Mineral controls on molybdate (MoO42−) solubility were studied to better understand the fate of molybdenum (Mo) in neutral rock drainage. Batch and column experiments lasting from 46–110 days predict that molybdate (MoO42−) mobility is limited by powellite (CaMoO4) and wulfenite (PbMoO4) precipitation under neutral pH conditions with aqueous Ca and Pb present. Batch experiments demonstrate that wulfenite forms almost instantaneously and effectively removes Pb from solution to concentrations below detection limits. Powellite formation is kinetically limited, but has the capacity to significantly reduce Mo concentrations in the presence of calcite. 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−. A column experiment provides further evidence of rapid wulfenite formation and kinetically limited powellite formation. Both powellite and wulfenite have also been identified in molybdenum-bearing carbonate-rich waste rock sampled from barrel-sized field cell weathering experiments, providing direct evidence that these minerals affect Mo mobility under field conditions. In contrast to Pb and Ca, both Cu and Zn did not form distinct molybdate precipitates.

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