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-dm-3 NH3aq / 0.1 mol-dm-3 HNO3.

The seaweed biomass

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.

The shell biomass

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 pg-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 pm 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 [33]:

q = (С{ — Cf )V / W [mg x g-1] (1)

where q = metal uptake (mmol-g-1); Ci = initial metal concentration (mmol-dm-3); Cf = equilibrium metal concentration (mmol-dm-3); V = volume of the solution (dm3); and W = dry mass of seaweed (g).

The removal efficiency (RE, %) of the biosorbent on the metal in the solution was determined by the following equation [24]:

RE = (Ci — Cf) x 100 / Ct (2)