Proton donors soils

In chemical weathering, proton donors or acids of organic and inorganic origin are involved, where the proton acceptors are the minerals in the soil (Stumm et al., 1985). In the last 100 years, anthropogenic pollution has contributed to the natural source of protons and this in turn has affected weathering rates and types. 

As briefly mentioned in Section 7.1.1, the nature, abundance, and strength of proton donors are responsible for the soil processes and the formation of the different horizons. Figure 7-4 further illustrates this principle. The major proton sources and weathering reactions are shown in Table 7-7. 

Table 7-7 Major proton donors and weathering reactions occurring in different soil compartments"... 

This table is part of Fig. 7-4. It shows the major proton donors and the major weathering reactions occurring in the different compartments (horizons) of a soil profile. It is assumed that the parent material consists of unconsolidated granite, materials are freely drained, and pedogenic impeding layers are absent. Also, no perched or fluctuating water is present. 

The retention or removal of ions, molecules, and compounds in the weathering and soil environment depends on the nature, strength, and abundance of the proton donors. The biota directly or indirectly affects the production of the proton donors. 

Ugolini, F. C. (1987). The proton donor theory Part I and II. Soil Sci. Amer. Ann. Meet., Agronomy, November 28-December4, abstract, p. 233, Atlanta, Georgia,... 

The principle of hard and soft Lewis acids and bases, proposed by Pearson (1963), is useful to describe these reactions. A Lewis acid is any chemical species that employs an empty electronic orbital available for reaction, while a Lewis base is any chemical species that employs a doubly occupied electronic orbital in a reaction. Lewis acids and bases can be neutral molecules, simple or complex ions, or neutral or charged macromolecules. The proton and all metal cations of interest in subsurface aqueous solutions are Lewis acids. Lewis bases include H, O, oxyanions, and organic N, S, and P electron donors. A list of selected hard and soft Lewis acids and bases found in soil solutions is presented in Table 6.1. 

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