Clay surface reactivity and its interaction with trace elements

• Author / Creator

  Hao, Weiduo

• Clay minerals are ubiquitous at the Earth’s surface and they impart a significant influence on numerous geochemical cycles due to their high surface reactivity. Aqueous pH and ionic strength (hereafter IS) are two major factors that affect clay surface reactivity and subsequently govern trace elemental behavior on clay surfaces. In environmental settings with dynamic pH and IS (such as estuaries), the variation of clay surface reactivity significantly controls the fate and transport of trace elements. In this regard, this work explores the variation of clay surface properties under various aqueous conditions, and how this impacts their ability to accumulate trace metals.
  Chapter 1 is an introductory chapter, where I briefly outline some background information about clay surface reactivity, solution pH and ionic strength, and trace elemental behavior on clay surfaces.
  In chapter 2, the net surface proton charge of three clays (kaolinite, illite and montmorillonite) under a range of pH and IS was evaluated through acid-base titration. The observation that the point of zero net proton charge (pHPZNPC) varies with IS for the three clay minerals implies that the variation of clay surface reactivities with IS differs from simple oxides minerals (such as Fe-oxides, Al-oxides). A mathematical relationship was established between the charge property of clay minerals and solution IS which can be used to predict the clay surface proton charge.
  In chapter 3, the proton interaction constants on clay surfaces and their variation with IS was further elucidated within a surface complexation modeling framework. Previous studies are still unclear on whether solution IS affects clay surface reactivities through attenuation of clay surface electrostatic field or saturation of surface adsorption sites by electrolytes. The application of models to investigate Na competitive adsorption onto surface active adsorption sites failed to match the experimental results and theoretical assumptions, thus indicating that attenuation of the clay surface electrostatic field better explains the experimental behavior.
  In chapter 4, I described how solution pH influences clay surface functional group protonation and deprotonation. Interestingly, under extremely low pH, structural elements of clay minerals can be released and lead to differences in surface reactivity compared to normal clays. To determine this differences, variable degrees of acidic treatment were applied, and the results reveal an increase in surface area but decrease in Cd adsorption capacity after acidic treatment despite a lack of morphological and crystal structure changes.
  In chapter 5, once the clay surface properties as a function of solution pH and IS were characterized, Cd behavior onto the clay surfaces was evaluated under simulated estuary conditions with dynamic changes in pH and IS. Coupled with Cd adsorption isotherms, surface complexation modeling and Extended X-ray Adsorption Fine Structure (EXAFS), I demonstrated that Cd mainly forms outer-sphere complex onto clay surfaces under freshwater conditions, while inner-sphere adsorption was observed under marine conditions. My results show that once clays are transported from rivers to the oceans, Cd will be released from clay surfaces to water column in estuaries.
  In chapter 6, experimental results show that phosphate, as a bio-limiting nutrient, is readily shuttled by kaolinite from land to the oceans. The “kaolinite-shuttle” mechanism is subsequently used to explain the transport of nutrients to the Paleoproterozoic oceans following the Great Oxidation Event at ~2.5 Ga when intense continental weathering generated kaolinite on the paleo-Earth surfaces. I hypothesized that the transport of kaolinite from the paleosol to seawater at that period of time delivered nutrients to the ocean, thus facilitating enhanced primary productivity in marginal marine settings. These events ultimately contributed to the largest carbon burial event in Earth’s history, the so-called Lomagundi Event between 2.20 to 2.06 Ga.
  In chapter 7, I summarize our current state of knowledge about clay surface reactivity and propose future avenues of research.

• Subjects / Keywords

  • clay minerals
  • surface reactivity
  • trace elements

• Graduation date

  Spring 2020

• Type of Item

  Thesis

• Degree

  Doctor of Philosophy

• DOI

  https://doi.org/10.7939/r3-53pq-9w80

• License

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