IRON ISOTOPE VARIATIONS DURING OXIDATIVE DISSOLUTION OF ARSENOPYRITE (Invited Presentation)

Arsenopyrite (FeAsS) can be a major source of arsenic (As) contamination in ground water. Previous studies have investigated the rates of iron (Fe) and As release during arsenopyrite dissolution, but the detailed reaction mechanism is not fully understood.

We ran a series of leaching experiments coupled with Fe isotope analysis to investigate the release of As and Fe from arsenopyrite in oxidizing and acidic environments and to predict the potential for As release as a function of ground water chemistry. Powders of naturally occurring arsenopyrite were placed into centrifuge tubes with solutions of hydrogen peroxide of varying concentrations (0.20M, 0.16, 0.05, and 0.01) and acidified to pH 2 with hydrochloric acid (HCl) to prevent Fe adsorption to mineral surfaces. Samples were continuously agitated for up to two months, except for periodic decanting for analysis and addition of fresh reactant.

Initial results from several batch leaching experiments indicate that arsenopyrite dissolves nearly stoichiometrically until release of ~3 to 5 mol % S. After this point, the rate of S release increases rapidly relative to Fe and As, then begins to slow after ~20 to 40 mol % S leaches from the mineral. Fe and As are released into solution at the same rate as each other throughout the entire experiment, but the rate decreases after ~ 5 to 10 mol % Fe (or As) is in solution. This retardation of dissolution rates is consistent with previous studies that suggest that passivation surface layers can inhibit sulfide mineral dissolution. Iron isotope data from these experiments indicate that lighter Fe is preferentially released as the mineral dissolves. This apparent fractionation, expressed as Δ⁵⁶Fe_{(aq- min)} ‰ (δ⁵⁶Fe of dissolved Fe minus δ⁵⁶Fe of the bulk mineral), generally decreased with reaction progress, and reached minimum values of -0.72 to -1.50 (±0.24‰) at the end of the experiments. This decrease in Δ⁵⁶Fe_{(aq- min)} values could be due to either diffusive or solid state bonding effects associated with the formation of a passivation surface layer. Ongoing studies involve varying pH and oxidant type; our preliminary results suggest that Fe isotopes could be useful in determination of arsenopyrite dissolution mechanisms and to predict the potential for As release into solution.