4.1 Selective Collection of Rare Elements Dissolved in Trace Amounts in the Sea by Using Polymer Adsorbent

Fig. 4-1 Production process of adsorbent and schematic drawing of adsorption mechanism
(a) Nonwoven fabrics of polypropylene are grafted with acrylonitrile after electron beam irradiation and then the cyano groups of the grafted chains are converted into amidoxime groups by a reaction with a hydroxylamine.
(b) Uranium is dissolved in a form of uranyl carbonate in seawater and an uranyl ion (UO2+) is adsorbed on the grafted chains to form a complex compound with two amidoxime groups.

Fig. 4-2 Schematic view for the adsorption experiment in the sea
The wire-mesh vessels packed with adsorbent are immersed in the sea. Since the seawater can easily flow through the vessels, the surface of the adsorbent is able to be in contact with uranyl ions dissolved in seawater to form complex compounds on it.

Seawater is known to contain various kinds of elements, such as uranium, vanadium, cobalt, as well as sodium, magnesium, and so on. Uranium is dissolved in the ratio of about 3.3 mg per 1 ton seawater. The total amount of uranium dissolved in all the earth's seawater can be assumed to be more than 4 billion tons. If it were possible to selectively collect the uranium dissolved in the sea, an inexhaustible resource of uranium could be established.
By using a radiation grafting technique, we developed a polymer material to be used as the adsorbent for these elements which are dissolved in trace concentrations in seawater. The adsorbent, which consists of the grafted nonwoven fabrics of polypropylene, was found to effectively adsorb the valuable elements, such as uranium, cobalt, and vanadium, with a high selectivity.
Nonwoven fabrics are irradiated with ionizing radiation and grafted with acrylonitrile monomers. Then, the grafted fabrics are immersed in the hydroxyl amine solution to convert the cyano groups on the graft chains into amidoxime groups. The amidoxime groups can selectively adsorb uranium from uranyl carbonate to form a complex compound.
The adsorbent thus obtained was packed in wire-mesh vessels, through which seawater can easily flow, and these vessels were immersed in the sea at a depth of 40-45 m at a temperature of about 20 degrees for 20 days. The adsorbent was found to be able to collect uranium in the ratio of 1 g uranium to 1 kg adsorbent. What this means is that the adsorbent developed here is very promising in practical terms for the collection of uranium from the sea.

Reference
T. Sugo, Status of Development for Recovery Technology of Uranium from Seawater, Nippon Kaisui Gakkai-Shi, 51 (1), 20 (1997).

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