The use of chitosan beads for the adsorption and regeneration of heavy metals
Abstract
This work studied the removal of heavy metals from wastewater through the use of South
African chitosan beads produced from locally available raw materials. For this purpose,
chitosan beads were prepared from chitosan flakes that were synthesized from the chitin
derived from the exoskeleton of the Jasus lalandii. The molecular weight and degree of
deacetylation of the chitosan flakes were 9.4-10 4 g/mol and 83% respectively. When the flakes
were converted into non-cross-linked beads, the molecular weight decreased slightly to 7.8-10 4
g/mol. Different beads were prepared ranging in size from 0.9 to 3.8 mm and the amount of
glutaraldehyde used to crosslink the beads was varied between 0 and 4 vol%, in order to obtain
beads with a different degree of cross-linking.
The beads were used as an adsorbent for heavy metals and were characterized for equilibrium
and kinetic adsorption studies. The mine concentration, which is in direct relation to the
adsorption capacity of non-cross-linked beads was determined as 4.9 mmol/g. The amine
concentration decreased with an increasing glutaraldehyde concentration and a decreasing bead
size. Cross-linking was however necessary to make the chitosan stable in acidic media, and a
degree of cross-linking larger than 18% made the chitosan beads insoluble at a pH of 2.
Two models, the Langmuir isotherm model and a pH-model were used to fit equilibrium
adsorption data. Although the Langmuir model gave good fits, the obtained parameters were
pH dependent. On the other hand, the pH-model, which was derived from: i) the adsorption
equilibrium reaction between the chitosan and the metal; ii) the acid base properties of
chitosan; and iii), a mass balance of the different forms of nitrogen in the chitosan, could
satisfactory describe the adsorption using pH independent variables. When deriving the pH-model
the effect of pH on the degree of protonation of the adsorbent was considered. The
model was fitted with the maximum adsorption capacity, and the fitted values were in close
agreement with the amine concentration. The desorption of the metal from the chitosan could
also be predicted well with this model, indicating a reversible complexation of the metal on the
chitosan, making the recovery and possible re-use of the metal possible.
The kinetics of the adsorption process were described with a shrinking core model, where an
instantaneous adsorption reaction was assumed. From this model, effective diffusion
coefficients were determined from batch experiments.
The adsorption was also studied in a column and the experiments were modelled with a CSTR's
in series model, using the experimentally determined adsorption equilibrium data. The
breakthrough curve could be described reasonably well with this model, and the fitted effective
diffusion coefficient was close to the one determined in the batch experiments. The adsorption
capacity of the locally sourced and produced chitosan beads was high in comparison to the
values indicated in the literature for other adsorbents. It was also found to be higher than that
of either the commercially produced chitosan or the ion-exchange resin. The regeneration of
the metal from the chitosan was effective. Multiple adsorption/desorption experiments were
also carried out, and it was found that the adsorption increased for the second and third cycle,
but decreased for the fourth and fifth ones. After the fifth cycle, the chitosan was physically
damaged and could not been used anymore. This degeneration of the beads across multiple
adsorption/desorption cycles was found to be the major concern blocking the uptake of the
studied chitosan beads in industrial applications.
Collections
- Engineering [1395]