Experimental Modelling of As Coprecipitation With
Iron (III) Hydroxide in Hydrothermal Plumes

Alla V. Savenko Geographical Fac., Moscow State University, Lengory, Moscow 119899, Russia

Submarine hydrothermal solutions are characterized by high acidity (pH=3-4), high Fe2+ concentration (up to 1-2 mM) and strong reducers (H2, CH4) abundance. During the mixing of the submarine hydrothermal solution with the sea water, the sharp pH increase and oxidizing of Fe2+ to Fe3+ form take place. As a result the solid iron (III) hydroxide is formed and it leads to coprecipitation of some trace elements (Feely et al., 1994; German et al., 1991).

The experiments on modelling of As3+ and As5+ coprecipitation with iron (III) hydroxide under conditions of oxidation of dissolved Fe (II) in sea water were carried out. They allow to determine the relationship between element (i)/Fe ratio in solid phase and that of in initial solution:

(i/Fe)iron(III) hydroxide = k (i/Fe)initial solution

where i - As3+, As5+. The coefficient k is constant and is equal to 0.7, when i/Fe initial solution < 0.15. It proves the high efficiency of As3+and As5+ removal from the sea water with hydrothermal iron (III) hydroxide.

Assuming that As in metalloferrous deposits are connected with hydrothermal iron (III) hydroxide, the i/Fe ratio in
sediments was estimated on the base of experimental data. The results of calculations give the As/Fe ratio is 0.002-0.02. It is close to data, received from analysis of hydrothermal plume's suspended matter (0.002) and hydroxide fraction of metalliferrous deposits (0.001).


Feely, R.A., et. al., J.Geophys. Res. 99B, 4985-5006 (1994).

German, C.R., Campbell, A.C. & Edmond, J.M., Earth Planet. Sci. Lett. 107, 101-114 (1991).

Fig. 1: Dependence between atomic ratios (a) As(III)/Fe, and (b) As(Y)/Fe in iron(III) hydroxide deposit and that in initial seawater at pH = 7.7-7.8. Initial concentrations As(III), *M: 0.91 (Circles), 1.96 (Squares), 3.13 (Triangles). Initial concentrations As(Y), *M: 1.13 (Circles), 2.0 (Squares), 2.9 (Triangles).