Langmuir and Freundlich parameters for biosorption of lanthanides on three kinds of Ca-loaded biomass at pH 4.0 ( --- represents the missing data due to the lack of sample)
Contamination of toxic metals in the aquatic environment is one of the most debated problems in the world with industrial development. Thus, the minimization and recovery of harmful pollutants such as heavy metals in natural environment is very significant . Various treatment technologies such as ion exchange, precipitation, ultrafiltration, reverse osmosis and electrodialysis have been used for the removal of heavy metal ions from aqueous solution . However, these processes have some disadvantages, such as high consumption of reagent and energy, low selectivity, high operational cost, and difficult further treatment due to generation of toxic sludge .
Among environmentally friendly technologies for the removal of heavy metals from aquatic effluent, biosorption has attracted increasing research interest recently [4-5]. The major advantages of biosorption are its high effectiveness in reducing the heavy metals and the use of inexpensive biosorbents . Biosorption studies using various low cost biomass as adsorbents have been currently performed widely for the removal of heavy metals from aquatic effluent [7-18].
Among many biosorbents, marine seaweed can be an excellent biosorbent for metals because it is well known to concentrate metals [19-20]. Seaweeds are reported to accumulate hydrocarbons (as well as metals); and they are exposed to the ubiquitous presence of organic micropollutants and can work as suitable biomonitors . Furthermore, it is considered that the shell (usually treated as waste material) can be also an promising adsorbent. The shell has an internal structure comprised of three distinct layers. The innermost layer (i.e., hypostracum) consists of aragonite; the middle layer (i.e., ostracum), which is the thickest of the three, consists of various orientations interbedded with protein molecules (conchiolin); and the outermost layer (i.e., periostracum) consists of chitin, which is represented as (C8H13NO5)n . Particularly, it is considered that protein (called “conchiolin”) including amino acid group play an important role for collecting trace metal in shell .
Biosorption studies have been mainly focused on toxic elements such as Cd, Pb, Cu, As and Cr for subject elements . In our research, the objective elements are mainly rare earth elements (REEs) from the viewpoint of resources recovery, although REEs do not represent a common toxic threat.
Rare earth elements (REEs) find wide range of applications as functional materials in agriculture and as other industrial products, then the demand of REEs in modern technology has increased remarkably over the past years [25-26]. These elements and their compounds have found a variety of applications especially in metallurgy, ceramic industry and nuclear fuel control . For example, current applications of lanthanum as a pure element or in association with other compounds are in super alloys, catalysts, special ceramics, and in organic synthesis . However, the shortage of trace metals including REEs (and the problem of stable supply for these metals) has been a concern in recent years. Therefore, the establishment of the removal or recovery method for trace metals is important from the viewpoint of resources recovery.
It is known that alginate is an exopolymer extracted mainly from brown algae (and various bacteria) that has been used both as immobilization material and as biosorbent of several heavy metals . Then, biosorption studies using seaweed have been generally concentrated on brown algae so far [30-31]. Green and red algae as well as brown algae were also used for biosorbent of REEs in the present work.
Considering the above-mentioned, laboratory model experiments for confirming the efficiency of marine biomass (seaweed and shell) as sorbent for REEs was designed in present work. Furthermore, the surface morphology of the marine biomass used in this work was determined by SEM (Scanning Electron Microscope) before and after metal adsorption.
The crystal structure, and the specific surface area of the shell biomass were also determined by by XRD (X-ray powder diffraction), and BET (Brunaeur, Emmet and Teller) and Langmuir method, respectively.
2. Experimental work
Many kinds of seaweeds samples (10 species of green algae, 21 species of brown algae and 21 species of red algae) were taken along several coasts in Niigata Prefecture (referred to the figure in our previous paper ) since April, 2004. Among seaweed species, the seaweeds for biosorbent used in this work were
Based on Diniz and Volesky’s study , each sieved biomass sample was loaded with Ca2+ in a solution of 50 mmol・dm-3 Ca(NO3)2 (biomass concentration of 10 g・dm-3) for 24 h under gentle agitation in order to remove the original cations on seaweed. Later, the biomass was washed with ultrapure water to remove excess Ca2+ until the mixture was reached approximately pH 5. Finally, the washed biomass was dried again overnight at 50°C in an electric drying oven, and stored in desiccators (containing silica gel as a desiccant) before use.
2.2. Sorption experiment for lanthanides using seaweed and shell biomass
The following sorption experiments were performed using the above-mentioned marine biomass. Experimental conditions (i.e., pH, contact time and biosorbent dose rate) in this work were optimized and determined based on our preliminary experiments [e.g., 21] and other literatures [24, 31]. The pH of each solution was adjusted by using 0.1 mol
Samples of 0.4 g of the biomass were contacted with 200 cm3 of solution containing known initial each lanthanide (La, Eu or Yb) concentration ranging from 0.1 to 4 mmol・dm-3. Afterwards, the suspensions were shaken for 24 h in a water bath at ambient temperature (～25 °C) at pH 4.
Each sample of 0.2 g was contacted with 100 cm3 of multi-element standard solution (prepared by XSTC-1) including known initial lanthanide concentration (10 to 500 μg dm-3) in a 200 ml conical flask. Afterwards, the suspensions were shaken for 30 min in a water bath at room temperature at pH 5.
Following with each sorption experiment, the suspension containing biomass and lanthanides standard solution was filtered through a 0.10 μm membrane filter (Advantec Mixed Cellulose Ester, 47mm) to remove lanthanides that have been adsorbed into the biomass, and the concentration of these metals in the ﬁltrate was determined with ICP-MS or ICP-AES.
The metal uptake by the marine biomass was calculated using the following mass balance equation :
The removal efficiency (RE, %) of the biosorbent on the metal in the solution was determined by the following equation :
2.3. Langmuir and Freundlich isotherm model
Langmuir adsorption isotherm model was applied based on Tsui et al.  in this study, and the model assumes monolayer sorption onto a surface and is given as below.
The Freundlich equation is widely used in the field of environmental engineering, and was applied based on based Dahiya et al. [10-11]. Freundlich isotherm can also be used to explain adsorption phenomenon as given below.
3. Results and discussion
3.1. The seaweed samples
3.1.1. Metal sorption capacity at different species of seaweed
The equilibrium sorption isotherms of La, Eu and Yb by three kinds of Ca-loaded seaweed biomass are shown in Fig. 1. Sorption experiment of Eu using
From this table, it is found that
From Table 1, it is found that the data of
According to our previous work , in case of U, the mean concentration is the highest in brown algae and is the lowest in green algae among phyla (i.e., green, red and brown algae). However, as for the mean concentration of light REE (LREE) such as La, a slightly higher concentration is found in green algae; whereas the concentration of heavy REE (HREE) such as Yb or Lu in green algae is smaller than that in brown algae (as shown in the figure in our previous paper ).
Then, large sorption capacity of La by
According to SEM observation, the surface of
From the above observation, two kinds of biomass:
3.1.2. Removal efficiency and binding mechanism of seaweed biomass
The removal efficiency (RE) of 3 kinds of seaweed biomass as a function of initial metal concentrations (
From the viewpoint of recovering trace metals from aqueous environment such as seawater, the removal efficiency at low concentration of metal is particularly important. The coefficient before exponential function in each equation in Table 2 represents the value of RE at low
The amount (mmol g) of adsorbed lanthanide and released Ca from three kinds of Ca-loaded seaweed biomass is shown in Tables 3-5. Based on the data in these tables, relationship between the uptake of each lanthanide ion and calcium ion released from each biomass is shown both in terms of mill equivalent per gram (meq g-1) in Fig. 6. Good and linear relationship is generally found for these samples between the uptake of each lanthanide and Ca released from these biomasses into the solution as shown in Fig. 6. Particularly, in case of
3.2. The shell samples
3.2.1. Characteristics of Buccinum tenuissimum shell biomass
X-ray powder diffraction (XRD) patterns of the four kinds of
Furthermore, the measurement of specific surface area of the four kinds of sieved samples was performed in this study; and the results are shown in Table 6 along with the main crystal structure of these samples. Remarkably decrease of specific surface area (i.e., from 3.32m2/g to 0.390m2/g for BET, or from 5.35 m2/g to 0.612 m2/g for Langmuir) was found after heat-treatment (480°C, 6h). It is suggested that the crystal structure transformation (i.e., from aragonite (CaCO3) into calcite (CaCO3) phase) and also the difference of the surface morphology can be closely related to the remarkable decrease of specific surface area of the shell biomass. On the other hand, the surface area of “heat-treatment (950°C, 6h) sample” was 1.88m2/g for BET or 3.10m2/g for Langmuir respectively; and that of “heat-treatment (950°C, 6h) and water added sample” was 6.37m2/g for BET or 9.91m2/g for Langmuir, respectively.
3.2.2. Comparison for sorption capacity of lanthanides by four kinds of sieved biomass
The comparison for sorption capacity of lanthanides by four kinds of sieved
Prieto et al.  pointed that the sorption capacity of calcite is considerably lower than that of aragonite for Cd. In case of lanthanides, similar tendency of sorption capacity were suggested from our work.
3.2.3. Effect of competitive ions on the sorption of lanthanides
The percentage removal of lanthanides under the presence of common ions (Ca2+, Mg2+, Na+ and K+) at different concentrations 50, 100 and 200 mg dm-3 is shown in Fig. 10. From this figure, the remarkable decrease of sorption capacity of lanthanides was not observed. Even when the concentrations of common ions are 200 mg dm-3, the percentage removal of light REE (LREE) such as La or Ce decreased slightly (2-3%), whereas the removal decreased about 5% for heavy REE (HREE) such as Yb or Lu. This implies that the shell biomass can be an efficient adsorbent for lanthanides in aqueous environment such as seawater, although it requires further investigations to apply the shell biomass to use as an adsorbent for lanthanides more practically.
3.2.4. Characteristics of Buccinum tenuissimum shell biomass after adsorption of metals
X-ray diffraction (XRD) patterns of four kinds of sieved samples after adsorption of metals are shown in Fig. 11. Similar to the XRD patterns before adsorption of metals, aragonite and calcite were found as the main crystal structure in (a): Ground original sample and (b): Heat-treatment (480°C, 6h) sample, respectively. However, the decrease of peak and increase of noise were also observed in both patterns, particularly in the ground original material as shown in Fig. 11(a). Bottcher  pointed out that the natural powdered aragonite was transformed to mixed rhombohedral carbonates by the reaction with (Ca, Mg)-chloride solutions. Therefore, there is the possibility that the transformation of aragonite occurred by the reaction with lanthanides in our experiment.
Moreover, according to XRD analysis, the main crystal structure of (c): “Heat-treatment (950°C) sample” was transformed from calcium oxide (CaO) to the mixture of calcium hydroxide (Ca(OH)2) and calcite (CaCO3) after exposing metals; and that of (d): “Heat-treatment (950°C) and water added sample” was transformed from calcium hydroxide(Ca(OH)2) to calcite (CaCO3) after adsorption of metals. These changes may be due to the reaction with water or carbon dioxide in atmosphere.
SEM pictures of four kinds of shell biomass after adsorption of metals are shown in Fig. 12. By comparing SEM pictures in Fig. 8 with that in Fig. 12, it is found that the morphology of sample (a) and (b) has hardly changed even after exposing metals. From this observation, these sieved samples should be predicted to withstand the repeated use; and hence it can be a good adsorbent.
In contrast to sample (a), clear crystal structure (sizes are mostly 0.25-2.0μm) was observed in sample (b) even after adsorption of metal. In case of Cd conducted by Kohler et al. , the difference of procedure for reaction with metals between aragonite and calcite was suggested. According to their work, the precipitation of several distinct types of crystals was observed after exposing metals in the case of aragonite. Then, it is anticipated that similar phenomenon were occurred by adsorption of lanthanides in case of our samples. On the other hand, the surface of sample (c) and (d) after exposing metals have changed largely compared to that before adsorption of metals (Fig. 8). This is in good accord with the results of XRD patterns. Particularly, remarkable transformation was observed in the morphology of sample (d). The reaction of sample (d) with metal is supposed to proceed rapidly.
3.2.5. Adsorption isotherms of lanthanides by Buccinum tenuissimum shell biomass
The adsorption data obtained for lanthanides using
From this table, it is found that
On the other hand,
Futhermore, this result indicates the stronger the monolayer adsorption (the surface adsorption) on the heat-treatment sample relative to on the original sample (before heat-treatment)
The correlation coefficient (
As mentioned above, biosorption studies have been mainly focused on toxic metals elements such as Cd, Pb, As and Cr so far, and a few reports are focused on lanthanides. The sorption experiments using shell biomass in this work were carried out under low concentration of lanthanide (i.e., 100 cm3 of multi-element standard solution including known initial lanthanide concentration (10 to 500 μg dm-3)). Then, sorption experiment for three lanthanides (La, Eu and Yb) in single component system by this shell biomass is being planned using the solution individually prepared by each nitrate salt: La(NO3)3 6H2O, Eu(NO3)3 6H2O, or Yb(NO3)3 3H2O as the case of seaweed biomass in our work.
From this work, it was first quantitatively clarified that seaweed biomass could be efficient sorbents for lanthanides, and exhibit high ability of chemical adsorption. Particularly,
Biosorption characteristic of
The data obtained and the method used in this work can be useful tool from the viewpoint of resource recovery in future work.
The present work was partially supported by a Grant-in-Aid for Scientific Research (Research Program (C), No. 22510084) of the Japan Society for the Promotion of Science.
The author wish to express his thanks to Dr. M. Baga of the Marine Ecology Research Institute and Dr. N. Sugai, Dr. H. Handa, Dr. O. Sato and Dr. R. Ishikawa of Niigata Prefectural Fisheries and Marine Research Institute for giving helpful advice concerning sampling, identification and pretreatment of seaweed and shellfish. The author is also grateful to Dr. K. Satoh of Fac. of Sci., Dr. K. Fujii and M. Ohizumi of Office for Environmental and Safety, Mr. N. Saito and Mr. T. Hatamachi of Fac. of Eng. in Niigata University for permitting the use of instrument (ICP-MS, ICP-AES XRD, SEM and Surface Area Analyzer) and facilities and for giving helpful advice in measurement.