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for excitation wavelengths ranging from 240-320 nm, Albinsson et al. (
1999
) proposed
that energy absorbed by a range of chromophores (hence multiple excitation wavelengths)
is transferred to small quantities of phenylcoumarone type structures and subsequently
emitted by these fluorophores. Quinones and quinine methides have also been proposed
to act as acceptor functional groups (Olmstead and Gray,
1997
). Alternatively, Barsberg
et al. (
2003
) proposed that lignin optical properties, including emission quenching, are
influenced by charge-transfer interactions with quinones as receptors.
Fluorescence analyses of lignin and lignin sulfonates (byproducts of pulp mills) have
been employed to identify the presence of these compounds in water samples (Christman
and Minear,
1967
; Thruston,
1970
; Santos et al.,
2000
) and pulp mill liquors (Bublitz
and Meng,
1978
). Christman and Minear (
1967
) assessed fluorescence as a tool to detect
lignin sulfonates in paper mill waste waters. They noted that excitation at three wave-
lengths (
λ
ex
=253, 293, 340 nm) all resulted in emission at
λ
em
=400 nm. This observation
is similar to those noted for lignin by Albinsson et al. (
1999
) and others. Christman and
Minear (
1967
) also noted that emission maxima for these samples were not a function of
molecular weight as determined by gel permeation chromatography, although intensity
did vary between the fractions. The fluorescence behavior of lignin compounds appears to
be largely independent of wood source, with many samples having similar emission max-
ima, although different woods exhibit different fluorescence intensities (Bublitz and Meng,
1978
). Lignin sulfonate fluorescence can be detected in waters influenced by lignin waste
products. However, interferences from aquatic humic substances can be an issue (Wilander
et al.,
1974
). The presence of lignin sulfonates from pulp mills has been traced in marine
(Almgren et al.,
1975
) and freshwaters (Josefsson and Nyquist,
1976
).
A number of studies have reported strong positive correlations between DOM absor-
bance coefficients at various wavelengths and lignin phenol contents for a variety of sur-
face waters (e.g., Boyle et al.,
2009
; Spencer et al.,
2009
). Recent studies suggest that
partially oxidized products of lignin exert a strong influence on the optical properties of
DOM, especially at longer wavelengths. Del Vecchio and Blough (
2004
) provided evidence
that the long-wavelength absorption and emission properties of aquatic humic substances
largely resulted from the presence of intramolecular charge transfer interactions involving
hydroxyl-aromatic donors and quinoid acceptors formed from the partial degradation of
lignin precursors. These intramolecular interactions are proposed to involve both energy
transfer and charge transfer mechanisms (Boyle et al.,
2009
).
2.4.7 Quinones
Quinones are an important class of redox reactive molecules produced by microbes, fungi,
and higher plants whose presence in aquatic humic substances has been confirmed by elec-
tron spin resonance (ESR) (e.g., Scott et al.,
1998
) and nuclear magnetic resonance (NMR)
studies (Thorn et al.,
1992
). The group consists of molecules ranging from simple to poly-
nuclear quinones, with the anthroquinones being the largest group of naturally occur-
ring quinones (Robinson,
1991
). Quinones can exist in oxidized (quinone) and reduced
