Environmental Engineering Reference
In-Depth Information
Meyers-Schulte and Hedges
1986
; Ma et al.
2001
; McIntyre et al.
2005
; Frimmel
2004
; Aiken and Malcolm
1987
; Abbt-Braun and Frimmel
1990
; Abbt-Braun et al.
1991
; IHSS
2011
; Senesi
1990
).
Allochthonous fulvic acids are supramolecular structures composed of a
variety of functional groups or components such as benzene-containing car-
boxyl groups, ketones, methoxylate and phenolic groups (catechol-type),
carboxylic and di-carboxylic groups, ethers, esters, amides, aliphatic OH, car-
bohydrate OH, -C
=
C-, hydroxycoumarin-like structures, chromone, xan-
thone, quinones, flavones, O, N, S, and P-atom-containing functional groups
attached to aromatic and aliphatic C, indole groups, degraded lignins, and so on
(Malcolm
1985
; Dai et al.
1996
; Frimmel
2004
; Allard et al.
1994
; McKnight
et al.
1988
; Leenheer et al.
1995
,
1998
,
2001
; Brown and Rice
2000
; Haiber
et al.
2001
; Kujawinski et al.
2002
; Lambert and Lankes
2002
; Cook et al.
2003
; Stenson et al.
2003
; Leenheer and Croué
2003
; Leenheer
2007
; Killops
and Killops
1993
). Lignins are complex, high-mass, primarily ether-linked
phenylpropanoid biopolymers including only C, H, and O atoms in their molec-
ular structure. They are mostly found in wood cells, whereas the main build-
ing blocks for the phenyl portion of lignins are coumaryl, coniferyl, and sinapyl
alcohols that vary from plant to plant (Helm
2000
; Filley et al.
2002
; Lewis
and Yamamoto
1990
; Christman and Oglesby
1971
). The lignin biopolymer
is degraded by fungi and eventually bacteria through different pathways that
include depolymerization, demethylation, side-chain oxidation, and aromatic
ring cleavage (Lewis and Yamamoto
1990
; Nelson et al.
1987
; Grushnikov and
Antropova
1975
; Higuchi
1993
; Radnoti de Lipthay et al.
1999
; Leonowicz
et al.
2001
; Lowe and Bustin
1989
).
In humic substances, 60-90 % of the acid groups are carboxylic and the
remainder are phenolic (Leenheer et al.
1995
). S-XANES have shown that sulphur
is present in humic substances in many different oxidation states: organic sulfides
(R-S-R), thiol (-SH), di- and polysulfides (R-S-S-R), sulfoxide (R-SO-R), sul-
fone S compounds (R-SO
2
-R), sulfonate (HSO
3
-R), and sulfate esters (HSO
4
-R)
(Frimmel
2004
; McKnight et al.
1988
; Morra et al.
1997
; Xia et al.
1998
,
1999
;
Schnitzer and Khan
1978
).
Depending on the major elemental composition of C, H, O and N disregarding
S, an average empirical formula for fulvic acid has been considered as C
12
H
12
O
9
N
(Steelink
2002
; Leenheer et al.
1998
; Paciolla et al.
1998
; Schnitzer
1985
). Based
on accurate mass measurements, molecular formulas have been assigned to 4626
individual Suwannee River fulvic acids with molecular masses between 316
and 1098 Da, which led to plausible structures consistent with degraded lignin
(Leenheer and Croué
2003
).
Hummel (Fimmen et al.
2007
) has shown that a fulvic molecule (i) contains
on average 5.5 mmoles of carboxyl groups per gram, which corresponds to one
carboxylic group per six carbon atoms, or one group per aromatic ring if distrib-
uted evenly; (ii) has an average phenolic group content of 1.2 mol per gram, which
means one phenolic group per 30 carbon atoms, or only two phenolic groups per
