10
1 Heavy Metals in the Marine Environment—An Overview
Zn compounds are used in the pharmaceuticals industry as ingredients in some
common products, such as vitamin supplements, sun blocks, diaper rash ointments,
deodorants and antidandruff shampoos (ASTDR 2011; McComb et al. 2014). Zn and
Cu are used in shipping activities such as sacrificial anodes and antifoulants (Negri
et al. 2002).
Further uses of Zn include galvanizing iron, preparation of certain alloys, roofing
and gutter in building construction; whilst zinc oxide is used as an activator in rubber
industry, and as a pigment in plastics, cosmetics, photocopier and paper wallpaper
and Zn based fungicides (Lenntech 2006).
At low concentrations, Zn contributes to the normal metabolism of most living
organisms through its role as a cofactor in > 300 enzymatic reactions (Morel et al.
1994). Zn does not show biomagnification characteristics (De Pooter 2013), excessive
levels of Zn in the environment lead to bioaccumulation of Zn in organisms.
1.3.6 Mercury (Hg)
Mercury (Hg) is a toxic, persistent, non-essential metal that exists in the environment in various chemical forms (i.e. elemental, organic and inorganic). Hg is released
into the environment from emission sources that can be natural or anthropogenic.
These forms of Hg are interconvertible and produce toxicity and bioavailability (Jaishankar et al. 2014). Organic forms of Hg include methylated Hg compounds such
as monomethyl mercury (MMHg), dimethyl mercury (DMHg); and phenyl mercury
(PhHg). Whereas elemental mercury (Hg
0 ) is a highly volatile liquid at room temperature and released as a vapour in the atmosphere while inorganic Hg occurs in the
form of mercuric (HgCl 2 , HgS) and mercurous (Hg 2 Cl 2 ) salts. Gaseous elemental
Hg has lifetime of about 1 year and once emitted into the atmosphere, it has the
potential to be transported on hemispheric and global scales (Slemr et al. 2018).
Hg is persistent in the environment and dispersed over great distances through
atmospheric, oceanic, aquatic and biotic transport vectors (Brosset 1981; Bank 2012;
Reichelt-Brushett et al. 2017). It is well documented that Hg can easily be transported
in the atmosphere as gaseous elemental Hg and deposits to aquatic and terrestrial
ecosystems as oxidised and particulate-bound Hg via dry and wet deposition (Selin
2009). The ultimate fate of all Hg emitted to the atmosphere is to deposit to ecosystems (Lyman et al. 2020). Hence, the constant transfer of Hg between the aquatic,
atmospheric, biological, and terrestrial reservoirs has made it ubiquitous throughout
the environment.
When deposited in anoxic environments, Hg (HgS, HgS 2 ) is transformed to methyl
mercury by microbial systems in the water column, surface sediments and wetlands
(Clark 2001; Solomon 2008; Kwon et al. 2020). Organic forms of Hg are more toxic
than the inorganic salts (Clark 2001). Methyl Hg (MHg) is the most common form of
organic mercury in the environment (Gworek et al. 2016), is a neurodevelopmental
toxicant (Obi et al. 2015); and is the most toxic form of Hg (Henriques et al.2015).
Clark (2001) reported that an outbreak of MHg poisoning occurred in the small
1 Heavy Metals in the Marine Environment—An Overview
Zn compounds are used in the pharmaceuticals industry as ingredients in some
common products, such as vitamin supplements, sun blocks, diaper rash ointments,
deodorants and antidandruff shampoos (ASTDR 2011; McComb et al. 2014). Zn and
Cu are used in shipping activities such as sacrificial anodes and antifoulants (Negri
et al. 2002).
Further uses of Zn include galvanizing iron, preparation of certain alloys, roofing
and gutter in building construction; whilst zinc oxide is used as an activator in rubber
industry, and as a pigment in plastics, cosmetics, photocopier and paper wallpaper
and Zn based fungicides (Lenntech 2006).
At low concentrations, Zn contributes to the normal metabolism of most living
organisms through its role as a cofactor in > 300 enzymatic reactions (Morel et al.
1994). Zn does not show biomagnification characteristics (De Pooter 2013), excessive
levels of Zn in the environment lead to bioaccumulation of Zn in organisms.
1.3.6 Mercury (Hg)
Mercury (Hg) is a toxic, persistent, non-essential metal that exists in the environment in various chemical forms (i.e. elemental, organic and inorganic). Hg is released
into the environment from emission sources that can be natural or anthropogenic.
These forms of Hg are interconvertible and produce toxicity and bioavailability (Jaishankar et al. 2014). Organic forms of Hg include methylated Hg compounds such
as monomethyl mercury (MMHg), dimethyl mercury (DMHg); and phenyl mercury
(PhHg). Whereas elemental mercury (Hg
0 ) is a highly volatile liquid at room temperature and released as a vapour in the atmosphere while inorganic Hg occurs in the
form of mercuric (HgCl 2 , HgS) and mercurous (Hg 2 Cl 2 ) salts. Gaseous elemental
Hg has lifetime of about 1 year and once emitted into the atmosphere, it has the
potential to be transported on hemispheric and global scales (Slemr et al. 2018).
Hg is persistent in the environment and dispersed over great distances through
atmospheric, oceanic, aquatic and biotic transport vectors (Brosset 1981; Bank 2012;
Reichelt-Brushett et al. 2017). It is well documented that Hg can easily be transported
in the atmosphere as gaseous elemental Hg and deposits to aquatic and terrestrial
ecosystems as oxidised and particulate-bound Hg via dry and wet deposition (Selin
2009). The ultimate fate of all Hg emitted to the atmosphere is to deposit to ecosystems (Lyman et al. 2020). Hence, the constant transfer of Hg between the aquatic,
atmospheric, biological, and terrestrial reservoirs has made it ubiquitous throughout
the environment.
When deposited in anoxic environments, Hg (HgS, HgS 2 ) is transformed to methyl
mercury by microbial systems in the water column, surface sediments and wetlands
(Clark 2001; Solomon 2008; Kwon et al. 2020). Organic forms of Hg are more toxic
than the inorganic salts (Clark 2001). Methyl Hg (MHg) is the most common form of
organic mercury in the environment (Gworek et al. 2016), is a neurodevelopmental
toxicant (Obi et al. 2015); and is the most toxic form of Hg (Henriques et al.2015).
Clark (2001) reported that an outbreak of MHg poisoning occurred in the small