| dc.description.abstract | The utilisation of South African produced chitosan membranes for the removal of
heavy metal ions from contaminated water was explored.
South African produced chitosan was used to manufacture membranes for the
adsorption of heavy metals by phase inversion. The optimum process parameters
were determined as 7 mass% chitosan in 5 mass% acetic acid solution in the
dissolution stage, a 4 mass% sodium hydroxide solution in the precipitation stage,
and a crosslinking time of 6 hours in the stabilisation stage. The adsorption
properties of the membranes were studied for the transition metal ion Zn(ll), and
followed a Langmuir isotherm. The maximum adsorption capacity determined, was
135 mg.g-1 dry chitosan, at a temperature of 303-313 K, the affinity parameter
increased according to temperature from 0.016-0.020 L.mg-1. The adsorption
characteristics were influenced by temperature, co-ions, membrane thickness, and
pH.
Chitosan membranes contain only 4-6% chitosan and can be visualised as hydrated
polymeric network, in which the chitosan forms a rigid honeycomb structure. The
water in the membrane is present as fixed water, that is integrated with the chitosan,
and free water, that can be removed from the membrane by applying a pressure
difference. The free water content equals the porosity of the membrane. The
physical properties of the chitosan membranes are: a wet density of 1100 kg.m-3; a
chitosan content of 5.2 mass%; a free water volume of 65 mass%; a fixed water of
30 mass%; a maximum pore radius of 40 nm; and a total surface area of 1.15-1-105
m2.kg-1.
The transport through chitosan membranes can be described analogous to
ultrafiltration membranes. The clean water flux of the membranes is in the order of
12 L.m-2.hr-1.bar-1, and the transport of solute and solvent could well be modelled, at
low solute concentrations, with a generic membrane model derived from irreversible
thermodynamics. Concentration polarisation occurred at high zinc concentrations
and here the transport model deviates from experiments.
Recoveries obtained with zinc were up to 90% from the loaded membrane. The
membranes were stable for 2 regeneration cycles.
In comparison with other adsorbents chitosan formulations, the SA produced
chitosan membranes have good adsorption characteristics and a good possibility of recovery of the zinc from the loaded membrane. However, it was also found that the
long term stability of the membranes still has to be improved. | |
