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theses

The sorption of divalent metals onto Al-oxide sorbents may lead to the formation of mixed metal-aluminum layered double hydroxide (Me(II)-Al(III)-LDH) phases. The formation of these secondary phases may represents a significant pathway of sequestration of various divalent metals, including Ni(II), Zn(II), Co(II), Fe(II) and Mn(II), in aqueous geochemical environments such as soils and sediments. Despite of their importance in metal sequestration there is not enough thermodynamic data available to model their behavior in geochemical environment. Thus this study was designed to estimate the solubility product values of Me(II)-Al(III)-LDH precipitates formed by Ni(II), Zn(II), Co(II), Fe(II) and Mn(II) during sorption onto Al-oxide at near neutral and alkaline pH value in NaCl electrolyte. Detailed kinetic studies were conducted to monitor the sorption of these divalent metals ¬¬onto γ-Al2O3. Equilibrium aqueous metal concentrations were used to calculate the solubility products (Ksp) of the Me(II)-Al(III)-LDH phases for the aqueous chemical equilibrium described by: Me(II)2/3Al(III)1/3(OH)2Cl1/3(s) ↔ 2/3 Me(II)(aq) + 1/3 Al(III) (aq) + 2OH- (aq) +1/3 Cl- (aq) The Ksp values of the five metals span three orders of magnitude, indicating substantial differences in the favorability of Me(II)-Al(III)-LDH precipitation, which increases in the order Mn(II)<Fe(II)<Co(II)<Zn(II)<Ni(II). A linear relation is observed between the Ksp values of the Me(II)-Al(III)-LDH and those of the corresponding β-Me(OH)2 phases,indicating that the substitution of Al(III) in the brucitic β-Me(OH)2 lattice influences the stability of the resulting Me(II)-Al(III)-LDH phase in a fashion that is independent of the size of Al(III) relative to Me(II). The thermodynamic data obtained from our study demonstrate that the chloride-interlayered Me(II)-Al(III)-LDH phases characterized here have a higher solubility than MeCO3 phases in a model groundwater system and in a calcitic system. Additional thermodynamic studies are needed to determine the impacts of environmentally relevant anions, particularly carbonate and silicate, on the formation and stability of Me(II)-Al(III)-LDH phases.

ETD_8260

M.S.

Includes bibliographical references

by Lasita Bhattacharya

doi:10.7282/T34X5BVW

ETD graduate

The author owns the copyright to this work.