U. p.DryU.   p.Alginic acidLa4.694.565.035.01U5.615.564.768.35In this model experiment, filtered
seawater was employed. In other words, the effect of suspended solid (in which the
concentration of La is large) is removed. That might be one reason for detecting
no large difference of adsorption capacity of La between two species.From Table4, it can be also observed
that the adsorption capacity of U was higher than that of La for both two
algae. It is reported that alginate adsorbs divalent metal ion selectively [14, 25]. It is known that U exists in different forms depending on pH, and that at
pH  ≤  4.3, U exists predominantly as monomeric species, UO22+,
and small amounts of UO2 (OH)+. At pH ≥ 5, colloidal or
oligomeric species, that is, (UO2)2(OH)22+,
(UO2)3(OH)5+, (UO2)4(OH)7+,
and (UO2)3(OH)7−, are formed [1, 26–28]. On the other
hand, it is considered that REEs usually exist as trivalent ions in environmental waters and that
organic complexes dominate at pH from 4 to 8, whereas carbonate complexes are
the predominant species at alkaline pH ≥ 8 and REEs relatively tend to be precipitated
at higher pH [29, 30].Thus, higher adsorption
capacity of U (than that of REEs) particularly found in alginate might be
attributable to the chemical behavior of element in solution (e.g., pH-dependence
of elements or stability of compounds), although further investigation of the uptake mechanism of elements is
needed.From this work, the behavior (or the uptake
method) of REEs and U on seaweed could be clarified to some extent. The data
obtained and the method used in this work can be useful for investigating the
marine environment.


## 4.1. Behavior of Rees and U in Marine Environment

The
concentrations of lanthanides and U in five
kinds of brown algae taken on the coast of Niigata
Prefecture are shown in Table1. The
relative standard deviation (RSD) of three replicated analyses of each sample
was less than 10%. In Table 1, the concentrations
of elements in seaweed are expressed on the basis of dry weight (in addition to
ash weight). From this table, it is noted that U. p. showed highest concentration of U in five samples. In regard to lanthanide
elements, the concentrations of elements of even atomic numbers (i.e., Ce, Nd,
Sm, Gd, Dy, Er, and Yb) were generally larger than those of neighboring  elements of odd atomic numbers (i.e., La, Pr,
Eu, Tb, Ho, Tm, and Lu), obeying Oddo-Harkins’ law.Table 1
The concentrations (ng g-1) of lanthanide and U in brown algae taken on the coast of Niigata  prefecture.
Concentration/ng g-1*Dictyota dichotoma(n=2)Ecklonia stolonifera(n=1)Sargassum hemiphyllum(n=5)Sargassum honeri(n=5)Undaria pinnatifida(n=5)La(1.1–1.3) × 1021.0 × 102(1.9–2.5) × 102(1.4–3.0) × 102(0.93–1.3) × 10235–382038–5333–6630–48Ce(2.4–2.6) × 1024.4 × 102(3.3–4.0) × 102(3.1–4.4) × 102(1.9–2.8) × 10274–799271–1.0 × 10277–9567–1.0 × 102Pr38–4026(1.9–4.4) × 10237–60(2.1–3.3) × 10212-135.44.9–9.79.3–135.3–11Nd(2.1–2.4) × 1021.3 × 102(1.9–2.2) × 102(15–23) × 10292–1.5 × 10-268–702740–5134–4923–44Sm85–998040–5336–5214–3428-291610–138.2–123.6–12Eu41–493.1 × 10212–1512–225.0–1113-14642.7–3.92.6–4.81.9–3.9Gd(1.8-1.9) × 1021.5 × 10249–5837–7016–3955–623110–158.0–153.8–12Tb30–361.1 × 1027.1–106.1–102.4–8.79.7–10231.4–2.41.3–2.26.0 × 10-1–3.1Dy(2.3–2.8) × 1021.4 × 10250–6634–699.6–2977–802811–169.4–153.4–9.9Ho53-541.2 × 1028.5–128.9–163.5–8.416-17241.9–3.22.0–3.59.0 × 10-1–2.8Er(1.5–1.8) × 1021.3 × 10231–4013–2213–2450–53287.8–9.22.9–4.83.8–7.4Tm22–271.2 × 1022.8–3.63.0–7.89.5 × 10-1–2.87.2–7.825(5.9–9.1) × 10-16.5 × 10-1–1.6(2.4–9.8) × 10-1Yb(1.4–1.8) × 1021.4 × 10230–3224–2912–2448–53306.2–7.96.1–7.33.8–8.5Lu21–281.4 × 1024.4–6.12.8–7.22.7–6.37.0–9.0289.0 × 10-1–1.67.1 × 10-1–1.57.6 × 10-1–2.3U(1.4–1.7) × 1031.3 × 103(1.4–1.8) × 103(1.0–1.5) × 103(1.8–2.5) × 103(4.6–5.0) × 1022.7 × 102(2.9–4.8) × 102(2.4–4.2) × 102(4.9–8.2) × 102*The concentrations of elements are
expressed on the basis of both ash weight (the upper line) and dry weight
(the lower line) of each brown algae species.The concentrations of lanthanides in each sample
were normalized to Leedey chondrite, and the relative concentrations are
plotted in logarithmic scale against atomic numbers (i.e., REEs patterns). REEs
patterns for five species of brown algae in this work were shown
in Figure2. In this figure, the average concentrations of each sample are indicated.Figure 2
Normalized REE concentrations for brown algae samples (∘: Dictyota dichotoma,   Δ: Ecklonia stolonifera, □ Sargassum hemiphyllum, ⋄: Sargassum honeri, •: Undaria pinnatifida).From this figure, the tendency of light REE
(LREE) enrichment was generally found for many samples exceptDictyota
dichotoma. It is
suggested that these samples are affected by crustal source (i.e., soil origins)
to some extent. Moreover, large deviation of Eu in Ecklonia stolonifera is found. Most of the lanthanide
elements have the valency of +3 on the earth, while only Ce and Eu are known to
take different valencies; Eu may have valency of +2 besides +3 [24]. Then, the
deviation of Eu in Ecklonia stolonifera may be closely related to the anomalous behavior of Eu
(e.g., difference in solubility or stability of compounds) due to different
valencies, although the cause of Eu deviation in this sample has yet to be
sufficiently clarified in our work.The concentrations of La, Eu, Yb, and U inS. h. and U.
p. bulk samples were
shown in Figure 3(a). Compared to U. p. sample, the concentrations
of REEs (i.e., La, Eu, and Yb) in S. h. were comparatively
higher, although the concentration of U is slightly smaller.The concentrations of REEs and U in brown algae sample. (a) Bulk sample; (): S. h., (□): U. p.; (b) each organ in S. h.; (): leaf, (□): main branch, (): vesicle.
(a)(b)The concentrations of La, Eu, Yb,
and U for each organ inS. h. are shown in
Figure 3(b). From this figure, it is found that the order of the concentration
of U is “leaf” > “vesicle” > “main branch.” On the other hand, the order
of the concentration of REEs (i.e., La, Eu, and Yb) is “main branch” > “leaf”
> “vesicle.” In case of REEs, the difference of accumulation among organs is
remarkably observed. It is suggested that the degree of uptake and/or
accumulation for each organ may strongly depend on the character of elements.It is considered that alginate is
significant for the uptake and/or accumulation of metals in brown algae. Hence,
the content of alginate inS. h. and U. p. was determined (Table 2).Table 2
Alginate content (%) in dryS. h. and U. p. samples.
SpeciesAlginate content  (%)S. h.(n=2)43.9±4.1U. p.(n=2)48.3±2.6As shown in Table2, no definite
difference of alginate content between these two kinds of algae was observed, although
the content of alginate in U. p. was slightly larger
than that in S. h. Furthermore, Table 2 shows that the content
of alginate comprises close to half of the representative brown algae.The
concentrations of La, Eu, Yb, and U in seawater samples are
shown in Figure4(a) (dissolved fraction) and Figure 4(b) (suspended solid
fraction). From Figure 4(a), it is found that the concentration of U in
seawater (dissolved fraction) is about 102-103 times higher than that of lanthanides. On the other hand, the concentration of La
in suspended solid fraction in seawater is larger than that of Eu, Yb, and U.
It is suggested that the concentration of REEs, particularly LREE (light REE)
such as La in S. h. may be strongly affected by suspended solid in seawater.The concentrations of REEs and U in seawater samples. (a) Dissolved fraction; (b) suspended solid fraction. (): Itoigawa city, (□): Izumozaki town, (): Kashiwazaki city.
(a)(b)In
this work, the concentration factor is defined as the ratio of “the
concentrations of elements in sample such as seaweed (ngg-1)” to
“the concentrations of elements in seawater (μg dm-3)” based on the work of Koyama et al.
[7]. The average concentration factor of La, Eu, Yb,
or U in S. h. and U. p. is shown
in Table 3.Table 3
The average concentration factor of La, Eu, Yb, or U
inS. h. and U. p.
LaEuYbUS.
h.2.4 × 1039.2 × 1022.8 × 1031.4 × 102U.
p.1.8 × 1037.9 × 1022.4 × 1032.5 × 102For both algae, the concentration factors of lanthanides (La, Eu, Yb) are much larger than those of U. In addition, it is noteworthy that the concentration
factors of lanthanides
are larger inS. h. than in U. p., whereas those of U are smaller in S. h.


## 4.2. Behavior of Rees and U in Model Experiment

Time dependency of the amount of REEs
(i.e., La, Eu, and Yb) adsorption to dryU. p. is shown in Figure 5. From this figure, it is found that 12 hours were required to achieve
equilibrium. The amount of adsorption varies depending on each element, and the
order of adsorption capacity was Eu ≥ La > Yb. This order was in accordance with
that of Diniz’s work [11]. They suggested that it may be attributable to the distribution
coefficient and affinity to biomass. However, the adsorption behavior is similar among three elements.
Based on this result, La was used as the representative of REEs in model
experiment hereinafter.Figure 5
The amount of REEs adsorption to driedS. h. sample. (∘): La, (Δ): Eu, (□): Yb.The adsorption amount of La or U to
each sample is shown in Figure6
(a) for La and Figure 6
(b) for U. Figure 6(a) is replotted in logarithmic
scale until the equilibrium is attained (Figure 6(c)). The adsorption amount of
La on living biomass was similar
to that on dry biomass. However, the difference of reaction rate between
living biomass and dry biomass was remarkably observed. On the other hand, 24
hours were required to achieve equilibrium in case of the adsorption of U (Figure
6(b)).  Figure 6(b) is also replotted in
logarithmic scale until the equilibrium is attained (Figure 6(d)). From this
figure, the difference of reaction rate of U among these samples was slightly
found, however the difference was not as large as that of REEs.The amount of (a) La or (b) U adsorption to each sample. (∘): S. h., (Δ): U. p., (□): dry U. p., (⋄): alginic acid. The amount of La or U adsorption to each sample. (∘): S. h., (Δ): U. p., (□): dry U. p., (⋄): alginic acid   ((c): La in logarithmical scale, (d): U in logarithmical scale).
(a)(b)(c)(d)The adsorption capacity of La
and U on each sample is shown in Table4. No large difference of adsorption capacity
between S. h. and U. p. was observed for both
elements, although S. h. showed slightly
higher adsorption capacity than U.
p. As shown in Figure 3(a), the concentration of La in S. h. is larger than that of U.
p. However, large difference of adsorption capacity of La between two species was
not observed in this model experiment.Table 4
Adsorption capacity
(nmol g-1/dry wt) of La and U on each sample.
S. h.U. p.DryU.   p.Alginic acidLa4.694.565.035.01U5.615.564.768.35In this model experiment, filtered
seawater was employed. In other words, the effect of suspended solid (in which the
concentration of La is large) is removed. That might be one reason for detecting
no large difference of adsorption capacity of La between two species.From Table4, it can be also observed
that the adsorption capacity of U was higher than that of La for both two
algae. It is reported that alginate adsorbs divalent metal ion selectively [14, 25]. It is known that U exists in different forms depending on pH, and that at
pH  ≤  4.3, U exists predominantly as monomeric species, UO22+,
and small amounts of UO2 (OH)+. At pH ≥ 5, colloidal or
oligomeric species, that is, (UO2)2(OH)22+,
(UO2)3(OH)5+, (UO2)4(OH)7+,
and (UO2)3(OH)7−, are formed [1, 26–28]. On the other
hand, it is considered that REEs usually exist as trivalent ions in environmental waters and that
organic complexes dominate at pH from 4 to 8, whereas carbonate complexes are
the predominant species at alkaline pH ≥ 8 and REEs relatively tend to be precipitated
at higher pH [29, 30].Thus, higher adsorption
capacity of U (than that of REEs) particularly found in alginate might be
attributable to the chemical behavior of element in solution (e.g., pH-dependence
of elements or stability of compounds), although further investigation of the uptake mechanism of elements is
needed.From this work, the behavior (or the uptake
method) of REEs and U on seaweed could be clarified to some extent. The data
obtained and the method used in this work can be useful for investigating the
marine environment.


## 5. Conclusion

The
concentrations of REEs and U in
some brown algae samples taken on the coast of Niigata
Prefecture were determined. In
addition, chemical analysis per each organ is performed inSargassum
hemiphyllum (S. h.). Consequently, it is found that the
concentrations of REEs (i.e., La, Eu, and Yb) in S. h. were comparatively higher than those in Undaria pinnatifida (U. p.), although the concentration of U in S. h. is slightly smaller. The order of lanthanides
concentration in S. h. is “main branch” > “leaf”
> “vesicle,” however for U, the order is “leaf” > “vesicle” > “main branch.”
The concentration of REEs such as La in S.
h. may be strongly affected by suspended solid in seawater.Moreover,
laboratory model experiment to uptake these elements
using living and dried algae (S. h. and U. p.) was also carried out to survey the uptake and bioaccumulation mechanism of REEs
and U in algae. Consequently, it is suggested
that the uptake and/or
accumulate mechanism of REEs in brown algae may be different from that of U due
to the chemical behavior of element.



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*Source: 706240-2008-12-01.xml*