Environmental Engineering Reference
In-Depth Information
Photodegradation of Fulvic Acid and Humic Acid
The fluorescence of standard or extracted fulvic and humic acids is typically
decreased by photoinduced degradation under sunlight. The fluorescence of SRFA
dissolved in Milli-Q water is photolytically decreased under simulated sunlight
(by 42 % in 1 mg L
−
1
SRFA for 10 h, 23 % for 3 mg L
−
1
and 3 h, 20 % for
5 mg L
−
1
and 3 h, and by 22 % with 1 mg L
−
1
SRFA
+
50
μ
M NO
2
−
for 3 h)
(Table
4
). Extracted fulvic acid from Göta River shows a decrease in fluorescence
(32 %) in alkaline samples (6.3 mg L
−
1
fulvic acid in 0.5 M NaOH solution) after
13 days UV-B irradiation (Table
4
). A 6-h summer sunlight exposure of fulvic
and humic acids extracted from lake, pond and marsh showed that the decrease
of humic acid fluorescence was relatively higher (64-100 %) compared to ful-
vic acid (48-83 %) (Table
4
). It is reported that the fluorescence of humic acid
is highly depleted in acidic samples, and undergoes a more pronounced decrease
compared to fulvic acid even at higher pH (Wu et al.
2005
). Correspondingly,
photoirradiation can decompose 35 % of extracted Nordic Reference humic
acid (NoHA) and 24 % of extracted Nordic Reference fulvic acid (NoFA) from
humus-rich pond water in photoexperiments conducted using a solar simulator
(Corin et al.
1996
). The reported results suggest that the photoinduced degrada-
tion of humic acid is pH and concentration dependent, but the reason behind this
phenomenon is still unknown. However, the relatively high photolability of humic
acid can be in agreement with the high level of aromaticity (30-51 %), in particu-
lar when compared to fulvic acid (14-21 %) (Malcolm
1985
; Gron et al.
1996
;
Wu et al.
2005
).
The rate constants for the decrease in fluorescence and for DOC loss are signifi-
cantly higher for humic than for fulvic acid, as obtained by photoexperiments car-
ried out at different pH levels on extracted humic and fulvic acid from upstreams
(Wu et al.
2005
). Thus, photodegradation of the humic acid fraction is significantly
higher than the fulvic acid fraction and is more sensitive to pH. Interestingly, the
higher photolability of humic compared to fulvic acid correlates well with the
higher production rate of H
2
O
2
upon irradiation of Suwannee River humic Acid
(179
×
10
−
2
M s
−
1
) than for Suwannee River fulvic Acid (69
×
10
−
2
M s
−
1
)
(Mostofa and Sakugawa
2009
). This might imply that the production of H
2
O
2
is
a primary step for the photoinduced degradation of DOM in aqueous solution.
Humic acid could thus be the primary target of DOM photodegradation in natural
waters (Wu et al.
2005
). In contrast, fulvic acid is photolytically more stable than
humic acid in aqueous media and may play a vital role in biogeochemical pro-
cesses due to its longer lifetime in natural waters.
It can be noted that the fluorescence intensity of humic acid is increased by
irradiation in some particular cases. Thus, increases have been observed of ca.
70 % for Suwannee River Humic Acid (SRHA) (1 mg L
−
1
, 10 h), 5 % for SRHA
(5 mg L
−
1
, 3 h), and 4 % in alkaline samples (6.5 mg L
−
1
Göta River humic acid
in 0.5 M NaOH) (Table
4
). The reason behind such a phenomenon may be the
generation of aromatic photoproducts upon irradiation of humic acid (Corin et al.
1996
). Some of these photoproducts may show fluorescence at peak C-region, for
