heavy metal is a metal or metalloid with a relatively high density and associated with
pollution and toxicity (Duffus 2002). Other criteria as the atomic weight or mass
were used to give consistency to the term “heavy”, but not prospered. With respect to
pollution and toxicity, heavy metals are highly toxic to very low concentrations. The
heavy metals tend to accumulate with respect to time, exceeding their permissible
concentration in the environment; the metallic accumulation unchains damages on
the biological systems (human, animals, microorganisms and plants), provoking
important problems of toxicity for environmental health and safety (Velea et al.
2009). Also, the term “heavy metal” in the legal aspects implies that the pure metal
and all its compounds have the same physicochemical, biological and toxicological
properties, which is false, as will mention below.
Heavy metals can be classified as (1) toxic metals, (2) essential metals for living
organisms and (3) radionuclides such as uranium (Gadd 2010; Wood and Wang
1983). (1) Toxic metals – the toxicity of the heavy metals can define as the ability of
a metal to cause negative effects on living organisms and depends on the bioavailability of the metals, and it is aggravated by their long-term persistence in the
environment. Arsenic, fluorine, cadmium, mercury, chromium and lead are some
examples of extremely toxic elements to biota, even at very low concentrations.
Lead is one metal with the most retention time in soil (150–5000 years). (2) Essential
metals, of all elements in the periodic table, 30 are required for microbial life,
although not all are necessary for the growth and cell division of every microbial
species. Among them, the carbon, nitrogen, hydrogen and oxygen are the bulk
elements, and 26 of them are required in intermediate to trace amounts. Twentytwo of the 26 elements are found to be essential for life in higher organisms; see
Fig. 13.1. An overabundance of any of these elements can cause build-up to an
intracellular toxic level, which can result in death (Wood and Wang 1983).
(3) Radionuclides, the radioactive elements, are formed by chemical elements
whose atomic nuclei are unstable. As a consequence of this instability, its atoms
emit subatomic particles intermittently and randomly. The imbalance is corrected by
the release of excess neutrons or protons, in the form of α particles that are really
helium nuclei and β particles that can be electrons or positrons. Among the radioactive elements are polonium, astatus, radon, francium, radio, actinium, thorium,
protoactinium, uranium, neptunium, plutonium, americium, lawrencio, curio,
berkelio, california, einsteinium, fermium, mendelevian, and nobel.
The contamination for heavy metals is caused by natural phenomena and human
activities. The human activities demand the development of new chemicals,
materials and enormous quantities of energy and exploit natural resources and
discharge of wastewater from metal-related industry, which result in environmental
pollution, mainly of water-body (Haferburg and Kothe 2010; Thakare et al. 2021).
Generally, the heavy metals are present in form of soluble salts in water, that these
cannot be separated by ordinary physical separation systems. Physicochemical
processes such as chemical precipitation, chemical oxidation or reduction, electrochemical treatment, evaporative recovery, filtration, ion exchange and membrane
technologies have been widely used to remove metallic ions from industrial wastewater (Das et al. 2008; Li et al. 2018). But these processes may be ineffective or
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M. I. Neria-González and R. Aguilar-López
pollution and toxicity (Duffus 2002). Other criteria as the atomic weight or mass
were used to give consistency to the term “heavy”, but not prospered. With respect to
pollution and toxicity, heavy metals are highly toxic to very low concentrations. The
heavy metals tend to accumulate with respect to time, exceeding their permissible
concentration in the environment; the metallic accumulation unchains damages on
the biological systems (human, animals, microorganisms and plants), provoking
important problems of toxicity for environmental health and safety (Velea et al.
2009). Also, the term “heavy metal” in the legal aspects implies that the pure metal
and all its compounds have the same physicochemical, biological and toxicological
properties, which is false, as will mention below.
Heavy metals can be classified as (1) toxic metals, (2) essential metals for living
organisms and (3) radionuclides such as uranium (Gadd 2010; Wood and Wang
1983). (1) Toxic metals – the toxicity of the heavy metals can define as the ability of
a metal to cause negative effects on living organisms and depends on the bioavailability of the metals, and it is aggravated by their long-term persistence in the
environment. Arsenic, fluorine, cadmium, mercury, chromium and lead are some
examples of extremely toxic elements to biota, even at very low concentrations.
Lead is one metal with the most retention time in soil (150–5000 years). (2) Essential
metals, of all elements in the periodic table, 30 are required for microbial life,
although not all are necessary for the growth and cell division of every microbial
species. Among them, the carbon, nitrogen, hydrogen and oxygen are the bulk
elements, and 26 of them are required in intermediate to trace amounts. Twentytwo of the 26 elements are found to be essential for life in higher organisms; see
Fig. 13.1. An overabundance of any of these elements can cause build-up to an
intracellular toxic level, which can result in death (Wood and Wang 1983).
(3) Radionuclides, the radioactive elements, are formed by chemical elements
whose atomic nuclei are unstable. As a consequence of this instability, its atoms
emit subatomic particles intermittently and randomly. The imbalance is corrected by
the release of excess neutrons or protons, in the form of α particles that are really
helium nuclei and β particles that can be electrons or positrons. Among the radioactive elements are polonium, astatus, radon, francium, radio, actinium, thorium,
protoactinium, uranium, neptunium, plutonium, americium, lawrencio, curio,
berkelio, california, einsteinium, fermium, mendelevian, and nobel.
The contamination for heavy metals is caused by natural phenomena and human
activities. The human activities demand the development of new chemicals,
materials and enormous quantities of energy and exploit natural resources and
discharge of wastewater from metal-related industry, which result in environmental
pollution, mainly of water-body (Haferburg and Kothe 2010; Thakare et al. 2021).
Generally, the heavy metals are present in form of soluble salts in water, that these
cannot be separated by ordinary physical separation systems. Physicochemical
processes such as chemical precipitation, chemical oxidation or reduction, electrochemical treatment, evaporative recovery, filtration, ion exchange and membrane
technologies have been widely used to remove metallic ions from industrial wastewater (Das et al. 2008; Li et al. 2018). But these processes may be ineffective or
370
M. I. Neria-González and R. Aguilar-López