Chemical Reactions in Soil

Soils are multicomponent, multiphase, open systems that sustain a myriad of interconnected chemical reactions, including those involving the soil biota. The multiphase nature of soil derives from its being a porous material whose void spaces contain air and aqueous solution. The solid matrix (which itself is multiphase), soil air, and soil solution—each is a mixture of reactive chemical compounds—hence the multicomponent nature of soil. Transformations among these compounds can be driven by flows of matter and energy to and from the vicinal atmosphere, biosphere, and hydrosphere. These external flows, as well as the chemical composition of soil, vary in both space and time over a broad range of scales. 

The complexity of soil notwithstanding, the principal features of its chemical behavior can be understood on the basis of well-established principles and methods for the description of reactions in aqueous systems. Reactions that occur exclusively in the gaseous phase or the solid matrix of soil less often control its chemical behavior than reactions involving the aqueous phase. The basic terminology associated with the latter chemical reactions will be reviewed in the present chapter to provide an initial context for the discussion of equilibria and kinetics to follow. 

A chemical reaction is termed elementary if it occurs in a single step, with no intermediate species appearing before the products of the reaction have formed. An elementary reaction takes place on the molecular level exactly as written in terms of reactants and products. A reaction that is not elementary is composite or overall.1 An example of an elementary reaction is the hydration of dissolved carbon dioxide in a soil solution to form the neutral species HjCO ( true carbonic acid )  

Another elementary reaction of molecularity 2 is the combination of true carbonic acid with hydroxide ion to form bicarbonate ion and water  

The concept of molecularity thus is not applied to the reaction in Eq. 1.3, since it does not display the actual molecular mechanism involving the intermediate species, H2C03, OH, and H20. Therefore, it would be incorrect to interpret the reaction in Eq. 1.3 as the combination of one C02 molecule with a water molecule to form H+ and HC03 ions. The error in this line of reasoning is brought into sharper focus after noting that the elementary bimolecular reaction  

In both cases, the crucial chemistry governing the process occurs at the interface between soil solution and the solid phase. Various mechanisms control the equilibrium between chemical species in the two phases, with movement between them being controlled by changes in solution concentration. 

---

Secondary minerals. As weathering of primary minerals proceeds, ions are released into solution, and new minerals are formed. These new minerals, called secondary minerals, include layer silicate clay minerals, carbonates, phosphates, sulfates and sulfides, different hydroxides and oxyhydroxides of Al, Fe, Mn, Ti, and Si, and non-crystalline minerals such as allophane and imogolite. Secondary minerals, such as the clay minerals, may have a specific surface area in the range of 20-800 m /g and up to 1000 m /g in the case of imogolite (Wada, 1985). Surface area is very important because most chemical reactions in soil are surface reactions occurring at the interface of solids and the soil solution. Layer-silicate clays, oxides, and carbonates are the most widespread secondary minerals. 

The fact that diffusion models describe a number of chemical processes in solid particles is not surprising since in most cases, mass transfer and chemical kinetics phenomena occur simultaneously and it is difficult to separate them [133-135]. Therefore, the overall kinetics of many chemical reactions in soils may often be better described by mass transfer and diffusion-based models than with simple models such as first-order kinetics. This is particularly true for slower chemical reactions in soils where a fast reaction is followed by a much slower reaction (biphasic kinetics), and is often observed in soils for many reactions involving organic and inorganic compounds. 

Virtually all chemical reactions in soils are studied as isothermal, isobaric processes. It is for this reason that the measurement of the chemical potentials of soil components involves the prior designation of a set of Standard States that are characterized by selected values of T and P and specific conditions on the phases of matter. Unlike the situation for T and P, however, there is no strictly Ihermodynamic method for determining absolute values of the chemical potential of a substance. The reason for this is that p represents an intrinsic chemical property that, by its very conception, cannot be identified with a universal scale, such as the Kelvin scale for T, which exists regardless of the chemical nature of a substance having the property. Moreover, p cannot usefully be accorded a reference value of zero in the complete absence of a substance, as is the applied pressure, because there is no thermodynamic method for measuring p by virtue of the creation of matter. 

Chemical reactions in soils are generally heterogeneous solid-liquid reactions involving a solid component of the soil and the soil solution. The reaction includes chemical processes involving formation or rupture of chemical bonds,... 

The importance of pH as a master variable controlling chemical reactions in soils has been stressed in previous chapters. However, soils subjected to fluctuations in water content come under the influence of another master variable the reduction-oxidation (or redox) potential Under conditions of water saturation, the lack of molecular oxygen can result in a sequence of redox reactions that changes the soil pH. In this sense the redox state of the soil exerts control over the pH. The nature of redox reactions will be discussed in this chapter, as these reactions profoundly influence metal ion solubility and the chemical form of ions and molecules dissolved in soil solution. The reader is referred to section 1.2f in Chapter 1 for a review of the basic chemical principles necessary for the understanding of redox reactions. 

Cover figure Conceptual diagram of chemical reactions in soils. Provided by H. Magdi Selim. 

Organic materials undergo microbial enzymatic and chemical reactions in soils to form colloidal polymers called humus (Fig. 6.1). Humus is a complex and rather microbially resistant mixture of brown to almost black, amorphous and colloidal substances modified from the original plant tissues or resynthesized by soil organisms. Humus contains approximately 10% carbohydrates, 10% nitrogen components (proteins, amino acids, and cyclical N compounds), 10% lipids (including alkanes, alkenes, fatty acids, and esters), and 70% humic substances. 

The kinetics and mechanisms of chemical reactions in soils have been broadly studied, and comprehensive mathematical models for the particular soil conditions have been presented (Bolt 1979, Huang 2000, Sauve 2001, Schmitt and Sticher 1991, Sparks 1999, Tan 1998). The diversity of ionic species of trace elements and their various affinities to complex inorganic and organic ligands make possible the dissolution of each element over a relatively wide range of pH and Eh. In most soil conditions the effect of pH on the solubility of trace cations is more significant than that of redox potential (Chuang et al. 1996). However, redox potentials of soils also have a crucial impact on the behavior of trace elements (Bartlett 1999). 

Short-range-ordered or noncrystalline Al precipitation products are ubiquitous in soil environments (Parfitt, 1980 Kampf et al., 2000). They dominate the chemical reactions in soils because of their extremely small particle sizes and highly reactive surfaces. In pure form they are not stable, but in the presence of chelating anions they may remain unchanged indefinitely. Short-range-ordered Al precipitates and soluble OH-Al species often coat crystalline minerals in soils or may be interlayered into the interlamellar spaces of vermiculites or smectites, altering the surface properties of these phyllosificates. OH-Al interlayered vermiculites and smectites are particularly abundant in Ultisols and Alfisols (Bamhisel and Bertsch, 1989). 

Flutson, J.L. and Wagner, R.J. (1992) Leaching Estimation and Chemistry Model. A Process Based Model of Water and Solute Movement, transformation, Plant Uptake and Chemical Reactions in the Unsaturated Zone. Version 3. Dept, of Soil, Crop and Atmospheric Sciences, Series No. 92-3, Cornell University, Ithica, New York. 

System failure for in situ bioremediation efforts is often the result of ineffective transport of nutrients and electron acceptors due to channeling into preferential flow paths, heterogeneities, adsorption, biological utilization, and/or chemical reactions in the soil. Many of these problems can be overcome using electric fields for transport and injection instead of conventional groundwater injection by hydraulic techniques. 

In short, much future research on kinetics of soil chemical processes is needed. Areas worthy of investigation include improved methodologies, increased use of spectroscopic and rapid kinetic techniques to determine mechanisms of reactions on soils and soil constituents, kinetic modeling, kinetics of anion reactions, redox and weathering dynamics, kinetics of ternary exchange phenomena, and rates of organic pollutant reactions in soils and sediments. 

Wu and Gschwend (1986) reviewed and evaluated several kinetic models to investigate sorption kinetics of hydrophobic organic substances on sediments and soils. They evaluated a first-order model (one-box) where the reaction is evaluated with one rate coefficient (k) as well as a two-site model (two-box) whereby there are two classes of sorbing sites, two chemical reactions in series, or a sorbent with easily accessible sites and difficultly accessible sites. Unfortunately, the latter model has three independent fitting parameters kx, the exchange rate from the solution to the first (accessible sites) box k2, the exchange rate from the first box to the... 

Kinetics of reactions in soil and aquatic environments is a topic that is of extreme importance and interest. Most of the chemical processes that occur in these systems are dynamic, and a knowledge of the mechanisms and kinetics of these reactions is fundamental. Moreover, to properly understand the fate of applied fertilizers, pesticides, and organic pollutants in soils with time, and to thus improve nutrient availability and the quality of our groundwater, one must study kinetics. 

There are many reactions in soil-water systems pertaining to nutrient availability, contaminant release, and nutrient or contaminant transformations. Two 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. 

The rate at which a particular reaction occurs is important because it could provide real-time prediction capabilities. In addition, it could identify a particular reaction in a given process as the rate-controlling reaction of the process. For example, chemical mobility in soils, during rain events, is controlled by the rate at which a particular species desorbs or solubilizes, Similarly, the rate at which a particular soil chemical biodegrades is controlled by the rate at which the soil chemical becomes available substrate. 

The movement of chemicals undergoing any number of reactions with the soil and/or in the soil system (e.g., precipitation-dissolution or adsorption-desorption) can be described by considering that the system is in either the equilibrium or nonequilibrium state. Most often, however, nonequilibrium is assumed to control transport behavior of chemical species in soil. This nonequilibrium state is thought to be represented by two different adsorption or sorption sites. The first site probably reacts instantaneously, whereas the second may be time dependent. A possible explanation for these time-dependent reactions is high activation energy or, more likely, diffusion-controlled reaction. In essence, it is assumed that the pore-water velocity distribution is bimodal,... 

It should be noted that lime reacts with clay particles. This leads to strength increase by pozzolanic and carbonation cementation processes. Cation exchange and pozzolanic reactions result in strength increase. The level of reactivity and hence strength gained in soil-lime mixtures depends on the level of pozzolanic product created. The chemical reaction between soil and lime can be presented as below ... 

Subsurface solute transport is affected by hydrodynamic dispersion and by chemical reactions with soil and rocks. The effects of hydrodynamic dispersion have been extensively studied 2y 3, ). Chemical reactions involving the solid phase affect subsurface solute transport in a way that depends on the reaction rates relative to the water flux. If the reaction rate is fast and the flow rate slow, then the local equilibrium assumption may be applicable. If the reaction rate is slow and the flux relatively high, then reaction kinetics controls the chemistry and one cannot assume local equilibrium. Theoretical treatments for transport of many kinds of reactive solutes are available for both situations (5-10). 

Santore, R.C. and Driscoll, C.T. (1995). The CHESS Model for Calculating Chemical Equilibria in Soils and Solutions, Chemical Equilibrium and Reaction Models. The Soil Society of America, American Society of Agronomy... 

---