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5 Bioremediation of Heavy Metals

and immobilize metals, whereas others actually enhance metal solubility in the

environment, thus converting them to non-toxic forms [6].

The bio-concentration factor (BCF) of several heavy metals in the crop–soil

interface, particularly in major global staple crops such as wheat and corn, has

been documented earlier [7]. The intake of metal-contaminated vegetables has

been previously reported to cause gastrointestinal cancer, fragile immunological

mechanisms, mental growth retardation, malnutrition, etc. [8]. Human health

hazards are closely linked to the intake of metal-contaminated food crops. Heavy

metals can accumulate in human bones or fatty tissues through dietary intake,

thereby leading to the depletion of essential nutrients and weakened immunological

defenses. Certain heavy metals (e.g. Al, Cd, Mn, and Pb) are further suspected to

cause intrauterine growth retardation [9]. Lead contamination adversely affects

mental growth, causing neurological and cardiovascular diseases in humans,

especially children [10]. Certain heavy metals can also lead to bone fractures and

malformation, cardiovascular complications, kidney dysfunction, hypertension,

and other serious diseases of the liver, lung, nervous system, and immune system.

Excessive levels of As in soil, food crops, and groundwater can cause cancer, dermal

problems, respiratory complications, and many other diseases in the cardiovascu-

lar, gastrointestinal, hematological, hepatic, renal, neurological, developmental,

reproductive and immune systems [1].

Excess Zn levels in the human body can affect the concentration levels of

high-density lipoproteins and disturb the immune system. Likewise, excess Cu

intake can induce liver damage and other gastric-related problems in humans

[11]. Heavy metals (e.g. Cr, Cu, and Zn) in soil can cause non-carcinogenic human

health hazards such as neurologic complications, headaches, and liver disease

[11]. Chromium(VI) is more hazardous than Chromium(III) and other ionic forms

in terms of its stability. As such, the former form is suspected to have enhanced

potential to cause lung cancer compared with the latter form [11]. Cadmium is

highly carcinogenic, typically ingested by humans through contaminated food

crops, especially rice, and causes postmenopausal breast cancer [12].

The inhalation of soil and dietary intake of fruits, crops and vegetables contam-

inated with metals or metalloids can lead to gastrointestinal cancer. The concen-

trations of heavy metals were measured in several leafy (e.g. lettuce and spinach)

and non-leafy vegetables (e.g. radishes and carrots) to determine the bioavailability

of the metals in the human gastrointestinal tract. Health risk studies on the intake

of food crops in a developing country were conducted to assess 30 agro-ecological

zones in terms of health indices. The results revealed that the consumption of veg-

etables contaminated by heavy metals (especially Mn and Cu) was more deleterious

to human health than the consumption of contaminated fruits [13]. Scientific stud-

ies earlier reported that vegetables grown near a Pb–Zn mine were contaminated

with heavy metals, especially Pb and Mn, which can lead to Alzheimer’s disease and

manganism, a toxic condition resulting from chronic exposure to manganese.

Heavy metal contamination influences human health in a negative manner

through the alteration of food chain even at very low concentrations. The health

hazards caused by heavy metals often mediated by oxidative stress through the

formation of free radicals [14], for instance, the enhanced generation of reactive