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Cadmium and mercury uptake by galena crystals under hydrothermal growth

A.P. Vinogradov Institute of Geochemistry, Siberian Branch of Russian Academy of Sciences, 664033 Irkutsk, Favorski street 1a, Russia

^* Corresponding author, e-mail: vltauson{at}igc.irk.ru

Cadmium and mercury speciation in galena crystals grown under hydrothermal conditions at 400°C and 500 bar is studied using X-ray photoelectron and Auger electron spectroscopy, and atomic absorption spectrometry with temperature control of element release. The main binding forms of Cd and Hg are identified as chemically adsorbed compounds, structurally bound elements in solid solution in galena, and non-autonomous surface phases produced by interaction of a component with altered (oxidized) surface of the growing crystal. The sulfur fugacity (f_S2) has been found to be the most important parameter affecting Cd and Hg speciation and distribution. Specifically, adsorbed Cd species are abundant at low f_S2, whereas the maximum of adsorbed Hg occurs at the highest f_S2 in equilibrium with S⁰. This suggests different mechanisms of Cd and Hg uptake by the surface of galena crystals. Under elevated P, T conditions, the element retention requires its interaction with surface defects (Pb vacancies in the case of mercury at high f_S2), or with active centers containing oxidized sulfur species (cadmium at low f_S2). The incorporation of minor components into non-autonomous phases is considered to be an important mechanism of trace-element uptake. It is proposed that these phases could be mixed phases of lead sulphate and chloride or lead thiosulphate and chloride (sulfoxychlorides). The crystal/solution distribution coefficients are much higher for Hg than for Cd. Mercury is nearly quantitatively extracted from the solution by the galena crystals. The bulk distribution coefficients are appreciably higher than the distribution coefficient of structurally bound impurities for both elements, demonstrating the necessity of evaluation of structural species even far below the region of mineral saturation with the trace element. It is shown that the element thermo-release atomic absorption spectrometry can be applied as a useful complement to surface spectroscopic methods.

Key-words: galena, hydrothermal growth, cadmium, mercury, XPS, AES, atomic absorption spectrometry, element thermo-release, speciation, distribution coefficients.