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Vol. 22 No. 1 - January 2016

Heavy metals and food security

By: Ashita Sharma¹, Simran Kaur² and Avinash Kaur Nagpal¹*

Introduction

The Sustainable Development Goals (SDGs) as framed by the United Nations focus primarily on reducing global hunger and providing food for all. There has been an exponential rise in the chronically hungry population around the world with approximately 800 million people suffering from chronic undernourishment, according to UNFAO (United Nations Food and Agriculture Organization). With the world’s human population estimated to be reaching an enormous 9.1 billion in 2050 (UN data), the need to make substantial change in  the current agricultural systems to feed the already suffering hungry millions and the additional 2 billion people expected by 2050, is the clear and present challenge facing humanity. According to recent FAO projection, the agricultural outcome is expected to increase by a phenomenal 70% to feed the ever expanding human population. Food security, which includes continuous availability of “safe” food and its access to all, is a condition that poses a big problem not only for developing countries but for developed nations too. In the efforts to increase the global food production new agricultural practices (often wrong, short term solutions with long term negative consequences) are being introduced. To meet the land requirement for sufficient crop growth, shifting landscape policies are being adopted which again are not the best practices. The recent surge in the usage of urban, industrial or the polluted land, which is certainly not suitable for agriculture as it may lead to the addition of toxins to food-crops, is a practice that needs serious reconsideration. The growing irrigation demands and insufficient water availability as a result of change in rainfall patterns and decrease in water level is another serious problem challenging food security. To meet the irrigation demands, wastewater from industries is being used for crop production. Though this practice has increased crop output, but the use of industrial wastewaters or other similar inappropriate resources produce a crop, which generally cannot be considered as “safe” food. Various research investigations conducted world-over have found significant concentrations of heavy metals in crops cultivated through the use of such inappropriate resources beyond acceptable daily intake. It is quite evident that there can be an increase in crop production by adopting certain agricultural and irrigational practices (as a few mentioned above) but they don’t necessarily help in attaining global food security. According to a statement made by Dr. Manmohan Singh (former Prime Minister of India), “if I don’t have enough food in my country how can I think of saving the environment”, the issue of food security is nevertheless deeply linked with the environment and “safe food”.

Heavy Metals: What are they?

Naturally occurring metals with density higher than 3.5 g/cm³ are considered as heavy metals.  Some heavy metals are essential for normal metabolism of most living organisms, unfortunately, the term “heavy metals in soils” has almost become synonymous to “toxic elements in soil”. A few elements categorized as heavy metals and of concern are: mercury (Hg), cadmium (Cd), arsenic (As), chromium (Cr), thallium (Tl), Zinc (Zn) and lead (Pb). Mercury (density14.6 gcm^-3); lead (density 11.4 g cm^-3) and cadmium (density 8.65 g cm^-3) are known as the “big three” heavy metals due to their highly toxic behavior with no known essential biological function. Mercury, essentially released in the environment through mining, coal combustion and burning of fossil fuels, is popularly used in thermometers, dental fillings, and batteries. Lead is usually found in nature in combination with other elements like sulfur and oxygen and particularly used in construction of plumbing essentials, lead storage batteries and in an alloy form with other metals. Cadmium is predominant in all types of soil, rocks and released primarily as a by-product of lead refining. It is used extensively in batteries, utensil coating, PVC and many electrical components. Select heavy metals are also known to move along with fertilizers, detergents and many petroleum products as an impurity.

Sources

Heavy metals exist naturally in the Earth’s core. Due to natural weathering processes and catastrophic events like floods, tsunamis and volcanoes these metals accumulate and increase in concentration in the biospheric zone as well.  Anthropogenic activities that lead to soil disturbances like mining, drilling and extraction of fossil fuels, leakage through discarded products in improperly protected landfills, use of leaded gasoline and lead based paints, indiscriminate use of fertilizers, mismanagement of industrial wastes, combustion of fossil fuels, metallurgy technology, electroplating and production of sewage sludge and industrial and automotive emissions (to name a few among innumerable practices adopted by humans to sustain civilization) further increase the concentration of heavy metals in biosphere beyond the general background level. These metals are then translocated between biotic and abiotic components of any ecosystem and undergo various types of chemical transformations into different ionic states. Soil pH and oxidation conditions, microbial activity, temperature fluctuations and various other environmental geochemical factors are responsible for this heavy metal speciation

Table 1 provides a brief description of sources that contribute to the increase in the concentration of heavy metals in soil.

 Heavy Metal Toxicity

Soil contamination from various sources (as identified above) leads to an imbalance between essential metals (required as micronutrients for normal cellular functioning) and non-essential metals that are harmful for life. Trace elements like Zinc, Chromium, Boron, Copper, Manganese, Iron, Molybdenum, Selenium and Cobalt are required for normal functioning of plants. Boron aids carbohydrate transport while manganese, iron and zinc participate in chlorophyll formation. Deficiency of copper and iron leads to chlorosis in plants. Natural metabolic processes allow heavy metals present in biosphere to enter the food chain through producers. Once absorbed, they exhibit toxicity at all trophic levels and thus their increasing and persistent presence in soil is of concern. Absorption of heavy metals from soil or water is through the root zone and from air through stomata (openings in the epidermis of leaf tissue). Uptake of metals is related to size, concentration, mobility and its ionic state. The toxicity associated with heavy metals can be classified as both physical and chemical toxicity. Dependent upon size of metal ion it can reach up to the organ, tissue and cellular level. Physical toxicity is the result of clogging of cellular channels, which in turn inhibits water transport. Chemical toxicity results due to binding of heavy metals with essential biomolecules leading to inhibition of catalytic activities, disruption in cell membrane functions, damage to nucleic acid structure and production of reactive oxygen species. Toxicological effects of heavy metals are known at all trophic levels and shown in table 2.

 Human-The Consumer:

Humans are continuously exposed to heavy metals through the food we ingest, air we inhale or through the pollutants we touch. The Minamata Bay (Japan) tragedy that involved toxicity due to high mercury content in food resulted in neurological disorders and death of large number of people. Mercury was slowly creeping (from 1932-1968) through industrial wastewaters into marine ecosystems and accumulating in shellfish popularly consumed by local communities living around the bay.  The astronomical accumulation of arsenic (due to natural geochemical processes) in the ground waters of the Hoogly region (West Bengal, India) is linked to number of cases of arsenic toxicity in the eastern states. Seafood is staple diet for most of West Bengal and arsenic in the food chain is reason for much concern. It is reported to cause skin damage, decrease in production of white blood cells and irregular heart rhythm. Cadmium, another toxic metal, considered as potential carcinogen by US Environmental Protection Agency is used in many agriculture fertilizers. The consumption of this metal with food crops leads to renal and gastrointestinal failures. The chemical homology of cadmium with zinc and copper ions has been reported to hinder metabolic functioning leading to breathing problems and recently cadmium has been linked to degenerative diseases like lung and stomach cancer.

In India, the practice of using industrial waste waters for irrigation purposes and growing food crops on land that is not appropriate for agriculture has increased the incidence of exposure of Indians to toxic levels of soil heavy metals. The hazard quotient calculated on the basis of concentration of metal ingested through food and body weight of individual, exceeded the acceptable limit in food crops from many areas in India, including, Varanasi, Andhra Pradesh, Rajasthan and many other parts of north India. The main source of heavy metals in these food crops has been reported to be wastewater irrigation and cultivation on contaminated land.

Though generating sufficient food to satisfy global hunger is the major issue facing both developed and developing nations, using inappropriate resources and practices to do so is not the solution. Moreover anthropogenic activities which have led to major disturbances in the delicate elemental and mineral composition of soil is not only harming human health but is proven to be detrimental to the well-being of all forms of life inhabiting Earth.

Remedial Measures:

The reduction in heavy metals in food chain for attaining food security can be a two-pronged approach. This is discussed as follows.

1) The prevention of further deterioration of land by using the following practices:

• Use of treated waste-water (if used for irrigation purposes).

• Prohibiting use of agricultural chemicals with heavy metal content.

• Sealing landfill sites to prevent the leaching of heavy metals in surrounding areas.

• Management of solid waste (municipal, household, industrial, biomedical etc.)

2) Make amendments to soil: Amendments (change or addition) to soil can be done to restore agricultural land in order to improve its physico-chemical properties. Amendments reduce the mobility and availability of these metals and in turn decrease the transfer of metals to plants and food chain. Mobility can be decreased due to pH alterations, chemical bonding of metal ions with substrate to form less mobile complex or by physical adsorption of metal ions on substrate. Both inorganic chemicals and organic products are used as soil amendments. 

• Inorganic amendments:

Inorganic amendments include addition of calcium, magnesium, phosphates, and nitrates to the soil. These amendments are known to increase the soil pH thus decreasing the mobility of metals. Apart from alteration in pH, certain amendments also bind with the metal ions. Limestone when added to soil releases carbonate ions (CO₃^2-) in the presence of CO₂. These CO₃^2- ions can then combine with certain heavy metals forming stable compounds, for example, CuCO₃ and CdCO₃.

• Organic amendments:

The amendments like vermicompost, peat, solid waste, biochar and plant residues have significant effect in lowering the concentration of heavy metals in crops. These amendments act as substrate decreasing the heavy metal mobility due to adsorption or precipitation of metals on these particles. They are also known to alter the pH of soil further decreasing the heavy metal mobility.

Human population growth is a threat to food security. To meet the enormous task of feeding the citizens of their countries, nations are implementing inappropriate agricultural and irrigational practices that are inadvertently disrupting the delicate elemental composition of soil. The disastrous situation of continuous buildup of heavy metals in food crop farms needs to be addressed. As explained earlier, either irrigational malpractice should be stopped immediately or low cost amendments should be adopted for the reclamation of land so as to somewhat control the mobility of heavy metals up the food chain. Use of such amendments not only helps in reclamation of land but also proves to be an effective strategy of waste management.

¹Department of Botanical and Environmental Sciences, Guru Nanak Dev University, Amritsar-143005, Punjab, India.

²Department of Biological Sciences and Geology, Queensborough Community College, Bayside, New York - 11364, USA, *E-mail: [email protected]

This article has been reproduced from the archives of EnviroNews - Newsletter of ISEB India.