Metals in the environment occur in several forms: dissolved in surface and ground waters, incorporated in or sorbed on the surface of minerals in rock, sand, and soils or as well as bound to soil organic matter. Cadmium (Cd), copper (Cu), nickel (Ni) and zinc (Zn) are trace metals widely used in industry, transportation, agriculture and are being released into the environment. Cadmium, Cu, Ni, and Zn enter surface waters both in particulate and in dissolved forms from natural and anthropogenic sources. Natural processes contributing trace metals into soils, water and air include soils mineral weathering, water and wind erosion of rocks and soils, volcanic activity, and biological transfer of elements. The major anthropogenic sources of trace metals are mining and smelting, agricultural application of fertilizers, pesticides, and
ameliorants containing trace metals, transportation, and municipal and industrial wastes.
Contamination of the environment with trace metals is a serious problem in many regions of the USA and all over the world. Soils can sorb trace metals and
through leaching and in the particulate form through erosion (Bergkvist, 1986; McColl et al., 1986; Rasmussen, 1986; Bergkvist, 1987; Bergkvist et al., 1989). Much of the trace metals entering the surface waters are usually released from contaminated solid particles. The accumulation of the metals in the waters is highly dependent on the weathering of these solid particles. The trace metals leaching out of the soil particles into soil solutions (pore water) can become available to plants and animals in soils.
Both sorption and desorption processes are important for controlling the metal behavior in the soil and solution systems. The sorption process directly affects the metal distribution among different soils components and thus the future desorption from soils. The equilibrium of trace metals partition between soils and solutions has been studied extensively and several equilibrium models have been developed to predict the partitioning of trace metals between solid and solution phases. However, the sorption and desorption of trace metals on the soil particles appear to be a slow process and the equilibrium between solid and solution may not be attained in soils (Sparks, 1989 and 2001). There have been extensive equilibrium studies of metal sorption on soils for many years (Jenne, 1968; Shuman, 1975; McBride and Blasiak, 1979; Elrashidi and O’Connor, 1982; Zachara et al., 1992; Weng et al., 2001; Tipping et al., 2003), but much fewer studies have focused on the kinetics of trace metal sorption/desorption on soils (Yin et al., 1997; Strawn and Sparks., 2000;
Sukreeyapongse et al., 2002; Zhang et al., 2004).
Knowledge of the kinetics reactions for metals between soils and solutions can be important for predicting metal behavior since an equilibrium assumption may
not be appropriate. The kinetic behavior of metals in the field may be affected by different processes including metal reactions between soils and solutions, the mobility of solutions, solution diffusion, or uptake by organisms. The metal reactions between soils and solutions do not achieve equilibrium instantaneously (Yin et al., 1997;
Strawn and Sparks, 2000; Zhang et al., 2004). In laboratory column studies, leaching of trace metals from soils is kinetically controlled rather than by instantaneous
equilibrium (Kedziorek et al., 1998). However, the importance of this kinetic reaction to control the metal behavior may differ for different chemistry and hydrological conditions (Zhang et al., 1998; Ernstberger et al., 2005; Voegelin et al., 2001). Thus a quantitative understanding of the rates of metal sorption and desorption on different soils at varying solution chemistry would be quite useful for developing models to accurately predict the fate and transport of trace metals in the environment for the levels of contamination available in the soils.
One challenge for the kinetics modeling in the soil system is the
heterogeneity of soils, and little progress has been made in previous studies. In soils trace metals are bound to different components including organic matter, clay minerals, and metal oxides-hydroxides to different extents. So, sorption and desorption of the metals may include various chemical reactions proceeding at different time scales with different mechanisms. Taking into account all the main processes controlling the metals sorption and desorption on soils is most important to predict the impact of soil compositions on the metal contamination of surface waters.
2000), metals reactions with soils may depend on metals content, SOM, iron and manganese oxides-hydroxides and clay fraction. It is essential to assess the role of different sorbents in soil components to control the reactions of metals with soils at different conditions.
In natural conditions, the solution compositions can be very different.
Solution pH, DOM concentration and other cations (e.g. Ca2+, Al3+) may be the principal solution properties affecting the metals reactions. How these solution parameters affect metal partition equilibrium has been extensively studied, and some speciation models have been developed to calculate the metal speciation at different conditions, such as MINEQL+ (Schecher and McAvoy, 1998) and WHAM (Tipping, 1994). However the understanding of the kinetics effect of solution chemistry is still insufficient. A systematical study on how these factors affect the kinetics of metal sorption and desorption on soils is necessary in order to develop predictive models that can be applied to different solution chemistry conditions.
Another important concept, which has been overlooked by most of the previous studies, is the speciation of metals in the reaction systems. Even in the solutions, the metal speciation can be different at different pH, DOM concentrations.
The reactivity of different species may differ greatly. Thus the total dissolved metals may not be a good indicator for the reactive metals in the solutions (Allen et al., 1980). For example, copper availability to plants and animals is found to be related not to total copper concentration in solutions but to the copper ion concentration (Di Toro et al., 2001; McBride, 2001). In the soils, due to their heterogeneity, the metal
may be bound by a variety of sorbents and form different metal species. Recently spectroscopic techniques have been used to identify the speciation of metals in soils, which demonstrated that the speciation of metals can vary considerably among soils (Manceau et al., 1996; Roberts et al., 2002). Thus, in the kinetics modeling, the metal speciation should be carefully taken account of since different metal species may have very different kinetic behaviors.
Some kinetics models that have been used to describe metal sorption and desorption kinetics in soil systems (Sparks, 1989 and 1999) are reviewed in Chapter 2 in detail. Briefly, the rates of the reactions on soil constituents have been described using various models, such as ordered models (zero-, first-, and second-order kinetics equations), parabolic diffusion, two-constant rate, Elovich, and differential rate
equations. However, the rate constants obtained in these models were not constant but changed with experimental conditions. Most of previous modeling was mainly based on individual curve fitting and then correlation analysis. The mechanisms controlling metal sorption and desorption in these models was not explicit and their usefulness is limited. A new kinetics model, which specifically incorporates the reaction chemistry, is needed in order to better predict the metal behavior in the environment.
Furthermore, it has been recognized that the reactivity of different trace metals was different in the environment. The cation properties, such as electron configuration, water exchange rate, and the first hydrolysis constant, etc., vary among metals, which may be the reasons accounting for their different behaviors. For
Zn binding is very linear. Cu can form strong complexes with DOM but Zn does not.
The pH can also affect Cu and Zn binding to humic substances differently. All these factors make kinetics modeling complicated. A robust kinetics model should be able to predict different metal behaviors according to different chemical reactions.
Overall, little progress has been made on the kinetics modeling of trace metals in soil systems compared the extensively used equilibrium models. The predictive kinetics models, which can be used at different solution chemistry and soil compositions, are highly desired. This will improve the existing approaches based on equilibrium assumptions.