Heavy metals are a group of metals that have a density greater than 4g/cm3 that is many times more than water. They have more atomic number, atomic weight and have relatively high density.
They become hazardous when they exceed the optimum required limit in the consequences of anthropogenic activities. They found naturally and are not destroyed by biological methods. Fe, Cu, Zn are such heavy metals that are essential for plant growth but when they exceed from their limited required amount then they will cause toxicity and may stunt the growth
Hazards of Heavy Metals
- If Cu exceeds its permitted limit then it will be toxic for living cells of plant, retard root growth, and reduce seed production and mass of plant and chlorosis of leaves.
- Similarly, Zn applied beyond its limit then it will cause a reduction in the growth, induce stress conditions, altering the enzymatic efficiency and also can inhibit the metabolic activities of the plant.
- Cadmium reduces the length of seedling and affects dry weight, minimizes the photosynthetic activity, respiration in leaves become low, reduce conductance through stomata and affect the structure of chloroplast. Similarly, other micronutrients when exceed their limit they will cause many types of problems for plant growth and development. But relatively fewer amounts of Ni, Pb and Cd restrain the respiration and photosynthetic activities in sunflower.
Toxicity of Heavy Metals
Toxicity of each heavy metal is not the same for all types of plants it differs from plant to plant, concentration, pH and chemical composition of the soil. As we know that there is a shortage of water in Pakistan so mostly in urban areas sewage water is applied to irrigate crops and vegetables that are grown around the cities. Heavy metals show toxic and harmful effects because they are persistent in nature and do not degrade easily containing long half-lives and they have the ability to accumulate in the different parts of living organisms.
Many heavy metals show very extreme toxic effects because they are readily soluble in water. Very low amount of these heavy metals is very hazardous for humans and animals because they have not a system to get rid or remove them from their bodies similarly plants also have no mechanism for this purpose. In man, they damage the nervous system directly and disturb the metabolic activities of the body. It also affects the reproduction, enzymatic activities, digestion and intestinal processes.
The entry of heavy metals in Food Chain (Bio accumulation &Bio magnifications)
As waste water contains many beneficial elements, inorganic material and organic matter may also present in sewage water that is good for better yield of crops and vegetables but many heavy metals that are harmful to human life also present. When we use waste water for irrigation then these heavy metals accumulate in the soil and uptake by plants. Bio accumulation of these heavy metals occurs in plants and this leads to the bio magnifications when humans and animals consume these crops and vegetables for the purpose of food in this way they enter the food chain. In agriculture during irrigation by waste water from industries and domestic use just soil is not polluted it also affects the quality of food, its safety and security.
Permissible amount of elements like Ni, Fe, Mn etc is fairly low but in waste water, they exceed from their optimum limit so then they show toxic effects. Heavy metals cause toxicity in plants is the result of the formation of complexes of heavy metals with the ions of plants that may be essential or not. Some researches show that there is less accumulation of them in soil as compared to the vegetables that are grown by the irrigation of waste water and they accumulate more in leaves as compared to fruit. It reveals that may be plats absorb them directly from waste water. There also some ways by which heavy metals accumulate in soil that are mining and smelting, fossil fuel burning, the formation of batteries and the use of chemicals to prevent pest and insect attack and reduce the growth of fungi in agriculture.
Remediation of heavy metals
There are a lot of threats of heavy metals for plants, humans and animals so there is a need to remediate them at least from the soil when they cause a problem for any type of life. There are many biological, physical and chemical methods for this purpose. But the biological method is most encouraging method because this is environment-friendly and attained by using biological methods.
Treatment of contaminated soil with the help of plants or microbes is called bioremediation. It is a more valid method because this is done by natural processes and not expensive. It is estimated that 65% of savings can be attained to remediate the one-acre soil that is contaminated by Pb. During bioremediation heavy metals cannot degrade but change occurs in their oxidation state. Heavy metals become less toxic, easily change in vapors and their bioavailability also reduce due to change in their oxidation states. This is done by microbes, plants or by the combination of both.
- Bioremediation by bacteria
Bacteria mostly used for the purpose of bioremediation. A more toxic form of Cr is converted to less toxic Cr with the action of bacteria. Some bacteria produce siderophore that form complexes with heavy metals so then they are not available to the plants. Some bacteria reduce sulfate into hydrogen sulfate and react with heavy metals to produce their insoluble forms this is called precipitation. There is also some Hg reducing bacteria that convert Hg into a more volatilize form that also helps in bioremediation.
- In phytoremediation plants are used to recover the soil from the contamination of heavy metals. This is most effective when we want to treat the wide-area and pollutants have reached to the root zone.
- A method of phytoextraction is used for this purpose. In this technique plants are grown in polluted soil and all the heavy metals are accumulated in the shoot and root of that plant after that harvested these plants and burn them. Plants use for this purpose having high biomass, rapidly growing and they have the ability to bear heavy metals.
- Another process is phytostabilization that is used when phytoextraction is not applicable or impossible. This is more useful when soil is heavily contaminated with heavy metals because this method immobilizes them. The efficiency of this method depends on amendments that are used for plant and soil. Due to these factors, this is important to select the suitable plants for this purpose. Organic materials and other amendments are used to inoperative the heavy metals. Amendments that are used for phytostabilization are litter, lime, and biosolids.
- Another one is phytovolatilization in which pollutants and heavy metals are uptake by plants and then change them into vapors and transpire them from the surface of the plant to the environment. This is mostly used for those soils that are contaminated due to Hg and this method coverts more toxic Hg into less toxic. But its drawback is that volatilize new products of Hg may be condensed and redeposit into lacks or on the soil.
If we use both plants and microorganisms for the treatment of polluted soil at the same time then the remediation process will be faster and more productive. The accomplishment of combined use of both microbes and plants relies on the type of species of plants and microorganisms and also on the number of heavy metals in soil.
Physiological changes occur in plants that grow on soil that is contaminated by heavy metals. They affect photosynthetic pigments and the growth of plants. So, there is need to eliminate them from the soil that can be done with the help of hyperaccumulators. Moreover, in humans, it damages the functioning organs like the liver, kidneys and brain. Some plants have the ability to uptake these heavy metals and store them in an inedible portion. When these plant portions come into use of humans or animals then it will cause some serious diseases like cancer or other bone diseases.