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 o f this element and its speciation in As-bearing solids and in run-off water s. Mineral precipitates and water samples were collected and analyzed to as sess processes controlling As mobility at this site. The integrated study o f a contaminated industrial area allowed identification of the source of th e 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 wast e leaching on carbonate rocks. At the upper end of a topographic transect a cross the site, run-off waters (pH = 7.9) interact with surficial waste pil es (containing arsenolite, arsenopyrite and pyrite), becoming acidic (pH=2. 2) and concentrated in dissolved arsenate species (As5+) (Sigma As ranging from 0.961 to 3.149.10(-3) mol/l). Those acidic waters interact with the li mestone substratum, providing dissolved Ca which reacts with As to precipit ate 1:1 Ca arsenates (weilite CaHAsO4, haidingerite CaHAsO4.H2O and pharmac olite CaHAsO4.2H(2)O) and, in minor amounts, Ca-Mg arsenates (picropharmaco lite (Ca,Mg)(3)(AsO4)(2) 6H(2)O). The 1:1 Ca arsenates identified are known to precipitate at low pH (3-6) and seem to be stable in media with high di ssolved CO2, in comparison with other types of Ca arsenates. However, due t o their high solubilities, they are not strictly relevant candidates to imm obilize As in contaminated surficial environments. Although reported solubi lities 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 sta bility and their efficiency in rapidly immobilizing As in contaminated surf icial environments. (C) 1999 Elsevier Science Ltd. All rights reserved.