3.2.2. Elution study of dye-adsorbed chitosan under
caustic conditions
In addition, the hydroxyl group of chitosan might be
attached with the reactive dye by covalent bonding under
caustic pH, as in the case of cellulose in dyeing processes
(Fig. 5). Therefore, water should not be able to elute SRDW
from chitosan polymer. However, Table 4 shows that water
could elute SRDW from chitosan, after SRDW adsorption
under caustic conditions, to 33%, while 1 M NaOH
could elute only 14%. Not only were covalent linkages involved
in the adsorption mechanism, but also van der Waals
forces occurred. This explanation follows the dyeing theory,
which mentions the ionic forces, hydrogen bonds, van
der Waals forces, and covalent linkages involved in the
dyeing process [13]. Van der Waals forces are weak bond-ings of physical adsorption. They occur between azo groups
(–N=N–) of reactive dye and hydroxyl groups (–OH) of chitosan.
Therefore, water and NaOH could destroy some of
the bonding and elute the dye out. However, covalent linkages
are actual chemical bonds between dyes and hydroxyl
groups of chitosan, resulting in the total elution of only 47%.
The results imply that the mechanism of SRDW adsorption
by chitosan under caustic conditions involved both chemical
and physical adsorption.