METAL SELECTIVITY BY CLAY MINERALS USING HARD-SOFT ACID-BASE PRINCIPLE

1. HOMO-LUMO Energy Gaps • Further Studies • Repetition of thermodynamic analysis after CaCO3 removal from Hectorite • General characterization of clays • TEM imaging and EELS measurements to obtain HOMO-LUMO energy gap • Modeling and UV-Vis absorbance measurements to obtain metal ion HOMO-LUMO energy gaps Acknowledgements Special Thanks to the Kearny Foundation of Soil Science, Klaus van Benthem, Sanjai Parikh and members of the Horwath lab • Goal of Study • Understand the different components contributing to the enthalpy of metal cation sorption onto clay minerals Montmorillonite and Hectorite through Pearson’s Hard-Soft Acid-Base (HSAB) principle • Thermodynamic studies and van’t Hoff analysis • Acid-base titration measurements • TEM-EELS measurements and modeling of clay mineral HOMO-LUMO energy gap • UV-Vis Absorption measurement and modeling of metal cation HOMO-LUMO energy gap • General clay mineral characterization (CEC, surface area, chemical analysis) METAL SELECTIVITY BY CLAY MINERALS USING HARD-SOFT ACID-BASE PRINCIPLE Yan Ling Liang, William R. Horwath, Timothy A. Doane. Land, Air and Water Resources Dept.; University of California, Davis; 1 Shields Rd.; Davis, CA 95616. • Experimental • Thermodynamic studies performed on Hectorite and Montmorillonite clay minerals (supplied by Clay Mineral Society, Chantlilly, VA) • Size of clays were homogenized • Clays were saturated with Ca to ensure subsequent exchange involved Ca • Adsorption equilibrium measured at 25, 35, 45 and 55 ˚C • Equilibrium: allowed 48 hrs to establish, shaking performed at 250 rpm in 50 mL polypropylene centrifuge tubes • Metal ions tested were Mg2+, Mn+2+, Pb2+, Cu2+, Cd2+, and Ag+(solutions made from its nitrate forms at a concentration of 0.005 mol/L) • Masses of Hectorite and Montmorillonite used for experiments varied between 0.15-1 gram • Blanks containing only metal solution and only clay were used, control to ensure inter-experiment reproducibility used • pH of exchange solutions measured before and after equilibrium • Metal concentrations quantified using UV-Vis absorbance spectroscopy VU-Vis Ab. Spec. trial1 Energy gap 4-5 eV Energy gap 4-5 eV Selectivity Sequences Background Relevant properties of metal ions (table below, CRC handbook) • Pearson’s Hard-Soft Acid-Base (HSAB) Principle • What are hard and soft acids and bases? • Pearson’s principle states that there exist an additional thermodynamic stability for reactants with similar Hardness (η) or Softness (σ), this is in additional to acid-base strength of reactants • △H = -(SASB + σA σB) TEM-EELS Acid-base titrations • SA and SB are the acid and base strengths and σA and σB are the acid and base softness respectively • The Hardness/Softness can be approximated by equation below • η ≈ (I - A)/2 ≈ (-EHOMO + ELUMO)/2 • η is the hardness (inverse of softness, η=1/σ), I is the ionization energy, A is the electron affinity, EHOMO is the energy of the highest occupied molecular orbital, ELUMO is the energy of the lowest unoccupied molecular orbital Hectorite CASTEP DOS model • Clay minerals are ubiquitous in nature • Clay minerals have many uses • Proposed properties to explain metal selectivity • Metal ion radius • Electronegativity • Hydrolysis constant • Hydrated radius • Hard-Soft Acid-Base Principle • All were successful to some extent, but none completely • Thermodynamics quantify the extent of an reaction (physical or chemical) • More in depth understanding of individual contributors for enthalpy will broaden our knowledge Acronyms & Abbreviations HSAB = Hard-Soft Acid-Base EN = Electronegativity CEC = Cation Exchange Capacity DOS = Density of States EELS = Electron Energy Loss Spectroscopy HOMO = Highest Occupied Molecular Orbital LUMO = Lowest Unoccupied Molecular Orbital NDA* = No data available TEM = Transmission Electron Microscopy VU-Vis Ab. Spec. = Ultraviolet-Visible Absorbance Spectroscopy Energy gap ~4eV Observed order of selectivity for Hectorite and Montmorillonite were temperature dependent (table below) VU-Vis Ab. Spec. Observed selectivity sequences for Hectorite and Montmorillonite were not successfully predicted using any of the relevant properties shown above. Exception being the ionization potential predicted the observed sequence at 45 ˚C for Montmorillonite. Thermodynamic Studies and van’t Hoff Analysis Montmorillonite CASTEP DOS model • Neither enthalpy or entropy alone explained observed adsorption sequence • Metals can be entropically or enthalpically driven, or both • Adsorption of Pb and Cu on Hectorite were abnormally large in comparison to Montmorillonite, possibly due to CaCO3 contributing to adsorption of acidic metals • No pattern of soft or hard metal ions being preferentially adsorbed by Hectorite or Montmorillonite • Temperature dependence of selectivity sequence : • Results from crossing of van’t Hoff curves • Not often reported in literature for mineral adsorption studies • Often observed in other chemical systems No Energy gap Clay Minerals of Interest UV-Vis. Spectroscopy indicate a possible energy gap of approximately 4-5 eV for both Hectorite and Montmorillonite. CASTEP modeling of the DOS for Hectorite also indicate a energy gap of approximately 4 eV. CASTEP modeling of the DOS for Montmorillonite indicate no band gap. This discrepancy between experimental and model can be resolved by TEM-EELS analysis. • Hectorite Mineral • Na0.4Mg2.7Li0.3Si4O10(OH)2 • Montmorillonite Mineral • (Na,Ca)0.33(Al,Mg)2(Si4O10)(OH)2·nH2O • 2:1 clays, phyllosilicates in Smectite family • Different CECs Acid-BaseTitrations High sorption of Pb and Cu onto Hectorite results partially from the basic component of Hectorite (CaCO3) and acidic nature of these metal cations. This is in accordance with the acid/base term of Pearson’s equation: △H = -(SASB + σA σB)