annual report annual report annual report annual report 2005

66
RADIOCHEMISTRY, STABLE ISOTOPES,
NUCLEAR ANALYTICAL METHODS, GENERAL CHEMISTRY
of chemical and physical interactions between the
heavy metals present in solution and functional
groups of the biosorbent: carboxylic, phosphate,
sulfate, ammino, amidic or hydroxylic [10-12].
Divalent cation binding by calcium alginates was
investigated using a number of techniques. Potentiometric
titration revealed that CA exhibits two
distinct pK a
values: (i) similar to that characteristic
of carboxylic groups, and (ii) comparable to that
of saturated alcohols. Esterification of the biosorbent
species resulted in a significant reduction in
metal sorption (up to 10 times), indicating that
mainly carboxylic groups are responsible for the
sorption. Also the ion-selective electrode (ISE)
studies suggest that sorption of cadmium is mainly
due to an ion exchange mechanism [13]. X-ray photoelectron
spectroscopy (XPS) and the infrared
(FTIR) results indicated that both alcoholic (-OH)
and carboxylic (-COO – ) functional groups present
in the carbohydrate moieties play an important role
in the metal removal from aqueous solutions [14].
Because of the difficulties in direct X-ray investigations
of the carbohydrate derivatives, FTIR
absorption spectra of selected M(II)-alginate complexes
and CA bed (Fig.1) were recorded as a continuation
of our studies [15,16] on species formed
by alginic biosorbent with different metal cations.
Vibrational spectroscopy is the most widely used
technique for studying natural products, being fast,
non-destructive, and demanding small sample
amounts. So, the aim of the presented work was to
continue our studies on transition metal complexes
with biosorbents on the alginate origin.
Main vibrational modes: IR spectra of the investigated
species, especially in the fingerprint region,
are rather complex and reflect the complex nature
of the biomass. Analysis of the main features of the
spectra shows that:
- In the fingerprint spectral window all spectra
exhibit the absorbance bands at approximately
1605, 1420, 1085, 1030, 890 and 820 cm –1 . Proposed
assignment of the bands forming the fingerprint
spectral window has been revised since
publication of the previous paper [16] and is presented
in Table 1.
- The difference between the spectra of CA and
M(II)-alginate is mainly in their absorbance intensities.
While the band intensities of the metal-
-loaded CA in the region of the symmetric carboxylate
stretching mode (1450-1300 cm –1 ) are
significantly higher than that of CA, in the asymmetric
carboxylate stretching modes (about 1610
cm –1 ) a slight decrease in the intensity can be
observed for the transition metal cations upon
the exchange of the calcium cation. The absorbance
bands in pure alginic acid are 1740 and
1240 cm –1 .
- The distance between the ν sym and ν asym absorbance
bands for CA is significantly smaller than
those for M(II)-alginate. It can be related to
the higher symmetry in CA in respect to other
M(II)-alginates, which occurred due to complexation
with metal cation.
- The ν asym (COO) band of the M(II)-alginates are
narrower and sharper than that in the CA spec-
trum. It could be linked with orthodox coordination
spheres, wherein both oxygen-M(II)
bonds have a very similar strength.
- Lack of displacement of the bands related to
vibrations associated with the C-O (both alcoholic
and ether groups) at 1085 and 1030 cm –1 is
observed when calcium cation is replaced by another
divalent cation. Such displacement could
result from different coordination strengths of
the metal and calcium cations to both alcoholic
or ether groups.
- Broad absorption peaks in the region of 3250-3500
cm –1 indicate the existence of hydroxyl groups
involved in H-bond network. The peak observed
at 2930 cm –1 can be assigned as vibration of the
CH group [17,18].
Table 2. Dependence of the ν(COO) sym and ν(COO) asym
peak positions for the Mn(II)-CA on the equilibrium
pH.
The dependence of the main peaks positions
on the equilibrium pH has been analyzed for all
the cations studied. The results are as follows:
- ν(COO) sym
and ν(COO) asym
: Table 2 and Fig.2
present the data obtained for the Mn(II)-alginate.
The complexation mode does not change
within the whole pH range studied [19]. All other
cations behave similarly – neither position nor
shape of both main peaks describing the complexation
mode depends on the solution acidity.
So, the results obtained for other cations are
not shown neither in Table 2, nor in Fig.2.
Fig.2. Spectra of the CA loaded with manganese(II) of
different pH.
- The significant decrease in the intensity of the
3429 cm –1 peak may be explained in terms of
decreasing number of H-bonds, caused by the
dissociation of the hydroxyl groups present in
the uronic moieties.
Because the carboxylate group in the alginate
resin plays an important role in binding metal ions,
the percentage of ionic bonding (PIB) was introduced
to characterize qualitatively the cation-anion
bond formed by the biosorbents [14]: