Evaporative mineral precipitates from a historical smelting slag dump, Rio Tinto, Spain

Abstract

This work reports the chemistry and mineralogy of mineral efflorescences associated with slag deposits at the historical Rio Tinto smelter site, southwest Spain. The slags have been subject to weathering since dumping in the 19 th and 20 th century, and a series of evaporative mineral efflorescences has been observed. The efflorescences commonly occur as powdery or cemented salt precipitates at seepage points at the base of the slag dump and as solid aggregates in protected overhangs facing the Tinto river. The mineral salt types include Ca and Mg sulfates (gypsum, epsomite, hexahydrite, bloedite) as well as mixed Fe2+ – Fe3+ hydrated sulfates (copiapite, roemerite). The salt mixtures have variable metal concentrations, including major (> 1 wt %) concentrations of Zn, minor Cu (> 1000 ppm), sub-minor (100–1000 ppm) to traces (< 100 ppm) of As and Co as well as traces (< 100 ppm) of Ag, Bi, Cd, Mo, Ni, Pb, Sb, Sn, Tl and W. Copiapite-rich samples exhibit the highest As, Cd and Cu, epsomite–hexahydrite rich samples have the highest Zn, and the gypsum-rich samples show the lowest metal and metalloid concentrations. Dissolution experiments show that all salt mixtures are acid generating due to Fe and Al hydrolysis and resultant pH decrease in the solution. Thus, weathering and leaching of metalliferous smelting slags are accompanied by the mobilisation of metals, metalloids, alkali earth elements and sulfate into pore and seepage waters. Evaporation of seepage waters emanating from the slag dump causes the precipitation of mobilised elements and compounds and leads to their temporary fixation in secondary soluble minerals. Dissolution of the efflorescences during the next rainfall and flushing event and associated Al3+ and Fe3+ hydrolysis contribute to the acidification and metal and sulfate contamination of Rio Tinto waters.