species is the fundamental feature of metals as the concentration of ligands in the
water alters. The form of the dissolved metal can vary considerably as in an estuary
transition of freshwater to seawater is caused by the metal [91].
Iron, one of the earth’s most plentiful metals, occurs in various oxidation states
while Fe (II) and Fe (III) are its two common forms in water. Fe (OH) 3 (oxidizing
environment), Fe
+2 and FeHCO 3 (reducing environment) are the species observed
in water at neutral pH [103]. For the normal functioning of living organisms, Iron is
an vital element and its deficiency as well as overload could be very harmful for the
plants and animals [108]. The WHO has set a standard limit of 0.3 mg/L for iron in
the drinking water. So the increased concentration of iron in natural water reservoirs
poses a potential risk for human health system as well as the surroundings. Though
compared to iron insufficiency, the overload or the overexposure of iron occurs less
commonly but poses several serious health issues related to heart and liver [60],
diabetes [29], cancer [6, 68], along with neurodegenerative disorders [9].
Manganese (Mn) occurs naturally in surface water and groundwater; however,
its addition to water is largely contributed by human activities also [110]. It is an
indispensable micro nutrient for all forms of life [30] since it combines to and/or
control a number of body enzymes [20]. Manganese, tenth most abundant element
in earth’s, crust is a redox sensitive and is found in different oxidation states. In the
aqueous environment, Mn
+2 (reducing environment), MnO 2 and Mn 2 O 3 (oxidizing
environment) are the most prominent species. The exposure to too much doses of
Mn results in severe nervous disorders and prominently targets brain [20] and in
worst case scenario may permanently lead to a neurological problem which shows
similar symptoms like Parkinson’s disease [49]. Moreover, it has been observed
that the exposure to Mn from food is generally higher than the intake from drinking
water [110].
Chromium forms one of the most frequent elements in both earth’s crust as well
as water. The WHO permits 0.05 mg/L value of chromium in drinking water. In
water it is unable to occur as freely as cation and appears as polyatomic anions
chromate and dichromate (CrO 4
2− and Cr 2 O 7
2− , respectively) [105]. Though this is
not poisonous in itself and holds an significant role in the carbohydrate metabolism
in our body, however, certain compounds of chromium mainly in its hexavalent
state-chromium (VI) (carcinogenic form) cause irritants and diseases related to the
digestive, excretory, respiratory and reproductive system, skin diseases ultimately
cancers [41].
Nickel is a broadly dispersed component present in air, water and soil, and was
dubbed as the “Allergen of the Year 2008” [26]. It can naturally enter the environment through processes such as weathering of minerals and rocks as well as
from anthropogenic sources. In the aquatic environment more than 90% of the
nickel is linked with particulate matter of sediments [45]. In natural waters, nickel
occurs in the +2 valence state. Ni
+2 form of nickel has been considered to be mainly
responsible for evoking a toxic effect in aquatic organisms. The WHOs has put the
limit of nickel concentration in water at 0.02 mg/L. Its compounds risk causing
26
A. Ahamad et al.
water alters. The form of the dissolved metal can vary considerably as in an estuary
transition of freshwater to seawater is caused by the metal [91].
Iron, one of the earth’s most plentiful metals, occurs in various oxidation states
while Fe (II) and Fe (III) are its two common forms in water. Fe (OH) 3 (oxidizing
environment), Fe
+2 and FeHCO 3 (reducing environment) are the species observed
in water at neutral pH [103]. For the normal functioning of living organisms, Iron is
an vital element and its deficiency as well as overload could be very harmful for the
plants and animals [108]. The WHO has set a standard limit of 0.3 mg/L for iron in
the drinking water. So the increased concentration of iron in natural water reservoirs
poses a potential risk for human health system as well as the surroundings. Though
compared to iron insufficiency, the overload or the overexposure of iron occurs less
commonly but poses several serious health issues related to heart and liver [60],
diabetes [29], cancer [6, 68], along with neurodegenerative disorders [9].
Manganese (Mn) occurs naturally in surface water and groundwater; however,
its addition to water is largely contributed by human activities also [110]. It is an
indispensable micro nutrient for all forms of life [30] since it combines to and/or
control a number of body enzymes [20]. Manganese, tenth most abundant element
in earth’s, crust is a redox sensitive and is found in different oxidation states. In the
aqueous environment, Mn
+2 (reducing environment), MnO 2 and Mn 2 O 3 (oxidizing
environment) are the most prominent species. The exposure to too much doses of
Mn results in severe nervous disorders and prominently targets brain [20] and in
worst case scenario may permanently lead to a neurological problem which shows
similar symptoms like Parkinson’s disease [49]. Moreover, it has been observed
that the exposure to Mn from food is generally higher than the intake from drinking
water [110].
Chromium forms one of the most frequent elements in both earth’s crust as well
as water. The WHO permits 0.05 mg/L value of chromium in drinking water. In
water it is unable to occur as freely as cation and appears as polyatomic anions
chromate and dichromate (CrO 4
2− and Cr 2 O 7
2− , respectively) [105]. Though this is
not poisonous in itself and holds an significant role in the carbohydrate metabolism
in our body, however, certain compounds of chromium mainly in its hexavalent
state-chromium (VI) (carcinogenic form) cause irritants and diseases related to the
digestive, excretory, respiratory and reproductive system, skin diseases ultimately
cancers [41].
Nickel is a broadly dispersed component present in air, water and soil, and was
dubbed as the “Allergen of the Year 2008” [26]. It can naturally enter the environment through processes such as weathering of minerals and rocks as well as
from anthropogenic sources. In the aquatic environment more than 90% of the
nickel is linked with particulate matter of sediments [45]. In natural waters, nickel
occurs in the +2 valence state. Ni
+2 form of nickel has been considered to be mainly
responsible for evoking a toxic effect in aquatic organisms. The WHOs has put the
limit of nickel concentration in water at 0.02 mg/L. Its compounds risk causing
26
A. Ahamad et al.