2.4 Result and Discussion
35
loss was observed. The last third degradation stage at 500 °C recorded about 10%
weight loss. However, this weight loss in different decomposition stages corresponds
to the decomposition of new acetylated and deacetylated units, surface water elimination and depolymerization [27, 28]. For grafted cross-linked chitosan (G/CRCS), decomposition started at 51 °C and 10% weight loss occurs as the temperature increases to 150 °C during the first stage of degradation. 24% weight loss was
observed in the 2nd stage which started at 280 °C up till 398 °C the third stage which
occurs above 500 °C recorded 15% weight loss. Just as previously observed, the
loss of weight in the different stages of G/CR-CT degradation corresponds to the
decomposition of new acetylated and deacetylated units, surface water elimination
and depolymerization [27, 28]. Therefore, the result obtained from TGA analysis
indicates increase in thermal stability after grafting or cross-linking the chitosan.
2.5 Application of Modified Chitosan for Metal Ions
Adsorption
2.5.1 Effect of Solution pH on Metal Ions Adsorption
The solution pH affects adsorption process because it controls the surface charge of
the adsorbent, the degree of ionization of the adsorbate in solution and the dissociation of various functional groups on the active sites of the adsorbent (chitosan) [29].
Hence, different concentrations of metal ions bind on chitosan surface at different pH
value as shown in Fig. 2.7a. At lower pH value of 1–3, the adsorption capacity was low
because various metal ions present in the medium competes with excessive amounts
of hydrogen ions for the available adsorption sites, with an apparent preponderance
towards H + ion uptake. However, as the pH value increases, deprotonation of the
binding adsorption sites increases as competition between hydrogen ions with metal
ions decreases; hence, adsorption of metal ions increases. This increase in adsorption process with decreasing H + ion concentration (relatively at high pH values) is
an indication that ion exchange is an important parameter controlling adsorption
processes [30]. Thus, at pH values of 5, 5, 6, 6 and 7, optimal removal efficiency of
91, 95, 93 and 97% was recorded for Cu(II), Pb(II), Zn(II) and Ni(II), respectively.
The result obtained is consistent with previous studies such as those by [4, 24, 31],
where it was reported that heavy metal ions compete with hydrogen ions in solutions
at a low pH value.
2.5.2 Effect of Contact Time
Contact time is one of the major parameters used for the evaluation of the kinetics
of the adsorption process. The removal of metal ions from aqueous solution depends
35
loss was observed. The last third degradation stage at 500 °C recorded about 10%
weight loss. However, this weight loss in different decomposition stages corresponds
to the decomposition of new acetylated and deacetylated units, surface water elimination and depolymerization [27, 28]. For grafted cross-linked chitosan (G/CRCS), decomposition started at 51 °C and 10% weight loss occurs as the temperature increases to 150 °C during the first stage of degradation. 24% weight loss was
observed in the 2nd stage which started at 280 °C up till 398 °C the third stage which
occurs above 500 °C recorded 15% weight loss. Just as previously observed, the
loss of weight in the different stages of G/CR-CT degradation corresponds to the
decomposition of new acetylated and deacetylated units, surface water elimination
and depolymerization [27, 28]. Therefore, the result obtained from TGA analysis
indicates increase in thermal stability after grafting or cross-linking the chitosan.
2.5 Application of Modified Chitosan for Metal Ions
Adsorption
2.5.1 Effect of Solution pH on Metal Ions Adsorption
The solution pH affects adsorption process because it controls the surface charge of
the adsorbent, the degree of ionization of the adsorbate in solution and the dissociation of various functional groups on the active sites of the adsorbent (chitosan) [29].
Hence, different concentrations of metal ions bind on chitosan surface at different pH
value as shown in Fig. 2.7a. At lower pH value of 1–3, the adsorption capacity was low
because various metal ions present in the medium competes with excessive amounts
of hydrogen ions for the available adsorption sites, with an apparent preponderance
towards H + ion uptake. However, as the pH value increases, deprotonation of the
binding adsorption sites increases as competition between hydrogen ions with metal
ions decreases; hence, adsorption of metal ions increases. This increase in adsorption process with decreasing H + ion concentration (relatively at high pH values) is
an indication that ion exchange is an important parameter controlling adsorption
processes [30]. Thus, at pH values of 5, 5, 6, 6 and 7, optimal removal efficiency of
91, 95, 93 and 97% was recorded for Cu(II), Pb(II), Zn(II) and Ni(II), respectively.
The result obtained is consistent with previous studies such as those by [4, 24, 31],
where it was reported that heavy metal ions compete with hydrogen ions in solutions
at a low pH value.
2.5.2 Effect of Contact Time
Contact time is one of the major parameters used for the evaluation of the kinetics
of the adsorption process. The removal of metal ions from aqueous solution depends