Kinetics and mechanisms of the precipitation of layered Fe(II)-hydroxides during Fe(II) sorption onto common soil minerals

This dissertation studies the formation kinetics and thermodynamics of secondary Fe(II) precipitates during aqueous Fe(II) sorption onto Al and/or Si containing minerals under various lab-based anoxic model systems. In Chapter 2, sorption of Fe(II) in anoxic aqueous suspensions of γ-Al2O3, smectitic clay and amorphous silica was studied under various pH values. In Chapter 3 and 4, the impacts of As and organic compounds on Fe(II) sorption kinetics and mechanisms onto γ-Al2O3 and/or clay were investigated at pH 7.5. Uv-Vis spectroscopy and ICP-OES were employed to determine the aqueous concentrations of Fe, As and Si in the supernatants sampled during the macroscopic batch experiments, while the XAS was applied to characterize the solid-phase Fe and As sorption products. The Fe(II)-Al(III)-LDH formed at pH ≥ 7.0 during Fe(II) sorption onto γ-Al2O3, and at pH 7.0 and 7.5 during Fe(II) sorption onto clay. The poorly crystalline trioctahedral Fe(II)-phyllosilicates formed at pH ≥ 7.5 during Fe(II) sorption onto amorphous silica, and at pH 8.0 during Fe(II) sorption onto clay. Greater sorption rate and extent were observed with increasing pH. Significantly slower Fe(II) sorption kinetics in clay systems compared to γ-Al2O3 and SiO2 is due to the relatively low dissolution of substrate-derived Al and Si. While As(III) did not interfere with the Fe(II)-Al(III)-LDH formation and Fe(II) sorption kinetics onto γ-Al2O3 at pH 7.5, the presence of As(V) slowed down the Fe(II)-Al(III)-LDH formation at low As(V) concentrations, and fully shut it down at high As(V) concentrations, leading to the formation of Fe(II) surface complexes instead. The inhibitive effects of As(V) is attributed to the interference of adsorbed As(V) with the Al needed for Fe(II)-Al(III)-LDH formation. On the other hand, the presence of Fe(II) did not affect the sorption kinetics and mechanisms of As(III) onto γ-Al2O3, however enhanced As(V) sorption rate and extent and did not change the As(V) adsorption mode onto γ-Al2O3. The presence of humic substances (HS) generally hindered the formation of Fe(II)-Al(III)-LDH during Fe(II) sorption onto γ-Al2O3 and clay substrates at pH 7.5. In Fe(II) reacted γ-Al2O3 systems, HS slowed down the formation kinetics of Fe(II)-Al(III)-LDH precipitate. Larger inhibitive impacts of HS were observed when it cosorbed with Fe(II) onto γ-Al2O3 than when it pre-coated onto γ-Al2O3 before Fe(II) sorption. In Fe(II) reacted clay systems, humic acid (HA) coating on clay primarily altered the main Fe(II) sorption product from Fe(II)-Al(III)-LDH into poorly crystalline Fe(II)-phyllosilicate. The effects of HS on Fe(II) sorption onto mineral sorbents were associated with Al dissolution capability from mineral substrates, HS formation of organo-Al complexes and HS masking on mineral surfaces limiting the Al and/or Si needed to form secondary Fe(II) precipitates. The results from this work suggest substantial complexity in the composition and structure of Fe(II) sorption products, and the occurrence of which may represent significant pathways for Fe(II) sequestration under reducing geochemical environments. This work develops our understanding of the fate and transport of Fe(II) under anoxic and suboxic environments such as natural riparian soils and sediments.Ph.D.Includes bibliographical referencesby Ying Zh