Environmental Engineering Reference
In-Depth Information
5.3 Effects of pH
Complexation between DOM and trace elements is highly dependent on pH
(Fig.
5
) (Zhang et al.
2009
,
2010
; da Silva et al.
1998a
,
b
,
2002
; Sonke and Salters
2006
; Shcherbina et al.
2007
; Christoforidis et al.
2010
; Iskrenova-Tchoukova et
al.
2010
; Liu and Cai
2010
; Cao et al.
2004
; Takahashi et al.
1997
; Naka et al.
2006
; Ghassemi and Christman
1968
; Glaus et al.
2000
). Fe can strongly form
complexes with colored humic substances at low pH (Ghassemi and Christman
1968
) and the iron-holding capacity of color increases with pH up to a value of
10, after which it decreases rather abruptly (Shapiro
1964
). The complexation of
Aldrich humic acid with As
3
+
shows that the stability constants for the first bind-
ing site are maximum under acidic conditions (log
10
K
1
=
6.9-7.2 at pH 5.2),
gradually decrease under neutral conditions (6.2-7.1 at pH 7.0) and become
lowest at basic pH (5.8-6.2 at pH 9.3) (Table
1
) (Liu and Cai
2010
). In contrast,
in the case of the second binding sites the constants remain similar or increase
a little, from log
10
K
1
=
4.5-5.0 (mean
=
4.6) at pH 5.2 to log
10
K
1
=
4.7-5.3
(mean
=
4.9) at pH 9.3 (Table
1
) (Liu and Cai
2010
). As
3
+
complexation to
Aldrich humic acid increases with pH (particularly at and above pH 9.3), whereas
the conditional stability constants decrease for the strong binding sites and remain
approximately constant for weak binding sites (Liu and Cai
2010
). The interac-
tion of Cu(II), Ni(II), and Fe(III) with extracted soil fulvic acids results into quite
stable soluble complexes in the acidic pH range from 3 to 6 (da Silva et al.
1998b
,
2002
). The pH effect can change the complexation capacity toward metals. In fact,
the metal order at pH 6 is Pb > Cu > Cd and at pH 7 and 8 it is Cu > Pb >> Cd
(Comte et al.
2008
). The conditional stability constants (
K
) between lanthanide
series (14 elements) and humic substances (standard fulvic and humic acids)
increases with increasing pH in waters (Sonke and Salters
2006
).
The fluorescence intensity of EPS at both peak T- and T
UV
-regions is strongly
dependent on the solution pH in the absence and presence of Hg(II), with the
maximal fluorescence intensity at neutral pH (Fig.
2
) (Zhang et al.
2010
). EPS
shows higher fluorescence intensity at the peak T
UV
-region than at peak T, and
the trend resembles that of a tryptophan standard (Mostofa et al.
2009a
; Zhang
et al.
2010
). The effects of pH on M-DOM complexation in water imply two
things. First, the pH variation (low-pH or high-pH) generates protonation-depro-
tonation phenomena in functional groups of DOM (or of organic ligands) that sub-
sequently alters the complexation capacity between DOM and trace metals. An
increase in pH generally increases the binding capacity between the trace metals
and DOM in aqueous media. Second, the presence of cations (ca. Mg
2
+
) can sig-
nificantly influence the generation of the high-pH forms of functional groups in
DOM, even under circumneutral conditions, which increases M-DOM complexa-
tion (Fig.
9
) (Christoforidis et al.
2010
). For example, at neutral pH the resonance
configuration of the amino-carboxylic group [-CH(NH
2
)-COOH] in tryptophan
can exist in the highest form that can require the lowest energy for the excita-
tion of electrons (Fig.
2
). This effect can result in high fluorescence intensity at
