Page 159 - Enzymes in Tropical Soils

Enzymes in Tropical Soils 147

Richards et al., 2011). The use of zeolites (for example Analcin NaAl 2 O 6 .H 2 O) may
increase the adsorption of heavy metal cations by soils, and may decrease the
solubility of heavy metals (Salam et al., 1998b; Setyawan, 2002; 2003; Penn et al.,
2010). However, Moseley et al. (2008) states that even though the reduction in Pb
bioaccessaility with P amendments are technically possible, it requires large
amendment masses to achieve relatively modest reductions in bioaccessibility.
It was reported that the soil CEC is not only affected by soil pH but also by P-
fertilizers. Several workers reported that phosphates may react with the soil
colloids and may increase the soil adsorption capacity towards cations, including
heavy metals cations (Saeed and Fox, 1979; Xie and McKenzie, 1990). Additon of 1
-1
mmol pyrophosphate increased the soil CEC as high as 0.52 – 0.89 cmol c kg (Xie
and McKenzie, 1990). This process was related to increased Zn adsortion
equivalent to 0.3 – 12% of the pyrophosphate induced increase in CEC. Therefore,
plant roots were not able to absorb P and may induce deficiency symptom in plants
(Friesen et al., 1980; Norvell et al., 1987). This process may result in the decrease
of the availability of heavy metal micronutrients (Brown et al.,1970; Saeed and Fox,
1979; Kuo and McNeal,1984; Xie and McKenzie, 1990). The decrease may also be
attributed to the increase in soil pH as affected by addition of P and the increase in
soil CEC induced by pH increase.
Other method that can be used without modifying the solubility of heavy
metals by employing chemical reactions through increasing the process of
adsorption and precipitation and their modifications is by extracting the heavy
metals out of the soil system contaminated with heavy metals. The heavy metals in
the soil system are collected and disposed in safer forms or are used for other
purposes after purification. One of the method to extract the heavy metals is by
using bioaccumator plants, which can easily grow and absorb heavy metals in soil
contaminated by heavy metals.
Several vegetations were indicated to be able to efficiently absorb heavy
metals and, therefore, can be used as bioaccumulator plants with the technique of
phytoremediation (Sriyani and Salam, 1998; Siregar and Siregar, 2010; Laidlaw et
al., 2012). Some of these vegetations are amaranth, sunflowers, and cabbage, that
can be used in the phytoextraction technique to extract heavy metal contaminants
in contaminated soils, sediments, and muds. For example, Kambhampati and Vu
(2012) suggest that chickpea (Cicer arietinum L.) seems to be a cost-effective and
environmentally friendly hyperaccumulator for Cu at 100 ppm Cu and 10 mM
EDTA.
The are several other vegetations indicated to be good metal accumulators.
Salam et al. (1999b) reported that Cu intake by amaranth was higher than that by

Abdul Kadir Salam - 2014