Geoscience Reference
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Fig. 16.20 Fluorescence quenching (FQ) of 1-naphthol in the presence of HA as a function of
pH and reaction time (1-naphthol = 8 lmol/L; HA = 11 ppm C; ionic strength of 0.1 M LiCl);
F
0
and F denote fluorescence intensities in the absence and in the presence of the quencher (HA),
respectively. Reprinted with permission from Karthikeyan and Chorover (
2000
). Copyright 2000
American Chemical Society
with humic acid (HA) was studied during seven days of contact, as a function of
pH (4-11), ionic strength (0.001 and 0.1 M LiCl), and dissolved O
2
concentration
(DO of 0 and 8 mg/L) using fluorescence, UV absorbance, and equilibrium dial-
ysis techniques. In a LiCl solution, even in the absence of HA, oxidative trans-
formation of 1-naphthol mediated by O
2
was observed. In addition, the presence of
humic acid in solution, in the absence of DO, was found to promote 1-naphthol
oxidation. These reactions are affected by the solution chemistry (pH, ionic
strength, and cation composition).
Because the mechanisms of 1-naphthol complexation with HA obtained by
using these three techniques exhibit similar pathways, we present the results only
from fluorescence spectroscopy. The ratio of fluorescence intensity in the absence
(F
o
) and in the presence (F) of the quencher (HA) over time, as affected by pH and
ionic strength, is illustrated in Fig.
16.20
. The fluorescence intensity of a fluoro-
phore in the absence of a quencher is directly proportional to its concentration in
solution, and therefore time-dependent changes in F
o
can be used to assess the
stability of 1-naphthol under different pH and ionic strength. Quenching (FQ) of
1-naphthol fluorescence by humic acid increased with equilibration time from one
to seven days. This time-dependent relationship was found to result from weak
complexation of 1-naphthol by humic acid and by its oxidative transformation
(slow reaction), resulting in the formation of secondary products that are more
