Mineralogically-induced metal partitioning during the evaporative precipitation of efflorescent sulfate salts from acid mine drainage

Abstract

Efflorescent sulfate salts constitute a transient storage of acidity and metals during the dry season in mining areas affected by acid mine drainage, especially under semiarid climates. The main goal of this work was to study the metal partitioning among the dissolved and solid phases through the evaporative precipitation sequence of extremely metal-rich mine waters. The evaporative sequence was induced in the laboratory under controlled conditions for 24 days. The loss of water caused a progressive decrease of pH values (from 2.71 to 1.33) and increase of metal concentrations (e.g., from 1826 to 17800mg/L of Al, 836mg/L to 9783mg/L of Fe and 301 to 2879mg/L of Zn), which caused the precipitation of efflorescent sulfate salts. The precipitated salts were mainly composed of a mixture of minerals of Ca (gypsum, CaSO4·2H2O), Al (alunogen, Al2(SO4)3·17H2O), Fe (copiapite, Fe2+Fe3+4(SO4)6(OH)2·20H2O and melanterite Fe2+SO4·7H2O) and Mg (hexahydrite, MgSO4·6H2O), although the proportion of each mineral phase varied throughout the experiment. A preferential precipitation of Al and Mg sulfate salts, together with melanterite, was observed until the second week of the experiment, with evaporation rates lower than 70%. The precipitation of gypsum predominated with higher evaporation rates, reaching values higher than 80% of the mineral assemblage. The evaporative precipitation sequence was modeled using PHREEQC, obtaining good agreement with experimental data for gypsum, but failing in turn to accurately reproduce the evaporative sequence of other minerals such as hexahydrite, alunogen and melanterite. A metal partitioning pattern was observed during the evaporative precipitation sequence. Fe-sulfate minerals (e.g., copiapite and melanterite) present a higher affinity for Cu, Y and Th (and other trace metals such as Mn, Cd or Sc) while Al and Mg sulfate salts would retain Zn, Co, Ni and to a lesser extent Cr. In the case of gypsum, found prominently in the mineral assemblage during the experiment, it seems to have affinity for REE. This metal partitioning pattern is not observed in field data reported in literature. Such discrepancies, as well as those observed between modeled and experimental results, must be investigated in further studies.