The speciation, kinetics, and adsorption of tungsten in sulfidic natural waters
Description
Redox-sensitive trace metals tend to be more soluble under oxidizing conditions and less soluble under reducing conditions resulting in authigenic enrichments in oxygen-depleted sedimentary facies (Algeo and Rowe, 2012; P Ho et al., 2017; Tribovillard et al., 2006). Because of that, redox sensitive trace elements are used as paleoredox proxies to reconstruct redox status of the environment (Tribovillard et al., 2006). Unfortunately, we lack the quantitative understanding of many of these trace elements. One of the most important trace elements is tungsten. To contribute a better understanding of the biogeochemistry of tungsten to our community, I utilize laboratory experiments, geochemical modeling, and statistical methods to investigate the speciation, kinetics, and adsorption of tungsten in the environment. This thesis includes three major chapters. In the first major chapter, I performed a series of chemical experiments to explore the particle reactivity of tungstate and tetrathiotungstate in sulfidic solutions. I found that pyrite is a strong scavenger of W in aquatic environments. Our results indicate that the difference of specific adsorption between WO42– and WS42– may be attributed to their different inner-sphere complexation on the pyrite surface. Our results also show that WS42– is less particle reactive with respect to pyrite than MoS42–. In the second major chapter, I examined effect of acid on tungsten (W) sulfidation process as well as developed the Brønsted acid relationship, which provides a tool to predict the effect of acids on the kinetics of the thiolation reaction of W in natural waters. The results of laboratory experiments show that thiotungstate formation is first order with respect to both H2S and WO42- concentrations, and is catalyzed by acids. Therefore, low pH and high H2S concentrations both favor W thiolation. However, compared to molybdenum (Mo), thiolation of W is kinetically “sluggish”. The modeling results show that full thiolation of Mo requires ca. 110 days, whereas full thiolation of W requires ca. 50 years under a persistent euxinic condition such as the Black Sea. Our results indicate that the longer the period of euxinia, the higher chance of WS42- species in solutions and subsequently be incorporated into euxinic sediments as W-S species. In the last chapter, I successfully tested whether protonated mineral surfaces also catalyze the hydrolysis of tetrathiotungstate anions. Our results show that kaolin (Al2Si2O5[OH]4), aluminum oxide (γ-Al2O3), and titanium dioxide (TiO2) exert an appreciable catalytic effect on tetrathiotungstate hydrolysis. The data suggest that the pH dependent hydrolysis rate of WS42- for kaolin, γ-Al2O3, and TiO2 fall into two distinct groups, which consist two reaction pathways. The pH dependence of the mineral-catalyzed reactions suggest that acid surface sites on the mineral surfaces promote WS42- hydrolysis reactions. In the presence of UV-light, TiO2 substantially enhanced the hydrolysis rate of WS42- compared to identical experiments that were conducted in the absence of UV-light, we suggest the increased hydrolysis rate of WS42- in the presence of UV-light reflects the production of reactive oxygen species by TiO2. Due to the rapid development of nanotechnology, more engineered nanomaterials like TiO2 are introduced into the environment, which can impact the speciation and mobility of trace elements. Combined, the results in this thesis advance our understanding of mechanisms for W biogeochemistry in euxinic systems and will allow to facilitate reconstruction of paleodepositional conditions, paleoproductivity, and paleoredox.