Page 152 - Enzymes in Tropical Soils

140 Enzymes In Tropical Soils

pertaining conditions to reach new equilibrium values. Through these processes,
the heavy metals in compexes, chelates, adsorbed sites, and/or precipitates will
buffer the concentrations of free ionic heavy metals based on the equilibration
principles (Lindsay, 1979).
The importance of each chemical processes in controlling the concentrations
of heavy metals in soil water is debatable. Abd-elfattah and Wada (1981)
-7
2+
2+
-2
2+
2+
-3
2+
suggested that Zn , Pb , Cu , Co , and Cd at concentrations 10 to 10 in 10
-2
to 10 M CaCl 2 were not precipitated as hydroxides but were adsorbed by cation-
exchange sites. Cavallaro and McBride (1980) previously reported that the free
2+
(uncomplexed) Cu concentration were greatly pH-dependent in the soil system,
but the concentrations were below the levels needed for precipitation processs to
occur.
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; Hernandez-Soriano et al., 2012). It can be concluded that adsorption
processes, chelation, or precipitation 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 funcional groups, both organic and inorganic and, thereby, the holding
capacity of soils towards heavy metal cations also increase. 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 metal may increase at high pH
with high concentrations of heavy metals, particularly if the concentrations of the
precipitating agents like carbonate and sulfate ions in soil water are high (Singh
and Sekhon, 1977; Brummer et al., 1983)
Concerning the negative effect of heavy metals on soil enzymes and the
environment, efforts must be conducted to regulate the concentrations of
contaminant heavy metals in soils. Currently there are several methods to do this
efforts (Rabinowitz, 1993; Ma et al., 1993; 1994; 1995; Ruby et al., 1994; Suryanto
and Susetyo, 1997; Salam et al., 1997g; 1997k; 1998j; 2000; Salam, 2000; 2001).
The most important and effective method is, of course, processing industrial
wastes before disposal to the environment so that the heavy metal concentrations
entering the environment are maintained low. However, problem may be more
complicated if soils are already contaminated by heavy metals.
One of the various method available to cope with this problem is utilizing
lime and/or organic matters/biosolids (Elliott, 1983; Salam, 1993; 1995a; 1995b;
Salam and Maswah, 1994; Helmke et al., 1995; Salam et al., 1997i; 1997l; 1997m;

Abdul Kadir Salam - 2014