Environmental Engineering Reference
In-Depth Information
Allochthonous Humic Acids
Allochthonous humic acids in surface waters can be defined as molecularly het-
erogeneous and supramolecular, with molecular weight ranging from less than
500 to over 300,000 Daltons. The largest fraction is found in the range larger
than 300,000 Daltons. They are optically active, typically refractory to microbial
degradation, photolytically reactive, biogenic, and yellow-colored organic acids.
They are insoluble and form precipitates at pH < 2 (MacFarlane
1978
; Hayase and
Tsubota
1983
; Sutton and Sposito
2005
; Steelink
2002
; Aiken and Malcolm
1987
;
Aiken and Gillam
1989
; Schulten and Schnitzer
1998
). Allochthonous humic acids
of various origin (soil, bog peat, sewerage sludge) have relatively high contents
of organic N to organic C, i.e. they have relatively low C:N atomic ratio (8-51).
Standard SRHA (1S1011H and 2S101H) have C:N
=
44-45. Allochthonous
humic acids also have relatively low contents of O and organic P, high contents
of S, relatively high aromaticity (30-40 % of total C) and relatively low contents
of aliphatic C (~30-47 %) compared to fulvic acids (Malcolm
1985
; Wetzel
1983
;
McKnight et al.
2001
; Meyers-Schulte and Hedges
1986
; Ma et al.
2001
; McIntyre
et al.
2005
; Frimmel
2004
; Aiken and Malcolm
1987
; Abbt-Braun and Frimmel
1990
; Abbt-Braun et al.
1991
; IHSS
2011
; Senesi
1990
). It has been shown that
the contents of aromatic and other functional groups are very variable depending
on the different sources of humic acids and their photobiogeochemical changes
in natural waters. The aromaticity of humic acids is very low (~15 %) in marine
waters (Malcolm
1990
).
Allochthonous humic acids have a supramolecular structure composed of a
variety of functional groups (or fluorophores), such as aromatic carboxylic and di-
carboxylic acids, aromatic OH groups including phenols (or catechols) and phe-
nolic acids, aliphatic or carbohydrate OH, aldehyde or aliphatic ketones, amide/
amino groups, peptides, esters (COOR) or benzene-containing methoxylates, poly-
methylenes (-CH
2
-), hydroxycoumarin-like structures, chromone, xanthone, qui-
none, O, N, S, and P-atom-containing functional groups attached to aromatic and
aliphatic carbon, methylated forms of para-coumaric, ferrulic, vanillic and syringic
acids, pyrrole, indole, imidazole and pyridine groups (Malcolm
1985
; Sutton and
Sposito
2005
; Steelink
2002
; Lambert and Lankes
2002
; Leenheer and Croué
2003
;
Stevenson
1982
; Schulten and Schnitzer
1998
; Laane
1984
; Mao et al.
1998
; Hu
et al.
2000
; Mahieu et al.
2000
,
2002
; Zang et al.
2000
; Kujawinski et al.
2009
;
Piccolo
2002
; Vairavamurthy and Wang
2002
; Abe and Watanabe
2004
; Schmidt-
Rohr et al.
2004
; Guignard et al.
2005
; Fiorentino et al.
2006
). A typical humic acid
containing 0.2 % reduced sulphur has only 63
μ
mol g
−
1
of thiol sites (Bloom et al.
2001
). Amino acids, amino sugars, ammonium (NH
4
+
) and nucleic acid bases make
up 46-53 % of the N associated with humic acids (Schnitzer
1985
). Depending
on the elemental compositions of C, H, O, and N, an empirical formula for humic
acids has been proposed as C
10
H
12
O
5
N and a representative molecular formula as
C
72
H
72
O
30
N
4
·
8H
2
O (Steelink
2002
; Schnitzer and Khan
1978
; Paciolla et al.
1998
).
The stable carbon isotope (
δ
13
C) fractionation of standard SRHA is -27.7 ‰,
which indicates that they are most likely derived from higher plant matter (IHSS
