Pathways of Heavy Metal Access

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In order to cause any effect in a living organism, heavy metals have to come into contact with this organism. There are three principal ways, through which this might happen. The first pathway is through the atmosphere or through atmospheric deposition to water and soil, the second is through drinking contaminated water or using it for cooking and crop irrigation, and the third is through accumulation in the food web.

3.1.1. Respiration

Heavy metals can enter organisms by respiration of natural and anthropogenic emissions. The sources of these emissions have been discussed in the first chapter of this book. Heavy metals can be volatile (mostly Hg) or particulate. These substances are released into the atmosphere in the order of magnitude of several thousands of tons annually [208]. These numbers can be expected to grow due to the increasing world population and industrial activities in countries striving to increase their industrial bases (e.g., China and India).

Respiration of metal pollutants through dust is one of the most serious threads to humans working in industrial workplaces. Health problems such as "black lung" disease, silicosis, and radiation sickness have been recognized early [209], Table 8 lists some health problems arising from the respiration and contact of heavy metals with humans in the workplace. They may cause a variety of health damages including cancer, liver, and kidney diseases, abortions, neurological and visual damages, negative effects on the immune system, allergies, cardiovascular toxicity, and anaemia.

The second pathway of entering organisms is through drinking water contaminated with heavy metals, which can be ingested directly by drinking, or indirectly by using this water for cooking and irrigation. These contaminations can be both of natural and of anthropogenic origins. One should be aware that to date more than one third of the world population have no access to clean water for cooking, drinking, personal hygiene, and sanitation, which is threatening especially for infants and children. Contaminated drinking water is one of the major hazards in West India and Bangladesh, where more than 20 million of the 120 million people living in this country are affected by arsenicosis (see also subchapter 4.1.3. Ecotoxi-cological Effects of Arsenic).

The third direct pathway is through foods with high natural or bioac-cumulated contents of heavy metals. One of the main routes of entering the food chain is plant uptake. If soils contain high natural metal contents, are amended with metal-bearing sludges, or are irrigated with water contaminated with metals, then some plants will hyperaccumulate those metals, which results in polluted food crops and animal forage. Then the heavy metals are transferred through higher trophic levels to humans. The access of an individual heavy metal to the food web is determined by how the metal is bound to a soil, the soil phase it is bound to, and its chemical form. Pollutants can be present in soils as particulates, liquid films, absorbed ions, adsorbed ions, and liquid phases in pores [210]. Fig. 26 shows an example of the distribution of some heavy metals in solid phases of some Polish soils. Five solid phases have been identified, from which heavy metals (i.e. Mo, Zn, Cd, Cu, Pb, Ni, and Cr) are able to enter the food web [211, 212]. These are readily soluble phases, exchangeable sites, Fe and Mn oxy/hydroxides, organic matter, and residual phases. While metals from the first four phases are easily released, this is not the fact for metals bound to the residual phase for most environmental conditions encountered. Each of the metal studied is unique in the way it is bound to the different phases. For example, chromium can be found mainly in the residual phase with only 2% in an easily soluble phase. More than 20% of cadmium, on the other hand, is available from easily soluble and exchangeable phases.

] Residual | | Associated wilh oxides ^^ Easily soluble

] Bound organic matter | j Exchangeable

Fig. 26. Distribution of some heavy metals in solid phases of soils in Poland (redrawn after Ref.209).

3.2. Bioavailability and Bioaccumulation

In order to be assimilated, heavy metals will have to be mobile, to be transported to the organism, and be bioavailable to it. Contrary to organic contaminants, heavy metals can not be degraded. The term "bioavailability" has different meanings when different disciplines are concerned, and must therefore be defined carefully.

3.2.1. Definition

In ecotoxicology, bioavailability can be defined as "the portion of a chemical in the environment that is available for biological action, such as uptake by an organism" [213]. In sediment-associated contaminant, bioavailability can be defined as "the fraction of the totai contaminant in the interstitial water and on the sediment particles that is available for bioaccumulation", whereas bioaccumulation is "the accumulation of contaminant via all routes available to the organism" [214]. When dealing with pharmacology, bioavailability refers to "the fraction of an orally administered dose that reaches the blood of an animal" [215], a definition, which is most commonly used by mammalian toxicologists [216],

Table 8

Health effects of some heavy metals

Carcinogens which may occur in the workplace

Human carcinogens: Possible human carcinogens: Lung carcinogens include:

Some reported cancers caused by or associated with certain occupations and industries

As: lung, skin: pesticides, others

Cr: lung: metals, welders

Ni: lung, nose: metals, smelters, engineering

Substances linked to occupations liver disease

As: Cirrhosis, angiosarcoma, hepatocellular carcinoma: pesticides, wood, vinters, smelters

Be: granulomatous disease: ceramics

Substances reported to have damaged the kidney in the workplace Cd: nephrotoxicity: welding, engineering Pb: nephrotoxicity: chemicals, paint, batteries Hg (inorganic): nephrotoxicity: chemicals, paints

Possible factors influencing reproductivity outcomes based on experimental data As: fetotoxic, teratogen, transplacental carcinogen: agriculture, wood preserving Cd: spontaneous abortions, impaired implantation, teratogen

Male and female damage: engineering, chemicals, batteries, paints, smelting Cr: teratogen: chemicals, engineering

Pb: decreased fertility, fetotoxic, impaired implantation, teratogen, sperm damage, hormonal alterations: various

Mn: decreased fertility, impaired implantation: various Hg: fetotoxic, teratogen, menstrual disorders: chemical, pesticides Se: fetotoxic Tl: : fetotoxic

Triethyl Pb: spontaneous abortions

Substances found in the workplace reported to have caused neurological damage AS: peripheral neuropathy: metal production, pesticides Pb: encephalopathy and peripheral neuropathy: general

Mn: encephalopathy, ataxia, later Parkinson disease-like symptoms occur, acute psychosis: engineering, aircraft industry, steel, aluminium, magnesium, and cast iron production

Hg: tremor, weakness, peripheral neuropathy is uncommon, chronic exposure leads to ataxia, mental impairment: chemicals, pharmaceuticals, dentistry, plastic, paper, various Ni: headache: engineering Tl: encephalopathy, ataxia (high doses) Sn (organic): encephalopathy

Table 8 (continued)

Health effects of some heavy metals

Substances known or associated with visual damage in the workplace Pb: optical neuropathy: foundry industry Hg: cranial nerve palsies: chemicals

Substances reported to have caused immune system effects in the workplace Ni: hypersensivity: metals engineering

Reported respiratory effects of certain workplace substances:

Metals: especially Pt, Ni, Cr, Co, and V: occupational asthma, metal and engineering workers

Substances known to be absorbed through or damaging to the skin in the workplace As: skin cancer: agriculture, lead workers, dyers, copper smelters, brass makers, chemicals, textiles, painters, pesticide users

Cr: allergic contact dermatitis: percutaneous absorption: engineering and chemicals Ni: allergic contact dermatitis: metals, engineering, jewellery

Sustances linked with cardiovascular toxicity:

Arsine: cardiac arrhythmia

As: myocardial injury

Sb: hypertension

Cd: hypertension

Co: myocardial injury

Pb: myocardial injury, hypertension

Reported adverse effects of chemicals on the blood

As: aplastic anaemia: glass, paints, enamels, pesticides, tanning agents

Cu: red blood cells: engineering

Pb: red blood cells, porphyria: general

Modified after Ref. 209.

The biological response and risk is a function of the dose of the heavy metal. For metals, which are essential for metabolism, there can be three ranges: first, the deficiency range, where biological activities can be increased by increasing the dose, second, the buffering or normal range, where biological functions are optimal, and third, the toxicity range, where further increases in concentration inhibit metabolism and may be even lethal to the organism. The range of concentration depends on the physical and chemical nature of the individual metal, the sensivity or tolerance of the receptor organism, and the nature and properties of the environmental medium concerned, e.g., soil or aquatic systems [183].

3.2.2. Bioavailability in the Soil-Plant System

The uptake and bioaccumulation of heavy metals by plants is of importance because of impact of soils by anthropogenic emissions and its consequences for human uptake. Fig. 27 shows the main three categories, into which plants can be grouped: excluders, indicators, and accumulators. Excluders include members of the grass family such as sudangrass, bromegrass, and others. These plants are insensitive to heavy metals over a wide concentration range. Indicators include grain and cereal crops such as corn, soybean, wheat, oats, etc., and accumulators include the mustard and Com-positae families such as lettuce, spinach, etc. and tobacco. Extreme accumulators are known as hyperaccumulators, which can be found on heavily contaminated soils and near ore deposits. These plants have developed a tolerance mechanism and can be used for the phytoremediation, i.e. the use of plants for the removal of heavy metals in soils, sediments, sludges and waters. Phytoremediation and its basic physiological plant mechanisms are discussed in detail in the last chapter of this book. In contrast to hyperaccumulators, excluders have developed avoidance or exclusion mechanisms. Indicators are plant species that respond to soil metal concentration displaying linear curves, while accumulators and excluders display logarithmic curves [217].



Uptake Intensity



Uptake Intensity

Klipp Vningar

Metal Concentration in Soil

Fig. 27. Relative uptake and bioaccumulation potential among plant species (redrawn after Ref. 183).

Metal Concentration in Soil

Fig. 27. Relative uptake and bioaccumulation potential among plant species (redrawn after Ref. 183).

In order to assess bioavailability of individual metals for certain plant species, chemical extraction techniques are used most commonly. In general, the readily soluble plus the weakly adsorbed (i.e. exchangeable) metal fraction is considered to be bioavailable. Organisms such as worms can also be used to assess bioavailability. The main categories used as extrac-tants are dilute acids such as HC1 or H2S04, and chelating agents such as EDTA and DTPA. The method most widely used is described in Ref. 6. This method and variations of it include in general the selection of chemical reagents from the least to the most aggressive in sequential fashion and from the least to the greatest extremes in temperature and stirring. The various fractions can be described as follows: water soluble (metal exists in soil solution either in free ionic or complexed form), exchangeable (metal is sorbed by electrostatic attraction to negatively charged exchange sites), sorbed (adsorption onto specific exchange sites on colloidal surfaces), organic (complexed with soil organic matter), oxide-crystalline or amorphous Fe/Mn oxides (specific adsorption onto Fe/Mn oxides), carbonate (precipitated and/or occluded in soils high in free CaCOs), sulfide (highly insoluble compounds of metal sulfides in poorly aerated soils), and residual (fixed within the crystalline lattices of alumosilicate particles).

3.2.3. Bioavailability in Aquatic Systems

Similar to heavy metal uptake by plants, aquatic organisms vary in their metal uptake. They can be grouped into two categories: regulators (excluders) and accumulators (non-excluders) [218]. Fig. 28 shows some examples for aquatic organisms and their different strategies for uptake, accumulation, and excretion of heavy metals [219]. Regulators are characterized by their low metal uptake, while accumulators are characterized by their high metal uptake. While regulators are able to control metal accumulations and keep their intracellular metal concentrations within a narrow range over a broad external concentration range of heavy metals, accumulators are capable of adopting a detoxification system with an elevated metal body level even in noncontaminated environments.

The most important factors influencing bioaccumulation in aquatic systems are compound characteristics (e.g. solubility), sediment characteristics (e.g. CEC, pH), water quality (e.g. temperature), biological characteristics (e.g. organism behaviour, modes of feeding), source of water, and age and size length of the individual organism [218, 220]. In general, bioavailability and toxicity of heavy metals correlates directly to concentrations of the free metal ion rather than to total or complexed metal concentrations.


Extreme Uptake

Barnacles Ascidians Bivalve mollusks Gastropod mollusks Isopods, amphipods Polychaetes

Macro algae Mussels/bivalves Polychaetes Decapod crustaceans Finfîsh

Bivalves (rare) Macroalgae Polychaetes Decapod crustaceans Finfîsh

Cu, Fe, Mn, Pb, Zn in granules V in vanadocytes Cu, Fe, Mn, Pb, Zn in granules Cu, Zn in granules Cu, Fe, Pb, Zn in granules Cu in granules most metals most metals, metallothioneins Cd, Pb Cd, Pb Cd, Pb


Low Uptake

Fig. 28. Aquatic organisms employing different strategies for the uptake, accumulation, and excretion of heavy metals (redrawn after Ref. 219).

The most common mechanisms to limit uptake of heavy metals are altering the chemical speciation in the surrounding environment to reduce bioavailability, complexing the metal at the organism surface, decreasing the permeability of epithelial surfaces by introducing extracellular barriers, reducing transport into the cell, and undertaking behavioural avoidance activity [183]. As bioavailability is influenced mainly by solubility and mobility of the individual metal, all factors influencing these properties will of course be of importance for bioavailability as well. As for soils, the master variables are pH, soil organic matter, redox potential, presence of Fe/Mn oxyhydroxides, etc. As for aquatic systems, such as fresh and salt water, the most important variables include pH, dissolved organic matter, suspended particulate matter, ionic strength, alkalinity, and salinity [218], Once heavy metals are absorbed, taken up or assimilated by an organism, they may unfold both adverse and positive effects depending on the kind and concentration of metal. Some metals are essential for organisms, while others are not. Among essential elements are Fe, Zn, Cu, Mn, Se, Cr, Co, and Mo [212]. Toxicological effects to humans are well known, especially those of Cd, As, Hg, and Pb. As for the toxicological effects of individual heavy metals, the reader is referred to the following chapter.

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  • Ken
    What is the pathway of heavy metal?
    3 years ago

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