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

Abstract

Background

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.

Methods

In this work, chitosan aerogel adsorbents were produced by supercritical CO₂ assisted drying, working at 200 bar, 35 °C, and CO₂ 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 CO₂ and the liquid, and large supercritical fluid diffusivity. Methyl Orange (MO) and Crystal Violet (CV) were used as model anionic and cationic dyes, respectively.

Significant findings

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.