The behaviour of thallium in the (sub) surface environment in terms of Eh and pH

Abstract

A revised Eh-pH diagram for thallium compounds is presented, which is very different from previously presented diagrams. Calculations are, therefore, given in detail. To obtain the correct diagram for “normal” Eh-pH space, i.e. between the water stability boundaries and between pH-values from 0 to 14, one has to move far beyond these boundaries into “unrealistic” areas. These areas of pH's, varying from 14 to 25 and strongly negative Eh-values, provide the key to the situation in the “normal” areas. Almost all Eh-pH space is occupied by the ionic species Tl +, and to a much lesser extent by solid phases as shown in previously published diagrams. This would indicate that thallium is a very mobile metal in most natural environments, rather than being locked-up in solid oxides. This is a theoretical confirmation of the empirical observation that thallium disperses easily during oxidation of thallium-bearing sulphide deposits.