Abstract:

Redox gradients are common features in mine tailings systems, with oxidizing conditions near the surface and sub-oxic to reducing conditions below. The impact of these gradients on mineral reactivity (i.e., oxidative sulfide dissolution) and the mobility of redox-sensitive solutes remains poorly quantified and is highly site-specific.

We investigated the occurrence of incompletely oxidized (intermediary) sulfur [S] and arsenic [As] in various legacy tailings systems in Ontario and Nova Scotia, Canada. Our analysis included trace-level aqueous As and S speciation, bulk and automated quantitative mineralogy, as well as careful probing of the in-situ hydrogeochemical conditions.

Sulfate and arsenate were major solutes under oxidizing conditions, but significant fractions of other species were detected under sub-oxic to reducing conditions, including thiolated As (up to 17% of the total dissolved As), thiosulfate and tetrathionate (up to 40% of total dissolved S). The bulk As and S contents of the tailing’s materials were strongly decoupled from their concentrations in solution, reflective of the different controls of adsorption, dissolution, and re-precipitation reactions along redox gradients. The relative abundance of intermediary As and S was higher for select mineralogical hosts (e.g., arsenosulfides), but mostly dictated by the prevailing redox conditions and porewater hydrochemistry, particularly dissolved Fe content.

Our work shows that “unconventional” intermediate As and S compounds play a pivotal role in their cycling in sub-oxic mine waste systems and should be explicitly considered in long-term reactivity predictions and management strategies.

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Matias Silva