Page 219 - Soil Degradation, Conservation and Remediation

208 6 Soil Pollution

Table 6.9 Toxicities of heavy metals in plants
Heavy metals Common toxicities of heavy metals on plants
Arsenic Detrimental to mung bean seed germination
Cadmium Inhibition of the biochemical processes – photophosphorylation, ATP
synthesis, mitochondrial NADH oxidation, and electron-transport system;
it affects seed germination and seedling growth. Rice plants are
particularly vulnerable
−1
Chromium Toxic at a tissue concentration of ~0.1 mmol kg ; toxic effects mainly on
roots and a direct or indirect effect on leaves resulting in intense growth
inhibition; symptoms of Cr toxicity appear as severe wilting and chlorosis
of plants

Copper Concentrations of <4 ppm are deficient, and >20 ppm are toxic. Cu is a
constituent of a number of plant enzymes, which trigger a variety of
physiological processes in plants such as photosynthesis, respiration, cell
wall metabolisms, and seed formation
Lead Inhibition of seed germination and seedling growth
Mercury Toxic effects of Hg on plants include retardation of growth and premature
senescence. Hg has been shown to inhibit synthesis of proteins in plant
leaves and to reduce photosynthetic activity as it has a strong affi nity for
sulfhydryl or thiol groups, which are involved in enzymatic reactions. Hg
also impairs cell division
Molybdenum Mo is an important component of several enzymes that catalyze unrelated
reactions. In plants, the most important functions of Mo are associated
with N metabolism, e.g., with nitrogenase and nitrate reductase enzymes.
Mo toxicity in plants has not been observed under fi eld conditions
Nickel Ni toxicity causes pale yellow stripes longitudinally in leaf. In extreme cases,
the entire plant may turn white with marginal necrosis (burn). In dicots, Ni
toxicity causes an interveinal chlorosis that looks very similar to
manganese defi ciency

Toxicity of Heavy Metals to Human

Toxicity of heavy metals involves either compounds with intrinsic toxicity or
activated metabolites. It may be acute or chronic depending on the type of exposure.
Changes in physiological and biochemical processes may be manifested in impaired
central nervous system (CNS) function and oxidative metabolism, injury to the
reproductive system, or altered DNA leading to carcinogenesis. Heavy metals can
induce adverse effects in human by disruption or destruction of cellular structure,
chemical combination with a cell constituent, inhibition of enzymes, initiation of
a secondary action, free-radical-mediated reactions and disruption of reproductive
function (Yu 2005 ). Toxicities due to most common heavy metal contaminants are
listed in Table 6.10 .

6.2.12.6 Heavy Metal Remediation of Soil

A variety of methods are employed for heavy metal remediation of soils. For the
sake of brevity, a brief description of the methods is given in the following sections.