Deprotonated mineral surface

Two types of mineral surface constants are considered by the CCM. One is a protonation-deprotonation constant and the other is an adsorbate complexation constant. The model is based on the consideration that the formation of an inner-sphere... 

Here, [=SOH2] or [=SO ] represents the concentration of protonated or deprotonated surface sites, respectively, on the mineral surface, and the exponents are constants for each mineral. According to this model, the rate of dissolution of most oxides is slowest in solutions where pH = pHppzc, the pH of the pristine point of zero charge where the surface charge of the mineral of interest equals zero (Figures 3 and 4). Some authors include a separate rate term describing dissolution at near-neutral pH (= h2o[=SOH]). Above and below the pHppzc, oxides are predicted to show enhanced dissolution due to protonated and deprotonated surface sites, respectively. 

Metal cations in the solid sorbent surface are symbolized by S in MINTEQA2. At mineral surfaces exhibiting amphoteric behavior, that behavior is often attributed to the successive dominance of surface SOH, SOH, and SO species with increasing pH (Fig. 10.4). In MINTEQA2, surface protonation and deprotonation reactions that are basic to the three SC models are written in the form... 

As mentioned earlier, complex formation reactions at hydrous metal oxide surfaces can be treated as an extension of classic coordination chemistry metal centers on mineral surfaces participate in inner-sphere and outer-sphere coordination reactions with molecules adsorbed from overlying solution, including H2O, OH , O, and solute molecules (Schindler, 1981 Schindler and Stumm, 1987). A variety of protonation/deprotonation and complex-formation reactions determine the speciation of surface sites. A few... 

Here, and are protonation and deprotonation constants at the interface, and is electric potential of the formed charge. Equilibrium constants in these reactions characterize acid-alkali properties of the mineral surface as amphoteric substance and are called surface acidity constants. 

In the adsorption of a humic molecule to a mineral surface, several reactions may occur simultaneously formation of inner-sphere complexes between carboxylate groups and the mineral through ligand-exchange reactions, formation of outer-sphere complexes with carboxylate (RCOO ) and hydroxyl (RCOH) groups, and protonation-deprotonation reactions. The overall adsorption reaction for an HS—for example a fulvic acid molecule—is written as (Filius et al. 2001, 2003)... 

All of the important classes of ligand-directed labilization are represented at mineral surfaces, including the deprotonation of a terminal ti -OH2 site. One therefore expects a similar increase in dissolution of oxide minerals as the surface sites become deprotonated. For a mineral siuface, these deprotonations form negative surface charge [cf Eqs (5) and (10)] and the rates correspondingly increase with pH or charge. The actual location of the deprotonations is unknown, but a likely scenario is that ri -OH2 sites on monomolecular steps deprotonate with increases in pH. 

Some naturally occurring mineral oxides exhibit a high degree of isomorphous substitution of metal ions. These complex oxides make the interpretation of the adsorption study very difficult, and a detailed pretreatment of the mineral is often required before any adsorption study. With inert nonaqueous solvents, the need for a detailed pretreatment of the mineral is substantially reduced. This can be explained from the fact that the protonation and deprotonation of the mineral surface in aqueous solutions must be controlled by adjusting the pH, while this adjustment is not necessary in most organic solutions due to the lack of smface protonation. On the other hand, the molecular behavior of many amphiphilic solute molecules in apolar... 

The weathering of silicates has been investigated extensively in recent decades. It is more difficult to characterize the surface chemistry of crystalline mixed oxides. Furthermore, in many instances the dissolution of a silicate mineral is incipiently incongruent. This initial incongruent dissolution step is often followed by a congruent dissolution controlled surface reaction. The rate dependence of albite and olivine illustrates the typical enhancement of the dissolution rate by surface protonation and surface deprotonation. A zero order dependence on [H+] has often been reported near the pHpzc this is generally interpreted in terms of a hydration reaction of the surface (last term in Eq. 5.16). 

As mentioned in Sections 1.3.2.1 and 1.3.2.2, the CEC and specific surface area (both internal and external) are higher than those of clay minerals. The functional groups of soil organic matter (Table 1.5) can be deprotonated or protonated, depending on pH. It means that they have pH-dependent charges. The major functional groups can be deprotonated at pH values characteristic of soils (pH = 6-8) so that they can sorb cations. It has been estimated that the CEC of soils comes from the soil organic matter in 20%-70% (Stevenson 1982). 

It is well-known that the amphoteric properties of hydroxyl groups that exist on the surface of oxide particles play an important role in adsorption phenomena. These groups are characterized by two acidity constants, one for the protonation reaction and the other for the deprotonation reaction, which are functions of the surface potential created by adsorbed ions (Atkinson et al., 1967 Davis et al., 1978). These types of surface reactions on soil minerals have been studied using transient relaxation methods. 

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