Chemical speciation of heavy metals and arsenic in tailings and soils contaminated by mining activities （Northern Portugal）

A chemical sequential speciation of heavy metals （Fe, Mn, Cu, Zn, Pb, Cd, Sn, W, Bi, Ni, Cr, Mo, Co） Al, Ti and As in tailings and soils around five W/Sn mines （Adoria Mine, Ervedosa Mine, Regoufe Mine, Rio de Frades Mine and Tarouca Mine-Northern Portugal） was realized using a 7-steps fractionation procedure. Each of the chemical fractions was operationally defined as （Dold and Fontbote 2001）： （1） water-soluble; （2） exchangeable; （3） bound to Fe oxyhydroxides （or easily reducible）; （4） bound to Fe oxides （or moderately reducible）; （5） bound to organic matter and secondary Cu-sulfides; （6） bound to primary sulfides; and （7） residual. The extracted element contents at each step were measured by ICP-MS. Samples were also analyzed for pH, electrical conductivity and organic matter. The results allowed us to notice that： （1） the pH was the main factor controlling the geochemical distribution of the studied elements. The tailing and soil samples were very acid, with an average pH of approximately 4.37. Some metal cations （Mn, Cd, Cu, Zn, Pb, Co, Cr, Ni） behave in a similar way, revealing important enrichments in the most bioavailable fractions （water-soluble and exchangeable fractions）. In contrast, oxyanions such as Mo and As show low mobility through adsorption to Fe（Ⅲ） oxyhydroxides dissolved in the two reducible fractions. These results reflect the pH dependent adsorption on the clay minerals, Fe and Mn oxyhydroxides and the co-precipitation with these secondary minerals; （2） residual fraction was the most important fraction for Sn, Ti, Al, Mn, V, Cr and Zn. These results suggest that in these soils these elements are in a detrital, non-available form; （3） large amounts of Fe, Mn, Cu, Zn, Pb, Cd, Bi, Cr, Ni, Co and As were present in the most bioavailable fractions in all sampling sites as consequence of the oxidation and acidification processes in sulfidic mine wastes;