3.1. Fourier transform infra red analysis
The FTIR spectra of the yields un-extracted and extracted dyes
of mangosteen shell are shown in Fig. 2a and b, respectively. It
was clear that the carboxylate ions gave rise a strong asymmetrical
stretching band at 1616cm−1 and a weaker symmetrical
stretching band at 1419cm−1, respectively. The double bands of
the carboxylate ion were identical with the observations made
on yields extracted of mangosteen shell (Fig. 2b). The bands at
1225cm−1 (Fig. 2a) and 1227cm−1 (Fig. 2b) were due to the –C–O
stretching of ether groups; the bands at 1052cm−1 (Fig. 2a) and
1054cm−1 (Fig. 2b) were assigned to the –C–O stretching of alcoholic
groups. An interesting phenomenon was the sharp decrease
in the band intensity of the ether groups after metal binding for
both algal materials. From the changes of band areas, it was reasonable
to assume that most of the ester groups had been converted
to carboxylic and alcoholic groups. The FTIR spectroscopic analysis
indicated broad bands at 3437–3511cm−1, representing bonded
–OHand–NHgroups. The bands observed at about 2930cm−1 could
be assigned to the–CHstretch. The spectral analysis before and after
dyes extracted indicated that the –NH was also involved in metal
biosorption. There were clear band shifts and intensity decrease
of the –NH band at 1516–1535cm−1. The change in the intensity
of the bands at 3370–3410cm−1 also suggested changes in the
amino groups present in the mangosteen shells. The bands at about
1250cm−1, representing –SO3 stretching, could be observed in the
FTIR spectrum of both un-extracted and extracted dyes mangosteen
shells. Since these peaks presented approximately the same frequency
before and after metal binding, it is likely that –SO3,mainly
present in sulfonic acids of polysaccharides, such as fucoidan.