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

Abstract

Most studies on the weathering of mine waste rock focus on the generation of acidic drainage with high metal concentrations, whereas metal(loid) release under neutral-rock drainage (NRD) conditions has received limited attention. Here, we present geochemical and mineralogical data from a long-term (>10 years) kinetic testing program with 50 waste-rock field barrels at the polymetallic Antamina mine in Peru. The weathering of most rock lithologies in the field experiments generated circumneutral to alkaline drainage (6 < pH < 9) but with concentrations of the oxyanion-forming metal(loid)s As, Mo, Se, and Sb in the mg/L range. The mobilization of As and Sb was particularly efficient from intrusive, marble and hornfels rocks that contained labile As- and Sb-sulfides, irrespective of bulk elemental content or waste-rock reactivity. High-alkalinity drainage from these materials sustained neutral-pH conditions that are unfavorable to oxyanion adsorption onto Fe-(oxyhydr)oxides and, therefore, enhanced As and Sb leaching. The release of Mo and Se from sulfidic skarn and intrusive waste rock was more proportional to elemental content but equally enhanced by pH-inhibited adsorption and negligible secondary mineral precipitation under NRD conditions. Our results demonstrate that oxyanion concentrations of environmental concern may be conveyed by neutral- to alkaline-pH waste-rock drainage and should be a focus of mine wastewater monitoring programs.

Mine Waste Rock: Insights for Sustainable Hydrogeochemical Management

“…This introduces important ramifications as to the timescales required for effective acid-neutralization, for instsance when transport times are fast or experimental durations short (e.g., static tests; Section 2.6.1). In addition to consuming protons, dissolution of carbonate, oxide, or aluminosilicate minerals introduces additional solutes to a leachate, including Ca, Mg, Mn, Al, and Fe, and possibly metal impurities if present at considerable levels [128,129]. Mobilization of such solutes and its effect on drainage quality must be considered if these neutralizing materials are to be employed to prevent or remediate acid drainage [13].…”

“…Contaminant loads in neutral drainage can be controlled by their occurrence in sulfidic minerals as well as carbonate or silicate phases, or through the dissolution of (secondary) salts and oxides (Figure 1). For instance, elevated As levels in high-alkalinity drainage have been mostly attributed to the oxidation of arsenopyrite or As-bearing pyrite that had been neutralized [137,138] but may also originate from natively As-rich carbonate phases in the absence of Fe-oxides [128]. With the increased interest in the development of rare earth element (REE) mines, there is an increasing interest in REE geochemistry and ecotoxicity [139][140][141].…”

“…While adsorption is highly surface-and solute-specific, the general relevance of adsorption for mine waste drainage quality has been widely acknowledged [2]. Field and laboratory waste-rock studies have demonstrated that sorption can be a dominant attenuation mechanism for various mine waste-relevant solutes, from metal cations [155] to metal(oid) oxyanions [128]. ) through covalent bonding or electrostatic interaction.…”

“…Finally, the surface properties of waste rock may evolve as weathering progresses, e.g., through precipitation of amorphous secondary Fe-oxide coatings with elevated sorption capacity. As a result, adsorption modeling for waste rock often relies on estimated parameters, adoption of synthetic idealized phases (e.g., hydrous ferric oxide [128]), or extrapolation of generalized behavior (the Irving-Williams series) in stable drainage types. Adsorption is often considered a precursor for secondary mineral formation (Section 2.4.2) but the contribution of adsorption versus that of secondary minerals to overall attenuation remains to be quantitatively resolved.…”

“…Static testing is often complemented by kinetic testing to overcome the discrepancy between conditions for (short-term) laboratory testing on crushed, sieved samples and the long-term behavior of coarser waste rock observed in the field. Examples of kinetic tests include laboratory humidity cell tests with artificial wetting-and drying cycles [129,132], column tests [68], as well as field cell tests of a variety of sizes [128,177]. The benefits and limitations of humidity cell tests have been previously reviewed [176,223].…”