22
K. M. G. Mostofa et al.
functional groups such as the benzene ring in phenolic structures with the addition of hydrophilic sulfonic, hydroxyl or trimethylammonium functional groups
(Klavins and Purmalis 2010). This effect can be used for the development of
biopolymers with surfactant properties (Klavins and Purmalis 2010; Heinze and
Liebert 2001). Humic substances might influence plankton food chains in lakes in
two ways (Jones 1992): (i) By altering the physical or chemical environment and
thus modifying autotrophic primary production and the dependent food chains;
and (ii) By acting as a direct carbon/energy source for food chains.
4.1.1 Redox Behavior of Fulvic and Humic Acids
Fulvic and humic acids (humic substances) can act as reductants and oxidants in
aqueous media (Lovley et al. 1996; Richardson 2007; Wilson and Weber 1979; Nash
et al. 1981; Skogerboe and Wilson 1981; Österberg and Shirshova 1997; Scott et al.
1998; André and Choppin 2000; Steelink 2002; Kuczewski et al. 2003; Shcherbina
et al. 2007). They are capable of reducing Fe 3+ , Sn 4+ , V 5+ and Cr 4+ . The +IV oxidation states of the redox-sensitive actinides (e.g. Pa, Np, U, Pu) are stabilized by
complexation with fulvic and humic acids. Fulvic and humic acids are thus capable
of detoxifying surface water and soils contaminated with toxic organic and inorganic
chemicals. Some examples are (i) reduction of metals from toxic valence states to
non-toxic states, such as Cr 4+ to Cr 3+ , V 5+ to V 4+ , or UO 2
2+ and UO 2 OH + to U 4+
(Steelink 2002; Wittbrodt and Palmer 1995; Markich 2002; Freyer et al. 2009); (ii)
reductive cleavage of halogenated hydrocarbons such as trichloroethylene, a common
pollutant in soil and groundwater, which can be degraded to ethylene and hydrochloric acid (Steelink 2002); (iii) abiotic reduction of mercury in the presence of a competing ion as well as methylation of the carboxylic groups of humic and fulvic acids,
which can consume methylmercury (Allard and Arsenie 1991); and (iv) reduction of
organic nitro groups to amines. For instance, trinitrotoluene (TNT) is reduced to compounds such as aminodinitrotoluene that can form complexes with fulvic and humic
acids (Steelink 2002). Note that TNT is an explosive that can migrate to and pollute
groundwater.
On the other hand, it has also been observed that the functional groups in fulvic and humic acids can be oxidized, as is the case of catechol moieties (oxidized
to quinones), aldehydes (to carboxylic acids), alcohols (to aldehydes or carboxylic acids) and so on (Steelink 2002). These redox processes account for the presence of intermediates such as semiquinones in fulvic and humic acids. A typical
redox process involving fulvic acids (FA) and humic acids (HA) can be depicted
as below (Wilson and Weber 1979; Skogerboe and Wilson 1981):
For instance, SRHA has standard reduction potential E° = 760 ± 6 at pH
5–7. The E° values are variable depending on the pH (Wilson and Weber 1979;
(2.1)
FA (ox) + e
− + H
+ = FA (red) , E
◦ = 500 mV (FA) and 700 − 794 mV (HA)
K. M. G. Mostofa et al.
functional groups such as the benzene ring in phenolic structures with the addition of hydrophilic sulfonic, hydroxyl or trimethylammonium functional groups
(Klavins and Purmalis 2010). This effect can be used for the development of
biopolymers with surfactant properties (Klavins and Purmalis 2010; Heinze and
Liebert 2001). Humic substances might influence plankton food chains in lakes in
two ways (Jones 1992): (i) By altering the physical or chemical environment and
thus modifying autotrophic primary production and the dependent food chains;
and (ii) By acting as a direct carbon/energy source for food chains.
4.1.1 Redox Behavior of Fulvic and Humic Acids
Fulvic and humic acids (humic substances) can act as reductants and oxidants in
aqueous media (Lovley et al. 1996; Richardson 2007; Wilson and Weber 1979; Nash
et al. 1981; Skogerboe and Wilson 1981; Österberg and Shirshova 1997; Scott et al.
1998; André and Choppin 2000; Steelink 2002; Kuczewski et al. 2003; Shcherbina
et al. 2007). They are capable of reducing Fe 3+ , Sn 4+ , V 5+ and Cr 4+ . The +IV oxidation states of the redox-sensitive actinides (e.g. Pa, Np, U, Pu) are stabilized by
complexation with fulvic and humic acids. Fulvic and humic acids are thus capable
of detoxifying surface water and soils contaminated with toxic organic and inorganic
chemicals. Some examples are (i) reduction of metals from toxic valence states to
non-toxic states, such as Cr 4+ to Cr 3+ , V 5+ to V 4+ , or UO 2
2+ and UO 2 OH + to U 4+
(Steelink 2002; Wittbrodt and Palmer 1995; Markich 2002; Freyer et al. 2009); (ii)
reductive cleavage of halogenated hydrocarbons such as trichloroethylene, a common
pollutant in soil and groundwater, which can be degraded to ethylene and hydrochloric acid (Steelink 2002); (iii) abiotic reduction of mercury in the presence of a competing ion as well as methylation of the carboxylic groups of humic and fulvic acids,
which can consume methylmercury (Allard and Arsenie 1991); and (iv) reduction of
organic nitro groups to amines. For instance, trinitrotoluene (TNT) is reduced to compounds such as aminodinitrotoluene that can form complexes with fulvic and humic
acids (Steelink 2002). Note that TNT is an explosive that can migrate to and pollute
groundwater.
On the other hand, it has also been observed that the functional groups in fulvic and humic acids can be oxidized, as is the case of catechol moieties (oxidized
to quinones), aldehydes (to carboxylic acids), alcohols (to aldehydes or carboxylic acids) and so on (Steelink 2002). These redox processes account for the presence of intermediates such as semiquinones in fulvic and humic acids. A typical
redox process involving fulvic acids (FA) and humic acids (HA) can be depicted
as below (Wilson and Weber 1979; Skogerboe and Wilson 1981):
For instance, SRHA has standard reduction potential E° = 760 ± 6 at pH
5–7. The E° values are variable depending on the pH (Wilson and Weber 1979;
(2.1)
FA (ox) + e
− + H
+ = FA (red) , E
◦ = 500 mV (FA) and 700 − 794 mV (HA)