PRINCIPLES OF TOXICOLOGY

Most of the nonessential metals also may be considered toxic, although some (e.g., titanium) appear
to be relatively nontoxic, even at high exposure levels.
14.2 SPECIATION OF METALS
Nonbiological Factors Affecting Speciation
14.2 SPECIATION OF METALS 327
Understanding the ways in which environmental parameters determine the speciation of a metal is the
first step in assessing its toxicological potential. These conditions, which are essentially physical and
chemical parameters, might be broadly categorized as nonbiological factors. A property common to
all metals is the tendency to ionize in solution, giving up one or more electrons to become a cation, or
species with a net positive charge (e.g., Cd → Cd 2+ + 2e – ). The degree to which this process is carried
out will, in turn, affect various other behaviors of the metal and ultimately its toxicity. As pH decreases,
metals typically become more mobile in the environment and more available to organisms.
One form in which a metal may exist is the uncharged, or elemental, form. This state is often
designated by a “0,” representing “zero charge” [i.e., Hg(0) or Hg 0 for elemental mercury]. In aqueous
solution, metals may exist in a variety of ionization states or valences that differ in the number of
missing electrons and therefore exhibit various net positive charges. The standard designation for the
first ionization state of many metals is the name of the metal with an ous suffix, such as mercurous ion
for Hg + , or cuprous ion for Cu + . The next level of ionization is sometimes designated with an ic suffix,
such as mercuric for Hg 2+ and cupric for Cu 2+ . Other ionization states may exist for some metals, but
usually the number of states most commonly encountered under normal conditions is limited to two
or three.
Ionization is not a uniform process across all metal types. For instance, tin gives up two electrons
in its first ionization state (the stannous ion, or Sn 2+ ), while mercury and copper give up only one. In
contrast to these examples, the primary valences of interest for chromium are Cr 3+ (trivalent chromium)
and Cr 6+ (hexavalent chromium), for which the toxicity varies dramatically. Consideration of a metal’s
ionization state is important because it may substantially affect that metal’s toxicity. Trivalent (3 + )
chromium compounds, for example, which are noncorrosive and noncarcinogenic, are less of a concern
than hexavalent (6+) chromium compounds, which are quite corrosive and may cause cancer on
sufficient exposure by the inhalation route only.
These different patterns of metal ionization are influenced by environmental conditions, as well as
by the inherent properties of the metals themselves. Since metal cations carry a charge, they will react
readily with other charged species, thus making their behavior heavily dependent on parameters such
as pH (the concentration of H + ions), pOH (the concentration of OH – ions), the presence of other
charged particles or ligands, oxidizing or reducing conditions, temperature, and whether the medium
of concern is air, soil, or an aqueous solution. Simply put, metals aren’t just metals.
The degree to which these factors may affect the biological hazards posed by metals is illustrated
by copper. Copper has an especially high affinity for organic ligands. In aquatic systems, copper readily
binds to organic matter and may thus settle out of the water column and be unavailable to most
organisms. Likewise, copper binds to organic matter in soil and tends to accumulate in the upper layers
of soil if applied at the surface. This decreases the likelihood of oral exposure to copper through
contaminated groundwater, but could increase the likelihood of exposure through soil contact.
When a metal becomes ionized, it may form a variety of compounds by combining with available
negatively charged groups, such as chlorides, sulfates, nitrates, carbonates, and acetates. These metal
complexes may have toxicological properties that differ from those of the uncomplexed metal.
Alternatively, metal complexes may behave in different ways that affect their absorption by and
distribution within living organisms.