Factors controlling the concentrations of thirteen rare metals in sea-water

The following factors controlling the concentrations of thirteen metals (Zn, Cu, Pb, Bi, Cd, Ni, Co, Hg, Ag, Cr, Mo, W, V) in sea-water were studied:

1. precipitation of insoluble compounds with ions normally present in aerated sea-water,
2. precipitation of sulphides locally in reducing environments,
3. adsorption by ferrous sulphide, hydrated ferric oxide, hydrated manganese dioxide, apatite, clay, and organic matter. The precipitation reactions were investigated first by calculations based on solubility data, and then by experiments in which equilibrium concentrations of the metal ions were determined in sea-water in contact with their precipitates. Adsorption was measured by adding or precipitating the various adsorbents in sea-water samples containing an excess of one of the metal ions.

The principal conclusions are:

1. Sea-water is greatly undersaturated with respect to all thirteen metals; in other words, precipitation of compounds with the ions normally present in aerated sea-water, even under extreme conditions of temperature and pH, cannot be responsible for the observed concentrations.
2. Local precipitation of sulphides is a possible control mechanism for seven of the elements (Cu, Zn, Hg, Ag, Cd, Bi, Pb), but is probably not the chief control, because the concentrations are unrelated to sulphide solubilities.
3. Adsorption is a possible mechanism for all elements except V, W, Ni, Co, and Cr; if Cr is assumed to be removed by local reduction and precipitation of the hydroxide, and the other four metals by organic reactions, the existing concentrations can be fairly adequately accounted for. Adsorption processes supplemented by organic reactions also furnish a qualitative explanation for the distribution of rare metals in sedimentary rocks of marine origin. These conclusions can probably be extended to other metals in the middle of the periodic system, but not to those on either side.