The adsorption of metal elements on hydrated ferric oxide, clay minerals, and marine sediments from artificial seawater is studied using radioactive multitracers produced by RIKEN Ring Cyclotron. Removal mechanisms of elements from the ocean have been discussed by many researchers [1][2][3][4][5][6][7][8][9]. It was suggested that the partition of elements between particulate solid phases (pelagic clays, manganese and iron oxides, organisms, etc.) and seawater is, for the first approximation, controlled by the affinity of cations and oxyanions for the hydrolyzed particulate surfaces [2,9]. Adsorption of metal elements on hydrated ferric oxide, clay minerals (montmorillonite and kaolinite), and marine sediments (deep-sea and nearshore sediments) from artificial seawater is studied by a radioactive multitracer method. On the basis of the distribution coefficients of metal elements for the particulates, the ultimate removal mechanism of metal elements from seawater is discussed.
MATERIALS AND METHODSAdsorbents Hydrated ferric oxide was prepared by hyrolyzing ferric ions at pH 7.5. Deep-sea and nearshore sediments were obtained from the Penrhyn Basin (12°26.44'S, 157°57.2(YW, depth 5351 meters) and Suruga Bay (34°44.52'N,169° 27.05'E, depth ca. 600 meters), respectively. The adsorbents were stored in artificial seawater as a suspension (2 mg cm'3). Multitracer The multitracers were separated from gold and copper foils irradiated with a 14N beam of 135 McV/nucleon accelerated by the RIKEN Ring Cyclotron in a earner-and salt-free state [10]. The multitracers were shed as 6 mol dm 3 HC1 solutions.Also tion procedure Artificial seawater and a small amount of a multitracer solution were put in a polyethylene bottle. After the pH value was adjusted to 7.5 with a 1 mol dm`3 Na2CO3 solution, the adsorbent suspension was added to the solution. Its pH value was readjusted to 7.5 when necessary. The suspension was shaken in an 8-shape mode with a shaker at 25° C. After centrifugation, a portion of the supernatant solution was pipetted. The y-ray spectrum of radioactive nuclides in the solutions were measured with a Ge detector. The distribution coefficients (Kd) for each nuclide were calculated as follows: Kd = ( Aads/m )/( Asoln/V) = ((Ai-Af)/Af)(V/m).Aads and Asoln are the radioactivity in the adsorbent and the solution after adsorption equilibrium, respectively. V is volume of the solution (cm'3 ). m is the amount of the adsorbent (g). Ai and AS are the radioactivity in the solution 1106 before and after adsorption equilibrium , respectively.
RESULTS AND DISCUSSIONThe distribution coefficients (Kd) of Co . Ga, Rb, Sr, Y, Zr, Nb, Mo, Ru, Rh, Pd, Ag, In, Sn, Eu, Gd, Tb, Tm, Yb, Lu, Hf, Re and Hg at pH 7 .5 and at 25°C are given in TABLE 1. The order of Kd for each element is summarized as follows: hydrated femc oxide > deep-sea sediment > montmorillonite > nearshore sediment kaolinite. The deep-sea sediment used in this study is red clay containing about 1 .5 % iron oxide and 0.5 %