Aluminum fractionation and speciation in a coal mine dump: Twenty years of time-course evolution

We study the time-course evolution of Al fractions and species in the solid and liquid phases of soils developed over spoil materials rich in sulfides, situated in the mine dump of As Pontes (NW Spain), over a twenty-year period. Three plots (CSA, CSP and P206), subjected to different reclamation practices, were monitored. Plot P206 was established by selecting spoil materials, trying to avoid pyrite oxidation by atmospheric contact, whereas in plots CSA and CSP spoils were placed randomly, CSA being richer in sulfides. Samples were taken at two depths (0-15 and 15-30 cm) in autumn 1992 and 2012. In 1992, plot CSA showed the lowest pH (2.8) and the highest SO42 − concentration (845 mg L− 1). This plot presented the lowest Al concentrations in the solid phase (corresponding to total non-crystalline Al, Al bound to organic matter, and inorganic non-crystalline Al), and the highest concentrations of total Al in solution. Most of this soluble Al was reactive Al, concretely labile Al, mainly represented by Al3 + (the most toxic species) and Al-SO4. Contrary, plot P206 (where spoil material was selected) was the less acidic (pH 6.0), showed the lowest SO42 − concentration (17 mg L− 1) and the highest levels of Al in the solid phase, whereas Al in solution was very low. Twenty years later, no significant variations were found for pH in any of the plots, while there was a decrease of organo-aluminum complexes in the solid phase, as well as of SO42 −, total Al and labile Al in the liquid phase, and some new (previously undetected) soluble organo-aluminum forms were quantified. As regards the labile Al species, Al3 + and Al-SO4 significantly decrease. However, plot CSA maintained the most acidic pH values and the highest SO42 −, total Al, reactive Al, labile Al, Al-SO4 and Al3 + concentrations. The time-course evolution during the twenty-year period situated plots CSA and CSP in better conditions, closer to that of natural soils in the area.