Page 156 - Enzymes in Tropical Soils

144 Enzymes In Tropical Soils

(Helling et al., 1964; Tan and Dowling, 1984; McGrath et al., 1988; Alloway 1990b;
Parfitt et al., 1995; Rodella et al., 1995; Bang and Hesterberg, 2004; Brown et al.,
2004; Stuczynski et al., 2007; Buss et al., 2012; Smolders et al., 2012). It was also
reported by Saidy and Badruzsaufari (2009) that the increase in soil organic matter
contents decreases the soil Cr (VI) concentration. The functional groups or organic
matters in soils might have dehydrogenized and arised negative charges that then
complexed or adsorbed free ionic heavy metals particularly at alkaline pH. Rodella
et al. (1995) reported that the CEC of Horizon A and Horizon B in some New
Zealand soils were originated from the soil organic matter carboxylic groups. The
maximum capacity of heavy metals can be adsorbed by soil organic matters was
equivalent to the amount of the soil carboxylics groups (Alloway, 1990b).

Table 9.8. The relationship between Cd, Cu, and Zn
solubilities with soil pH.

No Heavy Metal Equation

2+
1 Cd Log (Cd ) = 6.50 – 2 pH
2+
2 Cu Log (Cu ) = 2.80 – 2 pH
2+
3 Zn Log (Zn ) = 5.80 – 2 pH
Taken from Lindsay (1979), Workman and Lindsay (1990), Ma
and Lindsay (1990), El-Falaky et al. (1991)

Organic matter contributes about 32% of the soil CEC (Rodella et al., 1995).
If the soil pH is also considered, the contribution of organic matters to soil CEC
increases to be 78%, indicating that soil pH is a very important factor to consider to
immobilize heavy metals in soils. The importance of pH is not only related to the
non-specific adsorption of heavy metals, but also to the specific adsorption
involving the formation of covalent bonding. However, the specific adsorption of
heavy metals is more important because the soil capacity related to this
mechanisms is higher than do the non-specific adsorption. The decrease in heavy
metal solubility as affected by lime (that increase the soil pH) and organic matter
(that increase the soil adsorptive capacity) is shown in Fig. 9.11.
Fig. 9.11 clearly shows that heavy metals of industrial wastes available to
plant root absorption are lowered because part of the heavy metals are adsorbed
by soil clay and organic matter colloids with increased adsorption capacities as a
result of lime and/or organic matter treatments. This phenomenon is more clearly

Abdul Kadir Salam - 2014