62nd Annual Report on Research 2017

1. Annual summary

As from the time of abiogenesis, Earth's oceans were void of oxygen and abundant in H₂S. As a result, most metal ions would have existed primarily in the form of extremely insoluble sulfides. Different metal has different available abundance, and the available abundance of them would have been as a result of the dissolution of sulfide complexes and desorption/adsorption from sulfide minerals. For example, one of the most important processes is the reductive dissolution of Fe(III)/Mn(IV) oxides/oxyhydroxides within the aquifer sediments, which can lead to increase in toxic trace element concentration in solution as sorbed trace elements are released by the dissolving metal oxides/oxyhydroxides. Also, in groundwater and sediment porewaters, minerals that can absorb or desorb trace elements play important roles in controlling the concentration and speciation of trace elements. Furthermore, the relative pH value of groundwater/porewaters is also a critical factor in the subsequent release of sorbed trace element to the groundwaters. For example, metals that chiefly occur as cations in aqueous solution generally exhibit more adsorption onto mineral surfaces as pH increases. Conversely, metals that form oxyanions like W and Mo tend to desorb from mineral surfaces as pH increases.

Our present work has successfully evaluated the particle reactivity of thiotungstate anions, in aqueous solutions. We investigated the kinetics, equilibrium, and effects of pH and ionic strength on tungstate (WO₄^2–) and tetrathiotungtate (WS₄^2–) adsorption onto pyrite under anoxic conditions. The measured adsorption constants enable us to evaluate the adsorption process and particle reactivity of these two important tungsten species with respect to pyrite. The adsorption of WO₄^2– and WS₄^2– onto pyrite is compared to the adsorptive behavior of both anions onto goethite, which occurs very widely as colloidal materials in natural waters or as coatings on the surfaces of detrital minerals. Our results showed that WO₄^2– and WS₄^2– adsorption onto pyrite increased with decreasing pH. The greatest amounts of WO₄^2– and WS₄^2– adsorbed onto pyrite were measured at pH 4.95 and 5.2, respectively. Kinetic experiments indicate that WO₄^2– adsorption onto pyrite occurs more rapidly than WS₄^2–. The kinetic behavior of the adsorption of both W species onto pyrite is well described with a pseudo-second-order Langmuir model. More specifically, rapid external adsorption followed by intraparticle diffusion were the rate-controlling steps during WO₄^2– and WS₄^2– adsorption onto pyrite, and intraparticle diffusion of both W species onto pyrite accounted for > 99% of time of the adsorption process, suggesting that it was the major rate-limiting step. Application of the mono-surface Langmuir model provided the best fit to our adsorption data indicating that WO₄^2– and WS₄^2– were mainly adsorbed onto one specific site on the pyrite surface. Our model calculations suggest that the specific adsorption of WO₄^2– onto pyrite was greater than the corresponding adsorption of WS₄^2– onto pyrite in all cases. The difference of specific adsorption between WO₄^2– and WS₄^2– may be attributed to their different inner-sphere complexation on the pyrite surface. The adsorption experiments also showed that W species were less adsorbed onto pyrite than goethite. Current work demonstrates that pyrite plays an important role in determining the fate and transport of WO₄^2– and WS₄^2– in natural waters. Specifically, it indicates that WS₄^2– is less particle reactive with respect to pyrite than MoS₄^2–, which we propose can explain, in part, the apparent stability of W in sulfidic waters compared to Mo as well as the elevated Mo/W ratios in Black Sea sapropels

2. The importance of current work

This study demonstrates that pyrite plays an important role in determining the fate and transport of W in natural waters.

WO₄^{2 −} has greater adsorption kinetics and higher level of adsorption capacity onto pyrite than WS₄^{2 −}.

The different specific adsorption of W may be attributed to the different inner-sphere complexation on the pyrite surface.

3. Future work

Laboratory investigations have presented evidence that, like Mo, W undergoes sulfidation in four steps that conserve tungstate and lead to the formation of tetrathiotungstate. Our on-going investigation of the kinetics of tungstate thiolation reactions indicates that tungsten speciation in natural waters will be more sensitive to seasonal anoxia that molybdenum speciation. To further investigate the fate and transport processes of W in the environment, we will continue our kinetic investigations of tungstate thiolation reactions to provide a quantitative understanding of the formation of these important aqueous species (i.e., thiotungstate anions). In the following year, we will focus on these kinetic experiments, which will include investigations of the general acid catalysis of thiotungstate formation, and the development of Brønsted relationships, which can subsequently be applied broadly across different natural waters. Again, our current preliminary data indicates that the sulfidation reactions of W are indeed acid catalyzed. We suggest that in environments such as sediment porewaters and the presence of Brønsted acids, like carbonic acid and hydrogen sulfide, will promote conversion of tungstate to thiotungstates. However, the conversion of the predominant anion from a hard to a soft base will also alter W’s geochemical behavior, increasing its susceptibility to scavenging by adsorption reactions. Thus, an important product of this research will be an improved understanding of the scavenging pathways of W in euxinic environments, which are important regions where petroleum source rocks are thought to form.