Arsenic Release from a Natural Rock under Near-natural Oxidizing Conditions

The effects of carbonate concentration and the presence of iron hydroxide phases on the process of arsenic release from an ore material were investigated under experimental oxic conditions and in the pH range from 6.0 to 9.0. These experimental conditions are pertinent to arsenic leaching from tailings and mining wastes. The leaching tests lasted for <= 99 days and were performed with materials of five different particle sizes (<= 2 mm). Carbonate ions were produced in-situ by dolomite dissolution or were contained in used waters (0 to 30 mM as HCO3-) Iron hydroxide phases were formed in situ by oxidative dissolution of metallic iron (Fe-0) or pyrite (FeS2). Non-disturbed batch experiments and air-homogenized experiments were conducted with a constant amount (10 g/L) of an arsenic-bearing rock (ore material) of a given particle size and different types of water (deionized, tap and mineral water). For comparison, experiments were conducted with 0.1 M EDTA, 0.1 M Na2CO3, and 0.1 M H2SO4. Neither the use of dolomite nor the use of water containing various carbonate (HCO3-) concentrations could confirm the recent results on the favorable role of As-III-carbonate complexes on the arsenic transport in the environment. On the other hand, iron hydroxide phases (from Fe-0 and FeS2) univocally delayed the As release in both experimental procedures. Furthermore, the theoretically expected effects of the particle size of the ore material was observed. If one takes into consideration that the used HCO3- concentrations were up to six times larger then those of natural surface waters (<= 5.5 mM) but up to five times lower than those currently used in the literature (>= 100 mM), it is concluded that the reported conflicting results for As leaching from sediments may be a misinterpretation of processes occurring in the sediment and yielding increased As release with increasing HCO3-/CO32- concentration.