Heavy Metals in the Environment: Impact, Assessment, and Remediation

By Vinod Kumar

Heavy Metals in the Environment: Impact, Assessment, and Remediation synthesizes both fundamental concepts of heavy metal pollutants and state-of-the-art techniques and technologies for assessment and remediation. The book discusses the sources, origin and health risk assessment of heavy metals as well as the application of GIS, remote sensing and multivariate techniques in the assessment of heavy metals. The various contamination indices like contamination factor, geoaccumulation index, enrichment factor, and pollution index ecological risk index are also included to provide further context on the state of heavy metals in the environment.

Covering a variety of approaches, techniques, and scenarios, this book is a key resource for environmental scientists and policymakers working to address environmental pollutants.

• Covers state-of-the-art techniques for the assessment and remediation of heavy metals
• Presents the interdisciplinary impacts of heavy metals, including human health, ecosystems and water quality
• Includes various contamination indices, such as contamination factor, geoaccumulation index, enrichment factor, pollution index and ecological risk index

• Language: English
• Publisher: Elsevier Science
• Release date: Nov 21, 2020
• ISBN: 9780128216606

Book preview

India

1

Heavy metals in the ecosystem: Sources and their effects

Mamta Pujari and Dhriti Kapoor,    School of Bioengineering and Biosciences, Lovely Professional University, Phagwara, India

Abstract

Heavy metals (HMs) are considered crucial toxicants of the environment as they are very toxic, tenacious, easy to accumulate, and nondegradable in nature. HM pollution include some natural sources such as breakdown of rocks possessing metals, eruption of volcanoes and certain human activities like the increase in urbanization, establishment of industries, and smelting and extraction processes. These anthropogenic activities and their consequent application in the sectors of industries, agriculture & economic growth lead to deterioration of biogeochemical cycles. HM pollution in different ecosystems like terrestrial or aquatic ecosystems is a major environmental hazard for people’s health. HMs pollute the food chain as they get deposited in the environment due to their persistent nature. Drastic health hazards are caused by the accretion of lethal HMs in biota that pose threats to each trophic level. This chapter expansively analyzes various facets of HMs as harmful substances with particular attention on their environmental existence, noxiousness, and biomagnification in organisms. Such toxic metals include Cu, Ni, As, Pb, Cd, and so forth, which transfer from one trophic level to another and thus remain persistent in the food chain and adversely affect the health of living beings. The negative effects of HMs must be evaluated in context with its doses, which may lead to enhancement in toxicity in the environment. Certain strategies must be incorporated to combat the adverse effects of HMs that may pose threats to the health of living beings and the environment.

Keywords

Heavy metals; sources; toxicity; bioaccumulation

1.1 Introduction

Toxicity caused by heavy metals (HMs) may spoil vegetables, which cannot be ignored as vegetables contribute a very important part of our dietary requirements. Nutrient contents of vegetables like fibers, minerals, vitamins and antioxidants contribute in combating the adverse effects of environmental stress (Sharma et al., 2008XXX). Consumption of vegetables exposed to HM toxicity may lead to damaging effects on the health of living beings. Food contamination due to heavy metals is the major facet that should be focused as HMs are considered as main environmental toxicant and these cannot be degraded and are perishable in nature. These metals get adsorbed on the surfaces and tissues so that vegetables easily accumulate them. Large amounts of fertilizers that contain certain toxins are used in agricultural practices and hence negatively influence plant growth (Schroeder and Balassa, 1963XXX). In certain developing and developed nations, doses of metals have been observed and evaluated (Sharma et al., 2009XXX). Serious threats and health risks are caused to living organisms when subjected to HMs like Pb, Cu, As, Cr, Zn, and others, in their surrounding environment. Deposition of HMs in soil is largely due to the application of fertilizers, pesticides, and insecticides that are bioaccumulated by plants, hence affecting the quality of food, growth of plants, and are then transferred to further trophic levels (Fergusson, 1990XXX). In the geochemical cycle of HMs, transportation of these metals into the different spheres is due to the various anthropogenic processes and is therefore an important aspect to study. There are various steady and mobile factors that allow the release of HMs into the atmosphere including soil specifically in urban areas and nearby industries where there is an enhanced rate of their release (Bilos et al., 2001XXX).

HMs also lead to mutations in living organisms at very high rate and these metals also can trigger harmful effects at gene level. Long-term exposure to HMs and their bioaccumulation in the bodies of living organisms may cause deteriorating effects and it is difficult to eliminate such metals once they have been accumulated. HMs like As, Pb, Cd, Hg, and others are poisonous in nature and enhance toxicity in extended levels (Erisman et al., 2008XXX). Vegetables uptake the HMs from the soil that are deposited in the soil through agricultural practices, industrial effluents, and mining or smelting operations. Possibly these HMs also exist in food products, which are transported from soil and consumed by living beings. Prediction of level of these HMs in food stuffs is difficult and complicated as various factors must be taken into account, such as the different characteristics of the soil and the physiological conditions of the plants.

HMs are those metals that have an atomic density of more than 5 g/cm³ and are highly toxic even at low doses (Garbarino et al., 1995XXX). HMs are categorized on the basis of their influence on the physiological processes of plants (Duruibe et al., 2007XXX). HMs are classified into essential and nonessential HMs, where Cu, Fe, Mg, and Zn are considered as essential and Cr, Ni, Pb, and As are nonessential metals and have deleterious effects on plants. The specific HM unveils particular emblems of noxiousness, these have been observed as overall marks supplemented with Pb, Hg, Zn, As, and so forth.

1.2 Metal and their essentiality for life

The chemical definition of metals states that metals are elements that have the capacity to conduct electricity, are lustrous, have malleability and ductility, can form cations, and form alkaline oxides. Metals play an important role in society and have various applications in human life. Some critical physiological and biological functions are performed by some metals in biological systems and it has been seen that either the deficiency or excess of metals lead to metabolism problems, and as a result of this, to a number of diseases. Some essential metals and metalloids play physiologically and biochemically important roles in the human body as they are a part of biomolecules like enzymes that help in catalyzing biochemical reactions in the human body.

Csuros and Csuros define a heavy metal as a metal having a density greater than 5 g/cm³ (i.e., specific gravity greater than 5). Duffus concluded that the term heavy metals is generally used for metals and semimetals or metalloids as a group name which are associated with contamination and ecotoxicity (Duffus, 2002XXX).

1.3 Sources of heavy metals in the environment

In the environment heavy metals are natural, geogenic, lithogenic, or anthropogenic. Weathering of metal-bearing rocks and volcanic eruptions are natural sources of heavy metals. The anthropogenic share of heavy metals in the environment has increased due to the global level of industrialization and urbanization.

Heavy metals are found in insoluble forms in mineral structures or are precipitated in complex forms that are not readily available for uptake in plants. A great absorption capacity is shown by naturally occurring heavy metals in soil therefore these are not readily available for living organisms. Compared with anthropogenic sources, naturally occurring heavy metals show higher bonding energies. Comets, erosions, volcanic eruptions, and weathering of minerals are examples of natural processes that cause the presence of heavy metals in the environment.

1.4 Toxicity mechanisms

In arsenic biotransformation the methylation of harmful arsenic compounds takes place by bacteria, algae, fungi, and humans in the form of monomethylarsonic acid (MMA) and dimethylarsinic acid (DMA). In the process of biotransformation, enzymatic conversion of the inorganic arsenic species (iAs) into methylated arsenicals are known as the biomarkers and the end metabolites of chronic arsenic exposure, as shown in the formula:

The toxicity of lead in living cells is caused by the process of oxidative stress and ionic mechanisms. According to many researchers oxidative stress in living cells is caused by the imbalance produced between the production of free radicals and antioxidant generation for the detoxification of the reactive intermediates or for the repair of the resulting damage.

1.5 Uptake and bioaccumulation of heavy metals in plants

From the various sources, HMs enter into the environment, uptaken by plants and the different organisms, hence accumulated in the food chain. It has been reported that absorption of HMs in the cells and tissues of living organisms are dependent on several factors. Availability of HMs in the soil, type of HM, pH of soil, and organic substances present in the soil are various factors on which accumulation of HMs depend. HMs deposited in soil are easily accumulated in plants as their accumulation capacity is more. Measurement and evaluation of HMs absorption by plants can be utilized for the valuation of availability of these HMs in soil. This type of evaluation can provide information regarding the toxicity status of the environment. Plants, being sensitive organisms, are drastically affected by such alterations in the environment due to HM stress (Dursun et al., 2003XXX).

There are several plant species that act as indicators of oxidative stress caused by HMs. Several animals can also give an indication of HM toxicity. Plants such as Lithophaga lithophaga are significant indicators of seawater pollution (Akar et al., 2005XXX). Absorption of HMs in living systems is significantly important up to a certain extent and also has repercussions for human health and that of other organisms. Pollution of land and water reservoirs leads to toxicity in the food chain and food web, which further triggers major risks on the health of organisms in both the ecosystems.

Tobacco leaves are used for cigarettes, hence absorption of toxins like HMs in tobacco may cause health risks. In these plants mainly the leaf part uptakes the HMs and the uptake by leaves depend on the plant species and also its geographical conditions (Regassa and Chandravanshi, 2016XXX). This crop has been grown with the treatment of certain inorganic fertilizers, mainly phosphate, along with considerable doses of toxic HMs. A small amount of HMs are absorbed and reach the lungs of tobacco smokers.

Entry of HMs in the environment is also due to the smoke of tobacco, which contains a large amount of HMs. Children get exposed to lead stress when they come into contact with active smokers through passing smoking. These HMs are first inhaled during tobacco smoking, enter the blood from the lungs, and then through the agency of oxygen it is transferred to the different parts of body. Therefore more concentrations of HMs can be observed in the blood of those people who smoke in comparison to those who did not consume tobacco.

As human beings are omnivorous they are subjected to different toxins through a large variety of food groups like cereals, vegetables, sea food, and so forth. HM pollution in water reservoirs like ponds, lake, rivers, and oceans increases their toxicity level in various living organisms and plants as these metals are accumulated in cells and tissues. Enhancement in the toxicity level in the food chain and food web trigger the threats to health of the organisms in each trophic level. Reports related to HM triggered diseases like Minamata and itai-itai disease, which occurred in Japan, found that mercury and cadmium, respectively, were the major causal agents of these diseases (Barwick and Maher, 2003XXX).

Apart from this, amplification in the level of these HMs in each trophic level is also a provocative issue in the ecotoxicology of metals. There are various reports indicating HM magnifications in food chains. Due to the biomagnification of HMs in different food chains, organisms in each trophic level face greater health risks. HMs transferring from first trophic level (plants) to the next three or four trophic levels lead to enhancement of health hazards as every trophic level experiences higher biomagnification of metals in comparison to the previous one, affecting development, hampering physiological activities, and can prove lethal if the concentration is high (Balkhair, 2016XXX).

1.6 Impact of heavy metals on human life

The major antioxidants of cells, mainly antioxidants and enzymes of the thiol group (–SH) are depleted by some heavy metals like Cd, Pb, Hg, and As. Generation of reactive oxygen species (ROS) such as hydroxyl radical (OH), superoxide radical (O2) and hydrogen peroxide (H2O2) may be increased by such metals. Oxidative stress is a condition in which devastation of the inherent antioxidant defense of cell due to increased generation of ROS (Ercal et al., 2001XXX). Some heavy metals are nephrotoxic, accumulating especially in the renal cortex, including Cd, Pb, and Hg. One of the most important things concerning the toxicity of heavy metals is their chemical form.

1.7 Analysis of heavy metals in environment

For the assessment and control of pollution, monitoring and analysis of concentrations of heavy metals in the environment are very important (Elzwayie et al., 2017). The monitoring of the levels or concentrations of highly toxic metals and metalloids should be done on a regular bases in various environmental media (e.g., water, sediments, and soils) and in the resident biota. Some useful information about principal sources, distribution, and the fate of these heavy metals in the environment and accumulation of them in the food chains can be determined by environmental analysis.

Heavy metals are released during mining and the extraction of various elements from their ores. During mining smelting and other industry related processes, HMs are released into the atmosphere, and then through wet and dry deposition, return to the land. The addition of heavy metals into the environment is done by the discharge of wastewaters like industrial effluents and domestic sewage. Anthropogenic input of heavy metals into the environment is also through the use of chemical fertilizers and fossil fuel combustion.

1.8 Nature and scope of heavy metal studies

Observations on various facets related to HMs and metalloids indicate that these are associated with different fields such as ecology, environmental chemistry, and ecotoxicology, therefore knowledge of these fields is mandatory. This interdisciplinary area has great scope in context of health. These analyses also incorporate aquatic chemistry. According to Johnston aquatic chemistry is an ultimate section for the health of people (Elzwayie et al., 2017). For health risk assessment, deposition of HM doses in the tissues of plants and animals must be assessed and monitored.

1.9 Conclusion

Various HMs are utilized by human beings such as mercury, aluminum, lead etc. for the synthesis of different products which can be used in thermometer, utensils, accumulators etc.. The toxins produced by these HMs can trigger life-threatening risks, and cannot be ignored. There are various strategies that can be applied to handle these HMs such as certain precautions, the proper way of dispensing them, and appropriately hygienic conditions. While the toxic effects of HMs must be assessed and monitored clinically to ameliorate their toxicity and their hazardous effects on environment and living organisms.

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