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 Marek Kosmulski *, Edward Ma ˛czka Department of Electrochemistry, Lublin University of Technology, P.O. Box 189, ul. Nadbystrycka 38 A, Lublin 20-610, Poland Received 27 November 2002; accepted 5 May 2003 Abstract 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. # 2003 Elsevier B.V. All rights reserved. Keywords: Sea water; Dilatometry; Adsorption; Precipitation 1. Introduction Krauskopf [1] pointed out that actual concen- trations of heavy metals in sea water are substan- tially lower than the calculated concentrations of these metals in solution in equilibrium with their hydroxides, basic carbonates and other sparingly soluble salts that may potentially form in sea water. Therefore, biological processes and adsorp- tion have been invoked to explain the spontaneous removal of heavy metal cations from sea water. Adsorption and precipitation are in principle two different phenomena, but they often constitute two consecutive stages of the overall sorption process. According to the recent spectroscopic studies [2], adsorption (which is the initial stage of the uptake of heavy metal cations by the solid * Corresponding author. E-mail address: kosmulsk@hermes.umes.lublin.pl (M. Kosmulski). Colloids and Surfaces A: Physicochem. Eng. Aspects 223 (2003) 195  /199 www.elsevier.com/locate/colsurfa 0927-7757/03/$ - see front matter # 2003 Elsevier B.V. All rights reserved. doi:10.1016/S0927-7757(03)00194-8