Mineral weathering begins with mineral dissolution, typically as induced by protons or by ligands that form strong complexes with metals (Section 1.4). Proton-induced dissolution begins with H+ adsorption, exemplified in Eq. 3.2 for a metal oxyhydroxide mineral. In the absence of ligands that could replace the positively charged water molecule resulting from this rapid reaction, proton adsorption is followed by slow detachment of the metal, which then equilibrates as a soluble species in the soil solution, as illustrated in Fig. 5.1 (pathway 1) for gibbsite [Al(OH)3; see Fig. 2.7] at a pH value low enough that the detached Al3+ does not hydrolyze. Ligand-induced dissolution is also illustrated in Fig. 5.1 (pathway 2). The ligand is a fluoride anion, which forms a strong complex with Al3+ (see problem 3 in Chapter 4). Adsorption in this case occurs by ligand exchange, which is illustrated for carboxylate in Eq. 3.3. A similar reaction occurs for F-:...Slow detachment of the AlF2+ complex then follows. Whenever a mineral dissolution reaction induced by either of these two-step mechanisms is far from equilibrium, it is not influenced by the very low concentration of the constituent released from the dissolving mineral and its rate can be described by zero-order kinetics (Table 4.2). Accordingly, if [A] is the concentration of a constituent released, then the rate law can be expressed as...where kd is a rate coefficient independent of [A] , but a function of temperature, pressure, pH, the chemical properties of the mineral, and, if appropriate, the concentration of the ligand inducing dissolution via the second mechanism in Fig. 5.1. The mineral dissolution rate on the left side of Eq. 5.2 can be mass-normalized to express it in moles per mole of mineral per second by dividing the molar concentration [A] with the solids concentration of the mineral expressed in units of moles per liter. This mass-normalized rate does not depend on the amount of mineral dissolving. For proton-induced dissolution, the rate is then a function of temperature, pressure, pH, and the chemical nature of the mineral.
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