Environmental Engineering Reference
In-Depth Information
functional groups such as the benzene ring in phenolic structures with the addi-
tion 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 oxi-
dation 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
); (iii)
reductive cleavage of halogenated hydrocarbons such as trichloroethylene, a common
pollutant in soil and groundwater, which can be degraded to ethylene and hydrochlo-
ric acid (Steelink
2002
); (iii) abiotic reduction of mercury in the presence of a com-
peting 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 com-
pounds 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 ful-
vic and humic acids can be oxidized, as is the case of catechol moieties (oxidized
to quinones), aldehydes (to carboxylic acids), alcohols (to aldehydes or carbox-
ylic acids) and so on (Steelink
2002
). These redox processes account for the pres-
ence 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
):
FA
(
ox
)
+
e
−
+
H
+
=
FA
(
red
)
,E
◦
=
500 mV
(
FA
)
and 700
−
794 mV
(
HA
)
(2.1)
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
;
