The removal efficiency (RE) of 3 kinds of seaweed biomass as a function of initial metal concentrations (Ci) for 3 lanthanides is shown in Fig. 5((a): La, (b): Eu, (c): Yb). With increasing Ci, the RE generally decreased exponentially; and at high Ci, similar RE (i. e., about 40%) occurred for each lanthanide even with any biomass. These data are well fitted into an exponential function (R2 ranging from 0.866 to 0.994) shown in Fig. 5; and the equations and R2 for each lanthanide in each biomass are shown in Table 2.

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 Ci near approximately zero mmol-dm-3. From Table 3, the coefficient for each lanthanide in Sargassum hemiphyllum and especially that for La in Ulva pertusa is large. This implies that U. p. could be an efficient adsorbent for La as well as S. h. for lanthanides in aqueous environment such as seawater.

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 S. h. and U. p., the slope of the line is about one with the y-intercept of the graphs almost passes through the origin. It indicates that ion-exchange process is found to be the main mechanism responsible for the sorption of lanthanide ion onto the seaweed as Tsui et al. [24] and Diniz & Volesky [31] also pointed out.

(a)

(b)

(c)

Figure 5. Removal efficiency of lanthanides ((a) :La, (b): Eu, (c) :Yb) by Ca-loaded seaweed biomass at different initial concentrations,^: Sargassum hemiphyllum,| : Schizymenia duby, ▲:Ulva pertusa. Each exponential function is also shown (S. h.; solid curve, S. d. : broken curve and U. p. : dotted curve ). Data are mean±standard deviation (n=3).