Environmental Engineering Reference
In-Depth Information
of a variety of functional groups (or fluorophores) such as benzene-containing car-
boxyl, methoxylate and phenolic groups (catechol-type), carboxylic and di-carboxylic
groups, alcoholic OH, carbohydrate OH, -C
=
C-, hydroxycoumarin-like structures,
fluorophores containing Schiff-base derivatives, chromone, xanthone, quinoline, O, N,
S, and P-atom-containing functional groups including aromatic carbon (17-30 %) and
aliphatic carbon (47-63 %) (Malcolm
1985
; Senesi
1990
; Morel and Hering
1993
;
Leenheer et al.
1998
; Steelink
2002
; Leenheer and Croue
2003
; Stenson et al.
2003
;
Fimmen et al.
2007
). In humic substances, 60-90 % of the acidic groups are carboxylic
and the remainder are phenolic (Morel and Hering
1993
). S-XANES measures have
shown that sulphur is present in humic substances in many different oxidation states:
thiol, thiophene or disulfide, sulfoxide, sulfone, sulfonate and sulfate esters (Morra et
al.
1997
; Xia et al.
1998
; Fimmen et al.
2007
). A typical humic acid containing 0.2 %
reduced sulphur has only 63
μ
mol g
−
1
of thiol sites (Bloom et al.
2001
).
Fulvic and humic acids can be roughly classified into two categories: minor
(approximately 1-10 %), strong sites, and major (approximately 90-99 %), weak
sites (Mandal et al.
1999
; Chakraborty et al.
2007
; Buffle and Filella
1995
).Wu
and Tanoue
2001c
The strong binding sites are first occupied entirely, after which
the weak binding sites are occupied (Mandal et al.
1999
; Chakraborty et al.
2007
;
Buffle and Filella
1995
; Wu and Tanoue
2001c
). The major sites are less diverse
in type, but in number they represent the majority of the complexation sites (e.g.,
carboxylate and phenolate groups in humic substances). The minor sites, which
are fewer in number, include all those which exhibit a wide range of binding ener-
gies, including strongly complexing nitrogen- and sulphur-containing sites (Filella
2008
). Major sites represent the largest proportion of dissociable groups. They
play an important role in the polyelectrolytic properties of the complexant (Filella
2008
). The distinction between major and minor sites also reflects a chemical
selectivity: since the major sites have oxygen-containing groups, they will prefer-
ably react with hard metals such as calcium and magnesium (Filella
2008
).
EEM spectroscopy (EEMS) during the complexation between extracted fulvic
acid and several metals [Cu(II), Ni(II), Co(II), Cd(II) and Ca(II)] also showed two
major kinetically distinguishable binding sites on fulvic acid, 'fast' and 'slow',
having reaction half-lives of 1.3-3.9 and 34.7-69.3 s, respectively (Wu et al.
2004c
). The 'fast' sites in fulvic and humic acids are susceptible to be the fluo-
rophores (or functional groups) bound at the longer wavelength peak C-region,
which are rapidly complexed with metal ions. In contrast, the 'slow' sites are con-
sidered to be the fluorophores at the shorter wavelength peak A-region.
Another time-resolved fluorescence study demonstrates that fulvic acid during
its compexation with metal ions has three binding sites, which can be assigned
lifetimes and wavelength maxima as follows: ~50 ps at 392 nm, ~430 ps at
465 nm and 4.2 ns at 512 nm (Cook and Langford
1995
). The functional groups
in humic acid exhibit different proton binding properties as their concentration
increases from 20 to 100 mg L
−
1
. The acidic functional group content values
suggest that 40 % of the total acidity is accounted for by carboxylic-type sites
in humic acids extracted from peat and soil (Vidali et al.
2009
). Proton affinity
constants to humic acids extracted from various sources suggest that carboxylic
