Silicate surface chemistry and dissolution kinetics in dilute aqueous systems

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Date

2004

Authors

Choi, Wan-joo

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Abstract

Organic acids from the biosphere are important reactants in a number of weathering environments. Organic acids accelerate silicate dissolution, increase silicate solubility, mobilize aluminum and silica, and alter the equilibrium between the solution and precipitated secondary phases. The chemical dynamics of the weathering environment was examined by investigating the interaction between mineral surfaces and organic/inorganic electrolyte solutions. Organic acids, analogs of microbially generated siderophores, were examined for their effects on aluminosilicate dissolution kinetics at multiple temperatures in various electrolyte solutions. Mineral surface titrations were performed for six mineral samples: quartz, gibbsite, feldspars, microcline, andalusite, and kyanite. Mineral powder/distilled water mixture samples were titrated by 0.1 N HCl in the basic pH region, and by 0.1 N NaOH in the acidic pH region. UV-difference spectroscopic analysis was performed on dissolved silica-organic acid mixtures to characterize solution complexes. Mineral dissolution experiments were performed using temperature controlled, continuous-flow mixed reactors. For inorganic dissolution experiment, the solution ionic strength was controlled using LiCl while solution pH was adjusted using either dilute HCl or LiOH solutions. Reagent grade citric acid, tropolone, and 3,4-dihydroxybenzoic acid were used for organic ligand dissolution experiments. A constant flow rate was maintained by using a peristaltic proportioning pump. The mineral surface titration results revealed important surface properties that can be critical to interpreting dissolution kinetics in natural environments. One of the most important results would be that the amount of active surface sites can vary in different solution pH conditions which have been normally assumed to be fixed numbers based on surface area measurements. The UV-difference spectroscopy result shows that some siderophores form stable solution complexes with silica as well as Al and Fe. The results imply that dissolution of aluminosilicate minerals can be significantly enhanced in natural environments by bacterial siderophores, as suggested by previous researchers. Dissolution results in inorganic electrolyte solutions showed that the net effect of solution ionic strength on the aluminosilicate dissolution reactions is a decrease in the overall dissolution rates, opposite to the effect of ionic strength on quartz dissolution. When the solution ionic species interact with feldspar surfaces, the mechanism of lowering the dissolution rates may be by inhibiting the ion exchange reaction. However, when the solution ionic species do not interact with the silicate surfaces or no ion-exchangeable species are available on the mineral surface, the mechanism of lowering the dissolution rate may be attributed to the effect of activity changes in the neutral species in the solution. Microcline dissolution increased in organic ligand solutions relative to inorganic electrolyte solutions while andalusite and kyanite dissolution rates decreased in organic ligand solutions. The increased dissolution rate of microcline suggests that feldspar dissolution may be a SN2 mechanism. The decreased or unaffected dissolution rates from kyanite and andalusite suggest that the mechanism for these minerals may be a SN1 mechanism. The effect of organic ligands on dissolution rate was greater in pH 5 solutions than in pH 3. This result suggests that the dominant reaction mechanism in the pH 3 region is proton-promoted, while it is ligand-promoted in the pH 5 region. Lower activation energies in organic ligand solutions suggest that: (1) the metal-organic complex is more stable at lower temperatures; or (2) the dominant reaction mechanism at a high temperature region may be proton-promoted and is ligand-promoted at a lower temperature region.

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