Page 2 Plant-Bacteria Interactions Edited by Iqbal Ahmad, John ...

284j 15 The Use of Rhizospheric
15.2
Overview of Metal Ion Pollution
Heavy metals are generally defined as a group of approximately 65 metallic elements
with density greater than 5 g cm 3 [31]; for example, cadmium, chromium, copper,
mercury, nickel, lead and zinc, plus the metalloids arsenic and selenium, which have
long been contaminants in sewage [32]. These metal ions possess diverse physical,
chemical and biological properties and have the ability to exert toxic effects on
microbial and other life forms [33,34]. The ultimate cause for concern about metal
ions in the environment is their toxicity toward humans [35,36] and other biota.
Owing to their nondegradable nature, they will persist and cycle in different compartments
of the environment. In addition to the accumulation of metals within food
chains, elevated metal levels in the environment pose serious threats to human
health [37]. Although some metal ions such as Cr 6+ ,Cu 2+ ,Ni 2+ and Zn 2+ are
essential to life, they are required only in trace amounts and become toxic once
they exceed the threshold bioavailable level [31].
Metal ion toxicity may be manifested from either an acute, single high exposure or
chronic, long-term exposure to low concentrations [1]. Metals exert toxicity in man
and animals in variable degrees by deactivating enzymes, replacing essential metal
ions such as Ca 2+ and Mg 2+ or destabilizing biomolecules such as nucleic acids,
which results in genotoxic or mutagenic effects producing heritable genetic disorders
and cancers [1].
The rapid, unbridled industrialization without environmental controls has led to
the serious contamination of our environment by metal ions [38]. The US Environmental
Protection Agency (USEPA) estimates that there are 30 000 candidate sites
for hazardous waste treatment services in the United States alone, including industrial
sites that contain liquid and solid wastes contaminated with metals ions [39].
Current cleanup costs for all Superfund (National Priority) sites are estimated to be
$16.5 billion [39–41]. Approximately 15% of these sites are contaminated by
metals only, whereas 64% of the sites contain metals mixed with organic wastes
[39,42].
Metals enter the aquatic environment by various sources that are divided into two
main categories: anthropogenic and nonanthropogenic [1]. Natural geological processes
release metal ions into the environment by weathering, erosion and runoff
from the lithosphere. In the United States, it has been estimated that $1 million is
spent every day to clean up 12 000 miles (31 080 km) of rivers and 180 000 acres
(728 km 2 ) of lakes contaminated by mining wastes in which metal ions are the major
pollutant, and the total cleanup costs of acid mine drainage have been estimated to
be as high as $70 billion [6]. The major source of metal ions is human activities,
which include mining, agriculture and industrial processes such as electroplating,
oil refining, paper and textiles manufacturing [43]. For the major metal bearing
industrial waste streams, the market for treatment of metals in the United States is
estimated to be in the range of $1–2 billion per year [6,44]. To maintain a sustainable
use of our water resources, an efficient and cost-effective technique must be developed
to achieve a high effluent standard.