Proton-promoted dissolution, minerals

A weakening of the critical metal-oxygen bonds occurs as a consequence of the protonation of the oxide ions neighboring a surface metal center and imparting charge to the surface of the mineral lattice. The concentration (activity) of D should reflect that three of such oxide or hydroxide ions have to be protonated. If there is a certain numer of surface-adsorbed (bound) protons whose concentration (mol nr2) is much lower than the density of surface sites, S (mol 2), the probability of finding a metal center surrounded with three protonated oxide or hydroxide ions is proportional to (CJ/S)3. Thus, as has been derived from lattice statistics by Wieland et al. (1988), the activity of D is related to (C )3, and the rate of proton-promoted dissolution, Rh (mol nrr2 lr1), is proportional to the third power of the surface protonation ... 

These three concentration-time equations describe the proton-promoted dissolution reaction in Eq. 4.51 under the assumption that the selenate detachment reaction is already at equilibrium. The case K < < K corresponds to the approximate equality in Eq. 4.52f. Note that the rate of mineral dissolution, d[Mn2+]/dt, will be constant for observation times much smaller than 1/k, . 

To exemplify inhibition effects, we choose a few case studies with Fe(III)(hydr)oxides because these oxides are readily dissolved with protons, ligands, and reductants and are of great importance in the iron cycles in natural waters. The reductive dissolution of Fe(III) minerals by a reductant such as H2S is much faster than ligand- or proton-promoted dissolution. The dissolution reaction, as shown by Dos Santos-Afonso and Stumm (1992), is initiated by the formation of =FeS and =FeSH surface complexes the subsequent electron transfer within the complex leads to the formation of Fe(II) centers in the... 

In the absence of organic ligands, the rate of dissolution of most silicate minerals depends on pH as shown in Fig. 5. In the acid region, we have proton-promoted dissolution ... 

Fig. 10.8. Simple biogeochemical model for metal mineral transformations in the mycorhizosphere (the roles of the plant and other microorganisms contributing to the overall process are not shown). (1) Proton-promoted (proton pump, cation-anion antiport, organic anion efflux, dissociation of organic acids) and ligand-promoted (e.g. organic adds) dissolution of metal minerals. (2) Release of anionic (e.g. phosphate) nutrients and metal cations. (3) Nutrient uptake. (4) Intra- and extracellular sequestration of toxic metals biosorption, transport, compartmentation, predpitation etc. (5) Immobilization of metals as oxalates. (6) Binding of soluble metal species to soil constituents, e.g. clay minerals, metal oxides, humic substances.

Figure 13.10. Schematic representation of the oxide dissolution processes [exemplified for Fe(III) (hydr)oxides] by acids (H ions), ligands (example oxalate), and reductants (example ascorbate). In each case a surface complex (proton complex, oxalato and ascorbato surface complex) is formed, which influences the bonds of the central Fe ions to O and OH on the surface of the crystalline lattice, in such a way that a slow detachment of a Fe(III) aquo or a ligand complex [in case of reduction an Fe(ll) complex] becomes possible. In each case the original surface structure is reconstituted, so that the dissolution continues (steady-state condition). In the redox reaction with Fe(III), the ascorbate is oxidized to the ascorbate radical A . The principle of proton-promoted and ligand-promoted dissolution is also valid for the dissolution (weathering) of Al-silicate minerals. The structural formulas given are schematic and simplified they should indicate that Fe(III) in the solid phase can be bridged by O and OH.

However, the major recent advance in the modeling of mineral dissolution has been the presentation by Stumm and coworkers (Furrer and Stumm, 1986 Zinder et al., 1986 Stumm arid Furrer, 1987) of a surface coordination approach to explain the proton- and ligand-promoted dissolution of simple oxides (A1203, BeO, a-FeOOH). This approach, which relies on surface titrations and double layer concepts for characterizing the chemical speciation on mineral surfaces, offers several advantages ... 

There is clear evidence that the dissolution of oxide minerals is promoted by the specific sorption of solutes at the mineral-solution interface. Moreover, it has been found that comparatively simple rate laws are obtained if the observed rates are plotted against the concentrations of adsorbed species and surface complexes (Pulfer et al., 1984 Furrer and Stumm, 1986). For example, in the presence of ligands (anions and weak acids) surface chelates are formed that are strong enough to weaken metal-oxygen bonds and thus to promote rates of dissolution proportional to their surface concentrations. Simple rate laws have been also observed with H+—or OH —promoted dissolution of oxides in a manner that can be predicted from knowledge of the oxide composition and the surface concentrations of protons and hydroxyl radicals. 

Weathering of minerals Proton- and ligand-promoted dissolution Reductive dissolution of Fe(III) and Mn(III,IV) oxides... 

Metals and metalloids on the surface of silicate minerals are also connected by oxide or hydroxide ion bridges, which undergo acid-base reactions with the adjacent aqueous solution. The results of these acid-base reactions are measurable as surface charge, which varies in concentration with solution pH. (The situation is a little more complicated than this, especially for minerals that have a structural charge due to uncompensated cation substitutions. The reader is directed to, for example, Schindler and Stumm 1988.) The enhancement of dissolution rates via adsorption of hydrogen ions is referred to as the proton-promoted pathway for dissolution (Furrer and Stumm 1986) and is analogous to the proton-promoted pathway for the dimer dissociation discussed above. 

For minerals crystaUizing from low-temperature aqueous solutions, the primary controls on their stability should be (a) the activity of the species in solution and G ) protonation reactions between solid and solution at the edges of polyhedron chains deprotonation of edge anions promotes attachment of aqueous cation species (i.e., crystallization), whereas protonation of edge anions weakens their bonds to the bulk structure and promotes dissolution. With regard to crystallization, the character and activity of the aqueous species is of interest as these provide groups of atoms that may attach to the solid during crystallization. 

Whatever the cause of the changes in rhizosphere pH, the corresponding increase or decrease of proton concentration will promote the dissolution or precipitation of a range of soil minerals the direct implication of root-induced release of protons in the dissolution of phosphates, silicates, or oxides has been reported (Hinsinger et al., 1993 Hinsinger and Gilkes, 1996 Bertrand and Hinsinger, 2000). 

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