Cadmium, copper, lead, and zinc in secondary sulfate minerals in soils of mined areas in Southeast Spain

Soils in mined areas in southeastern Spain are commonly characterized by extreme acidity, high salinity, and metals. These present challenges to establish vegetation as a management option for these environmentally-problematic landscapes. We collected salt efflorescence and the corresponding soil materials to better understand the geochemical cycling of Cd, Cu, Pb, and Zn in soils of mined areas. Mineral composition was identified and quantified using powder X-ray diffraction analysis while the morphology of minerals was observed using scanning electron microscope with energy-dispersive system. Chemical composition was determined using atomic absorption spectroscopy and inductively coupled plasma mass spectrometry. Salt efflorescence is dominated by hydrated sulfate minerals of Al, Fe, Mg, Mn, and Zn (or halotrichites). Apjohnite, copiapite, zinc sulphate hydrate, and hexahydrite are observed in all salt samples. Apjohnite are fibrous of ~ 1 μm wide and ~ 10 μm long. In some cases, apjohnite fibers coalesced to form mat-like accumulations. Euhedral copiapite crystals have tabular {010} habits of ~ 3 μm long and < 0.5 μm thick and stacked in layers consisting of 5–10 plates. Zinc sulphate hydrate has crumb-like morphology. Other sulfate minerals observed in the present study are epsomite, starkeyite, goslarite, jarosite, natrojarosite, mirabillite, coquimbite, and hohmannite. Cadmium, Cu, and Zn enrichment in sulfate salts were highest for Red Brown salts and ranged from ~ 10 for Cu to > 400 for Cd compared to surface soils. We calculated that salt harvesting (or removal) during the summer months can extract up to 20 times more Zn than the most efficient plant species tested in phytoextraction of Zn in mined soils. This information is important to the management and the control of salinity and metals in soils affected by mine tailings.