METAL BEHAVIOUR IN ACID MINE WATERS FROM IBERIAN PYRITE BELT: FROM SPECIATION TO PRECIPITATION

Acidic mine waters collected in mines from Portuguese zone of the Iberian Pyrite Belt were analyzed in order to evaluate the potential mechanisms governing metals mobility, speciation and complexation, the interaction with colloidal particles and finally its adsorption and (co-)precipitation with minerals phases, namely with sulfate phases.

These waters are characterized by high or very high acidity, usually with pH that range from 1 to 4, high electrical conductivity and with high SO₄ concentrations and metals that exceed the background values. These acid mine waters corresponds to the Fe-Mg-SO₄-(Al) type, which explains why Fe and Mg sulfates are one of the most important phases precipitating during water evaporation. Zinc are the most mobile metal relative to Cu and Pb and remains in waters in high amounts, instead of Pb which is only stable in extreme acid pH values (pH<1.5). For this reason the ratio sediment:water concentration show a sequence Pb>Cu>Zn.

Speciation calculus indicates the predominance of Me²⁺ species in solution for majority of the acid waters analyzed. Although, since SO₄ concentration increases significantly at pH<2, Me-SO₄ type species gain more relevance.

Attenuated total reflectance infrared spectroscopy (FTIR-ATR) allowed the identification of S-O and Fe-O-OH (goethite) complexes in aqueous solution. The divalent metals form complexes with Fe-O-OH and SO₄-Me with monodentate (pH<2) and bidentate (2<pH<4) structural coordination.

These complexes represent precursor stages from which by polymerization processes leads to precipitation of schwertmannite, goethite or simple sulfates during evaporation. The precipitation of simple and more complex sulfates leads to sequestration of Cu²⁺ and Zn²⁺. There is a tendency for Mg-sulfates retain more the Zn while the Fe-sulfates show a great affinity for Cu. Under high acidic conditions (pH~1.5) jarosite is crystallized and the Pb is fixed in its structure, fact that is reinforced by the correlation of Pb with K.