Environmental Engineering Reference
In-Depth Information
4.24-4.63 (log
10
K
1
) for Cd
3
+
-HA; 4.27 (log
10
K
1
) for UO
2
2
+
-HA; 3.3-3.7 (log
10
K
1
) for Np
5
+
-HA; and 6.22 (log
10
K
1
) for Cm
3
+
-HA in aqueous solution except
lanthanides (Table
1
) (Liu and Cai
2010
; Cao et al.
2004
; Antunes et al.
2007
;
Tipping
1994
; Buckau et al.
1992
; Kim et al.
1991
). Lanthanides (Ln) are strong
complexing agents for humic acid, showing relatively high conditional stability
constants that range from 10.34 to 16.50 for Ln-LHA (Leonardite coal humic acid
standard) and from 12.17 to 16.22 for Ln-EHA (Elliot soil humic acid standard) at
pH 7-9 in aqueous media (Table
1
) (Sonke and Salters
2006
). The stability con-
stants of the standard Aldrich humic acid show significantly low values (2.65-2.75
at 5 mg L
−
1
) for lanthanides. These values are greatly increased with increasing
the humic acid concentration (e.g. La ranges from 2.65 to 3.85 for humic acid
concentrations of 5 and 20 mg L
−
1
, respectively) for each lanthanide (Pourret and
Martinez
2009
). For the concentration of 20 mg L
−
1
of the standard Aldrich humic
acid, the stability constants increase from La (3.85) to Eu (4.15) and then decrease
from Gd (4.06) to Lu (3.95).
The stability constants of the M-HA complexes reached high values (log
10
K
Pb
=
5.81, log
10
K
Cu
=
5.55, log
10
K
Cd
=
4.63 and log
10
K
Zn
=
4.43) for the
peak C (Ex/Em
=
440/510 nm) of humic acid extracted from the corresponding
soil (nonamended and control) compared to those of sewage sludge (4.95, 4.65,
4.24 and 4.08, respectively) and to soil amended with sewage sludge at a rate of
40 tons ha
−
1
(5.53, 5.36, 4.47 and 4.31, respectively), for metal ions such as Pb
2
+
,
Cu
2
+
, Cd
2
+
, and Zn
2
+
in aqueous solution at pH 8.1 (Table
1
) (Plaza et al.
2006
).
The overall stability constant for M-HA complexes can follow the order Pb(II)
> Cu(II) > Cd(II) > Zn(II) (Plaza et al.
2006
). These results suggest that the fluo-
rophores or functional groups of humic acid of different origin are highly vari-
able and may affect the metal-DOM complexation and transport in soil and natural
water ecosystems.
The conditional stability constants for complexation between humic acid and
alkaline earth metal ions follow the order Be > Ca > Sr, Ba humate. It can be
hypothesized that the humates of alkaline earth metal ions with smaller ionic radii
are more stable, and that humic acid is a 'hard' ligand (Takahashi et al.
1997
).
Correspondingly, Sc and heavy lanthanide elements (Yb and Lu) can form more
stable complexes than light lanthanide elements (Ce and Eu), depending on the
ionic radii (Takahashi et al.
1997
). In addition, humic acid complexes of trivalent
Fe and Ga are more stable than those of rare earth elements (REEs) except for
Sc, while the Cr(III)-humic acid complex is less stable than REE-humic acid com-
plexes (Takahashi et al.
1997
). It has also been shown that the conditional stability
constant (log
10
K
) between lanthanides and humic substances (fulvic and humic
acids) of varied sources significantly increases from La to Lu and increases with
increasing pH and decreasing ionic strength (
I
) of the solution (Fig.
5
a-c) (Sonke
and Salters
2006
). The log
K
values also significantly increase with decreasing
ionic radius (Fig.
6
) (Sonke and Salters
2006
). The strength of M-DOM compl-
exation between lanthanides and humic substances (fulvic and humic acids) thus
follows the order Lu > Yb > Tm > Er > Ho > Dy > Tb > Gd > Eu > Sm > Pm >
Nd > Pr > Ce > La).
