Page 155 - Ilmu Tanah Book

142 The Chemistry and Fertility of Soils under Tropical Weeds

High

Concentration

Low
Concentration

Precipitation

A B

pH

Fig. 8.3. The precipitation of heavy metal cation as a function of pH
(Salam, 2017).

2+
Table 8.1. The precipitation of Fe(OH) 2 at different concentration of Fe *.

2+
[Fe ]
pH Calculation
M
- 2
2+
0
10 7.15 K = [Fe ][OH ]
-3
10 8.65 K = 10 -13.7
2
2+
-6
10 10.15 [OH] = K/[Fe ]
pH + pOH = 14
*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

Abdul Kadir Salam and Nanik Sriyani – 2019