Mechanism of arsenic removal using chitosan and nanochitosan

• An initial short arsenate adsorption followed by desorption at low pH was observed.
• A novel pseudo-first order reversible model to describe the process was developed.
• Effect of pH on the equilibrium and kinetic studies on adsorption was examined.
• The speciation of arsenate and protonation of chitosan at various pH were studied.
• Both interactive effects revealed the adsorption/desorption mechanism.

Chitosan, a natural polysaccharide copolymer of glucosamine and N-acetyl-glucosamine, possesses one free primary amine and two free hydroxyl groups on each glucosamine unit. It has a polycationic nature and an abundance of amine functional groups. The sorption equilibrium and kinetics of arsenate onto chitosan flakes have been studied. The effect of pH on the adsorption capacity and the uptake kinetics is an important parameter to investigate the adsorption mechanism of anionic species such as arsenate ions on the protonated amine groups of chitosan. The equilibrium sorption and batch kinetic studies of arsenate ions on chitosan were performed at initial As concentration of 250–11,000 μg L−1 and initial pH ranging from pH = 3.50–5.50. The experimental results showed that initially for approximately the first 30 min there is a rapid and high adsorption of arsenate ions onto the chitosan leading to a maximum uptake capacity after this short time. However, this stage is followed by a slow desorption of arsenate from the chitosan with a steady increase in solution pH. A novel reversible pseudo-first order kinetic model was developed and applied to correlate this newly reported adsorption–desorption phenomenon. The physical and chemical properties of chitosan were studied and presented in terms of its surface and structural properties such as the degree of deacetylation, crystallinity, surface charge and its swelling properties.

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