Geoscience Reference
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increase in averaged molecular weight and in molecular area when the pH of humic acids
(HA) or fulvic acids (FA) was increased, an effect attributed to coiling or homolytic bond-
ing. Such effects were observed when either neutral electrolyte (i.e., NaCl) or hydrogen ion
concentration was high.
HA and FA extracts from soils, sediment, and natural waters have been studied by fluo-
rescence under varying pH. Generally, fluorescence changes nonlinearly as a function of pH
(
Figure 7.3A
; Smart et al.,
1976
; Laane,
1982
; Willey and Atkinson, 1982; Willey,
1984
).
Laane (
1982
) and Senesi (
1990
) described both “acid” and “alkaline” regions of greatest
impact (
Figure 7.3A
) and ascribed the increased fluorescence intensity to ionization of
low p
K
a
moieties (e.g., carboxylic acids) in the acid region and high p
K
a
(e.g., phenols or
amino groups) in the alkaline region. Senesi (
1990
) summarized that most FA fluorescence
emission studies found either decreasing intensity with increasing pH or an emission peak
between pH 5 and 7. Over pH ranges of 3-9, FA excitation intensity increased for aquatic,
peat, and microbial FA, yet decreased for soil FA. Senesi (
1990
) postulated that these con-
trasting results occur from the wide variety of acidic functional group ionization constants
and molecular rearrangements. Fluorophore deprotonation (phenolic hydroxyls), inter- and
intramolecular hydrogen bond disruption, and proton quenching were suggested as mecha-
nisms for excitation intensity increase and emission intensity reduction. Red shifts in emis-
sion wavelengths up to 10 nm were also noted. These effects support the supposition that
acidic and basic functional group ionization has occurred with changing pH. Emission
spectra from extracted HA and FA as well as DOM all respond similarly to pH changes.
In addition to emission and excitation scans, pH effects on DOM fluorescence has been
extensively studied using synchronous-scan fluorescence (SF). At lower pH, soil fulvic
acids exhibit a shoulder peak at 360 and more pronounced peak at 394 nm (18 nm syn-
chronous-scan offset). As pH is increased, these peaks decrease and merge into a single
relatively flat peak at 400 nm (pH 10). A peak at ~460 nm is reasonably pronounced at
low pH (4.5) but decreases, flattens and blue shifts to ~440 nm at pH 10 (Senesi,
1990
).
Using excitation-emission matrices (EEMs) is another means used to assess humic and
fulvic acid fluorescence shifts due to pH variation. Both soil humic and aquatic humic
substances have been analyzed with this technique. Fluorescence intensity has been shown
to increase as pH is increased for soil and aquatic fulvic and humic acids (Mobed et al.,
1996
). In longer wavelength regions (~450 nm), an emission peak was red shifted for soil
and aquatic humic and fulvic acids as pH increased. Aquatic humic acids had a lower
wavelength emission peak (~320 nm) which blue shifted as solution pH was increased.
Two separate pH-dependent wavelength shift behaviors (short and long wavelength emis-
sion peaks) were attributed to two general processes: the two fluorescence maxima found
in phenolic compounds and conformational changes that might expose different functional
groups to the solvent.
Humic and fulvic acids (or their spectra within natural DOM) have been the primary
focus of studies assessing optical property changes with respect to pH. Ma et al. (
2010
)
found no significant changes in the optical properties of humic and fulvic material as pH
was increased from 7 to 10. Several recent studies have assessed changes in amino acid
