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Most soil reactions of interest are heterogeneous solid-liquid reactions and take place by a multistep mechanism that comprises transport processes as well as chemical reactions. When the chemical reactions at the solid phase are rapid and are not associated with solid-phase transport processes, the liquid-phase transport processes determine the rate of the overall reaction, e.g., transport in the bulk of the liquid phase, diffusion across the liquid film surrounding the solid particles, diffusion in liquid-filled macropores. In these cases, the kinetics are accounted for by applying the Fick equation or the Nernst-Planck equation with suitable boundary conditions. When the processes taking place at the solid phase are rate determining, it is often observed that a plot of the reciprocal of the rate against the time is S-shaped, and the kinetics are approximated by a sequence of simple equations, each one valid during a limited range of time: the fractional power equation at the beginning of the experiment, the Elovich equation at an intermediate range of time, and the pseudo-first-order equation when saturation is approached. These kinetic phenomena indicate that the reactions at the solid phase are associated with activated diffusional processes, such as surface diffusion or bulk penetration, in which chemical bonds are formed and ruptured along the diffusion path. From the experimentally determined plots of the amount absorbed against time it is possible to deduce whether the rate-determining process is diffusion in a homogeneous medium or diffusion in a heterogeneous medium, and to estimate the parameters of the diffusion process.
Methods of obtaining and analyzing kinetic data for soil systems are examined in this review chapter. Relaxation methods are used to obtain kinetic data for very fast ion association reactions. Batch methods are used to obtain data for intermediate rate and slow sorption/desorption and precipitation/dissolution reactions. Reactor design, mixing, and separation of solid and liquid phases are all important considerations. Type and rate of mixing are critical in minimizing mass transfer processes (diffusion). Separation of phases is done by centrifugation and filtration, although some types of batch reactors allow in situ measurements. Flow methods and hybrid stirred-flow methods are also used to obtain data for intermediate and slow reactions. Flow rates and reactor design are important considerations. Thin disk methods do not minimize diffusion processes so that reaction kinetics alone cannot be studied by this method. Fluidized bed and stirred-flow reactors overcome many of the limitations and keep many of the desired features of batch and flow methods. The effects of reactant concentrations (solid phase and solute), temperature, pH, ionic strength, and other solution composition variables on rate functions provide valuable clues for deducing reaction mechanisms. Methods of analyzing data include initial rate, isolation, graphical, rate coefficient constancy, fractional lives, and parameter optimization techniques. Inferring reaction mechanisms from kinetic data is subject to many pitfalls, not the least of which is that many models produce results that can adequately describe the data, but are statistically indistinguishable and therefore the correct explanation is unknown.
A number of soil chemical phenomena are characterized by rapid reaction rates that occur on millisecond and microsecond time scales. Batch and flow techniques cannot be used to measure such reaction rates. Moreover, kinetic studies that are conducted using these methods yield apparent rate coefficients and apparent rate laws. since mass transfer and transport processes usually predominate. Relaxation methods enable one to measure reaction rates on millisecond and microsecond time scales and to determine mechanistic rate laws. In this chapter, theoretical aspects of chemical relaxation are presented. Transient relaxation methods such as temperature-jump, pressure-jump, concentration-jump, and electric field pulse techniques will be discussed and their application to the study of cation and anion adsorption/desorption phenomena, ion-exchange processes, and hydrolysis and complexation reactions will be covered.
The assessment of the fate (dispersion, retention, degradation), the quantification of transport phenomena, and the relative concentrations of a chemical in the different natural compartments (air, water, soil, and biota) is one of paramount importance in evaluating the chemical “mobility” in and through different phases. Among others, ion-exchange phenomena at the water-soil interface are of primary importance in soil and environmental chemistry, and research from both thermodynamic and kinetic viewpoints is necessary. With regard to kinetic aspects, despite the huge amount of work performed on reactive polymers, (e.g., ion-exchange resins and/or membranes) insufficient literature in reference to inorganic soil constituents (e.g., clays, oxides, and zeolites) exists. In general, due to the extreme subdivision of the constituent particles of natural exchangers, they are usually considered as “quasi-homogeneous” with the liquid phase. Thus homogeneous kinetic theory can be used to describe their reaction mechanisms. Recent studies have shown that mass transfer phenomena, either in the liquid and/or the solid phase, could play a relevant role in determining general kinetic behavior of these systems in addition to those strictly related to the pure chemical reactions. After a general overview of the basic principles of ion-exchange kinetics on reactive polymers, we shall apply these principles to clays, oxides and zeolites, which serve as models for heterogeneous soils.
Toxic heavy metal ions and radionuclides can be deposited on the soil surface where they are sorbed to a considerable extent also by soil organic matter. To improve predictions of the behavior of these ions in soils we investigated in stirred batch experiments the sorption rates of Pb2+, Cu2+, Cd2+, Zn2+, Ca2+ and, at trace concentrations (≈10−9 M) 137Cs+, 85Sr2+, 65Zn2+, 109Cd2+ and 57Co2+ on sphagnum peat and humic acid. Interruption tests showed that film diffusion is the rate-determining step for sorption of these metals on the humic substances. Accordingly, corresponding rate equations were derived for various initial and boundary conditions. The experimental and theoretical results show: (i) the half-time (t1/2) for sorption increases with decreasing concentration of the metal ions in solution, except at very low concentrations, where it becomes constant; (ii) at a given concentration of the metal ions in solution, t1/2 increases, the more the adsorbent prefers the sorbed ion (high separation factor α). At very low concentrations, however, t1/2 becomes independent of α; (iii) the t1/2 for sorption decreases if the adsorbent initially contains the metal ion that will be sorbed. The magnitude of this decrease is small, however, when the ion is sorbed preferentially (high values of α); and (iv) the t1/2 for sorption of radionuclides at trace concentrations, but at finite concentrations of the supporting electrolyte, increases with increasing distribution coefficients (Kd). At very high Kd values, however, t1/2 is almost independent of the Kd value. As a result, the t1/2 for the sorption of 85Sr, 65Zn, 109Cd and 57Co (Kd above 15 000 L kg−1) were almost identical ( ≈55 s) while 137Cs (Kd = 220 L kg −1) is sorbed much faster (t1/2 = 8 s).
Kinetic techniques are increasingly being used to characterize soil sorption/desorption processes and results of such studies are being used as sorption model input. There are benefits and limitations to the approach, and to avoid misuse of kinetics researchers should be aware of both. The initial choice among the many techniques should be based on appropriateness of the technique for modeling a process within the soil system. Without such basis, it is more difficult to develop a modeling strategy. Given an appropriate model, empirical data such as rate of sorption and reaction half-times and calculated information such as rate constants and thermodynamic quantities are assessable. In interfacing data from kinetics studies with models, one must always remember, however, that the heterogeneous nature of soils makes proper assignment of sorption mechanisms tenuous. It appears that the rate-determining processes during metal sorption by soil may be exchange reactions for the first few minutes then intraparticular diffusion until an equilibrium is established, but complete characterization of soil sorption kinetics is not so easily attained. For example, the effects of certain quantities (e.g., temperature) commonly varied in kinetics experiments are not always attributable to the sorption reaction itself, but may also alter the sorbent. Given these constraints, it is possible to make some tentative mechanism assignments and to calculate apparent rate coefficients for the reactions.
Under earth surface conditions between pH 4 and 10, the rates of primary silicate and oxide dissolution are controlled by surface reactions. For many oxides and hydroxides as well as some silicates, dissolution reactions can be modeled by surface complexation theory, which states that reaction rates are proportional to the population of surface complexes with H+, OH−, or ligands. The rate of release is controlled by the detachment of the complexes from the surface. This theory, however, fails to explain many observations including the dissolution of oxides at low pH, which is first-order with respect to solution H+, the dependence of rates on ionic strength, and the incongruent nature of the initial dissolution of most silicates. Rapid hydrolysis of charge balancing cations in silicates results in rapid release from surfaces. Even after removal of surficially exposed cations, the reaction is commonly incongruent. Much, but not all, of the nonlinear rates observed in silicate dissolution can be explained by the presence of high energy sites, such as dislocation outcrops, twinning planes, or damaged sites on ground mineral surfaces. These sites dissolve more rapidly than the bulk of the mineral, causing the high initial rates and producing etch pits, which results in increased surface area. The exact nature of the material remaining on reacted mineral surfaces (indicated by incongruence) is the subject of debate.
Manganese oxides are very reactive components in soils and associated environments. The objective of this chapter is to integrate the existing information on the kinetics of redox reactions on the surface of Mn oxides pertaining to transformations of certain metalloids, metals, and organics common in soils and sediments. Manganese oxides can oxidize the toxic As(III) to the less toxic As(V). The rate constants of Mn oxides to deplete As(III) vary with their crystallinity, specific surface, point of zero charge, and surface coatings. Trace metals such as Cr(III), Pu(III), and Co(III) have been shown to be oxidized by Mn oxides. Oxidation of trace metals can substantially influence their solubility, mobility, and toxicity. Further, the oxidation of Fe(II) by Mn oxides has been proven. Manganese oxides, which have different structural and surface properties, differ in their ability to influence the crystallization processes of hydrolytic products of Fe. The surface of Mn oxides catalyzes the oxidative polymerization of many polyphenolics, the polycondensation of pyrogallol and glycine, and the formation of humic substances. The rate and degree of the abiotic polymerization of phenolic compounds varies with the kinds of Mn oxides, the chemistry of phenolic compounds, and the pH of the systems. Many organics can be oxidatively decomposed during the reduction of Mn oxides that can lead to the mobilization of Mn in nature. The kinetics and mechanisms of these redox reactions on the surfaces of Mn oxides, thus, deserve increasing attention in the study of soil and environmental quality.
Adsorption of ionizable organic pollutants onto hydrous metal oxide surfaces in soils, sediments, and aquifers can have an important impact on pathways and rates of chemical transformations. In some instances, a particular degradative pathway can only occur at the oxide/water interface, for example, because of the low solubility of these higher-valent metals in most natural waters. In other instances, the unique chemical microenvironment of the oxide/water interface may catalyze transformations that otherwise would have occurred in solution. Hydrolysis of two carboxylic acid esters catalyzed by hydrous metal oxides is discussed. A detailed understanding of adsorption phenomena provides the basis for assessing the nature and importance of surface chemical transformations.
Modeling techniques are discussed that are appropriate for describing time-dependent adsorption, transformation, diffusional mass exchange, and precipitation reactions of inorganic cations and anions with soil. Included are the inorganic cations NH4, K, Ca, Mg, Cr, and Al as well as the inorganic anions NO3, F, and H2PO4. Experimental column breakthrough curves (BTC) for various types of inorganic reactions in soil are presented where the local equilibrium assumption is not valid and tracer migration is controlled by physical, chemical, and/or biological nonequilibrium. Such processes require that the reaction pathway be modeled during the approach to equilibrium. Proposed reaction schemes based on empirical expressions and the microscopic properties of the porous media are discussed. The effect of nonlinear adsorption and dispersion on the interpretation of time-dependent transport reactions are stressed, since these very different phenomena may lead to similar experimental BTC.
Rate-limited or, nonequilibrium, sorption of organic chemicals by natural sorbents (i.e., soils, sediments, and aquifer materials) has been a topic of interest for quite some time. The impact of nonequilibrium sorption on transport of organic chemicals in the subsurface has recently come under increased scrutiny as groundwater contamination has become a major issue. The purpose of this paper is to provide a brief review of the rate-limited sorption of organic chemicals by natural sorbents. The proposed processes held responsible for nonequilibrium sorption will be presented, as will a discussion of recent experiments whose results provide elucidation of rate-limiting mechanisms. Several models have been proposed to simulate sorption kinetics, and the transport of solutes influenced by nonequilibrium sorption; these will be reviewed. A large array of techniques are available for the study of sorption kinetics. However, much of this work has been oriented towards the study of inorganic chemicals. We will discuss two techniques that, in addition to the standard batch time study, have received the greatest amount of use in investigating nonequilibrium sorption of organic chemicals.