The adsorption data obtained for lanthanides using Buccinum tenuissimum shell biomass were analyzed using Langmuir and Freundlich equations. The correlation coefficient (R2) of Langmuir and Freundlich isotherms for lanthanides using ground original shell biomass is shown in Table 7 along with other relevant parameters.

From this table, it is found that R2 value for lanthanides is comparatively high. It indicates the applicability of these adsorption isotherms satisfactorily for lanthanides in this sample. The dimensionless parameter Hall separation factor (RL) for lanthanides is in the range of 0<Rl<1, which means that the sorption for lanthanides by this shell biomass is favorable. Furthermore, the negative value of ^G indicates that the sorption is spontaneous. The higher R2 value for Freundlich model rather than for Langmuir isotherm (0.638-0.886 for Langmuir isotherm and 0.844-0.932 for Freundlich one) suggests that the adsorption on this sample is due to multilayer coverage of the adsorbate rather than monolayer coverage on the surface. It is noted that the value of 1/n less than unity indicates better adsorption mechanism and formation of relatively stronger bonds between adsorbent and adsorbate [10]. That is to say, favorable adsorption for lanthanides by this shell biomass is presented.

On the other hand, R2 and other parameters of Langmuir and Freundlich isotherms for lanthanides using "heat-treatment (480°C) sample" is shown in Table 8. It is noteworthy that R2 value for REEs in this sample is still more large (0.947-0.982 for Langmuir isotherm and 0.948-0.975 for Freundlich one), compared with the original ground sample (Table 7).

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). Judging from the value of Rl or 1/n in Table 4, the heat-treatment (480°C) sample also exhibits the favorable property for lanthanides adsorption.

The correlation coefficient (R2) and other parameters of Langmuir and Freundlich isotherms for lanthanides using "heat-treatment (950°C) sample" is shown in Table 9. It is found that R2 value for lanthanides in this sample is fairly small compared with the values of "ground original sample" or "heat-treatment (480°C) sample" (In case of La, Ce, Yb and Lu, R2 can not be estimated due to the lack of sorption data at low initial concentration). The low correlation coefficient (R2) in this "heat-treatment (950°C) sample" may indicate that the removal of lanthanides occurred not by adsorption mechanism,

Particularly R2 value is remarkably small for Langmuir isotherm, and then other relevant parameters can not be estimated. As for Freundlich one, not only R2 value is relatively small (0.157-0.625), but the value of 1/n for most lanthanide is more than unity. That is to say, the almost perfect removal of lanthanides for this sample may be due to other mechanism rather than the adsorption on the biomass. However, the cause or mechanism of lanthanides removal on this sample has yet to be sufficiently clarified in our work, and further investigation to survey the mechanism is needed.

Finally, R2 and other parameters of Langmuir and Freundlich isotherms for lanthanides using "heat-treatment (950°C) and water added sample" is shown in Table 10. It is found that R2 value for lanthanides in this sample is fairly large particularly for Langmuir isotherm (0.992-0.999 for Langmuir isotherm and 0.885-0.951 for Freundlich one). This result is similar to that for "heat-treatment (480°C) sample", and indicates the stronger the monolayer adsorption on this sample. Judging from the value of Rl or 1/n in Table10, this sample also exhibits the favorable conditions for lanthanides adsorption.

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 qg-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)3-6№O, Eu(NO3)3-6H2O, or Yb(NO3)3-3H2O as the case of seaweed biomass in our work.

4. Conclusion

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, Ulva pertusa is found to be a promising biosorbent for removing La. It is also suggested that the adsorption on seaweed biomass is mainly due to monolayer sorption because of well-fitting for Langmuir model.

Biosorption characteristic of Buccinum tenuissimum shell biomass was also studied for lanthanides. Sorption isotherms were analyzed using Langmuir and Freundlich equations to confirm the efficiency of shell biomass as sorbent. The shell biomass samples showed excellent sorption capacity for lanthanides under our experimental condition, even the presence of diverse ions (Ca2+, Mg2+, Na+ and K+) up to the concentration of 200 mg-dm-3.

From these results, it was quantitatively clarified to some extent that shell biomass can be an efficient sorbent for lanthanides. It is very significant information from the viewpoint of environmental protection that the shell (usually treated as waste material) can be converted into a biosorbent for lanthanides.

The data obtained and the method used in this work can be useful tool from the viewpoint of resource recovery in future work.