Recebido em 21/6/07; aceito em 26/10/07; publicado na web em 15/5/08Acid drainage results from exposition of sulfides to the atmosphere. Arsenopyrite is a sulfide that releases arsenic (As) to the environment when oxidized. This work evaluated the As mobility in six sulfidic geomaterials from gold mining areas in Minas Gerais State, Brazil. Grained samples (<2 mm) were periodically leached with distilled water, during 70 days. Results suggested As sorption onto (hydr)oxides formed by oxidation of arsenopyrite. Low pH accelerated the acid generation, dissolving Fe oxihydroxides and releasing As. Presence of carbonates decreased oxidation rates and As release. On the other hand, lime added to a partially oxidized sample increased As mobilization.Keywords: arsenopyrite; acid mine drainage; contamination. INTRODUCTIONAcid mine drainage (AMD) results from a set of chemical, electrochemical and microbiological reactions, risen from exposure of sulfide bearing geological materials to the atmospheric oxygen and water.1 Under such conditions there is a net generation of sulfuric acid, decreasing pH of the drainage water and increasing solubility and leaching of toxic elements as heavy metals and metalloids.2-4 Consequently, there are increasing risks for toxic metals and metalloids incorporation in biological systems and so, bioaccumulation and biomagnification processes in the trophic chain.Arsenopyrite is a sulfide (AsFeS) commonly found in gold mine areas. 5,6 Its oxidation produces arsenic and sulfur oxiacids (e.g. H 3 AsO 3 , H 3 AsO 4 , H 2 SO 4 ). Arsenic is toxic at low concentrations and sulfuric acid is responsible by the pH drop. 7 The rate of sulfides oxidation depends on several factors, including the amounts of arsenopyrite (e.g. AsFeS), grain size distribution, temperature and the exposure time of the material to the atmospheric water and oxygen. Catalytic action of the Acidithiobacillus aggravates the problem under acid medium, at pH < 3, which stimulates bacterial activity. 8Ingestion of water is considered the main source for As contamination. The most noticeable reports are mass contamination cases that took place in Bangladesh, 9,10 Taiwan 11,12 and Chile. 13 In Brazil, the Iron Quadrangle is one of the affected regions in Minas Gerais State. Most impacted areas are close to gold mines, as in Nova Lima, Raposos, Mariana and Santa Bárbara districts. [14][15][16] Several researchers showed that As adsorption onto iron oxides decreases as the pH increases. [16][17][18][19][20] On the other hand there are reports of increasing As (III) sorption as the pH increases from 3 to the neutral, when the adsorbent is a sulfide as troillite (FeS) or pyrite (FeS 2 ). 21 Release of As from arsenopyrite structure, by oxidation, also decreases at increasing pH, mainly due to the drop of bacterial activity. 8,9 Face to the need for controlling As dispersion in the environment, this research was performed in order to study the mobility of As in samples of sulfidic materials from gold mine areas in different geological sc...