Lead on the Plate: Phytoremediation a Promising Approach
By: Chandra Shekhar Seth*
How heavy metals can be defined?
In light of no general and exact definition of heavy metals, it is defined as those chemical elements having specific gravity at least 5 times than those of water. There are 53 metals with a density above 4-5 g/cm3 reported as heavy metals, but it's better to consider them from their physiological effects and toxicity. At high concentrations, metals form unspecific complex compounds in the cell, which lead to various toxic effects. As per the various definitions, lead comes under heavy metals category with a wide range of toxicity to all life forms. Heavy metals are categorized into three groups based on their toxicity impacts (i) Less toxicity: Fe, Mo, Mn (ii) Average toxicity Zn, Ni, Cu, V, Co, W, Cr and (iii) High toxicity As, Ag, Sb, Cd, Hg, Pb, U.
Sources of lead contamination to environment:
Heavy metals including lead come in the environment from the natural, but anthropogenic activities are the major cause for their pollution. The major Pb releasing sources to environment are categorized into aerial emissions from combustion of leaded fuel, batteries waste, insecticide and herbicides. According to the latest report of Central Pollution Control Board (CPCB) New Delhi, the prevailing Pb contaminated areas in India are Ratlam (M.P.), Korba (Chhatisgarh), Vadodara (Gujrat) and Bandalamottu Mines (Andhra Pradesh).
As per the current UK air quality standards, 2.5 ug/m3 lead is present in air, which is equivalent to 0.3 parts per billion (ppb) if present in elemental form. Earlier in the 20th century, there has been a dramatic rise in the release of lead in the atmosphere because of the widespread use of leaded petrol (tetraethyl lead). This can increases the octane number while increases engine performance but now a day government of many countries found other alternative ways and put some stringent laws to curb its usage so that a remarkable drop in lead pollution can be achieved.
How passage of Pb into environment is a historical problem?
Lead is present all around us comprising old buildings, soil, workplaces and in thousands of products being used in our everyday life. Some common sources of lead poisoning are at home, where people, especially children, unknowingly swallow and breathe fine particles of dust emanating from lead paint, toys and fumes from lead-acid batteries. Lead damages children's developing brains and nervous system, and the damage caused is almost permanent because our neurons lack the ability of regeneration. Lead enters in the environment long back as historical manner where hunters in old days shoot animals with lead shot and leave the carcass without recovering the dead or injured animals, that shot having toxic lead is ultimately ingested by other wildlife animals. This lead is being magnified as it passes up the food chain, and the top predators, especially vultures, eagles, hawks, get poisoned with lead and many states now banned the use of lead shots and made stringent guidelines for use of steel shot.
Secret of Pb entry into human body and its consequences:
Iron deficiency in our food is one of the major factor that helps in enhanced Pb toxicity, which raises concern that pregnant women and young children in whom chances of iron deficiency are high, may be more susceptible to Pb toxicity. The Pb absorption also increased considerably with fasting or in those persons whose diet is deficient in calcium, iron, phosphorous or zinc. Absorption of Pb from different sources is dependent on the amount of Pb and the physical and chemical state in which Pb is present and is also influenced by other factors such as age and physiological condition of the body. In adults, almost 30% and in children almost 50% lead is absorbed through the Gastro-Intestinal tract. Depending on its particle size, lead can also enter through lungs. Organic lead can be easily absorbed through the skin but inorganic lead is generally not. Since lead is chemically similar to calcium present inside our body therefore it is treated like calcium and thus lead will be distributed throughout teeth, liver, lungs, brain; bone (a major place for Pb accumulator).
Blood lead level (BLL) > 10 μg dl-1 is considered as poisoning in children. Recent studies from Hyderabad also shows the abnormal cognitive functions in children at levels > 10 μg dl-1. Generally, Pb is excreted very slowly from the body and its biological half-life is estimated at 10 years, facilitates accumulation in the fatty tissues of the body. Almost 90% lead is bound to red blood cells, shows high affinity for thiols (-SH) groups and hence impairs the activity of many enzymes. Lead can also cross blood-brain barrier and placental barrier to harm the fetus growth and development, however its excretion occurs through urine and faeces.
Maggi noodles contain lead: a Recent Controversy
Maggi noodles are a popular and favorite choice of many children and if lead present in their food, then certainly it has a very severe effects on their mental as well as physical development. The recent controversy arises in light of alarming reports of Food Safety Regulators from Uttar Pradesh found that samples of Maggi 2 minutes noodles contain traces of lead beyond permissible limits and added monosodium glutamate. Permissible amount of lead content in any food material is 0.01 ppm while the Maggi noodles samples were found to contain 17 ppm by the FSDA, Lucknow. Monosodium glutamate are flavor enhancing compounds which provide a savory taste to food but excess intake can cause headaches and other feelings of discomfort collectively known as Chinese Restaurant Syndrome. Following the reports of FSDA, Lucknow, on June 3, 2015, New Delhi Government banned the sale of Maggi in New Delhi state for 15 days; some of India’s biggest retailers like Future Group, Big Bazaar, Easyday and Nilgiris imposed a nationwide ban on Maggi. Army issued advisory to its personnel asking them not to eat Maggi noodles; directed its canteens not to sell till further orders. Food Safety and Standards Authority of India collected samples of Maggi Noodles from all states for testing while Union Government referred the Maggi issue to National Consumer Disputes Redressal Commission to take appropriate action.
How soil health Impacted by Lead Pollution?
Soil is an important part of our life where food production begins and for proper food production, it should remain in substantial healthy conditions. Soil health can be defined as its capacity to act as a living system which can support diverse community of its organisms from bacteria, fungi, earthworms, insects, etc that help to control plant diseases, pests and weeds by forming beneficial symbiotic associations with plants. The soil health is greatly deterioted by the uncontrolled release of lead along with the other toxic agents. The major route in which the soil can became polluted are given below
Rupture/leakage of underground storage tanks
Discharge of untreated industrial wastes directly into the soil
Percolation of contaminated waste water into the soil
Seepage from a landfill
Excess spraying of pesticides and other fertilizers
Phytoremediation: A promising remediation approach
There are various methods to remove the pollutants from soil which are categorized into three main sections: Chemical, Physical and Biological methods. While conventional methods of soil clean-up including containment, solidification, air-stripping, vitrification, pump-and-treat, electro-kinetic, excavation, soil washing, etc. have proven to be effective in small areas but need special equipment’s and are very labor intensive. However, due to the side effects and highly costs of physical and chemical techniques, the biological methods especially phytoremediation, seems to be promising alternative techniques to traditional methodologies. Phytoremediation is defined as an eco-friendly, cheap and large scale method which implies plants and their associated microorganisms to degrade, reduce and/or remove organic and inorganic pollutants from the soil. Although phytoremediation is a “green technology” has shown many positive results by saving money and energy but often it takes longer time to reach desired remediation levels and it also encompasses the problem of biotic and abiotic stresses. Looking on the cost effective and eco-friendly remedial measure, its application cannot be ignored. Phytoremediation practices can be achieved in several ways including phytoextraction, phytomining, rhizofiltration, phytostabilization, phytodegradation, phytovolatilization and phytorestoration.
It is the process where heavy metals can be removed by plant roots with subsequent transport to shoots. Plants which can grow in heavy metal contaminated soils are categorized as “tolerant”, “indicators” and “hyperaccumulators”. A tolerant species can grow in contaminated soils while other plants cannot. For indicator species, there is a linear correlation between metal concentration in growth media and plant tissues. Hyperaccumulators have a high potential to uptake and accumulate heavy metals which may be more than 100 fold in comparison with other common plants. Some famous examples are Indian mustard, Duckweed, Sunflower, Hydrilla, etc.
This technique implies growing of suitable plant species in a metal contaminated area. After harvesting the biomass, it is burnt and finally bio-ores obtained which are further processed to extract the precious metal for its use in different socio-economical benefits.
It is a key process for removal of heavy metals from aquatic environments using suitable aquatic plants which could accumulate metals in their roots and shoots.
In this process, rather than cleaning pollutants from the soil, plants are used to stabilize metals in their root parts so that they can be prevented for their movement to surface and/or groundwater.
5) Phytodegradation and rhizosphere-enhanced degradation:
Plant enzymes are substantially being used for degradation of organic pollutants, which can be easily entered into the plant tissues or in the rhizosphere. The similar phenomenon occurred in plant rhizosphere by enhancing the activity of some degrading microorganisms through the release of root exudates, called as rhizosphere-enhanced degradation, which in fact is achieved by microbial enzymes rather than plant enzymes.
Some of the soluble contaminants of soil are taken up by the roots, transported to the leaves, and volatized into the atmosphere through the stomata. Certain heavy metals like mercury and arsenic get inactivated once translocated from the soil into the atmosphere by bonding to free radicals present in the air.
It is a complete remediation of contaminated soil to fully functional and healthy soil which is an attempt to return the land to its original natural state.
Natural environment has been severely polluted in the form of heavy metals, inorganic and organic toxicants by unprecedented human activities, industrialization, population growth, urbanization etc. Several organic pollutants, such as polycyclic aromatic hydrocarbons, polychlorinated aromatic compounds and nitrogen containing aromatic compounds are resistant to degradation and represent an ongoing toxicological threat to both wildlife and human beings. Phytoremediation is an eco-friendly and cost effective alternative to traditional physico-chemical techniques for the remediation of these hazardous toxicants from contaminated sites. It can selectively degrade the pollutants without damaging the soil physic-chemical properties as well as its micro-flora.
*Assistant Professor, Department of Botany, University of Delhi-110007, India <firstname.lastname@example.org>