Environmental Engineering Reference
In-Depth Information
The C-region peak does not appear because of relatively strong fluorescence
intensity at the peak A-region, which explains why this component can be
denoted as humic acid (A-like) (Fig.
3
g; Table
2
). The allochthonous humic
acid (A-like) has been identified at Ex/Em
=
~ 230/436 nm, with a minor peak
at 265/436 nm, in the peak A-region of SRHA dissolved in Milli-Q water;
at <250/400 nm in stream waters; at <250/450-470 in estuaries and coastal
marine waters; at 240/483 nm in water extracted from sugar maple leaves; and
at 220/432 nm in municipal leachate samples (Fig.
3
g; Table
2
) (Mostofa et al.
2005a
; Hunt et al.
2008
; Chen et al.
2010
; Fellman et al.
2009
,
2010
; Wu et al.
2011
; Murphy et al.
2008
). The EEM images and the fluorescence properties
of allochthonous humic acid (A-like) are apparently similar to those of alloch-
thonous fulvic acid (A-like). Another component of allochthonous humic acid
(M-like) is presumably occurring in soil samples together with allochthonous
humic acid (C-like). The allochthonous humic acid (M-like) is often detected
at Ex/Em
=
300/416 nm in the peak C-region and at 240/416 nm in the peak
A-region in soil; at 295/414 and <250/414 nm in stream waters and soil; at
330/412 and 240/412 nm in municipal leachate samples; at 295-300/465 and
>240/465 nm in plant biomass, manure and soil (Ohno and Bro
2006
; Fellman et
al.
2008
,
2009
; Wu et al.
2011
).
The allochthonous humic acid (C-like) can show a fluorescence peak at longer
Ex/Em wavelengths in the peak C-region, whereas allochthonous fulvic acid
(C-like) often shows fluorescence at shorter Ex/Em wavelengths. Allochthonous
humic acids (C-like, A-like and M-like) are not often detected in natural waters,
particularly in waters with low DOC concentration. Allochthonous fulvic acid is
generally more concentrated than humic acid in natural water, with a ratio of ful-
vic to humic acid that is typically 9:1. In some cases, especially for high-DOC
waters, the ratio decreases to 4:1 or less (Malcolm
1985
; Peuravuori and Pihlaja
1999
). For this reason, allochthonous humic acids are not often observed. The
molecular structure of humic acid is not yet defined due to its complex chemical
composition. It is generally highly aromatic in nature, with 30-51 % of aromatic
carbon compared to 14-20 % of aromatic carbon in fulvic acid (Malcolm
1985
;
Steelink
2002
; Gron et al.
1996
). The high aromaticity and the functional groups
of humic acid are responsible for the appearance of several fluorescence peaks
at peak C-regions and A-regions (Tables
1
,
2
). The fluorophores associated with
the low molecular weight fraction of humic acid (<10 kDa) exhibit relatively high
fluorescence intensity. Fluorescence sharply decreases in the molecular weight
fractions of 100-300 kDa and further increases for humic acid fractions >300 kDa
(Hayase and Tsubota
1985
; Levesque
1972
; Gosh and Schnitzer
1980
; McCreary
and Snoeyink
1980
; Visser
1984
). The fluorophores bound to humic acid are func-
tional groups with relatively high molecular weight. They show fluorescence in
the peak C-region as well as peak A-region (Hayase and Tsubota
1985
; Mostofa
et al.
2009a
,
2005a
; Kowalczuk et al.
2009
; Santín et al.
2009
; Yamashita and
Jaffé
2008
). Humic acid undergoes photoinduced decomposition by sunlight in
natural waters.
