The effect of pressure on the sorption/precipitation of metal cations, and its possible role in spontaneous removal of heavy metal cations from sea water

The actual concentration of heavy metals in sea water is low compared with the amount delivered on the one hand, and with saturated solution of sparingly soluble hydroxides and salts on the other. We show here that the high pressure on the ocean floor induces dissolution of sparingly soluble metal hydroxides and release of metal cations pre-sorbed on iron hydrous oxides. The concentration of divalent cations of a heavy metal in equilibrium with the metal hydroxide at a 5 km depth is four-times higher than on the ocean surface (at atmospheric pressure, and otherwise the same conditions), and this result is practically independent of the nature of the heavy metal. Then the solid particles, which are brought from the surface onto the ocean floor by sedimentation or by thermohaline circulation are potential carriers of heavy metals. The heavy metals bound by the solid particles at atmospheric pressure can be mobilized at sufficiently high pressures and then built into the marine ferromanganese nodules or bottom sediment. (C) 2003 Elsevier B.V. All rights reserved.