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Ebook Environmental soil and water chemistry - Principles and applications: Part 2

(BQ) Part 2 book "Environmental soil and water chemistry - Principles and applications" has contents: Reaction kinetics in soil water systems; organic matter, nitrogen, phosphorus and synthetic organics; soil colloids and water suspended solids; water quality; soil and water decontamination technologies,.and other contents. | 7 Reaction Kinetics in Soil-Water Systems 7.1 INTRODUCTION There are many reactions in soil-water systems pertaining to nutrient availability. contaminant release, and nutrient or contaminant transformations. 1\vo processes regulating these reactions are chemical equilibria (Chapter 2) and kinetics. The specific kinetic processes that environmental scientists are concerned with include mineral dissolution, exchange reactions, reductive or oxidative dissolution, reductive or oxidative precipitation, and enzymatic transformation. This chapter provides a quantitative description of reaction kinetics and outlines their importance in soil-water systems. To understand reaction kinetics one needs to understand the difference between kinetics and equilibria. Generally, equilibria involves forward and reverse reactions and it is defined as the point at which the rate of the forward reaction equals the rate of the reverse reaction. Consider the mineral AB (Reaction 7.1), where A denotes any cation (A+) and B denotes any anion (B-). Upon introducing H 20, the mineral undergoes solubilization (forward reaction) until precipitation (reverse reaction) becomes significant enough so that the two rates (forward and reverse) are equal: (7.1) The parameters kf and kb denote rate constants for the forward and reverse reactions, respectively. Reaction 7.1 demonstrates mineral equilibrium through two elementary reactions-one describes the forward reaction, while a second describes the reverse reaction. When the reverse reaction is inhibited, the forward reaction is termed dissolution (e.g., acid mineral dissolution). Reaction 7.1 at the equilibrium point is described by where dA+/dt denotes the rate of the overall reaction, kIAB) describes the rate of the forward reaction, and kb(A +)(B-) describes the rate of the reverse reaction. At equilibrIum, 272 7.1 273 INTRODUCTION (7.3) and (7.4) where Keq denotes the equilibrium product constant (note, in the example above Keq = K .