A kinetic study of the oxidation of arsenopyrite in acidic solutions: implications for the environment Yu Yunmei a, *, Zhu Yongxuan a , A.E. Williams-Jones b , Gao Zhenmin a , Li Dexian a a InstituteofGeochemistry,ChineseAcademyofSciences,Guiyang550002,PRChina b DepartmentofEarthandPlanetaryScience,McGillUniversity,Montreal,Quebec,H3A2A7Canada Received 2 January 2003; accepted 31 May 2003 Editorial handing by A.H. Welch Abstract Arsenopyrite is an important component of many ore deposits and dissolves in the O 2 -rich, acidic surface waters that are commonly found in the vicinity of active mines, releasing As, Fe and S to the environment. However, despite the potentially serious effect of this pollution on the human and animal population, the rate at which such oxidation occurs is poorly known. Kinetic experiments were therefore conducted in a mixed flow reactor to investigate the oxidation of arsenopyrite in Fe 2 (SO 4 ) 3 solutions (pH=l.8) having a concentration of ll0 2 to 1 l0 5 mol kg 1 at temperatures of 45, 35, 25 and 15  C. The results of these experiments show that the rate of oxidation of arsenopyrite increases with increasing concentration of dissolved Fe 2 (SO 4 ) 3 and temperature. They also show that As released during the oxidation of arsenopyrite has the form As(III), and that the rate of conversion of As(III) to As(V) is relatively low, although it tends to increase with increasing concentration of dissolved Fe 2 (SO 4 ) 3 and temperature. In the presence of Cl  , oxi- dation of arsenopyrite is accelerated, as is the conversion of As(III) to As(V). These findings indicate that exploitation of arsenopyrite-bearing ores will cause contamination of groundwaters by As at levels sufficient to have a major negative effect on the health of humans and animals. # 2003 Elsevier Ltd. All rights reserved. Keywords: Arsenopyrite; AMD; Mine wastes; Mine drainage; Arsenate and arsenite 1. Introduction Arsenopyrite is a major component of many mineral deposits, and is the principal raw material for the industrial production of As. However, arsenopyrite also presents a serious health hazard, if major quantities of it are exposed to oxidizing waters that are subsequently used for human consumption. In such waters, arseno- pyrite will decompose, releasing Fe, As, and S. Some of this solute will be adsorbed on solid phases near the site of dissolution, e.g., in a mine, but much of it will find its way into the surrounding environment and potentially into the drinking water of nearby towns and villages. Dissolved As occurs dominantly as inorganic As(III) (arsenite) and As(V) (arsenate) ions, and if consumed in large amounts may poison the nervous and digestive systems and cause a variety of skin diseases. The acute health risk is particularly high if As(III) is dominant, as this form of dissolved As is 60 times more toxic than As(V) (Bottomley, 1984). During the past more than 20a, a number of Carlin- type Au deposits have been found in the Yunnan– Guizhou–Guangxi golden triangle area of southwestern China, in which arsenopyrite and associated pyrite are the main ore minerals (Lixian et al., 1993). Extensive exploration and mining activity has led to widespread exposure of the sulfide minerals to the atmosphere. As a result, these sulfides are being oxidized due to interaction with O 2 -rich surface waters, and are decomposing, 0883-2927/$ - see front matter # 2003 Elsevier Ltd. All rights reserved. doi:10.1016/S0883-2927(03)00133-1 Applied Geochemistry 19 (2004) 435–444 www.elsevier.com/locate/apgeochem * Corresponding author. E-mailaddress: yum@public.gz.cn (Y. Yunmei).