3.2. Role of pH on TCF bio-catalyst surface
Zeta potential (pHpzc) of TCF bio-catalyst was measured to
obtain the surface property. The significance of pHpzc of the photocatalyst
on pollutant removal has been discussed in many previous
works (Chong et al., 2015). In this work, the pHpzc of TCF is 6.2.
In addition, the pKa of chitosan ranges from 6.3 to 7.2 (Gerente et al.,
2007), depending on factors including the degree of acetylation and
ionic strength (Crini and Badot, 2008). The protonated amino group
(-NH3+) on the sorbent in a TCF structure tentatively plays a significant
role in decreasing of pHpzc of TCF into a positive region. The
adsorption of Cr(VI) onto TCF surface across a pH range was followed
electrostatic interaction between the sorbentesorbate systems
as represents by the Cr(VI) adsorption efficiency in Table 1.
Charges of TCF and Cr(VI) are also represented in the same table.
The highest adsorption of TCF is occurred in acidic solution at pH
3. This could be explained that at low pH (acidic solution), most of
the functional groups such as -NH3+, -OH and -CH=N- in the TCF
were protonated and presented in the positively charged form,
while the Cr(VI) existed in the form of HCrO4 or Cr2O72. The
mechanism of the adsorption process of Cr(VI) on TCF in acidic pH
are likely to be the ionic interactions of the negative charge of Cr(VI)
with the amino groups of the chitosan. In aqueous solution at pH 3,
Cr(VI) exists in the form of HCrO4 and the amino groups of chitosan
(R-NH2) became protonated.