Environmental Engineering Reference
In-Depth Information
phenolic groups, carboxyl groups, alcoholic OH, carbohydrate OH, -C
=
C-,
hydroxycoumarin-like structures, chromone, xanthone, quinoline, and O-, N-, S-
and P-containing functional groups. They include aromatic carbon (17-30) and
aliphatic carbon (47-63 %) (Malcolm
1985
; Senesi
1990
; Leenheer and Croue
2003
; Steelink
2002
). All of these functional groups can be considered as key
chromophores in fulvic and humic acids in natural waters.
2.3 Theory of CDOM Absorbance
Photon absorption by a CDOM chromophore in aqueous solution firstly induces
the excitation of an electron from its ground state to an excited one (Fig.
2
),
(Mostofa et al.
2009
; Senesi
1990
). Three types of electronic transitions occur
with the CDOM chromophores in natural waters due to the absorption of UV or
visible radiation: (i) transitions involving
π
,
σ
, and
n
electrons; (ii) transitions
involving charge-transfer electrons, and (iii) transitions involving
d
- and
f
- orbital
electrons in metals. The CDOM chromophores (e.g. fulvic acid, humic acid
and tryptophan) mostly undergo transitions involving
n
or
π
electrons to the
π
*
excited state or charge-transfer electrons, and excitation of unpaired electrons in
d
- and
f
- orbitals (e.g. fulvic acid complexes with transition metals such as Cu(II)
and Fe(III), having unpaired electrons) (Senesi
1990
; Schulman
1985
; Voelker
and Sulzberger
1996
; Senesi
1990
; Fox
1990
; Morales et al.
1997
; Grabowski
et al.
2003
). The chromophores of CDOM absorb radiation in the wavelength
range 200-700 nm and control the penetration along the water column of UV-B
(280-320 nm), UV-A (320-400 nm) and total UV radiation (280-400 nm), as
Rotational
electronic levels
Excited singlet state
Vibrational electronic levels
Energy
Ground state
Fig. 2
A schematic energy level diagram for an organic molecule showing their rotational and
vibrational electronic levels
