Effects of surface properties and pore size on transition metal adsorption, isotopic fractionation, and redox reactions [electronic resource]
Keywords: Geological Sciences.

Fundamental investigations of geologic systems are critical to understanding how physical and chemical variables govern reactions occurring in nature. Such research provides insight into complex geochemical networks, and data for constructing predictive models of contaminant remediation, climate shifts, nutrient transport, and natural resource formation. Here, I examine how two material parameters (i.e. surface ordering and pore size) affect three geochemical processes (i.e. transition metal adsorption, isotopic fractionation associated with sorption, and the kinetics of oxidation-reduction reactions involving sorbed ions). Specifically, I show how surface (dis)order of quartz versus amorphous silica (SiO2(am)) and (nano)pore size from 10-328 nm diameters dictate divalent zinc (Zn(II)) adsorption and attendant isotopic fractionation. Employing findings about the Zn(II)-silica system to guide redox experiments, I discuss how surface disorder and pore size affect ferrous iron (Fe(II)) sorption on silica and reduction of toxic, hexavalent chromium (Cr(VI)) by various Fe(II) species. For metal ions whose mobility is affected by sorption, stable isotope signatures may provide a tool for tracking the fate and transport of these metal ions in the environment. Metal ion-mineral surface interactions and the attendant isotopic fractionation depend on the properties of the mineral surface and the local atomic-level chemical environment. However, these factors have not been systematically examined for phases of the same composition with different levels of surface disorder. Combining sorption edges, x-ray absorption spectra, and isotopic measurements illustrates the effects of surface disorder on Zn(II) surface complexation and isotope fractionation. Outer-sphere, octahedral Zn(II) adsorption complexes on quartz do not present significant isotopic fractionation (Δ66/64Znaqueous-sorbed = -0.01 ±0.06‰), whereas inner-sphere octahedral and tetrahedral Zn(II) complexes on quartz yield indistinguishable isotopic fractionation (Δ66/64Znaqueous-sorbed = -0.60 ±0.11‰). Outer-sphere Zn(II) adsorption complexes are not observed on SiO2(am), and inner-sphere octahedral and tetrahedral Zn(II) complexes on SiO2(am) result in significantly larger isotopic fractionation (Δ66/64Znaqueous-sorbed = -0.94 ±0.11‰) than on quartz. Differences in Zn isotope fractionations from sorption on quartz compared to SiO2(am) are attributed to differences in the bonding environments on the two types of silica surfaces rather than to a change in the coordination number of Zn(II). This work highlights that the dominant chemical bond in isotopic fractionation during equilibrium adsorption is the direct attachment to a solid surface, not electrostatic interaction with surrounding water ligands. Nanoporosity (< 100 nm diameter pores) is abundant in subsurface environments and often contributes significantly to the total surface area available for reactions. Although confined spaces in natural and synthetic porous media are ubiquitous, we lack a framework for evaluating nano-confinement phenomena in environmentally relevant systems (i.e. mineral surfaces in aqueous solutions) as a function of pore size. Batch experiments of Zn(II) and variably porous SiO2(am) particles serve as model systems for nanoscale studies. Similar to sorption complexes on quartz, Zn(II) sorbs to all porous SiO2(am) surfaces as a monodentate, corner-sharing complex with a silica tetrahedron. As pore size decreases from 328 to 10 nm, the level of surface loading above which Zn(II) transitions from octahedral to tetrahedral coordination with oxygen decreases, such that in the smallest of nanopores studied Zn(II) sorbs solely as tetrahedral complexes. Change in the coordination geometry of sorption complexes across pore sizes from 10-113 nm compared to 328 nm macropores suggests that nano-confinement phenomena may occur across a larger range of pore sizes than traditionally believed (i.e. < 10 nm). Despite these molecular-level differences with respect to pore size, there is no quantifiable difference in the macroscopic structure of sorption edges or observed equilibrium Zn isotope fractionation, which is attributed to the small energetic difference between octahedral and tetrahedral coordination of Zn(II). Hexavalent chromium (Cr(VI)) is a very soluble and toxic, groundwater and surface water contaminant derived from anthropogenic and natural sources. Choosing effective remediation strategies, such as in-situ reduction of Cr(VI) to insoluble, trivalent chromium (Cr(III)) by Fe(II), depends on our ability to model relevant subsurface geochemical reactions. However, species-specific rate constants for iron-chromium oxidation-reduction reactions are unknown for many systems, especially in the presence of sorbing surfaces. Mineral surfaces have been shown to increases the rate of Cr(VI) reduction by Fe(II), but this enhancement has been ascribed to multiple physiochemical processes. I present batch sorption and redox experiments of the Cr(VI)-Fe(II)-quartz and Cr(VI)-Fe(II)-SiO2(am) systems to decipher the role of silica surfaces in mediating abiotic reduction of Cr(VI) by aqueous and sorbed Fe(II) species. Sorption edges indicate outer-sphere (Fe(II)ads, OS) and inner-sphere (Fe(II)ads, IS) complexes are present on all silica substrates, but their abundance depends on pH, ionic strength, and surface disorder. The rate constants of Cr(VI)aq reduction by Fe(II) species increase in the following order: kaq < < kads, OS, quartz < kads, OS, SiO2(am) < kads, IS, quartz < kads, IS, SiO2(am). As the redox reaction proceeds and Cr(III)-Fe(III) precipitates form on the silica surface, a portion of the surface-bound Fe(II) is sequestered (Fe(II)s) into the precipitate. Therefore, when devising remediation strategies in Fe(II) limited systems, the balance between increases in the rate and decreases in the total amount of Cr(VI)aq reduction by various sorbed Fe(II) species must be considered. In summary, this work demonstrates how surface disorder and pore size affect transition metal adsorption, isotopic fractionation, and oxidation-reduction reactions. Zinc isotopic fractionation during adsorption to quartz and SiO2(am) surfaces provides an enlarged perspective on what drives equilibrium isotope partitioning during adsorption. The change in Zn(II) surface complexes with decreasing (nano)pore size suggests that the size range of nano-confinement effects is larger than previously thought. Finally, the sorption of Fe(II) mediates Cr(VI) reduction kinetics and demonstrates the importance of quantifying species-specific reduction rates. These fundamental investigations of surface disorder and pore size effects on metal ion sorption, isotope partitioning, and redox reactions enable development of better geochemical models and highlight complexities important to subsurface engineering practices.