Mineral surface charged

Once the molecules have been captured inside the protocell, the concept of a concentration gradient and semipermeable membrane becomes important. Near a negatively charged mineral surface, such as silica, the surface concentration... 

Barrow, N.J. Bowden, J.W. Posner, A.M. Quirk, J.P. (1981) Describing the adsorption of copper, zinc and lead on a variable charge mineral surface. Aust. J. Soil Res. 19 309-321... 

The theory is based on the autotrophic metabolism of low-molecular-weight constituents in an environment of iron sulfide and hot vents. Figure 2.4 gives an illustration of one reaction pathway. It is worthwhile to consider that the metabolism is a surface metabolism, namely with a two-dimensional order, based on negatively charged constituents on a positively charged mineral surface. Actually Wachtershauser sees this as an interesting part of a broader philosophical view (Huber and Wachtershauser, 1997). 

Bowden, J. W., Nagarajah, S., Barrow, N. J., Posner, A. M. Quirk, J. P. 1980. Describing the adsorption of phosphate, citrate and selenite on a variable charge mineral surface. Australian Journal of Soil Research, 18, 49-60. 

Adsorption of Ionized Organic Compounds from Aqueous Solutions to Charged Mineral Surfaces... 

Bowden, J. W., A. M. Posner, and J. P. Quirk. 1977. Ionic adsorption on variable charge mineral surfaces Theoretical charge development and titration curves. Aust. J. Soil. Res. 15 121-136. 

Figure 3. Schematic representation of mechanisms responsible for ion sorption on charged mineral surfaces. Key O.S., outer sphere I.S., inner sphere.

A schematic representation of the locations of these species relative to a charged mineral surface is given in Figure 6. 

Barrow, N.J. and Bowden, J.W., A comparison of models for describing the adsorption of anions on a variable charge mineral surface, J. Colloid Interf. Sci., 119, 236, 1987. 

There are good reasons to believe that the application of the Nemst equation and the DDL model to oxides and other variable-charge mineral surfaces is inappropriate. To begin with, cations and anions may adsorb on oxides by direct coordination to the charged surface group. Even for monovalent cations and anions, a high percentage of the bonds with oxide surfaces are believed to be of the inner-sphere type. 

Buffering by aluminosilicate mineral decomposition. In moderately to strongly acid soils (pH < 5.5), variable-charge mineral surfaces as well as layer silicate edges accept protons to generate anion exchange sites, for example at A1 sites ... 

A more detailed and quantitative analysis of organic acid bonding on variable-charge mineral surfaces is possible. Representing all organic acids by the chemical symbol RH,... 

Figure 10.12. Reaction scheme for the ligand exchange reaction of organic acid, RH, at protonated sites of a variable-charge mineral surface.

I. Indifferent salts do not strongly compete with the organic anion for adsorption sites on variable-charge mineral surfaces. Competing anions, such as sulfate and phosphate, are not considered indifferent and would have to be explicitly accounted for in the model, since their presence in solution would suppress the adsorption of the organic acid. 

Like calcium, strontium has moderate mobility in soils and sediments, and sorbs moderately to metal oxides and clays (Hayes and Traina 1998). The Sr2- ion is strongly hydrated and is firmly coordinated with six or more water molecules in aqueous solution. When Sr2- ions sorb on negatively charged mineral surface sites, the hydration sphere is retained (O Day et al. 2000). Strontium sorbs as hydrated ions on the surface of clay minerals (kaolinite), weathered minerals (amorphous silica), and iron oxides (Sahai et al. 2000). Sorbed carbonate on iron oxides enhances the sorption of Sr2- and permits the nucleation of Sr2- as strontium carbonate (Sahai et al. 2000). On calcite (calcium carbonate), Sr2 sorption occurs by electrostatic attraction as hydrated ions. However, at higher concentrations, precipitation of strontianite (strontium carbonate) occurs and strontium is likely to be less mobile (Parkman et al. 1998). 

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