considered in the experiment and modeling.
2.3.1 pH
The soil pH ranges in most soils from 4.5 to 7.8 (Sparks, 1995), and is influenced by the presence of carbonates, iron, aluminum, etc. The pH of acid soils
with pH less than 5.6 are controlled mainly due to the buffering action of Al. For the soils with pH higher than 7.4, carbonate is commonly found. The soil pH can be measured by different methods, typically with de-ionized water or Ca salts solution (Sims and Heckendorn, 1991).
The pH may affect metal sorption and desorption both directly and indirectly. Higher pH, meaning a decrease in proton activity, may suppress ion- exchange displacement of metal ions by H+. Some trace metals (e.g. Ni and Zn) compounds precipitate at higher pH values. At low pH, proton competition will
dominate since protons have very strong binding ability with soils, for example, SOM.
Thus, pH strongly affects distribution of trace metals between solid and solution phases (Buchter et al., 1989). Sauvé et al. (2000b) reported that distribution
coefficients of metals between solid and solution phases (Kp) for several soils may be predicted using empirical linear regressions with pH or with pH and either the log of SOM or the log of total metal. Desorption of Zn decreased with increasing pH (Rupa and Tomar, 1999; Rupa et al., 2000). At higher pH, hydrolyzed ZnOH+ sorption by soil may be responsible for the abrupt decrease of Zn desorption. The sensitivity of Cd desorption from soils to pH was also reported by Gray et al. (1998) and higher soil pH greatly decreased Cd desorption. Indirectly pH may affect metal sorption and desorption by influencing the partition of SOM (dissolution and precipitation of SOM), dissolution of iron oxides-hydroxides, etc. For example, the dissolution of SOM may enhance metal release from soils due to the enhanced solubility of metals
The proton binding and effect of pH on metal binding by humic substances have been extensively studied with speciation models (Christl and
Kretzschmar, 2001; Christl et al., 2001; Kinniburgh et al., 1996; Tipping et al., 1995).
In WHAM VI, the proton-metal exchange is an inherent part of the model and bidentate and tridentate binding sites were postulated. The effect of pH on metal binding can be related to the proton/metal exchange ratio (Kinniburgh et al., 1999;
Cernek et al., 1995). Overall, these mechanistic equilibrium models gave relatively good fitting of proton binding and proton-metal competition data.
Generally, the available data reveal that pH is one of the most important parameters controlling metal partition between soils and solutions. However, the effect of pH on the kinetics of trace metals sorption and desorption has not been extensively quantitatively studied.
2.3.2 DOM
Concentrations of DOM in soil solutions usually vary from 1 to 70 mg/L, increasing in organic-rich soils. In surface waters the concentration of DOM is reported to be from less than 0.5 mg/L to as high as 100 mg/L (Tipping, 2002). Like SOM, DOM can complex metal ions and thus affect their solubility and behavior.
The role of DOM affecting the reactions of metals can be mainly attributed to two aspects: (a) metal and DOM complexation and (b) DOM
sorption/desorption on soil particles. The complexation reaction can affect the metal
speciation and sorption/desorption of DOM to soil particles may affect the reactivity of soil particles.
The complexation reactions between metals and DOM have been
extensively studied. Desorbed Cu was strongly correlated with DOM concentration in soil suspensions (Hsu and Lo, 2000; Impellitteri et al., 2002; Rửmkens et al., 1996;
Temminghoff et al., 1997; Yin et al., 2002). Fractionating DOM into three
operationally defined fractions, namely humic acids (HA), fulvic acids (FA), and so- called hydrophilic acids (Hyd), Impellitteri et al. (2002) revealed that HA is able to leach Cu and Pb from soils. Compared with Cu, Zn has less tendency to form soluble complexes with DOM. Solubilization of Zn occurring at higher pH was attributed to the greater amount of DOM at high pH, and Ni behaves quite similarly to Zn (Yin et al., 2002). So, the effect of DOM on the reaction of different trace metals may vary and is highly dependent on pH.
DOM concentration in soil suspensions is affected by both pH and Ca concentrations (Rửmkens and Dolfing, 1998; You et al., 1999). The enhanced
dissolution of SOM, especially of high molecular weight material at elevated pH, was attributed to either soil particles dispersion or the repulsion effect caused by the increasing of negative charges on both organic matter and soil inorganic solid surfaces (You et al., 1999). Partitioning of FA and HA also correlated with the equilibrium pH (Impellitteri et al., 2002). Elevated Ca concentration can cause coagulation of DOM.
Furthermore, DOM can sorb onto soil particles which will affect the solubility of
metals and reactivity of soil particles, especially for the metals forming strong complexes with DOM.
Overall, the effect of DOM may be complex due to a number of possible reactions. It is thus important to identify and control different reactions in order to incorporate the effect of DOM into the kinetics modeling.
2.3.3 Dissolved Calcium
Calcium (Ca) concentrations in soil solutions usually range from 0.27 mM to 12.86 mM (Sparks, 1995). Ca can (i) displace the exchangeable fractions of the metals from soils through ion exchange process, (ii) compete with the trace metal for the binding sites in DOM and soils, and (iii) precipitate DOM from solutions thus influencing trace metals speciation in the reaction system.
An extensive study has been done for the Ca-DOM reactions. The complexation of Ca by DOM was reported to be much weaker than Cu and only a small portion of Ca is bound by DOM (Lu and Allen, 2002). Moreover, the most part of Ca and Mg is bound by carboxylic groups while Cu tended to be bound principally by phenolic groups of DOM (Lu and Allen, 2002).
Ca induces coagulation of humic acid and thus decreases concentration of DOM (Oste et al., 2002). Removal of DOM by Ca may be more important for the release of Cu and Pb forming strong complexes with DOM, than Ca-metal exchange.
In batch experiments copper concentrations in solution phase of soil suspensions decreased almost linearly with decreasing DOM level (Rửmkens and Dolfing., 1998).
At near neutral pH values, a large part of the available DOM is in a Ca form and sorption of Ca-DOM complexes may be more important to control Cu solution concentration than cation exchange displacement of Cu (Rửmkens et al., 1996).
Higher Ca concentrations enhanced Zn desorption due to decrease negative electrostatic potentials and thus decrease Zn retention (Rupa et al., 2000).
2.3.4 Other Factors
Besides the typical solution chemistry parameters, other factors may affect the metal reactions with soils, such as reaction time, hydrological residence time, and particle concentrations.
There is so-called aging effect for metal sorption and desorption
processes, in which with longer reaction time the sorbed metals are more difficult to release. Some mechanisms have been proposed to explain these phenomena including metal diffusion in soil particles, phase changes, e.g. from surface complexation to precipitation.
Soil:solution ratio and hydrological conditions may affect trace metals sorption and desorption reactions. It was reported that the higher soil:solution ratio enhanced soluble metal concentrations (Yin et al., 2002). This may be caused by higher concentrations of organic colloids at high pH or particle concentration effect at lower pH (Di Toro et al., 1986; Benoit, 1995; Benoit and Rozan, 1999). The different flow rates in the flow system can also affect the rate of metal desorption from soils