Speciation and solubility control of Al and Fe in minesoil solutions

Solutions obtained from minesoils originating from the opencast lignite mine dump at As Pontes (Galicia, NW Spain) were studied. Spoils were very heterogeneous and there were a wide range of environments. Generally, solutions showed high electrical conductivity, often with large quantity of weathering products derived from pyrite oxidation (Fe, SO42−, H+), as well as Al and Si originating from mineral alteration. The Al fractioning showed (a) in the most acid areas (pH < 5.0) AlSO4 was the dominant complexeand Al3+ was the second most common; (b) at pH values between 5.0 and 6.0, AlF monomers were the most abundant complexes; (c) at pH values > 6.0, AlOH species were most common. The distribution of Fe species was chiefly determined by the EhpH conditions and SO42− contentss in the solution. Generally, in areas with a pH < 4.0 and Eh > 600 mV, Fe was found as FeSO4 complexes, while in areas with a pH > 6.0 the only species present were Fe-hydroxy-monomers. The Al3+ activity in soil solution seems to be controlled by a jurbanite-type mineral at pH < 4.0, while amorphous Al(OH)3 seems to exercise this control at pH > 5.0. With regards to Fe, in the most acid (pH < 4.0) and oxidizing (pe < 9) environments, none of the minerals studied, could control their solubility, FeSO4·7H2O being the mineral phase closest to the equilibrium.