Effects of Tropical Weeds on Heavy Metals
8.1 The Chemistry of Heavy Metals in Soils
Heavy metals presence in soils may have different forms that may include free ions, complexes and chelates, exchangeable, secondary minerals (precipitates),
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primary minerals, and organics which are mostly in equilibrium among each other.
The existing equilibria between these various forms of heavy metals in the soil environment are shown in Fig. 8.1. Free ionic form of heavy metals in the soil- water is the central of heavy metal dynamics due to several reasons: (a) the availabilities of heavy metal elements to plant root absorption and heavy metal toxicities are related to this form (Allen et al., 1980; Checkai et al., 1987a; 1987b;
Hernandez-Soriano et al., 2012), (b) the rates of heavy metal movement and leaching in the soil system are related to this form, (c) all chemical mechanisms controlling the concentrations of heavy metals (compelexation-decomplexation, chelation-dechelation, precipitation-dissolution, and adsoption-desorption processes) in the soil environment are also related to this form (Bowman and O’Connors, 1982; Sanders, 1982; Salam and Helmke, 1998; Hernandez-Soriano et al., 2012).
Free ion of heavy metals is in a direct relationship with their absorption by plant roots. The free ionic form is also in chemical equilibria with the processes of chelation-dechelation, complexation-decomplexation and precipitation-dissolution, as well as adsorption-desorption processes. The status of free ionic forms of heavy metals is also greatly affected by the emission of heavy metals from industrial wastes, fertilizers, and pesticides. Numerous reports shows that these materials contribute great amounts of heavy metals in the soil environment (Lagerwerff, 1982; Kardoz et al., 1986; Leung, 1988; Hegstrom and West, 1989; Alloway, 1990b;
Davies, 1990; Kiekens, 1990; Rivai, 1990; Dowdy et al., 1991; Boon and Soltanpour, 1992; Jing and Logan, 1992; Wang et al., 1992; Herrero and Martin, 1993; Sweet et al., 1993; Cabrera et al., 1994; Nicholson et al., 1994; Tsoumbaris and Tsoukali- Papadopoulou, 1994; Schuhmacher et al., 1994; Bilski and Alva, 1995; Flegal and Smith, 1995; Vile et al., 1995; Gimeno-Garcia et al., 1996; Salam et al., 1996; Yeh et al., 1996; Salam et al., 1997a; Juracek and Ziegler, 2006; Biasioli et al., 2007; Benke et al., 2008; Berenguer et al., 2008; Lin et al., 2008; Hobara et al., 2009; Benn et al., 2010; Cakmak et al., 2010; Kien et al., 2010; Wang et al., 2010; and Tu et al., 2012).
Similarly, the status of free ionic of heavy metals in soil water is also affected by absorption by plant roots and/or leaching by percolating water, which may cause the decrease in heavy metal free ionic form concentration in the soil water.
Fig. 8.1 clearly shows that the heavy metals in complexes, chelates, adsorbed sites, and/or precipitates will buffer the concentrations of free ionic heavy metals based on the equilibration principles (Lindsay, 1979). This suggests that the related equilibrium constants are of great importance. Whenever the concentration of free ionic heavy metals in soil water is lowered by some mechanisms such as plant
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140 The Chemistry and Fertility of Soils under Tropical Weeds
root absorption or leaching, the heavy metals in complexes, chelates, adsorbed sites, or precipitates will be released to compensate the respective equilibrium constants through equilibrium processes. On the other hand, if the concentrations of free ionic heavy metals in soil water are high enough due to external addition such as waste and fertilizer applications, parts of the free ionic heavy metals in soil solution will be complexed, chelated, adsorbed, or precipitated, depending on the pertaining conditions to reach new equilibrium values.
Fig. 8.1. The relationships between forms of heavy metals in the soil environment (Adapted from Salam, 1997a; Salam, 2017).
Cavallaro and McBride (1980) reports that the free ionic Cu2+ concentration in the soil system was greatly pH-dependent, but the concentration was below the levels needed for the precipitation processes to occur. Abd-elfattah and Wada (1981) also suggest that Zn2+, Pb2+, Cu2+, Co2+, and Cd2+ at concentrations 10-7 to 10-
2 M in 10-3 to 10-2 M CaCl2 were not precipitated as hydroxides but were adsorbed by cation-exchange sites. Salam and Helmke (1998) also report that the logarithmic of Cu2+ concentration was linearly related to the soil pH and was controlled by adsorption-desorption process (Fig. 8.2). The importance of each chemical
Free Ions Complexes
Precipitates
Chelates Plants Wastes
Fertilizers
Leaching Organics
Pesticides Minerals
Adsorbed
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processes in controlling the concentrations of heavy metals in soil water is debatable, but are in general depending on the heavy metal cation concentration and soil pH.
Fig 8.2. The relationship between logaritmic (Cu2+) and total dissolved Cu (CuT) with soil pH (Adapted from Salam and
Helmke, 1998).
Salam (2017) shows the possibility of hydroxide precipitation at lower pH may occur when the concentration of heavy metal cation is higher (Fig. 8.3). Fig. 8.3 shows that at high concentrations, heavy metal cations precipitate at lower pH (A) and not at low concentrations (B). A good example is probably exemplified by Fe2+
that may precipitates at lower pH when the concentration of [Fe2+] is higher. The calculation is shown in Table 8.1. At [Fe2+] = 10-6 M the precipitation of Fe(OH)2
occurs at pH = 10.15 while at [Fe2+] = 10-3 M or 1,000 times higher, the Fe(OH)2
precipitation occurs at pH = 8.65, about 1.85 unit pH lower. At [Fe3+] 1,000,000 times higher or [Fe3+] = 1 M, the precipitation may occur at even lower pH i.e. pH = 7.15.
-9 -8 -7 -6 -5
4.4 4.8 5.7 6.2
Log (Cu)
pH
Free ionic Cu Total Cu
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Fig. 8.3. The precipitation of heavy metal cation as a function of pH (Salam, 2017).
Table 8.1. The precipitation of Fe(OH)2 at different concentration of Fe2+*.
[Fe2+]
M pH Calculation
100 7.15 K = [Fe2+][OH-]2 K = 10-13.7 [OH]2 = K/[Fe2+] pH + pOH = 14 10-3 8.65
10-6 10.15
*Salam (2017)
However, in general soil workers agree that all mechanisms greatly depend on soil pH (Ma and Lindsay, 1990; Workman and Lindsay, 1990; Salam and Helmke, 1998; and Hernandez-Soriano et al., 2012). Complexation, chelation, precipitation, or adsorption processes increase with the increase in soil pH. The soil adsorption capacities increase with the increase in soil pH due to H+ ionization from various soil functional groups, both organic and inorganic and, thereby, the holding capacity of soil solids towards heavy metal cations also increases. By this process, the concentrations of heavy metals in soil water decrease with the increase in soil pH (Workman and Lindsay, 1990; El-Falaky et al., 1991; Salam and Helmke, 1998).
Some other soil workers also believe that the heavy metal precipitation is important at high soil pH. The precipitation of heavy metals may increase at high
High
Concentration Low
Concentration
pH
Precipitation
A B
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pH with high concentrations of heavy metals, particularly if the concentrations of the precipitating agents like carbonate and sulphate ions in soil water are high (Singh and Sekhon, 1977; Brummer et al., 1983).
Some soil properties may affect the magnitude of the respective forms of heavy metals in the soil environment. Soil pH is repeatedly reported to be the most important factor (Salam and Helmke, 1998; Salam, 2017; Salam, 2019). As discussed previously the increase in soil pH may lower the concentrations of free ions and total dissolved heavy metals in soil solution (Fig. 8.2). At high concentration of heavy metals, parts of the heavy metals are precipitated forming secondary minerals due to the increase in the concentration of OH- as shown in Fig.
8.2. At low concentration, heavy metal adsorption may dominate due to increased CEC as a result of adsorbed H neutralization by OH- ions, shown in Fig. 8.4 and Fig.
8.5. The increase in the soil enzymatic activities may also indirectly enhance the