Arsenic speciation: Involvement in evaluation of environmental impact caused by mine wastes

Since large areas have been contaminated by arsenic-rich industrial and min ing by-products in several countries, knowledge of arsenic (As) behavior in soils and water is of major concern for reclamation of polluted sites and evaluation of environmental risk. Cheni's tailings (France) contains averag e concentrations up to 7000 mg kg(-1) As and was listed by the authority as an important potentially pollutant site. Because As speciation determines its mobility, it was investigated in surface and ground waters of the site and a study of the solid As-bearing phase was performed on suspended partic ulate material (SPM). Results of sequential extraction, carried out on the SPM coned-ed in drainage waters, show that most of total extracted As (78%) and Fe (77%) is bound to an iron hydrous oxide phase, which was determined to be lepidocrocite (gamma-FeOOH) by x-ray diffraction and infrared spectr ometry. Particulate material was >220 times more concentrated than dissolve d As. Conditions For the stability of dissolved (<0.45 mu m) arsenite [As(I II)] and arsenate [As(V)] are discussed on the basis of thermodynamical dat a and field measurements of pH and redox potentials. The theoretical stabil ity of As(V) corresponds to ii large association between As and particulate material, and large amounts of the metalloid are released in solution when As-rich particles come from oxidized ground waters to moderately reduced c onditions. These results are important for evaluating, modeling, and rehabi litating As-polluted sites.