Comparison of effects between kaolinite and hydrogen peroxide on tungsten and molybdenum speciation and implications for their geochemistry in aquatic environments

Abstract

Tungsten (W) and molybdenum (Mo) speciation greatly influences their biogeochemical cycling in natural waters. However, the factors that affect their speciation in the environment are poorly constrained. Here, we critically evaluate the impacts of kaolinite and hydrogen peroxide (H2O2) on the speciation of W and Mo in aquatic environments. We present data that constrain rates, environmental controls, and mechanisms for tetrathiotungstate (WS42−) and tetrathiomolybdate (MoS42−) hydrolysis catalyzed by kaolinite and H2O2. The laboratory experiments show that the hydrolysis rate constants for WS42− and MoS42− are first order with respect to kaolinite surface areas and H2O2 concentrations. Hence, high abundances of kaolinite and reactive oxygen species (ROS) like H2O2 in aquatic environments greatly facilitate WS42− and MoS42− hydrolysis. In addition, our results indicate that pH has a substantial impact on the catalytic ability of H2O2, whereas pH has a limited impact on the catalytic ability of kaolinite. In comparison, SO42− has a limited impact on catalytic ability of H2O2, whereas it has a moderate impact on the catalytic ability of kaolinite. Although the rate constants for H2O2–catalyzed WS42− and MoS42− hydrolysis reactions are substantially larger than those for kaolinite–catalyzed WS42− and MoS42− hydrolysis reactions, implying a higher decomposing potential of H2O2 than that of kaolinite, modeling results suggest that the impact of H2O2 on W and Mo speciation is less important compared to that of kaolinite due to the low abundance of H2O2 in natural aquatic environments. Because the equilibrium time for WS42− and MoS42− hydrolysis reactions in the presence of kaolinite is substantially longer than that for W and Mo adsorption onto minerals, these results also suggest that various aqueous W and Mo species, including W and Mo thioanion intermediates, are likely to be scavenged via adsorption onto minerals and possibly organic phases, within aquatic sediments. Our investigation highlights the importance of minerals in regulating the chemical speciation of W and Mo in natural systems, which can improve our ability to predict W and Mo cycling in the environment.