pH effects on the uptake
of RhB onto RH was investigated
and the highest percentage RhB uptake
was obtained at pH 3 (Fig. 4).
Rapid adsorption was observed between pH 2 and 3 while percentage adsorption decreased gradually above pH 3.
RhB with pka of 3.7 may exists as cationic, lactonic or zwitterionic form depending on the
solution media.
Zwitterionic form of RhB dominates at pH greater than that of RhB pka hence attraction between the xanthene and the carboxyl groups of RhB monomers occurs resulting into RhB dimers and subsequently a decrease in RhB adsorption [33].
Optimum adsorption at pH of 3 has been previously reported by researchers [34].
Subsequent adsorption studies were carried out at the optimum pH.