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Attenuation mechanisms for molybdenum in neutral rock drainage Conlan, Michael Joseph William

Abstract

Solubility controls and adsorption of molybdate (MoO₄) were studied to better understand the fate of molybdenum (Mo) in neutral rock drainage. Batch and column experiments predict that MoO₄ mobility is limited by powellite (CaMoO₄) and wulfenite (PbMoO₄) precipitation under neutral pH conditions and that adsorption of molybdate is relatively limited. Batch experiments demonstrate that wulfenite forms almost immediately and effectively removes Pb and Mo from solution to concentrations below detection limits whereas powellite formation is kinetically limited and reduces Mo concentrations to ~10 mg L−¹ in systems containing Ca. An initial inhibition of powellite formation is observed, likely due to a lack of available nucleation sites. After the nucleation phase, powellite formation from supersaturated conditions follows a second order rate expression with linear dependence on Ca²⁺ and MoO₄²−. Column experiments provide further evidence of rapid wulfenite formation and kinetically limited powellite formation. Both powellite and wulfenite have also been identified in samples extracted from barrel-sized field experiments designed to study the weathering of carbonate-rich waste rock material, providing direct evidence that these minerals contribute to controlling Mo mobility under field conditions. In contrast to Pb and Ca, both Cu and Zn did not form molybdate precipitates, even under highly supersaturated conditions as predicted by PHREEQC. Additional experiments indicate that Mo attenuation by adsorption under neutral pH conditions is limited and is dependent on the amount of available sorption sites. Desorption experiments on existing waste rock indicate that some Mo has likely been adsorbed to iron hydroxides, but further analyses are required to determine the extent.

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