Abstract
The elemental sulfur formed at the arsenopyrite surface after oxidation by ferric iron was quantitatively measured by extraction in perchloroethylene and subsequent quantitative analysis by HPLC. Reactions with ferric iron in perchloric acid solutions or in sulfuric acid solutions (both at pH = 1 and 42°C, which approximate extreme acid mine drainage conditions) produced elemental sulfur in quantities greater than 50% of the total reacted sulfur. The controversy surrounding the mechanism of the oxidative dissolution of arsenopyrite is discussed in light of these measurements. Based on the observation of greater than 50% production of elemental sulfur, a mechanism by which all the sulfur from the mineral proceeds through thiosulfate can be eliminated as a possible description of the dissolution of arsenopyrite. Instead, it is likely the other constituents of the mineral lattice, Fe and As, are leached out, leaving behind a S0 lattice. Nucleation reactions will then result in the formation of stable S8 rings.
Highlights
Arsenopyrite, FeAsS, is the most common arsenic-bearing mineral
The table summarizes the number of moles of elemental sulfur extracted from the mineral surface, the total aqueous sulfur content, the total reacted sulfur, the percentage of the total reacted sulfur that is found as elemental sulfur, and the total aqueous arsenic
Our results show that the amount of elemental sulfur found at the arsenopyrite surface cannot be accounted for by the decomposition of thiosulfate alone, and indicate that an additional direct pathway to the formation of elemental sulfur at the arsenopyrite surface must be taken into account
Summary
Arsenopyrite, FeAsS (a derivative of the marcasite structure), is the most common arsenic-bearing mineral. Either occurring naturally or as a result of mining processes, the mineral produces arsenite (AsO332), arsenate (AsO432), and sulfate (SO422),[1,2,3] contributing to the acidification of water as well as the release of soluble arsenic species. The rate of sulfide mineral dissolution is typically limited by the supply of ferric iron, Fe3z; in the presence of iron-oxidizing microorganisms, the supply of ferric iron is continuously replenished by microbial oxidation of the ferrous iron released from sulfide minerals.[4] Despite the importance of oxidation by ferric iron in natural systems, many of the fundamental details of the oxidation of arsenopyrite by Fe3z under acidic conditions still remain unclear. The literature is divided about whether the majority of the sulfur from the mineral is released into solution as sulfoxy anions5,6 [in a scheme similar to eqn (1)], or whether a substantial amount of the sulfur remains as insoluble elemental sulfur (S8) at the mineral surface7,8 [as shown in eqn (2)]: FeAsS 11Fe III ?12Fe II As III S VI (1)
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