Remobilization of arsenic from buried wastes at an industrial site: mineralogical and geochemical control

An industrial area contaminated by As was studied to determine the source of this element and its speciation in As-bearing solids and in run-off waters. Mineral precipitates and water samples were collected and analyzed to assess processes controlling As mobility at this site. The integrated study of a contaminated industrial area allowed identification of the source of the As and of the nature of secondary As-bearing phases. The results obtained both on solid and water samples were used to model As behavior during waste leaching on carbonate rocks. At the upper end of a topographic transect across the site, run-off waters (pH=7.9) interact with surficial waste piles (containing arsenolite, arsenopyrite and pyrite), becoming acidic (pH=2.2) and concentrated in dissolved arsenate species (As5+) (ΣAs ranging from 0.961 to 3.149·10−3 mol/l). Those acidic waters interact with the limestone substratum, providing dissolved Ca which reacts with As to precipitate 1:1 Ca arsenates (weilite CaHAsO4, haidingerite CaHAsO4.H2O and pharmacolite CaHAsO4.2H2O) and, in minor amounts, Ca–Mg arsenates (picropharmacolite (Ca,Mg)3(AsO4)2 6H2O). The 1:1 Ca arsenates identified are known to precipitate at low pH (3–6) and seem to be stable in media with high dissolved CO2, in comparison with other types of Ca arsenates. However, due to their high solubilities, they are not strictly relevant candidates to immobilize As in contaminated surficial environments. Although reported solubilities decrease to values close to the French and US drinking standards in Ca-rich solutions, a thorough examination of the precipitation/dissolution kinetics of Ca arsenates should be undertaken to assess their long-term stability and their efficiency in rapidly immobilizing As in contaminated surficial environments.