Page 144 - Enzymes in Tropical Soils

132 Enzymes In Tropical Soils

The increasing trend of heavy metal contamination is associated with the
increase in the global production in heavy metals for various purposes. For
example, the global production of Cu was 1,611,000 ton in 1930. The production
significantly increased to 7,660,000 ton in 1980 or within 50 years. This trend
increased the global emission of heavy metals into the soil system as high as
954,000 ton in 1980 (Alloway, 1990a).
In general, as long as their concentrations in soils are below the allowable
levels, the emission of heavy metals into the soil system will not endanger the living
things. Ross (1994) shows that the toxic levels of total Cu concentration in soils is
-1
-1
60 mg kg , of total Zn concentration is below 70 mg kg , and those for Cd and Pb
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are < 9 and < 100 mg kg , respectively. In fact, some of the metal elements such as
Cu and Zn, may be used as micronutrients for plants if managed at lower
concentrations. Particular industrial waste with heavy metal concentrations
relatively low are possible to be used as fertilizers precursor to enhance soil fertility
(Salam et al., 2000). Pereira et al. (2012) reported that the irrigation with
reclaimed waste water increased the availabilities and total concentrations of
nutrients and non-essential elements in soils, and also soil salinity and sodicity by
two to three times compared with that irrigated well water. The major parts of
nutrients in reclaimed waste water were free ionic species readily avaliable for
+ − + 2+ 2+ 2− − 2+ 2+
plant absorption such as: NH 4 , NO 3 , K , Ca , Mg , SO 4 , H 3 BO 3 , Cl , Fe , Mn ,
2+ 2+ 2+ −
Zn , Co , and Ni . More than 80% of Cu, Cr, Pb, and Al were complexed with CO 3 ,
−
OH , and/or organic matters.

9.1 Toxicities of Heavy Metals

Heavy metals are in general toxic and carcinogenic. The toxicities of heavy
metals to the living things including human beings have been documented in
current literature (Alloway, 1990d; Steinnes, 1990; Baker, 1990; Daoust et al., 2006;
Oporto, 2007). For example, Oporto (2007) reported an estimation of Cd intake
from potato consumption by local population in soil irrigated with contaminated
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river water in Potosi Bolivia was about 100 mg d , exceeding the WHO
recommended total daily intake. Health problem caused by Cd accumulation in
human tissue also occurred as a result of contamination of river and paddy field
water in Jintsu Valley, Toyama, Japan, in 1940 is included in current literatures
(Alloway, 1990d). This phenomenon caused a desease called Itai-Itai. In 1950s a
similar case also occurred in Minamata Bay, Japan, caused by methyl mercury

Abdul Kadir Salam - 2014