Dye removal from wastewater using nanostructured chitosan aerogels produced by supercritical CO2 drying

Abstract

Dyes abundance in wastewaters poses environmental threats, and a cost-effective strategy to remediate dyed water is adsorption: this process could be enhanced using nanostructured sorbents, that expose high surface areas. In this work, chitosan aerogel adsorbents were produced by supercritical CO 2 assisted drying, working at 200 bar, 35 °C, and CO 2 mass flow rate of 0.8 kg/h. FESEM images proved that the delicate biopolymeric network was intact thanks to near-zero surface tension at the interface between CO 2 and the liquid, and large supercritical fluid diffusivity. Methyl Orange (MO) and Crystal Violet (CV) were used as model anionic and cationic dyes, respectively. Changing dye initial concentration, chitosan aerogel showed different behavior at low and large driving forces, for both CV and MO. Adsorption experiments proved that MO could be not completely removed by aqueous solutions: whereas, CV removal was successful with removal efficiencies up to 89.0 ± 1.1 % from a 10 ppm solution, using 50 mg of aerogel. FT-IR analysis proved that chitosan active sites were more effectively involved with CV rather than MO. Kinetic analysis, carried out for both dyes, showed that the process follows a pseudo-second order kinetics, related to internal mass transfer resistances and diffusion phenomena. • Chitosan aerogels were produced using supercritical CO 2 drying at 200 bar/35 °C. • 50 mg of 3 % w/w nanostructured chitosan aerogels were used for dye adsorption. • Crystal Violet removal was enhanced with respect to Methyl Orange. • The system dye-aerogel behaves differently at low and large driving forces. • Both dyes follow Freundlich isotherm and a pseudo-second order kinetics.

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