Environmental Engineering Reference
In-Depth Information
Fig. 8
Changes in the
fluorescence intensities
of algal-originated
autochthonous fulvic
acids (C-like and M-like
component) identified in
lake DOM (Lake Hongfeng,
China) and its pH effect. The
fluorescent components are
identified using PARAFAC
model on sample's EEM
spectra.
Data source
Mostofa
KMG et al. (unpublished
data)
pH
C-region is less affected by pH: there is a 24 % increase at pH 2, a decrease at pH
6 and a further increase up to pH 10. In contrast, the fluorescence intensity of the
C-like component is significantly affected at the peak C-region: compared to the
initial pH of 7.5 there is a 67 % increase at pH 2, then a gradual decrease up to
the lowest intensity observed at pH 10. In the case of the peak A-region, one sees
a 10 % decrease followed by a gradual increase up to a value that is 18 % higher
compared to the initial one (Mostofa KMG et al., unpublished data). Therefore,
the effect of pH on algal-originated autochthonous fulvic acids (M- and C-like) is
quite different compared to allochthonous fulvic acids.
An increasing fluorescence intensity of humic substances has been detected as
pH increases from 4 to 5.5, above which the increase is less important (Vodacek
and Philpot
1987
). In fulvic acid standards and wastewater treatment plant sam-
ples, when lowering the pH from 7 to 3 the decrease of the fluorescence intensity is
30-40 % at peaks C- and A-regions as well as over most of the EEM range, includ-
ing the peak T- and T
UV
-region (Westerhoff et al.
2001
). For the fulvic acid-like
component, the excitation-emission wavelengths for peak C undergo a red shift
with increasing pH (Mostofa KMG et al., unpublished data; Westerhoff et al.
2001
;
Spencer et al.
2007
). The pH effect on the complex formation of trace elements
with DOM shows that, for DOM, the fluorescence index (f
450/500 at Ex370 nm
) has a
decreasing trend with increasing pH. For DOM
+
Hg(II) complexation, one sees
an increase of the fluorescence index till pH 8 followed by a decrease up to pH
10 (Fu et al.
2007
). This suggests that the fluorescence properties of DOM might
be affected by several factors such as pH, coexisting metal ions and other organic
substances. In addition, the fluorescence properties of three fluorescent whitening
agents (FWAs) are modified by pH in the 3-7 range, and the largest pH effect has
been detected for the distyrylbiphenyl (DSBP) (Westerhoff et al.
2001
).
