Molybdenum (Mo) stable isotopic variations as indicators of Mo attenuation in mine waste-rock drainage

Mobilization of Metal(oid) Oxyanions through Circumneutral Mine Waste-Rock Drainage

“…The discrete secondary minerals wulfenite and powellite could be identified in small amounts in the marble- and skarn waste rock (Table 1). In line with observations of these minerals in previous field- and laboratory column experiments with Antamina waste rock, 13,27,52 these minerals constitute Mo solubility controls under NRD conditions where Mo sorption is weak. Molybdenum attenuation through secondary molybdate precipitation was particularly efficient in the Pb- and Mo-rich skarn barrel S3, for which a Mo-content >900 mg/kg (Table S1) did not translate into release of appreciable dissolved Mo (<50 μg/L).…”

“…64−66 To prevent such detrimental environmental effects, the processes controlling oxyanion concentrations in mine drainage need to be understood. Although selected studies have investigated the occurrence, (bio)geochemical dynamics, and toxicity of As, Mo, Sb, and Se in mine wastes (e.g., for As, 22−25 Mo, 14,26,27 Sb, 28,29 and Se 30−32 ), the majority of these laboratory and field studies has focused on their behavior under acidic conditions, with relatively little attention given to neutral- or alkaline-pH waters (e.g., for As 15,33 ). Therefore, the potential sources and environmental fates of these elements in NRD systems remain poorly explored.…”

“…30 Once dissolved, As, Mo, Sb, and Se may co-precipitate in the form of solid solutions as secondary (oxyhydr)oxides and (hydroxy)sulfates or form discrete secondary mineral phases, both of which provide attenuation mechanisms in mine wastes. 30,38−40 Examples of identified secondary phases in mine wastes include, for As: various Ca- and Fe-arsenates (e.g., scorodite [FeAsO 4 ·2H 2 O] 8,36,41 ), for Mo: powellite [CaMoO 4 ], wulfenite [PbMoO 4 ], and ferrimolybdite [Fe 2 (MoO 4 ) 3 ], 13,27,42 and for Sb: stibiconite [Sb 3 O 6 (OH)] and kermesite [Sb 2 S 2 O]. 37 To the authors’ knowledge, no discrete secondary Se phases have been characterized in mine wastes, even though various metal–selenites are quite insoluble.…”

Mine Waste Rock: Insights for Sustainable Hydrogeochemical Management

“…Yet, the application of quantitative sorption models in drainage predictions is scarce. This may be because the sorption properties of less-abundant solutes remain poorly studied (e.g., Mo [161]) and stability constants deduced in laboratory studies may not be representative for variable field conditions. Furthermore, the data required to inform sorption models, i.e., mineral surface characteristics such as sorption-site density or solution chemistry, is typically unavailable (or semi-quantitative at best) for large, heterogeneous waste-rock piles.…”

“…The identification, let alone quantification, of secondary minerals in waste rock, e.g., with X-Ray diffraction (XRD), remains challenging for dispersed and poorly crystalline phases. Yet, even secondary phases with low overall bulk abundance can be substantial attenuating sinks: e.g., the scarce but rapidly precipitating wulfenite (PbMoO 4 ) has been shown to present an important control on Pb and Mo drainage levels [161,166,167]. For successful drainage management, it is thus critical that the secondary mineral assemblage is quantified and monitored over time.…”