Chitosan membranes for the removal of zinc from simulated wastewater

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.