HumBioEnvLogy: Heavy Metal Pollution, Bioaccumulation, And Effect...

HumBioEnvLogy: Heavy Metal Pollution, Bioaccumulation, And Effect...: The school of thought  that Technology brings more harm to us than good , may be as a result of our failure to apply best practices in o...

Heavy Metal Pollution, Bioaccumulation, And Effect To The Environment

The school of thought  that Technology brings more harm to us than good , may be as a result of our failure to apply best practices in our utilization of  the technological innovations in all areas of life, e. g. Exploration of both solid and liquid mineral resources, commercialisation of agriculture, construction of roads/other developmental infrastructure and their uses, industrialisation, expansion in the form of reclamation of lands, etc., in one way or the other has effect to the environment and to us that ought to manage our environment for sustenance.

The rusting of iron and steel used in construction and as ornaments, the application of inorganic fertilisers to the soil to enhance productivity for our agricultural produce, exploration of minerals that affects our atmosphere, hydrosphere and leaves soil seriously polluted, emission of carbon and other gasses which settles and react with water to become toxic to both plants and animals, and improper disposal/management of waste all are responsible for the pollution of our environment with one heavy metal or the other.

Heavy metals are members of loosely defined subset of elements that exhibit metallic properties. Lots of definitions have been proposed, some based on density (6.0g/cm³ or more (much higher than the average particle density of soils which is 2.66g/cm³)), some on atomic number or atomic weight, and some on chemical properties. These metals are alternatively called toxic metals, irrespective of their atomic mass or density.

Heavy metals include ill-defined subset of elements that exhibit metallic properties, which are transition metals, some metalloids, lanthanides, and actinides. These metals cause environmental pollution from sources such as leaded petrol, industrial effluents, and leaching of metal ions from the soil into lakes and rivers by acid rain. Other sources of heavy metal pollutants are metal mining, metal smelting, metallurgical industries, waste disposal, and corrosion of metals in use, agriculture and forestry. Heavy metals contamination affects large areas world-wide, but hot spots of pollution are locations close to the landfills, industrial sites, around large cities and in the vicinity of mining and smelting plants. This makes Agriculture within the areas to face major problems due to heavy metal transfer into crops and subsequently into the food chain.

The most important heavy metals with regard to potential hazards and occurrence in contaminated soils are:- Arsenic (As), Cadmium (Cd), Chromium (Cr), Mercury (Hg), Lead (Pb), Zinc (Zn), Copper (Cu), Selenium (Se), Nickel (Ni), Silver (Ag). Others that are less common metallic contaminants include Aluminium (Al), Caesium (Cs), Cobalt (Co), Manganese (Mn), Molybdenum (Mo), Strontium (Sr), and Uranuim (U).

Beryllium (Be) and aluminium (Al) which is light metals sometimes are counted as heavy metals in view of their toxicity. Exposure to Beryllium (Be) can result in lung and heart disorder and possibly death while aluminium is a major inhibitor of crop growth in acid soils.

The most prevalent heavy metal known so far is Lead. Lead, is part of the component of tetra-ethyl lead that is extensively used in gasoline. Lead levels in the aquatic environments of industrialized societies have been estimated to be two to three times those of pre-industrialized areas. Although the use of leaded gasoline is largely been phased out in some advanced countries, but soils within the roads still retain high concentrations of lead.

BIOACCUMULATION OF HEAVY METALS

Bioaccumulation is the accumulation of substances or chemicals in an organism. There are small numbers of plants that easily absorb high levels of metals from the surrounding soil and other organisms that feed on plants which already has absorbed the metals and soil surroundings. These are called hyperaccumulators and accumulators respectively. If these plants and animals are harvested or collected for human use, exposure to harmful levels of metals can occur. This is normally concerned only to organisms collected from arrears with high concentration of the metals in the soil.

The uptake of metals by plants is dependent on the acidity of the soil (_PH). The higher the acidity, the more soluble and mobile the metals become, and the more likely they are to be taken up and accumulated in organisms.

In general, humans are more likely to be exposed to metal contamination from soil that sticks to plants and animals than from bioaccumulation. This is because it is very difficult to wash all soil particles off of plant and animal materials before preparing and ingesting those.

Organisms that feed and grow near the soil as well as the plant parts that have closer contact to the soil are at higher risk for exposure to metal contamination than the higher portions of plants and animals that stay further away. Animals can accumulate metals by eating plants, fish, soil, or drinking water with elevated metal concentration. These metals are not excreted by animals; rather, they accumulate mostly in organs as well as skin, hair, and bones.

EFFECTS OF HEAVY METALS ON LIVING ORGANISMS

Varying amount of heavy metals is required by living organisms. Iron, cobalt, copper, manganese, molybdenum, and zinc are required by humans. All metals are toxic at high concentrations. Excessive levels can be damaging to the organism. Other heavy metals such as mercury, plutonium, and lead are toxic metals and are of no importance or beneficial effect on organisms, and their accumulation over time in the bodies of animals can cause serious illness. The types of heavy metals and their effect on human health with their permissible limits are enumerated in the table below.

Permissible limits of heavy metals in respect to food

POLLUTANTS	MAJOR SOURCES	EFFECT ON HUMAN HEALTH	PERMISSIBLE LEVEL (mg/kg)
Arsenic	Pesticides, fungicides, metal smelters	Bronchitis, dermatitis, poisoning	0.02
Cadmium	Welding, electroplating, pesticide, fertilizer, Cd and Ni batteries, nuclear fission plant	Renal dysfunction, lung diseases, lung cancer, bone defects (Osteomalacia, Osteoporosis), increase blood pressure, kidney damage, bronchitis, gastrointestinal disorder, bone marrow, cancer	0.06
Lead	Paint, pesticide, smoking, automobile emission, mining, burning of coal	Mental retardation in children, development delay, fatal infant encephalopathy, congenital paralysis, sensor neural deafness and, acute or chronic damage to the nervous system, epilepticus, liver, kidney, gastrointestinal damage	0.1
Manganese	Welding, fuel addition, ferromanganese production	Inhalation or contact causes damage to central nervous system	0.26
Mercury	Pesticides, batteries, paper industry	Tremors, gingivitis, minor psychological changes, acrodynia characterized by pink hands and feet, spontaneous abortion, damage to nervous system, protoplasm poisoning	0.01
Zinc	Refineries, brass manufacture, metal plating, plumbing	Zinc fumes have corrosive effect on skin, cause damage to nervous membrane	15
Chromium	Mines, mineral sources	Damage to the nervous system, fatigue, irritability	0.05
Copper	Mining, pesticide production, chemical industry, metal piping	Anaemia, liver and kidney damage, stomach and intestinal irritation	0.1

 (Source: Singh et al., 2011)

Heavy metals disrupt metabolic functions in two ways:

·       They accumulate and thereby disrupt function in vital organs and glands such as the heart, brain, kidneys, bone, liver, etc.

·       They displace the vital nutritional minerals from their original place, thereby, hindering their biological function. It is, however, impossible to live in an environment free of heavy metals. There are many ways by which these toxins can be introduced into the body such as consumption of foods, beverages, skin exposure, and the inhaled air.

EFFECT OF HEAVY METALS IN THE ENVIRONMENT

Metal concentrate in soil typically ranges from less than one to as high as 100,000 mg/kg. Heavy metals are the main group of inorganic contaminants. Considerable large areas of land is contaminated with heavy metals due to use of sludge or municipal compost, pesticides, fertilisers, and emission from municipal wastes incinerates, exudates, residues from metalliferous mines and smelting industries.

Irrespective of origin of the metals in the soil, consequential effect of its high level occurrence can lead to soil quality degradation, crop yield reduction, and poor quality of agricultural products, posing significant hazards to human, animal, and ecosystem health.

BIOTOXICITY OF HEAVY METALS AND ITS POISONING

Heavy metals consumed beyond the bio-recommended limits become harmful and show some effect to life, this phenomenon is referred to as biotoxicity. Although individual metals exhibit specific signs of their toxicity, the following have been reported as general signs associated with cadmium, lead, arsenic, mercury, zinc, copper and aluminium poisoning: Gastrointestinal (GI) disorders, diarrhoea, stomatitis, tremor, hemoglobinuria causing a rust-red colour to stool, ataxia, paralysis, vomiting and convulsion, depression, and pneumonia when volatile vapours and fumes are inhaled. The nature of effects could be toxic (acute, chronic or sub-chronic), neurotoxic, carcinogenic, mutagenic or erotogenic.

Lead is the most significant toxin of the heavy metals, and the inorganic forms are absorbed through ingestion by food and water, and inhalation. A notably serious effect of lead toxicity is its teratogenic effect. Lead poisoning also causes inhibition of the synthesis of haemoglobin; dysfunctions in the kidneys, joints and reproductive systems, cardiovascular system and acute and chronic damage to the central nervous systems (CNS) and peripheral nervous system (PNS). Other effects include damage to the gastrointestinal tract (GIT) and urinary tract resulting in bloody urine, neurological disorder and can cause severe and permanent brain damage. While inorganic forms of lead, typically affect the CNS, PNS, GIT and other biosystems, organic forms predominantly affect the CNS. Lead affects children by leading to the poor development of the grey matter of the brain, thereby resulting in poor intelligence quotient (IQ). Its absorption in the body is enhanced by Ca and Zn deficiencies. Acute and chronic effects of lead result in psychosis.

Cadmium is toxic at extremely low levels. In humans, long term exposure results in renal dysfunction, characterized by tubular proteinuria. High exposure can lead to obstructive lung disease, cadmium pneumonitis, resulting from inhaled dusts and fumes. It is characterized by chest pain, cough with foamy and bloody sputum, and death of the lining of the lung tissues because of excessive accumulation of watery fluids. Cadmium is also associated with bone defects, viz; osteomalacia, osteoporosis and spontaneous fractures, increased blood pressure and myocardic dysfunctions. Depending on the severity of exposure, the symptoms of effects include nausea, vomiting, abdominal cramps, dyspnea and muscular weak-ness. Severe exposure may result in pulmonary odema and death. Pulmonary effects (emphysema, bronchiolitis and alveolitis) and renal effects may occur following sub-chronic inhalation exposure to cadmium and its compounds.

Zinc has been reported to cause the same signs of illness as does lead, and can easily be mistakenly diagnosed as lead poisoning. Zinc is considered to be relatively non-toxic, especially if taken orally. However, excess amount can cause system dysfunctions that result in impairment of growth and reproduction. The clinical signs of zinc toxicosis have been reported as vomiting, diarrhoea, bloody urine, icterus (yellow mucus membrane), liver failure, kidney failure and anaemia.

Mercury is toxic and has no known function in human biochemistry and physiology. Inorganic forms of mercury cause spontaneous abortion, congenital malformation and GI disorders (like corrosive esophagitis and hematochezia). Poisoning by its organic forms, which include monomethyl and dimenthylmecury presents with erthism (an abnormal irritation or sensitivity of an organ or body part to stimulation), acrdynia (pink disease, which is characterised by rash and desquamation of the hands and feet), gingivitis, stomatitis, neurological disorders, total damage to the brain and CNS and are also associated with congenital malformation.

Just like lead and mercury, arsenic toxicity symptoms depend on the chemical form ingested. Arsenic acts to coagulate protein, forms complexes with coenzymes and inhibits the production of adenosine triphosphate (ATP) during respiration. It is possibly carcinogenic in compounds of all its oxidation states and high level exposure can cause death. Arsenic toxicity also presents a disorder, which is similar to and often confused with Guillain-Barre syndrome, an anti-immune disorder that occurs when the body’s immune system mistakenly attacks part of the PNS, resulting in nerve inflammation that causes muscle weakness.

Copper though plays a vital role in facilitating the uptake of iron, but its deficiency can produce anaemia like symptoms, neutropenia, bone abnormalities, hypopigmentation, and impaired growth, increased incidence of infections, osteoporosis, hyperthyroidism, and abnormalities in glucose and cholesterol metabolism. Severe deficiency can possibly be found by testing for low plasma or serum copper levels, low ceruloplasmin, and low red blood cell superoxide dismutase levels, although, these are not sensitive to marginal copper statue. Copper salts are known to be toxic to humans possibly due to redox cycling and generation of reactive oxygen species that damage DNA. Corresponding amount of copper salts (30 mgkg-¹) are toxic to animals. Chronic copper toxicity does not normally occur in humans because of transport systems that regulate absorption and excretion.

Cobalt is an essential element for life when in minute quantity which is estimated to be between 150 and 500 mgkg^-1. However, chronic cobalt ingestion has caused serious health problems at doses far less than the lethal dose. Cobalt compound has led to peculiar form of toxin induced cardiomyopathy.

Manganese has a greater bioavailability in water than in diet. Higher levels of exposure to manganese in drinking water increases intellectual impairment and reduced intelligent quotients in school age children. Manganism is a rare neurological disorder associated with excessive manganese ingestion or inhalation. People at higher risk of exposure are workers are workers at manganese alloy production/processing plants. Manganism has a biphase disorder with early stage resulting to depression, mood swings, compulsive behaviour and psychosis if intoxicated. This early neurological symptoms give way to late stage manganism that resembles Parkinson’s disease. Symptoms include weakness, monotone and slow speech, an expressionless face, tremor, forward leaning gait, inability to walk backward without falling, rigidity and general problems with dexterity, gait and balance.

Iron toxicity occurs when there is free iron in the cell and it generally occurs when iron levels exceed the capacity of transferring to bind the iron. Iron when ingested in large amounts can cause excessive levels of iron in the blood thus reacting with peroxides to produce free radicals that are highly reactive and damages the DNA, proteins, lipids and other cellular components. Damage to the gastrointestinal tract prevents the regulated iron absorption leading to further increase in blood levels. Iron typically damages cells in the heart, liver, and elsewhere, which can cause significant adverse effects, including coma, metabolic acidosis, shock, liver failure, coagulopathy, adult respiratory distress syndrome, long time organ damage, and even death.

Nickel sulphide fume and dust are believed carcinogenic and various other nickel compounds may be as well. Nickel carbonyl (Ni (CO)₄) is an extremely toxic compound of nickel in gaseous state. The minimal risk level of nickel and its compounds is set to 0.0002 mgm^-3 for inhalation during 15-364 days while the tolerable upper dietary limit is 1 mgday^-1 and average ingestion is 0.069-0.162 mgday^-1 in US. Individuals that are sensitive to nickel may show allergy affecting their skin known as dermatitis. It is an important cause of contact allergy which are often marked by itchy red skin, partly due to its use in jewellery.

The poisoning effects of heavy metals are due to their interference with the normal body biochemistry in normal metabolic processes. When ingested, in the acid medium of the stomach, they are converted to their stable oxidation states (Zn²⁺, Pb²⁺, Cd²⁺, As²⁺, As³⁺, Hg²⁺, and Ag⁺) and combine with the body’s biomolecules such as proteins and enzymes to form strong and stable chemical bonds.

CONCLUSION

There is growing concern on effects of heavy metals on human health and hence this has led to increase in research upon the flora and fauna upon which humans feed.

In my research, THE USE OF GIANT AFRICAN SNAILS (Archachatina maginata) AND (Achatina folica) AS BIO-INDICATORS OF HEAVY METAL POLLUTION, the results confirmed that:

1.      They were concentration of heavy metals in both the flesh and shell of the organisms for copper, zinc, manganese, lead, cadmium, etc. It is an indication that organisms must have contacted the heavy metals throws the developmental process of hatching of the deposited egg, crawling for food on soils loaded with heavy metals, and feeding on plants that have already accumulated the same metals in the cells and tissues.

2.      The concentration varied from region and metals accounting for the effect of spillage, emission of hydrocarbon and exploration within the area which has found itself into the food chain that can affect us human while consuming the snails.

3.    Some areas in the south east have results that show that pollution during exploration of solid minerals, industrial activities like welding, pollution via landfills, addition of fertilisers to the soil that sometimes leach or washed by run off contributes to the pollution and are contacted by absorption, body surface contact like the case of Snails.

4.    Again, though some of these metals are vital nutrients in the body but when they are found beyond the quantity that the body needs it, it obviously become toxic or harmful.

RECOMMENDATION

Our food should be properly washed to ensure that the soil remains on the food (organisms) we intend to ingest which may contain the metals are washed off.

Though these snails are processed (heating, cooking) before consumption, the effect of processing could be minimal, since the heavy metals are non-degradable. Hence, it is advised that the source of snail to be consumed must be scrutinized.

Also, land pollution should be minimized and effluents should be treated before discharge unto land.

Application of manure and fertilizers as well as emission of gases should be controlled in order to reduce intentional heavy metal application to the soil.

More proactive measures should be employed or policies to regulate exploration of minerals that often spills and pollute the environment.