ELABORATION OF CHITOSAN/ACTIVATED CARBON COMPOSITES FOR THE REMOVAL OF ORGANIC MICROPOLLUTANTS FROM WATERS

Abstract

Composite hydrogels were prepared by a wet‐casting process by blending a biopolymer, chitosan, with activated carbon (AC) for use in water treatment. Adsorption properties of the composite gels for an organic micro‐pollutant (phenol) which may be encountered in wastewaters was studied with an experimental design approach as a function of:

	the concentration of raw materials and thus the AC weight within the chitosan matrix.
	the accessibility of AC in the polymeric matrix, which is assumed to be related to the coating and thus to the pH of the immersion bath. ESEM observations showed that at a higher pH of gelation (pH = 14), AC particles were entrapped at the surface of the polymer matrix because of a faster gelation kinetic than at a lower pH (13.3).

Adsorption kinetic tests showed that phenol adsorption occurred according to two mechanisms. During the first step, phenol molecules were adsorbed by the AC particles located at the surface. The second step corresponded to a slow diffusion through chitosan chains leading to an adsorption by AC particles entrapped within the polymeric matrix coupled to an adsorption on to the chitosan. A mass transfer model was used to describe this two‐step adsorption phenomenon. However, due to a heterogeneous coating of AC by chitosan, this phenomenon was not supported by experimental design results: the initial kinetic coefficients were associated with a high experimental error which didn't allow for an analysis of the influence of elaboration parameters on kinetic coefficients. Regarding equilibrium adsorption properties, it was shown that composite gels were good adsorbents for phenol with removal ranging from 94% to 98% corresponding to adsorption capacities from 30 to 41 mg g⁻¹. The pH of the immersion bath had no influence on equilibrium adsorption properties, contrary to the AC weight within the chitosan matrix which was demonstrated to influence significantly adsorption capacities. Because carbon particles may improve mechanical properties, mechanical tests were carried out on the composite gels. For a total amount of dried matter in the composite kept constant, the increase in chitosan content led to an increase in the mechanical properties, because of an increase in the number of interactions between chitosan chains. The influence of sonication during the stirring step, leading to a better homogenisation of AC particules within the polymer matrix, was also examined.

Keywords:

• Chitosan
• active carbon
• composite hydrogel
• adsorption
• mechanical properties

ACKNOWLEDGEMENTS

The authors would like to thank Jean Michel Lucas (UMR CNRS 5223) and Serge Lagerge (Institut Gerhardt, UMR CNRS 5253) for BET surface area determination and Mickaël Charpentier (graduate student) for his help in the adsorption experiments.