Hg(II), Cd(II), and Pb(II), the concentrations at which they produce observable
negative effects are quite low. Their high toxicity thus serves as an example and
warrants special consideration for treatment in this chapter.
3.1.1 Chromium
Chromium has been used extensively in industrial processes such as leather tanning,
electroplating, negative and film making, paint and pigment processing, and wood
preservation (Beszedits 1988). Additionally, chromium has been used as a metallurgical additive in alloys (such as stainless steel) and metal ceramics. Chromium
plating has been widely used to give steel a polished silvery mirror coating. The
radiant metal is now used in metallurgy to impart corrosion resistance. Its ornamental
uses include the production of emerald green (glass) and synthetic rubies. Due to its
heat-resistant properties, chromium is included in brick molds and nuclear reactor
vessels (Dakiky et al. 2002).
Through the above and many other industrial uses, a large amount of chromium
(approximately 4500 kg/d) is discharged into the environment making it the most
voluminous metallic pollutant on earth. Almost all chromium inputs to the natural
systems originate from human activities. Only 0.001% is attributed to natural
geologic processes (Merian 1984).
Chromium from the anthropogenic sources is discharged into the environment
mainly as hexavalent chromium [Cr(VI)]. Cr(VI) – unlike Cr(III) – is a severe
contaminant with high solubility and mobility in aquatic systems. Cr(VI) is a
known carcinogen classified by the US EPA as a Group A human carcinogen
based on its chronic and subchronic effects (Federal Register 2004). It is for this
reason that most remediation efforts target the removal of Cr(VI) primarily.
3.1.2 Uranium
Uranium exists in the environment mainly as oxides, organic or inorganic
complexes, and rarely as a free metal ion (Mtimunye and Chirwa 2013). Free
elemental uranium primarily exists in higher oxidation states typically bound to
oxygen. The oxygen bound uranium exists mainly as uraninite (UO 2 ), triuranium
octaoxide also known as pitchblende (U 3 O 8 ), and uranium trioxide (UO 3 )
(Stefaniak et al. 2009). U 3 O 8 is relatively insoluble in water and relatively stable
over a wide range of environmental conditions. UO 2 on the other hand is not as
stable as U 3 O 8 in the environment as it may undergo alteration under various
environmental conditions (Senanayake et al. 2005). Upon exposure to air, UO 2 is
subjected to oxidation and as a result produces a secondary mineral (UO 2
2+ ) which
complexes easily with phosphates, carbonates, silicates, and sulfates (Senanayake
et al. 2005; Stefaniak et al. 2009).
24
E. M. Nkhalambayausi-Chirwa et al.
negative effects are quite low. Their high toxicity thus serves as an example and
warrants special consideration for treatment in this chapter.
3.1.1 Chromium
Chromium has been used extensively in industrial processes such as leather tanning,
electroplating, negative and film making, paint and pigment processing, and wood
preservation (Beszedits 1988). Additionally, chromium has been used as a metallurgical additive in alloys (such as stainless steel) and metal ceramics. Chromium
plating has been widely used to give steel a polished silvery mirror coating. The
radiant metal is now used in metallurgy to impart corrosion resistance. Its ornamental
uses include the production of emerald green (glass) and synthetic rubies. Due to its
heat-resistant properties, chromium is included in brick molds and nuclear reactor
vessels (Dakiky et al. 2002).
Through the above and many other industrial uses, a large amount of chromium
(approximately 4500 kg/d) is discharged into the environment making it the most
voluminous metallic pollutant on earth. Almost all chromium inputs to the natural
systems originate from human activities. Only 0.001% is attributed to natural
geologic processes (Merian 1984).
Chromium from the anthropogenic sources is discharged into the environment
mainly as hexavalent chromium [Cr(VI)]. Cr(VI) – unlike Cr(III) – is a severe
contaminant with high solubility and mobility in aquatic systems. Cr(VI) is a
known carcinogen classified by the US EPA as a Group A human carcinogen
based on its chronic and subchronic effects (Federal Register 2004). It is for this
reason that most remediation efforts target the removal of Cr(VI) primarily.
3.1.2 Uranium
Uranium exists in the environment mainly as oxides, organic or inorganic
complexes, and rarely as a free metal ion (Mtimunye and Chirwa 2013). Free
elemental uranium primarily exists in higher oxidation states typically bound to
oxygen. The oxygen bound uranium exists mainly as uraninite (UO 2 ), triuranium
octaoxide also known as pitchblende (U 3 O 8 ), and uranium trioxide (UO 3 )
(Stefaniak et al. 2009). U 3 O 8 is relatively insoluble in water and relatively stable
over a wide range of environmental conditions. UO 2 on the other hand is not as
stable as U 3 O 8 in the environment as it may undergo alteration under various
environmental conditions (Senanayake et al. 2005). Upon exposure to air, UO 2 is
subjected to oxidation and as a result produces a secondary mineral (UO 2
2+ ) which
complexes easily with phosphates, carbonates, silicates, and sulfates (Senanayake
et al. 2005; Stefaniak et al. 2009).
24
E. M. Nkhalambayausi-Chirwa et al.