Impacts of manganese oxides on molybdenum and tungsten speciation and implications for their geochemistry in aquatic environments

Abstract

Molybdenum (Mo) and tungsten (W) concentration patterns and isotope signatures are widely used to study changes in ocean redox conditions. This is in part due to the differential adsorption of Mo and W onto various types of minerals that are abundant under different redox regimes. The speciation of Mo and W, which also varies under different redox conditions, greatly controls their particle reactivities in the environment. Manganese (Mn) oxide minerals can not only provide surfaces for Mo and W adsorption, but also catalyze Mo and W speciation change. Thus, the Mn oxides that are often present at redox interfaces can play an important role in Mo and W speciation change. However, we lack knowledge of how rates for Mn oxide catalyzed Mo and W speciation transformation compare to rates for their adsorption and isotope fractionation processes, which creates uncertainty in our ability to interpret Mo and W concentration and isotope patterns in sediments and sedimentary rocks. Here, we present data that constrain rates, environmental controls, and mechanisms for tetrathiomolybdate (MoS42–) and tetrathiotungstate (WS42–) hydrolysis on Mn oxide surfaces, which transforms the thioanion species to oxyanion species. The experimental results show that hydrolysis rate constants for these Mo and W thioanions are first order with respect to Mn oxide surface areas. Hence, high abundance of Mn oxides in aquatic environments greatly catalyzes MoS42– and WS42– hydrolysis. In addition, experimental results indicate that crystal structure differences of Mn oxides contribute to differences in rates for Mo and W thioanion hydrolysis reactions. Solution pH and ionic strength have limited impacts on Mo and W thioanion hydrolysis rates catalyzed by Mn oxides, while sulfate has a moderate impact. Geochemical modeling results indicate that equilibrium times for MoS42– and WS42– hydrolysis reactions in the presence of Mn oxides at abundances found in the modern ocean are three orders of magnitude longer than those for adsorption and isotope fractionation of Mo and W on Mn oxide surfaces. This implies that Mn oxides play a larger role in affecting adsorption and isotope fractionation processes than Mo and W speciation transformation in the modern, well-oxygenated ocean. In contrast, the potentially high levels of dissolved Mn(II) in the Archean ocean would have shortened the equilibrium time for Mo or W speciation transformation such that this process could compete with their adsorption and isotope fractionation processes, and subsequently affect sedimentary Mo and W isotope records. The impact of dissolved Mn(II) catalyzed Mo and W speciation change should be considered in future studies of Archean Mo and W isotope records.