Environmental Engineering Reference
In-Depth Information
3.7 Homogeneous and Heterogeneous Complexation of
Fulvic and Humic Acids to Metals
Homogeneous complexation is operationally defined as a chemical binding equi-
librium behavior (approximately in the ratio of 0.8-1:1) between metal ion and
a ligand, when they are mixed up under specific conditions in aqueous media.
Heterogeneous complexation is operationally defined as a chemical binding equi-
librium behavior with a <0.8:1 ratio between metal ion and a ligand.
It has been shown that fulvic and humic acids (humic substances) can behave
as homogeneous and heterogeneous complexants to trace metals in waters (Filella
2008
; Chakraborty et al.
2007
; Pinheiro et al.
1994
; Town and Filella
2000
;
Murimboh
2002
). A comparison of the heterogeneity parameter (
Γ
, termed Gamma:
Greek Capital Letter) for Zn(II), Cd(II), Pb(II) and Cu(II) complexes in differ-
ent model solutions of SRFA shows that
Γ
Cd >
Γ
Zn >
Γ
Pb >
Γ
Cu (Chakraborty
et al.
2007
). The results show that the value of
Γ
remains about the same for a
given metal: the value of
Γ
for Cu(II) is ~0.50 in all metal to SRFA solutions with
C
M
/
C
SRFA
=
0.01, 0.001, and 0.0005, suggesting that SRFA behaves as an hetero-
geneous complexant for Cu(II) (Chakraborty et al.
2007
). The value of
Γ
for Pb(II)
is ~0.70 in comparable solutions, suggesting that SRFA behaved as a less hetero-
geneous complexant for Pb(II) compared to Cu(II). The
Γ
value obtained for Cd
is ~0.94, suggesting that SRFA almost behaved as a homogeneous complexant for
Cd, and a bit lower for Zn (
Γ
~ 0.86) (Chakraborty et al.
2007
). These results in
combination with other studies suggest that SRFA behaves as a relatively homoge-
neous complexant for Zn(II) and Cd(II), as a relatively heterogeneous complexant
for Pb(II) and as an even more heterogeneous complexant for Cu(II) (Chakraborty
et al.
2007
; Town and Filella
2000
; Murimboh
2002
). Coherently, it has also been
shown that humic acid derived from peat performs more heterogeneous complexa-
tion of Pb(II) compared to Cd(II) (Pinheiro et al.
1994
).
4 Binding Sites (or Functional Groups) in Fulvic Acid,
Humic Acid and Other Ligands
Humic substances (fulvic and humic acids), autochthonous fulvic acids of
phytoplankton origin (see chapters
“
Dissolved Organic Matter in Natural Waters
” and
“
Fluorescent Dissolved Organic Matter in Natural Waters
”
for detailed discussion),
polysaccharides, proteins, peptides, nucleic acids, extracellular polymeric substances
and amino acids, which have properties of polyfunctionality, polyelectrolyticy, size
polydispersity, physical heterogeneity and structural lability, are natural organic ligands
with binding sites (or functional groups) that can form complexes with trace met-
als (Malcolm
1985
; Mostofa et al.
2009a
,
b
; Zhang et al.
2009
,
2010
; Merroun and
Selenska-Pobell
2008
; Filella
2008
; Mandal et al.
1999
; Xue et al.
1995
; Hatch et al.
2009
). The molecular structure of allochthonous fulvic and humic acids is composed
